Research and Development of Ammonia-Fueled SOFC Systems

Abstract

Hydrogen is the primary fuel source for fuel cells. However, the low volumetric energy density and difficulty in storage and transportation are major obstacles for the practical utilization. Thus, the on-site generation of hydrogen from its carrier is an effective method for the fuel supply. Among various hydrogen carriers, ammonia is one of the promising candidates because of its high hydrogen density, low production cost, and ease in liquefaction and transport. Ammonia decomposes into nitrogen and hydrogen through a mildly endothermic process. The ammonia decomposition temperature is close to the operating conditions of solid oxide fuel cells (SOFCs). Therefore, the integration of these two devices is beneficial in terms of efficient heat and energy managements and will lead to the development of simplified generation systems. Furthermore, hydrogen and nitrogen produced from ammonia fuel are expected to have little negative effect on fuel cell performance, while hydrocarbon fuels draws some severe problems at electrodes, such as CO poisoning and carbon deposition in low- and high-temperature fuel cells, respectively. We have investigated three types of ammonia-fueled SOFC systems. In one system, ammonia is directly supplied to the anode chamber. Ammonia decomposes into nitrogen and hydrogen over Ni-based cermet anode, and subsequently the generated hydrogen is electrochemically oxidized to produce steam and electron. The second system consists of ammonia cracker and SOFC. In this case, ammonia is catalytically decomposed through the ammonia cracker, and hydrogen produced is electrochemically oxidized on the anode. Ruthenium catalysts are well-known to promote the decomposition of ammonia, but the limited availability of ruthenium requires the development of new catalyst systems. Then, we have developed nickel-based catalysts and applied them for the ammonia cracker due to their relatively high activity for ammonia decomposition among base metals. The last one is the system combined with auto-thermal ammonia cracker, which will be suitable for the rapid start-up. In this study, we introduce the development of ammonia-fueled SOFC systems. 200 W-class stacks as well as button-type cells were applied as SOFCs. The performance of the three systems were evaluated and compared with each other. This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST).