In particular, hybridization of Ni/MH secondary battery with a fuel cell, called Fuel Cell/Battery (FCB), is considered to be a promising method. This can be accomplished by fabricating the positive electrode with the mixture of nickel hydroxide and a small amount of manganese dioxide, a well-known oxygen reduction catalyst. Thus, this hybridized system can function both as a secondary battery and as a fuel cell. In this system, electric power is produced as a battery when it is charged. Furthermore, it can also produce electric power by consuming hydrogen and oxygen gases in a similar way to a fuel cell system when it is discharged. In addition, this system can also function in the following way: Water is electrolyzed when the Ni/MH secondary battery is overcharged, producing hydrogen in the negative electrode and oxygen in the positive electrode, respectively. Thus, the Ni/MH secondary battery can function as a fuel cell by using the stored gases when the battery is overcharged without the supplies of fuels from the outside of the system. Therefore, this leads to improved energy density per weight compared with conventional batteries.
The performances of the Ni/MH based FCB were investigated with cyclic voltammetry (CV) and X-ray Diffraction (XRD). The characteristics of full cell performances were measured with cell-voltage (V) - current density (I) curves. The reaction mechanisms of the positive electrode were also investigated. It was observed that the discharge curves had two plateaus; one at 1.1 V from the reactions of nickel oxyhydroxide, the other at 0.7 V from the fuel cell reactions of manganese dioxide. It was also found that manganese dioxide only functioned as oxygen reduction catalyst, which was confirmed from XRD and CV curves. These experimental results showed the improvement of the overall energy density of the hybrid system compared with conventional batteries.
Fuel Cell/Battery (FCB) systems, Ni/MH secondary battery, Water electrolysis