In situ exsolution synthesis of composite cathodes for protonic ceramic fuel cells
MetadataShow full item record
Protonic ceramic fuel cells (PCFCs) have the potential to operate with hydrogen as a fuel in an environmentally friendly manner with no emissions and high electrical efficiency. The lower operation temperature of PCFCs due to the use of proton conducting electrolytes, compared to traditional solid oxide fuel cells (SOFCs) integrating oxygen ion conducting electrolytes, alleviates thermal and chemical challenges of SOFCs and enables the use of less expensive materials for manifolds, housing and interconnects. Nevertheless, the slower reaction kinetics remain an issue at lower temperatures reducing performance of PCFCs. The cathode has become the key performance-limiting component of PCFCs, and new alternative cathodes are needed to achieve sufficiently low polarization resistance. Typical SOFC cathodes are mixed oxygen ion and electronic conductors, which enable to extend the reaction sites into the whole cathode. The protonic conduction in PCFCs occurs from the anode to the cathode with the consequent formation of water at this side and therefore, the cathode reactions become more complex compared to SOFCs. Hence, new alternative cathodes with novel design approach are needed to achieve low polarization resistance. The aim of this thesis was to develop novel cathode materials compatible with BaZrO3- based electrolytes, the state-of-the-art ceramic proton conducting oxide materials. La0.5Ba0.5CoO3-δ-based materials have extensively been studied as cathodes for SOCFs and recently for PCFCs due to their excellent electronic and oxygen ion conductivities. However, single phase cathode materials with no proton conduction will restrict the reaction sites to the cathode/electrolyte interface. An alternative is to develop composite cathodes composed of a mixed oxygen ion and electronic conductor material and a proton conductor material as this configuration will extend the reaction sites to the entire cathode-gas interface. In this work, cathode composites based on La0.5Ba0.5CoO3-δ - BaZrO3 system have been synthesized using an original exsolution synthesis approach and shown to be chemically compatible with BaZrO3-based electrolytes. The synthesis route based on a modified Pechini method followed by oxidation driven in situ exsolution of a single phase material was developed as an alternative route to prepare composite cathodes. The single phase and the exsolved composite materials were prepared by controlling the oxidation state of the transition metal. The model system 0.6 La0.5Ba0.5CoO3-δ - 0.4 BaZrO3 (LB-BZ) with four cations, was chosen to design the exsolution synthesis protocol. The nanocrystalline single phase La0.3Ba0.7Zr0.4Co0.6O3-δ was firstly synthesized by applying the modified Pechini method with a thermal treatment carried out at 715 °C in N2. The single phase was then exsolved into a two-phase composite by further annealing at 900 °C in air. An alternative composite with the same nominal composition was prepared by the same modified Pechini method followed by a direct calcination at 1000 °C in air. The mechanisms for the formation of the exsolved composite and the direct calcination composite were analyzed and compared by several in situ and ex situ techniques. The modified Pechini method allowed to prepare amorphous materials, with the exemption of crystalline BaCO3, after removal of the organic part at 450 °C. EDS analysis in combination with Rietveld refinements of the final composites revealed the solid solubility of Co in the BZ phase for both composites and the chemical composition of the composites was determined to be 0.4 La0.8Ba0.2CoO3- δ - 0.6 BaZr0.6Co0.4O3-δ for the exsolved composite (Cexs) and 0.5 La0.6Ba0.4CoO3-δ - 0.5 BaZr0.8Co0.2O3-δ for the composite obtained by direct calcination (Cdir). High temperature X-ray diffraction (HT-XRD) revealed the different formation mechanisms of the Cexs and Cdir composites, leading to different properties. Cdir showed larger grains with a high contiguity of the LB phase, leading to a higher electrical conductivity of the composite, as the conductivity of LB phase is higher than the BZ phase. On the other hand, the exsolution method yielded a nanostructured composite consisting of a matrix of BZ phase with 45 nm grain size embedding the LB phase, with 20 nm average grain size. The electrical conductivity of Cexs composite was lower than Cdir but both composites fulfilled the requirement for PCFC cathodes of 1 S/cm. HT-XRD also showed the possibility to tailor Cexs chemical composition unlike Cdir. Similar electrochemical performance was observed for both composites at high temperatures. However, Cexs showed lower polarization resistance at lower temperatures, e.g. 18 Ω∙cm2 at 400 °C in 3 % moist synthetic air. The exsolution synthesis protocol was further extended to a more complex system with seven cations, demonstrating the adaptability, flexibility and robustness of this synthesis route. La0.3Ba0.7Zr0.4-wYwM0.6O3-δ single phases, where M corresponds to a mixture of Fe, Mn and Co (3M, 0.6 La0.5Ba0.5Co1/3Mn1/3Fe1/3O3-δ - 0.4 BaZr1-zYzO3-δ) or only Co (CoxY, 0.6 La0.5Ba0.5CoO3-δ - 0.4 BaZr1-zYzO3-δ) were successfully synthesized with variable Y content, and the single phases were further exsolved into two-phase composites. The electrochemical performance of the cathodes in symmetrical cell configuration was analyzed by electrochemical impedance spectroscopy (EIS). The 3M composites revealed a clear effect of the Y content on the electrochemical performance with a minimum polarization resistance of 0.44 Ω∙cm2 at 600 °C in 3 % moist synthetic air, obtained for 0.45 La0.6Ba0.4MO3-δ - 0.55 BaZr0.79Y0.12M0.08O3-δ composite. The electrical conductivity of the composites was found to be dependent on the LB phase contiguity and content. The electrical conductivity increased with increasing LB phase in the composite from 0.42 to 0.47 mole fraction. A higher LB/BZ ratio directly increased the contiguity of the moreconductive LB phase, and hence the electrical conductivity. The optimal microstructure of PCFC cathodes was investigated in CoxY composites (x = 5 and 10 mol% Y doped BZ phase of the model system, while z = 0 corresponds to Cexs). The cathode microstructures obtained by spray coating and screen printing on the electrode microstructure were analyzed as well as the effects of the firing temperature and the ex situ and in situ exsolution of the composite cathodes. The cathodes deposited by spray coating were highly porous (~50 %) with uneven thickness and poor adhesion to the electrolyte. However, these cathodes showed the lowest activation energies (0.6 eV) of all electrodes prepared in this work. The screen printed cathodes had homogeneous thickness, lower porosity (~25 %) than the spray coated cathodes and increasing to 31 % with decreasing firing temperature, and were well-adhered even when firing at temperatures as low as 600 °C. The in situ exsolved cathodes, where the single phase was deposited on the electrolyte and the cathodes were fired and exsolved at the same time, showed web-like microstructure with large channels and high tortuosity. This would promote fast gas diffusion and increase the reaction sites. 0.38 La0.62Ba0.38CoO3-δ - 0.62 BaZr0.68Y0.07Co0.25O3-δ composite cathode deposited by screen printing, in situ exsolved and fired at 1100 °C showed the best electrochemical performance of 4.02 Ω∙cm2 at 400 °C and 0.21 Ω∙cm2 at 600 °C in 3 % moist synthetic air. These findings motivated further compositional engineering of a La1-xBaxCoO3-δ - (1-a) BaZr0.9Y0.1O2.95 (LBZ) composites. The influence of the LB phase content (a) and the Ba content in the LB phase (x) on the electrochemical performance were investigated. The electrochemical analysis revealed that response of all composite cathodes was dominated by the oxygen diffusion and surface related processes. The higher LB phase content as well as the higher Ba content in the LB phase showed a decrease of the cathode polarization resistance attributed to the hydrogen diffusion process. The disappearance of this contribution may indicate that Ba content increase in LB phase as well as LB phase increase in the composite is beneficial for the hydrogen diffusion. All LBZ cathodes showed excellent electrochemical performance with polarization resistance below 12 Ω∙cm2 at 400 °C and below 0.59 Ω∙cm2 at 600 °C in 3 % moist synthetic air, comparable to the best cathodes reported in the literature. Finally, this work has demonstrated the versatility of the modified Pechini method followed by in situ exsolution of a single phase to prepare composite materials consisting of a wide range of elements. The prepared composite cathodes showed excellent electrochemical properties and have the potential to be utilized as cathodes in PCFCs.