The concept of overpotential

It is worth emphasizing that although overpotentials are usually associated with electrode-electrolyte interfaces, in reality they refer to, and are measured as, deviations of the potential (

associated with an electrode and not with an electrode-electrolyte interface, although the nature of this interface will, in general, dictate the magnitude of the measured overpotential. 

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. 

Driven vs. Self-Driven Cells and the Concept of Overpotential.1763... 

It should be stated here that the concept of overpotential is related to the reversible potential of a pure metal in a given solution. In the case of codeposition of two metals and the formation of a phase this potential has no physical... 

The concept of overpotential is a powerful means for quantifying the performance of an electrochemical system. It applies at multiple levels the overall cell, the individual electrodes, and the processes occurring at those electrodes. All components of overpotential can be measured, so the method is of value in any electrochemical application. In most cases, component overpotentials may be considered independently. This simplification greatly aids the development of an electrochemical system as it enables isolated study of a part of the overall process. Since overpotentials are a deviation from ideality, which is the condition of maximum work, they quantitatively express the work lost in a system that results from a particular electrode process. In so doing, they express the inefficiency of electrochemical systems. 

The importance of the method in corrosion testing and research has stimulated other work, and since Stern s papers appeared there have been a number of publications many of which question the validity of the concept of linear polarisation. The derivation of linearity polarisation is based on an approximation involving the difference of two exponential terms, and a number of papers have appeared that have attempted to define the range of validity of polarisation resistance measurements. Barnartt" derived an analytical expression for the deviations from linearity and concluded that it varied widely between different systems. Leroy", using mathematical and graphical methods, concluded that linearity was sufficient for the technique to be valid in many practical corrosion systems. Most authors emphasise the importance of making polarisation resistance measurements at both positive and negative overpotentials. 

We have seen already that an absolute potential at an electrode cannot be known, so, in accord with all other electrochemistry, it is the potential difference between two electrodes which we measure. However, if the potential of the electrode of interest is cited with respect to that of a second electrode having a known, fixed potential, then we can know its voltage via the concept of the standard hydrogen electrode (SHE) scale (see Section 3.1). We see that a reliable value of overpotential requires a circuit containing a reference electrode. 

This theoretical framework accounts for the concept of surface poisoning by adsorbed OH and at low overpotentials (values of U close to 1.23 V) high OH and coverages result from the reduced activation barriers associated with the backward reactions of steps (26) and (28) according to... 

There is another interesting result of the concept of an rds. If all the exchange-current densities except that for the rds are very large, it means that the overpotentials due to all other steps are negligibly small [cf. Eq. (7.131)]. Since the magnitude of the overpotential for a step is a measure of how far the step is from equilibrium, then if Tl - 0 [j r], one concludes that the7th step is almost in equilibrium, i.e., it is in quasi-equilibrium. Hence, the existence of a unique rds usually implies that other steps are virtually in equilibrium. 

The concept of limiting current density permits a simple derivation of a relation between the steady-state concentration overpotential T]c and the current density i if the reaction is such that other forms of overpotential are negligible. One starts from the expression for the concentration overpotential T c [cf. Eq. (7.198)]... 

The link between the current density and the concentration overpotential under steady-state conditions for systems in which the exchange-current density is relatively large compared with the limiting current density (hence, the activation overpotential is negligible) was established through the concept of a limiting current iL arising from the fact that there is a maximum rate at which electron acceptors can move to an... 

Now we come to the concept of quasi-equilibrium. If there is a distinct rate-determining step in a reaction sequence, then all other steps before and after it must be effectively at equilibrium. This comes about because the overall rate is, by definition, very slow compared to the rate at which each of the other steps could proceed by itself, and equilibrium in these steps is therefore barely disturbed. To see this better, consider the specific example given earlier for chlorine evolution. Assume, for the sake of argument, that the values of the exchange current density i for steps 8F and 9F are 250 and 1.0 mA/cm, respectively. Assume now that we apply a current density of 0.5 mA/cm. We can calculate the overpotential corresponding to each step in the sequence, using Eq. 6E, namely... 

For the LSV and CV techniques, the concept of reversibility/irreversibility is therefore very important. Electrochemists are responsible for some confusion about the term irreversible, since a reaction may be electrochemically irreversible, yet chemically reversible. In electrochemistry, the term irreversible is used in a double sense, to describe effects from both homogeneous and heterogeneous reactions. In both cases, the irreversible situation arises when deviations from the Nernst equation can be seen as fast changes in the electrode potential, E, are attempted and the apparent heterogeneous rate constants, /capp, for the O/R redox couple is relatively small. The heterogeneous rate constant can be split into two parts a constant factor in terms of the standard rate constant, k°, and an exponential function of the overpotential E - Eq), as expressed in Eq. 59, where only the reductive process is considered (see also Eq. 5). 

The better understanding of the concept effective overpotential can be realized by taking into account the fact that the time of dendritic growth initiation depends on used deposition overpotentials. Increasing deposition overpotentials lead to decreasing times for... 

The Concept of Effective Overpotential Applied for Metal Electrodeposition Under an Imposed Magnetic Field... 

The application of the concept of effective overpotential for the case of the change of hydrodynamic conditions caused by magnetic field effects means that morphologies of nickel and copper deposits obtained under parallel fields (the largest magnetohydro-dynamic (MHD) effect) should be, at macro level, similar to those obtained at some lower overpotentials or potentials without imposed magnetic fields. This assumption can be confirmed by the following consideration ... 

The thickness of the diffusion layer is strongly related to the fluid flow conditions and only in rather simple cases 6 can be calculated as function of characteristic hydrodynamic parameters. An order of magnitude is about one tenth of the hydrodynamic laminar boundary layer thickness. Because the diffusion layer is very small, one can totalize the phenomena occurring in it and suppose that the diffusion layer belongs to the electrode. This will lead to the concept of concentration overpotential (see section 1.ii.3). 

The concept of useable potential is even more important in the case of PEC water-splitting devices. The useable potential of a PEC device must be sufficiently high to drive both half reactions in addition to the overpotential losses. This can typically require over 1.6-1.9 V. To achieve this in a single junction device, semiconductor bandgaps over 3.0 eV are typically necessary, which severely restricts optical absorption, saturated photocurrent, and therefore, STH conversion efficiency. 

Mechanism of Formation of the Honeycomh-Like Structure The Concept of Effective Overpotential ... 

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