Flexible electrochromic tape using steel foil with WO3 thin film

Journal Pre-proof Flexible electrochromic tape using steel foil with WO3 thin film Martin Rozman, Boštjan Žener, Lev Matoh, Regina Fuchs Godec, Argyroula Mourtzikou, Elias Stathatos, Urban Bren, Miha Lukšič PII:

S0013-4686(19)32201-7

DOI:

https://doi.org/10.1016/j.electacta.2019.135329

Reference:

EA 135329

To appear in:

Electrochimica Acta

Received Date: 30 August 2019 Revised Date:

21 October 2019

Accepted Date: 17 November 2019

Please cite this article as: M. Rozman, Boš. Žener, L. Matoh, R.F. Godec, A. Mourtzikou, E. Stathatos, U. Bren, M. Lukšič, Flexible electrochromic tape using steel foil with WO3 thin film, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135329. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Flexible electrochromic tape using steel foil with WO3 thin film b ˇ Martin Rozmana , Boˇstjan Zener , Lev Matohb , Regina Fuchs Godeca , Argyroula Mourtzikouc , Elias Stathatosd , Urban Brena , Miha Lukˇsiˇcb,∗ a Faculty

of Chemistry and Chemical Technology, University of Maribor, Smetanova ulica 17, SI-2000 Maribor, Slovenia of Chemistry and Chemical Technology, University of Ljubljana, Veˇcna pot 113, SI-1000 Ljubljana, Slovenia c BRITE Solar Technologies, Patras Science Park, Stadiou Str. Platani, GR-26504, Patras Greece. d University of the Peloponnese, Electrical and Computer Engineering Department, Nanotechnology and Advanced Materials Lab., GR-26334 Patras, Greece. b Faculty

Abstract Intercalation of lithium ions into the crystal lattice of tungsten trioxide is one of the most researched electrochromic mechanisms. Conventional electrochromic devices (ECDs) utilize electrodes made of thin layer coatings of WO3 on optically transparent conductive polymer or metal oxide substrates. A majority of Li+ -WO3 based ECDs is built in the so-called ”sandwich” configuration, where the optically transparent electrode (OTE) is placed directly on top of the Li+ -based electrolyte which separates it from the counter electrode. It is shown here that stainless steel foils can replace conventional electrode substrates with the advantage of higher electrical conductivity and mechanical flexibility. In addition, the so-called ”inverted sandwich” ECD architecture is introduced which allows for fabrication of devices without the use of at least one OTE. Electrochromic tape with two stainless steel based electrodes, coated with WO3 and pencil lead carbon, is presented and characterized both electrochemically and optically. Keywords: electrochromic cell device, stainless steel, Li+ -WO3 intercalation, inverted sandwich architecture 1. Introduction

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Electrochromic devices (ECDs) have the capability to reversibly change color upon applied electric current or potential and can sustain the coloration state also after the electric source has been disconnected [1–3]. As such, they present an alternative to liquid crystal displays (LCDs) and light emitting diode displays (LED) [4, 5]. Electrochromic effect has been known for more than half a century [6] and has found its use in multiple applications, such as e-readers, smart windows, dimmable mirrors and other low power consumption devices capable of changing coloration states [7–10]. Depending on the number of optically transparent electrodes (OTEs), the ECD can be used as a transmissive device [11], such as smart window, or as an reflectance device [3], for instance ereader. In general, these devices are most commonly assembled from at least one OTE [12], a counter-electrode, electroactive substance and electrolyte. Mostly all electrochromic devices are built in the so-called ”sandwich” configuration [13, 14]. This implies vertical arrangement of working and counterelectrodes: OTE is placed directly on top of the electrolyte layer which separates it from the counter-electrode, forming thus a sandwich-like structure. The active part of the OTE is in most cases made in the form of a conductive thin film applied onto a non-conductive substrate (glass or plastic). The film can be, for example, a conductive polymer (e.g. polyethylene dioxytiophene (PEDOT) [15] or polyaniline [16]), conductive metal ox∗ Corresponding

author Email address: [email protected] (Miha Lukˇsiˇc)

Preprint submitted to Electrochimica Acta

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ide [17] (e.g. SnO2 ) or thin metal layer deposited using physical or chemical principle. Various ECDs differ also in the number of coloration states which the device can display. Simple devices use one dye (e.g. viologen or redox dye) or utilize the intercalation principle (e.g. Li+ into WO3 lattice). This type of devices normally exhibit only two coloration states, so-called bleached and tinted states. On the other hand, multiple dye systems–for example PEDOT: PSS polymer devices [18] with incorporated multiple viologen dyes or vanadium compounds–can show various coloration states depending on the redox state of the dye, with the option of fine-tuning the displayed colors. However, finding optimal working parameters for achieving multicolor electrochromic effect in a single ECD is difficult, mainly due to the complexity of the redox reactions and overpotentials. Compared to two color states devices used in consumer electronics, the multicolor ECDs do not have a widespread commercial success. There are numerous types of electrochromic mechanisms [19, 20], however, the most researched ECDs are likely based on the Li+ intercalation into the metal oxide thin layer [21, 22], specifically devices which use WO3 [8, 23, 24]. Their mechanism is well understood, and the ECDs work under a wide variety of modifications [25, 26]. In a ”sandwich” ECD configuration at least one OTE is required [12], because otherwise the electrochromic effect cannot be observed. Certain attempts to bypass the necessity of using the OTE were presented. For instance, a metal electrode in the form of a thin net can be used instead of the OTE [27]. Here, a part of the electrochromic response is visible due to the perforation of the metal mesh electrode. This approach retains the speed at which the color is November 18, 2019

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changed with a slight expense of total visible area. An alternative to the net electrode is to use a transparent glass U-tube with 115 metallic wire or plate electrodes put into each sections of the Utube and filled with liquid EC mixture [28]. While this option leaves the screen unobstructed, it slows down the pace at which the device can change the coloration states and its use is limited to only certain electrochromic mechanisms. Additional options include devices where two metal electrodes, placed sideby-side, are connected on top with material that normally func120 tions as OTE (e.g. fluorine-doped tin oxide (FTO) [29] or PEDOT) [30]. Since these devices must use pseudo-electrode material, no advantage is made compared to traditional ”sandwich” ECDs. In the last few years, steps forward to construct flexible, 125 stretchable or foldable EC devices have been made. Their use ranges from robotic skin, biometric devices, energy devices to soft electronics and optoelectronic devices as well as to adaptive camouflage, biomimicry, wearable displays and fashion. An overview on the progress, current challenges in the fab130 rication and future prospects in the filed of flexible ECDs is given in a recent review paper [31]. Such devices, made in the form of an electrochromic tape or string, can change color of the entire surface relatively quickly [32]. However, due to the use of conductive polymer materials for electrode substrates 135 these devices possess certain limitations regarding their flexibility and durability. We propose here a novel alternative approach for constructing reflectance electrochromic tapes based on conventional electrochromic mechanisms (Li+ intercalation into WO3 ) but without the need of optically transparent elec140 trodes. WO3 represents an interesting study material, since its coloration mechanism is similar to Li+ based power storage devices, such as lithium-ion or litium-sulfur devices [33, 34] Currently, there is a large interest in flexible ECDs and optoelectronic devices combining energy storage and electrochromism 145 [35–37]. For recent advances in emerging (flexible) electrochromic energy conversion and storage technologies the reader is advised to see reviews [38, 39]. Although there exist much more efficient coloration systems, such as, for example, viologen dyes [40] or multicolor devices based on vanadium compounds [41], 150 the Li-WO3 remains to be the most investigated system and as such ideal for use as a reference electrochromic dye. First, we present a method of preparing optically non-transparent electrodes with WO3 thin film deposited onto stainless steel foil. Such electrodes are much more electrically conduc155 tive and mechanically flexible compared to conventional WO3 based OTEs. Next, we use a novel geometrical approach, called the ”inverted sandwich” architecture [42], to make electrochromic tape with such WO3 -stainless steel working-electrode. We show that the inverted sandwich geometry (electrodes sandwiched in160 between the electrolyte) enables the construction of ECDs that exhibit similar response times and coloration efficiency as the classical sandwich ECDs using OTEs with Li+ -WO3 intercalation mechanism. Assembly of long electrochromic tapes or fibres is possible with no impact on the coloration response, 165 while in classical ECDs electrochromic tapes become unfeasible to build. Morphological features of the prepared thin layer deposits onto stainless steel were investigated via scanning elec2

tron microscope imaging, while the performance of the presented ECD was tested electrochemically (cyclic voltammetry, chronoamperomety, impedance spectroscopy) and optically (UVVis reflectance). 2. Experimental 2.1. Materials and electrode preparation Chemicals. Sulfolane (99.9%), anhydrous lithium perchlorate (99.99%), hydrogen peroxide (30% aqueous solution), hydrochloric acid (10% aqueous solution), ethanol (96%), isopropanol (98%), acetone (99.9%), polyoxypropylene-polyoxyethylene block copolymer (Pluronic P-123, 99.0%), and tungsten powder (99.8%) were obtained from Sigma-Aldrich. Stainless steel foil AISI 316 was purchased from Goodfellow. Fluorine doped tin oxide (FTO) TECTM A10 (10 Ohm/square) transparent electrodes were purchased from Pilkington NSG Group. HB Color Express graphite pencil was produced my Marker. Stainless steel foil preparation. Stainless steel foil AISI 316 (Goodfellow) was first washed with demineralized water and finally with acetone to remove any corrosion or organic stains on the foil surface. WO3 coating solution. WO3 solution was prepared using a modified procedure [43]: 1.5 g of tungsten powder was put into a 100 mL beaker, followed by the addition of 10 mL of 30% H2 O2 . The beaker was covered with parafilm in which several punctures were made to prevent any gas build-up during the reaction. The solution was briefly stirred for 15 s and then left to react until the entire tungsten powder dissolved in the solution. The solution was then diluted with 10 mL of isopropanol and 5 mL of acetone. Finally, 0.2 g of Pluronic P-123 was added to act as a foaming reactant during the sintering process. WO3 coating procedure. Stainless steel foil was first scrubbed with 10% HCl solution and then washed first with demineralized water and finally with acetone, to remove any corrosion or organic stains on the foil surface. Coating was done using inkjet printing technique. Printing work was carried out on the DMP2831 printer. For each printing output, the electrochromic layer (WO3 coating) was printed on the stainless steel foil with a resolution of 1016.00 dpi. DMP 2831 is equipped with a silicon print-head cartridges having 16 nozzles, each with a nominal drop volume of 10 pL. The temperature of the solution was set at 28 ◦C. The printing process was conducted 3 times to obtain the printed films. The printed layers were annealed and sintered at 500 ◦C for 10 min to completely remove any organic components and form a steady film every time after printing process. FTO glass was cut in dimensions of 2 × 2 cm2 and the edges secured with an insulation tape of dimensions 5 × 20 mm2 . The active ares of the glass for coating was 15 × 20 mm2 . WO3 was then printed onto the conductive side of the FTO glass in the same manner as described above. Carbon pencil lead coating procedure. Cleaned stainless steel foil was manually coated with a HB graphite grading scale pencil using wider side to achieve larger surface area of coating. The process of coating was repeated until the entire surface of

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the foil was uniformly colored in shiny dark gray. Any remaining pencil powder was removed with compressed air. Coating of the conductive side of the FTO glass was done in the same manner as described above. Preparation of the electrolyte. A solution of lithium perchlorate in sulfolane was prepared by dissolving 17.13 g of anhydrous LiClO4 powder in 50 mL of sulfolane. The solution was mixed for 4 hours with magnetic stirrer. 2.2. Device assembly 2.2.1. Inverted sandwich ECD using optically non-transparent stainless steel based electrodes A piece of the insulation tape (Tesa, article no. 56192-10) was placed on the bottom of a WO3 top-coated stainless steel electrode. The tape was then cut to appropriate width of 2 mm. The total length of the electrode measured 60 mm, where 50 mm were used as an active surface of the electrode and edge 10 mm served as the connectors. The same procedure was used to insulate the un-coated side of the pencil lead coated stainless steel foil (2 × 60 mm2 ). Onto the insulation tape of the WO3 coated electrode, a layer of double sided insulation tape with the width of 2 mm was mounted. The pencil lead coated electrode was glued with the insulated side to the other side of the double sided tape. Both electrodes formed the so-called ”inverted sandwich” geometry structure, leaving 10 mm of the surface of both electrodes un-bound (connectors). The electrode assembly was wrapped with a cellulose paper (Paloma MultiFun), starting at the WO3 coated side, and continuously twisted around both electrodes through the entire active area of the electrodes. This formed a cellulose-based electrolyte holder. Excess paper was cut off with scissors. Next, the electrolyte was applied with a syringe and equally dispersed over the entire electrolyte holder. Excess electrolyte was adsorbed with a paper towel. Electrode assembly was then encapsulated with a plastic foil (HERMA 7010) in order to assure that the soaked electrolyte holder was hold in place and to prevent access of air and water moisture. Finally, the ECD device was placed between two microscope glasses and bound together with paper clippers for 1 hour in order to assure that the cellulose electrolyte holder was well-placed on the surface of both electrodes as described above. Devices with electrodes of different widths (d = 1, 4, 6, 8, and 10 mm) were prepared in the same manner. 2.2.2. Sandwich ECD using FTO based optically transparent electrodes Devices were assembled using a modified procedure from the literature [20]: WO3 coated FTO glass was first placed on a firm surface with conductive side facing upwards. A piece of cellulose paper with dimensions 15 × 20 mm2 was placed on top of the WO3 electrode forming an electrolyte holder. The electrolyte solution was applied with a syringe and equally dispersed over the electrolyte holder. Next, pencil lead carbon coated FTO electrode was then put on top of the assembly, with the conductive side facing downwards. Lastly, the electrode

assembly was bound with clippers to assure a firm contact between the electrodes and the electrolyte.

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2.3. Electrochemical and optical methods for ECD characterization Cyclic Voltammetry. Cyclic voltammetry of the ECDs was performed with PalmSens 4 Potentiostat/Galvanostat. All voltammograms were recorded in the potential range from −3.7 V to +3.7 V with scan rate of 100 mV/s starting at 0.0 V in the positive direction. All tested ECDs were connected in such a way that the WO3 coated stainless steel electrode was attached to the negative pole, coupled with the reference channel, while the pencil lead carbon coated electrode was connected to the positive pole. Chronoamperometry. Chronoamperometric stability testing was performed with Palmsens 4 Potentiostat/Galvanostat. The pulses on the working electrode were switching between −3.5 V and +3.25 V. Each pulse lasted for 5 s followed by a 30 s long resting interval. In total, 100 cycles were performed. The experimental error in current, recorded at the end of the pulse, did not exceed 5%, estimated from multiple repetitions of the experiment. UV-Vis spectrophotometry. UV-VIS reflectance spectroscopy in the range form 400 to 800 nm was combined with a chronoamperometric cycling of the ECD (same parameters as described above). The spectra were recorded during the pause intervals after the 1st and 100th CA cycle for both tinted and bleached states. Cary 50 (Agilent Technologies) instrument, connected to reflectance fibre optic probe (Agilent Technologies), was used. Scanning speed for stainless steel ECD was 400 nm/s with the exposure time of 0.4 s. For the FTO-based ECD the exposure time was 0.2 s. The experimental error was estimated to be less than 10%. Electrochemical impedance spectroscopy. Impedance spectroscopy spectra were recorded with a PalmSens 4 Potentiostat/Galvanostat at Eocp (open circuit potential) in the frequency range from 10 mHz to 100 kHz. The amplitude of the sinusoidal voltage signal was 10 mV (peak to peak). Scanning electron microscopy. For SEM analysis of prepared WO3 and pencil lead carbon coating, the coated electrodes were cut in dimensions 5 × 5 mm2 and analyzed without any further modification. SEM images were obtained using Carl Zeiss Ultra plus (Germany) field emission scanning electron microscope.

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3. Results and discussion

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The working electrode was made using inkjet printing technique to form WO3 thin layer coating on the surface of the stainless steel foil as described in section 2.1. Pencil lead carbon coated stainless steel foil served as the counter-electrode, and the details for its construction are also reported in section 2.1. Morphological features of WO3 and pencil lead carbon coatings on stainless steel foils were investigated through scanning electron microscope (SEM) imaging at two magnifications. The uniformity of coating along with any potential anomalies, e.g.

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cracks which could be formed during the preparation procedures, was checked at 10.000x magnification, while the inspection of the nanostructures of the prepared films was investigated at 200.000x magnification.

The mode of device’s operation is presented Fig. 2b. Initially, in the bleached state (top left) the Li+ ions are dispersed evenly in the electrolyte carrier. When voltage is applied be+ 315 tween the two electrodes, Li ions start to migrate to negatively charged working electrode and intercalate into the WO3 crystal lattice (top right). After 5 s, the power supply is disconnected from the electrodes resulting in a full coloration (tinting) of working electrode (bottom right). To bleach the working elec320 trode, polarity of the applied potential is reversed (bottom left). Finally, after 5 s, the power supply is again disconnected and the ECD is again in original bleached state (top left). Both the tinting and bleaching processes start at the edges of the working electrode and continue to the center of the electrode. The 325 current flow at the edges of the device is made possible through a continuous electrolyte bridge (designated by blue arrows in Fig. 2). Photographic images of the tapes in the bleached and tinted states are given in Fig. 2c. Comparison with the FTO based conventional cell is given in Fig. S1 in the Supporting In330 formation (SI) file. The flexibility of the device is demonstrated by coiling the ECD tape (2 mm wide and 50 mm long) around glass rod. The required times to achieve the full change in the Fig. 1. SEM images of WO3 thin films printed on stainless steel (a, b) and coloration state were 5 seconds (+3.25 V for tinting and −3.5 pencil lead carbon deposits on stainless steel (c, d) shown at 10,000x (left) and 200,000x (right) magnification. V for bleaching), which is compatible with conventional ECDs. 335 Video demonstrating the operation of a longer electrochromic At low magnification, shown in Fig. 1a, it was observed that tape (2 mm × 20 cm) is given as the Supporting Information. the WO3 coating was uniform and without any surface anomaOnce the tinted state was fully reached, the coloration effect lies. This suggests that the methods and materials used to prewas strongly visible for at least 3 hours before it slowly started pare conventional WO3 thin films on FTO substrates are suitto fade off and needed approximately 12 hours before it became able also in the case of stainless steel surfaces. Data obtained at 340 completely bleached. high magnification (Fig. 1b) revealed that the WO3 particles reCyclic voltammetry (CV) combined with visual inspection semble closely packed pebble-like structures, which increases of the viewable ECD surface was used to choose optimum voltthe surface area of the active layer and improves physical abilage and width of the device. Regardless of the width of the ities of the prepared layers, such as potential bending of the electrodes (from 1 to 10 mm), the general shape of the voltamelectrode. This suggests that the prepared electrodes should be 345 mograms was similar with peaks remaining at the same posidurable enough to sustain bending or twisting of the electrodes, tions (cf. Fig. S2a in the SI file). The only difference regarding which is required in case of electrochromic tapes. Deposits of the voltammograms were differences in the current intensities. pencil led carbon on stainless steel are less uniform (Fig. 1c). Fig. S2b in the SI file shows the current changes at +3.2 V with However, as seen in Fig. 1d smaller pieces of pencil lead carincreasing width of the electrodes. The current is first increasbon with dimensions of less than 200 nm are present on the 350 ing due to the ability of electrolyte to carry Li+ ions over the surface of the stainless steel substrate. These smaller chunks electrolyte carrier (cellulose paper) to the working electrode. most likely improve relative surface area of the electrode, since The maximum current is reached at 6 mm, and then starts to digraphite is a conductive material and acts as a reducing agent minish due to diffusion limit of the electrolyte and the increased [44]. path that the lithium particles have to travel to reach the center WO3 and pencil lead carbon coated stainless steel optically 355 of the working electrode. This was confirmed visually where it non-transparent electrodes were used to construct sandwichcould be seen that at width of 8 mm the center of the electrode like electrochromic tapes. In the inverted-sandwich architecwas barely tinted, while at 10 mm the center of the electrode ture, the electrolyte is not sandwiched in-between the two elecremained in bleached state. CV study confirmed that the intertrodes, but rather wrapped around them [42]. Since we used calation process begins at the sides of the electrodes and conliquid electrolyte (LiClO4 dissolved in sulfolane) a solid elec- 360 tinues with time to the center of the electrode, suggesting that trolyte carrier (kitchen cellulose paper) impregnated with the narrower devices exhibit faster response. Even though the CV electrolyte solution was used to hold the electrolyte around the data promoted the use of 4-6 mm wide tapes, we decided to use electrodes. Electric short circuit was prevented by first insulat2 mm wide electrodes since this device had lower required time ing the contact sides of the electrodes with an insulation tape. A to achieve fully tinted state. transparent non-conductive plastic foil was used to encapsulate 365 Chronoamperometry (CA) measurements at optimal workthe device from external influences. Fig. 2a gives the schematic ing conditions were done in order to test the durability of the representation of the inverted-sandwich electrochromic tape struc- system. The ECD was cycled between +3.25 and −3.5 V at ture. least hundred times, each cycle lasting for 5 seconds. CA data 4

Fig. 2. Schematics of the inverted sandwich electrochromic tape device assembly (a) and its principle of operation (b). Working electrode (WE, yellow) is stainless steel with printed WO3 thin film, while counter-electrode (CE, gray) is stainless steel with carbon pencil lead coating. The electrodes are separated via an insulating tape (red). Assembly is wrapped with a cellulose paper holder soaked in sulfolane/LiClO4 electrolyte (blue dotted) and sandwiched between the transparent plastic foil encapsulator (pink). Arrows indicate the Li+ migration during the tinting/bleaching cycle. Photographic images (c) show the bleached (left photo) and tinted (right photo) states of the WO3 –Li+ based intercalation ECD. Comparison with standard FTO sandwich device is given in Fig. S1 in the Supporting Information.

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presented in Fig. 3a show that there are no major differences in the current profile between the first and hundredth cycle. Initial current for the tinting process (+3.25 V per pulse) was between 3.5 and 4 mA, and reached the value of approximately 1 mA 400 after 5 seconds. For the bleaching cycle (−3.5 V per pulse), the starting current reached up to −5 mA and dropped below −1 mA after 5 s. This differences are most likely due to different potentials used for bleaching and tinting process. Coulometry results presented in Table S1 in the SI file show that there is 405 a difference of 5% in used charge. This is most likely due to the intercalation process itself: more charge is needed to pack Li+ ions into WO3 crystal lattice than for excretion of Li+ ions out of the lattice. It is also possible that during coloration, an amount of Li+ remains inside the WO3 crystal lattice. In gen- 410 eral, coulometry results showed around 2% difference in charge during 1st and 100th cycle which could be contributed to experimental error, since no decline or incline trend in the current consumption could be reliably detected. Chronoamperometry data suggests that over the period of 100 cycles the device was 415 completely stable. This is true for most WO3 -based systems described in literature. CA measurements were coupled with UV-Vis spectroscopy measurement to assess coloration efficiency of the demonstrated ECDs. As seen in Fig. 3b, there is a clear difference in trans- 420 mittance signal from the fibre probe between the tinted and bleached state. Coloration difference between the states is clearly visible in the range from 500 to 800 nm, gradually increasing with the increasing wavelength. There is no visible change in 5

the recorded transmittance of the 1st and 100th cycle, both for the bleached and tinted states. The estimated coloration efficiency [42] at 700 nm (cf. Table S1 in the SI file for coulometry data) of the FTO-based classical sandwich device was 74.4 cm2 /C while for the novel WO3 coated stainless steel ECD tape the coloration efficiency was 60 cm2 /C. The optical density contrast of the ECD was 0.39 which is slightly above the threshold for the applications in electrochromic display devices. Since stainless steel tape device actually had better absorbance differential than FTO device (cf. Fig. S3 in the SI file) the lower coloration efficiency of the OTE-free device can be contributed to longer reaction path and increased current. Presented data confirm that (i) the device shows detectable electrochromic response, comparable to existing OTE-based ECDs, and (ii) the device is stable during the investigated cycling period. No quantitative differences in the transmittance spectra of the presented electrochromic tapes and FTO-based ECDs were observed (cf. Fig. S3 in the SI file). The kinetic aspect of the tinting process is addressed in the SI file. In Fig. S4 it can be seen that the difference in UVVis transmittance between the 2nd and 3rd chronoamperometric pulse was negligible, suggesting that the intercalation process is limited by the number of open places in the WO3 crystal lattice. A linear relation of the chronoamperometric current on the inverse square root of time, I ∝ t−1/2 , was observed (Fig. S5) which is characteristic for a diffusion-controlled redox process observed at a macro-electrode. Snapshots of the SI video are shown in Fig. S6 where tinting process starts after 1 second

cies which corresponds to predominantly resistive role that the charge movement has in the reactions involved during the natural operation of the device, i.e. at frequencies lower than 100 Hz.

Fig. 4. Bode plot showing the magnitude of the impedance, |Z| (black), and phase angle (red) as a function of the frequency for bleached (squares) and tinted (triangles) state. Corresponding Nyquist plot is given in Fig. S7 in the SI file.

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Fig. 3. Chronoamperograms (a) and UV-Vis transmittance spectra (b) of the 1st (continuous lines) and 100th (dashed lines) cycle for the tinting (black) and bleaching (red) process. Potential on the working electrode was +3.25 V (tinting) and −3.5 V (bleaching). Transmittance spectra were recorded at the end of the CA cycle.

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and is fully developed at the end of a 5 second pulse. Electrochemical impedance spectroscopy (EIS) measurement of electrochromic devices provides information about the electrode processes taking place at the electrode interface, such as, for example, charge transfer and accompanying diffusion pro- 465 cesses. For the latter a broad frequency range measurements are needed. In Fig. 4 we show the Bode plot, i.e. the variation of the impedance vector modulus, |Z|, and capacitive phase angle with respect to frequency, ν, for the bleached and tinted states of the tested x × y mm2 ECD tape. The corresponding Nyquist repre- 470 sentation (Z 00 vs. Z 0 ) is given in Fig. S7 in the SI file. As seen in Fig. 4 |Z| values at all frequencies increase with decreasing frequency for both states. The increase in |Z| was prominent especially in the low frequency region (below ∼ 300 Hz) and was more pronounced for the tinted state of the ECD (black triangles). When the ECD is in the tinted state, the Li+ ions dope into the WO3 film, consequently decreasing the concentration 475 of Li+ ions within the electrolyte. This leads to an increased resistance (impedance) of the tinted compared to the bleached state (cf. also Fig. S7 in the SI file). From the |Z| profile we can observe mainly resistive behavior in low and medium frequen6

At high frequencies (> 100 Hz) the Bode plot of the ECD in the bleached (red squares) and tinted (red triangles) states shows a steady increase of the capacitive phase angle from approximately −4 to −25° with decreasing frequency. In the medium frequency region (∼ 0.2 − 50 Hz), we observe a broad phase angle change between −50 and −65°. In the lower frequency region (< 0.2 Hz), however, the phase angle of the bleached state starts to decreases with decreasing frequency from −60 to approximately −30°. Such behavior could be related to the porosity of the oxide film. Contrary to the bleached state, the phase angle of the tinted state in the medium to low frequency region increases up to −70°. This phenomenon indicates that intercalation of Li+ into WO3 leads to intricate structural changes and it could be speculated that Lix WO3 is therefore more stable [45]. In the tinted state the phase angle at a given frequency was higher form the corresponding values of the bleached state in the whole range of frequencies due to already mentioned higher resistivity of the tinted state. The presented electrchromic fibre devices showed good reproducibility. This was tested by constructing devices many times from scratch (electrode preparation and ECD assembly) and performing chronoamperometric, UV-VIS transmittance and electrochemical impedance experiments several times. No noteworthy differences were observed between various devices tested. 4. Conclusions An electrochromic tape with two optically non-transparent stainless steel based electrodes in the ”inverted-sandwich” configuration, operating on a Li+ -WO3 intercalation mechanism, was presented. Time needed to obtain either fully bleached or

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tinted state was ∼ 5 seconds. The tape showed excellent electrochemical stability upon cycling and good coloration differential. Compared to conventional ECDs which employ OTEs, the presented device uses low cost materials along with simple assembly procedure. While the width of the device had an impact on the tinting/bleaching times, the assembled electrochromic device can be made in almost arbitrary lengths (a video of a 20 cm long electrochromic tape is shown in the SI). Unlike many other types of ECDs, several parts of the demonstrated assembly procedure are easy to scale up (for example: electrode, electrolyte and coating solution) while for the rest it offers potentials to implement assembly procedure into already known industrial processes (for example: roll-to-roll, tape connecting etc. [46, 47]). This options improve production capabilities compared to existing systems that use conductive polymers. The presented device is more durable compared to the existing sandwich solutions due to geometry workaround which prevents any possible direct contact between the electrodes. Using liquid electrolyte in combination with cellulose holder makes the assembly procedure much simple compared to devices that use conductive polymers. In addition, due to the use of metal substrate (stainless steel), the electrochromic coating can be easily replaced with other metal oxides, for example titanium dioxide [48, 49], or the device can use other electrochromic components in the electrolyte solution [50]. It is known that TiO2 requires substrates which can be processed at higher temperatures and stainless steel is on of them. The color contrast of the tape could be increased by using mixed electrochromic substrate, for example a WO3 -doped Nb2 O5 films. Such electrochromic layer would help the Li+ intercalation, avoid water adsorption and improve the lifetime of the ECD [51]. Miniaturization of the device presents another advantage that can enable the construction of electrochromic strings and therefore electrochromic fabric [52]. This could lead to the development of new wearable consumer electronics and could be developed into adaptive camouflage uniforms [53, 54]. Additionally, since the length of the device is mainly limited only by the specific conductivity of the stainless steel, it would be possible to construct low-coat large-scale screens which could replace traditional posters and paper sheets.

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Supplementary data to this article can be found online at https://doi.org/XXX/j.electacta.XXX References References 535

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Appendix A. Supplementary data

ˇ L.M., R.F.G., U.B. and M.L. acknowledge the M.R., B.Z, financial support from the Slovenian Research Agency, ARRS, 580 (research core funding No. P2-0046 (M.R., U.B.), P2-0006 (R.F. ˇ L.M.), P1-0201 (M.L.)). M.R. and U.B. G.), P1-0134 (B.Z., thank Slovenian Ministry of Education, Science and Sports for support through grants F4F and AB FREE. A.M. has been co- 585 financed by the European Union and Greek national funds through the Operational Program: T1EDK-01846. E.S. acknowledges the financial support from RIS3 project n. 5021436, co-financed 590 by Greece and the European Union.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: