Stabilization of electrochemical

Recently, room-temperature ionic liquids (molten salts) have been extensively studied in order to replace volatile organic solvents by them in electrochemical devices such as batteries. Interest in these materials is stimulated by their properties (e.g., high ionic conductivity, good electrochemical stability, and low volatility). Among these properties, the low volatility is the most critical for ensuring the long-term stability of electrochemical devices. Room-temperature ionic liq-... 

Another contemporary and noteworthy review article by Koper follows yet another concept. Koper first stresses the importance of the electric circuit by evaluating, in a rigorous way, the stability of electrochemical systems by frequency response methods. He then thoroughly discusses the dynamics of selected examples, including some semiconductor systems, which are not included in this chapter, with special emphasis on how they relate to the frequency response theory. 

Ahlgren and Krische [92] reported the stability of electrochemically deposited polypyrrole on pristine polyacetylene (A-type) or predoped (B-type) with... 

Erlandsson et al. [42] reported the most detailed study of the stability of electrochemically prepared polypyrrole containing Bp4 who used x-ray photoelectron spectroscopy to study the reactions of polypyrrole, exposed to various atmospheres at room temperature. They observed electrical conductivity decay on exposure to various atmospheres in the order of O2 -1- H2O 5- O2 > Atm. > At and Ar -L H2O over a period of 500 hours as shown in Figure 16.16. Exposure to dry air or oxygen produced a measurable electrical conductivity decay by a factor of two over 500 hours. Water and oxygen had the most significant synergic effect where the electrical conductivity decreased by a factor of 7 over the same period. XPS studies showed that the concentration of the counter-ion decreased on storage in all cases and the formation of BF3 and HF could be detected in mass spectroscopic studies at elevated temperatures. They found little correlation between loss of BF " ions with the loss of electrical conductivity. 

Power Sources, 196, Pognon, G. et al., Performance and stability of electrochemical capacitor based on anthraqninone modified activated carbon, 4117-4122, Copyright 2011, with permission from Elsevier.)... 

J.F. Clos and J.G. Dorsey, Enhanced Stability of Electrochemical Detection with Surfactant Containing Mobile Phases in LC and Flow-Injection Analysis, Anal. Lett., 23 2327 (1990). 

In any case, the thermal stability of electrochemically prepared polypyrrole can be improved by ion exchange of the dopant counterion [90,95]. The degree of stabilization achieved depends on the counterion (a greater effect is observed for sulfate and bisulfate anions), temperature, and duration of treatment. Although the mechanism for improved stability is not yet clear, it is apparent that the original polymer microstructure is important. 

Bouzek K, Schmidt Ml, Wragg AA (1999) Influence of anode material composition on the stability of electrochemically prepared ferrate (VI) solutions. J Chem Technol Biotechnol 74 1188-1194... 

Non-Aqueous Electrolytes. One approach to increasing the stability of electrochemical photovoltaic cells is to use non-aqueous electrolytes. Work on this approach (40) is divided between organic-based electrolytes (methanol, ethanol, N,N-dimethyl-formamide, acetonitrile, propi lene carbonate, ethylene glycol, te-trahydrofuran, nitromethane, tienzonitrile), and room temperature molten salts (AlCls-butyl pyridinium chloride). These studies are relatively new and final conclusions concerning the relative merits of aqueous vs. non-aqueous electrolytes have not yet been made. 

Britz D, 0steiby O (1994) Some numerical investigations of the stability of electrochemical digital simulation, particultirly as affected by first-order homogeneous reactions. J Electroanal Chem 368 143-147... 

Km and Elsenbaumer [203] reported the synthesis of N-substituted polypyrrole, where FeCl3-doped poly(l-hexylpyrrole) and poly(l-dodecylpyrrole) (XXIII) showed electrical conductivities of 4.3 X 10 and 1.2 x 10 Scm , respectively (four-in-line probe). Both polymers are stable in air for a long time. The air stability of electrochemically doped poly(l-alkylpyrrole)s [204] and polypyrrole [205] was already reported in previous literature. However, the poly(l-butyl-2,5-pyrroIe) of this series is an... 

Lai RY, Seferos DS, Heeger AJ, Bazan GC, Plaxco KW (2006) Comparison of the signaling and stability of electrochemical DNA sensors fabricated from 6- or 11-carbon self-assembled monolayers. Langmuir 22 10796-10800... 

Xie Q, Perez-Cordero E and Echegoyen L 1992 Electrochemical detection of and enhanced stability of fullerides in solution J. Am. Chem. Soc. 114 3978-80... 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

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. 

Catalytic oxidation reactions in ionic liquids have been investigated only very recently. This is somewhat surprising in view of the well loiown oxidation stability of ionic liquids, from electrochemical studies [11], and the great commercial importance of oxidation reactions. Moreover, for oxidation reactions with oxygen, the nonvolatile nature of the ionic liquid is of real advantage for the safety of the reaction. While the application of volatile organic solvents may be restricted by the formation of explosive mixtures in the gas phase, this problem does not arise if a nonvolatile ionic liquid is used as the solvent. 

The overall pattern of behaviour of titanium in aqueous environments is perhaps best understood by consideration of the electrochemical characteristics of the metal/oxide and oxide-electrolyte system. The thermodynamic stability of oxides is dependent upon the electrical potential between the metal and the solution and the pH (see Section 1.4). The Ti/HjO system has been considered by Pourbaix". The thermodynamic stability of an... 

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

Electrochemical methods 78,79 and reactions under high pressure have also been investigated.80 Due to the high resonance stabilization of the macrocyclc, the formation of the phthalocyanine is strongly exothermic. Nevertheless, a high thermal activation and therefore usually a high temperature is necessary. 

This review focuses on the structural stability of transition metal oxides to lithium insertion/extraction rather than on their electrochemical performance. The reader should refer to cited publications to access relevant electrochemical data. Because of the vast number of papers on lithium metal oxides that have been published since the 1970s, only a selected list of references has been provided. 

The good cycling stability of the tin in TCO is quite unusual, because the electrochemical cycling of Li ASn and also of other Li alloy electrodes is commonly associated with large volume changes in the... 

Lithium carbonate and hydrocarbon were identified in XPS spectra of graphite electrodes after the first cycle in LiPF6/EC-DMC electrolyte [104]. Electrochemical QCMB experiments in LiAsF6/EC-DEC solution [99] clearly indicated the formation of a surface film at about 1.5 V vs. (Li/Li+). However the values of mass accumulation per mole of electrons transferred (m.p.e), calculated for the surface species, were smaller than those of the expected surface compounds (mainly (CF OCC Li ). This was attributed to the low stability of the SEI and its partial dissolution. 

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