Electrochemical window.

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. 

It must be noted that impurities in the ionic liquids can have a profound impact on the potential limits and the corresponding electrochemical window. During the synthesis of many of the non-haloaluminate ionic liquids, residual halide and water may remain in the final product [13]. Halide ions (Cl , Br , I ) are more easily oxidized than the fluorine-containing anions used in most non-haloaluminate ionic liquids. Consequently, the observed anodic potential limit can be appreciably reduced if significant concentrations of halide ions are present. Contamination of an ionic liquid with significant amounts of water can affect both the anodic and the cathodic potential limits, as water can be both reduced and oxidized in the potential limits of many ionic liquids. Recent work by Schroder et al. demonstrated considerable reduction in both the anodic and cathodic limits of several ionic liquids upon the addition of 3 % water (by weight) [14]. For example, the electrochemical window of dry [BMIM][BF4] was found to be 4.10 V, while that for the ionic liquid with 3 % water by weight was reduced to 1.95 V. In addition to its electrochemistry, water can react with the ionic liquid components (especially anions) to produce products... 

Figure 3.6-1 shows the electrochemical window of a 76-24 mol % [BMMIM][(GF3S02)2N]/Li[(GF3S02)2N] ionic liquid at both GG and Pt working electrodes [15]. For the purposes of assessing the electrochemical window, the current threshold for both the anodic and cathodic limits was set at an absolute value of 100 pA cm . ... 

Figure 3.6-1 The electrochemical window of 76-24 mol % [BMMIM][(CF3S02)2N]/Li [(Cp3S02)2N] binary melt at a) a platinum working electrode (solid line), and b) a glassy carbon working electrode (dashed line). Electrochemical window set at a threshold of 0.1 mA cm. The reference electrode was a silver wire immersed in 0.01 m AgBp4 in [EMIM][BF4] in a compartment separated by a Vicor frit, and the counter-electrode was a graphite rod.

Ideally, one would prefer to compare anodic and cathodic potential limits instead of the overall ionic liquid electrochemical window, because difference sets of anodic and cathodic limits can give rise to the same value of electrochemical window (see Figure 3.6-1). However, the lack of a standard reference electrode system within and between ionic liquid systems precludes this possibility. Gonsequently, significant care must be taken when evaluating the impact of changes in the cation or anion on the overall ionic liquid electrochemical window. 

The electrochemical windows exhibited by the chloroaluminates tend to fall into three ranges that correspond to the types of chloroaluminate ionic liquids basic. 

V, respectively) generated by this same group in a later publication [13], cause us to treat these impressive electrochemical windows as suspect. Consequently, they have not been included in Table 3.6-1. 

Germanium In situ STM studies on Ge electrodeposition on gold from an ionic liquid have quite recently been started at our institute [59, 60]. In these studies we used dry [BMIM][PF<3] as a solvent and dissolved Gel4 at estimated concentrations of 0.1-1 mmol 1 the substrate being Au(lll). This ionic liquid has, in its dry state, an electrochemical window of a little more than 4 V on gold, and the bulk deposition of Ge started several hundreds of mV positive from the solvent decomposition. Furthermore, distinct underpotential phenomena were observed. Some insight into the nanoscale processes at the electrode surface is given in Section 6.2.2.3. 

This is only one of some very promising potential non-synthetic applications of ionic liquids that have emerged recently. Many others - some more, some less fully documented in patent or scientific literature - have been published. Table 9-1 gives a few examples, showing that most of the non-synthetic applications of ionic liquids can be grouped into three areas. Electrochemical applications benefit from the wide electrochemical window of ionic liquids and/or from the distinct variation of con-... 

The wide electrochemical windows of ionic liquids, in combination with their ability to serve as solvents for transition metal catalysts, opens up new possibilities for a combination of electrochemistry and transition metal catalysis. A very exciting first example has recently been published by Bedioui et al. [27]. 

Table 7 lists the electrochemical windows or the anodic stability limits of several nonaqueous electrolytes, or their anodic stabilty, the reference electrodes Rref used, the working electrode material Ew, the experimental conditions, and the references. It shows the following features ... 

Recently, a eutectic mixture of choline chloride and urea (commercially known as Reline) was used as a medium from which CdS, as well as CdSe and ZnS, thin films were electrodeposited for the first time [53]. Reline is a conductive room-temperature ionic liquid (RTIL) with a wide electrochemical window. The voltammetric behavior of the Reline-Cd(II)-sulfur system was investigated, while CdS thin films were deposited at constant potential and characterized by photocurrent and electrolyte electroabsorbance spectroscopies. 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Figure 1 provides several electrochemical windows of important, relevant processes, including the reduction of alkyl carbonates, ethers, Li insertion into graphite, and Li metal deposition. Recent studies revealed two major failure mechanisms of graphite electrodes in repeated Li insertion/ deinsertion processes 21... 

Figure 1. Schematic representation of the electrochemical windows of the processes, occurring during cathodic polarization of graphite electrodes in nonaqueous solutions.

The thin-layer configuration and its associated diffusion problems means that it is possible to oxidise (or reduce) all of the electroactive species in the thin layer before they can be replenished to any marked degree. Consider, for example, the 0"+/0 couple, with a standard redox potential well within the "electrochemical window of the solvent, so that the current in the absence of the couple is small and can easily be accounted for. With the electrode pushed against the window the potential is stepped cathodic enough to ensure the rapid reduction of the 0" + and the current measured as a function of time, the concentration such that the time for the current to reach zero, or a steady residual value, is small. If the area under the I ft curve is A ampere seconds, then the charge passed Q = A coulombs. Thus, the number of moles of 0"+ reduced, N0, is given by ... 

The useful electrochemical windows of the acidic AlCh-EtMelmCI and AlCh-NaCI melts are approximately 2.2 V. The positive limit of each melt is most likely due to the oxidation of AICI4... 

Chemical reactivity of unfunctionalized organosilicon compounds, the tetraalkylsilanes, are generally very low. There has been virtually no method for the selective transformation of unfunctionalized tetraalkylsilanes into other compounds under mild conditions. The electrochemical reactivity of tetraalkylsilanes is also very low. Kochi et al. have reported the oxidation potentials of tetraalkyl group-14-metal compounds determined by cyclic voltammetry [2]. The oxidation potential (Ep) increases in the order of Pb < Sn < Ge < Si as shown in Table 1. The order of the oxidation potential is the same as that of the ionization potentials and the steric effect of the alkyl group is very small. Therefore, the electron transfer is suggested as proceeding by an outer-sphere process. However, it seems to be difficult to oxidize tetraalkylsilanes electro-chemically in a practical sense because the oxidation potentials are outside the electrochemical windows of the usual supporting electrolyte/solvent systems (>2.5 V). 

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