Triggering three-step relay mechanism over Cu-based electrocatalysts for nitrate reduction

Electrocatalytic nitrate reduction reaction (NO₃RR) is a promising approach for pollutant appreciation. Cu-based catalysts have attracted extensive attention due to their favorable adsorption with nitrate. At present, the continuous hydrogenation is considered as a single reaction mechanism for NO₃RR over Cu, in which the high energy barrier of NO₃^--to-NO₂^- conversion and insufficient active hydrogen (*H) supply lead to high onset potential and low faradaic efficiency. The three-step relay (TSR) mechanism can solve these issues, but it is retarded by the low driving force of spontaneous redox reaction between Cu and NO₃^-. Herein, we design P-doped Cu cluster catalyst to trigger and enhance TSR. The onset potential and faradaic efficiency of NH₃ are -0.15 V (vs. reversible hydrogen electrode) and 99.17% over P-doped Cu cluster, outperforming the reported Cu-based catalysts. The electrochemical in situ characterizations, isotope-labeling experiments and theoretical calculations prove that the low-coordinated Cu in cluster promotes the rate of spontaneous redox reaction as well as P doping provides sufficient *H, which can trigger and enhance TSR pathway for NO₃RR, achieving excellent NO₃RR performance.