A detailed kinetic study of selective electrochemical nitrate reduction to ammonia

The electrochemical nitrate reduction reaction (eNO₃RR) offers a dual benefit of converting nitrate pollutants into valuable ammonia while circumventing the limitations of the nitrogen reduction reaction. However, this complex multi-step proton-electron transfer process with stable intermediates necessitates highly active and selective catalysts. Despite extensive progress in catalyst development, a fundamental understanding of eNO₃RR kinetics remains limited. Herein, we present a detailed kinetic evaluation of the individual eNO₃RR steps using accessible electrochemical approaches, including impedance analysis, diffusion coefficient calculations, and multistep Bulter Volmer kinetic analysis. Spinel NiCo₂S₄ nanoparticles were employed as a model catalyst, showing an ammonia yield rate of 3223.9 ± 168.3 μg h⁻¹ cm⁻² at −0.30 V_RHE with a Faradaic efficiency of 64.0% in 0.1 M KOH containing 0.5 M KNO₃ electrolyte. The kinetic dependence on NOx reactants and electrolyte concentration was further analyzed to evaluate the contribution of individual reaction steps. Combined with in situ spectroscopic identification of surface intermediates, the kinetic analysis enabled identification of the rate-determining step in eNO₃RR. This study provides mechanistic and kinetic insights into eNO₃RR and proposes a practical working model that can be extended to other eNO₃RR catalysts.