Electrochemical cells continued galvanic

Chapter 1 by Joachim Maier continues the solid state electrochemistry discussion that he began in Volume 39 of the Modem Aspects of Electrochemistry. He begins by introducing the reader to the major electrochemical parameters needed for the treatment of electrochemical cells. In section 2 he discusses various sensors electrochemical (composition), bulk conductivity, surface conductivity, galvanic. He also discusses electrochemical energy storage and conversion devices such as fuel cells. 

Hence, two kinds of electrical current exist in an electrochemical cell the electronic current and the ionic current. According to Kirchhoffs law, no current can accumulate at any point of the circuit. As a result, the question arises concerning the nature of the phenomenon permitting the continuity of the current. It is clear that the junction between the two kinds of current cannot be located anywhere else than at the electrode-solution interface since each member of the interface possesses its own kind of current. The answer to the question is that the current s continuity is achieved by the reactions taking place at the interfaces, that is, by the electrochemical reactions. Indeed, electron transfer between two phases is the fundamental act of electrochemistry. In the case of Daniell s galvanic cell, reactions (13.2) and (13.3) take place. During the course of reaction (13.2) ... 

If the mixture of all reactants and products is in a same reactor, the above chemical reaction will proceed from left to right with a driving force equivalent to the free energy change of AG,. The reaction will continue until AG, = 0. If a galvanic cell is constructed as shown below, the same reaction will occur, but in an electrochemical manner. 

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. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

The main attractiveness of fuel cells follows from the definitions given above. It comprises the high theoretical efficiency associated with direct conversion of chemical energy into electrical energy by means of galvanic cells [10] the selectivity of the electrochemical process and the advantage of a continuous metabolism by using the ambient air to oxidize the steadily supplied fuel. 

In the literature on electrochemical power sources, semi-fuel cells are generally regarded as a variety of ordinary batteries (galvanic cells or storage batteries), rather than as a variety of fuel cells (which have the distinguishing feature of a continuous supply of all reactants). In this section, brief information was given on these half-fuel cells to show the connections between their development and the development of real fuel cells. 

Materials of construction for the zinc/mercuric oxide cells are limited not only by their ability to survive continuous contact with strong caustic alkali, but also by their electrochemical compatibility with the electrode materials. As far as the external contacts are concerned, these are decided by corrosion resistance, compatibility with the equipment interface with respect to galvanic corrosion, and, to some degree, cosmetic appearance. Metal parts may be homogeneous, plated metal, or clad metal. Insulating parts may be injection-, compression-, or transfer-molded polymers or rubbers. 

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