Proton-conducting crystalline solids are important electrolytes. Polyoxometalates (POMs) are promising building blocks, because of their intrinsically high proton conductivity; however, their acidic salts are generally water-soluble and unsuitable as solid electrolytes. To overcome this limitation, we have developed a series of crystalline POM-polymer composites. Here, we adopt a chemical functionalization strategy using urea, a small molecule with strong hydrogen-bonding capability, to reinforce proton-conduction pathways. We report crystalline composites composed of a Na-encapsulated Preyssler-type POM (Na), K+ ions, and urea-modified poly(allylamine) (UPAA) with modification ratios of 25%, 50%, and 75%, designated as Na-UPAA25, Na-UPAA50, and Na-UPAA75, respectively. Among them, Na-UPAA50 exhibits the highest proton conductivity of 8.0 × 10-3 S cm-1 at 75 °C and 75% relative humidity. Na-UPAA50 shows the highest packing ratio of water and UPAA within the framework, promoting an extended hydrogen-bonding network. Spectroscopic studies further reveal partial in-situ degradation of UPAA in Na-UPAA50, generating additional proton carriers such as ammonium ions. This synergistic effect of a urea-derived hydrogen-bonding network and partial polymer degradation enhances proton transport, providing a new strategy for designing high-performance proton-conducting POM frameworks.
Iwano et al. (Thu,) studied this question.