Electrode kinetics current density

In those cases where i. (region A in Eig. 6.6), the real current density i essentially coincides with the kinetic current density i 4, and the electrode reaction is controlled kinetically. When 4 ik (region C), we practically have i 4, and the reaction is diffusion controiled. When 4 and 4 have comparable values, the electrode operates under mixed control (region B). The relative valnes of these current densities depend on the kinetic parameters and on the potential. 

The basic relationships of electrochemical kinetics are identical with those of chemical kinetics. Electrochemical kinetics involves an additional parameter, the electrode potential, on which the rate of the electrode reaction depends. The rate of the electrode process is proportional to the current density at the studied electrode. As it is assumed that electrode reactions are, in general, reversible, i.e. that both the anodic and the opposite cathodic processes occur simultaneously at a given electrode, the current density depends on the rate of the oxidation (anodic) process, ua, and of the reduction (cathodic) process, vc, according to the relationship... 

This is the Tafel equation (5.2.32) or (5.2.36) for the rate of an irreversible electrode reaction in the absence of transport processes. Clearly, transport to and from the electrode has no effect on the rate of the overall process and on the current density. Under these conditions, the current density is termed the kinetic current density as it is controlled by the kinetics of the electrode process alone. 

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

All thermodynamic and kinetic parameter values needed in calculating the electrode reaction current densities are listed in Table 1. 

Figure 17 Potentiodynamic (0.1 V/s) oxidation current densities for several H2/CO mixtures on a PtRu-colloid/Vulcan electrode (7 pg/cm ) at 60°C and 2500 rpm. The upper abscissa gives the kinetic current density while the lower uses a scale-up factor of 143 to simulate the performance of a fuel-cell electrode. The electrooxidation of pure H2 at the same electrode is shown for reference. (From Ref. 58.)...

Figure 10 Dependency of kinetic current density on the alloy composition at Pt-Ni, Pt-Co and Pt-Fe electrodes in 0.1 M HCIO4 solution saturated with O2 at room temperature. 

The main objectives of the electrocatalytic reaction kinetics are to find j0, p, and (3, which are characteristics of a given electrode system. In general, the procedure is as indicated above. We can define the kinetic current density, jk, as that current that is not affected by mass transfer effects ... 

The kinetic current densities were believed to have two major contributors (1) the chemical reaction between O2 and the catalyst active site to form an adduct prior to the electron-transfer reaction, and (2) the electron-transfer reaction in the RDS. Therefore, the ORR kinetic current density (fk) may be expressed as a function of electrode potential ( ), as in Eqn (7.4) ... 

Table 7.6. ORR Kinetic Current Densities and Overall Electron Transfer Numbers. Obtained Based on tbe Data in Figure 7.19, Collected on a Co-N/C Coated Glassy Carbon Electrode Rotating at Various Rotation Rates. Electrolyte 02-Saturated 0.5 M H2SO4 Solution Co Loading in the Coating Layer ...

Electrode Potential. E(V vs. RHE) Kinetic Current Density, ik (mA cm ) Overall Electron Number, n... 

Jiang et al. [15] reported that the ORR activities in alkaline medium of a Pd-coated Ag/C were three times higher than the corresponding activities on the Pt/C, as measured in their rotating disk electrode tests. Piana et al. [16] reported that the specific current of their K18 non-PGM catalyst is about three times higher than Pt/C and also its Tafel slope is lower, as it is observed for other non-Pt catalysts [17]. He et al. [18] reported that the kinetic current density of their non-noble metal catalyst based CuFe-Nx/C material was comparable or even higher than a commercial Pt/C catalyst. 

Under current flow, the kinetics of the electrode reactions comes into effect. The Butler-Volmer equation describes the dependency of the current passing through the electrode interface (current density per unit geometric area) on a small voltage excursion (called overvoltage q) from the respective equilibrium potential E . ... 

As the potential of the electrode is increased or reduced, other electrochemical reactions may become possible and may therefore occur in competition with each other at the same electrode surface. Since electrochemical reactions are surface reactions, the rate of each reaction is conveniently referred to the unit surface area of the electrode. In fact, the rate of an electrochemical reaction is given as the current density and is expressed in the SI units of A m-, though multiples and submultiples are widely employed in practice. The kinetic behavior of a given electrochemical system, that is the electrode-electrolyte system, is experimentally characterized by recording the polarization curve at the electrode, i.e., the electrode potential-current density relationship. 

The first law of electrode kinetics, observed by Tafel in 1905 [197], is that overvoltage i) varies with current density i according to the equation... 

In electrode kinetics a relationship is sought between the current density and the composition of the electrolyte, surface overpotential, and the electrode material. This microscopic description of the double layer indicates how stmcture and chemistry affect the rate of charge-transfer reactions. Generally in electrode kinetics the double layer is regarded as part of the interface, and a macroscopic relationship is sought. For the general reaction... 

The exchange current density, depends on temperature, the composition of the electrolyte adjacent to the electrode, and the electrode material. The exchange current density is a measure of the kinetic resistance. High values of correspond to fast or reversible kinetics. The three parameters, a, a. ... 

Diffusion overpotential. When high current densities j exist at electrodes (at the boundary to the electrolyte), an impoverishment of the reacting substances is possible. In this case the reaction kinetics are determined only by diffusion processes through this zone, the so-called Nernst layer. Without dealing with the derivation in detail, the following formula is obtained for the diffusion overpotential that occurs (with as the maximum current density) ... 

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