Electrode kinetics density

The first law of electrode kinetics, observed by Tafel in 1905 [197], is that overvoltage i) varies with current density i according to the equation... 

In electrode kinetics a relationship is sought between the current density and the composition of the electrolyte, surface overpotential, and the electrode material. This microscopic description of the double layer indicates how stmcture and chemistry affect the rate of charge-transfer reactions. Generally in electrode kinetics the double layer is regarded as part of the interface, and a macroscopic relationship is sought. For the general reaction... 

The classification of methods for studying electrode kinetics is based on the criterion of whether the electrical potential or the current density is controlled. The other variable, which is then a function of time, is determined by the electrode process. Obviously, for a steady-state process, these two quantities are interdependent and further classification is unnecessary. Techniques employing a small periodic perturbation of the system by current or potential oscillations with a small amplitude will be classified separately. 

J. Srmivason, et al., "High Energy Efficiency and High Power Density Proton Exchange Membrane Fuel Cells - Electrode Kinetics and Mass Transport," Journal of Power Sources, p. 36, 1991. 

To summarize, one can say that the electrochemical performance of CNT electrodes is correlated to the DOS of the CNT electrode with energies close to the redox formal potential of the solution species. The electron transfer and adsorption reactivity of CNT electrodes is remarkably dependent on the density of edge sites/defects that are the more reactive sites for that process, increasing considerably the electron-transfer rate. Additionally, surface oxygen functionalities can exert a big influence on the electrode kinetics. However, not all redox systems respond in the same way to the surface characteristics or can have electrocatalytical activity. This is very dependent on their own redox mechanism. Moreover, the high surface area and the nanometer size are the key factors in the electrochemical performance of the carbon nanotubes. 

Boltzmann term, 1078 computer simulation. 1161 current density, 1078, 1081 current potential relation, 1082 doping, 1073 effect of light on, 785 -/junction, 1081 electrode kinetics of, 170 electrodeposition on, 1344... 

One general method may always be used to reduce the effect of impurity adsorption on electrodes, and that is to work only for shorl times. Impurities take substantial times to adsorb. If the time in which the measurement is made is short enough, the adsorption aspect of impurity interference with electrode kinetic measurements can be reduced. Many of the techniques for doing this are described in Chapter 8 (transients). However, this approach does not eliminate the difficulty that at low current densities impurities in the solution may compete with electrons from the electrode. Further, although transient measurements may greatly reduce the adsorption of impurities during the measurement, it is difficult to arrange techniques so that the electrode is in contact with the solution for seconds only. 

The introduction of 0 in the equations for current density need by no means refer only to the adsorbed intermediates in the electrode reaction. What of other entities that may he adsorbed on the surface For example, suppose one adds to the solution an oiganic substance (e.g., aniline) and this becomes adsorbed on the electrode surface. Then, the 0 for the adsorbed organic substance must also be allowed for in the electrode kinetic equations. So, in Eq. (7.149), the value of 0 would really have to become a 0, where the summation is over all the entities that remain upon the surface and block off sites for the discharging entities. Many practical aspects of electrodics arise from this aspect of the Butler-Volmer equation. For example, the action of organic corrosion inhibitors partly arises in this way (adsorption and blocking of the surface of the electrode and hence reduction of the rate of the corrosion reaction per apparent unit area).67... 

Use of the lower time would give a big advantage in respect to the upper limits of current density at which an electrode kinetic measurement can be made See of diffusion control. At 0.1 ms, the current density will be free of diffusion effects because it is 100 times higher than that at 1 s, when diffusion will in any case affect the measurement (Fig. 8.4). 

Although the kinetic variable in electrode reactions in the current density, extensive use of the overpotential concept has been made in the electrochemical literature to indicate the departure from equilibrium [7]. Depending on the particular rate-determining process, in the overall electrode kinetics ohmic, charge transfer, reaction, concentration or mass transport, and crystallization overpotentials are described in the literature. Vetter [7] distinguished the concept of overpotential from that of polarization in the case of mixed potentials when the zero current condition does not correspond to an equilibrium potential as will be discussed in Sect. 8. 

The structure of the double layer can be altered if there is interaction of concentration gradients, due to chemical reactions or diffusion processes, and the diffuse ionic double layer. These effects may be important in very fast reactions where relaxation techniques are used and high current densities flow through the interface. From the work of Levich, only in very dilute solutions and at electrode potentials far from the pzc are superposition of concentration gradients due to diffuse double layer and diffusion expected [25]. It has been found that, even at high current densities, no difficulties arise in the use of the equilibrium double layer conditions in the analysis of electrode kinetics, as will be discussed in Sect. 3.5. 

Figure 12(b) shows the local current distribution of first and second order reactions and applied over potentials ° for the coupled anode model without the mass transfer parameter y. The figure also shows the effect of a change in the electrode kinetics, in terms of an increase in the reaction order (with respect to reactant concentration) to 2.0, on the current distribution. Essentially a similar variation in current density distribution is produced, to that of a first order reaction, although the influence of mass transport limitations is more severe in terms of reducing the local current densities. 

An aim of the model is to determine the influence of the various mass transport parameters and show how they influence the polarization behavior of three-dimensional electrodes. In the model we have adopted relatively simple electrode kinetics, i.e., Tafel type, The approach can also be applied to more complicated electrode kinetics which exhibit non-linear dependency of reaction rate (current density) on reactant concentration. 

Fig. 13.27. Potential vs. current density plots for state-of-the-art fuel cells, o, proton exchange membrane fuel cell , solid oxide fuel cell , pressurized phosphonic acid fuel cell (PAFC) a, direct methanol fuel cell, direct methanol PAFC , alkaline fuel cell. (Reprinted from M. A. Parthasarathy, S. Srinivasan, and A. J. Appleby, Electrode Kinetics of Oxygen Reduction at Carbon-Supported and Un-supported Platinum Microcrystal-lite/Nafion Interfaces, J. Electroanalytical Chem. 339 101-121, copyright 1992, p. 103, Fig. 1, with permission from Elsevier Science.)...

Butler-Volmer equation — The Butler-Volmer or -> Erdey-Gruz-Volmer or Butler-Erdey-Gruz-Volmer equation is the fundamental equation of -> electrode kinetics that describes the exponential relationship between the -> current density and the -> electrode potential. Based on this model the -> equilibrium electrode potential (or the reversible electrode potential) can also be interpreted. 

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