Preparation And Research On Cathode Materials For Proton-Conducting Solid Oxide Fuel Cells

Energy and environmental issues are major challenges for the sustainable development of the earth.Solid oxide fuel cells(SOFCs)are environmentally friendly electrochemical devices with simple structure and high energy conversion efficiency that can directly convert chemical energy into electricity.The operating temperature of traditional SOFCs is relatively high,which would result a series of problems,such as electrodes sintering and interfacial diffusion.Thus reducing the operating temperature of the fuel cells is the urgent demand for its development.Developing SOFCs with proton conductors as electrolytes is an important way to reduce the working temperature.Hence,investigating cathode materials that can match the proton conductor electrolyte is of great significance.This paper focuses on the cathode materials that can output high performance in the proton conducting SOFCs.Chapter 1 in the thesis simply describes the basic concepts and principles of SOFCs,also introduces research progress and prospect in SOFCs.Emphasis is put on the electrolyte and cathode materials.In Chapter 2,electrochemical properties of iron and bismuth co-doped BaCeO3 as cathode material in proton conducting SOFCs are investigated.BaCe0.5Fe0.5-xBixO3-?(x=0.1,0.2,0.3)powders are synthesized by a citric acid-nitrate sol-gel combustion method.XRD analysis shows that crystal structure of BaCe0.5Fe0.3Bi0.2O3-? and BaCe0.5Fe0.2Bi0.3O3-? belongs to orthorhombic crystal system,while BaCe0.5Fe0.4Bi0.1O3-? structure is the combination of orthorhombic and cubic perovskite structure.Single cells with the structure of NiO-BaZro.1Ce0.7Y0.2O3-?(NiO-BZCY)anode substrate| NiO-BZCY anode functional layer| BZCY electrolyte membrane| BaCe0.5Fe0.5-xBixO3-? are fabricated by die-pressing.BaCe0.5Fe0.3Bi0.203-?shows the best performance,which outputs the maximum power density of 736 mWcrrm-2 at 700?.The polarization resistance of BaCe0.5Fe0.3Bi0.2O3-? cathode is as low as 0.1?cm2.Moreover,no degradation was observed during a 40-h short-term testing under discharge condition.Considering the mixed conducting property which can enlarge the triple-phase boundary,thermal compatibility with the BaCeO3 based electrolyte and cobalt-free that can increase chemical stability,BaCe0.5SFe0.3Bi0.2O3-? is a good cathode material candicate for proton conducting SOFCs.In Chapter 3,electrospinning is introduced to prepare cathode materials with nanofiber-structured morphology in SOFCs.Smooth and uniform La0.7Sr0.3FeO3-?(LSF)nanofibers are fabricated by electrospinning.The as-prepared nanofibers are calcined at 800? to remove the organic content and promote crystallization.Cathode slurry is prepared with the fibers and screen-printed on the surface of the electrolyte in proton conducting SOFC.SEM showed that short fibers are connected and intertwined with each other for webs in the cathode morphology.The NiO-BZCY|BZCY| LSF single cell shows good electrochemical performance which output maximum power density of 563 mWcn-2 with the polarization resistance of 0.15?cm2 at 700?,indicating LSF cathode with nanofiber structure is a good choice for SOFCs.Chapter 4 investigates the Bi-doped BaFe03 in B-site as the cathode material in BZCY proton conducting SOFCs.XRD analysis shows B-site doped BaFe0.8Bi0.2O3-?powders synthesized by a citric acid-nitrate sol-gel combustion method are pure cubic perovskite structure.Single cell outputs the maximum power density of 821 mWcm-2 and the polarization resistance of BaFe0.8Bi0.2O3-? cathode is as low as 0.1 ? cm2.The excellent electrochemical performance shows the good application prospect of BaFe0.8Bi0.2O3-? for proton conducting SOFCs.Chapter 5 presents a summary of the paper and some recommendations for future research on proton conducting SOFCs.