Investigating the Factors Influencing the Activity of Iridium-Based Perovskite Oxide Electrocatalyst for Acidic Water Oxidation By Operando XAS
Authors: Cao Weijie , Watanabe Toshiki , Yamamoto Kentaro , Matsunaga Toshiyuki , Thakur Neha , Kumar Mukesh , Kuroda Yoshiyuki , Mitsushima Shigenori , Uchimoto Yoshiharu 
Published: December 2024

To mitigate global warming, the integration of renewable energy sources into energy conversion/generation systems is fundamental. Currently the dominant alkaline water electrolysis (AWE) technology suffers from challenges including the crossover of product gases, high ohmic resistance, limited current density and low operating pressure, mainly because of utilization of a diaphragm and a liquid electrolyte in the AWE device. Using a polymer-based proton exchange membrane (PEM) for efficient proton transfers, PEM water electrolysis (PEM–WE) technology can effectively tackle the above challenges with markedly enhanced performance, and is thus attracting broad research interest,1 but relies on scarce iridium oxide at the anode. In the quest for efficient oxygen evolution reaction (OER) electrocatalysts, iridium-based perovskite oxides have attracted attention due to their unique AxIryO3 structures and cost-effectiveness, with the diversity of these structures determining their catalytic activity.2 The geometric arrangement of [IrO6] octahedra and their interaction with A-site cations influence their OER performance, particularly those with distorted [IrO6] octahedra showing higher activity due to the proximity of π- and π* orbital structures to the Fermi level.3 Furthermore, these materials undergo surface reconstruction and metal ion dissolution under electrochemical conditions, indicating a close connection between structural evolution and catalytic activity.4 This dynamic evolution process reflects changes on the catalyst surface under various external conditions, emphasizing the importance of utilizing in-situ or operando analytical techniques to track active structures for understanding catalytic mechanisms and guiding the design of new catalysts.

In this research, four iridium-based perovskite oxides with distinct crystal structures (9R-BaIrO3, 6H-SrIrO3, 3C-SrIrO3, and BCC-SrIrO3) were synthesized through the solution calcination method, aimed to act as catalysts for the OER in PEM–WE. The material characteristics and electrocatalytic performances of the surface IrOx layers post-acid treatment was rigorously analyzed utilizing ex-situ X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and electrochemical measurement. Among these catalysts, 9R-BaIrO3 demonstrated superior OER activity and durability, attributed to its exclusive face-shared [IrO6] octahedral trimer structure alongside extensive surface hydroxylation. The operando Ir L3-edge XAS and high-energy-resolution fluorescence-detected XAS (HERFD-XAS) measurement revealed changes in the oxidation state of the Ir (3+ to 4+σ) with increasing potentials, highlighting a lower Ir-O coordination environment and longer bond length as crucial factors for enhanced OER activity. Moreover, operando O K-edge XAS analysis provided in Figure 1 provides further insight into the selective oxygen binding sites during the OER, which are influenced by the degree of distortion in the [IrO6] octahedra. Specifically, oxygen species showed a preference for binding to the μ1 sites in catalysts with less symmetrical monoclinic structures, whereas, in more symmetrical tetragonal/cubic structured catalysts, oxygen species predominantly bonded to μ2 sites. Moreover, in perovskite-structured catalysts, activation at the μ3 sites leads to lattice oxygen involvement, resulting in a disordered surface structure. This disorder surpasses that observed in conventional amorphous IrOx catalysts, thereby enhancing the catalyst's efficiency in OER by promoting more dynamic and effective surface interactions. By deepening our comprehension of the operational dynamics of iridium-based perovskite oxides in OER, this research not only enriches the fundamental knowledge base but also forges a path toward the creation of more efficient and stable catalysts.

Acknowledgment

This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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Figure 1