Half-Cells, Reversible and Reference Electrodes

When a metal is in contact with an electrolyte solution, a dc potential occurs which is the result of two processes. These are (1) the passage of metallic ions into solution from the metal, and (2) the recombination of metal ions in the solution with free electrons in the metal to form metal atoms. After a metal electrode is introduced into an electrolyte, equilibrium is eventually established and a constant electrode potential is established (for constant environmental conditions). At equilibrium, a dipole layer of charge (electrical double layer) exists at the metal-electrolyte interface. There is a surface layer of charge near the metal electrode and a layer of charge of opposite sign associated with the surrounding solution. Although diffuse, this dipole layer produces an effective electrical capacitance (Cp) which accounts for the low-frequency behavior of the electrode polarization impedance as discussed in Chapters 2, 3, and 4. 

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The electrochemical properties of the microma-chined electrode arrays have been determined for several of the electrode materials. In these half-cell experiments, the electrode array served as the working electrode and lithium as the counter and reference electrodes. The first experiments were carried out on carbon arrays composed of powders of Ketjen Black. Reversible intercalation and deintercalation of lithium were obtained, and reversible capacities in the range of 0.4—0.5 mA h cm were reported. 

If we choose a set of standard conditions (cf. Section 2.3) and one convenient half-cell to serve as a reference for all others, then a set of standard half-cell EMFs or standard electrode potentials E° (Appendix D)1-9 can be measured while drawing a negligible electrical current, that is, with the cell working reversibly so that the equations of reversible thermodynamics... 

The potential of an electrode measured relative to a standard, usually the SHE. It is a measure of the driving force of the electrode reaction and is temperature and activity dependent (p. 230). By convention, the half-cell reaction must be written as a reduction and the potential designated positive if the reduction proceeds spontaneously with respect to the SHE, otherwise it is negative. If the sign of the potential is reversed, it must be referred to as an oxidation potential. 

In order to satisfy the necessary criteria, a reversible redox couple is utilized in the reference electrode half-cell reaction. The potential of a reversible reference electrode is thermodynamically defined by its standard electrode potential, EP (see for example Compton and Sanders, 1996, for further discussion). Currently, the most commonly used reference electrode in voltammetric studies is the silver/silver chloride electrode (3), which has overtaken the calomel electrode (see for example Bott, 1995) for which the reaction is (4). 

For relative electrode potential data to be widely applicable and useful, we must have a generally agreed-upon reference half-cell against which all others are compared. Such an electrode must be easy to construct, reversible, and highly reproducible in its behavior. The standard hydrogen electrode (SHE) meets these specifications and has been used throughout the world for many years as a universal reference electrode. It is a typical gas electrode. 

Figure 10.6 shows the CV of a LiMn2O4 electrode on a cell with Li foil for both the reference and auxiliary electrodes in ethylene carbonate plus dimethyl carbonate solution of LiAsFg (1 M) (Sinha and Munichandraiah, 2008). The pair of peaks at larger potential corresponds to the deintercalation/intercalation of Li in the range 0 < X < 0.5 for Li Mn2O4, whereas the pair of peaks at lower potentials is attributable to this process for 0.5 < x < 1, both accompanied by reversible Mn(lV)/Mn(lll) redox reactions. Following Xia and Yoshio (1996), the later electrochemical process corresponds to the removal/addition of Li+ ions from/into half of the tetrahedral sites in which the lithium intercalation occurs. The former couple is then attributed to this process at the other tetrahedral sites where lithium intercalations do not occur. 

In order to simplify, the quantified concept half-cell potential or electrode potential has been introduced. This has been done by choosing a certain electrode reaction as reference, and defining the equilibrium potential of this electrode to be equal to zero. The numerical value of the equihbrium potential of any other electrode reaction X is given by the reversible cell voltage of the combination X-reference electrode, see Figure 3.7. 

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