Novel mechanism of Fe⁴⁺/Ni³⁺ synergistic effect via exchange energy gain for boosting water oxidation

Synergistic effects are often used for interpreting the enhanced electrochemical oxygen evolution reaction (OER) activity of catalysts with two or more elements. However, the mechanism behind this synergy remains ambiguous. Here, we report an Fe-doped Ni₂Mo₃O₈ (Ni_2−xFe_xMo₃O₈, where x = 0.1, 0.3, 0.5, 0.7, and 1.0) series with high OER activity (only 196 mV at 10 mA cm⁻² for x = 0.5) and high stability over 200 h at a high current density of 500 mA cm⁻². Our in situ X-ray absorption spectroscopies indicated a valence-state transition from Fe²⁺/Ni²⁺ to Fe³⁺(Fe⁴⁺)/Ni³⁺ and a structural transition from corner-sharing Fe(Ni)–O–Fe(Ni) to an edge-sharing network. The highest catalytic activity of Ni_1.5Fe_0.5Mo₃O₈ among this series is interpreted as a gain in the exchange energy (ca. 1 eV) of the Fe⁴⁺/Ni³⁺ pair facilitated by intersite hopping owing to its highest Fe valence state. Our results reveal a previously unreported mechanism for the Ni-Fe synergistic effect that enhances OER activity.