Thermodynamic studies of barium zirconate-based electrolytes and kinetic studies of barium zirconate-based triple conducting electrodes for protonic ceramic electrochemical device applications

Abstract

In contrast to the electrode and electrolyte materials deployed in conventional oxygen-ion conducting solid oxide fuel cells (SOFCs), the materials used in protonic ceramic fuel cells (PCFCs) tend to exhibit considerably more complex defect chemistry, as protons, oxygen vacancies, and electron-holes often play a role in the behavior of both protonic ceramic electrolytes and electrodes. If each charge carrier behaves straightforwardly, describing their behavior and obtaining optimized materials would be easy; however, the reality is often not as simple as expected. The relative influence of these various mobile charge carriers on the performance of PCFC devices varies because the dominating carrier type and concentration can change dramatically depending on the operating conditions. This complexity hinders the current fundamental understanding of protonic ceramic materials and devices. Although fundamental studies focused on the thermodynamic and kinetic properties of protonic ceramic materials are increasing, further systematic investigation is needed to comprehensively understand these complex systems.

Most high-performing PCFCs today use cathode electrodes based on triple conducting oxides (TCOs), which are unique oxide materials that accommodate the simultaneous transport of protons, oxygen ions, and electron holes. One prominent TCO of particular interest is BaCo0.4Fe0.4Zr0.1Y0.1O3-d (BCFZY4411). In this thesis, we first explore the potential to tune the transport properties and catalytic activity of the broader BaCo0.8-xFexZr0.1Y0.1O3-d (BCFZY) compositional family by varying the Co/Fe ratio. The oxygen ion tracer diffusion properties of this compositional series are obtained by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Co-rich BCFZY7111 shows the highest oxygen ion tracer diffusivity among all TCO and mixed oxygen-electron conducting cathode materials reported in the literature so far. Moreover, button cells employing a BCFZY7111 cathode exhibit significantly higher peak power density (695 mW/cm2 at 600 °C) in fuel cell mode and higher current density (1976 mA/cm2 at 600 °C) in electrolysis mode compared to cells fabricated from lower Co-content compositions.

In a second study, we focus on the proton transport kinetics of the most well-studied and widely deployed BCFZY composition: BCFZY4411. Here, ToF-SIMS was used to measure the proton tracer diffusion properties of BCFZY4411. The use of isotope exchange in this study, rather than conductivity relaxation enabled, for the first time, quantification of the kinetic properties of protons in BCFZY4411 without the confounding effects of ambipolar diffusion. This allows extracting the kinetic properties tangled by the coupled multi-species transport effects. The proton tracer diffusivity obtained under wet air showed excellent proton transport in BCFZY4411 compared to other proton-conducting electrode materials and that of the best-known protonic-ceramic electrolyte material. While BCFZY4411 showed excellent proton tracer bulk diffusion compared to other proton-conducting materials, its surface exchange kinetics was found to be modestly lower than a number of other well-known PCFC-relevant electrode materials. Nevertheless, BCFZY exhibits competitive electrochemical performance in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) applications, on par if not higher than many of the other well-known PCFC-relevant electrode materials. This indicates that the oxygen ion kinetics may play an important, if not limiting role, in the overall ORR/OER reaction process, and that fast proton exchange may be less crucial. Finally, we observed a significant change in the temperature response of the diffusivity and surface exchange kinetics depending on the pO2, indicating a potential change in mechanism from the low to high-pO2 transport regime.

The last portion of this thesis undertakes a thermodynamic study of an archetypical protonic ceramic electrolyte material: BaCe0.7Zr0.1Y0.1Yb0.1O3-d (BCZYYb7111) via thermogravimetric analysis. To establish the defect thermodynamics in these systems, the mass changes of samples were obtained under various temperatures and atmospheres and directly fitted to a defect model. With the modeling study, the thermodynamic properties of each reaction were obtained, which were further applied to estimating the defect concentrations for the protonation reaction. Lastly, the total conductivity was calculated by applying predicted defect concentrations and the mobility that had already been reported. This study examined the thermodynamic behavior of, especially the oxygenation and protonation reactions that have not been deeply investigated by traditional conductivity and modeling studies. Moreover, the oxygenation/hydration level for protonation was firstly evaluated in the proton-conducting materials study. Thereby, this study mainly contributes to providing a piece of the puzzle for a better understanding of BCZYYb7111 for its further development.