Electrode kinetics application

A. A. Pilla [1970] Transient Impedance Technique for the Smdy of Electrode Kinetics Application to Potentiostatic Methods, J. Electrochem. Soc. 117,... 

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

As with previous kinetic applications of SECM, it should be noted that experimental measurements can be tuned to the kinetic region of interest by varying the radius of the electrode [Eq. (33)] and the separation between the tip and interface. In essence, the smaller the UME, and/or tip-interface separation, the higher the diffusion rates that may be generated and, consequently, the greater the tendency for interfacial kinetic limitations. 

Oxygen reduction can be accelerated by an application of electrodes with high surface area, e.g. the porous electrodes [9, 13]. The porous electrodes usually consist of catalysts, hydrophobic agent (polytetrafluoroethylene-PTFE) and conductive additive. Electrode kinetics on the porous electrodes is complicated by the mass and charge transfer in the pores and is called the macrokinetics of electrode processes . 

The most well known work that Conway and his colleagues completed in Ottawa was on the analysis of potential sweep curves. I had been critical of the application of potential sweep theory to reactions which involved intermediates on the electrode surface and, working particularly early with Gilaedi and then with Halina Kozlowska, and to some extent with Paul Stonehart, Conway developed an analysis of the effect of intermediate radicals on the shape and properties of potential sweep showing how interesting electrode kinetic parameters could be thereby obtained. 

The Alkaline Fuel Cell (AFC) was one of the first modern fuel cells to be developed, beginning in 1960. The application at that time was to provide on-board electric power for the Apollo space vehicle. Desirable attributes of the AFC include its excellent performance on hydrogen (H2) and oxygen (O2) compared to other candidate fuel cells due to its active O2 electrode kinetics and its flexibility to use a wide range of electrocatalysts, an attribute which provides development flexibility. 

Irreversible reaction, 1251, 1419 Isoconic, definition, 933, 978, 982 Isotherm, 932, 964, 1197 applicability, 941 and charge transfer, 954, 955 Conway and Angersein-Kozlowska, 943 definition, 933 in electrode kinetics, 1197 Flory—Huggins type, 941,942, 944, 965 Frumkin, 938, 942, 965 Frumkin-Temkin, 1197, 1198 Habib-Bockris, 943... 

Laminar flow. 1226, 1227 Landau, 1499, 1503 Lange and Miscenko, 823, 1059 Langmuir isotherm. 936. 937. 938. 942. 965. 1196 applicability at high coverages, 1197 in electrode kinetics, 1200 Langmuir equation, electrochemical version of. 1191... 

The great importance of the Tafel relation—because it is too widely observed to be applicable in electrode kinetics—does not seem to have been appreciated during the time (about 1960-1980) in which Gaussian concepts were frequently used to present a quantal approach to electrode kinetics. Supporting a theoretical view that does not yield what is in effect the first law of electrode kinetics is similar to supporting a theory of gas reactions that does not lead to the exponential dependence of rate on temperature. It represents a remarkable historical aberration in the field. Thus the... 

S. U. M. Khan, P. Wright, and J. O M. Bockris, Elektrokhimya 13 914 (1977). The first application of time-dependent perturbation theory to quantum electrode kinetics redox reactions. 

It is probably the complexity of these theories that prohibited this particular aspect of electrode kinetics from being attractive for application in the study of homogeneous reaction kinetics per se. Yet it must be clear that the electrochemical techniques, together providing an extremely wide range of time scales, should be preeminently suited for investigations of both slow and (very) fast homogeneous reactions. This is the more true since, nowadays, the problem of the non-availability of a closed-form expression for the response—perturbation or response—time relation has been overcome by numerical analysis procedures conducted with the aid of computers. 

The goal of this chapter is to describe the application of hydrodynamic electrodes to the study of electrode kinetics and the kinetics of electrode and coupled homogeneous reactions. In order to do this, it is important to describe first the mass transport and how to fulfil experimentally the conditions described by the mass transport equations, i.e. electrode construction and operation. 

Three-electrode control systems are widely available in the market and there are also four-electrode systems for double working electrodes. The construction is either integral or modular. It is perfectly possible to construct the necessary electronics in-house and, in this case, modular construction is suggested as being more flexible. Operational amplifiers and other components of high quality should be used, particularly for kinetic applications. The elements of a bipotentiostat (independent control of two working electrodes) and a galvanostat are described in ref. 139. 

Application of hydrodynamic electrodes to electrode kinetics 4.1 INTRODUCTION... 

For an experimental system in which the initial concentration profiles are flat, the unperturbed current is zero and the electrode potential is constant. Any changes in current or potential are due to the new perturbation. This situation is only encountered when one controls the surface concentrations by adjusting the bulk concentrations of the members of the electrochemical couple. This method is employed in faradaic impedance studies of electrode kinetics (detailed below) but is not generally applicable (particularly for analytical purposes). 

In the previous edition of this book, Dryhurst and McAllister described carbon electrodes in common use at the time, with particular emphasis on fabrication and potential limits [1]. There have been two extensive reviews since the previous edition, one emphasizing electrode kinetics at carbon [2] and one on more general physical and electrochemical properties [3]. In addition to greater popularity of carbon as an electrode, the major developments since 1984 have been an improved understanding of surface properties and structure, and extensive efforts on chemical modification. In the context of electroanalytical applications, the current chapter stresses the relationship between surface structure and reproducibility, plus the variety of carbon materials and pretreatments. Since the intent of the chapter is to guide the reader in using commonly available materials and procedures, many interesting but less common approaches from the literature are not addressed. A particularly active area that is not discussed is the wide variety of carbon electrodes with chemically modified surfaces. 

The primary objective of the discussion that follows is to establish a basis for choosing and applying carbon electrodes for analytical applications. As with any electrode material or electroanalytical technique, the choice depends on the application there is no ideal electrode for all situations. We first discuss the criteria that drive the chemist s choice of electrode or procedure. These criteria include background current, potential limits, and electrode kinetics, and may be considered dependent variables that are ultimately controlled by the properties of the carbon surface. Then we consider the independent variables that determine electroanalytical behavior. These include the choice of carbon material, surface roughness, cleanliness, etc. By considering the dependence of electroanalytical behavior on surface variables that the user can control, it should be possible to make rational choices of electrodes and procedures to lead to the desired analytical objective. 

Carbon electrodes exhibit a wide range of electron transfer rates for benchmark redox systems, depending on carbon material and surface history. Two examples are shown in Figure 10.2, which compares two carbon surfaces with very different k° for Fe(CN) /4. In some cases, the variations in electrode kinetics have been particularly important to analytical applications. For example, carbon paste and carbon fiber electrodes have been used to monitor neurotransmitters in living animal brains [5,6]. The determination of catechol transmitters in the presence of relatively large amounts of interferents (e.g., ascorbate) de-... 

Applications have been reported for photoelectrochemical experiments, for example, splitting of water [11], local generation of photoelectrodes by spatially selective laser excitation [12], and steady-state electrochemiluminescence at a band electrode array [13,14]. Band electrodes prepared from very thin films approaching molecular dimensions have been used to assess the limits of theory describing electrode kinetics at ultramicroelectrodes [9]. Spectroelectrochemical applications have been extensively reviewed [1], In an intriguing approach, thin, discontinuous metal films have been prepared on a transparent semiconductor substrate they are essentially transparent under conditions in which a continuous metal film containing the same quantity of metal would be expected to substantially absorb [15]. 

The distribution of current (local rate of reaction) on an electrode surface is important in many applications. When surface overpotentials can also be neglected, the resulting current distribution is called primary. Primary current distributions depend on geometry only and are often highly nonuniform. If electrode kinetics is also considered, Laplace s equation still applies but is subject to different boundary conditions. The resulting current distribution is called a secondary current distribution. Here, for linear kinetics the current distribution is characterized by the Wagner number, Wa, a dimensionless ratio of kinetic to ohmic resistance. 

Weppner W, Huggins RA. Determination of the kinetic parameters of mixed-conducting electrodes and application to the system Li3Sb. J Electrochem Soc 1977 124 1569-1578. 

Fleig reviews fundamental aspects of solid state ionics, and illustrates many similarities between the field of solid state electrochemistry and liquid electrochemistry. These include the consideration of mass and charge transport, electrochemical reactions at electrode/solid interfaces, and impedance spectroscopy. Recent advances in microelectrodes based on solid state ionics are reviewed, along with their application to measuring inhomogeneous bulk conductivities, grain boundary properties, and electrode kinetics of reactions on anion conductors. 

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