Morphological Anisotropy and Proton Conduction in Multiblock Copolyimide Electrolyte Membranes

Abstract

Performance improvement of advanced polymer electrolyte membranes requires control over the morphology as it plays an important role for mechanical, thermal, and proton transport properties. The ionic domain orientation of films cast from sulfonated block copolymer solutions is generally anisotropic, and to maximize proton conductivity in fuel cell applications, hydrophilic channel alignment is desirable. In this work, a series of multiblock copolymers based on sulfonated copolyimides were synthesized and characterized by NMR, TEM and AFM microscopy, and impedance spectroscopy. For constant ion exchange capacity, the higher the block length, the higher the proton conductivity and water uptake for a given relative humidity. A random copolymer exhibited the lowest performance, in particular at low relative humidity, caused by a reduced phase separation as derived from AFM and TEM measurements. Orientational order probed by 2 H NMR on absorbed D 2 O showed preferential alignment in the through-plane direction. However, proton pulsed-field-gradient (PFG) NMR along the two orthogonal membrane directions revealed water diffusion to be faster in-plane than in the through-plane direction. This difference in diffusion is attributed to a lamella-like structure composed of rather short, through-plane hydrophilic channels in our systems. For the block copolyimide with the highest block length, two distinct diffusion processes could be identified. This is ascribed to a superimposed morphology on the micrometer scale, leading to an opaque appearance of the membrane.

High Electrokinetic Energy Conversion Efficiency in Charged Nanoporous Nitrocellulose/Sulfonated Polystyrene Membranes

“…31 The relative small size of the cavity (∼10% of the active NMR volume) required the measurements to be performed in 1 M LiCl. The more concentrated membrane sample was obtained by leaving the sample holder in air overnight.…”

“…30 On the other hand, the tortuosity can be measured directly (ϑ exp ) from the ratio of the Li + diffusion coefficients in the membrane (D*) and in the bulk solution (D). For this purpose, the 7 Li pulse-field gradient (PFG) NMR technique with a pulsed-gradient stimulated-echo (PGSTE) sequence and a previously developed sample holder 31 32 To investigate the concentration dependence, the wetted membranes were allowed to evaporate overnight. Hereby, the LiCl concentration increased and the sample was measured again (Figure 2d), here D* = 2.1 ± 0.1 × 10 −11 m 2 s −1 and D = 3.8 ± 0.2 × 10 −10 m 2 s −1 was found in agreement with literature values for concentrated solutions.…”

ePTFE reinforced, sulfonated aromatic polymer membranes enable durable, high-temperature operable PEMFCs

“…Similar anisotropic swelling was previously observed in other sulfonated aromatic polymer membranes ( Shin et al., 2017 ; Peckham and Holdcroft, 2010 ), which exhibited uniaxial alignment of the polymer chain in the in-plane direction. The anisotropic morphology often resulted in the anisotropy in water diffusion and proton conductivity ( Blachot et al., 2003 ; Hou et al., 2010 ; Li et al., 2009 ; Park and Balsara, 2010 ; Kins et al., 2014 ). To reduce the excess swelling and improve the mechanical durability of the high IEC membrane, reinforcement strategy was adopted.…”

Sulfonated Fluorocarbon Polymers as Proton Exchange Membranes for Fuel Cells

“…However, to a certain degree of sulfonation, the PEMs tend to absorb too much water or even become soluble in water. To avoid this, a variety of functional block copolymers, consisting of hydrophilic and hydrophobic blocks for the PEMs, have been reported (Hou et al, 2010;Takamuku and Jannasch, 2012;Kins et al, 2014;Seh et al, 2017). With the formation of ion channel structures, polymers can achieve e ective proton conduction because the hydrophilic sulfonated polymer segments form an interconnected three-dimensional network and the hydrophobic segments form a reinforced complementary network, which can maintain water stability and enhance the mechanical properties (Roy et al, 2009;Hou et al, 2010;Li et al, 2012;Takamuku and Jannasch, 2012;Kins et al, 2014).…”