Andrzej Lewandowski

Ionic liquids as electrolytes

Salts having a low melting point are liquid at room temperature, or even below, and form a new class of liquids usually called room temperature ionic liquids (RTIL). Information about RTILs can be found in the literature with such key words as: room temperature molten salt, low-temperature molten salt, ambient-temperature molten salt, liquid organic salt or simply ionic liquid. Their physicochemical properties are the same as high temperature ionic liquids, but the practical aspects of their maintenance or handling are different enough to merit a distinction. The class of ionic liquids, based on tetraalkylammonium cation and chloroaluminate anion, has been extensively studied since late 1970s of the XX century, following the works of Osteryoung. Systematic research on the application of chloroaluminate ionic liquids as solvents was performed in 1980s. However, ionic liquids based on aluminium halides are moisture sensitive. During the last decade an increasing number of new ionic liquids have been prepared and used as solvents. The general aim of this paper was to review the physical and chemical properties of RTILs from the point of view of their possible application as electrolytes in electrochemical processes and devices. The following points are discussed: melting and freezing, conductivity, viscosity, temperature dependence of conductivity, transport and transference numbers, electrochemical stability, possible application in aluminium electroplating, lithium batteries and in electrochemical capacitors.

Supercapacitor based on activated carbon and polyethylene oxide–KOH–H2O polymer electrolyte

For the first time, a totally solid state electric double layer capacitor has been fabricated using an alkaline polymer electrolyte and an activated carbon powder as electrode material. The polymer electrolyte serves both as separator as well as electrode binder. The capacitor has a three-layer structure: electrode–electrolyte–electrode with a final thickness of ca. 1.5–2 mm, diameter of 1.8 cm (surface of 2.5 cm2) and a mass of ca. 300–500 mg. Cyclic voltammetry, galvanostatic cycling and impedance spectroscopy have been used for the determination of the electrochemical performance of prototype capacitors and a capacity of ca. 1.7–3.0 F for a 1 V voltage range has been measured.

Electrochemical capacitors with polymer electrolytes based on ionic liquids

Abstract A series of electrochemical capacitors, based on carbon powders (specific surface area 830 and 2600 m2/g) as an electroactive material, applying polymer–ionic liquid (IL) solid electrolytes were prepared and tested. Both polymer electrolytes as well as capacitor electrodes, in the form of thin foils, were prepared by a casting technique. All-plastic capacitors, having a coin-like shape, a diameter of 18 mm and a mass of ca. 200–900 mg, were constructed by sandwiching a polymer electrolyte between two electrodes. Ionic liquids: 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-1-methylpyrrolidinium bis(trifluoromethane sulfonyl) imide and 1-butyl-3-methylimidazolium hexafluorophosphate served both as sources of ions as well as polymer plasticisers. In some cases, sulpholane was added to the system as an additional plasticiser. The performance of capacitors was determined by cyclic voltammetry, impedance spectroscopy and galvanostatic charging and discharging. Specific capacity was in the range of 4.2–7.7 μF/cm2, calculated per total surface of the carbon materials, and up to 200 F/g expressed versus the mass of carbon material.

Relative molar Gibbs energies of cation transfer from a molecular liquid to ionic liquids at 298.15 K

Molar Gibbs energies of Ag+, Cu2+, Zn2+ and Cd2+ transfer from dimethylsulfoxide (DMSO), a reference molecular liquid, to a number of ionic liquids (IL), ΔtG(DMSO → IL), were obtained from M|Mn+ electrode potentials at 298.15 K. The ionic liquids consisted of various tetraalkylammonium cations and Cl−, Br−, BF4−, PF6− or N(CF3SO2)2− anions. The measured M|Mn+ (0.01 M, IL) potentials depend both on the tetraalkylammonium cation as well as on the anion. The transfer of cations from DMSO to ionic liquids brings about positive or negative changes of the molar Gibbs energy. The most important factor influencing the transfer molar Gibbs energy is the anionic component of the ionic liquid, which solvates the cation. In general, the molar Gibbs energy of cations in ionic liquids having N(CF3SO2)2− anion is lower than in those having halide or tetrafluoroborate anions.

Ionic solvation—III. free energies of transfer of copper(II) ion from water to sulfolane, tetrahydrofuran, acetone, dioxan and dimethylsulphoxide and to their mixtures with water

The emf of the cell: Cu/Cu2+, water + organic co-solvent/water, Cu2+//Cu with sulfolane, dioxan, tetrahydrofuran, acetone and dimethylsulphoxide (DMSO) as organic co-solvents has been measured for different solvent compositions. To decrease the liquid junction potential the salt bridge filled with the methanolic solution of tetrabutylammonium picrate has been applied. The emf data were used to calculate the Gibbs free energy of copper(II)cation transfer from water to nonaqueous or mixed solvents. Determined free energies of transfer are negative for DMSO and positive for other organic solvents.

Copper transport properties in polymer electrolytes based on poly(ethylene oxide) and poly(acrylonitrile)

Abstract Transport properties of copper salts were studied in a solvent-free polymer electrolyte PEO–Cu(CF 3 SO 3 ) 2 (where PEO is poly(ethylene oxide)) as well as in gel-type polymer electrolytes: polymer–solvent–CuX 2 (where X=Cl − , Br − or CF 3 SO 3 − , polymer is PEO or poly(acrylonitrile) (PAN), solvent is dimethylsulfoxide (DMSO), sulfolane or propylene carbonate). Generally, low values of the cationic transference numbers ( T + ≈0.02–0.08) for the polymer electrolytes were obtained. However, in the systems based on PAN plasticised with DMSO, the T + value increases significantly ( T + ≈0.1–0.4); the highest value is reached in the system containing an excess of the Br − anions (PAN–DMSO–CuX 2 –KBr, T + =0.4).

Polyacrylonitrile-DMSO-AgClO4 solid polymer electrolyte

This paper reports on the preparation and properties of a new type of thin-film, polymer-solvent-salt solid electrolyte, based on polyacrylonitrile (PAN)-dimethyl sulfoxide (DMSO; 30–80 wt.%)-Ag-ClO4 system that has a high ionic conductivity of 7 × 10−4Ω−1 cm−1 at room temperature. A good level of mechanical properties, dimensional stability and high conductivity are reached at the DMSO content of ca 70 wt.%. At lower DMSO content in the solid-electrolyte ( 5 × 10−2 mA cm−2 and, therefore, the resistance of the charge transfer reaction at room temperature at the order of kΩ cm2.

Apparent molar volumes of divalent transition metal chlorides and perchlorates in dimethyl sulfoxide solutions

Densities and apparent molar volumes for DMSO solutions of manganese(II), cobalt(II), nickel(II), copper(II), zinc(II) perchlorates and chlorides are reported. The limiting partial molar volumes of the hexakis-(DMSO) cations are derived and discussed in terms of the ligand field effects. The differences between volumetric properties of DMSO solutions of perchlorates and chlorides are interpreted in terms of formation of the respective chloride-complexes of the divalent transition metal cations.

A new polymer electrolyte poly(acrylonitrile)–dimethylsulphoxide–salt for electrochemical capacitors

Abstract The PAN–DMSO–Et4NBF4 and PAN–DMSO–Et4NTf (Tf is triflate ion) electrolytes were prepared as white, turbid foils with a thickness in the range of 0.1–0.5 mm, using the casting technique. Room temperature conductance of the electrolytes, detected from ac impedance experiments, was at the level of 8 and 14 mS cm−1 for Et4NBF4 and Et4NTf salts, respectively. The electrochemical stability window of approximately 2.6–2.8 V was estimated using a glassy carbon electrode. Temperature dependence of the conductivity is of the Arrhenius-type for both electrolytes, with an activation energy of approximately 34 kJ mol−1. The double-layer capacitors built with these electrolytes, serving both as separators and activated carbon powder (ACP) binders, showed a specific capacity of 50 F g−1 of carbon material. Capacitors were assembled by sandwiching the PAN–DMSO–salt electrolyte between two PAN–salt–DMSO–ACP–AB electrodes and pressing across the system; the resulting devices had a coin-like shape with 8 mm diameter and thickness between 2.0 and 2.5 mm.

New polymer electrolyte poly(acrylonitrile)–sulpholane–(C2H5)4NBF4 for chemical capacitors

Abstract A series of ion conducting, thin-film polymer electrolytes, based on poly(acrylonitrile), tetraethylammonium tetrafluoroborate and sulpholane (tetramethylene sulphone, TMS), PAN–(C2H5)4NBF4–TMS, were prepared using the casting technique. The polymer electrolytes were transparent with rubberlike elasticity. Maximum conductivity was at the level of 10 mS/cm at room temperature. The temperature dependence of the conductivity indicates the Arrhenius type of the behaviour, with the activation energy of 12 kJ/mol. The electrochemical stability window of the electrolyte, determined at the glassy carbon electrode, was of ca. 3 V. Capacitor electrodes, in the form of thin foils, were prepared by the casting technique, using activated carbon powder (ACP) mixed with acetylene black (AB) or graphite (G) as an electrode active material. The performance of capacitors, having a coin-like shape, a diameter of 18 mm and a mass of 180–350 mg, was determined by cyclic voltammetry, impedance spectroscopy and galvanostatic charging and discharging.