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Design of catalysts for a direct ammonia solid oxide fuel cell at intermediate temperatures

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vom Stein,  Julia Mareike
Research Department Schüth, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Citation

vom Stein, J. M. (2021). Design of catalysts for a direct ammonia solid oxide fuel cell at intermediate temperatures. PhD Thesis, Ruhr-Universität Bochum, Bochum.


Cite as: https://hdl.handle.net/21.11116/0000-000F-D45F-8
Abstract
Besides hydrogen, ammonia is considered to be one of the most promising fuels for solid oxide fuel cells (SOFCs). For the hydrogen generation by ammonia decomposition at a temperature of 500 °C a catalyst is needed. Thus, in this work, highly active ammonia decomposition catalysts made of ruthenium and potassium impregnated on yttrium-stabilized zirconia (YSZ) hollow spheres are synthesized and tested and an ammonia conversion of nearly 100 % with a space velocity of 15 000 ml gcat-1h-1 at 500 °C is achieved. This material has high catalytic stability under reaction conditions and has similar characteristics to the ceramic electrolyte, and it is therefore highly suitable as one additional component applied at the anode of a direct ammonia solid oxide fuel cell operated at intermediate temperatures (500 °C). However, the results of the operating fuel cell including the hollow sphere catalysts showed that the ammonia decomposition and corresponding hydrogen generation at the anode is not sufficient to investigate the correlation between catalyst activity and fuel cell performance. It turned out, that the coating procedure significantly reduces the catalyst activity. The fuel cell performance using the hollow spheres as a catalyst at the anode resulted only in maximum power densities of 11 µW cm-2 at 500 °C, if fueled with ammonia. The performance of the cell fueled with ammonia is lower than compared to the cell fueled with hydrogen, giving 15 µW cm-2 at 500 °C. Besides the reduced activity, the major reduction of the fuel cell performance could be attributed to a low ionic conductivity, generated by the polycrystalline nature of the hollow spheres, and low ionic conductivity of the YSZ itself at the denoted temperature.
In the second step the electrolyte membrane powder (bulk powder with the same composition as the used ceramic electrolyte) is also impregnated with ruthenium and potassium, to avoid ionic conduction restrictions related to the YSZ hollow spheres and to increase the ammonia decomposition activity of this material. By analyzing the anode-off gas no increased ammonia decomposition at the anode side of the fuel cell could be detected, but a significant, positive impact on the fuel cell performance was observed. By modifying the anode and cathode with 5 wt. % ruthenium and potassium, an increased maximum power density of 2.7 mW cm-2 at 500 °C, if fueled with ammonia, could finally be reached. By applying hydrogen as a fuel using the same membrane electrode assembly, only a slight improvement of the fuel cell performance with 3 mW cm-2 at 500 °C could be measured. Although, the ammonia decomposition could not be significantly increased by the ruthenium and potassium modification, an interesting positive effect of the impregnation on the electrode behavior and interaction with the electrolyte could be discovered, finally increasing the overall fuel cell efficiency. However, no further improvement could be achieved at this point any more, since the ohmic resistance of the ceramic electrolyte is restricting and limiting the fuel cell performance.