Electrodes aprotic

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

Film-forming chemical reactions and the chemical composition of the film formed on lithium in nonaqueous aprotic liquid electrolytes are reviewed by Dominey [7], SEI formation on carbon and graphite anodes in liquid electrolytes has been reviewed by Dahn et al. [8], In addition to the evolution of new systems, new techniques have recently been adapted to the study of the electrode surface and the chemical and physical properties of the SEI. The most important of these are X-ray photoelectron spectroscopy (XPS), SEM, X-ray diffraction (XRD), Raman spectroscopy, scanning tunneling microscopy (STM), energy-dispersive X-ray spectroscopy (EDS), FTIR, NMR, EPR, calorimetry, DSC, TGA, use of quartz-crystal microbalance (QCMB) and atomic force microscopy (AFM). 

FIGURE 8. Voltammetries in DMF-TBAP 0.1m, stationary mercury electrode, sweep rate lOmVs 1, concentration of sulphones 5 x 10 3m (1) in aprotic DMF, (2) DMF with phenol (10 2m) (after Reference 26). 

FIGURE 10. Voltammetric curves of fully aliphatic allylic sulphones (c = 3 x 10-3 M) in DMF/TBAP 0.1 m electrolyte, stationary mercury electrode, sweep rate 10 mV s—1 (a) and (b) curves in aprotic DMF (c) response of the sulphone, (b) with phenol 10-2 m (after Reference 26). 

The application of PANI as active electrode material in a commercially available polymer lithium battery is described by Nakajima and Kawagoe In aprotic solvents... 

The electrochemical reduction and oxidation of sulfur and of polysulfide dianions at inert electrodes has been studied in aprotic solvents and in liquid ammonia. In the latter case, sulfur-nitrogen compounds are involved and these systems [90] will not be discussed here. 

Aprotic polar solvents such as those listed in Table 8.1 are widely used in electrochemistry. In solutions with such solvents the alkali metals are stable and will not dissolve under hydrogen evolution (by discharge of the proton donors) as they do in water or other protic solvents. These solvents hnd use in new types of electrochemical power sources (batteries), with hthium electrodes having high energy density. 

The corrosion resistance of lithium electrodes in contact with aprotic organic solvents is due to a particular protective film forming on the electrode surface when it first comes in contact witfi tfie solvent, preventing further interaction of the metal with the solvent. This film thus leads to a certain passivation of lithium, which, however, has the special feature of being efiective only while no current passes through the external circuit. The passive film does not prevent any of the current flow associated with the basic current-generating electrode reaction. The film contains insoluble lithium compounds (oxide, chloride) and products of solvent degradation. Its detailed chemical composition and physicochemical properties depend on the composition of the electrolyte solution and on the various impurity levels in this solution. 

A point meriting attention is the voltage difference above. Doped polymers are rather electropositive (up to more than 4 V vs. a lithium electrode in the same solution), so much so that charging may have to be limited in order not to exceed the stability limits of the electrolyte (typically, propylene carbonate or acetonitrile as aprotic nonaqueous solvents). 

Electrochemical noise studies have also been beneficial in lithium battery research. The lithium electrode sitting in the aprotic electrolyte is covered by a passivating film... 

This reduction step can be readily observed at a mercury electrode in an aprotic solvent or even in aqueous medium at an electrode covered with a suitable surfactant. However, in the absence of a surface-active substance, nitrobenzene is reduced in aqueous media in a four-electron wave, as the first step (Eq. 5.9.3) is followed by fast electrochemical and chemical reactions yielding phenylhydroxylamine. At even more negative potentials phenylhydroxylamine is further reduced to aniline. The same process occurs at lead and zinc electrodes, where phenylhydroxylamine can even be oxidized to yield nitrobenzene again. At electrodes such as platinum, nickel or iron, where chemisorption bonds can be formed with the products of the... 

The reduction of [Pt(bipy)2]2+ in water by metallic iron or at a platinum electrode gives monomeric [Pt(bipy)2]N03-2I I20 that forms as black-green needles. 25The needles have a relatively high electrical conductivity and a Pt - Pt separation of 3.563(1) A.125 In contrast, reduction in aprotic solvents such as DMF or DMSO results in the formation of dimeric [Pt2(bipy)2(/x-bipy)]21 (bipy = 2,2 -bipyridine) with a Pt- -Pt separation of 2.527,2(5) A.126... 

The reaction is complicated in aprotic media by polymerization of the olefin at the electrode 132> apparently because anions such as 134 or 136 can initiate anionic polymerization of the activated olefin. Steric hindrance about the double bond can retard polymerization yields of hydrodimer from 132 in di-methylformamide as a function of the size of R are R = hydrogen or methyl, 0%, R % n-propyl, 25% R % i-propyl, 65% R % /-butyl, 95%, 32). Saturation of the double bond to produce, e.g., 136 from 132, is a side reaction in neutral... 

The reduction of cycloheptatriene was studied in aprotic solvents at a platinum electrode. A reversible wave at —2.5 V for the production of the radical anion was observed in ammonia containing 0.1 M KI. Quasi-reversible or irreversible reduction was observed in acetonitrile and in A,A-dimethylformamide (equation 34)103. 

The role of nonaqueous solvents in adsorption processes can be exemplified by the adsorption of thiourea. A number of systematic studies ofTU adsorption on Hg electrodes fromprotic as well as aprotic solvents have been published. The results of TU adsorption from water, methanol,ethanol, ethylene glycol, acetone, and ni-... 

The situation is different with aprotic solvents. The strength of interaction between the solvent and the metal, particularly at the positively charged electrode, may be significant in the aprotic solvent. For example, in the case of TU adsorption in acetone solution, the weaker the interaction of the metal with the solvent, the greater the TU adsorption observed. In spite of some differentiation of the AG° values (Table 1) for different solvents, for every one, AC° is greater than -10 kJ/mol, indicating the enhanced contribution of a specific interaction of TU with the metal. Nevertheless, the data in Table 1 show that the AG value in water is always greater than in methanol for all metals studied. 

A special problem can be the passivation of the electrode surface by insulating layers, for example, formation of oxides on metals at a too high anodic potential or precipitation of polymers in aprotic solvents from olefinic or aromatic compounds by anodic oxidation. As a result, the effective surface and the activity of the... 

The most widely studied examples are cyclooctatetraene (COT, 1) and its derivatives. In such conventional aprotic solvents as DMF, dimethyl sulfoxid (DMSO), or acetonitrile containing tetraalkylammonium salts, two distinct one-electron reduction waves are observed at approximately —1.64 V and —1.80 V vs. saturated calomel electrode (SCE), with separations... 

Electrolytes are ubiquitous and indispensable in all electrochemical devices, and their basic function is independent of the much diversified chemistries and applications of these devices. In this sense, the role of electrolytes in electrolytic cells, capacitors, fuel cells, or batteries would remain the same to serve as the medium for the transfer of charges, which are in the form of ions, between a pair of electrodes. The vast majority of the electrolytes are electrolytic solution-types that consist of salts (also called electrolyte solutes ) dissolved in solvents, either water (aqueous) or organic molecules (nonaqueous), and are in a liquid state in the service-temperature range. [Although nonaqueous has been used overwhelmingly in the literature, aprotic would be a more precise term. Either anhydrous ammonia or ethanol qualifies as a nonaqueous solvent but is unstable with lithium because of the active protons. Nevertheless, this review will conform to the convention and use nonaqueous in place of aprotic .]... 

---