Examples of Electrode Materials

In the following section, examples of electrode materials for application as anode and/or as cathode, and then some electrode types of practical interest are discussed. A comprehensive overview 

---

An electrochemical cell consists of two electrodes that are electrically interconnected by an electrolyte. Each electrode consists of an electric conductor in which the electric current is transmitted by electrons. As examples of electrode material, metallic copper, Cu, and zinc, Zn, can be mentioned. The electrolyte connecting the electrodes contains mobile electrically charged ions that serve as carriers of electric current. The positively charged ions are named cations and the negatively charged ions are named anions. As examples of electrolytes, water, aqueous salt solutions, and melts of ionically bound substances can be mentioned. 

In the first of the four examples, the electrode material (metallic silver) is chemically involved in the electrode reaction hence it becomes more [less] as a function of time. Such electrodes are called reacting [or consumable] electrodes. 

In the other examples, the electrode materials are not involved in the reactions chemically, but constitute the source [sink] of electrons. Such electrodes are called nonconsumable. The term inert electrodes sometimes used is unfortunate insofar as the electrode itself is by no means inert rather, it has a strong catalytic effect on the electrode reaction. For reactions occurring at such electrodes, the terms oxidation- reduction... 

What can be learnt from XPS about electrochemical processes will be demonstrated and discussed in the main part of this chapter by means of specific examples. Thereby a survey of new XPS and UPS results on relevant electrode materials will be given. Those electrode materials, which have some potential for a technical application, are understood as practical and will be discussed with respect to the relevant electrochemical process. The choice of electrode materials discussed is of course limited. Emphasis will be put on those materials which are relevant for technical solid polymer electrolyte cells being developed in the author s laboratory. 

The objective of this chapter is to study some essential practical aspects, which have to be considered. First, as necessary background information, the different alternatives for electrochemical cell operation are discussed in general. Then follows an overview of properties of electrode materials, electrolyte components, and cell separators. Finally, examples of cell constructions are shown. 

Look up, for example, in Reference [1], the conductivities of metallic platinum, gold, silver, mercury and tungsten, and hence explain why graphite is not the best choice of electrode material. 

Clay is just one example of a material used to modify the electrochemical properties of electrodes to form a chemically modified electrode (CME) (as described belovt/). A porous-clay CME has an area of 5 cm, and charging the double-layer requires a charge of 1.43 C per square centimetre. Repeat the calculations shown above in Worked Example 5.3 to determine the respective faradaic efficiencies. 

The term modified electrodes encompasses a broad variety of electrode materials obtained by attaching a monomolecular layer of a specific compound on the surface of a conducting solid [338]. In electrocatalysis, modified electrodes are common in the field of oxygen reduction, where carbon materials can be modified for example by attaching layers of macrocycles. Modified electrodes are very common in the field of molecular or supramolecular electrochemistry, especially in the organic area. 

To form an electrode concentration cell the electrode material must have a variable concentration. Amalgam and gaseous electrodes frill into this classification. An example of electrode concentration cells is the one in which two amalgam electrodes of different concentrations dip into a solution containing the solute metal ions. 

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

Finally, the narrow emission lines can be attributed to excitation of electrode materials. In fact, such emission lines have been observed from A1 and Mg Ag cathodes of the PPV-derivatives-based SL LEDs when operated under strong electrical pulse excitation [472]. (see also Sec. 5.3). For example, the reabsorption-shifted characteristic A1 emission at 395 nm could explain the relatively narrow line at about 400 nm observed in the Nile Blue (NB)-based edge emitting LED provided with an A1 cathode as shown in Figs. 145 and 146. 

Only simple outer-sphere (25) redox reactions involving, for example, complex or aquo ions of transition or certain rare earth elements do not experience electrocatalysis, and their standard rate constants are independent of electrode material. This is because neither the oxidized nor the reduced species are chemisorbed at the electrode. However, practically, many redox systems do experience electrocatalysis on account of significant adsorption of their ions or through mediation of electron transfer by adsorbed anions, in which case the processes are no longer strictly of the outer-sphere type. 

Finally, the conversion of the primary metal into the product and the form which are actually utilized in the battery system should be considered. For example, the electrode materials in lead acid batteries are normally cast lead or lead-alloy grids. The materials utilized in NiCd batteries are cadmium oxide and high surface area nickel foams or meshes. Technically, all of these factors should be considered to produce a detailed life cycle analysis. However, again, these differences are generally quite small compared to the principal variables - composition, performance and spent battery disposal option. 

---