1. Introduction

Electrochromic (EC) materials, capable of reversibly switching between different molecular configurations through electrochemical redox reactions induced by the intercalation and extraction of charge carriers, are currently being utilized to construct efficient ECDs, which fundamentally operate by manipulating the reflection or absorption of incident light rather than emitting light [1–3]. The ECD is now emerging as a novel light-weight all-solid configuration with slight energy consumption and is being introduced to the fields including architecture, interior design, vehicles, and military defense as smart windows, displays, sunglasses, antiglare rearview mirrors, and camouflages, etc [4–6]. With the escalating demands of stretchable and skin-conformable electronics, it is envisioned that bendable, stretchable and attachable ECDs could potentially be implanted on clothes or skin for dynamic or static displaying to extend its application in wearable displays, fashion, biomimicry, and adaptive electronic skins. Generally, ECDs are composed of transparent conducting electrodes (TCEs), EC chemicals and ion-gel electrolytes with a symmetrical sandwiched structure [7,8]. It has been a great challenge to realize such an extremely bendable, stretchable, and skin-conformable platform, as it requires a delicate design of each component to be ultra-thin, flexible, stretchable, and a sophisticated optimization of the assemblage to be compatible with intense deformation. Accordingly, the realization of novel TCEs to enable well-functioning skin-conformable ECDs is in urgent demand, which presents stringent requirements in following aspects. First, The TCE should exhibit a high electrical conductivity to enable the fast diffusion of charge balancing ions while still maintaining a high optical transmittance for not sacrificing the optical contrast. Second, the thickness and mechanical robustness of the conducting material and the supporting substrate should be optimized and balanced to alleviate the obstacle in the construction of highly flexible ECDs.

Initially, the indium tin oxide (ITO) on traditional plastic substrates by vacuum deposition was prepared as the TCE to obtain flexible ECDs in either the multiple layered structure [9,10] or the single EC gel configuration [11,12], which shows satisfying performance in terms of respond speed, optical contrast and multi-color operation [13]. In addition, ITO deposited on glass substrates treated with selective thinning has been utilized to fabricate ECDs with better environmental stability and optical transparency compared with ITO-deposited plastic substrates [14]. Nevertheless, the intrinsic brittleness of the ITO material, in addition with the high substrate thickness, uneven color distribution, and damped optical modulation during bending operation, have extremely limited its use in skin-conformable ECDs. Further, the vacuum deposition fabrication approach of the ITO based TCEs would severely add the cost of manufacturing to realize large-area and fast-response ECDs. Accordingly, solution or printing processing techniques have been developed to enable large-area, low-cost, and flexible ECDs. For example, conducting polymers such as the PEDOT: PSS have been spin-coated onto PET substrates to demonstrate all-organic solid-state ECD with outstanding mechanical flexibility and bendability [15,16], preserving the improvement property through chemical engineering to expand its working potential window, while the low electrical conductivity is a major limitation for polymers working as conducting films. In this regard, various solution-based processing techniques including spin-coating, blow spinning, electrochemical growing, and self-assembly have been developed to deposit the silver/gold nanowires or nanofibers onto PET substrates to yield ECDs with fast-response and conformal-attaching performance [17–20]. Moreover, silver nanowires have been integrated onto PDMS substrates as the elastomeric TCEs to enable wearable [21,22] and deformable [23] EC platform with outstanding capabilities in tolerating severe tensile and bending strains [24,25]. However, the bonding strength between the silver nanowires and the flexible substrates is low and the silver nanowires are performance instable due to oxidation, when exposed in atmosphere and during repeated redox reactions for coloration process. As a result, post-processing measures such as flash-induced nanowelding [26], reduced-graphene-oxide coating [27] and Pt nanowire doping [28] are often taken to overcome these limitations which intensely increase additional fabrication complexity and cost. Recently, the metal grid electrodes on the PET substrates by flexoprinting [29], inkjet printing [30] and self-assembly [31,32] techniques have been adopted as novel TCEs to demonstrate flexible ECDs where the pure metal wires provide excellent electrical conductivity and the spacing among the metal wires allows complete light transmission. The resultant flexible devices were capable of pixelated picture addressing [33] and exhibited excellent electrochromic performances including large optical contrast [34], fast switching speed [35], decent mechanical flexibility. Moreover, stretchable active-matrix ECD array which retained performance after repetitive bending and biaxial stretching deformations was fabricated by inkjet printing the liquid metal onto the elastomeric substrate as the TCE [36]. In order to further reduce the physical thickness of the TCEs, with the aim of improving the flexibility and bendability of the ECDs, researchers are developing novel supporting materials to replace the aforementioned PET and PDMS substrates. For example, ultra-thin ultraviolet (UV)-curable resin was utilized as the supporting film for the silver nanowire to enable the deformable EC device which showed performance retention of 90% after 8000 bending cycles [37]. Furthermore, a novel nanopaper transfer method was developed to obtained nanocellulose percolated with silver nanowires and the color neutral paper electrodes were used to obtain ultra-thin ECDs with outstanding foldability [38–40].

Despite the encouraging progresses was performed to obtain perfectly flexible ECDs with stretchability and bendability, as well as brilliant EC performance of high optical modulation and fast respond speed, the flexible TCE substrates which prop up conducting material, such as polyethylene terephthalate (PET), PDMS and glass, present as a great obstacle to accomplish ultrathin skin-conformable devices, which heavily increases the overall thickness of the device and reduces the strain deformation accommodation capability of the architecture. In this work, we have demonstrated a high-performance freestanding Ni grid as a novel TCE without the supporting of any flexible substrates and the resultant ECD with attractive features of ultra-thin thickness and skin-conformal attachability. The freestanding Ni network electrode was transferred onto the EC material and the ion gel to construct the ultra-thin ECD with an asymmetric working structure, which exhibited excellent EC performance endurance to mechanical deformation, like bending, folding and even stretching. In addition, the as-fabricated ECD could be seamlessly attached on human skin, botany, tortuous surfaces, which was further exploited to use as the visible camouflage device by merging with surroundings, providing a feasible method for wearable electronics implanted on clothes, skin and other irregular objects.

2. Experiment

2.1 Fabrication of the self-supporting transparent Ni network electrodes

We have developed a novel two-step manufacturing process with the potential of volume production to obtain the self-supporting Ni electrodes. Briefly speaking, the first step defines the grid pattern using lithography and the subsequent step to form the pure metal grid via electroplating. In the first step, the cleaned conducting ITO glass was treated with adhesion agent (RZN-6200, Suzhou Rui Hong Electronic Chemicals CO. LTD) and spin-coated with the positive photoresists (AZ P4620, EMD Performance Materials Corp.). The thickness of the photoresist could be controlled between 500 nm to 10 μm by adjusting the rotation speed, which is important in modulating the thickness of the resultant Ni grid film. Subsequently, the grid pattern was written into the photoresist using the laser direct write equipment (iGrapher 820, SVG Optronics Co. LTD), leaving the relief photoresist microstructures on the ITO glass substrate after being rinsed in the developer, with the grooves and ridges corresponding to the exposed and unexposed regions, respectively. Then the sample was placed in the Ni plating solution as the conducting electrode (cathode) in a plating station to induce the deposition of pure Ni metal in the trenches of the photoresist, where the surface was conductive. The thickness of the resultant Ni grid could be adjusted by the electrodeposition time, but should not surpass the depth of the photoresist trench to ensure the high optical transmittance. After that, the photoresist on the ITO substrate was removed by immersing the sample in the sodium hydroxide solution, leaving the bare Ni grid. The Ni grid film could be peeled off directly from the ITO substrate as a free-standing film using a tweezers or transferred to integrate with other functional layers to complete the functional device.

2.2 Preparation of the highly transparent conducting ion-gel electrolyte

Lithium chloride (1.1 g, LiCl, Beijing Innochem) was added into deionized water (5 ml), and the solution was magnetically stirred for 20 minutes. Subsequently, acrylamide monomer (0.8 g, AAM, Beijing Innochem Technology) ammonium persulfate (13 mg, AP, Shanghai Aladdin Biochem), and methylene double acrylamide (4.7 mg, MBAA, Shanghai Aladdin Biochem) were added into the solution and the mixture was magnetically stirred for additional 2 h to obtain a homogeneous solution. Finally, tetramethyl ethylenediamine (2 mg, TEMED, Beijing Innochem) as the catalyst to activate the cross-linking reaction was blended into the solution to obtain the liquid state ion-gel electrolyte.

2.3 Assembly of the ultra-thin ECD

A stable mixture was obtained by blending the PEDOT: PSS solution (10 ml, 0.8 wt% PEDOT, 0.5 wt% PSS, Heraeus Deutschland GmbH & Co. KG) with ethylene glycol (0.7 g, Shanghai Aladdin Biochem) and Triton-X100 surfactant (0.025 g, Shanghai Aladdin Biochem). The metallic Ni network on the ITO substrate was treated with oxygen plasma to improve the surface hydrophilic property. The prepared PEDOT: PSS mixture was then spin-coated onto the sample and was subsequently annealed at 120 ℃ for 20 min for solution evaporation and morphology improvement. After that, the liquid hydrogel was scrap-coated onto the PEDOT: PSS film, where the distance between the scraper and the substrate is set as 100 μm, and was further put on the heating plate at 60 °C for 1 hour to solidify the hydrogel. Finally, the triple-layered EC device consisting of the self-supporting Ni grid, the PEDOT: PSS EC layer and the solid ion-gel electrolyte was obtained by stripping it from the ITO substrate.

2.4 Characterization

The measurement of the surface morphology and the thickness of the metallic Ni network electrode was carried out with the field emission scanning electron microscopy (SEM) (JEOL, JSM-5400, USA). The sheet resistance of the self-supporting Ni grid electrodes was measured by a four-point probe (CMT SR2000, A. I. T.). Optical performance of the freestanding Ni grid film and the ion-gel electrolyte was conducted on a UV-VIS spectrometer (SPECORD 210 PLUS, Analytikjena). Electrochromic properties and electrochemical behaviors of the ECDs were measured on the same spectrometer triggered by an electrochemical workstation (CHI 760E, Shanghai CH Instrument).

3. Results and discussion

Traditional multi-layer ECDs have extremely limited flexibility and poor coordination between adjacent layers. In order to simplify the working architecture of the ECD, which is beneficial to reduce the overall thickness and improve the mechanical reliability, we have adopted a triple-layered asymmetrical electrochemical coloration structure based on the freestanding properties of the Ni grid (Fig. 1). The ECD could be regarded as the Ni grid (for electron injection) being partly buried in the hydrogel film (for ion storage and ion conduction), where the hexagon shaped EC material PEDOT: PSS was in area that contact with the hydrogel and was in line contact with the Ni wires. When a negative voltage was applied, the electrons were injected into the PEDOT: PSS film at these corner sites connecting the PEDOT: PSS film and the Ni grids to induce the redox reaction, accompanied by a balanced Li ion flow from the hydrogel into the EC material (Fig. 1(a)), and vice versa for the positive voltage functioned on the PEDOT: PSS film, exhibiting the reversible coloration and bleaching appearance (Fig. 1(b)). Thus, the free-standing Ni grid would be regarded as the motif for the injection and extraction of electrons and ions from the adjacent PEDOT: PSS regions. As electrons in the conjugated polymer film have a very limited diffusion length, these metal grids should be placed dense enough to enable a fast and uniform coloration and bleaching process of the adjacent EC material. In this regard, the spacing between the metal grids functions as an important parameter in controlling the performance parameters of the ECD including the optical contrast, the switching time and the coloration uniformity, in addition to the parameters of the free-standing Ni grid film including the optical transmittance and the conductivity, which indicated a delicate optimization of the Ni grid pattern and the pattern periodicity.

 

Fig. 1. Schematic illustration of the triple-layer architecture of the ultra-thin ECD based on the free-standing Ni grid electrode. The inset illustrates top view and cross-section view of the device. (a) and (b) the microstructure of the self-supporting ECD and the ionic migrations in coloration and bleached processes.

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The fabrication procedure of the free-standing Ni grid electrode and the corresponding ultra-thin ECD is shown schematically in Fig. 2. Briefly speaking, the relief microstructure in the photoresist on the ITO substrate, where the trenches and the ridges correspond to conductive and unconducive regions, respectively, was first defined via photolithography. After that, Ni metal wires were grown in the trenches via the electroplating process, which was followed by dissolving the photoresist in the sodium hydroxide solution, leaving the Ni grid electrode on the substrate. The PEDOT: PSS film and the hydrogel film were subsequently coated onto the grid film to construct the ECD (displayed in Fig. 7(a) of Appendix A). After solidification of the hydrogel, the triple-layered ECD was peeled off from the ITO substrate, resulting the ultra-thin ECD without supporting substrates.

 

Fig. 2. Schematic illustration of the fabrication process of the device which mainly consists of steps of photolithography, electroplating, coating and transferring.

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The optical, electrical and mechanical properties of the self-supporting Ni grid were investigated to feature its advantages as a substrate-free conductive electrode to be implemented in the ECD. The Ni grid on top of the ITO substrate could be directly paper transferred with the aid of water after the removal of the photoresist, resulting in an ultra-thin Ni grid electrode film with the self-supporting capability. The top-view SEM image indicated that the Ni grid consisted of uniformly distributed periodic hexagons with a metal wire linewidth of 5.5 μm (Fig. 7(b) of Appendix A) and a periodicity of 100 μm (Fig. 3(a)), as initially determined by photolithography. The cross-section view SEM image at a higher magnification revealed that the thickness of the grown metal grid was 3.7 μm (Fig. 7(C) of Appendix A). Besides, the inerratic Ni grid shows prominent flexibility which could be bent to a radius of 50 μm without compromising the electrical properties as shown in Fig. 3(b) and Fig. 8 of Appendix A. The ultra-thin Ni grid film is mechanically robust which could support a weight with 50 g (Fig. 3(d)), while the grid film stayed unbroken and intact. Finally, the ultra-thin metallic Ni grid showed brilliant resistance to mechanical deformation, like bending, folding, or even stretching, for example, it retained a low sheet resistance of 0.49 Ω/sq after being bent to a radius of 0.7 mm for 5000 times (Fig. 3(e)). The large space ratio of the self-supporting Ni grid shows satisfying optical transmittance of 90%. As shown in Fig. 3(g), the hydrogel acting as the electrolyte to transport anions so as to balance the charge during coloration and bleaching, had a high optical transmittance of 95% in the visible spectrum. Both the Ni electrode and the hydrogel can light the LED when connecting with electricity.

 

Fig. 3. (a), (b) The SEM images of the Ni grid electrode with a periodic hexagonal pattern at plane and bending states (with a bending radius of 50 μm). (c) The cross-section view SEM image of the ECD on top of a paper. (d-f) Photos of the self-supporting Ni grid electrode implying the properties of high optical transmittance, ultra-low weight, high mechanical toughness and excellent conductivity. (g) The transmittance curves of Ni gird electrode and hydrogel at the wavelength range of 400 nm -800 nm. The LED could be lighted with either the Ni grid or the hydrogel working as the conductive electrode. (h) The transmittance modulation of the ultra-thin ECD at the spectrum range of 400 nm -800 nm, and (i) the photos of the ECD in bleached and colored states.

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The PEDOT: PSS EC material and the hydrogel were then coated in sequence onto the Ni grid electrode to complete the ECD. The cross-sectional SEM image of the EC device on a paper indicated that the overall device thickness was around 113 μm (Fig. 3(c)), where the hydrogel film accounted for 90% of the device thickness to maintain sufficient mechanical reliability. In addition, the deeply buried Ni grids in the solidified hydrogel film were beneficial to achieve an intact device architecture with high resistance to mechanical deformation. The electrochromic performance of the ultra-thin ECD was investigated under alternating bias, accompanied by the intercalation and extraction of charge carriers to trigger the redox reactions in the conjugated polymer. Determined by the potential gradient of charge carriers, the yield of the isomerized EC material and thus the optical contrast of the device was affected by the applied voltage. As shown in Fig. 3(h), the bleached and colored transmittance of the ECD was measured at the spectrum range of 400 nm - 800 nm, having a contrast of 35% at 640 nm. At the peak modulation wavelength of 640 nm, the optical contrast was 22.2%, 31.8% and 39.4% for the applied voltage of -1 V, -1.5 V and -2 V, respectively (Fig. 9 of Appendix A). The optical contrast was merely deteriorated in the ultra-thin device architecture and the EC film exhibited the pale blue and dark blue appearance in the bleached and colored states, respectively (Fig. 3(i)), corresponding optical modulation in physical device is shown with the Visualization 1. The in situ transmittance response of the device implied stable device performance (both in the optical contrast and the response time) with the operating circles (Fig. 10 of Appendix A). The response time for the coloration and the bleaching process was deduced to be 3.3 s and 3.7 s according to the in situ response curve of the ultra-thin ECD (Fig. 11 of Appendix A). The device switching speed was not very fast, as compared with the previous conjugated polymer EC device based on the ITO or Ag nanowire conducting substrates, however was sufficient to implement in applications such as camouflage and smart window. Considering the high conductivity of the free-standing Ni grid electrode, the increase in the switching speed was attributed to the particular working architecture of the ECD in Fig. 1, where the contact area between the conducting metal wire and the EC film was small and the electron diffusion in the conjugated polymer to active the redox reaction required a quite long time. The intact working architecture of the ultra-thin ECD was advantageous in terms of working stability and the optical contrast at 640 nm attenuated from 35.07% to 29.24% after a cyclic number of 600 (Fig. 12 of Appendix A), giving a high retention of 83.4%. The slight deterioration in ECD performance was attributed to irreversible decomposition of the EC material during the cyclic operation, rather to the ultra-thin architecture based on the free-standing Ni grid film.

Flexibility and stretchability are crucial factors to dramatically expand the applications of ECDs used in conformal displays, stretched electronics, and wearable e-skin, etc. Using the self-supporting Ni grid electrode and transparent hydrogel with highly flexibility and stretchability can commendably solve the abovementioned problems. The ultra-thin and highly flexible ECD can be comfortably attached to cylinders with different radiuses, as shown in Figs. 4(a), 4(b). The tender flower won’t be broken off after being attached with the device in Fig. 4(c) due to the lightweight and ultrathin characteristics of the ECD. In addition, the assembled ultra-thin and flexible ECD can be conformally attached to people’s skin (Fig. 4(d)), satisfying the gradually growing demand for flexible wearable electronics. Depending on the extremely flexibility of Ni grid electrode and hydrogel, the assembled ultra-thin ECD exhibits excellent bending performance in Fig. 4(e). The optical contrast of the ECD only have a slight reduction when being 1, 000 circles. At the wavelength of 640 nm, the contrast decreased to 30.7% from beginning 35.9% after bending 1, 000 times (Fig. 4(f)), retaining 85.5% of the initial contrast in Fig. 13 of Appendix A. Because exogenous process prompts the deformation of materials (e.g., metals and polymers), so both the Ni gird and the electrochromic material layer produced irreversible structure change and crake which caused the impaired electrical channel. And thus the decreased electrochromic performance, which was exhibited through the attenuated optical transmittance spectrum in Figs. 4(e) and 4(f). The optical contrast of the ECD decreased slightly when the bending radius increased from 0.8 cm to 1.0 cm, 1.3 cm (Fig. 4(g)). Because the effective detect area of the UV-VIS spectrophotometer is invariable, so different bending radiuses of the ECD introduce discrepant detected area. Besides, the bleached transmittance shows bigger modulating space in optical contrast, compared with colored transmittance of the ECD, which introduced discrepant transmittances in bending radiuses of 0.8 cm, 1.0 cm, 1.3 cm.

 

Fig. 4. (a) Photos of the ultra-thin and highly flexible ECD in bleached and colored state under different bending radiuses of 0.8 cm, 1.0 cm, 1.3 cm respectively. (b-d) The conformal attachment of ultra-flexible ECDs on pen, flower, and glove as the wearable device. (e) The transmittance curve of the device with different bending times with a bending radius of 0.8 cm. (f) the transmittance modulation of the ECD after different bending times (at the wavelength of 640 nm with a voltage of -1.5 V and 1.0 V, respectively. (g) The transmittance modulation (at 640 nm) of the ECD in bending radiuses of 0.8 cm, 1.0 cm, 1.3 cm respectively.

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Stretchable ECDs have rigorous requirements on the characteristics of each component to meet the stretchability, under the premise that the performance of functional layers after stretching will not attenuate. We demonstrate that the ingenious combination among the self-supporting Ni gride electrode, EC material, and the flexible hydrogel is potentially applicable to develop the stretchable ECD. the honeycomb Ni grid electrode shows various structure shapes when stretched along different directions. Considering only regular deformation, the stretching of Ni grid electrodes can be divided into along sides and angles of the hexagon. The initial distance between parallel edges of the regular hexagon electrode was 99.15 μm. The distance of the parallel edges was 107.2 μm, 112.5 μm, and 116.6 μm, respectively, after being stretched by 8%, 13.5%, and 17.6% in Figs. 5(a)–5(d). We also showed the stretched Ni grid SEM pictures when pulling along angles of the hexagon (Fig. 14 of Appendix A) and photos of Ni network flake in initial and stretched states (Fig. 15 of Appendix A). The angle between the two adjacent sides changed from 120° to almost 180°, and the grid hexagon still maintained good integrity, without any fracture in each side, the schematic structure is shown in Fig. 5(e). The hydrogel shows highly transmittance, good electricity and brilliant flexibility, after being stretched by 300% and 450% (Fig. 5(f)). The photos of the stretchable ECD are shown in Fig. 5(g), where the color distribution is uniform in both initial and stretched states of the ECD. In stretched condition, the amount of PEDOT: PSS located in the detect area of the UV-VIS spectrophotometer is decreased, which has a thin coloration under applied voltage, so the colored transmittance has a slight rising as shown in Fig. 5(h).

 

Fig. 5. (a)-(d) The SEM photos of the ultra-thin and stretchable Ni grid with stretching sublevels of initial, 8%, 13.5%, 17.6%, respectively. (e) Schematic exhibition of the self-supporting and stretchable ECDs. (f) The photos of transparent hydrogel with a stretchability of 300% and 450%, respectively. (g) Photos of the ultra-thin and stretchable ECD in initial and stretched states. (h) Optical transmittance modulation of the ECD in initial and stretched (114%) states at 640 nm.

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Visual camouflage is a constant pursue in the military research field. Traditional camouflage uniforms are random fit ambient circumstances, so as to hiding from human eyes. As shown in Figs. 6(a)–6(d), the blue paper cuttings were attached on the cup as the background, where the middle of the paper was sheared to the shape of elk and plane. The flexible and attached ECD is exposed to the sheared area, having an obvious contrast when the ECD is being in the bleaching state. Nonetheless, the attached ECD has a similar appearance with the color changing from transparency to blue so as to achieve the goal of visual camouflage. Correspondingly, the reflection of the self-supporting and flexible ECD in coloration and bleaching was also measured in Figs. 6(e), 6(f). The reflection in coloration is lower compared with bleaching at spectrum ranges of 400 nm - 800 nm, which is because the optical absorption of the colored ECD. At spectrum of 600 nm - 700 nm, the reflection drop in coloration is more obvious for the EC material has a sensitive absorption peak.

 

Fig. 6. (a-d) show electrochromic images of elk and plane in visible light camouflage. (e, f) show the reflection changes in colored and bleached ECDs among spectrum ranges of 400 nm – 800 nm.

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4. Conclusion

In summary, a self-supporting metal Ni gird electrode is mentioned, which discarded solid or flexible polymeric substrates, having outstanding features of extremely foldability, stretchability, and conductivity, highly transmittance, ultra-thin thickness. By assembling the metal electrode and the hydrogel, a paper-thin, ultra-flexible, and stretchable ECD with an overall thickness of 113 μm was prepared, which could be attached to manifold and undulating surface of things and also as stretchable ECD realized the wearability. The ultra-thin and flexible ECD has brilliant durability, exhibiting a high retention of 83.4% after a cyclic number of 600, and excellent bending stability, retaining 85.5% of the initial contrast after bending 1, 000 times. The self-supporting, flexible, and stretchable ECD as futuristic electronics is supposed to be used in multipurpose, like flexible displays, camouflage wearables and medical sensors, etc.

Appendix A

1. SEM of the ultra-thin and self-supporting ECD and the Ni grid

 

Fig. 7. The SEM of the ultra-thin and self-supporting ECD and the Ni grid. (a) The surface morphology of the device. (b) the micro-structure of the hexagonal Ni grid. (c) the thickness of the Ni gird.

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Please note that three faces of Ni gird electrodes are embedded in the hydrogel to form a stable configuration, avoiding the separation of Ni electrodes from the hydrogel. Besides, the width and height of the self-supporting electrode are on the micro-scale ensuring the high bleached transmittance in the condition of good conductivity. The face of depositing nickel is undulating which will increase the contacting area with materials.

2. SEM of the ultra-flexible inerratic Ni grid in the bending state

 

Fig. 8. The SEM of the ultra-flexible inerratic Ni grid in bending with a bending radius of 50 μm(a) and a overlapping state(b).

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Please note that the self-supporting Ni grid electrode showing ultra-flexibility has a bending radius on micro-size for the advantage of metal ductility. The excellent bendability of Ni grid electrodes will accelerate the study of fabricating flexible electronics. The method of superposing multiple layers of Ni electrodes creates the possibility of configurating multi-colored ECD.

3. Optical transmittance property of the ECD under changing voltages

 

Fig. 9. the bleached and colored transmittance curves of the ECD at the spectrum range of 400 nm - 800 nm.

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The self-supporting ECD has a biggest contrast of 35% at 640 nm. At the peak modulation wavelength of 640 nm, the optical contrast was 22.2%, 31.8% and 39.4% for the applied voltage of -1 V, -1.5 V and -2 V, respectively.

4. Cycle-lives test of the ultra-thin and flexible ECD

 

Fig. 10. The optical transmittance modulations of the self-supporting ECD at wavelength of 640 nm.

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Please note that the dislodge of polymer substrates did not dramatically impact the cycle stability of the ultra-thin ECD, which still has many challenges to overcome for commercial applications.

5. Respond speeds of the ECD in colored and bleached processes

 

Fig. 11. The respond speed for the coloration and the bleaching process of the self-supporting ECD.

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Compared with inorganic electrochromic materials, organic materials usually have shorter respond time. The self-supporting ECD used PEDOT: PSS as the electrochromic material has a coloration time of 3.3 s and a bleaching time of 3.7 s according to the in situ response curve of the ECD.

6. Contrast attenuation of the ECD with the increase of cycle times

 

Fig. 12. The contrast attenuation curves of the ECD with the cycle times increased from 1 to 600.

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The intact working architecture of the ultra-thin ECD was advantageous in terms of working stability and the optical contrast at 640 nm attenuated from 35.07% to 29.24% after a cyclic number of 600, giving a high retention of 83.4%.

7. Contrast attenuation of the ECD with the increase of bending times

 

Fig. 13. The contrast attenuation of the ECD with the increase of bending times.

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The optical contrast of the ECD with the bending times increased to 1, 000 circles, At the wavelength of 640 nm. The contrast decreased to 30.7% from beginning 35.9% after bending 1, 000 times, retaining 85.5% of the initial contrast.

8. SEM of the stretched Ni grid pulling along angles of the hexagon

 

Fig. 14. The SEM of the stretched Ni grid pulling along angles of the hexagon.

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The honeycomb Ni grid electrode shows various structure shapes when stretched along different directions. As shown in Figs. 14(a), 14(b), and 14(c), the degrees of the angle became smaller from 54° to 25° gradually with the stretch extent increases.

9. Photos of the self-supporting Ni gird electrode in initial and stretched lengths

 

Fig. 15. The photos of the self-supporting Ni gird electrode in initial and stretched lengths.

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Please note that stretching the Ni grid electrode will not cause the breakage in some ranges, which was approved in the manuscript by the SEM of the Ni grid electrode in different stretching ratios. Adopting optimized periodic structure will improve the stretching ratio of the stretchable ECD.

Funding

National Natural Science Foundation of China (61974100); Natural Science Foundation of Jiangsu Province (BK20181166); Natural Science Research of Jiangsu Higher Education Institutions of China (20KJA510040).

Acknowledgments

Y. L., W. H. and L. C. conceived the project. S. Z. designed the experiments and analyzed the data. Z. J. performed material synthesis, structural characterization, devices fabrication, and electrochromic measurements. All the authors commented on the manuscript.

Disclosures

The authors declare no conflicts of interest.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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