pi-Conjugated polymers that are electrochemically cycled in ionic liquids have enhanced lifetimes without failure (up to 1 million cycles) and fast cycle switching speeds (100 ms). We report results for electrochemical mechanical actuators, electrochromic windows, and numeric displays made from three types of pi-conjugated polymers: polyaniline, polypyrrole, and polythiophene. Experiments were performed under ambient conditions, yet the polymers showed negligible loss in electroactivity. These performance advantages were obtained by using environmentally stable, room-temperature ionic liquids composed of 1-butyl-3-methyl imidazolium cations together with anions such as tetrafluoroborate or hexafluorophosphate.

Electrolytes play an important role in determining the performance of conducting polymer electrochromic devices. Good electrolytes should have high conductivity, large electrochemical windows, excellent thermal and chemical stability, and negligible evaporation. Room-temperature ionic liquids are ideal electrolytes to satisfy these requirements. In the present work, we explored the applications of ionic liquids as electrolytes in electrochemical synthesis of conducting polymers, in electrochemical and electrochromic characterization of both electrochemically and chemically synthesized conducting polymers and in fabrication of conducting polymer electrochromic devices. In ionic liquids, highly stable electroactivity has been obtained for polyaniline in a wide potential range covering its entire redox process of leucoemeraldine ↔ emeraldine ↔ pernigraniline for Ͼ1,000,000 cycles. During the fabrication of electrochromic devices, electrochemically synthesized polymers were employed for displays, while chemically synthesized polymers ͑via spin-coating͒ were preferable for large-area electrochromic windows. We have successfully fabricated the prototypes of alphanumeric displays and large-area (5 ϫ 5 cm) electrochromic windows. High device performance of low operation voltages ͑Ͻ1.5 V͒, high coloration contrast ͑Ͼ50%͒, fast coloration speed ͑Ͻ100 ms͒, and high coulombic efficiency ͑Ͼ98%͒ has been realized.

The electrochemical linear actuation of polyaniline fiber actuators has been studied in a variety of acidic aqueous electrolytes. Experimental results show that the linear strain changes significantly but nonlinearly with the anion volume. For anions smaller than Br-, a larger strain was obtained for a larger anion, that is, Br- > Cl- > F-, while once the anion was larger than Br-, a larger anion produced a smaller strain, that is, BF4- > ClO4- > CF3SO3-. On the basis of the definition of the ECR (elongation/charge ratio), that is, the contribution of a unit charge to fiber elongation, the maximum linear strain can be estimated by assuming the electrochemical efficiency is 100%. Furthermore, under isotonic conditions and the application of a constant voltage, the energy efficiency without energy recovery was shown to be proportional to the ECR and applied force and inversely proportional to the applied voltage. Under the same conditions, the highest energy efficiency is obtained in HBr. By assuming that ion and solvent insertion contributes mostly to the fiber expansion, a simple mathematical description is developed for the linear strain to show how it is determined by the volume and carried charge of the insert complex and the anisotropicity of the fiber. The difference between the theoretical and experimental results suggests that due to the crystallite structure, not all exchanged charge contributes to the fiber expansion. As the anion becomes larger, it may become more difficult for the anions to be inserted into the polymer fiber.

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