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 H3O+ 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.