Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the

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Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution Patrick Steegstra, Michael Busch, Itai Panas, and Elisabet Ahlberg J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp407030r • Publication Date (Web): 09 Sep 2013 Downloaded from http://pubs.acs.org on September 17, 2013

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The Journal of Physical Chemistry

Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution Patrick Steegstra,† Michael Busch,† Itai Panas,‡ and Elisabet Ahlberg∗,† Department of Chemistry and Molecular Biology, University of Gothenburg, SE-41296 Gothenburg, Sweden, and Department of Chemistry and Biotechnology, Chalmers University of Technology, SE-412 96, Gothenburg, Sweden E-mail: [email protected]

∗

To whom correspondence should be addressed Department of Chemistry and Molecular Biology, University of Gothenburg, SE-41296 Gothenburg, Sweden ‡ Department of Chemistry and Biotechnology, Chalmers University of Technology, SE-412 96, Gothenburg, Sweden †

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Abstract The electrochemistry of hydrous iridium oxide films (HIROF) is revisited. Cyclic voltammograms of HIROFs display two reversible redox couples commonly assigned to the Ir(III)/Ir(IV) and Ir(IV)/Ir(V) transitions, respectively. However, compared to the first, the second redox couple has significantly less charge associated to it. This effect is interpreted as partial oxidation of Ir(IV) as limited by nearest neighbor repulsion of resulting Ir(V) sites. Thus, the redox process is divided into two steps: one preceding and one overlapping the oxygen evolution reaction (OER). Here, the “super-nernstian” pH dependence of the redox processes in the HIROF is used to expose how pH controls the overpotential for oxygen evolution, as evidenced by the complementary increased formation of Ir(V) oxide. A recently formulated binuclear mechanism for the OER is employed to illustrate how hydrogen bonding may suppress the OER, thus implicitly favoring Ir(V) oxide formation above the thermodynamic onset potential for the OER at low pH. Keywords: DFT, IrO2 , hydrous, oxygen evolution, OER, electrocatalysis

Introduction The quest for sustainable energy based on converting solar energy into energy carrying molecules has renewed interest in tailored catalysts for guiding endergonic reactions. 1 A key process in this quest is the electrolytic splitting of water into oxygen and hydrogen. Research has focused mainly on catalysis of the oxygen evolution reaction (OER) because of its sluggishness compared with hydrogen evolution. 2,3 In vivo, the OER is catalyzed by a manganese oxide cluster. 4,5 In vitro, a number of different oxides are known to display remarkable electrocatalytic efficiency towards the OER. 6–8 Recently, a binuclear test rig has been described and used for density functional theory (DFT) calculations to obtain mechanistic insight into the OER on metal oxide electrodes. Calculations were performed on homo- and heteronuclear metal oxides of among 2


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others Fe, Co, Mn, Cr and a binuclear mechanism was proposed. 9,10 Another good electrocatalyst for the OER is iridium oxide. Hydrated iridium oxide films (HIROF) were recently studied for application in in situ pH measurements during electrochemical experiments. 11 HIROF are easily prepared by potential cycling of an iridium electrode in acid or by electrodeposition from solution on a variety of substrates. 12,13 Their pH sensing behavior is characterized by a response from 65 to 85 mV, which is sometimes referred to as “super-nernstian” and can be tuned through the average oxidation state of the film. 13,14 The relatively high sensitivity to pH is also reflected in the cyclic voltammetry (CV), which shows two redox couples shifting with 80 and 90 mV. These two couples are generally ascribed to the Ir(III)/Ir(IV) and Ir(IV)/Ir(V) transitions, but interestingly the first couple carries significantly more charge than the second. The origin of the two transitions has been studied experimentally with X-ray photoelectron spectroscopy (XPS), 15 ultraviolet photoelectron spectroscopy (UPS) 16 and X-ray absorption spectroscopy (XAS). 17–21 Hüppauff and Lengeler presented an in situ XAS study on anodic iridium oxide films (AIROF), grown on iridium foil in strong acids. 18,19 From measuring the shift in the L3 absorption edge of iridium it was concluded that the oxidation state of iridium changed between 3 and 4.8 in the redox active potential region. These results agree well with the ex situ XPS and UPS analysis of AIROF made by Kötz et al. 15,22 There, detailed analysis of the O1s signal revealed that the ratio between oxygen and iridium (O/Ir = 3) was independent of potential, but that the oxide content increased with increasing positive potential, indicating a corresponding increase in the oxidation state of iridium. Based on these results a model for the redox behavior was proposed starting with Ir(III)(OH)3 , which is oxidized sequentially to Ir(IV)O(OH)2 and Ir(V)O2 (OH). Later in situ XAS studies on electrodeposited iridium oxide films (EIROF) 20 showed a clear correlation between the oxidation state of iridium and the shift of the L3 edge. In the potential range across the first redox couple a 1 eV shift was observed. With support from coulometric analysis it was concluded that this concerns the Ir(III)/Ir(IV) redox transition 3



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involving the entire layer. Upon further oxidation in alkaline solution, across the second redox couple, a further shift of 0.5 eV was observed. 20 Recently, Hillman et al. 17 proposed a two-site model for EIROF, in which the redox reaction involves one electron per iridium and the average oxidation state changes from 3.5 to 4.5 for both redox couples. Interestingly, the experimental data are very similar to those reported earlier 18–20 but the interpretation is clearly different. The findings for strongly hydrated films, (AIROF and EIROF) are contrasted by those for sputtered iridium oxide films (SIROF). For these more anhydrous and much denser films XANES only provided evidence for oxidation states up to 4. 21 However, the accompanying cyclic voltammetry did show a second feature near the onset of oxygen evolution, hinting at a second oxidation step. While the experimental data are in general well founded, the interpretations differ. Based on recent experimental work on the HIROF system, 11,13,23 the present study attempts to provide a coherent understanding of the redox properties of the HIROFs by combining CV and DFT calculations. From the experimental side, the “super-nernstian” pH dependence becomes instrumental as is the pH dependence in the reduction peak close to the OER, the latter clearly showing increased oxide formation upon decreasing pH. From the modelling side the binuclear reaction channel (vide supra), recently employed to describe the OER, 9,10,24,25 on Ir(IV) and Ir(V) sites is used. An analysis supporting a potential dependent mechanism for OER, binuclear at low and mononuclear at high overpotentials, is provided.

Experimental and computational details Experimental All electrochemical measurements were carried out using either a Gamry Reference 600 or a Solartron (1287A) potentiostat with a standard three electrode setup. The cell was equipped with a double junction Ag/AgCl (sat’d KCl, E = 0.197 V vs. NHE) reference electrode, platinum gauge counter electrode and a gold rotating disk working electrode with a diameter 4


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of 3 mm. All potentials are given relative to the Ag/AgCl electrode. Before use, the working electrodes were polished to a mirror shine on a polishing cloth (DP-DUR, Struers) with diamond paste (Struers 6, 3, 1 and 0.25 µm) and cleaned by sonication in Milli-Q water (Millipore Milli-Q Academic). Electrodeposition of iridium oxide was performed by cyclic voltammetry (CV) as described by Steegstra et al., 12,13 using 600 cycles between the limits −500 and 650 mV at 1 V s−1 . Cyclic voltammetry on the deposited layers was measured in 0.1 M KOH (Sigma, KOH pellets, 99.99 %, semiconductor grade) at scan rates from 5 to 200 mV s−1 . The solutions were deaerated for a minimum of 20 minutes by N2 bubbling, prior to the CV measurements. Cyclic voltammetry on the iridium oxide films was measured on the Solartron and Gamry potentiostats for sweep rates below 100 mV s−1 and above 50 mV s−1 , respectively. A 5 Hz low pass filter was used for the measurements on the Gamry potentiostat while an analog ramp was employed for the Solartron potentiostat. The overlap in sweep rates (50 to 100 mV s−1 ) was used to ensure the validity of the measurements. Since the voltammetry of HIROFs involves surface processes the choice of potentiostat is not trivial. The Gamry potentiostat applies staircase instead of a linear scan, when measuring CV. For solution processes the current response to a staircase potential approaches that of a linear scan with decreasing step size. At step sizes below 0.26 mV no distinction can be made between the two. 26,27 For surface processes this is not the case, since mass transport is not involved the establishment of a new redox equilibrium can proceed at a very high rate, similar to double layer charging. In the staircase experiment the current is sampled shortly after each potential step. For reversible process the decay time can be sufficiently short for the system to relax before the current sampling takes place. In this scenario no or very low currents can be measured using a staircase input. The situation can be mended to some extent by using a low pass filter on the input signal, which smears the staircase, making it resemble a linear scan more closely. 26 In the present experiments an intermediate situation was recognized. At low sweep rates a staircase input gave lower currents than a linear scan (analog input signal). All CV 5



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below 50 mV s−1 was therefore measured on the Solartron potentiostat. For higher sweep rates the Gamry was used with a 5 Hz low-pass filter. Between 50 and 100 mV s−1 the scans were measured on both potentiostats, to ensure their validity. No significant difference was observed in this range of scan rates. A series of experiments was performed in which the pH response of an iridium oxide film was followed during the titration of a universal buffer solution, for different average oxidation states of the film. To modify the oxidation state of the film, the electrode was conditioned at constant potential for 2 minutes in 0.1 M KOH, prior to each titration. After conditioning, the electrode was allowed to equilibrate for at least 15 minutes. The conditioning potentials were chosen to span the electrochemically active region determined by CV in 0.1 M KOH, Fig. 2 The universal buffer was prepared according to Britton & Robinson 28 and contained 0.04 M boric, acetic and phosphoric acid (all three Merck, pro analyze). Titrations were carried out by stepwise addition of 1 M KOH solution. During the titrations the ocp of the iridium oxide film was measured against the Ag/AgCl reference electrode. In addition, the solution pH was measured with a glass electrode (Metrohm, 827 pH Lab) to facilitate relating the ocp to the solution pH.

Computational details All DFT calculations were performed using the CASTEP plane wave code. 29 A 1-dimensional 2 monolayer thick MgOx (OH)y rig with two adjacent magnesium ions replaced by iridium ions, thus producing a binuclear site, was chosen as model system (see Figure 1). The periodic test rigs were separated by 7 Å of vacuum, which was found sufficient in a previous study. 30 A second set of calculations employed a crossed rig system with the two Ir-dimers pointing towards each other (see Fig. 1b). The crossed rig systems were separated by a vacuum of 14 Å. The validity of this model was checked by comparing the energy profile with previous calculations on a single rig system. 30 No significant differences were found (see Supplementary 6


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Information).

a)

b)

Figure 1: The employed model systems (a) Single rig system, and (b) Crossed rig system. All electronic structure calculations employed the PBE 31 exchange-correlation functional. The core electrons were modelled using ultrasoft pseudo potentials. 32 The valence electrons were modelled assuming a low spin configuration at the Ir atoms. Gamma point calculations in conjunction with a 400 eV cutoff energy were performed throughout. The convergences with respect to k-points and cutoff energy were checked for this model system in a previous study. 30 No improvments upon increasing either the number of k-points nor the cutoff energy were found. Geometry optimisations were performed using the BFGS algorithm as implemented into CASTEP. All atoms were allowed to relax and the super-cell dimensions were kept constant.

Results and discussion Cyclic voltammetry Fig. 2 shows the CV of a HIROF in 0.1 M KOH, measured at 10 mV s−1 . Two reversible redox couples are observed, commonly attributed to the Ir(III)/Ir(IV) and Ir(IV)/Ir(V) transitions. 15,18,20,22 The charge under the two anodic waves at −0.3 V and 0.05 V was calculated after subtracting the contribution for oxygen evolution at the most positive 7



0.6 0.4

j / mAcm-2

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0.2 0.0 −0.2 −0.4 −0.6

−0.6

−0.4

−0.2

0.0

E vs (Ag/AgCl) / V

0.2

0.4

Figure 2: Cyclic voltammogram (v = 10 mV s−1 ) of an EIROF in 0.1 M KOH. The anodic scan was fitted using 3 Gaussians transitions (prepeak + 2 IrOx transitions) and one exponential function (OER) potentials, see Fig. 2. Two things can be noted. First, the charge involved in the second peak is smaller than the charge of the first peak, in this particular case the ratio is 0.7. The ratio depends on pH, electrolyte composition and concentration but is always less than 1. Second, the half width potential is larger for the second peak, 243 mV compared to 128 mV. These half width potentials are large compared with the 90.6 mV, expected for a reversible electron transfer under Langmuir conditions. 33 Broadening of the peaks can have different origin such as interactions within the layer, distribution of formal potentials in the film 34 and coupled ion-electron transfer. 35 For thinner films, deposited with 100 cycles, the peak separation and broadening observed in Fig. 3 are smaller, indicating mass transport in the film plays a role. However, other causes cannot be excluded, as the peak width at half maximum remains greater than 90.6 mV. Irrespective of the origin, the effect is clearly larger in the potential region of the second redox couple. Fig. 3 shows a series of cyclic voltammograms for the same film and in the same solution as those in Fig. 2, measured at 5 to 200 mV s−1 . The currents were normalized by the electrode 8


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60 40

jv-1 / mA cm-2 V- 1

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20 0 −20 −40 −60

−0.6

−0.4

−0.2

0.0

E vs (Ag/AgCl) / V

0.2

Figure 3: Cyclic voltammograms of an EIROF in 0.1 M KOH at a scan rate from 5 to 200 mV s−1 , normalized by the scan rate, showing the relative charge under the first and second transition is independent of the scan rate. area and the scan rate. With increasing sweep rate the peak width increases for both redox couples. However, the ratio between the charges under the two couples remains constant in the sweep rate range used.

pH dependence The linear shift with pH of the HIROF’s CV is illustrated in Fig. 4, where a number of voltammograms is plotted as a function of pH on the reversible hydrogen electrode (RHE) scale. The shift to more positive potentials with decreasing pH shows the “super-nernstian” behavior (pH sensitivity larger than 60 mV). The influence of the oxidation state of iridium in the film on the pH response was investigated by subjecting the electrode to different conditioning potentials prior to titration experiments, see Experimental Section. More details and discussion on these experiments can be found in. 13 Besides an influence on the pH sensitivity, defined as dE/dpH, the intercept of the titration curves showed an interesting trend as well. 13 This intercept was defined as the 9



0.4 0.2

j / mA cm-2

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0.0 −0.2 −0.4 0.2

0.4

0.6

0.8

1.0

E vs RHE / V

1.2

1.4

Figure 4: Cyclic voltammetry of an EIROF in a universal buffer solution at pH ranging from 2 to 12. potential at pH = 0, calculated from linear fits through the titration curves in the pH range of 6 to 11. In Fig. 5, the intercept is plotted as a function of condition potential together with the normalized charge obtained by integrating the CV measured in the same solution. From −400 to 75 mV the slope of the intercept vs. the conditioning potential is unity, indicating that the film can fully adapt to the applied potential. At higher potentials, the slope declines to merely 0.3, implying that not all iridium sites are further oxidized to Ir(V). Gaussian fits to the first and second anodic peak showed that the charge under the first peak did not vary with pH. However, the charge associated with the second couple increased with increasing pH and the ratio between the two increased from 0.3 at pH 2 to 0.5 at pH 12.

DFT Calculations The differences in charge between the two redox processes observed in the HIROF CV and the enhanced pH dependence provide an ideal context for mechanistic analysis of the OER. Recently, a test rig for transition metal hydroxides was described to address critical aspects

10


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1.3

1.0

1.2

E vs (Ag/AgCl) / V

0.8

1.1

0.6

1.0

0.4

0.9

θ

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0.8

0.2 0.0 −0.8

0.7 −0.6

−0.4

−0.2

0.0

0.2

E vs (Ag/AgCl) / V

0.4

0.6 0.6

Figure 5: N EIROF potential at pH 0, calculated from linear regression to titration curves, as a function of the conditioning potential applied in 0.1 M KOH, ( ) Normalized charge under a cyclic voltammogram of an EIROF in 0.1 M KOH. of electrocatalytic water oxidation. It was evaluated for binuclear systems and predictive power for mixed oxide catalysts was claimed. 10,24,30 The role of the electrocatalyst, within the binuclear paradigm, is to offer sufficiently unstable intermediate di-oxo moieties. After the oxidation steps, which can always be enforced and whereby two TM–OH2 moieties are converted into two adjacent TM=O/TM–O• sites upon removal of 4e– and 4H+ , a series of purely chemical steps remain for the catalyst to be recovered. The catalyst recovery starts with formation of the decisive TM–O–O–TM intermediate from the nearest-neighbor TM=O/TM–O• moieties, and eventually results in the release of an oxygen molecule. 9,10,24 What distinguishes the binuclear mechanism from the mononuclear is that for the latter, besides the TM=O/TM–O• oxidation step there is a second, subsequent, electrochemical step comprising formation of TM–OOH at this site. 7,36 In case of iridium oxide the O–O bond formation step was proposed to be potential determining. 7 Lundberg et al. proposed that a requirement for an efficient O–O bond formation is the appearance of radical character at the TM=O/TM–O• moiety. 37 Thus, this property may be employed as a descriptor for 11



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the onset of OER in the mononuclear mechanism paradigm.

a)

+ 2 H+ + 2 e-

b)

+ H+ + e-

c)

+ H+ + e-

Figure 6: The electrochemical steps used to model the OER at HIROF are shown. The electrochemical reactions proceed through oxidation of adsorbed water to hydroxo moieties (A; not modelled explicitly). The dihydroxo intermediate is oxidized to a oxo-hydroxo intermediate (b). In the final electrochemical step the remaining hydroxo moieties are oxidized to oxo species (c). In the present study a set of calculations was performed to clarify the dichotomy between the two mechanisms and how pH may control the amount of Ir(V) oxide formed by blocking the OER and increasing the overpotential (see pH dependence in reduction branch of Fig. 4 at 1.4 V). Thus, the binuclear concept was applied to the Ir(III-V) system with focus on the Ir(IV)/Ir(V) transition, see Fig. 6. The Ir(III)/Ir(IV) couple, the first redox feature in Fig. 2, is attributed to the oxidation of an adsorbed water to hydroxide, i.e. 2 Ir(III)–OH2 are oxidized to 2 Ir(IV)–OH, Fig. 6a. This step is not modelled explicitly here. In the next step, at the second peak, an oxo-hydroxo (Ir(IV)Ir(V)) or di-oxo (Ir(V)Ir(V)) intermediate could be formed. However, ensemble effects are expected to suppress the possibility of two adjacent Ir(V)=O moieties, leading to the oxidation of only a part of the surface hydroxyl groups. This is supported experimentally by the lower charge observed for the second couple in the cyclic voltammetry, Fig. 2, as well as the suppressed increase in the intercept at high conditioning potentials, Fig. 5. Indeed, assuming a binuclear mechanism, 9 the onset of oxygen evolution is found when 12


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neighboring Ir(IV)–OH become oxidized to Ir(V)=O. The thus formed di-oxo species displays two weak double bonds and may readily form the crucial µ-peroxo intermediate, Fig. 6c. [Ir(IV)–OH Ir(V)=O] displays only one such weak bond and does not support the OER according to the binuclear mechanism. The appearance of an Ir(IV)Ir(IV)/Ir(IV)Ir(V) oxidation process preceding the OER therefore supports the binuclear mechanism. Interestingly, GGA calculations on the [Ir(IV)–OH Ir(V)=O] system produce radical character on the oxy species at potentials where oxygen evolution is not allowed thermodynamically. This would contradict a mononuclear mechanism, since a single Ir(IV)–O• site should already be sufficient to trigger oxygen evolution. However, pure GGA-DFT is often problematic for strongly localized systems, 38 due to the self-interaction error. This is partially taken care of by employing a GGA+U ansatz, 39 which introduces an onsite repulsion penalty for electron delocalisation. At this level of theory the radical character on oxygen in Ir(V)=O is suppressed by the Ir(IV)–OH in the [Ir(IV)–OH O=Ir(V)] binuclear site and vanishes (see Table 1). Thus again, the mononuclear and binuclear mechanisms would make the same prediction as to [Ir(IV)–OH Ir(V)=O] preceding OER. While clearly unstable, any radical character on the mixed Ir(IV)/Ir(V) HIROF system could not be ruled out by the calculations on the binuclear model. An attempt to utilize the possibility that the radical character at an Ir(V)=O site would be quenched by neighboring Ir(IV)–OH sites was undertaken. Thus, the model was extended by including a second rig, which was placed perpendicular to the first, Fig. 1b. This crossed rig system allows for including ensemble effects without changing the overall model. The configuration was chosen, such that it reflects the flexibility of HIROF and allows for hydrogen bonding between the layers. Strong hydrogen bonding between the layers was indeed found for all structures containing hydroxo moieties. Upon subsequent removal of e– /H+ couples from Ir(IV)–O• , moieties with mostly strong radical character at the oxygen were again obtained at the GGA level of theory, thus supporting the findings obtained for the single-rig system. However, for the single rig system the Ir-O• was found unstable towards quenching of the oxygen radical 13



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character by hydrogen bonding. It is likely that such an effect exists also in the crossed rig configuration, owing to the hydrogen bonds between IrOx layers. Support for the existence of such an instability towards removal of the radical character on the oxy group is obtained for the [(Ir(IV)−OH)(Ir(V)−O)3 ]. Table 1: The Mulliken spin densities of the Ir-O• oxo-moieties of the crossed rig system shown in Fig. 1 are summarized. Spin densities close to 0 indicate a double bonded TM=O structure while spin densities close to 1 are a sign of radical Ir-O• intermediates. Ir1 −O1 and Ir2 −O2 belong to rig 1 and Ir3 −O3 and Ir4 −O4 to rig 2. All values are obtained employing the PBE-GGA functional. system

O1

(Ir−OH)3 (Ir−O) (Ir−OH)2 (Ir−O)2 (Ir-OH) same rig (Ir-OH) different rigs (Ir−OH)(Ir−O)3 (Ir−O)4

O2

O3

O4

0.84 -0.4

-0.74 -0.04 0.4

0.74 0.8 -0.9

-0.8 0.72 -0.82 0.88

Emerging understanding from experiment and modelling According to the calculations the Ir(IV)/Ir(V) couple can be considered split in two. First the system is oxidized from Ir(IV)Ir(IV) to a mixed Ir(IV)Ir(V) state. At this point stabilisation of the Ir(IV)–O• /Ir(V)=O moiety by hydrogen bonding from a neighboring hydroxo group inhibits the OER process. The appearance of a single Ir(V)=O moiety prior to oxygen evolution is in full agreement with the binuclear mechanism where two adjacent Ir(IV)–O• sites are required. Existence of any radical character in the Ir(IV)–O• /Ir(V)=O mixed system at potentials that do not support the OER would render that descriptor invalid, while not affecting the binuclear mechanism. Although, radical character was indeed found at the GGA level of theory, it was quenched at the GGA+U level. Hence, no conclusive contradiction to the mononuclear mechanism was found. Indeed, while the binuclear pathway is expected to 14


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dominate at the onset potential for OER, at higher potentials increasingly the mononuclear route will take over. On approaching the onset for oxygen evolution from below, at more positive potentials the Ir(V) coverage increases until the conditions for the binuclear OER mechanism are satisfied. The experimental data support this interpretation in the following way. Once the conditions in the film are suitable to support catalytic activity, thermodynamics determine whether the OER will occur. Since the CV of HIROFs shift with pH at 80 to 90 mV and the OER at 60 mV, the thermodynamic conditions for OER will not be met at the same position in the CV at all pH, see Fig. 4. At low pH the OER overlaps strongly with the Ir(IV)Ir(IV)/Ir(IV)Ir(V) peak. By inspecting the oxidation branch of the CV it is difficult to discern how much current is owing to oxygen evolution and how much to Ir oxidation. Moving towards alkaline conditions the CV of the HIROFs shift negative to a greater extent than the OER and oxygen evolution and the Ir(IV)Ir(IV)/Ir(IV)Ir(V) process become well separated, Fig. 2. Thus, the mixed oxidation state system is allowed to saturate at approximately a 1:1 ratio between Ir(IV) and Ir(V). The role of pH may be formulated as follows, under alkaline conditions the bare interactions between Ir(V) sites effectively prohibit further oxidation beyond said 1:1 ratio. Upon lowering pH, the screening of Ir(V) sites, owing to hydrogen bonding between [Ir(V)=O ←−→ Ir(V)−OH] and Ir(IV)–OH sites, allows for increased surface concentration of Ir(V) sites beyond 1:1. While increasing overlap with the OER occurs, owing to the super-nernstian pH dependence, it is possible to conclude that the effect of the increased proton activity is to cause increased overpotential for the OER and consequently the formation of a larger amount of Ir(V) oxide. This conclusion is based on the charge associated with the reduction peak at 1.4 V in Fig. 4. This peak can be attributed to the one electron reduction of Ir(V) and tells about the preceding formation of larger amount of Ir(V) oxide at lower pH than at higher. The ability of protons to redirect the oxidation path from the OER to Ir(IV) oxidation 15



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comes out naturally from the DFT calculations. Protons counter the radical character on the Ir(V)=O oxygens and block the binuclear OER channel by intersite hydrogen bonding. This appears as a suppression of the OER tail in the vicinity of 1.23 V (RHE). Thus, lowering pH may be said to contribute an “anti-catalytic” effect. This pH sensitivity may be further clarified as follows: the Ir(V)=O bond can be characterized by a linear combination of three mesomeric structures, i.e. [Ir(V)=O ←−→ Ir(IV)–O• ←−→ Ir(V)–O- ]. The radical character in the double bond of Ir(V)=O, (i.e. [Ir(V)=O ←−→ Ir(IV)–O• ]) is associated with the OER. The third mesomeric structure (i.e. [Ir(V)=O ←−→ Ir(V)–O- ]) is, similar to Ir(V)=O, a non-OER active electronic structure of Ir(V) and suppresses the OER. This OER suppressing candidate structure is enhanced by protonation owing to the formation of Ir(V)–O- -H+ or more chemically: Ir(V)–OH. Through hydrogen bonding this moiety stabilizes the non-OER active electronic structures, such as Ir(V)–OH—- O–Ir(V), suppressing the OER and favoring the formation of an Ir(V) oxide. In contrast to a mononuclear mechanism, for the binuclear mechanism the predicted emergence of an Ir(IV)/Ir(IV) −−→ Ir(IV)/Ir(V) peak, preceding the OER, does not depend on the existence of radical character on the binuclear Ir(IV)–Ir(V) site. This is because the binuclear mechanism requires a biradical site, and intersite repulsion among Ir(V)=O sites suppresses the probability for finding such nearest-neighbor sites at less than 50 % coverage under alkaline conditions. Consequently, the existence of the pre-peak and its sensitivity to pH in the CVs for HIROF catalyzed water oxidation are in perfect agreement with the understanding that emerges from the binuclear mechanism.

Conclusions pH control of Ir(IV) oxidation was demonstrated and discussed in the context of a binuclear mechanism of the OER. The super-nernstian dependence of the Ir(IV)/Ir(V) redox couple, allowed for the resolution of a two step process for the oxidation of Ir(IV)O(OH)2 . The two

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steps are best seen under alkaline conditions, where the first emerges well before the onset of the OER, while the second overlaps with the OER. Under alkaline conditions, the complete oxidation of Ir(IV) to Ir(V) is inhibited by the efficiency of the OER already at the theoretical onset for the OER. This is attributed to the efficiency of IrO(OH)2 as electrocatalyst. The suppression of Ir(IV) oxidation is reflected in the smallness of the current observed near the switching point in the subsequent reduction sweep. Due to the super-nernstian response the first Ir(IV) oxidation step becomes less pronounced under increasingly acidic conditions. Therefore, it becomes difficult to resolve the Ir(IV) oxidation from the OER in the oxidation sweep. Instead, the reduction sweep was used, which implies a monotonic increase in Ir(V) production with lowering of pH. A binuclear model has previously been employed to illustrate the electrocatalytic efficiency of iridium oxyhydroxide towards the OER. Important in the reaction channel investigated is that it could not support the OER in spite of the presence of Ir(V) if the neighboring site was Ir(IV). This is in agreement with the separability of the two oxidation steps. In the present study control of the conditions for the OER, by bridging protons between nearest neighbor Ir(V)=O moieties, was arrived at, and argued to act anti-catalytic by blocking the binuclear OER channel. Thus enhanced oxidation of Ir(IV) to Ir(V) was expected, consistent with the experimental observation of increased current during the reduction sweep at the anodic switching potential. This implies pH control of the electrochemical oxidation of hydrous iridium oxide films.

Acknowledgement This work was supported by the European Commission through the FP7 Initial Training Network “ELCAT” (Grant Agreement No. 214936-2) and the platform “Nanoparticles in interactive environments” at the University of Gothenburg.

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Supporting Information Available Figure 7

This material is available free of charge via the Internet at http://pubs.acs.org/.

TOC image

Figure 8: TOC image

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Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the

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