Growth of carbon nanosheets on carbon nanotube arrays for the fabrication of three-dimensional micro-patterned supercapacitors

Carbon 155 (2019) 453e461

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Carbon journal homepage: www.elsevier.com/locate/carbon

Growth of carbon nanosheets on carbon nanotube arrays for the fabrication of three-dimensional micro-patterned supercapacitors Pingge He a, Zhengping Ding b, Xudong Zhao a, Jiahao Liu a, Qun Huang c, **, Jingjing Peng d, Li-Zhen Fan a, * a

Beijing Advanced Innovation Center for Materials Genome Engineering, Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing, 100083, China International Center for Quantum Materials, Electron Microscopy Laboratory, School of Physics, Peking University, Beijing, 100871, China c State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China d Beijing Institute of Aeronautical Materials, Beijing, 100095, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 June 2019 Received in revised form 23 August 2019 Accepted 1 September 2019 Available online 3 September 2019

Micro-supercapacitors provide high peak power, long cycle life, and high charge/discharge rates for practical applications in microsystems. However, current micro-supercapacitors generally suffer from low energy density and complicated fabrication process. Here, we report carbon nanosheet (CN) selectively grown on carbon nanotube (CNT) patterns as three-dimensional hybrid electrodes for highperformance micro-supercapacitor applications. The growth mechanism is revealed that CNs prefers to grow on CNT surface rather than on SiO2 substrate due to the high binding energy of carbon atom absorbed on CNT surface. The symmetric all-carbon CN/CNT micro-supercapacitors exhibit ultrahigh areal capacitance of approximately 110 mF cm
2 at a current density of 0.3 mA cm
2, outstanding longterm cyclic stability (z7% loss of initial capacitance after 10,000 cycles). Furthermore, flexible microsupercapacitors are achieved by peeling off microelectrodes using H3PO4/PVA electrolyte and showing high functional flexibility (capacitance loss less than 9% after 100-cyclic bending tests). The outstanding electrochemical performance and functional flexibility of present micro-supercapacitors show great potential applications in smart and miniaturized electronic devices. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction The rapid development of the miniaturized portable electronic devices in the modern life has driven the increasing demand for micro-power sources and micro energy storage units [1e3]. Considering the requirement of miniaturization and reduction of the complexity of the whole system, the design and fabrication of the micro-scale energy storage systems with long cycle life, high rate, high energy and power density is essential and meaningful [4e6]. Among them, micro-supercapacitors have attracted considerable attention because of their ultrahigh power density, excellent rate capability, long lifetime and environmental benignity [7e9]. Recent development in micro & nanofabrication and

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Huang), [email protected] (L.-Z. Fan). https://doi.org/10.1016/j.carbon.2019.09.003 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

additive manufacturing techniques such as inkjet printing [10], laser writing [11] and lithography-based techniques [12,13] provide an effective approach to the fabrication of micro-supercapacitors. Conventional lithography-based techniques with the ability to pattern various materials into interdigitated structures as microscale electrodes have realized the fast fabrication and manufacturing of planar micro-supercapacitors, providing an opportunity to develop new-generation integrated devices [13,14]. However, selected electrode materials are generally required to be deposited on the substrate beforehand, limiting the thickness of electrodes and leading to difficulty in producing gaps/separation between adjacent electrodes. Consequently, the existing microsupercapacitors generally suffer from low areal capacitance (low areal energy densities) and complicated fabrication process [15]. To solve these issues, three-dimensional (3D) hybrid electrodes could considerably enhance the electrochemical performance of micro-supercapacitor [3,16,17]. Numerous techniques including hydrothermal process [18], chemical co-precipitation [19], electrodeposition [20], chemical vapor deposition [21] and physical

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mixture [22] have been proposed to fabricate 3D hybrid electrodes. However, for micro-supercapacitor application, the subsequent patterning process could easily destroy the electrode structure [23]. Recently, a novel approach through selective growth of active materials on designed area or substrate has attracted considerable attention since it can achieve the growth and pattering simultaneously at specific places [24], widely applied in sensors, photocatalysts and photonic devices [25e27]. Particularly, Hiramatsu M et al. attempted to selectively grow carbon nanowalls on patterned Ti substrates, but the gap width has been limited to be larger than 100 nm (not controllable) [28]. Moreover, the Ti pattern fabrication process through e-beam lithography is highly expensive and less scalable, hindering its practical applications. To take full advantage of 3D hybrid structures applied in micro-supercapacitors, the associated micro-electrode preparation process is expected to be easy, controllable and scalable. In this work, carbon nanosheets (CNs) selectively grown on vertical-aligned carbon nanotube (VCNT) patterns were reported as 3D hybrid electrodes for on-chip micro-supercapacitors by microwave plasma chemical vapor deposition (MPCVD) process. CNT arrays were directly grown on insulating substrates through onestep MPCVD method by pre-patterning a catalyst layer on the substrate. The as-prepared CNT patterns are used as an ideal 3D scaffold (seed) [29,30] for further growth of CNs. Such a CN/CNT hybrid structure, also defined as “graphenated carbon nanotube”, has been widely applied in electrochemical energy storage systems [31,32] due to the combination of high surface-area three-dimensional framework of the CNTs with the high edge density of CNs [33]. Herein, the selective growth mechanism has been deeply discussed, and the electrochemical performance and thermal stability of such micro-supercapacitors are systematically characterized to reveal the superiority of 3D CN/CNT electrodes. Such 3D hybrid electrodes are also demonstrated as efficient nanotemplates for pseudocapacitive materials and integrated in asymmetric microsupercapacitors with improved working voltage window. Moreover, flexible microsupercapacitors were fabricated by simply peeling off CN/CNT electrode patterns using H3PO4/PVA electrolyte, and their electrochemical properties and functional flexibility have also been tested to demonstrate the applicational diversity of such CN/CNT electrodes. 2. Experimental

growth process. 2.3. Material characterization The morphology and microstructure of CN/CNT patterned electrodes were characterized by field emission scanning electron microscope (SEM, Hitachi S-4800) operated at 5 kV and transmission electron microscopy (TEM, Japan FEM-2100F) with an accelerating voltage of 200 kV. Raman characterization was performed with LabRAM HR spectrometer (HORIBA Scientific) with a fixed laser excitation wavelength of 532 nm, power of 5 mW, spot size of approx. 1 mm, and magnification of 50  . 2.4. Electrochemical measurements The electrochemical performance was evaluated using a LAND test system. The micro-supercapacitor devices were electrochemically characterized in 1 M H2SO4 aqueous electrolyte solution. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements (with an AC perturbation amplitude of 5 mV in the frequency ranging from 1 MHz to 0.1 Hz) were carried out an electrochemical workstation (CHI660D). The preparation details of MnO2/CN/CNT-CN/CNT asymmetric device and flexible microsupercapacitors are provided in Supplementary data. Moreover, the methods to calculate specific capacitances, energy and power densities are provided in Supplementary data. 2.5. Simulation details Ab-initio simulations based on DFT (density functional theory) were carried out via VASP (the Vienna Ab-initio simulation package) [34e36] to study the interactions between carbon atoms and CNTs. For comparison, interactions between carbon atoms and SiO2 substrate are also investigated. PAW method (the projector augmented wave method) [37] with PBE functional (Perdew, Burke, and Ernzerhof functional) [38] was adopted. The energy cutoff was set to 500 eV for structural optimization and k-point separation of 0.03 Å
1 was used. The convergence tolerance for the force and energy during relaxation were set to 0.01/Å and 10
5 eV, respectively. Vdw-inclusive DFT-D3 method with Becke-Jonson damping [39] was employed to accurately deal with the weak vdW force in the systems. The binding energies (Eb , eV) of carbon atom on CNTs were computed as

2.1. Growth of CNT arrays on Si/SiO2 patterned wafer

Eb ¼ ECNTs þ Ec
EC
CNTs Before the growth, Ti/Al/Fe tri-layer catalysts (nominally 30/10/ 5 nm) were deposited on Si/SiO2 wafer by e-beam physical vapor deposition at a base pressure of 5  10
7 Torr. Then, Ti/Al/Fe patterns were fabricated via optical lithography, followed by hydrofluoric acid (HF) etching and photoresist removal. The details regarding the preparation of catalyst patterns are provided in SI. As-prepared Ti/Al/Fe patterns were loaded in a MPCVD chamber for CNT array growth. Before the growth, the temperature was increased to 800  C. After that, H2 (50 sccm) and CH4 (10 sccm) were introduced as gas sources, with a total pressure of 10 Torr. The plasma power during the growth was 300 W, and the substrates were kept at that condition for 10 min. 2.2. Selective growth of CNs on CNT patterns For the growth of CNs on CNT patterns, as-prepared CNT array patterns on Si/SiO2 wafers were further subjected to the same MPCVD system with a condition of H2 (50 sccm) and CH4 (10 sccm) as the primary feed gases at total pressure of 30 Torr [27]. The CN growth time was 8 min and the plasma power is 600 W during the

(1)

where ECNTs and EC are the energy of CNTs and carbon atom, respectively. EC
CNTs is the energy of CNTs with carbon atom adsorbed on surface. 3. Results and discussion Fig. 1 schematically illustrates the fabrication process of 3D CN/ CNT patterned micro-supercapacitors. First, Ti/Al/Fe tri-layer patterns were fabricated on Si/SiO2 wafer as catalysts for CNT growth through lithography. The formation of CNT arrays on Si/SiO2 substrate involves many factors such as metal catalyst, growth duration and plasma. The Ti/Al/Fe metal tri-layer was deposited on the Si/ SiO2 substrate as catalyst for the CNT array growth and metal layers act different roles in the growth of CNT arrays. Ti is deposited to increase the surface roughness and to provide more active nucleation sites [40]. Al is expected to surround the catalyst particles, impeding aggregation of the catalyst and enabling long-term catalytic activity for CNT synthesis [41]. Fe is the active catalyst for the CNT growth. Meanwhile, under the MPCVD system, the thermal

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Fig. 1. Schematic of fabrication process of 3D CN/CNT patterned supercapacitors. (A colour version of this figure can be viewed online.)

micro-

treatment reduces the thin film or quasi-triangular particles of catalyst into spherical dots. The energy available due to plasma generation and heating procedure decomposes the carbon precursors on the outer surface of the catalyst particles. The carbon atoms diffuse through the catalyst particles and tubular structures are formed with a catalyst particle either at the base or tip [42]. The plasma also helps to etch amorphous carbon that may deposit on top of the catalyst particles, thus providing a steady supply of carbon atoms at the surface of the catalyst particle. Consequently, CNT arrays grow vertically with high orientation on the electrically insulating substrate [43]. Such thick VCNT patterns serve as 3D scaffolds for CN growth and the preferential CN growth on CNT arrays directly leads to gaps (CN-free zones) between adjacent electrodes. Finally, the as-prepared CN/CNT patterned microsupercapacitor consists of a Si/SiO2 substrate and interdigitated 3D CN/CNT hybrid electrode, and adjacent CN/CNT electrodes are separated by a gap (CN-free zone), which can be highly controllable and easily adjusted through the pattern design. Moreover, flexible microsupercapacitors were achieved by simply peeling off CN/CNT electrode patterns after dipping in polymer gel electrolyte. The microstructure of interdigitated CN/CNT patterned electrodes for on-chip micro-supercapacitors has been shown in Fig. 2. The width of electrodes and the gap between them are approx. 150 and 100 mm, respectively (see Fig. 2a), which could be easily controlled by the pattern design. Fig. 2b shows a cross-sectional image of CNT arrays, indicating a thickness of approx. 100 mm. CNTs are well aligned in the array and possess diameters of tens of nanometers (SEM images of bare CNT arrays are provided in Fig. S1). As shown in Fig. 2c, after CN growth for 8 min, interestingly, CNs only selectively grow on CNT arrays, decorating individual CNTs uniformly along the axial direction (morphologies of CN/CNT patterned electrodes corresponding to different CN growth durations of 4, 6 and 10 min are shown in Fig. S2). The selective growth leads to CN growth on CNT arrays rather than that on SiO2 surface, rendering gaps (CN-free zones) between electrodes, and the CNfree zones (gaps) avoid shorting between adjacent electrodes. Fig. 2d shows the middle part of the sidewalls of CN/CNT micropatterned electrodes at a relatively low magnification, showing the dense and uniform growth of CNs on CNTs. A higher magnification of the CN/CNT microelectrodes is shown in the inset of

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Fig. 2 d, in which typical span width of an unwrinkled 2D CN plane ranges from approx. 100 nme500 nm. A high-resolution TEM image in Fig. 2e clearly displays the hybrid structure with a typical CN grown on a CNT. The inset in Fig. 2e further demonstrates the uniform distribution of CNs on CNTs, and the CNs display a typical lateral size of approx. 100 nm. Moreover, the CNT exhibits a distinct hollow core and an outer diameter of approx. 30 nm. The fringes of the CN in TEM image indicates the graphitic nature of CNs and the thickness of several nanometers with an atomic interlayer distance of 0.35 nm reveals it contains few-layer graphene sheets (see Fig. 2e). The Raman spectra of bare CNT and CN/CNT structures are shown in Fig. 1f, both of which contain prominent bands near 1350, 1580, 1620 and 2700 cm
1 corresponding to the D, G, D0 and 2D (also called G0 ) bands, respectively. The spectrum of CN/CNT exhibits additional prominent bands located near 2450 and 2940 cm
1 corresponding to the G* and D þ D0 bands, respectively [44,45]. The peak intensity ratio of the D and G peaks (ID/IG) and that of the 2D and G peaks (I2D/IG) are approx. 1.22 and 0.41 for bare CNT arrays, respectively. However, the values of ID/IG and of I2D/IG are approx. 0.8 and 0.98 for CN/CNT structure, respectively. The lower ID/IG suggests less defects are present in CN/CNT structure and the higher I2D/IG value indicates that CNs are few-layer graphene nanosheets [44,45]. To deeply study the “selective growth” phenomenon, the interface between electrodes and the substrate has been investigated by SEM. Fig. 3a displays a SEM image of the main view of a CN/CNT micro-electrode finger at a low magnification, in which the substrate surface presents clean and smooth. A highermagnification image of the interfacial region is shown in Fig. 3b and shows that CNs intercalate with each other to form an interconnect and porous network, efficiently reducing internal resistance and improving electrical properties by forming conducting paths. Moreover, Fig. 3ced displays the side view of interfacial contact between the micro-electrodes and insulating substrates, which also shows the clean and smooth Si/SiO2 surface (without CN growth), demonstrating the selective growth of CNs on CNTs. Notably, as shown in the blue rectangles in Fig. 3bed, CNs would also grow on the bottom (the interface between CNT arrays and Si substrate), significantly enhancing the interfacial bonding between CNT arrays and current collector. The 3D hybrid electrodes with CNs selectively grown on VCNT patterns show great uniqueness in structure and superiority in performance: (1) CNs intercalate VCNTs, which provide strong contacts between CNTs, leading to good electrical conductivity and strong connection of active materials. (2) CNs enhance the mechanical robustness of VCNT arrays [46] and the orientation of VCNT arrays maintains when wetted by electrolytes, which facilitates ion diffusion during charge/discharge process. Bare CNT array micro-electrodes suffer from severe structural damage (see Fig. S3) once wetted by aqueous electrolytes, leading to poor electrochemical performance. However, CN/ CNT micro-electrodes remained intact after being immersed in concentrated strong acids consisting of H2SO4 and HNO3 (v/v ¼ 3:1) at 40  C overnight (see Fig. S4), indicating their outstanding mechanical robustness. (3) CNs further increase specific surface area and thus increase the specific capacitance for electrodes. The nitrogen (N2) adsorptionedesorption isotherms of bare CNT and CN/ CNT electrodes are provided Fig. S5, and the measured BET surface area of CN/CNT is 15.3 m2 g
1, which is nearly 1.5 times greater than that of the bare CNT array structure (10.2 m2 g
1), confirming the increased specific surface area of CN/CNT after CN growth. As been demonstrated in previous research work [28], at the nucleation stage, carbon species would condense to form nanoislands with dangling bonds. At these dangling bonds, disordered carbon nanosheets of smaller sizes would be nucleated, followed by the two-dimensional growth and subsequent formation of

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Fig. 2. (a) SEM characterization of interdigitated CN/CNT patterned electrodes at a low magnification. (b) Cross-sectional SEM image of CNT arrays in a micro-electrode finger. (c) Cross-sectional SEM image of a CN/CNT micro-electrode finger. (d) Magnified SEM image in (c) to show the CN/CNT microstructure (inset shows the sharp edges of CNs). (e) TEM image of the CN/CNT electrodes (the inset shows typical CNs grown on CNT). (f) Raman characterizations of the bare CNT arrays and CN/CNT. (A colour version of this figure can be viewed online.)

nanographene sheets. Meanwhile, in the MPCVD system, the ion irradiation is considered to be one of important factors for the nucleation of carbon nanowalls. Since the CNT arrays with a height of 70e100 mm serve as a raised platform for CN growth, and the flux of ions reaching the bottom surface of CNT arrays (Si/SiO2 substrate) would be reduced compared with that of ions irradiating the top surface, the nucleation of carbon nanosheets at the substrate would be inhibited [28,47]. Among the nucleated CNs with random orientations, those standing almost vertically on the substrate continued preferably to grow up faster owing to the difference in the growth rates along the strongly bonded planes of CNs expanding and in the weakly bonded stacking direction [28]. Reactive carbon species arriving at the edge of the CN layer are easily bonded to the edge, and eventually the CN layer would expand preferably along the direction of radical diffusion, perpendicular to the substrate (CNT) plane. Moreover, the binding energy of a carbon atom absorbed on CNT and SiO2 surfaces have been calculated and the related results are

shown in Fig. 3eef. Fig. 3e schematically shows the absorption process with a carbon atom on CNT (CNT-50) surface and the absorption processes of carbon atoms on CNTs with different diameters are shown in Fig. S6. Meanwhile, the calculated binding energy of carbon atoms absorbed on CNTs with different diameters has been shown in SI, Table S1 and Fig. 3f. The binding energy decreases with the increase of CNT diameter (the limiting case is the graphene with an infinite diameter), and the lowest binding energy is approx. 2.92 eV, which is also much higher than that of a carbon atom on SiO2 surface. The simulation results indicate carbon atoms are preferentially absorbed on CNT surface rather than SiO2 surface, consequently leading to the selective growth of CN on CNT surface. Prior to the electrochemical characterization of CN/CNT patterned micro-supercapacitors in 1 M H2SO4 aqueous electrolyte, chemical activation to make them hydrophilic is necessary. Details of the activation process are provided in the SI and the current-time curve is provided in Fig. S7. The electrochemical properties of CN/

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Fig. 3. (a) SME of a main view of a CN/CNT finger. (b) A magnified main-view SEM image. (c) SEM of a side view at a relatively low magnification. (d) A magnified side-view SEM image. The blue rectangles in (b)e(d) show the CN coverage of interfacial regions. (e) Schematic of a carbon atom absorbed on CNT (CNT-50) surface. (f) The relationship between the binding energy and CNT diameter. (A colour version of this figure can be viewed online.)

CNT patterned micro-supercapacitors were tested in 1 M H2SO4 aqueous electrolyte, and the characteristic electrochemical performance is provided in Fig. 4 (the electrochemical properties of CN/ CNT microsupercapacitor without electrochemical activation have been provided in Fig. S8). CV curves recorded at different scan rates ranging from 5 to 100 mV s
1 (see Fig. 4a) display a nearly ideal rectangular shape, even at scan rates up to 100 mV s
1, indicating fast ion diffusion and low internal resistance of the microsupercapacitor. Fig. 4b shows galvanostatic charge/discharge curves of the on-chip micro-supercapacitor at different current densities ranging from 0.3 to 1.5 mA cm
2 (curves at higher current densities from 1.8 to 3 mA cm
2 are shown in Fig. S9). Based on the charge/discharge curves, the areal capacitance of the microsupercapacitor calculated by the method described in SI reaches approx. 110 mF cm
2 at a current density of 0.3 mA cm
2, 1.6 to 1000 times higher than those of the state-of-the-art carbon-based micro-supercapacitors that fall in the range of 0.087e69.5 mF cm
2 (see SI, Table S2). Moreover, the areal capacitances of CN/CNT microsupercapacitors with different CN growth times (4, 6, 8 and 10 min) have been plotted as a function of scan rates in SI, Fig. S10. The results reveal a relationship between the CN growth time and capacitance of microsupercapacitors and indicate that an optimal

CN growth time exists for superior electrochemical performance because of a balance between surface area and ion transfer kinetics. The rate capability and EIS results of the CN/CNT patterned microsupercapacitors are provided in Fig. S11 and Fig. S12, respectively, which indicate the good rate capability and relatively low internal resistance of the present CN/CNT patterned microsupercapacitors. Cyclic stability of as-prepared micro-supercapacitor was measured over 10,000 galvanostatic charge/discharge cycles at a current density of 4.5 mA cm
2 in a voltage range from 0 to 1 V. As shown in Fig. 4c, the present micro-supercapacitor exhibits a slight increase in capacitance at the beginning of the cycling, which could be explained by the gradually ameliorated electrolyte penetrating/ wetting the electrodes. Thereafter, the capacitance gradually decreases through 10,000 cycles, with an overall capacitance retention of approx. 93% compared to the first cycle, which is substantially higher than those of the state-of-the-art carbonbased micro-supercapacitor devices reported in prior work (see SI, Table S2) [11,48,49]. Moreover, the micro-supercapacitor exhibits high coulombic efficiencies (>95%, calculated by the method described in Supporting Information) over the 10,000 cycles, indicating high charge transfer efficiencies over long-term cycling. The

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Fig. 4. Electrochemical characterization of CN/CNT patterned micro-supercapacitors in 1 M H2SO4 aqueous electrolyte: (a) CV curves at scan rates from 5 to 100 mV s
1. (b) Galvanostatic charge/discharge curves at different current densities from 0.3 to 1.5 mA cm
2. (e) Cyclic stability and coulombic efficiencies over 10,000 cycles. (d) Comparative Ragone plots of contemporary micro-supercapacitors [5,12,30,51e53] and the present CN/CNT patterned micro-supercapacitors. (e) CV curves at a scan rate of 20 mV s
1 from 2 to 80  C. (f) Nyquist plots under different temperatures from 2 to 80  C. (A colour version of this figure can be viewed online.)

SEM images of CN/CNT micro-electrodes after testing over 10,000 cycles have been provided in Fig. S13, demonstrating the structural stability of CN/CNT micro-electrodes over charge/discharge cycles. The excellent cyclic stability of the present micro-supercapacitor is mainly attributed to the substantially enhanced mechanical properties of the 3D CN/CNT micro-electrodes. To demonstrate the synergistic effects of CNs on the electrochemical performance of CN/CNT microsupercapacitors, bare CNT microsupercpacitors without CNs were characterized under the same conditions for comparison. As shown in Fig. S14, the CNT microsupercapacitors exhibit a capacitance of 8.15 mF cm
2 at a current density of 0.3 mA cm
2, which is an order of magnitude lower than that of CN/ CNT microsupercapacitor. Moreover, the bare CNT microsupercapacitor suffers poor rate capability and cyclic stability, further demonstrating the superior electrochemical performance of microsupercapacitors after growth of CNs.

A comparative Ragone plot (normalized by the active area of the CN/CNT patterned micro-supercapacitor) is given in Fig. 4d to compare the performance of the present micro-supercapacitor with various contemporary carbon-based micro-supercapacitors. Energy and power densities calculated from the galvanostatic charge/discharge measurement technique are provided here for comparison and should be complementary to reflect the overall performance of the device. The present micro-supercapacitor exhibits an energy density up to approx. 16 mWh cm
2 and delivers a power density up to approx. 3.6 mW cm
2. Moreover, as shown in Fig. S15, the volumetric energy density of the present microsupercapacitor reaches up to 2 mWh cm
3, which is almost 10 times higher than that of a commercial 3.5V/25-mF supercapacitor [50]. Furthermore, the device in this work deliver a power density of approx. 0.45 W cm
3, which is almost 75 times higher than that of a typical lithium thin-film battery [50]. These performance

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metrics are significantly higher than those of contemporary carbon-based micro-supercapacitors [5,12,30,51e53], indicating the outstanding overall performance of the present CN/CNT patterned micro-supercapacitor. Furthermore, to further increase the working voltage of the microsupercapacitors, MnO2/CN/CNTCN/CNT asymmetric devices are prepared, and their electrochemical performance is provided in Fig. S16, demonstrating the high superiority of 3D CN/CNT structures as nanotemplates for pseudocapacitive materials. The electrochemical performance of CN/CNT-based asymmetric microsupercapacitors could be further improved by choosing appropriate high-performance pseudocapacitive materials, electrolyte and optimizing the mass loading of active materials. Thermal influence on the electrochemical performance of CN/ CNT patterned micro-supercapacitors has been investigated by electrochemically characterizing the micro-supercapacitor at different temperatures (ranging from 2 to 80  C). Fig. 4e shows the CV curves of the micro-supercapacitor at a fixed scan rate of 20 mV s
1 under different temperatures. The area and shape of CV

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loop of the micro-supercapacitor change little with operating temperature rising from 2 to 80  C, demonstrating the operating temperature (ranging from 2 to 80  C) has few influences on the capacitance of the present micro-supercapacitor. Comparative Nyquist plots for the micro-supercapacitor at different operating temperatures are shown in Fig. 4f. All the measured impedance spectra display two semicircles, which generally corresponds to the charge transfer process (the first semicircle in the plot) and the migration within the surface layer (the second semicircle in the plot) [54,55]. The bulk electrolyte resistance, as shown in Fig. 4f, changes little under different temperatures, which could be explained by the high structural stability of carbon structures and the dominant double-layer capacitance in the present microsupercapacitors. The cyclic stability of as-prepared micro-supercapacitor under a temperature of 50  C was measured over 1,000 galvanostatic charge/discharge cycles at a current density of 3 mA cm
2 in a voltage range from 0 to 1 V. As shown in Fig. S17, the microsupercapacitor experiences a gradual decrease in capacitance and shows a capacitance retention of 91% over 1000 cycles at a

Fig. 5. Electrochemical characterization of the flexible micro-supercapacitor: (a) CV curves at different scan rates from 10 to 100 mV s
1 with a voltage range between 0 and 1 V. (b) Galvanostatic charge/discharge curves at different current densities from 0.8 to 2.4 mA cm
2. (c) Areal capacitances and capacitance retention as a function of different current densities. (d) Cyclic stability and coulombic efficiencies over 5000 cycles. (e) Capacitance retentions of the flexible micro-supercapacitor under different bending angles of 30 , 60 , 90 , 120 and 180 (the inset shows its CV curves at a scan rate of 50 mV s
1 under different bending angles). (f) Capacitance retention under different cyclic bending times (the inset shows its CV curves at a scan rate of 50 mV s
1 under different bending cycles). (A colour version of this figure can be viewed online.)

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temperature of 50  C. The electrochemical performance of flexible microsupercapacitor has also been systematically tested and the results are shown in Fig. 5. CV curves recorded at different scan rates ranging from 10 to 100 mV s
1 (see Fig. 5a) display almost rectangular shapes, only showing some shape deformation at scan rates up to 100 mV s
1. Fig. 5b shows galvanostatic charge/discharge curves of the flexible micro-supercapacitor at different current densities ranging from 0.8 to 2.4 mA cm
2. Based on the charge/ discharge curves, the areal capacitance of the flexible microsupercapacitor reaches approx. 48 mF cm
2 at a current density of 0.8 mA cm
2. Areal capacitances and capacitance retention of the flexible micro-supercapacitor are plotted as a function of corresponding currents in Fig. 5c. As the current density increases from 0.8 to 4 mA cm
2, areal capacitances of the micro-supercapacitor display a gradual attenuation from 48 mF cm
2 at a current density of 0.8 mA cm
2 to 36 mF cm
2 at a current density of 4 mA cm
2, with a capacitance retention of 75%, indicating a good rate capability. Cyclic stability of the flexible micro-supercapacitor was measured over 5,000 galvanostatic charge/discharge cycles at a current density of 2 mA cm
2 in the voltage range from 0 to 1 V. As shown in Fig. 5d, the present flexible micro-supercapacitor exhibits a gradual decrease in capacitance over 5,000 cycles, with an overall capacitance retention of approx. 92% compared to the first cycle. Furthermore, the flexible micro-supercapacitor exhibits high coulombic efficiencies >94% over 5,000 cycles, indicating high charge transfer efficiencies over long-term cycling. To test the structural flexibility and stability of the flexible micro-supercapacitor, CV curves of the present microsupercapacitor were recorded at different applied bending angles (a) of 30 , 60 , 90 , 120 and 180 . As shown in Fig. 5e (inset), under various bending states, CV curves show few changes as compared to the original state (without bending). Fig. 5e displays over 90% capacitance retention of the micro-supercapacitor even when the device is bent up to 180 , indicating the excellent flexibility of the present micro-supercapacitor. Moreover, to meet the deformation stability requirements for practical applications, the flexible microsupercapacitor was further subjected to a consecutive bending situation, and the cycling test result is provided in Fig. 5f. The CV curves of the flexible micro-supercapacitor under different cyclic bending times are shown in Fig. 5f (inset), and based on CV loops, the present micro-supercapacitor displays a slight decrease in capacitance with a final capacitance retention of 91% after 100 bending cycles as compared to the original state. All the results reveal that the flexible micro-supercapacitor possesses outstanding structural flexibility and stability, suggesting that it is a promising candidate for power supplies in flexible electronic devices. Moreover, the leakage current and self-discharge results of the flexible microsupercapacitors are provided in Fig. S18, corroborating that the devices are promising for use in future energy storage systems. 4. Conclusions In summary, 3D hybrid electrodes with CNs selectively grown on CNT patterns have been prepared for on-chip micro-supercapacitors through two-step MPCVD process. Due to the higher binding energy of carbon atom absorbed on CNT surface, CNs preferentially grow on CNT arrays, rendering gaps (CN-free zone) between adjacent electrodes. The selective growth of CNs on CNT patterns provides a novel micro-supercapacitor fabrication technique with highly controllable gap width and high utilization efficiency of electrode materials along the axial direction, which will be almost impossible to achieve by the conventional multistep micro-supercapacitor fabrication approaches. The as-prepared micro-supercapacitors exhibit high areal capacitance, high rate

capability, long cycle life and outstanding thermal stability, which are attributed to the synergistic effect between CNs and CNT arrays, and the improved mechanical properties of such hybrid electrodes. Moreover, the flexible micro-supercapacitors were easily obtained by peeling off the H3PO4/PVA electrolyte containing the microelectrodes. The electrochemical performance and functional flexibility have been investigated to demonstrate its great potential as power supply for flexible electronic devices. The CN/CNT hybrid structures perfectly serve as 3D electrodes for on-chip microsupercapacitors, also showing tremendous potential as nanotemplates for deposition of pseudocapacitive materials in asymmetric systems to further improve the electrochemical performance of micro-supercapacitors. Acknowledgements The authors gratefully acknowledge the Financial supports from University Basic Research Fund of China (06500105), Postdoctoral Science Fund of China (11175360, 2019M650333), National Natural Science Foundation of China (51532002, 51872027and 51802297), Beijing Natural Science Foundation (L172023), and National Basic Research Program of China (2017YFE0113500). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.003. References [1] M. Beidaghi, Y. Gogotsi, Capacitive energy storage in micro-scale devices: recent advances in design and fabrication of micro-supercapacitors, Energy Environ. Sci. 7 (3) (2014) 867e884. [2] Z.L. Wang, Toward self-powered sensor networks, Nano Today 5 (6) (2010) 512e514. [3] N.A. Kyeremateng, T. Brousse, D. Pech, Microsupercapacitors as miniaturized energy-storage components for on-chip electronics, Nat. Nanotechnol. 12 (1) (2017) 7. [4] P.F. Ribeiro, B.K. Johnson, M.L. Crow, A. Arsoy, Y. Liu, Energy storage systems for advanced power applications, Proc. IEEE 89 (12) (2001) 1744e1756. [5] M.F. El-Kady, R.B. Kaner, Scalable fabrication of high-power graphene microsupercapacitors for flexible and on-chip energy storage, Nat. Commun. 4 (2013) 1475. [6] G. Xiong, P. He, B. Huang, T. Chen, Z. Bo, T.S. Fisher, Graphene nanopetal wire supercapacitors with high energy density and thermal durability, Nano Energy 38 (2017) 127e136. [7] C. Kang, D. Cao, Y. Liu, Z. Liu, R. Liu, X. Feng, D. Wang, Y. Ma, High loading carbon nanotubes deposited onto porous nickel yarns by solution imbibition as flexible wire-shaped supercapacitor electrodes, J. Energy Chem. 27 (3) (2018) 836e842. [8] Q. Yang, Z. Xu, C. Gao, Graphene fiber based supercapacitors: strategies and perspective toward high performances, J. Energy Chem. 27 (1) (2018) 6e11. [9] X. Mu, J. Du, Y. Li, H. Bai, H. Zhao, Z. Wei, B. Huang, Y. Sheng, Z. Zhang, E. Xie, One-step laser direct writing of boron-doped electrolyte as all-solid-state microsupercapacitors, Carbon 144 (2019) 228e234. [10] X. Li, G.-L. Xu, F. Fu, Z. Lin, Q. Wang, L. Huang, J.-T. Li, S.-G. Sun, Room-temperature synthesis of Co (OH) 2 hexagonal sheets and their topotactic transformation into Co3O4 (1 1 1) porous structure with enhanced lithium-storage properties, Electrochim. Acta 96 (2013) 134e140. [11] W. Gao, N. Singh, L. Song, Z. Liu, A.L.M. Reddy, L. Ci, R. Vajtai, Q. Zhang, B. Wei, P.M. Ajayan, Direct laser writing of micro-supercapacitors on hydrated graphite oxide films, Nat. Nanotechnol. 6 (8) (2011) 496. [12] M.S. Kim, B. Hsia, C. Carraro, R. Maboudian, Flexible micro-supercapacitors with high energy density from simple transfer of photoresist-derived porous carbon electrodes, Carbon 74 (2014) 163e169. [13] K.U. Laszczyk, K. Kobashi, S. Sakurai, A. Sekiguchi, D.N. Futaba, T. Yamada, K. Hata, Lithographically integrated microsupercapacitors for compact, high performance, and designable energy circuits, Adv. Energy Mater. 5 (18) (2015). [14] N. Kurra, Q. Jiang, H.N. Alshareef, A general strategy for the fabrication of high performance microsupercapacitors, Nano Energy 16 (2015) 1e9. [15] G. Xiong, C. Meng, R.G. Reifenberger, P.P. Irazoqui, T.S. Fisher, A review of graphene-based electrochemical microsupercapacitors, Electroanalysis 26 (1) (2014) 30e51. [16] Z.-S. Wu, Y. Sun, Y.-Z. Tan, S. Yang, X. Feng, K. Müllen, Three-dimensional

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