Determining the degradation mechanisms of oxygen evolution reaction (OER) catalysts is fundamental to design improved proton-exchange membrane water electrolyzer (PEMWE) devices but remains challenging under the demanding conditions of PEMWE anodes. To address this issue, we introduce a methodology combining identical-location transmission electron microscopy (IL-TEM), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements, and apply it to iridium nanoparticles (NPs) covered by a thin oxide layer (IrO x ) in OER conditions. The results show that, whatever the initial OER activity of the IrO x nanocatalysts, it gradually declines and reaches similar values after 30 000 potential cycles between 1.20 and 1.60 V versus the reversible hydrogen electrode (RHE). This drop in OER activity was ascribed to the progressive increase of the Ir oxidation state (fast change during electrochemical conditioning, milder change during accelerated stress testing) along with the increased concentrations of hydroxyl groups and water molecules. In contrast, no change in the mean oxidation state, no change in the hydroxyl/water coverage, and constant OER activity were noticed on the benchmark micrometer-sized IrO 2 particles. In addition to chemical changes, Ir dissolution/redeposition and IrO x nanoparticle migration/agglomeration/detachment were made evident during the conditioning stage and in OER conditions, respectively. By combining the information derived from IL-TEM images and XPS measurements, we show that Ir(III) and Ir(V) are the best performing Ir valencies for the OER. These findings provide insights into the long-term OER activity of IrO x nanocatalysts as well as practical guidelines for the development of more active and more stable PEMWE anodes.

Metal oxides are important functional materials with a wide range of applications, especially in the field of electrocatalysis. However, quick and accurate assessment of their real electroactive surface area (ECSA), which is of paramount importance for the evaluation of their performance, remains a challenging task. Herein, we present a relatively simple strategy for an accurate in situ determination of the ECSA of commonly used metal oxide catalysts, namely Ni-, Co-, Fe-, Pt-, and Ir-based oxides. Similar to the well-established practice in electrocatalysis, the method is based on the phenomenon of specific adsorption. It uses the fact that at electrode potentials close to the onset of the oxygen evolution reaction, specifically adsorbed reaction intermediates manifest themselves through so called adsorption capacitance, which is unambiguously detectable using electrochemical impedance spectroscopy. We determined and calibrated these capacitances for common catalyst metal oxides using model thin films. Therefore, with simple impedance measurements, experimentalists can acquire the adsorption capacitance values and accurately estimate the real electroactive surface area of the above-mentioned oxide materials, including nanostructured electrocatalysts. Additionally, as illustrative examples, we demonstrate the application of the method for the determination of the ECSA of oxide catalyst nanoparticles.

Implementing iridium oxide (IrO x) nanocatalysts can be a major breakthrough for oxygen evolution reaction (OER), the limiting reaction in polymer electrolyte membrane water electrolyser devices. However, this strategy requires developing a support that is electronically conductive, is stable in OER conditions, and features a large specific surface area and a porosity adapted to gas-liquid flows. To address these challenges, we synthesized IrO x nanoparticles, supported them onto doped SnO 2 aerogels (IrO x /doped SnO 2), and assessed their electrocatalytic activity towards the OER and their resistance to corrosion in acidic media by means of a flow cell connected to an inductively-coupled mass spectrometer (FC-ICP-MS). The FC-ICP-MS results show that the long-term OER activity of IrO x /doped SnO 2 aerogels is controlled by the resistance to corrosion of the doping element, and by its concentration in the host SnO 2 matrix. In particular, we provide quantitative evidence that Sb-doped SnO 2 type supports continuously dissolve while Tadoped or Nb-doped SnO 2 supports with appropriate doping concentrations are stable under acidic OER conditions. These results shed fundamental light on the complex

Advanced materials are needed to meet the requirements of devices designed for harvesting and storing renewable electricity. In particular, polymer electrolyte membrane water electrolyzers (PEMWEs) could benefit from a reduction in the size of the iridium oxide (IrOx) particles used to electrocatalyze the sluggish oxygen evolution reaction (OER). To verify the validity of this approach, we built a library of 18 supported and unsupported IrOx catalysts and established their stability number (S-number) values using inductively-coupled plasma mass spectrometry and electrochemistry. Our results provide quantitative evidence that (i) supported IrOx nanocatalysts are more active towards the OER but less stable than unsupported micrometer-sized catalysts, e.g. commercial IrO2 or porous IrOx microparticles; (ii) tantalum-doped tin oxides (TaTO) used as supports for IrOx nanoparticles are more stable than antimony-doped tin oxides (ATO) and carbon black (Vulcan XC72); (iii) thermal annealing under air atmosphere yields depreciated OER activity but enhanced stability; (iv) the beneficial effect of thermal annealing holds both for microand nano-IrOx particles, and leads to one order of magnitude lower Ir atom dissolution rate with respect to non-annealed catalysts; (v) the best compromise between OER activity and stability was obtained for unsupported porous IrOx microparticles after thermal annealing under air at 450°C. These insights provide guidance on which material classes and strategies are the most likely to increase sustainably the OER efficiency while contributing to diminish the cost of PEMWE devices.

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Lowering the noble metal (Ir or Ru) loading of Oxygen Evolution Reaction (OER) catalysts while maintaining both a high activity and a long-term stability for Proton Exchange Membrane Water Electrolysis (PEMWE) cells is a challenging topic for industry and academia. A possible strategy is the use of support materials (Figure 1) that are stable under OER conditions (> 1.4 V vs. the standard hydrogen electrode). Due to its large specific surface area and high electrical conductivity, carbon black is a popular and widely used catalyst support for electrochemical applications.[i] However its use is limited by the high anodic potentials required for the OER which would, due to corrosion, cause detachment of the supported catalyst.[ii] Therefore, non-carbon based supporting materials are required. Titanium dioxide (TiO2)[iii] or tin dioxide (SnO2)[iv] based materials are promising candidates due to their good corrosion resistance and strong interaction with noble metals catalysts. Wang and co-workers, reported enhanced OER activity and stability of antimony doped tin oxide (ATO)[v] aerogels supported Ir oxide nanoparticles.[vi] Uchida et al. also reported improved utilization of Ir deposited on tantalum doped tin oxide (TaTO) catalysts used at a PEMWE anode.[vii] However, Claudel et al. underlined that the stability of the doping element is a key issue for doped tin oxide to be implemented in PEMWE anodes.[viii] In this work[ix], our 3D highly porous aerogel materials were revisited to prepare Sb or Ta-doped tin oxide based catalyst support. IrOx nanoparticles were in-situ deposited over the different doped tin oxide based aerogels. After synthesis and characterization of both the support and the IrOx nanoparticles, the OER activity was measured on glassy carbon rotating disk electrodes under conditions simulating PEMWE anode operation. Investigations were based on N2 sorption, Scanning Electron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) and electrochemical characterization to analyse the physicochemical properties of the support and their impact on the catalytic activity and the stability of the catalysts. Our results show that supported Ir oxide nanoparticles are both more active and more stable than unsupported ones. Consistently with the results reported by Uchida et al.,[vii] despite very different electronic conductivities of the supports, the OER mass activities of supported IrOx nanoparticles were found similar in thin-film electrode configuration. Figure 1: Graphical representation of the designed anode for a PEWE cell, where the IrOx NPs are supported on a 3D porous aerogel material based on doped SnO2. Acknowledgements The authors wish to thank Pierre Ilbizian for supercritical drying, Frédéric Georgi for XPS analysis, Suzanne Jacomet for SEM/EDX analysis and Gabriel Monge for XRD analysis. This work was funded by the European Union's H2020 Program within the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement 779478 (FCH-JU project PRETZEL) and the French National Research Agency (ANR-17-CE05-0033 project MOISE). It was supported by Capenergies and Tenerrdis. [i] (a) A. Dicks, J. Power Sources, 2006, 156, 128; (b) E. Antolini, E. Gonzalez, Appl. Catal., A, 2009, 365, 1; (c) E. Antolini, Appl. Catal., B, 2012, 52,123. [ii] (a) H.-S. Oh, K. H.Lim, B. Roh, I. Hwang, H. Kim, Electrochim. Acta, 2009, 54, 6515; (b) H.-S. Oh, J.-G. Oh, S. Haam, K. Arunabha, B. Roh, I. Hwang, H. Kim, Electrochem. Commun,. 2008, 10, 1048; (c) S. Maass, F. Finsterwalder, G. Frank, R. Hartmann, C. J. Merten, Power Sources, 2008, 176, 444. [iii] G. Chen, S. R. Bare, T. E. Mallouk, J. Electrochem. Soc., 2002, 149, 1092 [iv] H. S. Oh, H. N. Nong, D. Teschner, T. Reier, A. Bergmann, M. Gliech, J. Ferreira de Araujo, E. Willinger, R. Schloegl, P. Strasser, J. Am. Chem. Soc., 2016, 138, 12552. [v] G. Ozouf, Ch. Beauger, J. Mater. Sci., 2016, 51 (11), 5305-5320 [vi] L. Wang, F. Song, G. Ozouf, D. Geiger, T. Morawietz, M. Handl, P. Gazdzicki, Ch. Beauger, U.Kaiser, R. Hiesgen, A. Gago, K. Friedrich, Journal of Materials Chemistry, 2017, 5, 3172 [vii] H. Ohno, S. Nohara, K. Kakinuma, M. Uchida, H. Uchida, Catalysts, 2019, 9, 74 [viii] F. Claudel, L. Dubau, G. Berthomé, L. Sola-Hernandez, C. Beauger, L. Piccolo, F. Maillard, ACS Catal, 2019, 9 (5), 4688-4698 [ix] L. Solà-Hernández, F. Claudel, F. Maillard, C. Beauger, Int. J. Hydrog. Energy, 2019, 44 (45), 24331–24341. Figure 1

Determining the relationships between structure, oxidation state and oxygen evolution reaction (OER) activity is fundamental to design improved anode catalysts for polymer electrolyte membrane water electrolyser (PEMWE) devices. To date, Ir-based electrocatalysts are the best compromise between activity and stability. According to the scarcity of Ir on the Earth crust, the deployment of PEMWE technology cannot overlook an optimized utilization of Ir loading. Following the successful approach in proton-exchange membrane fuel cells (PEMFC) consisting of using carbon-supported Pt nanoparticles instead of Pt-blacks, we investigated supported Ir oxide nanoparticles. The nature of the support appears to be crucial since high-surface area carbon supports are rapidly degraded in the operating conditions of a PEMWE anode (E > 1.6 V vs. the reversible hydrogen electrode, T = 80 °C). We opted for alternative support materials based on doped tin oxide. In this contribution, a wide and sound library of Ir oxide materials differing from each other by the presence/absence of support and by the Ir architecture has been studied by identical location transmission electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry and electrochemical techniques (Figure 1). It allowed us to establish trends on the viability of supported Ir oxide catalysts in the harsh PEMWE operating conditions [1]. [1] F. Claudel, L. Dubau, G. Berthomé, L. Sola-Hernandez, C. Beauger, L. Piccolo, F. Maillard, ACS catal., 2019, 9 (5), 4688–4698.