Unveiling the Dual Role of Oxophilic Cr⁴⁺ in Cr−Cu₂O Nanosheet Arrays for Enhanced Nitrate Electroreduction to Ammonia

Abstract Cuprous oxide (Cu 2 O)‐based catalysts present a promising activity for the electrochemical nitrate (NO 3 − ) reduction to ammonia (eNO 3 RA), but the electrochemical instability of Cu + species may lead to an unsatisfactory durability, hindering the exploration of the structure‐performance relationship. Herein, we propose an efficient strategy to stabilize Cu + through the incorporation of Cr 4+ into the Cu 2 O matrix to construct a Cr 4+ −O−Cu + network structure. In situ and quasi‐in situ characterizations reveal that the Cu + species are well maintained via the strong Cr 4+ −O−Cu + interaction that inhibits the leaching of lattice oxygen. Importantly, in situ generated Cr 3+ −O−Cu + from Cr 4+ −O−Cu + is identified as a dual‐active site for eNO 3 RA, wherein the Cu + sites are responsible for the activation of N‐containing intermediates, while the assisting Cr 3+ centers serve as the electron‐proton mediators for rapid water dissociation. Theoretical investigations further demonstrated that the metastable state Cr 3+ −O−Cu + favors the conversion from the endoergic hydrogenation of the key *ON intermediate to an exoergic reaction in an ONH pathway, and facilitates the subsequent NH 3 desorption with a low energy barrier. The superior eNO 3 RA with a maximum 91.6 % Faradaic efficiency could also be coupled with anodic sulfion oxidation to achieve concurrent NH 3 production and sulfur recovery with reduced energy input.