Controllable Syntheses of Proton-Conductive {P2Mo5}- and {P4Mo6}-Based Crystalline Materials

The development of crystalline proton-conducting materials with well-defined structures is crucial for exploring the proton transport mechanism and advancing proton exchange membrane fuel cells. Polyoxometalates (POMs), specifically the {P2Mo5} and {P4Mo6} clusters, have emerged as promising building blocks owing to their high charge density, rich phosphate groups, and synthetic accessibility. This work presents two novel POM-based coordination polymers with the formulas of [Na(H2O)2(HP2Mo5O23)(Me2NH2)4]·2H2O (1) and [Na5(H2O)5(H6.5P4Mo6O31)2(Me2NH2)6]·4H2O (2), controllably synthesized by a solvothermal method. Compound 1 exhibits a three-dimensional (3D) framework through various hydrogen bonding interactions among the “W”-shaped chains constructed from the linkage of Na+ ions and {P2Mo5} units, while the 3D architecture of compound 2 is built from one-dimensional chains linked by Na+ ions and hourglass-type {Na[P4Mo6]2} dimers, with these chains further interconnected via multiple hydrogen bonds. Compounds 1 and 2 exhibit good proton conduction performance with conductivities of 2.46 × 10–4 and 1.18 × 10–3 S·cm–1 measured at 65 °C and 85% RH. Furthermore, the composite membranes formed by incorporating compounds 1 and 2 into sulfonated poly(ether ether ketone) (SPEEK) deliver significantly improved proton conductivity relative to pristine SPEEK, demonstrating significant application potential.