Metal/electrolyte

The equations of electrocapillarity become complicated in the case of the solid metal-electrolyte interface. The problem is that the work spent in a differential stretching of the interface is not equal to that in forming an infinitesimal amount of new surface, if the surface is under elastic strain. Couchman and co-workers [142, 143] and Mobliner and Beck [144] have, among others, discussed the thermodynamics of the situation, including some of the problems of terminology. 

Kolb D M 1996 Reconstruction phenomena at metal-electrolyte interfaces Prog. Surf. Sc/. 51 109... 

Kolb D M and Franke C 1982 Surface states at the metal-electrolyte interface Appl. Phys A 49 379-87... 

Gordon J G, Melroy O R and Toney M F 1995 Structure of metal-electrolyte interfaces copper on gold(111), water on silver(111) Electrochim. Acta 40 3-8... 

In some cases, particularly with iaactive metals, electrolytic cells are the primary method of manufacture of the fluoroborate solution. The manufacture of Sn, Pb, Cu, and Ni fluoroborates by electrolytic dissolution (87,88) is patented. A typical cell for continous production consists of a polyethylene-lined tank with tin anodes at the bottom and a mercury pool (ia a porous basket) cathode near the top (88). Pluoroboric acid is added to the cell and electrolysis is begun. As tin fluoroborate is generated, differences ia specific gravity cause the product to layer at the bottom of the cell. When the desired concentration is reached ia this layer, the heavy solution is drawn from the bottom and fresh HBP is added to the top of the cell continuously. The direct reaction of tin with HBP is slow but can be accelerated by passiag air or oxygen through the solution (89). The stannic fluoroborate is reduced by reaction with mossy tin under an iaert atmosphere. In earlier procedures, HBP reacted with hydrated stannous oxide. 

This handbook deals only with systems involving metallic materials and electrolytes. Both partners to the reaction are conductors. In corrosion reactions a partial electrochemical step occurs that is influenced by electrical variables. These include the electric current I flowing through the metal/electrolyte phase boundary, and the potential difference A( = 0, - arising at the interface. and represent the electric potentials of the partners to the reaction immediately at the interface. The potential difference A0 is not directly measurable. Therefore, instead the voltage U of the cell Me /metal/electrolyte/reference electrode/Me is measured as the conventional electrode potential of the metal. The connection to the voltmeter is made of the same conductor metal Me. The potential difference - 0 is negligibly small then since A0g = 0b - 0ei ... 

Polarization can be divided into activation polarization and concentration polarization , Activation polarization is an electrochemical reaction that is controlled by the reaction occurring on the metal-electrolyte interface. Figure 4-418 illustrates the concept of activation polarization where hydrogen is being reduced over a zinc surface. Hydrogen ions are adsorbed on the metal surface they pick up electrons from the metal and are reduced to atoms. The atoms combine to... 

The thermodynamic and electrode-kinetic principles of cathodic protection have been discussed at some length in Section 10.1. It has been shown that, if electrons are supplied to the metal/electrolyte solution interface, the rate of the cathodic reaction is increased whilst the rate of the anodic reaction is decreased. Thus, corrosion is reduced. Concomitantly, the electrode potential of the metal becomes more negative. Corrosion may be prevented entirely if the rate of electron supply is such that the potential of the metal is lowered to the value where it is found that anodic dissolution does not occur. This may not necessarily be the potential at which dissolution is thermodynamically impossible. 

Current enters through the metal-electrolyte interface of the anode, which is usually made from the same metal as is plated on the cathode. The anode dissolves replacing the metal lost at the cathode ... 

The shortcomings of such a code are mainly attributable to the limited number of environments considered because specific behaviour is generally related to specific metal/electrolyte behaviour which can result in substantial potential changes or even polarity reversals. A polarity reversal implies a large potential at one or both metal electrodes and may be attributed to two main factors ... 

In this scheme, a single vertical line represents a metal-electrolyte boundary at which a potential difference is taken into account the double vertical broken lines represent a liquid junction at which the potential is to be disregarded or is considered to be eliminated by a salt bridge. 

The alkali-metal/electrolyte interphase was named LI] the solid-electrolyte interphase (SEI). A good SEI must have the following properties ... 

The model more generally accepted for metal/electrolyte interfaces envisages the electrical double layer as split into two parts the inner layer and the diffuse layer, which can be represented by two capacitances in series.1,3-7,10,15,32 Thus, the total differential capacitance C is equal to... 

Barker et al.m have developed a photoemission method to obtain metal/electrolyte interfaces. Later, the method was applied by Brodsky etal.m 205 to Pb, Bi, Hg, Cd, and In good agreement (A ff=0 = 0.02 V) with impedance data10 was found. 

Figure 16 shows the effect of the potential of passivated electrode and the interfacial tension of film-free metal/electrolyte interface on the activation barrier for film breakdown. From Eq. (22), the minimum potential for film breakdown AE corresponding to A b = is given by... 

This potential depends on the interfacial tension am of a passivated metal/electrolyte interface shifting to the lower potential side with decreasing am. The lowest film breakdown potential AEj depends on the surface tension of the breakdown site at which the film-free metal surface comes into contact with the electrolyte. A decrease in the surface tension from am = 0.41 J m"2 to nonmetallic inclusions on the metal surface, will cause a shift of the lowest breakdown potential by about 0.3 V in the less noble direction. 

Figure 16. Activation barrier A for the formation of a breakthrough pore in a thin surface oxide film on metal as a function of electrode potential at two different surface tensions, om, of the metal/electrolyte interface.7The solid lines indicate the values of A b against Aand the dotted lines correspond to die critical potentials for the pore formation. ACd= 1 F m-2, a = 0.01 J m-2, h = 2 x 10-9 m, a, am = 0.41 J m 2 b, am 0.21 J m 2 (From N. Sato, J. Electmchem. Soc. 129, 255, 1982, Fig. 3. Reproduced by permission of The Electrochemical Society, Inc.)...

Although the term non-Faradaic process has been used for many decades to describe transient electrochemical processes where part of the current is lost in charging-discharging of metal-electrolyte interfaces, in all these cases the Faradaic efficiency, A, is less than 1 (100%). Furthermore such non-Faradaic processes disappear at steady state. Electrochemical promotion (NEMCA) must be very clearly distinguished from such transient non-Faradaic processes for two reasons ... 

Figure 1.5. Schematic representation of a metal electrode deposited on a 02 -conducting (left) and on a Na -conducting (right) solid electrolyte, showing the location of the metal-electrolyte double layer and of the effective double layer created at the metal/gas interface due to potential-controlled ion migration (backspillover).

The non-zero value of e Fw-e FR in Eq. (5.35) implies that there are net surface charges on the gas exposed electrode surfaces. These charges (q+,q.) have to be opposite and equal as the cell is overall electrically neutral and all other charges are located at the metal-solid electrolyte interfaces to maintain their electroneutrality. The charges q+ = -q. are quite small in relation to the charges, Q, stored at the metal-electrolyte interface but nevertheless the... 

In aqueous electrochemistry electrochemical (charge transfer) reactions take place over the entire metal/electrolyte interface. 

In solid electrochemistry electrochemical (change transfer) reactions take place primarily at the three-phase-boundaries (tpb) metal-electrolyte-gas, e.g. ... 

S. Trasatti, Structure ofthe metal/electrolyte solution interface New data for theory, Electrochim. Acta 36, 1659-1667 (1991). 

Does the concept of absolute electrode potential, defined in chapter 7, allow one to measure the absolute electrical potential difference, A(p, at a metal/electrolyte interface, one of the famous unresolved problems in electrochemistry ... 

Figure 1. The electric potentials assumed to exist in a metal-electrolyte solution system.

Non-situ and ex situ studies can provide important information for understanding the properties of metal/electrolyte interfaces. The applicability of these methods for fundamental studies of electrochemistry seems to be firmly established. The main differences between common electrochemical and UHV experiments are the temperature gap (ca. 300 vs. 150 K) and the difference in electrolyte concentration (very high concentrations in UHV experiments). In this respect, experimental research on double-layer properties in frozen electrolytes can be treated as a link between in situ experiments. The measurements of the work functions... 

---