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 (Ni2−xFexMo3O8, 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 Ni1.5Fe0.5Mo3O8 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.