IrO2 bifunctional materials as catalysts for the

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Activity and stability of Pt/IrO2 bifunctional materials as catalysts for the oxygen evolution/reduction reactions Gabriel C da Silva, Mauro R Fernandes, and Edson A. Ticianelli ACS Catal., Just Accepted Manuscript • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Activity and stability of Pt/IrO2 bifunctional materials as catalysts for the oxygen evolution/reduction reactions Gabriel C. da Silva, Mauro R. Fernandes and Edson A. Ticianelli* São Carlos Institute of Chemistry, USP, C.P. 780, São Carlos – SP, 13560-970, Brazil. Corresponding author. Tel.: +55 1633739945 E-mail address: [email protected]

Abstract Pt/IrO2 bifunctional catalysts synthesized with varying Pt:Ir ratios and characterized using several techniques, including energy dispersive X-ray spectroscopy, transmission electrons microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, were investigated for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in acid media. Three stability testing protocols are used to evaluate the catalysts stabilities, comprising electrode cycling in the ORR, OER, and ORR-OER potential ranges. Electrochemical results evidence that a Pt/IrO2 1:9 material exhibit better balance between the OER and ORR mass activities and that cycling in the ORR-OER potential window is the most aggressive aging protocol for the Pt/IrO2 materials. Identical location transmission electron microscopy is used to investigate the aging processes taking part in the Pt/IrO2 catalysts. In addition to dissolution processes, particles coalescence, growth, and detachment are confirmed as responsible for the Pt/IrO2 instability.

Keywords Oxygen reduction, oxygen evolution, bifunctional, oxygen electrode, platinum, iridium oxide.

Introduction Unitized regenerative fuel cells (URFC) are single unit electrochemical devices that operate in both electrolyzer and fuel cell modes, and are currently pointed as viable alternatives to conventional secondary batteries, since URFCs do not require long time charging process, besides having a higher specific energy1, 2. Just like in the case of fuel cells, URFC can be classified according to the electrolyte they employ; among the different types of URFC, the unitized regenerative alkaline3 and the unitized regenerative proton exchange membrane fuel cells have the most developed status4. However, the sluggish kinetics of the oxygen electrode reactions and the limited stability of the catalysts remain as a bottleneck for URFC technology, which makes the development and optimization of bifunctional oxygen electrocatalysts a key issue. 1 ACS Paragon Plus Environment

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Non-noble catalysts with good activity and stability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), such as transition metal oxides5-7, perovskites8-10, and carbonaceous materials11-13 have been reported for use as bifunctional oxygen catalyst in alkaline media. Nonetheless, the highly corrosive environment of the proton exchange URFC still demands the use of noble-metal-based electrocatalysts. The most used catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells cathodes are based on platinum and platinum alloys nanoparticles supported on high surface area materials, usually carbon black (Pt/C and Pt-M/C)14. Nonetheless, despite the improvements achieved for Pt-based materials as ORR electrocatalysts, platinum provides an unsatisfactory performance as catalyst for the oxygen evolution reaction (OER); besides, the carbon support is unstable under the electrolyzer operation conditions, since at potential above 0.207 vs. NHE it can be converted to CO215, impeding its use in the oxygen electrode of URFC. In the electrolyzer mode operation, the URFC oxygen electrode works in the electrooxidation of water. Cherevko et al.16 studied the performance of several noble metals towards the OER; the activities found by authors followed the order Ru > Ir > Rh > Pd > Pt > Au. Danilovic et al.17 observed a similar trend, with osmium showing an activity higher than that of ruthenium (Os >> Ru > Ir > Pt >> Au). However, since the OER occurs at high potential, oxide materials are usually chosen as electrocatalysts for this reaction. Man et al.18 described the activity of different oxides for the OER as a function of the binding energies of the reaction intermediates; the authors obtained a volcano plot in which the noble metals oxides are located on its top. In this context, RuO2 and IrO2 are the conventional catalysts for electrolyzer anodes. Cherevko et al.19 compared the activity of Ru and Ir to those of their respective oxides, RuO2 and IrO2, in a scanning flow cell. Both metals exhibited higher activity compared to the respective oxides, however a higher dissolution rate was observed. The authors also reported a dissolution rate for IrO2 30 times smaller than that for RuO2. The instability of RuO2 is assigned to the formation of RuO420, whereas the higher stability of IrO2 justifies its choice as the benchmark OER catalyst in acidic media and it’s use in proton exchange membrane water electrolyzers21, 22. However, iridium oxide is not active for the oxygen reduction reaction, hindering its use as an oxygen bifunctional catalyst. In face to the limitations of Pt-based catalysts for the OER and of IrO2 for the ORR, electrocatalysts obtained by the combination of Pt and IrO2 have been proposed for use in the oxygen electrode of URFC. Zhigang et al.23 obtained a bifunctional catalyst by mixing 50 wt.% of Pt and 50 wt.% of IrO2; a good catalytic performance was obtained even with a catalytic load of just 0.4 mg cm-2. Ioroi et al.24 also prepared a bifunctional catalyst thorough the mixture of Pt and IrO2. The oxygen electrode operating with this bifunctional catalyst was more active for the water electrolysis than the one in which only platinum was used. However, in the fuel cell operation, the IrO2/Pt electrode exhibited poorer activity in comparison to the Pt electrode. This is assigned to the lower activity of IrO2 for the ORR. Ioroi et al.25 prepared a IrO2/Pt catalyst by depositing an iridium oxide colloidal precursor on platinum nanoparticles. Results showed that the mass activity of this 2 ACS Paragon Plus Environment

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material for both OER and ORR was higher than that of the mixture of the separated components, resulting in a higher round-trip energy conversion efficiency. The better efficiency of the deposited IrO2/Pt catalyst was attributed to the microstructure of the electrocatalyst. Other works also report the synthesis of oxygen bifunctional catalysts by the reduction of a platinum precursor over iridium oxide (Pt/IrO2). Yao et al.26 synthesized a Pt/IrO2 catalyst with better activity for the OER compared to the mixture of Pt and IrO2, but with a poorer activity for the ORR. The authors justified these results suggesting the presence of a stronger adsorption of oxides on the platinum surface in the case of the bifunctional electrocatalyst. Kong et al.27 reported the synthesis of Pt/IrO2 bifunctional catalyst by the deposition of Pt on the surface of IrO2 particles through a microwave-assisted polyol method. The Pt/IrO2 catalyst showed higher currents for the ORR and OER compared to a Pt supported on commercial IrO2 catalyst, or pure IrO2. Despite these promising results, the stability of Pt/IrO2 catalysts is still little accessed. To the best of our knowledge, only two works on this issue has been conducted before by Kong et al.28,29 In the first, the authors employed an accelerated potential cycling test to evaluate the stability of a Pt/IrO2 material, and observed an electrochemical surface area reduction of 35.5 % after 2,000 potential cycles between 0.05 – 1.2 V; in addition, after 2,000 potential cycles between 1.2 – 1.6 V, the OER current peak reduced by 15.1 %28. In the second work, after a longer aging protocol consisting of 5,000 cycles, the electrochemical surface area of the Pt/IrO2 material decreased by 29 %, while for the OER current peak a reduction of 20.4% was observed29. In none of these works, the effect of cycling on the ORR was characterized. Herein, the activities of Pt/IrO2 bifunctional catalysts synthesized with Pt:Ir contents of 1:9, 3:7 and 1:1 were comprehensively investigated for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in acid media, while their stabilities had been evaluated by employing three aging testing protocols, comprising electrode cycling in the ORR, OER, and ORR-OER potential ranges. Prepared catalysts had been characterized by several techniques, including energy dispersive X-ray spectroscopy, transmission electrons microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Identical location transmission microscopy (ILTEM) technique was used in order to better understand the structural modifications of the Pt/IrO2 materials after each aging procedure. This approach allows the identification of the processes responsible for the degradation of the catalytic layer, such as the particles growth/shrinking and a possible dissolution/re-precipitation phenomena, and that should be considered in the development of bifunctional oxygen electrocatalysts.

Experimental section Catalysts synthesis Initially, the IrO2 material was prepared accordingly to the hydrothermal method proposed by Bestaoui and Prozet, as reported before30. Briefly, 5 g of H2IrCl6.xH2O (99 3 ACS Paragon Plus Environment

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wt.%, Alfa Aesar) are dissolved in 100 mL of ultrapure water (18.2 MΩ cm, Milli-Q). To 10 ml of the iridium precursor solution, a 1M LiOH solution (99 wt.%, SigmaAldrich) was added at a rate of 0.2 mL h-1, under stirring, until the [OH-]/[Cl-] = 6 ratio has been reached. The obtained blue suspension was transferred to a polytetrafluorethylene (PTFE) autoclave bottle compartment, which was heated at 180 °C for 24 hours. The black precipitate was filtered and washed with boiling water to remove any lithium contaminant, and dried at 50 °C for 1 h. Finally, a heat treatment at 400 °C for 1 hour under air atmosphere was performed in order to obtain the IrO2. To prepare the Pt/IrO2 catalysts with Pt:Ir atomic ratios of 1:9, 3:7 and 1:1, a formic acid synthesis method was used. The IrO2 material was ultrasonically dispersed in a 2 M solution of formic acid (98 wt.%, Sigma-Aldrich), then the dispersed solution was heated to 80 °C and the platinum precursor solution of 5g/100 mL of H2PtCl6.xH2O (99 wt.%, Alfa Aesar) was added dropwise with stirring. Five minutes after completing the addition of the platinum precursor, the solution was let to cool down and the catalyst was collected in a 0.2 µm PTFE membrane, washed with boiling water, and dried at 50 °C for 1h.

Physicochemical characterization The catalysts mass and atomic compositions were evaluated by energy dispersive Xray spectroscopy (EDX) in a Leica-Zeiss LEO 440 scanning electron microscope with an accelerating voltage of 40 kV. X-ray diffraction was performed in a Bruker D8 Advance diffractometer with a 1,5406 Å KαCu radiation; data was collected for 2θ values between 10 and 100° at a rate of 0.075° s-1. The structural parameters of the catalysts were obtained using the Rietveld refinement method31. Transmission electron microscopy (TEM) images were obtained in a JEOL JEM 2100 microscope fitted with a lanthanum hexaboride (LaB6) filament and accelerating voltage of 200 kV. The scanning transmission electron microscopy (STEM) and EDX mapping images were acquired in a field emission FEI Tecnai G² F20 microscope with accelerating voltage of 200 kV. For both cases the catalysts were dispersed in isopropanol and deposited single-drops on carbon-film copper grids (EMS, 400 mesh). The catalysts’ surface compositions and chemical environments were studied by Xray photoelectron spectroscopy (XPS) on a Thermo Scientific K-Alpha+ spectrometer, using a 1487 eV X-ray energy and an aluminum monochromator; the binding energies were calibrated according to the C 1s energy at 284.8 eV. The experiments were carried out at the Laboratório Nacional de Nanotecnologia (LNNano, Campinas – Brazil).

Electrochemical experiments Electrochemical characterization The electrochemical measurements were performed in a three electrodes glass cell, with a reversible hydrogen electrode (RHE) and a graphite rod as reference and counter 4 ACS Paragon Plus Environment

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electrodes, respectively. A glassy carbon disc (A = 0.196 cm²) was used as substrate for holding the investigated catalyst layers. For preparing these layers, a catalyst ink was prepared by mixing in an ultrasonic bath 3 mg of the catalyst powder to 15 µL of a Nafion ionomer dispersion (5 wt.%, Alfa Aesar) and 1 mL of ultrapure water (18.2 MΩ cm, Milli-Q). The catalyst dispersion was deposited in the glassy carbon substrate to a final content of 0.38 mgmetal cm-2 (Pt+Ir) and dried with a N2 gas flow. All electrochemical tests were done in a 0.5 M H2SO4 electrolyte solution at 25 °C. The catalysts were firstly characterized by cyclic voltammetry (CV) obtained between 0.05 – 1.2 V at 50 mV s-1. The platinum electroactive surface areas (Aecsa) were estimated by CO stripping. For this, the working electrode was kept at 0.1 V for 5 min in the presence of the CO saturated electrolyte solution, then the electrolyte was purged with argon to remove any non-adsorbed carbon monoxide and after that, three CVs between 0.1 – 1.0 V were recorded. The Aecsa were obtained considering a charge density of 420 µC cmPt-2 for the oxidation of a CO adsorbed monolayer32, 33. OER and ORR activities evaluation The OER and ORR activities were tested in a rotating disc electrode setup using a Pine rotator system. The OER activities were studied using linear sweep voltammetries (LSV) between 1.1 and 1.6 V at 5 mV s-1; a rotation speed of 1600 rpm was used to avoid the accumulation of O2 bubbles on the electrode surface. The ORR activity measurements were performed in O2 saturated electrolyte solution; LSV at rotation speeds varying from 100 to 2500 rpm were collected between 0.1 – 1.1 V at 5 mV s-1. For comparison purposes, the OER and ORR activities of pure IrO2, commercial Pt black (HP 6000, Engelhard), and Pt/C (40 wt.%, Etek) were also tested. Accelerated stability tests Three protocols were used to evaluate the Pt/IrO2 catalysts stabilities: i) 1,000 potential cycles in the ORR region, i.e between 0.1 – 1.1 V at 100 mV s-1; ii) 1,000 potential cycles in the OER region, i.e between 1.1 – 1.6 V at 100 mV s-1; iii) 1,000 potential cycles in a potential range of both ORR and OER regions, i.e 0.1 – 1.6 V at 100 mV s-1. In these tests, the Pt/IrO2 ORR and OER activities were monitored using the same procedure described in the previous section. The potential ranges used in the stability tests were chosen based on the potential window that is usually employed for investigation bifunctional ORR/OER catalysts in three-electrode cells. In fuel cell operation mode, the fuel cell potential usually does not reach such a reducing potential (0.1 V).

Identical location transmission electron microscopy (IL-TEM) experiments IL-TEM experiments were conducted in a JEOL JEM 2100 microscope with a LaB6 filament and accelerating voltage of 200 kV. The sample grids were prepared by 5 ACS Paragon Plus Environment

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depositing a single drop of the catalyst ink prepared for the electrochemical measurements in a carbon-lacey gold grid (EMS, 300 mesh). Initial TEM micrographs were collected at different regions of the TEM grid; then the TEM grid was used as the working electrode of the three electrodes cell setup and one of the aging protocols were employed. The effects of the electrochemical aging on the catalysts were evaluated by collecting new images at the same previously chosen regions of the grid.

Results and discussion Pt/IrO2 catalysts characterization The final mass and atomic compositions of the synthesized Pt/IrO2 catalysts with nominal compositions of PtIr9O18, Pt3Ir7O14, and PtIrO2 were obtained by EDX and the results are shown in Table S1. These EDX data show that the platinum deposition on the iridium oxide through the formic acid synthesis method has provided catalysts with atomic compositions of PtIr7.2O11.8, Pt3Ir7O13 and PtIrO1.7. It is seen that a good correlation between the Pt/Ir nominal and real compositions of the catalysts with the higher platinum amounts, while for Pt/IrO2 1:9 somewhat lower iridium oxide content is observed. Regarding the Ir/O atomic ratio, instead of IrO2, the formula resulted IrO1.6, IrO1.9, IrO1.7, for increasing Pt contents. The more obvious explanation for this would be the occurrence of some reduction of this oxide forming metallic Ir and some Ir3+ specie, but the presence of metallic Ir was not evidenced by XRD and XPS data. In this way, this fact may be due to the presence of Ir3+ species in the same way as observed previously for IrO2 in absence of Pt34. The XRD profiles of the bifunctional Pt/IrO2 catalysts are displayed in Figure 1, which also includes the diffractograms of pure IrO2 and commercial Pt black materials. The diffractograms of the Pt/IrO2 catalysts exhibit the coexistence of peaks related to the tetragonal rutile IrO2 (PDF 15-870) reflections and to the face centered cubic (fcc) Pt (PDF 4-802) structures. Similar results were observed for a IrO2/Pt catalyst25, i.e, a material obtained through the deposition of iridium oxide on platinum, and also for a Pt/IrO2 obtained by a polyol method27, and a Pt-Ir/IrO2-PtO2 prepared by the reduction of the metallic precursors by a glycol35.

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Figure 1. XRD patterns of the Pt/IrO2 9:1, 3:7, 1:1, IrO2 and Pt black catalysts. The vertical lines correspond to the tetragonal IrO2-rutile (PDF 15-870) and Pt fcc (PDF 4802) crystallographic structure patterns. Yao et al.26 reported the synthesis of a Pt/IrO2 catalyst through the reduction of a platinum precursor with NaBH4 over the iridium oxide; the XRD analysis of this catalyst exhibited only the presence of Pt fcc reflection peaks, which can be attributed to the low crystallinity of the IrO2 support. The authors also observed a shift of the Pt fcc reflection peaks to higher angles, assigned to the interaction between Pt and IrO2 causing a contraction of the platinum crystal lattice structure. Here, no shift in the Pt reflection peaks can be observed in the XRD results in Fig.1. Besides, the use of the formic acid synthesis method does not cause measurable reduction of the IrO2 support, which is confirmed by the absence of metallic iridium reflections in the diffractograms. The structural parameters of the Pt/IrO2 bifunctional catalyst were calculated using the Rietveld method31. The mean crystallite sizes and lattice parameters of the studied catalysts are shown in Table 1. The calculated mean crystallite size of the IrO2 phase of the Pt/IrO2 catalysts varies between 4.7 and 5.9 nm, while that for bare IrO2 is 3.4 nm, which indicates that the Pt deposition method in some way affect the IrO2 nanoparticles size. Regarding the platinum nanoparticles, the calculated Pt mean crystallite sizes of the bifunctional catalysts are much smaller than that of commercial Pt black catalysts (13 nm), varying between 1.9 nm for the Pt/IrO2 1:9 catalyst to 3.8 nm for the Pt/IrO2 1:1 material. Table 1 – Mean crystallite sizes (D) and lattice parameters (aexp) calculated for the Pt/IrO2, IrO2 and Pt black catalysts using the Rietvel method.

D / nm

IrO2 aexp / Å (a=b ; c)

Pt D / nm

aexp / Å (a=b=c) 7

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Pt black IrO2 Pt/IrO2 1:9 Pt/IrO2 3:7 Pt/IrO2 1:1

– 3.4 4.7 5.3 5.9

– 4.4895 ; 3.1450 4.4896 ; 3.1497 4.5071 ; 3.1407 4.4898 ; 3.1353

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13.0 – 1.9 3.7 3.8

3.9104 – 3.9138 3.9295 3.9229

Transmission electron micrographs of the Pt/IrO2 bifunctional catalysts are presented in Figure 2. In all images it is noted the presence of smaller high contrast spherical nanoparticles supported on another lower contrast phase composed of bigger particles (see arrows in the pictures). A high-resolution TEM (HRTEM) image of the Pt/IrO2 3:7 material, the catalyst with intermediate Pt:Ir ratio, is shown in Fig. S1; despite the particles agglomeration and overlap, two atomic lattice fringes can be observed: i) one for the darker spherical nanoparticles with a d-spacing value of 2.26 Å, corresponding to the Pt (111) crystallographic plane (PDF 4-802) and ii) another for the bigger grayish particles with a d-spacing value of 3.18 Å, corresponding to the IrO2 (111) crystallographic plane (PDF 15-870). This results confirms that the Pt/IrO2 catalysts are composed of small spherical platinum nanoparticles dispersed over the iridium oxide. For all the catalysts it is possible to note the presence of Pt nanoparticle with sizes in the range of 1–4 nm, but, due to the high agglomeration of the nanoparticles, the mean particle sizes could not be precisely obtained. To further investigate the Pt nanoparticles distribution over the IrO2 in the Pt/IrO2 bifunctional catalysts, STEM imaging together with EDX chemical mapping were obtained for all the catalysts. The results for the Pt/IrO2 3:7 material are presented in Figure 3, while those for the Pt/IrO2 1:9 and 1:1 compositions can be seen in Figures S2 and S3, respectively. All EDX mapping confirms that the Pt nanoparticles are finely dispersed on IrO2, with some Pt richer regions, probably due to some Pt nanoparticles agglomeration.

Figure 2. TEM micrographs of the Pt/IrO2 catalysts (a) 1:9, (b) 3:7 and (c) 1:1.


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Figure 3. STEM imaging and EDX chemical mapping of the iridium and platinum elements of the Pt/IrO2 7:3 catalyst. X-ray photoelectron spectroscopy was used to study the catalysts’ surface compositions. Survey spectra are displayed in Fig. S4; the C1s peak comes from the carbon tape used in the samples preparation. The surface compositions of the bifunctional catalysts were estimated based on these XPS data. The surface atomic ratios between Pt and Ir (Pt/Ir) calculated for the Pt/IrO2 1:9, Pt/IrO2 3:7, and Pt/IrO2 1:1 materials are 1/6, 3/4, and 2/1, respectively. The catalysts’ surface compositions differ from that of the bulk obtained by the EDX analysis (1/7.2, 3/7, 1/1, respectively, see Table S1) and, as expected, indicates an increase in the Pt content on the surface of the IrO2 support particles.


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Figure 4. High resolution XPS spectra of the Pt 4f and Ir 4f orbitals for the (a)1:9, (b) 3:7 and (c) 1:1 Pt/IrO2 catalysts. The high resolution spectra of the Pt 4f and Ir 4f orbitals of the Pt/IrO2 materials are displayed in Figure 4. The Pt 4f spectra can be deconvoluted into three doublets, corresponding to the Pt 4f7/2 and 4f5/2 spin-orbit components with an energy splitting of 3.3 eV. The Pt 4f7/2 centered at 71.4, ~72.3 and ~74.2 eV can be attributed to the Pt0, Pt2+ and Pt4+ oxidation states, respectively.36-38 Table S2 summarizes the Pt 4f7/2 components binding energies as well the percentage contribution of each platinum oxidation states in the surface of Pt/IrO2 materials. The data for the Pt black catalyst were also included for comparison purposes. All the bifunctional catalysts contain almost 60% of the platinum in the Pt0 form, while 30% are in the +2 oxidation state, corresponding to the PtO specie, and 10% in the +4 oxidation state, i.e, PtO2. Regarding the commercial Pt black catalyst, the XPS data shows that half of the surface platinum is in the Pt0 form and the other half is in the Pt2+/Pt4+ oxidation states. With respect to the Ir 4f XPS data, each spectrum can be deconvoluted into two doublets: the most intense, with Ir 4f7/2 and 4f5/2 components centered at 62.8 and 65.6 eV, respectively, corresponding to Ir4+, and the less intense, with Ir 4f7/2 and 4f5/2 components centered at ~61.7 and 64.7 eV, respectively, corresponding to Ir3+39-41. There is also an additional peak in the high resolution Ir 4f spectra that is attributed to satellite due to conduction-band interactions during the photoemission process39. The absence of a Ir 4f7/2 peak at 60.9 eV confirms that no Ir0 was formed during the deposition of Pt nanoparticles, in accordance to the XRD data in Fig.1. The Ir 4f7/2 components’ binding energies and atomic percentages are shown in Table S3; the 10 ACS Paragon Plus Environment

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majority (66%) of iridium atoms are in the Ir4+ oxidation state, whereas the remaining are in the less oxidized Ir3+ state.

Electrochemical characterization of Pt/IrO2 catalysts The cyclic voltammetric (CV) results of the bifunctional Pt/IrO2 catalysts with varying Pt/Ir ratios are presented in Figure 5a. All CV profiles exhibit peaks at the 0.05 – 0.4 V potential window corresponding to the adsorption/desorption of hydrogen on platinum, being these peaks more evident for the materials with higher Pt load. The socalled “hydrogen region” (H-region) is characteristic of Pt-based materials, and its voltammetric charge depends on both the platinum load and the platinum nanoparticles sizes, which define the total Pt exposed area; the Pt/IrO2 1:9 catalyst has the lower Hregion charge among the studied materials, while for the other two Pt/IrO2 catalysts the charges of the H-region are very similar. However, since the double-layer currents observed for the Pt/IrO2 1:1 are lower compared to that of the 3:7 material, the H-region charge of the Pt/IrO2 1:1 is actually higher than that of Pt/IrO2 3:7. The existence of capacitive processes relative to the presence of IrO2 in the CV profiles of Fig. 5a makes difficult the precise distinction between the double-layer and the oxygenated species adsorption/desorption processes on Pt; nonetheless, a peak at ca. 0.8 V relative to desorption of oxygenated species in Pt can be seen for the Pt/IrO2 3:7 and 1:1 catalysts.

Figure 5. (a) Cyclic voltammograms and (b) CO stripping profiles subtracted from a baseline for the Pt/IrO2 catalysts with varying compositions. Generally, the electrochemically active surface area (Aecsa) of Pt-base catalysts, such as Pt/C and Pt-M/C, can be estimated by calculating the charge of the H-region subtracted from the electrode’s double-layer charge contribution42. Here, the subtraction of the double-layer contribution, in particular in the case of the Pt/IrO2 1:9, is quite complicated, which makes the calculated Aecsa innacurate. To overcome this difficulty, CO stripping experiments were performed and the CO oxidation profiles are shown in Figure 5b. The CO stripping profiles for the Pt/IrO2 catalysts in Fig. 5b show the existence of two oxidation peaks. The presence of multiple peaks in the CO stripping curves in Fig. 5b cannot be attributed neither to the presence of CO adsorbed on IrO2, since the 11 ACS Paragon Plus Environment

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iridium oxide is not active to the electro-oxidation of carbon monoxide43, nor to the oxidation of CO at metallic iridium, as the presence of Ir0 was excluded by the XRD (Fig. 1) and XPS (Fig. 4) data. Usually, carbon supported Pt catalysts exhibit a characteristic single peak, related to the oxidation of a CO monolayer, centered at ~0.8 V44, 45; here, a single peak at a potential around 0.7 V is observed for the Pt black catalyst. Guerin et al.46 evaluated the CO oxidation for a series of Pt/C (10 – 78 wt.%) and for Pt black catalysts; a single peak at 0.79 V was obtained for the 10 and 20 wt.% Pt/C catalysts, while for the higher platinum loads, a second peak at 0.69 V was observed. When the platinum loading was increased beyond 70%, the peak at 0.69 V becomes the major peak, and for platinum black the peak at 0.79 V becomes just a shoulder. Multiple peaks in platinum CO stripping experiments have been associated to the degree of CO coverage on the electrode and the adsorption of CO on different Pt sites, such as terraces, edges and corners46-48. A single CO oxidation peak at ~0.7 V for pure Pt was also obtained in other works49, 50. The catalysts’ Aecsa calculated through the integration of CO stripping curves (Fig. 5b) are presented in Table 2. A decrease in the Aecsa values can be observed in line with the Pt content enhancement in the studied electrocatalysts (Pt/IrO2 1:9 > Pt/IrO2 3:7 > Pt/IrO2 1:1 > Pt black), varying from 28.6 mPt2 gPt-1 for the Pt/IrO2 1:9 material to 8.3 mPt2 gPt-1 for the Pt black catalyst. Similar trend was observed for Pt/C materials with various Pt loadings51, which has been assigned to the increase of the Pt particle sizes caused by the increase of Pt load, in agreement with the results in Table 1. Regarding the IrO2 prepared by the hydrothermal method, the surface area calculated by the Brunauer-Emmett-Teller (BET) is 55 m² g-1, as reported elsewhere34.

Pt/IrO2 activities toward ORR and OER The activities for the oxygen reduction reaction (ORR) of the Pt/IrO2 catalysts were studied using the rotating disc electrode technique. The ORR polarization curves of the Pt/IrO2 and Pt black electrodes with varying rotation speeds are shown in Figure S5. The poor activity of the IrO2 for the ORR has already been reported52 and thus these results were not included; the polarization curves in Fig. S5 exhibit the typical profiles of the ORR in rotating disc electrodes for Pt-based catalysts, which can be divided in two regions: i) the first one at the potential range between 0.7 – 0.95 V, whose ORR kinetics is controlled by both the activation energy and the O2 transport to the electrode surface and ii) the second one at potential values lower than 0.7 that is diffusion controlled53. The numbers of electrons (n) involved in the ORR were calculated through the slopes of the Koutecky-Levich diagrams (not shown) using the equation54: 1 i

=

1 iK

1

+ = iL

1 iK

+

1 2/3 -1/6

0.62nFAD ν

ω1/2Co0

(1)


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where n is the number of transferred electrons per oxygen molecule, F is the Faraday constant (46485.3 C mol-1), A is the electrode area (0.196 cm²), ν is the kinematic viscosity (1.19 x 10-2 cm² s-1) of the electrolyte, ω is the angular rotation speed of the electrode, and Co0 is the oxygen solubility (1.10 x 10-6 mol cm-3), and D is the diffusion coefficient (1.4 x 10-5 cm² s-1) of oxygen in the electrolyte55. These results are shown in Table 2. It is seen that the calculated numbers of transferred electrons for the Pt/IrO2 catalysts are n ~ 4, similar to that reported for Ptbased catalysts in acidic media53, 56. The 4-electrons transfer pathway indicates that the ORR using the Pt/IrO2 bifunctional catalysts occurs via a route producing H2O, i.e, without net formation of H2O2. Figure 6a compares the ORR polarization curves at the rotation speed of 1600 rpm for all catalysts, including those for Pt-black and Pt/C (40 wt.% Pt), commonly used on PEMFC cathodes. These results denote minimal differences in the limiting currents for the different catalysts. Additionally, since the magnitude of the lines in the limiting current regions are essentially the same, we may conclude that the O2 diffusion are not so different for the different catalyst layers. In Fig. 6a, the polarization curves for the Pt/IrO2 and Pt/C materials exhibit positive currents for potential above the ORR onset, which can be attributed to the capacitive currents of the iridium oxide and Pt oxides, respectively. All the bifunctional catalysts present an ORR onset of ~1.0 V, similar to those of the commercial Pt-black and Pt/C catalysts. Materials’ activity toward the ORR increase in the order Pt/IrO2 1:9 < Pt/IrO2 3:7 < Pt/IrO2 1:1 < Pt black < Pt/C, as observed from the half-wave potentials (E1/2) reported in Table 2. The trend in ORR activities of the IrO2-containing catalysts reflects the increase in the platinum content, and clearly evidences that the Pt/IrO2 1:1 catalyst would provide better performance under practical URFC operating conditions. As expected, the Pt/C ORR activity is higher than those of the bifunctional Pt/IrO2 and Pt black materials, which is attributed to the greater electrochemical active area of this catalyst (see Table 2) resulting from its reduced Pt particles size and the high surface area of the carbon support.

Figure 6. (a) ORR polarization curves, and (b) mass corrected Tafel plots at 5 mV s-1, in 0.5 M H2SO4 at 25 °C and rotation speed of 1600 rpm. The catalysts ORR mass activities obtained with the currents normalized by the Pt mass in the catalysts active layers are presented in the mass-transport corrected Tafel 13 ACS Paragon Plus Environment

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plots in Figure 6b. Normalization regarding the Pt mass in the electrode was made for evidencing eventual synergetic interactions of Pt with IrO2 for the catalysts with different Pt/IrO2 proportions. These results show that all the curves exhibit two linear regions with corresponding Tafel slopes close to 60 and 120 mV dec-1, similar to those reported for Pt-based catalysts57, 58, and indicating that the presence of IrO2 does not alter the ORR mechanism and/or the rate determining step. For the Pt-IrO2 materials some deviations regarding the slopes of the Tafel lines are apparent in the low current density range, and this is possibly related to the deviations caused by the IrO2 double layer charging. The Tafel plots show that for E < 0.9 V, the mass activity of the IrO2containing materials increases for the catalysts with lower Pt content, resulting in a tendency opposite to that reported from the analysis of the polarization curves in Fig. 6a, i.e, Pt/IrO2 1:9 > Pt/IrO2 3:7 > Pt/IrO2 1:1. Also, it has to be remarked that the mass activity of the Pt/IrO2 1:9 material for the ORR is higher than that of the Pt/C catalyst, commonly employed in PEMFC cathodes. More specific analysis of the catalysts’ activities were made by using the current densities normalized by the electrode’s geometric area (j) and the mass activities (jk) obtained through the Koutecky-Levich diagrams (not shown), both at 0.85 V, as presented in Table 2. As expected, for the IrO2-containing catalysts the current densities normalized by the geometric area denote a decrease of the overall activity as a consequence of the decrease in the platinum content. A striking point is that, although clearly smaller than for Pt/C, the j value for the Pt/IrO2 1:1 is almost identical to that of the Pt black material, even with the former having only half the platinum content. These results can be attributed to the higher specific areas of the Pt/C and the bifunctional Pt/IrO2 1:1 material, as compared to Pt black (Table 2). Usually, platinum-iridium-based bifunctional catalysts described in the literature have ratios of these metals of about 1:1. Regarding the mass activities (jk) for this proportion, Kong et al.27 reported jk values at 0.85 V of 38.9 mA mgPt-1 for a Pt catalysts supported on a template-synthesized IrO2, and 30.7 mA mgPt-1 for the material in which a commercial IrO2 support was used. In another work, Kong et al.35 obtained jk values of 15.1 mA mgPt-1 for a Pt/IrO2 catalyst and 27.4 mA mgPt-1 for a Pt-Ir/IrO2-PtO2 material. Here, the values obtained for the 1:1, 3:7, 1:9 bifunctional catalysts are 35.7, 44.7, and 107.4 mA mgPt-1, respectively. A further increase in the mass activity for the ORR might be obtained with lower platinum loadings, as indicated by the mass activities for ORR of Pt/C catalysts with various Pt loadings51. However, excessive decrease of the Pt load would drastically reduce the performance of the electrode, which may difficult its practical applications. Another point to be considered is that in a real URFC system and for a Pt:Ir 1:1 catalyst, the current generation in operational conditions is substantially higher for the OER than for the ORR23, 59, 60. Similar observation is made for the present catalysts, and this, as expected, is somewhat more significant for Pt/IrO2 1:9. Table 2 – Electrochemically active surface area (Aecsa), calculated numbers of transferred electrons (n), half-wave potentials (E1/2), and ORR/OER current densities at 0.85 V and 1.55 V for the studied Pt/IrO2 catalysts. 14 ACS Paragon Plus Environment

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Aecsa / mPt2 gpt-1 28.6 Pt/IrO2 1:9 18.9 Pt/IrO2 3:7 14.1 Pt/IrO2 1:1 8.3 Pt black 35.3 Pt/C – IrO2

n 4.0 4.0 3.9 3.8 3.9 –

E1/2 / mV 850 860 870 880 907 –

ORR 0.85 V j/ jk / -2 mA cm mA mgPt-1 1.64 107.4 2.05 44.7 2.47 35.7 2.55 14.6 3.14 48.3 – –

OER 1.55 V j/ jm / -2 mA mgIr-1 mA cm 8.42 25.0 5.41 20.3 3.62 18.8 – – – – 12.1 31.8

The catalysts activities for the OER are displayed in Figure 7. The polarization curves in Fig. 7a show that the OER activities follow an opposite trend to that obtained for the ORR, increasing with the elevation of the iridium oxide content in the studied materials. The values of current density per geometric area (j) at 1.55 V shown in Table 2 indicate a decrease of the j value to about 70%, comparing results for the Pt/IrO2 1:1 and IrO2. With respect to the values of mass activity (jm), all synthesized Pt/IrO2 catalysts were less active than the IrO2; the 1:9 ratio catalyst exhibited a current density of 25.0 mA mgIr-1, while for the 1:1 ratio catalyst the current density found was 18.8 mA mgIr-1, representing a drop of ≈21% and ≈41%, respectively, compared to the IrO2 material. The lower mass activity of the Pt/IrO2 catalysts versus OER may be associated to the larger crystallite sizes of the iridium oxide in the bifunctional materials (Table 1). Kong et al.61 have supported Pt nanoparticles on porous amorphous iridium oxide with a molar Pt:IrO2 ratio of 1:1; an activity of 29 mA mgIr-1 at 1.55 V was reported for this material, corresponding to an increase of 28% in comparison to a Pt/IrO2 catalyst in which a commercial iridium oxide was used. The higher catalytic activity of the Pt/IrO2 material reported by Kong et al. compared to that of the Pt/IrO2 1:1 catalyst synthesized here can be attributed to the amorphous structure of the iridium oxide, which is known to be more active than crystalline IrO262-64. Regarding Pt/IrO2 catalysts with crystalline IrO2, Kong et al.27 obtained a current density of 10.6 mA mg-1 at 1.55 V for a Pt/IrO2 1:1 material, which is 44% lower than the current density obtained for the Pt/IrO2 1:1 (Table 2). The OER and ORR activities reported for the Pt/IrO2 catalysts with varying Pt:Ir ratios show that the Pt/IrO2 1:9 material presents the best balance between the OER and ORR mass activities. However, to determine the best composition of Pt/IrO2 bifunctional materials, it is necessary to evaluate their stability in both OER and ORR operation conditions, and this is shown below.


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Figure 7. (a) OER polarization curves and (b) mass normalized OER polarization curves of the Pt/IrO2 catalysts at 5 mV s-1, in 0.5 M H2SO4 at 25 °C and rotation speed of 1600 rpm.

Stability of Pt/IrO2 catalysts Three aging test protocols at 100 mV s-1 were used to evaluate the Pt/IrO2 catalysts stabilities, comprising 1000 cycles in the ORR region (0.1 – 1.1 V), the OER region (1.1 – 1.6 V) and ORR-OER regions (0.1 – 1.6 V). The polarization responses for the ORR and OER of the Pt/IrO2 materials before and after cycling in the ORR-OER potential window (0.1 – 1.6 V) are shown in Figs. 8 and 9. Corresponding results obtained after 1,000 potential cycles in the ORR and OER regions are shown in Figures S6 to S9. Figures also include the average normalized currents and standard deviations obtained for three independent measurements representing the decrease of the activity after successive 200 potential cycles, at 0.8 V for the ORR and at 1.55 V for the OER. Contrarily to what is seen for the ORR and OER cycling protocols, the polarization curves in Fig. 8 show that the catalytic performance of the Pt/IrO2 for the ORR massively decreased after the 1,000 potential cycles between 0.1 – 1.6 V. At the end of this aging protocol, the ORR currents at 0.8 V of the Pt/IrO2 1:1 and 3:7 materials dropped to ≈50% and ≈40% of their initial values, respectively, while the Pt/IrO2 1:9 catalyst keeps only 5% of the initial current. A higher platinum content is therefore necessary in order to maintain the ORR activity of Pt/IrO2 bifunctional catalysts when cycling in the ORR-OER potential window. For the other aging protocols, the maximum decay of performance was about 10% (Figs. S6 and S8).


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Figure 8. (a-c) ORR polarization curves at the beginning and after 1,000 potential cycles between 0.1 – 1.6 V. Curves were obtained at 5 mV s-1, in 0.5 M H2SO4 at 25 °C and rotation speed of 1600 rpm. (d-f) ORR current ratio (IORR/IInitial,ORR) at 0.8 V after every 200 aging cycles.

Figure 9. (a-c) OER polarization curves at the beginning and after 1,000 potential cycles between 0.1 – 1.6 V. Curves were obtained at 5 mV s-1, in 0.5 M H2SO4 at 25 °C and rotation speed of 1600 rpm. (d-f) OER current ratio (IORR/IInitial,ORR) at 0.8 V after every 200 aging cycles. Kong et al. evaluated the stability of a Pt/IrO2 1:1 materials using a stability protocol of 5,000 potential cycles between 0.05 – 1.2 V; although the catalytic performance was 17 ACS Paragon Plus Environment

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not tested, the author observed a decrease in the Pt electroactive area of 28.8%29 and 35.47%28. Figure S10 displays the CV profiles obtained for the Pt/IrO2 1:1 at beginning and after the 1,000 potential cycles between 0.1 – 1.1 V. After this aging protocol, the Pt electroactive surface area calculated by the integration of the hydrogen desorption peaks reduces by 33%, a value close to those reported by Kong et al., but that is not enough to significantly reduce the ORR catalytic performance of the material. When the electrode is cycled in the ORR potential domain (Fig. S7), there is reasonably small change in the OER activity. However, when cycling includes the OER domain (Figs. 9 and S9), all the catalysts exhibit large drop of the OER activities, especially after the first 200 cycles, and then the current tend to become more stable. Nonetheless, after the 1,000 aging cycles between 0.1 – 1.6 V, the OER currents at 1.55 V presented by the Pt/IrO2 materials are lower than the final currents observed after cycling only in the OER potential window (≈45% for the Pt/IrO2 1:9, and 30% for the others compositions), confirming that the ORR-OER cycling is the most aggressive aging protocol for all Pt/IrO2 materials. The OER activity loss after the first 200 cycles was also observed in the study of IrO234 and this has been associated, at least in part, to a blockage of the catalysts pores by the O2 bubbles65. It has to be pointed that no changes in the glassy carbon substrate were observed during either the OER performance tests or the stability studies. This was confirmed by electrochemical impedance spectroscopy measurements performed before and after cycling in the OER potential window, which did not evidence any significant change in the in electrode resistance as seen from the higher frequency region of the Nyquist diagram34. Kong et al.29 also evaluated the stability of Pt/IrO2 1:1 using 5,000 potential cycles between 1.2 – 1.6 V as aging protocol. The authors only evaluated the OER peak current degradation and observed a decay of just 20.4% after the end of the 5,000 cycles. The higher stability can be attributed to the lower catalyst loading, which makes the catalytic layer less thick, facilitating the removal of the oxygen bubbles that block the active sites of the catalyst. In agreement to the presented results, a study of the stability of platinum nanorods decorated with iridium nanodots evidenced a greater decrease in the hydrogen desorption charge and OER currents after cycling in the ORR-OER potential window, compared to a protocol in which the catalyst was aged only in the OER region66. These differences in stabilities can be explained based on the instability of Pt when cycled between the ORR and OER potentials, which causes the repeated formation and reduction of hydroxide/oxide layer on Pt66. Cherevko et al. evaluated the dissolution of polycrystalline Pt, Ir, and other noble metals, during the oxygen evolution in acidic media using the inductively coupled plasma mass spectrometry (ICP-MS) coupled to a scanning flow cell16; all the noble metals exhibited a transient dissolution during oxide formation/reduction, with the dominant dissolution process following the reduction of irreversible Pt- and Ir-oxides. Cherevko et al.64 also evaluated the dissolution of an electrochemically grown hydrous iridium oxide. Both anodic and cathodic dissolutions were observed for the prepared electrode, and the cathodic dissolution was also the dominant process. One of the causes for the higher degradation of the Pt/IrO2 materials


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during the ORR-OER can therefore be attributed to the dissolution of the Pt and IrO2 nanoparticles. Further investigation of the aging processes of the materials during the three stability protocols was performed by identical location transmission electron microscopy (ILTEM) analysis for the Pt/IrO2 1:1 catalyst, the most common composition for this kind of catalyst found in the literature. The IL-TEM micrographs obtained after cycling in the ORR region (first protocol) are shown in Figures 10a,b and S11. Two main degradation processes are highlighted in these Figures: i) the particles growth as indicated by the larger particles marked by the red circles, and ii) the particles detachment signed by the green arrows. Both processes contribute to the reduction of the Pt electroactive surface area shown in Fig. S10. However, other process like Pt dissolution, and Ostwald ripening, cannot be discarded, but they are difficult to detect using the IL-TEM due to the agglomeration of the Pt/IrO2 material. Perez Alonso et al. evaluated the stability of Pt/C aged in the ORR conditions; after 3,000 cycles between 0.6 – 1.1 V, and the authors could observe some particle loss, migration and mild sintering, but the Pt nanoparticles have retained their shape67. Here, the particles growth may have been accentuated due to the lower potential limit used in the aging test, and also due to the large agglomeration of the particles, which facilitates the coalescence process. IL-TEM micrographs were also collected after cycling the Pt/IrO2 1:1 catalyst in the OER region (second protocol) and the images are displayed in Figs. 10c,d and S12. A more stable profile can be observed in the IL-TEM micrographs, corroborating with the hypothesis that the OER activity decreases seen in Fig. S9 are due mainly to the accumulation of O2 bubbles in the catalysts pores, rather than to a degradation process. However, although some stable regions are seen (highlighted by the blue circles), some particle loss (red arrows) and especially particles shrinking (green circles), which have already been reported for IrO2 materials cycled in the OER region34, can be included as causes of the catalyst degradation. Zana et al.68 evaluated the influence of the aging test conditions on the degradation mechanism of Pt/C using IL-TEM. One of the protocols used was 12,000 cycles between 1.0 – 1.5 V at 500 mV s-1. The authors observed that the particles detachment from the carbon support, most likely due to the carbon corrosion, is the main degradation mechanism. The authors also underline that the Pt dissolution may be inhibited due to the constant presence of oxide layer in the Pt surface at the potential window used in the protocol. Here, the relative small particle detachment particles can be associated to the higher stability provided by the IrO2.


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Figure 10. IL-TEM micrographs of the Pt/IrO2 1:1 catalysts (a,c,e) before and (b,d,f) the three aging protocols. Finally, the stability of the Pt/IrO2 1:1 material in the ORR-OER potential region was also investigated using IL-TEM, and these results are shown in Figs. 10e,f and S13. Figures 10e,f show low magnification images of an analyzed region before and after the ORR-OER aging protocol; as already evidenced by the results of the electrochemical experiments (Figs. 8 and 9), images in Figs. 10e,f confirm that the third protocol is the most aggressive for the Pt/IrO2 catalysts. After the aging process, the presence of large particles is detected, probably arising from the process of dissolution and re-deposition of platinum, which may influence negatively the activity of the bifunctional catalysts for the ORR. In a recent work, inductively coupled plasma mass spectrometry and identical location transmission electron microscopy measurements exclude the formation of metallic iridium when electrochemically- or thermally-prepared iridium oxides are cycled between 0.05 – 1.6 V vs. RHE69. Zana et al.68 also evaluated the stability of Pt/C after 3,600 potential cycles between 0.4 – 1.4 V. Besides some particle detachment, the authors noticed particle growth due to particles migration and coalescence and/or due to the Ostwald ripening process. Here, the potential limits (0.1 – 1.6 V) may increase the dissolution/re-deposition rates, making the Pt/IrO2 catalysts very unstable in the ORR-OER region.

Conclusions The activity Pt/IrO2 bifunctional catalysts with varying Pt:Ir ratios were investigated for the oxygen evolution/reduction reactions. Results had shown that the overall 20 ACS Paragon Plus Environment

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catalysts activity for the ORR increases as the platinum content in the Pt/IrO2 materials is raised, while an opposite trend in seen for the OER. Regarding the mass normalized activities, Pt/IrO2 1:9 material exhibit the better balance between the catalytic performances for the ORR and OER, while the Pt/IrO2 1:1 catalyst have a performance similar to others bifunctional catalyst reported in the literature. The stability of the Pt/IrO2 bifunctional catalysts were evaluated using three aging protocols. The ORR activities of the Pt/IrO2 materials are maintained while cycling in the ORR and OER regions, while a large drop in the activities are observed when the electrodes are cycled in the ORR-OER potential window. The Pt/IrO2 activities for the OER decreases when cycling in both the OER and ORR-OER regions, with the O2 bubbles accumulation playing an important role in the activity loss. IL-TEM analysis shows that the catalysts dissolution are not the only responsible for the catalysts instability. Degradation processes such particles growth, shrinking, and detachment were observed. However, IL-TEM micrographs obtained after cycling in the ORR-OER region confirms that Pt/IrO2 materials are very unstable in that condition.

Supporting information Energy dispersive X-ray spectroscopy (EDX) results; high-resolution TEM micrograph of the Pt/IrO2 3:7 catalyst; EDX elements mapping of the Pt/IrO2 1:9 and 1:1 materials; X-ray photoelectron spectroscopy (XPS) survey spectra, and the binding energies and atomic percentage of the Pt4f7/2 and Ir4f7/2 components; oxygen reduction reaction polarization curves; stability results for the studied catalysts.

Acknowledgments The authors acknowledge the São Paulo Research Foundation (FAPESP – 2013/16930-7), Brazil, for the financial support, and the Laboratório Nacional de Nanotecnologia – LNNano for the XPS measurements (Project XPS – 21273).

References (1) Pettersson, J.; Ramsey, B.; Harrison, D. J. Power Sources 2006, 157, 28-34. (2) Gabbasa, M.; Sopian, K.; Fudholi, A.; Asim, N. Int. J. Hydrogen Energy 2014, 39, 1776517778. (3) Wang, Y.; Leung, D. Y.; Xuan, J.; Wang, H. Renew. Sust. Energ. Rev. 2017, 75, 775-795. (4) Wang, Y.; Leung, D. Y.; Xuan, J.; Wang, H. Renew. Sust. Energ. Rev. 2016, 65, 961-977. (5) Gorlin, Y.; Jaramillo, T. F. J. Am. Chem. Soc. 2010, 132, 13612-13614. (6) Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J. Nat. Chem. 2011, 3, 79-84. (7) Malkhandi, S.; Yang, B.; Manohar, A.; Manivannan, A.; Prakash, G. S.; Narayanan, S. J. Phys. Chem. Lett. 2012, 3, 967-972. (8) Hardin, W. G.; Slanac, D. A.; Wang, X.; Dai, S.; Johnston, K. P.; Stevenson, K. J. J. Phys. Chem. Lett. 2013, 4, 1254-1259. (9) Jin, C.; Cao, X.; Zhang, L.; Zhang, C.; Yang, R. J. Power Sources 2013, 241, 225-230. 21 ACS Paragon Plus Environment

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ABSTRACT GRAPHICS


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Figure 1 201x140mm (300 x 300 DPI)


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Figure 2 299x100mm (96 x 96 DPI)


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Figure 4 160x213mm (300 x 300 DPI)


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ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 10 319x212mm (96 x 96 DPI)


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Graphical Abstract 212x97mm (96 x 96 DPI)


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IrO2 bifunctional materials as catalysts for the

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