Electrocatalysis for dioxygen reduction by a μ-oxo decavanadium complex in alkaline medium and its application to a cathode catalyst in air batteries
Introduction
The electrochemical reduction of oxygen by metal complexes has been widely studied with biological [1], [2], [3], [4], [5], [6], [7], polymer synthetic [8], [9], [10] as well as fuel cell [4], [11] and metal–air batteries applications [12], [13], [14], [15], [16], [17] via 2e− (acidic: O2+2H++2e−→H2O2; basic: O2+2H2O+2e−→HO2−+OH−) or 4e− (acidic: O2+4H++4e−→2H2O; basic: O2+2H2O+4e−→4OH−) electron transfer at chemically modified electrodes. Most of the electroreduction reactions of oxygen were studied under acidic conditions. Only a few catalysts were effective in both acidic and basic media, such as the planar MN4 cyclic chelates (cofacial Co-porphyrins and Cu-phenanthroline) which were found to be the four-electron reduction catalysts at various pH values [6], [15]. However, the catalyst that can react with oxygen under basic conditions (pH>12) is very rare to our knowledge. Here we describe the novel μ-oxo decavanadium complex as a catalyst that has a high efficiency for the electroreduction of oxygen under basic conditions.
In previous reports, the μ-oxo type decanuclear vanadium complex [(VO)10(μ2-O)9(μ3-O)3(C5H7O2)6] (1) was realized as a good multi-electron transfer catalyst in homogenous electrolytes and an active four-electron reduction catalyst with a very high yield in the conversion of O2 to H2O, in which the H2O2 by-product is limited to not more than 2%, at approximately 0.5 V versus saturated calomel electrode (SCE) under acidic solutions [16], [17]. In the present study, the redox behavior of various pH solutions and the electrocatalytic reduction of oxygen in oxygen-saturated alkaline aqueous solutions were studied by cyclic voltammetry and rotating-ring disk voltammetry (RRDV). The electrochemical results are discussed in view of the factors for determining the reduction pathway.
Furthermore, we describe the application of the complex 1 as an oxygen reduction catalyst in the cathode of zinc–air batteries for trial purposes. The R&D study of metal–air batteries has been done for more than 30 years, and the performance needed for metal–air batteries is the high theoretical energy density, but the output power density is still very low. Seemingly, it is due to the low activity of air electrodes that are included in the metal–air batteries. To solve this problem of cathode materials for air electrodes, the high catalytic activity and chemical stability of an oxygen reduction catalyst have been required. Many authors and their cited references have investigated various electrode catalysts, including noble metal, oxide, nitrides, and organo metallic compounds [13], [14], [18]. On the other hand, the high valent μ-oxo type vanadium complex in non-planar structure is capable of reducing O2 and is expected to be a new material for the cathode which is reactive in both acidic and alkaline media.
Section snippets
Experimental
Materials and procedures: All reagents and solvents from commercial sources were used without further purification. The decavanadium cluster 1 was synthesized and analyzed according to the procedures as already described in the literature [16], [20]. The electrode modified with complex 1 was built in the same manner as previously described [17]. The Sorensen buffer electrolytes (pH 4.48, 10.11) and the neutral solution containing NH4PF6 as a supporting electrolyte, which were prepared using
Redox behavior of decavanadium complex in alkaline solutions
The cyclic voltammogram obtained with a glassy carbon electrode coated with dichloromethane solution containing complex 1 at a scan rate of 50 mV s−1 in argon-saturated aqueous solution of various pHs are shown in Fig. 1. Fig. 1(a) was obtained in pH 4.5 buffer solution which showed the reduction potential of complex 1 at 0.1 V versus SCE with no oxidation potential. Fig. 1(b) was obtained in a neutral solution that showed a large reduction current appearing near 0.05 V versus SCE with a slight
Conclusions
The electrochemical properties of the decavanadium complex in acidic and alkaline media have been investigated. The data obtained from the electrochemical studies in various aqueous solutions reveal that the decavanadium cluster is capable of acting as an excellent oxygen reduction catalyst over a wide pH range. While the oxides of V, Cr, Mn, and Co seem to be the most promising solid-state cathode catalyst for high performance secondary batteries and also others with the heat treatment process
Acknowledgements
L.D. Eniya acknowledges Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (DC1-2000 No. 05709).
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