Potentials and Thermodynamics of Electrochemical Cells

In this section we lirsi consider the thermodynamics of electrochemical cells and the relationship between the activities of ihc participants in typical cell reactions and the observed potential of the cell. We then describe the source of the junction potentials thal occur in most electrochemical cells. In Section 22C we consider the ptitentials of individual electrodes making up cells. 

Determining the cell potential requites knowledge of the thermodynamic and transport properties of the system. The analysis of the thermodynamics of electrochemical systems is analogous to that of neutral systems. Eor ionic species, however, the electrochemical potential replaces the chemical potential (1). 

Fig. 3.1 Placement of reactant energies relative to the edges of the electrolyte conduction and valence bands in a thermodynamically stable electrochemical cell at flat-band potential.

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. 

Thermodynamics treats electrochemical cells in equilibrium and indeed such hoary material, going back to the work of the great German physical chemist Nernst, is apart of classical electrochemistry that is still being taught in universities to students as if it were representative of modern electrochemistry Consider then the chemical potential of a metal as in the solid electrode. 

The differences in the hydration of a solnte in H2O and D2O have been extensively stndied by measnring their thermodynamic properties, the change of free energy (AG°t), enthalpy (A//°t), and entropy (AY°t) at the transfer of 1 mol of solnte from a highly dilute solution in H2O to the same concentration in D2O under reversible conditions (mostly 25 °C and atmospheric pressure). Greyson measured the electromotive force (emf) of electrochemical cells of several alkali halides containing heavy and normal water solutions. The cell potentials had been combined with available heat of solution data to determine the entropy of transfer of the salts between the isotopic solvents. The thermodynamic properties for the transfer from H2O to D2O and the solubilities of alkali halides at 25° in H2O and D2O are shown in Table 4. 

It is necessary to distinguish between the concept of a potential and the measurement of a potential. Redox or electrode potentials (quoted in tables in Stability Constants of Metal-Ion Complexes or by Bard et al., 1985) have been derived from equilibrium data, thermal data, and the chemical behavior of a redox couple with respect to known oxidizing and reducing agents, and from direct measurements of electrochemical cells. Hence there is no a priori reason to identify the thermodynamic redox potentials with measurable electrode potentials. 

Thermodynamics is used in the analysis of electrochemical cells (1) to predict which electrode reactions occur spontaneously in the anodic and cathodic directions if the two electrodes are in equilibrium with their respective adjacent solutions and are connected to one another via an external wire, and (2) to quantify chemical potentials and activity coefficients in nonideal electrolytic solutions. 

Bale] and coworkers published a number of papers on the thermodynamics and the reversible potential of sodium amalgam in contact with Na" " [18-27]. They measured the reversible potential of electrochemical cells consisting of... 

Generally, the available cell voltage of electrochemical cells depends on the thermodynamics of the two electrode reactions in the prevailing electrolyte, hence the difference in the electrode potentials, and is confined, according to the series of electrochemical potentials, to a few volts [9]. According to the individual electrode potentials of the reaction (by lUPAC standard zero volt in the series of... 

The purpose of this book is to present the field of electrodics to the advanced student or the qualified chemist. It is assumed that the reader is conversant with thermodynamics, basic kinetics and the basic concepts of conductance. No familiarity with electrochemistry is assumed. The book starts with very basic concepts of electrochemical cells, of potential and of kinetics, and develops these according to the needs of the study of electrode processes. 

Electrochemical cells can be constructed using an almost limitless combination of electrodes and solutions, and each combination generates a specific potential. Keeping track of the electrical potentials of all cells under all possible situations would be extremely tedious without a set of standard reference conditions. By definition, the standard electrical potential is the potential developed by a cell In which all chemical species are present under standard thermodynamic conditions. Recall that standard conditions for thermodynamic properties include concentrations of 1 M for solutes in solution and pressures of 1 bar for gases. Chemists use the same standard conditions for electrochemical properties. As in thermodynamics, standard conditions are designated with a superscript °. A standard electrical potential is designated E °. 

In practice the situation is less favorable due to losses associated with overpotentials in the cell and the resistance of the membrane. Overpotential is an electrochemical term that, basically, can be seen as the usual potential energy barriers for the various steps of the reactions. Therefore, the practical efficiency of a fuel cell is around 40-60 %. For comparison, the Carnot efficiency of a modern turbine used to generate electricity is of order of 50 %. It is important to realize, though, that the efficiency of Carnot engines is in practice limited by thermodynamics, while that of fuel cells is largely set by material properties, which may be improved. 

The EMF values of galvanic cells and the electrode potentials are usually determined isothermally, when all parts of the cell, particularly the two electrode-electrolyte interfaces, are at the same temperature. The EMF values will change when this temperature is varied. According to the well-known thermodynamic Gibbs-Helmholtz equation, which for electrochemical systems can be written as... 

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