| Energy and environmental issues are two major challenges facing human sustainable development.Compared with traditional fire-power plants,solid oxide fuel cells as an electrochemical device,providing with some advantages,such as high chemical-to-electrical conversion efficiency,fuel flexibility,low emissions and so on.However,inferior long-term stability in high operation temperature is still a big obstacle for commercialization of SOFCs.As a result,decreasing the operation temperature is a very important step.Recently,H-SOFCs have become one of the important ways to solve the high operating temperature of oxygen-ion conductor-based SOFCs due to its excellent performance at intermediate-temperature.In this study,a series of novel cathode and electrochemical characterization for proton conducting solid oxide fuel cells(H-SOFCs)was discussed.In Chapter 1,the basic concepts,principles,research progress of proton conducting solid oxide fuel cells are briefly described.Focusing on the electrolyte materials,cathode materials and principle of cathodic reaction kinetics.In Chapter 2,A novel cobalt-free cathode Ba0.95Ca0.05Fe0.9-xSnxY0.1O3-δ-SDC(x<=0.1),with various ratios of Sn was proposed.Here,it is applied for proton-conducting solid oxide fuel cells,for the first time.Special attention is paid to the exploration of doping amount of Sn effect on the phase structure,electronic conductivity,thermal expansion coefficient(TEC)and electrochemical properties.TEC data shows that doping of Sn can effectively decrease thermal expansion coefficient,the average thermal expansion coefficient decreases from 17.8 × 10-6 K-1 for Ba0.95Ca0.05Fe0.9Y0.1O3-δto 12 × 10-6 K-1 for Ba0.95Ca0.05Fe0.8Sn0.1Y0.1O3-δ at the temperature range of room temperature to 850℃.The single-cell with Ba0.95Ca0.05Fe0.85Sn0.05Y0.1O3-δ-SDC cathode achieves the maximum power density of 949 mW cm-2 at 700℃.These results demonstrate that Ba0.95Ca0.05Fe0.85Sn0.05Y0.1O3-δ-SDC are promising cathodes for proton-conducting solid oxide fuel cells.In Chapter 3,an innovative cathode of Ba0.95Ca0.05Fe0.85Sn0.05Y0.1O2 9-δF0.1 was successfully developed by co-doping of anion F and cations Ca,Sn,Y.We studied the effect of F--doped on phase structure,electrical conductivity and electrochemical properties of the cell.Experiments show that the doping of F-ion can effectively improve the electrical conductivity of the materials.Composite cathode consisting of Ba0.95Ca0.05Fe0.85Sn0.05Y0.1O2.9-δF0.1 and Sm0.2Ce0.8O2-δwas applied in H-SOFCs with BaZr0.1Ce0.7Y0.2O3-δ electrolyte which achieved an encouraging performance with the maximum power density of 1050 mW cm-2 and polarization resistance of 0.039 Ω cm2 at 700℃.These results demonstrate that anions and cations co-doped strategy can provide a new horizon for the cathode in H-SOFCs.In Chapter 4,we explore the effects of Cl-ion on the phase structure,electrical conductivity and electrochemical properties of the cell.The potential of chlorine-containing materials Ba0.95Ca0.05Fe0.85Sn0.05Y0.1O3-x-δClx(X=0,0.05,0.1)as a cathode for H-SOFCs was investigated.It is found that the conductivity of Ba0.95Ca0.05Fe0.85Sn0.05Y0.1O3-x-δClx increases with the increase of Cl content,and when the Cl content is 0.1,the conductivity of Ba0.95Ca0.05Fe0.85Sn0.05Y0.1O2.9-δCl0.1 is the largest.The doping of anionic Cl can effectively improve the electrochemical performance of the cell,and when the cathode composition is Ba0.95Ca0.05Fe0.85Sn0.05Y0.1O2.9-δCl0.1-SDC,The cell achieve the best electrochemical performance,polarization resistance(Rp)is 0.025 Ω)cm-2 in 700℃.Experimental results indicate that chlorine-containing materials are promising cathode materials for H-SOFCs.In Chapter 5,We made a summary of the paper and gives a personal outlook on the future research work of proton-conducting solid oxide fuel cells. |
