A study of carbon-catalyst interaction in bifunctional air electrodes for zinc-air batteries

The activity of bifunctional air electrodes in alkaline electrolyte was investigated as a function of the carbon black and graphite materials used as catalyst supports. The perovskite La(0.6)Ca(0.4)CoO(3) was used as the bifunctional catalyst for all air electrodes tested in this work. The carbon substrates were Vulcan XC 72 (as received), graphitized Vulcan XC 72. Ketjenblack K-600, and two different Timcal graphites (HS-100 and HS-300). Ketjenblack and Vulcan XC 72 provide high surface areas of ca. 1200 m(2).g(-1) and 250 m(2).g(-1), respectively, but the surface area of the graphitized Vulcan is only 70 m(2).g(-1). Graphitizing the amorphous Vulcan carbon at 2700 degrees C in inert atmosphere improved the crystallinity and corrosion resistance of the material, buy its activity for oxygen reduction decreased because of the smaller surface area. Reheating the material to 600 degrees C in air improved its activity for oxygen reduction. The graphite HS-100 available commercially provides a surface area of 100 m(2).g(-1) and a service life under continuous cycling comparable with that of Vulcan XC 72 that had been graphitized and activated. The stability of the high-surface-area air electrodes containing Ketjenblack is low when they are exposed to positive oxygen evolution potentials. The service life of these electrodes could be markedly improved by increasing their catalyst loading. With air electrodes containing Ketjenblack and Vulcan XC 72, AC impedance measurements were carried out in order to gather information on the kinetics, diffusion, and adsorption features of the electrode processes involved.