The Key Steps and Distinct Performance Trends of Pyrrolic vs Pyridinic M–N–C Catalysts in Electrocatalytic Nitrate Reduction

The electrochemical nitrate reduction reaction (NO3RR) offers a sustainable route for ambient ammonia synthesis. While metal-nitrogen-carbon (M-N-C) single-atom catalysts have emerged as promising candidates for the NO3RR, the structure-activity relations underlying their catalytic behavior remain to be elucidated. Through systematic analysis of reported experimental data and pH-field coupled microkinetic modeling on a reversible hydrogen electrode (RHE) scale, we reveal that the coordination-dependent activity originates from distinct scaling relations governed by metal-intermediate interactions. M-N-Pyrrolic catalysts generally demonstrate higher turnover frequencies for ammonia production than M-N-Pyridinic catalysts. Meanwhile, the adsorption and protonation of nitrate, which is a step often dismissed and/or assumed to be simultaneous in many previous reports, are identified to be the rate-determining step (RDS) in the NO3RR. Remarkably, our subsequent experimental validation confirms the theoretical predictions under both neutral and alkaline conditions. This study offers a comprehensive mechanistic framework for interpreting the electrocatalytic activity of M-N-C catalysts in the NO3RR, showing that a classical thermodynamic "limiting-potential model" is not sufficiently accurate to capture the RDS and the catalytic performance trends of different materials (even on M-N-Pyrrolic and M-N-Pyridinic catalysts). These findings provide brand new insights into the reaction mechanism of the NO3RR and establish fundamental design principles for electrocatalytic ammonia synthesis.