Charge Transport and Transfer in Ionic Liquids for Advanced Electrochemical Devices

A new family of ionic liquids can be obtained by simply mixing glyme (triglyme (G3) or tetraglyme (G4)) with lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) in a molar ratio of 1:1 [1-4]. The equimolar complex [Li(G3 or G4)₁][TFSA] maintains a stable liquid state over a wide temperature range and can be regarded as a solvate ionic liquid consisting of a [Li(glyme)₁]⁺ complex cation and a [TFSA]^- anion, exhibiting high self-dissociativity (ionicity) at room temperature [1, 4]. The electrochemical oxidation of [Li(glyme)₁][TFSA] takes place at the electrode potential of ~5 V vs. Li/Li⁺, while the oxidation of solutions containing excess glyme molecules ([Li(glyme)_x][TFSA], x > 1) occurs at lower than 4 V [2]. This enhancement of oxidative stability is due to the donation of lone pairs of ether oxygen atoms to the Li⁺ cation, resulting in the highest occupied molecular orbital (HOMO) energy level lowering of a glyme molecule, which is confirmed by ab initio molecular orbital calculations. The solvation state of a Li⁺ cation and ion conduction mechanism in the [Li(glyme)_x][TFSA] solutions is elucidated by means of NMR and electrochemical methods. The experimental results strongly suggest that Li⁺ cation conduction in the equimolar complex takes place by the migration of [Li(glyme)₁]⁺ cations, whereas the ligand exchange mechanism is overlapped when interfacial electrochemical reactions of [Li(glyme)₁]⁺ cations occur. The ligand exchange conduction mode is typically seen in a lithium battery with a configuration of [Li anode | [Li(glyme)₁][TFSA] | LiCoO₂ cathode] when the discharge reaction of a LiCoO₂ cathode, i.e., de- solvation of [Li(glyme)₁]⁺ and insertion of the resultant Li⁺ into the cathode, occurs at the electrode-electrolyte interface. The battery can be operated for more than 200 charge-discharge cycles in the cell voltage range of 3.0–4.2 V, regardless of the use of ether-based electrolyte, because the ligand exchange rate is much faster than the electrode reaction rate [2].

Protic ionic liquid as a proton conductor for a fuel cell is another topic to be discussed here. The characterization of a protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), and the fabrication of a membrane-type fuel cell system using [dema][TfO] under non-humidified conditions at intermediate temperatures are described in detail [5-8]. In terms of physicochemical and electrochemical properties, [dema][TfO] exhibits high activity for fuel cell electrode reactions (i.e., the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR)) at a Pt electrode, and the open circuit voltage (OCV) of a liquid fuel cell is 1.03 V at 150 °C [5]. However, diethylmethylammonium bis(trifluoromethane sulfonyl)amide ([dema][NTf₂]) has relatively low HOR and ORR activity, and thus, the OCV is ca. 0.7 V, although [dema][NTf₂] and [dema][TfO] have an identical cation ([dema]) and similar thermal and bulk-transport properties. Proton conduction occurs mainly via the vehicle mechanism in [dema][TfO] and the proton transference number (t₊) is 0.5~0.6. This relatively low t₊ appears to be more disadvantageous for a proton conductor than for other electrolytes such as hydrated sulfonated polymer electrolyte membranes (t₊ = 1.0). However, fast proton-exchange reactions occur between ammonium cations and amines in a model compound. This indicates that the proton-exchange mechanism contributes to the fuel cell system under operation, where de-protonated amines are continuously generated by the cathodic reaction, and that polarization of the cell is avoided [6]. Six-membered sulfonated polyimides in the diethylmethylammonium form exhibit excellent compatibility with [dema][TfO]. The composite membranes can be obtained up to a [dema][TfO] content of 80 wt% and exhibit good thermal stability, high ionic conductivity, and mechanical strength and gas permeation comparable to those of hydrated Nafion. H₂/O₂ fuel cells prepared using the composite membranes can successfully operate at temperatures from 30 °C to 140 °C under non-humidified conditions, and a current density of 400 mA cm⁻² is achieved at 120 °C [7]. The protic ionic liquid and its composite membrane are a possible candidate for an electrolyte of a H₂/O₂ fuel cell that operates under non-humidified conditions.

References

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