Proton Hopping and Long-Range Transport in the Protic Ionic Liquid [Im][TFSI], Probed by Pulsed-Field Gradient NMR and Quasi-Elastic Neutron Scattering

Abstract

The management of proton conductivity in the protic ionic liquid imidazolium bis(trifluoromethylsulfonyl)imide ([Im][TFSI]) is investigated via the use of quasi-elastic neutron scattering (QENS) and pulsed-field gradient NMR. The introduction of excess neutral imidazole to [Im][TFSI] leads to enhanced conductivity. We find that proton dynamics in [Im][TFSI] with excess imidazole are characterized by proton hopping that is encompassed in the slower of two translational processes, as identified by QENS. This relatively slow process contributes to long-range diffusion more than the faster process. NMR diffusion measurements show that proton hopping decreases with increasing temperature, but significant proton hopping persists even at the maximum experimental temperature of 120 °C. This, in combination with minimal ion aggregation, leads to high proton conductivity and a high proton transference number over a wide temperature range.

Applying neutron diffraction with isotopic substitution to the structure and proton-transport pathways in protic imidazolium bis{(trifluoromethyl)sulfonyl}imide ionic liquids

“…Indeed, hydrogen-bonding is generally invoked as a structural feature in base-rich imidazolium PILs and an important mechanism for proton transfer. 12 Unfortunately, this static image of N−H···N hydrogen-bonded chains of imidazolium and/or imidazole groups propagating through the liquid does not appear to be supported by the -anion in the two simulation models.…”

“…Indeed, PILs have been extensively studied for uses in fuel cells, and the most promising cases have base-rich compositions. 8,10,11 Transport mechanisms in imidazole/H[NTf 2 ] PILs have been investigated using pulsed-field gradient NMR and quasi-elastic neutron scattering studies 12 identifying both short range (Grotthuss) proton hopping and long range (diffusive) pathways. Oligomeric im−H···[H−im + −H]···im−H chains were implicated in the extended proton transfer mechanism.…”

Ionic Liquid-Based Non-Aqueous Electrolytes for Nickel/Metal Hydride Batteries

“…It has been demonstrated that the Grotthuss mechanism (structural diffusion) plays an important role in proton transfer, and that the diffusion coefficient of protons subject to the Grotthuss (proton hopping) mechanism is greater than those subject to the vehicle mechanism in the case of imidazole and bis(trifluoromethanesulfonyl)imide acid (HTFSI) mixtures [23]. It has also been reported that proton hopping persists over a wide temperature range (up to 120 • C) and results in high proton conductivity and high proton transference numbers in imidazolium bis(trifluoromethylsulfonyl)imide ([Im][TFSI]) with excess imidazole [24]. Depending on the availability of exchangeable protons in the chemical structure, ILs can be categorized into protic ionic liquids (PILs) and aprotic ionic liquids (AILs).…”

Proton Diffusivity in the Protic Ionic Liquid Triethylammonium Triflate Probed by Quasielastic Neutron Scattering

“…Being particularly sensitive to hydrogen atoms, incoherent neutron spectroscopy is beneficial in investigating dynamical processes in hydrogen-rich materials, to which ILs can be ascribed as well, [14][15][16][17] and finally could be applied to obtain information on proton diffusivity in PILs. 18,19 The objective of our work was to employ QENS and deuterium labelling to explore transport properties in triethylammonium triflate (TEA-TF). This PIL is formed by a proton transfer from trifluoromethanesulfonic acid, which is considered to be one of the strongest acids, to triethylamine.…”

“…For example, the interplay of the vehicular and Grotthus mechanisms has been studied for[Im][TFSI] experimentally by means of QENS and PFG-NMR 19. In this work the linewidths of the QENS spectra were analysed in terms of the jump diffusion model (eq 4), the residence time τ 0 serving as a measure of the time between proton transfer events.…”