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Abstract

Concurrent and cathodic electrolysis of tris(cyanopyridinio)mesitylene and zinc chloride gave redox-active polyviologen, densely formed around the zinc oxide nanorod, on a substrate, which displayed a large chargingdischarging capacity and an excellent rate performance based on a large diffusivity of charge, and was useful as an anode-active material.Organic polymers bearing redox-active groups or redox polymers have attracted much attention for their potential applications in flexible paper-like electronic devices. 1 Among the redox-active groups, viologen (1,1¤-dialkyl-4,4¤-bipyridinium dications) derivatives have been widely studied because they possess the following characteristics: 2 (i) moderately negative redox potentials in the range of ¹0.4 ¹0.8 V vs. Ag/AgCl, (ii) two-electron redox reaction leading to a large redox capacity (Figure 1), (iii) robustness of the cation radical and neutral state (V + and V 0 in Figure 1), and (iv) resistivity against oxidative degradation. These characteristics have been utilized in studies of electrochromic displays 3 and sensors, 4 and are often examined as anode active materials in rechargeable devices. 5 Viologen derivatives are dialkylene salts of the bipyridinium unit, and are insoluble in organic solvents but soluble in water. They have to be modified for their immobilization on substrates in the use with aqueous electrolytes, because of their favorable low electric resistivity (10 ¹2 10 ¹3 S cm ¹1 ). The high equivalent electrical conductivity of aqueous electrolyte solutions could contribute to the decrease of cell resistance in supercapacitors and batteries. 6 The aqueous electrolyte also enhanced charge transport in the electrodes, which resulted in, e.g., an ultrafast chargeable hydrophilic cathode-active material in the aqueous electrolyte. 7 Viologen derivatives have been extended to their polymeric analogues to insolubilize while maintaining their swellable properties in aqueous electrolytes and to utilize them as a redox-active electrode. A hydrophilic polyviologen was converted to the corresponding water-insoluble one by complexing it with a polyanion such as poly(styrenesulfonate); 8 however, this was accompanied by the partial loss of its redox capacity per weight.Crosslinking via the electropolymerization of multifunctional viologens was also studied for insolubilization. 9 Advantages of electropolymerization lie in the potentially easy and fast fabrication, good reproducibility, and tunability of layer thickness. Crosslinked polyviologens were prepared by the reductive electropolymerization of trifunctional precursors. The polymers were deposited on substrates, which were swelled but insoluble in water regardless of the charged state (V 0 , V •+ , and V 2+ in Figure 1). They performed a more satisfactory redox capability than a liner polyviologen formed from a bifunctional precursor, probably because of the high density of the viologen moieties per unit.The efficiency of redox polymers as electrode-active materials, such as redox capacity and/or the ch...