Infrared spectroscopy of fluoroalkyl-phosphonic-acid-based proton conductors

The structural properties of two fluoroalkyl phosphonic acid proton conductors were investigated as a function of hydration state in both proton and sodium ion exchanged forms with the use of transmission infrared spectroscopy. The mid-infrared spectral bands were recorded and assigned for the first time. Water condensation and transport in membrane pores and channels was probed through comparative parallel measurements performed on the structurally similar and well-studied ionomer, Nafion (perfluorosulfonic acid ionomer). For the phosphonic acid based conductors, vibrational modes associated with the phosphate moiety were observed near 1290cm-1 and between 1100-1000cm-1. Similar to Nafion, the strongest ionomer spectral bands appeared in the region between 1350-1100 cm-1 and likely contain dominant contributions from C-F stretching modes. A key finding was related to the appearance of a feature near 970 cm-1, which for Nafion has been associated with the vibrations of the two ether groups on the ionomer side chain. Similar to Nafion, which contains a sulfonate as the ionomer side chain end group, the 970 cm-1 feature is split in spectra of the phosphonate ionomer. The splitting is not apparent in spectra of the phosponyl ionomer studied in the present work, behavior which been reported for fluoroalkyl sulfonyl and fluoroalkyl bis-sulfonyl imide species. Possible explanations for the splitting in relation to ionomer structure are discussed.