Mechanism of Ionization, Hydration, and Intermolecular H-Bonding in Proton Conducting Nanostructured Ionomers

Abstract

Water−ions interactions and spatial confinement largely determine the properties of hydrogen-bonded nanomaterials. Hydrated acidic polymers possess outstanding proton-conducting properties due to the interconnected H-bond network that forms inside hydrophilic channels upon water loading. We report here the first far-infrared (FIR) coupled to mid-infrared (MIR) kinetics study of the hydration mechanism in benchmark perfluorinated sulfonic acid (PFSA) membranes, e.g., Nafion. The hydration process was followed in situ, starting from a well-prepared dry state, within unprecedented continuous control of the relative humidity. A step-by-step mechanism involving two hydration thresholds, at respectively λ = 1 and λ = 3 water molecules per ionic group, is assessed. The molecular environment of water molecules, protonic species, and polar groups are thoroughly described along the various states of the polymer membrane, i.e., dry (λ ≈ 0), fully ionized (λ = 1), interacting (λ = 1−3), and H-bonded (λ > 3). This unique extended set of IR data provides a comprehensive picture of the complex chemical transformations upon loading water into proton-conducting membranes, giving insights into the state of confined water in charged nanochannels and its role in driving key functional properties as ionic conduction.

Fast Water Diffusion and Long-Term Polymer Reorganization during Nafion Membrane Hydration Evidenced by Time-Resolved Small-Angle Neutron Scattering

“…This finding highlights the absence of structural transition and strongly indicates the kinetically controlled character of the swelling process, in agreement with a recent time-resolved synchrotron IR investigation. 16 Therefore, it strongly supports the dilution of objects which do not evolve in shape. We must emphasize that a structural reorganization related to the growth and percolation of nanoobjects as postulated by the reverse model would necessarily imply a deviation from the aforementioned affine behavior because the particles form factor would evolve upon hydration.…”

Perfluorosulfonyl Imide versus Perfluorosulfonic Acid Ionomers in Proton‐Exchange Membrane Fuel Cells at Low Relative Humidity

“…The sulfonic acid block copolymer In X / Y exhibits H 3 O + features that were found in Nafion . The FIR band located at 250 cm −1 , attributed to H‐bonding between sulfonates and hydroniums, grows until about 3 % RH and then totally disappears on further hydrating the material (Figure a).…”

“…The analysis of FIR/MIR data was combined, on one hand, with the knowledge acquired on Nafion membrane using the same in situ protocol and experimental techniques [52] and,o nt he otherh and, with the data availableo nb is(trifluoromethanesulfonyl)imide acid (HTFSI) and bis(trifluoromethanesulfonyl)imide anion (TFSI À ). [28,53,54] As our goal was to investigate the mechanisms of proton dissociation, the formationo fp rotonic species and the role/state of water molecules( in particulari nt he very early stageso fh ydration), we focusedo ur attentiono nt he vibrational bands of H 2 O( connectivity band at 200 cm À1 ;b ending at 1740 cm À1 ;s tretching at % 3400 cm À1 ), [55] protonic species (stretching of OÀHb ond in H 3 O + at 2700 cm À1 ,i nter-molecular signature for SO 3 À ·H 3 O + interactions at 250 cm À1 ; [53,56] Zundel ChemSusChem 2020, 13,590 -600 www.chemsuschem.org ion (H 5 O 2 + )b ands at 150 cm À1 and 1740 cm À1 ), [57] andH TFSI (NÀHa nd SO 2 )and TFSI À (NÀHa nd SO 2 ). [53] Figure 4d isplays the in situ absorbance of Si 15/15i onomer in the MIR [top panels, ranges (a) 2400-3200 cm À1 and (c) 1100-1600 cm À1 ]a nd FIR [bottom panels, ranges (b) 700-910 cm À1 and (d) 450-650cm À1 ].…”

Some physicochemical aspects of water-soluble mineral flotation

“…Water-ion interactions create distorted H-bonded networks, inducing peculiar hydration phenomena [36]. Comprehensive information on the structures of hydrated ions in aqueous solutions is essential for understanding their roles in flotation processes.…”

“…On the contrary, kosmotropicity correlates with a high charge density, and small or multiple-charged ions tend to be kosmotropic, which interfere strongly with local hydrogen bonds [44]. Binding of chaotropes to water molecules is weaker than that of water molecules to each other, while binding of kosmotropes to water molecules is stronger than that of water molecules to each other [36,44].…”