Fast electrochemistry of conductive polymer nanotubes: synthesis, mechanism, and application

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.