Investigating the Electronic Structure of a Complex Oxide Catalyst for Alkaline Water Electrolysis By Operando X-Ray Absorption Spectroscopy
Authors: Lin Jiaming , Ma Ningchen , Thakur Neha , Kumar Mukesh , Watanabe Toshiki , Yamamoto Kentaro , Matsunaga Toshiyuki , Kuroda Yoshiyuki , Mitsushima Shigenori , Uchimoto Yoshiharu 
Published: December 2024

Environmental and energy shortage issues arise due to the excess consumption of fossil fuels, which requires immediate attention to realize the dream of an energy society with virtually zero carbon dioxide emissions. In this context, H2 generated by the water electrolysis technology is considered as a possible substitute for fossil fuels due to its prominent features, such as high gravimetric energy density and zero carbon emission.1 In water electrolysis, the anodic oxygen evolution reaction (OER) is a multielectron transfer reaction that makes overall water electrolysis kinetically sluggish and dominated by noble metal oxides such as IrO2 and RuO2.2 However, their high prices and poor stability under strongly acidic or basic conditions hindered their practical applicability. Therefore, to fulfill this goal, it is necessary to reduce the cost and increase the efficiency and durability of water electrolysis by developing alternative OER electrocatalysts with earth abundant elements. Among various transition metal oxides, Perovskite oxides emerged as a potential OER catalyst due to their low cost, flexibility,
and tailorable properties. However, the poor stability of typical BSCF during OER remains a significant issue that hindered its widespread application. Meanwhile, recent reports show that the complex oxide hex-BSCF exhibits superior OER performance than 3C-BSCF. 3However, the detailed information about the intermediates generated during the OER process and the relationship between OER activity and electronic changes of hex-BSCF remains unclear.
Therefore, in this study, we synthesized several types of composite oxide, water electrocatalysts Ba4Sr4(Co1-xFex)4O15 (x = 0 to 1.0), and clarified the relationship between the OER activity and the amount of Fe added. The electrochemical investigations showed a volcanic relationship between the OER activity and the amount of Fe added, and the catalyst with an added Fe amount of 0.2 (x=0.2) showed the highest OER activity (Figure 1a). To eliminate the effect of oxygen bubbles attached to the catalyst surface, a forced-flow cell was developed to determine the actual OER activity. The electronic structure change of the catalyst/electrolyte interface during water electrolysis, was analyzed in detail by operando soft X-ray absorption spectroscopy (XAS) by measuring oxygen K - edge. The characteristic peaks originated in the range of 531eV to 532eV, corresponding to the intermediate region between CoOOH and FeOOH.2 The intensity of this characteristic peak is strongly correlated with its OER activity. Further, the ex-situ hard X-ray absorption spectroscopy (XAFS) shown in Figure 1b revealed that the valence state of Co decreases when a small amount of Fe is added, and hex-BSCF82 has the lowest metal-oxygen coordination number and minimum octahedral sites occupation. Hence, the doped Fe pre-occupies the octahedral sites, and the Co is forced to the tetrahedral sites, consequently enhancing the OER activity.

Figure 1