A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels.

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A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels Jinli Qiao,b Yuyu Liu,*c Feng Hong*a and Jiujun Zhang*de This paper reviews recent progress made in identifying electrocatalysts for carbon dioxide (CO2) reduction to produce low-carbon fuels, including CO, HCOOH/HCOO, CH2O, CH4, H2C2O4/HC2O4, C2H4, CH3OH, CH3CH2OH and others. The electrocatalysts are classified into several categories, including metals, metal alloys, metal oxides, metal complexes, polymers/clusters, enzymes and organic molecules. The catalyts’ activity, product selectivity, Faradaic efficiency, catalytic stability and reduction mechanisms during CO2 electroreduction have received detailed treatment. In particular, we review the effects of electrode potential, solution–electrolyte

Received 9th September 2013

type and composition, temperature, pressure, and other conditions on these catalyst properties. The challenges

DOI: 10.1039/c3cs60323g

in achieving highly active and stable CO2 reduction electrocatalysts are analyzed, and several research directions for practical applications are proposed, with the aim of mitigating performance degradation, overcoming addi-

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tional challenges, and facilitating research and development in this area.

1. Introduction Carbon dioxide (CO2) is the most notorious greenhouse gas, released by both natural and artificial processes. It is also a necessary material for the growth of all earth’s plants and for many industrial processes.1–4 In an ideal scenario, the CO2 produced on Earth should be balanced with what is consumed, so that the level of CO2 remains constant to maintain environmental stability. Unfortunately, with the intensification of human industrial activities, this balance has gradually been disrupted, leading to more CO2 production and making global warming a pressing issue. Therefore, reducing CO2 production and converting CO2 into useful materials seems to be necessary, indeed critical, for environmental protection, and various governments worldwide have signaled their concern by increasing their investment in research to address the CO2 issue. The different proposed technologies follow one of two major approaches: to capture and geologically sequestrate CO2, or to convert CO2 into useful low-carbon fuels.5–8 In today’s world of a

College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, 2999 Ren’min North Road, Shanghai 201620, P. R. China. E-mail: f [email protected]; Tel: +86-21-67792649 b College of Environmental Science and Engineering, Donghua University, 2999 Ren’min North Road, Shanghai 201620, P. R. China c Multidisciplinary Research on the Circulation of Waste Resources, Graduate School of Environmental Studies, Tohoku University, Aramaki, aza Aoba 6-6-11, Aoba-ku, Sendai 980-8579, Japan. E-mail: [email protected]; Tel: +81-90-6008-9342 d Research Institute of Donghua University, Shanghai 201620, P. R. China e NRC Energy, Mining & Environment, National Research Council of Canada, 4250 Wesbrook Mall, Vancouver, B.C. V6T 1W5, Canada. E-mail: [email protected]; Fax: +1 604 221 3001; Tel: +1 604 221 3087

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high energy demands, CO2 conversion and utilization seems to be a more attractive and promising solution. Normally, CO2 conversion can be achieved by chemical methods,9–16 by photocatalytic and electrocatalytic reduction,17–25 and by a few other means.26–28 However, at the present time, certain barriers still hinder the practical application of CO2 capture, conversion, and utilization. These barriers include (1) the high costs of CO2 capture, separation, purification, and transportation to user sites; (2) the high energy requirements for CO2 chemical/electrochemical conversion; (3) limitations in market size and investment incentives; (4) lack of industrial commitment to enhance CO2-based chemicals; and (5) insufficient socio-economic driving forces.4,29 Despite such challenges, CO2 capture, conversion, and utilization is still recognized as a feasible and promising cutting-edge area of exploration in energy and environmental research. In recent years, CO2 conversion using electrochemical catalysis approaches has attracted great attention for its several advantages: (1) the process is controllable by electrode potentials and reaction temperature; (2) the supporting electrolytes can be fully recycled so that the overall chemical consumption can be minimized to simply water or wastewater; (3) the electricity used to drive the process can be obtained without generating any new CO2—sources include solar, wind, hydroelectric, geothermal, tidal, and thermoelectric processes; and (4) the electrochemical reaction systems are compact, modular, on-demand, and easy for scale-up applications.8 However, challenges remain, such as the slow kinetics of CO2 electroreduction, even when electrocatalysts and high electrode reduction potential are applied; the low energy efficiency of the process, due to the parasitic or decomposition reaction of the solvent at high reduction potential; and high energy consumption. Researchers

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have recognized that the biggest challenge in CO2 electroreduction is low performance of the electrocatalysts (i.e., low catalytic activity and insufficient stability). Electrochemical reduction of CO2 can proceed through two-, four-, six-, and eight-electron reduction pathways in gaseous, aqueous, and non-aqueous phases at both low and high temperatures. The major reduction products are carbon monoxide (CO), formic acid (HCOOH) or formate (HCOO) in basic solution, oxalic acid (H2C2O4) or oxalate (C2O42 in basic solution), formaldehyde (CH2O), methanol (CH3OH), methane (CH4), ethylene

(CH2CH2), ethanol (CH3CH2OH), as well as others. The thermodynamic electrochemical half-reactions of CO2 reduction and their associated standard electrode potentials are listed in Table 1.30 Note that the reactions listed in Table 1 are thermodynamic, only indicating each reaction’s tendency and possibility but giving no indication of the reaction’s kinetics, such as rate and mechanism. In addition, the standard potentials listed in Table 1 are for aqueous solutions only; the potential values in nonaqueous solutions are different from those listed in Table 1.31 The kinetics of CO2 electroreduction involve very complicated reaction

Dr Jinli Qiao is a Professor, PhD Supervisor and Scientific CoreCompetency Leader at Donghua University, China. She received her PhD in Electrochemistry from Yamaguchi University, Japan, in 2004 before joining the National Institute of Advanced Industrial Science and Technology (AIST), Japan, as a research scientist. From 2004–2008, she carried out 7 fuel cell projects including two NEDO projects of Jinli Qiao Japan on the development of novel proton-conducting membranes, new binders for MEA fabrication, and non-platinum catalysts. Since March 2008, she has carried out a total of 8 projects funded by Chinese Government including the National Natural Science Foundation of China. Dr Qiao has more than 20 years of scientific research experience, particularly in the area of electrochemical material development and energy storage and conversion.

Dr Yuyu Liu is an Associate Professor at the Graduate School of Environmental Studies, Tohoku University, Japan. Dr Liu received his PhD in Environmental Engineering from Yamaguchi University, Japan, in 2003. He then worked at Kyushu Environmental Evaluation Association, Osaka Institute of Technology, Tokyo University of Agriculture and Technology, and Yokohama National University as PostYuyu Liu doctoral and Research Fellow. Dr Liu has more than 10 years of experience in the environment science and technology, particularly in the areas of air quality monitoring, water and soil research, and their associated instrument development. He is also a member of the Japan Society on Water Environment and Japan Society of Material Cycles and Waste Management.

Dr Feng Hong is a Professor, PhD Supervisor and Scientific competency Leader at the College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, China. He received his PhD in engineering from Nanjing Forestry University in 1998. He then went to Sweden for his postdoctoral research at the Lund Institute of Technology/ Lund University, and then joined Karlstad University as a senior Feng Hong research scientist. His research interests include biomaterials, electrochemical energy materials and fuel cells. Prof. Hong is a research proposal referee for several programs hosted by the National Natural Science Foundation of China, and International Cooperation Funding of the Ministry of Science and Technology of China.

Dr Jiujun Zhang is a Principal Research Officer and Technical Core Competency Leader at the National Research Council of Canada Energy, Mining & Environment Portfolio (NRC-EME). Dr Zhang received his BS and MSc in electrochemistry from Peking University in 1982 and 1985, respectively, and his PhD in electrochemistry from Wuhan University in 1988. He then carried out three terms of Jiujun Zhang postdoctoral research at the California Institute of Technology, York University, and the University of British Columbia. Dr Zhang has over 30 years of scientific research experience, particularly in the area of electrochemical energy storage and conversion. He is also the Adjunct Professor at the University of British Columbia, the University of Waterloo, Peking University, and Donghua University.

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Table 1 Selected standard potentials of CO2 in aqueous solutions (V vs. SHE) at 1.0 atm and 25 1C, calculated according to the standard Gibbs energies of the reactants in reactions. Reprinted with permission from ref. 30. Copyright r 1985 CRC Press

Half-electrochemical thermodynamic reactions

Electrode potentials (V vs. SHE) under standard conditions

CO2(g) + 4H+ + 4e = C(s) + 2H2O(l) CO2(g) + 2H2O(l) + 4e = C(s) + 4OH CO2(g) + 2H+ + 2e = HCOOH(l) CO2(g) + 2H2O(l) + 2e = HCOO(aq) + OH CO2(g) + 2H+ + 2e = CO(g) + H2O(l) CO2(g) + 2H2O(l) + 2e = CO(g) + 2OH CO2(g) + 4H+ + 4e = CH2O(l) + H2O(l) CO2(g) + 3H2O(l) + 4e = CH2O(l) + 4OH CO2(g) + 6H+ + 6e = CH3OH(l) + H2O(l) CO2(g) + 5H2O(l) + 6e = CH3OH(l) + 6OH CO2(g) + 8H+ + 8e = CH4(g) + 2H2O (l) CO2(g) + 6H2O(l) + 8e = CH4(g) + 8OH 2CO2(g) + 2H+ + 2e = H2C2O4(aq) 2CO2(g) + 2e = C2O42(aq) 2CO2(g) + 12H+ + 12e = CH2CH2(g) + 4H2O(l) 2CO2(g) + 8H2O(l) + 12e = CH2CH2(g) + 12OH 2CO2(g) + 12H+ + 12e = CH3CH2OH(l) + 3H2O(l) 2CO2(g) + 9H2O(l) + 12e = CH3CH2OH(l) + 12OH

0.210 0.627 0.250 1.078 0.106 0.934 0.070 0.898 0.016 0.812 0.169 0.659 0.500 0.590 0.064 0.764 0.084 0.744

mechanisms, and the reaction rates are very slow, even in the presence of electrocatalysts. In general, the catalysts currently being employed are still not active enough. Furthermore, in some cases, the product of the electroreduction is not a single species but a mixed product containing several component species (for example, it could be a mixture of C, CO, HCOOH, H2C2O4, CH2O, CH3OH, CH4, CH2CH2, CH3CH2OH, and so on). The number of species and the amount of each species that is present are factors strongly dependent on the kind and selectivity of the electrocatalyst employed and what electrode potential is applied. This suggests that the currently employed electrocatalysts have insufficient catalytic selectivity and stability. In most cases, these catalysts can survive for fewer than 100 hours,32–34 which is far below the requirements for practical use and technological commercialization. Therefore, unsatisfactory catalysis, including low catalytic activity, selectivity, and stability, is the biggest challenge in CO2 electroreduction. In the past several decades, almost all the efforts in CO2 electroreduction studies have been focused on research and development of electrocatalysts to overcome the above challenges.35,36 Hence, while several review articles related to CO2 reduction have been published,35–37 a comprehensive review specifically focusing on electrocatalysts for CO2 electroreduction is definitely necessary to facilitate research and development in this area.

compounds, such as metal complexes. This is probably because these metals have vacant orbits and active d electrons, which are believed to be able to energetically facilitate the bonding between the metal and the CO2 for adduct formation and also facilitate the desorption of the reduction product(s), as discussed below. 2.1

Transition metals and related electrocatalysts

2.1.1 Titanium. Normally, titanium (Ti) on its own has no significant catalytic activity towards CO2 electroreduction.38 However, TiO2 has shown some activity in both photocatalysis and electrocatalysis for CO2 electroreduction.25,39 When TiO2 is used as an electrocatalyst, thin TiO2 films or mixtures of TiO2 and other metal oxides are usually deposited on a Ti electrode substrate to catalyze CO2 electroreduction. For example, in the preparation of a CO2 catalytic electrode, Monnier et al.40,41 prepared TiO2, TiO2–Ru (or RuO2), and TiO2–Pt thin film electrodes by thermal deposition on titanium rods. Bandi42 deposited metallic oxide mixtures (including RuO2, TiO2, MoO2, Co3O4, and Rh2O3) on titanium foil. Cueto and Hirata43 prepared a TiO2–indium-tin oxide (ITO) thin-film glass electrode via a fixed-potential bulk electrolysis process, in which 1-butyl-3methylimidazolium tetrafluoroborate (BMImBF4, an ionic liquid) was used as both solvent and supporting electrolyte. Several material characterization techniques were employed to analyze the coated electrode. X-ray diffraction (XRD) results showed that TiO2 films were suitable candidates for electrocatalytic processes since no phase changes could be observed (Fig. 1). XPS measurements revealed a significant dissociation of CO2 into chemisorbed CO32. Furthermore, observation using Auger electron spectroscopy (AES) indicated strong interactions between TiO2 and CO2 or CO32 and revealed a decrease in carbon content after the CO2 electroreduction process (Fig. 2). Recently, electrocatalytic synthesis of low-density polyethylene (PE) from CO2 on a nanostructured TiO2 (ns-TiO2) film electrode was carried out by

2. Elements and compounds used as electrocatalysts for CO2 electroreduction The most commonly explored electrocatalysts for CO2 electroreduction are transition metal elements and their associated


Fig. 1 XRD pattern of a TiO2–ITO thin film (a) before and (b) after a CO2 bulk electrolysis reduction reaction in BMImBF4. Reprinted with permission from ref. 43. Copyright r 2006 Springer.

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Fig. 2 XPS (C1s, Ti2p, and O1s chemical states) and AES spectrum of a TiO2–ITO thin film: (a) before electrochemical reduction of CO2; (b) CO2 saturated in BMImBF4; and (c) after 19 hours of CO2 bulk electrolysis reduction reaction in BMImBF4. PQRE: Pt(s), Aux: Pt(s).43 Reprinted with permission from ref. 43. Copyright r 2006 Springer.

controlled potential electrolysis in a mixture of H2O and EMImBF4 at room temperature and ambient pressure.44 The ns-TiO2 film appeared to be remarkably efficient and selective for the electrochemical reduction of CO2 when EMImBF4 was the solvent,45 as EMImBF4 maintained a high concentration of CO2 at the electrode surface. According to the mechanism described in reactions (1)–(5), high pressure in the nanopores of the ns-TiO2 film can lead to polymerization of: CH2 to form PE.44 CO2 + TiIII - CO2*(adsorbed Ti

IV

on ns-TiO2 film)

+ e - Ti

+ TiIV

III

(1) (2)

CO2*(ads) + H+ + e - CO(ads) + OH

(3)

CO(ads) + 4H + 4e -: CH2(ads) + H2O

(4)

:CH2(ads) - [CH2CH2]n

(5)

+

Indeed, TiO2 and carbon nanotubes have been explored as catalyst supports in the synthesis of nanostructured electrocatalysts

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for the selective reduction of CO2. Due to the unique structures of these catalysts, some selectivity towards the desired products has been achieved. For example, Qu et al.46 loaded RuO2 onto TiO2 nanotubes (NTs) or nanoparticles (NPs) to form RuO2–TiO2(NTs) or RuO2–TiO2(NPs), which were then coated onto a Pt electrode for the electrocatalytic reduction of CO2. The potentiostatic electrolysis of CO2 on the RuO2–TiO2(NT) coated Pt electrode showed the selective formation of methanol with a current efficiency of up to 65.5%, much better than was achieved on the RuO2–TiO2(NP) coated Pt electrode. 2.1.2 Molybdenum, chromium, and tungsten. Metallic electrodes of Cr, Mo, and W do not appear to show significant activity towards CO2 electroreduction. For example, Noda et al.38 tested electrodes of these metals at 1.6 V vs. Ag/AgCl in KClsaturated 0.1 M KHCO3 aqueous solution at 298 1C, and did not observe significant catalytic activity for the electrochemical reduction of CO2. However, in an earlier study,47 researchers using molybdenum metal electrodes in the electrolysis of


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Fig. 3 Cyclic voltammetry for a Mo electrode in (A) N2 and (B) CO2saturated 0.2 M Na2SO4 aqueous solution (pH 4.2). Electrodes (3.4 cm2) were pretreated with HCl and allowed to remain at open circuit for 1 hour before scanning. Sweep rates were (A) 0.5 to 10 and (B) 0.5 to 6 V min1. CO was formed at open circuit; CO2 reduction (Mo + 2CO2 - MoO2 + 2CO) and molybdenum oxidation (Mo + 2H2O - MoO2 + 4e + 4H+, E = 0.57 V vs. SCE at pH 4.2) occurred simultaneously. Reprinted with permission from ref. 47. Copyright r 1986 Elsevier.

CO2-saturated 0.2 M Na2SO4 solution (pH 4.2) at 20 1C in the potential range of 0.7 to 0.8 V vs. saturated calomel electrode (SCE) observed a Faradaic efficiency above 50%, with the main product being methanol. Even in 0.05 M H2SO4 at 20 1C, methanol was obtained at 0.57 to 0.67 V vs. SCE with a Faradaic efficiency of up to 46%. Moreover, cyclic voltammetric (CV) measurements indicated that the corrosion of molybdenum metal to molybdenum dioxide (MoO2) might be the source of electrons for the electroreduction of CO2 (Fig. 3). In the late 1990s, Mo-containing catalysts were found to be useful in the reduction of chlorate, bromate, and iodide anions.48–52 The electrochemistry of molybdenum and molybdenum oxides has been described in detail.53 Nakazawa et al.54 employed two types of iron–sulfur clusters, [Fe4S4(SR)4]2 (R = C6H5CH2 and (CH3)3C) and [M2Fe6S8(SCH2CH3)9]3 (M = Mo or W), to catalyze the electrochemical reduction of CO2. It was observed that the reduction potential was shifted by about 0.7 and 0.5 V, respectively, in the positive potential direction compared to what occurred without any catalyst, demonstrating the catalytic effect of these two catalysts. Bandi et al.42 thermally decomposed a mixed metal oxide consisting of 25% RuO2, 30% MoO2, and 45% TiO2 on Ti foil to catalyze CO2 electroreduction to methanol, but no striking performance was observed. Regarding Cr-related catalysts, Ogura and Yoshida55 employed (III) Cr –TPPCl (TPP = 5,10,15,20-tetraphenylporphyrin) for CO2 electroreduction in dimethylformamide (DMF) solutions and compared the results with those for Co(II)–TPP, Ni(II)–TPP, Fe(III)–TPPCl, and Fe(II)–TPP. Cr complexes with 4-v-tpy and 6-v-tpy (v-tpy = vinyl-terpyridine) have also been reported for


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CO2 electrocatalysts.56 Sende et al.57 electropolymerized several 4-v-tpy and 6-v-tpy complexes of transition metals (including Cr) onto glassy carbon electrodes (GCEs) for CO2 electroreduction. They found that in CO2-saturated 0.10 M aqueous NaClO4 solution, their electropolymerized [Cr(4-v-tpy)2]2+ film was at 0.86 V vs. Ag/AgCl, which was more positive than for Fe, Ni, Ru, and Os complexes in the potential range of 1.10 to 1.22 V vs. Ag/AgCl; it also yielded a considerable amount of formaldehyde (CH2O), up to 87% at 1.10 V (far higher than the 39% and 28% achieved with Co and Fe complexes, respectively). The turnover number (TON) for catalyzed CO2 electroreduction was up to 6100 for the [Cr(4-v-tpy)2]2+ complex, about 50% lower than for both the [Co(4-v-tpy)2]2+ (11 000) and [Fe(4-v-tpy)2]2+ (15 000) complexes. In all cases, virtually the only reaction product detected was CH2O, demonstrating the catalysts’ high selectivity. It is worth mentioning that Sende et al. also explored one Cr salt (diaquabis(oxalato)-chromate(III) (K[Cr(III)(C2O4)2(H2O)2]) as a homogeneous catalyst for the electrochemical reduction of CO2 as an Everitt’s salt, i.e., KFeII[FeII(CN)6]-modified platinum gauze electrode. Regarding tungsten-related electrocatalysis for CO2 electroreduction, Reda et al.58 obtained a tungsten-containing formate dehydrogenase enzyme (FDH1) from Syntrophobacter fumaroxidans and adsorbed it on a freshly polished, pyrolytic graphite edge electrode to catalyze CO2 reduction. As either a homogeneous or a heterogeneous catalyst, FDH1 catalyzed the electrochemical reduction of CO2 in 20 mM Na2CO3 (pH 6.5) to produce only formate, with a reduction rate more than two orders of magnitude greater than that achieved with other known catalysts for the same reaction. Although the mechanism of CO2 activation and reduction in Cr-containing enzymes such as FDH1 is not

Fig. 4 Proposed mechanism for CO2 electroreduction catalyzed by W-containing formate dehydrogenase enzyme (FDH1), in which two electrons are transferred from the cathode to the active site to reduce CO2 to formate, forming a C–H bond. Conversely, when formate is oxidized, the two electrons are transferred from the active site to the electrode. Reprinted with permission from ref. 58. Copyright r 2008 Proc. Natl. Acad. Sci. U. S. A.

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fully understood, Fig. 4 shows a proposed bioelectrocatalytic CO2 reduction reaction. In this mechanism, CO2 is known to insert into the W–H bond in [HW(CO)5] to form a stable formate adduct. This W–H insertion reaction is also seen in homogeneous organometallic catalysis.59 2.1.3 Rhenium and manganese. Normally, rhenium complexes have quite promising catalytic activity. In the present paper’s context, tricarbonyl rhenium(I) complexes are a large group of catalysts for catalyzing the photochemical and electrochemical reduction of CO2 to CO.60–62 Lehn et al.61 studied the homogeneous catalysis of CO2 electroreduction using 9.0 104 M Re(I)(bpy)(CO)3Cl (namely, Lehn catalyst; bpy = 2,2 0 -bipyridine) in 0.1 M (CH3CH2)4NCl DMF–H2O (9 : 1) solutions at 25 1C on a GCE with a potential at 1.25 V vs. normal hydrogen electrode (NHE), which was much less negative than 2.0 V in the absence of the catalyst. With the catalyst, the current efficiency reached 91%. The catalyst CV showed a reversible one-electron reduction process at 1.25 V vs. NHE, forming [Re(0)(bpy)(CO)3Cl]. O’Toole et al.63 electropolymerized another kind of tricarbonyl rhenium complex, Re(CO)3(vbpy)Cl (vbpy = 4-vinyl-4 0 -methyl-2,2 0 -bipyridine), on a Pt electrode to form a polymeric film that was used to heterogeneously catalyze the electrochemical reduction of CO2 to CO. Results indicated that the heterogeneous catalyst could enhance the TON 20–30 times more than was observed in the homogeneous case with Re(CO)3(bpy)Cl catalyst. When the Re(CO)3(vbpy)Cl complex was coated on metallic Pt or on p-Si and polycrystalline thin films of p-WSe2 semiconducting electrodes, the TONs were as high as ca. 600 and 450, respectively, for CO2 electroreduction to CO.64 Cosnier et al.65,66 investigated the effects that electrode material, film thickness, and the structure of bipyridyl ligands had on the catalytic activity, stability, and current efficiency for CO2 electroreduction catalyzed by electropolymerized facRe(L)(CO)3Cl complexes (L = pyrrole-substituted bpy) on metallic Pt and carbon felt electrodes (Fig. 5).65,66 The coatings obtained by electropolymerization of a monomer containing one pyrrole group (L1) seemed to be as stable as those prepared with monomers containing two pyrrole groups (L2), whereas polyRe(L)(CO)3Cl films (L3, L4, and L5) with lower reduction potentials were more stable but less active towards the electrochemical reduction of CO2. O’Toole et al.67 co-electropolymerized cis-[(bpy)2Re(vpy)2]2+ with fac-Re(CO)3(vbpy)Cl or fac-[Re(CO)3(vbpy)CH3CN]+ on electrodes to form thin polymeric films (heterogeneous catalysts). Metal sites on these films showed increased reactivity and stability toward CO2

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reduction compared to those catalyzed by fac-Re(CO)3(bpy)Cl in solution. CO and oxalate (on the pure poly-Re(CO)3(vbpy)Cl film) were found to be the main products. The results of electrochemical kinetic studies of poly-Re(CO)3(vbpy)Cl showed that as the film thickness was increased, the film’s rate-determining step could be changed from (1) the chemical reaction between reduced Re and CO2 to (2) electron transport to the catalytic sites. Schrebler et al.68 investigated the electrochemical reduction of CO2 on a Re film electrodeposited onto a polycrystalline Au support in a CH3OH solution with 0.1 M LiClO4 under atmospheric pressure of CO2. The CO2 electroreduction displayed a Tafel slope of 2RT/F, suggesting that the first electronation of the CO2 molecule to form CO2* was the rate-determining step. It was found that the product distribution was strongly dependent on the electrode potential at which the electrolysis was carried out, as well as on the hydrodynamic conditions. Under stirred conditions the Faradaic efficiency of CO production was 87% at 1.35 V, whereas under quiescent conditions, the Faradaic efficiency of CO and CH4 production was 57% and 10%, respectively. Schrebler et al. also prepared Re and Cu–Re microalloy polypyrrole (PPy) modified Au electrodes for the electrochemical reduction of CO2 in CH3OH solution with the same composition. Higher Faradaic efficiencies for CH4 were obtained at 1.35 V, with Au–PpyRe at 34% and Au–PpyCu–Re at 31%. Importantly, both Re and Cu–Re alloy could be highly dispersed on the PPy films, and the amount and selectivity of CO, CH4, and H2 were independent of the hydrodynamic conditions of the solution. The fac-Re(CO)3(vbpy)Cl was also electropolymerized onto a mesoporous TiO2 film coated on a SnO2-doped glass electrode for CO2 electroreduction.69 The nanoporous nature of TiO2 allowed an increase in the 2D number of redox sites per surface area and hence achieved a significant enhancement in catalytic yield. In an effort to improve the catalytic activity of CO2 electrocatalysts, Cheung et al.70 recently electropolymerized a poly-Re(CO)3(k2-N,N-PPP)Cl film onto a GCE. Their results showed that the modified electrode also exhibited electrocatalytic activity for the reduction of CO2 to CO. To understand the mechanism, Re(CO)3LCl complexes with different bpy ligands that contain different substitutions on the benzene ring have been employed as example catalysts for CO2 electroreduction. A systematic study of the electrochemical reduction of CO2 catalyzed by 1.0 mM Re(CO)3LCl complexes (L = bpy, dcbpy, dmbpy, 4,40 -di-tert-butyl-bpy, or 4,40 -dimethoxy-bpy) in CH3CN + 0.1 M tetrabutylammonium hydroxide (TBAH) on a glassy carbon working electrode revealed that the electron donating/withdrawing

Fig. 5 Structure of electropolymerized fac-Re(L)(CO)3Cl complexes. L = pyrrole-substituted bpy. Reprinted with permission from ref. 65. Copyright r 1986 Elsevier.

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substituents in the 4,40 positions of the bipyridine ligand had a significant effect on CO2 electroreduction.71 The catalytic activity was increased (with less negative reduction potentials) in the order OCH3 o C(CH3)3 o CH3 o H o COOH. When L = bpy, dcbpy, dmbpy, and 4,4 0 -di-tert-butyl-bpy, the Re complex gave the best catalytic activity, even 2.2 times higher than that of Lehn catalyst. Recently, fac-(5,5 0 -bisphenylethynyl-2,2 0 -bipyridyl)Re(CO)3Cl was explored as a catalyst for CO2 electroreduction. The results showed a 6.5-fold increase in the current density of CO2 to CO at 1.75 V vs. NHE compared to without CO2, and the Faradaic efficiency for CO production was around 45%.72 In this study, the supporting electrolyte solution was CH3CN with 0.1 M (n-CH3(CH2)3)4NPF6. Regarding the solution pH effect, Wong et al.34 tested the effect ¨nsted acids, CF3CH2OH, C6H5OH, CH3OH, and of four weak Bro H2O, on CO2 electroreduction catalyzed by [Re(CO)3(bpy)(py)]2+ in CH3CN. They found that the addition of these acids could enhance the rate of the catalytic process as well as improve the ¨nsted acid catalyst’s lifetime, suggesting that the acidity of Bro could increase the efficiency of the reduction process. Since the discovery of the Lehn catalyst ( fac-Re(bpy)(CO)3Cl),61 the mechanism of CO2 reduction has also been explored through chemical synthesis, electrochemical, and spectroscopic measurements.69–71,73 Sullivan et al.73 proposed catalytic pathways, including an initial one-electron reduction of the catalyst, which then catalyzed CO2 reduction to form CO, as expressed in reactions (6)–(14): fac-Re(bpy)(CO)3Cl + e " [Re(bpy)(CO)3Cl]*

(6)

[Re(bpy)(CO)3Cl]* - [Re(bpy)(CO)3]* + Cl

(7)

[Re(bpy)(CO)3]* + S - [Re(bpy)(CO)3S]*

Table 2

(S = solvent molecule)

(8)

2[Re(bpy)(CO)3]* - [ fac-Re(bpy)(CO)3]2

(9)

[Re(bpy)(CO)3]* + CO2 - Re(bpy)(CO)3CO2

(10)

Re(bpy)(CO)3CO2 + CO2 + 2e - Re(bpy)(CO)3 + CO32 + CO

(11)

[ fac-Re(bpy)(CO)3]2 + 2e 2 2[Re(bpy)(CO)3]

(12)

[Re(bpy)(CO)3] + CO2 - [Re(bpy)(CO)3CO2]

(13)

[Re(bpy)(CO)3CO2] + A + e - [Re(bpy)(CO)3] + CO + [AO] (A = an oxide ion acceptor)

(14)

In their experimental observation of the above CO2 electroreduction mechanism, Johnson et al.74 reported the reaction products using an infrared spectroelectrochemical (IR-SEC) method, employing an optically transparent thin-layer electrochemical cell. The catalysts used were [Re(CO)3(bpy)P(OEt)3]+, [Re(CO)3(bpy)CH3CN]+, Re(CO)3(bpy)Cl, and Re(CO)3(bpy)CF3SO3). They confirmed that the [Re(CO)3(bpy)Cl]* radical was only attacked by CO2 to form [Re(bpy)(CO)3CO2]* after the dissociation of Cl, and that [Re(CO)3(dmbpy)Cl]* tended to form the [Re(CO)3(dmbpy)]* radical for CO2 reduction. Scheiring et al.75 conducted a mechanism study using electron paramagnetic resonance spectroscopy (EPR); the catalysts employed were paramagnetic Re(CO)3(bpy)X complexes (X = Cl, CF3SO3, CH3O, H, tetrahydrofuran, CH3CN, CO, HCO2, HCO3, and CH3C(O)). Furthermore, sum frequency generation (SFG) spectroscopy and DFT calculations indicated that Re(CO)3Cl(dcbpy) could bind to a rutile TiO2 surface through the –COOH groups of dcbpy in bidentate or tridentate linkage motifs, and the Re atom was exposed to the solution in a configuration suitable for the catalysis of CO2 electroreduction.76 Table 2 summarizes all the mono, bi, and tridentate ligands employed in Re complexes. Manganese generally does not catalyze the electrochemical reduction of CO2. However, some catalytically active Mn carbonyl

Mono-, bi-, and tridentate ligands employed in Re complexes

Pyridine (py)34 [4, vinyl-]. 4-Vinyl-pyridine (vpy)67

2,2 0 -Bipyridine (bpy)61,62,71 [4, –CH2(CH2)3R; 4 0 , –CH3].65 (L1)66 [4,4 0 , –CH2R]. (L2)66 [4, –CH2CH(CH2R)2; 4 0 , –CH3]. (L3)66 [4,4 0 , –COOCH2(CH2)5R]. (L4)66 [4,4 0 , –COOCH(CH2R)2]. (L5)66 [5, –CH3; 5 0 , vinyl-]. 4-Vinyl-4 0 -methyl-bpy (vbpy)63,64,67,69 [4,4 0 , –OCH3]. 4,4 0 -Dimethoxy-bpy71 [4,4 0 , –C(CH3)3]. 4,4 0 -Di-tert-butyl-bpy71 [5,5 0 , C6H5CRC–]. 5,5 0 -Bisphenylethynyl-bpy72 [5,5 0 , –COOH]. 4,4 0 -Dicarboxyl-bpy (dcbpy)71,76 [4,4 0 , –CH3]. 4,4 0 -Dimethyl-bpy (dmbpy)71

R: pyrrolyl

[R, –CH2CH2CH2-N(pyrrole)]. N-(3-N,N 0 -Bis(2-pyridyl)propylamino)pyrrole (PPP)70


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complexes, i.e., [Mn(bpy)(CO)3]+, [Mn(dmbpy)(CO)3]+,77 and [Mn(bpyt-Bu)(CO)3]+,78 recently were found to have some activity toward CO2 reduction. X-ray crystallography of [Mn(bpy-tBu)(CO)3] (ref. 78) showed a five-coordinate Mn center, similar to its rhenium analogue and to the IR-SEC of Mn(bpy-tBu)(CO)3Br. 2.1.4 Iron, cobalt, and nickel 2.1.4.1 Fe/Co/Ni metal electrodes. In general, the electrochemical reduction of CO2 on electrodes of Group 8–10 metals, such as Fe, Co, and Ni, in aqueous solutions under ambient conditions produces H2 and CO or other hydrocarbons. However, these metals have high activities for the hydrogenation of CO and/or CO2 in heterogeneous catalytic reactions (e.g., the Fischer–Tropsch reaction). At 30 atm, the electrochemical reduction of CO2 on Fe electrodes at a constant current density of 120 mA cm2 produced HCOOH with a Faradaic efficiency of B60%.79 Several factors that might have affected the reduction process were proposed, including the diffusion of molecular CO2 to the electrode surface and the suppression of hydrogen evolution by CO adsorption on the electrode surface.80 In the electrochemical reduction of CO2 at Ni electrodes in aqueous media, H2 as well as some small hydrocarbons, such as CH4, C2H4, and C2H6, was produced. Under high CO2 pressure, the Faradaic efficiency for CO2 reduction on Ni electrodes could be increased by raising the CO2 pressure, lowering the temperature, and polarizing the electrode potential at a more negative potential.81 It was suggested that hydrocarbons were formed on Fe, Co, and Ni electrodes through pathways similar to the Fischer–Tropsch reaction of thermal catalysis.81 In CO2 electroreduction catalyzed by Fe and Ni metals, CO adsorption on these two electrodes was observed by IR spectroscopy, and the results suggested that the catalytic activity was strongly related to the bonding between CO and the metal surface.82 2.1.4.2 Fe/Co/Ni complex catalysts. To date, a large number of studies have been conducted on CO2 electroreduction catalyzed by iron, cobalt, and nickel complexes. Both the type of the metal and the structure of the ligands play important roles in the catalytic behavior of these complexes. Fe/Co/Ni complexes of porphyrin. In CO2 electroreduction catalyzed by metal porphyrin (M–P)-modified electrodes (M = Fe(II, III), Co(II), Ni(II), and Cr(III); P = tetraphenylporphyrin) in DMF solutions,55 methanol production can be expressed as follows: CO2 + 6M–P + 6H+ - CH3OH + 6M–P+ + H2O

(15)

M–P + e - M–P

(16)

+

It has been reported that Fe(II) porphyrin yields efficient, CO-selective, and durable catalysts for CO2 electroreduction when it is reduced to Fe(0)-porphyrin by two successive electrons during the reduction process. In an early study, Takahashi et al.83 found that electrodes modified by Fe-meso-tetracarboxyphenyl porphyrin (Fe-mTCPP) and Fe-tetraphenylporphine sulfonate (Fe-sTPP) had insignificant catalytic activity towards CO2 electroreduction in aqueous electrolytic solutions. However,

638 | Chem. Soc. Rev., 2014, 43, 631--675

Table 3 Simplified reaction scheme for CO2 reduction by Fe(0)-porphyrins (por = porous, AH = acid). Reprinted with permission from ref. 86. Copyright r 2012 Science

[(por)Fe(I)] + e " [(por)Fe(0)]2 [(por)Fe(0)]2 + CO2 + 2AH - [(por)Fe(II)CO] + H2O + 2A (K) [(por)Fe(II)CO] + [(por)Fe(0)]2 - [(por)Fe(I)] + CO (K 0 { K) CO2 + 2AH + 2e - CO + H2O + 2A If pK(CO2+H2O) { pKAH: 2A + 2(CO2 + H2O) - 2AH + 2CO3H 3CO2 + H2O + 2e - CO + 2CO3H

in a DMF solution with tetraalkylammonium (TAA) salts, three complexes of Fe-porphyrins, including Fe-tetraphenylporphyrin (FeTPP), showed electrocatalytic activity for CO2 reduction, mainly producing CO, with a Faradaic yield of over 94%. The ¨nsted acids to the solution, such as addition of weak Bro n-propanol, 2-pyrrolidone, or CF3CH2OH,84,85 considerably improved the reduction process catalyzed by Fe(0)–TPP. The catalytic currents and the lifetime of the catalyst were both increased without significant hydrogen evolution. In contrast, the HCOOH yield decreased as the acidity of the acid synergist was increased, and finally became negligible when CF3CH2OH was added. Most recently, Costentin et al.86 found that modification of FeTPP through the introduction of phenolic groups in all ortho and ortho’ positions of phenyl groups—to form iron 5,10,15,20tetrakis(2 0 6 0 -dihydrolphenyl) porphyrin, i.e., FeTDHPP, and iron 5,10,15,20-tetrakis(2 0 6 0 -dimethoxyphenyl) porphyrin, i.e., FeTDDMPP—could considerably speed up the rate of CO2 electroreduction to CO by an electrogenerated iron(0) complex on a GCE. The catalyst manifested a CO Faradaic yield above 90% through 50 million turnovers over 4 hours of electrolysis at low overpotential and 0.465 V, with no observed degradation. The reason for the enhanced activity appeared to be the high local concentration of protons associated with the phenolic hydroxyl substituents. The stoichiometric reactions involved in the catalytic reduction of CO2 to CO by Fe(0)-porphyrins are presented in Table 3. Using carbon nanotubes (CNTs) for support, an FeTPPCl (FeP-CNT)-modified GCE for CO2 electroreduction was studied using CV and CO2 electrolysis in 0.1 M NaHCO3 solution.87 The FeP-CNT exhibited a less negative cathode potential and much higher reaction rate than pure FeP or CNT electrodes. By adding FDH, NADH, and methyl viologen to the electrolyte, HCOOH became the only product, and the concentrations of HCOOH formed at the cathode followed the order FeP-CNT > FeP > CNT. This improved catalytic activity was attributed to synergistic catalysis and direct electron transfer between Fe-porphyrin and CNTs, demonstrated using both electrochemical impedance spectroscopy (EIS) and electron paramagnetic resonance (EPR) analysis. The p–p interaction between the porphyrin ring and sidewalls of the CNTs reduced the electron density around the Fe nuclei in FeTPPCl, which expanded the macrocyclic conjugated structure of FeTPPCl and further increased the potential for CO2 reduction.87 Regarding cobalt complexes as catalysts, Co porphyrin catalyzed the electroreduction of CO2, producing CO in high yield.88


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Fig. 6 Cyclic voltammograms of the Co(II)–TPP complex at different modification stages of the glassy carbon electrode in pH 6.8 phosphate buffer solution under a N2 atmosphere; py = aminopyridine; sweep rate = 50 mV s1. Reprinted with permission from ref. 90. Copyright r 1997 Elsevier.

In the presence of aquopentacyanoferrate(II) or 2-hydroxyl-1nitrosonaphthalene-3,6 disulphonatocobalt(II) as the homogeneous catalyst, the Pt plate electrodes modified by CoIITPP, NiIITPP, FeIIITPPCl, or FeIITPP showed decreasing CO2 reduction activity in an aqueous solution of methanol plus 0.1 M KCl (pH 3.5),55 indicating that the CoIITPP complex, as a heterogeneous catalyst, certainly had higher catalytic activity. Using a GCE modified by CoIITPP or CoTPP-py-NHCO, the current efficiency of CO2 electroreduction to CO in an aqueous phosphate buffer solution (a mixture of l/15 M NaH2PO4 and l/15 M Na2HPO4) was 92% at 1.1 V vs. SCE. A CoTPP-py-NHCO-modified GCE also showed high catalytic stability, with an overall TON exceeding 107.89 An amine cation radical method was explored to coordinate a CoII–TPP complex directly onto a GCE to catalyze CO2 electroreduction, and the results were quite promising (Fig. 6).90 Compared to a GCE physically attached to Co(II)–TPP, this GCE chemically bonded to Co(II)–TPP yielded enhanced CO2 electroreduction. In the exploration of new catalysts, 5,10,15,20-tetrakis (4-methoxyphenyl)porphyrinato cobalt(II) (CoTMPP) adsorbed on a nanoporous activated carbon fiber (n-ACF) electrode was found to be an effective catalyst, yielding CO with current efficiencies of up to 70%.91 The catalytic activity of a binuclear Co complex, Co(I)TMPyP–MTPPS (TMPyP = a,b,g,d-tetrakis(1-methylpyridinium-4-yl)porphyrin p-toluenesulfonate; TPPS = tetraphenylporphine tetrasulfonic acid; M = metal), was also tested, and significant catalytic activity was observed.92 The researchers believed that the catalytic activity should be strongly dependent on the central metal (M) of TPPS, which could serve as an electron mediator.92 Imaoka et al.93 prepared phenylazomethine dendrimers to bear CoTPP cores as homogeneous catalysts for CO2 electroreduction on a GCE in a DMF solution containing a strong Lewis acid. They observed that the


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ligand had a strong steric effect on the catalytic activity of the CoTPP core. Recently, catalyst-modified diamond surfaces, dubbed ‘‘smart’’ electrodes, have been explored, with reports of good stability and electrocatalytic activity for CO2 electroreduction to CO in CH3CN.94 In this research, a catalytically active Co complex was covalently attached to a B-doped, p-type conductive diamond. In addition, some fluorinated derivatives of CoTPP (the phenyl rings of the CoTPP were substituted with electron withdrawing groups, such as F and CF3) have also been explored as catalysts for CO2 electroreduction, yielding positive results.95 It should be noted that the catalytic behavior of a metal porphyrin for CO2 electroreduction can be affected by many factors. Even the same catalyst may display different activity when used as a homogeneous catalyst rather than a heterogeneous catalyst.55,96 CO2 pressure and solution temperature also have a significant effect on CO2 electroreduction. For example, when CO2 pressure was increased from 1 to 20 atm, the current efficiencies of CO2 electroreduction by Fe- and Co-meso-TPP-supported gas diffusion electrodes (GDEs) were increased by up to 97.45% and 84.6%, respectively.97 According to the literature, the strength with which a catalyst bonds to the electrode surface has significant effects on the catalyst’s activity,98 as has been demonstrated by photoelectron spectroscopy (PES) and scanning tunneling spectroscopy studies, as well as quantum-chemical calculations based on density functional theory (DFT). Fe/Co/Ni complexes of phthalocyanine. Metal phthalocyanine (MPc) complexes seem to have less catalytic activity toward CO2 electroreduction than metal porphyrin complexes. For example, iron phthalocyanine tetrasulfonate (FeTSPc), as distinct from CoTSP and NiTSP, did not show remarkable homogeneous catalytic activity for CO2 electroreduction in Clark–Lubs buffer solution.99 CoPc and NiPc complexes coated on a graphite electrode showed some catalytic activity for CO2 electroreduction, with HCOOH being the predominant product in aqueous solutions (pH 3–7).100–102 The electrocatalytic reduction of CO2 in an aqueous electrolyte catalyzed by a graphite electrode coated with a CoPc/poly-4-vinylpyridine (PVP) film was also studied, and

Fig. 7 Proposed mechanism for CoPc in an aqueous medium in the presence and absence of CO2. Reprinted with permission from ref. 103. Copyright r 1995 Elsevier.

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enhanced catalytic activity was observed compared to what occurred with a pure CoPc catalyst.103 As shown in Fig. 7, the peripheral N atom on the Pc ring can be protonated by the addition of a proton following the first reduction. The second reduction generates the active species for the catalytic reduction of both proton (upward arrow) and CO2 (downward arrow), generating H2 and CO, respectively, at pH 4.4. When CoPc was incorporated into a PVP film coated on a graphite electrode, the polymer-incorporated CoPc showed high catalytic activity towards CO2 electroreduction, with high selectivity from CO2 to CO, where PVP functioned as a coordinative and weakly basic agent.104 The coordination of PVP to CoPc was considered to have caused the increase in electron density on the central metal ion, facilitating the formation of the intermediate. Cobalt octabutoxyphthalocyanine (CoPc(BuO)8) coated on a graphite electrode was also explored as a catalyst for CO2 reduction; the results showed that this catalyst had higher activity and selectivity from CO2 to CO than non-substituted CoPc.105 Under typical conditions at pH 4.4, the most active and selective CO2 reduction was achieved at 1.30 V vs. Ag/AgCl; the production selectivity of CO/H2 was reported to be B4.2. In addition, Zhang et al.106 employed a rotating ring (platinum)-disk (graphite) electrode to analyze CO2 electroreduction to CO, catalyzed by N,N0 ,N00 ,N00 -tetramethyltetra-3,4pyridoporphyrazinocobalt(II) coated on a graphite disk and protected by a Nafion film. In a microbial electrolysis cell, cobalt tetra-amino phthalocyanine (CoTAPc) coated on multiwalled CNTs was reported to produce high current efficiency for HCOOH production.107 Besides phthalocyanine ligands, other kinds of hexaazamacrocycle ligands, such as phenanthroline and bipyridine, and their complexes with Co(II), Ni(II), and Cu(II) metal centers, have also been explored as CO2 electroreduction catalysts.108 For example, CV and UV-vis spectroscopy have been employed to study the electroreduction of CO2 using, as an electrocatalyst, hexaazamacrocycles derived from the condensation of 1,10phenanthroline and its Co(II) complex, dissolved in DMF solution. The results showed that the ligand had no catalytic activity, whereas its cobalt complex showed electrocatalytic activity toward the reduction of CO2, generating CO and HCOOH.109 Fe/Co complexes of corroles. Some iron and cobalt complexes with corroles have also been explored as CO2 electroreduction catalysts. For example, Ph3PCoIII(tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole), ClFeIV(tpfc), and ClFeIV(tdcc) (tdcc = 5,10,15-tris(2,6-dichlorophenyl)corrole) dissolved in CO2-saturated CH3CN solution were tested, using CV, for their catalytic activity towards CO2 electroreduction; the results indicated that the CoI and FeI complexes were both effective catalyst centers.110 Ni complexes of cyclams. Nickel complexes of cyclam (i.e., 1,4,8,11-tetraazatetradecane) and Ni(cyclam)2+ have been recognized as highly selective catalysts for the electroreduction of CO2 to CO in aqueous solution on a mercury cathode (as the working electrode).111,112 The stability of such catalysts was also found to be remarkable. Even after thousands of catalytic

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cycles, no significant deactivation was observed (the turnover frequency was B103 mol of CO produced per mole of nickel complex in 1 hour).112 A study of the reduction mechanism on mercury, using CV, polarography, and electrocapillarity, indicated that the adsorbed complex Ni(I)-cyclam on Hg was the active catalyst.113 Theoretical calculations performed by Sakaki114 provided a reasonable description of the catalytic mechanism, based on the oxidation states of the adsorbed complexes ([Ni-cyclam]2+ and [Ni-cyclam]+), which were strongly dependent on the electrode potentials. Regarding the mechanism, Fujihira113 observed that the desorption of Ni(I)-cyclam gradually slowed down with the formation of nonadsorbable Ni(I)-cyclam-CO by the reaction of Ni(I)-cyclam in solution with electrocatalytically generated CO from CO2. In the CO2 reduction process, Ni(cyclam)2+ was only weakly adsorbed over a limited potential range, and in quantities substantially less than one monolayer, whereas Ni(cyclam)+ could be strongly adsorbed on the electrode surface over a wide potential range.115 However, the high electrocatalytic activity of the Ni(cyclam)2+ complex for the reduction of CO2 at a static mercury electrode was severely diminished in the presence of CO when the mercury electrode was not stirred.116 The cause of the decrease in activity was proposed to be an insoluble complex of Ni(O) and Ni(cyclam)CO, which formed during the reduction of CO2. To explore this mechanism, in situ analyses of the products of CO2 electroreduction catalyzed by Ni-cyclam were carried out using differential electrochemical mass spectroscopy during CV on an amalgamated-gold mesh electrode.117 A binuclear nickel complex, Ni2(biscyclam)4+, was found to have similar catalytic activity to that of Ni(cyclam)2+ for CO2 electroreduction. In Ni2(biscyclam)4+, two Ni atoms are indirectly linked. When DMF with low water content was used as a solvent, high Faradaic yields of HCOO were observed (up to 75%) in addition to CO.118 Methyl substitution of the amines on the cyclam ring was also explored as a ligand for the Ni complex when used as a CO2 reduction electrocatalyst, and the result showed some catalytic effects. The Ni(II) complex of N-hydroxyethylazacyclam (i.e. 3-(20 -hydroxyethyl)-1,3,5,8,12-penta-azacyclotetradecane nickel(II) perchlorate) appeared to be more active than unsubstituted Ni(cyclam).119 Recently, Schneider et al.120 explored a series of materials that are structurally similar to [Ni(cyclam)]2+ to test them as electrocatalysts for CO2 reduction at a mercury pool working electrode in aqueous solution.120 Both [Ni(HTIM)]2+ (HTIM = C-RRSS-2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [Ni(DMC)]2+ (DMC = C-meso-5,12-dimethyl-1,4,8,11tetraazacyclotetradecane) showed better electrocatalytic activities than Ni(cyclam)2+. Schneider et al. suggested that (1) the catalyst’s geometry should be suitable for its adsorption onto the mercury electrode surface and (2) there should be electronic effects of methyl groups or cyclohexane rings on the cyclam backbone. Additional observations have also been made about the influence of methyl substitution on these catalysts’ activities (Fig. 8).120,121 Abba et al. found that the structural features of the cyclam and azacyclam framework played an important role in the enhanced catalytic efficiency of Ni-cyclam derivatives for CO2 electroreduction. Even small deviations from such a geometrical


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the macrocycle methyl groups could destabilize the adducts, depending on the nature of the individual complex catalyst. With a Ni complex, it was reported that both RRSS-NiIIHTIM(ClO4)2 (HTIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and NiIIDMC(ClO4)2 (DMC = C-meso-5,12-dimethyl-l,4,8,11-tetraazacyclotetradecane) showed electrocatalytic activity for CO2 reduction. However, the latter was even more active than Ni(cyclam)2+ catalysts, whereas the former was not.126 In addition, for Ni complex catalysts, the issue of catalyst poisoning during CO2 electroreduction seems to be a concern. For example, this issue was reported when researchers employed isomers of a tetraazamacrocyclic Ni(II) complex in solutions saturated with argon, CO, and CO2.125–127 Some Ni-macrocyclic complexes were found to have catalytic activity for CO2 electroreduction from CO2 to oxalate.128 For example, with a homogeneous electron-transfer rate constant of B105 M1 s1, the complex Ni-Etn(Me/COOEt)-Etn was tested as a selective catalyst for CO2 electroreduction to yield oxalate. In this test, electrolysis experiments were carried out using 1.5 105 mol of the Ni complex as the catalyst in CO2-saturated CH3CN + 0.25 M Bu4NClO4 solution. The overall reaction mechanism was interpreted in terms of an outersphere electron-transfer reaction followed by dimerization of CO2 radical anions. In Table 4, the structure of the Fe/Co/Ni complexes and their tetradentate ligands for CO2 electrochemical reduction catalysts are summarized.

Fig. 8 (A) Cyclic voltammograms of 1.0 mM Ni(cyclam)2+ in 0.1 M KCl(aq) (GC electrode; 100 mV s1 scan rate). (B) CVs of 1.0 mM Ni(cyclam)2+, Ni(DMC)2+, and Ni(TMC)2+ in a 0.08 M tetrabutylammonium hexafluorophosphate electrolyte (1 : 4 water–acetonitrile (CH3CN); GC electrode; 100 mV s1 scan rate). Reprinted with permission from ref. 120. Copyright r 2012 American Chemical Society.

arrangement caused the electrocatalytic effect to be drastically reduced or completely lost.122 For CO2 electroreduction catalyzed by Ni-cyclam complexes on non-mercury electrodes, a GCE was used as the electrode substrate on which a Ni-cyclam complex catalyst was coated, together with a Nafion film123 or with a poly-(allylamine) (PALA) backbone, and some effective electrocatalytic activity was observed.124 The orientation of the nickel-cyclam complex on the electrode surface was found to be critical to the catalytic effect.123 Ni/Co complexes of tetraazamacrocycles. Some nickel and cobalt complexes of tetraazamacrocycles, such as Co(I)-14membered tetraazamacrocycles, can also catalyze the electrochemical reduction of CO2 to CO and H2 in either CH3CN or water solution, with current efficiencies of greater than 90%.125 In the reduction process, charge transfer from Co to bonded CO2 was found to be an important factor in stabilizing the CO2 adducts. The H-bonding interactions between the bound CO2 and amine macrocycle N–H protons might serve to additionally stabilize the adduct in some cases, while steric repulsion by


Fe/Co/Ni complexes with tridentate ligands. Iron, cobalt, and nickel complexes with tridentate ligands such as 40 -vinyl-2,2 0 :6 0 ,200 terpyridine (v-tpy) can also be employed as catalysts for CO2 electroreduction.57,129–132 The structures of these catalysts are provided in Table 5-(T1). An iron complex, [Fe(4-v-tpy)2]2+, electropolymerized onto an electrode yielded a TON in excess of 15 000. It was observed that heterogeneous catalysis with v-tpy (electropolymerized on the electrode) gave a higher catalytic activity than homogeneous catalysis (dissolved in solution).132 Heterogeneous catalysis by iron complexes of 4-v-tpy and 6-v-tpy, which were electropolymerized onto a GCE, was very selective, yielding only formaldehyde.57 The geometric structure and degree of conjugation of the ligand, as well as the orientation and fixation of the macrocyclic metal complexes on the electrode surface, were found to be important for catalytic activity and stability in CO2 electroreduction.133 Furthermore, iron, cobalt, and nickel complexes of some diacetylpyridine-derived tridentate ligands also appear to be effective electrocatalysts for CO2 reduction.134 Fe/Co/Ni with bidentate ligands. Fe, Co, and Ni complexes with bidentate ligands such as H2dophen,135 H2salen,136 and salophen137 have also been employed to electrocatalyze CO2 reduction. In addition, polypyrrole Co(II) Schiff-base complexes were electropolymerized on a polished GCE to catalyze CO2 reduction.138 The corresponding structures of the above catalysts are given in Table 5-(T2). However, the most studied systems seemed to be iron-based complexes. For example, electrolysis of CO2 using (Fe(dophen)Cl)22HCON(CH3)2 and Fe(dophen)(N-MeIm)2ClO4 as catalysts at 2.0 V vs. a ferrocenium– ferrocene reference electrode gave a mixture of CO, HCOO

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Fe/Co/Ni complexes and their tetradentate ligands for CO2 electrochemical reduction catalysts


Ligand core structure

Substitution on the ligand core structure

Fe/Co/Ni-complexes

1,4,7,10-H are substituted by 4-carboxyphenyl 1,4,7,10-H are substituted by 4-sulfophenyl 1,4,7,10-H are substituted by phenyl

Fe-meso-tetra(carboxyphenyl)porphine83 Fe-tetraphenylporphine sulfonate83 Fe–L;32,33,55,84–86,97,98 Co–L;55,89,90,95,97,98 Ni–L,55,96 L = tetraphenylporphyrin (TPP) Fe–L;86 Co–L,91 L = 5,10,15,20-tetrakis(2 0 6 0 -dihydroxylphenyl)-porphyrin Fe-5,10,15,20-tetrakis(2 0 6 0 -dimethoxyphenyl)porphyrin86 Co-5,10,15,20-tetrakis (4-methoxyphenyl)porphyrin (CoTMeOPP)133 Co-5,10,15,20-tetra(p-ethynylphenyl)-porphyrin94 Co–L,95 L = T3FPP, T3CF3PP, and TF5PP

1,4,7,10-H are substituted by 2 0 6 0 dihydroxylphenyl 1,4,7,10-H are substituted by 2 0 6 0 dimethoxyphenyl 1,4,7,10-H are substituted by 4-methoxyphenyl (1) Porphyrin

1,4,7,10-H are substituted by p-ethynylphenyl 1,4,7-H are substituted by C6F3H2, C6H4CF3, and C6F5, respectively Unsubstituted (1 or 2)H, (3 or 4)H, (5 or 6)H, and (7 or 8)H are substituted by ‘‘SO3H’’ 1,2,3,4,5,6,7,8H are substituted by 8 of ‘‘nCH3CH2CH2CH2’’ (1 or 2)H, (3 or 4)H, (5 or 6)H, and (7 or 8)H are substituted by ‘‘NH2’’

Fe–L,100 Co–L,100–104,133 Ni–L,100,102 L = phthalocyanine M-phthalocyanine tetrasulfonate; M = Fe, Co, Ni99 Co-octabutoxyphthalocyanine105 M-tetrakis aminophthalocyanine, M = Fe, Co, Ni371 Naphthalocyanato cobalt(II)133

(2) Phthalocyanine

1,2,3 H are substituted by 2,6-dichlorophenyl

(C6H5)3PCo(III)L; ClFe(IV)L, L = 5,10,15tris(pentafluorophenyl)corrole110 ClFe(IV)L, L = 5,10,15-tris(2,6dichlorophenyl)corrole110 Hydrophobic vitamin B12133

Other (N–)H substituted cyclams

Ni(cyclam)Cl2111–116,118,120–124,126 Ni–L,119–121,123,136 L= (N–)H substituted cyclams

1,2,3 H are substituted by pentafluorophenyl

(3) Corrole

[Ni2(biscyclam)]4+ (ref. 118) Biscyclam (4) 1,4,8,11-Tetraazacyclotetradecane (cyclam) Ref. 119

Co,125 Ni125,127,128

(5) 14-Membered tetraazamacrocycles

—

Ni,108 Co108,109

(6) Hexaazamacrocycle (ref. 109)

(major product) and C2O42.135 In this catalysis process, the Fe(I) species seemed to play an important role. As well, the rate of CO2 reduction was enhanced by adding CF3CH2OH or CH3OH, as a proton source, to the electrolyte. Both iron carbonyl and iron formato species were detected as intermediates by

642 | Chem. Soc. Rev., 2014, 43, 631--675

in situ FTIR measurements, where the formation of an Fe–Z1CO2 intermediate led to the production of CO. The formation of oxalate was attributed to the dimerization of two reduced CO2 molecules. The overall reaction mechanism was proposed to be as follows:



2,6-Bis[1-(2-methylpyridylimino)ethyl]pyridine (DAPMP), Fe,Co134

2,3,5,6-Tetra-2-pyridylpyrazine [M(tppz)2], M = Fe,Co,Ni131

6-Bis-[1-(phenylimino)ethyl]pyridine (DAPA), 2,6-Bis[1-(benzylimino)-ethyl]pyridine (DAPB), Fe,Co,Ni134 Fe,Co,Ni134

Dimethylaminomethylphenylsulfanylphenylamine (dapa) [M(dapa)2], M = Fe,Co,Ni131

4-Vinylterpyridine (4-v-tpy) [M(4-v-tpy)2], M = Fe,Co,Ni57

2,6-Bis[1-(2-ethylpyridylimino)-ethyl]pyridine (DAPEP), Fe,Ni134

Vinyl-DAPA, Fe,Co,Ni134

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (tptz)213, [M(tptz)2], M = Fe,Co131

4 0 -Vinyl-2,2 0 :6 0 ,200 -terpyridine (v-tpy) [M(v-tpy)2], M = Fe,Co,Ni129–132

Br-DAPA Fe,Co,Ni134

n,n,n 0 ,n 0 -Tetrakis(2-pyridylmethyl)ethane1,2-diamine (tpen)131 [M(tpen)], M = Co,Ni

6-Vinylterpyridine (6-v-tpy)57 [M(6-v-tpy)2], M = Fe,Co,Ni

(T1) Fe/Co/Ni complexes and their tetradentate ligands for CO2 electrochemical reduction catalysts. (T2) Fe/Co/Ni complexes and their bidentate ligands for CO2 electroreduction catalysts

2,2 0 :6 0 ,200 -Terpyridine (typ) [M(tpy)2], M = Fe,Co,Ni131,132

(T1)

Table 5


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644 | Chem. Soc. Rev., 2014, 43, 631--675

(continued)

Polyaniline (PAn)159,161

Complex of Co and 2-(3-pyrrole-l-yl-propylimino-methyl)phenol138

2,9-Bis(2-hydroxyphenyl)-1,10-phenanthroline (H2dophen)135

(T2)

Table 5

[Fe4S4(SR)4]2 (ref. 54,139,140)

N,N 0 -Bis(salicylidene)-o-phenylenediamine (salophen)137

Ni(COD)2 = nickel dicyclooctadiene166

N,N 0 -Bis(salicylidene)ethylenediamine (H2salen)136


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FeIII(dophen)Cl + e " FeII(dophen) + Cl

(17)

FeII(dophen) + e " [FeI(dophen)]

(18)

[Fe (dophen)] + e " [Fe (dophen)]


I

I

2

(19)

(20) (21)

[Fe(dophen)(CO)]+ + e + S " Fe(dophen)S + CO

(22)

(S is a solvent molecule) [Fe(dophen)(CO2)] + S " Fe(dophen)S + CO2* 2CO2* - C2O42

(23) (24)

The mechanism of HCOO formation was proposed to be as follows: [Fe(dophen)] + AH " Fe(dophen)H + A

(25) (26)

(27) In the above mechanisms expressed by reactions (21) to (27), HA is the proton source.135 2.1.4.3 Other Fe/Co/Ni-containing catalysts Fe4S4 cluster catalysts. Cubane-type Fe4S4 clusters139 are a unique type of iron complexes in which the ‘‘Fe4 active site’’ structure plays a significant role in the electron-transfer reactions for CO2 electrocatalytic reduction (see Table 5-T2). In this regard, Tezuka et al.140 reported the electroreduction of CO2 catalyzed by two tetranuclear iron–sulfur clusters, [Fe4S4(SCH2C6H6)4]2 and [Fe4S4(SC6H6)4]2, in DMF solution. Their results showed that without the catalysts, oxalate was predominantly formed in CO2 reduction, together with small quantities of formate and CO, whereas formate was obtained preferentially in the presence of the catalyst clusters. It was found that CO2 could be generated through electron transfer from the reduced clusters to CO2 in the bulk solution. In another study of CO2 electroreduction catalyzed by [Fe4S4(SCH2C5H6)4]2, it was found that the cubane structure of the cluster could be collapsed, generating two main products, C6H5COO and HCOO.54 Adding C6H5CH2SH could preserve the cluster structure for a long time, producing more C6H5COO than HCOO. Recently, Yuhas et al.141 obtained enhanced electrocatalytic activity for CO2 electroreduction catalyzed by both ternary Ni–Fe4S4 and Co–Fe4S4-based biomimetic chalcogels.141


Carbon nanotube supported metal catalysts. Unlike the Fe/Co/Ni metal electrodes or their complex catalysts mentioned above, Fe nanoparticles supported on carbon nanotubes (Fe/CNTs) were found to show high electrocatalytic activity toward CO2 reduction to small liquid fuels. Under conventional liquid-phase operations, C2H4 was found to be the main product of the electrocatalytic reduction of CO2 at high operation potentials,7,142 whereas in the gas phase, isopropanol was the main product.143,144 In this case, Fe/CNTs even showed better activity than Pt/CNTs but were unstable. The explanation for their degradation was thought to be that the electrolyte, especially K ions, reacted with iron particles, causing their dissolution and migration. In the case of Pt/CNTs, K ions could cover the Pt particles, causing deactivation. Preliminary tests indicated that Fe–Co/CNT catalysts were more stable. When CNTs were further doped with nitrogen to form N-doped CNTs (NCNTs), a supported FeOx catalyst (FeOx/NCNTs) yielded enhanced electrocatalytic activity and selectivity compared to FeOx deposited on pristine or oxidized CNTs.145 These iron nanoparticles supported on NCNTs were able to selectively reduce CO2 to isopropanol. To study the mechanism, the researchers employed microcalorimetry to determine the chemisorption sites of adsorbed CO2; the results showed that NCNTs could cause the formation of small nanoparticles on which there were reversible sites (120 kJ mol1). This approach had, in fact, already been utilized in the photoelectrocatalytic synthesis of solar fuels from CO2.144,146 Furthermore, Bocarsly et al.147 employed two N-containing heteroaromatics (imidazole and pyridine) as homogeneous ‘‘aromatic amine catalysts’’ in the photoelectrochemical reduction of CO2 at some illuminated iron pyrite (FeS2) electrodes. In their study of the catalysis mechanism of a series of imidazole derivatives, CV measurements were carried out (over a scan rate range of 5 to 200 mV s1), and they found that imidazole could reduce CO2 to a mixture of CO and HCOOH at a moderate potential, while pyridine selectively produced formic acid. Fe complexes with CO and CN ligands. Rail and Berben148 reported that under appropriate conditions, Et4N[Fe4N(CO)12] could be a catalyst for both the hydrogen evolution reaction (HER) and CO2 electroreduction at 1.25 V vs. SCE using a GCE. Everitt’s salt (ES) coated on a Pt plate electrode was also found to have activity for CO2 electroreduction to methanol149–151 in the presence of either pentacyanoferrate(II) (Na3FeII(CN)5H2O) or amminepentacyanoferrate(II) (Na3FeII(CN)5NH3). The proposed reaction mechanism was that the ES (Prussian blue reduction product) was first reduced from Prussian blue (PB) (KFeIII[FeII(CN)6]) on the cathode to start the reduction by transferring electrons to CO2. In another study, it was found that CO2 could also be reduced to CH3OH at an ES-mediated electrode in the presence of the 1,2-dihydroxybenzene-3,5-disulfonate (tiron) ferrate(III) complex and ethanol.152 The proposed mechanism was as follows. First, a weak coordination bond was formed between the central metal and ethanol, followed by the insertion of CO2 to form an intermediate Fe(III)–tiron–ethyl formate complex; this complex was then finally reduced by ES, with the

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consumption of protons in the solution, to CH3OH and the initial metal complex: CO2 + 6ES + 6H+ " CH3OH + 6PB + 6K+ + H2O


6PB + 6K+ + 6e = 6ES

(28) (29)

Although catalyzed CO2 electroreduction to CH3OH was also feasible with both quinone-derivative-coated Pt and stainless steel electrodes,153 the current efficiency (>50%) was much lower than what the ES-coated Pt plate electrode yielded (>80%).154 The high performance of this ES catalyst was simultaneously confirmed for the photocatalytic conversion of CO and CO2,155 and the same catalyst was used to produce CH3OH fuel from CO2 for a direct liquid fuel cell.156 Prussian blue-based catalysts. Based on their observations regarding the catalytic activity of the Co-2-hydroxy-l-nitrosonaphthalene-3,6-disulfonate complex in the homogeneous catalysis of CO2 electroreduction using an ES-mediated electrode in CH3OH,157 Ogura et al.158 carried out a further study using a PB-polyaniline (PAn) dual film coated Pt/Co-2-hydroxy-lnitrosonaphthalene-3,6-disulfonate complex electrode in aqueous solution158 (the structure is presented in Table 5-T2). The PB film was first electrodeposited on a Pt plate from an aqueous ferric ferricyanate solution; the PAn film was then deposited on a thin PB-coated electrode by repeated potential cycling in KCl solution (pH 1) containing 0.1 M C6H5NH2. Lactic acid, ethanol, acetone, and methanol were detected at a low overpotential (0.6 V vs. SCE) under ambient conditions. Ogura et al.159 then developed Fe–L (L = 4,5-dihydroxybenzene1,3-disulfonate and 2-hydroxy-l-nitrosonaphthalene-3,6-disulfonate) complex-immobilized PAn/PB-modified electrodes. The results of CO2 reduction at dual-film Pt electrodes modified with and without Fe(II) complexes confirmed that the Fe-4,5-dihydroxybenzene-1,3disulfonate complex-immobilized PAn/PB-modified electrode yielded high catalytic activity and product selectivity for lactic acid. It was found that CO2 could be reduced at the active centers existing in the coated film as well as at the electrode/solution interface. Interestingly, in CO2 electroreduction catalyzed by Fe(II) compleximmobilized PAn/PB/Pt electrodes, formic acid was also one of the products, in addition to lactic acid.160 The formation process for products such as lactic acid, formic acid, methanol, ethanol, and so on was examined using FTIR reflection spectra.161–163 Multinuclear nickel complexes. Unlike iron and cobalt, nickel can form multinuclear complexes, which have also been explored as catalysts for CO2 electroreduction.164,165 Ni atoms were linked with each other either directly164,166 or indirectly.118,165,167 It has been found that in other multinuclear complexes, metal atoms are almost all indirectly linked.168–172 Lee et al.167 studied the electrocatalytic activity of a multinuclear nickel(II) complex, Ni3(L)(ClO4)6 (L = 8,80 ,800 -(2,20 ,200 -nitrilotriethyl)-tris(1,3,6,8,10,13,15heptaazatricyclo[11.3.1.13,15]octadecane)), towards CO2 reduction and compared these results with those from catalysis by a mononuclear complex, such as [Ni(cyclam)]2+, in CH3CN–H2O (9 : 1, v/v). Unfortunately, the catalytic efficiencies (TON) of [Ni3(L)]2+ were both lower than those of [Ni(cyclam)]2+ and monometallic [Ni(3)]2+.

646 | Chem. Soc. Rev., 2014, 43, 631--675

Fig. 9 Schematic of CODH-catalyzed CO2 electroreduction to CO. Reprinted with permission from ref. 173. Copyright r 2003 American Chemical Society.

ń-Manso et al.166 synthesized several kinds of dinuclear Simo nickel(0) complexes with dppa as a bridging ligand, expressed as Ni2(m-dppa)2(m-CNR)(CNR)2 (R = Me(CH3), n-Bu(CH3CH2CH2CH2), or 2,6-(CH3)2C6H3). The products of CO2 reduction electrocatalyzed by these complexes were mainly CO and CO32, with a small amount of HCOO formed when residual water was present. The steric effect of such large multinuclear Ni complex molecules should be a major factor in the efficiency of CO2 electroreduction.165 Typical candidates are dinickel complexes [Ni2L2–6]4+ or pentaazamacrocycles with (CH3OHCH2)n bridges (n = 2, 3, 4, 6) or a p-xylylenediamine linkage (L6). The redox potentials were remarkably constant, but the current peak separations increased, reflecting slower electron transfer due to the steric effect. The catalytic currents increased slightly as the linking chain length increased, due to improved stereochemical constraints. Enzyme catalysts. Shin et al.173 reported using carbon monoxide dehydrogenase (CODH) from Moorella thermoacetica to catalyze CO2 electroreduction to CO with a current efficiency of B100% at 0.57 V vs. NHE in a 0.1 M phosphate buffer solution (pH 6.3). CODH can in fact catalyze microbial interconversion between CO and CO2;174 Fig. 9 presents a schematic of the reaction pathway. There are three types of CODH: Mo-CODH, in which the active site is a CuMo-pterin; Ni-CODH, in which the active site is a [Ni4Fe-5S] cluster; and Ni-CODH/ACS, in which Ni-CODH is part of a larger complex structure for CO2 reduction. Parkin et al.175 studied rapid and efficient electrocatalytic CO2/CO interconversion by carboxydothermus hydrogenoformans CO dehydrogenase I (Ch Ni-CODH; see Fig. 10), which was attached onto a pyrolytic graphite ‘‘edge’’ (PGE) electrode. This approach provided a way to study the electrocatalytic activity of enzymes under strict potential control; at the same time, it set a standard for future studies of CO and CO2 electrochemical conversion. 2.1.5 Pt group metals 2.1.5.1 Pt group metal electrodes. Pt group metals are wellknown catalysts for CO2 electroreduction. CO2, CO, and H2 adsorption on Pt group metal-based electrode surfaces differs depending on the metal, leading to different catalytic activities/ stabilities and product selectivities.176–181 Ruthenium. Normally, Ru metal shows high catalytic activity in the gas phase conversion of CO2 to CH4.181 In the aqueous electrochemical process of CO2 reduction, supported Ru sponge electrodes did not show high catalytic activity.38,178 However, due to their high stability, Ru electrodes were used for long-term CO2 reduction at a constant potential.177


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Fig. 10 Structure of Ch CODH II showing the two subunits (blue and gray) and the arrangement of Fe–S clusters (black) that relay electrons to and from the [Ni4Fe-5S] active sites (red). Reprinted with permission from ref. 175. Copyright r 2007 American Chemical Society.

Palladium. The electrocatalytic activity of Pd for CO2 reduction was first investigated in a 1.0 M NaHCO3 solution.182 HCOOH and CO (main products) and small amounts of hydrocarbons (from methane to hexane) resulted from an electrolysis process catalyzed by a Pd electrode in CO2-saturated KHCO3 aqueous solution.183,184 It was observed that the current yield of CO could be increased substantially by increasing the pressure; when the electrode potential was held at 1.8 V vs. Ag/AgCl in 0.1 M KHCO3, the yield went from 5.3% at 1.0 atm to 57.9% at 50 atm.184 The evolution of H2 can be suppressed by hydrogen absorption on the Pd surface, and this absorbed hydrogen can react with the adsorbed reaction intermediates to change the electrocatalytic activity.185 Hydrogenated Cu-modified Pd electrodes also showed higher catalytic activities, producing HCOOH, CH4, and CH3OH.186,187 It was found that CO2 electroreduction on Pd electrodes could occur at potentials higher than the reversible hydrogen potential, suggesting that adsorbed hydrogen atoms might take part in the slow stage of the electroreduction of HCO3 (CO2) to HCOO.188 Ohkawa et al.189 studied the CO2 electroreduction reaction on a Pd electrode in a non-aqueous CH3CN solution and compared the results to those obtained in an aqueous solution; they demonstrated that the concurrent desorption of hydrogen could lead to enhanced catalytic activity for CO2 electroreduction. Platinum. CO2 electroreduction on a Pt electrode surface was studied early on by Eggins and McNeill,190 in water, dimethyl sulfoxide (DMSO), CH3CN, and propylene carbonate solutions, respectively. By applying differential electrochemical mass spectrometry (DEMS), Brisard et al.191 investigated the mechanism of CO2 electroreduction catalyzed by polycrystalline Pt in acidic media. The results showed that the main product was methanol. A gas-diffusion electrode with Pt electrocatalyst was also employed for CO2 reduction under high pressure (o50 atm); a Faradaic efficiency of 46% was obtained at a current density of 900 mA cm2, and CH4 and CH3CH2OH were found to be the major products.192 The Faradaic efficiency for CH4 formation


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Fig. 11 FTIR spectrum of adsorbed CO on a Pt electrode formed during the electrochemical reduction of CO2 in the CH3CN electrolyte with a water concentration of 10.5 mM. Potential with respect to Fe/Fc+: +1.42 and 0.08 V. Reprinted with permission from ref. 195. Copyright r 2000 American Chemical Society.

was increased by increasing the CO2 pressure, whereas the efficiency for hydrogen formation was decreased. In a waterfree electrolyte, CO2 electroreduction on a Pt electrode yielded oxalate as the main product. Since there was no water, H2 evolution was not a concern. In this case, the CO2 that diffused to the electrode might have been readily reduced to CO2, with the CO2 then reacting with CO2 to form oxalate. By increasing the H2O concentration in the solution, H2 evolution was enhanced, leading to the production of HCOO and CO rather than oxalate, which was confirmed by in situ FTIR reflection absorption spectroscopy (Fig. 11).193–195 Regarding CO2 electroreduction catalyzed by supported Pt nanoparticle catalysts, Centi et al.196 employed carbon-supported Pt nanoparticles (Pt/C) as the catalyst to convert CO2 to long carbon-chain hydrocarbons (>C5) at room temperature and ambient atmospheric pressure in a continuous flow cell. Feng et al.197 prepared a three-dimensional porous nanostructured electrode, composed of nanoporous CuPt composites, for CO2 electroreduction in ionic liquid BMIMBF4.197 When Pt nanoparticles were supported on either calcia-stabilized zirconia (Pt/CSZ) or MnO2 (Pt/MnO2) to form a high-temperature CO2 reduction catalyst at 300–900 1C, up to 100% selectivity for paraformaldehyde (PFA) production was achieved.198 More recently, Pt/C–TiO2 and Pt-Pd/C–TiO2 based nanocomposite cathodes were employed to catalyze the electrochemical reduction of CO2 to CH4 and isopropanol.199 In addition, electrochemical conversion of methanol and CO2 to dimethyl carbonate (DMC) was realized at a graphite-Pt electrode in a dialkylimidazolium ionic liquid (1-benzyl-3-methylimidazolium chloride) methanol system without any other additives.200 Single-crystal surfaces of Pt group metals. The surface structures of Pt single-crystal electrodes have long been found to have a significant influence on catalytic activity for CO2 reduction.201,202 A difference was observed between Pt(111) and Pt(110) single-crystal electrodes when researchers investigated the dynamic process of adsorbed CO formation from CO2 and adsorbed hydrogen; the rate of CO formation on Pt(110) was more than 10 times higher than

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on Pt(111).202 Hoshi et al.203 confirmed the order of activity for CO2 reduction: Pt(S)-[n(111) (111)] > Pt(S)-[n(100) (111)] > Pt(S)-[n(111) (100)] (where S represents a single-crystal electrode) in 0.1 M HClO4. They further found that CO2 reduction rates on Pt(S)-[n(110)–(100)] electrodes (n = 2, 9) were higher than those on Pt(110), and the rates on Pt(S)-[n(100) (110)] (n = 2, 3, 9) were over twice as high as on Pt(S)-[n(100) (111)].204 In addition, Pt(210) (= Pt(S)-[2(100) (110)]) yielded the highest rate of CO2 reduction. This remarkably high activity might derive from the kink site characteristics of Pt(S)-[n(110) (100)] and Pt(S)-[n(100) (110)]. The order of activity for the stepped surfaces was Pt(110) > Pt(S)-[n(111) (111)] > Pt(S)-[n(111) (100)] > Pt(S)-[n(111) (100)] > Pt(100) > Pt(111).205 The most active site in the stepped surfaces was derived from the pseudo-4-fold bridged site in Pt(S)-[n(111) (111)]. Kinked stepped surfaces showed a higher activity than unkinked ones. The electrocatalytic activity of Pt single-crystal electrodes towards CO2 reduction decreased in the order Pt(210) > Pt(310) > Pt(510).206 More recently, when evaluating 12 Bi–, Se–, Te–, and Sb–Pt(hkl) electrodes for the electrocatalysis of CO2 reduction in both acid and neutral aqueous media, Sanchez-Sanchez et al.207 found that only Bi–Pt(111), Te–Pt(111), and Sb–Pt(100) electrodes showed a visible current increase in the presence of CO2, whereas Se–Pt(100) and Te–Pt(100) had lower catalytic activities than the corresponding unmodified Pt(100) electrode. The surface structure effects of Rh, Pd, and Ir single-crystal electrodes were also examined, as shown in Table 6. It was found that the rate of CO2 electroreduction on a Pd single-crystal electrode was strongly dependent on the crystal orientation. The rate of CO2 reduction at 0.5 V vs. RHE on Pd(110) was two orders of magnitude higher than on Pt(110).208 2.1.5.2 Pt group metal oxide (mixture) catalysts. In an early study on the electroreduction of CO2 on various conductive oxide mixtures (RuO2, TiO2, MOO2, Co3O4, and Rh2O3), two metal oxide electrodes—i.e., RuO2 (35, mole percentage) + TiO2(65) and RuO2(20) + Co3O4(10) + SnO2(8) + TiO2(62)—showed high current efficiencies for methanol production when the electrode potential was controlled near the equilibrium potential of hydrogen evolution in a solution of ¨hne209 0.2 M Na2SO4 (pH 4) saturated with CO2.42 Later, Bandi and Ku investigated the electrocatalytic activities of mixed Ru/Ti oxide electrodes (titanium sheets); their results indicated that the overpotential for H2 evolution increased with increasing TiO2 content. In a comparison study of CO2 electroreduction in 0.5 M NaHCO3 solution, three electrodes—Ru, Cu–Cd-modified Ru, and Cu–Cd-modified RuOX + IrOX—were used for electrolysis for 8 hours while the potential was held at 0.8 V vs. SCE. Both methanol and acetone were produced.177 A RuO2-coated diamond Table 6 Orders of catalytic activity of CO2 electroreduction catalyzed by single-crystal electrode surfaces of Pt group metals

Rh Pd Ir Pt

Rh(110) > Rh(100) > Rh(111) (0.1 M HClO4)272 Rh(100) > Rh(110) > Rh(111) (0.5 M H2SO4)272 Pd(110) > Pd(111) > Pd(100) (0.1 M HClO4)208 Ir(110) c Ir(100) = Ir(111) (no reactivity)274 Pt(S)-[n(110)–(100)] > Pt(110) > Pt(S)-[n(111) (111)] > Pt(S)-[n(100) (111)] > Pt(S)-[n(111) (100)] > Pt(100) > Pt(111);202,203,205 Pt(210) > Pt(310) > Pt(510)206

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film on a Si(111) wafer was also used as the electrode for the electrochemical reduction of CO2.210 The main reduction products obtained in acidic and neutral media were HCOOH and CH3OH, with efficiencies of 40 and 7.7%, respectively. Qu et al.46 loaded RuO2–TiO2 nanotubes and RuO2–TiO2 nanoparticles, respectively, onto Pt electrodes for CO2 electroreduction. Compared with electrodes coated with RuO2 or with RuO2–TiO2 nanoparticles, the electrodes coated with RuO2–TiO2 nanotubes had a higher electrocatalytic activity for the conversion of CO2 to CH3OH, with a current efficiency of up to 60.5%, suggesting that the nanotube structure might be important in achieving high efficiency and selectivity for CO2 electroreduction. 2.1.5.3 Pt group metal complex catalysts Pt group metal complexes with tetradentate ligands. Becker et al.96 explored several Pd-tetradentate complexes, including PdII–porphyrins (PdII–TPP) and PdII-2,3,7,8,12,13,17,18-octaethylporphyrin (PdII–OEP) complexes, as electrocatalysts for CO2 reduction. CV results showed that only PdII–TPP and PdII–OEP displayed electrocatalytic activity towards CO2 reduction, producing oxalic acid. Sende et al.57 found that the reduction potential of CO2 at electrodes modified with electropolymerized films of [Fe(4-v-tpy)2]2+, [Ru(4-v-tpy)2]2+, and [Os(4-v-tpy)2]2+ tended to become more negative from Fe to Os (1.10, 1.20, and 1.22 V, respectively), which is consistent with their positions in the periodic table (the first, second, and third rows). Pt group metal complexes of polypyridine. Bolinger et al.211 studied CO2 electroreduction using [Ru(tpy)(dppene)Cl]+ (dppene = cis-1,2-bis(diphenylphosphino)ethylene) or cis-[Rh(bpy)2(TFMS)2]+ (TFMS = trifluoromethanesulphonate anion) as the target catalysts. Two Ru complexes—[Ru(tpy)(bpy)(S)]2+ (S = solvent) and Ru(tpy)(Mebin-py)(S)2+ (Mebim-py = 3-methyl-1-pyridylbenzimidazol-2-ylidene)—and their catalytic activities in CO2 electroreduction were recently reported.212 In addition, Rh(III) complexes of tptz have also been explored and showed effective catalytic properties in CO2 electroreduction.213 Regarding transition metal polyphosphine complexes as electrocatalysts for CO2 reduction, Slater and Wagenknecht214 investigated Rh(diphos)2Cl (diphos = 1,2-bis((diphenylphosphino)ethane2) in CH3CN. DuBois and Miedaner215 also observed the catalytic activity of M(PhP(CH2CH2PPh2)2)L(BF4)2 (M = Pd, Pt, Ni) for CO2 reduction to CO in acidic CH3CN solutions. Pd complexes (L = CH3CN, P(OMe)3, PEt3, P(CH2OH)3, or PPh3) exhibited significant CO2 catalytic activity, while Pt complexes (L = PEt3) and Ni complexes (L = P(OMe)3 and PEt3) did not. In another report, DuBois et al.216 confirmed the electrocatalytic activity of [Pd(tridentate)(CH3CN)](BF4)2 complexes in acidic DMF or CH3CN solutions; they found that if one or more of the phosphorus atoms of the tridentate ligand were substituted with a nitrogen or sulfur heteroatom, the resulting complexes would not show significant catalytic activity towards CO2 reduction. By comparing the rate constants of catalysts with different alkyl and aryl substituents on the terminal phosphorus atoms, they found that the reaction rate of Pd(I) intermediates with CO2 was increased by increasing the electron-donating ability of the R groups, and that the steric interactions were of less importance.


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Steffey et al.217 synthesized and characterized many types of Pd complexes containing tridentate ligands with PXP (X = C, N, O, S, As) donor sets—for example, the complex Pd(PCP)(PEt3)(BF4) (where PCP is 2,6-bis((diphenylphosphino)methyl)phenyl)—and also evaluated their catalytic activities towards CO2 electroreduction. They found that Pd(PCP)(CH3CN)(BF4) exhibited significant catalytic currents in the presence of acid and CO2. The dependence of the catalytic current on CO2 and acid concentrations was consistent with the formation of a hydroxycarbonyl intermediate, which decomposed in the presence of acid to form H2 as the catalytic product. Raebiger et al.171 explored a bimetallic Pd complex with bis(triphosphine) ligand (C6H4(P(CH2CH2P(C6H11)2)2)2) and found that this complex could electrocatalyze the reduction of CO2 to CO in acidic DMF solutions with a significantly higher TON than that achieved with other previous monometallic, bimetallic, and dendritic complexes of this catalyst class. Pt group metal complexes with bidentate ligands. Polypyridyl complexes of the second- and third-row transition metals have also been reported to act as efficient homogeneous catalysts for the electrocatalytic reduction of CO2. Ishida et al.218,219 conducted research on the controlled potential electrocatalysis of CO2 reduction in a saturated H2O–DMF (9 : 1, v/v) solution containing Ru(bpy)2(CO)22+ or Ru(bpy)2(CO)Cl+ at an electrode potential of 1.5 V vs. SCE, and observed different product selectivity at different pH values. At pH 6.0, both CO and H2 were the main products, whereas at pH 9.5, HCOO was produced. They considered [Ru(byp)2(CO)COO]+, i.e., [Ru(byp)2(CO)COO]0, to be an important intermediate formed through the following reactions: [Ru(bpy)2(CO)2]2+ + OH - [Ru(byp)2(CO)COOH]+

(30)

[Ru(byp)2(CO)COOH]+ + OH - [Ru(byp)2(CO)COO]+ + H2O (31) Moreover, the authors also found that the percentages of HCOO, CO, and H2 produced during the reduction were largely dependent on the pKa.218 For example, the current efficiency of HCOO formation was increased by increasing the pKa value and reached 84.3% in the presence of Me2NHHCl. Pugh et al.220 studied the catalytic activity of the complex cis[Ru(bpy)2(CO)H](PF6) (i.e., cis-[Ru(bpy)2(CO)H]+) and, by means of FTIR spectroscopy, observed the cis-[Ru(bpy),(CO)H]+, cis[Ru(bpy)2(CO)(OC(O)H)]+, and cis-[Ru(bpy)2(CO)(NCCH3)]2+ species in the solutions at the end of the electrolysis period. They suggested a different mechanism of HCOO formation that included the formation of cis-[Ru(bpy)2(CO)H]0 and the insertion of CO2, as shown in Fig. 12. Chardonnoblat et al.221 prepared a ‘‘[RuII(bpy)(CO)2]n’’ polymeric film electrode by the electrochemical reduction of a mono(bipyridine) complex, such as RuII(bpy)(CO)2(Cl)2 or RuII(bpy)(CO)2(CH3CN)2, for CO2 electrocatalytic reduction. Exhaustive electrolysis at 1.55 V vs. SCE produced CO with a current efficiency of 97% but yielded only 3% current efficiency for formate production. By comparing the electrochemical behaviors of isomers of Ru(bpy)(CO)2Cl2 (trans(Cl)-Ru(bpy)-(CO)2Cl2 and cis-(Cl)-Ru(bpy)(CO)2Cl2) and the behaviors of cis-(CO)Ru(bpy)(CO)2(C(O)OMe)Cl complexes, they found that for the


Fig. 12 Schematic of the CO2 electroreduction mechanism catalyzed by the cis-[Ru(bpy)2(CO)H](PF6) complex. Reprinted with permission from ref. 220. Copyright r 1991 American Chemical Society.

cis-(Cl)-Ru(bpy)(CO)2Cl2 and cis-(CO)-Ru(bpy)(CO)2(C(O)OMe)Cl complexes, catalyzed CO2 electroreduction could lead to a Ru–Ru dimer when a chloride ion was lost, while the trans(Cl)Ru(bpy)-(CO)2Cl2 complex could form a polymeric film of [Ru(bpy)(CO)2]n.222 This suggested that a choice could be made between homogeneous and heterogeneous systems using the same experimental setup simply by changing the stereochemistry of the precursor. Collombdunandsauthier et al.223 also confirmed the catalytic activity of [Ru(II)(bpy)(CO)2]n polymeric thin films. The (mononuclear) [(bpy)2Ru(dmbbbpy)](PF6)2 and (dinuclear) [(bpy)2Ru(dmbbbpy)Ru(bpy)2](PF6)4 (dmbbbpy = 2,20 -bis(1-methylbenzimidazol-2-yl)-4,4 0 -bipyridine) was synthesized for CO2 electroreduction by Ali et al.224 In CO2-saturated CH3CN, CO2 reduction catalyzed by [(bpy)2Ru(dmbbbpy)](PF6)2 yielded a current efficiency of 89% for HCOOH in the presence of H2O (B2.5%) and 64% for C2O42 in the absence of H2O; in a similar experiment, the current efficiencies for [(bpy)2Ru(dmbbbpy)Ru(bpy)2](PF6)4 were 90% and 70%, respectively. Tanaka and Mizukawa225 observed the highly selective formation of ketones (current efficiency of 20%) in the electrocatalytic reduction of CO2 by Ru(bpy)(napy)2(CO)22+ (napy = 1,8-naphthyridine-KN) in the presence of (CH3)4NBF4. Only CH3C(O)CH3 and CO32 were formed, according to the following reaction:

(32) For structural and spectroscopic characterization, Tanaka et al.226 employed Ru(II) complexes [Ru(bpy)2(CO)L] (L = CH3, C(O)H, and C(O)CH3) as the model catalysts for a multi-step CO2 reduction study using IR, Raman, 13C-NMR, and singlecrystal X-ray crystallography. To probe the mechanism, they further prepared a series of [Ru(bpy)2(CO)L]n+ (L = CO2, C(O)OH, CO, CHO, CH2OH, CH3, and C(O)CH3; n = 0–2) and

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determined their molecular structures by X-ray analyses.227 They did so because they thought that these complexes might be the reaction intermediates in the multi-electron reduction of CO2 in protic media. Not all ruthenium complexes of 2,2 0 -bipyridine have been found to be electrocatalytically active toward CO2 reduction.215 For example, Begum and Pickup228 experimentally confirmed that Ru(2,20 -bipyridine)2[2-(2-pyridyl)benzothiazole]2+ was a highly active catalyst whereas 1-methylbenzimidazole analogue was not. Bolinger et al.229 investigated the electrocatalytic reduction of CO2 by 2,2 0 -bipyridine complexes of both Rh and Ir (cis-[M(bpy)2Cl2]+ or cis-[M(bpy)2(TFMS)2]+, where TFMS is a trifluoromethanesulfonate anion). It was observed that cis[Rh(bpy)2X2]+ (X = Cl or TFMS) gave a current efficiency as high as 80% for CO2 electroreduction to formate at a potential of 1.55 V vs. SCE in CH3CN solution. The proton source for the formate formation was considered to be [(n-Bu)4N](PF4) via the Hofmann degradation: [Rh(bpy)2] + CO2 - [Rh(bpy)2CO2]

(33)

[Rh(bpy)2CO2] + (n-Bu)4N+ - [Rh(bpy)2CO2H] + H3CCH2CHQCH2 + (n-Bu)3N [Rh(bpy)2CO2H] - [Rh (bpy)2] + HCOO I

+

(34) (35)

The electrocatalytic activity of cis-[Os(bpy)2(CO)H](PF4) in the reduction of CO2 was observed by Bruce et al.230,231 They found that under anhydrous conditions, CO was the dominant product. However, in the presence of water, 25% HCOO could be formed. In the reduction process, [Os(byp)2(CO)H] was considered an important intermediate that could be coordinated by CO2 to produce CO and HCOO. Nallas and Brewer170 explored a new family of catalysts for CO2 electroreduction, namely, two mixed-metal trimetallic complexes: {[(bpy)2Ru(BL)]2IrCl2}5+ (BL = 2,3-bis(2-pyridyl)quinoxaline) and 2,3-bis(2-pyridyl)benzoquinoxaline. The two remote Ru centers served to tune the redox properties of the central catalytically active IrIII(BL)2Cl2 core. These catalysts should represent a new class of systems in which the redox properties of catalytic sites can be altered through remote metal coordination and variation without changing the coordination environment of the catalytic iridium site. Some Pt group metal complexes with bidentate ligands, such as Ir complexes, have also been explored for CO2 electrochemical reduction. For example, two [Ir2(dimen)4]Y2 complexes (dimen = 1,8-diisocyanomenthane; Y = (PF6) and [B(C6H5)2]) were studied using infrared spectroelectrochemistry; the results indicated that [Ir2(dimen)4]2+ first accepted two electrons to form [Ir2(dimen)4]0, then reacted with CO2 and H2O to form two main products, formate and bicarbonate.232 Pt group metal complexes with monodentate ligands. Hossain et al. reported the electrocatalytic reduction of CO2 on either a GCE or a Pt electrode using a series of PdL2Cl2 complexes (L = substituted pyridine and pyrazole) as catalysts in CH3CN containing 0.1 M tetraethylammonium perchlorate.233 These catalysts (L = pyrazole, 4-methylpyridine, and 3-methylpyrazole) gave

650 | Chem. Soc. Rev., 2014, 43, 631--675

HCOOH yields of 10–20%, while the current efficiency for hydrogen evolution was 31–54%. RhCl(CO)(PPh3)2 and IrCl(CO)(PPh3)2 (PPh3 = triphenylphosphine) showed different electrocatalytic behaviors in DMF solution. IrCl(CO)(PPh3)2 was found to be an efficient homogeneous catalyst for CO2 electroreduction to CO and HCOOH.234 In addition, the Pd–organophosphine dendrimer complex was tested as a CO2 reduction catalyst.235 2.1.5.4 Other catalysts containing Pt group metals. Other complexes containing Pt group metals have also been explored as CO2 reduction electrocatalysts.236–240 A typical candidate was an Ir complex, for example, Ir(Z5-C5Me5))2(Ir(Z4-C5Me5)CH2CN)(m3-S)2, as reported by Tanaka et al.237 They used it to catalyze CO2 reduction to produce oxalate in CO2-saturated CH3CN solution. Some water-stable iridium dihydride complexes supported by PCP-type pincer ligands in CH3CN–water mixtures were also explored as catalysts for CO2 reduction.240 2.1.6 Copper, silver, gold, zinc, cadmium, and mercury 2.1.6.1 Cu/Ag/Au/Zn/Cd/Hg metal electrodes. Several studies using metal electrodes for CO2 electroreduction in aqueous KHCO3 solution reported that CH4 was predominantly the product at a Cu cathode, CO at Ag and Au cathodes, and HCOO at a Cd cathode,38,241 as reviewed by Jitaru et al.242 and Gattrell et al.243 Cu electrodes. The metallic Cu electrodes thus far developed can be classified into several types: bulk Cu electrode, Cuelectrodeposited GCE, in situ electrodeposited Cu electrode,241,244,245 and Cu-coated GDE.246 Aside from low hydrocarbons such as CH4, C2H4, CO, HCOOH, alcohols (methanol, CH3CH2OH, and CH3CH2CH2OH), and esters, some relatively high hydrocarbons—such as paraffins and olefins containing up to 6 carbon atoms—can also be formed using Cu electrodes.38,241,247–251 The Faradaic efficiencies of these products were largely dependent on temperature, type and concentration of electrolytes, electrode potential, pH, crystal surface, and even the purity of the cathode material (Cu). For example, in the electrolysis of CO2-saturated 0.5 M KHCO3 aqueous solution with a 99.999% Cu sheet cathode, Hori et al.247 found that increasing the temperature (0 to 40 1C) caused the Faradaic efficiency of CH4 production to drop rapidly from 65% to nearly zero, while that of C2H4 gradually increased by up to 20%. In addition, they carried out experiments using a range of electrolyte strengths, from 0.03 to 1.5 M.249 Using voltammetric, coulometric, and chronopotentiometric measurements, they observed that CO was predominantly formed at potentials more positive than 1.2 V vs. NHE, while hydrocarbons (e.g., CH4 and C2H4) and alcohols (e.g., CH3CH2OH and CH3CH2CH2OH) were produced in greater abundance below 1.3 V, where the Faradaic efficiency of CO dropped. In fact, CO was a reaction intermediate that strongly adsorbed on the cathode, interfering with hydrogen formation. In KCI, K2SO4, KCIO4, and dilute HCO3 solutions, C2H4 and alcohols were found to be the main products, whereas CH4 was preferentially produced in relatively concentrated HCO3 and K2HPO4 solutions. In nonaqueous solutions, the electrochemical reduction of CO2 on a 99.999% Cu wire in CH3OH at 20–25 1C and 40 atm primarily yielded CO when tetrabutylammonium (TBA) salts (TBABF4 and TBAClO4) were


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Fig. 13 Current–potential characteristics of the electrochemical reduction of CO2 in a CO2 + methanol medium with various concentrations of TBABF4 as the supporting electrolyte. Concentrations of TBABF4: 33 mM (dotted line); 66 mM (dashed line); 0.1 M (solid line). Reprinted with permission from ref. 252. Copyright r 1995 Elsevier.

used as supporting electrolytes.252 The formation of HCOOCH3 became predominant when lithium salts (LiBF4, LiClO4, and NH4ClO4) were used. The current–potential curves for various concentrations of TBABF4 showed a large cathodic current and a shoulder wave (Fig. 13). Since the magnitude of the shoulder wave was dependent on the concentration of TBABF4, this wave was attributed to the reduction of TBA+, presumably to TBA : NR4+ + e - NR4

(36)

CO2 + NR4 - CO2 + NR4+

(37)

At 2.3 to 4.0 V vs. Ag/AgCl, the electrochemical reduction of CO2 at a 99.98% Cu electrode in a CH3OH solution containing 80 mM LiOH supporting salt at about 30 1C, CH4, C2H4, CO, and HCOOH were the main products.253 The best current efficiency for CH4 (the main product) was 63% at 4.0 V vs. Ag/AgCl. When Kaneco et al.254 studied CO2 electroreduction at a 99.98% Cu electrode in a CH3OH solution with 80 mM CsOH supporting salt at about 30 1C, CH4, C2H4, C2H6, CO, and HCOOH were the products. The maximum Faradaic efficiency of C2H4 (the main product) was 32.3% at 3.5 V vs. Ag/AgCl. The C2H4/CH4 current efficiency ratio was in the range 2.9–7.9. It was thought that small cations, such as Li+ and Na+, might not easily adsorb on the electrode surface due to their strong hydration, while a less hydrated, bulky cation, such as Cs+, might preferentially adsorb on the cathode, giving a less hydrated electrode surface. On such a surface, the conversion of the intermediate CuQCH2 to CH2CH2 might occur more easily than on a more hydrated one, producing more CH2CH2 in the presence of Cs+. Under a CO2 pressure of 10 atm at about 30 1C in CH3OH with 0.5 M CsOH supporting salt, when the potential was increased from 3.5 to 2.0 V, the Faradaic efficiencies of CO and C2H4 dropped slowly from 84% to 40% and 5% to 4%, respectively, but those of HCOOH, CH4, and H2 slowly increased.255 Kaneco et al.256 suspended 1 mm copper


particles in CH3OH and conducted the electrochemical reduction of CO2 with Pb and Zn electrodes. Results showed that without the addition of the copper particles, only HCOOH and CO could be detected, but after their addition, hydrocarbons were formed. The Faradaic efficiencies of CH4 and C2H4 rose gradually as the amount of Cu particles in the solution was increased, while the currency efficiencies of HCOOH and CO decreased. When B-370 Cu (99.9% pure) was employed at the cathode instead of high-purity Cu,257 the Faradaic efficiency of CH4 was found to be lower. In Hori’s study,248 which used pure Cu (99.999%) and different electrolytes—including KCl (0.1/0.5/1.5 M), KHClO4 (0.1 M), K2SO4 (0.1 M), and K2HPO4 (0.1, 0.5 M)—the Faradaic efficiency of C2H4 reached 38.2–48.1%, while that of CH4 was 11.5–17.0%. Kim et al.258 tried to enhance the formation rate of CH4; the highest rates they obtained were 8 105 mol cm2 h1 (22 1C, 17 mA cm2) and 1.1 105 mol cm2 h1 (0 1C, 23 mA cm2) at 2.0 and 2.3 V vs. SCE, respectively, on Cu foil in 0.5 M KCO3 (pH 7.6). They found that the formation rate of CH4 was even higher if the electrode surface was prepared by cleaning it using HCl rather than HNO3 or by oxidation in air. Indeed, electrode surface conditions can have a major effect on an electrode’s catalytic activity in CO2 electroreduction. For example, some significant performance differences were found between rough and smooth electrodes as well as between thermally and nonthermally treated electrodes.259 Ohta et al.260 found that under ultrasonic irradiation, the production rates of CH4, HCOOH, and CO were greatly affected.260 Cook et al.244,261 employed an in situ electrodeposited Cu electrode (glass carbon substrate with in situ deposited Cu) in 0.5 M KHCO3 aqueous solution. Although the main products were still CH4 and C2H4, Cu purity and morphology were found to be crucial for promoting a high rate of CO2 reduction. They achieved a Faradaic efficiency of 71.3% for C2H4 and CH4 production by employing a Cu-based GDE.246 Recently, a Cu nanoparticle-covered electrode was reported to give better selectivity towards hydrocarbons than the surfaces of electropolished and argon-sputtered copper electrodes.262 The copper nanoparticles were formed in two steps. In the first step, the potential at the electropolished copper electrode was scanned between 0.6 and +1.15 V vs. RHE at 20 mV s1 under N2-saturated KClO4. In the second step, the copper was redeposited on the electrode surface; this was performed under CO2saturated KClO4 with a constant bias of 1.3 V vs. RHE for 20 minutes. Scanning tunneling microscopy (STM), scanning electron microscopy (SEM) (Fig. 14), and CV (Fig. 15) were employed to compare various treated electrodes. A few layers of Cu nanoparticles with sizes of 50–100 nm covered the Cu surface, creating a surface area 2–3 times greater than the geometric surface area of the Cu electrode. However, CV measurements in CO2 (pH 6.0) showed that the current density of the nanoparticle-covered surface (at 0.75 V vs. RHE) was 10 times higher than that of the electropolished surface, indicating that surface morphology can contribute more to current density than just the effect of increased surface area. The morphological effect was explained by the roughened surface having a greater abundance of undercoordinated sites; this was demonstrated by DFT calculations. In addition, electrochemical quartz crystal

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Fig. 14 Scanning electron microscopy images for three types of surfaces: (a) electropolished, (b) copper nanoparticle covered, and (c) sputtered. Reprinted with permission from ref. 262. Copyright r 2012 The Royal Society of Chemistry.

microbalance (EQCM)—a high-resolution (Bnanograms) mass sensing technique—has also been used to probe electroformed and electroreduced products on a copper electrode in aqueous solutions containing NaHCO3 and Na2CO3.263 Li and Oloman264 investigated the electroreduction of CO2 in a laboratory bench-scale continuous reactor in which a flow-by

Fig. 15 Cyclic voltammograms (CVs) of the formation of copper nanoparticles in 0.1 M KClO4 purged with N2 at pH 10.5. (a) CVs of the electropolished copper surface, copper nanoparticle covered surface, and sputtered copper surface in 0.1 M KClO4 purged with (b) CO2 (c) and N2. The current density is normalized by the geometric area of the electrode surface. The overpotentials are corrected for ohmic resistance between the working and reference electrodes. Reprinted with permission from ref. 262. Copyright r 2012 The Royal Society of Chemistry.

652 | Chem. Soc. Rev., 2014, 43, 631--675

3D cathode of 30# mesh tinned-copper was used as the cathode. With currents of 1–8 A, feeding gas phase CO2 concentrations of 16–100 vol%, and operating times of 10–180 minutes, a current efficiency of 86% was achieved for HCOO. The efficiency was dependent on current density and CO2 pressure. In a study of copper-catalyzed CO2 electroreduction, Kuhl et al.265 reported an experimental methodology that allows for product identification and quantification with unprecedented sensitivity. Among all the possible products, CH4 and C2H4 had the largest current efficiencies; the remaining products were oxygenates and other C2 and C3 species. The researchers offered some possible reaction pathways to account for the production of all the C2 and C3 species observed (see Fig. 16). Regarding the catalytic stability of CO2 electroreduction on Cu electrodes, several other factors have been identified, including CO adsorption,266 electrode purity,257 the formation of carbon deposits,267 and the presence of other surfacepoisoning species.268,269 The carbon deposited film seems to be a major factor in the irreversible degradation of the electrode surface. For example, when the electrochemical reduction of CO2 to both CH4 and C2H4 was conducted in aqueous 0.5 M KHCO3 solution at a constant potential of 2.00 V vs. SCE, a black film formed on the surface of the Cu (99.999%) cathode.267 XPS and AES studies indicated that this film was graphitic carbon formed by CO2 reduction through HCOO. Graphitic carbon deposit was also found to cause a decline in Cu electrodes’ catalytic activity.270,271 To resolve this problem, the researchers developed a new method that could selectively convert CO2 to C2H4 at the threephase (gas/liquid/solid) interface on a CuIBr confined Cu-mesh electrode in an aqueous solution of KBr. The conversion percentages of CO2 and H2 were found to be about 90% and 2%, respectively. It was suggested that the immobilized CuIBr, acting as a heterogeneous catalyst, offered some adsorption sites for reduction intermediates such as CO and carbene (H2C:). The mechanism can be expressed as follows: CO2(g) + 2H+ + 2e - CO(g) + H2O

(38)

CuBr(s) + CO(g) - CuBr CO

(39)

CuBr CO + 4H+ + 4e - CuBr :CH2 + H2O

(40)

2CuBr :CH2 - CuBr :CH2QCH2 + CuBr(s) (41) Similar to Pt-group metals such as Rh,272 Pd,208,273 Ir,274 Pt, and Ag,275 single-crystal copper electrode surfaces 201


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Fig. 16 Proposed reaction pathways for C2 and C3 products with enol-like surface intermediates. Arrows between overlapping circles indicate the changes between the enol, keto, and diol forms of each product. Arrows between non-overlapping circles indicate the electrochemical reduction steps involved in the addition of 2H+ and 2e. For simplification, product names are intended to refer to all forms of the product. Reprinted with permission from ref. 265. Copyright r 2012 The Royal Society of Chemistry.

can normally enhance the electrocatalytic activity in CO2 electroreduction by introducing steps and kinks into atomically flat surfaces. For example, Hori et al.276–278 investigated the electrochemical reduction of CO2 at various types of copper singlecrystal electrodes in 0.1 M KHCO3 aqueous solution and found that the reaction selectivity could be greatly altered by changing the crystal orientation. The major product with electrodes based on (100) terrace surfaces (i.e., Cu(S)-[n(100) (111)] and Cu(S)-[n(100) (110)]) was C2H4. The formation of CH4 was promoted at Cu(111) or by the introduction of (111) or (110) step atoms to the (100) basal plane. A Cu(S)-[n(111) (111)] electrode yielded high amounts of C2+ substances (i.e., substances containing more than two carbon atoms), while a (110) electrode derived from Cu(S)-[n(111) (111)] uniquely produced high yields of CH3COOH, CH3CHO, and C2H5OH. CO seems to be the key intermediate in CO2 electroreduction to CH4 and CH2CH2 on Cu electrodes.279,280 Schouten et al.280 observed two reaction pathways: a C1 pathway leading to CH4 formation on single-crystal Cu(111) electrodes and a C2 pathway leading to ethylene formation on Cu(100) electrodes.281 The authors also proposed a mechanism based on reactions as a function of potential, using online mass spectrometry combined with mechanisms suggested in the literature. Regarding theoretical studies of CO2 electroreduction on Cu electrodes, some researchers have carried out studies to further our fundamental understanding of the effects of face-centered Cu facets—such as cubic fcc(111), fcc(100), and fcc(211)—on the energetics of CO2 electroreduction. Durand et al.282 reported that the intermediates in CO2 reduction could be mostly stabilized by the (211) facet, followed by fcc(100) and fcc(111). This implied that the (211) facet should be the most active surface in producing CH4, as well as the by-products H2 and CO. HCOOH production might be mildly enhanced on the more closely packed surfaces (i.e., (111) and (100)). Their theoretical prediction of the trends for voltage


requirements was consistent with experimental measurements. Liu et al.283 studied the electroreduction of CO2 to CO on Fe, Co, Ni, and Cu surfaces using a DFT method involving three reaction steps: adsorption of CO2, decomposition of CO2, and desorption of CO. Both the binding energies and the reaction energies were calculated. They found that the reaction energies and the total reaction energy barrier were strongly dependent on the type of electrode metal. Ag and Au electrodes. Similar to Cu, both Ag and Au electrodes also show considerable catalytic activity towards CO2 electroreduction when appropriate electrolytes are employed.284 In 0.1 KHCO3 aqueous solution at 25 1C, the Faradaic efficiencies for CO production from CO2 at Ag(99.98%) and Au(99.95%) electrodes at 1.6 V vs. Ag/AgCl saturated with KCl were found to be 64.7% and 81.5%, respectively.38 A recent study251 reported that when Ag-coated nanoporous Cu composites (NPC) were employed in CO2 reduction in BMImBF4, the electrosynthesis of dimethyl carbonate (DMC) could be realized. The highest yield of DMC was 80%; this was attributed to the electrodes’ high surface area, open porosity, and high efficiency. A sputtering deposition technique was used to prepare a Au electrode for CO2 reduction.285 The results indicated that Ar pressure had an effect on the Au surface’s geometrical structure and surface area, resulting in different CO2 reduction potentials in both KCl and KHCO3 solutions. A porous Au film electrode (a 200–260 nm Au film deposited on a porous hydrophilic polymer membrane) was also prepared by vapor deposition for the electrochemical reduction of CO2 to CO in 99.99% KHCO3 aqueous solution.286 Recently, ligandprotected Au25(SC2H4Ph)18 clusters were explored to promote the catalytic electroreduction of CO2, and high catalytic activity in the conversion of CO2 to CO was achieved.287 Zn, Cd, and Hg electrodes. Due to the high overpotentials of hydrogen evolution on Zn, Cd, and Hg electrodes, they

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were considered as suitable electrodes for CO2 reduction. In 0.1 M KHCO3 aqueous solution, the electrochemical reduction of CO2 on Zn, Hg, and Cd electrodes was found to be very selective for the formation of HCOOH and CO, with HCOOH seemingly the only product at the Hg cathode. On metallic Zn, Cd, and Hg electrodes, the Faradaic efficiencies of HCOOH were measured to be 20%, 39%, and 94%, while those of CO were 39.6%, 14.4%, and undetectable, respectively.38 Shibata et al.288 employed a Cd-loaded GDE to reduce CO2 and nitrite ions with various catalysts and found that the maximum current efficiency of urea ((NH2)CO) formation was about 55% at 1.0 V vs. SCE. In an organic solution such as DMSO, CO2 electroreduction was also studied using both Au and Hg electrodes.289,290 It should be mentioned that several decades ago, a mercury pool electrode was considered the best electrode for CO2 reduction.291 The results showed that in the neutral pH range, all the current was consumed in the production of HCOOH, while in acid solutions, both HCOOH and H2 were produced. The mechanism was proposed to be as follows: CO2 + e - CO2(ads)

(42)

CO2(ads) + H2O - HCO3(ads) + OH

(43)

HCO3(ads) + e - HCOO

(44)

H+ + e - Hads

(45)

H + CO2 - HCO2

(46)

H + H + + e - H 2

(47)

HCO2 + H + e - HCOOH

(48)

+

+

prepared a CuO–Zn composite electrode by pressing a mixture of Zn particles (B7 mm) and CuO or Cu2O powder (B100 nm) for CO2 reduction.296 It was found that without the copper oxide particles, only HCOOH and CO were formed, whereas with the CuO–Zn composite electrode, hydrocarbons such as CH4 and C2H4 could be obtained. The maximum formation efficiencies of CH4 and C2H4 were 7.5% and 6.8%, respectively. Le et al.142 examined the catalytic activity of an electrode electrodeposited as a cuprous oxide thin film and found that the Faradaic efficiency for CH3OH production was 38%. They believed that Cu(I) species should play a critical role in selectivity for CH3OH. More recently, Li and Kanan297 prepared Cu as Cu2O layers for CO2 reduction. The Cu2O layers formed at 500 1C exhibited a large surface roughness, resulting in the electrochemically active surface area (ECSA) of a reduced electrode being 480 times larger than that of a polycrystalline Cu electrode. This improved ECSA resulted in a 0.5 V lower overpotential for CO2 reduction than on a polycrystalline Cu electrode. An interesting result was that the SEM showed a dense array of rods with diameters of 100–1000 nm on the electrode surface. These rods were the outermost portion of a thick Cu2O layer coating the electrode. However, these rods were not necessary for efficient CO2 reduction. It was observed that the Cu particles formed by reducing mm-thick (B3 mm) Cu2O films at the potentials at which CO2 reduction occurred could catalyze the reduction of CO2 to CO and HCOOH with high Faradaic efficiencies and exceptionally low overpotentials (Fig. 17). At high overpotentials, this Cu particle electrode could produce C2 hydrocarbons exclusively. To obtain

With a Hg electrode, the formation of malate (OOCCH(OH)CH2COO) in the process of CO2 reduction was also observed in aqueous solutions containing quaternary ammonium salts.292 Since the observed coulombic yield was more than 100%, the overall reaction was believed to consist of not only reaction (49) but also possibly (50): (49)

(50) However, with other supporting electrolytes, such as NaHCO3, NaH2PO4–Na2HPO4, NaCl, NaClO4, Na2SO4, LiHCO4, and KHCO3 and their mixtures, the favorable product was found to be HCOO, and the Faradaic efficiency of HCOOH formation increased with increasing CO2 pressure.293,294 In 0.5 M KHCO3 aqueous solution, the Faradaic efficiency of HCOOH production was as high as 100% at a CO2 pressure of 20 atm.294 2.1.6.2 Cu and Au oxide-related catalysts Cu oxides. Cu oxides have been explored to catalyze CO2 electroreduction. For example, Chang et al.295 deposited as-prepared Cu2O particles onto a carbon cloth electrode for CO2 reduction and carried out CV measurements; the results showed that CH3OH was the predominant product. Ohya et al.296

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Fig. 17 SEM images (a) before and (b) after CO2 reduction electrocatalysis at 0.5 V vs. RHE; (c) Faradaic efficiencies for CO and HCO2H vs. potential on polycrystalline Cu and Cu annealed at 500 1C for 12 hours. Reprinted with permission from ref. 297. Copyright r 2012 American Chemical Society.


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insight into the mechanistic pathway(s) for CO2 reduction, Tafel plots were analyzed. The plot for annealed Cu was found to be linear over the range of overpotentials from 0.05 to 0.3 V with a slope of 116 mV dec1. This slope was consistent with a rate-determining initial electron transfer to CO2 to form a surface-adsorbed CO2* intermediate, which suggested that the Cu surfaces formed by reducing thick Cu2O layers could enable the formation of this CO2* intermediate while suppressing H2O reduction. Regarding the understanding of fundamentals, Wu et al.298 investigated the adsorption of CO2, H2CO3, HCO3, and CO32 on a Cu2O(111) surface by first-principles calculations based on DFT at the B3LYP hybrid functional level, in which the Cu2O(111) surface was modeled using an embedded cluster method. It was concluded that on the surface, H2CO3 was dissociated into an H+ ion and an HCO3 ion, which was the only activated CO2 species on the surface. Cu organic frameworks and Cu-based Perovskite-type catalysts. Metal organic frameworks with crystalline ordered structures, extra-high porosity, high thermal stability, as well as adjustable chemical functionality have been pursued for many purposes, including gas-storage applications.299–304 Recently, Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate), a Cu-based metal organic framework, was explored as an electrode for CO2 reduction in a

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CO2-saturated 0.01 M TBATFB–DMF solution (TBATFB = tetrabutylammonium tetrafluoroborate) by Kumar et al.305 It was observed that this catalyst has high selectivity towards CO2 reduction to oxalic acid, and the highly active site was believed to be a Cu(I) species (Fig. 18). Cu-based Perovskite-type A1.8A 0 0.2CuO4 (A = La, Pr, and Gd; A 0 = Sr and Th) were also explored for catalyzing CO2 electroreduction.306 For example, when these kinds of catalysts were incorporated into a GDE, the cumulative Faradaic efficiencies of CH3OH, CH3CH2OH, and CH3CH2CH2OH reached 40% in La1.8Sr0.2CuO4 GDE/0.5 M KOH aqueous solution under ambient conditions. Au oxide catalysts. Chen et al.307 reduced Au oxide films to Au nanoparticles on electrodes for CO2 reduction. High selectivity from CO2 to CO in water at overpotentials as low as 140 mV was observed. The high catalytic activity was thought to be due to the dramatically increased stabilization of the CO2* intermediate on the surfaces of the oxide-derived Au electrodes. The proposed mechanisms are shown in Fig. 19. 2.1.6.3 Cu/Ag/Au/Zn/Cd/Hg alloy catalysts. In CO2 electroreduction, metal alloys such as Cu alloys can exhibit both high electrocatalytic activity and product selectivity, and they yield products quite different from those produced on a pure Cu

Fig. 18 Cyclic voltammograms for (a) Cu3(BTC)2-coated GC (the red line represents GC background); (b) CV 1 = bare GC, 2 = bare GC in the presence of CO2, 3 = Cu3(BTC)2-coated GC, and 4 = Cu3(BTC)2-coated GC in the presence of CO2, in a solution containing 0.01 M TBATFB–DMF, at a scan rate of 50 mV s1; (c) FTIR spectra of (I) oxalic acid (authentic) and (II) oxalic acid (synthesized); (d) GC–MS spectrum of the bulk electrolysis product of CO2 at the Cu3(BTC)2 coated electrode surface. Reprinted with permission from ref. 386. Copyright r 2012 Elsevier.


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Fig. 19 Proposed mechanisms for CO2 reduction to CO on polycrystalline Au and oxide-derived Au. Reprinted with permission from ref. 307. Copyright r 2012 American Chemical Society.

electrode, where CH4 and C2H4 formation predominates. When using Cu alloy electrodes, HCOOH, CO, and CH3OH were found to be the products at less negative potentials and almost at the reversible potentials of their formation.308,309 In 0.05 M KHCO3 aqueous solution, Cu–Ni alloys could produce CH3OH and HCOOH at onset potentials of 0.38 V and 0.5 V vs. SHE, respectively, which were quite a bit lower than the 0.90 V required for CO production. Cu–Sn and Cu–Pb produced HCOOH and CO with an enhanced reaction rate at their reversible potentials. Both Cu single-crystal and Cu–Au alloy (CunAu100n, n = 99, 90, 80, 50, respectively) electrodes were also explored for CO2 electroreduction in aqueous KH2PO4–K2HPO4 buffer solution, and H2, CO, CH4, C2H4, and trace C2H6 were found to be the products.310 For the Cu single-crystal electrode, the fraction of CH4 in the product mixture was increased while that of CO was decreased, in the order Cu(poly) o Cu(100) o Cu(111). For the Cu–Au alloy electrode, the fraction of CO production increased markedly with increasing Au content, while the fraction of CH4 gradually diminished. Amongst all the examined electrodes, the Au50Cu50 alloy appeared to be the most efficient catalyst for the conversion of CO2 into carbon-containing gaseous products. Schizodimou and Kyriacou311 employed a Cu88Sn6Pb6 alloy cathode for CO2 reduction to investigate the effects of the supporting electrolyte and cathode potential on the reduction rate; the results showed that the fractions of H2, CO, CH3OH, HCOOH, CH4, CH3CHO, and C2H6 in the production mixture changed with the type of electrolyte and the cathode potential. 2.1.6.4 Cu/Ag/Au/Zn/Cd/Hg complex catalysts. The Cu/Ag/Au/ Zn/Cd/Hg complexes that so far have been explored for catalyzing CO2 electroreduction are those formed by metal cations with tetradentate ligands. In an early study, two Ag(II) porphyrin complexes, i.e., Ag–2,3,7,8,12,13,17,18-OEP and Ag–TPP, were employed to homogeneously catalyze the reduction of CO2 at a glassy carbon cathode in CH2Cl2 + 0.1 M tetrabutylammonium fluoroborate (TBAF) solution. It was found that only AgII(OEP) displayed electrocatalytic activity, and oxalic acid was the main product.96 Recently, Cu- and Zn-meso-TPP supported GDEs were fabricated for CO2 reduction in 0.5 M KHCO3 aqueous solution by Sonoyama et al.;97 their results showed that the current efficiencies for CO and HCOOH formation could be increased and H2 evolution depressed by increasing the CO2 pressure from 1 to 20 atm.

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In organic solutions, for example, a methanol with 0.1% KHCO3 and 0.05 M Bu4NBF4 solution, the Cu-2,9,16,23-tetratert-butyl-phthalocyanine (CuTBPC) complex coated on a graphite rod catalyzed CO2 reduction to CH4 with maximum current efficiencies of 29–35%, while an ‘‘electropolymerized’’ tetraaminosubstituted Cu-monophthalocyanine-coated electrode gave current efficiencies as high as 75–85%.312 However, when a Cu-2,9,16,23tetraaminophthalocyanine (CuTAPC) complex-coated electrode was used, the main product was found to be HCOOCH3, with small amounts of CO and CH4. The formation pathway for HCOOCH3 was assumed to be CO2 - CO2 - HCOOH HCOOCH3. In addition, Cu(II) complexes formed with other tetradentate ligands, such as dinaphthotetraaza[14]annulene and 5,5 0 -terpyridinophane macrocycles, have also been explored as catalysts for CO2 electroreduction.313–315 2.1.6.5 Other catalysts. Dinuclear Cu complexes have also shown activity towards CO2 electroreduction.168,169,172,316 For example, Field et al.168,169 synthesized one dinuclear Cu complex, Cu2(m-PPh2bipy)2(CH3CN)2(PF6)2 (PPh2bipy = 6-diphenylphosphino-2,2 0 -bipyridyl), by treating Cu(CH3CN)4PF6 with 6-diphenylphosphino-2,2 0 -bipyridine in CH3CN solution, and used it as the catalyst for homogeneously catalyzing the electrochemical reduction of CO2 to selectively produce CO and CO32 in 0.1 M tetra-N-butylammonium perchlorate–CH3CN solution. Two sequential single-electron transfers to [Cu2(m-PPh2bipy)2(CH3CN)2]2+ were observed at E1/2(2+/+) = 1.35 V and E1/2(+/0) = 1.53 V vs. SCE, respectively. Thus, two possible routes were proposed, as follows: [Cu2(m-PPh2bipy)2(CH3CN)2]2+ + e - [Cu2(m-PPh2bipy)2(CH3CN)2]+

(51)

[Cu2(m-PPh2bipy)2(CH3CN)2]+ + e - [Cu2(m-PPh2bipy)2(CH3CN)2]0 [Cu2(m-PPh2bipy)2(CH3CN)2]0 + CO2 - product

(52) (53)

Indeed, the PPh2bipy ligand offers the dual advantage of coordinated bipyridine and bridging phosphines. The p*-unsaturation of the bipy component of the PPh2bipy ligand could provide the ability to shuttle electrons in and out of a closed-shell d10–d10 binuclear complex. Kauffman et al.287 recently electrocatalyzed the reduction of CO2 to CO using ligand-protected [Au25(SC2H4Ph)18] clusters in aqueous solution at 1.0 V vs. RHE. The efficiency was approximately 100%, while the rate was 7–700 times higher than for larger Au catalysts and 10–100 times higher than for current state-of-the-art processes. 2.2

Aluminum, gallium, indium, and thallium

2.2.1 Al metal electrodes and Al-containing catalysts. Al has been tried for the catalysis of CO2 electroreduction, but unfortunately the catalytic activities have been extremely low.38,290 For example, it was reported that in the electrochemical reduction of CO2 by a metallic Al electrode at 1.6 V vs. Ag/AgCl in 0.1 M KHCO3 aqueous solution, the Faradaic efficiencies for CH4, C2H4, and C2H6 production were as low as 0.58%, 0.04%, and 0.11%,


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respectively, versus 99% for H2 evolution.38 In another experiment, at a constant potential of 2.2 V vs. SCE in a CO2-saturated 0.05 M KHCO3 aqueous solution, the sum of the typical current efficiencies of CH4, C2H4, C2H6, and HCOOH was less than 1%, compared with 95.7% for H2.290 2.2.2 Ga electrodes and Ga-containing catalysts. In a photoelectrochemical cell, p-type Ga-containing semiconductors—i.e., p-gallium phosphide (p-GaP),317,318 p-gallium arsenide (p-GaAs),319 and n-gallium arsenide (n-GaAs)318—were explored as catalysts; high selectivity for CH3OH formation (a 6e reaction) was observed but only at some exceptionally high overpotentials. Recently, Bocarsly et al.320 reported the highly selective reduction of CO2 to CH3OH at illuminated p-GaP photoelectrodes, with near 100% Faradaic efficiency for underpotentials greater than 300 mV at 0.52 V vs. SCE. 2.2.3 In metal electrodes and In-containing catalysts. Normally, CO2 electroreduction at an indium metal electrode in aqueous media predominantly produces formate,241,321,322 but in nonaqueous solutions, the main product is CO.322 Kapusta and Hackermant321 reported CO2 reduction to formate ions in a 0.5 M HCOOH + 0.5 M HCOONa solution with high current efficiency (about 95%), although the overall power efficiency was low due to the high overpotential of the reaction. Ikeda et al.322 also observed the electrochemical reduction of CO2 in 0.1 M tetraethylammonium perchlorate (TEAP) aqueous electrolytes at potentials of 1.8–2.4 V vs. Ag/AgCl; the current efficiencies for HCOOH formation were in the range of 80–90%.322 However, in 0.1 M TEAP/propylene carbonate (nonaqueous electrolytes) at 2.0 to 2.2 V vs. Ag/AgCl, the current efficiencies were up to 95%.323 Todoroki et al.294 achieved a Faradaic efficiency of 100% under 60 atm of CO2. They found that the efficiency of HCOOH formation became higher at high cathodic current densities (or more negative potentials), and that at less negative potentials, HCOOH formation could be suppressed and CO formation became relatively predominant. Mizuno et al.324 also found that the Faradaic efficiency for HCOOH was about 100% at 20–60 1C, compared with 44.5% at 100 1C. Recently, Narayanan et al.325 studied the conversion of CO2 to formate in an alkaline polymer electrolyte membrane cell in which In powder-coated porous carbon paper was the cathode. Three different aqueous solutions (CO2-saturated deionized H2O, 1 M NaHCO3, and 1 M Na2CO3) were used as the cathode feed. The instantaneous Faradaic efficiency of formate production achieved in NaHCO3 solution was as high as 80% (although it decreased to B10% over a period of 1.0 hour). Carbon mass transport was found to be the limiting factor for Faradaic efficiency. However, this mass transport limitation could be mitigated using a high bicarbonate concentration or high carbon dioxide pressure. In their experiments, high Faradaic efficiency could be maintained during continuous operation, even at moderately high current densities. An In-containing semiconductor (p-type indium phosphide, p-InP) electrode was used in the electrolysis of CO2-saturated Na2SO4 aqueous solutions; the photocurrent densities for CH3OH formation were found to be 60–100 mA cm2, and the current efficiencies were found to be 40–80%.319 Parkinson and


Review Article

Weaver326 tried to fix CO2 by combining a p-InP electrode with a biological catalyst (a formate dehydrogenase enzyme); they observed a 2e reduction of CO2 to HCOOH. Kaneco et al.327 carried out a series of experiments on the photoelectrochemical reduction of CO2 at a p-InP electrode in a methanol-based electrolyte (nonaqueous media). In CO2-saturated 80 mM LiOH–methanol solution, they observed the maximum current efficiencies to be B40% for CO and B30% for HCOOH in the potential range of 2.2 to 2.5 V vs. Ag/AgCl.327 They also found that metalmodified InP electrodes could give different product selectivity.328 For example, on Pb-, Ag-, Au-, and Cu-modified InP electrodes, the main reduction products of CO2 were CO and HCOOH (the maximum current efficiency of CO was 80.4% on Ag-InP); in comparison, a Pd-modified electrode yielded only CO, and a Ni-modified electrode produced hydrocarbons with low Faradaic efficiencies (0.7% for CH4 and 0.2% for CH2CH2). Recently, they tried to catalyze CO2 reduction by suspending Cu particles (1 mm diameter) in 0.10 M NaOH–methanol solution.329 After the addition of Cu particles, the current efficiencies for methane and ethylene improved. 2.2.4 Tl metal electrodes and Tl-containing catalysts. Tl metal electrodes in aqueous electrolytes have been found to favor the formation of formic acid, while in nonaqueous solutions, oxalic acid has been the dominant product.242,322 Unfortunately, the literature contains only these two cited studies of Tl metal electrodes as cathode catalysts for CO2 electroreduction. 2.3

Tin and lead

2.3.1 Sn metal electrodes and Sn-containing catalysts. Sn metal electrodes were reported to be most active towards CO2 electroreduction in aqueous electrolytes, producing HCOO. However, in nonaqueous electrolytes, the predominant product was CO, with small amounts of formic acid, oxalic acid, and glyoxalic acid.242,322 Some early studies indicated that Sn metal working electrodes could catalyze CO2 electrochemical reduction in aqueous inorganic salt solutions to exclusively produce HCOOH with a current efficiency as high as B95%.241,321 However, during the reduction reaction, organometallic complexes formed on the electrode surface, accelerating the rate of hydrogen evolution and leading to poor reaction efficiency.321 Increasing the temperature also can cause a decrease in Faradaic efficiency for formic acid production and an increase for hydrogen.324 Li and Oloman330,331 developed a scale-up reactor system for CO2 electroreduction, in which a granulated tin cathode (99.9 wt% Sn) and a feed gas of 100% CO2 were used. The results showed that the granulated tin cathode yielded better performance than the tinned-copper mesh cathode reported in their previous communications, in terms of both current efficiency and stability.264,330 The formate current efficiencies were up to 91%. When tin was electrodeposited on a GDE in a zero-gap cell for CO2 electroreduction, the electrode showed good stability.332 Chen and Kanan333 prepared a thinfilm catalyst by simultaneously electrodepositing Sn0 and SnOx onto a Ti electrode. Using an H-type cell reactor and CO2saturated aqueous NaHCO3 solution, the Sn0/SnOx catalyst exhibited up to eight-fold higher partial current density and

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Fig. 20 Tafel plots for SnB, SnG, and SnGDL, obtained from the corresponding voltammograms. Reprinted with permission from ref. 334. Copyright r 2013 Elsevier.

Table 7 Tafel parameters obtained from the Tafel plots. bc1 represents the slope for the lower overpotential region and bc2 for the high overpotential region. Reprinted with permission from ref. 334. Copyright r 2013 Elsevier

bc1 (mV) bc2 (mV) j0 (A cm2) Eeq (V)

SnB

SnG

SnGDL

116 430 1.2 106 0.10

180 480 2.8 105 0.030

185 458 1.6 104 0.053

four-fold higher Faradaic efficiency for CO2 reduction, more than a Sn electrode coated with a native SnOx layer. They suggested that metal–metal oxide composite materials were promising catalysts for sustainable fuel synthesis. Recently, Surya Prakash et al.334 compared three kinds of Sn electrodes for CO2 reduction in aqueous NaHCO3 solution: a Sn-powder-decorated gas diffusion layer (SnGDL) electrode, a Sn metal disc electrode (SnB), and a Sn powder-coated graphite (SnG) electrode. The exchange current densities ( j0, A cm2) of CO2 reduction on these electrodes, determined by Tafel plots from current–voltage curves, showed a five-fold increase on the SnGDL electrode as compared to that on the SnG electrode (Fig. 20 and Table 7), although the SnG electrode showed a j0 value two orders of magnitude higher than on the SnB electrode. The maximum current density obtained during electrolysis was 27 mA cm2 at 1.6 V vs. NHE, with 70% Faradaic efficiency for the formation of formate, which probably is one of the highest values to be found in the literature on Sn electrodes at ambient pressure. The electrolyte also has an effect on CO2 reduction on Sn-based electrodes. For example, Wu et al.335 found that both SO42 and Na+ gave higher Faradaic and energy efficiencies (as high as B95% for 0.1 M Na2SO4 at a potential of 1.7 V vs. SCE), while HCO3 and K+ yielded a higher rate of HCOOH production (0.5 M KHCO3 was found to be the optimal electrolyte for obtaining a high production rate of HCOOH, which reached over 3.8 mmol min1 cm2 at a potential of 2.0 V vs. SCE while maintaining a Faradaic

658 | Chem. Soc. Rev., 2014, 43, 631--675

Fig. 21 Tafel plots of the products by electrochemical reduction of CO2 at a Pb electrode in methanol at 15 1C. HCOOH, |; CO, J; H2, K. Catholyte/anolyte: 300 mM dm3 KOH in methanol. Reprinted with permission from ref. 338. Copyright r 1998 Elsevier.

efficiency of B63%). Normally, the Faradaic efficiency for CO2 reduction rises with increasing electrolyte concentration. 2.3.2 Pb metal electrodes and Pb-containing catalysts. In general, lead, glassy carbon, mercury, platinum, and gold are popular electrode materials for CO2 electroreduction.190 The earliest CO2 electroreduction was probably conducted on a lead cathode at 1.5 to 2.2 V vs. NHE in quaternary ammonium salt aqueous solutions, with a carbonate/bicarbonate buffer being used to maintain the pH at 8.3.336 Pb metal electrodes in aqueous electrolytes favored the formation of formic acid, while in nonaqueous electrolytes, oxalic acid was the dominant product.322 On a Pb electrode, the Faradaic efficiency could be increased by increasing the CO2 pressure and decreasing the temperature.294 For example, CO2 electroreduction on a Pb electrode in a CO2-satuarated 0.05 M KHCO3 solution at 0 1C gave a higher current efficiency than at higher temperature.290 At a high pressure of B50 atm and a high temperature of B80 1C, using Pb granule electrodes in a fixed-bed reactor fed with aqueous 0.2 M K2CO3 electrolyte solution, the only reaction product was found to be HCOOH, with a maximum Faradaic efficiency of 94% at 1.8 V vs. SCE.337 If the reduction was conducted at ambient temperature and pressure, CO and methane were also produced.338 The Tafel plots from the current–voltage curves for HCOOH and CO formation (Fig. 21) show a linearity in the entire potential range studied, indicating that the electroreduction of CO2 to formic acid and CO was not limited by mass transfer. Eneau-Innocent et al.339 employed techniques such as cyclic voltammetry, chronoamperometry, and in situ infrared reflectance spectroscopy to investigate the catalytic activity of lead electrodes towards CO2 electrodimerization in 0.2 M tetraethylammonium perchlorate–propylene carbonate (TEAP–PC) solution, and found that CO was not produced while oxalate was the main product. The CV measurements for CO2 reduction in aqueous medium on a lead plate in a filter-press cell showed that when the pH of the cathodic solution was 8, CO2 existed predominantly in the form of HCO3, and HCOOH was the exclusive product, with high Faradaic yields of 65–90%.340 The main reaction can be expressed as follows:


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HCO3 + H2O + 2e - HCOO + 2OH

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(54)

Subramanian et al.341 designed a flow type electrochemical membrane reactor to improve CO2 mass transfer in electrocatalytic reduction in a potassium phosphate buffer solution. The anode and cathode chambers in this reactor were separated by a composite perfluoropolymer cation exchange membrane (Nafions 961 and Nafions 430). With a lead-coated cathode, a maximum current efficiency of 93% for formate formation was achieved. 2.4

Alkaline metals and alkaline earth metals

Neither alkaline metals nor alkaline earth metals can be used as electrodes for CO2 catalytic reduction catalysts due to their instability in electrochemical systems. However, their salts, commonly used as supporting electrolytes in electrochemical cells for CO2 electroreduction, exhibit different effects on product selectivity, reduction reaction rate, and even catalyst stability. To study the effect of electrolytes on CO2 electroreduction, Murata and Hori342 tested several aqueous 0.1 M MHCO3 solutions, where M = Li+, Na+, K+, and Cs+. They found that H2 evolution prevailed over CO2 reduction in the Li+ electrolyte, whereas CO2 reduction was favorable in Na+, K+, and Cs+ solutions. In addition, they also observed that the magnitudes of the C2H4/CH4 ratio were in the order Li+ > Na+ > K+ > Cs+, indicating that the current efficiency for the formation of C2H4 was apparently related to the size of the cation radius, which should have a strong effect on the outer Helmholtz plane (OHP) potential of the electrode–electrolyte interface. The same conclusion was also reached by Kyriacou and Anagnostopoulos.343 In their study on CO2 electroreduction catalyzed by Fe(0)TPP in DMF with tetraalkylammonium (TAA) salts as the supporting electrolyte, adding Mg2+ ions into the solution dramatically improved the rate of CO2 reduction to CO. The catalyst’s stability was also improved, and a Faradaic efficiency of over 94% was achieved. The mechanism is presented in Fig. 22. Other salts have also been used as supporting electrolytes in CH3OH solution to study their effect on CO2 electroreduction on a Cu electrode. They include Li salts (LiBF4, LiClO4, LiCl, LiBr, LiI, LiClO4, and CH3COOLi),242,252 sodium salts (CH3COONa, NaCl, NaBr, NaI, NaSCN, and NaClO4),344 potassium salts (CH3COOK, KBr, KI, and KSCN),345 cesium salts (CH3COOCs, CsCl, CsBr, CsI, and CsSCN),346 and alkali hydroxides such as LiOH,253 NaOH,347 KOH, RbOH,348 and CsOH.347 Table 8 lists some typical cations of supporting electrolytes and their effects on the Faradaic efficiency of CO2 electroreduction. Interested in the effect of multivalent supporting cations, Schizodimou and Kyriacou311 investigated CO2 electroreduction on a Cu(88)–Sn(6)–Pb(6) alloy cathode in 1.5 M HCl and found that the reduction rate slightly increased in the presence of divalent cations such as Mg2+, Ca2+, and Ba2+. They also observed that the enhancement in the reduction rate was dependent on the charge number of the supporting electrolyte cation—the higher the charge number, the greater the rate enhancement, in the order Na+ o Mg2+ o Ca2+ o Ba2+ o Al3+ o Zr4+ o Nd3+ o La3+.


Fig. 22 Proposed mechanism for the enhancement of Mg2+ in CO2 electroreduction catalyzed by Fe(0)TPP catalyst on a Cu electrode in DMF + TAA solution. Reprinted with permission from ref. 32. Copyright r 1991 American Chemical Society.

Table 8 Typical Faradaic efficiencies of CH4 and C2H2 production in the electrochemical reduction of CO2 at a Cu electrode. Reprinted with permission from ref. 348. Copyright r 2007 Springer

Faradaic efficiency (%) CH3OH

H2O

Cation of supporting salts

CH4

C2H4

CH4

C2H4

Li Na K Rb Cs

63 63 16 4.6 4.1

14.7 17.6 37.5 31.0 32.7

26 19 16 — 15

4 11 14 — 13

2.5

Other catalysts

Besides the metals and metal complexes mentioned above, organic molecules can also be mediators and catalysts for CO2 electroreduction. Oh and Hu349 have recently published a review of the literature on this subject. Conducting polymer electrodes. Conducting polymer electrodes have also been developed for the heterogeneous electrocatalysis of CO2 reduction. For example, Koleli et al.350 developed a polyaniline (PAn) electrode. In methanol solution, the maximum Faradaic efficiencies were found to be 12% for formic acid and 78% for acetic acid. Aydin et al.351 developed a polypyrrole (PPy) electrode. In the electrocatalytic reduction of CO2 under high

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Fig. 23 Proposed overall mechanism for pyridinium-catalyzed CO2 reduction to three products: formic acid, formaldehyde, and methanol. Reprinted with permission from ref. 354. Copyright r 2010 American Chemical Society.

pressure in CH3OH at an overpotential value of 0.4 V vs. Ag/AgCl, the maximum Faradaic efficiencies at 20 bar were 1.9, 40.5, and 62.2% for HCHO, HCOOH, and CH3COOH, respectively. Smith et al.352 synthesized two organic polymers based on repeating benzimidazole and pyridine–bipyridine units, respectively. The pyridine-based polymer exhibited stable electrochemical behavior, while the bipyridine-based polymer gave large catalytic currents for CO2 reduction in CH3CN containing 1% H2O. Aromatic amine catalysts. Seshadri et al.353 found that the pyridinium cation and its substituted derivatives could be effective and stable homogeneous electrocatalysts for the multipleelectron, multiple-proton reduction of CO2 to methanol at low potentials, with Faradaic yields of up to 30%. The selectivity for pyridinium-catalyzed methanol production was found to increase significantly under photoelectrocatalytic conditions. Cole et al.354 observed CO2 reduction to HCOOH, HCHO, and CH3OH at 0.58 V vs. SCE when using a Pt disk electrode in a 10 mM aqueous solution of pyridine (Py) at pH 5.3. In the proposed overall mechanism, with an inner-sphere-type electron transfer, the pyridinium radical was believed to play a role in the reduction, as shown in Fig. 23. Ertem et al.355 proposed that the pyridinium cation ‘‘PyH+’’ could undergo a one-electron reduction, forming hydrogen atoms adsorbed on the Pt surface, ‘‘Pt–H’’. This ‘‘Pt–H’’ was susceptible to electrophilic attack by CO2, leading to a two-electron proton-coupled hydride transfer reaction: CO2 + Pt–H + PyH+ + e - Py + Pt + HCOOH

(55)

In a theoretical study of CO2 electroreduction in the presence of a pyridinium cation and its substituted derivatives, Keith and Carter356 employed first-principles quantum chemistry and the thermodynamic energies of various pyridine-derived intermediates, as well as energy barrier heights for key homogeneous reaction mechanisms. They predicted that the actual form of the co-catalyst was not the long-proposed pyridinyl radical in solution, but was more probably a surface-bound dihydropyridine species. Lim et al.357 investigated the mechanism of homogeneous CO2 reduction by pyridine (Py) in the

660 | Chem. Soc. Rev., 2014, 43, 631--675

Fig. 24 Two potential routes for the formation of PyCOOH0 in the homogeneous phase. Reprinted with permission from ref. 357. Copyright r 2013 American Chemical Society.

Py–p-GaP system. Based on ab initio quantum chemical calculations, they identified PyCOOH0 as an important intermediate, whose formation was believed to be the rate-determining step for CO2 reduction to CH3OH. As shown in Fig. 24, in the homogeneous phase, the formation of PyCOOH0 proceeds by two potential routes. In Route 1, Py is protonated to PyH+, which is then reduced to PyH0, which reduces CO2 to form PyCOOH0. In Route 2, Py and CO2 are combined to form PyCO2, which is reduced to PyCOO and, finally, protonated to PyCOOH0. Bocarsly et al.147 compared the catalytic activity of imidazole and pyridine in the photoelectrochemical reduction of CO2 at illuminated iron pyrite (FeS2) electrodes. The aqueous electrolyte was composed of 10 mM catalyst and 0.5 M KCl solution, and a 350–1350 nm light beam with an intensity of 890 mW cm2 was used to illuminate the electrode surface. The mechanism of imidazole-based catalysis was investigated using CV measurements (over a scan rate range of 5 mV s1 to 200 mV s1) to analyze the catalytic activity of a series of imidazole derivatives toward CO2 reduction. Results indicated that imidazole could reduce CO2 to a mixture of CO and HCOOH at a moderate potential, while pyridine selectively catalyzed the production of HCOOH.


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Radical anion catalysts. Gennaro et al.358,359 found that the anion radicals of certain aromatic esters (i.e., phenyl benzoate and methyl benzoate) and nitriles (i.e., benzonitrile) possessed remarkable catalytic activity toward CO2 reduction to oxalate. The reduction pathway was simply described as follows: (1) A (aromatic ester or nitrile) was first reduced to A (an aromatic radical anion); (2) A transferred the electron to CO2, forming CO2*; and (3) two CO2* dimerized to give oxalate. Ionic liquid (catalysts). CO2 has been dissolved in the ionic liquid EMIMBF3Cl and electrochemically reduced at ambient pressure and room temperature.360 The BF3Cl anion catalyzed the CO2 reduction by forming a Lewis acid–base adduct, BF3–CO2. The B–Cl bond was relatively weak in BF3Cl. Thermogravimetric analysis (TGA) results showed that the decomposition of EMIMBF3Cl required two steps. The first included the breaking of the B–Cl bond and then the release of BF3 at about 220 1C; the second involved the breaking of other bonds which started at about 330 1C. Enzyme catalysts. Two and a half decades ago, Yoneyama and coworkers361 used isocitrate dehydrogenase (IDH) as an electrocatalyst for the fixation of CO2 to isocitric acid (HOOCCH2CH(COOH)CH(OH)COOH) in oxoglutaric acid (HOOCCH2CH2C(QO)COOH). The reaction occurred selectively, with current efficiencies approaching 100% at 0.95 V vs. SCE in 0.2 M (HOCH2)3CNH2 (tris buffer, pH 7). They also fixed CO2 to yield pyruvic acid (CH3C(QO)COOH) in acetyl-coenzyme A using pyruvate dehydrogenase complexes as electrocatalysts.362 Solvent molecules seemed to be involved in the reactions. Kuwabat et al.363 reported that at potentials between 0.7 and 0.9 V vs. SCE, the electrolysis of CO2-saturated phosphate buffer solutions (pH 7) that contained formate dehydrogenase (FDH) and either methyl viologen (MV2+) or pyrroloquinolinequinone as an electron mediator yielded HCOO with current efficiencies of 90%. The FDH demonstrated considerable durability. They also found that the electrolysis of phosphate buffer solutions containing HCOONa in the presence of methanol dehydrogenase (MDH) and MV2+ at 0.7 V vs. SCE yielded HCHO when the enzyme concentration was low, whereas it produced both HCHO and CH3OH when the concentration became relatively high. The formation and accumulation of formaldehyde promoted methanol production.363 Addo et al.364

Review Article

Fig. 26 Steady-state electrochemical kinetics visualized using current– voltage curves. When both the oxidized and the reduced forms of a redoxactive species are present, a reversible electrochemical reaction (one with a large exchange current density) produces a single sigmoidal wave (blue) that cuts (without inflection) through the zero-current axis at the equilibrium potential (Eeq) and achieves a potential-independent limiting current in either direction at a relatively low overpotential. Conversely, if the exchange current density is low, the current is negligible around Eeq, and two sigmoidal waves (red), one for either direction, are separated in potential, emerging from the baseline with an exponential dependence on potential. A substantial overpotential is required to match the current produced by the reversible system. Reprinted with permission from ref. 365. Copyright r 2011 National Academy of Sciences.

found that the addition of carbonic anhydrase could efficiently accelerate CO2 reduction achieved by formate, aldehyde, and alcohol dehydrogenase, although the process could be catalyzed only by dehydrogenase. With respect to this, a comparison is provided in Fig. 25.364 Hansen et al.365 recently developed a model based on DFT calculations to describe (1) trends in catalytic activity for CO2 reduction to CO at a metal surface and (2) the active sites in CODH enzymes, in terms of the adsorption energy of the reaction intermediates, CO and COOH. Enzymes that were able to catalytically transform small molecules (e.g., CO, formate, or protons) were a special category of electrocatalysts. Due to the presence of enzymes (catalysts), those previously irreversible processes could become electrochemically reversible, as shown in Fig. 26.366 In the active sites, some elements (e.g., Ni, Fe, Cu, Se, Mo, and W) have been found.366–368

3. Product selectivity in the electrocatalytic reduction of CO2

Fig. 25 Schematic of the enzyme cascade reaction from CO2 to methanol on an electrode surface without carbonic anhydrase IV (left) and with carbonic anhydrase IV (right). Reprinted with permission from ref. 364. Copyright r 2011 American Electrochemical Society.


As discussed above, catalyst selectivity to produce desired products in catalyzed CO2 electroreduction is very important for practical applications. Normally, this selectivity is closely related to the reduction mechanism, with different reaction pathways or combinations of different pathways leading to different products. In the initial reduction step of a typical CO2 electroreduction mechanism, CO2 can obtain electrons either directly from the cathode surface (a bare electrode surface or a surface coated catalyst) or indirectly from a medium, such as a soluble catalyst, to produce an intermediate, such as CO2 , which then absorbs on the cathode for product

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Proposed reaction pathways in CO2 electroreduction252,312,322,370

CO2 + e - CO2(ads) or NR4+ + e - NR4 CO2 + NR4 - CO2 + NR4+ (ref. 252)


Chem Soc Rev

CO2(ads) + H+ + e - CO + OH

CO2(ads) + H2O - HCO2(ads) + OH 2 CO2(ads) - C2O42

formation. In general, which kind of pathways and how many pathways are required for the reduction process will be strongly affected by experimental conditions, such as the catalysts and/or electrodes, electrode potential, electrolyte solution, buffer strength, pH, CO2 concentration and pressure, as well as temperature.369 The possible pathways are summarized in Table 9.252,312,322,370 Effects of single metal electrode type. Regarding product selectivity, single metal electrodes seem to be the most popular type of electrocatalysts for CO2 reduction.241,290 Two groups can be roughly designated:176 (1) CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and (2) formate formation metals (Pb, Hg, In, Sn, Cd, and Tl). There are other types of catalysts that also have both high selectivity and high current efficiency. In addition, some electrocatalysts have been specifically designed for and show unique catalytic activities toward CO2 reduction to produce desired products with high current efficiencies. Effects of metal complex, metal center, and ligand type. It is well known that the catalytic performance of metal complex catalysts for CO2 reduction strongly depends on the chemical properties of the metal center and ligand. Therefore, it is expected that the distribution of the electrolysis products, the current efficiencies, and the reaction mechanism of CO2 electroreduction will also be strongly affected by the type of central metals and ligands in macrocyclic complexes. For example, CO2 electroreduction on a GCE modified with polymeric M-tetrakis aminophthalocyanines (M = Co, Ni, Fe) indicated that different metal centers produced different products.103 When M was Co, HCOOH was the only product; when M was Fe, a mixture of CH2O and H2 was produced, whereas when M was Ni, a mixture of HCOOH and CH2O was observed.371 Furuya and Matsui372 investigated the electrocatalytic reduction of CO2 on GDEs modified by 16 kinds of metal phthalocyanine (MPc) catalysts (where M = Co, Ni, Fe, Pd, Sn, Pb, In, Zn, Al, Cu, Ga, Ti, V, Mn, Mg, Pt) in 0.5 M KHCO3. They found that the distribution and current efficiencies of the electrolysis products were strongly dependent on the nature of the central metal coordinated to the phthalocyanines. With transition metals of Co- and Ni-phthalocyanines, the main electrolysis product was CO, with a current efficiency of B100%. On the other hand, HCOOH was the main product on phthalocyanines with Sn, Pb, or In metal centers. The highest current efficiency, B70%, was observed on SnPc around 1.6 V. In the case of Cu-, Ga-, and Ti-phthalocyanines, CH4 was the main product, with the highest current efficiencies being 30–40%. Analogously, in the simultaneous reduction of CO2 and NO3 with various MPc catalysts (where M = Ti, V, Cr,

662 | Chem. Soc. Rev., 2014, 43, 631--675

CO + 4H+ + 4e - CH2(ads) + H2O

CH2(ads) + 2H+ + 2e - CH4 2 CH2(ads) - C2H4 2 CH2(ads) + 2H+ + 2e - C2H6 HCO2 + CH3OH - HCOOCH3 + OH (ref. 312) HC(O)COO + 2e + 2H+ - H2C(OH)COO (ref. 322)

HCO2 (ads) + e - HCO2 C2O42 + 2H+ + 2e - HC(O)COO + OH

Mo, Fe, Ru, Co, Ni, Pd, Cu, Zn, Cd, Ga, In, Ge, Sn, Pb), the current efficiency of CO formation at CoPc and NiPc catalysts in the reduction of CO2 alone was further demonstrated to be far higher than at pure metal catalysts; hence, these M/Pc catalysts were expected to have a fairly high capacity for urea formation.373 The effects of various aza-macrocyclic ligands on the production of fuels from macrocyclic complexes were also investigated, including simple tetraazamacrocycles, porphyrin, phthalocyanine, and biphenanthrolinic hexaazacyclophanes.374,375 The mechanism for producing a particular product is normally related to the structural features of the azacyclam framework and its interaction with the central metal and CO2 or CO molecules, while replacement of a –CH2 group in the ligand backbone by an amide residue, for example, does not disturb the catalytic process. Effects of cations and anions in the electrolyte. As described previously, alkaline metals and alkaline earth metals cannot be used as electrodes for CO2 catalytic reduction catalysts. However, their salts have commonly been used as supporting electrolytes in electrochemical cells for CO2 electroreduction, and they have different effects on product selectivity. For example, in the case of FeTPP catalyst, the addition of Lewis acid cations such as Mg2+, Ca2+, Ba2+, Li+, or Na+ decreased HCOOH formation as the acidity increased; the order of reactivity of these Lewis acid synergists was Mg2+ = Ca2+ > Ba2+ > Li+ > Na+.33 Thorson et al.376 confirmed that the presence of large cations such as cesium (Cs) and rubidium (Ru) in the electrolyte could enhance the electrochemical conversion of CO2 to CO. This was explained by the interplay between the level of cation hydration and the extent of cation adsorption on the metal electrodes. The effects of anions in the electrolyte on the products of CO2 electroreduction were also investigated using a copper mesh electrode in aqueous solutions containing 3 M KCl, KBr, and KI as the respective electrolytes.377 The results showed that the bond between the adsorbed halide anion (e.g. Br, Cl, or I) and carbon helped the electron transfer from the adsorbed halide anion to the vacant orbital of CO2, promoting CO2 conversion.377 The stronger the adsorption of the halide anion to the electrode was, the more strongly CO2 was restrained, resulting in a higher CO2 reduction current. Furthermore, the specifically adsorbed halide anions suppressed the adsorption of protons, leading to a higher hydrogen overvoltage. This reaction mechanism was also confirmed by Schizodimou and Kyriacou,311 who showed that the rate of electrochemical reduction of CO2 increased in the order Cl o Br o I. Effects of supporting electrolytes. In fact, even for the same metal electrode with the same purity, different supporting electrolytes have a great effect on the final products. For


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example, in the electroreduction of CO2 at a copper electrode (99.999% purity) in methanol, Saeki et al.252 observed two different main products—CO (current efficiency 48.1–86.8%) and CH3COOH (54.5 and 46.7%)—when the supporting electrolyte was either tetrabutylammonium (TBA) salts (i.e., TBABF4 and TBAClO4) or lithium salts (LiBF4 and LiClO4). They proposed that the intermediate, CO2*, was stabilized by forming a TBA+–CO2* ion pair or by being adsorbed on the electrode surface. By contrast, NH4ClO4 as a supporting electrolyte in the same electrolysis only led to hydrogen evolution (84.6%). Other electrolytes have also been explored, such as tetraethylammonium perchlorate (TEA).339 Effects of solvent–CO2 concentration. It should be mentioned that CO2 utilization in aqueous solution can be limited by its low solubility in water at standard temperature and pressure.378 This is because there are relatively small amounts of CO2 available for the reaction to proceed at the electrode surface. To speed up the reaction process for industrial purposes, pressurized CO2 is normally required,79,379 which often causes a certain degree of change in product selectivity. Normally, in aqueous solutions, metallic catalysts (or electrodes), such as sp group metals (e.g., In or Pb), tend to give higher CO production at pH levels higher than 4, while d group metals (such as Pd and Cu) can promote HCOOH production.369 It was concluded that the main products obtained in aqueous media under ambient conditions were strongly dependent on the type of cathode: Cu electrodes mainly yielded mixtures of hydrocarbons (mostly methane and ethylene) and alcohols; Au, Ag, and Zn mainly produced CO, whereas other metals, such as In, Sn, Hg, and Pb, were selective for the production of formic acid/ formate.365 Compared to water solutions, the application of nonaqueous solvents is relatively popular due to their high solubility for CO2. For example, DMF, PC, and CH3OH may contain, respectively, up to 20, 8, and 5 times more CO2 than corresponding amounts of aqueous solutions. Among them, solvents having low proton availability, such as DMF, favor the formation of oxalate and CO, while aqueous solution favors formate.380 Strategies that have been demonstrated experimentally are further described below according to the selective generation of desired products.

Review Article

the large-scale electrochemical reduction of CO2 to HCOOH and HCOO has been discussed by Agarwal et al.8 Electrocatalytic reduction of CO2 by an enzyme catalyst, namely, formate dehydrogenase enzyme (FDH1), which was isolated from Syntrophobacter fumaroxidans, was found (as either a homogeneous or a heterogeneous catalyst) to produce formate exclusively.58 Two acetogenic bacteria, Moorella thermoacetica (Mt, formerly Clostridium thermoaceticum, ATCC 35608) and Clostridium formicoaceticum (Cf, DSM 92), were explored as catalysts for CO2 electroreduction in 1.0 atm CO2saturated 0.1 M phosphate buffer solution (pH 7.0) at 0.58 V vs. NHE; the results showed that these catalysts could efficiently convert CO2 to formate with current efficiencies of 80% for Mt and 100% for Cf.382 3.2

CO is one of the important products generated in CO2 electroreduction. Some metal cathodes, such as Ag, Au, and Zn, are highly selective for the electrocatalytic reduction of CO2 to CO in KHCO3 aqueous solutions.38 [NiII(cyclam)]2+ is a well-known catalyst for CO2 electroreduction to CO on a mercury cathode at 0.9 V in aqueous solutions.111 Two other electrocatalysts with a similar structure to [Ni(cyclam)]2+ were also found to be selective for CO production.120 In addition, metal polyphosphine complexes, such as Pd(triphosphine)L2+ (L = CH3CN, P(OMe)3, PEt3, P(CH2OH)3, and PPh3), exhibited high catalytic activity for the reduction of CO2 to CO in acidic CH3CN solutions.216 As an enzyme-based catalyst, a Ni- and Fe-containing metalloenzyme isolated from Moorella thermoacetica showed highly selective activity towards the conversion of CO2 to CO, with a current efficiency as high as B100% at 0.57 V vs. NHE in a 0.1 M phosphate buffer solution (pH 6.3).173 3.3

Selective production of formic acid (and formate)

Electrochemical reduction of CO2 in aqueous solution to formic acid and formate was reported as early as 1870.381 Noda et al.38 reported that Zn, Cd, Hg (Group 12), In (Group 13), Sn, and Pb (Group 14) metal cathodes in 0.1 M KHCO3 aqueous solution exhibited high production selectivity for HCOO formation. Chen et al.232 reported that HCOOH was the main product of CO2 electroreduction by both [Ir2(dimen)42+](PF6)2 and [Ir2(dimen)42+](B(C6H5)4)2 (dimen = 1,8-diisocyanomenthane) electrocatalysts. As previously mentioned, to optimize the largescale (even industrial-scale) electrocatalytic reduction of CO2 to formate, Li and Oloman331 investigated a series of condition variables that might affect the performance of a reactor. The current efficiency of formate formation was reportedly as high as 91%. Recently, the engineering and economic feasibility of


Selective production of formaldehyde

Senda et al.57 reported that [M(4-v-tpy)2]2+ and [M(6-v-tpy)2]2+ (M = Cr, Ni, Co, Fe, Ru, or Os), after being electropolymerized onto GCEs to form films, exhibited electrocatalytic activity toward CO2 reduction, with formaldehyde as virtually the only product. The current efficiency for films of Cr[(4-v-tpy)2]2+ was as high as 87%. 3.4

3.1

Selective production of carbon monoxide

Selective production of methanol

Some electrocatalysts have been found to be selective for methanol production in CO 2 electroreduction, as listed in Table 10. In addition, the photoelectrocatalytic reduction of CO 2 by semiconductor materials also showed selectivity for methanol production. The pyridinium cation on Pt also reduced CO 2 to methanol, with a Faradaic efficiency of 30%. 353,354 3.5

Selective production of oxalic acid (oxalate)

Electroreduction of CO2 in DMF solution showed a current efficiency of 73% for oxalic acid, with a little formate and CO production.140 The macrocyclic nickel complex Ni-Etn(Me/COOEt)-Etn was found to be one of the most active and persistent homogeneous catalysts for CO2 electroreduction selectively to oxalate.128 Dinuclear copper(I) complexes

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Summary of electrocatalytic reduction of CO2 to selectively produce several important low-carbon fuels

Product Electrode/electrocatalysts CO

Ag(99.98%) electrode Au(99.95%) electrode Glassy carbon electrode (WE)/Re(I)(bpy)(CO)3Cl


Chem Soc Rev

Platinum gauze electrode (WE)/Re(CO)3(vbpy)Cl

Electrode potential (V)

Temperature/ pressure and other conditions

Electrolyte

1.6 V vs. Ag/AgCl saturated with KCl 1.25 V vs. NHE 0.1 M Et4NCl DMF-H2O 25 1C (10%) solutions 1.55 V vs. CH3CN-Bu4NPF6 satusodium SCE rated with CO2 1.85 V CH3CN + 0.1 M TBAP

Platinum (WE), Ag/10 mM Ag+ (RE)/(1 mM) facRe(L)(CO)3Cl (L = pyrrole-substituted 2,2 0 bipyridine) Carbon felt (WE)/fac-Re(L)(CO)3Cl (L = pyrrolesubstituted 2,2 0 -bipyridine) Polymeric films formed by coelectropolymerization 1.55 V vs. SSCE 0.1 M TBAH in CH3CN of cis-[(bpy)2Ru(vpy)2]2+ with fac-[Re(CO)3(vbpy)Cl] saturated with CO2 or fac-[Re(CO)3(vbpy)CH3CN]+ Re film electrodeposited onto a polycrystalline gold 1.35 V 0.1 M LiClO4 CH3OH 1 atm CO2, stirred support solution conditions Quiescent conditions fac-(5,5 0 -Bisphenylethynyl-2,2 0 -bipyridyl)Re(CO)3Cl 1.750 V vs. NHE CH3CN with 0.1 M (n-CH3(CH2)3)4NPF6 1.70 V 0.1 M TBAP in MeCN At room [Mn(bpy)(CO)3]+ [Mn(dmbpy)(CO)3]+ temperature 2.2 V vs. SCE DMF/2 M H2O Glassy carbon (WE), Pt wire (CE), Ag/AgCl (RE)/ [Mn(bpy-t-Bu)(CO)3]+ Glassy carbon (WE); aqueous SCE electrode (RE); Pt DMF + 0.1 M n-Bu4NPF6 21 1C/? wire (CE) + 2 M H2O (1) Iron 5,10,15,20-tetrakis (2 0 ,6 0 -dihydroxyphenyl)- 1.333 V vs. NHE porphyrin (Fe0TDHPP) (2) Iron 5,10,15,20-tetrakis (2 0 ,6 0 -dimethoxyphenyl)- 1.69 V vs. NHE porphyrin (Fe0TDMPP) Carbon monoxide dehydrogenase (CODH) from Moorella thermoacetica Pd metal 1.8 V vs. Ag/AgCl 0.1 M KHCO3 aqueous 50 atm solution 1.5 V Ag/Ag+ TBAP (0.1 M) + CH3CN cis(Cl)-[Ru(bpy)-(CO)2Cl2], cis(CO)(0.01 M) [Ru(bpy)(CO)2(C(O)OMe)Cl]/ saturated with CO2 3.5 CH3OH with 500 mM 30 1C, CO2 CsOH supporting salt pressure of 10 atm Ag(99.98%) electrode 1.6 V vs. 0.1 M KHCO3 aqueous 25 1C Au(99.95%) electrode Ag/AgCl saturated solution saturated with KCl with KCl Metallic Zn electrode 0.1 M KHCO3 aqueous Metallic Cd electrode solution Metallic Hg electrode HCOOH/ Working electrode: metallic electrodes HCOO Zn Cd Hg Polished pyrolytic graphite edge electrode (WE)/ tungsten-containing formate dehydrogenase enzyme (FDH1) from Syntrophobacter fumaroxidans Fe wire (99.5%, 0.16–0.63 cm2) electropolished in 1.53 to 1.61 vs. Ag/AgCl HClO4-(CH3CO)O-H2O Ni(cyclam)2+ 1.3 V vs. SCE [Ru(bpy)2(CO)2]2+ or [Ru(bpy)2(CO)Cl]+ (homogeneous catalysts)/Hg pool [(bpy)2Ru(dmbbbpy)](PF6)2 (dmbbbpy = 2,2 0 -bis(1- 1.65 V vs. Ag/AgCl methylbenzimidazol-2-yl)-4,4 0 -bipyridine) 1.55 V vs. [(bpy)2Ru(dmbbbpy)Ru(bpy)2](PF6)4 Ag/AgCl Tinned-copper sheet Metallic Pb electrode

664 | Chem. Soc. Rev., 2014, 43, 631--675

2.4 V Ag/AgCl

0.1 M KHCO3 aqueous solution

64.7 81.5

38

98

61

92.3

63

92

65, 66

Max. 98.5

66

90–98

67

87

68

57 45

72

85 100 100

77 78

>90

86

173 57.9

184

95–97

222

84

255

64.7 81.5

38

39.6 14.4 ND

38

38

20 mM Na2CO3 (pH 6.5) 37 1C solution 0.1 M KClO4

Faradaic efficiency (%) Ref.

25 1C; 30 atm, 120 mA cm2

Saturated H2O/DMF (9 : 1, v/v) solution in the presence of Me2NH HCl, pH 9.5 CO2 saturated MeCN + 2.5% H2O CO2 saturated MeCN + 2.5% H2O 0.45 M KHCO3 0.22 kA m2, ambient conditions 100 1C, normal 0.1 M TEAP/H2O pressure

20 39 94 B100

58

59.5–59.6

79

75 84.3

118 218

89

224

90

224

86

264

78.9

322


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Review Article

(continued)

Electrode potential (V)

Product Electrode/electrocatalysts


Sn-powder-decorated gas diffusion layer (SnGDL) 1.6 V vs. NHE electrode Sn foil (Alfa Aesar, 99.998%) with an active surface 1.7 V vs. SCE area of 1 cm2 Pb granule electrodes 1.8 V vs. SCE CH3OH

RuO2–TiO2 nanoparticle (NP) composite electrodes 0.8 V vs. SCE (WE); SCE (RE); Pt plate (CE)–RuO2 RuO2–TiO2 nanotube (NT) composite electrodes (WE); SCE (RE); Pt plate (CE)–RuO2 Molybdenum metal 0.7 to 0.8 V vs. SCE Electrodeposited cuprous oxide film 1.1 V vs. SCE II

II

Platinum plate electrode/KFe [Fe (CN)6] Ru Ru/Cu Cu RuO2–TiO2 p-GaP Illuminated p-GaP photoelectrodes p-GaAs p-InP n-GaAs Pyridinium cation and substituted derivatives (homogeneous electrocatalysts)

0.54 V vs. SCE 0.8 V vs. SCE

C2H4

Aqueous NaHCO3 solution 0.1 M Na2SO4

27 mA cm2

70

334

B95

335

0.2 M K2CO3 aqueous solution

B80 1C, B50 atm

94

337

40.2

46

0.5 M NaHCO3 solution saturated with CO2

60.5 0.2 M Na2SO4 solution 20 1C/?/pH 4.2 saturated with CO2 0.5 M KHCO3 solution saturated with CO2 0.5 M NaHCO3 solution saturated with CO2

ACN + 0.25 M Bu4NClO4 saturated with CO2 CO2-saturated MeCN + 2.5% H2O CO2-saturated MeCN + 2.5% H2O 0.1 M TEAP/propylene 100 1C under carbonate (PC) normal pressure

Au electrodes PpyRe microalloy polypyrrole PpyCu–Re microalloy polypyrrole 99.999% Cu sheet cathode

1.35 V

99.999% Cu sheet cathode

0.5 M KHCO3 aqueous solution saturated with CO2 1.40 V vs. NHE, 0.1 M KClLO4, aqueous 2 5 mA cm , solution saturated with CO2 3.5 V vs. CH3OH with 80 mM Ag/AgCl CsOH supporting salt, 0.1 M KClO4 1.44 V 1.42 V 1.55 V 1.56 V vs. NHE

99.999% Cu electrode 99.98% Cu electrode Cu single-crystal electrodes Polycrystal (100) (110) (111) HCHO

Faradaic efficiency (%) Ref.

Electrolyte

1.1 V vs. SCE 0.95 V vs. SCE 1.4 V vs. SCE 0.52 V vs. SCE 1.3 V vs. SCE

H2C2O4/ Ni-Etn(Me/COOEt)Etn C2O42 [(bpy)2Ru(dmbbbpy)](PF6)2 (dmbbbpy = 2,2 0 -bis(1- 1.65 V vs. Ag/AgCl methylbenzimidazol-2-yl)-4,4 0 -bipyridine) [(bpy)2Ru(dmbbbpy)Ru(bpy)2](PF6)4 1.55 V vs. Ag/AgCl Metallic Pb electrode 2.6 V Ag/AgCl CH4

Temperature/ pressure and other conditions

0.1 M LiClO4 in CH3OH Atmospheric solution pressure of CO2

electrocatalyzed CO2 conversion selectively to oxalate in CH3CN with a soluble lithium salt, resulting in quantitative


0.10 M NaClO4 solution saturated with CO2

47

38

142

>80 42 41.3

154 178 177

40 30 60 B100 55 70 100 30

370 209 317 320 326

98

128

64

224

70

224

73.3

322

353

68 34 31 65

247

40 1C

20

247

19 1C/pH 5.9

48.1

248

30 1C

32.3

254

18 1C/pH 6.8

31.7

276

87

57

0.5 M KHCO3 aqueous 0 1C solution saturated with CO2

GCEs (WE); Ag/AgCl (saturated with sodium chlor- 1.100 V vs. ide) (+0.222 vs. NHE) electrode (RE); Pt plate (CE)/ Ag/AgCl electropolymerized [Cr(4-v-tpy)2]2+ film

>50

precipitation of lithium oxalate.172 In addition, anion radicals of aromatic esters such as phenyl benzoate and methyl benzoate, and

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of nitriles such as benzonitrile, in DMF at an inert electrode (e.g., mercury) were able to reduce CO2 exclusively to oxalate.358,359


3.6

Selective production of lactic acid

Lactic acid (CH3CH(OH)COOH) is an organic C3 compound that plays an important role in numerous industries, including food, medicine, and cosmetics. Ogura et al.159 found selective reduction of CO2 to lactic acid, catalyzed by Fe(II)-4,5-dihydroxybenzene-1,3-disulfonate immobilized on a PAn/PB-modified Pt electrode in 0.5 M KCl solution. However, if Co-4,5dihydroxybenzene-1,3-disulfonate was used, acetaldehyde was also produced. To summarize the selective generation of desired products in electrocatalytic CO2 reduction, Table 10 lists typical examples of several important low-carbon fuels, together with their generation conditions.

4. Catalyst stability, activity degradation, and mitigation Catalytic activity is normally evaluated by considering both the onset potential of reduction and the Faradaic efficiency, while catalyst stability (or durability) is assessed according to variations in catalyst behavior with electrolysis time.331 With respect to catalyst stability, the issue of deactivation has often been reported; the formation of poisonous intermediates and the deposition of inactive compositions on electrode surfaces are the main causes.233,245,259,268,383,384 Hori et al.268 put forth several possible factors, which can be summarized as (1) heavy metal impurities contained in reagent chemicals and introduced to the electrolyte solution; (2) very small amounts of organic substances possibly contained in water, and (3) intermediate poisoning species or products formed during CO2 reduction and adsorbed on electrodes. Besides these, electrolysis mode and condition can also affect catalyst stability.142,297,385,386 For example, deactivation of a Cu cathode was observed after only 3 hours of electrolysis in a constant potential mode, while the electrocatalytic activity of Cu remained constant for 7 hours if a superimposed potential method was applied.269 Using the latter, the surface structure of the copper electrode was changed with the formation of cuprous oxide (Cu2O), then the adsorption of amorphous graphite was prevented, leading to stable long-term electrolysis for CH4 production.269 The pulse electrolysis mode was also found to have a mitigating effect on electrode deactivation.387 Changing the electrolytic conditions led to the deposition of poisoning species on a Cu electrode being highly suppressed, while the selectivity for C2H4 formation was enhanced. Details of the reduction mechanism on Cu2O remained unclear, and further study of the electrodeposition of CO2 on the Cu2O cathode should be considered. An early study found that the deactivation of a Sn electrode was related to the formation of organometallic complexes on the electrode surface, which could accelerate the rate of hydrogen evolution.321 Recently, Agarwal et al.8 investigated the longterm performance of Sn, together with other proprietary

666 | Chem. Soc. Rev., 2014, 43, 631--675

Chem Soc Rev

catalysts, in the electrochemical reduction of CO2 (ERC) to HCOO/HCOOH at a gas/solid/liquid interface, using a flowthrough reactor. Although better durability was observed in Sn than in Cu, a color change appeared on the electrode surface, as well as slight deactivation. Wu et al.335 observed the effects of the electrolyte on selectivity and activity with a Sn electrode. For a pure Sn electrocatalyst, a decrease in performance could be caused by several factors:388 (1) cathodic degradation of the catalyst surface, (2) deposition of non-catalytic species from reaction intermediates in the reduction of the pollutant species, (3) deposition of non-catalytic metallic species from contaminants in the electrolyte,335 and (4) anodic degradation of the catalyst at sites where gas bubbles formed, preventing the cathodic polarization of the catalyst. Bujno et al.389 conducted experiments in diluted solutions and confirmed that the Ni(I) complex catalysts present at the electrode surface were transformed into a catalytically inactive Ni(0) carbonyl deposit, blocking the electrode surface against further catalysis. Benson and Kubiak390 investigated the deactivation pathway of the Lehn catalyst. One pathway was concluded to be the formation of thermodynamically stable and often catalytically inactive dimers.73,216,391 Pugh et al.220 found that the electrocatalytic activity of cis-[Ru(bpy)2(CO)H]+ decreased slowly over an extended period. Normally, active species (sites) in catalysts are always responsible for the catalytic activity and are indispensable for electrocatalytic CO2 reduction.61,216,297,392 Loss in catalytic activity is always associated with the disappearance of active sites. For example, during CO2 reduction, Ru-based complex catalysts gradually lost their carbonyl-containing complexes, and inactive species, such as [Ru(bpy)2(CO3)], were formed. The instability of the [Cl(CO)2-(bpy*)Ru–Ru(bpy)(CO)2Cl] species was confirmed by a voluminous black precipitate, produced by exhaustive electrolysis at 2.00 V.222 In a recent study of CO2 reduction to CO at low overpotential in neutral aqueous solution by a Ni(cyclam) complex attached to poly(allylamine), Saravanakumar et al.124 achieved a current efficiency of 92% during the initial 6 hours of electrolysis, but this dropped to 88% at 12 hours and then to 79% at 24 hours. Hence, to mitigate the degradation of catalyst activity and stability, two major factors should be considered: (1) the effect

Fig. 27 Schematic representation of the electrochemical process to convert CO2 into formate/formic acid. Reprinted with permission from ref. 8. Copyright r 2011 WILEY-VCH Verlag GmbH & Co. KGaA.


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of catalyst type, structure, and composite, and (2) the effect of catalyst operating conditions.


5. Technological challenges in CO2 electrocatalytic reduction Regarding CO2 electrochemical reduction technology, several technical challenges remain, mainly (1) low catalyst activity, (2) low product selectivity, and (3) insufficient stability. In terms of the practical application of CO2 reduction to produce usable low-carbon fuels, our technology still seems to be far from adequate. Particularly for industrial-scale implementation (Fig. 27), low catalyst stability seems to be the major limitation at present. Therefore, developing highly active, selective, and stable electrocatalysts for the reduction of CO2 is still the major focus of activity in this area. The several challenges can be summarized as follows: (1) Low catalyst activity. It can be seen from the above discussion that for every single type of catalyst developed in the literature, the overpotential for CO2 electroreduction is normally too high, indicating that these catalysts’ activities are still not good enough for practical applications in terms of energy efficiency. (2) Low product selectivity. Although some of the catalysts discussed above gave desirable product selectivity and stable yields under continuous operation, so far only a few attempts have resulted in selective production—for instance, the use of Sn, Pb, and Hg metal electrodes to produce formate/formic acid.8,18,264,330,331 Unfortunately, the stability of the catalysis processes on these electrodes/catalysts is too low to be practical. Although other types of catalysts, such as metal complexes, have been explored, and some high catalytic selectivity has been achieved, they suffer from the same low stability. For the majority of the catalysts explored, even if they show high activity, the product selectivities are low, with some undesired products also resulting. (3) Insufficient catalyst stability/durability. This is probably the single biggest challenge. In the literature, the normally reported stability tests are in the region of under 100 hours, while long-term tests have yet to be done. As discussed above, as the CO2 reduction reaction proceeds, the active electrode/ catalyst surface can gradually become covered by reaction intermediates and by-products (such as carbon films and poisonous species), blocking and poisoning the catalyst’s active sites and leading to rapid catalytic activity degradation.330 (4) Insufficient fundamental understanding. The literature contains attempts to fundamentally understand the CO2 reduction process on catalysts through both experimental and theoretical modeling approaches to catalyst downselection with respect to catalyst activity, new catalyst design, and catalyst operation optimization. However, the work in this area seems to be insufficient. (5) Non-optimized electrode/reactor and system design for practical applications. The scale-up of the electroreduction of CO2 for practical applications is a necessary step toward the success of this technology.263 Although there have been some attempts in this respect—such as the work by Li and Oloman18


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and Agarwal et al.8 on the engineering and economic feasibility of CO2 electroreduction to produce formate/formic acid with a planned conversion of 100 tonnes of CO2 per day---optimization of the electrode/reactor and system design seems to be the second biggest challenge, next to low catalyst stability. Therefore, efforts to optimize system designs and at the same time develop durable catalysts should be carried out.

6. Summary and proposed research directions To facilitate the research and development of CO2 electroreduction, this paper gives a comprehensive overview of several decades of development and recent trends in the catalysis of CO2 electroreduction. Various electrocatalysts explored and reported in the literature are summarized and classified into metals, metal complexes, and organic/bio-organic compounds. The composition of electrocatalysts, catalytic mechanisms of CO2 reduction, conditions of use (medium, electrolyte, concentration, electrolysis potential, temperature, and pressure), current or Faradaic efficiency, and product selectivity are reviewed in depth with respect to catalyst activity and stability. Some typical catalysts and their associated data for catalytic activity, product selectivity, and catalytic stability are summarized and presented in tables to help readers quickly locate the information they are looking for. Furthermore, industrial attempts to scale up the technology of CO2 electroreduction for practical applications, and achievements in this area over the past several decades, are also discussed to give a clear picture of the current state of technology. It seems that the maturity of CO2 electroreduction technology to produce low-carbon fuels is still far from reaching the requirements for commercialization, due to several major technological challenges, including low catalyst activity, low product selectivity, and insufficient catalyst stability. To overcome these challenges, we propose several future research directions: (1) Enhancement of catalytic activity and stability by exploring innovative electrocatalysts. Generally speaking, almost all of the possible pure metals and their associated compounds that show some catalytic activity toward CO2 electroreduction have been tried as electrocatalysts, and some progress has been achieved in terms of catalytic activity, product selectivity, and catalytic stability. However, these technological advances are still not sufficient for practical applications. Breakthroughs in catalysis are definitely desirable and, indeed, necessary. With respect to these, developing new material synthesis technology to give innovative new catalysts with optimal performance is the priority. Two important types of catalyst materials should be emphasized here: (i) composite catalyst materials, which are synthesized by combining several different materials together, and (ii) nanostructured catalyst materials. Composite catalyst materials should have different properties and catalytic performance than their individual components because the individual substances in the composites experience a synergistic effect; this comes about through optimizing particle size,

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specific surface area, porosity, and active sites, preventing particles from agglomerating, facilitating electron and proton conduction, and protecting active materials from chemical and mechanical degradation. As a result, the obtained composites may have high catalytic activity, high product selectivity, and high catalytic stability in CO2 electroreduction. Regarding nanostructured catalyst materials, such as nanoaerogels, nanotubes/rods, nanoplates/sheets, nanospheres, and so on, their unique properties—such as high specific surface area and 1-, 2-, and 3-dimensional structures—can provide easy transport/ diffusion pathways for substrates to access, leading to faster kinetics, more efficient contact for electrolyte ions, and more active sites for the catalytic process. Furthermore, these nanostructured materials can be used as catalyst supports to provide a synergistic effect between the catalyst and the support particles, resulting in highly active and stable electrocatalysts for CO2 electroreduction. (2) Further fundamental understanding through both experiments and theoretical modeling. For down-selecting catalysts, designing and optimizing new catalyst structures with respect to improving catalytic activity, product selectivity, and catalytic stability, better fundamental understanding through both experiments and theoretical modeling is necessary. For example, we need to fundamentally understand the mechanisms of CO2 electroreduction and their relationship to catalyst active site structures and composition, using both theoretical calculations (molecular/electronic-level modeling) and experimental approaches, to guide new catalyst development. To mitigate catalyst degradation, it is necessary to understand the degradation mechanisms and failure modes, which can be done using both experimental and theoretical modeling approaches. For instance, a variety of instrumental analysis methods (e.g., SEM, TEM, XRD, XPS, NMR, HPLC, GC, and so on) and electrochemical methods (CV, RDE/RRDE, and EIS) can be used to characterize catalysts before and after lifetime tests. With greater understanding, it should be possible to develop new mitigation strategies. (3) Optimizing electrodes, reactors, and system designs for practical applications. In the current state of technology for industrial-scale CO2 electroreduction to produce low-carbon fuels, the major limitations seemed to be rapid catalyst degradation and slow CO2 transfer to the electrode surface, both of which should be partially related to the electrode/reactor design. It is believed that catalyst degradation is mainly related to the material itself but also to its operating environment. Besides improving the catalyst material’s stability, improving the electrode/reactor design to optimize the operating conditions is also important for performance. For example, certain innovative designs for GDEs and electrolyte membrane-based electrochemical cells developed in recent years seem to be the right approaches for reducing internal resistance and improving the reactant mass transfer process. In summary, CO2 electroreduction to produce low-carbon fuels is an important research and development subject for overcoming excessive, environmentally harmful CO2 emissions; it also presents the option of producing useful fuels using

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undesirable CO2 to solve our energy shortage issues. The authors of this article deeply believe that with continued and extensive efforts focused on developing innovative composite and nanostructured catalyst materials to overcome the challenges of insufficient catalytic activity, product selectivity, and catalytic stability, the technology of CO2 electroreduction will become practical in the near future.

Acknowledgements This investigation was financially supported by the National Natural Science Foundation of China (grant no. 21173039), the Specialized Research Fund for the Doctoral Program of Higher Education, SRFD (20110075110001); the Innovation Program of the Shanghai Municipal Education Commission, the Program for New Century Excellent Talents in University (NCET-120828), the Science and Technology Commission of Shanghai Municipality (11230700600), the Fundamental Research Funds for the Central Universities, the State Environmental Protection Engineering Center for Pollution Treatment and Control in the Textile Industry of China and Sendai Kankyo Kaihatsu Corporation, Japan. All of the above financial support is gratefully acknowledged.

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