Ammonia as a green energy carrier: Electrochemical synthesis and direct ammonia fuel cell - a comprehensive review

To achieve carbon neutrality by 2050 is an emergent challenge facing the world in order to tackle the climate change. Ammonia, being one of the most promising media for hydrogen storage, is regarded as an ideal carbon-free energy carrier. Moreover, ammonia exhibits critical superiority in long-distance transportation and storage, which are the essential bottlenecks yet to be solved for hydrogen. Furthermore, the electrochemical synthesis of ‘green ammonia’ from renewable energy and its efficient utilization to generate electricity can be well integrated with reversible solid oxide cell (R-SOC), which is a promising technology that provides a potential route to decarbonize a number of applications dependent on fossil fuels. For electrochemical ammonia synthesis using both solid oxide electrolytic cell (SOEC) and proton ceramic electrolytic cell (PCEC), the state-of-the-art ammonia production rate can reach 8–9 × 10−9 mol s−1 cm−2 at temperatures as low as 400 °C. For direct ammonia conversion using solid oxide fuel cell (SOFC) and proton ceramic fuel cell (PCFC), the best power density reported is comparable to that of the hydrogen fuel cells, reaching >1.2 W/cm2. This review aims to provide a comprehensive summary of the latest advances in both aspects: electrochemical ammonia synthesis (also known as electrochemical Haber-Bosch Process) and direct ammonia fuel cells. The process mechanisms of two major types of electrochemical routes based on oxygen ion conductor and proton conductor have been reviewed, with a discussion on material development and stability issues.