A Versatile Material for a Symmetrical Electric Energy Storage Device: A Composite of the Polymer of the Ferrocene Adduct of C₆₀ and Single-Wall Carbon Nanotubes Exhibiting Redox Conductivity at Both Positive and Negative Potentials

Abstract

A (carbon nanotube)–(fullerene–ferrocene dyad polymer) composite, pyr-SWCNTs/(C60Fc-Pd), was devised and tested as an active material of a symmetrical device for electric energy storage. The composite was redox conducting at both positive and negative potentials due to the Fc/Fc+ and C60–/C60 electrode process of the ferrocene and fullerene moiety of the dyad, respectively. The composite was prepared, first, by electrophoretic deposition of a film of noncovalently modified with 1-pyrenebutyric acid stacked single-wall carbon nanotubes (pyr-SWCNTs). Then, this film was coated, by potentiodynamic electropolymerization, with a film of the palladium-doped polymer of 2′-ferrocenylfulleropyrrolidine (C60Fc-Pd). The AFM imaging showed that the C60Fc-Pd film was uniformly built of 40–150 nm diameter globules of C60Fc-Pd while the film formed a tangle of pyr-SWCNTs bundles wrapped with clusters of ∼45 nm diameter globules of C60Fc-Pd. The XPS measurements identified interactions between the Pd and C60 moieties in C60Fc-Pd. The electrochemical and viscoelastic properties of the pyr-SWCNTs/(C60Fc-Pd) film in a blank acetonitrile solution of tetra-n-butylammonium perchlorate favorably compared to those of the C60Fc-Pd film as disclosed by simultaneous cyclic voltammetry (CV) and piezoelectric microgravimetry (PM) measurements. Specific capacitance (Cs) of the pyr-SWCNTs/(C60Fc-Pd) film was higher than that of the C60Fc-Pd film and for positive charging its maximum value was Cs,max ≅ 300 F g–1. The EIS measurements revealed capacitive and mixed capacitive and pseudocapacitive behavior of the composite over the narrow and wide range of the applied potentials, respectively. Alternate galvanostatic +0.72 mA cm–2 charging and −0.72 mA cm–2 discharging and then opposite polarity charging of a device constructed of two identical composite film electrodes was reversible only for low (+0.6 and −0.6 V) voltage of charging and discharging. After multiple charging and discharging (1000 cycles), the capacity increased by ∼5%, but it decreased by ∼18% for the high (+1.2 and −1.2 V) voltage. Importantly, the average power of the device was ∼7.3 times higher for the latter.