In this study, two types of commercial 30 wt% CB- supported Pt catalysts (c-Pt/CB, TEC10E30E, Tanaka Kikinzoku Kogyo K. K.) and graphitized carbon black (GCB)-supported Pt catalyst (c-Pt/GCB, TEC10EA30E, Tanaka Kikinzoku Kogyo K. K.), and 30 wt% acetylene black (AB)-supported Pt catalysts, prepared in house by the nanocapsule method [9,10] (n-Pt/AB800 and n-Pt/ AB250, specific surface area of AB800 and AB250: 779 m2 g 1 and 219m2 g 1, Denki Kagaku Kogyo Co., Ltd.), were used as the cathode catalysts.