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IrO-TiO: a High-Surface Area, Active and Stable Electrocatalyst for Oxygen Evolution Reaction Emma Oakton, Dmitry Lebedev, Mauro Povia, Daniel F. Abbott, Emiliana Fabbri, Alexey Fedorov, Maarten Nachtegaal, Christophe Copéret, and Thomas J. Schmidt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03246 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017
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ACS Catalysis
IrO2−TiO2: A High-Surface Area, Active and Stable Electrocatalyst for Oxygen Evolution Reaction Emma Oakton,†,# Dmitry Lebedev,†,# Mauro Povia,‡ Daniel F. Abbott,‡ Emiliana Fabbri,‡ Alexey Fedorov,† Maarten Nachtegaal,‡ Christophe Copéret*,† and Thomas J. Schmidt*,†,‡ †
ETH Zürich, Department of Chemistry and Applied Biosciences, Vladimir Prelog Weg 1-5, CH-8093 Zürich, Switzerland ‡
Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
KEYWORDS: electrocatalysis, oxygen evolution reaction, water splitting, iridium oxide, operando X-Ray absorption spectroscopy
ABSTRACT: The utilization and development of efficient water electrolyzers for hydrogen production is currently limited due to the sluggish kinetics of the anodic process − the oxygen evolution reaction (OER). Moreover, state-of-the-art OER catalysts contain high amounts of expensive and low abundant noble metals such as Ru and Ir, limiting their large scale industrial utilization. Therefore, the development of low-cost, highly active and stable OER catalysts is a key requirement towards the implementation of a hydrogen−based economy. We have developed a synthetic approach to high surface area chlorine-free iridium oxide nanoparticles dispersed in titania (IrO2−TiO2), which is a highly active and stable OER catalyst in acidic media. IrO2−TiO2 was prepared in one step in molten NaNO3 (Adams fusion method) and consists of ca. 1-2 nm IrO2 particles distributed in matrix of titania nanoparticles with an overall surface area of 245 m2 g-1. This material contains 40 molM % of iridium and demonstrates improved OER activity and stability compared to commercial benchmark catalyst and state-of-the-art high surface area IrO2. Ex situ characterization of the catalyst indicates the presence of iridium hydroxo surface species, which were previously associated with the high OER activity. Operando X−ray absorption studies demonstrate the evolution of the surface species as a function of the applied potential, suggesting the conversion of the initial hydroxo surface layer to the oxo-terminated surface via anodic oxidation (OER regime).
INTRODUCTION Polymer electrolyte water electrolyzers (PEWEs) have been actively investigated in recent years for generating hydrogen using renewable energy.1 However, this technology is severely limited by the large overpotential associated with the oxygen evolution reaction (OER), a process that accompanies the hydrogen evolution reaction (HER) in water electrolysis.2 Among various electrode materials studied for PEWEs, IrO2 and RuO2, as well as their solid solutions feature reasonable stabilities and low overpotentials for OER2-4, but the high cost and low availability of Ru and especially Ir limit their large scale industrial application. To reduce the cost of OER catalysts, these noble metals can be partially substituted by other elements.5-8 Alternatively, Ru and Ir metal oxides can be combined with other robust oxides that effectively act as a catalyst support.3, 9 Advantages of the latter approach include the stabilisation of the small, active particles against sintering and increased catalytic activity, which in some cases can be attributed to the particle-support interactions.3, 10 Typical requirements for such catalyst supports include long−term stability, high surface area and sufficiently high electrical conductivity (approx.
> 0.1 S cm–1).10-11 Recent work has shown that titanium,12 tantalum13 and silicon14 carbides as well as titanium,10, 15 silicon,16 tin,17-18 and tantalum19 oxides are potentially suitable supports for OER catalysts. In the case of IrO2 supported on TiO2 it was shown that improving catalyst utilization is a compromise between the high surface area of the support and the minimal amount of IrO2 required to generate a conductive network of IrO2 nanoparticles.10 In that context, we have recently disclosed a synthesis route to high surface area iridium oxide dispersed in titania20 based on the reaction of titanium oxysulfate and iridium chloride in molten NaNO3 yielding metal oxides (socalled Adams fusion method).21 Such materials become sufficiently conducting from a nominal Ir loading of 40 molM% (55 wt%, conductivity 0.49 S cm–1).20 However, the use of IrCl3 leads to a chlorine contaminated material, which complicates its evaluation as an OER catalyst due to competing Cl2 evolution.22 Here, we report a Cl-free synthesis of iridium oxide nanoparticles dispersed in titania (IrO2−TiO2) and demonstrate the improved OER catalytic properties of this material when compared to both high surface area IrO223 and a benchmark commercial catalyst provided by Umicore GmbH & Co. KG.24-25 A combination of ex situ catalyst
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characterization and operando XAS studies suggest the presence of the iridium hydroxo surface layer, which transforms into an oxo-terminated surface upon anodic oxidation.
EXPERIMENTAL SECTION Materials and general methods All chemicals were used as received unless otherwise stated. Iridium(III) acetylacetonate (98%) was purchased from Strem Chemicals. Titanium (IV) oxysulfate (99.999%), concentrated H2SO4 and sodium nitrate (> 99.0 %) were purchased from Sigma Aldrich. Distilled water was further purified using a Merck Millipore Synergy Ultrapure Water System and had a resistivity of 18.2 MΩ cm at 25 °C. The commercial catalyst was provided by Umicore GmbH & Co. KG and used as received. Powder XRD experiments were performed on a STOE Padi Diffractometer in Debye−Scherrer Mode (2θ) equipped with a Dectris Mythen 1K area detector using Cu Kα1 radiation. N2 adsorption-desorption analysis was conducted using a BEL-Mini device supplied by BEL Japan Inc. All samples were pre-treated at 150 °C under vacuum for 16 h on a BEL-Prep instrument also from BEL Japan Inc. Results were fitted using BEL-Master program and BET theory.26 HAADF-STEM images and EDX maps were recorded on a Hitachi HDCS2700CS microscope. Electrical conductivity values were determined from chronoamperometric measurements of compressed powder disks with thicknesses in the range 100 – 1000 μm. The conductivities determined for a minimum of three disks were averaged to give the reported values. X−ray photoelectron spectroscopy (XPS) measurements were performed using a VG ESCALAB 220iXL spectrometer (Thermo Fischer Scientific) equipped with an Al Kα monochromatic source (15 kV/150 W, 500 µm beam diameter) and a magnetic lens system. The binding energies of the acquired spectra were referenced to the C 1s line at 284.6 eV. Background subtraction has been performed according to Shirley27 and the atomic sensitivity factors (ASF) of Scofield were applied to estimate the atomic composition.28 X−ray absorption (XAS) spectroscopy XAS spectra were collected at the SuperXAS beamline of the Swiss Light Source (SLS) (PSI, Villigen, Switzerland). The SLS operates under top up mode at 2.4 GeV electron energy and a current of 400 mA. The incident beam was collimated by a Ru−coated mirror at 2.8 mrad and passed through a channel−cut Si (111) monochromator. The beam intensity was ∼ 1012 ph s−1 and focused with a Rh coated toroidal mirror (at 2.8 mrad) down to 100 × 100 μm at the sample position. The beamline energy was calibrated with Pt reference foil to the Pt LIII−edge position at 11564 eV. Ionization chambers filled with N2 at 2 bars were used for XAS detection in transmission mode, where a Pt reference foil was measured congruently with the sample between the second and third ionization chamber. The SuperXAS beamline29 allowed for the rapid collection of 120 spectra during a measurement time of 60 seconds, which were then averaged. X−ray absorption spectra were
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analyzed using the Demeter program package30, which included energy calibration, background subtraction and edge step normalization. The resulting spectra were converted to the photoelectron wave vector k (in units Å−1) by assigning the photoelectron energy origin, E0, corresponding to k = 0, to the first inflection point. The resulting χ(k) functions were weighted with k2 to compensate for the dampening of the XAFS amplitude with increasing k. These χ(k) functions were Fourier transformed over 3−17 Å−1 for the dry catalyst samples and for the in situ measurements over 3−12 Å−1. The theoretical model used for the EXAFS fitting was generated from the rutile−type IrO2 structure using the FEFF6.2 library. A fitting window of 1–3.75 Å and 1–3 Å in R-space was used for the dry catalysts and in situ studies respectively. Details of the in situ XAS spectroscopy as well as EXAFS modeling have been previously described25 and are summarized in the Supporting information. Electrochemical measurements The electrochemical oxygen evolution activity of the prepared materials was evaluated in a standard single−compartment 3−electrode cell using a rotating disk electrode (RDE) setup (Pine Instruments, USA) and a BioLogic VMP−300 potentiostat following the thin-film RDE approach.31 All glassware was vigorously cleaned in a solution of 98% H2SO4 and 30% H2O2 then boiled in Milli−Q water several times before use. Catalyst ink suspensions were prepared using 10 mg of IrO2 catalyst, 1.0 mL Milli−Q H2O, 4.0 mL isopropyl alcohol, and 20 µL of 5 wt.% Nafion (Nafion 117, Sigma Aldrich) solution. The ink suspensions were sonicated for 30 minutes and spin coated at 50 rpm on freshly polished glassy carbon disk electrode (0.196 cm2) to give a total catalyst loading of 100 µgcat cm−2. A piece of platinum served as the counter electrode and a Hg/HgSO4 electrode served as the reference electrode. All potentials reported herein refer to the RHE scale and have been corrected for the ohmic drop in solution. The 0.1 M HClO4 electrolyte was prepared from 60% HClO4 (Kanto Chemical Co., Inc.) and was saturated with synthetic air during all measurements. Electrodes were cycled in the potential range of 1.0 to 1.4 V at 50 mV s−1 until a steady surface capacitance was measured prior to collecting polarization data. Polarization curves are derived from the steady−state chronoamperometric measurements. The potential was gradually stepped from 1.2 V to 1.6 V while holding for 1 minute at each potential. Electrochemical impedance spectroscopy measurements were recorded in the range of 15 kHz to 1 Hz with an amplitude of 10 mV. To test the electrochemical stability of the catalysts the RDE set up has been tilted of about 15° compared to its standard vertical configuration keeping a rotation speed of 2900 rpm for the whole stability test. The use of the rotational drying method (50 rpm) to deposit the catalysts on the glassy carbons results in uniform electrodes,32 while the fast rotation of the titled RDE set up allows removing most of the gas bubbles formed when the electrode is polarized in the OER regime. The use of both strategies allows a more reliable evaluation of the intrinsic electrochemical stability of the catalysts.
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During the stability measurements the potential was switched between 1.0 and 1.6 V for 500 cycles while holding for 10 seconds at each potential. The normalized current values are based on the current reading at 1.6 V after every 100 cycles. All reported measurements were repeated at least 3 times to ensure reproducibility. Synthesis of IrO2−TiO2 catalyst TiOSO4⋅0.6H2SO4⋅1.3H2O (0.307 g, 1.28 mmol) was dissolved in 15 mL of water with 1.5 mL concentrated H2SO4 followed by addition of Ir(acac)3 (0.408 g, 0.828 mmol) and NaNO3 (7.5 g, 88 mmol). After homogenizing and drying at 60 °C in vacuo (60 mbar), the solid mixture was further dried in a Muffle furnace in ambient atmosphere at 150 °C for 2 h before heating to 350 °C (3 °C min−1) and holding at this temperature for 1 h. The resulting powder was washed with water and dried in a vacuum oven at 150 °C for 16 h.
RESULTS AND DISCUSSION Preparation and characterization of IrO2−TiO2 Iridium acetylacetonate (Ir(acac)3) and titanium oxysulfate sulfuric acid hydrate (TiOSO4·xH2SO4·yH2O) were chosen as Cl-free molecular precursors for material synthesis. IrO2−TiO2 with 40 molM% of Ir was prepared by heating a homogenized mixture of these precursors and sodium nitrate at 350 °C for 1 h followed by washing and drying steps. The obtained black powder has a surface area of 245 m2 g−1 (the sample is denoted here as IrO2−TiO2−245), according to BET analysis of N2 adsorption-desorption isotherms (Fig. S1). Our previous results showed that synthesis of pure IrO2 using identical conditions23 results in the material with the surface area of 150 m2 g−1. This allows us to conclude that in IrO2−TiO2−245 iridium and titanium oxides have a comparable contribution to the final surface. The electrical conductivity of this material was 0.26 S cm–1, which is comparable to that of IrO2−TiO2 material with the same nominal Ir loading prepared from IrCl3 (0.49 S cm-1).20 Powder X-ray diffraction studies show the formation of rutile IrO2 and TiO2 as well as anatase TiO2 nanoparticles. However, the inherent similarity of the rutile IrO2 and TiO2 powder patterns complicates further interpretation (Fig. 1a). The lack of scattering intensity in the region of ca. 28 °2θ was previously observed for the small rutile particles.33 The broadness of peaks in the diffractogram indicates the formation of small particles. In line with the XRD data, high angle annular dark field scanning electron microscopy (HAADF-STEM) (Fig. 1b, c) reveals the presence of small bright particles (ca. 1-2 nm) assigned to IrO2. Elemental mapping by energy dispersive X-ray spectroscopy (EDXS) shows the formation of iridium-rich regions in IrO2−TiO2 sample. These regions are responsible for the sufficient conductivity of the material, consistent with previous reports on use the percolation theory to model the electrical conductivity of IrO2−TiO2 catalysts (Fig. S2).10, 20
Fig. 1. Powder X-ray diffraction pattern of IrO2−TiO2−245 34 with reference patterns from ICDD , the feature at ca. 22 °2θ originated from incomplete subtraction of sample holder background (a); HAADF-STEM images of IrO2−TiO2−245 (b, c).
According to X−ray photoelectron spectroscopy (XPS), the surface of IrO2−TiO2−245 contains 43 ± 1 molM% of Ir and 57 ± 1 molM% of Ti, which is in good agreement with the nominal iridium loading of 40 molM%. Ir 4f, Ti 2p, O 1s and the survey XPS spectra of IrO2−TiO2−245 are given in the Supporting Information (Fig. S3). XAS spectroscopy studies IrO2−TiO2−245 was further characterized by X-ray absorption spectroscopy and compared to the high surface area IrO223 and a benchmark industrial catalyst provided by Umicore GmbH & Co. KG with an iridium loading of ∼ 66 molM%;24-25 these catalysts are denoted here as IrO2−150 and IrO2−TiO2−Umicore with surface areas of 150 m2 g–1 and 34 m2 g−1 by BET analysis, respectively (powder XRD pattern of IrO2−TiO2−Umicore is shown in Fig. S4). X-ray absorption near-edge structure (XANES) spectra recorded at the Ir LIII edge (Fig. 2a) probe 2p to 5d electronic transitions and thus are sensitive to changes in the oxidation state of Ir. XANES spectra show that IrO2−TiO2−245 has an absorption edge energy of 11215.7 eV, which is close to that of IrO2−150 and lower than absorption edge energy of IrO2−TiO2−Umicore (see Table 1). This suggests that Ir centers in IrO2−TiO2−245 are reduced with respect to the commercial sample, which is rationalized by the presence of a larger fraction of Ir3+. The Fourier transformed extended X-ray absorption fine structure spectra (FT-EXAFS, Fig. 2b) of IrO2−TiO2−245 and IrO2−TiO2−Umicore catalysts significantly differ from each other. In the case of IrO2−TiO2−245, the larger surface to volume ratio of the small IrO2 particles (ca. 1-2 nm) results in minor contribution of the scattering shells after 4 Å.
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Table 1. XAS data of OER catalysts Catalyst
Absorption edge energy, eV
Ir–O length, Å
IrO2−TiO2−245
11215.7
2.023 (8)
IrO2−150
11215.9
2.031 (8)
IrO2−TiO2−Umico re
11216.4
1.999 (7)
The EXAFS spectra of the materials were fitted using a two-shell model based on the IrO2 structure including Ir−O and Ir−Ir scattering paths (details of the model and fits are given in the Supporting Information, Table S1 and Fig. S5). The refined Ir–O bond length within the first shell for IrO2−TiO2−245 is comparable to IrO2−150 and 0.02 (1) Å longer compared to IrO2−TiO2−Umicore (see Table 1 and fit of the real part of FT-EXAFS in Fig. S5). Since the Ir–O bond distance within the first shell is highly sensitive to the iridium oxidation state,23, 35 the longer Ir−O distance indicates a lower oxidation state of Ir, consistent with the presence of Ir3+ as determined by XANES spectroscopy. The average bond length for Ir−O in IrOx containing Ir3+ has been reported to be roughly 2.02 Å, whereas the bond length of Ir4+ in the IrO2 rutile oxide is closer to 1.98 Å.36-37 Because of the small IrO2 particle size in IrO2−TiO2−245, XAS is mostly probing the nature of the surface sites of the IrO2 nanoparticles. Therefore, similar to our previous observations,23 the shift in the iridium oxidation state toward +3 observed by both XANES and EXAFS could be explained by the formation of iridium hydroxo species (*OH) on the particle surface.
Fig. 2. Ir LIII-edge XAS data of OER catalysts: XANES spectra (a); Fourier transformed EXAFS spectra (solid line for experimental data and dash line for the fits) with k-weight 2 (b).
Electrochemical studies The activity and stability of IrO2−TiO2−245 towards OER electrocatalysis was investigated using rotating disk electrode experiments. The Tafel slope of IrO2−TiO2−245 is slightly lower in comparison to IrO2−TiO2−Umicore (Fig. 3a, Table 2) but similar to IrO2−150. While the rate determining step (and reaction mechanism) cannot unambiguously be identified based on the Tafel slope alone,38 the similar Tafel slope, synthetic approach and nature of the active oxide (IrO2) in IrO2−TiO2−245 and IrO2−150 indicate that the OER likely proceeds by the same mechanism, and presence of TiO2 does not affect
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the reaction pathway. In order to compare the OER activities, the potential required to achieve a current of 10 A gIr– 1 was taken as an activity descriptor with low potentials correlating to high activities.2 A significant reduction of the overpotential compared to the commercial Umicore catalyst was found for IrO2−TiO2−245 (Table 2). At 1.525 V, IrO2−TiO2−245 shows more than 5 times higher mass current density than the IrO2−TiO2−Umicore catalyst. Similar to the case of the high surface area IrO2,23 high OER activity of the IrO2−TiO2−245 could possibly be explained by the presence of surface iridium hydroxo species. Surface Ir hydroxyls were shown to be the key OER intermediates,39-41 and their increased presence was related to the high catalytic activity.23, 41 Slightly lower overpotential of the IrO2−TiO2−245 catalyst compared to the unsupported high surface area IrO2 may result from better distribution and accessibility of the IrO2 particles.
Table 2. Comparison of OER activities and stabilities of the catalysts Catalyst
Tafel slope, −1 mV dec
E vs. RHE at −1 10 A gIr , V
J at 1.525 V vs. RHE, −1 A gIr
IrO2−TiO2−245
42 (3)
1.485 (5)
70 (3)
IrO2−150
44 (2)
1.497 (4)
44 (3)
IrO2−TiO2−Umicore
52 (3)
1.520 (7)
14 (4)
The stability of each sample was studied in order to evaluate the long term performance of the catalysts. The stability tests are intended to simulate the start/stop behavior of an electrolyzer by varying potential stepwise between a potential close to the open circuit value, i.e. where no OER current is observed (1.00 V) and a potential where an appreciable OER current is observed, i.e. J > 10 A g−1 (1.60 V). Thus the stability protocol consists of stepping the applied potential between 1.0 and 1.6 V while holding for 10 seconds at each potential for 500 cycles. Further details of the measurement procedure are given in the experimental section. The normalized current values are based on the current reading at 1.6 V after every 100 cycles (Fig. 3b.). The IrO2−TiO2−245 catalyst shows a smaller decrease in current after 500 cycles in comparison to IrO2−150 and IrO2−TiO2−Umicore. This stability enhancement when compared to the Umicore catalyst may be assigned to a more homogenous distribution of the IrO2 nanoparticles within titania (lower loading, smaller particle size). Moreover, the improved stability of IrO2−TiO2−245 in comparison to the unsupported high surface area IrO2 (IrO2−150) could be a result of particle – support interactions between IrO2 and TiO2. Interestingly, the synthesis of pure TiO2 by the Adams fusion method leads to the formation of the pure anatase phase, while addition of the Ir precursor results in the predominant formation of the rutile TiO2.20 Such a transition suggests the presence of particle – support interactions, such as the epitaxial particle growth and / or partial iridium –
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titanium intermixing (supported by almost identical unit cell parameters of IrO2 and rutile TiO2).34
less pronounced compared to the in situ XANES data discussed above.
Fig. 3. Tafel plots (a) and stabilities (b) of the Ir OER catalysts.
Operando XAS spectroscopy studies In order to investigate the structural changes of the iridium oxide surface as a function of the applied electrode potential, the IrO2−TiO2−245 sample was further studied using operando XAS spectroscopy at the Ir LIII edge. Fig. 4 presents the normalized XANES and Fourier transformed EXAFS spectra recorded during a typical OER polarization measurement. Results of the operando XAS studies of IrO2−TiO2−245 material are similar to what is found for high surface area iridium oxide.23 As the electrode potential is stepped into the OER regime, the Ir−edge position gradually shifts towards higher energies. This effect is attributed to the fact that IrO2−TiO2−245 initially exists as a mixture of Ir3+ and Ir4+ sites and transitions to higher oxidation state upon anodic polarization. The Ir4+/Ir3+ and Ir5+/Ir4+ redox couples were reported to take place at ca. 0.9 V and 1.2 V vs. SHE respectively.42 The CV studies of IrO2−TiO2−245 in the region from 1.0 V to 1.4 V do not reveal the Ir5+/Ir4+ redox couple, but its presence was reported to strongly depend on the sample preparation procedure (i.e. precursors, the annealing process, etc.).43 The observed transition to higher oxidation states upon anodic polarization likely represents the transformation from a hydroxide to an oxide terminated surface. This observation is in a good agreement with previous XANES studies of hydrous iridium oxide films.44-45 The effect is partially reversible as the edge position shifts to lower energies upon returning to 1.00 V, indicating that the oxidized fraction of iridium (Ir oxide surface layer) can be partially reduced back to its initial state (Ir hydroxide surface layer) during the cathodic polarization (Fig. 4a and 4c). The in-situ EXAFS spectra were fitted using the same approach as for dry catalysts (Fig. 4b). Further information regarding the best fit parameters can be found in the Supporting Information (Table S2 and Fig. S6). The Ir−O bond length obtained from the EXAFS fitting (Fig. 4d) gradually changes from ca. 2.01(1) Å at 1.00 V to 1.99(1) Å at 1.50 V (Table S2). The shortening of the Ir−O bonds represents the transition of IrO2−TiO2−245 from a mixed Ir3+/4+ oxidation state to a structure more heavily populated by Ir4+, which is in line with the XANES data. Upon returning to 1.00 V during the cathodic polarization, the Ir−O bond length slightly increases, but the effect is much
Fig. 4. In situ XAS data of IrO2−TiO2−245 recorded at different electrode potentials: XANES spectra (a); Fourier transformed EXAFS spectra (solid line for experimental data and nd dash line for the fits) with k-weight 2 (b); 2 derivative of XANES absorption edge (c) and the EXAFS-determined Ir−O bond distance as a function of the applied potential (d).
Therefore, the observed partial oxo to hydroxo surface layer transformation upon cathodic polarization is different from fully reversible transformation observed previously for unsupported high surface area iridium oxide.23 This indicates that the presence of the TiO2 partially stabilizes higher iridium oxidation states. Since high Ir3+ content has been associated with lower stability,23, 46 the increased stability of IrO2−TiO2−245 when compared to IrO2−150 could result from the stabilisation of high iridium oxidation states by TiO2 support.
CONCLUSIONS The synthesis and OER activity of high surface area chlorine-free iridium oxide dispersed in titania has been described. The reported single-step synthetic approach allowed the preparation of an OER catalyst with reduced iridium loading, which is significantly more active than the benchmark commercial catalyst. The high catalytic activity towards the oxygen evolution reaction is rationalized by initial large quantities of surface iridium hydroxo (*OH) species, which most likely transform into the oxo species upon anodic oxidation, as supported by operando XAS studies. The observed similarities of the OER properties and operando behavior of IrO2−TiO2 and high surface area IrO2 indicate that the same OER mechanism is operating for these catalysts and the presence of TiO2 does not affect the reaction pathway. Nevertheless, the IrO2−TiO2 catalyst shows improved activity and stability compared to pure IrO2, likely due to the stabilisation of small IrO2 particles on TiO2 and possible particle – support interac-
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tions between IrO2 and rutile TiO2. Additionally, the observed partial stabilisation of high iridium oxidation states may also be correlated with improved stability of the catalyst. Overall this study shows that combining iridium oxide with another high surface area oxide, which effectively acts as a support (here TiO2, prepared in situ), is a valid approach to improve stability and activity of anodes for PEWEs.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. *E-mail:
[email protected].
Author Contributions #
E.O. and D.L. contributed equally to this work
Notes The authors declare no competing financial interest.
ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: Details of the in situ XAS spectroscopy and EXAFS modeling, N2 adsorption / desorption isotherms, HAADF−STEM images and EDX maps, powder XRD and XPS data
ACKNOWLEDGMENT The authors would like to thank and acknowledge the financial support of CCEM Switzerland (projects DuraCat and Renerg2), Umicore GmbH & Co KG, Swiss Electric Research, the Swiss Federal Office of Energy, CTI and the Swiss Competence Center for Energy Research (SCCER) Heat & Electricity Storage. We also thank ScopeM for the use of their electron microscopy facilities. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland for beamtime at the SuperXAS beamline.
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37. Pfeifer, V.; Jones, T. E.; Velasco Vélez, J. J.; Massué, C.; Arrigo, R.; Teschner, D.; Girgsdies, F.; Scherzer, M.; Greiner, M. T.; Allan, J.; Hashagen, M.; Weinberg, G.; Piccinin, S.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R. Surface and Interface Anal. 2016, 48, 261-273. 38. Reier, T.; Nong, H. N.; Teschner, D.; Schlögl, R.; Strasser, P. Adv. Energy Mater. 2017, 7, 1601275. 39. Matsumoto, Y.; Sato, E. Mat. Chem. Phys. 1986, 14, 397426. 40. Fierro, S.; Nagel, T.; Baltruschat, H.; Comninellis, C. Electrochem. Commun. 2007, 9, 1969-1974. 41. Chandra, D.; Takama, D.; Masaki, T.; Sato, T.; Abe, N.; Togashi, T.; Kurihara, M.; Saito, K.; Yui, T.; Yagi, M. ACS Catal. 2016, 6, 3946-3954. 42. Ouattara, L.; Fierro, S.; Frey, O.; Koudelka, M.; Comninellis, C. J. Appl. Electrochem. 2009, 39, 1361-1367. 43. De Pauli, C. P.; Trasatti, S. J. Electroanal. Chem. 1995, 396, 161-168. 44. Minguzzi, A.; Locatelli, C.; Lugaresi, O.; Achilli, E.; Cappelletti, G.; Scavini, M.; Coduri, M.; Masala, P.; Sacchi, B.; Vertova, A.; Ghigna, P.; Rondinini, S. ACS Catal. 2015, 5, 51045115. 45. Minguzzi, A.; Lugaresi, O.; Locatelli, C.; Rondinini, S.; D'Acapito, F.; Achilli, E.; Ghigna, P. Anal Chem 2013, 85, 70097013. 46. Cherevko, S.; Geiger, S.; Kasian, O.; Mingers, A.; Mayrhofer, K. J. J. J. Electroanal. Chem. 2016, 774, 102-110.
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TOC 82x44mm (300 x 300 DPI)
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Figure 1 85x83mm (300 x 300 DPI)
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Figure 2 197x89mm (300 x 300 DPI)
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Figure 4 191x178mm (300 x 300 DPI)
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