Recent developments in heterogeneous electrocatalysts for ambient nitrogen reduction to ammonia: Activity, challenges, and future perspectives

Ammonia (NH3) plays a significant role in fertilizer production to support the ever-increasing population and serves as a renewable energy carrier as well as a zero-carbon emission fuel. Currently, NH3 is produced by a non-sustainable and highly energy-intensive process. Among the available alternatives, the electrochemical nitrogen reduction reactions (NRR) has attracted attention due to the compact and on-site electrolytic cells that can operate from solar or wind power under ambient conditions but still suffer from relatively low Faraday efficiency and NH3 yields. For the commercialization of electrocatalytic NRR, a catalytic material demonstrating a FE ∼50% and an NH3 yield rate of ∼10−6 mol s−1 cm−2 is suggested. Various strategies, including amorphization, structural engineering, catalyst-support interactions and hydrogen evolution suppression over catalytic materials, have been presented to enhance the electrocatalytic NRR. In this review, the current progress of various identified NRR electrocatalysts (including metal carbides, nitrides, oxides, phosphides, sulfides, selenides, borides, bimetallic materials, metal-organic frameworks, and metal-free materials) is summarized by collectively focusing on both theoretical analysis and experimental investigations. For further development of rational catalysts, a collaboration of theoretical and experimental studies, advanced characterization techniques, understanding of the electrocatalytic mechanism, efficient screening systems, and precise detection methods are needed. Specially designed electrocatalysts can improve NRR activity by regulating the cathodic reactions. The challenges and future perspectives have been described with special emphasis on various transition metal-based electrocatalysts for N2 fixation.