Exchange current densities, hydrogen evolution reaction

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

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

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.)...

Table 5.5 indicates die range of exchange current densities that have been observed for the hydrogen evolution reaction on various metals. Note that the... 

TABLE 5.5 The Exchange Current Density y0 for the Hydrogen Evolution Reaction in 1 M H2S04... 

Estimate the corrosion potential corr and the corrosion current density icorr of Zn in a deaerated HC1 solution of pH 1 at 298 K. In this solution Zn corrosion is accompanied by the hydrogen evolution reaction (h.c.r.). The parameters (standard electrode potential E°, exchange current density i0, Tafel slope b of Zn dissolution and the h.e.r. on Zn are... 

Return to the Fe dissolution experiment discussed above, altering the solution to contain 5 pM M Fe2+. In addition, allow hydrogen evolution to occur on the iron surface with an exchange current density of 1(T5 A/cm2, whereas the exchange current density for the iron reaction is 1CT6 A/cm2. Assume that both reactions have Tafel slopes of 100 mV/decade. These conditions are illustrated graphically in the Evans diagram, named in honor of its creator, U. R. Evans, shown in Fig. 25. The lines represent the reaction kinetics of the two reactions considered. 

Fig. 7F The exchange current density for the hydrogen evolution reaction as a function of the metal-hydrogen bond energy. Reprinted with permission from Trasatti, J. Electroanal. Chem. 39, 163. Copyright 1972, Elsevier Sequoia.

The reaction kinetics is particularly sensitive to trace amounts of noble metal impurities in the solution such as Cu and Au, which tend to deposit on the silicon surface. For example, an increase of the exchange current density from 5 x 10" A/cm to 2 X 10" A/cm is measured with 15 ppb copper ions as contaminant in the solution. On the other hand, deposition of metals such as Pb and Cd, which have lower exchange currents than silicon, inhibits the hydrogen evolution. ... 

This particular volcano plot is indeed nearly symmetric and demonstrates that Pt, Rh, Ir, and Re have the optimal M—H bond strength for maximum activity (exchange current density) for the hydrogen evolution/hydrogen oxidation reaction (HER/HOR). 

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