Current densities electrode potential

Figure 3.3.14 displays the results of voltammetric ORR activity measurements of a dealloyed Pt-Cu and a dealloyed Pt-Ni core-shell catalyst under fuel cell relevant conditions. The typical sigmoidal current density electrode potential (i-E) shape (compare to Figure 3.3.10B) of the Pt-based catalyst is clearly evident. The large... 

The data are plotted in Figure 7. The anodic and cathodic Tafel slopes ("base 10") are 40 and 120 mV, respectively, and the exchange current density is 1.0 mA/cm". Therefore, the current density-electrode potential behavior of this activation controlled electrode reaction is described by... 

Current Density, Electrode Potential, and Current Efficiency... 

The dynamic EDL can be formulated with two closely coupled equations (e.g., given by Equations 2.2 and 2.3 by taking a spherical electrode), which describe, respectively, the interfacial structure (relation between the electrostatic potential and concentration distributions) and the voltammetric behavior (current density-electrode potential dependence) ... 

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

Each of these two procedures can be varied by proceeding from a low to a high current density (or potential) or from a high to a low current density (or potential) the former is referred to as forward polarisation and the latter as reverse polarisation. Furthermore, there are a number of variations of the potentiostatic technique, and in the potentiokinetic method the pwtential of the electrode is made to vary continuously at a predetermined rate, the current being monitored on a recorder in the pulse method the electrode is given a pulse of potential and the current transient is determined by means of an oscilloscope. 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

The influence of the charge-discharge current density on the specific capacity properties of the electrodes is depicted in Fig. 3. One can conclude that even at a 0.2 mA/cm2 current density the potentialities of the material are not developed fully. Further increasing the cycling current density up to... 

Conductivity and anodic charge density, current density and potential of peak recorded by cyclic voltammetry on a platinum electrode (scan rate 50 mV s-1) in commercial and homemade creams... 

The following relationship is experimentally observed between applied electrochemical current density and potential for a corroding electrode in the absence of competing reduction-oxidation reactions (1,2). The applicability of this relationship relies on the presence of a single charge transfer controlled cathodic reaction and a single charge transfer controlled anodic reaction. 

If for any reason, such as the application of an external current, the electrode potential of the metal is changed from the equilibrium value, or cannot assume this value, there will be a net anodic or cathodic current density according to the value of E. A change in the noble direction (more positive) will cause a net ionic current in the anodic direction, (ia) > equal to the difference between the total anodic and cathodic current. According to Eqs. (8) and (9) this change will be given by... 

In order to simplify this discussion. Fig. 1 schematically represents the main features of the current density / vs. potential K behavior of n- and p-type III-V electrodes in indifferent electrolyte solutions at (a) low and (b) high pH, in the dark and under illumination (Av > E ). This generalized representation is primarily based on experimental results reported for GaAs and GaP for InP, less detailed data are available. Examples of actual experimental i- V curves are found in this text, as well as in the papers cited. 

The geometry of an electrode frequently constrains the distribution of current density and potential in the electrol5rte adjacent to the electrode in such a way that both carmot simultaneously be uniform. The primary and secondary current and potential distributions associated with a disk embedded in an insulating plane, originally developed by Newman, are presented in Section 5.6. The potential distribution on the disk electrode is not uniform under conditions where the current density is uniform and, conversely, the current distribution is nonuniform under the primary condition where the solution potential is uniform. 

Following Hueing et al., ° a notation is presented in Section 7.5.2 that addresses the concepts of a global impedance, which involved quantities averaged over the electrode surface a local interfacial imgedance, which involved both a local current density and the local potential drop V — Oo(r) across the diffuse double layer a local impedance, which involved a local current density and the potential of the electrode V referenced to a distant electrode and a local Ohmic impedance, which involved a local current density and potential drop Oo(r) from the outer region of the diffuse double layer to the distant electrode. The corresponding list of symbols is provided in Table 7.2. 

Another problem in application of the basic theories is associated with surface geometry. Most theories are developed to describe the relationships among the area-averaged quantities such as charge density, current density, and potentials assuming a uniform electrode surface. In fact, the silicon surface may not be uniform at the micrometer, nanometer, or atomic scales. There can be great variations in the distribution of reactions from extremely uniform, for example, in electropolishing, to extremely nonuniform, for example, in the formation of porous silicon. 

Fig. 7.53 Interfacial current vs. electrode potential for 0.05 M and 0.05 M Fe + + at Sn02 electrodes of different doping densities in 0.5 M. H2SO4 dots, experimental values solid lines, theoretical curves.

In Section II. 1 the transfer coefficient a was introduced in the usual general way in relation to the definition of b, the Tafel slope. In electrode reaction mechanisms involving two or more consecutive steps, the dependence of a time-invariant current density on potential must be evaluated by one of the following ... 

Vigorous stirring, therefore, is necessary for several purposes. It helps to dislodge the gas bubbles that form at the electrodes. More importantly, it provides convective transport of the Cu ions to the cathode as the solution becomes increasingly dilute in Cu ion. At this point, if the applied voltage is raised to increase the current density, the potential could become sufficiently negative to cause the reduction of the next easiest reduced species present, in this case H ions ... 

In the region sufficiently far from the interface that electroneutrality holds and under the assumptions that the concentration is uniform and that the solution adjacent to the electrodes may be treated as equipotential surfaces, the potential distribution can be obtained through solution of Laplace s equation, V2 = 0, and is a function of current density. The potential drop in the region between the counterelectrode and the outer limit of the diffusion layer is given by... 

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

Simple, purely transfer-related electrode reactions give cumulative current density-potential curves of the type shown by the unbroken Une in Figure 20.11. At the points pi and pj, it swings into the overpotential curves of the respective part reactions because beyond these points, only the anodic or cathodic reaction exists. At these points, the equilibrium potentials of the reverse reaction exceeded and superimposition no longer occurs. These pure overpotential curves thus form linear Tafel lines, which, after reflection of the cathode curve in the x-axis, can be made to intersect by extrapolation in the direction of the abscissa. The ordinate section at the point of intersection is then log that is, the log of the corrosion current density, rest potential, from which the corrosion rate can be calculated by Faraday s law. 

Because most applications of (photo)elec-trochemical systems involve the transfer of electrons across an interface (Sect. 2.1.1), current density-potential techniques are commonly used in (photo)electrochemis-try. In this case, the difference in electrochemical potential of electrons across the interface of interest (accessible via the working electrode - reference electrode potential difference) and the current density through this interface are used as the perturbation and the response (or vice versa). Two approaches can be distinguished. When (quasi) steady state signals are used, one speaks of current density versus potential measurements whereas harmonically modulated signals, superimposed on a bias, are involved in electrochemical impedance spectroscopy (EIS). We introduce these two approaches on the basis of the kinetics of the simple system shown in Fig. 1. 

Current Density Versus Potential Measurements Figure 7 shows a current density versus potential curve for an illuminated n-GaAs electrode (A, = 480 nm) in a 0.1 M H2SO4 solution. As reported for various photoanodes in the Kterature,... 

Fig. 7 Current density versus potential curve, recorded at an illuminated n-GaAs electrode in a 0.1 M H2SO4 aqueous solution. Indicated are the flat band potential (as determined in the dark) and the different potential ranges (see text).

We have considered above the Butler-Volmer equation for the relationship between current density and potential under the situation when transport of ions in solution makes little or no difference to the rate of an electrode reaction. In order to considered the situation in which transport does control the flow we shall adopt a correspondingly simple counterassumption electron transfer at the interface no longer has control of the electrode reaction. 

All electrode reactions manifest this type of shape. Somewhat like the situation with the examination of the proverbial elephant, the situation may seem different depending upon the point at which one touches reality. If for example, one measures an extremely low current density very near the reversible potential, it may seem that the relationship between current density and potential is a linear one, but in the large range of potentials one may obtain a logarithmic relationship between the current density and... 

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