Infrared dynamics study of thermally treated perfluoroimide acid proton exchange membranes

Abstract

The temperature induced dehydration process of the 3M Brand perfluoroimide acid (PFIA), an advanced proton exchange membrane for fuel cells, was studied by in situ infrared spectroscopy to understand proton transport processes under conditions of low hydration levels. A comprehensive assignment of the vibrational bands of PFIA in the mid infrared region is provided. Investigation of the kinetics in conjunction with 2D correlation spectroscopy methods revealed the sequential process of the hydration and dehydration in a conclusive model. The results indicate that at a lower water content the sulfonate group of the PFIA side chain is preferentially ionised and involved in a hydrogen bonding structure with the sulfonyl imide acid group, until a sufficient amount of water is present to ionise the second ionic site. Comparison to the well-understood NAFION™ membrane revealed that under low humidity conditions a higher amount of water is retained in PFIA in a state most similar to liquid water. The results contribute to a better understanding of water retention capability and thus proton conductivity under high-temperature and low-humidity conditions.

Atomistic simulation study of the hydrated structure and transport dynamics of a novel multi acid side chain polyelectrolyte membrane

“…The Nafion vehicular proton conductivity values were calculated by using the previously published Nafion simulated vehicular diffusivities for hydronium ions [26][29] [30]. The PFIA vehicular proton conductivities are observed to be higher than those of Nafion at both T=300 K and T=353 K. This agrees qualitatively with the experimental trends seen at T=300 K [20] and T=353 K [22].…”

“…1(a)) [18] have been developed to serve this purpose. There have been experiments [20] which have shown that the average current through a PFIA membrane is significantly higher than that in a Nafion™ 212 membrane 2 , which has an EW of 1100 [21], at 300 K. It has also been observed in experiments [22] that PFIA has significantly higher proton conductivity than NAFION™ NR211 3 , which has an EW of 990-1050 [23], across a wide range of hydration levels at 353 K. For instance, PFIA and Nafion have a proton conductivity of 0.1 S/cm [22] and 0.05 S/cm [22] for 50% RH at 353 K respectively.…”

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

“…This important observation means that the proton dissociation process—and subsequent H‐bonding to water molecules to form protonic species—is completely different when the sulfonyl imide moiety is used instead of sulfonic acid (i.e., a peculiar hydronium‐free ionization–hydration mechanism characterizes the TFSI polymer). A very different dissociation process of sulfonic and sulfonyl imide acids was also reported with perfluoro ionene chain‐extended ionomers from 3 m even if the side‐chain chemistry promotes H‐bonds between acid functions and H‐bonding configurations of water that facilitate proton dissociation …”

“…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 ].…”

“…Avery different dissociation process of sulfonicand sulfonyl imide acids was also reported with perfluoro ionene chainextended ionomers from 3m even if the side-chain chemistry promotes H-bonds betweena cid functions and H-bonding configurations of water that facilitate protondissociation. [57,61] Additional information can be extracted from Figure 5, as the selected regions contain potentials ignatures of Zundel ions, together with the well-established water features.S toyanov et al [53] identified Zundel ion bandsa t1 740 and 3050 cm À1 in their IR study realized on successive addition of water molecules on triflic acid compounds. The difference spectra of Si 15/15 allow to exacerbate the potential observation of H 2 O 5 + features in the FIR (Figure 5a)a nd in the MIR ChemSusChem 2020, 13,590 -600 www.chemsuschem.org (Figure 5b,c)r egions of interest.…”