Recent advances and challenges of electrochemical ammonia synthesis

Abstract

Ammonia is essential for human activities because approximately 50% of global food production relies on ammonia-derived fertilizers. Great efforts have been devoted to develop a green and sustainable ammonia synthesis route powered by renewable energy under ambient conditions to replace the carbon-intensive and highly energy-consuming Haber-Bosch process. Here, we summarize recent advances and challenges in three electrochemical ammonia synthesis approaches: lithium-mediated nitrogen reduction reaction, electroreduction of nitrate, and NO x reduction. This review offers insight into reaction mechanisms and strategies to improve the activity, selectivity, and durability of the electrochemical ammonia synthesis processes as well as perspectives regarding future industrial applications. At present, ammonia is synthesized from nitrogen and methane-derived hydrogen via the Haber-Bosch process, which has high energy consumption, CO 2 emission, and high centralization. More than 10% of global ammonia products are further oxidized to produce nitrates and fertilizers. In the future, with a sustainable green energy system, electrochemical ammonia synthesis using air and water, powered by renewable energy under ambient conditions, is highly desirable for a carbon-neutral economy. Although ammonia synthesis via a direct nitrogen reduction reaction is highly appealing, it is still intangible but thermodynamically feasible. Using metallic lithium as a medium offers a practical version of nitrogen reduction. Ammonia synthesis via a nitrate pathway, which combines oxidative fixation and nitrate/NO x reduction, could be more energy efficient than the Haber-Bosch/Ostwald system and, therefore, potentially revolutionize the ammonia industry. Green ammonia synthesis is one of the key components to enable sustainability of new energy systems such as hydrogen economy; hence, the highly energy-consuming and carbon-emitting Haber-Bosch process using fossil fuel-derived hydrogen has to be improved or replaced if possible. Green ammonia synthesis directly from water and air is thermodynamically feasible but intangible. Li-mediated nitrogen reduction and a nitrate pathway that combines oxidative fixation and nitrate/NO x reduction might be viable candidates for a revolutionary replacement of the Haber-Bosch/Ostwald system.