Development of Samarium Doped Ceria (SDC) and SDC-based Composite Electrolytes for Applications in Intermediate Temperature Solid Oxide Fuel Cells (SOFCs)

Abstract

Solid oxide fuel cells (SOFCs) are the most efficient devices for clean energy that directly convert the chemical energy of fuels into electrical power. Traditional SOFCs employ yttria-stabilized zirconia (YSZ) as an electrolyte but YSZ exhibited sufficient ionic conductivities at high temperatures (800-1000°C). Due to this high temperature, the commercial utilization of the conventional SOFCs is limited because of the resistive causes, e.g. expensive materials, thermal stress and long start-up and shut-down time etc. In the last decade, the research of SOFCs had concentrated on decreasing the operating temperature through the evolution of novel materials, notably the electrolyte materials having sufficient ionic conductivity. Doped ceria based electrolytes has higher oxygen ion conductivity than YSZ, especially at lower temperatures. In the first part of the dissertation, samarium doped ceria (SDC), and samarium doped ceria-based composite electrolyte with addition of lithium & sodium carbonates, (LiNa)2CO3-SDC were prepared by co-precipitation route. Cubic fluorite structures have been observed in both electrolytes and crystallite sizes of SDC and (LiNa)2CO3-SDC were found to be 14 nm and 55 nm, respectively. SEM images showed that SDC nanoparticles appeared to be spherical in shape while (LiNa)2CO3-SDC composite nanoparticles are irregular in shape and consisted of agglomerated nanocrystallites. The conductivities of SDC and (LiNa)2CO3-SDC at 650 °C are 5.502×10-5 S/cm and 1.90×10-3 S/cm, respectively. Furthermore, the conductivity of (LiNa)2CO3-SDC is also calculated at 700 °C and found to be 2.94×10-3 S/cm. In the second part of the thesis, the effects of pH of medium on the microstructure of SDC have been studied and SDC-based composite with addition of potassium carbonate (SDC-K2CO3) has also developed to study the effect of calcination temperature on the microstructure of SDC- K2CO3. XRD and SEM studies showed that the crystallite size and particle size of SDC increases with the increase in pH. The SEM images of all the samples of SDC synthesized at different pH values showed the irregular shaped and dispersed particles. SDC-K2CO3 was calcined at 600 °C, 700 °C and 800 °C for 4 h and XRD results showed that crystallite size increases while lattice strain decreases with the increase in calcination temperature and no ii peaks were detected for K2CO3 because it is present in the electrolyte as amorphous phase. The conductivity of SDC-K2CO3 at 700 °C is found to be 3.2×10-3 S/cm. In the last part of the thesis, co-doped SDC with the addition of yttrium has been developed in order to find the effect of yttrium co-doping on the conductivity of SDC. The conductivity of YSDC at 650 °C is found to be 2.0×10-4 S/cm which is higher than the SDC electrolyte. At the end, chemical compatibility test of YSDC with lithiated nickel oxide cathode was performed at 800 °C for 3 h in order to check the compatibility of these two materials and it was found from XRD data that neither new reaction occurs nor new phases are formed between the YSDC electrolyte and lithiated nickel oxide cathode. So we may say that these two materials can be used as an electrolyte and a cathode in intermediate temperature SOFCs. The result in this dissertation may benefit the development of ITSOFCs and expand the related research to a new horizon.