Development of a Novel Manganese Dioxide/Nickel Oxyhydroxide Fibrous Positive Electrode for Fuel Cell/Battery (FCB) Systems

A novel MnO2/NiOOH fibrous positive electrode for Fuel Cell/Battery (FCB) systems has been proposed. FCB systems containing manganese dioxide and metal hydride (MH) as active materials for the positive and negative electrodes, respectively, have been found to be attractive energy storage and power generation systems because they can function as both fuel cells and secondary batteries with high energy and power density. Moreover, they can also be chemically charged by gaseous hydrogen and oxygen supply. However, the chemical charging rate of the positive electrode of the FCB systems is relatively low compared to the negative electrode. In our previous study, highly dissolved oxygen in an electrolyte was used in order to improve the chemical reaction rate of oxygen charging on a manganese dioxide electrode. The study showed that the charge rate will be 3.45 mAh/(g･min) if the oxygen is supplied with the limiting pressure. This result implies that the proton diffusion in the crystalline structure of the manganese dioxide or the dissociation of oxygen molecule on the manganese dioxide surface is the rate determining step in the oxygen charging. In the current study, therefore, we suggest two approaches for the fabrication of FCB positive electrodes to improve the oxygen charging rate: i) enlargement of active surface area by an electrodeposition method of active materials on carbon fibers, and ii) introduction of small amount of nickel oxyhydroxide to fibrous manganese dioxide electrodes in order to enhance the electrical conductivity of the positive electrode. Nickel oxyhydroxide is a well known positive electrode material of Ni-MH batteries and has higher electrical conductivity than manganese dioxide. Thus, the combination of nickel oxyhydroxide with manganese dioxide is expected to lead to a higher electrical conductivity of the FCB positive electrode. In the present paper, the characteristics of the proposed fibrous electrode will be studied by using galvanostatic measurement, electrochemical impedance spectroscopy, scanning electron microscopy analysis, and X-ray diffraction measurement.