State of water and ionic conductivity of solid polymer electrolyte membranes in relation to polymer actuators

This paper studies the state of the water and the ionic conductivity of solid polymer electrolyte membranes (SPM) of various ionic forms relating to polymer actuators. Perfluorinated polymers in both sulfonic and carboxylic forms were studied. From differential scanning calorimetry (DSC) measurements, the freezable water content in the SPM was estimated. From impedance measurements, the membrane conductivity of the SPM was estimated by equivalent circuit analysis. The counter cation in the SPM can be categorized in three groups from the freezable water content and the ionic conductivity of the membrane. Group A includes small hydrophilic cations such as those of the alkali and alkaline earth metals, for which the membrane conductivity increases with an increase in the freezable water content of the membrane. Group B includes hydrophobic cations such as alkyl ammonium ions except tetrapropyl- and tetrabutyl-ammonium ions in the perfluorosulfonic acid membrane. The ionic conductivity depends on the size of the ion and the membrane has a relatively large freezable water content. Group C includes the large hydrophobic cations such as TPrA and TBA in perfluorosulfonic acid membrane, which have a low conductivity and low freezable water content. In the light of these results, the deformation properties of the SPM/metal composites are discussed.