Furthermore, the low freezing temperatures in DSC results indicate that water in ZrHEDP is less movable as P/Zr increases, not favorable for conventional mechanisms of proton conduction. Nevertheless, the proton conductivity of the ZrHEDP with high P/Zr is enhanced, which also supports that the proton conduction mechanism in the ZrHEDP samples is the packed-acid mechanism. In addition, the Ea of the ZrHEDP pellet with P/Zr = 3.0 was measured to be 15.3-35.4 kJ/mol at 95% RH and 90 °C. These values are below 0.4 eV (= 38.6 kJ/mol), typically considered the Ea of the Grotthus mechanism. The Ea of the previous sample where proton moves through packed-acid mechanism exhibited 41 kJ/mol. It also means that the proton conduction mechanism in ZrHEDP samples can be the packed-acid mechanism. Therefore, it is possible that the low dependence on RH originated from the concentrated -PO3H2s through the packed-acid mechanism.