Enhancement of charge-transport rates by redox cross-reactions between reactants incorporated in Nafion coatings

The rate of charge transport within Nafion coatings by Co(typ)₂³⁺ (tpy = 2,2′,2″-terpyridine) is enhanced when it is measured in the presence of Ru(NH₃)₆²⁺ with which it undergoes a rapid electron-transfer cross-reaction. Similarly, the presence of Cp₂FeN(CH₃)₃²⁺ (Cp = cyclopentadienide) enhances the propagation rate of Co(tpy)₂²⁺. Cyclic voltammetry was used to demonstrate the effect qualitatively and chronocoulometry to document it quantitatively. The enhancement results because electron transfer between the more slowly diffusing complex (Co(tpy)₂³⁺ or Co(tpy)₂²⁺) and the more rapidly diffusing Ru(NH₃)₆²⁺ or Cp₂FeN(CH₃)₃²⁺ complexes allows the latter to carry charge between the electrode surface and the slower moving complex. A formula for the extent of the enhancement is derived for the case of potential step chronocoulometry or chronoamperometry and used to compare calculated enhancements with those measured experimentally. Some implications of electron-transfer enhancement of charge propagation rates for electrocatalytic applications with polymer-coated electrodes are pointed out.