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

17 January 2024, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


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.


mixed metal oxide
real active species
X-ray absorption spectroscopy

Supplementary materials

Supplementary Information
The supplementary information contains supporting experimental data and detailed descriptions for EXAFS data fitting. Supplementary information on synthesis of mixed metal oxide and its characterization is shown in Section 1. Section 2 focuses on fabrication of working electrode and electrochemical measurements. Section 3 shows fitting parameters on Ex-situ and in-situ XAS measurements along fitting in Imaginary R and K space to represent the goodness of the fit.


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