Low humidity dependence of proton conductivity in modified zirconium (IV)-hydroxy ethylidene diphosphonates

Proton conducting materials play an essential role in various fields of science and many applications, such as polymer electrolyte fuel cells (PEFCs). However, their proton conductivities suffer from the strong dependence on relative humidity (RH) and severely decrease at low RH conditions. Here, we synthesized zirconium (IV)-hydroxy ethylidene diphosphonates (ZrHEDP), which have phosphonic acid groups at a close distance. As the acidic groups are more concentrated in ZrHEDP, the proton conductivity exhibits lower dependence on the RH. In particular, the ZrHEDP with the largest amount of phosphonic acid groups among the examined samples showed the lowest RH dependence; the proton conductivity at 40% RH remained 2/3 of the conductivity at 95% RH, whereas a representative electrolyte, Nafion, at 40 % RH showed 1/5-1/20 of its conductivity at 95% RH.