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

Abstract

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

Theoretical Insights into the Structure of the Aminotris(Methylenephosphonic Acid) (ATMP) Anion: A Possible Partner for Conducting Ionic Media

“…The aim of this study was to show that by a suitable choice of the anion we can produce a proton conducting, highly ionised medium without the onset of neutral components. Even in the unlikely case that the system was not liquid under all circumstances, the results herein presented remain relevant, as they reveal the molecular mechanisms which are the basis of the conductive properties of well known ionic solid phases such as those previously reported [33].…”

“…ATMP in its completely deprotonated form has been utilised on only one occasion as an IL component of a hybrid catalytic nanomaterial to reduce nitro-anilines [32]. ATMP has also been employed as part of a protic ionic solid to capture protons in an anhydrous proton conducting material [33]. Lastly, ATMP was incorporated in an innovative hybrid ion-exchange material as a cation exchanger [34].…”

Water-free, Proton-Conducting Hybrid Materials for Elevated-Temperature Electrochemical Systems

“…On the other side, strong growth of the liquid-ionics area has inspired the development of protic liquid − and solid , ionics exhibiting apparently good proton conductivity in virtually fully anhydrous conditions. The way these materials work is basically analogous to the poly­(benzimidazole)/phosphoric acid system devised by Savinell and co-workers, , whereby the protons migrate through the matrix by a hydrogen-bonding, hopping-site mechanism involving a sequence of jumps between vicinal Bro̷nsted base sites.…”

“…Incidentally, nitrilo tris-(methylenephosphonic acid) had already been employed with some success in our laboratory to develop water-free proton conductors. 23,26 It is known that phosphate and phosphonate acid functionalities can coordinate a variety of metals, such as titanium, aluminum, zirconium, etc., to give many metal phosphate or phosphonate complexes. 27−29 These metal phosphates and phosphonates are crystalline and typically highly insoluble in water.…”

“…In this work we developed hybrid composite materials based on blends of a titanium–nitrilo tris­(methylenephosphonic acid) complex (Ti-NTMPA) and benzimidazole (BIm). Incidentally, nitrilo tris­(methylenephosphonic acid) had already been employed with some success in our laboratory to develop water-free proton conductors. , It is known that phosphate and phosphonate acid functionalities can coordinate a variety of metals, such as titanium, aluminum, zirconium, etc., to give many metal phosphate or phosphonate complexes. − These metal phosphates and phosphonates are crystalline and typically highly insoluble in water. The latter characteristic makes these materials attractive for some important applications.…”