A review on intermediate temperature electrochemical synthesis of ammonia

Ammonia and its derivatives, such as ammonium nitrite and urea, are important nitrogen sources for fertilizers that are extensively used in modern global agriculture. The current Haber Bosch process of ammonia synthesis contributes significantly to global CO2 emissions (∼450 million metric tons of CO2, which is around ∼1.2% of global CO2 emissions). Due to the increasing availability of renewable energy sources, electrochemical technologies to produce green hydrogen and ammonia have attracted worldwide attention. Using an electrochemical route, ammonia can be synthesized in a single reactor using air and water as the feedstocks, contributing significantly to reducing CO2 emissions. Technology provides various social, economic and environmental benefits if DOE ammonia synthesis rate targets can be achieved for commercialisation of this technology. Notably, it could also be a sustainable route to transport renewable energy in the form of hydrogen derivative liquid fuels in large quantities to areas lean in renewables. Intermediate temperature operations (400–600 °C) are widely recognised for facilitating improved kinetics and offering better energy efficiency for various electrochemical reactions. In this review, recent advancements in the electrochemical ammonia synthesis field have been explicitly covered on solid-state electrolyte materials and electrodes for intermediate temperature operations (400–600 °C). Some challenges regarding ammonia production rates, energy inefficiencies, unresolved issues such as reaction selectivity versus competing side reactions, mechanistic understanding, and lack of standardization in ammonia measurement protocols have been extensively covered, which are essential for further developments in this field.