In Situ and Operando S ynchrotron Characterization of Commercial Ir Oxide Catalysts for Water Electrolyzers

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To enable further adoption of proton exchange membrane water electrolysis (PEMWE), we must continue to advance our fundamental understanding of the activity and degradation mechanisms of state-of-the-art iridium-based oxide catalysts for the oxygen evolution reaction (OER). Here, we employ synchrotron-based X-ray diffraction (XRD), pair distribution function (PDF), in situ and operando X-ray absorption spectroscopy (XAS) of commercial iridium oxide catalysts to i) explore the potential dependence of the iridium oxidation state and local coordination structure and correlate these findings with dissolution data and a reaction model, and ii) understand the physiochemical nature of numerous commercially available products toward informing the selection of a catalyst to enable studies of materials integration and degradation mechanisms of PEMWEs within the framework of the U.S. Department of Energy H2NEW consortium. Among the sources of degradation of OER catalysts that currently hinder more widespread adoption of PEMWEs is the dissolution of Ir, resulting in the loss of catalytic activity over time. Kasian et al. proposed reaction mechanisms for Ir dissolution that vary with potential – at low overpotentials, Ir 3+ dissolution following O 2 release is in competition with re-oxidation to Ir 4+ O 2 . At high overpotentials, IrO 2 OH is oxidized to Ir 6+ O 3 , which may dissolve as IrO 4 2- after reacting with water. 1 In this work, we investigate the oxidation state and local coordination environment of Ir in two commercially available IrO x catalysts through in situ and operando Ir L 3 -edge XAS; we then correlate these results with Ir dissolution measured using inductively coupled plasma mass spectrometry for hydrous IrO x under varying applied potentials. We find that in hydrous IrO x , Ir continues to oxidize up to a certain potential, above which the oxidation state plateaus. We compare this finding with experimental dissolution data in addition to a reaction model developed from proposed mechanisms, both of which display a decrease in Ir dissolution above the same potential. This result diverges from the conventionally observed activity-stability tradeoff and affirms the theory that a passivating oxide is formed at high overpotentials. Furthermore, we have characterized six commercially available iridium oxide catalysts using ex situ XAS, synchrotron XRD, and PDF measurements to detect differences in crystallinity, structure, and oxidation state and correlate these differences with OER performance. From these techniques, there are two distinct groups of catalysts: i) amorphous or hydrous Ir oxides exhibiting local ordering, and ii) crystalline Ir oxides with varying crystallite sizes. These characteristics are found to correlate with electrochemical redox behavior and OER activity. This work improves our understanding of the dissolution mechanisms of iridium oxide catalysts and lays the groundwork for improved understanding of the impact of anode catalyst properties on the performance and durability of PEMWEs. References (1) Kasian et al., Angewandte Chemie International Edition 2018 , 57 (9), 2488-2491.

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