Kinetics electrochemical

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R ) and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to 

Atp absolute potential of A fig absolute potential of E2 U potential of E- (voltage between and E2 Uq voltage of the external current 

By comparison with Eq. (2-1) the measured value in Fig. 2-3 is too negative by according to Eq. (2-33) and correspondingly is too positive in the case of the anodic current. The error can be calculated for uniform current flow lines from Ohm s Law  

The term a is a symmetry factor for the energy threshold for the passage of electrons and is approximately equal to 0.5. In Fig. 2-4, the value of a was chosen as Vs for better distinction integer exponents are chosen for Tq, G and Gq for clarity, 

The time dependence of the changes in the measured values is important in determining J(U) curves. In the region of the Tafel lines, stationary states are reached 

Various characterization techniques can be applied to measure electrochemical properties and thus evaluate the fuel cell system performance. Below, we describe several of the most widely used characterization techniques applicable for photocatal)4 ic fuel cells. 

The processes that govern the electrode reaction rates are the mass transfer between the bulk solution and electrode surface, the electron transfer at the electrode, and the chemical reactions involving electron transfer. These processes are heterogeneous reactions between electrode and electrolyte and are characterized by both chemical and electrical changes. Several steps are involved in these reactions. For electron transfer to the electrodes, first electroactive species must be transported to the electrode surface by migration or diffusion. At the electrode, adsorption of electroactive material may be involved both before and after the electron transfer step. In the whole sequence of reactions, the slowest step determines the overall rate of the electrochemical process. In this chapter, we discuss electrochemical kinetics that governs the reaction rate and hence the rate of electrochemical energy output from a fuel cell. 

---

K. Vetter, Electrochemical Kinetics, Aca demic Press, New York, 1967. 

The literature [14] on electrochemical kinetics is extensive and specialized. Figure 2-4 shows a J(rjj) curve of a redox reaction according to Eq. (2-9) with activation and diffusion polarization. It follows from theory [4, 10] for this example ... 

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

Transport Phenomena in Electrochemical Kinetics Arvia, A. J. Marchiano, S. L. 6... 

Vetter, K. J. (1967). Electrochemical Kinetics Theoretical and Experimental Aspects , Academic Press. 

Vet Vetter, K.J. Electrochemical Kinetics, New York Academic Press, 1967, p. 73. 

Peter LM, Wright GA (1987) Electrochemical kinetics of bismuth sulphide formation on bismuth amalgam. Electrochim Acta 32 1353-1356... 

For thermodynamic reasons, an electrochemical reaction can occur only within a dehnite region of potentials a cathodic reaction at electrode potentials more negative, an anodic reaction at potentials more positive than the equilibrium potential of that reaction. This condition only implies a possibility that the electrode reaction will occur in the corresponding region of potentials it provides no indication of whether the reaction will actually occur, and if so, what its rate will be. The answers are provided not by thermodynamics but by electrochemical kinetics. 

Each of the intermediate electrochemical or chemical steps is a reaction of its own (i.e., it has its own kinetic pecnliarities and rules. Despite the fact that all steps occur with the same rate in the steady state, it is true that some steps occur readily, without kinetic limitations, and others, to the contrary, occur with limitations. Kinetic limitations that are present in electrochemical steps show up in the form of appreciable electrode polarization. It is a very important task of electrochemical kinetics to establish the nature and kinetic parameters of the intermediate steps as well as the way in which the kinetic parameters of the individual steps correlate with those of the overall reaction. 

It is the basic task of electrochemical kinetics to establish the functional relations between the rate of an electrochemical reaction at a given electrode and the various external control parameters the electrode potential, the reactant concentrations, the temperature, and so on. From an analysis of these relations, certain conclusions are drawn as to the reaction mechanism prevailing at a given electrode (the reaction pathway and the nature of the slow step). 

In electrocatalysis, in contrast to electrochemical kinetics, the rate of an electrochemical reaction is examined at constant external control parameters so as to reveal the influence of the catalytic electrode (its nature, its surface state) on the rate constants in the kinetic equations. 

Electric double layers are formed in heterogeneous electrochemical systems at interfaces between the electrolyte solution and other condncting or nonconducting phases this implies that charges of opposite sign accumnlate at the surfaces of the adjacent phases. When an electric held is present in the solntion phase which acts along snch an interface, forces arise that produce (when this is possible) a relative motion of the phases in opposite directions. The associated phenomena historically came to be known as electrokinetic phenomena or electrokinetic processes. These terms are not very fortunate, since a similar term, electrochemical kinetics, commonly has a different meaning (see Part 11). 

Another result of the cold-fusion epopee that was positive for electrochemistry are the advances in the experimental investigation and interpretation of isotope effects in electrochemical kinetics. Additional smdies of isotope effects were conducted in the protium-deuterium-tritium system, which had received a great deal of attention previously now these effects have become an even more powerful tool for work directed at determining the mechanisms of electrode reactions, including work at the molecular level. Strong procedural advances have been possible not only in electrochemistry but also in the other areas. 

Studies in the field of electrochemical kinetics were enhanced considerably with the development of the dropping mercury electrode introduced in 1923 by Jaroslav Heyrovsky (1890-1967 Nobel prize, 1959). This electrode not only had an ideally renewable and reproducible surface but also allowed for the first time a quantitative assessment of diffusion processes near the electrode s surface and so an unambiguous distinction between the influence of diffusion and kinetic factors on the reaction rate. At this period a great number of efectrochemical investigations were performed at the dropping mercury efectrode or at stationary mercury electrodes, often at the expense of other types of electrodes (the mercury boom in electrochemistry). 

---