Study And Application Of Perovskite La_1-xCa_xCoO₃Bi-functional Oxygen Electrode

In this dissertation, electrochemical performance of perovskite bi-functional oxygen electrodes are studied. Perovskite-type series of compounds La1-xCaxCoO3powders were synthesized by citrate-nitrate auto-combustion method and used as electrocatalyst for bi-functional oxygen electrodes. The influence of the ratio of La to Ca of La1-xCaxCoO3on their catalytic performance was investigated by polarization curves in alkaline electrolyte. Among these compounds, La0.6Ca0.4CoO3calcined at650℃exhibited the highest catalytic activity for oxygen reduction and evolution.In order to further improve the catalytic activities and electron conductivity of the perovskite-electrocatalyst, the La0.6Ca0.4CoO3powder were modified with silver by chemical reduction method. The silver-modified La0.6Ca0.4CoO3powder was used as binary electrocatalyst for bi-functional oxygen electrodes. By means of the physical and chemical examinations and electrochemical methods, the silver loading was found to have a significant impact on the electrode performance, which could facilitate or block the electrochemical processes of the gas diffusion electrodes. The binary catalyst electrodes exhibited higher electrocatalytic activities than that of the electrodes with only La0.6Ca0.4CoO3as the catalyst. The best performance was achieved when the silver loading was3.0wt.%.Well-dispersed nanoporous La0.6Ca0.4CoO3catalyst for bi-functional oxygen electrode was synthesized by a modified citrate-nitrate auto-combustion method using Vulcan XC-72R as a pore-forming material. The calcination temperature and the content of the pore-forming material were optimized by TG-DSC, XRD, particle size distribution and SEM analysis. Compared with the traditional citrate-nitrate auto-combustion method, the perovskite La0.6Ca0.4CoO3prepared by this modified technique illustrated higher electrocatalytic activity mainly due to the high surface area and the novel nanostructure, which would provide potential applications in metal-air batteries and fuel cells.The La0.6Ca0.4CoO3bifunctional oxygen electrode processes were investigated by electrochemical impedance spectroscopy (EIS). The overall impedance data were fitted by a complex non-linear least squares fitting program in Z-view2.0software. The impedance spectra were analyzed by an equivalent circuit containing an electrode intrinsic resistance at the high frequency and kinetic impedance through the porous electrode at the low frequency. Meanwhile, the equivalent circuit was used for studying the silver-modified La0.6Ca0.4CoO3binary electrocatalyst and the nanoporous Lao.6Cao.4Co03with high specific surface area how to improve their electrode processes. The EIS results confirmed that the silver-modified La0.6Ca0.4CoO3and the nanoporous La0.6Ca0.4CoO3with high specific surface area used as electrocatalysts for bi-functional electrode could reduce both intrinsic and kinetic impedance, indicating that their electrical conductivity and electrocatalytic activity were higher than the common La0.6Ca0.4CoO3catalyst.Well-dispersed nanoporous La1-xCaxCoO3were also synthesized by the modified a citrate-nitrate auto-combustion method. Their catalytic activities for hydrogen peroxide electroreduction in3.0mol·dm-3KOH at room temperature were first evaluated by cyclic voltammetry and galvanostatic measurement. The influence of annealing temperature and the ratio of La to Ca of La1-xCaxCoO3on their catalytic performance were investigated. Among the series of compounds, La0.6Ca0.4CoO3calcined at650℃exhibited the highest catalytical activity. An aluminum-hydrogen peroxide semi fuel cell using La0.6Ca0.4CoO3as cathode catalyst showed a peak power density of201mW·cm-2at150mA·cm-2and1.34V running on0.4mol·dm-3H2O2.