Constructing Ultrahigh-Capacity ZincâNickelâCobalt Oxide@Ni(OH)2

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Constructing Ultrahigh#Capacity Zinc#Nickel#Cobalt Oxide@Ni(OH)2 Core-Shell Nanowire Arrays for HighPerformance Coaxial Fiber-Shaped Asymmetric Supercapacitors Qichong Zhang, Weiwei Xu, Juan Sun, Zhenghui Pan, Jingxin Zhao, Xiaona Wang, Jun Zhang, Ping Man, Jiabin Guo, Zhenyu Zhou, Bing He, Zengxing Zhang, Qingwen Li, Yuegang Zhang, Lai Xu, and Yagang Yao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03507 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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Constructing Ultrahigh˗Capacity Zinc˗Nickel˗Cobalt Oxide@Ni(OH)2 Core-Shell Nanowire Arrays for High-Performance Coaxial Fiber-Shaped Asymmetric Supercapacitors Qichong Zhang+a,c, Weiwei Xu+b, Juan Sun+a,d, Zhenghui Pana, Jingxin Zhaoa, Xiaona Wanga, Jun Zhanga, Ping Mana, Jiabin Guoa, Zhenyu Zhoua,Bing Hea, Zengxing Zhangc, Qingwen Lia,d, Yuegang Zhanga, Lai Xu*b and Yagang Yao*a a. Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China b. Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199 Ren’ai Road, Suzhou, Jiangsu 215123, P. R. China c. Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, P. R. China d. School of Physical Science and Technology, ShanghaiTech University, Shanghai 200120, P. R. China

[*] Email:[email protected] [email protected] [+] These authors contribute equally to this work.

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ABSTRACT Increased efforts have recently been devoted to developing high-energy-density flexible supercapacitors for their practical applications in portable and wearable electronics. Although high operating voltages have been achieved in fiber-shaped asymmetric supercapacitors (FASCs), low specific capacitance still restricts the further enhancement of their energy density. This article specifies a facile and cost-effective method to directly grow three-dimensionally well-aligned zinc-nickel-cobalt oxide (ZNCO)@Ni(OH)2 nanowire arrays (NWAs) on a carbon nanotube fiber (CNTF) with an ultrahigh specific capacitance of 2847.5 F/cm3 (10.678 F/cm2) at a current density of 1 mA/cm2, These levels are approximately five times higher than those of ZNCO NWAs/CNTF electrodes (2.10 F/cm2) and four times higher than Ni(OH)2/CNTF electrodes (2.55 F/cm2). Benefiting from their unique features, we successfully fabricated a prototype coaxial FASC (CFASC) with a maximum operating voltage of 1.6 V, which was assembled by adopting ZNCO@Ni(OH)2 NWAs/CNTF as the core electrode and a thin carbon layer of coated vanadium nitride (VN@C) NWAs on a carbon nanotube strip (CNTS) as the outer electrode with KOH poly(vinyl alcohol) (PVA) as the gel electrolyte. A high specific capacitance of 94.67 F/cm3 (573.75 mF/cm2) and an exceptional energy density of 33.66 mWh/cm3 (204.02 µWh/cm2) were achieved for our CFASC device, which represents the highest levels of fiber-shaped supercapacitors to date. More importantly, the fiber-shaped ZnO-based photodetector is powered by the integrated CFASC and it demonstrates excellent sensitivity in detecting UV light. Thus, this work paves the way to construct ultrahigh˗capacity electrode materials for next-generation wearable energy-storage devices. KEYWORDS:

zinc˗nickel˗cobalt

oxide,

core-shell

nanostructure,

supercapacitor, wearable electronics


coaxial

asymmetric

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As efficient energy storage and supply devices for next-generation portable and wearable electronics, fiber-shaped supercapacitors (FSCs) are attracting tremendous attention as a result of the integration of the high power density, fast charge/discharge capability, long cycle life, light weight, tiny volume and extraordinary weavability.1-14 However, small specific capacitance and low operating voltage limit their practical applications in high-energy-density wearable devices. Constructing asymmetric supercapacitors is a very effective way to extend the voltage window and accordingly increase the energy density.15-25 Their use in fiber-shaped asymmetric supercapacitors (FASCs) has resulted in higher operating voltages, but their low specific capacitance still restricts further enhancement of their energy density. This limitation is largely due to the characteristics of their electrode materials. Thus, remarkably capacitive fibrous electrode materials with simple and cost-effective synthesis methods are highly desirable. Among the various pseudocapacitive materials currently under investigation, complex oxides of transitional metals have been considered the most promising candidates for high-performance electrode materials because of their high electronic conductivity, excellent electrochemical capability, low cost and environmental benignity.26-30 Nevertheless, further improving the specific capacitance of single-component complex oxides remains a stumbling block. A feasible approach to address this issue is to construct hierarchical core-shell heterostructures consisting of highly conductive materials as the core and well-known pseudocapacitive transitional metal oxides/hydroxides as the shell layer.31-35 Such a unique core-shell structure is expected to offer a convenient ion diffusion path and provide more electroactive sites for pseudocapacitive reaction. For example, zinc-nickel-cobalt oxide (ZNCO) nanowire arrays (NWAs) have better electronic conductivity and electrochemical performance than the corresponding mono-metal and binary-metal oxides.36 These prominent features make ZNCO NWAs an attractive secondary substrate for the


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growth of other active materials. Herein, we report a facile and cost-effective method to fabricate three-dimensionally well-aligned hierarchical ZNCO@Ni(OH)2 core-shell heterostructures directly grown on a carbon nanotube fiber (CNTF) as a novel binder-free electrode, where the well-aligned ZNCO NWAs act as the core and ultrathin Ni(OH)2 nanosheets serving as the shell layer. Benefiting from their intriguing structural features, the as-fabricated ZNCO@Ni(OH)2/CNTF electrodes exhibited an ultrahigh specific capacitance of 2847.5 F/cm3 (10.678 F/cm2), which is significantly higher than other reported fibrous electrode materials. In this work, we rationally constructed high-performance coaxial FASCs (CFASCs) with a maximum operating voltage of 1.6 V, which were assembled by adopting ZNCO@Ni(OH)2/CNTF as the core electrode and a thin carbon layer of coated vanadium nitride (VN@C) NWAs on carbon nanotube strip (CNTS) as the outer electrode with KOH poly(vinyl alcohol) (PVA) as the gel electrolyte. As a new type of wearable energy-storage device, our optimized CFASC exhibited the highest measured specific capacitance and energy density levels ever recorded for an FSC. Encouragingly,

the

as-fabricated

UV

photodetectors

were

stably

operated

by

our

high-energy-density CFASCs. Figure 1a shows a schematic fabrication process for the three-dimensionally well-aligned hierarchical ZNCO@Ni(OH)2 core-shell heterostructure NWAs directly grown on a CNTF as a novel binder-free electrode and the details are presented in the experimental section. As shown in Figure 1b, the entire CNTF surface was uniformly covered by the ZNCO NWAs after hydrothermal synthesis and subsequent high-temperature calcination. The magnified scanning electron microscopy (SEM) image in Figure 1c clearly demonstrates that the ZNCO NWAs were well distributed and highly aligned in the hybrid fiber. After the chemical bath deposition process, the ultrathin Ni(OH)2 nanosheets were homogeneously anchored onto the surfaces of the as-fabricated


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ZNCO NWAs, forming unique and three-dimensionally hierarchical core-shell heterostructures (Figure 1d). Uniform growth of the Ni(OH)2 nanosheets on the ZNCO NWAs was further demonstrated using transmission electron microscopy (TEM) (Figure 1e and 1f). The 0.256 nm lattice fringes in the high magnification image correspond to the (012) plane of α-Ni(OH)2. Furthermore, the X-ray diffraction (XRD) patterns of the ZNCO@Ni(OH)2 NWAs, pure ZNCO NWAs, and Ni(OH)2 nanosheets were also analyzed and compared(Figure 1g). The ZNCO NWAs exhibit diffraction peaks centered at 18.7°, 30.9°, 36.5°, 44.5°, 59.0° and 64.7° and slightly shifted in comparison with the spinel Co3O4, thusindicating forming Zinc/Nickel (Zn/Ni) co-doped Co3O4, which agrees with previously published results.36 The diffraction peaks at 11.4°, 23.1°, 34.0° and 60.4° can be indexed to the (003), (006), (012) and (110) phases of α-Ni(OH)2 (JCPDS card no. 38-0715). The XRD patterns of the ZNCO@Ni(OH)2 NWAs contain the major diffraction peaks of both the ZNCO and Ni(OH)2 without any extras, indicating that the fabricated core-shell heterostructures consist solely of ZNCO and α-Ni(OH)2. The chemical compositions and valence states of the ZNCO@Ni(OH)2 NWAs were investigated with X-ray photoelectron spectroscopy (XPS) (Figure S1), while the homogeneous distribution of Zn, Co, and Ni was evaluated using TEM mapping (Figure S2). These data further confirmed the ZNCO core/Ni(OH)2 shell hierarchical structure and composition.


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Figure 1. ZNCO@Ni(OH)2 NWAs fabrication and characterization. (a) Schematic fabrication process of the ZNCO@Ni(OH)2 NWAs on a CNTF. (b-c) SEM images of ZNCO NWAs/CNTF at increasing

magnifications.

(d)

SEM

image

of

ZNCO@Ni(OH)2

NWAs/CNTF.(e)

Low-magnification TEM image of the ZNCO@Ni(OH)2 NWA. (f) Higher magnification of the orange rectangle in (e). (g) XRD patterns of the ZNCO@Ni(OH)2 NWAs, pure ZNCO NWAs and Ni(OH)2 nanosheets. To investigate the influence in Co3O4 of doping with Zn/Ni, first principle calculations using density functional theory (DFT) were performed. First, the differential charge density of ZNCO was calculated. The optimized structure of ZNCO is shown in Figure 2a. As depicted in Figure 2b, there


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is a strong charge transfer on the surface: the Zn and Ni are wrapped around a layer of blue charge density, and the O is wrapped around a layer of yellow charge density. This phenomenon indicates that Zn and Ni have a strong charge depletion, while O has a charge accumulation, which means that the Zn/Ni serves as n-type dopants. More importantly, the Zn/Ni co-doping promotes surface activity and would further boost the growth of other active materials. Moreover, the density of states (DOSs) of the pristine Co3O4 and ZNCO were evaluated to compare their electronic properties. From Figure 2c, it can be clearly seen that the total DOS for pristine Co3O4 presents the character of a semiconductor, and the Fermi level (Ef) is located between the valance band (VB) and the conduction band (CB). The band gap is about 1.35 eV, which is a little smaller than the experimental value (1.6eV).37 This is because DFT calculation usually underestimates the energy gaps for its own limitation. In contrast, after the Zn/Ni is doped, the gap is narrowed to 0.87 eV because a new state is introduced to the band gap, which is close to the CB (Figure 2d). This indicates that the Zn/Ni is n-type doping and serves as the electron donor. This phenomenon may result from more electrons accumulating on the O atoms, leading to the downshift of the valance and conduction band edge of Co3O4 after the Zn/Ni is co-doped. Therefore, the narrowed band gap and the shift of the Ef towards CB demonstrate an enhancement of electron mobility and electrical conductivity, manifesting ZNCO NWAs a promising secondary substrate for the growth of other active materials. In order to verify these calculation results in theory, ZNCO NWAs/CNTF and CO NWAs/CNTF with the same mass loading of Ni(OH)2 nanosheets are measured with the GCD. Figures 2e and 2f compare these corresponding GCD curves at current densities of 1 mA/cm2 and 4 mA/cm2, demonstrating that ZNCO NWAs can effectively decrease voltage drop and enhance electrochemical capacitance.


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Figure 2. (a) The structure of ZNCO. (b) The isosurface of differential charge density. The blue and yellow colors represent the depletion and accumulation of electrons, respectively. (c) Calculated DOS of pristine Co3O4 (inset: the optimized structure of pristine Co3O4). (d) Calculated DOS of ZNCO (inset: the optimized structure of ZNCO). Ef represents the Fermi level. (e) and (f) ACS Paragon Plus Environment

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Comparisons of the GCD curves of ZNCO NWAs/CNTF and CO NWAs/CNTF with the same mass loading of Ni(OH)2 nanosheets at current densities of 1 mA/cm2 and 4 mA/cm2, respectively. A three-electrode measuring system was used to evaluate the electrochemical performance of the fabricated fibrous electrode materials in a 3M KOH aqueous solution. Figure 3a compares the typical galvanostatic charge/discharge (GCD) curves of the ZNCO@Ni(OH)2 NWAs/CNTF, ZNCO NWAs/CNTF, and Ni(OH)2/CNTF electrodes at the same current density. As expected, the discharge time for the ZNCO@Ni(OH)2 NWAs/CNTF electrode was significantly longer than that for the ZNCO NWAs/CNTF and Ni(OH)2/CNTF electrodes. This difference is ascribed to the increased electrical conductivity of the core electrode and the synergistic effects of the well-aligned ZNCO NWAs and ultrathin Ni(OH)2 nanosheets. Indeed, the fabricated ZNCO@Ni(OH)2 NWAs/CNTF electrode delivered an ultrahigh specific capacitance of 10.68 F/cm2 at a current density of 1 mA/cm2, which is approximately five times higher than that of the ZNCO NWAs/CNTF electrode (2.10 F/cm2) and four times higher than that of the Ni(OH)2/CNTF electrode (2.55 F/cm2). This value is, in fact, significantly higher than all other measurements reported for fibrous electrode materials in the current literature.38-40 The representative cyclic voltammetry (CV) curves of the ZNCO@Ni(OH)2 NWAs/CNTF electrode were determined at various scan rates from 1 to 10 mV/s (Figure 3b). It is important to note that a pair of redox peaks was clearly observed, demonstrating typical battery-type electrochemical behavior. Furthermore, the nonlinear and nearly symmetric GCD curves of the ZNCO@Ni(OH)2 NWA/CNTF electrode suggest enhanced pseudocapacitive properties and Faradaic reversibility (Figure 3c). The specific capacitances of the ZNCO@Ni(OH)2 NWAs/CNTF electrode were calculated from these GCD curves, and values as high as 2847.47 F/cm3 (10.68 F/cm2 ) were achieved at a current density of 1 mA/cm2 (Figure 3d). These levels were also maintained at 1826.67 F/cm3 (6.85 F/cm2 ) even when


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the current density was increased to 10 mA/cm2, demonstrating superior rate performance. A similar GCD analysis and specific capacitance calculations were performed for the ZNCO NWAs/CNTF electrode for comparison (Figure S3). The remarkable properties of the ZNCO@Ni(OH)2 NWAs/CNTF electrode make it a promising candidate for use in the construction of future wearable supercapacitors.

Figure 3. (a) Comparisons of the GCD curves of ZNCO@Ni(OH)2 NWAs/CNTF, ZNCO NWAs/CNTF and Ni(OH)2/CNTF electrodes at a current density of 1 mA/cm2. (b) CV curves of the ZNCO@Ni(OH)2 NWAs/CNTF electrode at different scan rates. (c) GCD curves of the ZNCO@Ni(OH)2 NWAs/CNTF electrode at different current densities. (d) Areal and volumetric specific capacitances of the ZNCO@Ni(OH)2 NWAs/CNTF electrode calculated from the GCD curves as a function of current density.


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Recently, we developed a feasible and effective method to directly grow VN@C NWAs on CNTFs and successfully applied it to create ultrahigh-energy CFASCs.15 In the present study, VN@C NWAs were also successfully fabricated on a highly conductive CNTS. The VN NWAs formed a uniform array structure when grown on the surface of the CNTS (Figure S4). Analysis of this structure using XRD demonstrates that all of the diffraction peaks correspond to cubic VN (Figure S5) (JCPDS card no. 35-0768). The electrochemical properties of the as-prepared VN@C NWAs/CNTS electrodes were also investigated using a 3M KOH aqueous solution with a three-electrode configuration. The CV curves of the VN@C NWAs/CNTS electrodes have pronounced redox peaks, typical pseudocapacitive behavior, when a potential window between -1.2 and -0.2 V and variable scan rates were used (Figure S6a). The nonlinear GCD curves of the VN@C NWAs/CNTS electrodes within the voltage range of -0.2 to -1.2 V are shown in Figure S6b, again confirming the pseudocapacitive characteristics of this electrode. In fact, the VN@C NWAs/CNTS electrode had an areal specific capacitance of 1543 mF/cm2 at a current density of 1.0 mA/cm2, far exceeding that of conventional carbon-based negative materials. Such exceptional electrochemical properties indicate that a VN@C NWAs/CNTS could be an outstanding negative electrode for high-energy-density flexible supercapacitors. To create a high-energy-density FSC for wearable energy storage applications, a prototype CFASC device was assembled using a ZNCO@Ni(OH)2 NWAs/CNTF as the core electrode, a VN@C NWAs/CNTS as the outer electrode and KOH-PVA as the gel electrolyte. The fabrication process of the CFASC is schematically illustrated in Figure S7. The schematic of the cross-sectional structure of the CFASC in Figure 4a clearly demonstrates that the aligned structure favored the rapid charge transport and diffusion of electrolyte ions. To wrap the VN@C NWAs/CNTS around the ZNCO@Ni(OH)2 NWAs/CNTF/KOH-PVA, the ends of the modified CNTF were fixed on two


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motors and the VN@C NWAs/CNTS was carefully attached to the surface and wrapped around the core while the two motors were running (Figure 4b). The area of the VN@C NWA/CNTS on the modified CNTF was controlled by the helical angle. The CV curves of the assembled VN@C NWAs/CNTS//ZNCO@Ni(OH)2 NWAs/CNTF CFASC device at a scan rate of 25 mV/s with different potential windows ranging from 0.4 to 1.6 V were rectangular-like without any obvious redox peaks (Figure 4c). Furthermore, the GCD curves of the CFASC at a current density of 9 mA/cm2 were symmetrically triangular in shape even at an operating potential as high as 1.6 V (Figure 4d), indicating that the device possesses ideal capacitive behavior and low equivalent series resistance.The areal specific capacitances and energy density of the assembled CFASC device were also calculated from GCD curves collected at 9 mA/cm2 as a function of the potential window (Figure 4e). Notably, the calculated areal specific capacitance significantly increased from 177.75 to 482.46 mF/cm2 when the operating potential was increased from 0.4 to 1.6 V. The areal energy density of the CFASC device was also similarly improved from 3.95 to 171.54 µWh/cm2.

Figure 4.(a) The cross-sectional structure of the CFASCs. (b) Wrapping of the VN@C NWAs/CNTS around the surface of the ZNCO@Ni(OH)2 NWAs/CNTF/KOH-PVA. (c) CV curves


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of the as-assembled CFASCs measured at different operating voltages at a constant scan rate of 25 mV/s. (d) GCD curves of the CFASCs collected over different voltages (0.4 to 1.6 V) at a current density of 9 mA/cm2. (e) Areal specific capacitance and energy density calculated based on the GCD curves obtained at 9 mA/cm2 as a function of the potential window. The CV curves of the CFASC device collected at scan rates ranging from 5 to 100 mV/s with a voltage window of 0 to 1.6 V exhibit quasi-rectangular shapes without noticeable redox peaks (Figure 5a). Thus, these CFASC devices appear to be remarkably reversible and exhibit desirable capacitive behaviors. The electrochemical performance of the as-assembled CFASC was further evaluated at different current densities from 3 to 15 mA/cm2 and the corresponding GCD curves were nearly linear and symmetric, again confirming the enhanced pseudocapacitive properties and Faradaic reversibility of our device (Figure 5b). As presented in Figure 5c, the specific capacitance of our CFASC was calculated to be as high as 94.67 F/cm3 (573.75 F/cm2) at a current density of 3 mA/cm2 and was maintained at 70.34 F/cm3 (423.75 F/cm2) when the current density was increased to 15 mA/cm2, indicating superior rate capability. A high energy density of 33.66 mWh/cm3 was achieved at a power density of 396 mW/cm3 and remained at 25.01 mWh/cm3 even at a high power density of 1,992 mW/cm3 (Figure 5d). These data considerably exceed previously reported FASCs, including

NiCo2S4/graphene

fiber//graphene

fiber,40

CuO@AuPd@MnO2/Cu

wire//Fe2O3@C/carbon fiber wire,23 MoS2-reduced graphene oxide/CNTF//reduced graphene oxide/CNTF,18

Co3O4/Ni

wire//reduced

MnO2/PEDOT:PSS/CNTF//ordered

microporous

graphene

oxide/carbon

carbon/CNTF42

and

fiber,41 graphene

nanosheets@TiN/carbon fiber//graphene nanosheets@Fe2N carbon fiber.43 The areal energy and power densities of the CFASC device are presented in Figure S8, and a Nyquist plot of the electrochemical impedance (measured in the frequency range from 10−2 to 105 Hz) is illustrated in


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Figure S9. The equivalent series resistance of our device was as low as 27.7 Ω and the nearlystraight line in the low frequency region demonstrates fast ion diffusion at the electrode/electrolyte interface. In addition, the as-assembled CFASC retained 90.3% of its initial capacitance after long-term charge/discharge cycling at 6 mA/cm2 for 3,000 cycles (Figure S10), revealing superior cycling stability. To demonstrate the potential application of the as-assembled CFASC in portable and wearable electronics, a series of mechanical flexibility tests were also performed. As illustrated in Figure 5e, there were negligible changes in the GCD curves at a current density of 10 mA/cm2 with different bending angles from 0° to 180°, highlighting the exceptional flexibility of our CFASC. Furthermore, 93.6% of the specific capacitance was retained after bending at 90° for more than 3,000 cycles (Figure 5f), and the corresponding comparison of electrochemical impedance spectroscopy was shown in Figure S11 , again confirming the mechanical stability of this CFASC device.

Figure 5. Electrochemicalcharacterization and flexibility analysis of the as-assembled CFASCs. (a) CV curves of the assembled CFASCs measured at different scan rates between 0 and 1.6 V. (b) GCD curves collected at different current densities between 0 and 1.6 V. (c) Areal and volumetric


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specific capacitances calculated from the GCD curves as a function of the current density. (d) Volumetric energy and power densities of our CFASCs compared with previously reported FASCs. (e) GCD curves of the as-assembled CFASCs bent at various angles at a current density of 10 mA/cm2. (f) Normalized capacitances of our CFASCs bent 90° for 3,000 cycles. In addition to enhancing the technical capabilities of wearable devices, increased practical functionality for the consumer should also be considered. For example, as the public becomes more aware of the harmful effects of ultraviolet (UV) light, real-time monitoring of UV exposure with wearable UV photodetectors will become increasingly important and sought after.44-45 The fabrication process of wearable UV photodetectors is schematically illustrated in Figure 6a, while a diagram of the final integrated configuration is shown in Figure 6b. SEM imaging shows that the ZnO NWAs were uniformly and perpendicularly grown on the CNTFs (Figure 6c). The aligned CNT sheet (ACNTS) dry-drawn from a spinnable array was both optically transparent (Figure 6d) and electrically conductive, supporting its use as a flexible transparent electrode. One end of the CNTF/ZnO NWAs were coated with polydimethylsiloxane (PDMS) before the ACNTS was wrapped around it to avoid contact between the ACNTS and CNTF. After an ethanol treatment, the ACNTS was tightly attached to the top of the ZnO NWAs leading to close contact (Figure 6e). The fiber-shaped supercapacitor-photodetector integrated configuration was fabricated by twisting the high-energy-density CFASC and the wearable UV photodetector together and leaving it overnight until the pre-coated gel PVA was solidified. The digital image and low-magnification SEM image of the as-assembled device are shown in Figure S12. As shown in Figure 6f, the fully charged CFASC was used to power the wearable UV photodetector without any external bias voltage and the photodetector exhibited a current increase upon UV light irradiation.


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Figure 6. (a) Schematic illustrations of the fabrication of the fiber-shaped UV photodetectors. (b) Schematic diagram of the integrated fiber-shaped supercapacitor-photodetector integrated configuration. (c) SEM images of the ZnO NWAs on a CNTF. The inset corresponds to the low-magnification SEM image. (d) Photograph of the transparent ACNTS (red rectangle). (e) SEM image of the ACNTSattached to the top of the ZnO NWAs.(f) The current responses of the fiber-shaped UV photodetector powered by a fully charged CFASC upon UV light irradiation. In summary, we have demonstrated the rational design and fabrication of three-dimensionally well-aligned hierarchical ZNCO@Ni(OH)2 core-shell heterostructures on a CNTF by employing highly conductive ZNCO NWAs as a secondary substrate with a facile and cost-effective method. Benefiting from the increased electrical conductivity of the core electrode and synergistic effects of the

well-aligned

ZNCO

NWAs

and

ultrathin

Ni(OH)2

nanosheets,

the

as-fabricated

ZNCO@Ni(OH)2 NWAs/CNTF exhibited an ultrahigh specific capacitance of 2847.5 F/cm3 (10.678 F/cm2). Furthermore, a high-performance CFASC device (maximum operating voltage of 1.6 V) was assembled by wrapping a VN@C NWAs/CNTS around the coreZNCO@Ni(OH)2 NWAs/CNTF. A high specific capacitance of 94.67 F/cm3 (573.75 mF/cm2) and an exceptional energy density of 33.66 mWh/cm3 (204.02 µWh/cm2) were achieved for our CFASC device, which represent the highest levels measured for an FSC to date. In addition, the device also delivered ACS Paragon Plus Environment

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exceeding flexibility in that its capacitance only decreased to 93.6% after bending 3,000 times. More importantly, the fiber-shaped photodetector was sensitive to UV light using the integrated CFASCs as the external power source. The successful construction of ultrahigh˗capacity ZNCO@Ni(OH)2 core-shell heterostructures opens the door to designing new electrode materials for next-generation wearable supercapacitors. Preparation of ZNCO@Ni(OH)2 NWAs/CNTF. The ZNCO NWAs on the CNTF were fabricated using a simple hydrothermal method followed by high-temperature calcination. The CNTFs were pretreated withO2 plasma for 5 min at 150 W. The fibers were then immersed in an aqueous solution (40 ml) containing0.873g Co(NO3)2·6H2O, 0.436g Ni(NO3)2·6H2O, 0.447g Zn(NO3)2·6H2O, 0.360g CO(NH2)2 and 0.074g NH4F that had been stirred for 40 min prior to transfer into a Teflon-lined stainless steel autoclave (50 ml inner volume). The autoclave was then sealed and kept at 130 °C for 5 h. After cooling to room temperature, the resulting CNTFs were washed repeatedly with distilled water and driedovernight at 60 °C. They were subsequently annealed into ZNCO NWAs/CNTF at 350 °C in air for 4 h with a heating rate of 5 °C/min. The synthesizedZNCO NWAs/CNTF was then suspended into a stirringsolution containing 20 g of Ni(SO4)2, 4 g of K2S2O8, 10 ml of aqueous ammonia (28%) and 200 ml of distilled water for 30 min. Finally, the ZNCO@Ni(OH)2 NWAs/CNTFwas washed with deionized water several times and dried at 60 °C in a vacuum overnight. Assemblingthe CFASC.The solidgel electrolyte was fabricatedby mixing 11.2 g KOH and 10g PVA in 100 ml deionized water under vigorous stirring at 95°C for 2 h. The ZNCO@Ni(OH)2 NWAs/CNTFelectrode was then coated with this PVA gel electrolyte and maintained at 60°C for 2h to evaporate the excess water. After drying, a VN@C NWAs/CNTSwas wrapped aroundaZNCO@Ni(OH)2 NWAs/CNTF.The area of the VN@C NWAs/CNTS could be easily


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adjusted by changing the helical angle. A second PVA gel electrolyte was then coated onto the constructed CFASCs. Fabrication of the fiber-shaped UV photodetectors. First of all, the ZnO seed solution was fabricated as previously reported. The CNFTs were fixed to a polytetrafluoroethylene (PTFE) framework using high-temperature adhesive tape and then repeatedly soaked in the ZnO seed solution and annealed at 60 °C for 2 h. Second, the resulting CNTFs attached to the PTFE framework were then transferred into a 50 ml Teflon-lined stainless steel autoclave containing 40 ml aqueous solution consisting of 0.03 M Zn(NO3)2·6H2O and 0.03 M (CH2)6N4. The autoclave was then sealed and maintained at 90 °C for 4 h. After cooling to room temperature naturally, the as-obtained CNT fibers were washed with distilled water several times and dried at 60 °C in a vacuum overnight. Finally, the ACNTSs were wrapped around the as-prepared ZnO NWAs and the wrapped ACNTSs were 3 layers. The ACNTS and CNTF serve as the electrodes of the fiber-shaped UV photodetectors. Fabrication

of

the

fiber-shaped

supercapacitor-photodetector

integrated

configuration.The CFASCs and fiber-shaped UV photodetectors were soaked in gel PVA for 10 min and annealed at 70 °C for 30 min. The fiber-shaped supercapacitor-photodetector integratedconfiguration was prepared by twisting the CFASC and fiber-shaped UV photodetector together and leaving it overnight until the gel PVA was solidified. Characterization of materials.The morphologies of the samples were characterized with a scanning electron microscope (Hitachi S-4800, 5 kV). X-ray diffraction patterns were obtained with a Rigaku D/MAX2500 V with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy was recorded on an ESCALab MKII X-ray photoelectron spectrometer with non-monochromatized Mg Kα X-rays as the excitation source. High-resolution TEM images were recorded on an FEI


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Tecnai G2 20 high-resolution transmission electron microscope at an acceleration voltage of 200 kV. Computational Methods.All the calculations were carried out by the “Vienna ab initio simulation package” (VASP 5.4.4).46 The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerh (PBE) function was chosen to describe the electronic interaction effect.47 A plane-wave basis set was employed within the framework of the projector augmented wave (PAW) method to describe the electron-ion interactions.48-49 To get accurate results, the cutoff energy was set to 450 eV. The convergence criteria of energy and force were 1.0×10-6 eV/atom and 0.05 eV/Å, respectively.To calculate the differential charge density, a three layer cell of the 001 surface of Co3O4 was built. A vacuum slab larger than 15Å was set to avoid the interaction between adjacent images. To acquire an accurate description of the electronic properties of the Co and Ni elements with 3d orbitals, the U was added by 6.5 eV and 4.5eV, respectively.50 Moreover, the k-point mesh was set as 7×7×7 to sample the Brillouin zone. Acknowledgements This work was supported by the National Key R&D Program of China (No. 2017YFB0406000), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No. QYZDB-SSW-SLH031), Collaborative Innovation Center of Suzhou Nano Science & Technology, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Natural Science Foundation of Shanghai (Nos. 16ZR1439400 and 17ZR1447700), the Natural Science Foundation of Jiangsu Province, China (Nos. BK20160399 and BK20140392), the Transformation of Scientific and Technological Achievements in Jiangsu Province (No. BA2016026), the Postdoctoral Foundation of Jiangsu Province (No. 1601065B), and the Science and Technology Project of Suzhou, China (Nos. SZS201508, ZXG201428 and ZXG201401).


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Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: Materials;Preparation of aligned carbon nanotube sheets; Electrochemical Performance Measurements; XPS spectra of the ZNCO@Ni(OH)2 NWAs;TEM mapping of Co, Zn and Ni in the ZNCO@Ni(OH)2 NWAs; Electrochemical characterization of the ZNCO NWAs/CNTF; SEM image of VN NWAs on the CNTF; XRD pattern of the VN NWAs; Electrochemical characterization of the VN@C NWA/CNTS; Schematic illustrations of the fabrication of the CFASC; Areal energy and power densities measured for our CFASC; Nyquist plot of the as-assembled CFASC; Cycling performance of the as-assembled CFASC; The corresponding comparison of electrochemical impedance spectroscopy of our CFASCs bent 90° for 3,000 cycles The digital image and low-magnification SEM of integrated configuration.These materials are available free of charge via the internet at http://pubs.acs.org.

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Constructing Ultrahigh-Capacity ZincâNickelâCobalt Oxide@Ni(OH)2

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