Selectivity Control in the Nitrogen Reduction Reaction over Mo2C MXene by a Nitrogen-Rich Environment

Abstract

Electrochemical ammonia formation through electrocatalytic nitrogen reduction (NRR) is an environmentally friendly route, but the low intrinsic activity of catalytic materials and competition with the hydrogen evolution reaction (HER) represent a major hurdle for large-scale implementation. Strategies to achieve high NRR selectivity in aqueous medium, which is promising for scale up due to their simplicity, nontoxicity, and low cost, are still quite limited. Using density functional theory calculations combined with descriptor-based analyses, we investigate NRR over Mo₂C, a two-dimensional material from the MXene class that is considered as a potential NRR catalyst based on previous experimental and theoretical work. We outline that several reaction mechanisms and elementary reaction steps contribute to the formation of ammonia for Mo₂C(0001), indicating that the conventional picture of a single rate-determining step is insufficient to describe the complex proton-coupled electron transfer steps of NRR. Our theoretical study shows that a nitrogen-rich environment around the catalytically active site allows selectivity to be directed toward ammonia formation due to increased NRR activity while simultaneously decreasing HER activity. This finding could serve as a guide for the synthesis of heterogeneous electrocatalysts with a nitrogen-rich environment near the active site to steer the selectivity problem of the competing NRR and HER to ammonia formation.