Study Of Cathode Process For Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is a clean and highly-efficient energy conversion device that converts the chemical energy in fuels to electricity with negligible emissions. At the present, there are still some obstacles to achieve its commercial applications. The traditional SOFCs are operated at high temperatures in the range of800to1000℃. The high temperatures not only make the cell difficult to seal but also cause the components seriously aged. However, it is not reasonable to directly reduce the operating temperature, since the cell performance decrease obviously due to the increase of both the electrolyte and the cathode resistances. Thus, it is necessary to develop new cathode materials and/or optimize the electrodes structure for SOFC, which is stable and highly performed at intermediate temperatures.In the first part of my thesis, the operating principle of SOFC and factors affecting cell performance are briefly introduced. Structures, electrochemical performance and application limitations of typical systems for the cathode, anode and electrolyte are subsequently summarized. Some characterization technique including AC impedance spectroscopy and conductivity relaxation are also introduced for studying a cathode reaction mechanism, which gives useful information to improve the cathode performance.In Chapter2, a series of LSM/YSZ/LSM symmetrical cells with various microstructures are obtained by changing the pre-firing and sintering temperatures of cathodes. XRD, SEM and laser particle analyzer are used to perform the microstructure evolvement during the preparation. Direct relationships between microstructural parameters from actual cathodes and the polarization resistance of the significant elementary steps of the cathodic reaction are established. It is found that the charge transfer polarization resistance and the adsorption polarization resistance display proportional relationship with LTPB-1.53and LTPB2.47, respectively.In Chapter3, effects of introducing the second phase to LSM on the oxygen surface exchange coefficients are investigated using the electrical conductivity relaxation (ECR) method. The oxygen surface exchange process only occurs in the place where the electron, the oxygen ion vacancy and the gas come together. For typical electronic conducting material, the ionic conductivity of LSM is significantly less than its electronic conductivity. Consequently, the surface exchange process is controlled by the concentration of ionic species. The introduction of either YSZ or SDC to LSM can significantly reduce the re-equilibration time, demonstrating the substantial promotion in the surface exchange kinetics. The coefficient at1000℃increases from9.00×10-5cm2s-1for pure LSM to2.45×10-4cm2s-1for LSM coated with yttria-stabilized zirconia, and further increases to7.92×10-4cm2s-1for LSM with samaria-doped ceria. The improved surface exchange coefficient supplies a reasonable explanation for the decreased polarization resistance in the middle frequency on YSZ or SDC impregnated LSM cathodes. The promotion depends on the ionic conductivity of the second phase and a higher conductivity results in high k value.In Chapter4, nano-structured electro-catalyst of layered-structure cobaltite PrBaCo2O5+x (PBC) has been developed as cathode for solid oxide fuel cells (SOFCs) and excellent electrochemical activity towards oxygen reduction has been achieved. PBC nano-particles are deposited into porous samaria-doped ceria (SDC) backbones with an impregnation method. The fabrication processing parameters including composition of precursor solution, PBC loading, and firing temperature have been investigated to optimize the cathode microstructure and further to minimize the cathode interfacial polarization resistance, leading to a cathode interfacial polarization resistance of only0.0820cm2at600℃, much lower than those other Co-based cathodes prepared under similar conditions. Performance of the impregnated PBC cathode is further investigated using single cells with Ni-SDC anodes and26μm-thick SDC electrolytes. A peak power density of600mW cm-2has been achieved at600℃using humidified H2as the fuel. The novel nano-structured PBC electrochemical reaction mechanism has found to be similar to that of a conventional PBC cathode. However, both oxygen ion incorporation and charge transfer steps are both greatly accelerated for the novel nano-structured PBC cathode.In Chapter5, BaCeO3based ceramics have demonstrated high proton conductivity and have been extensively investigated as electrolytes for solid oxide fuel cells (SOFCs). However, these materials are not chemically stable and are prone to reaction with CO2in air at the typical SOFC operating conditions. This work presents a new strategy of improving the stability of BCS electrolyte by Cl substitution using BaCl2as the partial precursors. Better tolerance against CO2in air for Cl doped BCS has been demonstrated from negligible mass gain when exposing BCSCl powders to air and stable OCV for cells using BCSCl electrolyte. Single cells with BCS and BCSCl electrolytes show similar cell power densities and polarization resistance. The proton conductivity drop as indicated by the hydrogen permeation test in humidified reducing atmosphere is no more than10%.