Mechanochemical synthesis of proton conductive cesium hydrogen salts of 12-tungstophosphoric acid and their composites

Cs-containing salt or solid acid (Cs2CO3 or CsHSO4) and dodecatungstophosphoric acid (H3PW12O40·nH2O:WPA·nH2O) were mechanically milled aiming at synthesis of chemically durable WPA salts and their composites. In the Cs2CO3-WPA system, XRD patterns suggested the formation of partially substituted CsxH3 − xPW12O40 after milling. After the formation of CsxH3 − xPW12O40, chemical durability of the Cs2CO3-WPA composites markedly improved. The proton conductivity of the Cs2CO3-WPA composites containing CsxH3 − xPW12O40 increased with an increase in the WPA content. CsxH3 − xPW12O40 obtained from 40Cs2CO3·60(WPA·6H2O) by milling showed 2 × 10− 2 S cm− 1 at 60 °C and 90% relative humidity (R.H.), while the conductivity drastically decreased under a dry N2 atmosphere. In the CsHSO4-WPA system, the formation of CsxH3 − xPW12O40 was not confirmed from XRD patterns after mechanical milling, whereas the water solubility of the CsHSO4-WPA composites decreased. The conductivity of 95CsHSO4·5(WPA·6H2O) with milling increased with increasing humidity and became 1 × 10− 2 S cm− 1 at 60 °C and 90% R.H. The mechanically milled 95CsHSO4·5(WPA·6H2O) composite exhibited a steep increase in conductivity at around 130 °C on heating even in a dry N2 atmosphere and the conductivity reached about 10− 3 S cm− 1 at 150 °C, which should be ascribed to the superprotonic phase transition of CsHSO4 in the composite.