Date of Award
Doctor of Philosophy (PhD)
School of Materials Science and Engineering
Ionic conducting ceramics attracted much attention in their applications as electrolytes in many energy conversion, storage, and harvesting devices based on their high singular ionic conductivity (H+ or O2-). Within these applications, protonic ceramic fuel cells (PCFCs) show promising performance on electricity generation with high efficiency and low emission at intermediate temperatures. However, the sluggish oxygen reduction reaction (ORR) at the cathode side and the potential coking issue at the anode side are still the crucial stumbling stones to the further development of PCFCs. Utilizing materials with mixed conductivity, including co-ionic conductivity (H+ and O2-) and triple conductivity (H+, O2-, and e-), could potentially mitigate these problems. For the cathode, the simultaneously conducting of H+ and O2- along with the sufficient electronic conduction, more sites could be activated for ORR than the situation for mixed ionic and electronic conductors (MIECs). Furthermore, the transport of a small amount of oxygen ion from the cathode to the anode through the proton conducting electrolyte is one of the common strategies for mitigating the coking effect by directly burning the deposited solid carbon, which utilizes the transferred O2- and generates water at the anode side to promote the reforming reaction of hydrocarbons. The phase-pure perovskite oxides of doped barium cerates and zirconates usually possess some degree of oxygen ion conductivity besides the predominant proton conductivity, behaved co-ionic conduction characteristic. However, it is not easy to simultaneously increase proton and oxygen ion conductivity by only adjusting perovskite dopants. The equilibrium between the protonation and deprotonation reactions determines the partial ionic conductivity, while the solubility-limited extrinsic dopant (e.g., Y) concentration decides the total ionic conductivity. Mechanically mixing two pre-prepared phases with independently controllable transport properties to form dual-phase composites allowed more flexibility to design the co-ionic conducting materials. However, the mechanically mixing resulted in poor homogeneity, large phase domain size, continuous solid reactions, and emergent interface impurities, which usually caused significant performance degradation. Thus, the one-pot fabrication method provided an effective route to achieve stable composites with homogenous phase distribution, stable phase composition, and stable microstructure to ensure the steady promising transport properties for the resulted co-ionic conducting composites. This Ph.D. study took advantage of composite materials and one-pot fabrication methods to develop mixed conducting materials for PCFCs and investigate the electrochemical properties. First, we investigated phase compositions and corresponding electrochemical properties of dual-phase materials BaCe0.5Fe0.5O3-δ (BCF) in Chapter III. Based on BCF, we developed novel triple-conducting nanocomposite cathode material BaCe0.4Fe0.4Co0.2O3-δ (BCFC) and demonstrated the promising cathodic performance in Chapter IV. Second, candidates of the electrolyte and the anode scaffold materials were developed based on the investigation of mixed conduction properties of BaCe0.5Zr0.4Y0.1O3-δ-Ce0.5Y0.5O2-δ (BCZY-YDC) composites with different phase ratios in Chapter V. Finally, proper demonstrations were given with these developed composites to confirm the feasibility as each component and the contribution of unique mixed conduction properties.
Zhao, Zeyu, "Mixed Conducting Composite Materials for Intermediate-Temperature Protonic Ceramic Fuel Cells" (2021). All Dissertations. 2883.