Influence of Supporting Electrolyte Concentration and Composition on Formal Potentials and Entropies of Redox Couples Incorporated in Nafion Coatings on Electrodes

摘要

The potentials (often termed Donnan potentials) presents at the interface between polyelectrolyte coatings on electrodes and the solutions in which they are used contribute to the formal potentials measured for redox couples incorporated in the coatings. The dependence of the Donnan potentials on the concentration of supporting electrolyte causes the apparent formal potentials to shift with concentration in predictable ways that are discussed. With pH-dependent redox couples, shifts in formal potential reflect both the concentration and the pH of the supporting electrolytes. Spectral and electrochemical measurements with pH-dependent couples incorporated in Nafion coatings allowed the changes in pH inside the coatings resulting from changes in the supporting electrolyte pH to be monitored. Comparisons with the changes calculated on the basis of Donnan equilibrium and ion-exchange selectivity coefficients are presented. The contributions of Donnan potentials to apparent formal potentials prevent simple correlations of differences in formal potentials of redox couples inside and outside polyelectrolyte coatings with their intrinsic strengths as reducing or oxidizing agents in the two environments. The temperature dependences of formal potentials measured in polyelectrolyte coatings include an entropy of transfer as well as the reaction entropy of the redox couple involved. The two components of the measured entropies can have comparable magnitudes under common experimental conditions.