Catalytic Water Electrolysis by Co-Cu-W Mixed Metal Oxides: In-sights from Operando X-ray Absorption Spectroscopy

Mixed Metal oxides (MMOs) are a promising class of electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolu-tion reaction (HER). Despite their importance for sustainable energy schemes, our understanding of relevant reaction pathways, catalytically active sites and synergistic effects is rather limited. Here, we applied synchrotron-based X-ray absorption spectrosco-py (XAS) to explore the evolution of an amorphous Co-Cu-W MMO electrocatalyst, shown previously to be an efficient bifunc-tional OER and HER catalyst for water splitting. Ex-situ K- and L-edge XAS measurements provided structural environments and the oxidation state of the metals involved, revealing Co2+ (octahedral), Cu+/2+ (tetrahedral / square-planar) and W6+ (octahedral) cen-ters. Operando XAS investigations, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), elucidated the dynamic structural transformations of Co, Cu, and W metal centers during OER or HER. The ex-perimental results indicate that Co3+ and Cu0 are the active catalytic sites involved in OER and HER, respectively, while Cu2+ and W6+ play crucial roles as structure stabilizers, suggesting strong synergistic interactions within the Co-Cu-W MMO system. These results, combined with the Tafel slope analysis, revealed that the bottleneck intermediate during OER is Co3+ hydroperoxide, whose formation is accompanied by changes in the Cu-O bond lengths, pointing to a possible synergistic effect between Co and Cu ions. Our study reveals important structural effects taking place during MMO-driven OER/HER electrocatalysis and provides es-sential experimental insights into the complex catalytic mechanism of emerging noble metal-free MMO electrocatalysts for full water splitting.