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.