The driving-force dependence of electrochemical rate parameters: Origins of anodic-cathodic asymmetries for metal-aquo redox couples

The consequences of differences in the intramolecular force constants and the ionic entropies between the oxidized and reduced states of aquo redox couples upon their electrochemical kinetics are examined as a function of the driving force. A generalized harmonic oscillator model is utilized that involves estimating the activation barrier from the individual force constants in the oxidized and reduced states rather than employing average ("reduced") values. Noticeable asymmetry in the anodic and cathodic Tafel plots is predicted for redox couples having large (ca. twofold) differences in force constants, the plots being more curved at anodic overpotentials. The calculated plots are in reasonable agreement with experimental Tafel plots determined previously for Cr(OH₂)₆^3+/2+ and Eu(OH₂)_n^3+/2+ at the mercury-aqueous interface. Comparisons are also presented between electrochemical "ideal" activation parameters determined for Cr(OH₂)₆^3+/2+ as a function of anodic and cathodic overpotentials and the corresponding parameters calculated from structural and thermodynamic data. Substantially smaller activation enthalpies are observed for Cr(OH₂)₆²⁺ oxidation than for Cr(OH₂)₆³⁺ reduction; both are close to the theoretical predictions. The anodic activation enthalpies approach zero at moderate overpotentials even though the activation entropies remain large and negative. Parallels are also drawn with corresponding results and data interpretations of the driving-force dependencies for related homogeneous reactions.