title: Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells and Reversible Solid Oxide Fuel Cells
Creators: name: Javed, Aroosa
Publisher: name: Graduate Studies
Year: 2025
Subjects: subject: Solid oxide fuel cell
subject: Cathodes
subject: Infiltration
subject: Reversible solid oxide fuel cell
subject: Electrochemical impedance spectroscopy
subject: Electrochemistry
subject: Materials Science

description: Solid oxide fuel cells are high-temperature devices that utilize solid electrolytes to convert the chemical energy of hydrogen or hydrocarbon-based fuels into electrical energy. Reversible solid oxide fuel cells function as both solid oxide fuel cells and solid oxide electrolysis cells, enabling dual functionality for energy storage and power generation. These fuel cells offer numerous advantages, including high efficiency (60–85% when integrated with combined heat and power systems), reduced CO2 emissions, and the flexibility to operate with diverse fuels such as hydrogen, natural gas, biogas, and other hydrocarbons. The objective of this thesis is to develop robust, cost-effective, and stable cathode materials, with a focus on mixed ionic and electronic conducting perovskites. These materials are explored for their potential use as cathodes in intermediate-temperature solid oxide fuel cells and as symmetrical electrodes in reversible solid oxide fuel cells. Interest in perovskites stemmed from prior research on Nd0.75Sr0.25Co0.8Fe0.2O3−δ, which demonstrated significant potential as a solid oxide fuel cell cathode. Consequently, in this thesis, we focused on studying the electrochemical performance of Nd1-xSrxCo0.8Fe0.2O3−δ (0 ≤ x ≤ 0.85) electrodes by varying the Sr doping content. The effect of Sr doping on the chemical and electrical properties of perovskite-type Nd1-xSrxCo0.8Fe0.2O3−δ (0 ≤ x ≤ 0.85) cathodes was evaluated. Electrochemical impedance spectroscopy results showed a decrease in polarization resistance of the cathode with increase in Sr-doping due to increase in electrical conductivity. Among the samples studied, composite electrode of Nd0.3Sr0.7Co0.8Fe0.2O3−δ – La0.8Sr0.2Ga0.8Mg0.2O3−δ showed the lowest area specific resistance of 0.1 Ω cm2 at 750 °C in air. To improve the stability of the perovskite under reducing conditions, perovskite-type Nd0.75Sr0.25Co0.1Fe0.9O3−δ was developed and used as a symmetrical electrode for CO2 electrolysis. The structural and chemical stability of the perovskite was investigated at 800 °C under pure CO2,70% CO2/30% CO and 5% H2/95% N2. The exposure of the perovskite to the reducing environment resulted in the formation of in situ exsolved metal nanoparticles forming a metal-oxide active interface on the perovskite surface and helped improve the catalytic activity by increasing the active sites. Another focus of the research was on the infiltration technique, a cost-effective approach for fabricating nanostructured electrodes. This method was employed to enhance the oxygen reduction reaction kinetics and improve the electrochemical performance of solid oxide fuel cells at intermediate temperatures (600–800 °C). Nanostructured praseodymium oxide and multiphase heterostructures containing perovskites with the nominal composition of Nd0.6Sr0.4CoO3-δ were developed via infiltration into the symmetric metal-supported backbone as binary layer composite and their electrochemical performance has been investigated. These binary-layer composites demonstrated superior electrochemical performance, achieving the lowest polarization resistance of 0.05 Ω·cm2 at 700 °C, compared to 0.1 Ω·cm2 for Nd0.6Sr0.4CoO3-δ alone under similar conditions. Electrochemical impedance spectroscopy combined with impedance spectroscopy genetic programming (ISGP) analysis was used to study the underlying electrochemical processes. Praseodymium oxide was found to enhance oxygen reduction reaction-related processes significantly. The full cell performance of the composite electrode achieved a peak power density of 329 mW·cm⁻2 at 700 °C with 3% H2O/H2 as fuel and air as the oxidant.