Charge Transport in Ferrocene-Containing Polyacrylamide-Based Redox Gels

Abstract

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 (Dap) 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 (NaH2PO4/Na2HPO4, NaClO4, or NaNO3). The range of measured Dap values [(0.6−6.0) × 10-7 cm2 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 Dap values are attributed to the large λ (range of molecular motion permitted to the ferrocene residues) and large kex (bimolecular electron self-exchange rate constant of the ferrocenes in the PVAB gels). It was further observed that Dap decreases on increasing the electrolyte (NaClO4 or NaNO3) 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 Dap 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.