Coupling Nitrate‐to‐Ammonia Conversion and Sulfion Oxidation Reaction Over Hierarchical Porous Spinel MFe2O4 (M═Ni, Co, Fe, Mn) in Wastewater

Abstract The construction of coupled electrolysis systems utilizing renewable energy sources for electrocatalytic nitrate reduction and sulfion oxidation reactions (NO 3 RR 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 MFe 2 O 4 (M═Ni, Co, Fe, Mn) dual‐functional electrocatalysts consisting of Mn‐doped porous NiFe 2 O 4 /CoFe 2 O 4 heterostructure networks and Ni/Co/Mn co‐doped Fe 3 O 4 nanosheet networks. The excellent NO 3 RR with high NH 3 Faradaic efficiency of 95.2% at ‐0.80 V versus reversible hydrogen electrode (vs RHE) and NH 3 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 MFe 2 O 4 . Key intermediates such as * NO, * NH 2 , and NH 3 are identified in the NO 3 RR process by in situ Fourier transform infrared spectroscopy (in situ FTIR). The MFe 2 O 4 ‐assembled two‐electrode coupling system (NO 3 RR||SOR) shows an ultra‐low cell voltage of 1.14 V at 10 mA cm −2 , much lower than the NO 3 RR||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.