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

Activation of HO cleavage for H* production by defect engineering eliminates the insufficient supply of protons in the NORR 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 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 NO and inhibits the binding of H*. A localized NO and H-rich environment is thus constructed to achieve the rapid hydrogenation of *NO and ensure a high NORR 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.