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.

协同效应通常用于解释具有两种或多种元素的催化剂增强的电化学析氧反应（OER）活性。然而，这种协同作用背后的机制仍然不明确。在这里，我们报道了具有高OER活性（在OER下仅为196 mV）的Fe掺杂Ni ₂ Mo ₃ O ₈ (Ni _2−x Fe _x Mo ₃ O ₈，其中x = 0.1、0.3、0.5、0.7和1.0)系列。 10 mA cm ^{-2 (x = 0.5)) 以及在 500 mA cm -2}的高电流密度下超过 200 小时的高稳定性。我们的原位X 射线吸收光谱表明从 Fe ²⁺ /Ni ²⁺到 Fe ³⁺ (Fe ⁴⁺ )/Ni ³⁺的价态转变以及从共享角 Fe(Ni)–O– 的结构转变Fe(Ni) 到边缘共享网络。该系列中 Ni _1.5 Fe _0.5 Mo ₃ O ₈的最高催化活性被解释为 Fe ⁴⁺ /Ni ³⁺对的交换能（约 1 eV）的增益，由于其最高的 Fe 而由位点间跳跃促进价态。我们的结果揭示了一种先前未报道的 Ni-Fe 协同效应增强 OER 活性的机制。