Carbon nitride in energy conversion and storage: recent advances and future prospects.

DOI: 10.1002/cssc.201403287

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Carbon Nitride in Energy Conversion and Storage: Recent Advances and Future Prospects Yutong Gong, Mingming Li, and Yong Wang*[a]

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Reviews With the explosive growth of energy consumption, the exploration of highly efficient energy conversion and storage devices becomes increasingly important. Fuel cells, supercapacitors, and lithium-ion batteries are among the most promising options. The innovation of these devices mainly resides in the development of high-performance electrode materials and catalysts. Graphitic carbon nitride (g-C3N4), due to structural and chemical properties such as semiconductor optical properties,

rich nitrogen content, and tunable porous structure, has drawn considerable attention and shown great potential as an electrode material or catalyst in energy conversion and storage devices. This review covers recent progress in g-C3N4-containing systems for fuel cells, electrocatalytic water splitting devices, supercapacitors, and lithium-ion batteries. The corresponding catalytic mechanisms and future research directions in these areas are also discussed.

1. Introduction

the building block.[7, 8] Although sharing similar microstructures, graphite and g-C3N4 exhibit greatly different physicochemical properties. The most obvious is that g-C3N4 is yellow, whereas graphite is black in appearance. For the electronic properties, the two materials show more distinctions.[9] Graphite has excellent conductivity in the dimensions of the layers, whereas gC3N4 is characterized as a wide-band semiconductor.[10] Ideally, condensed g-C3N4 consists of only carbon and nitrogen atoms with a C/N molar ratio of 0.75. Although different synthetic approaches[6, 11] were proposed for the fabrication of g-C3N4, no perfectly condensed sample was obtained and the existing gC3N4 architectures showed different degrees of condensation and defects. To date, the condensation pathways of liquid precursors (e.g., cyanamide, dicyandiamide, melamine, ethylenediamine, and CCl4) were seen as good synthetic strategies to generate slightly defective, polymeric species. The extraordinary properties of g-C3N4, such as its peculiar thermal stability, appropriate electronic structure, and nitrogen richness, have accelerated interest in extending its application as a metal-free catalyst and catalyst support. g-C3N4 has been successfully applied to oxidation reactions,[12] hydrogenation reactions,[13] photocatalytic degradation of pollutants,[14] photocatalytic water splitting,[10] and so forth. Compared with these intensively investigated areas, the prospect of using g-C3N4 and its hybrids as electrode materials for energy devices, such as fuel cells, supercapacitors, and electrocatalytic hydrogen evolution devices, is relatively new. Herein, we review the most advanced progress in the use of g-C3N4-based systems as electrode materials for energy conversion (e.g., for the oxygen reduction reaction (ORR), the alcohol oxidation reaction, electrocatalytic water splitting) and storage (supercapacitors, lithium-ion battery anode). As a semiconductor, g-C3N4 has shown great promise for photocatalytic water splitting, which undoubtedly is an important part of energy conversion. Given that this aspect has been intensively summarized and reviewed by many research groups,[4, 15] we do not discuss this field herein.

The global population explosion and economic expansion have been accelerating energy consumption dramatically. As Bilgen et al. noted, we each live with energy in various forms, such as for heating, cooking, lighting, television, working, and shopping, every day.[1] It is estimated that the world needs to double its energy supply by 2050.[2] Currently, fossil fuels (i.e., petroleum, natural gas, and coal) still contribute to most of the world’s energy demand and they are being depleted rapidly. Their combustion leads to emissions that are harmful to the environment. As a consequence, developing sustainable energy conversion and storage technologies and devices has been one of the hottest topics in the scientific community. There is no doubt that the applied material in these devices is a key factor determining the performance. Among the materials exploited, carbon materials have attracted intensive interest due to their outstanding characteristics, such as high surface area, easy processability, excellent stability, and low cost.[2, 3] Graphitic carbon nitride (g-C3N4), which is a unique carbon material, has shown broad prospects in energy applications.[4] It is regarded as the most stable allotrope of carbon nitrides under ambient conditions based on both theoretical studies[5] and its stability in air up to 600 8C. It is characterized as a stacked 2D structure, which could be regarded as a nitrogen heteroatom-substituted graphite framework consisting of pconjugated graphitic planes formed through sp2 hybridization of carbon and nitrogen atoms.[6] Two structures have been proposed as the elementary building block of g-C3N4. The first one is composed of condensed s-triazine units (ring of C3N3 ; Scheme 1 a) with a periodic array of single carbon vacancies. The second one is composed of condensed tri-s-triazine (triring of C6N7; Scheme 1 b) subunits connected through planar tertiary amino groups with larger periodic vacancies in the lattice. The latter was recently shown to be energetically favored and the most stable local connection pattern.[7] Therefore, tri-striazine is widely accepted as the basic unit for the g-C3N4 network and recent work has shown that the pyrolysis of cyanamide, dicyandiamide, or melamine yields g-C3N4 with melem as

2. Functionalization of g-C3N4

[a] Y. Gong, M. Li, Y. Wang Advanced Materials and Catalysis Group, ZJU-NHU United R&D Center Center for Chemistry of High-Performance and Novel Materials Key Lab of Applied Chemistry of Zhejiang Province Department of Chemistry, Zhejiang University Hangzhou (P.R. China) E-mail: [email protected]

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Carbon nitride (CN) materials collected directly after self-condensation of organic precursors are bulk materials with a very small surface area, normally below 10 m2 g1. The surface area, conductivity, band gap, mass transfer ability, and so forth have to be modified to meet the needs of the application. The func2


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Scheme 1. Structures of s-triazine (a) and tri-s-triazine (b) as tectons of g-C3N4.

Yutong Gong studied at Qufu Normal University and received his B.S. in 2010. Then he received his M.S. degree from the Department of Chemistry at Zhejiang University under the guidance of Prof. Haoran Li. He is currently a Ph.D. candidate at the Department of Chemistry, Zhejiang University, under the supervision of Prof. Yong Wang. His research interests concern the development of functional carbon nanomaterials and their applications in catalytic hydrogenation, catalytic oxidation, energy conversion, and energy storage.

tionalization of g-C3N4 would provide more chance for wider applications. Porosity is a basic requirement of nanocarbon materials for practical applications. The liquid condensation process enables the employment of nanocasting methods to introduce a porous structure. The nanocasting of silica templates is still the main way to obtain porous g-C3N4.[16] Only a few soft template and template-free protocols have been proposed for the generation of porous g-C3N4 to improve its applicability.[14, 17] In addition to textural modification, chemical functionalization is another effective way to tune the physicochemical properties of the parent materials and extend their applications. Both post-functional treatment and in situ heteroatom doping approaches[18] have been reported to introduce functional groups onto the surface or into the g-C3N4 matrix. The design of g-C3N4-based composites[19] was put forward as a feasible way to combine the advantages of different components, and thus, improve the performance of g-C3N4. Typical functionalized g-C3N4 compounds used for energy conversion and storage are summarized herein.

Mingming Li studied at Xinjiang University and received his B.S. in 2012. He is currently a Ph.D. candidate at the Department of Chemistry, Zhejiang University, under the supervision of Prof. Yong Wang. His current research interests are functional carbon nanomaterials and their applications as catalyst supports for catalytic hydrogenation and oxidation reactions.

3. g-C3N4 and Functionalized g-C3N4 for Fuel Cells Fuel cells are electrochemical devices that directly convert chemical energy into electrical power with higher efficiency and less greenhouse gas emissions than the well-established technologies based on fossil fuel combustion. Polymer electrolyte membrane fuel cells [e.g., proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC)] are particularly attractive because they can be operated at low temperature with a long service life. They are considered to be the most promising for fuel cell vehicles and portable devices. The fuel cells operate by the oxidation of fuel (e.g., H2 oxidation, alcohol oxidation) at the anode and the ORR at the cathode. The ORR always requires more energy input.[20] Platinumbased materials are known to be the most active catalysts for the ORR. However, sluggish kinetics, the high cost, and low reserves of platinum remain the greatest stumbling blocks that limit wider commercialization of fuel cells. The electrochemical ORR can occur through a four-electron (4 e) pathway to pro-

Prof. Dr. Yong Wang studied chemical engineering at Xiangtan University from 1998 to 2002. He received his Ph.D. degree from Zhejiang University in 2007. After a postdoctoral stay at the Department of Chemistry, Zhejiang University, he joined the Max Planck Institute for Colloids and Interfaces in Potsdam, Germany, in 2009. He rejoined Zhejiang University and became a Professor for Chemistry in 2011. His research focuses on carbon nanomaterials and their applications in heterogeneous catalysis, energy storage, and energy conversion.


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Reviews duce H2O or OH , or through a two-electron (2 e) pathway to produce H2O2 or HO2 (Scheme 2). The more efficient 4 e process is necessary for eligible fuel cells. The exploitation of effective catalysts for both anodic alcohol oxidation and cathodic ORR has been attracting significant scientific interest. So far, gC3N4 has been involved in both cases.

groups.[21, 22] From this viewpoint, nitrogen-rich g-C3N4, in principle, possesses an inherent ability to adsorb O2, which would facilitate the ORR. The ORR activity of g-C3N4 was first exploited by Lyth and co-workers.[23] Although g-C3N4 exhibited higher activity than pure carbon in acidic medium, the current density is still low relative to other nitrogen-containing carbon materials; this was attributed to the low surface area, and thus, limited active sites. A similar investigation was carried out in alkaline medium by Xia et al. and the ORR was characterized to undergo an inefficient 2 e pathway.[24] To counter the problem of insufficient active sites, an ordered mesoporous g-C3N4 (OMCN) replicated from SBA-15 was employed to catalyze the ORR and it indeed showed greatly enhanced catalytic activity.[16d] Lyth et al. also pyrolyzed carbon black as a support for g-C3N4 at different temperatures, and CN pyrolyzed at 1000 8C displayed the best oxygen reduction activity with a 4 e process.[25] The obtained material was no longer g-C3N4 because g-C3N4 would completely decompose before 750 8C.[15a] The nitrogen content of 1.8 at % indicated that the material was a kind of nitrogendoped carbon material. DFT calculations by Qiao et al. gave some convincing enlightenment about the main reasons for the low ORR activity of g-C3N4 and provided ideas to develop well-performed gC3N4-based catalysts.[19a] The results showed that the limited electron-transfer ability was responsible for the poor catalytic performance. Oxygen molecules cannot be reduced on the pristine g-C3N4 surface without electron participation (Scheme 3 b) due to the existence of two insurmountable free

Scheme 2. The reaction pathway of the ORR in alkaline and acidic media.

3.1. Oxygen reduction reaction Since the discovery by Dai et al.[21] that nitrogen-doped carbon nanotubes (CNTs) showed high catalytic activity, long stability, and excellent resistance to carbon monoxide poisoning for the ORR, various metal-free nitrogen-doped carbon materials have been fabricated and demonstrated surprising activity for the ORR. Although controversially, the theory that the high activity of nitrogen-doped carbon materials arose from the strong electronic affinity of nitrogen atoms, which led to a high positive charge density on the adjacent carbon atoms, resulting in very favorable adsorption of oxygen, was accepted by many

Scheme 3. a) Free energy plots of the ORR determined by DFT calculations and optimized configurations of adsorbed species on the g-C3N4 surface with participation from zero, two, and four electrons, as demonstrated by paths I, II, and III, respectively. The ORR pathway on g-C3N4 without electron participation (b), g-C3N4 with participation from two electrons (c), and the g-C3N4@CMK-3 (CMK-3: ordered mesoporous carbon) composite with participation from four electrons, respectively (d).[19a]

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Reviews energy barriers from O2 to OH . When insufficient electrons were introduced (e.g., two electrons, path II in Scheme 3 c), the free energy of intermediate OOH@g-C3N4 decreased to a comparable level with that of the initial state of O2@g-C3N4 ; this indicated that the first 2 e reaction could spontaneously proceed. However, a clear barrier still existed at the final state of OH/g-C3N4, which then blocked the occurrence of the second 2 e reaction and OOH accumulation. This barrier could be eliminated by introducing more electrons (e.g., four electrons); most of the initially adsorbed O2 molecules could be quickly reduced to OOH and further directly formed OH in solution without any barrier through an efficient 4 e pathway (Scheme 3 d). Because g-C3N4 is a semiconductor, electron exchange is limited to the electrode–electrolyte–gas three-phase boundaries. Therefore, the ORR over pure g-C3N4 could only proceed through the 2 e path. Inspired by this computational study, Qiao et al. addressed this issue by synthesizing gC3N4@CMK-3[19a] and mesoporous g-C3N4@C[19b] hybrids. Conductive CMK and C served not only as a mesoporous support, but also as a transmitter that offered adequate electrons for the favorable 4 e pathway. The composites exhibited competitive catalytic activities, superior methanol tolerance, and stability compared with the commercial Pt/C catalyst in alkaline medium. The 4 e pathway was indeed achieved in both cases. Two-dimensional graphene is the perfect choice for an electron collector and transporter to provide sufficient electrons to reduce oxygen at the active sites on g-C3N4. Yang et al. demonstrated the fabrication of graphene-based carbon nitride (GCN) nanosheets (NSs) with individual dispersion of graphene between the NSs by nanocasting technology.[19c] For comparison, pure CN NSs without graphene were also prepared by a similar process during which graphene was removed by air treatment (Scheme 4). Judging from both the onset potential and kinetic current density in 0.1 m KOH, the G-CN NSs showed much better electrocatalytic activity than that of CN for the ORR. More importantly, G-CN exhibited a high selectivity for the more efficient 4 e ORR, whereas the ORR over CN proceeded through the 2 e pathway. G-CN was also characterized by better durability and resistance to methanol than commercial Pt/C. Yang et al. stated that better electronic conductivity was not the only factor that determined the electrocatalytic activity and the content of pyridinic nitrogen was another vital factor that affected the electrocatalytic performance.[19c] g-C3N4 (GCN) was immobilized on the surface of chemically converted graphene (CCG) by Shi et al.[26] The ORR activity of the composite (G-GCN) was enhanced compared with pure CN and CCG, and even comparable to that of 23 % Pt/graphene. Theoretical calculations indicated that the ORR mechanism at GCN was similar to that at nitrogen-doped CNTs:[21] carbon atoms of GCN in the oxidized state were electrochemically reduced in the first step, and then the reduced carbon atoms were reoxidized by adsorbed O2 molecules. As a result, the oxygen molecules were reduced during this process. Until recently, the combination of CN with graphene was a frequently studied strategy to develop metal-free carbonaceous materials for the ORR. Mesoporous g-C3N4, with abundant nitrogen ChemSusChem 0000, 00, 0 – 0

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Scheme 4. Fabrication of G-CN and CN NSs for the ORR.[19c] TEOS = tetraethyl orthosilicate, CTAB = cetyl trimethylammonium bromide.

active sites, was implanted in nitrogen-doped graphene (NG) sheets through a sonochemical approach by Chen and coworkers.[19d] Compared with the single component, the hybrid (I-NG, the hybrid prepared by the implantation of nitrogen active sites to NG nanosheets with mpg-C3N4) showed an improved ORR activity and high durability in neutral phosphate buffer solution (PBS, 50 mm). Sun et al. reported a 3D porous supramolecular architecture (g-C3N4/rGO; rGO = reduced graphene oxide) derived from the self-assembly of g-C3N4 NSs with GO followed by photoreduction.[19f] The 3D hybrid possesses a high surface area, multilevel porous structure, good electrical conductivity, efficient electron-transport network, and fast charge-transfer kinetics at g-C3N4/rGO interfaces. The ultrathin g-C3N4 NS also allows effective electron tunneling through the g-C3N4 barrier, leading to rich electrode–electrolyte–gas three-phase boundaries and shortening of the electron diffusion distance from rGO to O2. The 3D porous hybrid as a metal-free ORR electrocatalyst thus exhibits superior catalytic performance over many other g-C3N4/C composites.[19f] Recently, we developed a series of g-C3N4/CNTs composites and proved that 1D CNTs could serve as the electron transporter that would provide sufficient electrons to the g-C3N4 layer and promote the dispersion of electrons (Scheme 5).[19e] These hybrids can be divided into two categories according to the gC3N4/CNTs ratio. When CNTs dominated in the prepared composites, the composites acted as a superior ORR catalyst due to the synergistic action between g-C3N4 and multiwalled CNTs, namely, g-C3N4 afforded oxygen adsorption and activation sites, whereas the CNTs delivered enough electrons for the 4 e ORR reaction (Scheme 5, left).[19a] It showed clearly a more positive onset potential and higher limiting current density than g-C3N4 and CNTs. The composites revealed excel5


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Reviews vides abundant nitrogen sites to trap transition metals and some reports have shed light on the possibility of developing M–g-C3N4 ORR catalysts. Li and co-workers illustrated that the Fe–g-C3N4/BP2000 carbon black catalyst showed higher activity than Fe–melamine/BP2000.[28] Zhang and Liu reported the synthesis of a cobalt-doped g-C3N4, which was supported on graphene (Co–g-C3N4@rGO) by a chemical doping strategy (Figure 1 a).[29] They demonstrated the formation of the CoN polymer instead of cobalt or cobalt oxide by XRD, FTIR spectroscopy, and Raman spectroscopy. The Co–g-C3N4@rGO-catalyzed ORR was characterized by a 4 e process in alkaline fuel cells. It exhibited comparable ORR activity to 20 wt % Pt/C in the view Scheme 5. Illustration of g-C3N4/CNTs as ORR catalysts (left) and photocurof both the onset potential and current density, which was rent generation material (right, not described in detail in this review).[19e] greatly enhanced compared with those of pure rGO and gC3N4@rGO (Figure 1 b). The apparent improvement in the ORR activity was ascribed to the abundant accessible CoNx active lent durability and resistance to methanol crossover. For gsites and fast charge transfer at the interfaces of Co–g-C3N4/ C3N4-rich composites, which were applied for photocurrent generation, the CNTs would contribute to the efficient disperrGO. Also, better durability and methanol tolerance ability than sion of the photoexcitons, suppress the recombination of elecPt/C were observed for the hybrid. trons and holes, and thus, lead to the enhanced photocurrent Jin et al. developed a Co–g-C3N4–graphene hybrid with (Scheme 5, right). However, this is an aspect we do not focus a completely different structure.[30] CoO particles were embedon herein. ded in g-C3N4, which was covalently supported on a 2D graphene sheet (GS), denoted as NCo–GS (Figure 2 a). TEM images To obtain nitrogen-doped, highly porous carbon materials indicated that the particles on GS featured a core–shell strucfor ORR, Kurungot and co-workers incorporated g-C3N4 into ture with a core of 3–8 nm in diameter encapsulated by a shell a highly mesoporous MOF-5 and then converted the hybrid 1–2 nm thick (Figure 2 b). A typical NCo–GS-0.5 (mass ratios of into a highly porous, nitrogen-rich carbon.[19g] In this process, g-C3N4 only acted as an intermediate that helped in the generprecursors GO to Co(OAc)2 = 0.5) electrode showed a kinetication of the desired nitrogen-doped active sites for the ORR limiting current density of 16.78 mA cm2 at 0.25 V, which ap(e.g., graphitic and pyridinic N). proached that of 20 % Pt/C at the same potential. The perNitrogen-doped, carbon-supporting, transition-metal materiformance gap between NCo–GS-0.5 and 20 % Pt/C in terms of als (MNxC, M = Co, Fe, Ni, Mn, etc.) were regarded as anoththe half-wave potential difference was 25 mV, which was er kind of electrocatalyst to act as a substitute for platinumbetter than most of the metal-free g-C3N4-based systembased ORR catalysts due to the utilization of abundant, inexs.[19a, b, 26] The Co–N–GS hybrid without a cobalt oxide core ex[27] pensive precursor materials and tunable catalytic activity. Inhibited a similar onset potential to that of NCo–GS-0.5 at 0.08 V, which suggested that they may possess the same spired by studies with respect to the ORR activity of metal– active sites as CoN species. In contrast, CoO–GS and N–GS macrocycle-based catalysts, MNx species were widely considdisplayed a very negative onset potential and inferior kineticered as the real active center for MNxC. Many researchers limiting current; this demonstrates the different nature of the even believed that the ORR activity of nitrogen-doped CNTs active sites. NCo–GS-0.5 also revealed a 4 e reaction pathway may stem from the residual transition metal used as the chemfor the reduction of oxygen, which was a combined action of ical vapor deposition (CVD) catalyst, which could not be comthe rich active sites and the efficient electron-transfer channel pletely removed.[22a] As a nitrogen-rich material, g-C3N4 proserved by the GS. More interestingly, the NCo–GS hybrid showed a self-healing ability, for which the oxide core functioned as an abundant cobalt source to slowly release cobalt ions to recover the disabled surface sites, ensuring the longterm stability of the electrocatalyst. Silver and silver alloys are used as catalysts in alkaline fuel cells due to their relatively low cost and similar ORR activities Figure 1. a) A schematic drawing of Co–g-C3N4@rGO. b) Linear sweep voltammetry (LSV) curves of rGO, gto those of platinum and platiC3N4@rGO, Co-g-C3N4@rGO, and 20 wt % Pt/C with a sweep rate of 5 mV s1 at 1600 rpm in O2-saturated 0.1 m num alloys.[31] However, the perKOH, adapted from Ref. [29].

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Reviews methanol has also been exploited. Yu and co-workers synthesized g-C3N4 with 3D-extended, highly ordered pore arrays, and porous g-C3N4 was utilized for the first time as a support for the Pt50Ru50 alloy catalyst to study the effect of the support on the anodic performance in direct methanol fuel cells.[34] The PtRu alloy nanoparticles were shown to be homogeneously dispersed as small, spherical, and uniform dark spots with an average diameter of 3 nm on the surface of gC3N4. The PtRu/C3N4 catalyst demonstrated more than twice the current density of commercial PtRu alloys supported on Vulcan XC-72 (Pt-Ru/E-TEK) at both 30 and 60 8C. According to the power density (77 vs. 42 mW cm2 at 30 8C, 180 vs. 103 mW cm2 at 60 8C), the PtRu/C3N4 catalyst showed 73– 83 % better performance than that of the PtRu/E-TEK catalyst under the same test conditions. PtRu/C3N4 also exhibited a good stability over 150 h. The improved catalytic performance was attributed to the specific properties of the supporting material. The 3D interconnected well-developed porosity of synthesized g-C3N4 provides an open highway network around the active catalyst for the facile diffusion of fuels and products towards and away from the catalyst, resulting in fast kinetics at the catalyst sites. The surface properties of the gC3N4 network, such as graphiticity and framework nitrogen atoms, facilitate electron transfer through the conduction layer. The metal-free g-C3N4 carbon nanotube (CNNTs) prepared by the thermal polymerization of (CH2NH2)2/CCl4 sol–gel by using an anodic aluminum oxide membrane as the template was applied to methanol electro-oxidation in H2SO4 by Zhang et al.[35] When evaluated by cyclic voltammetry (CV), the CNNTs showed two weak responses attributed to the oxidation of methanol. The performance study shows that the as-prepared CNNTs have durability towards methanol electro-oxidation in acid medium. The fundamental catalytic activity was currently considered to arise from the defect and lone pairs of electrons on nitrogen of six-membered rings. However, they demonstrated that CNNTs were not a good support for Pt towards methanol electro-oxidation because the Pt/CNNTs showed an inferior catalytic activity than that of Pt/C. In contrast, Brett et al. recently demonstrated that g-C3N4 could potentially replace conventional carbon supports (Vulcan XC-72R) for methanol oxidation.[36] They took three different CN materials as supports for Pt nanoparticles: polymeric carbon nitride (gCNM), poly(triazine imide) carbon nitride (PTI/ LiCl), and boron-doped graphitic carbon nitride (B-gCNM). The catalytic activity for methanol was investigated in 1 m methanol containing 0.1 m HClO4. All g-C3N4-supported catalysts exhibited lower overpotentials and higher peak current densities than those of Pt/Vulcan after being normalized to the electrochemical surface area (ECSA; Table 1). In addition, Pt/PTI–LiCl exhibits the lowest overpotential, whereas Pt/B-gCNM has the highest peak current density. Given that Pt/Vulcan has the smallest particle size, the effect that smaller Pt nanoparticles enhance the oxidation of poisoning intermediates, and thus, decrease the overpotential, can be eliminated. Therefore, the intrinsic catalytic enhancement for methanol oxidation was ascribed to the presence of nitrogen on/within the support material.

Figure 2. TEM images showing the microstructure of NCo–GS-0.5 (a, b). c) LSV curves of NCo–GS-0.5 and 20 % Pt/C in 0.1 m KOH. d) LSV curves of Co–N–GS, NCo–GS-0.5, CoO–GS and g-C3N4/GS in 0.1 m KOH, adapted from Ref. [30].

formance of silver-based catalysts is affected by the agglomeration of silver nanoparticles and the electrochemical corrosion of the catalyst supports.[32] Protonated g-C3N4 was claimed to be effective to suppress the aggregation of silver particles, and thus, lead to good catalytic activity (Figure 3).[33] Excellent dispersion of silver nanoparticles was observed at a silver loading as high as 80 wt %. Ag/g-C3N4 (80 wt %) showed a 4 e ORR pathway and good durability.

Figure 3. TEM images of Ag/g-C3N4 (80 wt %) at different magnifications (a, b).[33]

3.2. Electrocatalytic oxidation of methanol At present, Pt/C catalysts are widely adopted as electrocatalysts for alcohol oxidation at the anode in polymer electrolyte membrane fuel cells. The high cost and limited supply of platinum have accelerated the search for efficient alternative catalyst supports to promote the activity of noble metals. g-C3N4 was shown to be a positive support material for a variety of reactions, such as catalytic oxidation and hydrogenation, by improving the particle dispersion and electronic properties of noble metals.[12, 13] Its performance in the anodic oxidation of ChemSusChem 0000, 00, 0 – 0

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Reviews Table 1. Methanol oxidation peak potential (Epeak) and maximum methanol oxidation reaction current density (jmax) of supported platinum electrocatalysts in 1 m CH3OH containing 0.1 m HClO4 at 25 8C. Catalyst

Epeak [V]

jmax [mA cmECSA2]

Pt/Vulcan Pt/gCNM Pt/PTI–LiCl Pt/B-gCNM

0.903 0.850 0.842 0.858

0.821 3.21 174 209

4. Functionalized g-C3N4 for Electrocatalytic Water Splitting (HER and OER)

Scheme 6. Illustration of the synthetic procedure for depositing CN on different substrates (for simplification, only glass is shown).[44]

Hydrogen is seen as an ideal energy carrier and as a valid alter0.1 V in neutral and basic solutions, respectively, and the correnative to fossil fuels because of its advantage of being a clean sponding current densities reach 0.8 mA cm2 at 0.6 and 0.3 V, respectively (Figure 4 a and b). Bare FTO shows only the reducfuel when considering that its combustion produces only tion of SnO2 to SnO at about 0.1 V. For the bare TiO2 electrode water. Currently, hydrogen is mainly produced from natural in neutral media, a cathodic current is observed, which is relatgas through steam methane reforming, producing H2 and CO2, ed to the accumulation of electrons within TiO2 instead of the which exacerbates the greenhouse effect.[37] Comparatively, the electrocatalytic hydrogen evolution reaction (HER) from water HER (Figure 4 c). Under basic conditions, most of the TiO2 surface states are occupied by the basic electrolyte and no cathosplitting can be more renewable and environmentally frienddic current appears (Figure 4 d). After combining with C3N4, the ly.[38] A catalyst is needed to reduce the overpotential of the HER (2 H + + 2 e !H2). Platinum-based catalysts with high cost cathodic current under neutral conditions reduced and the intense anodic peak was significantly quenched. The strong and low reserves offer the best performance for the HER with quenching of the anodic peak implies that the cathodic cura near-zero overpotential.[39] In recent years, a variety of nonnoble-metal materials (e.g., metal nitrides,[40] carbides,[41] metal rent represents electron transfer to the solution (HER) instead alloys[42]) were investigated as catalysts for the HER. However, of electron accumulation within the TiO2. The overpotential the inherent corrosion and oxidation susceptibility hinder their was measured to be 0.3 V and the current density of use in acidic proton exchange membrane based electrolysis for 1 mA cm2 was observed at an overpotential of 0.6 V. In basic [43] hydrogen production. On the other hand, a few acidic-/alkasolution, the activity is increased. A lower overpotential was revealed with a notable current density of 1.3 mA cm2 at an line-resistant and -abundant carbon-based materials or hybrids have also been studied in the HER. Recently, two typical works overpotential of 0.3 V. A long-term cycling stability test of described the fundamental activity of g-C3N4-based materials 500 cycles was carried out in basic solutions. The C3N4/FTO for the electrocatalytic HER.[44, 45] system demonstrated the full degradation of the materials and substrate reduction (SnO2 to SnO) was also detected. In conShalom et al. reported a generic, facile, and easy method to grow highly ordered CN on different substrates (mainly glass, trast, the C3N4/TiO2 system exhibited a relatively high stability with about 15 % decrease in the current density, along with fluorine-doped tin oxide (FTO), and TiO2) by placing a supramolecular complex prepared by mixing cyanuric acid and melamine between two layers of substrates followed by heating at 550 8C under a nitrogen atmosphere (Scheme 6).[44] The existence of g-C3N4 on the substrate was confirmed by SEM, XRD, elemental analysis, FTIR spectroscopy, UV/Vis spectroscopy, and photoluminescence (PL) spectra. Both C3N4/FTO and C3N4/TiO2 showed fundamental activity for the HER in both neutral (0.1 m phosphate buffer solution, pH 6.9) and basic (0.1 m KOH, pH 13.1) media. For C3N4/FTO, catalytic currents are observed Figure 4. CV measurements of C3N4/FTO in 0.1 m PBS (a) and 0.1 m KOH (b). CV measurements of C3N4/TiO2 in at overpotentials of 0.25 and 0.1 m PBS (c) and 0.1 m KOH (d). The scan rate is 25 mV s1.[44]

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Reviews N-graphene to g-C3N4, leading to an electron-rich region on the g-C3N4 layer and a hole-rich region on the N-graphene layer. This change in projected density of states between pure gC3N4 and hybridized C3N4@NG indicates enhanced electron mobility in the latter, which is significant for the electrocatalytic HER. The polarization curve (i–V) recorded on C3N4@NG afforded an overpotential of about 240 mV to achieve an HER current density of 10 mA cm2 and a Tafel slope of 51.5 mV dec1; this was greatly enhanced relative to pure g-C3N4, N-graphene, and their physical mixture (Figure 6 a and b). The HER exchange current density (i0) for Figure 5. Electron microscopy characterization of the C3N4@NG NS. a) Aberration-corrected and monochromated C3N4@NG, as calculated from high-resolution TEM image of freshly prepared C3N4@NG hybrid. b) High-resolution TEM image of the same area as that shown in (a) after removal of the g-C3N4 layer by electron beam irradiation (under prolonged exposure of the Tafel plot by the extrapola 20 s). Scale bar 2 nm (a, b). c) Electron energy loss spectroscopy (EELS) results for C3N4@NG collected at two spetion method, is 3.5 cific sites as indicated in (e). Site 1 represents a g-C3N4-containing region with an apparent nitrogen K-edge 107 A cm2, which is comparaenergy loss peak at about 400 eV; site 2 represents a g-C3N4-free region. d) Fine structure of the carbon K-edge ble (within an order of magniEELS on C3N4@NG. e–g) High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the C3N4@NG NS and EELS mappings of overall f) carbon and g) nitrogen species in the red-line areas. tude) to those of well-develScale bar 2 mm (e). h–j) EELS mappings of various carbon species (peaks 1–3, respectively), as indicated in d). A oped nanostructured MoS2temperature color code is used in mapping images, in which the intensity increases as the color changes in the based metallic catalysts.[46] The order of black–blue–green–red–yellow–white.[45] C3N4@NG catalyst features strong stability in both acidic and basic solutions and much reduced Faradic resistance that a small shift in the overpotential (ca. 70 mV). Tafel analysis of is comparable to the C3N4/NG mixture (Figure 6 c). The excelthe J–V curves reveals that the HER on the two catalytic systems is controlled by a Volmer–Heyrovsky mechanism. lent performance for the ORR was attributed to intrinsic chemiA C3N4/NG hybrid was recently taken as a metal-free catalyst cal and electronic coupling synergistically promoting approprifor HER by Qiao et al.[45] The hybrid was produced by the therate proton adsorption and reduction kinetics. mal treatment of chemically exfoliated graphite oxide (GO) The other half-reaction of water splitting is the oxygen evowith dicyandiamide. The aberration-corrected high-resolution lution reaction (OER), the slow kinetics of which is another TEM image shows its multilayered structure formed by stackmajor factor that determines the water splitting rate. The OER ing multilayered g-C3N4 on N-doped GSs and g-C3N4 can be seis more complex because it involves a 4 e reaction, during lectively removed by TEM irradiation (Figure 5 a and b). which two water molecules form one oxygen molecule.[47] The high cost and scarcity of the most efficient precious metal catHAADF-STEM reveals that C3N4@NG consists of ultrathin NSs alysts, such as IrO2 and RuO2, endow the development of with a fairly uniform thickness (Figure 5 c). EELS spectra and EELS mappings of C3N4@NG demonstrate that g-C3N4 is grown earth-abundant metal oxides and carbon-based alternatives on the surface of N-graphene and there are new chemical with great significance. g-C3N4 has shown promise as a support bonds formed between the two components (Figure 5 c–j). The or composite component of OER catalysts. formation of in-plane C–N–C species was also characterized by For non-precious-metal OER catalysts, cobalt-based ones are synchrotron-based near-edge X-ray absorption fine structure most investigated.[48] Different carbonaceous materials were se(NEXAFS) and X-ray photoelectron spectroscopy (XPS). lected as supports to advance the performance of the active The coupling of C3N4 and N-graphene resulted in a great metal sites. Asefa et al. took g-C3N4 as an intermediate to impact on the electronic structure and electron-transfer propdirect the growth of Ni-doped Co3O4.[49] After removal of gerties, according to DFT calculations. Contrary to semiconducC3N4, residual Ni-doped Co3O4 exhibited highly porous nanotive pure g-C3N4, the C3N4@NG hybrid had no band gap. More networks composed of nanoparticles with an average size of about 30 nm. This catalyst showed superior OER activity to importantly, after coupling of g-C3N4 with N-graphene, the these synthesized without g-C3N4 or Ni-free Co3O4. There are charge density in the interlayer of the hybrid was redistributed in the form of an apparent electron transfer from conductive four main reasons for the enhancement of the catalytic activiChemSusChem 0000, 00, 0 – 0

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Reviews anodic current with an onset potential at approximately 1.53 V was observed; this suggests that the coupling of two components can significantly improve the catalytic activity (Figure 7 a). Although the onset potential is a little higher than that of IrO2– CNT (1.53 vs. 1.51 V), the OER current density of gC3N4 NS–CNT is higher than that of IrO2–CNT at potentials over 1.62 V, which implies that g-C3N4 NS– CNT performs better. By comparing the operating potentials to deliver a current density of 10 mA cm2, the conclusion can be drawn that gC3N4 NS–CNT (1.6 V) possesses a comparable ability to some state-of-the-art noble-metal catalysts, for example, IrO2/C (1.60 V, 0.1 m KOH),[50d] colloidal IrO2 nanoparticles (1.60 V, 0.5 m H2SO4),[52] and Ru0.2Ir0.8O2 (1.61 V, 0.5 m H2SO4),[53] as well as transition-metal catalysts, for example, Mn3O4/CoSe2 hybrids (1.68 V, 0.1 m KOH)[54] and Co3O4/N-graphene (1.63 V, 1 m KOH).[48b] The activity of g-C3N4 NS–CNT is also higher than that of previously reported N(5)-ethylflavinium ions[50a] and N-doped graphene–CNT composites (> 1.65 V, 0.1 m KOH).[50b] The lower Tafel slope value for g-C3N4 NS–CNT demonstrates more favoraFigure 6. Fundamental electrochemical relationships measured for HER on C3N4@NG. a), ble kinetics relative to that of IrO2–CNT (Figure 7 b). b) HER polarization curves and Tafel plots for four metal-free electrocatalysts and 20 % Pt/C (electrolyte: 0.5 m H2SO4, scan rate: 5 mV s1). The curve for C3N4@NG was recorded Higher stability was demonstrated by the chronoamfor the sample with 33 wt % g-C3N4 in the hybrid. c) Polarization curves recorded for the perometric response, with a slight current attenuaC3N4@NG hybrid before and after 1000 potential sweeps (+ 0.2 to 0.6 V versus a reversition within 10 h and a negligible change in operatble hydrogen electrode) under acidic and basic conditions. d) Electrochemical impedance ing potential (Figure 7 c). Over g-C3N4 NS–CNT, spectroscopy data for the C3N4@NG hybrid and C3N4/NG mixture in H2SO4 ; data were collected for the electrodes at an HER overpotential of 200 mV.[45] 91.2 % of the original catalytic current can be retained after 60 scan cycles at a scan rate of 5 mV s1; this provides further evidence of excellent stability (Figure 7 d). Studies on a rotating ring–disk electrode (RRDE) ty: 1) Co3O4 nanoparticles obtained with Ni dopants are smaller reveal that g-C3N4 NS–CNT favors a desirable 4 e water oxidain size and have a high surface area; 2) Ni-doped Co3O4 nanoparticles exhibit a greater ability to adsorb H2O, which should tion pathway with little formation of H2O2. be beneficial for the surface catalytic conversion of H2O to O2 ; Three characteristics of g-C3N4 NS–CNT are considered to be 3) nickel doping accelerates the OER kinetics (or charge transresponsible for the outstanding catalytic activity: 1) the high fer between the catalyst and reactant); and 4) nickel doping may improve the conductivity of Co3O4. The metal-free catalytic OER is a seldom reported field due to the inferior performance of metal-free catalysts. Until recently, few inspiring works concerning the metal-free catalytic OER were proposed and the nitrogen atom was regarded as the active site.[50] A g-C3N4 NS–CNT 3D porous composite was revealed to show a higher catalytic oxygen evolution activity and stronger durability than iridium-based noblemetal catalysts reported by Qiao et al.[51] The composite afforded the best performance among reported non-metal catalysts. The 3D g-C3N4 NS–CNT was fabricated through the self-assembly of the g-C3N4 NS and CNT and driven by p–p stacking and electrostatic interactions (Scheme 7). In alkaline solutions (0.1 m KOH), the purified oxidized CNTs and g-C3N4 NSs showed a negligible OER response, which indicated that trace metal residues in the CNTs barely contributed to the catalytic OER activity. For the g-C3N4 NS–CNT sample, a strong Scheme 7. Fabrication of the 3D g-C3N4 NS–CNT porous composite.[51]

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Reviews demonstrates excellent stability. RDE measurements show firstorder reaction kinetics toward water oxidation for both gC3N4/graphene and g-C3N4 NSs. The Tafel slopes are calculated to be 68.5 and 120.9 mV per log unit for g-C3N4/graphene and gC3N4 NSs, respectively; this implies more favorable OER kinetics over the g-C3N4/graphene composite. XPS studies suggest that the OER activity results from pyridinic N-related active sites.

5. Functionalized gC3N4 Hybrids for Supercapacitors In recent years, supercapacitors have attracted significant attention, mainly due to their high power density, long life cycle, and bridging function for the power/energy gap between traditional dielectric capacitors (which have a high power Figure 7. a) LSV results and b) Tafel plots for g-C3N4NS–CNT, IrO2–CNT, bulk g-C3N4–CNT, purified oxidized CNTs, output) and batteries/fuel cells and g-C3N4NSs on a rotating disk electrode (RDE; 1500 rpm) in an O2-saturated 0.1 m solution of KOH (scan rate: 5 mV s1). The calculation of the OER current density was based on the geometric surface area (GSA). c) Chro(which have high energy stornoamperometric response at a constant potential of 1.54 V. Inset: Chronopotentiometric response of g-C3N4 NS– age).[57] Usually, they can be di2 CNT compared with that of IrO2–CNT at a constant current density of 3.0 mA cm . d) Curve of the current density vided into two categories on of g-C3N4 NS–CNT at 1.60 V versus scan cycles. Inset: LSV results for g-C3N4NS–CNT before and after 60 scan cycles the basis of their charge storage (scan rate: 5 mV s1).[51] mechanisms, namely, electrical double-layer capacitors (EDLCs) nitrogen concentration (23.7 w %), which can result in high and pseudocapacitors. EDLCs show an electrostatic attraction with accumulation of charges at the electrode/electrolyte charge density on the neighboring sp2-bonded carbon double-layer interfaces, whereas pseudocapacitors exhibit faraatoms;[21] these atoms would facilitate adsorption of OH ions, dic redox reactions. For EDLCs, the electrode materials are promote electron transfer between the catalyst surface and remainly carbon materials, the large surface area, excellent elecaction intermediates (e.g., O2 ions), and assure an easy recomtronic conductivity, and tunable porous structures of which are bination of two adsorbed oxygen atoms for O2 evolution; suitable for charge transfer and accumulation.[58] 2) the highly porous 3D architecture, which favors the easy in filtration of electrolytes, efficient transfer of reactants (i.e., OH Reports have shown that the incorporation of nitrogen atoms in the carbon matrix would improve the electron-donor ions), and fast emission of products (i.e., O2); and 3) high conproperties, surface polarity, and electrical conductivity of the ductivity, which stems from coupling of highly conductive material. Also, the wettability of the material in the electrolyte CNTs with semiconducting g-C3N4 and promotes the smooth would be enhanced, which would lead to efficient charge transfer of the generated catalytic current through p–p stacktransfer.[59] From this perspective, nitrogen-rich g-C3N4 possessing between g-C3N4 NSs and CNTs. A metal-free ultrathin g-C3N4/graphene composite was rees a great advantage over other carbon materials. The graphitic C3N4 nanofiber (GCNNF) with an average diameter of cently synthesized and employed for the electrocatalytic OER.[55] The conductive composite was developed by the 100 nm and length of 20 mm was recently produced from melamine, which was pretreated with HNO3 in ethanol and heated mixing of ultrasonically exfoliated g-C3N4 sheets with graphene. It showed enhanced OER activity relative to that of at 450 8C for 2 h (Figure 8). The as-prepared GCNNF was tested bulk g-C3N4, pure graphene, and pure g-C3N4 NS, and was even as a supercapacitor electrode material.[60] comparable to some of the previously reported metal oxide The capacitance of the GCNNF was evaluated by galvanobased catalysts in an alkane electrolyte.[54, 56] g-C3N4/graphene static charge–discharge measurements in 0.1 m Na2SO4. The retains 100 % of its OER activity after 1000 CV cycles, which time duration difference for the charge–discharge of the 1st, ChemSusChem 0000, 00, 0 – 0

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Reviews tance loss of 1.82 % at 0.5 A g1 and 6.8 % at 1 A g1 (Figure 9 c). The excellent capacitance retention of 89.5 % at 10 Ag1 illustrates that the GCNNFs have a good rate capability (Figure 9 d). The enhanced performance of the GCNNF was ascribed to two factors: 1) the presence of a high degree of nitrogen; and 2) the large surface area of GCNNF, which provides the advantage of a large electrode–electrolyte contact and improves electrolyte ion transport. The presence of nitrogen can affect the capacitive performance in various ways: 1) it provides various active reaction sites; 2) it improves the surface wettability of GCNNF, which is useful for the electrolyte moving into the inner layer of the carbons; 3) it increases the electron donor/ acceptor characteristics; 4) it improves the wettability with electrolytes, and consequently, enhances mass transfer efficiency; and 5) it provides a large additional pseudocapacitance. When the solvent was changed from ethanol to ethylene glycol, tubular g-C3N4 with an average diameter of 0.8 mm and Figure 8. Low-magnification SEM images of GCNNFs (a and b) and higha length of 20 mm, instead of GCNNFs, was obtained magnification SEM images of GCNNFs (c and d).[61] (Figure 10).[61] The capacitance of tubular g-C3N4 was also evaluated by galvanostatic charge–discharge measurements in 6 m 1000th, and 2000th cycles is very small at current densities of KOH. The electrode shows good capacitive performance at cur0.5 and 1 A g1, which indicates good cycling stability (Figrent densities of 0.2 and 0.5 A g1. The calculated specific caure 9 a and b). The electrode composed of GCNNFs shows pacitance is 233 F g1 at a current density of 0.2 A g1 for the 1 a specific capacitance of 275 F g at a current density of 1st cycle and 212 F g1 for the 1000th cycle; this shows the 1 1 0.5 A g for the 1st cycle, and 270 F g for the 2000th cycle; high cycling stability of tubular g-C3N4. Similarly, tubular g-C3N4 this reveals remarkable long-term cycling stability. In contrast, maintains its high specific capacitance, even at a higher curbulk g-C3N4exhibits a very low specific capacitance of 71 F g1 rent density of 0.5 A g1, for which it is 204 F g1 for the 1st 1 cycle and 182 F g1 for the 1000th cycle. For bulk g-C3N4, the at a current density of 0.5 A g for the first cycle; this may be due to the low surface area. The excellent retention capability capacitance is only 81 F g1 at 0.2 A g1 and 70 F g1 at of the GCNNF electrode was also validated by a small capaci0.5 A g1; this reveals that the high surface area and specific tubular morphology are the main factors that enhance the capacitance. Additionally, better capacitive performance of tubular g-C3N4 than those of many other carbonaceous materials was also explained by the reasons as those outlined for the GCNNF. Hybridized g-C3N4/a-Fe2O3 hollow microspheres were designed and fabricated for the use of a supercapacitor electrode material.[62] The specific hollow spherical structure, high surface areas, and lower electronic resistance contributed to the good capacitive performance of the composite. In Li2SO4, the g-C3N4/a-Fe2O3 hollow microspheres did not show any redox peaks over the tested potential window (1.0 to 0 V); this means that the hybrid gives complete capaciFigure 9. 1st, 1000th, and 2000th charge–discharge curves within a potential window of 0.1 to 0.7 V at current tive behavior. The best specific densities of 0.5 (a) and 1.0 A g1 (b) for GCNNFs in 0.1 m Na2SO4. c) Cyclic performance of GCNNFs and bulk g-C3N4 capacitance was calculated to at 0.5 and 1 A g1 in 0.1 m Na2SO4. d) Capacitance of GCNNFs as a function of current density.[61]

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Reviews (Scheme 8). The calcination of diacyanamide-functionalized rGO at 550 8C would afford the hybrid. g-C3N4–rGO delivered a lithium insertion capacity of 3002 mA h g1 and a reversible charging capacity of 1705 mA h g1. The pure g-C3N4 electrode exhibited a very low reversible capacity of 68 mA h g1, whereas pure rGO delivered a specific capacity of 1200 mA h g1 during initial discharging and a reversible capacity of 426 mA h g1 during the first charging cycle.[69] The improved capacity revealed the synergistic effect between the two moieties. g-C3N4–rGO also showed a high Coulombic efficiency of 57 %, which was higher than that of pure g-C3N4 (45 %) and rGO (33 %). Durability tests showed that the g-C3N4–rGO hybrid exhibited a high, stable, and reversible capacity of 1525 mA h g1 at a current density of 100 mA g1 after 50 cycles. Even at a high current density of 1000 mA g1, a reversible capacity of 943 mA h g1 could be retained; this demonstrates a good rate capability. The enhanced lithium-storage capacity and excellent cyclability for g-C3N4–rGO were attributed to five main reasons: 1) Porous g-C3N4 can be used as an excellent template for the highly dispersed decoration of lithium ions, which favors the reversible capacity. The topological defects on the rGO NS resulted from covalent doping to provide access for the diffusion of Li ions from g-C3N4 to rGO, which further enhances the capacity. 2) Fully exfoliated rGO NSs offer sufficient adsorption and interaction sites for Li ions on both sides and inner mesopores; thus providing increased reversible storage capacity. 3) The high surface areas supply more Li-ion insertion/extraction sites to give higher reversible Li-ion storage capacity. 4) The covalent interactions between g-C3N4 and rGO stabilize the hybrid more, which leads to excellent cyclability. 5) The covalently coupled hybrid possesses much better conductivity than that of g-C3N4, which ensures efficient and continuous charge transfer between the material and current collector; thus affording a high rate capability.

Figure 10. a, b) SEM images of tubular g-C3N4 at different magnifications. c) TEM image of tubular g-C3N4. d) TEM image of a single tube of g-C3N4 (the inset shows a selected-area electron diffraction pattern of tubular g-C3N4).[62]

be 167 F g1, which was better than that of mesoporous aFe2O3 (116.25 F g1 in 1 m Li2SO4), a-Fe2O3 nanotube arrays (138 F g1 in 1 m Li2SO4), nanostructured Fe2O3/graphene composite (151.8 F g1 in 2 m KOH), and Fe2O3@C nanocomposites (155.2 F g1). Identical specific capacitances for the 180th and 1000th cycles reveal satisfactory cycling stability of the g-C3N4/ a-Fe2O3 composite.

6. Functionalized g-C3N4 for Lithium-Ion Batteries Among diverse technologies for energy storage, the lithiumion battery, due to factors such as high energy density, lightweight, and long life span,[63] has become an attractive system for portable electronic devices, such as cell phones, laptops, and digital cameras. A lithium-ion battery is composed of an anode (e.g., graphite), a cathode (e.g., LiCoO2), and a solidstate electrolyte.[64] Lithium ions are deintercalated from the cathode, pass across the electrolyte, and intercalate between the layers of the anode when charging and discharging undergo the reverse process. The commercially applied graphite anode has many advantages of low cost, high cyclability, and outstanding kinetics. However, lithium dendrite growth on the graphite anode during the overcharging process leads to severe safety issues.[65] Furthermore, the rate capacity of the graphite anode cannot match some newly developed highrate cathodes;[66] this becomes the key factor that limits the performance of lithium-ion batteries. As a nitrogen-substituted graphite with the highest nitrogen-doping level,[15a] the good performance of g-C3N4 as an anode material may be foreseen because nitrogen doping is an efficient way to improve the reversible capacity, the Coulombic efficiency, or cycle performance.[67] The poor conductivity of g-C3N4 is a main stumbling block for its use as an electrode material. Recently, Wang et al. tried to fix this issue by designing a g-C3N4–rGO hybrid.[68] The formation mechanism for g-C3N4–rGO was studied by FTIR spectroscopy, solid-state 13C NMR spectroscopy, and XPS ChemSusChem 0000, 00, 0 – 0

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7. Summary and Outlook Thanks to its structure and properties, g-C3N4 has been extended to a wide scope of applications, covering traditional heterogeneous catalytic reactions (e.g., Friedel–Crafts reaction, selective oxidation, and hydrogenation), photocatalytic water splitting, pollutant degradation, and so forth. From this perspective, we have presented the most advanced applications of gC3N4 in energy conversion and storage, mainly for use in fuel cells, electrocatalytic water splitting, supercapacitors, and lithium-ion batteries. These related reports have provided new opportunities for g-C3N4. As new research areas, they provide challenges, opportunities, and problems to resolve. For metal-free g-C3N4 hybrids, the observed activity for the ORR, HER, and OER is still too far away from requirements for practical applications. Although they show superior durability and robust resistance to poisoning, high activity is the first goal to aim for. Therefore, more attention should be given to improving the catalytic activity. For energy storage, inferior conductivity and the low sur13


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Scheme 8. Illustration of the formation process for g-C3N4–rGO.[69]

face area of g-C3N4 are key stumbling blocks that should first be addressed. From a fundamental point of view, improving the activity and stability relies on further understanding of the active-site structures in g-C3N4. At present, the location of the active sites is still a subject of controversy; this hinders efforts to control active-site formation. Fundamentally understanding the mechanisms and their relationship with the catalyst active-site structures and composition, by using more comprehensive theoretical calculations (molecular/electronic level modeling), is an important future research direction. From the viewpoint of the g-C3N4 structure, the well-developed nanoporous structure is undoubtedly a feature that favors the electrocatalytic activity and energy storage. Exploring high-surface-area g-C3N4 with a tunable pore size for the purpose of achieving more abundant active sites and better mass transfer is another important goal. Highly effective and easily scaled-up soft template or template-free methods are in urgent need to replace the most frequently applied nanocasting processes. As a common problem of metal-free, nitrogen-doped carbon materials for the ORR, poor activity and stability in acidic systems are also observed for g-C3N4 systems. This is

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a great challenge because most fuel cells usually operate in acidic electrolytes (particularly the PEMFC). In this sense, it is expected to develop g-C3N4-based catalytic systems with excellent performance in acidic environments.

Acknowledgements Financial support from the National Natural Science Foundation of China (U1162124&21376208), the Zhejiang Provincial Natural Science Foundation for Distinguished Young Scholars of China (LR13B030001), the Specialized Research Fund for the Doctoral Program of Higher Education (J20130060), the Fundamental Research Funds for the Central Universities, the Program for Zhejiang Leading Team of S&T Innovation, and the Partner Group Program of the Zhejiang University and the Max-Planck Society are greatly appreciated. Keywords: carbon · electrochemistry · energy conversion · fuel cells · water splitting [1] S. Bilgen, S. Keles, A. Kaygusuz, A. Sari, K. Kaygusuz, Renewable Sustainable Energy Rev. 2008, 12, 372 – 396.

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REVIEWS For use later: Favorable properties of graphitic carbon nitride (g-C3N4), such as semiconductor optical properties, rich nitrogen content, and tunable porous structure, have attracted attention for applications as electrode materials or catalysts. This review describes advanced applications of g-C3N4 in energy conversion and storage.


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Y. Gong, M. Li, Y. Wang* && – && Carbon Nitride in Energy Conversion and Storage: Recent Advances and Future Prospects

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