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^{[email protected]} 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^{[email protected]} 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.