Shifts in Redox Formal Potentials Accompanying the Incorporation of Cationic Complexes in Perfluoro Polycarboxylate and Polysulfonate Coatings on Graphite Electrodes

The formal potentials of several redox couples incorporated in coatings of a perfluoropolycarboxylate on graphite electrodes were measured and compared with the formal potentials of the same couples in homogeneous solution. The differences observed agreed with those calculated from the Nernst equation with the independently measured incorporation coefficients for both halves of the redox couples. The dependences of the shifts in formal potentials on the nature of the incorporating complex ion, the ionic strength, and the temperature were determined and indicated that the incorporation equilibrium is governed by electrostatic and hydrophobic interactions that act in opposite directions. The incorporation of most cation s examined was driven by large increases in entropy which overcame the usually unfavorable enthalpy changes. Redox reactants incorporated in polyelectrolyte coatings on electrode surfaces will exhibit formal potentials that are shifted from their values in solution whenever the two oxidation states of the reactant have different equilibrium constants for incorporation by the polyelectrolyte. This situation has been recognized in earl ier reports (1, 2) .and in a recent study this aspect of the electrochemistry of redox polymer coatings was examined in detail (3). In continuing studies of the electrochemical responses obtained from redox reactants at electrodes coated with Nation, I, and the related polyelectrolyte, II, [(CF2-CF2)m-'CFCF2.:.~ n