Revised Nitrogen Reduction Scaling Relations from Potential-Dependent Modeling of Chemical and Electrochemical Steps

The electrochemical nitrogen reduction reaction (NRR) is a promising route to enable carbon-free ammonia production. However, it is limited by the poor activity and selectivity of current catalysts. The rational design of superior NRR electrocatalysts requires a detailed mechanistic understanding of current material limitations to inform how these can be overcome. The current understanding of how scaling limits NRR on metal catalysts is predicated on a simplified reaction pathway that only considers proton-coupled electron transfer (PCET) steps. Here, we apply grand canonical density functional theory to investigate a more comprehensive NRR mechanism that includes both electrochemical and chemical steps on 23 metal surfaces in solvent under an applied potential. We applied Φmax, a grand canonical adaptation of the Gmax descriptor, to evaluate trends in catalyst activity. This approach produces a Φmax “volcano” diagram for NRR activity scaling on metals that qualitatively differs from the scaling relations identified when only PCET steps are considered. NH3* desorption was found to limit NRR activity for materials at the top of the volcano and truncates the volcano’s peak at increasingly reducing potentials. These revised scaling relations may inform the rational design of superior NRR electrocatalysts. This approach is transferable to study additional materials and reaction chemistries where both electrochemical and chemical steps are modeled under an applied potential.