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

Materials with proton-conducting properties have attracted much interest and play a key role in various processes and devices such as polymer electrolyte fuel cells. In these materials, the major issue is the strong dependence of the proton conductivity on the relative humidity (RH); conductivity severely decreases under low RH conditions. Here we report the synthesis of zirconium-hydroxy ethylidene diphosphonates (ZrHEDP), in which the phosphonic acid groups are placed at a closer distance. As the acid groups are more concentrated in ZrHEDP, the proton conductivity exhibits lower dependence on the RH. In particular, the ZrHEDP with the largest number 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 well-known proton-conducting material, Nafion, at 40% RH showed 1/5-1/20 of its conductivity at 95% RH.