A Wearable Supercapacitor Engaging with Gold Leaf Gilding Cloth

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Energy, Environmental, and Catalysis Applications

A Wearable Supercapacitor Engaging with Gold Leaf Gilding Cloth Towards Enhanced Practicability Yukun Wang, Zengxia Pei, Minshen Zhu, Zhuoxin Liu, Yan Huang, Zhaoheng Ruan, Yang Huang, Yan Zhao, Shanyi Du, and Chunyi Zhi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Materials & Interfaces

A Wearable Supercapacitor Engaging with Gold Leaf Gilding Cloth Towards Enhanced Practicability Yukun Wang † ‡, Zengxia Pei‡, Minshen Zhu‡, Zhuoxin Liu‡, Yan Huang§, Zhaoheng Ruan‡, Yang Huang* ∆, Yan Zhao†, Shanyi Du* †, and Chunyi Zhi* ‡. † School of Materials Science and Engineering, Beihang University, Beijing 100083, China. ‡ Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region, 999077, China § Sate Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology (Shenzhen); School of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China ∆ Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China. KEYWORDS: conductive cloth, gold leaf gilding, wearable supercapacitors, polypyrrole, low cost

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ABSTRACT

Flexible energy storage devices have attracted widely attention because of the increasing requirement of wearable electronics. However, comfortability, productivity, and feasibility, to name a few, are still far from satisfactory in current wearable supercapacitors (SCs). This is largely due to the missing of an ideal low-cost flexible substrate/current collector that should not only exhibit high conductivity, but also be compatible with modern textile technologies. Herein, we apply the traditional gilding technique to cloth and successfully convert the cloth to be an excellent current collector which is at a reasonable cost and compatible to textile technologies. Thanks to the strong electrostatic interaction, we find positive charged gold leaf could be laminated on negative charged polyester cloth intimately. This substrate could perfectly act as an integrated compact electrode after the electrodeposition of polypyrrole (PPy) nanorods. The resultant electrode is mechanical strong enough to withstand the tortures of repeated bending, cutting or puncturing, which is readily assembled into wearable SCs and energy cloth with outstanding practicability, for example, safety and breathability. It is foreseeable that our work will inspire a series design of wearable electronics based on the fascinating gilding art.

1. Introduction With a rapid development of smart electronic devices, the requirement of high-performance energy storage devices has increased dramatically.1-4 In the big family of these devices, supercapacitor (SC) is one important type considering its particular advantages, for example, good cyclic stability. Beyond improvements of energy storage capability, flexibility is another focus of developing SCs, which caters to the practical need of wearability in portable devices.5-8


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A number of attempts have been made to realize the above goals by selecting appropriate electrode active material and substrate/current collector, two indispensable components for a complete SC device.9-12 However, most of them are far from satisfactory, since comfortability of devices, productivity of fabrication processes, and feasibility of mass production, to name a few, are not taken into account. One of the key foundations to achieve an ideal wearable SC is to select or fabricate an appropriate substrate/current collector that is intrinsic flexible, readily accessible and easy to process. Table 1 shows the comparisons between the common used substrates for wearable SCs, in terms of their costs, densities, electrochemical stabilities, flexibility and textile compatibility. The traditional planar metal foil/meshes, such as stainless steel mesh, Ni foam and Ti mesh, were chosen to fabricate flexible SCs owing to its high conductivity and ready availability, which unavoidably brings excess weight and decreased comfortability, making them not favorable for wearable devices.13-17 Another group of substrate/current collector with one-dimensional appearance was also widely applied, namely, the conductive fiber/yarn, which could be woven or knitted into a two-dimensional fabric. Some of these special substrates/current collectors are commercial products, for example, stainless yarn, which is less desirable considering its extremely high density in wearable application.18-20 Nano carbon materials, including carbon nanotubes (CNT) yarns or films, graphene nanosheet (GN) films and paper/fabric immersed by nano-carbon, have also reported to be promising candidates for wearable SCs.21-25 However, high cost and complex processing are the two main challenging problems for the popularization of these materials, considering the fact that even the carbon paper costs $ 650 per square meter, not to mention CNT film ($ 12000 per square meter) and GN film ($ 3050 per square meter). Actually, the traditional cloths, commonly seen in daily life, should be another option with great potential, while taking account of their diversity and


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accessibility, let alone the outstanding comfortability and compatibility in wearable applications. There are a few studies constructed wearable SCs via the usage of normal cloths which are intrinsically conductive or non-conductive.26-31 Nevertheless, the conductive cloths are relatively expensive, for instance, carbon cloth costs $ 875 per square meter.32 Meanwhile, the nonconductive cloths are difficult to modify, or even success, the procedures and costs are somehow unsatisfied.33-37 For example, ion sputtering can make the cloth substrate conductive. But the requirements of special expensive equipment and its limited potential in fabricating large samples make this method only suitable for small-scale production in the laboratory. In fact, the only commercial conductive textile with low-cost is the one woven by silver coated yarn, but the poor electrochemical stability of silver makes it unsuitable for energy storage applications. Some polymer materials, saying ITO coated polyester (PET), seem to be conductive and bendable, but the poor textile compatibility and uncomfortable feeling make them far from satisfactory wearable SC substrates. Obviously, it is urgent to develop an appropriate substrate/current collector for wearable SCs via practical techniques that could tackle with a series of challenging problems. As well as the flexible substrate/current collector, electrode active materials should be intrinsically flexible at the same time, while involving in the design of wearable SCs. As one main group of electrode materials, conductive polymers are promising in high-performance wearable SCs in view of its pseudocapacitive performance and excellent flexibility when comparing to the other categories of active materials.38-42 But there are still some shortcomings of conductive polymers, such as unsatisfied electrical conductivity and low mechanical properties, leading to a capacitance lower than the theoretical value as well as unpromising stability under tensile loading,43,44 which limits their wide applications in wearable electronics.


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An effective method to solve the existing problems is to grow these conductive polymers on suitable substrates that could provide high electric conductivity and good mechanical strength simultaneously.30,45-47 Therefore, once again, a suitable substrate/current collector is crucial to the conductive polymer based wearable SCs. Herein, we applied the traditional gilding techniques to a PET cloth and successfully converted the cloth to be a wearable current collector at a reasonable cost. A high-performance wearable SC was then fabricated via the electrodeposition of a nanostructured conductive polymer, PPy nanorod, on the gold leaf gilding PET cloth. The pristine PET cloth was selected because it is readily accessible with low cost and widely used for clothing production. A historical fine art, gold leaf gilding, was applied to the non-conductive PET cloth endow the pristine cloth with a high electrical conductivity. Due to the strong interaction between the two components, gold leaf could be gilded on the cloth intimately, and thus ensured the successful fabrication of an integrated compact electrode after electrodeposition of PPy. Beyond the good mechanical performance, this gold leaf gilding cloth (GLGC) substrate/current collector is highly tailorable that can be easily bent, cut and sewn. As expected, the wearable SC based on these electrodes exhibits a series of distinctive practicability other than energy storage capability, such as safety, tailorability, and breathability. Moreover, multiple SC electrode modules could be fabricated on a same cloth by simply patterning the conductive gold leaf, allowing us to construct compact energy cloth that showed good compatibility to traditional textile. We believe our work would inspire other ingenious applications of the fine gold leaf gilding art in different wearable electronics. 2. Experimental section 2.1 Fabrication of gold leaf gilding cloth (GLGC) and integrated electrode


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The cloth was cleaned with type to remove the dust first, and then washed with acetone, ethanol and deionized water. After drying at 80 oC for 2 hours, the cloth was put on the gold leaf and being compressed for several seconds to ensure intimate contact of the two components. The PPy nanorods were electrodeposited on GLGC in a three-electrode configuration at 0 oC with a constant current of 0.8 mA cm-2 for 600 seconds. The electrodeposition solution contained 30 mL phosphor buffer solution (pH=6.8), 0.04M pTSA and 300 µL pyrrole monomer. GLGC, platinum mesh and saturated calomel electrode served as the working electrode, counter electrode and the reference electrode, respectively. 2.2 Preparation of solid-state supercapacitor The PVA/H2SO4 electrolyte was prepared by mixing 6 g H2SO4, 6 g PVA (M.W. = 100,000) and 60 mL deionized water together. The mixture was heated to 90 oC with continuous stirring for 15 minutes. A non-woven cloth was immersed in the electrolyte for 10 seconds and then put between the two electrodes acting as a separator to avoid short circuit. 2.3 Electrochemical measurement The electrochemical tests were carried out on an electrochemical work station (CHI760E). The electrochemical tests of single electrode were measured in a three-electrode configuration with a platinum mesh and a saturated calomel electrode serving as counter electrode and reference electrode. The electrochemical tests of SCs were conducted in a two-electrode system. All of the measurements are conducted at room temperature. 2.4 Characterization The pristine cloth, GLGC and integrated electrode samples was investigated by XRD using a Bruker D2 Phaser diffractometer with Cu Kα radiation (λ = 1.54 Å). XPS analyses were conducted on an ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific) at 1.2×


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10-9 mbar using Al Ka X-ray beam (1486.6eV). The XPS spectra were charge corrected to the adventitious C 1s peak at 284.6 eV. Raman spectra were measured with a multichannel modular triple Raman system with confocal microscopy at room temperature using 532 nm laser. The water contact angles were measured by a Micro Capture Pro coupled with Image J software. The microstructure and morphology of gold leaf and PPy nanorods were characterized by scanning electron microscope (SEM) (JEOL JSM-6355F) with an acceleration voltage of 15 kV. The static voltmeter R-4021 was used to measure the electronical resistance of the PET cloth. 3. Results and discussion As shown in Figure 1a, we have achieved a GLGC substrate that could effectively solve these challenging problems with a feasible scheme. Gold is selected for the traditional gilding technique because they are electrochemically very stable, which is essential for most electrochemistry based energy storage devices. In addition, although gold is considered as an expensive material, it is extremely ductile and possible to fabricate very thin gold leaves to dilute the cost ($ 168 per square meter). The gold leaves have been widely used for decoration and production of high-end alcohol and food. Non-conductive PET cloth was used as pristine substrate, offering high mechanical strength and outstanding flexibility. Because the cloth is made of PET fibers which carries negative charge, positive charged gold leaf could be tightly laminated on the pristine cloth by simply pressing due to their strong electrostatic interaction. The cost of PET cloth and GLGC substrate is ～$ 1.5 and $ 169.5 per square meter, respectively, which is much cheaper than common used substrates such as carbon cloth and CNT film (as shown in Table 1). Moreover, this simply gliding technique also significantly reduces the fabrication cost in large-scale production of conductive textile, more convenient than other modification methods (e.g. ion sputtering) with a low requirement on equipment. Thus, by using


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this fine gold leaf gilding art, we could fabricate a highly conductive flexible substrate for wearable SCs at a reasonable cost, which is comfortable and compatible with traditional textile industry. Cost Materials

$ Per square Density meter

Electrochemical Flexibility Stability

Textile compatibility

Stainless steel mesh

~9

High

Good

Bendable but not soft

Poor

Ni foam

~73

High

Good


Poor

Ti mesh

~190

High

Good


Poor

Stainless steel yarn/cloth

~160

High

Good

Soft

Good

Freestanding CNT film

~12000

Low

Good

Soft

Poor

Freestanding GN film

~3050

Low

Good

Soft

Poor

High (CNT or infiltrated by graphene used) Low nano-carbon

Good

Soft

Poor

Carbon paper

~650

Low

Good

Soft

Poor

Carbon cloth

~875

Low

Good

Soft

Decent

Paper/fabric

Silver coated yarn textile

~54

Low

Poor, not suitable for Soft electrochemical devices

ITO PET

~780

Low

Good


Poor

169.5

Low

Good

Soft

Good

coated

Gold leaves gilded cloth

Good


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(GLGC) (This work) Table 1. Comparison of different flexible current collector for wearable SCs. It should be noted that, beyond the PET cloth, we also tried to laminate the gold leaves on some other kinds of cloths as shown in Figure S-1a. PET cloth is the only one that could interact with gold leaf tightly and meanwhile fulfill the requirement of flexibility and comfortability (Figure S-1b). The obvious differences between different cloth substrates in interacting with gold leaf could be attributed to the existence of potential difference between PET cloth and gold leaf as proved in Figure S-1c, which will lead to the strong electrostatics interaction as demonstrated in supporting video 1. The static voltmeter R-4021 was used to measure the electronical resistance of the PET cloth, suggesting the resistance is nearly 1014 ohm. This would lead to a large tendency to carry charge and thus could easily attract the gold leaf as the demonstration (video 1). Other metal leaves, including the silver and copper ones, were also used to modify the PET cloth in order to further reduce the cost of gilding technique. However, gold is the only metal film that can be used in gilding technique. Because the metal film is attached to the cloth by electrostatic attraction, thus it should be thin, soft and light. Thanks to its great ductility, the thickness of laminated gold leaf could be made into about 0.1 µm (Figure S-2), which could stick to the cloth surface intimately (supporting video 1) and had very little influence on the total thickness of GLGC substrate, which was not achieved by other metal leaves. After the gilding process, gold leaf is firmly laminated on pristine PET cloth and exhibiting the specific diffraction peaks of Au, proved by the corresponding XRD analysis (Figure S-3a). Thus, a smooth and highly conductive planar substrate is formed (Figure 1b, c).


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Thanks to the outstanding conductivity and electrochemical stability of the GLGC, coral-like PPy nanorods with a length of 100-200 nm were easily electrodeposited on it (Figure 1d). To achieve better electrochemical performance, different concentration of p-toluenesulfonic acid (pTSA), a typical doping agent,44,48 and different deposition duration time were tried for the fabrication of PPy nanorods as shown in Figure S-4 and Table S1. The optimized pTSA concentration were 0.04M, while the deposition duration only changed the deposited amount of PPy. The follow-up work is all based on the above optimal process unless specified. After the successful electrodeposition of PPy, an integrated compact electrode for flexible SCs is finally realized. It’s noted that PPy nanorods are evenly grown on GLGC substrate and thus forming a continuous conductive network (Figure 1d), which is essential for the fast ion transportation and electrochemical reaction.49 In addition, the contact angle of GLGC decreased to 93.0o after PPy deposition which would further decrease after 10 cyclic voltammetry (CV) cycles, reaching a value of 59.7o as Figure S-3b shows. As a result, electrolyte can easily penetrate to the surface of integrated electrode, ensuring the potential of pseudocapacitive PPy nanorods can be fully developed. Obviously, the GLGC is the core subject of our design, which is targeting on wearable SCs. Thus, it should be endowed with both comfortability and mechanical stability. Indeed, thanks to the thin gold leaf (~ 0.1 µm, Figure S-2), this conductive GLGC maintained outstanding flexibility like pristine PET cloth after gilding process (Figure 1e, f). Moreover, it demonstrated excellent stability of repeated washing and bending due to the strong electrostatic force between gold leaf and PET cloth (Figure 1g-i). Even after up to 1000 bending cycles, gold leaf could still be firmly laminated on PET cloth, and the corresponding electrical resistances were almost


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unchanged as shown in Figure 1h and i. The above advantages make GLGC an ideal conductive substrate for a variety of wearable electronics besides SC application.

Figure 1. (a) Fabrication process of the integrated compact electrode. (b) SEM image of PET cloth. (c) SEM image of gold leaf laminated on PET cloth. (d) SEM image of carol-like PPy nanorods on GLGC. (e) Initial GLGC, (f) GLGC after bending, (g) GLGC after washing, (h) and


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(i) electrical resistance of GLGC before and after 1000 times bending. Inset in (h) and (i) SEM image of GLGC before and after 1000 times bending. As one typical conductive polymer, the electrochemical activity of PPy can be significantly affect by different doping agent and corresponding doping level.50 Based on our previous work, pTSA was chosen as a doping agent for PPy nanorods, the existence of which can be proved by the strong N 1s peak as indicated in the XPS spectra (Figure 2a).51,52 Figure 2b shows the C1s core level, which can be divided into five Gaussian peaks. The α carbons (C-N) and the β carbons (C-C) are located at 285.2 and 284.5 eV, respectively.53 The peak located at 286.2 eV and 288.1 eV stand for the C-OH and C=O,

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respectively, which are formed by the water

molecules attack since the polymerization occurred in the water environment.48 The N1s spectra is shown in Figure 2c, which can be divided into two Gaussian peaks. The peak located at 399.6 eV is the highest one, which corresponds to the neutral N on the pyrrole ring (-NH-).55 The low peak located at 397.8 is related to the basic structural defect.56, 57 Raman spectra (figure 2d) shows an obvious peak at 1586 cm-1, which corresponds to the backbone stretching mode of C=C bonds.46 The peak at 1390 cm-1 is assigned to the ring stretching mode, while the peaks at 940 cm-1 and 974 cm-1 represent the ring deformation of PPy. The peak at 1060 cm-1 and 1260 cm-1 are assigned to the symmetrical C-H in-plane bending and antisymmetrical C-H in-plane bending, respectively. The above analysis results suggest that those PPy nanorods as grew on GLGC were effectively doped with pTSA and would present excellent capacitive performance in line with previous study.


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Figure 2. (a) Full spectra of PPy based GLGC electrode. (b) and (c) High resolution spectra of C and N. (d) Raman spectra of PPy based GLGC electrode. The electrochemical performance of integrated compact electrode based on our flexible GLGC substrate was firstly evaluated by a three-electrode system. Because the as-synthesized PPy was N-doped and has a large tendency to interact with hydrogen ions, the acidic H2SO4 electrolyte was chosen here, which provided a better ion conductivity than neutral Na2SO4 electrolyte, leading to better capacitor performance (Figure S-5). As shown in Figure S-6a, the capacitance should be mainly attributed to PPy nanorods instead of GLGC substrate. Because of the highly conductive gold leaf, CV curves of the integrated electrode presented a quasi-


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rectangular shape at a wide range of scan rates (Figure 3a), from 5 mV s-1 to 200 mV s-1, suggesting the fast charge propagation. The symmetric galvanostatic charge-discharge (GCD) curves also indicated a fast electrochemical reaction of the integrated electrode (Figure 3b), while reaching a high capacitance of 43.43 mF cm-2 at a current density of 1 mA cm-2. Such good performance should be attributed to the formation of effective conductive network between GLGC substrate and PPy nanorods as observed in the corresponding SEM images. This conductive network can be further convinced by the electrical impedance spectroscopy (EIS). As shown in Figure S-6b, the charge transfer resistance of GLGC slightly increased from 1.61 Ω to 3.39 Ω after the electrodeposition of PPy nanorods, resulted from the synergistic effect of GLGC substrate and effective doping effect of pTSA, suggesting an existence of conductive network in the integrated compact electrode, which is helpful for the fast ion transportation during charging/discharging. Poor cycling stability is a tricky problem for PPy because of the repeated swelling and shrinking of backbone during charging/discharging process that would eventually cause structural pulverization. Gold leaf is a highly conductive metal that possesses excellent flexibility and ductility. Thus, it could act as a conductive buffer layer to alleviate the structural deterioration of PPy during repeated charging/discharging. In fact, that is the reason why our integrated electrode exhibited outstanding cycling stability, retaining 98.7 % of its original capacitance after 10000 charging/discharging cycles (Figure 3c). Considering its good capacitive performance and great flexibility, this PPy-electrodeposited GLGC is an ideal electrode for wearable SCs. As expected, solid-state SC constructed by the integrated electrode exhibits high capacitive performance and excellent cycling stability. As shown in Figure 3d, CV curves remain quasi-rectangular shapes at a variety of scan rates, indicating the high reversibility. Meanwhile, the GCD curves presented symmetric quasi-


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triangular shapes (Figure 3e), also suggesting the highly reversible electrochemical reaction. The areal capacitance reaches a high value of 17.14 mF cm-2 at 0.2 mA cm-2, which is largely attributed to the PPy nanorods (Figure S-7). With such a good capacitive performance, this symmetric wearable SC could deliver a volumetric energy density of 161.6 kW m-3 with a power density of 13.5 kW m-3 at the current density of 1 mA cm-2. Owing to the formation of conductive network in integrated electrode, the solid-state SC has favorable rate performance, which holds 57.7 % of its original capacitance with a high scan rate of 200 mV s-1; similarly, 76.6% of the original capacitance can be maintained with a current density of 1.0 mA cm-2 (Figure S-8). In addition, alike single integrated electrode, the solid-state SC shows excellent cycling stability, retaining 99.0 % of its original capacitance over 3000 charging/discharging cycles as shown in Figure 3f. This stable cyclic performance is derived from the effective conductive network between gold leaf and PPy, which is confirmed by the EIS curves before and after cycling test, since the charge resistance only increased by 7.76 Ω (Figure S-9).


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Figure 3. (a) CV curves; (b) GCD curves and (c) cyclic performance of the integrated compact GLGC electrode in 1 M H2SO4 solution electrolyte; (d) CV curves, (e) GCD curves and (f) cyclic performance of solid-state SC. As a wearable device, it is important to ensure its comfortability and safety in the first place.4 The solid-state SC based on our GLGC demonstrate excellent flexibility that can be bent or rolled up while maintaining most of its capacitance, as Figure 4a shows. Such flexibility can ensure our SC properly fitting different shapes in different usage scenarios that brings desired comfortability. Besides, for wearable SCs, safety issue is another important problem relates to practicability. Because our SC was sandwiched with a non-woven cloth as separator, it could effectively avoid the short circuit even after serious damage. As shown in Figure 4b, even after multiple cuttings, the SC worked well and remained most of its capacitance, indicating its


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excellent safety performance. Though the enclosed area of CV curve decreased with a small proportion, due to a loss of active material, it would still preserve the quasi-rectangular shape, suggesting the electrochemical reaction was not affected by such serious damage (Figure 4b). In virtue of the above flexibility and safety, we could develop a breathable SC by punching a number of through-holes that would endow our wearable device with enhanced comfortability. As vividly demonstrated in the supporting video 2, distinctive air permeability is realized in our breathable SC. These small holes with a diameter of about 60 µm (inset in Figure 4c) were punctured by needles, which would not affect the loaded active material significantly. Therefore, CV curve could maintain a quasi-rectangular shape and corresponding capacitance would remain nearly unchanged (Figure 4c).

Figure 4. (a) Photo of a bending SC and corresponding CV curves. (b) Photo of a SC before and after cutting and corresponding CV curves. (c) Photo of a breathable SC, schematic illustration and corresponding CV curves. Inset is the SEM image of through-hole.


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Narrow working potential window and relatively low energy density are two typical problems that wearable SCs will confront in practical applications. In order to achieve the intended voltage and current need, one feasible and universal solution is to connect these SCs in series or parallel.10 However, most of these connected modules can’t perfectly meet the requirement of wearability, let alone comfortability, because there are only loosely connections between them, especially considering they are usually connected by some thin wires on different planes,58 which will break apart easily in daily activities. As mentioned at the beginning, the gold leaf offers the conductivity, whereas the pristine PET cloth is non-conductive. This means we could easily control the conductive area and its shape on GLGC by simply patterning gold leaf, which is not so easily achieved by other metallization method. Thereafter, PPy nanorods, the active material, can be easily electrodeposited on targeted area with desired shape. Figure 5a shows three PPy modules electrodeposited on the same piece of PET cloth that was laminated with three separated columns of gold leaf. The PPy nanorods were directly grown on the gold leaf surface forming a clear boundary on the patterned GLGC as intended (inset in Figure 5a). Afterwards, another identical GLGC with three PPy modules were attached on the preceding one, while sandwiching them together with a separator immersed with gel electrolyte. Thus, three solid-state SCs were constructed on the same plane with a compact structure that would not loose easily after different manipulations (Figure 5a, b). By changing the connection methods of lead wires, different power need can be easily met. For example, by connecting these SC modules in series, a high voltage of 2.1 V was achieved, which could power a digital watch continually as shown in Figure 5c, d. Or, by connecting them in parallel, the capacitance would be trebled (Figure 5e). Owing to the great flexibility, good comfortability, and compact design, these planar SCs, or more exactly, the energy cloth, showed high compatibility to traditional textile, which could be readily sewed on a


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lab coat and even replace part of it, as Figure 5f shows. Of course, this integrated compact energy cloth containing 6 SC modules could power a digital watch without effort (Figure 5f). Obviously, our GLGC based SC shows great potential in the wearable energy storage device that possesses high practicability, for instance, safety and breathability as demonstrated previously.

Figure 5. (a) Three PPy electrode modules on one piece of PET cloth, inset is the SEM image of PPy/cloth boundary. (b) Assembly of three SC modules on one plane. (c) Three SCs connected in series and lighted a digital watch. (d) GCD curves of a single SC and three in series. (e) GCD curves of a single SC and three in parallel. (f) Compact energy cloth containing six SCs was sewed in a lab coat and lighted a digital watch. 4. Conclusion In conclusion, through a modified traditional gilding technique, gold leaf was laminated tightly on the PET cloth thanks to the strong electrostatic interaction between themselves, and thus obtaining a GLGC with high electrical conductivity and compatibility with traditional textile industry. Due to the application of PET cloth, this GLGC substrate is equipped with outstanding


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flexibility, stable electrochemical property and good comfortability, together with a reasonable cost, working excellently as a flexible current collector for wearable energy storage devices. After the electrodeposition of PPy nanorods, this GLGC based solid-state SC provides a high capacitance of 17.14 mF cm-2 and excellent cycling stability. Moreover, the as-assembled flexible SC exhibited enhanced practicability, such as safety and breathability, thanks to the application of GLGC and solid-state design. By patterning gold leaf on the cloth substrate, we could fabricate separated electrodes on a targeted area and desired shape, which allowed us to construct multiple SC modules on the same plane with compact structure. The integrated compact energy cloth containing multiple SC modules showed good compatibility to traditional textile that could be readily sewed on an optional position and act as a reliable power source. This work offers a new strategy to fabricate flexible and conductive substrate for wearable energy storage devices, which is easily fabricated, of low cost and with excellent comfortability, providing a possible solution for the production of other wearable electronics. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional digital photos, SEM images, XRD, contact angle, CV curves and EIS spectra of different samples and in different electrolytes, capacitance-scan rate and capacitance-current density curves. (PDF) Video of the interaction between the PET cloth and the gold leaf. Video of the breathable supercapacitor lighting digital watch. (WMV) AUTHOR INFORMATION


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Corresponding Author *(Y.H.) E-mail: [email protected] *(S.D.) E-mail: [email protected] *(C.Z.) E-mail: [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was also sponsored by the project 2017JY0088 supported by Science & Technology Department of Sichuan Province. This work was also supported by Natural Science Foundation of Shenzhen University (Grant No. 2017004) and a Grant from the City University of Hong Kong. The authors would like to thank Dr. WEI Wei in the School of Materials Science and Engineering of Southwest Jiaotong University for his valuable advice on writing this article. ABBREVIATIONS SC, supercapacitor; GLGC, gold leaf gliding cloth; PPy, polypyrrole; CNT, carbon nanotube; GN, graphene nanosheet; ITO, indium tin oxide; PET, polyester; pTSA, p-toluenesulfonic acid; PVA, polyvinyl alcohol. REFERENCES


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A Wearable Supercapacitor Engaging with Gold Leaf Gilding Cloth

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