Recent progress in electrocatalytic nitrogen reduction to ammonia (NRR)

Ammonia is an essential raw ingredient for fertilizers, a carbon-free hydrogen carrier, and an alternative fuel. The natural process of producing ammonia cannot meet the rapidly growing global demand. On the other hand, the conventional industrial Haber-Bosch process for ammonia production demands high energy and generates massive greenhouse gases. Recently, the research community has focused on designing environmentally friendly nanocatalysts for the electrocatalytic N2 reduction reaction (NRR) that involves less energy consumption. However, the NRR is still far from industrial implementation because of the low faradic efficiency and ammonia formation rate. In this comprehensive review, we start with a brief overview of the fundamentals of NRR related to thermodynamic perspectives and discuss the reaction mechanisms for ammonia synthesis. We then critically introduce the research methods for electrocatalytic NRR and various nanocatalysts, including transition metals, non-metals, main group elements, and noble-metal-based NRR electrocatalysts. Finally, some outlooks are provided on the research progress of the NRR electrocatalytic nanomaterials, methods and techniques toward ammonia production.