Recent Advances and Challenges of Electrocatalytic N2 Reduction to Ammonia

Global ammonia production reached 175 million metric tons in 2016, 90% of which is produced from high purity N-2 and H-2 gases at high temperatures and pressures via the Haber-Bosch process. Reliance on natural gas for H-2 production results in large energy consumption and CO2 emissions. Concerns of human-induced climate change are spurring an international scientific effort to explore new approaches to ammonia production and reduce its carbon footprint. Electrocatalytic N-2 reduction to ammonia is an attractive alternative that can potentially enable ammonia synthesis under milder conditions in small-scale, distributed, and on-site electrolysis cells powered by renewable electricity generated from solar or wind sources. This review provides a comprehensive account of theoretical and experimental studies on electrochemical nitrogen fixation with a focus on the low selectivity for reduction of N-2 to ammonia versus protons to H-2. A detailed introduction to ammonia detection methods and the execution of control experiments is given as they are crucial to the accurate reporting of experimental findings. The main part of this review focuses on theoretical and experimental progress that has been achieved under a range of conditions. Finally, comments on current challenges and potential opportunities in this field are provided.