Organic protic ionics based on Nitrilo(trimethylenephosphonic acid) as water-free, proton-conducting materials

We produced two solid protic ionics by stoichiometric acid-base reaction between Nonafluorobutanesulfonic or p-Toluenesulfonic acid with Nitrilotri(methylenephosphonic acid). The latter behaves as a Bronsted base by means of the nucleophilic nitrogen atom which captures the proton from the Nonafluorobutanesulfonic or p-Toluenesulfonic acid. Moreover, the Nitrilotri(methylenephosphonic acid) moiety possesses six POH terminating units. 1H MAS NMR evidenced hydrogen-bonding activity of these units, which enables proton transport through the lattice by a hopping-site mechanism. Homogeneous, transparent and mechanically and thermally robust disks from these materials were obtained by sintering the powders under mild pressure and temperature. We showed, using electrochemical impedance spectroscopy, that these protic ionics possess good proton conductivity, in excess of 10–2 –1 cm–1, under fully anhydrous conditions at 190 °C. As such, these materials appear potentially attractive for application in high-temperature electrochemical devices, such as polymer electrolyte fuel cells and water electrolyzers operating at elevated temperature, typically above 130 °C and up to 200 °C for fuel cells. The proton-transport mechanism is also discussed in the light of the NMR- and impedance-spectroscopy results.