Electrocatalyst exchange current density

For (AGh 0), these two regimes are approximately balanced, and a maximum in the exchange current density is predicted. This is the optimal value of the descriptor, and it immediately suggests that a reasonable goal for a computational, combinatorial electrocatalyst search is to find alloys with AGh values close to zero. 

It is often claimed that electrocatalysts in fuel cells are dependent on the exchange current density, i0 of the slowest reaction in the cell, (a) Make Tafel plots for i0 = 10 I,10 6, and 10-3 A cm-2 and bTaM = 0.12. (b) Then draw plots of the same type and the same i0, but with b values of 0.12, 0.05, 0.038, and 0.029 (T = 298 K). (c) Write out your conclusions concerning the interplay of /0 and b in the Tafel relation (B = RT/aF). (d) How does this relate to the choice of electrocatalytic surfaces for optimal fuel cell performance (Bockris)... 

Thus, it is possible for a material to be judged to be a better electrocatalyst than another on the basis of log ig values, but it may give a lower current density at, say, an overpotential of 200 mV than the other material if the b value for the latter is smaller. This is illustrated in Fig. 12 for a given process at two materials, I and II, at one of which the exchange current density is tg, and at the other /gj, with ig ig however, b, may be substantially lower than b , depending on the rate-controlling step. Thus actual currents at, say, rj = 200 mV may be substantially larger for process I than for process II (Fig. 12). 

The major dehciency of the oxygen electrode reaction is its low exchange current density (about 10 A/cm on a smooth surface) in acid electrolytes on even the best-known electrocatalyst (a platinum-chromium alloy). This value is about six orders of magnitude lower than that for the hydrogen electrode reaction in the same electrolyte. The reaction is about three orders of magnitude faster on smooth platinum or nickel oxide surfaces in an alkaline medium as compared to acid. The... 

In electrocatalysis, the activity of different electrocatalysts is usually expressed via the exchange current I0, and the specific activity, via the exchange current density, iQ (A cm-2), still often computed on the basis of the superficial electrode surface area. Only when the current is normalized using the true surface area of the electrode-electrolyte interface, the comparison between different electrocatalysts is truly meaningful. The determination of the true surface area of porous electrodes is discussed in Sect. 2.3.5. 

For a Pt(lll) surface, with a surface density of 1.5 x 1015 atoms cm-2, the current density corresponding to a TOF of Is-1 is 0.18 mA cm-2 for a one-electron charge-transfer process. Such exchange current densities based on the real electrocatalyst surface area are quite typical for a decent electrocatalyst for H2 evolution or oxidation. Thus, one may conclude that the order of magnitude of the TOFs of catalytic and electrocatalytic reactions are quite similar. In the latter... 

Electrocatalysts are produced in different ways, on different substrates, and for different purposes,10,64-72 but almost in all cases the electrochemical characterization was performed by using the cyclic voltammetry observations. In this way, it was not possible to analyze the effects of the mass-transfer limitations on the polarization characteristics of electrochemical processes. As shown recently,7,9 the influence of both kinetic parameters and mass-transfer limitations can be taken into account using the exchange current density to the limiting diffusion current density ratio, jo/ju for the process under consideration. Increased value of this ratio leads to the decrease of the overpotential at one and the same current density and, hence, to the energy savings. 

Fig. 9. Dependence of the exchange current density of various electrocatalysts on d-band character (112).

Again a primary current distribution can be obtained for small values of electrolyte conductivities, x- or better with large values of exchange current densities, j0. The latter is of interest in electrocatalysis, because by simply changing the nature or the composition of the electrocatalyst we can achieve small values of Wa, and then a uniform primary current distribution. For the case of the electrocatalytic agents used in the industry, the expression of the Equation 13.33 is not useful at all. By introducing large polarizations, we have... 

Fig. 13. Tafel relations for some electrode reactions on two electrocatalysts (land II). Though electrocatalyst II has higher exchange current density than I at most overpotentials, I is a better electrocatalyst than II at over potentials greater than jj. ...

Electrocatalysts for cathodic hydrogen evolution or its oxidation and catalysts for chemical hydrogenation are essentially the same platinum and the transition metals of group 10 of the periodic table. Hence, for catalysis and electrocatalysis the same correlation of catal5dic activity in terms of exchange current density (mA/cm ) and... 

Nonplatinum tungsten carbides have also been investigated for electrocatalytic ethanol oxidation. Pd/W2C-C and Pd/W2C-MWCNT (MW = multiwall) were tested in a half cell mode with 1 mol dm KOH and 1 mol dm ethanol, with Hg/HgO (1.0 mol dm KOH) reference electrode under various temperatures (61). Pd/WC-C and PdTWC-MWCNT electrocatalysts exhibited higher specific activities (mA/cm ), Tafel slope (mV/dec), and exchange current density (A/cm ) than Pd/C, Pd/MWCNT for ethanol oxidation. Especially, the WC-containing... 

Table 1.6. Exchange current densities of the hydrogen evolution reaetion at different electrode materials in aqueous 1 M H2SO4 solution at ambient temperature [80]. (Reprinted from Wendt H, Spinace EV, Oliveira Neto A, Linardi M. Eleetroeatalysis and electrocatalysts for low temperature fuel cells fundamentals, state of the art, researeh and development. Quim Nova 2005 28 1066-75. With pertiussion from Soeiadade Brasileira de Quimica.)...

Two electrochemical parameters define the electrochemical properties of an electrocatalyst material the intrinsic exchange current density fl and the Tafelparameter b. These parameters are determined from current-potential relationships measured by voltammetric methods of classical electrochemistry (Bard and Faulkner, 2000). One could write an expression for b of the form... 

The main basic parameter of catalyst evaluation is the specific exchange current density which, by definition, is normalized to the unit surface area of the electrocatalyst. This property is the target of many fundamental studies in electrocatalysis, too numerous to be listed (Adzic et al., 2007 Debe, 2013 Gasteiger and Markovic, 2009 Kinoshita, 1992 Paulus et al., 2002 Stamenkovic et al., 2007a,b Tarasevich et al., 1983 Zhang et al., 2005, 2008). 

The exchange current densities io 3 and io 3 of the anode and cathode, respectively, are of cracial importance as they determine the magnitude of 1133,3 and 1133, 3 via Equation (13.14). A good electrocatalyst is characterized by a high value of the exchange ciurent density io. 

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