Phase Behavior of a PVAL-Based Polymer Proton Conductor

Abstract

Solid protonic conductor gels were synthesized using poly (vinyl alcohol) (PVAL), hypophosphorous acid (H3PO2) and water as prime chemicals. The samples were characterized by means of impedance spectroscopy, fuel cell measurements, differential scanning calorimetry (DSC), thermogravimetry (TG) and X-ray diffraction. The electrical conductivity of the samples at room temperature showed a sensitive variation between 10—6 and 10—1 S/cm as the acid concentration was increased. Using the raw membranes as electrolytic separator in a fuel cell, voltages up to 726 mV were obtained. DSC thermograms showed a well-resolved step anomaly associated to a glass transition for samples with the highest acid concentrations: at about —130 °C for the first set of samples and at about —120 °C for the other sets, which indicates the amorphous character of the samples. TG traces confirmed that the membranes with higher acid concentrations have higher water contents and that the maximum rate of water removal is at about 50 °C for all samples. X-ray spectra for the raw samples at room temperature show a large peak at about 2θ = 20°, which is smaller for the higher acid content samples and increases when the samples are annealed at 70 °C, indicating that the amorphousness of PVAL complexes increases with the H3PO2 content and drops with the water removal. The results, then, indicated the presence of a separate acid/water phase in the raw samples and an increasing polymer chain intervention in the ionic mobility as the samples are thermally treated.