Sustainable ammonia production by non-thermal plasmas: Status, mechanisms, and opportunities

Ammonia is one of our most important industrial chemicals supporting the global food supply as the major crop fertilizer. Moreover, it is increasingly being promoted as a promising carbon-free fuel source, and an energy storage and transportation medium. However, the current approach for ammonia synthesis, known as the Haber-Bosch process, requires large-scale infrastructure preventing the designs of decentralization. The process consumes a large fossil fuel input leading it to be a major source of CO2 emissions. Plasma-enabled ammonia synthesis provides a clean, sustainable and flexible alternative, where the process is driven by the use of plasmas that activate the source gas(es). However, atmospheric plasmas are complex due to their highly reactive environment (energetic electrons, reactive oxygen and nitrogen species, UV photons, electric field effects, and others), resulting in a challenging scientific issue for both plasma researchers and chemical engineers. The review summarizes the current state-of-the-art of plasma-enabled ammonia synthesis, and provides insights into the fundamental physio-chemistry of plasma activation, including the excitation, dissociation and ionization of feedstocks, as well as the underlying mechanisms for the reaction dynamics of reactive species in the highly-reactive plasma environment. Finally, the opportunities and challenges for this plasma-enabled technology are outlined to approach a sustainable and flexible ammonia industry.