The role of adsorbed hydroxide reduction in hydrogen evolution and nitrogen reduction reactions in aqueous solution

Abstract

Understanding the reaction mechanisms of the hydrogen evolution reaction (HER) and nitrogen reduction reaction (NRR) in aqueous solution is important for securing a sustainable energy future. However, such mechanisms have not yet been fully elucidated. In this work, taking transition-metal (TM) single-atom catalysts supported on nitrogen-doped carbon (TMN₃ or TMN₄, denoted as *) for demonstration purposes, we employed density functional theory calculations to study their HER/NRR electrocatalytic activities, with H₂O as a proton donor. Our calculated results suggest that TMN_3/4 is likely to be oxidized by hydroxide adsorption (*OH) from H₂O dissociation, leading to the formation of TM(OH)N_3/4 without HER/NRR reactivity. In this regard, a particular electrode potential (U) is needed to drive *OH reduction (*OH → * + OH⁻ − e⁻, denoted as *OH-red) to keep the TMN_3/4 site ready for the delivery of HER/NRR activity. Therefore, *OH reduction plays a critical role in the electrocatalytic activity of HER/NRR in aqueous solution. Specifically, the limiting potential (U_L) of HER is −max(ΔG_*OH-red, ΔG_*H)/e, while the U_L of NRR is −max(ΔG_*OH-red, ΔG_*N2H, ΔG_*NH3)/e for the NRR-*N₂ mechanism and −max(ΔG_*OH-red, ΔG_*H, ΔG_*NHNH, ΔG_*NH3)/e for the NRR-*H mechanism. Our calculated results provide new insights into the chemical reaction mechanisms of HER/NRR in aqueous solution, which can promote the rational design of efficient electrocatalysts.