Oriented Morphology and Anisotropic Transport in Uniaxially Stretched Perfluorosulfonate Ionomer Membranes

Abstract

Relations between morphology and transport sensitively govern proton conductivity in perfluorsulfonate ionomers (PFSIs) and thus determine useful properties of these technologically important materials. In order to understand such relations, we have conducted a broad systematic study of H + -form PFSI membranes over a range of uniaxial extensions and water uptakes. On the basis of small-angle X-ray scattering (SAXS) and 2 H NMR spectroscopy, uniaxial deformation induces a strong alignment of ionic domains along the stretching direction. We correlate ionic domain orientation to transport using pulsed-field-gradient 1 H NMR measurements of water diffusion coefficients along the three orthogonal membrane directions. Intriguingly, we observe that uniaxial deformation enhances water transport in one direction (parallel-to-draw direction) while reducing it in the other two directions (two orthogonal directions relative to the stretching direction). We evaluate another important transport parameter, proton conductivity, along two orthogonal in-plane directions. In agreement with water diffusion experiments, orientation of ionic channels increases proton conduction along the stretching direction while decreasing it in the perpendicular direction. These findings provide valuable fodder for optimal application of PFSI membranes as well as for the design of next generation polymer electrolyte membranes.

Hot-Pressing Effects on Polymer Electrolyte Membrane Investigated by2H NMR Spectroscopy

“…For example, pulsed field gradient (PFG) methods have been used to detect self diffusions of water and to correlate them with proton conductivities. 3,12,15,19,20 Field-cycling NMR relaxometry was used to detect water dynamics in ionomer membranes.…”

“…Peak splitting data of 2 H NMR spectra 3,15,19,20,[22][23][24][25][26][27][28][29] have been used to investigate hydrophilic channel alignments, and they naturally detect the influence of water used as a probing molecule. Although similar information can be obtained with 1 H NMR spectra, 30 2 H NMR spectra are better since they do not have background signals.…”

“…Thus, to date, peak splitting of 2 H NMR spectra has been widely used to determine the anisotropic dynamics of probe molecules and/or anisotropic structures of the matrix. 3,15,19,20,[22][23][24][25][26][27][28][29] In this work, the average O-2 H director of 2 H 2 O in PEM was employed to extract the structural information from 2 H NMR data and applied to detect the structural change of Nafion induced by hot-pressing, which is one of the representative procedures for preparing MEAs for low temperature fuel cells. For the first time, the dependence of 2 H NMR data on the water content in PEM was systematically interpreted in terms of channel shape, size, and ordering.…”

“…This expansion indicates that the channels were forced to further align in each in-plane direction as predicted by the ST membrane results. 3,18,20 The peak splitting of each NS membrane was observed for the purpose of comparison with the starting materials. However, the peak splitting of each NS membrane saturated with 2 H 2 O (Figure S2(a) and S2(b) in Supporting Information) was too small for determination of the average channel orientation.…”

Relação morfologia-propriedades elétricas de eletrólitos compósitos de Nafion para célula a combustível de alta temperatura

“…As medidas de σ são geralmente realizadas ao longo do comprimento da amostra [44,55,56], enquanto que o transporte protônico em medidas de células a combustível ocorre através da espessura dos filmes. É bem sabido que a microestrutura do Nafion é anisotrópica, e diferentes valores de σ são obtidos nas diferentes direções [57]. Além disso, medidas de σ para T > 80 °C (RH = 100%) são raramente encontrados para estes materiais [48].…”