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

TL;DR

This study elucidates the structure-activity relationships of M-N-C catalysts in nitrate reduction, revealing distinct performance trends and the rate-determining step, supported by theoretical analysis and experimental validation.

Contribution

It provides a comprehensive mechanistic framework and design principles for electrocatalytic ammonia synthesis via nitrate reduction, highlighting the importance of coordination-dependent activity.

Findings

M-N-Pyrrolic catalysts have higher ammonia production rates.

M-N-Pyridinic catalysts show broader activity ranges.

Nitrate adsorption and protonation are the rate-determining step.

Abstract

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 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 modelling 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 demonstrate higher turnover frequencies for ammonia production, whereas M-N-Pyridinic catalysts exhibit broader activity ranges across the activity volcano plot. Meanwhile, the adsorption and protonation of nitrate, which is a step often dismissed and/or assumed to be simultaneous in many previous reports, is identified to be the rate-determining step (RDS) in 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 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 NO3RR and establish fundamental design principles for electrocatalytic ammonia synthesis.