Ru‐Modulated High‐Entropy Spinel Oxides Enable Efficient and Selective Electrochemical Nitrate Reduction to Ammonia

ABSTRACT Electrochemical nitrate reduction to ammonia is a highly desirable route for simultaneous nitrate remediation and renewable NH ₃ electro‐generation, but its kinetics of multistep proton–electron transfer and the competing reaction limit activity and selectivity. Here, we construct a Ru‐modulated high‐entropy spinel oxide, (Fe _0.2 Co _0.2 Ni _0.2 Cu _0.2 Ru _0.2 ) ₃ O ₄ (Ru‐HESO), to disrupt electronic averaging within high‐entropy oxides, realize site‐dependent catalytic modulation. Ru‐HESO achieves a high NH ₃ Faradaic efficiency of 94.9% at −0.3 V versus RHE and maximal NH ₃ yield rate of 66.5 mg h ⁻¹ cm ⁻² , maintaining stable operation for 60 h. In situ Raman spectroscopy, in situ FTIR spectroscopy, and online DEMS collectively observe the evolving NO _x ‐to‐NH _x intermediates and substantiate a stepwise hydrogenation process toward NH ₃ . Density functional theory reveals that Ru substitution alters the d‐band characteristics, lowers the kinetic bottleneck, and changes the rate‐determining step on Ru‐HESO while moderate H binding. This work establishes a modulator‐driven paradigm for high‐entropy spinel electrocatalyst design and provides mechanistic insights for the rational design of effective electrocatalysts for multi‐electron nitrogen conversions.