CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402079
Electrocatalysis of Water Oxidation by H2O-Capped Iridium-Oxide Nanoparticles Electrodeposited on Spectroscopic Graphite Naghmehalsadat Mirbagheri,[a] Jacques Chevallier,[a, b] Jakob Kibsgaard,[a, b] Flemming Besenbacher,[a, b] and Elena E. Ferapontova*[a]
Electrocatalysis of water oxidation by 1.54 nm IrOx nanoparticles (NPs) immobilized on spectroscopic graphite electrodes was demonstrated to proceed with a higher efficiency than on all other, hitherto reported, electrode supports. IrOx NPs were electrodeposited on the graphite surface, and their electrocatalytic activity for water oxidation was correlated with the surface concentrations of different redox states of IrOx as a function of the deposition time and potential. Under optimal con-
ditions, the overpotential of the reaction was reduced to 0.21 V and the electrocatalytic current density was 43 mA cm2 at 1 V versus Ag/AgCl (3 m KCl) and pH 7. These results beneficially compete with previously reported electrocatalytic oxidations of water by IrOx NPs electrodeposited onto glassy carbon and indium tin oxide electrodes and provide the basis for the further development of efficient IrOx NP-based electrocatalysts immobilized on high-surface-area carbon electrode materials.
1. Introduction Electrochemical oxidation of water is considered as an attractive way to provide fuel cells with a clean and sustainable supply of O2 and H2 fuel.[1–5] As a result of kinetic impediments, the water oxidation reaction (WOR) proceeds at potentials essentially exceeding the theoretical value of 1.23 V [versus a standard hydrogen electrode (SHE)].[6] To overcome the kinetic limitations, a number of catalytic systems have been proposed;[7–11] these include the metal oxides IrO2, RuO2, PtO2, Co3O4, and Mn2O3.[12–15] Of these, iridium oxide (IrOx) has been shown to exhibit high electrocatalytic activity and performance stability in the WOR over a wide pH range, thus enabling the four-electron electrooxidation of water at overpotentials (h) ranging between 0.20 and 0.38 V.[16–19] Significant improvement of the catalysis, as compared to solid IrOx electrodes, can be achieved with IrOx nanoparticles (NPs), which provide a higher surface-area-to-volume ratio. 1–2 nm-sized IrOx NPs have been shown to exhibit long-term stability and high electrocatalytic activity in both acidic and alkaline media and reduce the h of water oxidation to 0.20– 0.29 V.[18–22] The procedure of IrOx NP immobilization is likely to affect the electrocatalytic properties of IrOx NPs deposited onto a substrate electrode. [a] N. Mirbagheri, Dr. J. Chevallier, Dr. J. Kibsgaard, Prof. F. Besenbacher, Prof. E. E. Ferapontova Interdisciplinary Nanoscience Center (iNANO), Aarhus University Gustav Wieds Vej 1590-14, 8000 Aarhus C (Denmark) E-mail: [email protected] [b] Dr. J. Chevallier, Dr. J. Kibsgaard, Prof. F. Besenbacher Department of Physics, Aarhus University Ny Munkagade 120, 8000 Aarhus C (Denmark)
Electrocatalytically active high-surface-area IrOx films have been prepared both by chemisorption and electrodeposition of IrOx NPs from their colloidal solutions onto indium tin oxide (ITO), fluorine-doped tin oxide (FTO),[23] and glassy carbon (GC) electrodes.[19, 20] Hydroxide-capped IrOx NPs diffusing from the bulk solution to a Pt rotating disk electrode (RDE) surface have been shown to exhibit essential electrocatalytic activity.[21] 2 nm H2O-capped IrOx NPs have been electrochemically deposited onto GC electrodes from a pH 1 solution; their high catalytic activity between pH 1 and pH 13 has been shown previously.[20] In situ formation of H2O-capped IrOx·n H2O NPs on gold and GC electrodes has been shown to produce highly stable and electrocatalytically active IrOx NP electrode systems with significantly reduced water oxidation h of down to 0.2 V.[18] The relation between the deposition conditions and the efficiency of electrocatalysis, for similarly prepared NPs, has not yet been studied. In the present work, the electrocatalytic properties of H2Ocapped IrOx NPs electrodeposited onto spectroscopic graphite (Gr) were studied (Figure 1). Immobilization of IrOx NPs onto cost-effective and high-surface-area microstructured Gr was expected to produce a simple and electrocatalytically active electrode material. IrOx NPs with H2O molecules as surface ligands were synthesized, deposited onto the electrodes either by electrochemical deposition or physical adsorption, and investigated in the reaction of electrochemical oxidation of water at different pH values. The focus was on the analysis of the electrocatalytic reactivity of NPs as a function of the electrodeposition conditions.
Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402079.
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www.chemphyschem.org 2.2. Electrocatalytic Oxidation of Water at Graphite Electrodes Modified with IrOx NPs through Adsorption and Electrodeposition
Figure 1. Schematic illustration of graphite modification with IrOx NP at pH 1.
Gr electrodes were modified with IrOx·NPs by two different protocols, specifically, through electrochemical deposition and physical adsorption, and then studied in the electrocatalytic WOR. In both cases, the electrocatalytic activity of the IrOxmodified Gr electrodes in the WOR was greatly enhanced compared with that of the bare Gr electrode, and it was further seen that electrodeposited IrOx NPs exhibited a higher activity than physically adsorbed ones (Figure 3 and Figures S2 and S3 in the Supporting Information).
2. Results and Discussion 2.1. UV/Vis Spectroscopic and Transmission Electron Microscopy (TEM) Characterization of IrOx NPs As can be seen from the absorbance spectra of colloidal solutions of IrOx NPs shown in Figure S1 in the Supporting Information, the H2O-capped IrOx NPs exhibit a broad visible band around 576 nm, which is characteristic of IrIV oxides. Based on the reported value of the extinction coefficient (e574) of 630 50 m1 cm1 for IrOx··n H2O NPs,[20] the concentration of freshly synthesized IrOx NPs was estimated to be 2.4 0.2 mm. During the first step of the synthesis, under heating in basic media, K2IrCl6 is hydrolyzed to [Ir(OH)6]2 with a formation of OH-capped IrOx NPs. Addition of HNO3 leads to a protonation of OH-capped IrOx NPs, which makes their condensation with the rest of the [Ir(OH)6]2 more efficient and results in a high yield of IrOx·n H2O NPs. The reaction between [Ir(OH)6]2 and OH-capped IrOx NPs is reported to be rapid; however, longer reaction times (such as overnight storage of the reaction mixture) are required to achieve a more complete IrOIr bond formation within the NPs (Figure S1, curve 2 in the Supporting Information).[20] In the present work, aged solutions (not freshly prepared solutions) of IrOx·n H2O NPs, which were shown to be stable for at least one month when kept at 4 8C, were used. The average IrOx NP diameter was 1.54 0.20 nm, which is consistent with previous reports (Figure 2).[18, 20]
Figure 3. Representative CVs of electrocatalytic oxidation of water recorded with the IrOx-NP-modified electrode prepared by 10 min electrodeposition of IrOx NPs at 1.3 V (1–5, pH 1.5–13.2) and by drop-casting (1’–5’, pH 1.5– 13.2), and that of the bare graphite electrode (black dotted line, pH 1.5). Potential scan rate 20 mV s1. Inset: zoomed potential window corresponding to the IrOx NP redox peaks at pH 7. The axis titles in the inset are the same as in the main figure.
The varying electrocatalytic activities of the electrochemically deposited and physically adsorbed IrOx NPs correlated well with the surface coverage of the NPs, which was estimated by integration of the cyclic voltammetry peaks (at around 190 mV at pH 7, Figure 3 inset) that correspond to the IrIII/IrIV redox transformation. For physically adsorbed NPs, the IrOx surface coverage (GIrIII/IrIV) was undetectable and cyclic voltammograms (CVs) showed atypical crossing (Figure 3, inset) until they were stabilized during consecutive scanning; this stabilization was accompanied by the concomitant decrease in the H2O oxidation currents (Figure S2 in the Supporting Information), which correlated with the variation of the electrode surface properties, most likely owing to desorption of NPs. For electrochemical deposition, GIrIII/IrIV increased to 52.1 2.9 nmol cm2. Thus, electrodeposition of NPs provided higher loading of the electrode with the nanocatalyst and, as a result, a higher electrocatalytic activity for the IrOx-modified Gr electrodes in the WOR. Like with some other electrode materials, the drop-casting method was not very efficient for Gr.[23, 24] This was ascribed to the weak physical adsorption of IrOx NPs onto the Gr surface, unless the electric potential stimulation was applied. All further studies were performed with the electrodeposited IrOx NPs.
Figure 2. TEM image of synthesized 1.54 0.20 nm IrOx NPs. Scale bar: 5 nm.
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CHEMPHYSCHEM ARTICLES 2.3. Effect of NP Electrodeposition Time and Potential on Electrocatalysis of Water Oxidation Several redox processes can be distinguished in the CV of IrOx NPs immobilized on Gr (Figure 4 A): the redox couples at around 190 and 527 mV, pH 7, associated with the IrIII/IrIV and IrIV/IrV redox transformations, and the reduction peak at 725 mV, which can be attributed to electroreduction of the IrVI that is not consumed in the WOR proceeding at the same potentials.[19]
Figure 4. A) Representative CVs recorded with the IrOx-NP-modified electrodes prepared by electrodeposition of IrOx NPs at 1.3 V for 1) 0.6, 2) 6, 3) 60, and 4) 600 s. Potential scan rate 20 mV s1 and pH 7. Inset: Zoomed potential window corresponding to the IrOx redox peaks. The axis titles in the inset are the same as in the main figure. B) Dependence of GIrIII/IrIV and GIrIV/IrV (inset) on the electrodeposition time, data derived from the CVs in Figure S4 in the Supporting Information.
The surface concentration of the correspondingly electroactive IrOx sites at the electrode surface was estimated by integration of the IrIII/IrIV and IrIV/IrV CV peaks recorded in the NPfree solution[18, 19, 22, 23] (Figure 4 B) and correlated with the electrocatalytic efficiency of the IrOx-modified Gr electrodes, namely the catalytic current densities and h of the WOR, for different electrodeposition times (t) and potentials (E). As can be seen in Figure 4 A, for longer electrodeposition times, higher current densities for the electrocatalytic WOR and lower potentials for the electrocatalysis onset are observed, which is consistent with the increased surface coverage of IrOx NPs. The dependence of the surface amount (GIrIII/IrIV) of the IrIII/IrIV sites on time exhibited a typical saturation behavior, with a theoretical saturation limit of 57.3 2.4 nmol cm2 obtained from data fitting to the Langmuir adsorption isotherm. The experimental value of 52.1 2.9 nmol cm2 was obtained for a 600 s electro 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org deposition time; this corresponds to 31.3 1015 sites cm2 (Table S1 in the Supporting Information) or 4.74 1014 NP cm2 if the data are recalculated for the NP number by taking into account the 66 Ir sites per NP, based on the rutile lattice of IrOx. The absolute amount of these NPs will occupy 0.77 cm2, which roughly corresponds to the real surface area of a Gr electrode at its lower limit (a roughness factor of at least 10) and, thus, represents a (sub)monolayer NP coverage. Similarly, the electrocatalytic currents of the WOR consistently increased with increasing electrodeposition potential (Figure S5 in the Supporting Information). Analysis of the IrIII/IrIV and IrIV/IrV redox signals evidenced that the enhancement of the efficiency of electrocatalysis by NPs electrodeposited at higher potentials may also be correlated with the increased surface concentration of the electrodeposited nanocatalyst (Table S2 in the Supporting Information). In contrast to the Gt(electrodeposition) dependences that exhibited a typical saturation behavior at higher electrodeposition times (Figure 5), the GE(electrodeposition) dependences demonstrated an exponential increase in GIrIII/IrIV and GIrIV/IrV.
Figure 5. Dependence of GIrIII/IrIV and GIrIV/IrV (inset) on the electrodeposition potential for the IrOx-NP-modified electrodes prepared by 10 min electrodeposition at 1) 0.7, 2) 0.9, 3) 1.1, and 4) 1.3 V. Measurements were performed at pH 7 and potential scan rate 20 mV s1. Data derived from the CVs presented in Figure S4 D and S6 in the Supporting Information.
It is understood that not all iridium sites of the electrodeposited IrOx NPs may be electrochemically and electrocatalytically active; only a fraction of them undergo the IrIII/IrIV transformation and further consecutive IrIV/IrV and IrV/IrVI transformations to form higher oxidation states of Ir involved in electrocatalysis. Depending on experimental conditions, such as the IrOx NP capping ligand, NP size, the substrate used for NP deposition, and the deposition method, 3.2,[24] 12.5,[22] and 16 %[23] of the total amount of electrodeposited Ir atoms in the IrOx films have been reported to be electrochemically active, whereas IrOx NPs in solution have shown 100 % electrochemical activity.[25] For the latter case, despite the quite different electrochemical behavior of the freely diffusing NPs and the electrode-immobilized ones, they exhibited a very similar electrocatalytic activity, namely very similar h values for the WOR.[21] Our estimations of the number of electroactive IrIII/IrIV sites that are further oxidized to the IrV state show that only 3.3 % of the ChemPhysChem 2014, 15, 2844 – 2850
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Figure 6. Dependence of the amount of electrochemically active IrIV/IrV on the amount of electroactive IrIII/IrIV, for the IrOx-NP-modified Gr electrodes prepared by electrodeposition of IrOx NPs at 1.3 V for: 1’) 0.6, 2’) 6, 3’) 60, and 4’) 600 s and by 10 min electrodeposition at 1) 0.7, 2) 0.9, 3) 1.1, and 4) 1.3 V. Data obtained from CVs in Figure S4–S6 in the Supporting Information.
total amount of IrIV (52.1 2.9 nmol cm2) are converted into IrV (1.7 0.06 nmol cm2) for the electrodes prepared with 10 min electrodeposition at 1.3 V (Figure 6). Similar data were obtained from the slopes of the GIrIV/IrVGIrIII/ IrIV dependences constructed for different electrodeposition times (3.3 %) and potentials (3.4 %; Figure 6). The electrontransfer (ET) rate constants (kET) estimated by the Laviron approach[26] were 3.5 and 6.1 s1 for the IrIII/IrIV and IrIV/IrV ET processes, respectively, thus indicating a lower rate of electrochemical transformation of IrIV to IrIII (Figure S7 and comments in the Supporting Information). Both ET processes were coupled to a proton release in the course of Ir oxidation (1 e/1 H + reactions),[25, 27] with a formal potential–pH dependence exhibiting a slope slightly higher than the characteristic 58 mV pH1 (Figure S8 in the Supporting Information).[25] The efficiency of electrocatalysis was correlated with the concentration of IrIV/IrV sites in IrOx NPs electrodeposited onto the electrodes for different times and at different potentials (Figure 7). Although the apparent catalytic activity increased with the increasing IrOx NP loading (Figure 7 A, B), the specific catalytic activity decreased (Figure 7 A, B, inset, and Figure S9 in the Supporting Information), presumably owing to the decreased surface-area-to-volume ratio for NPs electrodeposited for longer times or at higher potentials. Two distinct regions, consistent with two different specific catalytic activities of IrOx NPs, can be observed at low and high surface coverage of active sites (Figure 8), that is, at GIrIV/IrV < 0.1 nmol cm2 with a slope of 148.34 mA nmol1and at GIrIV/IrV > 0.1 nmol cm2 with a slope of 12.79 mA nmol1. The same trend was followed with electrodes prepared by varying electrodeposition potentials (Figure 8), where 133.28 and 9.94 mA nmol1 specific catalytic activities (i.e. related to the number of the electrocatalytically active Ir sites) were obtained for low and high IrOx active-site surface coverage, respectively. In general, the observed drop in the specific activity of the NPs at a NP surface coverage exceeding 0.3 1014 NPs cm2 may be related either to surface aggregation of NPs or lower availability of IrOx for electrocatalysis, owing to, for example, the effect 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7. Dependence of GIrIV/IrV and catalytic current density at 1 V as a function of the electrodeposition time. Inset: the normalized current density at 1 V to the surface concentration of IrIV/IrV redox. Data obtained from CVs recorded at pH 7, with the IrOx-NP-modified Gr electrodes prepared by electrodeposition of NPs A) at 1.3 V for: 1) 0.6, 2) 6, 3) 60, and 4) 600 s and B) by 10 min electrodeposition at 1) 0.7, 2) 0.9, 3) 1.1, and 4) 1.3 V.
Figure 8. Dependence of the catalytic current density at 1 V on GIrIV/IrV. Data obtained from CVs recorded in PBS, pH 7, with IrOx-NP-modified Gr electrodes, prepared by applying 1.3 V for deposition times: 1) 0.6, 2) 6, 3) 60, and 4) 600 s and by 10 min electrodeposition at applied potentials: 1’) 0.7, 2’) 0.9, 3’) 1.1, and 4) 1.3 V.
of enhanced O2 evolution on adsorption of H2O, detrimentally affecting their electrocatalytic properties. However, the demonstrated high specific electrocatalytic activity of IrOx NPs at their lowest surface concentrations allow the development of cheaper (owing to the decreased amount of used Ir) and yet more efficient nanocomposite catalysts for water oxidation based on IrOx NPs and inexpensive high-surface-area carbon electrode materials. Thus, the electrodeposition potentials and times affected the amount of electrodeposited NPs and as a result the elecChemPhysChem 2014, 15, 2844 – 2850
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trocatalytic activity of NPs in the WOR. The potential of water oxidation measured at 3 mA cm2 (EWOR) decreased with increasing electrodeposition time and potential (Figure S10 in the Supporting Information). The most electrocatalytically active electrodes with the lowest EWOR were obtained by 10 min electrodeposition of NPs at 1.3 V, and these electrodes were used to evaluate the onset potentials of the WOR and the electrocatalytic current-density values at different pH values. The average pH-independent h related to the onset of oxygen evolution (determined by the Tafel method) was around 0.21 V (Figure 9). With increasing pH, the efficiency of electrocatalysis, namely the catalytic current density at 1 V increased (Figure S12, blue); this trend is consistent with a favorable release of protons in a basic medium. The current density at pH 13.2 could not be measured accurately, owing to vigorous evolution of oxygen. Nevertheless, the catalytic current density at a constant h of 0.4 V (Figure S12, red) had the highest value at pH 7 and 13.2.
2.4. The Electrocatalytic Activity of the IrOx-Modified Gr Electrode Based on the comparative analysis of the data available in the literature, IrOx NPs electrodeposited onto Gr may be considered as one of the most efficient IrOx-based catalysts for the WOR (Table 1 and Figure 9), with the catalytic current densities essentially exceeding those observed at NP-modified electrodes prepared both by adsorption and electrodeposition techniques by using a variety of electrode materials, such as GC, ITO, and FTO electrode supports.[18, 22–24, 28] The h of the WOR of 0.21 V may be also considered as one of the lowest hitherto reported, being only slightly higher than that of 0.20 V reported by Zhao et al. (obtained with IrOx·n H2O-NPmodified GC rotating-disk electrode). Also, IrOx-NP-modified Gr
Figure 9. pH dependence of the thermodynamic and experimental potential (grey) of the WOR at the IrOx-NP-modified electrodes prepared by 10 min electrodeposition of NPs at 1.3 V. Data were derived from CVs recorded at 20 mV s1.
exhibited electrocatalytic current densities that were several times higher (four–fivefold, depending on pH) than those shown with IrOx-NP-modified GC RDE, which may be connected both with the much higher surface area of the Gr electrodes used here, a roughness factor exceeding 10,[29] and the hydrophilic character of the graphite surface, favoring interactions with the hydrophilic surface of IrOx NPs. When catalytic current densities are referred to the surface concentration of IrV (Table 1), IrOx-NP Gr electrodes also demonstrate the highest specific catalytic activity, comparable only with NPs adsorbed on GC at a sub nmol cm2 level[22] (ca. 30 % less for the latter).
3. Conclusions 1.54 nm IrOx NPs electrodeposited onto the high-surface-area spectroscopic Gr electrode demonstrate one of the highest hitherto reported electrocatalytic activities in the reaction of
Table 1. The overpotentials (h), and current densities (i) reported for the WOR at the differently prepared IrOx-NP-modified electrodes. IrOx NP solution
Deposition conditions
Substrate
Surface coverage (G) [mol cm2]
h [V]
Current density at 1 V [mA cm2]
Ref.
2 mm IrOx·n H2O NP synthesized in acidic medium (pH 1); NP diameter of 1.54 0.2 nm 0.18 mm IrOx·n H2O NP synthesized in acidic medium (pH 1); NP diameter of 1–2 nm Anodic in situ synthesis of IrOx·n H2O NP from 2 mM [Ir(OH)6]2 ; NP diameter of 2–5 nm 2 mm IrOx·n H2O NP synthesized in acidic medium (pH 1); NP diameter of 2 nm 2.4 mm IrOx NP synthesized in basic medium (pH 13); NP diameter of 2 nm 2.5 mm IrOx NP synthesized in basic medium (pH 13); NP diameter of 1.6 0.6 nm 0.28 mm solution of citrate-capped IrOx NP, pH 3.5; NP diameter of 60–100 nm 0.28 mm solution of citrate-capped IrOx NP, pH 3.5; NP diameter of 60–100 nm 0.62 mm solution of citrate-capped IrOx NP synthesized at pH 3.5; NP diameter of 50–100 nm
electrodeposition at 1.3 V vs. Ag/AgCl (3 m) for 10 min dipping for 5 h
graphite electrode GC
0.21[a]
~ 43,[d] pH 7
0.28[b]
~ 3.5,[e] pH 7.2
this work [22]
electrodeposition at 1.3 V vs. Ag/AgCl (3 m) electrodeposition at 1.4 V vs. Ag/AgCl (3 m) for 15 min electrodeposition at 1.3 V vs. Ag/AgCl for 10 min –
GC RDE
GIrIII/IrIV = 52.1 109 GIrIV/IrV = 1.7 109 GIrIII/IrIV = 1.89 1010 (at pH 9.3) GIrIV/IrV = 3.5 108
0.20[c]
~ 18,[d] pH 7
[18]
GIrIV/IrV = 1.1 108 (at pH 1) GIrIV/IrV ~ 1.6 108
0.22[b]
~ 10,[d] pH 9
[20]
0.25[b]
~ 20,[d] pH 7
[19]
n.r.
0.29[b]
~ 22,[d] pH 13
[21]
dipping for 24 h
rotating Pt disc ITO
GIrIV/IrV = 1.5 109
n.r.
~ 0.1,[d] pH 5.3
[23]
dipping for 24 h
FTO
n.r.
n.r.
< 0.02,[d] pH 5.3
[23]
dipping for 1 h
ITO
GIrIV/IrV = 6.5 1010
n.r.
< 0.1,[d] pH 5.3
[24]
GC RDE GC
[a] The h of the WOR was measured by using the Tafel method. [b] the h of the WOR were measured at 0.5 mA cm2. [c] the h of the WOR was measured at 1.5 mA cm2. [d] CV at 20 mV s1. [e] CV at 50 mV s1. n.r. = not reported.
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CHEMPHYSCHEM ARTICLES electrocatalytic oxidation of water by IrOx, with an h of the oxidation reaction reduced to 0.21 V and electrocatalytic current density approaching 43 mA cm2 at 1 V and pH 7. The specific electrocatalytic activity of IrOx NPs (i.e. electrocatalytic currents related to the number of electrocatalytically active sites) were 133 and 148 mA nmol1 at GIrIV/IrV values less than 0.1 nmol cm2, which allows the development of cheap (owing to the lower amount of used nanocatalyst) and highly efficient nanocomposite electrocatalysts for water oxidation based on IrOx NPs and inexpensive high-surface-area carbon electrode materials.
Experimental Section Materials K2IrCl6 (99.99 %), NaOH (98 %), H2SO4 (95–97 %) were purchased from Sigma–Aldrich. H3PO4 (85 %) and HNO3 (68 %) were from Riedel-deHaen and VWR, respectively. Phosphate buffer solutions (PBS) were prepared from 1 m H3PO4 and pH was adjusted to 5, 7 and 10 using concentrated NaOH. Solutions at pH 1.5 and pH 13.2 were prepared from the corresponding H2SO4 and NaOH solutions, respectively. All experiments were performed at 22 1 8C if not stated otherwise; 18.2 MW de-ionized Milli-Q water (Millipore, Bedford, MA, USA) was used throughout the work.
Synthesis of IrOx. NPs An appropriate amount of K2IrCl6 was dissolved in water, and the pH of the solution was adjusted to pH 13 by adding 4 m NaOH under stirring. The solution was heated up to 90 8C for 20 min and then cooled in an ice bath under continuous stirring.[30] The 2 mm solution of H2O-capped IrOx NPs was prepared by adjusting the pH of the cooled solution to pH 1 by rapid addition of 4 m HNO3 under 80 min stirring until the solution color changed to deep blue.[20] The resulting solution was kept at 4 8C between the electrode modifications.
Spectrophotometric Characterization of NPs Spectrophotometric properties and concentration of IrOx NPs were estimated by monitoring the increase in absorbance within the 200–1000 nm range by using a Shimadzu UV/Vis/near-infrared spectrophotometer UV-3600 (Shimadzu Corporation, Japan).
TEM TEM imaging of the IrOx NPs in acidic and basic solutions used for the electrode modifications was done by a 200 keV TEM (CM20, Philips) and a CCD camera. Prior to the TEM imaging, the studied suspensions were diluted 2 times and a 2 mL drop of the examined suspension was placed onto the TEM grid (200 mesh copper grid with formvar/carbon support film, Ted Pella Inc.) and dried by a vacuum pump.
Electrode Modification and Electrochemical Measurements Graphite disk electrodes (Ø 2.97 mm rods of solid Gr, type RW001, Werk Ringsdorff, Germany, fitted in Teflon holders) were used as working electrodes. The surface of graphite rods was cut and polished on fine emery paper and further on Kimwipes wipers (Kimberly-Clark Professional). IrOx NPs were either adsorbed or electrochemically deposited onto Gr at different deposition potentials 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org (0.7, 0.9, 1 and 1.3 V) and for different times (0.6, 6, 60 and 600 s) from the stock (synthesized) IrOx NP solution. For physical adsorption, a 5 mL drop of the IrOx NPs solution was casted onto the surface of the Gr and air-dried; this protocol was found to result in the highest efficiency of electrocatalysis compared to prolonged adsorption from the drop and lower concentrations of NPs. Cyclic voltammetry and chronoamperometry were performed in a standard three-electrode electrochemical cell with an Ag/AgCl (3 m KCl) and a Pt wire as the reference and auxiliary electrodes, respectively. The electrodes were connected to a potentiostat/galvanostat AUTOLAB PGSTAT 300 (Eco Chemie, the Netherlands). The reproducibility of the data at each pH was verified by measurements with at least three equivalently prepared electrodes. The h values of water oxidation were estimated as a difference between the thermodynamic potential of the reaction (1.23 V versus SHE at pH 0) and the experimentally observed potentials obtained by the Nernst equation [Eq. (1)]: E SHE ¼ E Ag=AgCl=3 m KCl þ E 0 Ag=AgCl=3 m KCl þ 0:059 pH
ð1Þ
where ESHE is the converted potential corresponding to the reaction onset versus SHE, E0Ag/AgCl/3 m KCl is the standard potential of the used reference electrode (0.210 V vs. SHE at 25 8C), and EAg/AgCl/3 m KCl is the experimentally obtained potential against this reference electrode. All the current densities were calculated based on the geometric surface area of the graphite electrode (0.069 cm2).
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CHEMPHYSCHEM ARTICLES [17] T. Reier, M. Oezaslan, P. Strasser, ACS Catal. 2012, 2, 1765 – 1772. [18] Y. X. Zhao, N. M. Vargas-Barbosa, E. A. Hernandez-Pagan, T. E. Mallouk, Small 2011, 7, 2087 – 2093. [19] T. Nakagawa, C. A. Beasley, R. W. Murray, J. Phys. Chem. C 2009, 113, 12958 – 12961. [20] Y. X. Zhao, E. A. Hernandez-Pagan, N. M. Vargas-Barbosa, J. L. Dysart, T. E. Mallouk, J. Phys. Chem. Lett. 2011, 2, 402 – 406. [21] T. Nakagawa, N. S. Bjorge, R. W. Murray, J. Am. Chem. Soc. 2009, 131, 15578 – 15579. [22] M. C. Chuang, J. A. A. Ho, RSC Adv. 2012, 2, 4092 – 4096. [23] T. Kuwabara, E. Tomita, S. Sakita, D. Hasegawa, K. Sone, M. Yagi, J. Phys. Chem. C 2008, 112, 3774 – 3779. [24] M. Yagi, E. Tomita, S. Sakita, T. Kuwabara, K. Nagai, J. Phys. Chem. B 2005, 109, 21489 – 21491.
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www.chemphyschem.org [25] A. A. Gambardella, N. S. Bjorge, V. K. Alspaugh, R. W. Murray, J. Phys. Chem. C 2011, 115, 21659 – 21665. [26] E. Laviron, J. Electroanal. Chem. 1979, 101, 19 – 28. [27] M. E. G. Lyons, S. Floquet, Phys. Chem. Chem. Phys. 2011, 13, 5314 – 5335. [28] M. Yagi, E. Tomita, T. Kuwabara, J. Electroanal. Chem. 2005, 579, 83 – 88. [29] M. Sosna, A. Bonamore, L. Gorton, A. Boffi, E. E. Ferapontova, Biosens. Bioelectron. 2013, 42, 219 – 224. [30] L. Wçhler, W. Witzmann, Z. Anorg. Chem. 1908, 57, 323 – 352.
Received: March 2, 2014 Published online on July 8, 2014
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Electrocatalysis of Water Oxidation by H2O-Capped Iridium-Oxide Nanoparticles Electrodeposited on Spectroscopic Graphite Naghmehalsadat Mirbagheri,[a] Jacques Chevallier,[a, b] Jakob Kibsgaard,[a, b] Flemming Besenbacher,[a, b] and Elena E. Ferapontova*[a]
Electrocatalysis of water oxidation by 1.54 nm IrOx nanoparticles (NPs) immobilized on spectroscopic graphite electrodes was demonstrated to proceed with a higher efficiency than on all other, hitherto reported, electrode supports. IrOx NPs were electrodeposited on the graphite surface, and their electrocatalytic activity for water oxidation was correlated with the surface concentrations of different redox states of IrOx as a function of the deposition time and potential. Under optimal con-
ditions, the overpotential of the reaction was reduced to 0.21 V and the electrocatalytic current density was 43 mA cm2 at 1 V versus Ag/AgCl (3 m KCl) and pH 7. These results beneficially compete with previously reported electrocatalytic oxidations of water by IrOx NPs electrodeposited onto glassy carbon and indium tin oxide electrodes and provide the basis for the further development of efficient IrOx NP-based electrocatalysts immobilized on high-surface-area carbon electrode materials.
1. Introduction Electrochemical oxidation of water is considered as an attractive way to provide fuel cells with a clean and sustainable supply of O2 and H2 fuel.[1–5] As a result of kinetic impediments, the water oxidation reaction (WOR) proceeds at potentials essentially exceeding the theoretical value of 1.23 V [versus a standard hydrogen electrode (SHE)].[6] To overcome the kinetic limitations, a number of catalytic systems have been proposed;[7–11] these include the metal oxides IrO2, RuO2, PtO2, Co3O4, and Mn2O3.[12–15] Of these, iridium oxide (IrOx) has been shown to exhibit high electrocatalytic activity and performance stability in the WOR over a wide pH range, thus enabling the four-electron electrooxidation of water at overpotentials (h) ranging between 0.20 and 0.38 V.[16–19] Significant improvement of the catalysis, as compared to solid IrOx electrodes, can be achieved with IrOx nanoparticles (NPs), which provide a higher surface-area-to-volume ratio. 1–2 nm-sized IrOx NPs have been shown to exhibit long-term stability and high electrocatalytic activity in both acidic and alkaline media and reduce the h of water oxidation to 0.20– 0.29 V.[18–22] The procedure of IrOx NP immobilization is likely to affect the electrocatalytic properties of IrOx NPs deposited onto a substrate electrode. [a] N. Mirbagheri, Dr. J. Chevallier, Dr. J. Kibsgaard, Prof. F. Besenbacher, Prof. E. E. Ferapontova Interdisciplinary Nanoscience Center (iNANO), Aarhus University Gustav Wieds Vej 1590-14, 8000 Aarhus C (Denmark) E-mail: [email protected] [b] Dr. J. Chevallier, Dr. J. Kibsgaard, Prof. F. Besenbacher Department of Physics, Aarhus University Ny Munkagade 120, 8000 Aarhus C (Denmark)
Electrocatalytically active high-surface-area IrOx films have been prepared both by chemisorption and electrodeposition of IrOx NPs from their colloidal solutions onto indium tin oxide (ITO), fluorine-doped tin oxide (FTO),[23] and glassy carbon (GC) electrodes.[19, 20] Hydroxide-capped IrOx NPs diffusing from the bulk solution to a Pt rotating disk electrode (RDE) surface have been shown to exhibit essential electrocatalytic activity.[21] 2 nm H2O-capped IrOx NPs have been electrochemically deposited onto GC electrodes from a pH 1 solution; their high catalytic activity between pH 1 and pH 13 has been shown previously.[20] In situ formation of H2O-capped IrOx·n H2O NPs on gold and GC electrodes has been shown to produce highly stable and electrocatalytically active IrOx NP electrode systems with significantly reduced water oxidation h of down to 0.2 V.[18] The relation between the deposition conditions and the efficiency of electrocatalysis, for similarly prepared NPs, has not yet been studied. In the present work, the electrocatalytic properties of H2Ocapped IrOx NPs electrodeposited onto spectroscopic graphite (Gr) were studied (Figure 1). Immobilization of IrOx NPs onto cost-effective and high-surface-area microstructured Gr was expected to produce a simple and electrocatalytically active electrode material. IrOx NPs with H2O molecules as surface ligands were synthesized, deposited onto the electrodes either by electrochemical deposition or physical adsorption, and investigated in the reaction of electrochemical oxidation of water at different pH values. The focus was on the analysis of the electrocatalytic reactivity of NPs as a function of the electrodeposition conditions.
Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402079.
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www.chemphyschem.org 2.2. Electrocatalytic Oxidation of Water at Graphite Electrodes Modified with IrOx NPs through Adsorption and Electrodeposition
Figure 1. Schematic illustration of graphite modification with IrOx NP at pH 1.
Gr electrodes were modified with IrOx·NPs by two different protocols, specifically, through electrochemical deposition and physical adsorption, and then studied in the electrocatalytic WOR. In both cases, the electrocatalytic activity of the IrOxmodified Gr electrodes in the WOR was greatly enhanced compared with that of the bare Gr electrode, and it was further seen that electrodeposited IrOx NPs exhibited a higher activity than physically adsorbed ones (Figure 3 and Figures S2 and S3 in the Supporting Information).
2. Results and Discussion 2.1. UV/Vis Spectroscopic and Transmission Electron Microscopy (TEM) Characterization of IrOx NPs As can be seen from the absorbance spectra of colloidal solutions of IrOx NPs shown in Figure S1 in the Supporting Information, the H2O-capped IrOx NPs exhibit a broad visible band around 576 nm, which is characteristic of IrIV oxides. Based on the reported value of the extinction coefficient (e574) of 630 50 m1 cm1 for IrOx··n H2O NPs,[20] the concentration of freshly synthesized IrOx NPs was estimated to be 2.4 0.2 mm. During the first step of the synthesis, under heating in basic media, K2IrCl6 is hydrolyzed to [Ir(OH)6]2 with a formation of OH-capped IrOx NPs. Addition of HNO3 leads to a protonation of OH-capped IrOx NPs, which makes their condensation with the rest of the [Ir(OH)6]2 more efficient and results in a high yield of IrOx·n H2O NPs. The reaction between [Ir(OH)6]2 and OH-capped IrOx NPs is reported to be rapid; however, longer reaction times (such as overnight storage of the reaction mixture) are required to achieve a more complete IrOIr bond formation within the NPs (Figure S1, curve 2 in the Supporting Information).[20] In the present work, aged solutions (not freshly prepared solutions) of IrOx·n H2O NPs, which were shown to be stable for at least one month when kept at 4 8C, were used. The average IrOx NP diameter was 1.54 0.20 nm, which is consistent with previous reports (Figure 2).[18, 20]
Figure 3. Representative CVs of electrocatalytic oxidation of water recorded with the IrOx-NP-modified electrode prepared by 10 min electrodeposition of IrOx NPs at 1.3 V (1–5, pH 1.5–13.2) and by drop-casting (1’–5’, pH 1.5– 13.2), and that of the bare graphite electrode (black dotted line, pH 1.5). Potential scan rate 20 mV s1. Inset: zoomed potential window corresponding to the IrOx NP redox peaks at pH 7. The axis titles in the inset are the same as in the main figure.
The varying electrocatalytic activities of the electrochemically deposited and physically adsorbed IrOx NPs correlated well with the surface coverage of the NPs, which was estimated by integration of the cyclic voltammetry peaks (at around 190 mV at pH 7, Figure 3 inset) that correspond to the IrIII/IrIV redox transformation. For physically adsorbed NPs, the IrOx surface coverage (GIrIII/IrIV) was undetectable and cyclic voltammograms (CVs) showed atypical crossing (Figure 3, inset) until they were stabilized during consecutive scanning; this stabilization was accompanied by the concomitant decrease in the H2O oxidation currents (Figure S2 in the Supporting Information), which correlated with the variation of the electrode surface properties, most likely owing to desorption of NPs. For electrochemical deposition, GIrIII/IrIV increased to 52.1 2.9 nmol cm2. Thus, electrodeposition of NPs provided higher loading of the electrode with the nanocatalyst and, as a result, a higher electrocatalytic activity for the IrOx-modified Gr electrodes in the WOR. Like with some other electrode materials, the drop-casting method was not very efficient for Gr.[23, 24] This was ascribed to the weak physical adsorption of IrOx NPs onto the Gr surface, unless the electric potential stimulation was applied. All further studies were performed with the electrodeposited IrOx NPs.
Figure 2. TEM image of synthesized 1.54 0.20 nm IrOx NPs. Scale bar: 5 nm.
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CHEMPHYSCHEM ARTICLES 2.3. Effect of NP Electrodeposition Time and Potential on Electrocatalysis of Water Oxidation Several redox processes can be distinguished in the CV of IrOx NPs immobilized on Gr (Figure 4 A): the redox couples at around 190 and 527 mV, pH 7, associated with the IrIII/IrIV and IrIV/IrV redox transformations, and the reduction peak at 725 mV, which can be attributed to electroreduction of the IrVI that is not consumed in the WOR proceeding at the same potentials.[19]
Figure 4. A) Representative CVs recorded with the IrOx-NP-modified electrodes prepared by electrodeposition of IrOx NPs at 1.3 V for 1) 0.6, 2) 6, 3) 60, and 4) 600 s. Potential scan rate 20 mV s1 and pH 7. Inset: Zoomed potential window corresponding to the IrOx redox peaks. The axis titles in the inset are the same as in the main figure. B) Dependence of GIrIII/IrIV and GIrIV/IrV (inset) on the electrodeposition time, data derived from the CVs in Figure S4 in the Supporting Information.
The surface concentration of the correspondingly electroactive IrOx sites at the electrode surface was estimated by integration of the IrIII/IrIV and IrIV/IrV CV peaks recorded in the NPfree solution[18, 19, 22, 23] (Figure 4 B) and correlated with the electrocatalytic efficiency of the IrOx-modified Gr electrodes, namely the catalytic current densities and h of the WOR, for different electrodeposition times (t) and potentials (E). As can be seen in Figure 4 A, for longer electrodeposition times, higher current densities for the electrocatalytic WOR and lower potentials for the electrocatalysis onset are observed, which is consistent with the increased surface coverage of IrOx NPs. The dependence of the surface amount (GIrIII/IrIV) of the IrIII/IrIV sites on time exhibited a typical saturation behavior, with a theoretical saturation limit of 57.3 2.4 nmol cm2 obtained from data fitting to the Langmuir adsorption isotherm. The experimental value of 52.1 2.9 nmol cm2 was obtained for a 600 s electro 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org deposition time; this corresponds to 31.3 1015 sites cm2 (Table S1 in the Supporting Information) or 4.74 1014 NP cm2 if the data are recalculated for the NP number by taking into account the 66 Ir sites per NP, based on the rutile lattice of IrOx. The absolute amount of these NPs will occupy 0.77 cm2, which roughly corresponds to the real surface area of a Gr electrode at its lower limit (a roughness factor of at least 10) and, thus, represents a (sub)monolayer NP coverage. Similarly, the electrocatalytic currents of the WOR consistently increased with increasing electrodeposition potential (Figure S5 in the Supporting Information). Analysis of the IrIII/IrIV and IrIV/IrV redox signals evidenced that the enhancement of the efficiency of electrocatalysis by NPs electrodeposited at higher potentials may also be correlated with the increased surface concentration of the electrodeposited nanocatalyst (Table S2 in the Supporting Information). In contrast to the Gt(electrodeposition) dependences that exhibited a typical saturation behavior at higher electrodeposition times (Figure 5), the GE(electrodeposition) dependences demonstrated an exponential increase in GIrIII/IrIV and GIrIV/IrV.
Figure 5. Dependence of GIrIII/IrIV and GIrIV/IrV (inset) on the electrodeposition potential for the IrOx-NP-modified electrodes prepared by 10 min electrodeposition at 1) 0.7, 2) 0.9, 3) 1.1, and 4) 1.3 V. Measurements were performed at pH 7 and potential scan rate 20 mV s1. Data derived from the CVs presented in Figure S4 D and S6 in the Supporting Information.
It is understood that not all iridium sites of the electrodeposited IrOx NPs may be electrochemically and electrocatalytically active; only a fraction of them undergo the IrIII/IrIV transformation and further consecutive IrIV/IrV and IrV/IrVI transformations to form higher oxidation states of Ir involved in electrocatalysis. Depending on experimental conditions, such as the IrOx NP capping ligand, NP size, the substrate used for NP deposition, and the deposition method, 3.2,[24] 12.5,[22] and 16 %[23] of the total amount of electrodeposited Ir atoms in the IrOx films have been reported to be electrochemically active, whereas IrOx NPs in solution have shown 100 % electrochemical activity.[25] For the latter case, despite the quite different electrochemical behavior of the freely diffusing NPs and the electrode-immobilized ones, they exhibited a very similar electrocatalytic activity, namely very similar h values for the WOR.[21] Our estimations of the number of electroactive IrIII/IrIV sites that are further oxidized to the IrV state show that only 3.3 % of the ChemPhysChem 2014, 15, 2844 – 2850
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Figure 6. Dependence of the amount of electrochemically active IrIV/IrV on the amount of electroactive IrIII/IrIV, for the IrOx-NP-modified Gr electrodes prepared by electrodeposition of IrOx NPs at 1.3 V for: 1’) 0.6, 2’) 6, 3’) 60, and 4’) 600 s and by 10 min electrodeposition at 1) 0.7, 2) 0.9, 3) 1.1, and 4) 1.3 V. Data obtained from CVs in Figure S4–S6 in the Supporting Information.
total amount of IrIV (52.1 2.9 nmol cm2) are converted into IrV (1.7 0.06 nmol cm2) for the electrodes prepared with 10 min electrodeposition at 1.3 V (Figure 6). Similar data were obtained from the slopes of the GIrIV/IrVGIrIII/ IrIV dependences constructed for different electrodeposition times (3.3 %) and potentials (3.4 %; Figure 6). The electrontransfer (ET) rate constants (kET) estimated by the Laviron approach[26] were 3.5 and 6.1 s1 for the IrIII/IrIV and IrIV/IrV ET processes, respectively, thus indicating a lower rate of electrochemical transformation of IrIV to IrIII (Figure S7 and comments in the Supporting Information). Both ET processes were coupled to a proton release in the course of Ir oxidation (1 e/1 H + reactions),[25, 27] with a formal potential–pH dependence exhibiting a slope slightly higher than the characteristic 58 mV pH1 (Figure S8 in the Supporting Information).[25] The efficiency of electrocatalysis was correlated with the concentration of IrIV/IrV sites in IrOx NPs electrodeposited onto the electrodes for different times and at different potentials (Figure 7). Although the apparent catalytic activity increased with the increasing IrOx NP loading (Figure 7 A, B), the specific catalytic activity decreased (Figure 7 A, B, inset, and Figure S9 in the Supporting Information), presumably owing to the decreased surface-area-to-volume ratio for NPs electrodeposited for longer times or at higher potentials. Two distinct regions, consistent with two different specific catalytic activities of IrOx NPs, can be observed at low and high surface coverage of active sites (Figure 8), that is, at GIrIV/IrV < 0.1 nmol cm2 with a slope of 148.34 mA nmol1and at GIrIV/IrV > 0.1 nmol cm2 with a slope of 12.79 mA nmol1. The same trend was followed with electrodes prepared by varying electrodeposition potentials (Figure 8), where 133.28 and 9.94 mA nmol1 specific catalytic activities (i.e. related to the number of the electrocatalytically active Ir sites) were obtained for low and high IrOx active-site surface coverage, respectively. In general, the observed drop in the specific activity of the NPs at a NP surface coverage exceeding 0.3 1014 NPs cm2 may be related either to surface aggregation of NPs or lower availability of IrOx for electrocatalysis, owing to, for example, the effect 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7. Dependence of GIrIV/IrV and catalytic current density at 1 V as a function of the electrodeposition time. Inset: the normalized current density at 1 V to the surface concentration of IrIV/IrV redox. Data obtained from CVs recorded at pH 7, with the IrOx-NP-modified Gr electrodes prepared by electrodeposition of NPs A) at 1.3 V for: 1) 0.6, 2) 6, 3) 60, and 4) 600 s and B) by 10 min electrodeposition at 1) 0.7, 2) 0.9, 3) 1.1, and 4) 1.3 V.
Figure 8. Dependence of the catalytic current density at 1 V on GIrIV/IrV. Data obtained from CVs recorded in PBS, pH 7, with IrOx-NP-modified Gr electrodes, prepared by applying 1.3 V for deposition times: 1) 0.6, 2) 6, 3) 60, and 4) 600 s and by 10 min electrodeposition at applied potentials: 1’) 0.7, 2’) 0.9, 3’) 1.1, and 4) 1.3 V.
of enhanced O2 evolution on adsorption of H2O, detrimentally affecting their electrocatalytic properties. However, the demonstrated high specific electrocatalytic activity of IrOx NPs at their lowest surface concentrations allow the development of cheaper (owing to the decreased amount of used Ir) and yet more efficient nanocomposite catalysts for water oxidation based on IrOx NPs and inexpensive high-surface-area carbon electrode materials. Thus, the electrodeposition potentials and times affected the amount of electrodeposited NPs and as a result the elecChemPhysChem 2014, 15, 2844 – 2850
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trocatalytic activity of NPs in the WOR. The potential of water oxidation measured at 3 mA cm2 (EWOR) decreased with increasing electrodeposition time and potential (Figure S10 in the Supporting Information). The most electrocatalytically active electrodes with the lowest EWOR were obtained by 10 min electrodeposition of NPs at 1.3 V, and these electrodes were used to evaluate the onset potentials of the WOR and the electrocatalytic current-density values at different pH values. The average pH-independent h related to the onset of oxygen evolution (determined by the Tafel method) was around 0.21 V (Figure 9). With increasing pH, the efficiency of electrocatalysis, namely the catalytic current density at 1 V increased (Figure S12, blue); this trend is consistent with a favorable release of protons in a basic medium. The current density at pH 13.2 could not be measured accurately, owing to vigorous evolution of oxygen. Nevertheless, the catalytic current density at a constant h of 0.4 V (Figure S12, red) had the highest value at pH 7 and 13.2.
2.4. The Electrocatalytic Activity of the IrOx-Modified Gr Electrode Based on the comparative analysis of the data available in the literature, IrOx NPs electrodeposited onto Gr may be considered as one of the most efficient IrOx-based catalysts for the WOR (Table 1 and Figure 9), with the catalytic current densities essentially exceeding those observed at NP-modified electrodes prepared both by adsorption and electrodeposition techniques by using a variety of electrode materials, such as GC, ITO, and FTO electrode supports.[18, 22–24, 28] The h of the WOR of 0.21 V may be also considered as one of the lowest hitherto reported, being only slightly higher than that of 0.20 V reported by Zhao et al. (obtained with IrOx·n H2O-NPmodified GC rotating-disk electrode). Also, IrOx-NP-modified Gr
Figure 9. pH dependence of the thermodynamic and experimental potential (grey) of the WOR at the IrOx-NP-modified electrodes prepared by 10 min electrodeposition of NPs at 1.3 V. Data were derived from CVs recorded at 20 mV s1.
exhibited electrocatalytic current densities that were several times higher (four–fivefold, depending on pH) than those shown with IrOx-NP-modified GC RDE, which may be connected both with the much higher surface area of the Gr electrodes used here, a roughness factor exceeding 10,[29] and the hydrophilic character of the graphite surface, favoring interactions with the hydrophilic surface of IrOx NPs. When catalytic current densities are referred to the surface concentration of IrV (Table 1), IrOx-NP Gr electrodes also demonstrate the highest specific catalytic activity, comparable only with NPs adsorbed on GC at a sub nmol cm2 level[22] (ca. 30 % less for the latter).
3. Conclusions 1.54 nm IrOx NPs electrodeposited onto the high-surface-area spectroscopic Gr electrode demonstrate one of the highest hitherto reported electrocatalytic activities in the reaction of
Table 1. The overpotentials (h), and current densities (i) reported for the WOR at the differently prepared IrOx-NP-modified electrodes. IrOx NP solution
Deposition conditions
Substrate
Surface coverage (G) [mol cm2]
h [V]
Current density at 1 V [mA cm2]
Ref.
2 mm IrOx·n H2O NP synthesized in acidic medium (pH 1); NP diameter of 1.54 0.2 nm 0.18 mm IrOx·n H2O NP synthesized in acidic medium (pH 1); NP diameter of 1–2 nm Anodic in situ synthesis of IrOx·n H2O NP from 2 mM [Ir(OH)6]2 ; NP diameter of 2–5 nm 2 mm IrOx·n H2O NP synthesized in acidic medium (pH 1); NP diameter of 2 nm 2.4 mm IrOx NP synthesized in basic medium (pH 13); NP diameter of 2 nm 2.5 mm IrOx NP synthesized in basic medium (pH 13); NP diameter of 1.6 0.6 nm 0.28 mm solution of citrate-capped IrOx NP, pH 3.5; NP diameter of 60–100 nm 0.28 mm solution of citrate-capped IrOx NP, pH 3.5; NP diameter of 60–100 nm 0.62 mm solution of citrate-capped IrOx NP synthesized at pH 3.5; NP diameter of 50–100 nm
electrodeposition at 1.3 V vs. Ag/AgCl (3 m) for 10 min dipping for 5 h
graphite electrode GC
0.21[a]
~ 43,[d] pH 7
0.28[b]
~ 3.5,[e] pH 7.2
this work [22]
electrodeposition at 1.3 V vs. Ag/AgCl (3 m) electrodeposition at 1.4 V vs. Ag/AgCl (3 m) for 15 min electrodeposition at 1.3 V vs. Ag/AgCl for 10 min –
GC RDE
GIrIII/IrIV = 52.1 109 GIrIV/IrV = 1.7 109 GIrIII/IrIV = 1.89 1010 (at pH 9.3) GIrIV/IrV = 3.5 108
0.20[c]
~ 18,[d] pH 7
[18]
GIrIV/IrV = 1.1 108 (at pH 1) GIrIV/IrV ~ 1.6 108
0.22[b]
~ 10,[d] pH 9
[20]
0.25[b]
~ 20,[d] pH 7
[19]
n.r.
0.29[b]
~ 22,[d] pH 13
[21]
dipping for 24 h
rotating Pt disc ITO
GIrIV/IrV = 1.5 109
n.r.
~ 0.1,[d] pH 5.3
[23]
dipping for 24 h
FTO
n.r.
n.r.
< 0.02,[d] pH 5.3
[23]
dipping for 1 h
ITO
GIrIV/IrV = 6.5 1010
n.r.
< 0.1,[d] pH 5.3
[24]
GC RDE GC
[a] The h of the WOR was measured by using the Tafel method. [b] the h of the WOR were measured at 0.5 mA cm2. [c] the h of the WOR was measured at 1.5 mA cm2. [d] CV at 20 mV s1. [e] CV at 50 mV s1. n.r. = not reported.
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CHEMPHYSCHEM ARTICLES electrocatalytic oxidation of water by IrOx, with an h of the oxidation reaction reduced to 0.21 V and electrocatalytic current density approaching 43 mA cm2 at 1 V and pH 7. The specific electrocatalytic activity of IrOx NPs (i.e. electrocatalytic currents related to the number of electrocatalytically active sites) were 133 and 148 mA nmol1 at GIrIV/IrV values less than 0.1 nmol cm2, which allows the development of cheap (owing to the lower amount of used nanocatalyst) and highly efficient nanocomposite electrocatalysts for water oxidation based on IrOx NPs and inexpensive high-surface-area carbon electrode materials.
Experimental Section Materials K2IrCl6 (99.99 %), NaOH (98 %), H2SO4 (95–97 %) were purchased from Sigma–Aldrich. H3PO4 (85 %) and HNO3 (68 %) were from Riedel-deHaen and VWR, respectively. Phosphate buffer solutions (PBS) were prepared from 1 m H3PO4 and pH was adjusted to 5, 7 and 10 using concentrated NaOH. Solutions at pH 1.5 and pH 13.2 were prepared from the corresponding H2SO4 and NaOH solutions, respectively. All experiments were performed at 22 1 8C if not stated otherwise; 18.2 MW de-ionized Milli-Q water (Millipore, Bedford, MA, USA) was used throughout the work.
Synthesis of IrOx. NPs An appropriate amount of K2IrCl6 was dissolved in water, and the pH of the solution was adjusted to pH 13 by adding 4 m NaOH under stirring. The solution was heated up to 90 8C for 20 min and then cooled in an ice bath under continuous stirring.[30] The 2 mm solution of H2O-capped IrOx NPs was prepared by adjusting the pH of the cooled solution to pH 1 by rapid addition of 4 m HNO3 under 80 min stirring until the solution color changed to deep blue.[20] The resulting solution was kept at 4 8C between the electrode modifications.
Spectrophotometric Characterization of NPs Spectrophotometric properties and concentration of IrOx NPs were estimated by monitoring the increase in absorbance within the 200–1000 nm range by using a Shimadzu UV/Vis/near-infrared spectrophotometer UV-3600 (Shimadzu Corporation, Japan).
TEM TEM imaging of the IrOx NPs in acidic and basic solutions used for the electrode modifications was done by a 200 keV TEM (CM20, Philips) and a CCD camera. Prior to the TEM imaging, the studied suspensions were diluted 2 times and a 2 mL drop of the examined suspension was placed onto the TEM grid (200 mesh copper grid with formvar/carbon support film, Ted Pella Inc.) and dried by a vacuum pump.
Electrode Modification and Electrochemical Measurements Graphite disk electrodes (Ø 2.97 mm rods of solid Gr, type RW001, Werk Ringsdorff, Germany, fitted in Teflon holders) were used as working electrodes. The surface of graphite rods was cut and polished on fine emery paper and further on Kimwipes wipers (Kimberly-Clark Professional). IrOx NPs were either adsorbed or electrochemically deposited onto Gr at different deposition potentials 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org (0.7, 0.9, 1 and 1.3 V) and for different times (0.6, 6, 60 and 600 s) from the stock (synthesized) IrOx NP solution. For physical adsorption, a 5 mL drop of the IrOx NPs solution was casted onto the surface of the Gr and air-dried; this protocol was found to result in the highest efficiency of electrocatalysis compared to prolonged adsorption from the drop and lower concentrations of NPs. Cyclic voltammetry and chronoamperometry were performed in a standard three-electrode electrochemical cell with an Ag/AgCl (3 m KCl) and a Pt wire as the reference and auxiliary electrodes, respectively. The electrodes were connected to a potentiostat/galvanostat AUTOLAB PGSTAT 300 (Eco Chemie, the Netherlands). The reproducibility of the data at each pH was verified by measurements with at least three equivalently prepared electrodes. The h values of water oxidation were estimated as a difference between the thermodynamic potential of the reaction (1.23 V versus SHE at pH 0) and the experimentally observed potentials obtained by the Nernst equation [Eq. (1)]: E SHE ¼ E Ag=AgCl=3 m KCl þ E 0 Ag=AgCl=3 m KCl þ 0:059 pH
ð1Þ
where ESHE is the converted potential corresponding to the reaction onset versus SHE, E0Ag/AgCl/3 m KCl is the standard potential of the used reference electrode (0.210 V vs. SHE at 25 8C), and EAg/AgCl/3 m KCl is the experimentally obtained potential against this reference electrode. All the current densities were calculated based on the geometric surface area of the graphite electrode (0.069 cm2).
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Received: March 2, 2014 Published online on July 8, 2014
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