Complex impedance studies of proton-conducting membranes

Complex impedance studies have been carried out on Dow 800, Dow 1000 and Nafion 117 membranes at various water contents and a variety of temperatures and hydrostatic pressures. At room temperature and pressure the usual gradual decrease in electrical conductivity with decreasing water content is observed. For very low water content materials the variation of the conductivity with pressure from 0 to 0.2 GPa (2 kbar) is large and gives rise to apparent activation volumes, ΔV, as large as 54 cm3/mol. In addition, for low water content materials, there is a tendency for smaller equivalent weights (same side chains) or larger side chains to have larger activation volumes. At high water content, ΔV is relatively independent of the host polymer and negative values are observed at the highest water contents. These results provide support for the model where proton transport in high water content sulfonated fluorocarbons is similar to that for liquid water. All results are explained qualitatively via free volume. Ambient-pressure, variable-temperature 2H T1 and linewidth measurements imply a heterogeneous environment of the water molecules. Proton pulsed field gradient NMR studies in saturated Dow membranes verify the expectation that ionic conductivity is determined primarily by diffusion of water molecules.