Structure of and Interactions between P/SiWA Keggin Nanocrystals Dispersed in an Organically Modified Electrolyte Membrane

Hybrid organic−inorganic proton conducting gels (PWA/ICS−PPG or SiWA/ICS−PPG) were made from 3-isocyanatopropyltriethoxysilane and poly(propylene glycol)bis(2-aminopropyl ether), resulting in a hybrid silicon precursor (ICS−PPG) in which was dispersed phosphotungstic acid (PWA) or silicotungstic acid (SiWA) in ethanol. In this system, heteropoly acid takes on the additional role of catalyst, initiating the hydrolysis/condensation reactions of the sol−gel composite. The existence of amidonium ions [C(OH) NH + ] was established from the IR and Raman spectra of PWA/ and SiWA/ICS−PPG composites below and close to the crystallization limits (70−75%). The crystallization of the Keggin ions inside the ICS−PPG host was determined from the XRD spectra of the composites and confirmed also using vibrational spectroscopy, which shows the appearance of H 3 O + modes at 1700−1710 cm - 1 . The amidonium ions help in the immobilization of the negatively charged Keggin ions in the membranes, ensuring their uniform distribution in the sol−gel host. The proton conductivity of the composites, measured using impedance spectroscopy, increases with increasing concentration of the heteropoly acid from 10 - 6 S/cm (<10 wt % PWA or SiWA) to 10 - 3 S/cm for >70 wt % of PWA or SiWA in the ICS−PPG.