The use of manganese dioxide (MnO2) as a positive electrode material in Fuel Cell/Battery (FCB) systems is described. A positive electrode containing MnO2 was fabricated and its performance was evaluated for charge/discharge behavior in three different systems: (i) secondary battery positive electrode, (ii) positive electrode in an alkaline fuel cell, and (iii) positive electrode performance in an FCB system by performing half cell tests. MnO2 was observed to possess redox capabilities as the positive electrode of a secondary battery when it was subject to charge/discharge cycles. It was found that Mn3O4, which inhibits the discharge reaction, was produced during charge/discharge cycles. The I–V characteristics of MnO2 material were measured to check the feasibility of the fuel cell system by supplying H2 into the negative electrode and O2 into the MnO2 positive electrode, respectively. The MnO2 electrode showed similar performance to Ni electrode, which was fabricated by using a similar method to the MnO2 electrode. The MnO2 electrode also showed that it functioned as an FCB positive electrode, which was confirmed by continued production of current when the O2 supply was terminated. These results suggest that MnO2 is a good candidate for an FCB positive electrode material.