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.

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 Ϫ2 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.Electrochemical reduction of oxygen has been important in a number of applied aspects, such as air cathodes in fuel cells and metal/air batteries to generate electrical energy, and air cathodes in electrolytic processes to reduce power consumption. For reduction of oxygen at the cathodes at low and moderate temperatures, Pt and Pt-based catalysts have been most widely investigated because of their high catalytic activity and stability. However, nonplatinum catalysts have also attracted electrochemists since the activity for oxygen reduction of transition-metal macrocycles was reported in the 1960s. 1 Many efforts have been made to investigate nonplatinum catalysts for oxygen reduction prepared from transition-metal macrocycles 2-8 or from transition-metal salts and nitrogencontaining organics. 9-13 Despite such extensive studies, improvement in the catalytic activity and/or the development of new electrocatalysts based on nonnoble metal elements are still challenging targets.Recently, we have reported a catalyst prepared by heat-treating inorganic precursors ͑transition-metal hexacyanometallates, Prussian blue analogs͒ under an inert atmosphere, whose catalytic activity for oxygen reduction examined by a gas-diffusion electrode is comparable to that of a commercial platinum-black catalyst supported on carbon black. 14,15 The preparation procedure of the catalyst involves colloidal dispersion of the inorganic precursor on carbon black suspended in an aqueou...

A high-active oxygen reduction catalyst was prepared by heat-treatment of transition metal hexacyanometalate precursors containing cobalt and iron dispersed on carbon black under an inert atmosphere. The catalytic activity for oxygen reduction was insensitive to the presence of high concentration of methanol and was comparable to that of the platinum catalyst in the absence of methanol.

Remarkable enhancement in catalytic activity of a non-platinum catalyst for oxygen reduction in alkaline solutions has been achieved by an oxidative pretreatment of a high-area carbon support before dispersion of catalyst precursor (cobalt hexacyanoferrate), followed by pyrolyzing the precursor under an inert atmosphere. The polarization potential for oxygen reduction at substantial current densities at room temperature under air was higher than a potential corresponding to a limit of relative predominance for O2/HO2− couple.

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