Charge Transport in Ferrocene-Containing Polyacrylamide-Based Redox Gels

The charge transport properties of the redox gels formed from the copolymerization of vinylferrocene (VF) with acrylamide and N, N ‘-methylenebisacrylamide (PVAB gels) are described. The apparent electron diffusion coefficients ( D ap ) measured by cyclic voltammetry and chronocoulometry exhibit an upward bowlike curvature vs immobilized VF concentration ([VF] imm ) within the range 0.1−1.60 mM in 0.10 M supporting electrolyte (NaH 2 PO 4 /Na 2 HPO 4, NaClO 4, or NaNO 3 ). The range of measured D ap values [(0.6−6.0) × 10 -7 cm 2 s -1 ] can be interpreted in terms of the mean-field model developed by Blauch and Savéant (Blauch, D. N.; Savéant, J. M. J. Am. Chem. Soc. 1992, 114, 3323). The large D ap values are attributed to the large λ (range of molecular motion permitted to the ferrocene residues) and large k ex (bimolecular electron self-exchange rate constant of the ferrocenes in the PVAB gels). It was further observed that D ap decreases on increasing the electrolyte (NaClO 4 or NaNO 3 ) concentration from 0.10 to 0.50 M, especially at low (0.1−0.6 mM) and high (1.1−1.6 mM) [VF] imm, resulting in almost flat D ap vs [VF] imm profiles at 0.50 M electrolyte. This observation is attributed to increased nonbonded interactions between the polymer matrix, the redox sites, and the electrolyte upon increasing the electrolyte concentration.