A Cs 2.5 H 0.5 PW 12 O 40 /Nafion/polytetrafluoroethylene ͑PTFE͒ self-humidifying composite membrane was developed for proton exchange membrane fuel cells. It was prepared by recasting the Cs 2.5 H 0.5 PW 12 O 40 /Nafion self-humidifying layer onto the two sides of a Nafion/PTFE composite membrane with a solution-recast method. Due to the strong acidic, hydrophilic, and redox properties of Cs 2.5 H 0.5 PW 12 O 40 , the Cs 2.5 H 0.5 PW 12 O 40 /Nafion self-humidifying layer not only contributes to humidify the membrane but also contributes to the improvement of the open-circuit voltage of the fuel cell. When the self-humidifying composite membrane was employed as an electrolyte in H 2 /O 2 PEMFC, a higher maximum power density value ͑1.1 W/cm 2 ͒ was obtained than that of Nafion/PTFE composite membrane ͑0.6 W/cm 2 ͒, under the operating conditions of 70°C and at the humidified temperature of 20°C.The proton exchange membrane fuel cell ͑PEMFC͒ attracts much attention due to its high efficiency, high power density, zero emission, and low-temperature start-up as a power sources suitable for both stationary and mobile application. However, the typical membrane currently used for the PEMFC, such as Dupont's Nafion or perfluorosulfonic acid ͑PFSA͒ membrane, shows a significant loss in conductivity at a lower relative humidity ͑RH͒ environment. 1 Thus the reaction gas needs to be humidified in the PEMFC. But the humidifier will make the whole system complex and large. Therefore these needs promote the research and development selfhumidifying membrane. 2 Up to now, many materials such as Pt, 3 metal oxides, 4 heteropoly acid, 5 and ZrHPO 4 6 have been used to highly disperse in the Nafion or sulfonated poly ͑ether ether ketone͒ ͑SPEEK͒ resin to fabricate self-humidifying membrane. However, due to the poor mechanical strength and stability of those selfhumidifying membranes, the humidifier still has to be used in the PEMFC system. Cs 2.5 H 0.5 PW 12 O 40 is insoluble in water and organic solvents and has micro-and mesopores with a high surface area ͑around 130 m 2 /g͒. 7 In addition, the acidity per unit acid site of Cs 2.5 H 0.5 PW 12 O 40 was superior to Nafion-H as well as homogeneous acids, e.g., H 2 SO 4 , H 3 PW 12 O 40 , and p-toluenesulfonic acid. 8 These facts demonstrate that Cs 2.5 H 0.5 PW 12 O 40 is a promising solid acid to be used in PEMFCs. Furthermore, Cs 2.5 H 0.5 PW 12 O 40 possesses a redox property. Various catalyzed reactions such as alkylation, ester decomposition, acylation, Diels-Alder reaction, have been reported to proceed on Cs 2.5 H 0.5 PW 12 O 40 . 9-11 Recently, Limoges et al. 12 reported the heterployacides can be used as the catalyst of the anode and cathode in PEMFCs.In view of the above strong acidic, hydrophilic, and redox properties of Cs 2.5 H 0.5 PW 12 O 40 , a high performance Cs 2.5 H 0.5 PW 12 O 40 /Nafion/PTFE self-humidifying composite membrane with a sandwich structure was developed to improve the mechanical strength of the membrane and the cell performance under lower RH condit...