Sprecher
Beschreibung
The exceptional performance of polymer electrolyte membranes (PEMs) used in energy conversion technologies arises from their microphase-separated morphology that consists of hydrophilic ionic domains containing functional groups that facilitate proton transport under hydrated conditions, and hydrophobic crystalline polymer matrix that provides mechanical integrity and chemical stability. Nafion, a perfluorosulfonic acid (PFSA) ionomer, is widely regarded as the benchmark material for PEM fuel cell and water electrolysis applications. However, despite its advantages, Nafion exhibit several drawbacks. In particular, they belong to the broader class often referred to as “forever chemicals” and consequently, increasing concerns regarding the environmental and potential human health impacts associated with fluorinated compounds may lead to future regulatory restrictions on their use. Therefore, replacing perfluorosulfonic acid (PFSA) ionomers with environmentally friendly and less expensive hydrocarbon-based materials represents an attractive strategy for the development and optimization of polymer electrolyte membranes for energy conversion applications.
Semicrystalline syndiotactic polystyrene (sPS) exhibits complex polymorphic behavior including crystalline forms with polymer chains in either planar zigzag conformations (α and β forms) or TTGG helical conformations (γ, δ, and ε forms). In addition, sPS can form a variety of co-crystalline (clathrate) complexes with numerous small organic molecules, which can be incorporated as guest species within cavities located between the polymer helices in the δ and ε forms and can subsequently be exchanged without disrupting the crystalline framework. Exploiting this unique property, sPS membranes containing the δ crystalline form can be functionalized homogeneously through the so-called solid-state sulfonation method: sulfonation occurs selectively within the amorphous regions, which become hydrophilic and proton-conductive upon hydration, while the crystalline domains remain largely unaffected. Subsequent annealing at 180–200 °C promotes the conversion of the δ form into the thermodynamically stable α or β crystalline forms, yielding sulfonated sPS membranes with enhanced mechanical and chemical stability and properties suitable for fuel cell applications, as we recently have confirmed.
To understand and optimize the performance of sPS membranes, it is first necessary to establish correlations between their microscopic structural and dynamical properties and proton conductivity, as well as to gain insight into the water transport mechanisms operating over different length and time scales. Small-angle neutron scattering (SANS), performed over an extended Q range, and multi-resolution quasielastic neutron scattering (QENS), both benefiting from contrast variation achieved through the selective deuteration of components in the hydrated membranes, are particularly powerful and complementary techniques for investigating the structural organization and molecular dynamics of this material, as will be shown in this presentation.