Advances in electrocatalytic ammonia synthesis under mild conditions

Ammonia plays a crucial role in food production and industrial manufacturing, and it is predicted as the ideal energy carrier in the future. Typically, it is artificially synthesized from nitrogen and hydrogen through the Haber-Bosch process, which consumes ~5% of the global natural gas supply and emits ~1.5% of the global greenhouse gas. In recent years, electrocatalytic ammonia synthesis from nitrogen and water under mild conditions utilizing renewable electricity has drawn much attention from the scientific community owing to its potential in producing ammonia through a clean and sustainable route. Numerous achievements have been made during the past few years; nevertheless, two major challenges remain. These are the low catalytic activity due to the inertness of nitrogen molecule and the low selectivity attributed to the violent hydrogen evolution side reaction. This review will discuss the fundamentals of electrocatalytic ammonia synthesis at the atomic level relying on a thorough understanding of the mechanism involved in the Haber-Bosch process. The theoretical screening of the electrocatalysts and the experimental practice of rational electrocatalyst design with different strategies are reviewed. Importantly, the relations among design strategies, the catalytic activity and selectivity of the catalyst are evaluated. Additionally, key experimental techniques to conduct an insightful and efficient study of the subject area are discussed. Finally, current challenges and the strategies for overcoming these challenges are examined.