The ab initio calculation showed that the suitable acid group to induce the packed-acid mechanism is a hydrocarbon-based phosphonic acid group rather than a sulfonic acid group, generally employed for electrolytes in PEFCs due to its high dissociation constant of a proton. Based on the theoretical suggestion, the distance between hydrocarbon-based phosphonic acid groups is preferably ~13Å to induce the packed-acid mechanism. There have been many studies on polymers functionalized with dense-phosphonic acid groups. However, polymers are generally flexible and swollen by water under humid conditions, making it challenging to maintain a high-density acid structure.