Charge Transport in Nonstoichiometric 2-Fluoropyridinium Triflate Protic Ionic Liquids

Abstract

Pyridinium triflates are highly dissociated protic ionic liquids, and nonstoichiometric compositions have been used in protic energy devices such as the all-organic proton battery. Herein, we use a combination of pulsed field gradient NMR spectroscopy and electrochemical impedance spectroscopy to investigate the charge transport properties of the nonstoichiometric protic ionic liquid 2-fluoropyridinium triflate and the variation with the acid-doping level. While all diffusion coefficients decreased with the level of acid doping, the room-temperature conductivity increased because of the concurrent increase in charge carrier concentration. The maximum room-temperature conductivity was 7.33 mS/cm, obtained when 14% of the pyridine was protonated with triflic acid, while higher acid-doping levels lead to liquid/solid mixtures with low conductivity. PEDOT supercapacitor cells with this electrolyte demonstrated very high capacitance (83.9 F/g) and charge storage capacity (23.3 mA h/g). In addition, we predict that using an acid-doping level lower than before will result in superior electrolyte performance in proton batteries because of improvements in conductivity, processability, and electrochemical stability.

Protic ionic liquids for sustainable uses

“…Further work on these systems indicated that lower acid-doping level would likely result in improvements in conductivity, processability, and electrochemical stability, thus bolstering future proton battery performance. 253…”

“…The highest ion conduction is often found in non-stoichiometric mixtures of acids and bases. 50,56,57 Within the last decade, non-stoichiometric protic ionic liquids (NSPILs) have been of increasing interest as electrolytes and as Brønsted acids. Indeed, Coutinho and coworkers suggest that more attention should be paid to nonstoichiometric compositions, in particularly for "pseudo-protic ionic liquids", that may have higher ionicity and overall superior performance in certain electrochemical applications.…”

“…60 These results, although decades old, have lost nothing of their validity and are convergent with trends reported in recent papers on PILs. 50,56,57,62 It is therefore very valuable to screen older literature on acid-base mixtures.…”

Transport Properties of Protic Ionic Liquids Based on Triazolium and Imidazolium: Development of an Air-Free Conductivity Setup

“…Molecular dynamics (MD) simulations can be performed to gain complementary insights on this matter [ 8 ]. Studies performed so far indicate that the molecular structure, intermolecular interactions, composition and confinement can all affect the macroscopically observed transport properties [ 9 , 10 , 11 , 12 , 13 ]. Moreover, the current understanding is that the vehicular mechanism of charge transfer dominates in ionic liquids, even in protic ionic liquids that may, in principle and under certain circumstances, also support other means of proton transfer such as hopping or the Grotthuss mechanism [ 14 ].…”

Relationship between Structure and Properties of Nonstoichiometric Protic Ionic Liquids: n-Butylammonium Butyrate System

“…Most groups tune the properties of PILs through selecting new anions and cations or changing their alkyl chain length, − while in nonstoichiometric PILs, adjusting the ratio of acid to base also has an effect on their structure and properties which can effectively tune the viscosity, conductivity, and polarity of PILs. Thus, nonstoichiometric PILs draw much attention in their application in batteries, − chemical reactions, and purification of ingredients. , However, there is still a lack of deep study into the relationship between structure and properties of this kind of ionic liquids, which hinders the rational design for specific applications.…”