Complex impedance studies of proton-conducting membranes

Complex impedance studies have been carried out on Dow 800, Dow 1000 and Na fron 117 membranes at various water contents and a variety of temperatures and hydrostatic pressures. At room temperature and pressure the usual gradu al decrease in electrical conductivity with decreasing water content is obs erved. For very low water content materials the variation of the conductivi ty with pressure from 0 to 0.2 GPa (2 kbar) is large and gives rise to appa rent activation volumes, DeltaV as large as 54 cm(3)/mol. in addition, for low water content materials, there is a tendency for smaller equivalent wei ghts (same side chains) or larger side chains to have larger activation vol umes. At high water content, DeltaV is relatively independent of the host p olymer and negative values are observed at the highest water contents. Thes e results provide support for the model where proton transport in high wate r content sulfonated fluorocarbons is similar to that for liquid water. All results are explained qualitatively via free volume. Ambient-pressure, var iable-temperature H-2 T-1 and linewidth measurements imply a heterogeneous environment of the water molecules. Proton pulsed field gradient NMR studie s in saturated Dow membranes verify the expectation that ionic conductivity is determined primarily by diffusion of water molecules. (C) 2000 Elsevier Science B.V. All rights reserved.