Electrochemical techniques ionic liquids

Section 3.3. In this section we deal specifically with the electrochemical properties of ionic liquids (electrochemical windows, conductivity, and transport properties) we will discuss the techniques involved in measuring these properties, summarize the relevant literature data, and discuss the effects of ionic liquid components and purity on their electrochemical properties. 

The ionic conductivity of a solvent is of critical importance in its selection for an electrochemical application. There are a variety of DC and AC methods available for the measurement of ionic conductivity. In the case of ionic liquids, however, the vast majority of data in the literature have been collected by one of two AC techniques the impedance bridge method or the complex impedance method [40]. Both of these methods employ simple two-electrode cells to measure the impedance of the ionic liquid (Z). This impedance arises from resistive (R) and capacitive contributions (C), and can be described by Equation (3.6-1) ... 

These results are quite interesting. The initial stages of Al deposition result in nanosized deposits. Indeed, from the STM studies we recently succeeded in making bulk deposits of nanosized Al with special bath compositions and special electrochemical techniques [10]. Moreover, the preliminary results on tip-induced nanostructuring show that nanosized modifications of electrodes by less noble elements are possible in ionic liquids, thus opening access to new structures that cannot be made in aqueous media. 

After reviewing the properties and structure of ionic liquids, leading specialists explore the role of these materials in optical, electrochemical, and biochemical sensor technology. The book then examines ionic liquids in gas, liquid, and countercurrent chromatography, along with their use as electrolyte additives in capillary electrophoresis. It also discusses gas solubilities and measurement techniques, liquid-liquid extraction, and the separation of metal ions. The final chapters cover molecular, Raman, nuclear magnetic resonance, and mass spectroscopies. 

Shi et al. [70] were the first to demonstrate the use of an air and moisture stable ionic liquid, [C4mim][PF,s], for the electrochemical synthesis of poly(thiophene), grown onto a platinum working electrode by potentiodynamic, constant potential or constant current techniques. The use of growth potentials between 1.7 and 1.9 V (vs. Ag/AgCl) reportedly gave smooth, blue-green electroactive films, whereas potentials above 2 V resulted in film destruction by overoxidation. 

Endres et al. [82] have demonstrated the suitability of an air- and water-stable ionic liquid for the electropolymerization of benzene. This synthesis is normally restricted to media such as concentrated sulfuric acid, liquid SO2 or liquid HF as the solution must be completely anhydrous. The ionic liquid used, l-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, can be dried to below 3 ppm water, and this ionic liquid is also exceptionally stable, particularly in the anodic regime. Using this ionic liquid, poly(para-phenylene) was successfully deposited onto platinum as a coherent, electroactive film. Electrochemical quartz crystal microbalance techniques were also used to study the deposition and redox behavior of the polymer from this ionic liquid (Section 7.4.1) [83]. 

In Section 8.2 the basics of pulsed dectrodeposition (PED) will be described for the case of aqueous electrolytes which allow the deposition of comparatively noble metals like Cu, Ni, Pd, or Au less noble metals like Fe or Zn can still be electrodeposited from aqueous electrolytes because they exhibit a comparatively large overpotential for hydrogen evolution. However, the main limitation of aqueous dectrolytes, of course, is their narrow electrochemical window which adversely affects the electrodeposition of metals like A1 or Ta. Therefore, recently, the PED technique has been extended to ionic liquids as electrolytes. General electrochemical aspects of ionic liquids can be found in Ref. [44] here, in Section 8.3, we will only address the technical aspects with respect to PE D. Examples of nanometals and nanoalloys electrodeposited from chloroaluminate-based ionic liquids are given in... 

Like other salt melts ionic liquids are characterized by a specific combination of physicochemical properties high ionic conductivity, low viscosity, high thermal stability compared to conventional liquid solvents, wide electrochemical windows of up to 7 V and - in most cases - extremely low vapor pressures. Due to their low vapor pressure ionic liquids are not only well suited for the application of UHV-based analytical techniques (e.g. photoelectron spectroscopy [3]), but also for use in plasma reactors with typical pressures of the order of 1 Pa up to 10 kPa. Moreover, due to their high electrical conductivity, ionic liquids may even be used as electrodes for plasmas. To date there are just a few reports on the combination of low-temperature plasmas and ionic liquids available in the literature [4—6]. Therefore, the essential aspects of experiments with ionic liquids in typical plasma reactors are discussed in this section. 

Deposition of silver metal In order to exemplify the plasma electrochemical deposition (PECD) technique in ionic liquids we first deposited silver... 

The main contaminants in an ionic liquid will be introduced from the synthesis, absorbed from the atmosphere or produced as breakdown products through electrolysis (see above). The main contaminants for eutectic-based ionic liquids will be from the components. These will be simple amines (often trimethylamine is present which gives the liquid a fishy smell) or alkyl halides. These do not interfere significantly with the electrochemical response of the liquids due to the buffer behavior of the liquids. The contaminants can be effectively removed by recrystallization of the components used to make the ionic liquids. For ionic liquids with discrete anions the major contaminants tend to be simple anions, such as Li+, K+ and Cl-, present from the metathesis technique used. These can give significant difficulties for the deposition of reactive metals such as Al, W and Ti as is demonstrated below with the in situ scanning tunnelling microscope. 

Electrochemical methods can be powerful tools. They can be used to reveal the chemical and physical properties of room-temperature ionic liquids. Most of existing electrochemical techniques [1] developed in aqueous solutions are applicable for the ionic liquids, as demonstrated in the chloroaluminate ionic liquids. However, there are several procedures that must be observed if one is to obtain reliable data in electrochemical measurements. This section describes the procedures that are important for the ionic liquids. 

Electrochemical methods are sensitive to the extent that it is possible to detect a trace of electroactive species in electrolyte solutions. Because of this distinctive feature, electrochemical methods have been developed and utilized for analytical purposes. The detection method used is known as polarography. For the electrochemical study purification of the electrolyte solutions is therefore important. As for most aqueous and organic electrolyte solutions, there are various well-established techniques for purifying both solvents and electrolytes. In the case of room-temperature ionic liquids, it is especially important to purify the starting materials used for preparing the ionic liquids. 

Platinum, glasslike carbon, and tungsten are often used as inert working electrodes for the fundamental electrochemical studies in the ionic liquids. For such transient electrochemical techniques as cyclic voltammetry, chronoamperometry, and chronopotentiometry, it is safer to use the working electrode with a small active area. This is because most of the ionic liquids will have low conductivity, and this often causes the ohmic drop in the measured potentials by the current flowing between the working and counter electrode. Microelectrodes may be useful for the electrochemical measurements in the case of handling low conductive media. 

Palladium electrodeposition is of special interest for catalysis and for nanotechnology. It was reported that it can be deposited from basic chloroaluminate liquids, in the acidic regime the low solubility of PdCh and passivation phenomena complicate the deposition [52]. However, thick Pd layers are difficult to obtain from basic chloroaluminates. With different melt compositions and special electrochemical techniques at temperatures up to 100 °C Endres et al. succeeded in depositing mirror bright and thick nanocrystalline palladium coatings [10]. Stm et al. have reported that Pd-Au [53] and Pd-Ag [54] alloys can be electrodeposited in a basic l-ethyl-3-methylimidazolium chloride/ tetrafluroborate ionic liquid. 

---