Selectivity Tafel

Often, it will be found that currents for a given reaction cannot be measured at all metals at the same value of potential. At some metals the currents would be too low for a reliable, sufficiently accurate determination at others they might be too high for a satisfactory experimental realization. A comparison will then be possible only after an extrapolation of data obtained in a different region of potentials, to the value of selected for comparison. This extrapolation may not be sufficiently reliable where the Tafel section of the polarization curve is too short or indistinct. 

Tafel analysis of these mixtures indicated that pigmenting samples consisted of pure azurite, pure smalt, and azurite plus smalt mixtures, concentrated in smalt molar percentages of 55%, 72%, and 85%. This can be seen in Fig. 4.11, where a frequency diagram illustrative of the use of selected azurite/smalt dosages is presented. 

This claim becomes intelligible if we consider more carefully the nature of the electrode material. It is necessary to choose a certain theory among the various ones which have been proposed—with more or less justification—on the electrical mechanism of reaction. I select that one which seems to me to have the best foundation. The fundamental idea of this theory has been derived from Tafel.2 Its general usefulness... 

The logarithmic nature of the current density axis amplifies errors in extrapolation. A poor selection of the slope to be used can change the corrosion current density calculated by a factor of 5 to 10. Two rules of thumb should be applied when using Tafel extrapolation. For an accurate extrapolation, at least one of the branches of the polarization curve should exhibit Tafel (i.e., linear on semiloga-rithmic scale) over at least one decade of current density. In addition, the extrapolation should start at least 50 to 100 mV away from Ec[Pg.45]

Figure 1 Z -log(japp) data for hypothetical corroding interfaces with Rp = lOOand 10,000 ohm-cm2 and (5a = (5C = 60 mV/decade. The two cases produce corrosion current densities of 130.4 and 1.3 pA/cm2, respectively. The Tafel slope is obtained from -log(japp) data at high overpotential. The open circuit potential is arbitrarily selected to be 0 mV.

The above analysis is, of course, based on the assumption of simple order reactions under Tafel operation and on the availability of sufficiently accurate data ( 5-10%). With complex reaction kinetics, for example, those involving surface adsorption terms (Eq. 16), a nonlinear regression analysis would yield the best estimate of a, Uj, and for a possible kinetic model. In all cases, use of these parameters for predicting the performance of an electrochemical reactor or the selectivity of a reaction scheme should be restricted within the potential, concentration, and temperature range that they were determined. We should stress here that kinetic information is presently scanty for complex, multiple electrochemical reactions, yet it is essential for the design, optimization, and control of electrochemical processes. 

Many workers studied HER on various metal electrodes since Tafel s work. They attempted to obtain the correlation between the hydrogen overvoltage and the properties of the electrode material. During the course of these works, widely scattered voltage current relations were published by different workers. Kuhn et al. reviewed the previous works, and decided to lay down criteria to select the experimental data for their discussions. " They chose the data by only those workers who had used preelectrolysis for the purification of the electrolyte solution. Bockris described the importance of preelectrolysis of the electrolyte solutions in the study... 

Not only is the value of jQ important in electrocatalysis but also the experimental Tafel slope at the operating electrode potential. As expected in an electrocatalytic process, this complex heterogeneous reaction exhibits at least one intermediate (reactant or product) adsorbed species. Therefore, a single or simple Tafel slope for the entire process is not expected, but rather surface coverage and electrolyte composition potential dependent Tafel slopes within the whole potential domain are expected. Instead of calculating the most proper academic Tafel slope, the experimental current vs. potential curve is required for the selected electrocatalysts [4,6]. 

The Tafel colour reaction has been re-examined ° and a method, of potential value in synthesis, has been developed for the selective debenzylation of N-benzyl quaternary salts this involves the treatment of the salt at 0—5 C with lithium n-propylmercaptide (PrSLi) in HMPA. 

Figure 4. Ohmic Corrected Tafel Plots for Oxygen Reduction on Pt and Pt Alloy Electrocatalysts Prepared at Northeastern (note that PtCr/C is a system being considered only for ORR fundamentals it will not be under consideration for down-selection)...

Many different electrochemical and non-electrochemical techniques exist for the study of corrosion and many factors should be considered when selecting a technique. Corrosion rate can be determined by Tafel extrapolation from a potentiodynamic polarization curve. Corrosion rate can also be determined using the Stem-Geary equation from the polarization resistance derived from a linear polarization or an electrochemical impedance spectroscopy (EIS) experiment. Techniques have recently been developed to use electrochemical noise for the determination ofcorrosion rate. Suscephbility to localized corrosion is often assessed by the determination of a breakdown potenhal. Other techniques exist for the determinahon of localized corrosion propagahon rates. The various electrochemical techniques will be addressed in the next section, followed by a discussion of some nonelectrochemical techniques. 

Table 6.9 Corrosion Current Values of Selected Coatings Determined in 0.5 M H3BO3 Solution at 25 °C Using the Tafel Extrapolation Method...

In solution, the collisional interactions cause a blurring of the vibration-rotational states so that there is a semicontinuum of these states. This fact, together with the absence of a selection rule, causes the fulfillment of the Gurney condition at any energy, whereas the Bohr condition applies only for the situation where the transition moment is nonzero. The smooth relation between current density and overpotential, known as Tafel s law, " (in contrast to the peaked nature of the intensity-energy situation in spectroscopic phenomena) arises, therefore, from a combination of the absence of selection rules in the electrochemical case, and the presence in solution of a semicontinuum of acceptor and emitter states. 

Figure 3.5 (a) A typical potentiodynamic polarization plot for a Co electrode, graphically analyzed to determine E on and icon- (b) and (c) display coupled Tafel plots recorded in selected slurry solutions for the Co—A1 and Cu—Ta bimetallic systems, respectively. The juncture point (p) of the two plots in (h) corresponds to the galvanic parameters g and ig for the Co—A1 galvanic system. The plots in (c) represent a reversal of conventional galvanic polarities, where there are two crossover points (pi and P2). In all cases of (a), (b), and (c), the potential was scanned at a speed of 5 mV/s. 

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