Tracking Electron Flow in Aqueous Electrochemical Nitrogen Reduction to Ammonia on Ru(0001)

Abstract

Global ammonia production is dominated by the energy-intensive Haber-Bosch process, motivating the development of sustainable electrochemical alternatives. While lithium- and calcium-mediated systems in organic solvents have relatively high yields, aqueous electrochemical nitrogen reduction (N₂ER) remains inefficient. Density functional theory (DFT) studies have significantly advanced the understanding of N₂ER in aqueous solution; however, most have focused on thermodynamic and/or kinetic analyses to elucidate reaction mechanisms, while orbital-level insights remain largely unexplored. Here, we combine DFT calculations with intrinsic bond orbital analysis to examine orbital evolution during 13 protonation steps of N₂ER. This approach identifies the orbitals responsible for proton accommodation in each intermediate. We find that nine of the 13 steps involve nitrogen lone-pair orbitals. These results suggest that protonation kinetics in aqueous N₂ER are strongly influenced by interactions between nitrogen lone pairs and the local solvation environment, including cations and hydrogen bonding. Moreover, surface modifications, which profoundly affect the reactivity of the metal–nitrogen σ bond but have a lesser influence on nitrogen lone-pair orbitals are not expected to have a direct impact on the protonation kinetics of N₂ER. Thus, rational manipulation of the local electrochemical environment, rather than solely modifying the catalyst surface, may be essential for improving N₂ER performance.