New inorganic–organic proton conducting membranes based on Nafion® and [(ZrO2)·(SiO2)0.67] nanoparticles: Synthesis vibrational studies and conductivity

In this report is described the preparation of six nanocomposite membranes of formula {Nafion/[(ZrO2)dot operator(SiO2)0.67]ΨZrO2} with ΨZrO2 ranging from 0 to 1.79 based on Nafion® and [(ZrO2)·(SiO2)0.67] nanofiller. Morphology investigations carried out by SEM measurements indicate that the composition of membranes is asymmetric. Indeed, with respect to the direction of the films after casting procedure, the top side (A-side) and bottom side (B-side) present a different nanofiller concentration. The concentration of nanofiller increases gradually from A to B side. The membranes present thicknesses ranging from 170 to 350 nm and are studied by FT-IR ATR and micro-Raman measurements. The vibrational investigations permit us to reveal that: (a) the hydrophobic polytetrafluoroethylene (PTFE) domains of Nafion® are composed of a mixture of polymer chains with 157 and 103 helical conformations; (b) the concentration of chains with 103 helical conformation depends on the nanofiller concentration and is much higher in side A; (c) six different water domains are present in bulk membranes which are singled out as I, II, II, III, III and IV; (d) water uptake of membranes is correlated to the conformational transition 103 → 157 of PTFE chains occurring in hydrophobic domains of Nafion®. The conductivity of {Nafion/[(ZrO2)dot operator(SiO2)0.67]ΨZrO2} was determined by analyzing the complex conductivity plots measured in the frequency and temperature range of 10−2 Hz–10 MHz and 5–155 °C, respectively. Interestingly, the {Nafion/[(ZrO2)dot operator(SiO2)0.67]ΨZrO2} nanocomposite membranes with ΨZrO2=0.313 and 0.534 showed values of conductivity of 4.3 × 10−2 S cm−1 at 135 °C and of 3.5 × 10−2 S cm−1 at 115 °C, respectively.