| Summary: | 碩士 === 國立交通大學 === 材料科學與工程系所 === 92 === Metal-air fuel cells include the Zn-air and Al-air type fuel cells. They have been used as low-power portable electronics, because of their simple structure, high specific energy, low cost, stability and other advantages. To meet the future energy requirements of electrical motorcycles in this country, the cathode activities of the zinc-air fuel cells (or the reduction rate of O2) should be boosted to a high specific energy/power. The reduction mechanism of O2 in alkaline media prevails on the surfaces of conductive powders (e.g. carbon black loaded with an electrocatalyst) mixed in the cathode, which absorbs the reactants (O2, H2O and OH-) and releases the products (OH- and HO2-). The conductive powders act as the reaction carriers. The characteristics of cathode catalyst to facilitate the reduction of O2 would be critical to the activity of the cathodes. For economical reasons, low-cost catalyst would be the choice for practical mass production.
This study is to stress on non-noble oxide catalysts. A series of the transition metal oxides, La-Ca-Co-O, Y-Ba-Cu-O, Ba-Ru-O, Co-O, Cu-O and Ru-O compounds were prepared using the amorphous citrate precursor method. Catalytic activity for H2O2 decomposition reaction on all catalysts was found that the Y-Ba-Cu-O compound was calcined at 350℃ has better activity. The porous electrodes are made of slurry with various kinds of compositions. Manufacturing processes and optimal conditions for the air cathode are described as follow. The diffusion layer is consisted of 60wt% carbon black and 40wt% PTFE suspension, and the active layer is made of 60wt% carbon black, 20wt% catalyst and 20wt% PTFE suspension. The diffusion layer and the active layer are pressed together with a Ni net to form the air cathode.
The investigation is via the utilization of familiar electrochemical methods including the constant-current discharging measurement, the cyclic voltammetry and the Tafel polarization curves analysis. The various catalysts for air cathode were carried out the electrochemical testing. In the constant-current discharging measurement, the results show that the Co3O4, RuO2 and CuO compound were calcined at 350℃ have the best catalytic effects. Over a wide potential range, cyclic voltammograms of the air cathode would be observed the redox peaks. The Tafel curve analysis indicates that the equilibrium potential of the air cathode is difficult to approach to the theoretical value and is under the control of kinetics. It leads to the OCV’s difference of 0.3~0.65V for the air cathode. The resulting polarization curves show the performance of the oxygen reduction and oxygen evolution reaction. The electrode without catalyst was the most active in the oxygen evolution reaction which has relation to the specific surface area of carbon material.
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