Cathodic Interfacial Oxidation-Induced Rapid Synthesis of Nickel Phosphate Electrocatalyst from Wastewater for Acidic Nitrate-to-Ammonia Conversion

The electrochemical nitrate reduction reaction (NO₃⁻RR) presents a promising strategy for simultaneous wastewater remediation and ammonia synthesis. However, the fabrication of efficient, stable, and economically viable electrocatalysts remains a significant challenge. Addressing this, we report the rapid (14 min) electrodeposition of an amorphous nickel phosphate catalyst directly onto a metallic foam cathode, utilizing nickel plating wastewater as a precursor under high-current conditions. This catalyst exhibits exceptional performance in acidic media, achieving a Faradaic efficiency of 96.6% for ammonia at −0.5 V (versus RHE) in 10 mM NO₃⁻ solution, alongside remarkable electrochemical stability exceeding 20 h. We attribute the catalyst formation to the oxidation of hypophosphite at the cathode interface during high current polarization and its subsequent co-deposition with Ni²⁺ ions. When applied to actual nickel plating wastewater at 200 mA cm⁻², the system demonstrated relatively stable operation for 30 h and successfully recovered ammonium sulfate. Combined experimental and theoretical analyses reveal the origin of the superior NO₃⁻RR activity. This work provides an innovative waste-derived strategy for the rapid synthesis of high-performance catalysts and advances the sustainable recovery of nitrogen, phosphorus, and nickel from industrial waste streams.