A computational study of the electrochemical cyanide reduction for ambient ammonia production on a nickel cathode

Abstract

The dependence of the global population on ammonia means that the Haber–Bosch process is one of the most studied and optimised processes in chemistry. Despite this focus, the reaction still requires harsh conditions, which has prompted substantial research efforts to enable the electroreduction of N₂ at ambient conditions. The literature regarding this reaction is plagued with reports of competition with the hydrogen evolution reaction and large potentials required for N₂ dissociation. Cyanide, isoelectronic with N₂, has a significantly lower bond dissociation energy which could enable a route to sustainable ammonia production. Herein, we present the first mechanistic study of the cyanide electroreduction on a Ni cathode, experimentally shown to produce methylamine as a major product, and CH₄ and NH₃ as minor products. Theoretical calculations reveal that the dominance of methylamine in the products is due to the weak binding of this species, which desorbs from the surface preventing further hydrogenation. We also present an alternative CNRR pathway which involves the hydrogen atoms comprising the predicted surface coverage at relevant reaction conditions. Interestingly, this new pathway circumvents the formation of methylamine, exclusively producing CH₄ and NH₃, although it is hindered by the presence of a large energy barrier. Strategies to favour this alternative pathway and the prospects of CNRR for the sustainable production of ammonia under ambient conditions are also outlined.