Electrocatalytic reactions adsorption free energy

To illustrate the influence exerted by the energy of adsorption of an intermediate on the rate of an electrocatalytic reaction, consider a very simple two-step reaction of the type A —> X —> B where X, the intermediate, is reversibly adsorbed on the electrode (with a degree of surface coverage 9x). For the sake of simplicity, the electrode surface will be assumed to be homogeneous (i.e., conditions of Langmuir adsorption hold), while the system lacks adsorbed species other than X. The rate, of the adsorption step (the first step) is then proportional to the bulk concentration of the starting material, c, and to the free surface part (1 - 9x) (the part not taken up by species X), while the rate of further transformation of intermediate X, which is tied to its desorption, will be proportional to the surface fraction, 9x, taken up by it ... 

The hydrogen evolution reaction is an example where its electrocatalytic character shows that it is necessary for both the description of the adsorption process and the knowledge of the kinetic parameters. Most analysis of the electrocatalytic properties involve correlations from the estimated exchange current densities with characteristic electric potentials, free energies of adsorption, enthalpy of sublimations for the metal electrode, etc. [60]. 

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