Electrochemical Ammonia Synthesis

Abstract

Ammonia is essential for global food production as a component of fertiliser, and a potential energy carrier in the energy transition. Electrochemical ammonia synthesis faces numerous challenges as an alternative to the carbon intensive Haber-Bosch process, including competition from hydrogen evolution, and mass transport limitations. An unconventional cell design with a hydrogen permeable electrode could help to address these problems. The reaction mechanism and performance of ammonia synthesis using hydrogen permeable electrodes was investigated at elevated temperatures and pressures of up to 120 °C and 8 bar. Furthermore, a facile method for enhancing the electrochemical surface area of the electrode was developed and tested. Operation at elevated pressure resulted in a moderate increase in cell performance. However, replenishment of the nitrogen vacancies in the nickel nitride catalyst through dinitrogen adsorption is identified as the elementary step that limits activity and stability. The amount of pre-deposited N is found to significantly influence the ammonia production rate and its stability. Dynamics of ammonia desorption could also play a role in nitride regeneration. In future studies, the presence of a decomposition reaction of the nitride should be investigated. Usage of more stable nitride species or a combination of host nitride and dopants is recommended to improve stability and simultaneously promote dinitrogen activation and hydrogenation to ammonia. These findings expand the understanding of the mechanisms underlying the nitrogen reduction reaction, paving the way for the development of a more efficient green ammonia synthesis process.