PEDOT/WO3 Hybrid Nanofiber Architectures for High Performance Electrochromic Devices.

Electrochromic devices (ECDs) are of much current interest for a wide range of applications, including color displays, sensors, reflectance mirrors, vehicle sunroofs and smart windows. WO3 is perceived as a prospective candidate for ECDs since it has many advantages such as genuine color switching and, good chemical stability and exhibits the most efficient electrochromic properties both in the visible and near infrared regimes. These properties are substantially dependent on the crystal structure, which in turn affects the electronic properties of the ion injection/ejection during electrochemical reactions.

The crystal structure of WO3 can be controlled by synthetic methods or deposition processes. Several methods can be used to fabricate WO3 electrodes for EC applications, including sputtering, thermal evaporation, sol¢gel coating, hydrothermal electrochemical methods and electrospinning. In addition to bulktype electrodes, nano-structured WO3 has been increasingly investigated because its morphology provides a high surface area for electron and ion transport. In a recent study, nanobrick-like WO3 thin films have been synthesized via facile hydrothermal route. The WO3 film comprised of nanobricks exhibited high crystallinity with large surface area that affects ion insertion kinetics leading to enhanced EC performance. In another study, tungsten oxide nanowires grown on the ITO-coated glass substrates were prepared successfully by the thermal evaporation process and an ECD was assembled using the tungsten oxide nanowires as the electrochromic layer. Nanowires of WO3 were found to have larger transmittance change in visible light region and better electrochromic effect.

As an important one dimensional nanomaterial, nanofibers have extremely high specific surface area because of their small diameters, and a porous structure with excellent pore-interconnectivity. These unique characteristics plus the functionalities from the materials themselves impart nanofibers with desirable properties for a range of advanced applications in areas as various as sensors, optics, photocatalysts, biomedical engineering, wound healing, drug delivery and release control, catalyst and enzyme carriers, filtration, environment protection, composite reinforcement, energy conversion and storage and many others.

In our attempts to synthesize hybrid nanofibers, electrochemical polymerization of PEDOT onto nanoporous WO3 was made and subsequent electrospinning was performed. According to the best of our knowledge, there is no precedence in the literature for nanofibers of WO3 with PEDOT prepared through electrochemical method in ionic liquid media and it is the first time that this novel and convenient strategy to fabricate hybrid nanofibers by electrospinning for the assembly of electrochromic devices is proposed. Our aim was to study the feasibility of electrochromic device applications of PEDOT/WO3 hybrid nanofibers. The electrochemical, electrochromic and morphological characterization of the hybrid nanofibers are presented in this contribution.