Preparation of proton conductive solid acid composites by mechanical milling

Abstract

New proton conductive solid acid composites were prepared by mechanical milling. First, the formation of highly proton conductive cesium-containing sulfate-phosphate double salt composites, and their phase transition are described. Second, a mechanochemical synthesis of "cesium ortho-oxosalt-heteropoly acid composites," and the relationship between proton conductivity and hydrogen bonding network in the composites are discussed. Mechanochemical treatment of mixtures of cesium hydrogen sulfate (CsHSO4) and cesium dihydrogen phosphate (CsH2PO 4) using a planetary type of ball mill formed Cs3(HSO 4)2(H2PO4) and Cs5(HSO4) 3(H2PO4) 2. Cs3(HSO 4) 2(H2PO4) and Cs 5(HSO4) 3(H2PO4) 2 were transformed to the higher temperature phase of Cs 2(HSO4)(H2PO4) by heating at around 100°C. Proton conductivity of the treated compound remarkably in creased on heating to 2 x 10-3 S · cm-1 at around 180°C, whereas no steep decrease was observed on cooling. The high proton conductivity was ascribed to the presence of the high temperature phase of Cs2(HSO4)(H2PO4). Milling of.Cs-containing ortho-oxosalts (Cs2SO4, Cs 2CO3 or CsHSO4) and phosphotungstic acid (H3PW12O40. 6H2O: WPA-6) mixtures obtained partially substituted Csx H3-xPW12O40 composites. Chemical durability and proton conductivity of the resultant composites markedly improved under both humidified and dried conditions. In addition, mechanochemically prepared 90CsHSO4-10WPA-6 (mol%) composite maintained high proton conductivity from room temperature to 180 °C. Conductivity of the composite was 3.3 X10-3 S'cm-1 at 100°C under dry atmosphere, much higher than those of pure CsHSO4 (4 x 10-7 S.cm-1) and WPA-6 (1 x 10-7 S.cm-1). The-O(H)···O hydrogen bonding distance in the CsHSO4-WPA-6 composites was estimated from the 1H-isotropic chemical shift of 1H MAS NMR spectra. Proton conductivities of CsHSO4-WPA-6 composites under dry atmo sphere were strongly related to the bond distance.