Meanwhile, the "packed-acid mechanism" has been recently proposed as a derivative of Grotthuss mechanisms that occurs in materials consisting of concentrated acids. In the packed-acid mechanism, acidacid interactions weaken H-bonds and stimulate reorientation. Thus, the mechanism does not require water molecule movements. In fact, the proton in the material comprising of packed-acids diffuses even when water is frozen, whereas an ordinary proton conducting material cannot retain the proton diffusivity under the same condition. It suggests that the packed-acid mechanism might not be strongly influenced by RH.