A Review of Composite/Hybrid Electrocatalysts and Photocatalysts for Nitrogen Reduction Reactions: Advanced Materials, Mechanisms, Challenges and Perspectives

The electrochemical reduction of nitrogen to produce ammonia using sustainable and "green" materials and electricity has proven to be not only feasible, but promising. However, low catalytic activity and stability as well as poor product selectivity have hindered practical application. To address this, this review will provide a comprehensive presentation of the latest progress in the experimental investigation and fundamental understanding of nitrogen reduction reaction (NRR) for the production of ammonia as catalyzed by electrocatalysts and photocatalysts. In particular, the design, synthesis, characterization and performance validation of these catalysts are classified and analyzed in terms of their catalytic activity, stability and selectivity toward ammonia production. Reviewed electrocatalysts include metal/carbon, metal/metal oxide and metal oxide/carbon composites, and reviewed photocatalysts include semiconductor-semiconductor, semiconductor-metal, semiconductor-carbon and multicomponent heterojunctions. Furthermore, several challenges are discussed and possible research directions are proposed to facilitate further research and development to overcome the challenges in NRR toward practical application.