Nanostructure Evolution in High-Temperature Perfluorosulfonic Acid Ionomer Membrane by Small-Angle X-ray Scattering

Abstract

The high-temperature morphology of supported liquid membranes (SLMs) prepared from perfluorinated membranes such as Nafion and Hyflon and hydrophobic ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMI-TFSI) has been investigated by small-angle X-ray scattering (SAXS). Proton conductivity results of SLMs before and after leaching show an increase in conductivity with temperature up to 160 °C in an anhydrous environment. DSC results show that crystallites within perfluorinated membranes are thermally stable up to 196 °C. High-temperature SAXS results have been used to correlate structure and morphology of supported liquid membranes with high-temperature conductivity data. The ionic liquid essentially acts as a proton solvent in a similar way to water in hydrated Nafion membranes and increases size of clusters, which allow percolation to be achieved more easily. The cation of the ionic liquid interacts with sulfonate groups within ionic domains through electrostatic interactions and displaces protons. Protons can associate with free anions of the ionic liquid, which are loosely associated with cations and can transport by hopping from anion sites within the membrane. The ionic liquid contributes to proton conductivity at high temperature through achievement of long-range ordering and subsequent percolation.

Molecular dynamics simulations of side chain pendants of perfluorosulfonic acidpolymer electrolyte membranes

“…This peak is not observed in experimental studies, as SAXS and SANS can only detect crystallites. The peak positions of the side chain pendant ionomer and hydronium ions remain unaffected by temperature, which is found to be consistent with the high temperature SAXS study of Mistry et al 66 on Naon and Hyon polymer membranes.…”

Evolution of morphology, segmental dynamics, and conductivity in ionic liquid swollen short side chain perfluorosulfonate ionomer membranes

“…Besides, a scattering maximum associated with the crystalline region of the Aquivion backbone was observed at q ∼ 0.04 Å −1 which is called the matrix peak. Moreover, a very diffuse scattering maximum attributed to the aggregation of ionic clusters of Aquivion occurs at q excess of 0.1 Å −1 , and is called ionomer peak . As shown in Figure , compared with the pristine Aquivion (orange cross symbol), both the upturn region and the matrix peak of the swollen membranes are not significantly affected by the absorbed EMI‐Tf even up to λ = 1.26.…”

“…Perfluorosulfonic acid ionomer membrane, such as Nafion, is well known for its distinctive nanophase‐separated morphology . Analogous to Nafion, the morphology of Aquivion consists of a fluorinated hydrophobic backbone and hydrophilic domains arising from clustering of sulfonic acid groups . Figure plots the SAXS patterns of the EMI‐Tf swollen membranes measured at room temperature which reveals several general features.…”

“…It is generally believed that the electrostatic interactions among ionic groups reduce the mobility of these chains in the ionic cluster region, leading to the higher glass transition temperature ( T g ) of the ionic region than that of the ion‐poor matrix region of the perfluorosulfonic acid ionomers . Also, the morphology studies by small angle X‐ray scattering (SAXS) in previous reports suggest the linkage of the absorbed ILs to the ionic clusters of ionomers …”

Chemical modification of Nafion membranes by protic ionic liquids: the key role of ionomer–cation interactions

“…The result conrms that PILs have successfully immobilized within the ionic domains of Naon through electrostatic interactions between PIL cations and sulfonated groups. 33 It means that the ionic domains of the Naon/PIL composite membranes are lled with PILs and larger than the polar domains of Naon. The composite membranes doped by the PILs with linear cations have a similar location of q at $0.17 ÅÀ1 .…”