Electroactive Behavior of Adjustable Vinylferrocene Copolymers in Electrolyte Media

The redox-active properties of a series of ferrocene-containing vinyl polymers were investigated in aqueous and organic media. Each metallopolymer contained vinylferrocene (VFc) and a non-redox-active species (X), and was combined with carbon nanotubes (CNT) to generate P(VFc(n)-co-X1-n)-CNT composites for heterogeneous electrochemical analysis. Tunable pseudocapacitances spanning ca. 0.03-280 F/g VFc in aqueous solution were achieved by varying the copolymer composition, with P(VFc(0.11)-co-HEMA(0.89)) producing standardized values at ca. 160-180 F/g VFc even for differently hydrated anions. Additionally, the polymer-bound ferrocene/ferrocenium redox potential was seen to depend prominently on its electrolyte anion's Gibbs free energy of hydration. Although the hydrophilic chloride anion negatively influenced the electrochemical stability of the VFc units when in their PVFc homopolymer, copolymerizing them with 2-hydroxyethyl methacrylate (HEMA) and introducing perchlorate anions ameliorated their overall capacity retention by 64% and 38%, respectively. Lastly, the electrodes' responses in aprotic and protic solvents were examined for correlations with numerous solvent polarity metrics and solubility measures, with a notable observation being the stability and pseudocapacitive increase of the styrene (St)-containing P(VFc(0.27)-co-St(0.73))-CNT from 5 to ca. 190 F/g VFc when in methanol instead of water. This study can help provide insight regarding material design considerations for redox moiety implementation in electrochemical applications.