Electrochemical nonaqueous

Active metal electrodes are defined as electrodes which react spontaneously with the solution components at open circuit potential. With the exception of miscellaneous nonaqueous systems based on aromatic nonpolar solvents, described in Chapter 1 (e.g., Gileadi et al. [1,2]), most electrochemical nonaqueous systems are based on polar solvents. These contain C-halogen (Cl, Br), C—-O, C—N, C—S, or C—P bonds that can readily be reduced by alkali metal, and probably by alkaline earth metals as well. In addition, the commonly used salts comprise anions such as C104 , BF4 , AsF, PFeA SO3CF3A N(S02CF3)2 , C(S02CF3)3A which can also be attacked by alkali and alkaline earth metals. Thus, both classes of metals fit into the category of active electrode materials. 

New Synthesis. Many attempts have been made to synthesize oxaUc acid by electrochemical reduction of carbon dioxide in either aqueous or nonaqueous electrolytes (53—57). For instance, oxaUc acid is prepared from CO2 as its Zn salt in an undivided ceU with Zn anodes and stainless steel cathodes ia acetonitrile containing (C4H2)4NC104 and current efficiency of >90% (53). Micropilot experiments and a process design were also made. 

In the tradition of previous reviews [1-22], this section addresses various aspects of nonaqueous electrolytes, including intrinsic properties, such as local structures caused by ion-ion and ion-solvent interactions and bulk properties, such as ionic conductivity, viscosity, and electrochemical stability (voltage window), and their relationships to intrinsic properties. 

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... 

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

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 ... 

Most 2,5-unsubstituted pyrroles and thiophenes, and most anilines can be polymerized by electrochemical oxidation. For pyrroles, acetonitrile,54 or aqueous55 electrolyte solutions are normally used, while the polymerization of thiophenes is performed almost exclusively in nonaqueous solvents such as acetonitrile, propylene carbonate, and benzonitrile. 0 Polyanilines are generally prepared from a solution of aniline in aqueous acid.21 Platinum or carbon electrodes have been used in most work, although indium-tin oxide is routinely used for spectroelectrochemical experiments, and many other electrode materials have also been employed.20,21... 

Gal D, Hodes G (2000) Electrochemical deposition of ZnSe and (Zn,Cd)Se films from nonaqueous solutions. J Electrochem Soc 147 1825-1828... 

Colyer CL, Cocivera M (1992) Thin-film cadmium mercury teUuride prepared by nonaque-ous electrodeposition. J Electrochem Soc 139 406 09... 

Aruchamy A, Bmce JA, Tanaka S, Wrighton MS (1983) Characterization of the interface energetics for n-type cadmium selenide/nonaqueous electrolyte junctions. J Electrochem Soc 130 359-364... 

Electrochemical reduction of carbon dioxide is usually conducted in aqueous or nonaqueous electrolyte solutions at cathodes made of various materials. As a result, various organic substances can form. The most common reactions are as follows ... 

Another factor also contributed to the appearance of new concepts in electrochemistry in the second half of the twentieth century The development and broad apphca-tion of hthium batteries was a stimulus for numerous investigations of dilferent types of nonaqueous electrolytes (in particular, of sohd polymer electrolytes). These batteries also initiated investigations in the held of electrochemical intercalation processes. 

It has been pointed out that metals residing below the position held by manganese (and, therefore, much below hydrogen) in the electrochemical series (Table 6.11) cannot be electrodeposited from aqueous solutions of their salts. These metals are called base metals or reactive metals and can be electrodeposited only from nonaqueous electrolytes such as solutions in organic solvents and molten salts. As with an aqueous electrolyte, there is a minimum voltage which is required to bring about the electrolysis of a molten salt. 

Ito et al.40 examined the electrochemical reduction of C02 in dimethylsulfoxide (DMSO) with tetraalkylammonium salts at Pb, In, Zn, and Sn under high C02 pressures. At a Pb electrode, the main product was oxalic acid with additional products such as tartaric, malonic, glycolic, propionic, and n-butyric acids, while at In, Zn, and Sn electrodes, the yields of these products were very low (Table 3), and carbon monoxide was verified to be the main product even at a Pt electrode, CO was mainly produced in nonaqueous solvents such as acetonitrile and DMF.41 Also, the products in propylene carbonate42 were oxalic acid at Pb, CO at Sn and In, and substantial amounts of oxalic acid, glyoxylic acid, and CO at Zn, indicating again that the reduction products of C02 depend on the electrode materials used. 

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

Mann, C. and Barnes, K. (1970). Electrochemical Reactions in Nonaqueous Systems. Dekker, New York... 

Relatively little attention has been devoted to the direct electrodeposition of transition metal-aluminum alloys in spite of the fact that isothermal electrodeposition leads to coatings with very uniform composition and structure and that the deposition current gives a direct measure of the deposition rate. Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum is plated. Thus, it is necessary to employ nonaqueous solvents (both molecular and ionic) for this purpose. Among the solvents that have been used successfully to electrodeposit aluminum and its transition metal alloys are the chloroaluminate molten salts, which consist of inorganic or organic chloride salts combined with anhydrous aluminum chloride. An introduction to the chemical, electrochemical, and physical properties of the most commonly used chloroaluminate melts is given below. 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

Although the ECL phenomenon is associated with many compounds, only four major chemical systems have so far been used for analytical purposes [9, 10], i.e., (1) the ECL of polyaromatic hydrocarbons in aqueous and nonaqueous media (2) methods based on the luminol reaction in an alkaline solution where the luminol can be electrochemically produced in the presence of the other ingredients of the CL reaction (3) methods based on the ECL reactions of rutheni-um(II) tra(2,2 -bipyridinc) complex, which is used as an ECL label for other non-ECL compounds such as tertiary amines or for the quantitation of persulfates and oxalate (this is the most interesting type of chemical system of the four) and (4) systems based on analytical properties of cathodic luminescence at an oxide-coated aluminum electrode. 

One of the most significant applications of STM to electrochemistry would involve the application of the full spectroscopic and imaging powers of the STM for electrode surfaces in contact with electrolytes. Such operation should enable the electrochemist to access, for the first time, a host of analytical techniques in a relatively simple and straightforward manner. It seems reasonable to expect at this time that atomic resolution images, I-V spectra, and work function maps should all be obtainable in aqueous and nonaqueous electrochemical environments. Moreover, the evolution of such information as a function of time will yield new knowledge about key electrochemical processes. The current state of STM applications to electrochemistry is discussed below. 

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