Exchange current density definition

It was demonstrated, however, in 1947 by John O M. Bockris that between the exchange current densities of the hydrogen reaction at different metals and the values of the electron work function (into vacuum), a definite correfation does exist. Many workers have confirmed this correlation. An example of this correlation is shown as a plot of log f vs. X° in Fig. 28.2. 

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... 

The ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the ion of interest in the presence of various other ionic components of the sample. By definition, the ion-selective membrane forms a nonpolarized interface with the solution. If the interface is permeable to only one ion, the potential difference at that interface is expressed by the Nernst equation (6.6). If more than one ion can permeate, the interface can be anything between the liquid junction and the mixed potential. The key distinguishing feature is the absolute magnitude of the total exchange current density. 

The physical meaning of i, which is called the exchange current density, should be clear from its definition in Eq. 34E. It represents the rate at which the electrochemical reaction proceeds back and forth at equilibrium when the net reaction rate, observed as a current flowing through the external circuit, is zero. It is similar to the exchange rate discussed earlier in connection with Eq. 5E. We also note that i... 

Now we come to the concept of quasi-equilibrium. If there is a distinct rate-determining step in a reaction sequence, then all other steps before and after it must be effectively at equilibrium. This comes about because the overall rate is, by definition, very slow compared to the rate at which each of the other steps could proceed by itself, and equilibrium in these steps is therefore barely disturbed. To see this better, consider the specific example given earlier for chlorine evolution. Assume, for the sake of argument, that the values of the exchange current density i for steps 8F and 9F are 250 and 1.0 mA/cm, respectively. Assume now that we apply a current density of 0.5 mA/cm. We can calculate the overpotential corresponding to each step in the sequence, using Eq. 6E, namely... 

The second holds that metals passive by Definition 1 are covered by a chemisorbed film—for example, of oxygen. Such a layer displaces the normally adsorbed H2O molecules and decreases the anodic dissolution rate involving hydration of metal ions. Expressed another way, adsorbed oxygen decreases the exchange current density (increases anodic overvoltage) corresponding to the overall reaction M -1- ze. Even less than a monolayer on the surface is... 

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). 

By definition, the RPD In = elci is inversely proportional to the square root of the exchange current density (Equation 4.56). In contrast, the RPD la is independent of t. Physically, this means that the rate-determining process is the impeded proton transport through the CL. In other words, regardless of the available catalyst active surface, the conversion occurs close to the membrane, where the expenditure for proton transport is lower. 

From the previous section, it was seen that the activation losses depend primarily on the exchange current density, jo, and the charge transfer coefficient a. For a reaction with larger exchange current density, the required overpotential is smaller and hence there are lower electrode losses. Thus, increasing jo enhances the electrode kinetic performance. In order to understand how jo can be increased, we look at the definition of exchange current density. It is given by... 

From Equation 2.10 we can see that our two definitions of exchange current density occur in Equation 2.13, meaning it simplifies to yield... 

From the intuitive point of view, the concept of bond order is somewhat more ambiguous than that of atomic population. Whereas A a is usually construed as the number of electrons ascribed to an atom A, the bond order Bab niay be interpreted as a measure of bond strength, bond multiplicity, or the number of electron pairs involved in covalent interactions between A and B. Most of the currently known definitions of Bab stem from the last of these interpretations. Accordingly, they begin with the extraction of the exchange correlation term from the two-electron reduced density matrix y(ri, T2, r, rj). 

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