Electrocatalysis exchange current density

Erdey-Gruz, 1048, 1306 1474 Erschler, 1133, 1134, 1425 Ethylene oxidation, anodic, 1052 1258 Exchange current density, 1049, 1066 correction of, 1069 definition, 1053 electrocatalysis and, 1278 impedance and, 1136 interfacial reaction, 1047 and partly polarizable interface, 1056 Excited states, lifetime, 1478 Exothermic reaction, 1041 Ex situ techniques, 785, 788, 1146... 

Changes of A from one metal to another, for a given process (e.g. the HER), provide the principal basis for dependence of the kinetics of the electrode process on the metal and are recognized as the origin of electrocatalysis associated with a reaction in which the first step is electron transfer, with formation of an adsorbed intermediate. In the case of the HER, this effect is manifested in a dependence of the logarithm of the exchange current density, I o (i.e., the reversible rate of the process, expressed as A cm , at the thermodynamic reversible potential of the reaction) on metal properties such as 0 (Fig. 2) (14-16, 20). However, as was noted earlier, for reasons peculiar to electrochemistry, reaction rate constants cannot depend on under the necessary condition that currents must be experimentally measured at controlled potentials (referred to the potential of some reference... 

One of the most important phenomenological aspects of electrocatalysis is the dependence of standard rate constants or exchange current densities, Iq (see Section III), for the reaction concerned on the properties and chemical identity of the electrode metal (Fig. 15) and/or the state and orientation of its surface. In fact, this is the basis of the good deffnition of electrocatalysis proposed by Busing and Kauzmann (12). 

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. 

Fig. 5.39. (a) Extrapolation of semilogaiithmic current voltage curves to the equilibrium potential. Eg, for determining the exchange current density ig. (b) Increase of the exchange current density due to electrocatalysis, (c) Access of alternative mechanisms with decreased overpotential at high current densities due to electrocatalysis with the consequence of reduced slope of the semilogarithmic... 

It can be seen from the figure that, with the r.d.s. quoted, the stronger the H-M bonding, the smaller the heat of activation and the faster the reaction. If one makes a plot of the logarithm of the exchange current density, jQ, for hydrogen evolution, as a function of the bond strength of H to the metal, one can see that log iQ increases with an increase of M-H. This, then, is electrocatalysis in a very explicit way [10]. 

John Agar was a Cambridge man and for many years a stalwart of the university s Department of Chemistry, where he made contributions to physical electrochemistry. Why he is mentioned among those who founded the subject of electrocatalysis is because it was he who introduced the term exchange current density [15], and it is in this term that academics tend to define and measure electrocatalysis. 

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... 

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... 

The intrinsic exchange current density,/ , is not a mere materials constant, but it depends on size distributions of catalyst nanoparticles, their surface structure, as well as surface composition in the case of alloy catalysts like PtRu. In this section, we discuss modeling approaches that highlight particle size effects and the role of surface heterogeneity in fuel cell electrocatalysis. 

Many electrode reactions only occur at a measurable rate at a very high overpotential, i.e. the exchange current is low. The art of electrocatalysis is to provide alternative reaction pathways which avoid the slow step and permit the reaction to be carried out with a high current close to the reversible potential, i.e. to increase the exchange current density. 

The term electrocatalysis is, however, more commonly applied to systems where the oxidation or reduction requires bond formation, or at least a strong interaction of the reactant, intermediates, or the product with the electrode surface. The catalyst is the electrode material itself or a species adsorbed from solution. This chapter will discuss this more limited definition of electrocatalysis (note also that simple electron transfer reactions which are pictured as occurring by an outer sphere mechanism and may have very high exchange current densities, are not normally considered within electrocatalysis — in this book they are dealt with in Chapter 3). 

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). 

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