Cathodic hydrogen evolution volcano plots

FIGURE 2.5 Qualitative volcano plots for log j vs. AG lds on different metal electrodes at the reversible potential of the hydrogen evolution reaction. The cathodic polarization is plotted as a (3Fr shift to the right and upward. 

As a result, it is to be expected that the maximum rate of hydrogen evolution will occur at intermediate values of AG ds which lead to a significant but not monolayer coverage by adsorbed hydrogen atoms. This is indeed observed and Fig. 1.16 shows a volcano plot of exchange current density vs AG ds for a series of metal cathodes. Similar dependences of rate parameters on the free energy of adsorption of an intermediate are common in gas-phase catalysis. 

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