Hydrogen exchange current density

Fig. 3. Hypothetical Evans diagram and polarization curve for a metal corroding in an acidic solution, where point A represents the current density, /q, for the hydrogen electrode at equiUbrium point B, the exchange current density at the reversible or equiUbrium potential, for M + 2e and point...

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. 

FIGURE 28.2 Relation between the exchange current densities of hydrogen evolution and ionization at different metals and the electron work functions. 

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. 

Figure 4. Typical polarization curve of the electrooxidation of hydrogen and electroreduction of oxygen the exchange current density, io, determined by extrapolation of E vs, log i to the reversible potential...

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