Galvanic cells applications

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

Fig. 5.24. The electrochemical properties of the galvanic cell shown have been studied under high pressure shock compression. The cell is composed of anode, electrolyte, and cathode materials studied in independent applications of thermal batteries.

Although iron pipes suffer from the same corrosion risk as steel pipelines, associated with the generation of a galvanic cell with a small anode and a large cathode, the risk is mitigated for iron pipelines because the electrical continuity is broken at every pipe joint. For this reason long-line currents are uncommon in iron lines and cathodic protection is rarely necessary. It also accounts for the ability to protect iron lines by the application of nonadherent polyethylene sleeving . 

An interesting application of electrode potentials is to the calculation of the e.m.f. of a voltaic cell. One of the simplest of galvanic cells is the Daniell cell. It consists of a rod of zinc dipping into zinc sulphate solution and a strip of copper in copper sulphate solution the two solutions are generally separated by placing one inside a porous pot and the other in the surrounding vessel. The cell may be represented as ... 

As in aqueous electrochemistry it appears that application of a potential between the two terminal (Au) electrodes leads to charge separation on the Pt film so that half of it is charged positively and half negatively8 thus establishing two individual galvanic cells. The Pt film becomes a bipolar electrode and half of it is polarized anodically while the other half is polarized cathodically. The fact that p is smaller (roughly half) than that obtained upon anodic polarization in a classical electrochemical promotion experiment can be then easily explained. 

Lithium metal had few uses until after World War II, when thermonuclear weapons were developed (see Section 17.11). This application has had an effect on the molar mass of lithium. Because only lithium-6 could be used in these weapons, the proportion of lithium-7 and, as a result, the molar mass of commercially available lithium has increased. A growing application of lithium is in the rechargeable lithium-ion battery. Because lithium has the most negative standard potential of all the elements, it can produce a high potential when used in a galvanic cell. Furthermore, because lithium has such a low density, lithium-ion batteries are light. 

Another proposed procedure of finding the ionic data is the application of a special salt bridge, which provides practically constant or negligible liquid junction potentials. The water-nitrobenzene system, containing tetraethylammonium picrate (TEAPi) in the partition equilibrium state, has been proposed as a convenient liquid junction bridge for the liquid voltaic and galvanic cells. 

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. 

Internal electrolysis is the term applied by Sand1,2 to an electrogravimetric analysis proceeding spontaneously without the application of an external voltage, i.e., by the short-circuited galvanic cell. 

In this section, you learned about electrolytic cells, which convert electrical energy into chemical energy. You compared the spontaneous reactions in galvanic cells, which have positive cell potentials, with the non-spontaneous reactions in electrolytic cells, which have negative cell potentials. You then considered cells that act as both galvanic cells and electrolytic cells in some common rechargeable batteries. These batteries are an important application of electrochemistry. In the next two sections, you will learn about many more electrochemical applications. 

Tower, Stephen. All About Electrochemistry. Available online. URL http //www.cheml.com/acad/webtext/elchem/. Accessed May 28, 2009. Part of a virtual chemistry textbook, this excellent resource explains the basics of electrochemistry, which is important in understanding how fuel cells work. Discussions include galvanic cells and electrodes, cell potentials and thermodynamics, the Nernst equation and its applications, batteries and fuel cells, electrochemical corrosion, and electrolytic cells and electrolysis. 

Unfortunately, -alumina cannot be used as a fuel cell electrolyte since its oxygen ions are immobile and so any oxygen electrodes in the corresponding galvanic cells will not be reversible. However, under open circuit conditions, /1-alumina has been successfully used as an oxygen potential sensor. In this mode of application, the oxygen electrode equilibrates with the Na+ ions of the electrolyte as follows... 

In this section, we describe time-resolved, local in-situ measurements of chemical potentials /, ( , f) with solid galvanic cells. It seems as if the possibilities of this method have not yet been fully exploited. We note that the spatial resolution of the determination of composition is by far better than that of the chemical potential. The high spatial resolution is achieved by electron microbeam analysis, analytical transmission electron microscopy, and tunneling electron microscopy. Little progress, however, has been made in improving the spatial resolution of the determination of chemical potentials. The conventional application of solid galvanic cells in kinetics is completely analogous to the time-dependent (partial) pressure determination as explained in Section 16.2.2. Spatially resolved measurements are not possible in this way. 

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

Galvanic cells with solid Zr02 (+CaO) electrolyte are used as the oxygen sensor for high temperature applications. The oxygen sensor enables determination of the activity of a metal element in an alloy. Consider the following cell ... 

Use of the potential of a galvanic cell to measure the concentration of an electroactive species developed later than a number of other electrochemical methods. In part this was because a rational relation between the electrode potential and the concentration of an electroactive species required the development of thermodynamics, and in particular its application to electrochemical phenomena. The work of J. Willard Gibbs1 in the 1870s provided the foundation for the Nemst equation.2 The latter provides a quantitative relationship between potential and the ratio of concentrations for a redox couple [ox l[red ), and is the basis for potentiometry and potentiometric titrations.3 The utility of potentiometric measurements for the characterization of ionic solutions was established with the invention of the glass electrode in 1909 for a selective potentiometric response to hydronium ion concentrations.4 Another milestone in the development of potentiometric measurements was the introduction of the hydrogen electrode for the measurement of hydronium ion concentrations 5 one of many important contributions by Professor Joel Hildebrand. Subsequent development of special glass formulations has made possible electrodes that are selective to different monovalent cations.6"8 The idea is so attractive that intense effort has led to the development of electrodes that are selective for many cations and anions, as well as several gas- and bioselective electrodes.9 The use of these electrodes and the potentiometric measurement of pH continue to be among the most important applications of electrochemistry. 

One of the oldest and most important applications of electrochemistry is to the storage and conversion of energy. You already know that a galvanic cell converts chemical energy to work similarly, an electrolytic cell converts electrical work into chemical free energy. Devices that carry these conversions out on a practical scale are called batteries1. In ordinary batteries the chemical components are contained within the device itself. If the reac-tantsare supplied from an external source as they are consumed, the device is called a fuel cell. 

Batteries are a practical application of the galvanic cell in that an oxidation-reduction reaction generates an electric current. A battery that has an enormous impact on our lives is the automobile battery, shown in Figure 10.3. 

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