Hierarchical NiCo2S4@NickelâCobalt Layered Double Hydroxide

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Surfaces, Interfaces, and Applications

Hierarchical NiCo2S4@Nickel-Cobalt Layered Double Hydroxide Nanotube Arrays on Metallic Cotton Yarns for Flexible Supercapacitors Yi-Fan Wang, Hai-Tao Wang, Shi-Yi Yang, Yuan Yue, and Shaowei Bian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06317 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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ACS Applied Materials & Interfaces

Hierarchical NiCo2S4@Nickel-Cobalt Layered Double Hydroxide Nanotube Arrays on Metallic Cotton Yarns for Flexible Supercapacitors Yi-Fan Wang, Hai-Tao Wang, Shi-Yi Yang, Yuan Yue and Shao-Wei Bian* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, PR China *E-mail: [email protected] Abstract: Constructing high capacitance active materials and 3D conductive network inside textile yarn frames is a promising strategy to synthesize yarn supercapacitor electrodes. In this study, growing NiCo2S4@Ni-Co layered double hydroxide nanotube arrays on Au metalized cotton yarns yields a novel yarn supercapacitor electrode material. The resulting yarn electrode possesses mumerous merits, including high electrical conductivity from NiCo2S4 and Au metalized cotton yarns, high capacitance of Ni-Co layered double hydroxide nanosheets and 3D hierarchical electrode structure. The unique electrode structure leads to excellent electrochemical properties including high capacitance (5680 mF cm-2), excellent rate performance and stable cycling performance. A two-ply symmetric yarn supercapacitor assembled from the NiCo2S4/Ni-Co layered double hydroxide/Au/cotton yarn electrode reaches an areal energy density of 3.5 μWh cm-2. KEYWORDS: supercapacitor, NiCo2S4, yarn, fiber, layered double hydroxide

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1. Introduction The significant advancement in science and technology promotes various novel wearable electronics in our daily life in recent years.1-2 Integrating these devices into the clothing structure facilitates to perform their functions. However, it puts much higher requests on the energy storage devices.2-4 Yarn supercapacitors (also called fiber supercapacitors) have aroused extensive attention because they have the potential to greatly enhance the capacitance density and mechanical flexibility. Meanwhile, they have the potential to be integrated or woven into diverse shaped structure through the matured textile technology. These advantages are very important to the development of wearable electronics.5-9 To fill the requirement of practical application, yarn supercapacitors should have high charge storage capability, excellent mechanical flexibility and large-scale production.2, 4 Combining the flexible current collectors and high capacitance active materials is considered as the best way to achieve the above goals. In recent years, metal and carbon-based yarn (fiber) supercapacitors have been widely developed.10-12 However, the nonporous structure and high bulk density of metal fibers severely limit the ion diffusion path and active material loading, and decay the energy density. The compact structure of carbon yarns results in an ineffective electrolyte diffusion and low accessible active surface area. As a result, the low electrochemical performance, high cost as well as complicated preparation process of such metal and carbon yarn-based supercapacitors can not satisfy the practical purposes.12 Therefore, developing novel yarn electrodes is still a great challenge.


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Common textile yarns are widely used in fabric weaving. They are considered as the reasonable basic materials to design and fabricate yarn electrodes due to their outstanding merits including excellent mechanical flexibility, lightweight, low cost, scalable production and hierarchical porous structure.2, 13 Coating the fiber surface with the reduced graphene oxide/multiple-wall carbon nanotube layers formed a composite yarn electrode, which showed aspecific capacitance of 1.25 F g-1.14 A polypyrrolecotton yarn electrode was prepared by in situ polymerizing polypyrrole nanotubes on cotton yarns, achieving a capacitance of 74.0 mF cm-2.15 Growing polyaniline nanowire arrays on the cotton/graphene yarns formed a yarn electrode, which delivered a high capacitive property of 246 mF cm-2.4 Although these abovementioned textile yarn electrodes exhibited a lot of promising properties, however, they still face some key issues which block their electrochemical performance, such as the unsatisfied electric conductivity, low active surface area and ineffective electrolyte diffusion.16-18 To address these challenges abovementioned, reasonably designing 3D highly conductive network and hierarchical hybrid structure of textile-based yarn electrodes is the most promising way to greatly boost the electrochemical performance.9,

19

In

comparison with carbon materials (carbon nanotubes and reduced graphene oxide sheets) and conducting polymers, constructing a metallic layer on the textile fiber surfaces, forming a 3D highly conductive network, can combine the advantages of both metal layers and textile yarns. The resulting composite yarns as a flexible current collector can exhibit high electrical conductivity, lightweight and hierarchical porous structure.9



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Directly loading active materials on yarn current collectors facilitates to increase the capacitance. However, the inappropriate active material/current collector structure results in ineffective electrolyte diffusion, low active surface area and high internal resistance,

limiting

the

electrochemical

performance.20

Recently,

novel

heterostructured core-shell array electrode materials with a number of advantages including the enlarged active area, effective electrolyte diffusion channel, and significant synergistic effect is gaining intensive attention.19,

21-25

For example,

hierarchical NiCo2S4@Ni3V2O8 hybrids were grown on Ni foam. The specific capacitance of the resulted composite was 512 C g-1.22 Recently, a novel electrode was prepared by growing hierarchical NiCo2S4@CoSx core/shell nanotube arrays on Ni foam, which delivered an areal capacitance of 4.74 F cm-2.21 Based on the above work, it is believed that the core-shell structured NiCo2S4-based materials are an excellent candidate for yarn supercapacitor electrodes. Inspired by these above ideas, we successfully constructed a flexible 3D metallic conductive network inside cotton yarns by coating the cotton fibers with a compact and thin Au layer. The utilization of active materials and redox reaction kinetics could be significantly enhanced owing to the help from rapid electron transport. In this study, 3D core-shell structured NiCo2S4@Ni-Co layered double hydroxide (NiCo2S4@Ni-Co LDH) arrays were grown on the fiber surfaces inside the Au/cotton yarn (ACY) using a combined method of hydrothermal synthesis and electrodepositon. NiCo2S4 nanotubes not only possess high capacitance, but also serve as an ideal conductive skeleton for effective loading active materials. The resulting NiCo2S4@Ni-Co layered


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double hydroxide/Au/cotton (NiCo2S4@Ni-Co LDH/ACY) electrode showed superior electrochemical performance including high areal capacitance, stale cycling performance and superior mechanical flexibility. In addition, we assembled a two-ply structured yarn supercapacitor from the yarn electrode, which exhibits high energy density. These avbove electrochemical properties endow it with a promising future in flexible energy storage devices.

2. Experimental section 2.1 Chemicals The commercical cotton yarns were available in the local market. Chloroauric acid (HAuCl4·4H2O), urea, nickel chloride hexahydrate (NiCl2·6H2O), sodium hydroxide (NaOH), potassium hydroxide (KOH), trisodium citrate dehydrate (Na3C6H5O7·2H2O) and cobalt ( Ⅱ ) chloride hexahydrate (CoCl2·6H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl alcohol (PVA), used for gel eyectrolyte preparation, was a Sigma-Aldrich product. Ethanol and sodium sulfide nonahydrate (Na2S·9H2O) were provided by Shanghai Lingfeng Chemical reagent Co., Ltd. Dopamine hydrochloride (DA) was bought from Adamas Reagent Co., Ltd. 3aminopropyltrimethoxysilane was a commercial product from Alfa Aesar. All chemicals and regents were directly used in the material synthesis and characterization. 2.2 Synthesis of NiCo2S4 nanotube arrays on ACYs The typical synthesis process of grow ing Ni-Co precursor nanowire arrays on ACYs was carried out as follows. The ACY was firstly prepared using the electroless



deposition method.26 60 mL of deionized (DI) water containing 1.5×10-3 mol of NiCl2, 3×10-3 mol of CoCl2, and 5×10-3 mol of urea was stirred for 30 min. It was transferred to 100 mL Teflon-lined stainless steel autoclave. After immersing a 10 cm ACY into the above solution, the sealed autoclave was heated at 120 °C for 8 h. The resulting composite yarn was taken out of the autoclave, alternately washed with DI water and ethanol for three times. After that, the composite yarn was treated again in the sealed autoclave containing 60 mL of 2.08×10-2 mol/L Na2S solution at 180 °C for 6 h.27-28 The resulting NiCo2S4/ACY was cleaned by washing it thoroughly with DI water and ethanol for three times. The resulting yarn electrode had a NiCo2S4 mass loading of 5.6 mg cm-2. 2.3 Synthesis of hierarchical NiCo2S4@Ni-Co LDH /ACY electrodes The NiCo2S4 nanotube arrays on the fiber surface inside ACYs were used as the conductive skeleton for constructing Ni-Co LDH nanosheets. A typical three-electrode system was consisted of Pt wire as a counter electrode, Ag/AgCl electrode as a reference electrode and NiCo2S4/ACY electrode as a working electrode.29 In 20 mL of aqueous solution containing CoCl2 and NiCl2 wth the same concentration of 50×10-3 mol/L, a potential static technique at -1.0 V was used to electrodeposit Ni-Co LDH nanosheets at room temperature. After 10 min, the composite yarn electrode was rinsed three times with DI water and ethanol, and vacuum dried overnight. The mass loading of Ni-Co LDH nanosheets was determined to be 0.5 mg cm-2. 2.4 Supercapacitor assembly The gel electrolyte, PVA/KOH, was prepared according to our previous work, which


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was used as both the electrolyte and separator.26 Two NiCo2S4@Ni-Co LDH/ACY electrodes were immersed in the PVA/KOH electrolyte for 5 min and then allowed to dry at room temperature. After that, two yan electrodes were assembled to a two-ply yarn supercapacitor by carefully twisting them together with the gel electrolyte. The all-solid-state symmetric yarn supercapacitor (ASYS) was encapsulated in a heat shrinkable tube. The excess water in the PVA/KOHelectrolyte was removed using a hot-air blower. 2.5 Materials characterization The electrical resistance, chemical state of electrode surface, crystalline structure and morphology of yarn electrodes were characterized by UT 58 digital multimeter, X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi), powder Xray diffractometer (XRD, Rigaku D/Max-2550PC), transmission electron microscopy (TEM, JEOL 2100F, supplied with an energy-dispersive X-ray spectroscopy detector, EDS) and scanning electron microscopy (SEM, Hitachi S-4800). 2.6 Electrochemical characterization A three-electrode testing system was assembled to evaluate the electrochemical properties of NiCo2S4@Ni-Co LDH/ACY electrode, where the NiCo2S4@Ni-Co LDH/ACY electrode, Pt wire and saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. 1 mol/L KOH solution was employed as the electrolyte. The electrochemical measurements of ASYSs were performed using a two-electrode testing system.30 A CHI 660E electrochemical workstation was used to conduct all electrochemical measurements.



The electrochemical properties were calculated from the reported methods in the earlier literature.30

3. Results and discussion 3.1 Characterization of yarn electrodes Figure 1 depicts the NiCo2S4@Ni-Co LDH/ACY electrode preparation process. Coating the NH2- modified cotton fibers with a thin and uniform Au nanoparticle layer could form the flexible ACY with high electrical conductivity (1.3 Ω cm-1). After hydrothermally treating the ACY in the presence of urea, NiCl2 and CoCl2, the fiber surface was coated with a 3D Ni-Co precursor nanowire array layer, which was then transformed to the NiCo2S4 nanotube array layer by anion-exchanging with S2-.27-28 The NiCo2S4 nanotube arrays on the fiber surfaces were further served as the conductive skeleton for the electrodeposition of layered double hydroxides. These thin Ni-Co LDH nanosheets could provide abundant redox active sites and short electrolyte diffusion distance, which would have a positive impact on the capacitance. The NiCo2S4@Ni-Co LDH/ACY electrode could exhibit excellent electrochemical porperties in virtue of its unique electrode structure characteristics.


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Figure 1. Schematic of the NiCo2S4@Ni-Co LDH/ACY electrode preparation process.

Powder XRD analysis is widely used to characterize the crystalline structure. As shown in Figure 2a, several strong peaks were observed at around 44.4°, 64.5°, 77.5° and 81.7°, which are assigned to (111), (200), (220), (311) and (222) lattice plane of metallic Au layers (JCPDS No. 04-0784) on the cotton fiber surfaces.31 However, the low mass loadings of NiCo2S4 nanotubes and Ni-Co LDH nanosheets is beyond the XRD detection limit, which caused the characteristic diffraction peaks to be invisible. The NiCo2S4 nanotube arrays were further justified by the EDS spectrum. The strong peaks in Figure 2b indicate the existence of Ni, Co and S elements with an atom ratio of 1.3:2.1:4.5, which is very approach to the elemental composition of NiCo2S4 (1:2:4).



Figure 2. (a) Powder XRD patterns of the yans including ACY, NiCo2S4/ACY and NiCo2S4@Ni-Co LDH/ACY. (b) EDS spectrum of the NiCo2S4 nanotube arrays.

SEM and TEM were employed to characterize the morphology of all yarn electrodes (Figure 3 and S1). Lots of fiber bundles inside the cotton yarn was clearly observed in Figure 3a. The cotton fiber surfaces are wrinkled and rough (Figure 3b, c). The low electric conductivity of cotton yarns (~108 Ω cm-1) severely blocks their application as flexible current collectors in the electrodes.32 Nevertheless, numerous spaces among the adjacent fibers and fiber surfaces could serve as a reservoir for loading conductive and high capacitance materials. Figure 3d, e shows that the cotton yarns kept their initial state after the Au nanoparticle layer coating. No broken fibers were observed, indicating the initial mechanical flexibility is well retained. The high-magnification SEM image (Figure 3f) clearly shows a compact and uniform Au nanoparticle layer on the fiber surface, which offers high electrical conductivity (1.3 Ω cm-1), outperforming those previously reported textile yarn-based collectors.26,

32

The highly conductive ACY

favors the electron transportation, significantly promoting the utilization of active materials and electrochemical reaction kinetics. Figure 3g-i shows that the NiCo2S4 nanotubes with an average length of 2.7 μm were vertically aligned on the fiber surfaces. The inset in Figure 3i further discloses their hollow nanotubular structure. The average diameter and wall thickness are 80 and 18 nm, respectively. The hollow nanotube scaffold can provide more sites for loading active materials than the solid nanorod scaffold, which can significantly enhance the capacitance. Figure 3j-m displays the Ni-


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Co LDH nanosheets on the outer surface of NiCo2S4 nanotubes. These nanosheets with 36 nm thickness facilitate to greatly enhance the capacitance because they can expose more redox active sites. Figure 3m, n shows the TEM image and high-resolution TEM image of a NiCo2S4@Ni-Co LDH nantube, respectively. The calculated lattice spacing from the inset in Figure 3n is around 0.21 nm, which is in accordance to the (107) lattice plane of Ni-Co LDH.33 The circle shaped SAED pattern further reveals the polycrystalline nature of these nanosheets (Figure 3o).



Figure 3. SEM images of the yarn materials: (a-c) cotton yarn, (d-f) ACY, (g-i) NiCo2S4/ACY and (j-l) NiCo2S4@Ni-Co LDH/ACY. (m) TEM image and (o) SAED pattern of a detached NiCo2S4@Co-Ni nanotube. (inset in i) TEM image of NiCo2S4 nanotubes. (n and its inset) high-resolution TEM images of the marked red rectangular area (m).

The surface chemical state of NiCo2S4@Ni-Co LDH/ACY electrode was analyzed by XPS surface analysis technique. The strong peaks observed in survey spectrum (Figure 4a) reveals the Ni, Co, S, O and Au elements. This result is in accordance with the element composition of yarn electrode. With regard to the high-resolution XPS spectrum of Ni 2p, two spin-orbit doublets as well as two shakeup satellites (indicated as ‘‘Sat.”) were observed in Figure 4b. The peaks at 855.2 eV and 872.8 eV are indexed to the spin-orbit doublet of Ni3+, while the peaks at 853.4 eV and 872.8 eV are ascribed to the spin-orbit doublet of Ni2+.34 Figure 4c also shows two doublet, the first one at 780.8 eV and 795.9 eV, while the second one at 779.4 eV and 794.3 eV are related to Co2+ and Co3+, respectively.35-36 Figure 4d shows two peaks with binding energies of 162.0 eV and 163.4 eV and one shakeup satellite peak with a binding energy of 168.5 eV. The S 2p3/2 peak (162.0 eV) relates to the low-coordinated sulfur ion, while the S 2p1/2 peak (163.4 eV) results from the metal-sulfur bond.37


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Figure 4. (a) XPS survey spectrum of the NiCo2S4@Ni-Co LDH/ACY electrode. Highresolution XPS spectra: (b) Ni 2p, (c) Co 2p and (d) S 2p.

3.2 Electrochemical properties of the yarn electrodes The NiCo2S4@Ni-Co LDH/ACY electrode were tested to evaluate its electrochemical properties, which determine its potential in suprecapacitors. The typical CV curves of NiCo2S4@Ni-Co LDH/ACY, NiCo2S4/ACY, and Ni-Co LDH/ACY electrodes were compared in Figure 5a. All cyclic voltammetry (CV) curves contained two pairs of redox peaks, revealing the reversible Faradaic reactions.38-39 Compared to the NiCo2S4/ACY and Ni-Co LDH/ACY electrodes, the significant synergistic effect resulting from NiCo2S4 nanotubes and Ni-Co LDH nanosheets significantly enlarged the CV curve area of NiCo2S4@Ni-Co LDH/ACY electrode, which is further verified from the galvanostatic charge/discharge (GCD) measurements (Figure 5b).40-41 The NiCo2S4 nanotube arrays as a conductive skeleton could provide more sites for growing



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active electrode mateirals and induce additional capacitance.19 The ultrathin Ni-Co LDH nanosheets significantly enlarged the active material/electrolyte interface.42-43 Meanwhile, in comparation with the NiCo2S4/ACY and Ni-Co LDH/ACY electrodes (Figure 3g-i and s1), the hierarchical NiCo2S4@Ni-Co LDH nanotube array structure with abundant pores and spaces also created an effective diffusion channels for the electrolytes.43 As shown in Figure 5c, all CV curves exhibited two pairs of redox peaks within the testing range, which result from the redox processes of Ni2+/Ni3+ and Co2+/Co3+ and the redox couples of Ni2+/Ni3+ and Co3+/Co4+, respectively.19,

26

The

charge diffusion polarization within the electrode materials at high scan rates caused a potential shift phenomenon.44 Figure 5d displays the GCD curves at a seriers of current densities. At an areal current density of 2 mA cm-2, the electrode delivered 5680 mF cm-2 (Figure 5e). The capacitance value presented here is superior to those reported in the previous literatures (Table S1), such as the cotton/graphene/polyaniline yarn electrode (246 mF cm-2) and the polypyrrole-cotton yarn electrode (74.0 mF cm-2).4, 15 Meanwhile, an excellent rate performance was observed due to the rapid electron transportion and electrolyte ion diffusion. The yarn could remained 87 % of initial capacitance even at the largest testing current density (20 mA cm-2). In this study, the electrical impedance spectroscopy (EIS) measurements were conducted and the Nyquist plot is depicted in Figure S2. The typical Nyquist plots are composed of a semicircle and a linear component, which appear separately in the high- and lowfrequency ranges. They are related to the charge transfer resistance and the diffusive resistance. In comparation with the Co-Ni LDH/ACY and NiCo2S4/ACY electrodes,


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the NiCo2S4@Co-Ni LDH/ACY electrode exhibited a larger curve slope, indicating its superior capacitive nature and highly effective electrolyte ion diffusion. The semicircle reprents the internal resistance (Rs), which can be measured from the x axis intercept of the plots. Compared to the Co-Ni LDH/ACY (3.4 Ω) and NiCo2S4/ACY (3.9 Ω) electrodes, the 3D core-shell structured NiCo2S4@Ni-Co LDH arrays on the fiber surfaces inside the ACY only resulted in a slightly higher Rs (5.5 Ω). Since the cycling performance directly influences the supercapacitor applications, the NiCo2S4@Ni-Co LDH/ACY electrode was repeatedly charged and discharged at 20 mA cm−2. Figure 5f exhibits the gradual decrease in capacitances with the cycles. After 2000 charge/discharge cycles, a relatively high capacitance retention of 79 % was still achieved. The electrochemical measurements were also conducted under the mechanical bending condition (Figure 5g, h). The integrated area and shape CV curves did not vary obviously during 250 binding cycles, and the same is true for capacitance values, suggesting the good mechanical flexibility and strong contact between various interfaces inside the yarn electrode.



Figure 5. Electrochemical characterization of yarn electrodes: (a) CV curves at 5 mV s-1, (b) GCD curves at 4 mA cm-2 and (e) areal capacitances at different areal current densities. (c) CV curves, (d) GCD curves and (f) cycling performance of the NiCo2S4@Ni-Co LDH/ACY electrode. (g) CV curves at 5 mV s-1 and (h) corresponding capacitance retentions of the NiCo2S4@Ni-Co LDH/ACY electrode during 250 bending cycles.

3.3 Electrochemical Characterization of yarn supercapacitors A two-ply yarn supercapacitors made by our developed yarn electrodes was tested to further certify their potential application. No obvious distortion was found in the CV


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curve shape within a broad scan rate range (Figure 6a), demonstrating a good rate capacity, and rapid electrolyte and electron transports within the yarn electrode material. Figure 6b displays the GCD curves that were recorded at a series of current densities. The areal capacitance of 37.2 mF cm-2 was achieved at 0.4 mA cm-2, outperforming those recently reported metal- and carbon-based yarn supercapacitors, for instance, the RGO-GO-rGO suprecapacitor (0.45 mF cm-2), Ni wire/graphitic pen ink supercapacitor (19.5 mF cm-2), NiO/MW//CNF/MW fiber supercapacitor (12.5 mF cm-2) and GF@PEDOT supercapacitor (15.39 mF cm-2).45-48 The EIS tests were conducted and the corresponding Nyquist plots are shown in Figure 6c and its inset. A close observation of the inset in Figure 6c reveals the presence of a depressed semicircle and a high phase angle, reprenting the rapid ion transfer during the charge-discharge process. Figure S3 shows a simple equivalent circuit model to evaluate this assembled ASYS. The semicircle diameter represents the charge transfer resistance (Rct). The Rs and Rct were calculated to be 6.96 Ω and 109.7 Ω, respectively. The assembled supercapacitor was repeatedly charged and discharged to evaluate its long-term cycling stability. Figure 6d shows the capacitance varitions during the testing process. The capacitance retention only decreased by 9 % after the 3000th cycle, demonstrating the stable cycling performance. The mechanical bending effect on the electrochemical performance was evaluated at 5 mV s-1 (Figure 6e). These almost overlapped CV curves indicate the excellent flexibility and stable electrode structure of the assembled ASYS. The Ragone plot presented in Figure 6f discloses the highest areal energy density, which reached 3.6 μWh cm-2 at an areal power density of 163.2 μW cm-2, outperforming those earlier



reported yarn supercapacitors, such as the CNTs/Co3O4 fiber supercapacitor (1.1 μWh cm-2) and SWCNTs/PANI yarn supercapacitor (0.8 μWh cm-2). 49-50

Figure 6. Electrochemical characterization of the assembled ASYS: (a) CV curves, (b) GCD curves, (c and its inset) Nyquist plots, (d) cycling performance, (e) CV curves at different bending cycles and (f) Ragone plot.

The tow-ply ASYS was furher used as a flexible electrochemical energy storage device to power some small electronic devices (Figure 7). As shown in Figure 7a, the tandem ASYSs could light up a red light-emitting diode (LED). They could also power a calculator for the simple data calculation (Figure 7b). These above results further reveal that the NiCo2S4@Ni-Co LDH/ACY electrode has great potential application for


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supercapacitors.

Figure 7. Digital photograph of small electronics with the tandem ASYSs as the power source: (a) a red LED and (b) a small calculator.

4. Conclusions A novel textile yarn-based electrode for flexible supercapacitors was developed by growing hierarchical NiCo2S4@Ni-Co LDH nanotube arrays on the fiber surfaces inside the ACYs. In its unique electrode structure, the NiCo2S4 nanotube arrays play dual roles: a conductive scaffold for loading Ni-Co LDH nanosheets with high active surface area and an excellent active material for additional capacitance. The 3D conductive network fabricated by Au layers and NiCo2S4 nanotube arrays are capable of fast electron transportation. Moreover, the electrolyte diffusion path is shortened because of the hierarchically porous electrode structure. The NiCo2S4@Ni-Co LDH/ACY electrode exhibited superior electrochemical properties including high capacitance, good rate capacity and stable cycling performance. What is more, a high energy density was achieved by the as-assembled yarn supercapacitor. This work may provide a promising alternative strategy to develop excellent yarn supercapacitor electrodes.



Supporting Information Available: The additonal SEM images and Nyquist plots. The equivalent circuit of the assembled ASYS.

Acknowledgement: This work was financially supported by the National Natural Science Foundation of China (51402048), DHU Distinguished Young Professor Program, the Fundamental Research Funds for the Central Universities and the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University. Reference (1) Liu, Y.-N.; Zhang, J.-N.; Wang, H.-T.; Kang, X.-H.; Bian, S.-W. Boosting the Electrochemical Performance of Carbon Cloth Negative Electrodes by Constructing Hierarchically Porous Nitrogen-Doped Carbon Nanofiber Layers for All-Solid-State Asymmetric Supercapacitors. Mater. Chem. Front. 2019, 3, 25-31. (2) Zhi, J.; Reiser, O.; Wang, Y.; Hu, A. From Natural Cotton Thread to Sewable Energy Dense Supercapacitors. Nanoscale 2017, 9, 6406-6416. (3) Lee, S.-S.; Choi, K.-H.; Kim, S.-H.; Lee, S.-Y. Wearable Supercapacitors Printed on Garments. Adv. Funct. Mater. 2018, 28, 1705571. (4) Jin, C.; Wang, H.-T.; Liu, Y.-N.; Kang, X.-H.; Liu, P.; Zhang, J.-N.; Jin, L.-N.; Bian, S.-W.; Zhu, Q. High-Performance Yarn Electrode Materials Enhanced by Surface Modifications of Cotton Fibers with Graphene Sheets and Polyaniline Nanowire Arrays for All-Solid-State Supercapacitors. Electrochim. Acta 2018, 270, 205-214. (5) Ghosh, D.; Das, C. K. Hydrothermal Growth of Hierarchical Ni3S2 and Co3S4 on a Reduced Graphene Oxide Hydrogel@Ni Foam: A High-Energy-Density Aqueous Asymmetric Supercapacitor. ACS Appl. Mater. Interfaces 2015, 7, 1122-1131. (6) Winter, M.; Brodd, R. J. What Are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104, 4245-4269. (7) Pan, S. W.; Ren, J.; Fang, X.; Peng, H. S. Integration: An Effective Strategy to Develop Multifunctional Energy Storage Devices. Adv. Energy Mater. 2016, 6, 19. (8) Chen, X. L.; Qiu, L. B.; Ren, J.; Guan, G. Z.; Lin, H. J.; Zhang, Z. T.; Chen, P. N.; Wang, Y. G.; Peng, H. S. Novel Electric Double-Layer Capacitor with a Coaxial Fiber Structure. Adv. Mater. 2013, 25, 6436-6441. (9) Liu, L.; Yu, Y.; Yan, C.; Li, K.; Zheng, Z. Wearable Energy-Dense and PowerDense Supercapacitor Yarns Enabled by Scalable Graphene–Metallic Textile


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