A poly(3,4-ethylenedioxypyrrole)-Au@WO3 -based electrochromic pseudocapacitor.

CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402625

A Poly(3,4-ethylenedioxypyrrole)–Au@WO3-Based Electrochromic Pseudocapacitor B. Narsimha Reddy , P. Naresh Kumar, and Melepurath Deepa*[a] A poly(3,4-ethylenedioxypyrrole)–gold nanoparticle (Au)–tungsten oxide (PEDOP–Au@WO3) electrochromic supercapacitor electrode capable of optically modulating solar energy while simultaneously storing/releasing energy (in the form of charge) was fabricated for the first time. WO3 fibers, 50 to 200 nm long and 20 to 60 nm wide, were synthesized by a hydrothermal route and were electrophoretically deposited on a conducting substrate. Au nanoparticles and PEDOP were coated over WO3 to yield the PEDOP–Au@WO3 hybrid electrode. The inclusion of Au in the hybrid was confirmed by X-ray diffraction, Raman spectroscopy, and energy-dispersive X-ray analyses. The nanoscale electronic conductivity, coloration efficiency, and transmission contrast of the hybrid were found to be significantly

greater than those of pristine WO3 and PEDOP. The hybrid showed a high specific discharge capacitance of 130 F g1 during coloration, which was four and ten times greater than the capacitance achieved in WO3 or PEDOP, respectively. We also demonstrate the ability of the PEDOP–Au@WO3 hybrid, relative to pristine PEDOP, to perform as a superior counter electrode in a solar cell, which is attributed to a higher work function. The capacitance and redox switching capability of the hybrid decreases insignificantly with cycling, thus establishing the viability of this multifunction hybrid for next-generation sustainable devices such as electrochromic psuedocapacitors because it can concurrently conserve and store energy.

1. Introduction Multifunctional hybrid materials based on conducting polymers and inorganic transition metal oxides have sparked considerable research interest because the complementarity in their optical and electrical properties leads to devices that can outperform pristine polymer or oxide-based devices. Such hybrid materials find use as active electrodes in diverse applications that include chemical sensors, supercapacitors, electrochromic devices, batteries, and organic solar cells.[1–5] Conducting polymers are attractive because they are cheap, easy to process and can exhibit a range of colors as a function of the applied potential.[6–8] Among the conducting polymers, poly(3,4-ethylenedioxypyrrole) or PEDOP is potentially useful owing to a lower oxidation potential [1.2 V versus normal hydrogen electrode (NHE)] compared to its much studied thiophene analogue, poly(3,4-ethylenedioxythiophene) or PEDOT, an optical contrast between transmissive blue–grey and deep red hues contrasting strikingly against the pale to dark blue change offered by PEDOT, good environmental stability of the oxidized form and high electrical conductivity at room temperature.[9–11] Similarly, in transition-metal oxides, tungsten oxide (WO3), because of a very high redox switching stability (> 105 cycles), a large transmission modulation in the visible region [a] B. N. Reddy , P. N. Kumar, Dr. M. Deepa Department of Chemistry Indian Institute of Technology Hyderabad Ordnance Factory Estate Yedduaram-502205, Telangana (India) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402625.

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corresponding to the colorless-to-deep-blue transition, and a good ion storage capacity, is a versatile material useful for both electrochromic and supercapacitor applications.[12, 13] Pseudocapacitors store energy by undergoing Faradaic chargetransfer reactions[14] and electrochromic materials conserve energy by dynamically electro-modulating solar radiation by absorption.[15] A single material capable of performing both functions, undergoing charging (insertion of cations and deintercalation of anions) and coloration in tandem and on changing the polarity, discharging (removal of cations and insertion of anions) and bleaching simultaneously will permit the deduction of the charge-storage level in the film, simply on the basis of its color.[15] In a previous report, an electrochromic supercapacitor based on an ordered bi-continuous double-gyroid V2O5 network with a specific capacitance of 155 F g1 underwent a concurrent color change from green to yellow, which demonstrated the viability of using charge ingress/egress and color change together.[16] Similarly, MoO3 and Nb2O5 have also been used for supercapacitor and electrochromic applications, and a high coloration efficiency of 149 cm2 C1 was reported for MoO3 electrodeposited into Nb2O5 nanochanelled films.[17] In another study, a poly(aniline) or PANI/WO3 composite prepared by electropolymerizing aniline monomer onto a sol-gel-based spin-coated WO3 film showed higher coloration efficiency and specific capacitance compared to the pure components.[18] In a proof of concept study, PANI and poly(pyrrole) were photoelectrodeposited into one-dimensional nanoporous nanotubular WO3 or TiO2 arrays, which in turn were grown by anodization of W or Ti foil; illumination was observed to have an amelChemPhysChem 0000, 00, 1 – 14

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CHEMPHYSCHEM ARTICLES iorating effect on the interfacial properties.[19] Preparation of a WO3/PEDOT composite by co-electrodeposition from a tungstate/EDOT bath, followed by an evaluation of pseudocapacitive behavior of the electrodes, has been done earlier.[20] An asymmetric capacitor with co-electrodeposited WO3/PANI and PANI as electrodes exhibited a specific capacitance of 48.6 F g1 and a power density of 53 W kg1.[21] In another report, poly(pyrrole) or PPy/WO3metacomposites were synthesized by in-situ chemical polymerization of pyrrole on the surface of WO3 nanoparticles and needles and the electrical properties were superior for the nanorod based composites.[22] Onedimensional nanostructures of WO3 by the virtue of high aspect ratio and large surface area have a larger number of electrochemically addressable sites relative to a granular WO3.[23] Further, crystalline WO3 films are more robust compared to amorphous WO3, as the former have a denser structure and do not undergo dissolution in the electrolyte. But crystalline films owing to the absence of a porous open structure, show slower switching kinetics relative to amorphous WO3 films.[24] An optimal balance between switching speed and chemical stability is therefore attained in nanocrystalline WO3 films.[25] We found that PPy/Au nanocomposites, Au–PEDOT–PPy–Au nanowire structures,[26] and PEDOT/WO3, PPy/WO3 and PANI/ WO3 composites have been synthesized and characterized in the past; to the best of our knowledge, there are no reports on a PEDOP–WO3 hybrid yet. Here, we present the synthesis of a PEDOP/WO3 composite by using citrate capped Au nanoparticles as a bi-functional linker. WO3 fibers were prepared by a novel hydrothermal route[27] and coated on transparent conducting electrodes by electrophoresis. A monolayer of Au nanoparticles was applied, and PEDOP was electropolymerized onto the Au@WO3 fibers electrode. We present the effect of complementary characteristics of WO3 and PEDOP on electrochromism and electrochemical energy storage capacity of the PEDOP–Au@WO3 hybrid. In particular we focus on correlating the electro-optical behavior and capacitive response of the hybrid with its structure. We also furnish an insight into the mechanistic aspects of charge transport and transfer in the hybrid and how these phenomena differ from the pristine polymer or the oxide. The novel synthetic route presented herein, which lays emphasis on chemically connecting the two components of the hybrid as opposed to a generally used physical mixing process, paves the way for adapting this design strategy to other unexplored conducting polymer/ metal oxide composites.

2. Results and Discussion 2.1. Structural Aspects In a PEDOP–Au@WO3 hybrid, the complementary properties of the two materials, such as the high electrical conductivity of PEDOP, can make the WO3 matrix more conducting and the large surface area and the porous assembly afforded by the WO3 fibers can serve as a scaffold for coating the polymer to yield a fibrillar morphology, as opposed to a granular one. The 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org coloration efficiency of the hybrid is expected to be higher than the individual electrochromes, as they will both undergo color change in response to the external stimulus. Both PEDOP and WO3 are pseudocapacitive materials, as they undergo charge/discharge via reversible Faradaic charge-transfer processes, and as a result the specific capacitance of the hybrid will be high because it will include contributions from both of them. Furthermore, pristine conducting-polymer electrodes suffer from a poor cycling stability that results from their brittle texture and low mechanical strength.[28, 29] Coupling PEDOP with WO3, where WO3 has excellent cycling lifetime and good mechanical strength, is an effective approach to overcome the limitations posed by PEDOP. The affinity of the carboxylate groups on citrate capped Au nanoparticles towards the positively charged tungsten in WO3 (thus mimicking the role of carboxylate from mercaptopropionic acid in binding to TiO2 in dye-sensitized solar cells)[30] and the strong propensity of the Au core to attach to sulfur, which is a part of poly(4-styrene sulfonate) or PSS, the counter ion we used for doping PEDOP ensure that PEDOP coated uniformly on the WO3 fibers and also inhibits the polymer from leaching out in electrolytes. The X-ray diffractograms of Au nanoparticles, pristine WO3, PEDOP and Au@WO3 electrodes are shown in Figure 1 (i). The XRD pattern of the Au nanoparticles shows four prominent Bragg reflections at around 38.28, 44.78, 64.58, and 78.38, which match well with the d-values corresponding to the face-centered cubic lattice of Au (JCPDS file No. 04-0784). The intensity of the (111) plane is much higher compared to the (200), (220), and (311) diffraction peaks, suggesting that the (111) plane is the predominant orientation. The XRD pattern of the PEDOP electrode shows a broad hump centered at 2q = 26o and d = 1.76 , confirming the amorphous structure of the PEDOP:PSS electrode. The XRD pattern of WO3 showed dominant peaks at d = 3.5, 3.36, and 3.35 nm, which were indexed to the monoclinic crystal structure of WO3 (ICDD-PDF2-2004 01-072-0677). These peaks were assigned to the (002), (020), and (200) planes of WO3, respectively. The XRD pattern of Au@WO3 showed the same cluster of three peaks corresponding to WO3 and in addition peaks at d = 2.3, 2.0, 1.4, and 1.2 were also observed, which match with the (111), (200), (220), and (311) reflections of Au with an fcc structure, thus indicating that Au nanoparticles are attached to WO3, as expected. The Raman spectra of WO3, PEDOP, Au nanoparticles, Au@WO3, and PEDOP–Au@WO3 are shown in Figure 1 (ii). For WO3, the bands centered at 812 and 663 cm1 arise from the OWO stretching vibrations, in concurrence with the characteristic frequencies reported for monoclinic WO3.[31] A strong band at 1050 cm1 with a shoulder at 1024 cm1 corresponds to the n(W=O) stretching mode of the terminal bonds. It is upshifted compared to the literature values of hydrated WO3 (910-1010 cm1),[12, 31] which could be due to a shorter bond length, rendered by the synthetic route. A strong band seen at 100 cm1 arises from the lattice vibrational mode of monoclinic WO3, which is close to the reported value of 93 cm1.[31] Weak bands at 272 and 330 cm1 can be assigned to the bending d(OWO) mode.[31] In the Au@WO3 film, medium-intensity peaks are observed at 132 and 274 cm1 because of lattice and ChemPhysChem 0000, 00, 1 – 14

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1200–1300 cm1 region, and these originate from the CN stretching vibrations of the pyrrole rings. A medium intensity band at ~ 715 cm1 is ascribed to the NH out-of-plane bending mode and it is observed at ~ 711 cm1 in the hybrid. In the hybrid, this peak position also overlaps with the n(OWO) mode. Au nanoparticles show peaks at 1380 and 1590 cm1 corresponding to the symmetric and asymmetric stretching of CO bond from the citrate cap.[33] The PEDOP–Au@WO3 film shows distinct peaks at 350, 1370, and 1400 cm1 attributed to the lattice vibrational mode of Au, CbCb and Ca = Cb(O) stretching modes, thus confirming the inclusion of PEDOP and Au in the hybrid film. The intensities of the Raman peaks in the hybrid are not significantly enhanced by the plasmonic effect of Au, as observed in Au@WO3, possibly because the interaction is swamped by the colored polymer’s absorptions. In the hybrid, a strong peak is observed at 1024 cm1, which is flanked by two peaks on either side, at 1055 and 995 cm1. These involve mixed contributions from COC deformation and the n(W=O) stretching modes. The Figure 1. i) X-ray diffraction patterns of: a) PEDOP, b) WO3, c) Au nanoparticles and d) Au@WO3 films. ii) Raman peak at 86 cm1 is attributed to spectra of: a) PEDOP film, b) WO3 film, c) Au nanoparticles, d) Au@WO3 film and e) PEDOP–Au@WO3 hybrid film. iii) Schematic of WO3–Au–PEDOP:PSS interactions in the hybrid film. W and A in (i) represent WO3 and Au, respectivethe lattice mode of WO3. The ly. The insets of (ii) are the enlarged views of the low wavenumber response. peaks at 239 and 135 cm1 are due to the d(OWO) and lattice modes of WO3 ; the positional difference compared to Au@WO3 or WO3 is in all likelihood d(OWO) modes. The lattice mode peak at 100 cm1 in WO3 caused by interaction with PEDOP. and the n(OWO) stretch peak are downshifted to 86 and 803 cm1. This downshift is indicative of interactions between the WO3 and citrate-capped Au nanoparticles, which weaken 2.2. Morphology and Nanoscale Conduction the WO bonds, thus causing the downshift of the Raman freThe SEM micrographs and energy-dispersive X-ray analysis quencies. Furthermore, there is an enhancement in the overall (EDX) plots of the WO3, Au@WO3, PEDOP, and PEDOP–Au@WO3 peak intensities, roughly by an order of magnitude, ongoing from WO3 to Au@WO3. The surface plasmons produced by the films are shown in Figure 2. The low- and high-magnification interaction between Au nanoparticles and the laser beam, inimages of WO3 reveal the presence of a densely packed and interact with WO3, and increase the Raman scattering intensity. terlinked network of fibrillar shapes with tapered ends. The morphology is akin to the topography seen in the correspondFor PEDOP, the CbCb vibrational stretching mode is seen in ing AFM image (Figure 3), thus confirming that the film is comthe range of 1330–1350 cm1 and it arises from the pyrrole posed of randomly organized but highly compact framework rings on the polymer backbone.[32] The symmetric stretching of WO3 fibers with distinct grain boundaries. The fiber lengths mode of the Ca = Cb(O) groups of PEDOP produces a peak at 1400 cm1. A cluster of low-intensity peaks is observed in the vary between 100 and 250 nm. The WO3 fibers acquire distort 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.chemphyschem.org force microscopy (AFM), by running an AFM (Pt–Ir) tip across a step formed at the film/FTO interface; they were ~ 705, 695and 700 nm, respectively (Figure S2). The topography and current images recorded concurrently over scanned areas of 1 mm 1 mm for the WO3, PEDOP and PEDOP–Au@WO3 films are shown in Figure 3. The corresponding cross-section profiles recorded exactly along the middle of the images are shown in the Supporting Information (SI, Figure S3). The WO3 electrode film comprises elongated particles but the topography is distorted as in C-AFM, the tip contacts the surface, which adversely affects the image. Both PEDOP and WO3 showed featureless topographies. The currents flowing through films were mapped by applying a dc bias (50 mV) to the tip, whilst the tip ran horizontally over the surface (Figure 3 a, c, e). The bright regions in the current images represent high currents and dark regions are characteristic of low currents. The currents are color-scaled on the right side of the current maps. The maximum currents achieved for the WO3, PEDOP, PEDOP–Au@WO3 films were 0.03, 12 and 24 nA, thus indicating that the hybrid film is capable of conducting higher currents relative to PEDOP or WO3. Point contact I–V curves were recorded at fifteen equidistant points on each current image, and the average I–V profile for each film is shown (Figure 3 b, d, f). The room-temperature electronic conductivities were determined from straight-line fits within the voltage windows where the I–V response was almost linear by using Equation (1):

Figure 2. SEM micrographs of: a, b) WO3, c,d) Au@WO3, e) PEDOP and f) PEDOP–Au@WO3 films and EDX plots of: g) WO3, h) Au@WO3, i) PEDOP and j) PEDOP–Au@WO3 hybrid films.

ed shapes and appear as blobs, 50 to 200 nm in size, when the film is coated with Au nanoparticles, as in the Au@WO3 film (Figure 2 c, d). The particles now have irregular shapes with indistinctive grain boundaries but continue to retain their mingling characteristics, which is useful for ion and electron transport during charge/discharge or color-bleach process. While the EDX pattern of WO3 revealed distinct signals from W and O, the EDX of the Au@WO3 film shows signals from W, Au and O, thus confirming the inclusion of Au in the film. The morphology of the pristine PEDOP film is relatively featureless, and the film appears to be composed of agglomerated polymer particles of no particular shape. Signals from C, N and O are seen in the corresponding EDX plot, which are the elements that constitute the backbone of the polymer. The PEDOP– Au@WO3 hybrid film is composed of an interconnected framework of aggregated particles with indistinctive shapes. The film is highly porous and the pore sizes vary from 0.5 to 1 mm. The EDX pattern shows signals from C, N, O, Au and W, thus indicating the presence of the polymer and Au along with WO3 in the hybrid, which is in line with the rationale used for fabricating the hybrid. Since only the WO3 morphology was distinctive, and the subsequent incorporation of Au or PEDOP led to relatively featureless morphologies, the AFM images of pristine WO3 were recorded over different length scales (Figure S1).The thicknesses of PEDOP, WO3 and PEDOP–Au@WO3 films were obtained from the height profiles generated using atomic 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

sRT ¼ ðI=VÞ ðd=pr 2 Þ

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In Equation (1), r is the radius of the conducting tip and d is the thickness of the film. The average nanoscale electronic conductivities of the WO3, PEDOP and PEDOP–Au@WO3 films deduced from the ohmic regimes were 0.64, 0.015 and 0.88 S cm1, respectively. The superior electronic conductivity of the hybrid compared to the pristine polymer or oxide is expected to be advantageous for facilitating charge transport through the bulk of the film during oxidation and reduction. The presence of metallic gold nanoparticles in the hybrid is responsible for the enhanced nanoscale conductivity. 2.3. Electrochemical Behavior The cyclic voltammograms of WO3, PEDOP and PEDOP– Au@WO3 hybrid films recorded at different scan rates in the ionic liquid 1-butyl-3-methyl imidazoliumtriflate with a Pt rod as the counter electrode are shown in Figure 4. The CV plot of WO3 film recorded at a scan rate of 2 mV s1 shows a broad oxidation peak 0.24 V in the anodic branch corresponding to de-intercalation of lithium ions and electrons from WO3. This is followed by a shoulder at 0.46 V in the cathodic sweep signifying reduction involving concomitant intercalation of lithium ions from the electrolyte and electrons from the external circuit into the WO3 film. The CV profile is retained as the scan rate is successively raised from 2 to 100 mV s1, albeit the fact that the cathodic shoulder is not perceptible at scan rates above 30 mV s1. The reversible redox process operational in ChemPhysChem 0000, 00, 1 – 14

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under the voltammograms recorded at 100 mV s1 are 0.0032, 0.05 and 0.2 mC cm2 for WO3, PEDOP and PEDOP–Au@WO3 hybrid films. The higher ion-storage capacity obtained for the hybrid illustrates the combined enhancement in its ion uptake capability due to PEDOP and WO3. The mechanism for charge storage in WO3, PEDOP and PEDOP–Au@WO3 films is by electrochemical oxidation and reduction or pseudocapacitance, and in the hybrid the charge storage capacity is increased as the WO3 fibers by the virtue of their structure comprising of elongatedshapes permit more ions to intercalate and the high electrical conductivity of PEDOP allows facile electron transport in the hybrid, an advantage which cannot be realized in pristine WO3 films, which are largely ion conducting but electronically inFigure 3. Concurrent topography and current images of: a, b) WO3, d, e) PEDOP, g, h) PEDOP–Au@WO3 hybrid films sulating. Although in PEDOP, the recorded over scanned areas of 1 mm 1 mm. Resultant current–voltage curves of: c) WO3, f) PEDOP, i) PEDOP– benefit of good electronic conAu@WO3 hybrid films. Each curve is averaged over 15 I–V curves recorded at 15 spots on each current image ductivity of PEDOP contributes shown in (b), (e) and (h). to improving its capacitance, but the ion intercalation–deintercalation capacity via electrochemical oxidation and reduction is adWO3 is shown in Equation (2) and the CV curves observed by versely affected compared to the PEDOP–Au@WO3 hybrid, us are similar to the ones reported for sol-gel-derived WO3 owing to the regular granular morphology of the pristine polyfilms:[34] mer. In the hybrid, the polymer PEDOP is coated over the WO3 ð2Þ WO3 þ xe þ x½BmImþ Ð ½BmImx WO3 fibers and is also linked to the oxide through the Au monolayer. The poly(styrene sulfonate) dopant in PEDOP flanks to the The CV curve for the PEDOP film is featureless in the cathoAu monolayer and this link is the interconnect between the dic branch, but two shoulders are observed at 0.2 and two pseudocapacitive layers: WO3 and PEDOP. As a consequence, the fibrillar morphology of PEDOP–Au@WO3 hybrid 0.43 V in the anodic sweep (at 2 mV s1). These peaks indicate the stepwise insertion of triflate ions and extraction of allows for greater ion insertion compared to pristine granular [BmIm] + ions from the neutral polymer to yield the oxidized PEDOP. PEDOP. The voltammograms do not exhibit well-defined Galvanostatic charge–discharge curves (at 1 A g1) of the anodic/cathodic peaks in the voltage range at any scan rate, WO3, PEDOP and PEDOP–Au@WO3–PEDOP hybrid electrodes and similar featureless curves have been observed for PEDOT with 1-butyl-3-methyl imidazoliumtriflate as the electrolyte are doped by Szymanska et al. in the past.[20] The CV plot of the displayed in Figure 5 a. The polymer and WO3 exhibit almost PEDOP–Au@WO3 hybrid film recorded at 2 mV s1 shows an oxsymmetric charge–discharge curves and their specific capacitances are 14 and 30 F g1, respectively. The PEDOP–Au@WO3– idation peak at + 0.33 V followed by a reduction peak at + 0.11 V in the reverse sweep. Both the reduction and oxidaPEDOP hybrid shows an SC of 130 F g1, which is four and ten tion peaks shift to more negative potentials as a function of intimes greater than that attained for the WO3 and polymer creasing scan rate. The reduction peak is observed at 0.11 V films. This excellent capacitive behavior of the PEDOP– in a plot recorded at 100 mV s1. The oxidation peak ceases to Au@WO3 film is attributed to the combined pseudocapacitanexist at scan rates above 30 mV s1, as it is seen at + 0.17 V and ces of WO3 and PEDOP. An IR drop at the turning point of the 20 mV s1. The oxidation peak can be ascribed to the convercharge–discharge line was observed only for the PEDOP– Au@WO3, probably arises from Au nanoparticles, and the magsion of PEDOP from the neutral to the radical cation state and to the transformation from tungsten oxide to tungsten bronze. nitude of the IR drop was enlarged at the higher current densiThe charge-storage capacities obtained by integrating the area ty, as can be judged from the rate capability curves. The rate 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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Figure 5. Galvanostatic charge–discharge curves of: a) WO3, PEDOP and PEDOP–Au@WO3 hybrid films at a fixed current density of 1 A g1, b) rate capability of a PEDOP–Au@WO3 hybrid film (at different current densities between 0.2–1.6 A g1), c) cycling response (at 1 A g1) and d) specific capacitance versus current density of WO3 (*), PEDOP (&) and PEDOP–Au@WO3 hybrid (~) films.

Figure 4. Cyclic voltammograms of: a) WO3, b) PEDOP and c) PEDOP– Au@WO3 hybrid films recorded at different scan rates of 5, 10, 20, 30, 40, 50 and 100 mV s1. Pt was used as the counter electrode and the ionic liquid 1butyl-3-methyl imidazolium triflate was used as the electrolyte.

performance of the PEDOP–Au@WO3 films was evaluated by charging/discharging at different current densities ranging between 0.2 and 1.6 A g1 (Figure 5 b). The specific capacitance decreased from 260 to 95 F g1 upon increasing current density from 0.2 to 1.6 A g1. The capacitive retention of PEDOP– Au@WO3 was about 36 % in the current loading range of 0.2 to 1.6 A g1. These observations revealed that the combination of PEDOP with WO3 leads to improved rate performance and capacitance, which can be explained by the synergic effect of the high surface area provided by the WO3 fibers, which not only allows for increased loading of PEDOP, but also affords 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

a greater number of electrochemically addressable sites available for ion uptake upon application of electric voltage or current. The uniform coating of PEDOP on WO3 fibers also prevents them from aggregating, resulting in improved accessibility for ingress of electrolyte ions. In a previous work, an SC of 115 F g1 was achieved for PEDOT spheres[35] and in yet another study, Szymanska et al. reported a nSC of 205 mF cm2 for a WO3/PEDOT composite.[20] A WO3/PANI composite film exhibited good pseudocapacitive performance over a wide potential range of 0.5 to 0.7 V versus SCE with an SC of 168 F g1 at a current density of 1.28 mA cm2.[36] The cycling stability of the films recorded at a current density of 1 A g1 is shown in Figure 5 c. At the end of 100 cycles, the PEDOP–Au@WO3 hybrid film showed a capacitance of 90 F g1 compared to the much lower values of 10 and 4 F g1 obtained for WO3 and PEDOP. The hybrid film retained 70 % of its original capacitance upon repetitive cycling, thus indicating the suitability of this film for supercapacitor applications. The SC of the three films was also calculated from CV (Figure S4) and they were found to be 44, 18 and 214 F g1 at n = 5 mV s1 and 13, 3 and 78 F g1 at n = 100 mV s1 for WO3, PEDOP and PEDOP– Au@WO3-PEDOP hybrid electrodes. The values were calculated using Equation (5). 2.4. Electro-Optical Performance Characteristics The absorption spectra of the as-fabricated films and the Au colloid are shown in Figure 6 a. The Au colloid shows an absorption peak at 531 nm, which originates from the surface plasmon resonance of the Au nanoparticles.[37] The absorption of Au on WO3 fibers is affirmed from retention of this peak in the Au@WO3 film, albeit a slight blue shift by 4 nm. The pristine WO3 film does not show any absorption peaks in the visible region and pristine PEDOP shows a broad absorption wave ChemPhysChem 0000, 00, 1 – 14

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Figure 6. a) Absorbance versus wavelength spectra of a WO3 film (*), an Au colloid (~), a PEDOP film ( ! ), an Au@WO3 film (^) and a PEDOP–Au@WO3 film (&). In situ optical spectra as a function of the applied potential. b) Transmittance (%) versus wavelength of a WO3 film, c) optical density versus wavelength of a PEDOP film and d) transmittance (%) versus wavelength of a PEDOP–Au@WO3 film. In (c, d), the electrolyte and the counter electrode were 1-butyl-3methyl imidazolium triflate and Pt, respectively, and each potential was applied for a duration of 60 s. The insets of (b–d) show photographs of the films under different bias values.

above 700 nm attributed to bipolaronic transitions. The absorption profile of the PEDOP–Au@WO3 hybrid film is akin to the absorption of curve of pristine PEDOP and is relatively featureless in the as-fabricated state. The variation in the absorbance of a PEDOP film and the transmittance of WO3 and PEDOP–Au@WO3 films under the influence of an applied bias in the range of + 1.5 to 1.5 V as a function of the wavelength are shown in Figure 6 b–d. The measurements were performed in situ in the IL 1-butyl-3methyl imidazoliumtriflate. For all the three films the reduction potential was varied in steps of 0.1 V from 0.1 to 1.5 V and oxidation potentials were applied over longer intervals between + 0.2 and + 1.5 V. The transmission spectra of the WO3 films in their colored (under E = 0.2 to + 1.5 V) and bleached (under E = 0.1 to 1.5 V) states in the 300 < l < 1700 nm wavelength region are displayed in Figure 8 b. In the fully oxidized state (E = + 1.5 V), the percent transmittance of WO3 film varies between 70 %–85 % in the visible region and the film is colorless and transparent. The percent transmission decreases monotonically in the NIR region and reaches a value of ~ 30 % at 1650 nm. The transmittance in the visible and NIR region decreases systematically at less positive potentials (when E is decreased from + 1.0 to 1.5 V) because of the formation of [BmIm]xWO3 which is blue. This absorptive modulation of WO3 in the photopic region has been attributed to an intervalence 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

charge transfer between W + 5 and W + 6 sites[38] or to small polaron absorption.[39] For the pristine polymer film, DOD is defined as the optical density change at a monochromatic wavelength: DOD (l) = OD(l, x V)OD(l, + 1.5 V), where x = 0.1 to 1.5 V. For the PEDOP film, the broad band observed in the NIR region under all oxidation potentials stems from bipolaronic transitions (Figure 6 c). With increasing reduction potential, the broad NIR absorption loses intensity and paves the way for a peak corresponding to p p* transitions in the visible region, which progressively gains intensity upon raising negative bias signaling the formation of neutral reduced polymer. When the film is fully reduced (at E = 1.5 V), the film is insulating and shows a lmax at 500 nm. The PEDOP film has a transparent blue–black hue in the oxidized state and is deep red in the reduced form. The DODmax value for PEDOP was deduced to be 2.33 at lmax = 500 nm. The variation in transmittance with wavelength and applied bias in the PEDOP–Au@WO3 hybrid is shown in Figure 6 d. The significant dip in the transmittance curve with Tmin = 5.2 % at 500 nm in the fully reduced/colored (reddish–blue) state of the hybrid, especially in the 400– 600 nm wavelength range achieved under E = 1.5 V is ascribed to the increased absorption due to p p* transitions of PEDOP complemented by [BmIm]xWO3 formation. These two phenomena impart the film with an intense hue, which cannot be achieved either in pristine PEDOP or WO3 films. In the fully ChemPhysChem 0000, 00, 1 – 14

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Tb and Tc are the transmittances of the film in the bleached and colored states, respectively. The coloration efficiencies for the films were determined from the slopes of the DOD (for PEDOP) or log(Tb/Tc) (for WO3 and the PEDOP–Au@WO3 hybrid) versus the inserted charge density at different wavelengths (Figure 7 b–d). The coloration efficiency plots for all the films can be divided into one or two linear dependence regimes. At low values of the inserted charge densities, CE is low, but in the higher charge density region, the CE values are high. The coloration efficiencies of pristine WO3 in the visible region at lmax values of of 441 and 572 nm are 66 and 88 cm2 C1. Figure 7. a) Transmission modulation: DT (l) = T (+ 1.5 V)T [1 V (*) or 1.5 V (&)] of a WO3 film and DT (l) = T (+ 1.5 V)T [1 V (^) or 1.5 V (~)] of a PEDOP–Au@WO3 hybrid film. b) Log (Tb/Tc) versus q/A of a WO3 film at PEDOP shows a coloration effi441 (&), 572 (*) and 800 (~) nm. c) DOD versus q/A of a PEDOP film at 500 (&) and 1100 (*) nm. d) Log (Tb/Tc) ciency of 176 cm2 C1 at lmax = versus q/A of a PEDOP–Au@WO3 film at 460 (&), 500 (*) and 1100 (~) nm. The Tb value at + 1.5 V was taken as ref500 nm. Compared to the polyerence for the WO3 and PEDOP–Au@WO3 films in (b) and (d), respectively, and the OD value at + 1.5 V was taken mer and WO3 films, the PEDOP– as reference for the PEDOP film in (c). Au@WO3 hybrid shows outstandingly high coloration efficiencies of 523 and 707 cm2 C1 at lmax values of 460 and 500 nm. The bleached/oxidized (transparent) state attained at E = + 1.5 V, the hybrid retains good transmission characteristics in the visihighest coloration efficiency achieved for the PEDOP–Au@WO3 ble–NIR range with a Tmax of about 78 % (at 460 nm), primarily hybrid is four and eight times greater than the coloration effidue to the good reversibility of the oxidation–reduction prociency maxima shown by pristineWO3 and pristine PEDOP films cess. The transmittance in the fully bleached state is slightly for the same values of the applied bias. In fact, the intercalated less for the hybrid compared to WO3, due to the presence of charge-density range is much wider for WO3 (0.65– PEDOP, which shows a pale color even in the oxidized state. 7.0 mC cm2) compared to PEDOP or the PEDOP–Au@WO3 hybrid (0.025–2.2 mC cm2). The CE in the hybrid is larger as The transmission modulation (DT), a parameter that deterPEDOP–Au@WO3 is a dual electrochrome electrode wherein mines the suitability of an electrode for electrochromic both PEDOP and WO3 undergo coloration concurrently when window applications, was calculated for both WO3 and PEDOP–Au@WO3 films using the following equation: DT(l) = subjected to reduction potentials and bleach simultaneously T(l, + 1.5 V)T(l, x V), where x = 0.1/1.5 V (Figure 7 a). The when oxidation potentials are applied. A higher degree of colhybrid shows a remarkably enhanced maximum transmission oration results in the fully reduced state and likewise a quasimodulation of 71 % at lmax = 476 nm, which is 1.5 times greater transparent hue is realized in the bleached state. In pristine than the maximum modulation of 47 % offered by WO3 film at PEDOP, the bleached state is dark and in pristine WO3, in the lmax = 650 nm, for the same values of applied bias. In bulk of fully reduced state (E = 1.5 V), the film does not darken significantly, it continues to be transmissive as can be judged from the spectral range under consideration, between 400– a high transmittance of 53 % (lmax = 441 nm). In the NIR region 1650 nm, the maximum modulation offered by the hybrid is 50 %, which makes it an excellent candidate for electrochroat 1100 nm, pristine PEDOP shows a slightly higher CE of mic smart window applications. It is obvious that the hybrid about 407 cm2 C1 compared to 328 cm2 C1 obtained for the film has greater electrochemical capacity than the pure oxide PEDOP–Au@WO3 hybrid. In the hybrid, the decreasing transfilm and the additional optical modulation in the hybrid arises mittance of WO3 in the reduced states causes a drop in CE. from the contribution of PEDOP to color change. Coloration efHowever, the best trade-off between high CE in the visible and ficiency (CE) at a given wavelength is the change in the optical NIR regions is attained only in the PEDOP–Au@WO3 hybrid, density (DOD) for the charge (q) consumed per unit electrode which establishes its suitability for smart window applications. area (A). It has been calculated in the present report for all In an earlier report, Wang et al. obtained a maximum optical films using the following Equation (3): modulation of ~ 66 % after applying a voltage of 3.0 V for WO3 nanorods,[40] and in another study they achieved a contrast CE ðhÞ ¼ DODðlÞ=ðq=AÞ ¼ logðT b ðlÞ=T c ðlÞÞ=ðq=AÞ ð3Þ of ~ 33.9 % after applying a 1.0 V voltage bias for 300 s.[41] In 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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a report on a PEDOT film, a contrast of 71 % was attained in a LiBF4/PC solution.[42] In another report, the contrast for an electropolymerized PEDOT film was found to be 32 %.[43] Our values for contrast and CE render the hybrid suitable for electrochromic applications. The coloration-bleaching kinetic plots of WO3 (lmax = 650 nm), PEDOP (lmax = 500 nm) and the PEDOP–Au@WO3 hybrid (lmax = 476 nm) films are shown in Figure 8 with half cycle times of 3 and 10 s respectively. The time required for absorbance to attain 90 % of the total contrast in a half cycle is the coloration time and the time taken by absorbance to decrease by 90 % of the total contrast in a half cycle is the bleaching time. The coloration and bleaching times are calculated as 7.4 and 1.7 s, respectively, for the WO3 film under a step time of 10 s, and 2.1 and 1.6 s under a step time of 3 s. Under a step time of 10 s, tc and tb values of 6.6 and 5.9 s— and under 3 s 1.7 and 1.6 s, respectively—were observed for PEDOP. The PEDOP–Au@WO3 hybrid achieved a contrast considerably higher than that of WO3 and PEDOP for the same step times under the same applied potential, thus reaffirming the cumulative contributions from PEDOP and WO3 in increasing the color contrast. For the hybrid, under a step time of 10 s, tc and tb values of 6 s—and under 3 s of 2 s—were obtained. In the past, switching times of 5.1 s at 700 nm were obtained for a WO3 nanosheet-based ECD.[44] For a PEDOP film, switch times of 2.14 and 1.37 s were observed at 490 nm.[45] Although there are no reports on a WO3-PEDOP combination to date, nonetheless, our switching times are comparable to the reported values for electrochromic PEDOP or WO3.

2.5. Electrochromic Supercapacitor To evaluate the potential of the PEDOP–Au@WO3 hybrid as a dual-function electrochromic supercapacitor electrode, a 3D visualization of the variation of transmittance at a monochromatic wavelength of 460 nm and a potentiometric response is presented in Figure 9. At the onset, under a reduction potential of 1.5 V, the film is fully colored/reduced with a deep red–blue hue corresponding to the fully discharged state and upon linearly increasing the potential from 1.5 to + 1.5 V through 0 V, the film undergoes oxidation or charging concomitant with a gradual optical transition from a red–blue color to a transparent blue–grey color. The progressive increase in transmittance from ~ 9 % to 78 % (l = 460 nm) as E is raised from 1.5 to + 1.5 V, simultaneously accompanied by charging or intercalation of anions in the film, is followed by a systematic drop in transmittance from 78 % to 9 % when the potential is swept in the reverse direction from + 1.5 to 1.5 V and this optical change is accompanied by discharging or ejection of anions from the film. The charge and discharge capacitances are almost 100 F g1 at a current density of 1 A g1. This plot clearly shows that the PEDOP–Au@WO3 hybrid can easily work as an electrochromic supercapacitor as it can perform two linked functions of charging/discharging and optically modulating visible radiation at the same time. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 8. Absorbance versus time curves for: a) WO3, b) PEDOP and c) PEDOP–Au@WO3 hybrid films at l = 650, 500 and 476 nm, respectively, under a square-wave dc potential of 1.5 V at different step times of 3 and 10 s. The electrolyte and the counter electrode were 1-butyl-3-methyl imidazolium triflate and Pt, respectively.

2.6. Charge-Transport Dynamics The electrochemical impedance spectra of symmetric WO3, PEDOP and PEDOP–Au@WO3 hybrid films recorded before and after 100 charge–discharge cycles are shown in Figure 10. A perturbation amplitude of 5 mV was superimposed over a 0 V dc potential in a frequency range of 1 MHz to 0.01 Hz. The charge/discharge process of a pseudocapacitive electrode such as PEDOP involves the diffusion of counterions into/out of the polymer (doping and dedoping) or the extraction and insertion of cations in WO3 to balance the charges generated during the ChemPhysChem 0000, 00, 1 – 14

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Figure 9. In situ optical transmittance spectra as a function of applied potential and time for a PEDOP–Au@WO3 film, illustrating the simultaneous electrochromic and pseudocapacitve capabilities of this film.

Figure 10. Nyquist plots of WO3 (&), PEDOP (*) and PEDOP–Au@WO3 hybrid (~) films: a) before and b) after 100 charge–discharge cycles (performed at 1 A g1), recorded in the frequency range of 0.01 Hz to 1 MHz. The inset of (a) shows the equivalent circuit used for fitting the experimental data.

redox reaction. The ion insertion/extraction rates should be fast to provide constant current during the redox reaction. The process not only involves electron transport but also ion transport. In any of the three electrodes: WO3 or PEDOP or PEDOP– 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org Au@WO3, the ion-transport rate determines the charge/discharge rate of the electrode because it is slow compared to the electron transport. The impedance data were fitted into an equivalent circuit with the following components to a good approximation: Rs(solution resistance), CF (Faradaic capacitance), RCT (charge transfer resistance) and Zw (infinite length Warburg diffusion element). Before cycling, in the high-frequency regime, a well-defined semicircle is not seen in any of the films, indicating that the electron-transfer resistance is small and the systems are kinetically fast. As a consequence, the electrons are readily transferred into the solid phase (WO3 or PEDOP or PEDOP–Au@WO3) at the FTO/film interface, thus mimicking an Ohmic contact. The first intercept on the abscissa corresponds to the uncompensated solution resistance (Rs), and the values are very close for all the three films (~ 25–27 W). In the low-frequency region, while the Z“ versus Z’ curves are almost capacitive for the WO3 and PEDOP–Au@WO3 films, a skewed semi-circular response flowed by a slanted Warburg diffusion line is observed for the pristine PEDOP film. The impedance offered by the hybrid film to ion/electron transport and transfer over the entire frequency range spans from 25 to 245 W cm2 whereas it is significantly greater for the pristine WO3 film (~ 906 W cm2). This is also reflected in the abnormally high RCT observed for WO3 and PEDOP compared to the PEDOP–Au@WO3 hybrid. The hybrid has a low internal resistance due to PEDOP, and therefore, electron transport through the bulk of the electrode is facile. Ion transport is also promoted, possibly because of the fibrillar morphology. The Faradaic capacitance of the WO3 and PEDOP films was found to be 0.015 and 0.0019 F cm2 and was significantly lower than that of the PEDOP–Au@WO3 hybrid film (0.042 F cm2). Upon cycling, the Faradaic capacitance of the hybrid reduced to 0.023 F cm2 and the Z” versus Z’ curve exhibits a slightly increased charge-transfer resistance, possibly due to structural deterioration caused by the repeated intercalation/deintercalation of ions and electrons. The capacitances of WO3 and PEDOP after cycling were 0.008 and 0.0026 F cm2, respectively. Wei et al. reported a capacitance of 0.025 F cm2 at 5 mV s1 for a WO3/PANI composite,[18] and for a PEDOT–ionic liquid based supercapacitor, an SC of 10.9 mF cm2 was observed.[46] Our value for the hybrid is comparable to the state of the art values. The use of PEDOT instead of Pt as a counter electrode in dye-sensitized solar cells is well-reported[47] and here we applied the PEDOP film and the PEDOP–Au@WO3 hybrid film to a quantum-dot solar cell (QDSC) to study how the solar-cell parameters are influenced by the counter electrode. The counter electrode in a QDSC should have high electronic conductivity and an appropriate work function for facile reduction of the electrolyte species.[48] We have already observed that the nanoscale conductivity of the hybrid (0.88 S cm1) is greater than that of the polymer (0.015 S cm1). The work functions of the two electrodes were determined by Kelvin probe force microscopy. The topography and the corresponding surface potential maps of PEDOP film and the PEDOP–Au@WO3 hybrid film are shown in Figure 11 a. Experimental details and work-functionrelated calculations are provided in the SI. The surface potenChemPhysChem 0000, 00, 1 – 14

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Figure 11. a) Kelvin probe atomic force microscope (KPFM) topography and surface potential maps of PEDOP and PEDOP–Au@WO3. b) Energy band diagrams for the CdS/TiO2/S2/PEDOP or PEDOP–Au@WO3 configurations; and c) J–V characteristics of solar cells with CdS/TiO2 as the photoanode, 0.1 m aqueous Na2S as the electrolyte and PEDOP (&) or PEDOP–Au@WO3 (*) as the counter electrode (l > 300 nm, 1 sun illumination).

tials of the PEDOP film and the PEDOP–Au@WO3 hybrid film are 270 and 100 mV, respectively, and the corresponding work functions were 4.73 and 4.9 eV. The work function of the hybrid is close to that of Pt, which is widely used in dye-sensitized solar cells. The higher nanoscale conductivity of the hybrid can facilitate the reduction of the sulfide species at the counter electrode, which can lead to better solar-cell performance. Quantum dot solar cells were assembled using a CdS/TiO2 electrode as the photoanode, a pristine PEDOP film or the PEDOP–Au@WO3 hybrid film as the counter electrode and aqueous Na2S (0.1 m) as the electrolyte (see SI for details). The energy-band diagram illustrating photocurrent generation is shown in Figure 11 b. The J–V characteristics, under illumination, in terms of open circuit voltage (VOC) and short circuit current density (JSC) are shown in Figure 10 c. A JSC of 0.85 mA cm2 was registered for the cell when a PEDOP– Au@WO3 hybrid electrode was used as the counter electrode, which is higher than the JSC of 0.31 mA cm2 obtained for a cell with pristine PEDOP as the counter electrode. The short circuit current density is 2.7 times higher for the PEDOP–Au@WO3 hybrid cell, indicating that the hybrid improves the photocurrent collection ability of the cell. The open circuit voltages of the cells based on the PEDOP film and the PEDOP–Au@WO3 hybrid are 636 and 748 mV. Since VOC is the difference between the Fermi level of CdS/TiO2 and the redox potential of the S2/ S couple, it is apparent that the electrolyte potential moves to more negative potentials with respect to NHE (normal hydrogen electrode) when the PEDOP–Au@WO3 hybrid is used as the counter electrode. More studies are required for a deeper understanding of this behavior. The solar cell parameters are provided in Table S1. For instance, for a CdS/TiO2 cell with a Pt 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

alternate like Cu2S/graphene oxide as the counter electrode, VOC = 615 mV, JSC = 2.33 mA cm2 and h = 0.87 % were observed.[49] In our study, for the hybrid based cell, h was 0.70 %. Our values for the PEDOP–Au@WO3 hybrid film based cell are comparable to the literature values for quantum dot solar cells of CdS and show that this hybrid is a promising low-cost alternative to costly Pt-based counter electrodes in solar cells.

Experimental Section Chemicals 3,4-Ethylenedioxypyrrole (EDOP), poly(4-styrenesulfonic acid) (PSS) and auric chloride (HAuCl4) were purchased from Sigma–Aldrich and tungsten metal powder (particle size < 20 mm) was obtained from Alfa Aesar. 1-butyl-3-methyl imidazoliumtrifluoromethanesulfonate or triflate ([BmIm][CF3SO3]), hydrogen peroxide (H2O2, 30 %), ethyl alcohol, tri-sodium citrate, acetonitrile and acetone were obtained from Merck. Ultrapure water (~ 18.2 MW cm) or deionized water (DIW) was obtained through a Millipore Direct-Q3 UV system. SnO2 :F (FTO) coated glass substrates with a sheet resistance of about 14 W sq1 were procured from Pilkington, washed with soap solution, flushed with large amounts of distilled water and cleaned with acetone before use.

Preparation of WO3, PEDOP and the Hybrid Films WO3 powder was prepared by a hydrothermal process.[27] Tungsten powder (1 g) was weighed in a 50 mL beaker and H2O2 (15 mL) was added gradually. The mixture was stirred for 20 min and after the effervescence had subsided, the resulting colorless sol of peroxotungstic acid was transferred to an autoclave of 50 mL capacity and heated for 12 h at 180 8C. A light yellow precipitate of WO3 ChemPhysChem 0000, 00, 1 – 14

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was obtained which was washed with deionized water repeatedly and dried at room temperature. WO3 powder (0.1 g) and ethyl alcohol (20 mL) were taken in a 25 mL beaker and this mixture was sonicated for 15 min. A white suspension was obtained; two FTO/ glass electrodes were immersed in the suspension and a dc potential of 10 V was applied for 5 min using a Tarsons electrophoresis power supply unit. A thin transparent layer of WO3 was deposited onto one of the two FTO/glass electrodes. The WO3 films were washed with deionized water, heated at 80 8C for 30 min in an oven and subsequently annealed at 500 8C in a furnace for 1 h and stored in a desiccator.

sured under a square-wave potential at a fixed monochromatic wavelength using the same spectrophotometer in the kinetic mode. The electrochemical charges intercalated/deintercalated during redox switching of the films were determined by chronoamperometry. Galvanostatic charge–discharge measurements were performed on a battery testing unit (Arbin Instruments, BT 2000) at a current density of 1 A g1 in the voltage range of 0.4 to + 0.8 V in a two-electrode cell configuration. Specific capacitance (SC, F g1) was determined using galvanostatic charge/discharge curves by applying the following expression to the linear part of the SC versus time [Eq. (4)]:

Gold nanoparticles were prepared by using a citrate reduction method.[50] An aqueous solution of auric chloride was prepared in a 50 mL beaker with HAuCl4 (6 mg). In a separate beaker, a solution of tri-sodium citrate (4.4 mg) was prepared in ultrapure water (2 mL). The citrate solution was added dropwise to the auric chloride solution with constant stirring over a period of 10 min. A deep-pink-colored colloid of Au nanoparticles was obtained and used without any further treatment.

SC ¼ i Dt=DV

The transparent WO3 film was immersed in a gold colloid for 2 h to enable the Au nanoparticles to tether to the oxide vis--vis the electrostatic attraction between the carboxylate groups on citrate capped Au and the W6 + centers. The resulting WO3@Au film was dried in air. A 0.1 m EDOP solution was prepared in 20 mL of acetonitrile and PSS (0.2 g) was dissolved in the solution. A three-electrode electrochemical cell with a FTO/glass electrode as the counter electrode, Ag/AgCl/KCl as the reference electrode and an Au@WO3 film as the working electrode was used for the oxidative electropolymerization of 3,4-EDOP. Upon application of a dc potential of + 1.4 V for 500 s to the working electrode, a PEDOP– Au@WO3 hybrid film was obtained. Pristine blue–black colored PEDOP films were obtained when FTO/glass was employed as the working electrode. The films were washed in acetonitrile and dried in air. The current–time transients for the electropolymerization of 3,4-EDOP are shown in Figure S5.

ð4Þ

In Equation (4), i is the current density and Dt is the time interval required for the change in voltage DV. The mass values were determined for WO3, PEDOP and WO3@Au-PEDOP films with a Sartorius microbalance CPA2P with a 1 mg resolution. Electropolymerization, cyclic voltammetry, chronoamperometry and electrochemical impedance spectra were recorded on an Autolab PGSTAT 302N potentiostat/galvanostat equipped with a NOVA 1.9 software. The ionic liquid 1-butyl-3-methyl imidazoliumtrifluoromethanesulfonate was used as the electrolyte for all electrochemical and spectroelectrochemical measurements. From CV plots, the SC was calculated using Equation (5): SC ¼

Z

E2

E1

iðE ÞdE 2ðE2 E1Þmv

ð5Þ

where E1 and E2 are the cutoff Rpotentials in cyclic voltammetry, i(E) is the instantaneous current, E1E2 iðE ÞdE is the total voltammetric charge obtained by integration of the positive and negative sweeps in the cyclic voltammograms, (E2E1) is the potential window width, m is the mass of the individual sample, which is the mass difference of the working electrode before and after active material deposition, and n is the scan rate.

Instrumental Methods

3. Conclusions

Raman spectra were recorded for the electrodes on a Bruker Senterra Dispersive Raman Microscope spectrometer; the laser excitation wavelength was fixed at 785 nm. X-ray diffraction patterns were recorded on an XRD, PANalytical, X’PertPRO instrument with Cu-Ka (l = 1.5406 ) radiation. Surface morphology analysis was performed using a field emission scanning electron microscope (Carl Zeiss Supra 40 FE-SEM). Atomic force microscopy (AFM), and conducting-AFM (C-AFM) measurements were performed on the electrodes using a Veeco, Multimode 8 with ScanAsyst (Nanoscope 8.10 software) microscope. The conductive probes used in this study were coated with platinum–iridium on the front and back sides. The probe tip had a radius of 10 nm, a spring constant of 0.2 N cm2, and a current sensitivity of 1 nAV1. For C-AFM, the sample deposited on FTO-coated glass (area ~ 9 mm2) was affixed on a stainless steel disk with a conducting carbon tape. A thin strip of pin-hole-free silver paste was used for taking contacts. The contact tip is scanned in contact with the sample surface. Both topography and the current flowing through the sample are imaged at the same time. A 50 mV bias was applied to the tip during imaging.

A dual-function PEDOP–Au@WO3 hybrid film was prepared for the first time by firstly synthesizing WO3 fibers with a monoclinic crystal structure by a hydrothermal route, followed by their electrophoretic deposition onto a conducting electrode. Citrate-capped Au nanoparticles served as electrically conductive linkers between WO3 and PEDOP wherein PEDOP was electropolymerized using poly(styrene sulfonate) as the dopant onto an Au@WO3 film. The inclusion of Au in the PEDOP–Au@WO3 hybrid film was confirmed by XRD, Raman, and EDX studies. The nanoscale electrical conductivity of the PEDOP–Au@WO3 hybrid film was found to be 0.88 S cm1, which is 60 times and 1 fold times greater than that of the pristine polymer and WO3, respectively. An exceptionally high maximum electrochromic color contrast of 71 % (l = 476 nm) was achieved in the hybrid, which underwent a color change from a transparent blue to bluish–red to a deep red hue as opposed to a maximum contrast of 47 % (at 650 nm) obtained in pristine WO3, which experienced a colorless-to-blue transition. The coloration efficiency of the hybrid was 707 cm2 C1 (at 500 nm) and it also switched in about 6 s; both coloration efficiency and kinetics were substantially higher and faster than that achieved in pristine WO3 or polymer. The specific capacitance of the PEDOP–

The optical density and transmittance of the Au colloid and the films were measured on a Shimadzu UV-Visible-NIR 3600 spectrophotometer under dc potentials of different magnitudes (applied for a 60 s duration). Electrochromic switching responses were mea 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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CHEMPHYSCHEM ARTICLES Au@WO3 hybrid film was 130 F g1 (at 1 A g1) and decreased by 30 % after 100 cycles, indicating its suitability for practical applications. A proof-of-concept demonstration for the PEDOP–Au@WO3 hybrid film, illustrating the superiority in photoconversion efficiency (when applied to solar cells) relative to pristine PEDOP films, was also accomplished. The concurrent electrochromic and pseudocapacitive roles of a PEDOP– Au@WO3 hybrid film have been established and assessed with respect to pristine PEDOP and WO3 films, thus opening up avenues to integrate this conducting polymer with other electroactive oxide nanostructures for the realization of electrochromic psuedocapacitors. Supporting Information: The SI includes AFM images, height profiles, section profiles of C-AFM, specific capacitance variations with the scan rate, details of KPFM and solar cell fabrication, solar cell parameters and electropolymerization curves.

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Received: September 9, 2014 Published online on && &&, 2014


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ARTICLES B. N. Reddy , P. N. Kumar, M. Deepa* && – && A Poly(3,4-ethylenedioxypyrrole)– Au@WO3-Based Electrochromic Pseudocapacitor


All-in-one hybrid: A multifunctional PEDOP–Au@WO3 electrode that is useful for energy conservation (electrochromic), storage (pseudocapacitive), and conversion (photovoltaics) is presented [PEDOP: poly(3,4-(ethylenedioxypyrrole)].


&14&


Graphene based flexible electrochromic devices.

A fast electrochromic polymer based on TEMPO substituted polytriphenylamine.

A Paper-Based Electrochromic Array for Visualized Electrochemical Sensing.

High performance pseudocapacitor based on 2D layered metal chalcogenide nanocrystals.

Buckling Structured Stretchable Pseudocapacitor Yarn.

Electrochromic fiber-shaped supercapacitors.

Electrochromic graphene molecules.

Stretchable and wearable electrochromic devices.

A plate-based electrochromic approach for the high-throughput detection of electrochemically active bacteria.

An Electrochromic Bipolar Membrane Diode.

Hydrogenated NiO nanoblock architecture for high performance pseudocapacitor.

Optical constants of electrochromic films and contrast ratio of reflective electrochromic devices.

Visualization of energy: light dose indicator based on electrochromic gyroid nano-materials.

High performing smart electrochromic device based on honeycomb nanostructured h-WO3 thin films: hydrothermal assisted synthesis.

Electrochromic bis(terpyridine)metal complex nanosheets.

Photopatternable electrochromic materials from oxetane precursors.

Coordination-based molecular assemblies as electrochromic materials: ultra-high switching stability and coloration efficiencies.

Reversible modulation of gold nanoclusters photoluminescence based on electrochromic poly(methylene blue).

A Resettable Keypad Lock with Visible Readout Based on Closed Bipolar Electrochemistry and Electrochromic Poly(3-methylthiophene) Films.

Self-organized cobalt fluoride nanochannel layers used as a pseudocapacitor material.

Integrated smart electrochromic windows for energy saving and storage applications.

Self-powered visual ultraviolet photodetector with Prussian blue electrochromic display.

Enhanced performance of nano-Bi2WO6-graphene as pseudocapacitor electrodes by charge transfer channel.

Great improvement in pseudocapacitor properties of nickel hydroxide via simple gold deposition.