Electrochemical Synthesis of Ammonia: Status, Challenges and Opportunities

Abstract

Ammonia (NH3) is an extremely important feedstock producing nitrogen fertilizers that provide essential nutrition for crop vitality. The worldwide production of ammonia is more than 100 million tons per year. The conventional Haber–Bosch ammonia production process requires high pressure and temperature (up to 300 atm and 500°C) and is, as such, extremely energy-intensive.Electrochemical synthesis of ammonia via an ion exchange membrane (IEM) may proceed at lower temperature and pressure thus potentially reducing the energy consumption 1. However, the application of this technology has been restricted by lower ammonia production (< 10-7 mol/s-cm2) and Faradic efficiency, making the initial capital cost very prohibitive. This work will present current status and challenges of electrochemical synthesis of ammonia, based on our technology development and other noteworthy researches in this area. The strategies to improve the ammonia production rate, efficiency and stability have been elaborated, which include advanced catalyst development, alternative membranes 2 (e.g. alkaline membrane, proton or oxygen ion-conducting metal oxide electrolyte) and optimized operating conditions (e.g., temperature, pressure, and humidity). In particular, the electrochemical synthesis of ammonia using water (free of H2) and nitrogen as reactants is analyzed. Finally, the integration of this technology with off-peak renewable energy towards building distributed ammonia plants will be evaluated. The successful application of this technology may lead to ammonia synthesis via a more environmentally-benign and economically viable approach, compared to the conventional Haber-Bosch process. The synthesized ammonia using renewable energy can be widely used as a feedstock for fertilizers in agriculture. The ammonia can also be considered as a means of storing surplus renewable energy during off-peak hours. Reference: Marnellos, G., and M. Stoukides, “Ammonia synthesis at atmospheric pressure,” Science, 282(2), 98-100 (1998). Licht, S, Cui, B., Wang, B., Li, F-F, Lau, J., and Liu, S., “Ammonia syhthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3”, Science, 345, 637-640 (2014)