| Author: | Bello, Idris Temitope |
| Title: | The power of perovskites : advancing ceramic electrochemical cell technology with novel cathode materials for clean energy |
| Advisors: | Ni, Meng (BRE) Yam, Michael (BRE) Leu, Shao-yuan (CEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Cathodes Perovskite materials Electric batteries -- Materials Electrochemical cells Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building and Real Estate |
| Pages: | xiii, 245 pages : color illustrations |
| Language: | English |
| Abstract: | Perovskites have emerged as a promising class of materials for use as cathodes in ceramic electrochemical cells (CECs), which offer a clean and efficient means of energy conversion. The conventional CECs have operated at elevated temperatures (800 – 1000 °C), which has resulted in rapid material degradation and poor stability, impeding their commercial viability. Reducing the operating temperatures of CECs to 450 – 650 °C has therefore been proposed as a potential solution. Still, it has often led to suboptimal electrocatalytic reactions at the cathode, necessitating further research and development efforts to overcome them. Therefore, this research aimed to enhance the oxygen reduction reaction (ORR) kinetics and stability of cathode materials for CECs, focusing on solid oxide fuel cells (SOFCs) and protonic ceramic fuel cells (PCFCs) at reduced operating temperatures (450 – 650 °C). The objectives of this study were to: -examine the structure-function relationship between lattice strain and ORR activity/CO2 tolerance in ABO3 perovskite-based cathodes; -evaluate self-assembly engineering as a viable cathode material developmental strategy in comparison to state-of-the-art methods; and - understand the electrokinetic triple ionic and electronic conductivity (TIEC) dynamics in self-assembled cathodes that enabled their superior performance using electrical conductivity relaxation (ECR) and protonation enthalpy. The present study addressed these objectives to improve the performance and viability of CECs as a clean and sustainable energy generation technology. The first research objective was accomplished by applying lattice engineering to improve the ORR and CO2 tolerance of Ba0.5Sr0.5Co0.7Fe0.3O3-δ (BSCF) functional material for intermediate temperature SOFCs. The introduction of copper-transition metal (Cu-TM) and zinc-transition metal (Zn-TM) caused a contraction and expansion in the lattice of the BSCF, respectively. Enhanced catalytic ORR and CO2 tolerance were also correlated with lattice contraction, which was attributed to the shortened pathway for oxygen mobility in the lattice of BSCF and the weakened adsorption energy of the constitutive reactive intermediates. The BSCFC5 cathode, in which lattice contraction occurred, demonstrated the best performance with an area-specific resistance (ASR) of 0.0247Ωcm2 and an exceptional peak power density (PPD) of 1715 mW cm-2 at 650 ℃ for symmetrical and single cells, respectively. In addition, the material displayed enhanced tolerance to CO2 infusion, with good recoverability when switched intermittently between pure air and 10-vol% CO2 infusions for 100 hours. The findings conclude that Cu-TM-doped BSCF (BSCFC5) is a superior substitute for BSCF material in SOFC applications. The second research objective was actualized by developing a thermodynamically stable self-assembled nanocomposite cathode material with a unique composition BaCo0.5Ce0.3Fe0.1Yb0.1O3-δ (BCCFYb). Upon calcination, the precursor material separates into a host cubic and ancillary rhombohedral phase. The performance of the TIEC (O2-/H+/e-) BCCFYb was compared to that of cobalt-rich oxide-ion and electron (O2-/e-) conducting BaCo0.833Yb0.167O3-δ (BCYb), cerium-rich proton and electronic (H+/e-) conducting BaCe0.75Fe0.25O3-δ(BCF), and Co-Ce-rich triple ionic and electronic (O2-/H+/e-) conducting BaCo0.833Yb0.167O3-δ-BaCe0.75Fe0.25O3-δ (BCYb-BCF) traditional composite materials. The BCCFYb demonstrates a low TEC, good operational stability, and superior cathodic performance in oxygen ion- and proton-conducting ceramic fuel cell (CFC) modes, making tailored self-assembly engineering a promising cathode material developmental method for CFCs. In the final objective, the electrokinetic phenomena in a self-assembled cathode material were investigated. A low-cobalt-content self-assembled cubic-rhombohedral TIEC nanocomposite cathode material with the composition BaCe0.4Co0.4Fe0.1Zr0.1O3-δ (BCCFZ) was synthesized for this purpose. The proton conduction mechanisms of the material were elucidated via hydrogenation and hydration reactions using the ECR method. The BCCFZ exhibited good TIEC properties with a low protonation enthalpy of -30 ± 9 kJ/mol compared to state-of-the-art proton-conducting materials. The material's high cerium and low cobalt content also contributed to its low thermal expansion coefficient (TEC) of 9.6 × 10-6K-1. The PPD of the anode-supported single cell based on a BCCFZ air electrode reached 1054 mW cm-2 at 650 °C, and the material demonstrated good operational stability over 500 h at 550 °C. The findings from this study suggest that low enthalpy enabled enhanced proton uptake, and the synergistic phases enabled improved TIEC and low TEC, providing valuable insights as to how tailored self-assembly engineering can revolutionize PCFC cathode material development. The present study has made significant contributions to the discipline of CEC technology, theoretically and practically. The research outcomes have provided valuable insights, practical strategies, and recommendations for addressing the limitations of current CECs. The results of this investigation will be crucial in improving the performance and viability of CECs as a clean energy generation technology, both regionally and globally. To sum up, the findings of this study have the potential to significantly advance the field through the development of innovative cathode materials for clean energy generation. |
| Rights: | All rights reserved |
| Access: | open access |
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