Coupling Nitrate‐to‐Ammonia Conversion and Sulfion Oxidation Reaction Over Hierarchical Porous Spinel MFe2O4 (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 (NO3RR 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 MFe2O4 (M═Ni, Co, Fe, Mn) dual-functional electrocatalysts consisting of Mn-doped porous NiFe2O4/CoFe2O4 heterostructure networks and Ni/Co/Mn co-doped Fe3O4 nanosheet networks. The excellent NO3RR with high NH3 Faradaic efficiency of 95.2% at -0.80 V versus reversible hydrogen electrode (vs RHE) and NH3 yield rate of 608.9 µmol h-1 cm-2 at -1.60 V vs RHE, and impressive SOR performance (100 mA cm-2@0.98 V vs RHE) is achieved for MFe2O4. Key intermediates such as *NO, *NH2, and NH3 are identified in the NO3RR process by in situ Fourier transform infrared spectroscopy (in situ FTIR). The MFe2O4-assembled two-electrode coupling system (NO3RR||SOR) shows an ultra-low cell voltage of 1.14 V at 10 mA cm-2, much lower than the NO3RR||OER (oxygen evolution reaction, 10 mA cm-2@2.62 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.