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.

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The hierarchical structures of poly(styrenesulfonic acid)‐grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes (ETFE‐PEMs) with different scale ranges including lamellar spacing, interfacial thickness, and intra‐structure of conducting layers were evaluated by small‐angle X‐ray scattering in terms of background scattering (I B(q)). First, I B(q) was roughly estimated by modifying Ruland's method and then optimized to avoid overestimation using a “contribution factor,” which is defined as the contribution of I B(q) to the observed scattering intensities over the entire q range. Then, I B(q) was optimized again by using the model for the deviation from Porod's law also proposed by Ruland to select proper q‐range for interfacial thickness evaluation. The lamellar spacing, which is observed in the low‐q range, was not altered by background correction. In contrast, in the high‐q range, the interfacial thickness and the internal structures can be estimated only after correction for the background scattering data. The interfacial thickness of the final membranes (ETFE‐PEMs) is affected by both the graft‐polymerization and sulfonation processes and because the change in membrane structures is observed during propagation steps at lower ion exchange capacity (IEC) range (IECs < 2.4 mmol/g), which should affect the mechanical strength of graft‐type PEMs.

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