Challenges and advancement in direct ammonia solid oxide fuel cells: a review

Abstract

Finding solutions to tackle climate change and decarbonizing the energy sector are emerging as significant challenges for the global community. Carbon dioxide (CO₂) emission is the primary driver of global climate change, and therefore the world needs to urgently reduce CO₂ emissions by producing clean energy. Among the family of fuel cells, a solid oxide fuel cell (SOFC) is a high-efficiency power generation device with zero emission if hydrogen produced from renewable energy sources is used as a fuel. However, the utilization of hydrogen is restricted by the challenges related to its compression, storage, and transportation. Green ammonia contains 17.6 wt% hydrogen and is considered a suitable medium for hydrogen storage/carriage, facilitating CO₂-free energy systems, and so can play a critical role in the transition to clean energy. Therefore, green ammonia is an ideal carbon-free fuel for power generation in direct ammonia SOFCs (DASOFCs). In DASOFCs, ammonia cracks into hydrogen and nitrogen in situ in the anode chamber, and hydrogen is consumed in the electrochemical process; hence, there is no need for a separate ammonia cracking and hydrogen/nitrogen separation unit. Key technological challenges, though, must be addressed to realize the potential of green ammonia as a fuel in SOFCs. Therefore, the current review focuses on the role of ammonia as an energy carrier for power generation and discusses technological challenges such as ammonia safety, open circuit voltage (OCV) stabilization, thermal shocks, NO_x/N₂O emission, nitride formation, and nickel coarsening, limited power densities, and sealing issues, and the recent advancements in DASOFC technology. This review will also cover the performance of state-of-the-art anode materials and newly emerging proton-conducting materials for DASOFCs, along with the various approaches being used for the high-efficiency and long-term stability of DASOFCs in kilowatt (kW) scale systems. Finally, we address remaining challenges, discuss new opportunities, and suggest future recommendations for developing efficient DASOFC systems for the single-step conversion of ammonia to electricity.