Basic Research Based On Solid Oxide Fuel Cells For Application

Energy crisis and environmental issues have become two major challenges for the sustainable development of human society. Solid oxide fuel cells (SOFCs) are a kind of clean and highly-efficient energy conversion device that converts the chemical energy in fuels directly to electricity with negligible emissions. To meet the commercialization requirement on cost and reliability, the operating temperatures of SOFC has to be lowered from traditional800-1000℃to intermediate and low-temperatures ranging400-800℃. Unfortunately, the lowered operating tempreture bring forth new challenges to SOFCs, including1) much lowered electro-activity of tranditional cathodes, which depress the discharging output of SOFCs;2) the electronic conduction in doped ceria, which the most promising intermediate-temperature electrolyte materials, decreased the open circuit voltage of cell and thus energy conversion efficiency;3) the carbon depositing on tranditional Ni anode when directly using hydrocarbon fuels, which greatly shorten the lifetime of SOFCs.Aimed to develop IT-SOFCs, several topics are involeved in this thesis, including:1) exploring suitable intermediate temperature cathode materials based on the investigation of electrode reaction mechanism;2) developing in-situ reaction layer to block the electronic conduction in doped ceria electrolyte;3) exploring novel carbon-tolerant anode materials that can be used directly in hydrocarbons fuels. The main results are shown as follows:Chaper1:A brief introduction of the research background, the basic working principle as well as general research progress of SOFC. Major challenges in key materials for low-temperature SOFC are also addressed. Based on these analysises, the main topics of this thesis are issued.Chapter2; To improve the electro-activity of cathode at intermediate temperature, several new cathode materials are explored and applicated, including double perovsikite-type LaBaCuCoO5+x (LBCC) and LaBaCuFeO5+x (LBCF), Sm0.5Sr0.5Fe0.8Cu0.2O3-δ (SSFCu), SrFe0.9Sb0.1O3-δ(SFSb) and Ni0.7Co0.3O (NC3O). Investigations on these new cathode materials suggest:1) Co doped LaBaCuO5+x has better electrochemical activity than Fe doped one;2) Copper substitution in SSFCu improved its oxygen ionic conductivity at the cost of electronic conductivity. The improved ionic conductivity benefits the cathode reaction process;3) Cr-tolerant Ni0.7Co0.3O can be directly used as the cathode material. A maximum power densities of204mW cm2with the electrolyte BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) was achieved at700℃.Chapter3:An easy in situ electronic-blocking reaction layer fabrication technique is proposed to slove the electronic conduction in doped ceria electrolyte. A NiO-BaZr0.1Ce0.7Y0.2O3-δ (NiO-BZCY) composite was proposed as the anode substrate for doped ceria electrolyte. During the co-sintering process of anode and electrolyte, Ba partially migrates from anode to electrolyte and formed a thin electronic-blocking layer for doped ceria. Intensive study also suggested that the formed BaCeO3-based reaction layer could largely improve the proton transferring number of La2Ce2O7(LCO). The activation energy of the LCO electrolyte conductivity differed with anode materials, approximately52.51kJ mol"1with NiO-BZCY anode and95.08kJ mol-1with NiO-LCO anode, respectively.Chapter4:A low cost carbon-tolerant anode material NiTiO3(NTO), which is reduced to Ni/TiO2with nano-network structure in reducing atmosphere, is designed and applied for SOFCs using hydrocarbon fuels. Within the tested40h’s long time test in humidied methane fuel, no decay in discharging performance was found, suggesting that NTO is a good carbon-tolerant anode. By introducing NTO-SDC active layer, the maximum power output of the cell with NTO-10%SDC/NTO-SDC/SDC/LSCF-SDC structure is413mWcm-2at700℃with humidified CH4(～3%H2O) as fuel. Low ohmic resistance and polarization resistance of0.176Ω2cm2and0.064flcm, respectively, are achieved, suggesting that NTO has good electrochemical activity to anode reaction.Chapter5:Solid oxide electrolysis cell (SOEC) is the reverse reaction system of SOFCs, and could be used to adjust the peak shaving of power grid along with SOFCs. In this work, Sr0.95Y0.5TiO3+δ was used as Ni substitute in SOECs to improve the cell performance. The XRD Rietveld refinement and electron paramagnetic resonance investigation suggests that Sr0.95Y0.5TiO3+δ powders reduced in H2has high-spin Ti3+with unpaired electron. Impedance study of a cell with SYT-SDC/YSZ/LSM-YSZ structure show that the polarization resistance in SOEC mode is much lower that that in SOFC mode, suggesting that the applied voltages help to improve the electrical conductivity and catalytic activity of SYT under a reducing atmosphere.