Heat-Treated Transition Metal Hexacyanometallates as Electrocatalysts for Oxygen Reduction Insensitive to Methanol

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Highly active catalysts for electrochemical reduction of oxygen insensitive to the presence of methanol were prepared from transition metal hexacyanometallate precursors by heat-treatment with carbon black under an inert atmosphere. The catalytic activity for oxygen reduction was examined with the floating electrode technique under an air atmosphere at room temperature. The electrolyte used in most of the measurements was 1 M sodium phosphate buffer solution (pH 7.5), whereas acid and alkaline solutions were also used in addition to the neutral buffer solution to examine the catalytic activity of the prepared catalyst over a wide range of pH. Remarkable enhancements in the catalytic activity were observed for samples heat-treated at temperatures higher than 500°C. Among several 3d-transition metals incorporated in the inorganic precursor, the combination of cobalt and iron incorporated at neighboring sites gave the highest activity, comparable to that of platinum black catalyst (Pt/C). The catalytic activity for oxygen reduction was not affected by the presence of 2.5 M methanol in the electrolyte, while that of Pt/C was severely impaired by the presence of methanol. The catalysts prepared from the inorganic precursors were characterized by X-ray diffraction (XRD), infrared (IR), and X-ray photoelectron spectroscopy (XPS) measurements. The XRD and IR data indicated that the cyanide structure of the inorganic precursor was decomposed when heating beyond 500°C. The XPS data indicated that the oxidation states of cobalt and iron are close to metallic ones and two types of nitrogen forming new bonding are present in the heat-treated samples. The same structural and spectral changes were observed for samples heat-treated without carbon black. From these results, the evolution of the high catalytic activity by heat-treating the inorganic precursors is discussed. © 2004 The Electrochemical Society. All rights reserved.