Peroxide Elimination Activity of Heat-Treated Transition Metal Hexacyanoferrate Catalysts for Oxygen Reduction in Alkaline Solution

Abstract

Nonprecious metal catalysts for oxygen reduction were prepared by heat-treating transition metal ͑Mn, Fe, Co, Ni, Cu, and Zn͒ hexacyanoferrate precursors ͑Prussian blue analogs͒ dispersed on carbon black under a nitrogen atmosphere. Polarization measurements for oxygen reduction on the prepared catalysts were carried out with a gas-diffusion electrode floating on the surface of alkaline solution under a stream of oxygen. The catalytic activity toward decomposition of peroxide generated during oxygen reduction at the gas-diffusion electrode was examined in situ by monitoring the open-circuit potential recovery after interruption of steady-state cathodic current. The catalytic activity for oxygen reduction on the carbon-based electrode was well correlated with the activity toward peroxide decomposition. A linear relation between the electrode potential at constant current and the logarithm of the peroxide decomposition rate constant was observed. Among the catalysts consisting of several combinations of 3d transition elements, the catalysts containing cobalt or copper with iron exhibited high operation potential of the electrode, and the catalysts containing copper or zinc with iron exhibited high activity toward peroxide decomposition. When the normalized rate constants toward peroxide decomposition were compared with those reported for the conventional catalyst such as transition metal oxides, the catalytic activity of the present catalysts was comparable to or higher than the conventional catalysts. From these results, the possibility of cost-effective catalysts for air cathodes in alkaline solution is discussed.Electrochemical reduction of oxygen is of great importance in fundamental and applied fields, such as air cathodes of fuel cells and metal/air batteries to generate electrical energy, and air cathodes in electrolytic processes to reduce power consumption. 1-4 It is well known that the polarization for oxygen reduction at low and moderate temperatures is rather large, even with the most active platinum-based catalyst. Although the oxygen reduction reaction may appear simple at first glance, operation of oxygen electrode at small polarization has been a challenging target for over a century, as reviewed in the literature. 5-7 For oxygen reduction in acid solutions, noble metal catalysts are normally used. However, for oxygen reduction in alkaline solutions there is a possibility to use low-cost catalysts employing abundant non-noble elements, such as transition metal oxides, 8-11 transition metal macrocycles, 12-14 and alternative catalysts. [1][2][3][15][16][17] Recently, we have reported highly active, nonprecious metal catalysts prepared from the inorganic precursors based on abundant 3d transition elements. [18][19][20][21][22][23] The catalysts were prepared by heattreating the nitrogen-containing inorganic compounds ͑transition metal hexacyanometallates, Prussian blue analogs, PBs͒ dispersed on carbon black. The advantage of using such inorganic precursor is that a series of compounds consisting of v...

Bifunctional Oxygen Reduction Reaction Mechanism on Non-Platinum Catalysts Derived from Pyrolyzed Porphyrins

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Electrocatalytic Oxygen Reduction in Alkaline Medium at Graphene-Supported Silver-Iron Carbon Nitride Sites Generated During Thermal Decomposition of Silver Hexacyanoferrate

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