Recent Advances in Tungsten Oxide/Conducting Polymer Hybrid Assemblies for Electrochromic Applications

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1 See discussions, stats, and author profiles for this publication at: Recent Advances in Tungsten Oxide/Conducting Polymer Hybrid Assemblies for Electrochromic Applications Chapter December 216 DOI: 1.12/ ch3 CITATIONS READS 27 2 authors: Cigdem Dulgerbaki T.C. Alanya Alaaddin Keykubat 9 PUBLICATIONS 43 CITATIONS Aysegul Uygun Oksuz T.C. Süleyman Demirel Üniver 14 PUBLICATIONS 933 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Nanomotors View project Electrochromic nanofibers prepared in the presence öf ionic liquids View project All content following this page was uploaded by Cigdem Dulgerbaki on 27 December 217. The user has requested enhancement of the downloaded file.

2 Recent Advances in Tungsten Oxide/ Conducting Polymer Hybrid Assemblies for Electrochromic Applications Cigdem Dulgerbaki and Aysegul Uygun Oksuz* Department of Chemistry, Faculty of Arts and Science, Suleyman Demirel University, Isparta, Turkey 3 Abstract Much effort is currently devoted to implementing new materials in electrodes that will be used in electrochromic (EC) technology. Tungsten oxide ( ) has emerged as one of the key materials for EC devices (ECDs) since it shows the best EC activity among transition metal oxides. However, hybrid nanostructures have been investigated in order to enhance the EC properties. The introduction of /conducting polymer-based hybrid materials has prompted the development of nanocomposites with properties unmatched by conventional counterparts. Combined with the intrinsic properties and synergistic effect of each component, it is anticipated that these unique organic inorganic heterostructures pave the way for developing new functional materials. In the current chapter, some of these recent results on /conducting polymer-based hybrid films are discussed, with selected examples chosen from among the deposition of layer-by-layer assembled hybrids, spin-coated, dip-coated materials, surface-initiated-polymerized, chemical bath-deposited films, solvothermal, and electropolymerized materials. In addition to discussing film deposition techniques, an attempt will also be made to indicate how the resulting films might be useful for ECD applications. These newgeneration materials are evaluated as an electrode material of ECDs and exhibit improved optoelectronic properties. Keywords: Tungsten oxide, conducting polymer, hybrid, electrochromic *Corresponding author: aysegul.uygun@yahoo.com Ashutosh Tiwari, Filiz Kuralay and Lokman Uzun (eds.) Advanced Electrode Materials, (61 12) 217 Scrivener Publishing LLC 61

3 62 Advanced Electrode Materials 3.1 Introduction A large fraction of the energy delivered to buildings is wasted because of inefficient building technologies. Energy savings can be made not by reducing the standard of living, but by utilizing more efficient technologies to provide the same, or higher, levels of comfort and convenience, we have come to enjoy and appreciate [1]. Smart windows can make use of a range of chromogenic technologies where the term chromogenic is used to indicate that the optical properties can be changed in response to an external stimulus. The main chromogenic technologies are thermochromic (TC) (depending on temperature), photochromic (depending on ultraviolet irradiation), and electrochromic (EC) (depending on electrical voltage or charge) [2]. The chromogenic technologies are seen to be very advantageous: specifically, TC fenestration gives low cooling energy, photochromic fenestration can lead to low electric lighting energy, whereas ECs yields superior performance with low energies both for electric lighting energy and cooling energy [3]. EC smart windows are able to vary their throughput of visible light and solar energy by the application of an electrical voltage and are able to provide energy efficiency and indoor comfort in buildings [4]. EC materials manifest reversible and visible change in optical properties as the result of electrochemical oxidation or reduction at different potentials. For its particular properties, EC materials can be interesting candidates for smart windows, rearview mirrors, e-papers, and low-cost displays [5]. EC materials can be classified into three groups: inorganic materials (transition metal oxides) [6], organic small molecules [7], and conjugated conducting polymers [8]. Among those inorganic EC materials, has many advantages, including genuine color switching, good chemical stability, and strong adherence to the substrate. However, single color change and slow switching speed limit its application [9]. As a comparison, organic EC materials ( conducting polymers) show many advantages such as multicolor, fast switching speed, flexibility, and easy to optimize their EC properties through molecular tailoring [1]. The synergistic combination of the merits of conducting polymers and inorganic materials may provide an opportunity to deploy a hybrid EC material with higher coloration efficiency, shorter response time, and outstanding device lifetime [11]. This chapter will focus on the recent advancements on tungsten oxide/conducting polymer hybrid materials that exhibit visible electrochromism. The emphasis is to correlate the structures and morphologies of the hybrid EC materials to their electronic and ionic properties and illustrate how these influence EC

4 Advances in Tungsten Oxide/Conducting Polymer Hybrid 63 properties of the materials and offer advantages. A future outlook for the tungsten oxide/conducting polymer hybrids will also be presented. 3.2 History and Technology of Electrochromics Electrochromism is the reversible change of a chemical species between two redox states with distinguishable absorption or reflection spectra, such redox change is being induced by application of an electrical current or a potential difference [12]. Much of the EC technology is being developed for building and automotive windows, as well as mirrors, but the history of ECs dates back to 174, when Diesbach discovered the chemical coloration of Prussian Blue. In the 193s, electrochemical coloration was noted in bulk. Twenty years later, Kraus observed electrochemical coloration in thin films. The first ECDs were made by Deb in By the mid-197s, ECDs were being developed for displays. ECs based on viologens and followed in the 198s for switchable mirrors in cars, which continues as a viable product to this day. In the 199s, several companies began developing devices for glazing applications and the work still continues [13]. 3.3 Electrochromic Devices In fact, the suitable integration of EC materials into devices makes it possible to take advantages of these materials in practical applications, making it easier to define standards when investigating the characteristics of the EC materials. The most practical design for testing and commercializing ECDs is the solid-state design. An ECD is composed of a working electrode, a counter-electrode, and an electrolyte (in solid/gel forms). A very thin layer of electrolyte is usually placed between these two electrodes. Other than the EC materials, the electrolyte is an indispensable component in the ECDs. It is the ionic conduction medium between the electrodes [14]. An ECD contains three principally different kinds of layered materials: The electrolyte is a pure ion conductor and separates the two EC films (or separates one EC film from an optically passive ion storage film). The EC films conduct both ions and electrons and hence belong to the class of mixed conductors. The transparent conductors, finally, are pure electron conductors. Optical absorption occurs when electrons move into the EC film(s) from the transparent conductors along with charge-balancing

5 64 Advanced Electrode Materials Electron flow Ion flow TCO Electrochromic material Solid or gel electrolyte Counter-electrode Transparent conductor (TCO) Figure 3.1 Schematic of the ECD. Electrons flow through an external circuit into the EC material, while ions flow through the electrolyte to compensate the electronic charge. Reprinted with permission from Ref. [16]. Copyright 214, Royal Society of Chemistry. ions entering from the electrolyte. This very simplified explanation of the operating principles for an ECD emphasizes that it can be described as an electrical thin-film battery with a charging state that translates to a degree of optical absorption [15]. Figure 3.1 illustrates a principle EC design which is convenient for introducing basic concepts and materials types. The shown device contains five superimposed layers on a transparent substrate [16]. The key parameters of ECDs include the following Electrochromic Contrast EC contrast is probably the most important factor in evaluating an EC material. It is often reported as a percent transmittance change (%T) at a specified wavelength where the EC material has the highest optical contrast. For some applications, it is more useful to report a contrast over a specified range rather than a single wavelength. To obtain an overall EC contrast, measuring the relative luminance change provides more realistic contrast values since it offers a perspective on the transmissivity of a material as it relates to the human eye perception of transmittance over the entire visible spectrum Coloration Efficiency The coloration efficiency (also referred to as EC efficiency) is a practical tool to measure the power requirements of an EC material. In essence, it determines the amount of optical density change (ΔOD) induced as a

6 Advances in Tungsten Oxide/Conducting Polymer Hybrid 65 function of the injected/ejected electronic charge (Q d ), i.e. the amount of charge necessary to produce the optical change. It is given by the equation η = ΔOD/Q d = [log(t b /T c )]/Q d where η (cm 2 /C) is the coloration efficiency at a given λ, and T b and T c are the bleached and colored transmittance values, respectively. The relationship between η and the charge injected to the EC material can be used to evaluate the reaction coordinate of the coloration process, or the η values can be reported at a specific degree of coloration for practical purposes Switching Speed Switching speed is often reported as the time required for the coloring/ bleaching process of an EC material. It is important especially for applications such as dynamic displays and switchable mirrors. The switching speed of EC materials is dependent on several factors such as the ionic conductivity of the electrolyte, accessibility of the ions to the electroactive sites (ion diffusion in thin films), magnitude of the applied potential, film thickness, and morphology of the thin film. Today, sub-second switching rates are easily attained using polymers and composites containing small organic electrochromes Stability EC stability is usually associated with electrochemical stability since the degradation of the active redox couple results in the loss of EC contrast and hence the performance of the EC material. Common degradation paths include irreversible oxidation or reduction at extreme potentials, ir loss of the electrode or the electrolyte, side reactions due to the presence of water or oxygen in the cell, and heat release due to the resistive parts in the system. Although current reports include switching stabilities of up to 1 6 cycles without significance performance loss, the lack of durability (especially compared to Liquid Crystal Displays (LCDs)) is still an important drawback for commercialization of ECDs. Defect-free processing of thin films, careful charge balance of the electroactive components, and air-free sealing of devices are important factors for long-term operation of ECDs Optical Memory One of the benefits of using an EC material in a display as opposed to a light-emitting material is its optical memory (also called open-circuit memory), which is defined as the time the material retains its absorption

7 66 Advanced Electrode Materials state after the electric field is removed. In solution-based EC systems such as viologens, the colored state quickly bleaches upon termination of current due to the diffusion of soluble electrochromes away from the electrodes (a phenomenon called self-erasing). In solid-state ECDs, where the electrochromes are adhered to electrodes, the EC memory can be as long as days or weeks with no further current required [17]. EC films are being developed for application in dynamic or smart window technologies that are at the forefront of emerging energy saving advances in building technologies [18]. Svensson and Granqvist coined the term smart window to describe windows that own electrochromism character, meaning they can change transmittance under different voltage [19]. The appeal for smart windows is both in economic and environmental angles: if mature, they can be employed to properly modify sunlight into a room or a building for saving energy or preclude much solar radiation to avoid light pollution [2]. Figure 3.2 describes the mechanism of EC window. In the EC window design, the window is an electrochemical cell in which two conducting glass panes are separated by an electrolyte material. At open circuit voltage, the window is in Bright Mode, that is, both Bright mode Cool mode Universal smart window Dark mode Visible light Visible light Visible light Infrared In bright mode windows allow natural light and heat to enter room Infrared In cool mode windows allow natural light to enter room but block heat from entering Infrared In dark mode windows limit the amount of heat and natural light that enter the room Eletrochromic layer in glass Nanocrystal counter electrode Ion conducting electrolyte Nanocomposite working Current turned on. electrode Electrons and ions flow to nanocrystals in working electrode Nanocrystal blocks infrared light Current turned on. Electrons and ions flow to matrix in working electrode Matrix blocks visible light Figure 3.2 Design of EC window. Reprinted with permission Ref. [21]. Copyright 213, Nature.

8 Advances in Tungsten Oxide/Conducting Polymer Hybrid 67 working and counter electrodes are transparent to solar radiation, allowing heat and natural light to enter the room. When the voltage is reduced to an intermediate value, the window switches to Cool Mode, blocking heat while allowing natural light to enter the room. At lower potentials, the window switches to Dark Mode, limiting the amount of heat and natural light that enter the room. These three switching modes enable the window to operate at different weather conditions, which is helpful for energy savings and comfort [21]. 3.4 Transition Metal Oxides Many different EC transition metal oxides have been discovered over the years, e.g. iridium, rhodium, ruthenium, manganese, and tungsten oxide. They are renowned for their intense optical absorptions, when partially reduced, which are a result of inter-valence charge transfer processes. This is when an electron is excited to a similar, vacant orbital on an adjacent ion or molecule [22]. 3.5 Tungsten Oxide Peter Woulfe was the first to recognize a new element in the naturally occurring mineral, Wolframite (W, tungsten) during the 18th century. In 1841, Robert Oxland first gave the procedure of preparing compound. The powder appears yellow in color having density of 7.16 g/cm 3. The melting temperature of is ~1473 C, but its sublimation starts at nearly 9 C. Among transition metal oxides, is one of the most interesting materials exhibiting a wide variety of novel properties particularly in thin film form useful for advanced technological applications. exhibits a cubic perovskite-like structure based on the corner sharing of regular octahedra with the oxygen atoms at the corner and the tungsten atoms at the centre of each octahedron. The crystal structure of is temperature dependent. It is tetragonal at temperatures above 74 C, orthorhombic from 33 to 74 C, monoclinic from 17 to 33 C, and triclinic from 5 to 17 C [23]. The most common monoclinic crystal structure of is represented in Figure 3.3. The discovery of EC effect in transition metal oxides opened a new window for research and development of employing such material. is a material of high interest in the transition metal oxides not only for ECDs but in many other related applications [24]. It is found in the form

9 68 Advanced Electrode Materials W O Figure 3.3 Monoclinic crystal structure of tungsten oxide. Reprinted with permission from Ref. [23]. Copyright 213, Journal of Non-Oxide Glasses. of hydrates in the nature. It has been of great interest during the past few years due to its enormous attractive structural, optical, and electrical properties. The material ability to sustain reversible and persistent changes of its optical properties under the action of a voltage was discovered in 1969 by Deb. The coloration of from transparent to dark was shown in highly disordered thin films. Since then, extensive studies have been carried out for in smart window applications [25]. has a nearly cubic structure which may be simply described as an empty-perovskite type formed by WO 6 octahedra that share corners. The empty space inside the cubes is considerable, and this provides the availability of a large number of interstitial sites where the guest ions can be inserted., with all tungsten sites as oxidation state W(VI), is a transparent thin film. On electrochemical reduction, W(V) sites are generated to give the EC (blue coloration to the film) effect. Although there is still controversy about the detailed coloration mechanism, it is generally accepted that the injection and extraction of electrons and metal cations (Li +, H +, etc.) play an important role. is a cathodically ion insertion material. The blue coloration in the thin film of can be erased by the electrochemical oxidation. In the case of Li + cations, the electrochemical reaction can be written as Eq. (3.1) [26].

10 Advances in Tungsten Oxide/Conducting Polymer Hybrid 69 (k) + x(li + (aq) + e ) Li x (k) (3.1) has received much attention among transition metal oxides with chromogenic properties because of its potential to be used in thin film ECDs, such as smart windows and mirrors with controllably variable transmission and/or reflection, electro-optical displays, variable-emittance surfaces, and gas sensors. EC variable transmittance glazings which permit dynamic control of radiative properties are of particular interest nowadays concerning energy conservation, temperature and lighting control in buildings and vehicles [27]. Interest in the use of for chromic applications arose from its optical properties in the visible wavelengths region, which are dominated by the absorption threshold. The threshold is defined by the bandgap energy (Eg) of nanostructures, which ranges from 2.6 to 3.25 ev. These properties make the films generally transparent in nature [28]. 3.6 Conjugated Organic Polymers In the recent years, conjugated polymers (CPs) have gained a lot of attention for ECDs. This is due to the fact that all electroactive and CPs are potentially EC materials and are more processable than inorganic EC materials and offer the advantage of a high degree of color tailorability. This tailorability has been achieved through the modification of various polymer systems via monomer functionalization and copolymerization as well as with the use of blends, laminates, and composites. Complex colors are achieved by mixing two existing colors in a dual polymer device. In CPs, EC changes are induced by redox processes which are accompanied by ion insertion/expulsion and result in a modification of the polymer s electronic properties giving rise to changes in color of the material [26]. Electrochromism in CPs occurs through changes in the CPs π-electronic character accompanied by reversible insertion and extraction of ions through the polymer film upon electrochemical oxidation and reduction. In their neutral (insulating) states, these polymers show semiconducting behavior with an energy gap (Eg) between the valence band (HOMO) and the conduction band (LUMO). Upon electrochemical or chemical doping ( p-doping for oxidation and n-doping for reduction), the band structure of the neutral polymer is modified, generating lower-energy intraband transitions and creation of charged carriers (polarons and bipolarons), which are responsible for increased conductivity and optical modulation [17].

11 7 Advanced Electrode Materials All conjugated organic polymers are potentially EC in thin-film form, redox switching giving rise to new optical absorption bands in accompaniment with transfer of electrons/counter anions [29]. 3.7 Hybrid Materials With technological breakthroughs increasingly happening around the globe, the need for novel materials which are cost effective, light weight, and energy efficient is increasing as ever. Scientists and engineers realized that many well-established materials like plastics, ceramics, or metals cannot fulfill the technological needs required for various new applications and found that the combination of certain materials to form hybrids can show extraordinary properties when compared with their original components [3]. The main motivation behind creating a hybrid material is to utilize the electrical, mechanical, thermal, and structural properties of the inorganic material and flexibility, functionality and templating ability of the organic material. Organic inorganic hybrid materials are not only useful for the design of new compounds for academic research, but their unusual features and versatile characteristics open up promising applications in many fields such as electronics, optics, optoelectronics, mechanics, environment, and medicine [31]. For EC technology, the discovery of new hybrid materials and creating new combinations of EC materials for use in novel operational devices is fundamental to research in this field [32]. The main advantages of inorganic materials are the relatively fast color switching, durability, and long-term stability, but their use is hampered by their narrow color variation and low coloration efficiencies. This latter, together with the high contact resistance in the device, results in the need of high electrical power input to reach the required color change. On the other hand, CPs exhibit high coloration efficiencies at relatively lower redox switching potentials, on a short timescale. Their relatively low environmental stability (especially in the oxidized state) and mechanical strength, however, are important drawbacks from an application perspective [33]. Research in the topic of hybrid materials entails challenges and opportunities. The main challenge is managing to synthesize hybrid combinations that keep or enhance the best properties of each of the components while eliminating or reducing their particular limitations. Undertaking this challenge provides an opportunity for developing new materials with synergic behavior leading to improved performance or to new useful properties [34]. It was soon recognized that in hybrids, the complementary properties can be exploited, and the synergies fully utilized. Such synergies

12 Advances in Tungsten Oxide/Conducting Polymer Hybrid 71 predominantly stem from the combination of the flexibility and functionality of the CP with the mechanical strength and chemical stability of the inorganic material. In addition to combining distinct characteristics, new or enhanced phenomena can also arise as a result of the interface between the organic and inorganic components [35]. 3.8 Electrochromic Tungsten Oxide/Conducting Polymer Hybrids Ling et al. employed layer-by-layer assembly method to fabricate multilayer hybrid films based on poly (styrenesulfonate)-doped poly(3,4- ethylenedioxythiophene) (PEDOT:PSS) and tungsten oxide nanoparticles ( NPs). Polyethylenimine (PEI) is deposited in between to introduce electrostatic force between the components. Since both NPs and PEDOT:PSS colloidal particles have negatively charged surfaces, to facilitate the electrostatic adsorption of the components, polycationic PEI was used as intermediate layers to attract the anionic species, as illustrated in Scheme 3.1. To compare the EC properties of the hybrid films with those of their PEDOT:PSS and NP counterparts, spectro-electrochemical characterization were conducted on 1-layer PEDOT:PSS and NP films, and 5-layer hybrid films. The transmittance of NPs, PEDOT:PSS, and hybrid thin films were recorded at constant potentials of +.8,, and 1. V, respectively. The optical transmittances against the wavelength of all three films are shown in Figure 3.4 (a c). All the three films exhibit maximum transmittance differences (DT) between the bleached and colored states at wavelength of around 633 nm, which is defined as optical contrast. With comparable thickness of each film, the optical contrast of Repeating cycles PEI + PEDOT:PSS NPs 1 Hybrid layer Scheme 3.1 Scheme for the formation of EC multilayer hybrid film [PEI/PEDOT:PSS/ -NPs] n (Hn).

13 72 Advanced Electrode Materials V V V V V V V V 6 1. V (a) Wavelength (nm) (b) Wavelength (nm) (c) Wavelength (nm) Transmittance (%) Transmittance (%) Figure 3.4 UV Vis spectrum of 1 layers of (a) NP, (b) PEDOT:PSS, and 5 layers of (c) hybrid thin films under different potentials of +.8,, and 1. V. Reproduced with permission from Ref. [36]. Copyright 215, Electrochimica Acta. Transmittance (%) (11)W (2)W Intensity (a.u.) WO 2 (2) WO 2 (2) WO2 (2) (22) WO 2 Annealed Intensity (a.u.) P3HT (1) 1 2 (c) As-formed theta (degree) (d) 2 theta (degree) Figure 3.5 X-ray diffraction (XRD) patterns of (c) and (d) P3HT/. the hybrid film (DT = 2%) is significantly higher than that of NP (DT = 7.3%) and PEDOT:PSS (DT = 9.6%) films. Owing to the efficient charge transfer between the two active components and complementary electrical conductivity of the two components in the redox switching process, the coloration efficiency of the hybrid film is significantly improved to cm 2 /C at wavelength of 633 nm [36]. Kim et al. investigated the enhanced electrochemical and EC properties of P3HT (poly 3-hexylthiophene)/ composites. Nanoporous layers were prepared using electrochemical anodization. P3HT was spin coated on these layers to obtain hybrid P3HT/ composites. After annealing at 3 C for 1 h, the monoclinic phase of the layer and selforganized lamella structure of P3HT were examined. The P3HT/ composites exhibited a crystalline structure after heat treatment (Figure 3.5) and enhanced current densities (Figure 3.6) and three different reflective colors (Figure 3.7) with a combination of pristine P3HT and during the redox reaction. Furthermore, the composites exhibited faster switching speeds compared with layers, which might be attributed to the

14 Advances in Tungsten Oxide/Conducting Polymer Hybrid (a).1.5. Current density (ma/cm 2 ) Current density (ma/cm 2 ).5 (b) Current density (ma/cm 2 ) P3HT P3HT/ (c) Voltage (V vs. Ag wire) Figure 3.6 Cyclic voltammogram of (a), (b) P3HT, and (c) P3HT/ composites performed between.7 and 1. V with a scan rate of 5 mv/s in propylene carbonate solution with.4 M LiClO 4. easy Li + insertion/extraction resulting from the incorporation of P3HT. Therefore, it can be concluded that the combination of P3HT and yields a promising EC material exhibiting multicolor electrochromism and faster response [37]. Cai et al. prepared /PANI core/shell nanowire array by the combination of solvothermal and electropolymerization methods. The core/shell nanowire array film shows remarkable enhancement of the EC properties.

15 74 Advanced Electrode Materials Reflectance (a.u.) Reflectance (a.u.) (a) V Wavelength (nm).2.7 V P3HT/ (c) Wavelength (nm).2 1. V V V.1 V.2 V.3 V.4 V.5 V.7 V 1. V.9 V.7 V.5 V.3 V.1 V.1 V.2 V.3 V.4 V.5 V.7 V Reflectance (a.u.) V (b) Wavelength (nm) Normalized reflectance (a.u.) P3HT.9 V.8 V.6 V.5 V.3 V.7 V to.2 V.6 65 nm 6 nm 55 nm (d) Voltage (V vs. Ag wire) Figure 3.7 Reflectance spectra of (a), (b) P3HT, and (c) P3HT/ composites at applied voltages from.7 to 1. V and (d) reflectance changes of P3HT/ at specific wavelengths as a function of voltages from.7 to 1. V. Reproduced with permission from Ref. [37]. Copyright 215, Electrochemistry Communications. (2) ** (2) - FTO * - Intensity/a.u. a b (112) * (22) (222) (4) (24) (42) * * * * θ/ degree Figure 3.8 XRD patterns of (a) and (b) /PANI nanowire arrays. Except for the peaks of FTO glass, all the diffraction peaks of both the films can be indexed as the monoclinic phase in XRD patterns (JCPDS no ). No obvious diffraction peaks of PANI are observed, indicating the amorphous nature of PANI deposited by the CVs (Figure 3.8). The CV

16 Advances in Tungsten Oxide/Conducting Polymer Hybrid PANI Current density/ma cm Potential/V (vs. Ag/AgCl) 1. Figure 3.9 The 1th CV curves of, PANI, and /PANI films. 1. V.2 V.2 V.7 V Figure 3.1 Photographs of a /PANI core/shell nanowire array sample (2 4 cm 2 in size) under different applied potentials. curve of the /PANI nanowire array exhibits both characteristic peaks of nanowire and PANI film. In addition, the /PANI nanowire array shows significantly high exchange current densities compared to the and PANI film (Figure 3.9). In particular, a significant optical modulation (59% at 7 nm) (Figure 3.11), fast switching speed (Figure 3.12), high coloration efficiency (86.3 cm 2 /C at 7 nm) and excellent cycling stability are achieved for the core/shell nanowire array film. The improved EC properties are mainly attributed to the formation of the donor acceptor system, and the porous space among the nanowires (Figure 3.13), which can make fast ion diffusion and provide larger surface area for charge-transfer reactions. Due to the non-overlapping of the coloration and bleaching between PANI and, the dual-electrochromism effect is obtained for the /PANI core/shell nanowire array. It is a great promise for the /PANI core/shell nanowire array as a potential multicolor EC material (Figure 3.1) [38]. Enlightened by Cai et al. s work, Zhang et al. synthesized ultra-thin nanorods (NRs)-embedded polyaniline (PANI) composite thin films by

17 76 Advanced Electrode Materials Transmittance/(%) V.7 V (a) Wavelength/nm 8 Transmittance/(%) (b) V.2 V.2 V 1. V Wavelength/nm Transmittance/(%) V.2 V.2 V 1. V (c) Wavelength/nm Figure 3.11 Visible transmittance spectra of (a), (b) PANI, and (c) /PANI films under different applied potentials. embedding NRs into PANI using a surface-initiated polymerization method, followed by spin-coating deposition. The ultra-thin NRs with length of 6 nm and diameter of 4 nm were prepared by a solvothermal method and were used as nanofillers reinforced into the PANI matrix

18 Advances in Tungsten Oxide/Conducting Polymer Hybrid 77 Transmittance/(%) Current density/ ma cm 2 (a) Time/sec Transmittance/(%) Current density/ ma cm 2 (b) Time/sec Transmittance/(%) Current density/ ma cm 2 (c) Time/sec Transmittance/(%) Current density/ ma cm (d) Time/sec Figure 3.12 EC response of (a) nanowire array (.7 to 1. V), (b) PANI film (.7 to 1. V), (c) /PANI core/shell nanowire array (.2 to 1. V), and (d) /PANI core/shell nanowire array (.7 to.2 V). Reproduced with permission from Ref. [38]. Copyright 214, Solar Energy Materials & Solar Cells. to form an organic/inorganic nanocomposite with excellent processability (Figure 3.15). Diffraction peaks of NRs match well with hexagonal (JCPDS ). XRD patterns of the NRs PANI composite shows a similar profile with the NRs, except for weaken intensity of the characteristic peaks (Figure 3.14). The CVs of the NRs PANI composite film exhibit both characteristic peaks of NRs and PANI (Figure 3.16). The composite film, being a dual EC material, varied from purple to green, light yellow, and finally dark blue (Figure 3.17). The durability of the hybrid film was enhanced compared with neat NRs film (Figure 3.19). As is well known, is colored at negative potential, and PANI is colored at positive potential. The combination of the two materials with different coloration mechanisms leads to a dual electrochromism. The effect is due to that the coloration of and bleaching of PANI are not entirely overlapped, and vice versa. Moreover, the two materials are strongly complementary to each other in conductivity. That is at

19 78 Advanced Electrode Materials 4 nm (a) 1 µm (b) 2 nm (c) 1 µm (d) 2 nm 2 nm (e) (f) 4 nm 2 nm Figure 3.13 Scanning electron microscope (SEM) images of (a and b) nanowire, (c and d) /PANI core/shell nanowire array, and (e and f) PANI film. Intensity/a.u PANI NRs PANI NRs # θ/ Figure 3.14 XRD patterns of NRs, PANI, and NRs PANI composite.

20 Advances in Tungsten Oxide/Conducting Polymer Hybrid 79 (a) 1 nm (b) 1 nm (c) 1 nm Figure 3.15 SEM images of (a) NRs, (b) PANI, and (c) NRs PANI composite. positive potential the PANI shows excellent conductivity, and at negative potential is conductive because of the formation of hydrogen tungsten bronze. Conductivity is a key factor to EC switching speed. Therefore, fast response is expected in the composite film. It is found that NRs show a response time longer than 5s. The response times of PANI and NRs PANI composite films are.6 and.9 s, respectively (Figure 3.18). Comparing with the neat NRs film, a much faster switching speed is obtained for the NRs PANI composite film [39]. Wei et al. prepared poly(dntd,n,n-di[p-phenylamino(phenyl)]- 1,4,5,8-naphthalene tetracarboxylic diimide) and its nanocomposite film

21 8 Advanced Electrode Materials NRs Current density/ma cm (a) Potential/V vs. Ag/AgCl 3 PANI Current density/ma cm (b) Potential/V vs. Ag/AgCl 3 NRs/PANI Current density/ma cm (c) Potential/V vs. Ag/AgCl Figure 3.16 Cyclic voltammograms of (a) NRs, (b) PANI, and (c) NRs PANI composite films in.5 M H 2 SO 4 electrolyte at a potential scanning rate of 5 mv s 1.

22 Advances in Tungsten Oxide/Conducting Polymer Hybrid 81 Transmittance/(%) V.2 V.1 V.5 V Wavelength/nm Figure 3.17 Visible transmittance spectra of the NRs PANI composite film at different bias potentials. Transmittance/% Current density/ma cm 2 2 NR 1. PANI 3 NR-PANI Current density/ma cm (a) Time/s (b) Time/s (c) Transmittance/% Transmittance/% Current density/ma cm Time/s Figure 3.18 Current transient response and corresponding optical switching at 55 nm for (a) NRs, (b) PANI, and (c) NRs PANI composite films in.5 M H 2 SO 4 electrolyte applied potential steps of.5 V (5 s) and.5 V (5 s). Reproduced with permission from Ref. [39]. Copyright 213, Solar Energy Materials & Solar Cells.

23 82 Advanced Electrode Materials Current/a.u. (a) 5 1 Cycle number 15 Current/a.u. Current/a.u. (b) Cycle number (c) Cycle number Figure 3.19 Cyclic stability test using chronoamperometry (CA). incorporated with nanoparticles by a facile electropolymerization method on an indium tin oxide (ITO)-coated glass slide from the DNTD monomer and nanoparticles suspended methylene chloride solution. The SEM image shows that the nanoparticles are uniformly embedded

24 Advances in Tungsten Oxide/Conducting Polymer Hybrid 83 (a) 1 µm (b) 2 µm 1 µm Figure 3.2 SEM images of thin films of (a) pure poly(dntd) and (b) poly(dntd)/ nanocomposites grown onto ITO-coated glass. Current (ma) Poly(DNTD) Poly(DNTD)/( ) Potential vs Ag/AgCl(V) Figure 3.21 Cyclic voltammograms of thin films of (a) pure poly(dntd) and (b) poly(dntd)/ nanocomposites in.1 M TBAPF 6 CH 2 Cl 2 DNTD-free solution with a scan rate of 2 mv/s. in the polymeric matrix (Figure 3.2). For the poly(dntd)/ nanocomposite film, the oxidation potential is the same as that of pure polymer, but the films were reduced at.34 V in the negative sweep (Figure 3.21). The composite film exhibits multiple colors at both the cathodic and anodic potentials, i.e. light blue at 1.4 V, orange red at.8 V, colorless at V, orange green at.8 V, light blue at 1. V, and deep blue at 1.2 or 1.4 V vs Ag/AgCl in propylene carbonate containing 1. M LiClO 4 electrolyte (Figure 3.23). The UV visible-incorporated electrochemical spectroscopy coupled with amperometry was also employed to study the composite film under different potentials in the range of 1.4 to 1.4 V vs Ag/AgCl (Figure 3.22). The composite film also shows stable electrochromism even after 1 scans [4]. Nwanya et al. prepared PANI and its nanocomposite /PANI films deposited on fluorine-doped tin oxide (FTO) glass slides by simple chemical bath deposition (CBD) method. The film shows spherical

25 84 Advanced Electrode Materials Absorbance (a.u.) 1.4 V 1.2 V 1. V.8 V V.6 V.8 V 1. V 1.2 V 1.4 V Absorbance (a.u.) 1.4 V 1.2 V 1. V.8 V V.6 V.8 V 1. V 1.2 V (a) 5 6 Wavelength (nm) (b) Wavenumber (cm 1 ) 7 8 Figure 3.22 Transmittance spectra change of thin films of (a) poly(dntd) and (b) poly(dntd)/ under applied potentials ranging from 1.4 to +1.4 V in propylene carbonate containing 1 M LiClO 4 as the electrolyte. 1.2 V 1. V.8 V.6 V.4 V V.4 V.6 V (a).8 V 1. V 1.2 V 1.4 V 1.4 V 1.2 V 1. V.8 V.6 V.4 V V.4 V (b).6 V.8 V 1. V 1.2 V 1.4 V Figure 3.23 Color change of (a) poly(dntd) and (b) poly(dntd)/ composite thin films upon different potentials. Reproduced with permission from Ref. [4]. Copyright 212, The Journal of Physical Chemistry.

26 Advances in Tungsten Oxide/Conducting Polymer Hybrid 85 grains spread irregularly all over the surface while the /PANI shows micro aggregates with a larger active surface area than that of pure (Figure 3.24). It should be seen from the comparative CVs that the peakto-peak separations ( Ep) between the anode and cathodic waves for the PANI film are much larger than for the nanocomposite /PANI film, indicating that the composite film exhibits enhanced reversible redox reactions than the PANI alone. The CV at various scan rates for the composite film shows that the peaks get more pronounced with increased scan rate (Figure 3.25). The /PANI nanocomposite exhibited multiple colors (electrochromism) during the CV scans, from brownish green to transparent to light green then back to brownish green (Figure 3.26). Surprisingly, the integration of the PANI with the led to synergistic performance of nanohybrid wherein a true electrochemical double layer capacitor was obtained. Also, interestingly and unlike literature reports, the CBD method led to excellent capacitance retention (>98%) of the PANI even at 1 continuous cycles (Figure 3.27). This work demonstrates that simple CBD can be used to get /PANI films that give good electrochromism and pseudo-capacitance comparable to the ones obtained by other methods. Hence, the obtained nanocomposite film of /PANI can be a promising material for EC and energy storage applications [41]. PEDOT/ composite films were electrochemically prepared using different ionic liquids as electrolytes and synthesis media. A series of ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF 6 ), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (BMIMTFSI), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMPTFSI) were used as both electrochemical growth media and electrolytes for the synthesis of composites. The peak currents corresponding to the redox process of PEDOT and in the cyclic voltammograms of PEDOT/ composite films are much higher than that of either pure PEDOT or, which reflects the fact that proton insertion/extraction is facilitated and the composite films exhibit enhanced (a) (b) Figure 3.24 SEM images of (a) and (b) /PANI.

27 86 Advanced Electrode Materials 1.2 /PANI PANI Current density/ma cm (a) Potential/V (vs. Ag/AgCl) Current density/ma cm 2 (b) /PANI Potential/V (vs. Ag/AgCl) 5 mv/s 1 mv/s Figure 3.25 (a) CV curves for the films at 5 mv s 1 and (b) the CV at 5 and 1 mv s 1 for the composite film. Bleached cross section of /PANI Bleached cross section of PANI Figure 3.26 Color change of the composite film during cyclic voltammetry.

28 Advances in Tungsten Oxide/Conducting Polymer Hybrid 87 4 /PANI PANI Capacitance/mF cm Cycle number Figure 3.27 Comparative cycle stability of the films at.16 ma/cm 2. Reproduced with permission from Ref. [41]. Copyright 214, Electrochimica Acta. Current (A) (a) BMIMPF 6 PEDOT PEDOT/ Potential (V) (vs. Ag/AgCl) Current (A) (b) BMIMTFSI PEDOT PEDOT/ Potential (V) (vs. Ag/AgCl) (c) Current (A) BMPTFSI PEDOT PEDOT/ Potential (V) (vs. Ag/AgCl) Figure 3.28 (a) Cyclic voltammograms of films in BMIMPF 6. (b) Cyclic voltammograms of films in BMIMTFSI. (c) Cyclic voltammograms of films in BMPTFSI. reversible redox reactions than the PEDOT or alone (Figure 3.28). All three composite films prepared from different ionic liquids and observed on the SEM exhibited distinctly different morphologies. The SEM analysis of PEDOT/ nanocomposite synthesized by BMPTFSI

29 88 Advanced Electrode Materials (a) (b) (c) (d) Figure 3.29 (a) SEM image of PEDOT/ composite synthesized in BMIMPF 6. (b) SEM image of PEDOT/ composite synthesized in BMIMTFSI. (c) SEM image of PEDOT/ composite synthesized in BMPTFSI. (d) SEM image of film. ITO coated glass (counter electrode) Gel electrolyte PEDOT/WO3 (EC electrode) + ITO coated glass Scheme 3.2 Construction of ECD. was interesting to observe since this composite displayed the best EC properties. The appearance of pores with increased diameter could account for the good electrochemical behavior, from the perspective of ions which can be injected/ejected easily into/out of the polymer matrix (Figure 3.29). In order to carry out optical and EC measurements, ECDs were fabricated (Scheme 3.2). For the composite synthesized with BMIMPF 6, optical contrast was found as Δ%T = 32.8, for the composite synthesized with BMIMTFSI Δ%T = 22.3 and for the composite prepared with BMPTFSI,

30 Advances in Tungsten Oxide/Conducting Polymer Hybrid 89 PEDOT/ synthesized 16 in BMIMPF V V V (a) Wavelength (nm) Transmittance (T %) Transmittance (T %) PEDOT/ synthesized in BMIMTFSI V +2. V 2. V (b) Wavelength (nm) PEDOT/ synthesized in BMPTFSI V V 9 2. V (c) Wavelength (nm) Transmittance (T %) Figure 3.3 (a) Transmittance change of PEDOT/ film synthesized with BMIMPF 6 for applied potentials of, +2, 2 V. (b) Transmittance change of PEDOT/ film synthesized with BMIMTFSI for applied potentials of, +2, 2 V. (c) Transmittance change of PEDOT/ film synthesized with BMPTFSI for applied potentials of, +2, 2 V. maximum optical modulation Δ%T was measured as 41.3 (Figure 3.3). These three ECDs presented stable and reproducible redox processes between +2. V and 2. V even after a thousand scans (Figure 3.31). The electrochemically prepared composite nanoporous films in the presence of RTIL can be also applied to photovoltaic cells, photocatalytic composites, CP-based batteries, and photo EC cells [42]. Polypyrrole (PPy)/tungsten oxide ( ) composites were electrosynthesized in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF 4 ), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF 6 ), 1-butyl- 3-methylimidazolium bis (trifluoromethylsulfonyl) imide (BMIMTFSI), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMPTFSI) for fabrication of ECDs. The intensity of the electrochemical signal is largely increased in PPy/ composites compared to PPy or which indicates that PPy/ materials have higher electrochemical activity than PPy or (Figure 3.32). XRD patterns of the / PPy composites show a similar profile with the, except for weaken intensity of the characteristic peaks that may result from the interaction

31 9 Advanced Electrode Materials Current density (ma cm 2 ).2.1 PEDOT/ synthesized in BMIMPF 6 PEDOT/WO3 synthesized in BMIMTFSI st cycle 5 th cycle 1 th cycle Current density (ma cm 2 ) st cycle 5 th cycle 1 th cycle (a) Time (sec) Current density (ma cm 2 ) (c) (b).6 PEDOT/ synthesized in BMPTFSI Time (sec) Time (sec) 1 st cycle 5 th cycle 1 th cycle Figure 3.31 (a) Current response for the BMIMPF 6 -based device during repeating CA. (b) Current response for the BMIMTFSI-based device during repeating CA. (c) Current response for the BMPTFSI-based device during repeating CA. Reproduced with permission from Ref. [42]. Copyright 214, Electroanalysis. between PPy and. In PPy/ hybrid nanocomposites, crystalline structures decrease compared to, and more amorphous arrangements have been introduced into the hybrid nanocomposites (Figure 3.33). The highest contrast between the colored and the bleached forms in the visible range was observed at 65 nm, with a transmittance variation of and for BMIMBF 4 - and BMIMPF 6 -based devices. Optical contrasts of the BMIMTFSI and BMPTFSI devices were found as and 22.16, respectively (Figure 3.34). After 5 and 1 cycles, the current curves responding to cyclic voltage remained almost the same as the beginning in BMIMBF 4 and BMPTFSI devices. However, ECDs synthesized by BMIMPF 6 and BMIMTFSI mediums exhibited weaker stability, relatively (Figure 3.35). CA results are in accordance with XRD results in a way that the weakest crystalline-structured composite in BMIMTFSI medium has the weakest cyclic stability [43]. Hybrids of tungsten trioxide titanium dioxide ( TiO 2 ) and tungsten trioxide poly(3,4-ethylenedioxythiophene) ( PEDOT) were

32 Advances in Tungsten Oxide/Conducting Polymer Hybrid 91 Current density (ma/cm 2 ) (a).6 Current density (ma/cm 2 ) (c) PPy PPy/ PPy PPy/ BMIMBF BMIMPF Potential (V) (vs. Ag/AgCl) (b) Potential (V) (vs. Ag/AgCl) BMIMTFSI PPy PPy/ 1.5 PPy PPy/ Potential (V) (vs. Ag/AgCl) Current density (ma/cm 2 ) Current density (ma/cm 2 ) (d) BMPTFSI Potential (V) (vs. Ag/AgCl) Figure 3.32 (a) Cyclic voltammogram comparison between, polypyrrole (PPy), and PPy/ films in BMIMBF 4 at scan rate of 1 mv/s. (b) cyclic voltammogram comparison between, PPy, and PPy/ films in BMIMPF 6 at scan rate of 1 mv/s. (c) Cyclic voltammogram comparison between, PPy, and PPy/ films in BMIMTFSI at scan rate of 1 mv/s. (d) Cyclic voltammogram comparison between, PPy, and PPy/ films in BMPTFSI at scan rate of 1 mv/s. PPy/ by BMPTFSI PPy/ by BMIMTFSI PPy/ by BMIMPF 6 PPy/ by BMIMPF 4 Intensity (a.u.) θ (degree) Figure 3.33 XRD patterns of and PPy/ synthesized by BMIMBF 4, BMIMPF 6, BMIMTFSI, and BMPTFSI.

33 92 Advanced Electrode Materials Transmittance (T %) (a) Transmittance (T %) (c) V +2. V 2. V Wavelength (nm) (b) Transmittance (T %) V V V Wavelength (nm) (d) Transmittance (T %) V +2. V 2. V Wavelength (nm) V +2. V 2. V Wavelength (nm) Figure 3.34 (a) Transmittance change of ECD based on PPy/ /BMIMBF 4 for applied potentials of and ±2 V, (b) transmittance change of ECD based on PPy/ /BMIMPF 6 for applied potentials of and ±2 V, (c) transmittance change of ECD based on PPy/ / BMIMTFSI for applied potentials of and ±2 V, and (d) transmittance change of ECD based on PPy/ /BMPTFSI for applied potentials of and ±2 V. prepared by an rf rotating plasma modification method. Voltammetric cycles of hybrid films exhibit higher current densities than that of the current peak at.233 V, and their onset potentials of the cathodic current shifted significantly in the positive direction (Figure 3.36). The transmittance variations (ΔT%) were obtained as 66.86% of TiO 2 and 6.3% of PEDOT at 7 nm. These values are higher than that of (5.73%) (Figure 3.37). The color switching times of solid-state ECDs of TiO 2 and PEDOT from the bleached state to the colored state are found to be 1.4 and 1.5 s (for the reverse process, it takes longer times for bleaching of 1.1 and 9.5 s), respectively (Figure 3.38). After subjecting the samples during 2 cycles, the peak currents remained stable and were not affected much by the air exposure, particularly for the. ECDs of hybrids showed weaker stability, relatively. The cyclic stability of the hybrids was damaged relatively because of the decreased crystallinity

34 Advances in Tungsten Oxide/Conducting Polymer Hybrid 93 Current density/ (ma/cm 2 ) st cycle 5 th cycle 1 th cycle Current density/(ma/cm 2 ) st cycle 5 th cycle 1 th cycle (a) Current density/(ma/cm 2 ) Time/sec 1 st cycle 5 th cycle 1 th cycle 4 (b).8.4 Current density/(ma/cm 2 ) Time/sec 1 st cycle 5 th cycle 1 th cycle (c) Time / sec (d) Time / sec Figure 3.35 (a) CA measurement of the ECD based on PPy/ /BMIMBF 4 during 1 cycles against an applied cyclic potential of ±2 V, (b) CA measurement of the ECD based on PPy/ /BMIMPF 6 during 1 cycles against an applied cyclic potential of ±2 V, (c) CA measurement of the ECD based on PPy/ /BMIMTFSI during 1 cycles against an applied cyclic potential of ±2 V, and (d) CA measurement of the ECD based on PPy/ /BMPTFSI during 1 cycles against an applied cyclic potential of ±2 V. Reproduced with permission from Ref. [43]. Copyright 216, Polymers for Advanced Technologies. after modification depending on the unstable proton-capturing sites (Figure 3.39) [44]. Hybrid nanofibers of PEDOT/ were prepared through electrochemical polymerization of PEDOT onto nanoporous and subsequent electrospinning for the assembly of ECDs. Different ionic liquids media; BMIMBF 4, BMIMPF 6, BMIMTFSI, and BMPTFSI were used for the synthesis of hybrids. Both the support and PEDOT exhibit well-defined and reversible electroactivity in the hybrid configuration. Although electrochemical behaviors are similar, the PEDOT/ nanofiber synthesized in BMIMPF 6 medium has the highest current values in cyclic voltammograms (Figure 3.4). Optical contrasts of BMIMTFSI- and BMPTFSI-based fibers were determined as

35 94 Advanced Electrode Materials TiO 2 -PEDOT Current density/ A cm Potential/V vs Ag/AgCl Figure 3.36 CV curves of, TiO 2, and PEDOT films in 1M LiClO 4 (in PC) at a potential scanning rate of 5 mv/s versus Ag/AgCl. Transmittance/(%) Wavelength/nm -TiO 2 -PEDOT 7 8 Figure 3.37 Optical transmittance spectra of solid-state ECDs of, TiO 2, and PEDOT under potentials of +3 and 3V, respectively.

36 Advances in Tungsten Oxide/Conducting Polymer Hybrid 95 Transmittance/ (%) -TiO 2 -PEDOT -TiO 2 -PEDOT Time/sec Time/sec Figure 3.38 Color switching speed of ECDs. Current density/ma cm and 18.57, respectively. However, BMIMBF 4 - and BMIMPF 6 - based fibers reached contrast values of 4.58 and 47.89, respectively (Figure 3.41). The smallest switching times were achieved for PEDOT/ /BMIMTFSI-based ECD which shows 2. s for coloring and 1.5 s for bleaching process (Figure 3.42). The device was transparent in its oxidized state (3. V) while in its fully reduced state ( 3. V), it became light-brown tint (Figure 3.43). Thinner and dense fibers decrease the probability of extinction of polarons due to the shorter diffusion path length. This effect is evidenced by higher EC efficiency and optical modulation as seen in BMIMBF 4 - and BMIMPF 6 -based fibers (Figure 3.44). The present results should open new perspectives for the application of hybrid nanofibers in ECDs [45]. 3.9 Conclusions and Perspectives This chapter has presented the conceptual and materials-oriented basis of ECs with special attention to hybrids of tungsten oxide and conjugated polymers. In view of the growing demand for functional materials of various types, strategies to tune properties and design novel materials have become increasingly important. Introduction of metal oxides into CPs, or deposition of CPs on metal oxide surfaces aims to result in advanced properties in various aspects. We have outlined recent progress

37 96 Advanced Electrode Materials.3 1st cycle 2th cycle Current density/ ma cm Time/sec Current density/ ma cm TiO 2 1st cycle 2th cycle Time/sec Current density/ ma cm PEDOT 1st cycle 2th cycle Time/sec Figure 3.39 CA measurements of solid-state devices during 2 cycles against an applied cyclic potential of ±3 V with the time interval set to.1 s at 5 mv/s scan rate. Reproduced with permission from Ref. [44]. Copyright 214, Industrial & Engineering Chemistry Research.

38 Advances in Tungsten Oxide/Conducting Polymer Hybrid 97 Current (ma) Current (ma) Current (ma) PEDOT/WO PEDOT/ /BMIMBF 3 /BMIMPF Potential (V) PEDOT/ /BMIMTFSI Potential (V) Current (ma) Potential (V) PEDOT/ /BMPTFSI Potential (V) Figure 3.4 Cyclic voltammograms of PEDOT/ /BMIMBF 4, PEDOT/ / BMIMPF 6, PEDOT/ /BMIMTFSI, and PEDOT/ /BMPTFSI hybrid nanofibers in 1 M Li-PC during 1 cycles. Transmittance (%) Transmittance (%) 1 1 V V 3V 3V 3V 8 3V PEDOT/WO3/BMIMBF4 PEDOT/WO3/BMIMPF Wavelength (nm) Wavelength (nm) 1 V 4 V 3V 3V 8 3V 3V Transmittance (%) Transmittance (%) PEDOT/WO3/BMIMTFSI PEDOT/WO3/BMPTFSI Wavelength (nm) 2 1 Wavelength (nm) Figure 3.41 Visible transmittance spectra of PEDOT/ /BMIMBF 4, PEDOT/ / BMIMPF 6, PEDOT/ /BMIMTFSI, and PEDOT/ /BMPTFSI nanofiber-based ECD for applied potentials of and ±3 V.

39 98 Advanced Electrode Materials Current density (ma/cm 2 ) PEDOT/ /BMIMBF 4 PEDOT/ /BMIMPF 6 PEDOT/ /BMIMTFSI PEDOT/ /BMPTFSI Time (sec) Figure 3.42 Current densities monitored for the hybrid nanofiber based ECDs stepped between ±3 V. Red x (a) (b) Figure 3.43 Photographs of the PEDOT/ hybrid nanofiber-based ECD in the two extreme states (a) in its bleached state at +3V (b) in its colored state at 3V. related to durability and material rejuvenation for ECDs containing films based on tungsten oxide/conjugated polymer hybrids. It is clear that the coloration efficiency, switching kinetics, and stabilities of conjugated polymers can be significantly improved by the hybrid approaches owing to the enhanced electron and ion transport as well as donor acceptor interactions. The attractive attributes of these novel materials have resparked much attention on such hybrid assemblies to be deployed in EC applications.

40 Advances in Tungsten Oxide/Conducting Polymer Hybrid 99 (a) (b) (c) (d) Figure 3.44 SEM micrographs of as electrospun hybrid nanofibers (a) PEDOT/ / BMIMBF 4, (b) PEDOT/ /BMIMPF 6, (c) PEDOT/ /BMIMTFSI, and (d) PEDOT/ /BMPTFSI. Reproduced with permission from Ref. [45]. Copyright 216, Electroanalysis. Acknowledgements The authors would like to acknowledge the TUBITAK Project-with number 114Z321 and SDU Project with number 3193-D2-12 for the financial support of this research. References 1. Granqvist C. G., Electrochromics for smart windows: oxide-based thin films and devices. Thin Solid Films, 564, 1 38, Granqvist C.G., Lansaker P.C., Mlyuka N.R., Niklasson G.A., Avendano E., Progress in chromogenics: new results for electrochromic and thermochromic materials and devices. Sol. Energy Mater. Sol. Cells, 93, , Granqvist C.G., Green S., Niklasson G.A., Mlyuka N.R., Kræmer S. von, Georén P., Advances in chromogenic materials and devices. Thin Solid Films, 518, , Azens A., Granqvist C.G., Electrochromic smart windows: energy efficiency and device aspects. J Solid State Electrochem., 7, 64 68, 23.