On the other hand, inorganic materials with a rigid framework, such as zirconium phosphonates, possess high stability toward swelling and keep the densified acidic circumstance. Moreover, zirconium phosphonates have a 2D structure and can consist of acid groups at a certain distance, 4.5-5.5 Å continuously along the layer (Fig. ). There is no research to regulate phosphonic acid groups at such a close distance and examine the humidity dependence of the electrolyte.