Solvation effects in the electrochemical reduction of hydrogen cyanide for ambient ammonia production on a Ni cathode

Abstract

The large-scale production of ammonia via the Haber–Bosch process is an integral part of maintaining global populations, yet it is dependent on the harsh reaction conditions and hydrogen sourced via steam reforming. The electrochemical reduction of hydrogen cyanide (HCNRR), a fixed form of nitrogen, has shown itself to be a promising route for ammonia synthesis at ambient conditions, offering a path to contribute to a circular nitrogen economy. While the HCNRR is still an understudied area of catalysis, few experimental reports have identified nickel as a promising catalyst, outperforming precious metals such as platinum. On a Ni cathode, two sets of HCNRR products have been observed, namely methylamine (major product) and ammonia/methane (minor products). Recent computational studies have rationalized this product distribution with the desorption of methylamine and hinted on the electrolyte playing a role in the selectivity towards ammonia production. Herein, we investigate the HCNRR mechanism on a Ni cathode using different solvation models in a bid to account for the influence of the electrolyte. Our findings reveal that the presence of an explicit solvent environment has indeed a drastic effect on the HCNRR, resulting in different binding modes and an unexpected metastable intermediate which ultimately leads to a different potential limiting step. These results highlight the necessity of including explicit solvent molecules for the effective modelling of physisorbed intermediates in the HCNRR process, although it may also be generalizable to other important electrochemical processes.