High-Performance Flexible In-Plane Micro-Supercapacitors Based on

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Functional Nanostructured Materials (including low-D carbon)

High-Performance Flexible In-Plane Micro-Supercapacitors Based on Vertically-Aligned CuSe@Ni(OH) Hybrid Nanosheet Films 2

Jiangfeng Gong, Jing-Chang Li, Jing Yang, Shulin Zhao, Ziyuan Yang, Kaixiao Zhang, Jianchun Bao, Huan Pang, and Min Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12543 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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ACS Applied Materials & Interfaces

High-Performance

Flexible

In-Plane

Micro-Supercapacitors

Based on Vertically-Aligned CuSe@Ni(OH)2 Hybrid Nanosheet Films Jiangfeng Gong,†,* Jing-Chang Li,† Jing Yang, ‡ Shulin Zhao, ‡ Ziyuan Yang,† Kaixiao Zhang,† Jianchun Bao,‡ Huan Pang, ┴, §,* and Min Han ‡, §,*

† ‡

College of Science, Hohai University, Nanjing 210098, P. R. China Jiangsu Key Laboratory of New Power Batteries, and Jiangsu Key Laboratory of

Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China ┴

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002,

Jiangsu, P. R. China §

State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Solid

State Microstructures, Nanjing University, Nanjing 210093, P. R. China

Abstract: Orientation and hybridization of ultrathin two-dimensional (2D) nanostructures on interdigital electrodes is vital for developing high-performance flexible in-plane micro-supercapacitors (MSCs). Despite great progress has been achieved, integrating CuSe and Ni(OH)2 nanosheets to generate advanced nanohybrids with oriented arrangement of each component and formation of porous frameworks remains a challenge, and their application for in-plane MSCs has not been explored. Herein, the vertically-aligned CuSe@Ni(OH)2 hybrid nanosheet films with hierarchical open channels are skillfully deposited on Au interdigital electrodes/polyethylene

terephthalate

substrate

via

a

template-free

sequential

electrodeposition approach, and directly employed to construct in-plane MSCs by choosing 1

ACS Paragon Plus Environment

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polyvinyl alcohol-LiCl gel as both the separator and solid electrolyte. Due to the unique geometrical structure and combination of intrinsically conductive CuSe and battery-type Ni(OH)2 components, such hybrid nanosheet films not only can resolve the poor conductivity and re-stacking problems of Ni(OH)2 nanosheets but also create the three-dimensional electrons or ions transport pathway. Thus, the in-plane MSCs device fabricated by such hybrid nanosheet films exhibits high volumetric specific capacitance (38.9 F cm-3). Moreover, its maximal energy and power density can reach 5.4 mW h cm-3 and 833.2 mW cm-3, superior to pure

CuSe

nanosheets,

and

most

of

reported

carbon

materials

and

metal

hydroxides/oxides/sulfides based in-plane MSCs ones. Also, the hybrid nanosheet films device shows excellent cycling performance, good flexibility and mechanical stability. This work may shed some light on optimizing 2D electrode materials and promote the development of flexible in-plane MSCs or other energy storage systems. Keywords: In-plane micro-supercapacitors, vertically-aligned hybrid nanostructures, CuSe nanosheets, Ni(OH)2 nanosheets, flexibility

1. INTRODUCTION The ever-increasing demand for flexible and wearable electronics have stimulated the development of miniaturized energy storage systems or small-scale energy storage devices.1,2 Among them, in-plane micro-supercapacitors (MSCs) have attracted substantial attention because they possess higher power density, faster charge-discharge rate, longer life spans and better safety tolerance than batteries. Moreover, MSCs can be easily integrated into electronic circuits/systems, advantageous to design next-generation power devices for meeting the 2


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requirements in flexible or wearable electronics or nano-electronic devices.3,4 To fabricate high quality in-plane MSCs, the key lies on finding proper active materials and efficiently integrating them on pre-designed interdigital electrodes. By far, several nanostructured carbon materials (e.g. onion-like carbon,5 carbide derived carbons,6,7 carbon nanotubes,8-10 and graphene11-17), have been employed as active materials to fabricate desired in-plane MSCs with good flexibility and mechanical stability. However, limited by the electric double-layer energy storage mechanism, those carbon-based MSCs exhibit a low energy density. Contrarily, due

to

the

fast

reversible

surface

Faradic

redox

reactions,

transition

metal

oxides/hydroxides/sulfides and conducting polymers nanostructures, such as RuOx,18 MnO2 nanoflakes,19 Ni(OH)2 nanoplates,20 Co3O4,21 VS2 nanosheets (NSs),22 MoS2 NSs,23 polypyrrole,24,25 polyaniline,26 poly(3,4-thylenedioxythiophene),27 and so on, are proven to be ideal candidates for improving the energy density of MSCs. Nonetheless, using conventional sputtering or printing method,1 those active materials are randomly stacked on interdigital electrodes, causing a part of them can’t contact with gel electrolyte that will reduce the available active surfaces. Thus, their performance can’t be maximally manifested. To fully utilize desired active materials, new strategy is needed for assembling them into special geometrical motifs with oriented spacial arrangement of active units. In numerous geometrical configurations, the vertically-aligned porous frameworks are highly appealing because they can offer more accessible surfaces, contribute to the ion diffusion, and facilitate electrolyte penetration and electron transfer,28-34 helping to enhance energy storage capacity. Besides geometrical structures, integrating diverse active materials to form heterostructures or nanohybrids facilitates to improve the performance of in-plane MSCs yet. For example, 3



Moosavifard and co-authors integrated laser scribed graphene with CoNi2S4 NSs and obtained 3D graphene/CoNi2S4 NSs nanohybrids.35 The in-plane MSCs device constructed by such nanohybrids exhibit enhanced performance in relative to pure graphene and CoNi2S4 NSs based ones. Additionally, Shen’s group synthesized reduced graphene oxides/Fe2O3 hollow spheres and constructed on-chip MSCs with the energy density of 1.61 mWh cm–3, which had been used for driving the photodetectors.36 Moreover, Wu’s group fabricated phosphorene/ graphene based in-plane MSCs, whose energy density can approach 11.6 mWh cm−3 that superior to pure phosphorene and graphene based ones.37 These outstanding works demonstrate that combination of intrinsically conductive and pseudo-capacitive active materials to obtain advanced nanohybrids actually can improve the performance of specific in-plane MSCs. Unfortunately, except for graphene,35-42 other intrinsically conductive materials (e.g. CuSe43) are rarely “married” with pseudo-capacitive components to fabricate desired in-plane MSCs. In previous work, we have synthesized the vertically-oriented and interpenetrating CuSe NS frameworks and constructed stacked flexible all-solid-state supercapacitors,44 whose energy density is very low. To improve the energy density of CuSe NSs based device, combining with other active materials to produce vertically-aligned hybrid NS films and fabricating in-plane MSCs may be a better choice. As a battery-type electrode, Ni(OH)2 has high theoretical specific capacitance (2082 F g-1),45 but its conductivity is poor (10-5-10-9 S cm-1).46 To foster the strength of CuSe NS frameworks and Ni(OH)2 and circumvent their drawbacks, combining them to generate vertically-aligned hybrid NS films and exploring their applications for in-plane MSCs attract our interest. Here, we report the synthesis of vertically-aligned CuSe@Ni(OH)2 hybrid NS films with 4


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hierarchical open channels on Au interdigital electrode via a template-free sequential electrodeposition method. Such CuSe@Ni(OH)2 hybrid NS films possess the following features: (1) The CuSe NSs can well adhere and vertically grow on interdigital electrodes (current collector) due to the good affinity or interplay of CuSe and Au, helpful to reduce the contact resistance; (2) The intrinsic resistance of CuSe is much lower than that of Ni(OH)2. Thus, the CuSe NSs component can serve as a conductive scaffold to support Ni(OH)2 NSs and enhance their conductivity; (3) The Ni(OH)2 NSs perpendicularly grow and interweave on the surfaces of CuSe NSs, which can efficiently inhibit the re-stacking of Ni(OH)2 NSs and fully expose their edge or surface active sites; (4) The hierarchical open channels are generated between the hybrid NSs, which can ensure the gel-electrolyte adsorbed on whole surfaces of the active materials, beneficial to diffuse electrolyte ions. Using such CuSe@Ni(OH)2 hybrid NS films as the active materials, the in-plane MSCs device has been successfully fabricated by selecting polyvinyl alcohol (PVA)/LiCl gel as both the separator and solid electrolyte. The constructed device displays a high volumetric capacitance (38.9 F cm-3). And its maximal energy and power density are 5.4 mW h cm-3 and 833.2 mW cm-3, respectively, outperforming pure CuSe NSs, and most of reported carbon materials and transition metal hydroxides/oxides/sulfides based in-plane MSCs. Moreover, the fabricated device exhibits robust cycling performance, excellent flexibility and mechanical stability yet. This work demonstrates that geometry and component dual-modulation strategy can be employed to optimize 2D electrode materials for fabrication of high-performance flexible in-plane MSCs.

2. EXPERIMENTAL SECTION 5



2.1. Reagents and Materials. The CuCl2·2H2O (99%), SeO2 powder (99%), KCl (99%), and LiCl (97%) were bought from Alfa Aesar. And the Ni(NO3)2 (98%), KNO3 (99%) and HCl (37%) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai). All reagents were used as received without further purification. The polyvinyl alcohol (PVA) powder and poly (ethylene terephthalate) (PET) pieces were purchased from Alfa Aesar. 2.2. Preparation of Au Interdigital Electrodes. The Au interdigital electrodes with different electrode width and inter-electrode distance on flexible PET substrate were pre-processed through classical photolithography method of photoresist spin-coating, patterning, exposure, development and fixation. Then, a thin layer of Au (50-60 nm) was sputtered on the patterned PET substrate. Finally, the photoresist was removed by dipping the PET substrate into acetone solution for 10 minutes. 2.3. Synthesis of Vertically-Oriented and Interpenetrating CuSe NS Frameworks. A potentiostatic mode was adopted to deposit vertically-oriented and interpenetrating CuSe NS frameworks on Au interdigital electrodes following previous report with a slight modification.44 In brief, a three-electrode system was assembled by using the Au interdigital electrodes on PET, a piece of platinum sheet, and a saturated calomel electrode (SCE) as the working electrode, counter electrode and reference electrode, respectively. The electrolyte was made up of 2.5 mM CuCl2·2H2O, 4.5 mM SeO2 and 0.1 M KCl. The pH of the electrolyte was adjusted to 1.5 by using HCl solution. The electrodeposition was carried out under the potential of -0.10 V (vs. SCE) at 60 oC for 30 min. After the electrodeposition, the working electrode was taken out and rinsed with deionized water for several times, and then dried at 25 oC for 4h. After that, it was used as a platform to further deposit Ni(OH)2 NSs. 6


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2.4. Synthesis of Vertically-Aligned CuSe@Ni(OH)2 Hybrid NS Films. By using the pre-synthesized vertically-oriented and interpenetrating CuSe NS frameworks on Au interdigital electrodes as the working electrode, the vertically-aligned CuSe@Ni(OH)2 hybrid NS films could be obtained by secondary electrodeposition growth of Ni(OH)2 NSs. The electrolyte was consisted of 0.1 M Ni(NO3)2 and 0.1 M KNO3. The impact factors including deposition potential, temperature and time, were carefully studied to optimize the electrodeposition parameters. The best conditions for secondary growth of Ni(OH)2 NSs were under the potential of -0.5 V (vs. SCE) at 70 oC for 20 min. 2.5. Fabrication of In-Plane MSCs. To fabricate in-plane MSCs, a drop of PVA-LiCl gel solution was covered on the surface of the hybrid NS films that deposited on Au interdigital electrodes, and then pressed with a PET sheet to form a thin layer of solid electrolyte. Finally, the PVA-LiCl gel coated hybrid NS films were dried in vacuum at 50 °C for 6 h to remove the residual water. The PVA-LiCl gel electrolyte was prepared according to the following procedure: 6 g of LiCl was dissolved in 60 mL of deionized water, and then 6 g of PVA powder was added into the solution. Subsequently, the whole mixture was heated to 85 oC under violently magnetic stirring until the solution became clear. 2.6. Materials and Electrochemical Characterization. Scanning electron microscopy (SEM, helios 600i and Quanta 200, FEI) and transmission electron microscopy (TEM, Tecnai F20, FEI) equipped with energy-dispersive X-ray spectroscopy (EDS) were used to study the morphology, microstructure and elemental composition of the films. X-ray diffraction (XRD) patterns were recorded on Bruker AXS D8 ADVANCE diffractometer. X-ray photoelectron spectroscopy (XPS) were performed on an ESCALAB 250 Xi XPS system. And the binding 7



energies were calibrated using C1S peak as a standard. The cyclic voltammetry, galvanostatic charge-discharge, cycling performance and flexibility, and electrochemical impedance spectroscopy (EIS) tests were executed on CHI 660D electrochemical workstation (Chenghua Co. Shanghai) in a two-electrode system at room temperature. The EIS data were recorded at the frequency ranging from 10-1 to 105 Hz.

3. RESULTS AND DISCUSSION Scheme

1

depicts the

whole procedure for preparation of vertically-aligned

CuSe@Ni(OH)2 hybrid NS films. The Au interdigital electrodes with different electrode width and inter-electrode distance are pre-processed on PET substrate via conventional photolithography, which are employed as the working electrode for growth of CuSe@ Ni(OH)2 hybrid NS films by a template-free sequential electrodeposition approach. More specifically, the CuSe NSs are firstly electrodeposited on Au/PET substrate under the optimized potential of -0.10 V (vs. SCE) at 60 oC for 30 minutes. From the SEM image shown in supporting information (SI), Figure S1, large-areaed highly uniform CuSe NSs with the average thickness of about 15 nm can be observed, which vertically grow and interpenetrate to form porous frameworks. Such CuSe NSs on Au/PET substrate is served as the platform for further integrating Ni(OH)2 NSs via the secondary electrodeposition. To identify the optimal parameters on deposition of Ni(OH)2 NSs components, a series of conditional experiments have been carried out (SI, Figure S2-3). The applied potential and deposition time as well as the electrolyte temperature are found to greatly influence the growth of Ni(OH)2 NSs. By taking into account the experimental results, the high-quality ultrathin Ni(OH)2 NSs can be 8


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integrated on CuSe NS frameworks to generate vertically-aligned CuSe@ Ni(OH)2 hybrid NS films under the potential of -0.50 V (vs. SCE) at 70 °C for 20 minutes.

Scheme 1. The schematic diagram for illustrating the preparation of vertically-aligned CuSe@Ni(OH)2 hybrid NS films on Au interdigital electrodes/PET substrate.

Figure 1A shows the low-magnification SEM image of the typical CuSe@Ni(OH)2 hybrid NS films, from which we can see that the morphologies of the formed hybrid nanostructures are highly uniform, and there are no hybrid NSs peeled from the Au/PET substrate. The related high-magnification SEM image is given in Figure 1B. It can be clearly observed that many tiny Ni(OH)2 NSs are perpendicularly grown on the surface of CuSe NSs to form the network-like structure or open frameworks, which can maximally expose the basal planes and edge sites of Ni(OH)2 NSs that beneficial to electrochemical energy storage. By statistical analyses, the lateral size and thickness of the generated Ni(OH)2 NSs are approximately 50 nm and less than 10 nm, respectively. The corresponding cross-section SEM image (Figure 9



1C) reveals that the Ni(OH)2 NSs are covered on the entire surfaces of CuSe NSs, which are interwoven to form free-standing hybrid NS films with the height of about 3 µm.

Figure 1. A-C) The low- (A) and high- (B) magnification as well as cross-section (C) SEM images of the typical CuSe@Ni(OH)2 hybrid NS films that deposited on Au/PET substrate, respectively. D) The representative TEM image of the CuSe@Ni(OH)2 hybrid NSs. E-F) The corresponding SAED pattern (E) and HRTEM image (F). G) EDS mapping images of the elements: copper, selenium, nickel and oxygen.

The microstructure of the typical CuSe@Ni(OH)2 hybrid NS films is further examined by TEM. Figure 1D shows the representative TEM image of an individual hybrid NSs that obtained by ultrasonic-assisted exfoliation from Au/PET substrate. Compared with pure CuSe NSs, the surfaces of CuSe@Ni(OH)2 hybrid NSs are relatively rough and many subtle textures can be observed, corresponding to the thin irregular Ni(OH)2 NSs that assembled on 10


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the surfaces of CuSe NSs. The related selected area electron diffraction (SAED) pattern is provided in Figure 1E. The sets of bright diffraction spots result from the CuSe NSs component, revealing the single crystal nature of CuSe NSs. Whereas the couple of weak concentric rings confirm the presence of Ni(OH)2 NSs component in the obtained nanohybrids. The corresponding d-spacings are about 2.12 Å and 1.52 Å, which can be indexed to the interplanar separation of (103) and (300) planes of α-Ni(OH)2 (JCPDS-22-0444), respectively. The enlarged high-resolution TEM (HRTEM) image for the outmost Ni(OH)2 NSs shell layer is shown in Figure 1F. The intermittent or distorted lattice fringes indicate the existence of abundant lattice defects in α-Ni(OH)2 NSs component. The measured lattice spacing is approximately 2.12 Å, corresponding to the interplanar separation of (103) plane of α-Ni(OH)2 and consistent with the SAED results. Tong and co-authors have proven that introducing oxygen vacancies in transitional metal oxides/hydroxides can improve their donor density and surface properties.47 Here, the tiny and ultrathin Ni(OH)2 NSs show the similar defect features, which are expected to improve the electrochemical energy storage performance. Further EDS mapping analyses also identify the formation of CuSe@Ni(OH)2 hybrid NSs. Figure 1G gives the elemental distribution images of Cu, Se, Ni and O, respectively. The Cu and Se elements show clear boundary while Ni and O elements diffuse around their edges, further confirming the successful preparation of continuous Ni(OH)2 nanostructures on CuSe NSs. The component, crystallinity and chemical states of various elements for the typical CuSe@ Ni(OH)2 hybrid NS films are further identified by XRD and XPS. Figure 2A shows the XRD pattern of CuSe@Ni(OH)2 hybrid NS films that deposited on Au/PET substrate. The 11



two broad and strong diffraction peaks at 26.0o and 53.7o result from the flexible PET substrate. And the three sharp diffraction peaks at 38.2o, 44.4o and 77.6o originate from the Au interdigital electrodes, which can be indexed to the (111), (200), and (311) planes of face-centered cubic phase Au (JCPDS-04-0784). Due to very strong diffraction signals of PET and Au as well as the vertically-oriented growth of CuSe NSs on Au interdigital electrodes and surface covered by Ni(OH)2 NSs, only one weak diffraction peak for CuSe NSs component can be observed at 50.2o, assigning to the (108) plane of hexagonal phase CuSe (JCPDS-34-0171). Similarly, owing to the vertically-oriented growth of Ni(OH)2 NSs on the surfaces of CuSe NSs and the relatively low crystallinity of Ni(OH)2, only one middle diffraction peak for Ni(OH)2 can be found at 33.2o, corresponding to the (110) planes of α-Ni(OH)2 (JCPDS-22-0444). To further identify the component and chemical states of various elements, XPS tests have also been carried out by peeling off the hybrid NSs from Au/PET substrate through ultrasonic treatment. The survey XPS spectrum is given in Figure 2B. Since the probing depths of XPS is only several nanometers and the CuSe NSs are encapsulated by Ni(OH)2 NSs shell layer, the signal intensities for Cu 2p and Se 3d peaks are very weak (SI, Figure S4). As for Ni 2p core-level XPS spectrum (Figure 2C), the two major peaks centered at 855.7 eV and 873.4 eV are the characteristic Ni 2p3/2 and 2p1/2 peaks for

α-Ni(OH)2, and the spin-orbit splitting energy between the Ni 2p peaks is 17.5 eV, which is consistent with previous reports.48-50 The other two featured satellite peaks located at 861.3 eV and 879.4 eV further confirm the presence of α-Ni(OH)2. While for O 1s core-level XPS spectrum (Figure 2D), it is slightly asymmetric and can be deconvoluted into two peaks. The peak located at 528.4 eV is attributed to the lattice oxygen in α-Ni(OH)2. And the other peak 12


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centered at 532.1 eV can be assigned to the -OH groups that adsorbed on α-Ni(OH)2 surfaces.48,49

Figure 2. A) The XRD pattern of the typical CuSe@Ni(OH)2 hybrid NS films. B) The survey XPS spectrum of CuSe@Ni(OH)2 hybrid NS films. C-D) Corresponding core-level XPS spectra for Ni 2p (C) and O 1s (D) peaks, respectively.

By coating a thin layer of PVA-LiCl gel to the surfaces of CuSe@Ni(OH)2 hybrid NS films, the in-plane MSCs can be easily fabricated, whose optical image is given in SI, Figure S5. Here, the PVA-LiCl gel is served as both the separator and solid-state electrolyte. The

13



electrode width and inter-electrode distance are kept at 100 µm, and the effective working area of the device is 2 cm2. Figure 3A illustrates the schematic configuration of the as-fabricated in-plane MSCs device. The adjacent poles represent contrary electrodes that can be considered as some line-shaped capacitors connected in parallel. Due to the CuSe@Ni(OH)2 hybrid NSs intercrossed to form hierarchical open channels, such configurations can efficiently inhibit the re-stacking of electrode materials and introduce more available active surfaces for absorption/desorption of electrolyte ions, facilitating to generate the flowing ion channels during the charge-discharge process (Figure 3B). Moreover, the CuSe NSs possess good intrinsically electrical conductivity, which can efficiently delivery electrons to each interdigital electrode. Hence, such CuSe@ Ni(OH)2 hybrid NS films based in-plane MSCs is expected to manifest excellent electrochemical energy storage performance. The corresponding cyclic voltammetry (CV) curves under different scanning rates are shown in Figure 3C. As a battery-type electrode material component, Ni(OH)2 stores energy through the redox reactions of metal ions in the crystalline framework, which should show apparent redox peaks during the CV scans. However, in our system, no obvious redox peaks can be observed from the CV curves. The possible reason may be that the Ni(OH)2 NSs in our hybrid NS films are tiny (lateral size only 50 nm) and ultrathin (less than 10 nm), making their intercalation/de-intercalation rates of ions much faster than bulk Ni(OH)2 and thereby the energy storage mechanism more like intercalation pseudo-capacitance.51 To further examine the electrochemical behaviors of such hybrid NS films based in-plane MSCs, the galvanostatic charge-discharge (GCD) tests are performed yet. The related discharge branch curves at the current density of 0.05, 0.10, 0.15, 0.20 and 0.25 mA cm-2 are presented 14


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in Figure 3D. At various current densities, the volumetric specific capacitances can be calculated according to their discharge branch curves. As illustrated in Figure 3E, the maximum volumetric specific capacitance of CuSe@Ni(OH)2 hybrid NSs based in-plane MSCs can reach 38.9 F cm-3 at the current density of 0.05 mA cm-2, which is much higher than that of pure CuSe and pure Ni(OH)2 NSs based devices (SI, Figure S6-7). In addition, to investigate the conductivity or electron transport properties of fabricated in-plane MSCs, electrochemical impedance spectroscopy tests are further carried out. The Nyquist plot of the hybrid NS films based MSCs (SI, Figure S8) shows obvious straight line section at low frequency region, indicating the ideal capacitive behavior. From the intercept with real axis at high frequency region, the equivalent series resistance (ESR) of CuSe@Ni(OH)2 hybrid NS films based MSCs is identified to be 92.2 Ω, which is slightly higher than pure CuSe NSs based MSCs (72.5 Ω) but much lower than pure Ni(OH)2 NSs based MSCs (155.5 Ω). This result confirms that the CuSe NSs component can reduce the ESR and promote the electron transport of our hybrid NS films based device. It should be mentioned that the width of interdigital electrodes and their inter-spacing also affect the energy storage of in-plane MSCs. As revealed in SI, Figure S9, the volumetric specific capacitance of CuSe@Ni(OH)2 hybrid NS films based MSCs will greatly decrease when both the electrode width and inter-electrode distance increase to 150 µm. Besides specific capacitances, the long-term cyclic performance, flexibility and mechanical stability of the device are also examined through a cyclic GCD process at a constant current density of 0.3 mA cm-2 under different bending angles or folding states. As shown in Figure 3F, the specific capacitance of the device shows a minor increase in the first 4000 cycles, which may result from the gradual activation of redox active species 15



by exposing to the solid electrolyte. Subsequently, the capacitance of the device exhibits a slight decline that may result from the partial dissolution of active materials during the electrochemical process. After continuously working for 10000 cycles, the device can retain almost 100% of its initial capacitance, showing good cycling stability. Additionally, the specific capacitance is nearly unchanged by varying the bending or folding angles, revealing that the fabricated in-plane MSCs device possesses excellent flexibility and mechanical stability yet.

16


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Figure 3. A-B) The schematic illustration for the configuration (A) and the ion transport (B) of CuSe@Ni(OH)2 hybrid NS films based in-plane MSCs device. C) Cyclic voltammetry curves at different scanning rates. D) Galvanostatic discharge branch curves at various current densities. E) The plot of volumetric specific capacitance versus current density. F) Cycling performance at the current density of 0.3 mA cm-3 under different bending or folding states. G) Ragone plot of our in-plane MSCs device in comparison with electrolytic capacitors, Li-ion 17



batteries, and recently reported some in-plane MSCs.

Except for the above-mentioned aspects, the energy density and power density are another two important parameters for determining the practical application of desired in-plane MSCs. Figure 3G presents the energy density versus power density (Ragone) plot of the CuSe@ Ni(OH)2 hybrid NS films based in-plane MSCs. For comparison, the data of pure CuSe NSs frameworks based in-plane MSCs and other energy storage devices are also inserted in Figure 3G. As can be seen, our in-plane MSCs can deliver a maximal volumetric energy density of 5.4 mW h cm-3, which is one order higher than that of commercial electrolytic capacitors (< 0.1 mW h cm-3).13, 20 Additionally, its maximal volumetric power density can reach 833.2 mW cm-3, which is two orders higher than Li-ion batteries (1~10 mW cm-3).13, 20 Moreover, its energy storage properties are much higher than pure CuSe NSs based in-plane MSCs, and close or superior to recently reported carbon nanotubes,10 Ni(OH)2 nanoplates,20 graphene oxide/Fe2O3 hollow spheres,36 phosphorene/graphene,37 graphene,52 sulfur-doped graphene,53 polyaniline/ graphene,54 graphene nanowalls/Ni heterostructure,55 and carbon56 based in-plane MSCs. These results disclose that the vertically-aligned CuSe@Ni(OH)2 hybrid NS films are promising active materials for fabricating high-performance in-plane MSCs. Moreover, portable equipment often requires the energy storage units or cells packaged either in series or in parallel with the aim to meet operating voltage or current requirements in some situations. To demonstrate the potential use of CuSe@Ni(OH)2 hybrid NS films based in-plane MSCs, two MSCs units or devices (denoted as Devices 1 and 2) are manufactured on one flexible PET substrate, which have the same effective area and be built under the identical

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condition. The corresponding optical image for the integrated two devices is shown in Figure 4A. Four terminals are generated and extended out, which are marked as P1, P2, P3, P4, respectively. When terminals P2 and P3 are adopted to discharge, the devices have a series relationship. For the parallel relationship, terminals P1 and P4 are connected as one pole, and terminals P2 and P3 as the other pole. Figure 4B-C illustrate the electrochemical responses of the two tandem in-plane MSCs devices. It is clear that the working or output voltage has raised to 2.0 V under this case, which is double of its original one (single in-plane MSCs). Whereas the current density and discharge time (GCD) are kept unchanged. As for in parallel assembly (Figure 4D-E), the output current density has increased by a factor of 2, but the voltage is the same as the single in-plane MSCs. In relative to individual Device 1 (4.34 mF) and Device 2 (6.97 mF), the capacitances of the two devices assembled in series and in parallel are calculated to be 3.04 mF and 11.86 mF, respectively, approaching the predicated values for the two combinations.

19



Figure 4. A) Optical image of the integrated two in-plane MSCs devices on PET substrate. B-C) Cyclic voltammetry (B) and galvanostatic charge-discharge (C) curves for the two in-plane MSCs devices connected in series. D-E) Cyclic voltammetry (D) and galvanostatic charge-discharge (E) curves for the two in-plane MSCs devices connected in parrallel.

4. CONCLUSIONS In summary, the vertically-aligned CuSe@Ni(OH)2 hybrid NS films with hierarchical open channels have been skillfully integrated on Au/PET substrate via a template-free sequential electrodeposition method, and directly employed to construct flexible in-plane MSCs. Microstructure analyses demonstrate that the CuSe NSs with the average thickness of 15 nm are vertically-oriented and interpenetrated on Au interdigital electrodes to form 3D porous frameworks, and the Ni(OH)2 NSs with the thickness less than 10 nm perpendicularly grow and interweave on the surfaces of CuSe NS frameworks to generate free-standing hybrid NS films. Such hybrid NS films combine intrinsically conductive CuSe and battery-type Ni(OH)2 components, which not only can resolve the poor electrical conductivity of Ni(OH)2 NSs and inhibit their re-stacking for exposing more available active sites but also create the 3D electrons or ions transport channels. Thus, the in-plane MSCs device fabricated by such CuSe@ Ni(OH)2 hybrid NSs films, manifests very high volumetric specific capacitance (38.9 F cm-3) with the maximal energy and power density of 5.4 mW h cm-3 and 833.2 mW cm-3, respectively, superior to pure CuSe NS frameworks, and most of reported carbon materials and transition metal hydroxides/oxides/sulfides based in-plane MSCs. Moreover, the constructed device exhibits good cycling performance, excellent flexibility and mechanical 20


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stability yet. This work offers an efficient avenue for optimizing 2D electrode materials by geometry and component dual-modulation, and fabricates high-performance flexible in-plane MSCs with both high energy and power density, showing great promise as a flexible power supply to use in wearable electronics or nanoelectronic devices.

ASSOCIATED CONTENT Supporting Information Available. This information contains the characterization of CuSe NS frameworks, control experiments for optimizing electrodeposition parameters, XPS spectra of the hybrid NS films, and optical image and additional electrochemical data for the in-plane MSCs devices. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Phone/ Fax: 86-25-85891051 E-mail: [email protected]

(Dr. J. F. Gong)

[email protected]

(Prof. Dr. H. Pang)

[email protected]

(Prof. Dr. M. Han)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

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The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China for the project (Nos. 21671106, 21671170, and 21541007), the Fundamental Research Funds for the Central Universities (Grant Nos. 2018B19714), research fund from the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the opening research foundations of State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Solid State Microstructures, Nanjing University.

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