Naoki Tachikawa

Energy applications of ionic liquids

Ionic liquids offer a unique suite of properties that make them important candidates for a number of energy related applications. Cation–anion combinations that exhibit low volatility coupled with high electrochemical and thermal stability, as well as ionic conductivity, create the possibility of designing ideal electrolytes for batteries, super-capacitors, actuators, dye sensitised solar cells and thermo-electrochemical cells. In the field of water splitting to produce hydrogen they have been used to synthesize some of the best performing water oxidation catalysts and some members of the protic ionic liquid family co-catalyse an unusual, very high energy efficiency water oxidation process. As fuel cell electrolytes, the high proton conductivity of some of the protic ionic liquid family offers the potential of fuel cells operating in the optimum temperature region above 100 °C. Beyond electrochemical applications, the low vapour pressure of these liquids, along with their ability to offer tuneable functionality, also makes them ideal as CO2 absorbents for post-combustion CO2 capture. Similarly, the tuneable phase properties of the many members of this large family of salts are also allowing the creation of phase-change thermal energy storage materials having melting points tuned to the application. This perspective article provides an overview of these developing energy related applications of ionic liquids and offers some thoughts on the emerging challenges and opportunities.

Oxidative-Stability Enhancement and Charge Transport Mechanism in Glyme–Lithium Salt Equimolar Complexes

The oxidative stability of glyme molecules is enhanced by the complex formation with alkali metal cations. Clear liquid can be obtained by simply mixing glyme (triglyme or tetraglyme) with lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) in a molar ratio of 1:1. The equimolar complex [Li(triglyme or tetraglyme)(1)][TFSA] maintains a stable liquid state over a wide temperature range and can be regarded as a room-temperature ionic liquid consisting of a [Li(glyme)(1)](+) complex cation and a [TFSA](-) anion, exhibiting high self-dissociativity (ionicity) at room temperature. 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 around 4 V vs Li/Li(+). 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 nuclear magnetic resonance (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]|LiCoO(2) cathode] when the discharge reaction of a LiCoO(2) cathode, that is, desolvation 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.

Reversibility of electrochemical reactions of sulfur supported on inverse opal carbon in glyme-Li salt molten complex electrolytes

Electrochemical reactions of sulfur supported on three-dimensionally ordered macroporous carbon in glyme-Li salt molten complex electrolytes exhibit good reversibility and large capacity based on the mass of sulfur, which suggests that glyme-Li salt molten complexes are suitable electrolytes for Li-S batteries.

Electrode Kinetics of Ferrocenium/Ferrocene in Some Amide-Based Room-Temperature Ionic Liquids

The electrode kinetics of ferrocenium/ferrocene ([Fe(Cp) 2 ] + /Fe(Cp) 2 ) was investigated in some room-temperature ionic liquids based on bis(trifluoromethylsulfonyl)amide. The diffusion coefficients of Fe(Cp) 2 were inversely proportional to the viscosity of the ionic liquids. The diffusion coefficients of [Fe(Cp) 2 ] + were smaller than those of Fe(Cp) 2 probably because of the coulombic interaction between charged species and the ionic liquids. The apparent heterogeneous rate constants of [Fe(Cp) 2 ] + /Fe(Cp) 2 in the ionic liquids were smaller than those in organic solvents. The heterogeneous rate constants correlate with the viscosity of the electrolytes probably due to the slow dynamics of the ions composing the ionic liquids.

Electrode Kinetics of Ferrocenium/ferrocene in Room-temperature Ionic Liquids

INTRODUCTION The electron transfer kinetics of the outer-sphere redox reaction is influenced by the nature of the electrolytes. In conventional solvents, the heterogeneous rate constants (k) are represented [1] by the following equation; k = KPκelνnexp(-∆G/RT) (1) where ∆G is the activation Gibbs energy, KP is the precursor equilibrium constant, νn is the nuclear frequency factor, κel is the electronic transmission coefficient, R is the gas constant and T is the absolute temperature. Recently, in the ionic liquid, the activation energy for the k was discussed [2] using eq. (1). It was found that the apparent activation energy for the k is mainly related to the activation energy for the viscosity of the ionic liquid. In the present study, the electrode kinetics of ferrocenium/ferrocene ([Fe(Cp)2] /Fe(Cp)2) redox system was investigated in several roomtemperature ionic liquids.