There is another reason for the decrease in conductivity in a low RH environment. With less H2O, the conventional proton conduction mechanism, structural diffusion, which inevitably requires water movement, is less likely to occur - . In contrast, the packed-acid mechanism, the mechanism of the proton conduction in a material consisting of a high-density acid structure, does not need water movement and is considered to be less humidity-dependent , , . Therefore, the packed-acid mechanism improves the conductivity under low RH conditions. Moreover, membranes, where protons conduct via the packed-acid mechanism, are expected to be applied not only to PEMFCs but also to other fields, such as ammonia electrochemical synthesis . The high contents of inorganics are suitable to induce the packed-acid mechanism, expected to overcome the severe dependence of proton conductivity on RH.