Coupling Nitrate-to-Ammonia Conversion and Sulfion Oxidation Reaction Over Hierarchical Porous Spinel MFeO (M═Ni, Co, Fe, Mn) in Wastewater.

The construction of coupled electrolysis systems utilizing renewable energy sources for electrocatalytic nitrate reduction and sulfion oxidation reactions (NORR and SOR), is considered a promising approach for environmental remediation, ammonia production, and sulfur recovery. Here, a simple chemical dealloying method is reported to fabricate a hierarchical porous multi-metallic spinel MFeO (M═Ni, Co, Fe, Mn) dual-functional electrocatalysts consisting of Mn-doped porous NiFeO/CoFeO heterostructure networks and Ni/Co/Mn co-doped FeO nanosheet networks. The excellent NORR with high NH Faradaic efficiency of 95.2% at -0.80 V versus reversible hydrogen electrode (vs RHE) and NH yield rate of 608.9 µmol h cm at -1.60 V vs RHE, and impressive SOR performance (100 mA cm V vs RHE) is achieved for MFeO. Key intermediates such as NO, NH, and NH are identified in the NORR process by in situ Fourier transform infrared spectroscopy (in situ FTIR). The MFeO-assembled two-electrode coupling system (NORR||SOR) shows an ultra-low cell voltage of 1.14 V at 10 mA cm, much lower than the NORR||OER (oxygen evolution reaction, 10 mA cm V), simultaneously achieving two expected targets of value-added ammonia generation and sulfur recovery, and also demonstrating high durability of 18 h. This work also demonstrates the great potential of spinel ferrite-based catalysts for environmental remediation.