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The rate of charging of supercapacitor and battery electrodes is often limited by the transport of ions through the active electrode material. One way to accelerate this transport is to increase the porosity of the electrode, at the cost of electrode capacity. This tradeoff is studied through the fabrication and characterization of porous carbon nanofibre/polypyrrole films used as storage electrodes. Electrospun poly (acrylonitrile-co-acrylamide) fibres were carbonized and then electrochemically coated with variable amounts of polypyrrole. The resulting hybrid materials were characterized using both conventional methods such as cyclic voltammetry, electron microscopy and conductivity, as well as ionic conductivity and pulsed-field-gradient nuclear magnetic resonance which directly probe ion transport. It is found that with modest porosity, these materials retain much of the capacitance (∼50%) of bulk polypyrrole with dramatically increased (∼300 times) charge and discharge rates, suggesting the potential of the approach for increasing the useful frequency range of polypyrrole-based supercapacitors, as well as other storage materials whose porosity can be varied. Simple transport models based on series or parallel arrangements of electrolyte and active electrode are developed to explain the results and identify the factors limiting charge/discharge rates.

Electrochemically driven actuation of polypyrrole in aqueous sodium hexafluorophosphate (NaPF 6 ) solution has been shown to produce repeated large strains (>6%) at low voltages and with high conductivity, making it one of the most promising electroactive conducting polymers. Little is known about the voltage dependent stiffness of this version of the polymer. This information is important in determining the strain as a function of load. In this paper the complex Young's modulus (storage and loss components) of a hexafluorophosphate-doped polypyrrole film in aqueous NaPF 6 electrolyte at different oxidation states, under various loads and as a function of the frequency of the applied load, is investigated. Uniformity of doping was ensured by allowing enough time to reach steady state charge levels, and the creep during measurements was minimized by using preconditioning cycles. The results of this study show that storage modulus decreases (from 1 GPa to 0.80 GPa) as the polypyrrole oxidation potential increases (from −0.4 V to +0.4 V versus Ag/AgCl reference electrode). The loss modulus, on the other hand, increases from 55 MPa to 80 MPa. An increasing trend in the Young's modulus is also observed with the applied load. The storage modulus increases from 0.65 GPa to 1 GPa by increasing the applied load from 0.2 MPa to 2.5 MPa. The modulus is found to increase with time through the experiment, which may be due to stretch alignment of the polymer. It is also observed that complex Young's modulus increases in proportion to the logarithm of frequency.

A variety of NMR techniques are employed to examine the abundance, dynamics, and translational diffusion of PF 6 − ions in doped polypyrrole films in various oxidation states. NMR is employed as a non-invasive alternative to elemental analysis for quantifying ion content, which is found to decrease linearly with decreasing electrochemical potential, directly confirming the ion intercalation mechanism of polymer actuation, and demonstrating the capacitive nature of the polypyrrole film/electrolyte system. With known ion content, sample mass, and deposition current, the relative amounts of pyrrole, ions, and solvent in the films can be determined. A T 1 relaxation study along with 1D nuclear Overhauser effect (NOE) difference experiments reveal that the rotational correlation time and solvent accessibility of PF 6 − ions in the oxidized films are similar to those in the solvent, indicating the ions experience a solvated environment, and do not sit at stable sites in the polymer matrix. A drastic decrease of the NOE enhancement, along with changes in relaxation behavior and electrical conductivity in reduced films implies that polypyrrole undergoes a significant structural change when reduced. This change leads to a much less solvated ion environment, and may be responsible for the expansion of films sometimes observed upon reduction. Translational motion of the PF 6 − ions in the oxidized films is probed via diffusion measurements made using pulsed-field gradient NMR. The diffusion coefficients found ( ∼ 5 × 1 0 − 9 cm2/s) are an order of magnitude smaller than those typically extracted from impedance spectroscopy measurements; this is explained in terms of field driven ion migration in the impedance spectroscopy case.

The electronic, structural, and chemical properties of polypyrrole doped with the hexafluorophosphate ion are investigated as a function of oxidation state. These properties are found to be highly correlated; specifically, they all experience a rapid transition at approximately −0.1 V versus SCE. The electronic conductivity of the film changes by two orders of magnitude through the transition potential, in the well-known doping-induced metal–insulator transition. Also at −0.1 V versus SCE, X-ray diffraction and macroscopic actuation measurements reveal a structural change in the polymer. Results suggest a loss of pi-stacking in the polymer crystals, and a reordering of the dopant ions in the matrix. The dopants appear to exist in an isotropic liquid-like state in the highly oxidized film, changing to a channel structure at the transition point. This change in structure is also consistent with the transition observed in the electrochemical impedance spectrum. The equilibrium charge on the polymer with oxidation state is consistent with a constant DC capacitance, however the AC impedance spectrum shows increased constant phase element behavior in the reduced state. This could be due to a greater range of diffusion constants in the reduced state, which we associate with the heterogeneous ion configuration.

Conducting polymers demonstrate effective capacitances of more than 100 Farads per gram, 5 orders of magnitude higher than traditional capacitors. However polymer discharge times tend to be on the order of seconds, as opposed to the milli or microseconds of conventional capacitors, so that the overall power density is still at least an order of magnitude lower. The polypyrrole devices are essentially electrolytic capacitors in which charging times appear to be limited by rates of ionic mass transport and RC charging times. Electrochemical impedance measurements suggest diffusion time constants of 33 ms in 158 nm thick polypyrrole films with volumetric capacitances of 107 F/m3. The impedance of a highly porous polypyrrole/carbon composite was measured to investigate the achievement of similarly fast response times in much thicker materials. It is shown that increasing the polypyrrole content of the film increases capacitance up to 60 F/g, but also increases the charging time constant. Analysis of the rate limiting factors suggests a method of optimizing capacitor geometry in order to maximize rates, including the prediction of device geometries that will lead to power delivery matching those of traditional capacitors.

Conducting polymer-based actuators are capable of producing at least 10 times more force for a given cross-sectional area active . stress than skeletal muscle, and potentially 1000 times more, with strains typically between 1% and 10%. Low operating voltages make them particularly attractive for use in micro-electromechanical systems, in place of electrostatic and piezoelectric actuators. A drawback of conducting polymer actuators is their relatively slow speed, and hence low power-to-mass ratio. In this paper, shaped voltage pulses are applied to generate strain rates of up to 3% s y1 , with peak power to mass ratios of 39 W kg y1 of polymer, nearly matching mammalian skeletal muscle. Results are obtained from polypyrrole linear and bilayer actuators and employ both liquid and gel electrolytes. q 2000 Elsevier Science S.A. All rights reserved.

Conducting polymer-based actuators undergo volumetric changes as they are oxidized or reduced, from which mechanical work can be obtained. Polypyrrole [Q. Pei, O. Inganas, Advanced Materials 4 (1992) 277; Q. Pei, O. Inganas, Journal of Physical Chemistry 96 (1992) 10508; E. Smela, O. Inganas, Q. Pei, I. Lundstrom, Advanced Materials 5 (1993) 630; E. Smela, O. Inganas, I. Lundstrom, Science 268 (1995) 1735; J.D. Madden, S.R. Lafontaine, I.W. Hunter, Proceedings – Micro Machine and Human Science 95, Nagoya, Japan, October 1995] and polyaniline-based actuators have attracted recent interest because of the high forces per cross-sectional area (stress) and relatively large strains generated at low activation voltages; however, with the notable exception of some bilayers, the operation of these actuators has largely been constrained to bulk liquid environments. Operation out of solution is clearly desirable for many potential applications. Linear actuators that contract in length like muscle fibers fully exploit the high forces produced by conducting polymers. Results presented here demonstrate the operation in air of polypyrrole linear actuators. These actuators are capable of generating stresses exceeding those of mammalian skeletal muscle.