Brønsted Acid−Base Ionic Liquids as Proton-Conducting Nonaqueous Electrolytes

Abstract

A new series of Brønsted acid−base ionic liquids were derived from the controlled combination of a monoprotonic acid with an organic base under solvent-free conditions. Appropriate amounts of solid bis(trifluoromethanesulfonyl)amide (HTFSI) and solid imidazole (Im) were mixed at various molar ratios to have compositions varying from an equimolar salt to HTFSI- or Im-rich conditions. The mixture at equivalent molar ratio formed a protic neutral salt with a melting point of 73 °C, which was thermally stable at temperatures even above 300 °C. The melting points of other compositions were lower than those of the equimolar salt and Im or HTFSI, giving eutectics between the equimolar salt and HTFSI or Im. Some of the compositions with certain molar ratios of Im and HTFSI were liquid at room temperature. For Im excess compositions, the conductivity was found to increase with increasing Im mole fraction, and the 1H NMR chemical shift of the proton attached to the nitrogen atom of Im was shifted to a lower magnetic field. On the contrary, the conductivity decreased with increasing HTFSI mole fraction, and the 1H NMR chemical shift of the proton attached to the TFSI imide anion also shifted to a higher magnetic field. Self-diffusion coefficients, measured by pulsed-gradient spin−echo NMR (PGSE-NMR) methods in Im- or HTFSI-rich compositions, indicated that fast proton exchange reactions between the protonated Im cation and Im take place in excess Im. The proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms. Direct current polarization measurements were used for the confirmation of proton conduction in Im-rich compositions. Furthermore, reduction of molecular oxygen could be observed at the interface between a Pt electrode and these ionic liquids. This introduces the Brønsted acid−base ionic liquid system as a new candidate for proton conductor such as a fuel cell electrolyte to operate under anhydrous conditions and at elevated temperature.

Protic Ionic Liquids: Preparation, Characterization, and Proton Free Energy Level Representation

“…In case of acid or base excess compositions, the TGA curve would show a two-step process, with the initial loss corresponding to the excess of acid or base in the system. 25 Thermophysical Properties. Some representative DTA scans for protic ILs of this study are shown in Figure 3 to indicate the variety of signals that can be obtained for different cases and the manner in which the transition temperatures are defined from these signals.…”

Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

“…Early in our IL studies, we applied protic ILs for use as H + -conducting electrolytes for non-humidified H 2 /O 2 fuel cells operating at temperatures greater than 100°C based on the expectations that cations having active protons (typically ammonium cations) are highly mobile and electrochemically active for use in fuel cell reactions, i.e., the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). 43,44 In the case of acidic aqueous systems (including polymer electrolyte membrane fuel cells), water molecules act as a base, accepting protons to form hydronium cations (H 3 O + ). H + can be conducted through such media by the migration of H 3 O + cations themselves or by proton exchange between H 3 O + and H 2 O, the so-called vehicle and Grotthuss mechanisms, respectively.…”

“…45 Our initial interest in H + -conducting electrolytes concerned the transport properties of H + in protic ILs. [43][44][45][46][47] At that time, K.-D. Kreuer at the Max Planck Institute for Solid State Research was carrying out pioneering research with non-aqueous proton carriers; [48][49][50][51] his research focused on the autoprotolysis reactions of neutral species such as imidazole and pyrazole, which form proton defects in the systems and accelerate Grotthuss transport. The remarkable changes in conductivity that occur on changing the composition of systems of imidazole or pyrazole and H 2 SO 4 opened a new route to the design of non-aqueous proton conductors.…”

“…The remarkable changes in conductivity that occur on changing the composition of systems of imidazole or pyrazole and H 2 SO 4 opened a new route to the design of non-aqueous proton conductors. 50 This 43,44 However, in the base-rich conditions, the thermal stability is disappointingly low due to evaporation of the neutral species. 43,47 The thermal stability and electrochemical activity of protic ILs for fuel cell reactions are greatly affected by the chemical structure of protic ILs.…”

“…50 This 43,44 However, in the base-rich conditions, the thermal stability is disappointingly low due to evaporation of the neutral species. 43,47 The thermal stability and electrochemical activity of protic ILs for fuel cell reactions are greatly affected by the chemical structure of protic ILs. The most important factor affecting the thermal stability of protic ILs is the difference in pK a values ("pK a ) between the constituent acids and the conjugate acids of the constituent bases.…”