Conductive polymers exhibit several interesting and important properties, such as metallic conductivity and reversible convertibility between redox states. When the redox states have very different electrochemical and electronic properties, their interconversion gives rise to changes in the polymers' conformations, doping levels, conductivities, and colors, useful attributes if they are to be applied in displays, energy storage devices, actuators, and sensors. Unfortunately, the rate of interconversion is usually slow, at best on the order a few hundred milliseconds, because of the slow transport of counterions into the polymer layer to balance charge. This phenomenon is one of the greatest obstacles toward building rapidly responsive electrochemical devices featuring conductive polymers. One approach to enhancing the switching speed is decreasing the diffusion distance for the counterions in the polymer. We have found that nanotubular structures are good candidates for realizing rapid switching between redox states because the counterions can be readily doped throughout the thin nanotube walls. Although the synthesis of conductive polymer nanotubes can be performed using electrochemical template synthesis, the synthetic techniques and underlying mechanisms controlling the nanotube morphologies are currently not well established. We begin this Account by discussing the mechanisms for controlling the structures from hollow nanotubes to solid nanowires. The applied potential, monomer concentration, and base electrode shape all play important roles in determining the nanotubes' morphologies. A mechanism based on the rates of monomer diffusion and reaction allows the synthesis of nanotubes at high oxidation potentials; a mechanism dictated by the base-electrode shape dominates at very low oxidation potentials. The structures of the resulting conductive polymer nanotubes, such as those of poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole, can be characterized using scanning electron microscopy and transmission electron microscopy. We also discuss these materials in terms of their prospective use in nanotube-based electrochemical devices. For example, we describe an electrochromic device incorporating PEDOT nanotubes that exhibits an ultrafast color switching rate (<10 ms) and strong coloration. In addition, we report a supercapacitor based on PEDOT nanotubes that can provide more than 80% of its own energy density, even at power demands as high as 25 kW/kg.

We have investigated the electrochemical synthetic mechanism of conductive polymer nanotubes in a porous alumina template using poly(3,4-ethylenedioxythiophene) (PEDOT) as a model compound. As a function of monomer concentration and potential, electropolymerization leads either to solid nanowires or to hollow nanotubes, and it is the purpose of these investigations to uncover the detailed mechanism underlying this morphological transition between nanowire and nanotube. Transmission electron microscopy was used to characterize electrochemically synthesized PEDOT nanostructures and measure the extent of their nanotubular portion quantitatively. The study on potential dependency of nanotubular portion shows that nanotubes are grown at a low oxidation potential (1.2 V vs Ag/AgCl) regardless of monomer concentration. This phenomenon is attributed to the predominance of electrochemically active sites on the annular-shape electrode at the pore bottom of a template. The explanation was supported by a further electrochemical study on a flat-top electrode. We elaborate the mechanism by taking into account the effect of electrolyte concentration, temperature, and template pore diameter on PEDOT nanostructures. This mechanism is further employed to control the nanotube dimensions of other conductive polymers such as polypyrrole and poly(3-hexylthiophene).

We report the fast charging/discharging capability of poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes during the redox process and their potential application to a high-powered supercapacitor. PEDOT nanotubes were electrochemically synthesized in a porous alumina membrane, and their structures were characterized using electron microscopes. Cyclic voltammetry was used to characterize the specific capacitance of the PEDOT nanotubes at various scan rates. A type I supercapacitor (two symmetric electrodes) based on PEDOT nanotube electrodes was fabricated, and its energy density and power density were evaluated by galvanostatic charge/discharge cycles at various current densities. We show that the PEDOT-nanotube-based supercapacitor can achieve a high power density of 25 kW kg(-1) while maintaining 80% energy density (5.6 W h kg(-1)). This high power capability is attributed to the fast charge/discharge of nanotubular structures: hollow nanotubes allow counter-ions to readily penetrate into the polymer and access their internal surfaces, while the thin wall provides a short diffusion distance to facilitate the ion transport. Impedance spectroscopy shows that nanotubes have much lower diffusional resistance to charging ions than solid nanowires shielded by an alumina template, providing supporting information for the high charging/discharging efficiency of nanotubular structures.

and those measured on PDMS were 100.1 and 77.7. Surface energies of indium tin oxide (ITO) and Alq 3 and interfacial energy from the literature [14] which will bring revolutionary advances in the display technology, owing to attributes such as thin and flexible materials, fast switching times, and low-power consumption. However, current electrochromic technologies need to be improved in order to play moving images due to their slow color-switching rates.[1,2,5±11] Poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives are an ideal electrochromic material of conducting polymers for electronic paper due to their good color, mechanical stabilities, and facile fabrication.[5±11] Much work has been performed in order to improve contrast ratios and color switching rates by synthetic approaches.[5±11] It appears, however, that there are no examples of their use in electrochromic displays with moving-image speeds (24 frames/s; switching times of < 40 ms). This is due to the fact that the color-switching rate of PEODT is limited by the diffusion rate of counter-ions into the film during the redox process. The diffusion time, t, of ions required to reach a saturation concentration in a polymer film, that implies switching time, is proportional to the square of film thickness, x: t µ x 2 /D, where D is the diffusion coefficient of an ion in a polymer film. [12,13] Therefore, the simplest way to overcome the slow switching rates is to decrease the diffusion distance of ions, that is, to reduce film thickness. Based on the reported switching time of 2.2 s for a 300 nm thick PEDOT film, [5] we expect the switching time to be approximately 10 ms for a 20 nm thick film. However, the coloration of such a thin film is never sufficient for display applications. An array structure of PEDOT nanotubes provides an attractive solution to both of these limitations, slow switching rates and extent of coloration. Figure 1 explains that the wall thickness of PEDOT nanotubes can provide ions with short COMMUNICATIONS

We describe a new electrochemical synthetic method and characterization of poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes in a polycarbonate membrane. The very thin nanotubular structures in the highly flexible template provide fast electrochromic response (20 ms for oxidation or decoloration) even in a rolled state.

A structure comprising nanotubes on a transparent substrate is a key to fabricating fast window-type electrochromic devices. We describe the fabrication of a thin porous anodic aluminium oxide (AAO) structure as a template and poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes. The AAO template with small pore diameters (about 70 nm) is prepared by sputtering and subsequently anodizing a thin layer of aluminium on an indium-doped tin oxide/glass substrate. The PEDOT nanotubes are electrochemically synthesized on flat electrodes in the AAO nanopores. This requires a low monomer concentration and a high applied potential, mainly because of the increased monomer flux into the pores of the thin template due to the steeper concentration gradient. Fast colour changes (50 and 70 ms response times for oxidation and reduction, respectively) are achieved using partially-filled PEDOT nanotubes in a window-type electrochromic device.

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