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

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 N₂ adsorption (*N₂) 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 *N₂ protonation step. This asymmetry originates from the proton acceptor geometry: proton transfer to a flat metal site requires a close donor approach to the surface and is increasingly blocked by bulky substituents, whereas the distal N atom of adsorbed *N₂ protrudes from the surface and remains comparatively accessible. Consistent with this picture, the effective symmetry factor for the HER decreases with donor steric bulk, suppressing the HER in both the barrier height and potential response. Microkinetic simulations predict that bulky proton donors widen the cathodic potential window over which high NRR faradaic efficiency is maintained, while intrinsic NRR activity remains largely catalyst-limited. These findings identify donor-acceptor accessibility as a molecular design principle for selective interfacial PCET in nonaqueous electrocatalysis.