Solid oxide fuel cells (SOFCs) are one of the most promising energy conversion devices for the next generation. This is mainly a result of the low emission of environmental hazardous species associated with SOFC’s, high fuel conversion efficiency, and fuel flexibility. However, current ‘state-of-the art’ catalysts used in SOFCs are suffering from several drawbacks that encumber a commercial application of SOFCs in the direct conversion of hydrocarbons. In this application, high operating temperatures are required due to low oxygen reduction reaction (ORR) activity at the cathode, which limits the use of less expensive materials. In addition, ‘state of the art’ anode catalysts are very susceptible to sulfur poisoning and carbon depositions when it is operated with carbonaceous fuels. Since SOFCs aim to use the relatively abundant fuel sources, such as coal-derived syngas and natural gas, high activity and stable catalysts are required. Therefore, the need in developing and testing new catalysts formulations for both cathode and anode catalysts are crucial.
The current research aims to find solutions for these problems through development of novel electro-catalysts based on perovskite materials. The objective is to develop perovskite materials which are capable of enhancing ORR activity as well as possessing a high tolerance to sulfur poisoning and carbon laydown.
New formulations of cerium-doped perovskite material were synthesized with varying concentrations of cerium. The bulk structure, oxygen mobility, and electro-catalytic performance of the catalysts were examined using in-situ X-ray diffraction (XRD), oxygen temperature-programmed desorption (O2-TPD), X-ray absorption fine structure (XAFS), CO2 temperature programmed oxidation (TPO) and button cell/impedance measurements.
Cerium-doped perovskites exhibit a cubic structure at room temperature and no apparent structural changes were observed with increasing temperature. An additional CeO2 phase was observed when cerium concentration exceeded 15% in the A-site of perovskite. Thermal compatibility of the catalyst gets closer to that of galdolinia-doped ceria (GDC) electrolyte by addition of cerium. Oxidation states of both Fe and Co were shown to be very close to their valence state with different cerium dopant levels at room temperature, indicating that the charge imbalance is compensated by the creation of oxygen vacancies. The oxygen vacancy was generated mainly from the reduction of Co where the Fe contribution was minimal. It was observed that the oxygen vacancy generation was inversely proportional to the dopant level of Ce. However, the electrocatalytic activity showed that the intermediate concentration of Ce doping has the best unit cell performance, which suggests that the secondary ceria phase at higher cerium dopant levels has a detrimental effect on the performance. The trend of the unit cell performance followed the CO2-TPO experiment results and found to be a good probe for button cell performance.
On the anode side, the structural stability of the new formulations was examined under anodic conditions. As the temperature was raised the catalyst with lower cerium loadings changed from its initial cubic structure to a lower symmetry structure while the higher loading cerium samples remained in the original cubic structure. The catalytic activity of cerium-doped catalysts for the methane oxidation showed fairly matched to that of state of the art anode Ni-YSZ catalyst. The sulfur tolerance was significantly enhanced with cerium-doped perovskites showing no deactivation up to 10 hrs of operation while Ni-YSZ catalysts deactivated almost immediately upon introduction of H2S. Surface analysis using XPS on poisoned samples showed that the sulfur exists as strontium sulfate where no significant changes for the transition metals, which are critical for oxidation catalytic activity, were observed. The button cell performance of intermediate cerium-loading catalyst matched to that of Ni-YSZ, while the highest cerium-loading samples showed lower performance due to the presence of secondary phase (CeO2). Lowest cerium-loading showed instability issues during the performance test. Therefore, the catalyst with intermediate cerium-loading catalyst is suitable for the anode catalyst.