Tafel slope change

Because of the logarithmic relation, polarization depends more strongly on parameter a than on parameter b. The parameter a, which is the value of polarization at the unit current density (1 mA/cm ), assumes values which for different electrodes and reactions range from 0.03 to 2-3 V. Parameter b, which is called the Tafel slope, changes within much narrower limits in many cases, at room temperature b 0.05 V and 0.115 V (or roughly 0.12 V). 

The steady-state Tafel lines for methanol oxidation in acid solution are 55-60 mV/decade over the potential range 0.4-0.5 (SHE). When the potential of the working electrode on this scale is made more positive, the Tafel slope changes and becomes 110 mV/decade. These two numerically stated Tafel slopes can readily be reexpressed in electrode kinetic terms to correspond, respectively, to ... 

It should be noted that the Tafel slopes just given were calculated for the combined isotherm under Langmuir conditions, namely at very low coverage. The same type of calculation can be repeated to obtain the kinetic parameters for different mechanisms both at low and at intermediate values of the coverage. The effect on the Tafel slope of competition with water is rather small for small molecules. Thus, for n = 1 the Tafel slope changes only by about 2 mV for the two mechanisms just discussed. This is within experimental error in most cases, perhaps explaining why the need to use the combined adsorption isotherm is not... 

Carbon dioxide is the main reaction product for < 1.1 V (vs reversible hydrogen electrode (RHE)) on pyrolytic graphite. For a pH between 1 and 9, the Tafel slope changes from 0.150 to 0.240 V per decade, depending on the solution composition and electrode preparation. 

The Tafel slope changes from 30 mV at lower overpotentials to 40 mV at the higher,... 

The two limiting cases just discussed are approximately applicable for 0 < 0.1 and 0 > 0.9, respectively. In the intermediate region, one could readily solve the equation by substituting 0 from Eq. (6.16) into Eq. (6.17), but the result (besides being cumbersome) leads to a transfer coefficient that decreases gradually with increasing overpotential, from Ugn = 1 + Pan Oan = Pan- This also means that the Tafel slope changes with potential. In other words, the Tafel plot (which is the plot of... 

Hi) Surface blockers. Type 1 tlie inliibiting molecules set up a geometrical barrier on tlie surface (mostly by adsorjDtion) such as a variety of ionic organic molecules. The effectiveness is directly related to tlie surface coverage. The effect is a lowering of tlie anodic part of tlie polarization curve witliout changing tlie Tafel slope. 

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

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. 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

A qualitative measure of the corrosion rate can be obtained from the slope of the curves in Fig. 2. Z INT is given in units of s ohm" . Owing to the presence of the uncompensated ohmic resistance and lack of values for Tafel slopes [Eq. (2)], data in Fig. 2 should be viewed as indicative of significant changes in corrosion rates. Corrosion loss remained low during the first 2 months, followed by a large increase for both flushed samples and controls. The corrosion rate increased when the surface pH reached values of 1 or less. Total corrosion loss as determined from integrated Rp data was less for the control than for the flushed sample. 

The relevance of Pt-OH formation to the change in the Tafel slope has been demonstrated by varying the content of water in the electrolyte [Murthi et al., 2004]. The experiments were performed in H20/trifluoromethanesulfonic acid (TFMSA) mixtures with several water/acid molar ratios. Whereas at high water contents the usual change in the Tafel slope from —112 to —59 mV/dec observed in aqueous solutions of H2SO4 and HCIO4 took place, at low water contents no change in the Tafel slope was observed. This corroborates the involvement of water in the formation... 

In this notation, anodic current is positive, while cathodic current is negative. As the later section on oxygen reduction will show, the Tafel slope can change with overpotential. This is because the Butler-Volmer law only applies to outer-sphere reactions. Although it can describe electrode reactions, the equation does not account for repulsive interactions of the adsorbates or changes in the reaction mechanism as potential is changed. 

At low coverage, the Tafel slope will be 2RT/3F or c. 40 mV, as observed on high at.% Ru electrodes. As the at.% Ru decreases, the number of Ru sites decrease, resulting in more coverage of the active Ru sites by 0ac]. Hence 0ad will approach 1 and the Tafel slope will tend to reach values of 2RT/F or 120 mV, thus potentially explaining the results in Fig. 5.3. Alternatively, this change in the Tafel slope may arise from an increase in the electrical resistivity of the low at.% Ru electrodes, during the course of the chlorine evolution reaction [35]. 

Because of the different potential distributions for different sets of conditions the apparent value of Tafel slope, about 60 mV, may have contributions from the various processes. The exact value may vary due to several factors which have different effects on the current-potential relationship 1) relative potential drops in the space charge layer and the Helmholtz layer 2) increase in surface area during the course of anodization due to formation of PS 3) change of the dissolution valence with potential 4) electron injection into the conduction band and 5) potential drops in the bulk semiconductor and electrolyte. 

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