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.

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