Solid metal electrodes

Expansion of the field to include electrode metals other than mercury is necessary in order to learn about the relation between charge transfer kinetics and (a) the nature of the metal and (b) the surface properties of the metal. 

For a long time, the possibility of obtaining valuable data about fast reactions has been hampered by the problem of how to prepare a pure and reproducible interface between a solid metal electrode and an electrolyte solution free of impurities. Without going into details, it can be said 

More than at mercury, it makes a difference whether the electrode is inert or not. In the first case, the electrode reaction is of the type Fe3+/ Fe2+ etc. and the modelling of processes is the same as with mercury. However, if the electrode reaction is of the type Zn2+/Zn, e.g. at a gold electrode, at least the electrode surface will be modified by the deposited zinc, Frequently, it is observed that the first monolayer of the foreign metal is deposited at a potential substantially positive to its standard potential. This phenomenon is named underpotential deposition and bears some resemblance to an electrode reaction that involves adsorption of the reacting species (see Sect. 6). 

In electrode reactions of the type H+/H2, 02/H20, and probably many organic redox systems, the electrode surface may be involved by virtue of the presence of adsorption sites where intermediates in the reaction mechanism, e.g. atomic hydrogen, are located. Generally, the reaction rate is higher at metals with a larger adsorptive capacity. This is a particular form of electrocatalysis, which is a subject of still-growing interest. 

When transient techniques are employed for fundamental research on these and other subjects, the effect of double-layer charging has to be accounted for in the analysis procedures. It has been observed frequently that at solid—solution interfaces, this process does not obey the capacitive behaviour predicted by double-layer theories. For example, the doublelayer admittance, Fc, cannot be represented by Yc = jciCd, but rather follows the relation [118] 

---

A new technique based on electrocapillary phenomena at partially immersed solid metal electrodes has been developed by Jin-Hua et al. 146,147 jhe involves the detection of the rise of a solution... 

Mercury electrodes require far less maintenance than solid metal electrodes. Especially for the dropping mercury electrode, a noticeable amount of impurities present in the solution at low concentrations (<10-5mol dm-3) cannot appreciably reach the surface of the electrode through diffusion during the drop-time (see Section 5.7.2). 

Solid metal electrodes with a crystalline structure are different. The crystal faces forming the surface of these electrodes are not ideal planes but always contain steps (Fig. 5.24). Although equilibrium thermal roughening corresponds to temperatures relatively close to the melting point, steps are a common phenomenon, even at room temperature. A kink half-crystal position—Fig. 5.24c) is formed at the point where one step ends and the... 

Solid metal electrodes are usually polished mechanically and are sometimes etched with nitric acid or aqua regia. Purification of platinum group metal electrodes is effectively achieved also by means of high-frequency plasma treatment. However, electrochemical preparation of the electrode immediately prior to the measurement is generally most effective. The simplest procedure is to polarize the electrode with a series of cyclic voltammetric pulses in the potential range from the formation of the oxide layer (or from the evolution of molecular oxygen) to the potential of hydrogen evolution (Fig. 5.18F). 

A typical adsorption process in electrocatalysis is chemisorption, characteristic primarily for solid metal electrodes. The chemisorbed substance is often chemically modified during the adsorption process. Then either the substance itself or some fragment of it is bonded chemically to the electrode. As electrodes mostly have physically heterogeneous surfaces (see Sections 4.3.3 and 5.5.5), the Temkin adsorption isotherm (Eq. 4.3.46) is suitable for characterizing the adsorption. 

The inhibition of electrode processes as a result of the adsorption of electroinactive surfactants has been studied in detail at catalytically inactive mercury electrodes. In contrast to solid metal electrodes where knowledge of the structure of the electrical double layer is small, it is often possible to determine whether the effect of adsorption on the electrode process at mercury electrodes is solely due to electrostatics (a change in potential 02)... 

Most earlier papers dealt with the mercury electrode because of its unique and convenient features, such as surface cleanness, smoothness, isotropic surface properties, and wide range of ideal polarizability. These properties are gener y uncharacteristic of solid metal electrodes, so the results of the sohd met electrolyte interface studies are not as explicit as they are for mercury and are often more controversial. This has been shown by Bockris and Jeng, who studied adsorption of 19 different organic compounds on polycrystaUine platinum electrodes in 0.0 IM HCl solution using a radiotracer method, eUipsometry, and Fourier Transform Infrared Spectroscopy. The authors have determined and discussed adsorption isotherms and the kinetics of adsorption of the studied compounds. Their results were later critically reviewed by Wieckowski. ... 

The shift of the potential of zero charge toward the negative direction induced by the contact adsorption of chloride ions has been found not only with liquid mercury electrodes but also with solid metal electrodes such as gold [Jiang-Seo-Sato, 1990]. 

To appreciate that the most common causes of contamination for solid metallic electrodes are coatings of oxide and adsorbed organic materials. 

This can be accomplished by means of two different processes (1) an electrodeposition process in which z electrons (e) are provided by an external power supply, and (2) an electroless (autocatalytic) deposition process in which a reducing agent in the solution is the electron source (no external power supply is involved). These two processes, electrodeposition and electroless deposition, constitute the electrochemical deposition. In this book we treat both of these processes. In either case our interest is in a metal electrode in contact with an aqueous ionic solution. Deposition reaction presented by Eq. (1.1) is a reaction of charged particles at the interface between a solid metal electrode and a liquid solution. The two types of charged particles, a metal ion and an electron, can cross the interface. 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

Mercury is not a typical electrode material it is liquid, and there is constant movement of atoms on the surface in contact with solution. A solid electrode has a well-defined structure, probably polycrystalline and in some cases monocrystalline. In a solid metallic electrode conduction is predominantly electronic owing to the free movement of valence electrons, the energy of the electrons that traverse the interface being that of the Fermi level, EF (Section 3.6), giving rise to effects from the electronic distribution of the atoms in the metallic lattice already mentioned. 

Much has been written about solid metal electrodes, which have now largely displaced liquid mercury. Those most often used as redox ( inert ) electrodes for studying electron transfer kinetics and mechanism, and determining thermodynamic parameters are platinum, gold, and silver. However, it should be remembered that their inertness is relative at certain values of applied potential bonds are formed between the metal and oxygen or hydrogen in aqueous and some non-aqueous solutions. Platinum also exhibits catalytic properties. 

Fuel cell researchers deal primarily with interfaces between solid electrolyte materials and solid metallic electrodes. The characterization of electrochemical systems with solid-solid interfaces has become a major issue in the study of fuel cells. It is generally believed that the interface of a solid electrode and solid electrolyte is similar to the electrode/liquid electrolyte interface but more complicated [4],... 

The most recent experimental work has involved studies of organic adsorption at the single crystal faces of polarizable solid metal electrodes [57]. These experiments provide details of the role of the metal in organic adsorption. By examining these data within the context of the new molecular descriptions of interfacial adsorption the theory of this important process will be greatly advanced. 

Writing such a chapter on solid metal electrodes is a challenge when, at every moment, the latest developments of surface science give rise to new possibilities and new results in this area of electrochemistry. However, an attempt will be made to give an up-to-date snapshot of the state of affairs for the double layer at singlecrystal electrodes of metals which are not of the transition series. 

It will be useful to emphasize the practical aspects of the problem which are twofold the solution side and the metal side. On the solution side at the interphase, a level of impurities which does not interfere with dl measurements over the time scale of a mercury-drop lifetime, which is 4 s, could completely hinder observations of significant current-potential curves [i( )] or meaningful differential capacity-potential curves [C(E)] at a solid metal electrode which will stay 2, 3, or 4 h in the same solution. Not only must the water, salts, and glassware be kept clean, but also the gas used to remove oxygen and the tubing for the gas. Of course, conditions are less drastic for studies of strong adsorption than in the case of no adsorption also bacteria develop less in acid solutions than in neutral ones (which cannot be kept uncontaminated more than one or two days). This aspect will not be discussed in this chapter. 

All this is clear as long as the charge is uniformly distributed on the surface of the solid metal electrode (as it is for mercury) and then the thermodynamic analysis can be reasonably carried out.t... 

The electrocapillary curve of a solid metal electrode is more difficult to measure than that of a liquid electrode, because of problems of surface cleanliness. The most widely used approach has been the bending-beam method, which was originally developed by Fredlein et al. [28] using large samples. More recently Raiteri and Butt [29] have used gold electrodes deposited on an AFM cantilever to record electrocapillary curves. 

Despite some rare exceptions, the material used as an electrode is not supposed to react with the solvent and the supporting electrolyte. This requirement is best satisfied by the noble metals, glassy carbon, and graphite. Solid metal electrodes are made primarily of platinum and gold. Mercury satisfies the above requirement only partly, but it is widely used because it is liquid and possesses a large overvoltage for hydrogen evolution. 

Rammelt U. and Reinhard G., On the applicability of a constant phase element of solid metal electrodes, Electrochim. Acta, 1990,35,1045-1049. 

---