Direct ammonia solid-oxide fuel cells: A review of progress and prospects

With the rapidly declining cost of renewable energy, efficient ways are needed for its transportation between different regions. Hydrogen is becoming a major energy vector, with the key challenges of its storage and transportation commonly overcome by using ammonia for chemical storage of hydrogen energy. Ammonia, which is more energy dense than hydrogen and easier to transport, is a carbon-free alternative fuel that can be used in a variety of ways to generate power. Owing to their robustness and efficiency, solid-oxide fuel cells (SOFC) stand out as one of the most promising technologies that convert ammonia to electricity. Unlike other fuel cells, such as polymer electrolyte membranes, SOFCs do not require the fuel to be cleaned by energy-intensive external cracking and extensive cleaning; their high operating temperature provides the flexibility to crack the ammonia inside the anode or to use it directly. Here, we discuss experimental and numerical studies of ammonia SOFCs and critically review the status and opportunities for ammonia-fuelled SOFC technology. In the first section, we briefly outline the potential cathode and electrolyte materials for SOFCs. Only the anode component poses additional challenges with ammonia over the well-established hydrogen-fuelled SOFC technology, and this topic has been addressed in detail. Anode catalysts for ammonia decomposition, parameters affecting ammonia decomposition and anode catalyst degradation are also discussed. In the second section, we review the modelling studies for ammonia SOFCs. Finally, we run through the major commercial initiatives and demonstrations in green ammonia production and ammonia SOFCs.