Understanding the fingerprint region in the infra-red spectra of perfluorinated ionomer membranes and corresponding model compounds: Experiments and theoretical calculations

Abstract We present an ATR-FTIR study of three major perfluorinated ionomers that are used as proton exchange membranes in fuel cells (FCs) and that differ in their side chains: Nafion, Aquivion and 3M membrane. The choice of the following low-molecular-weight model compounds (MCs) that mimic the membrane side chains was essential for FTIR band assignment: perfluoro(3-methyl-2,4-dioxahexane)sulfonic acid for Nafion, perfluoro(2-ethoxyethane)sulfonic acid for Aquivion, and perfluoro(2-ethoxybutane)sulfonic acid for the 3M membrane. The major goal was to identify spectral bands that can be used for recognizing bonds involved in membrane fragmentation. A major focus was on the signals from the C–O–C bonds, which were assigned in some papers for Nafion and Aquivion membranes. Our ATR-FTIR results for 3M membranes did not conform to these assignments, and DFT calculations of the vibrational frequencies for the MCs were used to resolve this conundrum. The ATR-FTIR spectra of membranes and MCs and the DFT calculations led to an understanding of the fingerprint region of all membranes, and to a re-examination and re-assignment of results for Nafion and Aquivion membranes. The low intensity of the spectral bands for the ether link connected to the backbone (for all membranes) and also in the side chain (for Nafion) suggests that these bands cannot be used for the determination of the extent of degradation.