Electrochemical reactions concentration overpotential

The classical electrochemical methods are based on the simultaneous measurement of current and electrode potential. In simple cases the measured current is proportional to the rate of an electrochemical reaction. However, generally the concentrations of the reacting species at the interface are different from those in the bulk, since they are depleted or accumulated during the course of the reaction. So one must determine the interfacial concentrations. There axe two principal ways of doing this. In the first class of methods one of the two variables, either the potential or the current, is kept constant or varied in a simple manner, the other variable is measured, and the surface concentrations are calculated by solving the transport equations under the conditions applied. In the simplest variant the overpotential or the current is stepped from zero to a constant value the transient of the other variable is recorded and extrapolated back to the time at which the step was applied, when the interfacial concentrations were not yet depleted. In the other class of method the transport of the reacting species is enhanced by convection. If the geometry of the system is sufficiently simple, the mass transport equations can be solved, and the surface concentrations calculated. 

Electrochemistry is in many aspects directly comparable to the concepts known in heterogeneous catalysis. In electrochemistry, the main driving force for the electrochemical reaction is the difference between the electrode potential and the standard potential (E — E°), also called the overpotential. Large overpotentials, however, reduce the efficiency of the electrochemical process. Electrode optimization, therefore, aims to maximize the rate constant k, which is determined by the catalytic properties of the electrode surface, to maximize the surface area A, and, by minimization of transport losses, to result in maximum concentration of the reactants. 

Fig. 8.5. In electrochemical reactions involving one or more adsorbed reaction intermediates (sometimes involved in the rate-determining step), the steady-state concentration of the intermediate changes with the potential. However, each intermediate has a time constant to reach the surface coverage corresponding to a given overpotential. The downside of too low a pulse time, or too fast a sweep rate, is that the intermediate concentration does not relax to its appropriate concentration in time. The Tafel slope (sometimes a significant mechanism indicator) may then differ from that calculated for the assumed path and rate-determining step.

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. 

In electrode kinetic studies, reactant concentrations are, in general, in the millimolar range and double layer contributions for such low ionic concentrations may become very important. If excess of inert or supporting electrolyte is used, the relative variation in the ionic concentration at the double layer due to the electrochemical reaction is at a minimum at high concentration of an inert z z electrolyte, most of the interfacial potential drop corresponds to the Helmholtz inner layer and variations of A02 with electrode potential are small (Fig. 3). In addition, use of supporting electrolyte prevents the migration of electroactive ionic species from becoming important and also reduces the ohmic overpotential. 

These examples are based on both electrodes operating in the activation polarization regime, in which the logarithm of the current is proportional to the overpotential. However, there are situations - particularly at low concentrations - in which the electrochemical reaction is limited by mass transport to the electrode surface. This is referred to as concentration polarization, and is illustrated in Figure 13.2d. In this case, above a critical overpotential the current becomes constant, which appears as a vertical line in the plot. A new mixed potential is established at the intersection of this vertical line and the cathode polarization for the oxygen reduction. This potential depends on the gas concentration, and thus can be used for the chemical sensor signal. 

The driving force for all electrochemical processes at conventional electrodes (i.e. mass transport to electrodes and electrochemical reactions) is the difference between the electrode potential at operation and at equilibrium, which is the overpotential rj. It is theoretically a difficult term to work with because it combines the effects of many physical processes (e.g. Stern layer potential gradient change and concentration polarization) into a single quantity. Yet it is accessible experimentally, and hence a practical quantity for the better understanding of complicated electrochemical systems such as clays. 

Because all electrochemical reactions involve anodic and cathodic reactions, polarization will have components for both reactions. As will be explained later, the electrode potentials have two terms for each electrode surface overpotential ija or ijc and concentration overpotential Apart from these overpotentials, electrical energy will also be expended due to the electrical resistance of the cell components such as electrolyte, diaphragm, busbar, etc. Thus the practical cell voltage (, when a net current is flowing through the cell, is the sum... 

As in any heterogeneous reaction, two major controlling regimes are also possible in electrochemical reactions, surface reaction and external mass transfer, referred to specifically as surface overpotential (or charge transfer) and concentration overpotential, respectively, in electrochemical terminology. 

Mass transport in an electrochemical reactor occurs by three mechanisms migration in the electrical field, film diffusion, and convection. The first of these is a special feature of electrochemical reactions, whereas the other two are common to all reactions that have a solid phase. However, where an inertsupporting electrolyte is used, the effect of migration can be neglected. With this assumption, let us consider a single electrode reaction given by reaction 21.3. When a finite current is passed through the cell and conditions are perfectly reversible, the concentration overpotential can be expressed as (Pickett, 1979)... 

Measurement of the Tafel lines for different concentrations, the partial electrochemical reaction orders, can be detennined from Eqs. (6.34) and (6.35). This implies that the concentration at the electrode surface remains approximately equal to the bulk concentration (elimination of the diffusion overpotential). It should be pointed out that for this evaluation the logarithm of current must be plotted versus the electrode potential and not versus the overpotential. 

Concentration overpotential arises from the transport limitations within the porous electrode structure. The maximum potential that can be derived during the fuel cell operation depends on the partial pressure of electrochemically active reactant species, and the product of electrochemical charge transfer reaction at the reaction site (TPB) as described by Nemst equation... 

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