Electrical spectroscopy studies of two new siloxanic proton conducting membranes

This contribution is focused on the conductivity study and the protonic transfer investigation of two new siloxanic membranes. The conductivity of the systems has been studied within the temperature range 5 °C ≤ T ≤ 145 °C, both for pristine and hydrated membranes. Membrane A has been hydrated up to 33.12% in weight, while in B up to 27.76%. The conductivity of these membranes has shown a temperature dependence of the Arrhenius type variable in the interval 1.6 × 10−4 ≤ σA ≤ 2.3 × 10−3 S cm−1 and 1.3 × 10−5 ≤ σB ≤ 2.9 × 10−4 S cm−1, respectively, for A and B. In particular, conductivities of 2 × 10−3 S cm−1 (A) and of 2 × 10−4 S cm−1 (B) at 125 °C were observed.The conductivity mechanism was investigated by using broad band electrical spectroscopy in the region between 40 Hz and 10 MHz. This study, for both the materials has shown the presence at low frequencies (102 ≤ fβ ≤ 104 Hz) of β relaxations related to the sulphonic side chain dynamics. The activation energy measured for this molecular dynamics is about ≅30 kJ mol−1 and corresponds to the typical interaction energy associated with hydrogen bonding. Furthermore, it was observed that the activation energies determined from the conductivity measurements are 12 and 14 kJ mol−1, respectively, for A and B. This shows that the protonic conductivity is strongly influenced by the side chain dynamics and that the charge migration occurs through an ion hopping mechanism between different regions, consisting of micro-clusters of hydration water coordinated with the polar sulphonic groups of the side chains. The comparable activation energies and the values of the conductivity demonstrate that in these systems the conductivity is proportional to the concentration of the sulphonic groups. This shows also that these kinds of membranes, with a high concentration of SO3H are necessary in order to obtain materials with a high protonic conductivity with the capacity to retain water in bulk up to 145 °C.