Reacting Metal Electrodes

Electrochemical systems with reacting metal electrodes are widely used in batteries, electrometallurgy, electroplating, and other areas. Corrosion of metals is a typical example of processes occurring at reacting metal electrodes. 

At most metals that are in contact with an electrolyte containing their own ions, the equilibrium potential of the metal s discharge and ionization reaction, 

When the metal is in contact with an electrolyte solution not containing its ions, its equilibrium potential theoretically will be shifted strongly in the negative direction. However, before long a certain number of ions will accumulate close to the metal surface as a result of spontaneous dissolution of the metal. We may assume, provisionally, that the equilibrium potential of such an electrode corresponds to a concentration of ions of this metal of about 10 M. In the case of electrodes of the second kind, the solution is practically always saturated with metal ions, and their potential corresponds to the given anion concentration [an equation of the type (3.35)]. When required, a metal s equilibrium potential can be altered by addition of complexing agents to the solution (see Eq. (3.37)]. 

Some metals are thermodynamically unstable in aqueous solutions because their equilibrium potential is more negative than the potential of the reversible hydrogen electrode in the same solution. At such electrodes, anodic metal dissolution and cathodic hydrogen evolution can occur as coupled reactions, and their open-circuit potential (OCP) will be more positive than the equilibrium potential (see Section 13.7). 

Fundamentals of Electrochemistry, Second Edition, By V. S. Bagotsky Copyright 2006 John Wiley Sons, Inc. 

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Depending on electrolyte composition, the metal will either dissolve in the anodic reaction, that is, form solution ions [reaction (1.24)], or will form insoluble or poorly soluble salts or oxides precipitating as a new solid phase next to the electrode surface [reaction (1.28)]. Reacting metal electrodes forming soluble products are also known as electrodes of the first kind, and those forming solid products are known as electrodes of the second kind. 

The reactions occurring at reacting metal electrodes are associated with structural changes lattice destruction or formation of the metal and, in certain cases, of other solid reaction components (oxides, salts, etc.). One should know the metal s original bulk and surface structure in order to analyze the influence of these structural changes. 

Forced convection can be achieved in a variety of ways, e.g. by agitating the solution (stirring mechanically, sparging with gas bubbles, ultrasonic radiation, etc.) or by moving or rotating the metal electrode, and this will result in more rapid transport of the reacting species to the electrode than when the solution is unagitated. 

The reorganization energy of the slow polarization for the reactions at metal electrodes can be calculated with the use of Eqs. (34.11). For a spherical model of the reacting ion, it is equal approximately to... 

These ideas can be applied to electrochemical reactions, treating the electrode as one of the reacting partners. There is, however, an important difference electrodes are electronic conductors and do not posses discrete electronic levels but electronic bands. In particular, metal electrodes, to which we restrict our subsequent treatment, have a wide band of states near the Fermi level. Thus, a model Hamiltonian for electron transfer must contains terms for an electronic level on the reactant, a band of states on the metal, and interaction terms. It can be conveniently written in second quantized form, as was first proposed by one of the authors [Schmickler, 1986] ... 

Concerted Reduction of O and Cu+ or Acr+. Figure 5 illustrates the cyclic voltammograms for O2 in MeCN(0.1M TEAP) at glassy carbon, Cu, Ag, and Au electrodes (each polished immediately prior to exposure to O2). The drawn out reduction waves and the absence of significant anodic peaks upon scan reversal for the three metal electrodes indicate that 02 reacts with the surface prior to electron transfer. 

It follows that metal electrodes of the second kind not only serve as indicator electrodes for their own cations but also react to changes in metal ion activity resulting from precipitation or complexation reactions. For example, a Cl- ion electrode responds to the activity of the... 

The equilibrium situation can be achieved with the reacting electrons coming from the Fermi level Ep of a metal electrode in contact with the solution of the redox couple. The free energy change in the respective redox reaction is then zero. 

Data on the standard potentials for inorganic redox systems in aqueous solutions have been compiled by IUPAC [1], The standard potentials for some M"+/M and Mn+/M(Hg) couples are shown in Table 4.1 [2]. For alkali metals, the standard potentials of M+/M(Hg) are about IV more positive than those of M+/M. This is because alkali metals have strong affinities to mercury and are stable in the amalgams. It is impossible to measure the potentials of alkali metal electrodes directly in aqueous solutions, because alkali metals react with water. In order to determine the potential of an alkali metal electrode in an aqueous solution, we measure the potential of the corresponding amalgam electrode in an aqueous solution and then the difference between the potentials of alkali metal and alkali metal amalgam electrodes using an appropriate non-aqueous solution [2].2 ... 

Cathodic currents are stimulated by the illumination of metal electrodes [55-59]. These currents are often strongly enhanced by the presence in solution of known electron scavengers such as N20 and H30+. Research carried out by Barker [59— 61], Pleskov [62-64], Delahay [65], and their co-workers indicates rather strongly that the impinging photons eject electrons from the electrode surface. They appear to travel some distance ( 50 A) before becoming solvated [55,59,62,65]. If a scavenger is present, the solvated (usually aquated) electrons may react with it irreversibly. For example,... 

As COR and OER occur simultaneously in the cathode, their kinetics are particularly important in evaluating carbon-support corrosion. The kinetics of OER is material-specific, dependent on catalyst composition and electrode fabrication.35,37 -39 A number of OER kinetics studies were done on Pt metal electrodes.37-39 However, there is a lack of OER kinetics data on electrodes made of Pt nano-particles dispersed on carbon supports. Figure 2 shows the measured OER current density with respect to the overpotential defined by Eq. (6).35 The 02 concentration was measured at the exit of a 50-cm2 cell using a gas chromatograph (GC). The 02 evolution rate (= 02 concentration x cathode flow rate) was then converted to the OER current density, assuming 4e /02 molecule. Diluted H2 (10%) and a thicker membrane (50 p,m) were used in the measurement to minimize H2 crossover from anode to cathode, because H2 would react with 02 evolved at the cathode and incur inaccuracy in the measured OER current density. Figure 2 indicates that the OER... 

Mobile 0 states must also manifest themselves in electric conductivity and impedance measurements. The experimental difficulties encountered in such types of measurements are threefold. First, 0 states which diffuse to the surface generate a positive surface charge which wraps around the whole sample. When metal electrodes are put in contact with the surface and a potential is applied, the surface current will short-circuit the charge. Second, 0 states may chemically react with the the sample/electrode interface leading to polarization at the contact. Third, 0 states may react with each other at the surface or interface forming peroxy which decompose 0 + 0" - 022 => 02 + 1/2 02. This irreversibly removes 0 charge carriers from the system. 

It is well known that ACN reacts with active metals (Li, Ca) to form polymers [48], These polymers are products of condensation reactions in which ACIST radical anions are formed by the electron transfer from the active metal and attack, nucleophilically, more solvent molecules. Species such as CH3C=N(CH3)C=N are probably intermediates in this polymerization. ACN does not react on noble metal electrodes in the same way as with active metals. For instance, well-re-solved Li UPD peaks characterize the voltammograms of noble metal electrodes in ACN/Li salt solutions. This reflects a stability of the Li ad-layers that are formed at potentials above Li deposition potentials. Hence, the cathodic limit of noble metal electrodes in ACN solutions is the cation reduction process (either TAA or active metal cations). As discussed in the previous sections, with TAA-based solutions it is possible that the electrode surfaces remain bare. When the cations are metallic (e.g., Li+), it is expected that the electrode surfaces become covered with surface films originating from atmospheric contaminants reduction if the electrodes are polarized below 1.5 V (Li/Li+). As Mann found [13], in the presence of Na salts the polarization of metal electrodes in ACN solutions to sodium deposition potentials leads to solvent decomposition, with evolution of H2, CH4 and sodium cyanide (due to reaction with metallic sodium). 

A typical example is the electrochemistry of active metals, e.g., Li electrochemistry. Active metal electrodes react continuously, not only with water and protonic... 

Because of the possible complexity of the electrode process no general treatment is possible, but some of the main concepts may be illustrated in terms of a simple model in which we shall assume a definite kinetic path. Let us consider an electrolysis system that consists of an inert metal electrode in contact with oxidized and reduced forms of a dissolved ionic species which can react at the electrode according to the stoichiometric equation... 

Corresponding to this is the idea of biosensors that could be implanted in the body for the electroanalysis of conceivably any chemical in the body. Thus, it may become possible to adsorb enzymes on the surface of electrodes and then tune these enzymes to react with appropriate biomolecules, as represented in Fig. 4.114. How would conducting polymers figure in such devices They might be useful as the biosensor itself, since, being organic, they are more likely to interact positively with enzymes and biochemicals than metal electrodes would. 

Usually, the working electrode (W) is a porous metallic electrode in PEVD. Thus, reactant (B) in the vapor phase can reach the surface of the solid electrolyte for initial electrochemical reaction at a three-phase boundary of solid electrolyte (E), working electrode (W) and sink vapor phase (S) as shown in Eigure 3 (location II). All reactants for the sink side electrochemical reaction (1) or (2) are only available there. Subsequent reaction and deposition of the product (D) requires both electrons and ions to travel through product (D) to the surface to react with vapor phase reactant(s) electrochemically at location III in Eigure 3. 

Because radical ions are known to react through a number of possible reaction pathways, [59], photoelectrochemistry provides, in principle, a possible method for choosing among competitive routes. The formation of a cation radical by hole trapping by an organic donor on an irradiated semiconductor surface often gives rise to products different from those obtained on a poised metal electrode or those derived from the same cation radical when produced in homogeneous solution (Eq. 5) [60]. 

Metal electrodes of this kind require that neither the metal nor the metal cation reacts chemically with the solvent reactive metals may, however, be used as their amalgams. 

The Ag/Ag" " electrode is reversible [200] in many solvents. The main advantage of this and similar reference electrodes is that it allows working in a system containing a single solvent. Silver ions react with certain organic solvents, such as DMF, and other metal electrodes must then be used however, a silver cryptate electrode [Ag/0.005 M Ag Cryp(2.2)004] is stable in DMF [201]. 

Electrochemical reactions in a PEMFC occur at the Three-phase interface, where the three necessary components for the reaction, i.e., reactant (e.g., gas), electron conductor (e.g., metal electrode), and ionic conductor (e.g., solid polymer electrolyte), meet. Fig. 6 schematically illustrates this in the PEMFC situation. The three components can actually meet, not only on the line defining the three-phase contact, but also in an area of two-phase contact, where gas molecules dissolve and diffuse through the electrolyte onto the electrode surface. As the electrochemical reaction depends on the concentrations of reacting species at the interface, the diffusion and solubility of the reactants and products in the electrolyte are important parameters in determining the overall rate of the electrochemical reactions. In practice, the polymer electrolyte, which is dissolved in solution, is mixed with electrode... 

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