<i>(Invited) Operando</i> Insights into Anionic Effects and Mechanistic Understanding of Iridium Oxide Catalysts for PEM Water Electrolysis Using XAS and Theoretical Approaches

The transition to sustainable green energy production aims to replace fossil fuels, mitigate global warming, and meet the increasing global energy demand. Hydrogen has emerged as a promising renewable energy carrier, offering high energy density and zero CO₂ emissions, thereby enhancing energy security while minimizing greenhouse gas emissions. 1 Water electrocatalysis utilizing intermittent renewable energy sources is a popular method for producing purified hydrogen. Proton-exchange membrane (PEM) water electrolysis is currently the most advanced water electrolysis technology, utilizing commercial Pt/C as the cathodic catalyst for the hydrogen evolution reaction (HER) and noble metal-based materials as anodic catalysts for the oxygen evolution reaction (OER). However, the inherently sluggish four-electron transfer kinetics of OER, coupled with the harsh acidic conditions in PEM electrolyzers, significantly limit the selection of suitable anodic catalysts. Iridium-based materials, particularly IrO₂, are regarded as the most viable OER electrocatalysts due to their superior stability and catalytic activity under these conditions. 2 To advance the development of cost-effective PEM water electrolysis catalysts, a comprehensive understanding of the reaction mechanisms governing the activity of highly efficient iridium oxides is crucial. 3 In this study, a comprehensive investigation was conducted on a series of IrO x catalysts with varying degrees of crystallinity (SA103, SA58, SA14.6, and SA58), with their structural characteristics analyzed using atomic pair distribution function (PDF) analyses. 4 Operando X-ray absorption spectroscopy (XAS) revealed that the SA3.5 sample, which exhibits a combination of orthorhombic ( Pbcn ) and monoclinic ( C 2 /m ) phases with high monoclinic content, features a structure with corner- and edge-sharing octahedra and highly undercoordinated sites, resulting in structural defects. This promotes the generation of electrophilic O I⁻ species, which serve as highly reactive centers for water nucleophilic attack, thereby enhancing oxygen evolution reaction (OER) activity. Furthermore, operando surface-enhanced infrared absorption spectroscopy (SEIRAS) demonstrated that potential-driven oxygen coupling at Ir sites facilitates the formation of crucial *O-OH intermediates in the SA3.5 sample, enabling an efficient and rapid 4-electron OER under acidic conditions. To further elucidate the mechanistic influence of crystal structure on catalytic performance, density functional theory (DFT) calculations were performed, providing deeper insights into the underlying reaction pathways of IrO x catalysts with different structural configurations. In the SA3.5 sample, the structural configuration, comprising a combination of orthorhombic and monoclinic phases, exhibits increased susceptibility to anion poisoning by species such as SO₄²⁻ and PO₄³⁻. This susceptibility highlights a fundamental trade-off between catalytic efficiency and long-term stability. In contrast, the SA58 sample, which adopts a rutile-type structure, demonstrates greater structural stability; however, this stability comes at the expense of reduced OER activity. To gain deeper insights into electrolyte composition's influence on catalytic performance, this study systematically evaluated the effects of varying H₂SO₄ and H₃PO₄ concentrations on these catalysts. Operando XAS and DFT calculations were employed to elucidate the mechanistic impact of anion adsorption on the catalytic sites. Furthermore, the correlation between electrolyte-derived anions and OER activity was extensively analyzed across IrO x catalysts with distinct crystal structures, providing a comprehensive understanding of anion effects on catalyst performance and stability. Acknowledgments This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Reference: S. Chu and A. Majumdar, Nature , 2012, 488 , 294-303. Y. Wang, M. Zhang, Z. Kang, L. Shi, Y. Shen, B. Tian, Y. Zou, H. Chen and X. Zou, Nat. Mater. , 2023, 14 , 5119. L. An, C. Wei, M. Lu, H. Liu, Y. Chen, G. G. Scherer, A. C. Fisher, P. Xi, Z. J. Xu and C.-H. Yan, Adv. Mater. , 2021, 33 , 2006328. N. Thakur, et. al., 244th ECS meeting abstract.