Medium‐Entropy Hydroxides Enable Bridge‐Bound NO Stabilization and Hydrogen Spillover for Nitrite‐to‐Ammonia Electroreduction

ABSTRACT Electroreduction of nitrite provides a route to couple aqueous nitrite remediation with ammonia synthesis, but high selectivity requires coordinated nitrite deoxygenation and surface‐hydrogen delivery. Here, we report ultrathin NiFeCoCu medium‐entropy layered double hydroxide (ME‐LDH) nanosheets that promote selective nitrite‐to‐ammonia conversion by coupling charge‐polarized bridge‐bound * NO stabilization with hydrogen‐spillover‐assisted hydrogenation. In neutral K 2 SO 4 electrolyte containing nitrite, the catalyst delivers NH 4 + Faradaic efficiencies above 90% from −0.5 to −0.9 V vs. the reversible hydrogen electrode (RHE), reaches a max i mum Faradaic efficiency of 99%, and achieves an NH 4 + yield rate of 7417 µg·h −1 ·cm −2 . Quasi‐in situ x‐ray photoelectron spectroscopy (XPS), in situ Fourier‐transform infrared (FT‐IR) spectroscopy, isotope‐resolved spectroscopy, differential electrochemical mass spectrometry (DEMS), kinetic isotope effect (KIE) analysis, and electron paramagnetic resonance (EPR) collectively support dynamic charge redistribution, bridge‐bound * NO formation, and rapid generation and consumption of surface active hydrogen, consistent with hydrogen‐spillover‐assisted hydrogenation. Density functional theory (DFT) calculations further reveal the preferential stabilization of bridge‐bound * NO at multimetal sites and the thermodynamically unfavorable accumulation of isolated H on the fully hydroxylated surface, providing theoretical support for the proposed adsorption–hydrogen‐transfer pathway. These findings highlight adsorption‐spillover coupling as a promising design strategy for entropy‐engineered hydroxides in nitrogen‐cycle electrocatalysis.