Steric Hindrance of Proton Donors Modulates Heterogeneous Electrochemical Nitrogen Reduction Reaction Selectivity in Nonaqueous Electrolytes

The electrochemical nitrogen reduction reaction (NRR) in aqueous electrolytes is severely limited by the early buildup of adsorbed H (*H) at cathodic potentials, which suppresses N 2 adsorption (*N 2 ) and promotes the hydrogen evolution reaction (HER). Nonaqueous electrolytes containing molecular proton donors offer a route to mitigate this competition, but molecular design principles that suppress the HER without comparably slowing the NRR remain poorly defined. Here, density functional theory calculations, potential-dependent free energy analysis, and microkinetic modeling show that proton donor steric hindrance selectively gates interfacial proton-coupled electron transfer (PCET) on Ru(0001) in acetonitrile. Increasing donor steric bulk substantially raises the barrier for the HER Volmer step, while only weakly perturbing the rate-determining *N 2 protonation step. This asymmetry ori…