However, proton exchange membranes (PEMs) used in PEMFCs have a fatal drawback: proton conductivity severely decreases at low relative humidity (RH) conditions, resulting in reduced energy efficiency. Thus, a lower operating temperature is employed to avoid the excessive evaporation of water. However, lowering the operating temperature leads to CO poisoning of the catalyst . Thus, highly pure fuels are utilized to suppress catalyst poisoning, and water and temperature must be managed to alleviate the decrease in conductivity. These result in extra work and equipment, increasing the cost and decreasing energy efficiency. For this reason, much effort has been devoted to developing membranes to overcome the conductivity dependence on RH - . In particular, it has been demonstrated that inorganic particles in the membrane can adsorb moisture and retain it at high temperatures , . Therefore, an organic-inorganic composite membrane is one solution to improve efficiency. Especially zirconium phosphate (ZrP) is promising as a proton-conducting inorganic. ZrP is a layered crystalline, represented as Zr(HPO4)2・H2O, which is water-insoluble and thermally stable up to about 450℃ , . ZrP has a moderate conductivity of 10 -4 ~10 -3 S cm -1 when it is humidified , and it can retain hydration water up to about 270℃, depending on crystalline morphology .