SAXS Investigation on Morphological Change in Lamellar Structures During Propagation Steps of Graft‐Type Polymer Electrolyte Membranes for Fuel Cell Applications

Abstract

their advantages over other fuel cells in terms of the clean and efficient power generation and lower operating temperature. [1] They are expected to reduce the fossil fuel consumption, which is believed to be the primary sources causing the climate change. PEM, which consists of super acid groups (i.e., sulfonic acid), has been considered as one of the key components in achieving the high fuel cell performance because of its unique fuel cell properties such as ionic conductance, mechanical strength, and thermal and chemical stabilities. When a dry PEM is immersed in water, the hydrophilic chains with sulfonic ion groups can absorb water and consequently form the interconnected ion channels inside the hydrated regions. The macro and microphase separations, viz., crystalline morphologies, conducting layers (ion channels), characteristic domain sizes, and the connection and distribution of the ionic groups and water in the conducting layers and around the crystalline phases are believed to play an important role in the conductivity and mechanical integrity of the PEMs. [2][3][4] Currently, the main challenges to improve the conductivity and mechanical strength of the PEMs are the lack of detailed informationThe changes of the lamellar periods (L 1D ), thickness of lamellar crystals (L c ), and amorphous layers (L a ) within the stacked lamellae of poly(styrenesulfonic acid)-grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs), induced by the preparation and water-absorbing steps are investigated using the small-angle X-ray scattering method. The L 1D values of all the samples quickly increase at a grafting degree (GD) range of less than 19% and then level off. The solvent-induced recrystallization is observed at the early stage of grafting (GD < 10%) and at successive sulfonation and hydration steps. The L 1D , L a , and L c of dry and hydrated PEMs show similar values at higher GD ranges (>34%), leading to the conclusion that most water molecules in the PEMs with higher GDs exist at the outside of the lamellar stacks. Accordingly, for the PEMs with low GD (<19%), all the hydrophilic graft-polymers (ion-channels) locate in the lamellar stacks and are strongly restricted by lamellar crystalline layers, which suppress the swelling of the PEMs. The unique lamellar structures of ETFE-PEMs characterized by L a and L c are well connected with the high conductance and mechanical properties of the membranes, and are suitable for fuel cell applications.

Investigation of the Lamellar Grains of Graft-type Polymer Electrolyte Membranes for Hydrogen Fuel Cell Application using Ultrasmall-angle X-ray Scattering

“…The pronounced peaks seen in the low-q region (q < 1.510 -1 nm) are observed even at high GD values (GD  59%), which indicate the strong development of the grain structures (lamellar grains) with the increase of GD. The new shoulder-like peak observed in the very low-q region (q < 1.510 -1 nm) implies the presence of the lamellar grains at a scale higher than that seen in the lamellar stacks [7][8][9]. At the localized scale, the peak corresponds probably to the average correlation distance between the two grains, which are composed of the crystalline regions and graft chains incorporated in the amorphous ETFE regions [7,18].…”

“…Based on these characterizations, the styrene was confirmed to be grafted onto the ETFE film and the grafted film was sulfonated by introduction of the −SO3H group. Figure 1 presents the Lorentz-corrected SAXS profiles of the original SAXS data [7][8][9]. This figure shows that the shoulder-like peaks to be pronounced.…”

“…Thus, it is generally assumed that the above phase presents a new two-phase structure, which also has a layered configuration (lamellar grain), at least at a localized scale. Thus, we also utilize the one-dimensional correlation function in order to determine the characteristic parameters of the lamellar grains using the same procedures as employed for the lamellar stacks [7,8]. Since the ETFE films are one-dimentionally oriented due to the extrusion drawing step in the film preparation, the one-dimensional correlation function, 1D(r), were calculated by Equation (1) (see Experimental) using the SAXS data with the extrapolation to both low-and high-q regions and listed in Figure 2.…”

“…The equipment and detailed procedures for the SAXS measurements were described in our previous work [7][8][9]. Briefly, the SAXS measurement was performed using two in-house SAXS spectrometers (NIMS-SAXS-II and NIMS-SAXS-III) at the National Institute of Material Science (NIMS) and USAXS at Super Photon ring-8 GeV (SPring-8), Japan.…”

“…The obtained results showed that the ETFE-PEMs have the proton conductivities that are less dependent on the relative humidity (RH) and their mechanical strength and conductivity relate strongly to the PEM crystallinities. The hierarchical structures of the membranes were then investigated using the small-and ultrasmall-angle X-ray scattering (SAXS/USAXS) [7][8][9]. It was found that, for the ETFE-PEMs with high IECs (> 2.7 mmol/g), a higher conductivity at 30% RH and compatible tensile strength at 100% RH and 80 C, in comparison with Nafion-212, were originated respectively from the well-interconnected ion channels around the lamellar grains and the remaining lamellar crystals and grains as well.…”