Phase behavior of a PVAL-based polymer proton conductor

Solid protonic conductor gels were synthesized using poly (vinyl alcohol) ( PVAL), hypophosphorous acid (H3PO2) and water as prime chemicals. The sampl es were characterized by means of impedance spectroscopy, fuel cell measure ments, differential scanning calorimetry (DSC), thermogravimetry (TG) and X -ray diffraction. The electrical conductivity of the samples at room temper ature showed a sensitive variation between 10(-6) and 10(-1) S/cm as the ac id concentration was increased. Using the raw membranes as electrolytic sep arator 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 s amples with the highest acid concentrations: at about -130 degrees C for th e first set of samples and at about -120 degrees C for the other sets, whic h indicates the amorphous character of the samples. TG traces confirmed tha t the membranes with higher acid concentrations have higher water contents and that the maximum rate of water removal is at about 50 degrees C for all samples. X-ray spectra for the raw samples at room temperature show a larg e peak at about 2 theta = 20 degrees, which is smaller for the higher acid content samples and increases when the samples are annealed at 70 degrees C , indicating that the amorphousness of PVAL complexes increases with the H3 PO2 content and drops with the water removal. The results, then, indicated the presence of a separate acidi water phase in the raw samples and an incr easing polymer chain intervention in the ionic mobility as the samples are thermally treated.