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Proton-exchange membrane water electrolyzers (PEMWEs) electrodes use scarce and costly platinum group metals (PGMs), but only such devices are able to meet the requirements associated with renewable energies (large amplitude and frequent and rapid variations in the current applied to the cell). Catalysts represent only 8 % to the overall stack cost, 6 % being associated with the high iridium (Ir) loading used to electrocatalyze the anodic oxygen evolution reaction (OER). High surface area supported Ir oxide (IrOx) catalysts thus represent a promising strategy to reduce the cost of this technology and limit the geological pressure on Ir. However, the Gibbs-Thompson effect, which controls the electrochemical stability of nanomaterials casts a doubt on the viability of this approach. To shed light into the benefits and limitations of supported and unsupported IrOx catalysts, we benchmarked commercial materials (unsupported IrO2, Ir/C), unsupported porous IrOx microparticles, and IrOx nanoparticles (NPs) supported on carbon black or on doped tin oxide aerogels (AG)/nanofibers (NFs). Transmission electron microscopy (TEM) and identical-location transmission electron microscopy (IL-TEM) provided changes in morphology during accelerated stress testing. Complementarily, a flow cell connected to an inductively-coupled mass spectrometer (FC-ICP-MS) was used to assess their stability number (S-number, see Figure 1). The results show that supported IrOx nanocatalysts are extremely active toward the OER because they feature mixed Ir oxidation states and a high density of active sites (small crystallite/particle size). The lack of robustness of their supports, however, prevents the nanocatalysts from sustaining this high OER activity [1, 2]. Similar to what was observed on extended surfaces, we report that mild thermal annealing (450°C) leads to lower Ir atom dissolution rate. Overall, the best compromise between OER activity and stability was obtained for unsupported porous IrOx microparticles after mild thermal annealing under air at 450°C [2]. On the cathode side, IL-TEM measurements revealed mild changes in morphology for Pt/C nanoparticles [3]. Ackowledgements This work was supported by the French National Research Agency in the frame of the MOISE project (grant number ANR-17-CE05-0033). References S. Abbou, R. Chattot, V. Martin, F. Claudel, L. Solà-Hernández, C. Beauger, L. Dubau, F. Maillard, ACS Catal. 10 (2020) 7283-7284. C. Daiane Ferreira da Silva, F. Claudel, V. Martin, R. Chattot, S. Abbou, K. Kumar, I. Jiménez-Morales, S. Cavaliere, D. Jones, J. Rozière, L. Solà-Hernandez, C. Beauger, M. Faustini, J. Peron, B. Gilles, C. Beauger, L. Piccolo, F. H. Barros de Lima, L. Dubau, F. Maillard, ACS Catal. 11 (2021) 4107-4116. A. Viola, L. Dubau, F. Maillard, in preparation. Figure 1. S-number values calculated for supported and unsupported IrOx electrocatalysts during a galvanostatic accelerated stress test (j = 10 mA cm-2 geo, T = 80 °C, U cut-off = 2 V vs. RHE) performed in Ar-saturated 0.05 M H2SO4. Reprinted with permission from ref. [2]. Copyright 2021 American Chemical Society. Figure 1