Effect of adsorption on electrode reactions

Adsorption influences the rate of electrode reaction in several ways (a) change of electrostatic potential at the reaction site, (b) geometric effect of coverage of the interface by the adsorbate, which may not necessarily be electroactive, and (c) change in the free energy of solvent or adsorbate at the electrode—solution interface with change in electrode potential and concentration of adsorbate in solution. 

The simplest effect of pure electrostatic ionic adsorption on electrode reaction rates of ions is the Frumkin double layer effect already been discussed in Sect. 3.5. 

On the other hand, electroinactive but surface active substances (SAS) adsorbed at the electrochemical interface affect the rate of electrode reactions and clear examples are organic additives used in metal deposition and inhibitors of metal corrosion. In a few cases, these substances accelerate the rate of electrode reactions [118, 119]. 

The simplest treatment of this problem considers that, for a given potential, the electrode reaction rate coefficient is a linear function of the coverage by adsorbate. The overall electrode reaction rate coefficient is thus expressed as a weighted linear combination of the rate coefficients at the covered sites, fex, and at the adsorbate free surface, kQ 

If the rate of the electrode reaction is negligible when the electrode is totally covered by the adsorbate, kx k0, then a blocking effect by the adsorbate is observed 

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