Proton conductivities of the ZrHEDP powders were measured at various RH conditions (Fig. ) as pellet samples (diameter: 13 mm, thickness: ~0.1 mm), made by uniaxial pressing the respective powders. Figure shows the enhanced proton conductivity values and low dependence on RH in ZrHEDP with a high P/Zr ratio (see Fig. for cole-cole plots). In particular, the proton conductivity of the ZrHEDP pellet with P/Zr = 3.0 at 95% RH only decreased to 2/3 at 40% RH. On the other hand, Nafion at 40% RH exhibits 1/5-1/20 of the proton conductivity compared to its value at 95% RH. Hence, the dependence on RH in the synthesized ZrHEDP is low. The Ea of the ZrHEDP pellet with P/Zr = 3.0 was 15.3-35.4 kJ/mol at 95% RH and 90 °C (see Fig. ). The origin of these conductivities will be discussed in Section 4.2.