Electrochemical Nitrogen Reduction to Ammonia at Ambient Condition on the (111) Facets of Transition Metal Carbonitrides.

Abstract

We conducted Density Functional Theory calculations to investigate a class of materials with the goal of enabling nitrogen activation and electrochemical ammonia production under ambient conditions. The source of protons at the anode could originate from either water splitting or H2, but our specific focus was on the cathode reaction, where nitrogen is reduced into ammonia. We examined the conventional associative mechanism, dissociative mechanism, and Mars-van Krevelen mechanism on the (111) facets of the NaCl-type structure found in early transition metal carbonitrides, including Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, Sc, Y, and W. We explored the catalytic activity by calculating the free energy of all intermediates along the reaction pathway and constructing free energy diagrams to identify the steps that determine the reaction's feasibility. Additionally, we closely examined the potential for catalyst poisoning within the electrochemical environment, considering the bias required to drive the reaction. Furthermore, we assessed the likelihood of catalyst decomposition and the potential for catalyst regeneration among the most intriguing carbonitrides. Our findings revealed that the only carbonitride catalyst considered here exhibiting both activity and stability, capable of self-regeneration and nitrogen-to-ammonia activation, is NbCN with a low potential-determining step energy of 0.58 eV. This material can facilitate ammonia formation via a mixed associative-MvK mechanism. In contrast, other carbonitrides of this crystallographic orientation are likely to undergo decomposition, reverting to their parent metals under operational conditions.