Critical review on electrocatalytic reduction of nitrogen and nitrate to ammonia

Among the various strategies for ammonia production, the electrocatalytic reduction of nitrogen is promising method and various heterogeneous electrocatalysts are being developed to facilitate the nitrogen reduction reaction (NRR) for ammonia synthesis. However, these methods are not suitable for large-scale commercialization due to the high stability and nonpolar nature of the N2 bond, which is further challenged by competing hydrogen evolution reactions. Elevated temperatures enhance the Faradaic efficiency (FE) but also increase the reverse reaction rate, leading to the dissociation of ammonia. Thus, nitrate has been used as an alternative reagent for electrocatalytic ammonia synthesis because it shows higher solubility and more feasible bond dissociation. Electrocatalytic nitrate reduction achieves high FE and NH3 yields, surpassing those of the competitive hydrogen evolution reactions experienced in the NRR. Various electrocatalysts have been used for effective nitrate reduction, including metal oxide-based, metal alloys, non-oxide, metal phosphides, metal sulfides, metal carbides, and carbon-based electrocatalysts. Previously, researchers addressed these methods' advantages but lacked a comparative assessment. This review addresses this gap, thoroughly examines the mechanisms involved in the electrocatalytic synthesis of NH3, including various nitrate and NRR pathways, and proposes a reliable protocol for detecting ammonia. This work provides important insights on recent advancements in innovative electrocatalysts.