Design And Characterization Of Novel Perovskite-like Electrode Materials

Solid oxide fuel cells (SOFC) are environmentally friendly energy conversion devices with high efficiency. SOFCs can directly convert the chemical energy of the fuels into electrical energy. As the reverse operating of SOFC, solid oxide electrolytic cells (SOECs) can transform the electrical power into chemical energy to store in the fuels. The combination of SOFCs and SOECs is one of the best way to solve the fluctuation of the renewable clean energy, such as solar, wind and geothermal, to make them suitable for grid connection. Unfortunately, there are still some difficulties that hinder the commercial application of SOFCs, mainly including the unavailability of carbon tolerant anode and the high catalytic-active and stable cathode.Aiming to provide the possible solutions of these problems, several studies have been carried out in this thesis, including (1) Exploring a novel single phase cathode material Sr3Fe2O7-δ for proton conducting SOFCs (P-SOFCs);(2) Optimizing the performance of Sr3Fe2O7-δ using the doping strategy with La and Co as dopants, respectively;(3) Fabrication and characterization of integrated Reversible Solid Oxide Cells (RSOCs) with YSZ supporting framework and Sr2Fe1.5Mo0.5O6-δ(SFM) symmetrical impregnated electrodes;(4) Exploring NiTiO3and Co2TiO4as novel anode internal reforming material and anode material for hydrocarbon fueled SOFCs. The main results are listed as follows:Chapter One:A brief introduction of the background, operating mechanism and development of SOFCs were presented. The demands of commercial SOFCs were mainly expounded. At last, the research goals and objectives were outlined.Chapter Two:To promote the electrochemical performance of P-SOFC, Sr3Fe2O7-δ(SFO) with R-P structure was proposed as a novel single phase cathode for P-SOFCs. The main results are summarized as follows:(1) Sr3Fe2O7-δ was a good oxygen ion-electronic conductor with high surface exchange coefficient in dry atmosphere. The oxygen flux of Sr3Fe2O7-δ reached 6.06*10-8molcm-2s-1measured at800℃.(2) The DFT calculation results demonstrated that the special R-P structure of SFO leaded to the low proton formation and the low proton migration energy, which made it a potential single phase cathode material for P-SOFC.(3)For a cell with Sr3Fe2O7-δ-5wt.%BZCY as cathode, the maximum power density of683mWcm-2was achieved at700℃, which was one of the best discharging performance up to now. Furthermore, great discharging stability of the single cell with Sr3Fe2O7-δcathode had been demonstrated through a100hours’ discharging test.All these results suggested that SFO was an excellent proton and electronic mixed conducting cathode material for P-SOFC.Chapter Three:Doping strategy was used to optimize the electrochemical performance of SFO. The results suggested that Co dopant could promote the electronic conductivity and the oxygen surface exchange coefficient of SFO at the expense of stability in the air-electrode atmosphere. While La dopant could improve the stability of SFO at the high water content atmosphere, which was more suitable as air electrode material for SOEC. With Sr2.8Lao.2Fe207-δ-5wt.%BZCY as air-electrode, the current density of single cell was406mAcm-2at550℃under the overvoltage of0.3V with the interfacial polarization resistance of0.14Ωcm2. Notedly, the cell exhibited stable electrolyzing performance with little degradation in25hours’ operation.Chapter Four:The mechanical differences of the SOFCs components, including cathode, electrolyte and anode, may cause large performance degradation during the long-term discharging operation. This problem can be solved using an integrated cell configuration. Here, a cell based on YSZ framework with impregnated Sr2Fe1.5Mo0.5O6-δ(SFM) symmetric electrodes was fabricated and characterized. The maximum power density of the cell was288mWcm-2at800℃with the impregnated SFMO quantity of9.7wt%. At the same impregnate quantity of SFMO, the cell operated for60hours under the discharging voltage of0.72V at750℃. Under the SOEC model to electrolyze water, the current density was454mAcm-2at the over voltage of0.4V.Chapter Five:To promote the stability of cell in hydrocarbon fuels, an anode internal reforming layer of NiTiO3was applied. With NTO reforming layer, the maximum power densities of single cells were236mWcm2and270mWcm2at700℃using methane and hydrogen as fuel, respectively. Moreover, the cell operated smoothly with no degradation for90hours using methane as fuel and discharging at0.6V and700℃. Similarly, CO2TiO4can largely promote the carbon tolerance of the cell fueled with hydrocarbons. The cell using Co2TiO4anode discharged steadily for more than20hours at700℃using methane as fuel.