Highly active nonplatinum catalyst for air cathodes

Nonplatinum electrocatalysts for air cathodes showing higher activity than the conventional Pt/C catalysts were prepared by heat-treating cobalt hexacyanoferrate precursors dispersed on carbon support under an inert atmosphere. The activity for oxygen reduction was examined by polarization measurements with a gas-diffusion electrode floating on the surface of an electrolyte under an air atmosphere at room temperature. The electrolyte used was a neutral phosphate buffer solution. To investigate the effect of carbon support on the catalytic activity, different types of carbon supports were used with and without pretreatment with nitric acid prior to dispersion of the inorganic precursor. Among several carbon supports examined, the high-area carbon pretreated with nitric acid at higher concentrations than 8 M led to an enhanced performance for oxygen reduction in the gas-diffusion electrode. The effect of Co/Fe ratio in the precursor on the catalytic activity was also investigated. The catalyst prepared by heat-treating the inorganic precursor with a Co/Fe ratio of 1.2 dispersed on the high-area carbon showed the highest activity. Concerning the polarization voltage for oxygen reduction with the floating electrode, the best catalyst prepared in this study exhibited ca. a 200 mV higher potential at 100 mA cm(-2) than the carbon-supported Pt-black catalysts. The remarkable enhancement in the performance of the air cathodes seems to be due to high dispersion on the high-area carbon, leading to effective utilization of the catalyst in addition to its intrinsic activity. (C) 2004 The Electrochemical Society.