Phase transition and proton conductivity of CsHSO4–WPA composites prepared by mechanical milling

A typical solid acid of CsHSO4 and phosphotungstic acid (H3PW12O406H2O: WPA6H2O) were mechanically milled, and non-humidified proton conductivity of the obtained composite was investigated. The 90CsHSO410WPA (mol%) composite showed high proton conductivities of 10− 2–10− 4 S/cm from 160 to 30 °C under dry nitrogen atmosphere, while the conductivity of pure CsHSO4 dropped abruptly at around 145 °C due to the phase transition from superprotonic CsHSO4(I) phase to CsHSO4(II) phase. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy measurements were performed at various temperatures to study the phase transition behaviour as well as hydrogen bonding structure. The phase transition was not observed for the 90CsHSO410WPA composite. FTIR measurements revealed that a peak for OH stretching shifted to a higher wavenumber with increasing temperature. The O–H•••O hydrogen bond distance was calculated from the OH band wavenumber. The proton conductivities in dry conditions were related to the bonding distance.