Hydrogen reaction exchange current density

The slope of the Tafel curve drj/d log / is only one of the criteria that are required to determine the mechanism of the h.e.r., since different mechanisms, involving different r.d.s. often have the same Tafel slope. Parameters that are diagnostic of mechanism are the transfer coefficient, the reaction order, the stoichiometric number, the hydrogen coverage, the exchange current density, the heat adsorption, etc. 

Table 21.13 Exchange current densities for the hydrogen evolution reaction...

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. 

One of the many published correlations between the logarithms of exchange current density of the hydrogen reaction at different metals and the values of hydrogen-bond energy on the same metals is reported in Fig. 28.3. Considering all the points mentioned, one can certainly conclude that a certain bell shape does exist in this plot, but it would not be justified to draw any quantitative conclusions going beyond this assertion (Petrii and Tsirlina, 1994). 

Figure 9.4 Exchange current density for the hydrogen evolution reaction at various metals data taken from Trasatti [5].

The concentration of the 0.25N NaOH was selected because it has equivalent conductivity to the 3.5% NaCl solution. The initial current levels in the NaCl Versus NaOH tanks were different despite the equivalent conductivities and the equal applied potential. The difference was due to the difference in pH. The more basic solution reduced the exchange current density for the hydrogen reaction. At this potential, the principal reaction is the hydrogen reaction. 

Electrocatalytic Oxidation of Hydrogen The rate constant of the hydrogen oxidation reaction (HOR), as measured by the exchange current density jo (i.e. the current... 

Given that the rates of oxidation and reduction of the half-reactions are controlled by activation polarization only, that = 4-0.07 and = —0.08, and that the exchange current densities for both the oxidation of Fe and reduction of hydrogen in acidic solution are identical, use the data in Tables 3.3 and 3.4 to determine the following quantities. Recall that the potential for each half-cell is the sum of the equilibrium potential and the corresponding overpotential, in this case, r]a-... 

Since different surfaces will catalyze a given reaction to different degrees, it follows that io values are specific for a particular electrode surface. The exchange current densities for the evolution of hydrogen from 1 mol L-1 HC1, for example, range over some 11 powers of 10 (Table 15.1). 

Fig. 7.114. A plot of exchange current density for hydrogen evolution reaction vs. M-H bonding strength. (Reprinted hom S. U. M. Khan, Some Fundamental Aspects of Electrode Processes," in Modern Aspects of Electrochemistry, Vol. 15, R. E. White, J. O M. Bockris, and B. E. Conway, eds., Plenum 1983, p. 339.)...

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