Fabrication and Electrochemical Investigation of Composite Cathodes for Solid Oxide Fuel Cells

SAEED UR REHMAN University of Science and Technology 2018 국내박사

Solid oxide fuel cell (SOFC) technology has made significant progress in the past 50 years for a wide range of power generation applications. The SOFC technology is attractive due to its efficiency for the direct conversion of the chemical energy stored in fuels such as hydrogen and hydrocarbons into electricity. The SOFC features an all solid-state construction and high-temperature operation. The combination of these characteristics provides a number of advantages for SOFCs, including flexibility in cell and stack designs, manufacturing processes, fuel type and generator sizes. However, for successful commercialization, it is essential to greatly improve the performance and durability of the SOFC technology. In this regard, SOFC cathodes need special attention because of their lower activity for oxygen reduction reaction (ORR) and gradual degradation over time. The composition and microstructure of cathode materials have a large impact on the performance of SOFCs. The purpose of this dissertation is fabrication and characterization of composite SOFC cathodes by using the existing SOFC materials to extend the triple-phase boundaries through microstructure optimization. Chapter 1 and 2 provide an introduction and current status of the SOFC technology. Basic principles, thermodynamics, and state of the art materials for SOFCs are discussed. The designs of SOFC (tubular and planar) and their characteristics, benefits, and shortcomings are explained. New methods and strategies are discussed to improve the SOFC designs for better performance. Chapter 3 discusses the effect of Gd0.1Ce0.9O2-δ (GDC) addition on the phase stability, sintering behavior, thermal expansion, and porosity of La0.65Sr0.3MnO3-δ/(Y2O3)0.08(ZrO2)0.92 (LSM/YSZ) composite. The sintering temperature and the porosity of the LSM/YSZ composite were observed to increase with an increase in the amount of GDC. An LSM/YSZ/GDC tri-composite with optimized properties was selected to fabricate the tubular cathode-supported DCFCs (LSM/YSZ/GDC|YSZ|NiO/YSZ) through extrusion, slurry coating, and co-firing. A special chamber was designed for the in situ steam gasification of carbonaceous fuels and operation of the tubular SOFC. Electrochemical characterization was done by measuring the polarization curves and electrochemical impedance spectroscopy, using the syngas produced by in situ steam gasification of carbon black. Chapter 4 discusses the effect of GDC addition method on the properties of LSM-YSZ composite cathode support for SOFCs. Equal amounts of GDC were added to LSM/YSZ powder either by physical mixing or by a sol-gel process, to produce a highly porous cathode support for SOFCs. The effect of the GDC mixing method was analyzed in view of sinterability, thermal expansion coefficient, microstructure, porosity, and electrical conductivity of the LSM/YSZ composite. GDC infiltrated LSM/YSZ (G-LY) composite showed a highly porous microstructure when compared with mechanically mixed LSM/YSZ (LY) and LSM/YSZ/GDC (LYG) composites. The cathode support composites were used to fabricate the button SOFCs by a slurry coating of YSZ electrolyte and a nickel/YSZ anode functional layer, followed by co-firing at 1250 °C. The G-LY composite cathode-supported SOFC showed maximum power densities of 215, 316, and 396 mW cm-2 at 750, 800, and 850 °C, respectively, using dry hydrogen as fuel. Results showed that the GDC deposition by a sol-gel process on LSM/YSZ powder before sintering is a promising technique for producing porous cathode support for the SOFCs. Planar design of SOFCs is very attracted as an alternative to tubular design due to its higher power density, short current path, and lower operating temperature. Chapter 5 explains a new fabrication method of nanofibrous composite cathodes for planar SOFCs. The proposed method involves chemically assisted electrodeposition (CAED) of mixed metal hydroxide onto a carbon nanotube (CNT) template, followed by a low-temperature heat-treatment process. The CNT template is first fabricated on porous zirconia-based ion-conducting scaffolds (ICS) by catalytic chemical vapor deposition (CCVD) of C2H4. Perovskite-type LCO is then fabricated on the CNT template by CAED process of mixed La-Co hydroxide combined with thermal conversion of hydroxide to perovskite oxide. The method proposed here allows for the fabrication of LCO perovskites with a unique nanofibrous structure at reduced temperatures (~900 °C) while avoiding the formation of pyrochlore phases (e.g., La2Zr2O7), which are typically observed during conventional high-temperature sintering processes of LaCoO3 with zirconia-based electrolytes. The new method also provides the precise control needed to achieve desired oxide loadings without the need for repeated deposition-annealing processes. The anode-supported SOFCs with nanofibrous LCO cathodes on zirconia and ceria scaffolds show high and stable electrochemical performance of 0.95 and 1.27 W cm-2, respectively, at 800 °C. In addition to the absence of insulating pyrochlore phases, the unique nanostructure of the LCO cathode is believed to play a beneficial role in improving the electrochemical properties by providing a large number of active reaction sites and by facilitating mass transport through the porous nanofibrous structure Solid oxide fuel cell (SOFC) technology has made significant progress in the past 50 years for a wide range of power generation applications. The SOFC technology is attractive due to its efficiency for the direct conversion of the chemical energy stored in fuels such as hydrogen and hydrocarbons into electricity. The SOFC features an all solid-state construction and high-temperature operation. The combination of these characteristics provides a number of advantages for SOFCs, including flexibility in cell and stack designs, manufacturing processes, fuel type and generator sizes. However, for successful commercialization, it is essential to greatly improve the performance and durability of the SOFC technology. In this regard, SOFC cathodes need special attention because of their lower activity for oxygen reduction reaction (ORR) and gradual degradation over time. The composition and microstructure of cathode materials have a large impact on the performance of SOFCs. The purpose of this dissertation is fabrication and characterization of composite SOFC cathodes by using the existing SOFC materials to extend the triple-phase boundaries through microstructure optimization. Chapter 1 and 2 provide an introduction and current status of the SOFC technology. Basic principles, thermodynamics, and state of the art materials for SOFCs are discussed. The designs of SOFC (tubular and planar) and their characteristics, benefits, and shortcomings are explained. New methods and strategies are discussed to improve the SOFC designs for better performance. Chapter 3 discusses the effect of Gd0.1Ce0.9O2-δ (GDC) addition on the phase stability, sintering behavior, thermal expansion, and porosity of La0.65Sr0.3MnO3-δ/(Y2O3)0.08(ZrO2)0.92 (LSM/YSZ) composite. The sintering temperature and the porosity of the LSM/YSZ composite were observed to increase with an increase in the amount of GDC. An LSM/YSZ/GDC tri-composite with optimized properties was selected to fabricate the tubular cathode-supported DCFCs (LSM/YSZ/GDC|YSZ|NiO/YSZ) through extrusion, slurry coating, and co-firing. A special chamber was designed for the in situ steam gasification of carbonaceous fuels and operation of the tubular SOFC. Electrochemical characterization was done by measuring the polarization curves and electrochemical impedance spectroscopy, using the syngas produced by in situ steam gasification of carbon black. Chapter 4 discusses the effect of GDC addition method on the properties of LSM-YSZ composite cathode support for SOFCs. Equal amounts of GDC were added to LSM/YSZ powder either by physical mixing or by a sol-gel process, to produce a highly porous cathode support for SOFCs. The effect of the GDC mixing method was analyzed in view of sinterability, thermal expansion coefficient, microstructure, porosity, and electrical conductivity of the LSM/YSZ composite. GDC infiltrated LSM/YSZ (G-LY) composite showed a highly porous microstructure when compared with mechanically mixed LSM/YSZ (LY) and LSM/YSZ/GDC (LYG) composites. The cathode support composites were used to fabricate the button SOFCs by a slurry coating of YSZ electrolyte and a nickel/YSZ anode functional layer, followed by co-firing at 1250 °C. The G-LY composite cathode-supported SOFC showed maximum power densities of 215, 316, and 396 mW cm-2 at 750, 800, and 850 °C, respectively, using dry hydrogen as fuel. Results showed that the GDC deposition by a sol-gel process on LSM/YSZ powder before sintering is a promising technique for producing porous cathode support for the SOFCs. Planar design of SOFCs is very attracted as an alternative to tubular design due to its higher power density, short current path, and lower operating temperature. Chapter 5 explains a new fabrication method of nanofibrous composite cathodes for planar SOFCs. The proposed method involves chemically assisted electrodeposition (CAED) of mixed metal hydroxide onto a carbon nanotube (CNT) template, followed by a low-temperature heat-treatment process. The CNT template is first fabricated on porous zirconia-based ion-conducting scaffolds (ICS) by catalytic chemical vapor deposition (CCVD) of C2H4. Perovskite-type LCO is then fabricated on the CNT template by CAED process of mixed La-Co hydroxide combined with thermal conversion of hydroxide to perovskite oxide. The method proposed here allows for the fabrication of LCO perovskites with a unique nanofibrous structure at reduced temperatures (~900 °C) while avoiding the formation of pyrochlore phases (e.g., La2Zr2O7), which are typically observed during conventional high-temperature sintering processes of LaCoO3 with zirconia-based electrolytes. The new method also provides the precise control needed to achieve desired oxide loadings without the need for repeated deposition-annealing processes. The anode-supported SOFCs with nanofibrous LCO cathodes on zirconia and ceria scaffolds show high and stable electrochemical performance of 0.95 and 1.27 W cm-2, respectively, at 800 °C. In addition to the absence of insulating pyrochlore phases, the unique nanostructure of the LCO cathode is believed to play a beneficial role in improving the electrochemical properties by providing a large number of active reaction sites and by facilitating mass transport through the porous nanofibrous structure