Electrochemical and Thermal Characterization of Doped Ceria Electrolyte with Lanthanum and Zirconium

Abstract

There is an urgent need to explore affordable and renewable energy resources because of the decline in reserves of fossil fuels. Biomass is an abundantly available resource in nature and can be used to generate energy in a sustainable manner. Fuel cells deliver a combination of advantages and make use of renewable energy sources. Solid oxide fuel cells (SOFCs), specifically, overcome the petroleum scarcity issue by using biofuel. The aim of this thesis is the development of nanocomposite electrolytes and anode composite catalysts for low-temperature SOFCs fuelled with biogas for clean energy applications. In the present work, Sr/Sm-doped ceria (Sr-SDC) nanocomposite electrolytes with a core shell structure are synthesized with different compositions for low temperature SOFCs. A co-doping technique is successfully used to achieve a significant enhancement in the ionic conductivity of 0.50 S/cm at 600 ˚C for the nanocomposite electrolyte Sr0.1Sm0.1Ce0.8O2-δ-carbonate. The carbonate phase (shell layer) acts as a barrier and protects the SDC (core) from the partial reduction by the fuel. This carbonate shell introduces an interface between these two phases, which is the key to realizing the interfacial super-ionic conduction pathways. This work also describes the development of ceria electrolytes that are doped and co-doped with lanthanum (La) and zirconium (Zr) and show excellent thermal stability. The ionic conductivity of La0.2Ce0.8O2-δ (LDC), Zr0.2Ce0.8O2-δ (ZDC) and Zr0.2La0.2Ce0.6O2-δ (ZLDC) has been measured in the temperature ranges of 450 °C to 650 °C and LDC achieved a high ionic conductivity of 0.81 × 10-2 S/cm. Thermal expansion coefficients (TECs) of these electrolytes have also been found to have good concurrence and compatibility with commonly used electrolytes and electrodes. The main objective of this work is the development of stable and active anode catalysts that run over biogas as well as hydrogen for low temperature SOFCs. The anode composite Ni0.6Zn0.4- Gd0.2Ce0.8O2-δ (NiZn-GDC) has been developed that exhibits semiconductor conductive behaviour, and a maximum conductivity of 1.37 S/cm has been achieved at 600 ˚C. This composite anode is found to have excellent thermally stability as well as being carbon resistant to coking during testing with biogas. A maximum power density of xii 820 and 548 mW/cm2 has been reported with hydrogen and biogas fuels, respectively, at 600 ˚C. This thesis also describes Ni-based and Ni-free anode catalysts NiLiCu-oxide with LDC for SOFCs fuelled with biogas. The anode composite NLC622-LDC has reported a maximum DC conductivity of 3.47 S/cm with Pmax of 650 and 390 mW/cm2 for hydrogen and biogas, respectively, at 600 ˚C. A Ni free anode catalyst Zn0.2Li0.2Cu0.6O2-δ (ZnLiCu-oxide) is also developed as a potential candidate for biogas-based SOFCs that bypasses the difficulty of carbon deposition and has a maximum conductivity of 4.0 S/cm at 600 ˚C. An open circuit voltage (OCV) of 0.96 V is achieved with maximum power density of 600 mW/cm2 with biogas (50% methane) at 650 ˚C. In the present work, a theoretical model of FC system has been designed using MATLAB software, and it makes use of biomass (animal waste, redwood, rice husk and sugar cane). In the last part of the thesis, a partial research work has been conducted to cast the tapes of NiO-GDC (NiO-Gd0.1Ce0.9O1.95) as anode and GDC (Gd0.1Ce0.9O1.95) as electrolyte via aqueous tape casting method. The aqueous tape casting is an emerging and cost-effective technique for the commercialization of SOFCs but faces challenges with ceria tapes due to its poor mechanical strength and co-sintering of half-cells.