In a NiMH-based Fuel Cell/Battery (FCB) system, the performance of the positive electrode composed of a mixture of alpha-Ni(OH)(2) and MnO2 was compared with that of another electrode composed of a mixture of beta-Ni(OH)(2) and MnO2. When Al-doped alpha-Ni(OH)(2) was used to replace beta-Ni(OH)(2) for the active material of the positive electrode, more stable operation of Fuel Cell/Battery (FCB) system was achieved. Cyclic voltammetry (CV) results for the Al-doped alpha-Ni(OH)(2) positive electrode showed that the electron transfer number of alpha-Ni(OH)(2) was 1.5 times larger than that of beta-Ni(OH)(2), which led to an improved discharge capacity. Furthermore, the plateau voltage in the second working voltage for alpha-Ni(OH)(2)/MnO2 was more stable compared to that for beta-Ni(OH)(2)/MnO2 in the fuel cell reaction when the cell was overcharged to an SOC of 300%. In addition, the plateau voltage in the first working voltage was higher than that of beta-Ni(OH)(2) by 0.2 V, which could be attributed to better contact between the polymer gel electrolyte and the electrode in the case of Al-doped alpha-Ni(OH)(2) due to the negligible volume change of Al-doped alpha-Ni(OH)(2) during the overcharge/discharge.