Charge-transfer overpotential, electrode

However, under working conditions, with a current density j, the cell voltage E(j) decreases greatly as the result of three limiting factors the charge transfer overpotentials r]a,act and Pc,act at the two electrodes due to slow kinetics of the electrochemical processes (p, is defined as the difference between the working electrode potential ( j), and the equilibrium potential eq,i). the ohmic drop Rf. j, with the ohmic resistance of the electrolyte and interface, and the mass transfer limitations for reactants and products. The cell voltage can thus be expressed as... 

Charge-transfer overpotential — The essential step of an - electrode reaction is the charge (- electron or - ion) transfer across the phase boundary (- interface). In order to overcome the activation barrier related to this process and thus enhance the desirable reaction, an - overpotential is needed. It is called charge-transfer (or transfer or electron transfer) overpotential (f/ct). This overpotential is identical with the - activation overpotential. Both expressions are used in the literature [i-iv]. Refs. [i] Bard A], Faulkner LR (2001) Electrochemical methods. Wiley, New York, pp 87-124 [ii] Erdey-Gruz T (1972) Kinetics of electrode processes. Akademiai Kiadd, Budapest, pp 19-56 [Hi] Inzelt G (2002) Kinetics of electrochemical reactions. In Scholz F (ed) Electroanalytical methods. Springer, Berlin, pp 29-33 [iv] Hamann CH, Hamnett A, Viel-stich W (1998) Electrochemistry. Wiley VCH, Weinheim, p 145... 

We first consider the case in which the charge transfer is the slow step. In this case, the rate of the electrode reaction is determined by the charge-transfer overpotential, rj = rjct. It is assumed here that... 

Electrodes The AFCs use porous carbon electrodes. Platinum is a typical catalyst for both the anodic and cathodic reaction, but Ni and Ag are also used for the anodic and the cathodic reactions, respectively [6]. The kinetics of both reactions, but especially of oxygen reduction, is favorable in the basic environment. The charge-transfer overpotentials for the pure gases are less than 0.1 V for both electrode reactions. 

In this paper we describe a model of a cup plater with a peripheral continuous contact and passive elements that shape the potential field. The model takes into account the ohmic drop in the electrolyte, the charge-transfer overpotential at the electrode surface, the ohmic drop within the seed layer, and the transient effect of the growing metal film as it plates up (treated as a series of pseudo-steady time steps). Comparison of experimental plated thickness profiles with thickness profile evolution predicted by the model is shown. Tool scale-up for 300 mm wafers was also simulated and compared with results from a dimensionless analysis. 

Each value of current density, j, is driven by a certain overpotential, 17. This overpotential can be considered as a sum of terms associated with the different reaction steps r/mt (the mass-transfer overpotential), (the charge-transfer overpotential), (the overpotential associated with a preceding reaction), etc. The electrode reaction can then be represented by a resistance, R, composed of a series of resistances (or more exactly, impedances) representing the various steps R i, R, etc. (Figure 1.3.7). A fast reaction step is characterized by a small resistance (or impedance), while a slow step is represented by a high resistance. However, except for very small current or potential perturbations, these impedances are functions of E (or /), unlike the analogous actual electrical elements. 

Consider a cell composed of two ideal nonpolarizable electrodes, for example, two SCEs immersed in a potassium chloride solution SCE/KCl/SCE. The i-E characteristic of this cell would look like that of a pure resistance (Figure 1.3.8), because the only limitation on current flow is imposed by the resistance of the solution. In fact, these conditions (i.e., paired, nonpolarizable electrodes) are exactly those sought in measurements of solution conductivity. For any real electrodes (e.g., actual SCEs), mass-transfer and charge-transfer overpotentials would also become important at high enough current densities. 

The overpotentials at the anode qAnode (oxygen overpotential) and cathode qcathode (hydrogen overpotential), also referred to as charge transfer overpotentials, result from the inhibition of electron transport in the separate electrochemical reactions (see Fig. 11.2). In order for current to flow through the electrolysis cell, the resistance polarization must also be overcome. It is caused by the ohmic resistance of the ceU (electrolytes, separator and electrodes). The ohmic voltage drop can be calculated from the current density i in A cm and the surface-specific resistance R of the ceU in Q cm. ... 

In principle, the polarization at each electrode may have a contribution from charge transfer, mass transport, nucleation and passivation overpotentials. The major contribution will normally be from the charge transfer overpotential since mass transport control has a catastrophic effect on the battery voltage (see Fig. 10.3) and one would not normally design a battery to operate in such conditions. Examples of nucleation and passivation overpotentials do occur. The former occur when the electrode reaction requires the formation of a new phase although the nucleation overpotential is normally a transitory phenomenon since, once nuclei of the new phase are present in numbers, the overpotential will disappear. The... 

The radius of a nucleation exclusion zone can be calculated on the basis of the following discussion, taking into account the charge transfer overpotential also. If there is a half-spherical nucleus on a flat electrode, the extent of the deviation in the shape of the equipotential surfaces which occurs around it depends on the crystallization overpotential, current density, a resistivity of the solution and on the radius of the nucleus, r. If the distance from the flat part of the substrate surface to the equipotential surface which corresponds to the critical nucleation overpotential, rj, is /n, then this changes around defect to the extent where A is a number, as is presented in Fig. 2.18. 

Mien current is drawn from a galvanic cell, or piassed through an electrolytic cell, the electrodes generally dep>art from their equilibrium pxitentials and manifest charge-transfer overpotential due to a significant activation barrier to the faradaic process. 

SO low that the concentrations at the site of the electrochemical reactions (electrode surface) do not substantially differ from the bulk values. Precisely, the equation can be used in a region of the charge transfer overpotential so that in Equation 6.6 q,., should be used instead of the total overpotential q. Let us discuss two key parameters, 7o and p, introduced in Equation 6.6. 

At the moment, the fuel cell technology is under extensive development and is considered as one of the viable options for high-efficiency power generation. As shown in Figure 8.1, four main components of a fuel cell are (1) electrolyte, (2) electrodes (anode and cathode), (3) gas diffusion layers, and (4) chemicals (fuel and oxidizer). The electrolyte [e.g., KOH(aq)] is needed to conduct OH (aq) ions from the cathode to the anode. The electrodes are needed to speed up electrochemical reactions and reduce the charge transfer overpotentials. The gas diffusion layers are needed to provide the desirable mass transport of chemicals to (from) the electrodes, reduce... 

Multiple select For a polarized electrode only under the influence of a charge transfer overpotential, which parameters can be changed in value with the electrode material ... 

Multiple select For a polarized electrode with both mass and charge transfer overpotentials, which parameters will change in value with changes in the bulk solution concentration ... 

Charge transfer overpotential on an electrode [V] Overpotential of electrolytic cell [V]... 

Charge transfer overpotential The charge transfer overpotential is caused by the fact that the speed of the charge transfer through the phase-boundary electrode/electrolyte is limited. It generally depends on the kind of substances that are reacting, the conditions in the electrolyte, and the characteristic of the electrode (for example, what kind of metal). The formulae which deal with this form of overpotential are called the Butler-Vohner equation and the Tafel equation [10]. 

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 activation overpotential, and hence the activation energy, varies exponentially with the rate of charge transfer per unit area of electrode surface, as defined by the well-known Tafel equation... 

The activation overpotential Tiac,w is due to slow charge transfer reactions at the electrode-electrolyte interface and is related to current via the Butler-Volmer equation (4.7). A slow chemical reaction (e.g. adsorption, desorption, spillover) preceding or following the charge-transfer step can also contribute to the development of activation overpotential. 

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