This serious proton conductivity dependence on RH is explained by the inevitable characteristic of the conventional proton conduction mechanisms: the vehicle mechanism and the structural diffusion. In the vehicle mechanism, a proton transfers as H3O + via molecular diffusion. Structural diffusion is typically regarded to be the same as the well-known "the Grotthus mechanism." The structural diffusion involves necessary coordination constructed by the following two procedures: "hopping," in which a proton hops from a proton donor to an acceptor along a hydrogen bond (H-bond), and "reorientation," in which a H-bond is cleaved, and the proton reorients to another proton acceptor. The diffusion occurs through coordination originating from reorientation in the second hydration shell, followed by successive hopping events involving the movement of H5O2 + (the Zundel cation) and H9O4 + (the Eigen cation), with several water molecules per proton. The ratedetermining step is the reorientation The movement of water molecules is indispensable because a thermal fluctuation of water breaks a H-bond to cause reorientation. Obviously, water movement is also essential for the vehicle mechanism. Therefore, both mechanisms inevitably require water movements, and thus water molecules (i.e., high RH condition) are crucial.