Study On Preparation And Properties Of Copper Cobalt Spinel Based Cathode Of Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?is a kind of high efficiency and environmentally friendly electrochemical energy conversion device.Currently,intermediate-temperature solid oxide fuel cell?IT-SOFC?represents theresearch mainstream in international SOFC field.However,the lower temperature reduce catalytic activity of cathode materials,which the one of key bottlenecks confine the development of IT-SOFC.Although cobalt-based perovskite materials reveal good catalytic activity at intermediate-temperature,its thermal and chemical matching are poor with the electrolyte.To improve the performance of cathode,a new type of SOFC cathode was developed in this thesis,spinel CuCo₂O₄,which has a considerable electrocatalytic activity.The physical and chemical properties,the compatibility with the electrolyte,the electrocatalytic activity for oxygen reduction reaction?ORR?and the output performance of the battery were studied in detail.Furthermore,a variety of microstructural composite cathodes were constructed by introducing the high oxygen ion conducting electrolyte.Therefore,the relationship between reaction mechanism,microstructure and electrochemical properties were investigate d.Firstly,CuCo₂O₄ powders were prepared by sol-gel method.The results show that the pure cubic phase spinel Cu Co 2O4 is obtained when the molar ratio of CA to EDTA increases to 1:1.5.The grain size of the material is approximately 70-200 nm and the particle size is uniform.The CuCo₂O₄ material has good chemical compatibility with SSZ electrolyte.The thermal expansion coefficient of CuCo₂O₄ is 11.76 × 10-6 K-1 in the range of 50-900 ?C which matches with SSZ electrolyte.According to the influence of cathode sintering temperature and pore forming agent on CuCo₂O₄ cathode,the optimum conditions were obtained as follows: the sintering temperature at 1000 ?C and the starch content of 25%.The polarization resistance was 0.12 ??cm2 and the maximum power density of single cell was 972 m Wcm-2 at 800 ?C.In order to further improve the electrocatalytic reactivity and increase the three-phase reaction interface length in CuCo₂O₄ cathode,10 mol% Sc2O3 stabilized Zr O2?SSZ?and Ce0.9Gd0.1O1.95?GDC?oxygen ion conducting materials were introduced into the cathode,and the composite cathode s were prepared by mechanical mixing method.The thermal expansion coefficient of CuCo₂O₄-SSZ and CuCo₂O₄-GDC composite cathodes were 11.39-11.86 * 10-6 K-1 and 11.83-12.20 x 10-6 K-1,respectively,which matching well with other SOFC components.The electrochemical test results show that the length of the three-phase reaction interface in the cathode can be e ffectively increased and the polarization resistance of the composite cathodes were decreased with the increasing oxygen ion electrolyte content in the composite cathode.However,when the content is too high,the diffusion resistance increased leading to polarization resistance increase.Among them,the composite cathode has the best performance when the composite content of GDC is 60 wt.%.At 800 ?C,the polarization resistance is 0.09 ??cm2 and the cathode overpotential is 74 m V in current density0.5Acm-1of CuCo₂O₄-GDC60 composite cathode.The maximum output power of single cell is 1074 m Wcm-2 with good stability.To further improve the performance of CuCo₂O₄ cathode,nanostructure CuCo₂O₄/SSZ composite cathode was prepared by in situ ion impregnation method.The effects of the concentration of impregnation solution,impregnation amount and sintering temperature on the morphology of the impregnated cathode were studied.The results show that the CuCo₂O₄ particles were more uniform and the impregnation times was relatively less?4⁵?when the impregnation solution was 0.025 mol/L.The effect of impregnation CuCo₂O₄ is better in the impregnation amount 17.6 wt.%.It is found that the influence of sintering temperature is not significant on the growth of CuCo₂O₄ particles because of the low thermal decomposition?360 ?C?in the preparation of material by impregnation process.At the same time,SSZ porous skeleton effectively inhibited the growth process of CuCo₂O₄ that ensure the CuCo₂O₄ does not appear coarsening phenomenon and so on.Finally,the nano structure composite cathode is stable,and the SSZ porous framework effectively increased the interfacial area and length of the three-phase reaction,and enhances the catalytic activity of the nano CuCo₂O₄ cathod.The electrochemical properties were improved significantly.The optimum immersion amount was 17.6 wt.%,which show the smallest polarization resistance at 800 ?C for 0.08 ??cm2.The output power of single cell was 1136 m W cm-2.At the same time,the oxygen reduction reaction mechanism of the CuCo₂O₄ impregnation impregnated was studied.It is illustrated that the oxygen ion in the electrode or fr om the electrode to the diffusion process of three phase reaction interface electrolyte is the rate controlling step of the cathode reaction.