Sm_0.2Ce_0.8O_1.9 Electrolyte Film Fuel Cell And The Performance Optimization Of Its Electrodes

Solid oxide fuel cell (SOFC) is an energy conversion device that directly generates electricity power from the chemical energy of a fuel. It offers several advantages, such as high energy conversion efficiency and environment-friendliness. The conventional SOFC with an yttria-stabilized zirconia electrolyte is usually operated at a high temperature around 1000 oC. However, operating at such a high temperature can cause lots of serious problems, leading to a short cell span. It is therefore necessary to operate the SOFC at a low temperature below 650 oC. To obtain high cell performance at the reduced temperatures, significant efforts should be made to search for alternative electrolyte materials with higher ionic conductivities for low-to-intermediate temperature SOFC, such as to fabricate dense thin-electrolyte films and/or to improve the electrochemical performance of the electrodes.Samaria doped ceria (Sm0.2Ce0.8O1.9,SDC) is such a promising alternative electrolyte material for low-to-intermediate temperature SOFCs. We fabricate dense SDC films successfully using slurry spin coating method. Glycine-nitrate process (GNP) is used to synthesize the SDC powder. The effect of heat-treatment temperature and other treating conditions of the SDC powder on the performance of electrolyte films is studied. The results show that the grain size of the SDC powder could be easily controlled by changing the heat-treatment temperature. There are significant agglomerates existing in the as-prepared powder, of which the soft agglomerates could be broken up by the ball-milling treatment. The size of the secondary particles of the powder is different as the heat-treatment temperature changes. For example, there are two peaks at 0.2 and 1μm in the particle size distribution curves; a lower heat-treatment temperature causes smaller grain size and thereby a higher surface energy for easy bonding of the grains, leading to a larger height of the peak at 1μm. The sintering profiles shows that the shrinkage starts at a higher temperature and the overall shrinkage is smaller as the powder is heat-treated at a higher temperature. By comparing the sintering profiles between the electrolyte films and the anode supports and by observing the difference in the micrographs of the films, it is concluded that the SDC powder heat-treated at 800oC is most suitable for fabricating dense electrolyte films. The corresponding single cell exhibits the maximum power densities 1.39, 0.99, 0.59 and 0.31 W/cm2 at 650, 600, 550 and 500 oC, respectively.An anode functional layer (AFL) is fabricated on the Ni/SDC anode supports. The effect of the composition and the thickness of AFL on the performance of a single cell is studied. The results show that the existence of the AFL structure is beneficial to enhance the electrochemical performance of the anode and facilitates the fabrication of a dense electrolyte film. By comparing the performance of the cells based on AFLs with different compositions, it is concluded that a weight ratio of 6:4 for the NiO and SDC in the AFL results in the optimum microstructure and maximum reaction sites, thus significantly minimizing the ohmic loss and electrode polarization loss of the cell. The optimum AFL thickness, which is determined by the effects between the interactions of TPBs and gas transport on the anode performance, is different at different testing conditions. When stationary air is used as the oxidant, a 12μm thick or thicker AFL causes the better cell power; on the other hand, when pure oxygen is fed to the cathodes, an 8μm thick or thinner AFL causes the better cell power.To avoid the carbon deposition on Ni-based anode due to the pyrolysis reaction of CH4, nano Cu film is incorporated into porous Ni/SDC anodes by vacuum-assisted electroless plating method for the direct use of methane in intermediate temperature SOFCs (IT-SOFCs). The scanning electron microscopy (SEM) observation indicates the formation of a uniformly distributed nano-structured Cu network on the Ni/SDC anode structure after electroless plating. The maximum power density of the cell with the Cu electroless-plated Ni/SDC anodes is 0.84 and 0.54 W/cm2 in dry H2 and dry CH4 at 600 oC, respectively, enhanced by ~30% as compared to the cell with conventional Ni/SDC anodes. The increase in the cell performance after Cu electroless-plating may be attributed to the enlargement of effective three-phase boundaries by interconnecting the isolated Ni particles with the Cu network. The stability testing shows that cell degradation in dry methane due to carbon deposition is effectively suppressed by the Cu electroless-plating, suggesting the existence of the Cu network effectively limits the direct exposure of the Ni to the methane.A novel nano-structured cathode is fabricated by incorporating a mixed ionic and electronic conducting (MIEC) perovskite, Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCF) via ion impregnation into a commonly used and highly electronic conducting La0.8Sr0.2MnO3-δ(LSM) porous cathode skeleton. The introduction of nano-sized MIEC BSCF particles significantly improves the electrocatalytic activity of the LSM for the oxygen reduction reaction of SOFCs at an intermediate temperature range 600-800oC. The electrode polarization resistance of a 1.8 mg/cm2 BSCF-impregnated LSM cathode is ~12 times lower than that of pure LSM. A single cell with yttria-stabilized zirconia (YSZ) electrolyte film and the nano-structured BSCF-LSM cathode exhibits maximum power densities of 1.21 and 0.32 W/cm2 at 800 and 650 oC, respectively.It can be concluded in this thesis that the operating temperature of SOFC is reduced from a high temperature region to a low one after the fabrication of dense SDC films using slurry spin coating; the cell performance is significantly enhanced by fabricating and then optimizing the fine AFL structure; carbon deposition on the anode is effectively suppressed by preparing a homogeneous nano Cu film into the anode substrates by electroless plating; and the electrochemical performance of the LSM cathode for high-temperature SOFC is significantly enhanced by impregnating nano-scale BSCF particles into the porous LSM skeletons.