Promoting defect formation and inhibiting hydrogen evolution by S-doping NiFe layered double hydroxide for electrocatalytic reduction of nitrate to ammonia

Activation of H₂O cleavage for H* production by defect engineering eliminates the insufficient supply of protons in the NO₃^-RR process under neutral conditions. However, it remains challenging to precisely control the defect formation for optimizing the equilibrium between H* production and H* binding. Here, we propose a strategy to boost defect generation through S-doping induced NiFe-LDH lattice distortion, and successfully optimize the balance of H* production and binding. The Faraday efficiency of the Sx-NiFe-LDH-Ov@CuO/CF electrode for treating 100 mg-N L^-1 nitrate wastewater at -0.4 V vs. RHE is up to 97.8 %, which is superior to the reported advanced catalysts for the treatment of low nitrate concentrations. In situ characterization and theoretical calculations show that the sulfur-mediated defect leads to the d-band center displacement of Ni and Fe sites, which efficiently promotes the enrichment of NO₃^- and inhibits the binding of H*. A localized NO₃^- and H⁺-rich environment is thus constructed to achieve the rapid hydrogenation of *NO and ensure a high NO₃^-RR activity. This work provides several insights for modulating structural defects and analyzing intrinsic active sites to achieve high-performance electrocatalysts for the treatment of nitrate wastewater with low carbon-to-nitrogen ratio.