Usefulness of Electrochemical Cells

As far as electrochemical cells relevant for applications or electrochemical measurements are concerned, we must distinguish between polarization cells, galvanic cells and open-circuit cells, depending on whether an outer current flows and, if so, in which direction this occurs. Table 1.1 provides examples of the purposes for which such cells may be used. In terms of application, we can distinguish between electrochemical sensors, electrochemical actors and galvanic elements such as batteries and fuel cells. These applications offer a major driving force for dealing with solid-state electrochemistry. 

Cell type Measurement technique Technological application 

Polarization cell Measurement of kinetic data by polarization Electrochemical composition actors (electrolyzers, pumps, electrochromic windows), electrochemical composition sensors (amperometric, conductometric) 

Current-generating Measurement of kinetic data Electrochemical energy storage and 

Open-circuit cell Measurement of thermodynamic formation data, transport number of electrons Potentiometric composition sensors 

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This chapter deals with the design and use of electrochemical cells for cases in which the dimensions that define the electrode area (e.g., diameter, length, width) are large relative to the dimensions of the diffusion layer. The design of electrochemical cells for use with microelectrodes (i.e., electrodes with dimensions on the order of micrometers) may require consideration of different characteristics and are considered in Chapter 12. 

Describe the principles that underlie the use of electrochemical cells as pH meters (Section 17.3, problems 39-40). 

Our interest in this chapter is with the mass and energy balances for chemical reactors, and in electrochemical cells. We consider first the mass and energy balances for tank and tubular reactors, and then for a general black-box chemical reactor, since these balance equations are an important application of the thermodynamic equations for reacting mixtures and the starting point for practical reactor design and analysis. Finally, we consider equilibrium and the energy balance for electrochemical systems such as batteries and fuel-cells, and the use of electrochemical cells for thermodynamic measurements. 

Electrochemistry, certainly one of the most important areas of applied thermodynamics, is the study of the relationship between chemical change and electrical work. It is typically investigated through the use of electrochemical cells, systems that incorporate a redox reaction to produce or utilize electrical energy. 

Although the detection limit for ESR spectroscopy per se is extremely low, the use of electrochemical cells filled with solvents that have high dielectric constants results in considerable losses in the cavity of the ESR spectrometer. This in turn increases the limit of detection. In the case of electrode reactions that have only very small stationary concentrations of radicalic intermediates, detection may be impossible. The use of spin traps may help. These compounds are rather simple organic molecules that react easily with radicals forming adducts (see Fig. 5.118). The molecular structure of the intermediate may be deduced from the known structure of the spin trap and the observed ECESR spectrum. Unfortunately, this technique doesn t necessarily trap the major reaction intermediate rather, it only traps those which react easily with the spin trap. Consequently, misinterpretations are possible. 

Table 21-1 shows the amounts of several elements produced during electrolysis by the passage of 1 faraday of electricity. The use of electrochemical cells to relate the amount of reactant or product to the amount of current passed is called coulometry. 

Another troublesome aspect of the reactivity ratios is the fact that they must be determined and reported as a pair. It would clearly simplify things if it were possible to specify one or two general parameters for each monomer which would correctly represent its contribution to all reactivity ratios. Combined with the analogous parameters for its comonomer, the values rj and t2 could then be evaluated. This situation parallels the standard potential of electrochemical cells which we are able to describe as the sum of potential contributions from each of the electrodes that comprise the cell. With x possible electrodes, there are x(x - l)/2 possible electrode combinations. If x = 50, there are 1225 possible cells, but these can be described by only 50 electrode potentials. A dramatic data reduction is accomplished by this device. Precisely the same proliferation of combinations exists for monomer combinations. It would simplify things if a method were available for data reduction such as that used in electrochemistry. 

Unfortunately, it is far simpler to define pH than to measure it, despite the commercial availability of instruments that purport to do this. Most instruments use an electrochemical cell such as... 

A battery is a series of electrochemical cells. Electrochemical cells are devices that, whenever in use, can continuously and directly convert chemical energy into electrical energy. 

Secondaiy batteries consist of a series of electrochemical cells. The most popular types are the lead-acid type used for starting, lighting, and electrical systems in motor vehicles and the small rechargeable batteries used in laptops, camcorders, digital phones, and portable electronic appliances. 

Electrochemical cells are familiar—a flashlight operates on current drawn from electrochemical cells called dry cells, and automobiles are started with the aid of a battery, a set of electrochemical cells in tandem. The last time you changed the dry cells in a flashlight because the old ones were dead, did you wonder what had happened inside those cells Why does electric current flow from a new dry cell but not from one that has been used many hours We shall see that this is an important question in chemistry. By studying the chemical reactions that occur in an electrochemical cell we discover a basis for predicting whether equilibrium in a chemical reaction fa-... 

The usefulness of Table 12-1 is clear. Qualitative predictions of reactions can be made with the aid of the ordered list of half-reactions. Think how the value of the list would be magnified if we had a quantitative measure of electron losing tendencies. The voltages of electrochemical cells furnish such a quantitative measure. 

The majority of electrochemical cells to have been constructed are based on PEO, PAN, or PVdF [101]. Recently, the Yuasa Corporation have commercialized solid polymer electrolyte batteries, primarily for use in devices such as smart cards, ID cards, etc. To date, the batteries which have been manufactured and marketed are primary lithium batteries based on a plasticized polymer electrolyte, but a similar secondary battery is expected [120]. 

The term electrolyte was first introduced in Section I to refer to the solute. In the discussion of electrochemical cells, the term is commonly used to refer to the ionically conducting medium. 

The ionic potentials can be experimentally determined either with the use of galvanic cells containing interfaces of the type in Scheme 7 or electroanalytically, using for instance, polarography, voltammetry, or chronopotentiometry. The values of and Aj f, obtained with the use of electrochemical methods for the water-1,2-dichloroethane, water-dichloromethane, water-acetophenone, water-methyl-isobutyl ketone, o-nitrotol-uene, and chloroform systems, and recently for 2-heptanone and 2-octanone [43] systems, have been published. These data are listed in many papers [1-10,14,37]. The most probable values for a few ions in water-nitrobenzene and water-1,2-dichloroethane systems are presented in Table 1. 

Having introduced matters pertaining to the electrochemical series earlier, it is only relevant that an appraisal is given on some of its applications. The coverage hereunder describes different examples which include aspects of spontaneity of a galvanic cell reaction, feasibility of different species for reaction, criterion of choice of electrodes to form galvanic cells, sacrificial protection, cementation, concentration and tempera lure effects on emf of electrochemical cells, clues on chemical reaction, caution notes on the use of electrochemical series, and finally determination of equilibrium constants and solubility products. 

Methods employing X-rays and y-radiation are used less often in electrochemistry. The possibility of using X-ray diffraction for in situ study of the electrode surface was first demonstrated in 1980. This technique has long been used widely as a method for the structural analysis of crystalline substances. Diffraction patterns that are characteristic for the electrochemical interface can be obtained by using special electrochemical cells and elec-... 

Coupling an electrochemical cell to an analytical device requires that hindering technical problems be overcome. In the last years there has been a considerable improvement in the combined use of electrochemical and analytical methods. So, for instance, it is now possible to analyze on-line electrode products during the simultaneous application of different potential or current programs. A great variety of techniques are based on the use of UH V for which the emersion of the electrode from the electrolytic solution is necessary. Other methods allow the in situ analysis of the electrode surface i.e the electrode reaction may take place almost undisturbed during surface examination. In the present contribution we shall confine ourselves to the application of some of those methods which have been shown to be very valuable for the study of organic electrode reactions. 

The use of electrochemical transmittance spectroscopy in both the UV-visible and IR regions of the spectrum is elegantly shown by the work of Ranjith et al. (1990) who employed an OTTLE cell to study the reduction of benzoquinone, BQ. The authors were the first to report the UV-visible spectrum of BQ2- and to demonstrate the quantitative aspects of the technique by reporting extinction coefficients for the major bands of BQT and BQ2- in both the UV-visible and IR. 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

Ionic conductors, used in electrochemical cells and batteries (Chapter 6), have high point defect populations. Slabs of solid ceramic electrolytes in fuel cells, for instance, often operate under conditions in which one side of the electrolyte is held in oxidizing conditions and the other side in reducing conditions. A signihcant change in the point defect population over the ceramic can be anticipated in these conditions, which may cause the electrolyte to bow or fracture. 

Some new developments are also proposed such as a system based on the use of electrochemically active bacteria in combination with a microbial fuel cell [34], giving good responses over 60 days, or a biosensor developed for fast... 

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