Graphical abstract

The electrocatalytic reduction of nitrate (NO₃-RR) to ammonia presents a promising approach to removing nitrate pollutants from wastewater while offering an alternative to the Haber-Bosch process. Carbon materials are particularly appealing as metal-free catalysts for NO₃-RR due to their large surface area, low cost, and minimal environmental impact. In this study, P-doped carbons with different phosphorus (P) contents were prepared by doping activated carbon with phosphoric acid to investigate the influence of heteroatom doping on NO₃-RR performance. The results revealed the beneficial effect of P-doping, significantly enhancing both the NH3 yield rate and Faradaic efficiency (FE) compared to those for undoped carbon. Notably, the catalyst with optimal P-doping achieved a four-fold increase in NH₃ yield rate and 90.2% FE. However, excessive P led to the aggregation of surface phosphate species, diminishing the catalytic enhancement. Characterization studies revealed a positive correlation between the surface P concentration, which can generate positively charged P and C sites that facilitate NO₃^– adsorption, and improved NO₃-RR performance. This study emphasizes the importance of carefully controlling P-doping to optimize surface P species and highlights the potential of P-doped carbons as effective metal-free catalysts for NO₃-RR.