Synthesis and Characterization by Ellipsometry of Cationic Membranes for Fuel Cells

Abstract

In this study, cationic membranes based on poly (vinyl alcohol) (PVA) were synthesized, crosslinked with citric acid (carboxylic group) and 4-sulpho-phthalic acid (sulphonic group) for application as proton-conducting membranes of H3O+. The membranes were prepared from the mixture of PVA with the crosslinking agents (CA), previously dissolved and separated with the mixture of these. The CAs proportions used were 10%, 30% and 50%, based on the weight of PVA, and they were crosslinked at the temperatures of 120oC and 140oC. The films were characterized by thermal stability using TGA, spectroscopy ellipsometry (relative refractive index and real part of dielectric constant), swelling degree (water absorption) and conductivity, using electrochemical impedance spectroscopy (EIS). A variation in the refractive index as a function of CA concentration was observed. This fact was corroborated with the results of dielectric constant, because the higher concentration of hydrophilic group added to the film, higher the values of dielectric constant. The membranes which showed lowest swelling degree values were crosslinked with 4-sulpho-phthalic acid (sulphonic group), for example, the PVA membrane crosslinked with 50% of 4-sulpho-phtalic acid, at 140oC, showed 6.4% of water absorption, while the membrane PVA crosslinked with the same content of CA, but of citric acid, and temperature, presented 15.8%. The conductivity of some membranes, under the assay condition evaluated, was the same order of magnitude as the commercial membranes, Nafion®. The synthesized membranes have been showing potential application as cationic polymer membranes for the transport of H3O+.