Kinetic current densities

The kinetic and polarization equations described in Sections 6.1 and 6.2 have been derived under the assumption that the component concentrations do not change during the reaction. Therefore, the current density appearing in these equations is the kinetic current density 4. Similarly, the current density appearing in the equations of Section 6.3 is the diffusion current density When the two types of polarization are effective simultaneously, the real current density i (Fig. 6.6, curve 3) will be smaller than current densities and ij (Fig. 6.6, curves 1 and 2) for a given value of polarization. 

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. 

Curve 1 in Fig. 6.9 shows the influence of constant k, (or of parameters or which are proportional to it) on the current density at constant potential for a reaction with an intermediate value of k°. Under diffusion control (low values of/) the current density increases in proportion to/ . Later, its growth slows down, and at a certain disk speed kinetic control is attained where the current density no longer depends on disk speed. The figure also shows curves for the kinetic current density 4 and the diffusion current density /. 

Figure 9.14 Kinetic current density (squares) at 0.8 V for O2 reduction on the Pt monolayer deposited on various metal single-crystal surfaces in a 0.1 M HCIO4 solution, and calculated binding energies (circles) of atomic oxygen (BEq), as a function of calculated d-band center (relative to the Fermi level, ej — sp) of the respective surfaces. The data for Pt(lll) were obtained from [Markovic et al., 1999] and are included for comparison. Key 1, PIml/ Ru(OOOl) 2, PtML/Ir(lll) 3, PtML/Rh(lH) 4, PtML/Au(lll) 5, Pt(lll) 6, PIml/ Pd(lll). (Reproduced with permission from Zhang et al. [2005a].)...

Figure 9.16 ORR activity of two mixed-metal monolayer electrocatalysts supported on Pd(l 11), expressed as the kinetic current density at 0.85 V as a function of the M Pt ratio in the Pd-supported Pt-M monolayer. (Reproduced with permission from Zhang et al. [2005b].)...

Kinetic Data Catalyst Pt Dispersion (m /gpt) Half-Wave Potential at 1600rpm(V) Kinetic Current Density at 0.85 V (mA/cm ) Specific Kinetic Current Density at 0.85 V (A/m )... 

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. 

If transport were infinitely fast, the concentrations c x and c ed of nonadsorbing reacting species would be the same at the interface as in the bulk. The measured current density would solely be determined by the reaction, and would equal the kinetic current density. 

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... 

Determine the reaction order with respect to and the kinetic current density... 

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. 

Mass-transport-corrected currents at T = 30 °C, that is, the kinetic current densities evaluated from the measured T = 30 °C... 

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... 

Koslowski UI, Abs-Wurmbach I, Fiechter S, Bogdanoff P (2008) Nature of the catalytic centers of porphyrin-based electrocatalysts for the ORR a correlation of kinetic current density with the site density of Fe-N-4 Centers. J Phys Chem C 112(39) 15356-15366... 

Pt monolayer catalyst introduced by the Adzic group, is a particularly promising concept that allows ultra low Pt content [49]. The correlation between the position of i/-band center relative to the Fermi energy and measured kinetic current density has been reported by Zhang et al. for the Pt monolayer catalyst supported on single... 

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