Electrochemical Stability Range

The electrochemical stability range determines the usefulness of nonaqueous electrolytes for electrochemical studies as well as for applications. It indicates the absence of electrochemical oxidation or reduction of solvent or ions, and of faradaic current 

However, even if electrolytes have sufficiently large voltage windows, their components may not be stable (at least ki-netically) with lithium metal for example, acetonitrile shows very large voltage windows with various salts, but is polymerized at deposited lithium if this reaction is not suppressed by additives, such as SO2 which forms a protective ionically conductive layer on the lithium surface. Nonetheless, electrochemical stability ranges from CV experiments may be used to choose useful electrolytes. 

It is worth mentioning that the use of microelectrodes allows the investigation of the electrochemical stability range of solvents without addition of a salt [138J. These studies make it possible to discrimi- 

The use of liquid salts, based on anodi-cally stable cations such as 1,2-dimethyl-3-propylimidazolium (Dmpi) without added solvent, allows the investigation of the electrochemical stability of anions [75]. 

Assigned as oxidation voltage, the anodic stability limit is slightly lower. 

Tahle 7. Electrochemical stability ranges or anodic stability limits of several nonaqueous electrolytes 

However, in most cases merely a pseudo-reference with Hthium metal is used. Other options are alloy electrodes with Sn or Ati [239, 240] or the nonaqueous Ag/Ag -cryptand electrode by Izutsu et al. [241, 242]. This group recently investigated [243] such REFs, which are very interesting, especially for miniaturization. Electrochemical deposition of Li from a 0.470 mol solution liBOB in PC onto an Sn-wire (0.5 mm 0,20 h at +680 mV vs Li/Li+) produces Ii/Li2Sn5 REFs. Investigations show that the potential in 0.470 mol-solution liBOB in PC was stable at (0.741 0.0005 V) for 2.5 x 10 s before corrosion shifted the potential. 

To compare redox potentials of aqueous and nonaqueous systems, a variety of internal references were investigated. In 1984, Gritzner and Kuta recommended two systems for nonaqueous electrolytes that are also accepted by lUPAC [244]. The solvent-independent organometallic redox couples are fer-rocene/ferrocenium (Fc/Fc+) (E = 0.158 V vs saturated calomel electrode (SCE)) and bis(biphenyl)chromium(0)/(l) (BCr/BCr+) (E = —0.82 V vs SGE) [245]. Very stable electrode redox potentials Ei/i vs Ag/Ag+-cryptand electrode of 0.478 V for Fc/Fc+ and —0.616 V for BCr/BCr+ in EMIm tetrafluoroborate were measured [246]. 

Furthermore, many investigations of nonaqueous electrolytes have even been performed with a saturated calomel electrode despite obvious problems such as contamination by water. In addition, unknown liquid junction potentials and insufficient knowledge of electrode reactions must be taken into account in addition to differing experimental conditions for the interpretation of such data. 

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It is worth mentioning that the use of microelectrodes (dimensions of micrometers or less) [248] allows the investigation of the electrochemical stability range of solvents without addition of a salt [249]. These studies allow a distinction between chemical and electrochemical reactions at the electrodes. In contrast to the results of Ue et al. [247], THF is not reduced at potentials down to —2 V vs Li/Li+, but oxidizes already at +4 V vs Li/Li+, whereas PC is stable up to 5 V, but already reduces at potentials of less than 1 V vs Li/Li+. Nonetheless, electrochemical stability ranges from CV experiments may be used for screening useful electrolytes. Table 17.9 shows the electrochemical stability limits of several nonaqueous lithium salt electrolytes versus different REFs. Furthermore the used WEs and the experimental conditions, like scan rate v and the onset current density io, with their references are listed. 

Table 17.9 Electrochemical stability ranges of several nonaqueous lithium salt electrolytes.

Hie electrochemical stability range of the lithium-doped di-ureasils was determined by microelectrode cyclic voltammetry over the potential range between -1.5 and 6.5 In the anodic region, all ormolytes are stable... 

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for... 

Before the measurement of HOR activity, a pretreatment of the alloy electrode was carried out by potential sweeps (10 V s ) of 10 cycles between 0.05 and 1.20 V in N2-purged 0.1 M HCIO4. The cyclic voltammograms (CVs) at all the alloys resembled that of pure Pt. As described below, these alloy electrodes were electrochemically stabilized by the pretreatment. Hydrodynamic voltammograms for the HOR were then recorded in the potential range from 0 to 0.20 V with a sweep rate of 10 mV s in 0.1 M HCIO4 saturated with pure H2 or 100 ppm CO/H2 at room temperature. The kinetically controlled current 4 for the HOR at 0.02 V was determined from Levich-Koutecky plots [Bard and Faulkner, 1994]. 

Application of new types of graphite, found to be more oxidation-proof (in particular, TEG and TEG modified by boron), can largely increase the electrochemical stability of materials used in aqueous electrolyte media. Their high resistance to oxidation and enhanced long-term cycling stability create realistic prerequisites for wide range of applications for such graphite... 

Figure 10. Components of a three-state system and schematic representation of the ranges of electrochemical stability of the three states available to the system.

It can only be shielded by ion pairing that drives the formation of the tetraanion to a potential sufficiently positive for reduction to occur within the stability range of the solvent-electrolyte system. Thus, the electrochemical reduction of... 

Unfortunately, these aza-ethers showed limited solubility in the polar solvents that are typically preferred in nonaqueous electrolytes, and the electrochemical stability window of the LiCl-based electrolytes is not sufficient at the 4.0 V operation range required by the current state-of-the-art cathode materials. They were also found to be unstable with LiPFe. Hence, the significance of these aza-ether compounds in practical applications is rather limited, although their synthesis successfully proved that the concept of the anion receptor is achievable by means of substituting an appropriate core atom with strong electron-withdrawing moieties. 

The third aspect to consider is the electrochemical stability of the material used. For the oxygen reduction reaction, the electrode potential is highly anodic and at this potential, most metals dissolve actively in acid media or form passive oxide films that will Inhibit this reaction. The oxide forming metals can form non-conducting or semi-conducting oxide films of variable thickness. In alkaline solutions, the range of metals that can be used is broader and can include non-precious or semi-precious metals (Ni, Ag). 

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