Haixia Li

Date of Award

Fall 2023

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Fanglin (Frank) Chen

Abstract

Solid oxide cells (SOCs) can directly convert chemical energy to electricity in the fuel cell mode and store electricity to chemicals in the electrolysis mode. However, there are still critical barriers, such as energy efficiency and durability, for developing and commercializing SOCs. This dissertation aims to design electrode materials and optimize the cell fabrication process to address the critical barriers for SOCs in energy conversion and energy storage applications. Therefore, one primary focus of the dissertation is to develop robust fuel electrode material for solid oxide fuel cells (SOFCs) with improved sulfur tolerance. In addition, the design of novel fuel electrode material to improve energy conversion efficiency in solid oxide electrolysis cells (SOECs) for CO2 electrolysis was also pursued. Moreover, large-size fabrication techniques were explored within anode-supported proton-conducting solid oxide fuel cells (P-SOFCs). SOFCs offer a great promise to produce electricity with a wide variety of fuels such as natural gas, coal gas, and gasified carbonaceous solids; however, the conventional nickel-based anodes face grand challenges due to contaminants in readily available fuels, especially sulfur-containing compounds. Thus, developing new anode materials that can suppress sulfur poisoning is crucial to realizing flexible and cost-effective fuel SOFCs. In the first part of this dissertation, La0.1Sr1.9Fe1.4Ni0.1Mo0.5O6-δ (LSFNM) and Pr0.1Sr1.9Fe1.4Ni0.1Mo0.5O6-δ (PSFNM) materials have been synthesized using a sol-gel method in air and investigated as anode materials for SOFCs. Metallic nanoparticle-decorated ceramic anodes were obtained by reduction of LSFNM and PSFNM in H2 at 850 °C, forming a Ruddlesden−Popper oxide with exsolved FeNi3 bimetallic nanoparticles. The electrochemical performance of Sr2Fe1.4Ni0.1Mo0.5O6-δ (SFNM) ceramic anode is greatly enhanced by A-site La doping, resulting in 44% decrease of polarization resistance in the reducing atmosphere. The maximum power densities of LSGM (300 um) electrolyte-supported single cells with LSFNM as the anode reached 1.371 W cm⁻² in H2 and 1.306 W cm⁻² in 50 ppm H2S-H2 at 850 °C. Meanwhile, PSFNM shows improved sulfur tolerance, which can be fully recovered after six cycles from H2 to 50 ppm H2S-H2 operation. This part of the dissertation indicates that LSFNM and PSFNM are promising high-performance anodes for SOFCs. Solid oxide electrolysis cell (SOEC) is the reverse operation of SOFC and can directly convert electrical energy to chemical energy in fuels with high energy efficiency. The durability and performance of SOEC are highly related to the stability of the fuel electrode materials. Massive carbon dioxide (CO2) emission from recent human industrialization has affected the global ecosystem and raised significant concern for environmental sustainability. SOEC is a promising energy conversion device capable of efficiently converting CO2 into valuable chemicals using renewable energy sources. However, typical Sr/Ba-containing fuel electrode materials face the challenge of Sr/Ba carbonation during CO2 electrolysis, which dramatically affects the energy conversion efficiency and long-term stability. Thus, A site Ca doped La1-xCaxCo0.2Fe0.8O3-δ (0.2≤x≤0.6) (LCxCF) oxides are developed and presented in the second part of this dissertation. With a polarization resistance as low as 0.18 Ω cm2 in pure CO2 atmosphere, a remarkable current density of 2.24 A cm⁻² was achieved at 1.5 V with La0.6Ca0.4Co0.2Fe0.8O3-δ (LCCF64) as the cathode in La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM) electrolyte (300 µm) supported electrolysis cells using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as the air electrode at 800 °C. Furthermore, symmetrical cells with LCCF64 as the electrodes also show promising electrolysis performance of 1.78 A cm⁻² at 1.5 V at 800 °C. In addition, stable cell performance has been achieved on direct CO2 electrolysis at an applied constant current of 0.5 A cm⁻² at 800 °C. The easily removable carbonate intermediate produced during direct CO2 electrolysis makes LCCF64 a promising regenerable cathode. The outstanding electrocatalytic performance of the LCCF64 cathode is ascribed to the highly active and stable metal/perovskite interfaces resulting from the in situ exsolved Co/CoFe nanoparticles, and the additional oxygen vacancies originating from the Ca2Fe2O5 phase, synergistically providing active sites for CO2 adsorption and electrolysis. This section offers a novel approach to design catalysts with high performance for direct CO2 electrolysis. The above two parts are based on oxygen-ion conducting electrolyte solid oxide cells (O-SOCs). The high operation temperature of O-SOCs causes many problems like long-term stability issues and higher costs. Protonic ceramic electrochemical cells (PCECs) are a promising energy-conversion device capable of generating electricity with hydrogen or hydrocarbon as fuels. However, the lower-than-predicted performance of the PCEC and difficulty with scale-up make the PCEC challenging to commercialize, mainly accounting for the un-well controlled parameters during the cell fabrication process, including tape casting process, sintering process, sealing process, cell testing process, etc. In the third part of this dissertation, a binder-controlled tape casting technique was introduced, aiming to produce high-quality green tapes with suitable porosities and thickness for anode-supported cells. With the optimized binder amount, solid content with 66.14% was proved to be the most suitable for tape fabrication and a bubble-free along with the ideal thickness of anode supported cell was prepared, achieving the highest power density with 0.78 W cm⁻² at 700 °C. Based on this achievement, a new fabrication strategy was proposed by using symmetric double-sided electrolytes to offset the asymmetry of thermal stresses during the fabrication process in the fourth part of this dissertation. This new design has been successfully applied to fabricate large-scale anode-supported 5×5 cm² PCEC with electrochemical performance close to theoretical value. Moreover, the cell fabricated with this strategy shows good electrochemical performance. The cell fabrication strategy proposed in this dissertation points out a promising way to scale up PCECs in the future.

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© 2024, Haixia Li

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