Electrode potentials current densities and

Current Density and Electrode Potential under Different Operating Conditions on Smooth and Rough Surfaces... 

It should be noted, however, that for redox processes the relation between current density and electrode potential is a direct consequence of the general relationship (56.1) between classical activation energy and reaction heat. Therefore, the Tafel equation and the deviations from it predicted by this relation cannot be used for a test of the oscillator model assumed. Such deviations were first observed by FROTKIN et al./170/ for the case of reduction of Fe(CN) " ions on a mercury electrode. PARSONS et al./171/ have shown that the... 

The activity and performance of fuel cell electrocatalysts need to be evaluated in terms of electrochemieal parameters, including current density and electrode potential. Electrochemieal sereening methods have been identified as ideal direct approaches for combinatorial studies of fuel cell catalysts. Two types of electrochemical measurement systems have been developed for combinatorial screening of fuel cell catalysts the array half-cell system and the array single-cell system. 

The general relation between current density and electrode potential was given by Tafel in 1905 (Tafel s equation) ... 

The relationship of anode current density with electrode potential for mild steel in dilute aqueous soil electrolytes has been studied by Hoar and Farrer. The study shows that in conditions simulating the corrosion of mild steel buried in soil the logarithm of the anode current density is related approximately rectilinearly to anode potential, and the increase of potential for a ten-fold increase of current density in the range 10 to 10 A/cm is between 40 and 65 mV in most conditions. Thus a positive potential change of 20 mV produces a two- to three-fold increase in corrosion rate in the various electrolyte and soil solutions used for the experiments. 

Beside laminar flow created by e.g. a rotating disc electrode mrbulent flow provides a means of artificially enhanced transport. A consistent mathematical description and analytical treatment of this mode of transportation is not possible. Various approximations have been proposed and tested for correctness [84Barl], an experimental setup has been described [78Ber, 83Her, 831wa]. From comparisons of measured and calculated current density vs. electrode potential relationships exchange current densities are available. (Data obtained with this method are labelled TPF.)... 

At mercury and graphite electrodes the kinetics of reactions (15.21) and (15.22) can be studied separately (in different regions of potential). It follows from the experimental data (Fig. 15.6) that in acidic solutions the slope b 0.12 V. The reaction rate is proportional to the oxygen partial pressure (its solution concentration). At a given current density the electrode potential is independent of solution pH because of the shift of equilibrium potential, the electrode s polarization decreases by 0.06 V when the pH is raised by a unit. These data indicate that the rate-determining step is addition of the first electron to the oxygen molecule ... 

Polarization is much higher for the electrochemical reduction of hydrogen peroxide. The slope has the unusually high value of about 0.3 V. At a given current density the electrode potential in this reaction is again independent of solution pH. These and certain other data indicate that addition of the first electron to the peroxide molecule and simultaneous peroxide decomposition is the rate-determining step ... 

FIGURE 9.2 Current density vs. electrode potential curves for the H2-02 and the CH30H-02 fuel cells showing the reaction overvoltages T a and T c at different catalytic electrodes (Pt, Pt-Ru,...). 

Figure 3.3.8 Schematic illustration of the origin of activation overpotentials in a hydrogen-oxygen fuel cell. The solid curves represent exponential analytic current densities versus electrode potential of the hydrogen electrode (standard potential 0 V) and the oxygen electrode (standard potential 1.23 V). Relevant for a PEMFC fuel cell are the HOR (anode) and the ORR (cathode) branches. To satisfy a cell current (yceii), the anode potential moves more positive by riact,HOR> while the cathode potential moves more negative by iiact.oRR- As a result of this, the observed cell potential is V, which is smaller than 1.23 V. The shape of the individual characteristics is such that the cathode overpotentials are larger than those at the anode.

A. Constant-current-density electrolysis (electrode potential and current density)... 

A. Constant-Current-Density Electrolysis (Electrode Potential and Current Density)... 

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. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Figure 11.2 A current density versus electrode potential curve for a multiple reaction system, of oxygen reduction, Fe(III) ion reduction and metal deposition.

This equation provides a relation between the current density and the potential for an electron transfer reaction at an n-type semiconductor electrode. 

Anodic Reactions in Electrocatalysis -Methanol Oxidation, Fig. 2 Current density versus electrode potential curves for electrochemical reactions involved in a PEMFC and in a DMFC... 

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