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

Cuprous oxide (Cu₂O)-based catalysts present a promising activity for the electrochemical nitrate (NO₃ ^-) reduction to ammonia (eNO₃RA), 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 Cr⁴⁺ into the Cu₂O matrix to construct a Cr⁴⁺-O-Cu⁺ network structure. In situ and quasi-in situ characterizations reveal that the Cu⁺ species are well maintained via the strong Cr⁴⁺-O-Cu⁺ interaction that inhibits the leaching of lattice oxygen. Importantly, in situ generated Cr³⁺-O-Cu⁺ from Cr⁴⁺-O-Cu⁺ is identified as a dual-active site for eNO₃RA, wherein the Cu⁺ sites are responsible for the activation of N-containing intermediates, while the assisting Cr³⁺ centers serve as the electron-proton mediators for rapid water dissociation. Theoretical investigations further demonstrated that the metastable state Cr³⁺-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 NH₃ desorption with a low energy barrier. The superior eNO₃RA with a maximum 91.6 % Faradaic efficiency could also be coupled with anodic sulfion oxidation to achieve concurrent NH₃ production and sulfur recovery with reduced energy input.