(428h) Theoretical Insights into the Potential-Dependent Competition between Electrochemical Nitrogen Reduction Reaction and Hydrogen Evolution Reaction

The electrochemical N₂ reduction reaction (NRR) produces ammonia cleanly and sustainably at ambient conditions. The challenge in realizing NRR is the suppression of the hydrogen evolution reaction (HER), thus understanding the competition between NRR and HER is critical. One of the biggest obstacles for developing catalysts for NRR is a premature decrease of NRR activity with electrode potential before reaching the limiting potential which can facilitate NRR exothermically. Undesirably, the NRR activity has been consistently shown to reach a maximum quickly and then decrease with increasing reduction potential, but the exact mechanistic origin is unclear. In this talk, we discuss the potential-dependent competition between NRR and HER using the constant electrode potential model and the microkinetic modeling. We find a potential-dependent crossover between the H and N₂ binding energies, resulting in the crossover between the H and N₂ coverages. The simulated NRR activity vs. electrode potential trend is in great agreement with the experimentally observed trend. We find that the crossover originates from the larger charge transfer in *H formation than that of *N₂ and *NNH formation. Consideration of various catalysts shows that the larger charge transfer of *H than *N₂ and *NNH formation, and the potential-dependent activity crossover is a general trend, thus presenting a fundamental challenge to realizing NRR.