Nafion β-Relaxation Dependence on Temperature and Relative Humidity Studied by Dielectric Spectroscopy

Abstract

Nafion β-relaxation was studied by detailed dielectric spectroscopy measurements carried out in a wide range of both temperature and relative humidity. Cast and extruded Nafion were compared in the H+, Na+, and Cs+ forms to evaluate contributions arising from membrane morphology and ionic interactions. The experimental data indicated that β-relaxation is associated with the motion of side chains of the ionomer. The unusual shift to lower frequencies of the β-relaxation was shown to be more pronounced for samples containing high water content and at temperatures above the α-relaxation, evidencing that the electrostatic repulsions between bare sulfonic groups reduce the side chain mobility. A pronounced increase of relaxation times was observed at relative humidity >60% suggesting a change of the morphology of the ionomer, in agreement with recent descriptions of the Nafion structure by small angle X-ray scattering analyzes.

dcProton conductivity at low-frequency in Nafion conductivity spectrum probed by time-resolved SAXS measurements and impedance spectroscopy

“…Nonetheless, careful DS data showed that such dependence is not observed in Nafion, as recently reported . The high frequency proton conductivity ( f ∼ 10 6 Hz) continuously increases from ∼ 40 to 180 °C when relative humidity is kept at RH = 100%, accompanied by the high‐frequency relaxation shift to higher frequencies up to T ∼ 110 °C; however, above such temperature the relaxations unexpectedly reverse and displace to lower frequencies with further increasing temperature . This mismatch between the temperature dependence of proton conductivity and the high frequency relaxation is a compelling evidence that the high capacitance (associated with the high Δ ε values) cannot be directly ascribed to electrode polarization.…”

“…Careful dielectric spectroscopy (DS) measurements at very low frequencies ( f ∼ 10 −4 Hz) have identified a conductivity plateau for f < 10 −3 Hz, confirming the presence of a second dc conductivity at low f . In addition, the DS measurements of Nafion membranes with different thicknesses indicate that the relaxation observed at f ∼ 10 5 Hz is not associated with electrode polarization . It is also important to consider the temperature dependence of the relaxation observed at f ∼ 10 5 Hz.…”

“…Nyquist diagrams of Nafion membranes at T = 25 °C (RH = 100%) exhibit a frequency independent conductivity at high frequencies ( f ∼ 10 6 Hz), which was attributed to the transport of dissociated protonic charges occurring mainly via structural diffusion in the absorbed aqueous media . However, the low frequency tail observed for f < 10 6 Hz, representing a large increase of the capacitance, has received different interpretations such as electrode polarization or polarizations occurring within the polymer phase …”

“…In order to advance the understanding of the origin of Nafion dielectric relaxations a special sample holder for dielectric spectroscopy measurements was designed to perform measurements in a broad temperature and humidity range . Careful dielectric spectroscopy (DS) measurements at very low frequencies ( f ∼ 10 −4 Hz) have identified a conductivity plateau for f < 10 −3 Hz, confirming the presence of a second dc conductivity at low f . In addition, the DS measurements of Nafion membranes with different thicknesses indicate that the relaxation observed at f ∼ 10 5 Hz is not associated with electrode polarization .…”

Aplicação de catalisadores PtSn/C e membranas Nafion-SiO2 em células a combustível de etanol direto em elevadas temperaturas

“…Medidas de SAXS e SANS em soluções diluídas de Nafion sugeriram a presença de nanoparticulados cilíndricos com raios de ~2,5 nm e comprimento de ~100 nm [61,62] . [57,55,63] .…”