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RecentAdvancesinTungstenOxide/ConductingPolymerHybridAssembliesforElectrochromicApplications

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Ashutosh Tiwari, Filiz Kuralay and Lokman Uzun (eds.) Advanced Electrode Materials, (61–102) © 2017 Scrivener Publishing LLC

3

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

AbstractMuch effort is currently devoted to implementing new materials in electrodes that will be used in electrochromic (EC) technology. Tungsten oxide (WO3) 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 WO3/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 WO3/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, chemi-cal 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 new-generation 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

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 technolo-gies 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 thermo-chromic (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, photo-chromic 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 pro-vide 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 mate-rials 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, WO3 has many advantages, including genuine color switching, good chemical stability, and strong adherence to the substrate. However, single color change and slow switch-ing 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 [10]. The synergistic combination of the merits of conducting polymers and inorganic materials may provide an oppor-tunity 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 poly-mer 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

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 auto-motive windows, as well as mirrors, but the history of ECs dates back to 1704, when Diesbach discovered the chemical coloration of Prussian Blue. In the 1930s, electrochemical coloration was noted in bulk WO3. Twenty years later, Kraus observed electrochemical coloration in thin films. The first ECDs were made by Deb in 1969. By the mid-1970s, ECDs were being developed for displays. ECs based on viologens and WO3 followed in the 1980s for switchable mirrors in cars, which continues as a viable product to this day. In the 1990s, 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 pos-sible to take advantages of these materials in practical applications, mak-ing it easier to define standards when investigating the characteristics of the EC materials. The most practical design for testing and commercial-izing 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 elec-trodes. 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

64 Advanced Electrode Materials

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 transpar-ent substrate [16].

The key parameters of ECDs include the following.

3.3.1 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 con-trast. 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 mate-rial as it relates to the human eye perception of transmittance over the entire visible spectrum.

3.3.2 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

Electron �ow

Ion �ow TCO

Electrochromic materialSolid or gel electrolyte

Counter-electrodeTransparent 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 2014, Royal Society of Chemistry.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 65

function of the injected/ejected electronic charge (Qd), i.e. the amount of charge necessary to produce the optical change. It is given by the equation

η = ΔOD/Qd = [log(Tb/Tc)]/Qd

where η (cm2/C) is the coloration efficiency at a given λ, and Tb and Tc are the bleached and colored transmittance values, respectively. The relation-ship 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.

3.3.3 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 appli-cations 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.

3.3.4 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 106 cycles without significance performance loss, the lack of durability (espe-cially 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.

3.3.5 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

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 cur-rent due to the diffusion of soluble electrochromes away from the elec-trodes (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 environmen-tal 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 [20]. 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 mate-rial. At open circuit voltage, the window is in Bright Mode, that is, both

Bright mode

Visiblelight

Visiblelight

Visiblelight

Infrared

Eletrochromiclayer in glass

Nanocrystalcounter

electrode

Nanocrystalblocks infraredlight

Matrixblocksvisiblelight

Nanocompositeworking

electrodeCurrent turned on.Electrons and ions

�ow to nanocrystalsin working electrode

Current turned on.Electrons and ions

�ow to matrixin working electrode

Ionconductingelectrolyte

Infrared

Infrared

In bright modewindows allownatural lightand heat toenter room

In cool modewindowsallow naturallight to enterroom but blockheat fromentering

In dark modewindowslimit theamount ofheat andnatural lightthat enter theroom

Cool mode Dark mode

Universal smart window

Figure 3.2 Design of EC window. Reprinted with permission Ref. [21]. Copyright 2013, Nature.

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 win-dow 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 WO3 compound. The WO3 powder appears yellow in color having density of 7.16 g/cm3. The melting temperature of WO3 is ~1473 °C, but its sublimation starts at nearly 900 °C. Among transition metal oxides, WO3 is one of the most interest-ing materials exhibiting a wide variety of novel properties particularly in thin film form useful for advanced technological applications. WO3 exhib-its 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 WO3 is temperature dependent. It is tetragonal at temperatures above 740 °C, orthorhombic from 330 to 740 °C, monoclinic from 17 to 330 °C, and triclinic from –50 to 17 °C [23]. The most common monoclinic crystal structure of WO3 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. WO3 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

68 Advanced Electrode Materials

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 prop-erties. 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 WO3 from transparent to dark was shown in highly disordered thin films. Since then, extensive studies have been carried out for WO3 in smart window applications [25].

WO3 has a nearly cubic structure which may be simply described as an “empty-perovskite” type formed by WO6 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. WO3, with all tungsten sites as oxidation state W(VI), is a transparent thin film. On electrochemical reduction, W(V) sites are gener-ated 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. WO3 is a cathodically ion insertion material. The blue coloration in the thin film of WO3 can be erased by the electrochemical oxidation. In the case of Li+ cations, the electrochemical reaction can be written as Eq. (3.1) [26].

W O

Figure 3.3 Monoclinic crystal structure of tungsten oxide. Reprinted with permission from Ref. [23]. Copyright 2013, Journal of Non-Oxide Glasses.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 69

WO3(k) + x(Li+(aq) + e–) → LixWO3(k) (3.1)

WO3 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 nowa-days concerning energy conservation, temperature and lighting control in buildings and vehicles [27].

Interest in the use of WO3 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 WO3 nanostructures, which ranges from 2.60 to 3.25 eV. These proper-ties make the WO3 films generally transparent in nature [28].

3.6 Conjugated Organic Polymers

In the recent years, conjugated polymers (CPs) have gained a lot of atten-tion 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 dop-ing (“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].

70 Advanced Electrode Materials

All conjugated organic polymers are potentially EC in thin-film form, redox switching giving rise to new optical absorption bands in accompani-ment 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 can-not 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 com-ponents [30]. 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 applica-tions 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 lat-ter, 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 perspec-tive [33]. Research in the topic of hybrid materials entails challenges and opportunities. The main challenge is managing to synthesize hybrid combi-nations 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 prop-erties [34]. It was soon recognized that in hybrids, the complementary properties can be exploited, and the synergies fully utilized. Such synergies

Advances in Tungsten Oxide/Conducting Polymer Hybrid 71

predominantly stem from the combination of the flexibility and function-ality 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 mul-tilayer hybrid films based on poly (styrenesulfonate)-doped poly(3,4- ethylenedioxythiophene) (PEDOT:PSS) and tungsten oxide nanoparticles (WO3 NPs). Polyethylenimine (PEI) is deposited in between to introduce electrostatic force between the components. Since both WO3 NPs and PEDOT:PSS colloidal particles have negatively charged surfaces, to facili-tate 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 WO3–NP counterparts, spectro-electrochemical charac-terization were conducted on 10-layer PEDOT:PSS and WO3–NP films, and 5-layer hybrid films. The transmittance of WO3–NPs, PEDOT:PSS, and hybrid thin films were recorded at constant potentials of +0.8, 0, and –1.0 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 col-ored states at wavelength of around 633 nm, which is defined as optical contrast. With comparable thickness of each film, the optical contrast of

Scheme 3.1 Scheme for the formation of EC multilayer hybrid film [PEI/PEDOT:PSS/WO3-NPs]n (Hn).

Repeatingcycles

1 Hybrid layerWO3 NPsPEDOT:PSSPEI+ + +

+ + + + + + + + +

+ + −

− −

−

−

−

−

−−−−−−−

−−

−−−−−−−−

−−−−−−−−

+++++++++

+

++++++++

++++++++

++++++++

+++++++++

+++++++++

+++++++++

+++++++++

++++++++

72 Advanced Electrode Materials

the hybrid film (DT = 20%) is significantly higher than that of WO3–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 pro-cess, the coloration efficiency of the hybrid film is significantly improved to 117.7 cm2/C at wavelength of 633 nm [36].

Kim et al. investigated the enhanced electrochemical and EC proper-ties of P3HT (poly 3-hexylthiophene)/WO3 composites. Nanoporous WO3 layers were prepared using electrochemical anodization. P3HT was spin coated on these layers to obtain hybrid P3HT/WO3 composites. After annealing at 300 °C for 1 h, the monoclinic phase of the WO3 layer and self-organized lamella structure of P3HT were examined. The P3HT/WO3 com-posites 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 WO3 during the redox reaction. Furthermore, the composites exhibited faster switch-ing speeds compared with WO3 layers, which might be attributed to the

10 20(c) (d)

30 402 theta (degree) 2 theta (degree)

Inte

nsity

(a.u

.)

Inte

nsity

(a.u

.)

50 60As-formed

Annealed

(200)W

P3HT(100)

(110)W

WO2 (002)

WO2(020)WO2

(200)

(202) WO2

70 4 8 12 16 20

Figure 3.5 X-ray diffraction (XRD) patterns of (c) WO3 and (d) P3HT/WO3.

90

85

80

75

70

Tran

smitt

ance

(%)

Wavelength (nm)

65

60

55

9085807570

Tran

smitt

ance

(%)

656055

9085807570

Tran

smitt

ance

(%)

656055

300(a) (b) (c)

400 500 600 700 800Wavelength (nm)

300 400 500 600 700 800Wavelength (nm)

300 400 500 600 700 800

0 V

0 V

0 V

−1.0 V−1.0 V

−1.0 V

+0.8 V

+0.8 V

+0.8 V

Figure 3.4 UV–Vis spectrum of 10 layers of (a) WO3–NP, (b) PEDOT:PSS, and 5 layers of (c) hybrid thin films under different potentials of +0.8, 0, and –1.0 V. Reproduced with permission from Ref. [36]. Copyright 2015, Electrochimica Acta.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 73

easy Li+ insertion/extraction resulting from the incorporation of P3HT. Therefore, it can be concluded that the combination of P3HT and WO3 yields a promising EC material exhibiting multicolor electrochromism and faster response [37].

Cai et al. prepared WO3/PANI core/shell nanowire array by the combi-nation of solvothermal and electropolymerization methods. The core/shell nanowire array film shows remarkable enhancement of the EC properties.

0.2

0.1

WO3

P3HT

P3HT/WO3

0.0Cu

rren

t den

sity

(mA

/cm

2 )Cu

rren

t den

sity

(mA

/cm

2 )Cu

rren

t den

sity

(mA

/cm

2 )

Voltage (V vs. Ag wire)

0.10

0.05

0.00

−0.05

−0.1

−0.2

−0.3

−0.4

−0.4

−0.4 0.4 0.8 1.20−0.8

−0.2

0.2

0.4

(c)

(b)

(a)

0.0

−0.6

Figure 3.6 Cyclic voltammogram of (a) WO3, (b) P3HT, and (c) P3HT/WO3 composites performed between −0.7 and 1.0 V with a scan rate of 50 mV/s in propylene carbonate solution with 0.4 M LiClO4.

74 Advanced Electrode Materials

1.0

0.9

0.8

0.7

0.6

0.5

1.0

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0

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0.2−0.7 V −0.7 V

−0.7 V

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0400 500 600 700 800 900

400(a) (b)

(c) (d)

500 600 700 800

WO3

P3HT

P3HT/WO3

900 400 500 600 700 800 900

0.2Voltage (V vs. Ag wire)Wavelength (nm)

Wavelength (nm)

Ree

ctan

ce (a

.u.)

Ree

ctan

ce (a

.u.)

Ree

ctan

ce (a

.u.)

Nor

mal

ized

ree

ctan

ce (a

.u.)

Wavelength (nm)

0.4−0.6−0.4−0.2 0.6 0.8 1.0

1.0 V0.9 V0.7 V0.5 V0.3 V0.1 V−0.1 V−0.2 V−0.3 V−0.4 V−0.5 V−0.7 V

−0.1 V0 V0.2 V1.0 V 1.0 V

0.9 V0.8 V0.6 V0.5 V0.3 V−0.7 V to0.2 V

−0.2 V−0.3 V−0.4 V−0.5 V−0.7 V

1.2

650 nm

550 nm600 nm

1.4

Figure 3.7 Reflectance spectra of (a) WO3, (b) P3HT, and (c) P3HT/WO3 composites at applied voltages from −0.7 to 1.0 V and (d) reflectance changes of P3HT/WO3 at specific wavelengths as a function of voltages from −0.7 to 1.0 V. Reproduced with permission from Ref. [37]. Copyright 2015, Electrochemistry Communications.

10 20

a

(112

)

(202

)

(002

)

(200

)

(222

)

(400

)(2

40)

(420

)

b

30 40 502θ/ degree

Inte

nsity

/a.u

.

60 70 80

∀- FTO

∀ ∀ ∀∀

∀∀∀

* - WO3

**

* * * **

Figure 3.8 XRD patterns of (a) WO3 and (b) WO3/PANI nanowire arrays.

Except for the peaks of FTO glass, all the diffraction peaks of both the films can be indexed as the monoclinic WO3 phase in XRD patterns (JCPDS no. 72-0677). No obvious diffraction peaks of PANI are observed, indicating the amorphous nature of PANI deposited by the CVs (Figure 3.8). The CV

Advances in Tungsten Oxide/Conducting Polymer Hybrid 75

curve of the WO3/PANI nanowire array exhibits both characteristic peaks of WO3 nanowire and PANI film. In addition, the WO3/PANI nanowire array shows significantly high exchange current densities compared to the WO3 and PANI film (Figure 3.9). In particular, a significant optical modu-lation (59% at 700 nm) (Figure 3.11), fast switching speed (Figure 3.12), high coloration efficiency (86.3 cm2/C at 700 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–accep-tor 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 WO3, the dual-electrochromism effect is obtained for the WO3/PANI core/shell nanowire array. It is a great promise for the WO3/PANI core/shell nanowire array as a potential multicolor EC material (Figure 3.10) [38].

Enlightened by Cai et al.’s work, Zhang et al. synthesized ultra-thin WO3 nanorods (NRs)-embedded polyaniline (PANI) composite thin films by

0.5

WO3 PANI WO3@ PANI

−0.5

−0.5

0.5

0

−1.0

1.0

1.00Potential/V (vs. Ag/AgCl)

Curr

ent d

ensi

ty/m

A c

m−2

Figure 3.9 The 10th CV curves of WO3, PANI, and WO3/PANI films.

1.0 V 0.2 V −0.2 V −0.7 V

Figure 3.10 Photographs of a WO3/PANI core/shell nanowire array sample (2 × 4 cm2 in size) under different applied potentials.

76 Advanced Electrode Materials

embedding WO3 NRs into PANI using a surface-initiated polymerization method, followed by spin-coating deposition. The ultra-thin WO3 NRs with length of 60 nm and diameter of 4 nm were prepared by a solvother-mal method and were used as nanofillers reinforced into the PANI matrix

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1.0 V0.2 V

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Figure 3.11 Visible transmittance spectra of (a) WO3, (b) PANI, and (c)WO3/PANI films under different applied potentials.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 77

to form an organic/inorganic nanocomposite with excellent processability (Figure 3.15). Diffraction peaks of WO3 NRs match well with hexagonal WO3 (JCPDS 85-2460). XRD patterns of the WO3 NRs–PANI composite shows a similar profile with the WO3 NRs, except for weaken intensity of the characteristic peaks (Figure 3.14). The CVs of the WO3 NRs–PANI composite film exhibit both characteristic peaks of WO3 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 dura-bility of the hybrid film was enhanced compared with neat WO3 NRs film (Figure 3.19). As is well known, WO3 is colored at negative potential, and PANI is colored at positive potential. The combination of the two mate-rials with different coloration mechanisms leads to a dual electrochro-mism. The effect is due to that the coloration of WO3 and bleaching of PANI are not entirely overlapped, and vice versa. Moreover, the two mate-rials are strongly complementary to each other in conductivity. That is at

020

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Figure 3.12 EC response of (a) WO3 nanowire array (–0.7 to 1.0 V), (b) PANI film (–0.7 to 1.0 V), (c) WO3/PANI core/shell nanowire array (–0.2 to 1.0 V), and (d) WO3/PANI core/shell nanowire array (–0.7 to –0.2 V). Reproduced with permission from Ref. [38]. Copyright 2014, Solar Energy Materials & Solar Cells.

78 Advanced Electrode Materials

400 nm 200 nm

200 nm

200 nm

200 nm

400 nm

1 µm

1 µm

(a) (b)

(c) (d)

(e) (f)

Figure 3.13 Scanning electron microscope (SEM) images of (a and b) WO3 nanowire, (c and d) WO3/PANI core/shell nanowire array, and (e and f) PANI film.

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nsity

/a.u

.

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#85-2460

WO3 NRs

WO3 NRs–PANI

PANI

90

Figure 3.14 XRD patterns of WO3 NRs, PANI, and WO3 NRs–PANI composite.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 79

positive potential the PANI shows excellent conductivity, and at negative potential WO3 is conductive because of the formation of hydrogen tung-sten bronze. Conductivity is a key factor to EC switching speed. Therefore, fast response is expected in the composite film. It is found that WO3 NRs show a response time longer than 5s. The response times of PANI and WO3 NRs–PANI composite films are 0.6 and 0.9 s, respectively (Figure 3.18). Comparing with the neat WO3 NRs film, a much faster switching speed is obtained for the WO3 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

(a)

(b)

(c) 100 nm

100 nm

100 nm

Figure 3.15 SEM images of (a) WO3 NRs, (b) PANI, and (c) WO3 NRs–PANI composite.

80 Advanced Electrode Materials

0

WO3 NRs

PANI

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Figure 3.16 Cyclic voltammograms of (a) WO3 NRs, (b) PANI, and (c) WO3 NRs–PANI composite films in 0.5 M H2SO4 electrolyte at a potential scanning rate of 50 mV s–1.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 81

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2WO3 NR WO3 NR-PANIPANI

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Figure 3.18 Current transient response and corresponding optical switching at 550 nm for (a) WO3 NRs, (b) PANI, and (c) WO3 NRs–PANI composite films in 0.5 M H2SO4 electrolyte applied potential steps of –0.5 V (5 s) and 0.5 V (5 s). Reproduced with permission from Ref. [39]. Copyright 2013, Solar Energy Materials & Solar Cells.

82 Advanced Electrode Materials

1000Cycle number

Curr

ent/

a.u.

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a.u.

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ent/

a.u.

50(a)

(b)

(c)

1005

1000Cycle number

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1000Cycle number

50 1005

Figure 3.19 Cyclic stability test using chronoamperometry (CA).

incorporated with WO3 nanoparticles by a facile electropolymerization method on an indium tin oxide (ITO)-coated glass slide from the DNTD monomer and WO3 nanoparticles suspended methylene chloride solution. The SEM image shows that the WO3 nanoparticles are uniformly embedded

Advances in Tungsten Oxide/Conducting Polymer Hybrid 83

in the polymeric matrix (Figure 3.20). For the poly(DNTD)/WO3 nano-composite film, the oxidation potential is the same as that of pure polymer, but the films were reduced at 0.34 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 −0.8 V, colorless at 0 V, orange green at 0.8 V, light blue at 1.0 V, and deep blue at 1.2 or 1.4 V vs Ag/AgCl in propylene carbonate containing 1.0 M LiClO4 electrolyte (Figure 3.23). The UV–visible-incorporated electrochemical spectroscopy coupled with amperometry was also employed to study the composite film under differ-ent 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 100 scans [40].

Nwanya et al. prepared PANI and its nanocomposite WO3/PANI films deposited on fluorine-doped tin oxide (FTO) glass slides by simple chemical bath deposition (CBD) method. The WO3 film shows spherical

10 µm(a) (b) 10 µm20 µm

Figure 3.20 SEM images of thin films of (a) pure poly(DNTD) and (b) poly(DNTD)/WO3 nanocomposites grown onto ITO-coated glass.

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mA)

Potential vs Ag/AgCl(V)

Poly(DNTD)/(WO3)

Poly(DNTD)

0.26

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0.34

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Figure 3.21 Cyclic voltammograms of thin films of (a) pure poly(DNTD) and (b) poly(DNTD)/WO3 nanocomposites in 0.1 M TBAPF6 CH2Cl2 DNTD-free solution with a scan rate of 200 mV/s.

84 Advanced Electrode Materials

400(a) (b)

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Figure 3.22 Transmittance spectra change of thin films of (a) poly(DNTD) and (b) poly(DNTD)/WO3 under applied potentials ranging from −1.4 to +1.4 V in propylene carbonate containing 1 M LiClO4 as the electrolyte.

–1.2 V –1.0 V –0.8 V –0.6 V –0.4 V 0.4 V 0.6 V

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(b)

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0 V

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Figure 3.23 Color change of (a) poly(DNTD) and (b) poly(DNTD)/WO3 composite thin films upon different potentials. Reproduced with permission from Ref. [40]. Copyright 2012, The Journal of Physical Chemistry.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 85

(a) (b)

Figure 3.24 SEM images of (a) WO3 and (b) WO3/PANI.

grains spread irregularly all over the surface while the WO3/PANI shows micro aggregates with a larger active surface area than that of pure WO3 (Figure 3.24). It should be seen from the comparative CVs that the peak-to-peak separations (∆Ep) between the anode and cathodic waves for the PANI film are much larger than for the nanocomposite WO3/PANI film, indicating that the composite film exhibits enhanced reversible redox reac-tions 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 WO3/PANI nanocomposite exhibited multiple colors (electrochromism) during the CV scans, from brownish green to transpar-ent to light green then back to brownish green (Figure 3.26). Surprisingly, the integration of the PANI with the WO3 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 1000 continuous cycles (Figure 3.27). This work demonstrates that simple CBD can be used to get WO3/PANI films that give good electrochromism and pseudo-capacitance comparable to the ones obtained by other methods. Hence, the obtained nanocomposite film of WO3/PANI can be a promising material for EC and energy storage applications [41].

PEDOT/WO3 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 (BMIMPF6), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (BMIMTFSI), and 1-butyl-1-methylpyrrolidinium bis(trifluoro-methylsulfonyl) 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 WO3 in the cyclic voltammograms of PEDOT/WO3 composite films are much higher than that of either pure PEDOT or WO3, which reflects the fact that proton insertion/extraction is facilitated and the composite films exhibit enhanced

86 Advanced Electrode Materials

Bleached cross sectionof WO3/PANI

Bleached crosssection of PANI

Figure 3.26 Color change of the composite film during cyclic voltammetry.

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50 mV/s 100 mV/s

Potential/V (vs. Ag/AgCl)

Figure 3.25 (a) CV curves for the films at 50 mV s−1 and (b) the CV at 50 and 100 mV s−1

for the composite film.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 87

reversible redox reactions than the PEDOT or WO3 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/WO3 nanocomposite synthesized by BMPTFSI

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Figure 3.27 Comparative cycle stability of the films at 0.16 mA/cm2. Reproduced with permission from Ref. [41]. Copyright 2014, Electrochimica Acta.

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WO3PEDOTPEDOT/WO3

WO3PEDOTPEDOT/WO3

WO3PEDOTPEDOT/WO3

Figure 3.28 (a) Cyclic voltammograms of films in BMIMPF6. (b) Cyclic voltammograms of films in BMIMTFSI. (c) Cyclic voltammograms of films in BMPTFSI.

88 Advanced Electrode Materials

was interesting to observe since this composite displayed the best EC prop-erties. 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 fabri-cated (Scheme 3.2). For the composite synthesized with BMIMPF6, opti-cal 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,

(a) (b)

(c) (d)

Figure 3.29 (a) SEM image of PEDOT/WO3 composite synthesized in BMIMPF6. (b) SEM image of PEDOT/WO3 composite synthesized in BMIMTFSI. (c) SEM image of PEDOT/WO3 composite synthesized in BMPTFSI. (d) SEM image of WO3 film.

Scheme 3.2 Construction of ECD.

ITO coated glass (counter electrode)

Gel electrolyte

PEDOT/WO3 (EC electrode)

ITO coated glass+

–

Advances in Tungsten Oxide/Conducting Polymer Hybrid 89

160PEDOT/WO3 synthesized

in BMIMPF6

PEDOT/WO3 synthesizedin BMIMTFSI

PEDOT/WO3 synthesizedin BMPTFSI

150140130120110100

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Figure 3.30 (a) Transmittance change of PEDOT/WO3 film synthesized with BMIMPF6 for applied potentials of 0, +2, –2 V. (b) Transmittance change of PEDOT/WO3 film synthesized with BMIMTFSI for applied potentials of 0, +2, –2 V. (c) Transmittance change of PEDOT/WO3 film synthesized with BMPTFSI for applied potentials of 0, +2, –2 V.

maximum optical modulation Δ%T was measured as 41.3 (Figure 3.30). These three ECDs presented stable and reproducible redox processes between +2.0 V and –2.0 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 (WO3) composites were electrosyn-thesized in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), 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/WO3 composites compared to PPy or WO3 which indicates that PPy/WO3 materials have higher electrochem-ical activity than PPy or WO3 (Figure 3.32). XRD patterns of the WO3/PPy composites show a similar profile with the WO3, except for weaken intensity of the characteristic peaks that may result from the interaction

90 Advanced Electrode Materials

0.02

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1st cycle500th cycle1000th cycle

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1st cycle500th cycle1000th cycle

Figure 3.31 (a) Current response for the BMIMPF6-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 2014, Electroanalysis.

between PPy and WO3. In PPy/WO3 hybrid nanocomposites, crystalline structures decrease compared to WO3, 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 vis-ible range was observed at 650 nm, with a transmittance variation of 18.37 and 24.41 for BMIMBF4- and BMIMPF6-based devices. Optical contrasts of the BMIMTFSI and BMPTFSI devices were found as 33.25 and 22.16, respectively (Figure 3.34). After 500 and 1000 cycles, the current curves responding to cyclic voltage remained almost the same as the beginning in BMIMBF4 and BMPTFSI devices. However, ECDs synthesized by BMIMPF6 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 (WO3–TiO2) and tung-sten trioxide–poly(3,4-ethylenedioxythiophene) (WO3−PEDOT) were

Advances in Tungsten Oxide/Conducting Polymer Hybrid 91

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Figure 3.32 (a) Cyclic voltammogram comparison between WO3, polypyrrole (PPy), and PPy/WO3 films in BMIMBF4 at scan rate of 100 mV/s. (b) cyclic voltammogram comparison between WO3, PPy, and PPy/WO3 films in BMIMPF6 at scan rate of 100 mV/s. (c) Cyclic voltammogram comparison between WO3, PPy, and PPy/WO3 films in BMIMTFSI at scan rate of 100 mV/s. (d) Cyclic voltammogram comparison between WO3, PPy, and PPy/WO3 films in BMPTFSI at scan rate of 100 mV/s.

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PPy/WO3 by BMIMPF4

WO3

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nsity

(a.u

.)

40 50 60

Figure 3.33 XRD patterns of WO3 and PPy/WO3 synthesized by BMIMBF4, BMIMPF6, BMIMTFSI, and BMPTFSI.

92 Advanced Electrode Materials

prepared by an rf rotating plasma modification method. Voltammetric cycles of hybrid films exhibit higher current densities than that of the WO3 current peak at −0.233 V, and their onset potentials of the cathodic cur-rent shifted significantly in the positive direction (Figure 3.36). The trans-mittance variations (ΔT%) were obtained as 66.86% of WO3−TiO2 and 60.03% of WO3−PEDOT at 700 nm. These values are higher than that of WO3 (50.73%) (Figure 3.37). The color switching times of solid-state ECDs of WO3−TiO2 and WO3−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 10.1 and 9.5 s), respectively (Figure 3.38). After sub-jecting the samples during 2000 cycles, the peak currents remained stable and were not affected much by the air exposure, particularly for the WO3. ECDs of hybrids showed weaker stability, relatively. The cyclic stability of the hybrids was damaged relatively because of the decreased crystallinity

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500 600 700 800 900

Figure 3.34 (a) Transmittance change of ECD based on PPy/WO3/BMIMBF4 for applied potentials of 0 and ±2 V, (b) transmittance change of ECD based on PPy/WO3/BMIMPF6 for applied potentials of 0 and ±2 V, (c) transmittance change of ECD based on PPy/WO3/BMIMTFSI for applied potentials of 0 and ±2 V, and (d) transmittance change of ECD based on PPy/WO3/BMPTFSI for applied potentials of 0 and ±2 V.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 93

after modification depending on the unstable proton-capturing sites (Figure 3.39) [44].

Hybrid nanofibers of PEDOT/WO3 were prepared through elec-trochemical polymerization of PEDOT onto nanoporous WO3 and subsequent electrospinning for the assembly of ECDs. Different ionic liquids media; BMIMBF4, BMIMPF6, BMIMTFSI, and BMPTFSI were used for the synthesis of hybrids. Both the WO3 support and PEDOT exhibit well-defined and reversible electroactivity in the hybrid config-uration. Although electrochemical behaviors are similar, the PEDOT/WO3 nanofiber synthesized in BMIMPF6 medium has the highest current values in cyclic voltammograms (Figure 3.40). Optical con-trasts of BMIMTFSI- and BMPTFSI-based fibers were determined as

0.4

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1st cycle500th cycle1000th cycle

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Time / sec0 0.5 1.0 1.5 2.0

Time / sec

1st cycle500th cycle1000th cycle

1st cycle500th cycle1000th cycle

1st cycle500th cycle1000th cycle

Figure 3.35 (a) CA measurement of the ECD based on PPy/WO3/BMIMBF4 during 1000 cycles against an applied cyclic potential of ±2 V, (b) CA measurement of the ECD based on PPy/WO3/BMIMPF6 during 1000 cycles against an applied cyclic potential of ±2 V, (c) CA measurement of the ECD based on PPy/WO3/BMIMTFSI during 1000 cycles against an applied cyclic potential of ±2 V, and (d) CA measurement of the ECD based on PPy/WO3/BMPTFSI during 1000 cycles against an applied cyclic potential of ±2 V. Reproduced with permission from Ref. [43]. Copyright 2016, Polymers for Advanced Technologies.

94 Advanced Electrode Materials

0.005

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Potential/V vs Ag/AgCl

WO3

WO3-TiO2

WO3-PEDOT

–0.004

–0.005–0.6 –0.4 –0.2 0.2 0.4 0.6 0.8 1.0 1.20

Figure 3.36 CV curves of WO3, WO3–TiO2, and WO3–PEDOT films in 1M LiClO4 (in PC) at a potential scanning rate of 50 mV/s versus Ag/AgCl.

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Wavelength/nm

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Figure 3.37 Optical transmittance spectra of solid-state ECDs of WO3, WO3–TiO2, and WO3–PEDOT under potentials of +3 and −3V, respectively.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 95

100 0.04

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WO3 WO3-PEDOTWO3-TiO2 WO3 WO3-PEDOTWO3-TiO2

Figure 3.38 Color switching speed of ECDs.

33.71 and 18.57, respectively. However, BMIMBF4- and BMIMPF6-based fibers reached contrast values of 40.58 and 47.89, respectively (Figure 3.41). The smallest switching times were achieved for PEDOT/WO3/BMIMTFSI-based ECD which shows 2.0 s for coloring and 1.5 s for bleaching process (Figure 3.42). The device was transparent in its oxidized state (3.0 V) while in its fully reduced state (–3.0 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 mod-ulation as seen in BMIMBF4- and BMIMPF6-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 conju-gated polymers. In view of the growing demand for functional materials of various types, strategies to tune properties and design novel materi-als 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

96 Advanced Electrode Materials

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WO3 1st cycle2000th cycle

WO3-TiO2

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1st cycle2000th cycle

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A c

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ty/ m

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m–2

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ty/ m

A c

m–2

Figure 3.39 CA measurements of solid-state devices during 2000 cycles against an applied cyclic potential of ±3 V with the time interval set to 0.01 s at 50 mV/s scan rate. Reproduced with permission from Ref. [44]. Copyright 2014, Industrial & Engineering Chemistry Research.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 97

Curr

ent (

mA)

–6

–4

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4Cu

rren

t (m

A)

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–3 –2 –1 0 1 2 3Potential (V)

–3 –2 –1 0 1 2 3Potential (V)

–3 –2 –1 0 1 2 3Potential (V)

–3 –2 –1 0 1 2 3Potential (V)

PEDOT/WO3/BMIMBF4PEDOT/WO3/BMIMPF6

PEDOT/WO3/BMPTFSIPEDOT/WO3/BMIMTFSI

Figure 3.40 Cyclic voltammograms of PEDOT/WO3/BMIMBF4, PEDOT/WO3/BMIMPF6, PEDOT/WO3/BMIMTFSI, and PEDOT/WO3/BMPTFSI hybrid nanofibers in 1 M Li-PC during 10 cycles.

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PEDOT/WO3/BMIMBF4 PEDOT/WO3/BMIMPF6

PEDOT/WO3/BMIMTFSI PEDOT/WO3/BMPTFSI

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ance

(%)

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0V3V

–3V

0V3V

–3V

0V3V

–3V

0V3V

–3V

700 800 900 400 600500

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700 800 900

Figure 3.41 Visible transmittance spectra of PEDOT/WO3/BMIMBF4, PEDOT/WO3/BMIMPF6, PEDOT/WO3/BMIMTFSI, and PEDOT/WO3/BMPTFSI nanofiber-based ECD for applied potentials of 0 and ±3 V.

98 Advanced Electrode Materials

10 20

–2

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PEDOT/WO3/BMIMBF4PEDOT/WO3/BMIMPF6PEDOT/WO3/BMIMTFSIPEDOT/WO3/BMPTFSI

3

4

30 40 50 600

Curr

ent d

ensi

ty (m

A/c

m2 )

Time (sec)

Figure 3.42 Current densities monitored for the hybrid nanofiber based ECDs stepped between ±3 V.

(a) (b)

Red

0x

Figure 3.43 Photographs of the PEDOT/WO3 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 re-sparked much attention on such hybrid assemblies to be deployed in EC applications.

Advances in Tungsten Oxide/Conducting Polymer Hybrid 99

Acknowledgements

The authors would like to acknowledge the TUBITAK Project-with num-ber 114Z321 and SDU Project with number 3193-D2-12 for the financial support of this research.

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