Kinetics of the ferric ferrous electrode reaction on Nafion®-coated electrodes

Cyclic and convolution voltammetry is used to study the kinetics of the ferric/ferrous electrode reaction on Nafion(R) coated Au and Pt electrodes in a perchloric acid solution. The reaction is characterized by the standard rate constant k(s)(0) approximate to 10(-6) cm s(-1) of the electron transfer across the metal \ Nafion(R) interface and by the diffusion coefficient of the ferric ion in the Nafion film D-(m) = (6.7 +/- 1.4) x 10(-8) cm(2) s(-1). The Nafion(R) film apparently prevents the trace anions present in the perchloric acid solution (e.g. chlorides and sulfates) reaching the electrode surface and, thereby, influencing the ferric/ferrous electron transfer reaction via the inner-sphere catalysis, In addition, the polymer phase introduces a steric factor which causes the rate of the electron transfer reaction to decrease compared to the uncoated electrode, and proportionally the ion diffusion flux toward the metal surface, so that the values of the ratio k(s)(0)/D-1/2 for the coated and uncoated electrodes are comparable. It is proposed that the considerably higher rate of the electron transfer on the Nafion(R) coated electrode in the sulfuric acid solution is due to the catalytic effect of sulfate or hydrogensulfate anions, which are transported through the Nafion(R) film as the sulfate complexes of the ferric cation, i.e. FeSO4+ and FeHSO42+. The catalytic effect of oxides on the Nafion(R) coated electrodes is demonstrated. (C) 1999 Elsevier Science S.A. All rights reserved.