The air electrode used different size of carbon powders as the conducting material, Bi2-xRu2O7-z as the bi-functional catalyst , and PTFE as the binder. The size of carbon powders was changed from 200 nm to 60 μm. Carbon powders, oxide catalysts and aqueous dispersion of PTFE particles were directly mixed. Then, the air electrode using the mixture was prepared by two ways; the one was that the mixture was rolled and pressed on a nickel mesh by a non-heated roller to form a sheet, and the obtained sheet was finally calcined at 370 oC under nitrogen atmosphere [2-4], i.e., the conventional roll press and calcination method. The other was that the mixture was rolled and pressed on the nickel mesh by the hot roller heated at 80 oC without following the calcination, i.e.,the low temperature roll press (LTRP) method. The surface and cross-sectional structures of the air electrode were observed by SEM. The polarization behaviors of the air electrode were examined by cyclic voltammetry using a three-electrode cell, in which one side of the air electrode was exposed to air and the other side to 6 mol/L KOH solutions.
The size of carbon powders and preparation process significantly influenced on the internal structure and polarization behaviors of the air electrode. The polarization for oxygen reduction of the air electrode using 1 μm carbon powders was lower than that of the air electrode using 200 nm and 60 μm, because suitable pores for gas flow were created inside of the air electrode. The air electrode prepared by the LTRP method was good to reduce the polarization for oxygen reduction and evolution, and drastically reduced the polarization for oxygen reduction, especially at the high current density with oxygen supply compared to that by the conventional method, implying the enhancement in gas permeability. We will further present the improved performance of the metal hydride/air secondary battery using the air electrode prepared by the LTRP method.
This work was done under “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST). The authors also acknowledge FDK Corp. for supplying the negative electrode.
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