Exploring nitrogen reduction reaction mechanisms in electrocatalytic ammonia synthesis: A comprehensive review

Abstract

The electrochemical nitrogen reduction reaction (eNRR) holds significant promise as a sustainable alternative to the conventional large-scale Haber Bosch process, offering a carbon footprint-free approach for ammonia synthesis. While the process is thermodynamically feasible at ambient temperature and pressure, challenges such as the competing hydrogen evolution reaction, low nitrogen solubility in electrolytes, and the activation of inert dinitrogen (N2) gas adversely affect the performance of ammonia production. These hurdles result in low Faradaic efficiency and low ammonia production rate, which pose obstacles to the commercialisation of the process. Researchers have been actively designing and proposing various electrocatalysts to address these issues, but challenges still need to be resolved. A key strategy in electrocatalyst design lies in understanding the underlying mechanisms that govern the success or failure of the electrocatalyst in driving the electrochemical reaction. Through mechanistic studies, we gain valuable insights into the factors affecting the reaction, enabling us to propose optimised designs to overcome the barriers. This review aims to provide a comprehensive understanding of the various mechanisms involved in eNRR on the electrocatalyst surface. It delves into the various mechanisms such as dissociative, associative, Mars-van Krevelen, lithium-mediated nitrogen reduction and surface hydrogenation mechanisms of nitrogen reduction. By unravelling the intricacies of eNRR mechanisms and exploring promising avenues, we can pave the way for more efficient and commercially viable ammonia synthesis through this sustainable electrochemical process by designing an efficient electrocatalyst.