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.

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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

The use of high amounts of iridium in industrial proton exchange membrane water electrolyzers (PEMWE) could hinder their widespread use for the decarbonization of society with hydrogen. Nonthermally oxidized Ir nanoparticles supported on antimony-doped tin oxide (SnO 2 :Sb, ATO) aerogel allow decreasing the use of the precious metal by more than 70% while enhancing the electrocatalytic activity and stability. To date, the origin of these benefits remains unknown. Here, we present clear evidence of the mechanisms that lead to the enhancement of the electrochemical properties of the catalyst. Operando near-ambient pressure X-ray photoelectron spectroscopy on membrane electrode assemblies reveals a low degree of Ir oxidation, attributed to the oxygen spill-over from Ir to SnO 2 :Sb. Furthermore, the formation of highly unstable Ir(III) species is mitigated, while the decrease of Ir dissolution in Ir/ SnO 2 :Sb is confirmed by inductively coupled plasma mass spectrometry. The mechanisms that lead to the high activity and stability of Ir catalysts supported on SnO 2 :Sb aerogel for PEMWE are thus unveiled.

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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et al.. Nafion®-sepiolite composite membranes for improved proton exchange membrane fuel cell performance.