One-Pot Template-Free Strategy toward 3D Hierarchical Porous

Jun 14, 2018 - ... and a transmission electron microscopy (TEM) (JEM-2100 (HR) JOEL Ltd., Japan). ..... (45) Figure 6c shows CV curves within the pote...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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One-Pot Template-Free Strategy toward 3D Hierarchical Porous Nitrogen-Doped Carbon Framework in Situ Armored Homogeneous NiO Nanoparticles for High-Performance Asymmetric Supercapacitors Liya Ma, Guanglin Sun, Jiabing Ran, Song Lv, Xinyu Shen, and Hua Tong*

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Key Laboratory of Analytical Chemistry for Biology and Medicine, Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China S Supporting Information *

ABSTRACT: The composites based on graphitic carbon and transitional metal oxides are regarded as one of the most promising electrochemical materials owing to the synergistic combination of the advantages of both superior electrical conductivity and high pseudocapacitance. In this work, a simple one-pot templatefree strategy for the preparation of three-dimensional hierarchical porous nitrogendoped carbon framework in situ armored NiO nanograins (NCF/NiO) by an ammonia-induced method assisted by the pyrolysis of a decomposable salt is reported. Due to such unique architecture and homogeneously dispersed nanoparticles, the as-prepared NCF/NiO-2 hybrid exhibits a large specific surface area (412.3 m2 g−1), a high specific capacitance (1074 F g−1 at 1 A g−1), good rate capability (820 F g−1 at 20 A g−1), and outstanding cycling performance (almost no decay after 5000 cycles). Moreover, the solid-state asymmetric supercapacitor, assembled with NCF/NiO-2 and NCS electrodes, can achieve a high cell potential of 1.6 V and deliver a superior specific capacitance of 113 F g−1 at 1 A g−1 with a maximum energy density of 40.18 W h kg−1 at a power density of 800 W kg−1, consequently, giving rise to stable cycling performance (94.3% retention over 5000 cycles). The prepared devices are shown to power 20 green light-emitting diodes efficiently. These encouraging results open up a wide horizon for developing novel carbon-supported metal oxide electrode materials for high rate energy conversion and storage devices. KEYWORDS: one pot, template-free, hierarchical, nickel oxide, asymmetric supercapacitors

1. INTRODUCTION Due to the ever-growing global demand for energy together with the rapid depletion of traditional fossil fuels, the energy shortage has to be addressed urgently, and substantial efforts have, therefore, been made to develop advanced energy conversion and storage devices.1,2 Among diverse energy storage systems, supercapacitors with high energy density have been recognized as promising systems for power-based applications.3−5 Conventional supercapacitors (i.e., electric double layer capacitors (EDLCs) and pseudocapacitors) are far from satisfactory because of some intrinsic defects (low specific capacitance and poor cycling stability),6,7 so a rational hybridization of EDLCs and pseudocapacitors has been considered as an ideal alternative strategy to develop highperformance supercapacitors. Among various hybrid electrode materials, nanostructured carbon/metal oxide composites are very appealing candidates.8 Briefly, the superb electronic conductivity of the carbon matrix facilitates rate capability and power density of the hybrids at a high current density, whereas the superior electrochemical activity of metal oxides endow the composites with large specific capacity and energy density.9−11 According to this © XXXX American Chemical Society

design philosophy, researchers have done much corresponding work, and they primarily focused on zero, one, or twodimensional hybridizations of the carbon matrix and metal oxide.12−14 Resultantly, the as-prepared composites demonstrated a low surface area and inferior hierarchical porosity, which limits electrolyte penetration and ion diffusion in the internal structures. During the fast charging/discharging process, the sluggish ion diffusion can result in undesirable capacitance loss and lower the rate capability and power density.15 Recently, some studies have suggested that porous three-dimensional (3D) nanostructures can overcome these drawbacks. The 3D carbonaceous network with a highly opened structure and hierarchical porosity has a continuous 3D pathway to guarantee the rapid electron transfer and allow high accessibility to electrolyte ions across the large electrode− electrolyte interface.16,17 They have been widely used as substrates and backbones, combining with metal oxides of high capacitance. For instance, Hao et al. fabricated NiCo2O4 Received: April 13, 2018 Accepted: June 14, 2018 Published: June 14, 2018 A

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Here, we demonstrate a simple and novel one-pot templatefree strategy for the synthesis of robust 3D hierarchical porous nitrogen-doped carbon framework/nickel oxide (NCF/NiO) hybrids via an ammonia-induced method assisted by thermal decomposition of nitrates. The pyrolysis of ammonium nitrate released abundant gas to form macropores, mesopores, and even micropores in the as-prepared 3D nanostructure by the ammonia induction, which enhanced the hierarchical porosity of the composites. As expected, the NCF/NiO composite exhibited exceptional electrochemical performance. In the three-electrode cell, the NCF/NiO-2 hybrid had a high specific capacitance of 1074 F g−1 at 1 A g−1 and outstanding cycling stability with almost no decay over 5000 cycles at 5 A g−1. Moreover, the assembled asymmetric supercapacitor (ASC) delivered a maximum energy density of 40.18 W h kg−1 at a power density of 800 W kg−1 and had a long cycle life. On the one hand, such a versatile 3D porous carbon texture with hierarchical pores functions as a physical buffering matrix to reinforce the relation with NiO NPs, delays the coarsening evolution of NiO NPs at high temperatures, and improves the conductivity and formation of accessible EDLC.24,26 On the other hand, homogeneously distributed NiO NPs are beneficial to the highly active surface area and relatively high capacitance.27 Furthermore, the ammonia treatment and the addition of ammonium nitrate may enhance the doped nitrogen content in favor of the conductivity and reaction activity of the electrode.28 Considering the simplicity of the synthesis process and the excellent electrochemical properties of the NCF/NiO composite, the one-pot template-free method can be one of the solutions to tackle down very challenging energy storage issues for the large-scale practical applications with low cost and efficiency.

nanoneedle array/carbon aerogel and NiCo2S4 nanotube array/carbon aerogel composites by a two-step process of lyophilization followed by the hydrothermal method, and discovered their applications in asymmetric supercapacitors (ASCs).18 Kim et al. developed micro- and mesoporous silicon carbide substrates and coated their surface with Fe 3O4 nanoparticles (NPs) by chemical deposition for asymmetric supercapacitors with high energy density.19 However, the synthesis methods of these hybrid-structure electrode materials required separate preparation of the carbonaceous backbone before the fabrication of carbon/metal oxide composites. Also, multiple complex steps are involved in their preparation process, which contributes to high production costs, restricting their large-scale manufacturing. Moreover, the weak heterogeneous connections between the metal oxides and the carbon matrix via a multistep method inevitably induce low structural strength and high junction resistances, which harm the rate capability and cycling performance. In this regard, one-pot approaches become appealing. Yu et al. discovered a one-step approach for the preparation of porous carbon aerogel/NiO composites via sol−gel and freezedrying (ice template removal) methods and their application in supercapacitors.20 Besides, Yu et al. demonstrated an in situ synthesis of template-assisted graphene-enhanced 3D hierarchical porous carbon nanobelt networks uniformly anchored with polycrystalline Fe3O4 nanoparticles as a supercapacitor electrode.21 In these composites, the metal oxide nanoparticles were homogeneously dispersed into the 3D porous carbon frameworks, which contributed to their good electrochemical properties in three-electrode systems. Nevertheless, the employment of these templates or structure-directed agents not only boosts their production cost but also increases the complexity of the need for the template removal step. Additionally, these composites are not assembled into a supercapacitor device to explore their practical applications. Therefore, the development of a one-step and template-free fabrication proposal for preparing high-performance 3D metal oxides/carbon composite electrodes remains a scientific challenge. In our previous work, we have found that the ammonia treatment can induce the generation of anisotropic pores in chitosan hydrogels; whereas the alkaline condition provided by ammonia and the compartment effect of the hydrogel crosslinking system also facilitate preintroduced metal salts in the hydrogel to be converted into nanosized metal hydroxides.22 To the best of our knowledge, this approach has not been applied to the prepared supercapacitor electrode materials. The ammonia-induced method can not only efficiently yield plentiful anisotropic pores but also effectively promote the uniform formation of metal hydroxide nanoparticles. Moreover, the ammonia treatment can also simplify the preparation procedures and reduce the production costs without the use and removal of templates. Also, chitosan, a cost-effective natural biopolymer with abundant amino and amide groups, is considered as a sustainable precursor for the self-doped carbon framework.23,24 As has been noted, NiO, a typical transition metal oxide material, possesses many merits of superior theoretical specific capacitance (2584 F g−1 within 0.5 V), robust chemical/thermal stability, environmental benignity, easy attainability, and cost effectiveness.25 Herein, it was selected as a model of metal oxides to combine with carbon materials.

2. EXPERIMENTAL SECTION 2.1. Preparation of NCF/NiO Composites. At 45 °C, 0.5 g of chitosan was dissolved in 40 mL of acetic acid solution (2 vol %) with vigorous stirring to form a homogenous mixture. Subsequently, 0.5 g Ni(Ac)2·4H2O and 0.5 g NH4NO3 were dissolved in the previously mixed solution. Afterwards, genipin (0.05 g) was added to the above solution to form a hydrogel. Then, the solidified hydrogel was subjected to NH3 treatment (in a sealed box full of ammonia). Next, the hydrated samples were air-dried at room temperature. The resulting samples were annealed in a horizontal tubular furnace at 700 °C for 1 h at a ramp rate of 5 °C min−1 under a flow of N2, and then they were post-heat treated at 300 °C for 30 min in an ambient atmosphere, yielding NCF decorated with NiO NPs (NCF/NiO-2). For comparison, different composite materials labeled as NCF/NiO-1 and NCS were fabricated by the same procedures and conditions as that of the NCF/NiO-2 but by adding 0.25 and 0.0 g of Ni(Ac)2· 4H2O, respectively. At the same time, the raw CS/NiO was also prepared by a similar approach without ammonium nitrate and NH3 treatment. 2.2. Characterization Methods. The macroscopic pore structure of the hydrated sample was observed using a stereoscopic microscope (SM) (XTL-165, Phoenix, China). The morphologies and microstructures of the samples were investigated with a field emission scanning electron microscope (SEM) (Sigma, Zeiss, Germany) coupled with an energy-dispersive X-ray spectroscopy (EDS) (GENESIS, AMETEK) and a transmission electron microscopy (TEM) (JEM-2100 (HR) JOEL Ltd., Japan). The Raman spectra were recorded using a Raman microspectrometer (LabRam HR800, Jobin Yvon, France). The phase was examined from 10 to 90° by Xray diffraction (XRD) (PANalytical, Holland) with Cu Kα (1.5406 Å) radiation. Thermogravimetric analyses of the as-prepared samples were performed using a Perkin Elmer Pyris Diamond analyzer from room temperature to 800 °C with a heating rate of 10 °C min−1 in air. B

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Preparation Processes of NCF/NiO Composites

The energy density (E, W h kg−1) and power density (P, W kg−1) of ASCs were calculated by the equations as follows

The Fourier transform infrared (FT-IR) spectra were recorded with a Nicolet 5700 spectrometer (Thermo Scientific). The N2 adsorption− desorption isotherms were measured using a Micromeritics TriStar II 3020 system at 77 K. The surface composition and chemical states of the composites were confirmed by X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250Xi with an Al-monochromic X-ray source). 2.3. Electrochemical Measurements. Electrochemical tests were performed using a CHI 660E electrochemical workstation (CH Instruments, Shanghai, China). A three-electrode electrochemical set-up was employed to study the electrochemical properties of the individual electrode, whereby a counter electrode of Pt foil, a reference electrode of Ag/AgCl and an aqueous 2 M KOH electrolyte were utilized. The active material mass of an individual electrode is about 4 mg. In solid-state asymmetric cells, cyclic voltammetry (CV) tests at various scan rates and galvanostatic charge−discharge (GCD) profiles at different current densities were both measured from 0 to 1.6 V. Electrochemical impedance spectroscopy (EIS) were performed in a frequency range of 0.01−105 Hz. The specific capacitances (Cs, F g−1) of the electrode in the three-electrode cell and the ASC system were calculated by the following equation Cs =

I Δt mΔV

E=

1 1 × × Cs·ΔV 2 2 3.6

P=

3600E Δt

(2) (3)

Where, Cs, ΔV, and Δt are the same meaning as shown in eq 1.

30

3. RESULTS AND DISCUSSION The synthetic strategy of NCF/NiO composites is schematically displayed in Scheme 1, in which a one-pot and in situ growth process was proposed. Briefly, nickel salt and ammonium nitrate were dissolved in chitosan solution, followed by the addition of genipin as the crosslinker. Then, the solidified hydrogels were subjected to NH3 treatment. During this process, the abundant amino and amide groups of chitosan act as special active sites for the coordination of Ni2+ to form ion complexes.31,32 Afterwards, with the NH3 treatment the in situ nucleating NPs grew homogeneously in the crosslinked organic substrates. The following reactions are involved during the NH3 treatment

(1)

Where, I (A) is the charge−discharge current, Δt (s) is the time of discharge, and m (g) is the mass of the active material in electrodes (for the ASCs, m is the total active material mass of the two electrodes), and ΔV (V) is the voltage window, respectively.29

NH3 + H 2O ↔ NH3·H 2O

Ni2 + + x NH3·H 2O ↔ [Ni(NH3)x ]2 + + x H 2O C

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Figure 1. (a) Photograph of the final hybrid hydrogels of NCF/NiO-2 with different states (the inset presents the stereoscopic microscopy image of the hydrogel); (b−e) SEM images of the typical NCF/NiO-2 with different magnifications.

NH3·H 2O ↔ NH 4 + + OH−

of the NCF/NiO-2 samples, from which a three-dimensional continuous and hierarchical pore structure can be observed owing to the NH3 treatment and thermal decomposition. These special macropores can act as the ion-buffering reservoirs to shorten the diffusion distance of ions to the internal surfaces.33 From Figure 1d,e, it can be seen that NiO NPs were well-encapsulated and uniformly incorporated into the carbon matrix which efficiently prevents the aggregation of NiO particles, and the microsized pores around nanoparticles can provide the active materials with the channels to fully react with the electrolyte. However, the CS/NiO composite without the NH3 treatment and nitrates had no visible pore structures (Figure S1). As shown in Figure 2a,b, the monodispersed NPs of NCF/ NiO-2 with an average size of 10−50 nm are homogeneously embedded in the entire carbon framework, which is in agreement with SEM images. Particularly, the white spots of the porous structure (as marked by dashed circles in Figure 2a) can be observed to be uniformly distributed throughout the resulting sample, which is due to the release of gaseous contents during the pyrolysis of nitrates and acetates.34 Such a nanopore architecture increases the contact area between the electrode and the electrolyte, and supplies mesopore channels toward the rapid diffusion of ions as well. Moreover, no large

[Ni(NH3)x ]2 + + 2OH− ↔ Ni(OH)2 + x NH3

In addition, the anisotropic interconnective pores were simultaneously prepared under ammonia treatment. In our previous work, the rationale behind the pore formation process has been reported in detail. Under ammonia treatment, the solid−liquid interface migrates from the top to the bottom, leading to the formation of 3D honeycomb-like pores.22 The subsequent calcination process induced the graphitization of the organic matrix. Simultaneously, the as-obtained NiO NPs were uniformly decorated into the carbon framework, as well as more macro-, meso-, and micropores were generated by the thermal decomposition of ammonium nitrate and acetate. Thereby, a typical 3D hierarchical porous nitrogen-doped carbon framework in situ armored homogeneous NiO NPs was obtained. The photograph of the NCF/NiO-2 hydrogel before and after pyrolysis is shown in Figure 1a. The sample by NH3 treatment took on apparent honeycomb-like anisotropic pores (the SM image inset in Figure 1a), and the unique architecture was maintained well even after desiccation and calcination. Figure 1b−e demonstrate different magnification SEM images D

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a, b) TEM images with different magnifications, (c) HRTEM image and (d) the corresponding SAED pattern of the NCF/NiO-2 sample. (e) EDS spectrum and (f) elemental mapping revealing the homogenous distribution of C, N, and Ni element of the NCF/NiO-2 composite.

IR spectrum of NCS, an obvious peak at 460 cm−1 of NCF/ NiO-2 belongs to the vibration of the Ni−O.34 Furthermore, Raman spectroscopy was applied to further investigate the structural features of the samples. As shown in Figure 3c, the bands located at around 520 cm−1 and 1090 cm−1 are attributed to one-phonon and two-phonon scattering at NiO nanostructures.39 In addition, the other two broad peaks centered at about 1340 (labeled as D-band) and 1580 cm−1 (labeled as G-band) are associated with amorphous/disordered and graphitic carbons, respectively, which account for the conductivity of the composite electrodes.23 These consequences further prove that the composites consisted of NiO and graphitized carbon. The NiO contents in the typical samples were determined by thermogravimetric analysis (TGA) in air (Figure 3d). This indicates that the mass ratio of NiO NPs was estimated to be 22.5, 46.0, and 48.0 wt % for the NCF/NiO-1, NCF/NiO-2, and CS/NiO, respectively. To unveil the surface composition and chemical states of NCF/NiO-2, XPS was performed as shown in Figure 4. The full-survey-scan spectrum (Figure 4a) suggests the presence of C, N, O, and Ni in the NCF/NiO-2. The observation of N with the atom percentage of 6.6% elucidates the successful heteroatom doping. Moreover, the C 1s spectrum (Figure 4b) of NCF/NiO-2 exhibits three peaks at 284.6, 285.6, and 288.9 eV, corresponding to the C−C/CC, C−N/C−O, and O− CO/CO, respectively.40 As for the N 1s spectrum (Figure 4c), two major peaks are assigned to pyridinic-N and graphiticN at 398.8 eV and 400.7 eV, respectivity.41 Figure 4d exhibits

agglomeration can be observed, which contributes to superb electrochemical performances because of the large electroactive area. High-resolution TEM (HRTEM) observation (Figure 2c) indicates that the NiO NPs are wrapped with graphitic carbon. The lattice fringes with two inter plane distances of 0.21 nm were well-resolved, which correlate to the (200) planes of cubic NiO.35 The selected area electron diffraction (SAED) pattern (Figure 2d) displays a series of concentric rings which is readily indexed to the lattice planes of the NiO phase.36 Additionally, the EDS analysis (Figure 2e) demonstrates the presence of C, N, O, and Ni in NCF/NiO-2. The corresponding elemental maps of C, N, and Ni (Figure 2f) display a highly homogeneous element distribution. Thus, these results verify the formation of homogenously embedded NiO NPs into the 3D hierarchical carbon framework. The XRD data further confirm the crystal structure and phase composition of the as-prepared composites (Figure 3a). All of the diffraction peaks at about 37.2, 43.3, 62.9, 75.4, and 79.4° exactly correspond to (111), (200), (220), (311), and (222) reflections of NiO (JCPDS no. 047-1049)37 excluding the peaks (2θ = 26°) of the carbon matrix, respectively, which revealed the formation of NiO NPs and carbon in the resultant samples.38 The ingredients of NCF/NiO-2 and NCS were explored by FT-IR analysis (Figure 3b). The broad band at 3432 cm−1 corresponds to the O−H stretching vibration of the adsorbed water molecules. The weak peak at 1598 cm−1 should be attributed to the C−N, and the peak of the vibration of the C−O can be observed at 1099 cm−1. Compared with the FTE

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) XRD patterns of the CS/NiO, NCF/NiO-1, and NCF/NiO-2 hybrids; (b) FT-IR spectra of the NCS and the NCF/NiO-2 composites. (c) Raman spectra and (d) TG curves of the NCF/NiO-1, NCF/NiO-2, and CS/NiO composites.

increase from about 4.07 to 5.45 nm via the BJH method with the total pore volume of 0.12, 0.27, and 0.55 cm3 g−1 for CS/ NiO, NCF/NiO-1, and NCF/NiO-2, respectively. These results reveal the fact that the ammonia treatment and pyrolysis of nitrates increase both porosity and pore size. Meanwhile, with the increasing oxide mass loading for NCF/ NiO-1 and NCF/NiO-2, more gaseous products emerged from the carbon matrix due to the decomposition of acetates during the heat treatment, resulting in a larger BET surface area for NCF/NiO-2. Apparently, NCF/NiO-2 exhibits porous structures in accordance with the above SEM and TEM images, which can enhance the electrochemical properties. Motivated by these compelling structure characteristics, the electrochemical performance of the as-obtained composites was evaluated in a three-electrode set-up. As a comparison, the classic CV tests at 100 mV s−1 and GCD profiles at 5 A g−1 (Figure 6a,b) are carried out on several typical samples, indicating that NCF/NiO-2 possesses a significantly improved specific capacitance and fast redox reaction kinetic processes.45 Figure 6c shows CV curves within the potential range from 0 to 0.6 V of NCF/NiO-2 at various scanning rates. A couple of quasi-symmetric redox peaks (between 0.15 and 0.45 V) are identified, corresponding to a reversible redox reaction on the electrode−electrolyte interface as follows26

the XPS peaks of Ni 2p3/2 and Ni 2p1/2 located at 854.3 and 872.3 eV respectively, together with the two satellite peaks at the high energy side. The binding energy separation around 18.0 eV is consistent with the energy difference of NiO.42 Also, the O 1s spectrum (Figure S2) displays three deconvoluted peaks at 529.6, 531.4, and 533.4 eV related to Ni−O, CO, and C−O−C, respectively.43 All the investigation results verify the successful incorporation of NiO NPs into the N-doped carbon network. The porous characteristics of the three NiO/C composites were further estimated by N2 adsorption−desorption tests (Figure 5). All of the samples show a typical type IV isotherm and a pronounced H2-type hysteresis loop (at P/P0 of 0.5− 0.8) associated with the primary mesoporous structure and the narrow pore size distribution. The H3-type hysteresis loop at P/P0 between 0.8 and 1.0 indicates the presence of macropores. In addition, the isotherms exhibit an increase in adsorption at a low relative pressure, implying the existence of micropores and smaller mesopores. 44 Remarkably, the adsorption volume of NCF/NiO-2 is significantly larger than that of NCF/NiO-1 and CS/NiO, suggesting its higher specific surface area. The specific surface areas calculated based on the Brunauer−Emmett−Teller (BET) method, pore sizes, and pore volumes of the three samples are shown in Table 1. The total specific surface area of NCF/NiO-2 is about 412.3 m2 g−1, which become 281.0 m2 g−1 for NCF/NiO-1 and 137.4 m2 g−1 for CS/NiO. Additionally, the average pore sizes also

NiO + OH− ↔ NiOOH + e− F

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Full-scale XPS spectrum and high-resolution XPS spectra of (b) C 1s, (c) N 1s, and (d) Ni 2p for the typical NCF/NiO-2 composite.

Figure 5. (a) Nitrogen adsorption−desorption isotherm and (b) pore size distributions of the obtained samples.

the voltage window from 0 to 0.5 V collected at different current densities (Figure 6d) display similar shapes, and the existence of almost symmetric potential plateaus implies the typical pseudocapacitive behavior of NCF/NiO-2, which is highly consistent with the behavior of the NCF/NiO-1 and CS/NiO electrodes as shown in Figure S3. Figure 7a shows the relationship between the capacitance of the samples and the charge−discharge current densities. The specific capacitances of the NiO-based electrodes are calculated based on the GCD curves by eq 1. It is distinctly seen that NCF/NiO-2 possesses the largest specific capacitance of 1074 F g−1 at 1 A g−1 among the three composites. Moreover, it is also superior to most of the previously reported nickel-based electroactive materials as shown in Table S1. The

Table 1. Pore Structure Parameters of CS/NiO, NCF/NiO1, and NCF/NiO-2a sample

SBET (m2 g−1)

D (nm)

Vt (cm3 g−1)

Vmi (cm3 g−1)

CS/NiO NCF/NiO-1 NCF/NiO-2

137.4 281.0 412.3

4.07 5.15 5.45

0.12 0.27 0.55

0.04 0.06 0.10

a

SBET: total BET surface area; D: adsorption average pore width; Vt: total pore volume; Vmi: micropore volume.

When increasing the scan rate, the shapes of the CV curves are without obvious distortion and the current responses increase accordingly, suggesting the good rate capability and rapid redox reactions of the electrode. Moreover, GCD curves within G

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Electrochemical characterization of the samples in 2 M aqueous KOH electrolyte in a three-electrode system. (a) CV curves of the Ni foam, standard NiO, CS/NiO, NCF/NiO-1, and NCF/NiO-2 at a scan rate of 100 mV s−1; (b) galvanostatic charge−discharge curves of the standard NiO, CS/NiO, NCF/NiO-1, and NCF/NiO-2 at a current density of 5 A g−1; (c) CV curves of NCF/NiO-2 at various scan rates; and (d) galvanostatic charge−discharge curves of NCF/NiO-2 at different current densities.

specific capacitance of NCF/NiO-2 changes from 1012 F g−1 to 982, 896, 870, and 820 F g−1 at various current densities of 2, 4, 8, 10, and 20 A g−1, respectively. Impressively, the specific capacitance can still retain 820 F g−1 (about 77.7% capacitance retention) even at a current density as high as 20 A g−1. In addition, the specific capacitance of other electrodes, such as NCF/NiO-1 (624 F g−1 at 1 A g−1) and CS/NiO (452 F g−1 at 1 A g−1), is much lower than that of the NCF/NiO-2 electrode. Figure 7b shows the EIS data for the composite electrodes. The plot features a nearly vertical curve in the lowfrequency region, an almost 45° diagonal line in the intermediate-frequency region corresponding to the Warburg impedance and an arc in the high-frequency region.20,46 The intercept at the real axis and the diameter of the semicircle signify the intrinsic internal resistance (Rs) of the electrodes and charge transfer resistance (Rct) at the electrode/electrolyte interface, respectively. From the Nyquist plots, the almoststraight lines at low frequency indicate superior capacitive behavior of the NCF/NiO electrodes,30 and the NCF/NiO-2 electrode obviously has a smaller Rs (0.38 Ω) and Rct (0.49 Ω) than the behavior of CS/NiO (Rs = 0.48 Ω, Rct = 0.98 Ω), manifesting that the elaborate architecture of the 3D hierarchical porous N-doped carbon network remarkably improves the electroactive surface area and electroconductivity.

Figure 7c shows the cycling stability of the NCF/NiO-2 electrode at 5 A g−1. A conspicuous increase occurs in the early stages, whereas it gradually drops to 99.4% of the initial capacitance after 5000 cycles. The increase is due to the activation process of the electroactive materials. The NiO that tightly wrapped into the carbon matrix can be gradually exposed via the soakage and diffusion of the electrolyte during the initial cycles, whereas further exposure may lead to an inferior conductivity, which is a typical property of pseudocapacitive electrodes.34,40,47 Similar GCD curves of NCF/NiO-2 before and after 5000 charge−discharge cycles (Figure S4) also corroborate the good cycling stability. These remarkable electrochemical behaviors are due to better utilization of synergy of NiO nanoparticles and graphitic carbon. Notably, the coulombic efficiency of NCF/NiO-2 is always maintained over 95% during the cycles, displaying its favorable electrochemical capacitive behavior. To gain more insight into the capacitive performances of NCF/NiO-2 in a full cell, a solid-state ASC was assembled by utilizing NCF/NiO-2 as the cathode and the as-prepared carbon matrix (NCS) as the anode with the KOH/poly(vinyl alcohol) gel electrolyte. The NCS with a 3D interconnected porous structure was obtained by calcining chitosan-derived carbonaceous substances in the absence of Ni(AC)2 and H

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. (a) Comparison of specific capacitance with different current densities and (b) Nyquist plots of the CS/NiO, NCF/NiO-1, and NCF/ NiO-2 (the inset demonstrates the magnified high-frequency region of the impedance spectra and the equivalent circuit). (c) Cycling performance and coulombic efficiency of the as-synthesized NCF/NiO-2 electrode at a current density of 5 A g−1 (the inset shows the galvanostatic charge− discharge curves for the first 20 cycles).

symmetric GCD curves at different current densities (Figure 8f) suggest exceptional electrochemical reversibility. On the basis of GCD tests, the corresponding rate performances are calculated as shown in Figure 8g. As revealed, the ASC devices demonstrate a maximum specific capacitance of 113, 106, 98, 89, and 84 F g−1 calculated by eq 1 at the current densities of 1, 2, 4, 8, and 10 A g−1, respectively. Furthermore, the EIS measurement of the ASC device was performed (Figure 8h). From the enlarged data at a high frequency (see the inset in Figure 8h), the Rs and Rct values were calculated to be 0.64 and 0.58 Ω, respectively, demonstrating its superior electrochemical properties. Additionally, cycling performances of the NCF/NiO-2//NCS ASC was tested through GCD tests at a current density of 2 A g−1, and only ∼5.7% fading of the initial capacitance is observed after 5000 cycles (Figure 8i). From the Ragone plot (Figure 9a), the NCF/NiO-2//NCS ASC shows a high energy density of 40.18 W h kg−1 corresponding to a power density of 800 W kg−1. Even at a high-power density of 8 kW kg−1, the energy density still remains 29.87 W h kg−1. Noteworthily, the results are superior to those of the ASC devices shown in the previous literature including meso-NiO/Ni//CNCs (19.1 W h kg−1 at 700 W kg−1),49 CNT@NiO//PCPs (25.4 W h kg−1 at 400 W kg−1),38 GF-CNT@NiO//G-CNT (23.4 W h kg−1 at 1060 W kg−1),50 NiO/GF//HPNCNT (32 W h kg−1 at 700 W kg−1),51 NiO// rGO (23.25 W h kg−1 at 151 W kg−1),52 NOM//3DNG (34.4 W h kg−1 at 150 W kg−1),53 NiO/C-HS//AC (30.5 W h kg−1 at 193 W kg−1),54 NCNP//HPC (38.4 W h kg−1 at 90.9 W kg−1),55 and Ni,Co-HC//HPC (30 W h kg−1 at 8.69 kW

exhibits a typical carbonaceous character in the XRD pattern and the Raman spectrum (Figure S5).24 As depicted in Figure S6a,b, the CV and GCD tests of the NCS electrode display typical EDLC properties and good electrochemical stability. The specific capacitance of 149 F g−1 was obtained at 1 A g−1 and decreases with the increasing current density, and still attains a value of 107 F g−1 at 10 A g−1 (Figure S6c). These superior electrochemical results of NCS reveal that it is a good anode to pair the optimal NCF/NiO-2 anode for a highperformance ASC. To establish an ASC device with superb electrochemical performances, it is vital to balance the stored charge between the anode and the cathode. The mass ratio of NCF/NiO-2 to NCS was adjusted to be 0.28 according to eq 2 in the Supporting Information. Figure 8a depicts the schematic diagram of the structure of the NCF/NiO-2//NCS ASC device. According to the voltage range for NCF/NiO-2 (0−0.6 V) and NCS (−1.0−0 V) at 20 mV s−1, the operating voltage of the assembled ASC is expected up to 1.6 V (Figure 8b). The CV curves of the ASC recorded at different potential windows from 0.8 to 1.6 V in Figure 8c do not change too much, whereas GCD profiles present symmetrical charge−discharge curves in Figure 8d without distinct distortion, even at 1.6 V. Therefore, 1.6 V is identified as the operating voltage for further investigation.48 From a set of CV curves at various scan rates (Figure 8e), the ASC displays a typical electrical double layer capacitance and a prominent reversible redox reaction from NCS and NCF/NiO-2 electrodes, respectively.18 Moreover, the nearly I

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 8. (a) Schematic illustration of the as-assembled asymmetric supercapacitor device; (b) comparative CV curves of NCF/NiO-2 and NCS electrodes performed at a scan rate of 20 mV s−1 in a three-electrode cell; (c) CV curves of the asymmetric supercapacitor at different potential windows at a scan rate of 100 mV s−1; (d) galvanostatic charge−discharge curves of the asymmetric cell at different potential windows at a current density of 1 A g−1; (e) CV curves of the device at different scan rates; (f) galvanostatic charge−discharge curves of the asymmetric cell at various current densities; (g) specific capacitance of the device as a function of current density; (h) Nyquist plots of the device (the inset presents the magnified high-frequency region of the impedance spectra); (i) cycling stability of the solid-state device over 5000 cycles at a current density of 2 A g−1 (the inset shows the galvanostatic charge−discharge curves for the first five cycles and the last five cycles).

Figure 9. (a) Ragone plots of the NCF/NiO-2//NCS asymmetric supercapacitor. (b) Digital image of 20 green LEDs lighted by two supercapacitors in series.

J

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ACS Applied Materials & Interfaces kg−1).56 Moreover, 20 green light-emitting diode (LED) indicators (5 mm, 2.2 V) were successfully lightened by the two ASC devices in series after charging (Figure 9b), which indeed confirms the potential application of the device. On the basis of the above results, the as-prepared NCF/NiO composites of the superb electrochemical performances are believed to inherit the advantages of nanoscaled nickel oxide and the interconnected porous carbon framework. In one aspect, the partially graphitic carbon framework not only acts as the efficient conductive pathway for electron transfer but also hinders the inner nanoparticles from aggregation and pulverization during long-term cycling, resulting in outstanding cycling stability. In another aspect, the unique 3D morphology with the high surface area can help to accommodate the electrolyte ions to a large extent and tender unimpeded channels during the fast charge/discharge process, which can effectively decrease the charge transfer resistance and enhance charge storage and high rate capability. Furthermore, the nanoscaled and homogeneous distribution of NiO NPs can increase the surface area of the active material in contact with the electrolyte and offer sufficient electroactive sites for rapid redox reactions. Finally, the heteroatom doping results in extra defects on the carbon substrate, enhancing the electrochemical performance of the materials.

ORCID

Hua Tong: 0000-0001-7347-0968 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (31071265 and 30900297) and the Science and technology project of cultural relics protection of Zhejiang Province (no. 2013016).

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(1) Bonaccorso, F.; Colombo, L.; Yu, G.; Stoller, M.; Tozzini, V.; Ferrari, A. C.; Ruoff, R. S.; Pellegrini, V. Graphene, Related TwoDimensional Crystals, and Hybrid Systems for Energy Conversion and Storage. Science 2015, 347, No. 1246501. (2) Zhang, Y. Z.; Wang, Y.; Cheng, T.; Lai, W. Y.; Pang, H.; Huang, W. Flexible Supercapacitors Based on Paper Substrates: a New Paradigm for Low-Cost Energy Storage. Chem. Soc. Rev. 2015, 44, 5181−5199. (3) Yu, Z.; Tetard, L.; Zhai, L.; Thomas, J. Supercapacitor Electrode Materials: Nanostructures from 0 to 3 Dimensions. Energy Environ. Sci. 2015, 8, 702−730. (4) Choudhary, N.; Li, C.; Moore, J.; Nagaiah, N.; Zhai, L.; Jung, Y.; Thomas, J. Asymmetric Supercapacitor Electrodes and Devices. Adv. Mater. 2017, 29, No. 1605336. (5) Huang, P.; Lethien, C.; Pinaud, S.; Brousse, K.; Laloo, R.; Turq, V.; Respaud, M.; Demortiere, A.; Daffos, B.; Taberna, P. L.; Chaudret, B.; Gogotsi, Y.; Simon, P. On-Chip and Freestanding Elastic Carbon Films for Micro-Supercapacitors. Science 2016, 351, 691−695. (6) Li, Y.; Wang, G.; Wei, T.; Fan, Z.; Yan, P. Nitrogen and Sulfur Co-doped Porous Carbon Nanosheets Derived from Willow Catkin for Supercapacitors. Nano Energy 2016, 19, 165−175. (7) Zhao, Y.; Hu, L.; Zhao, S.; Wu, L. Preparation of MnCo2O4@ Ni(OH)2 Core-Shell Flowers for Asymmetric Supercapacitor Materials with Ultrahigh Specific Capacitance. Adv. Funct. Mater. 2016, 26, 4085−4093. (8) Zhi, M.; Xiang, C.; Li, J.; Li, M.; Wu, N. Nanostructured Carbon-Metal Oxide Composite Electrodes for Supercapacitors: a Review. Nanoscale 2013, 5, 72−88. (9) Cakici, M.; Kakarla, R. R.; Alonso-Marroquin, F. Advanced Electrochemical Energy Storage Supercapacitors Based on the Flexible Carbon Fiber Fabric-Coated with Uniform Coral-like MnO 2 Structured Electrodes. Chem. Eng. J. 2017, 309, 151−158. (10) Ma, H.; He, J.; Xiong, D. B.; Wu, J.; Li, Q.; Dravid, V.; Zhao, Y. Nickel Cobalt Hydroxide @Reduced Graphene Oxide Hybrid Nanolayers for High Performance Asymmetric Supercapacitors with Remarkable Cycling Stability. ACS Appl. Mater. Interfaces 2016, 8, 1992−2000. (11) Lai, F.; Miao, Y. E.; Zuo, L.; Lu, H.; Huang, Y.; Liu, T. Biomass-Derived Nitrogen-doped Carbon Nanofiber Network: a Facile Template for Decoration of Ultrathin Nickel-Cobalt Layered Double Hydroxide Nanosheets as High-Performance Asymmetric Supercapacitor Electrode. Small 2016, 12, 3235−3244. (12) Liu, M.; Wang, X.; Zhu, D.; Li, L.; Duan, H.; Xu, Z.; Wang, Z.; Gan, L. Encapsulation of NiO Nanoparticles in Mesoporous Carbon Nanospheres for Advanced Energy Storage. Chem. Eng. J. 2017, 308, 240−247. (13) Liu, L.; Lang, J.; Zhang, P.; Hu, B.; Yan, X. Facile Synthesis of Fe2O3 Nano-Dots@Nitrogen-doped Graphene for Supercapacitor Electrode with Ultralong Cycle Life in KOH Electrolyte. ACS Appl. Mater. Interfaces 2016, 8, 9335−9344. (14) Ning, P.; Duan, X.; Ju, X.; Lin, X.; Tong, X.; Pan, X.; Wang, T.; Li, Q. Facile Synthesis of Carbon Nanofibers/MnO2 Nanosheets as High-Performance Electrodes for Asymmetric Supercapacitors. Electrochim. Acta 2016, 210, 754−761.

4. CONCLUSIONS In summary, we demonstrate a facile one-pot template-free synthesis of a 3D hierarchical porous N-doped carbon framework armored nickel oxide (NCF/NiO) via the ammonia-induced method assisted by the pyrolysis of decomposable salts. Benefiting from such 3D interconnected porous architecture and the homogeneously decorated NiO NPs, the resulting NCF/NiO-2 electrode exhibits a high specific capacitance (1074 F g−1 at 1 A g−1) and superb cycling stability (capacitance retention of 99.4% after 5000 cycles) in the three-electrode cell. The corresponding ASCs deliver a maximum energy density of 40.18 W h kg−1 at a high-power density of 800 W kg−1 with a long cycle life, and can power 20 green LEDs efficiently. We confirm that the economical, facile, and green preparation method offers a new avenue for fabricating 3D porous carbon-supported metal oxide electrodes for high-performance supercapacitors or other energy storage systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05967. Experimental materials; fabrication of the NCF/NiO2//NCF solid-state asymmetric supercapacitor device; SEM and TEM images of CS/NiO; O 1s XPS spectrum of NCF/NiO-2; CV and GCD curves of NCF/NiO-1 and CS/NiO; GCD curves of NCF/NiO-2 before and after 5000 cycles; SEM image, XRD pattern, Raman spectrum, CV and GCD curves of NCS; and comparison of electrochemical properties of the reported nickelbased electrodes in three-electrode configuration (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +8602768764510. Fax: +8602768752136. K

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Research Article

ACS Applied Materials & Interfaces (15) Liu, T.; Zhang, F.; Song, Y.; Li, Y. Revitalizing Carbon Supercapacitor Electrodes with Hierarchical Porous Structures. J. Mater. Chem. A 2017, 5, 17705−17733. (16) Ma, F.; Ma, D.; Wu, G.; Geng, W.; Shao, J.; Song, S.; Wan, J.; Qiu, J. Construction of 3D Nanostructure Hierarchical Porous Graphitic Carbons by Charge-Induced Self-Assembly and Nanocrystal-Assisted Catalytic Graphitization for Supercapacitors. Chem. Commun. 2016, 52, 6673−6676. (17) Song, S.; Ma, F.; Wu, G.; Ma, D.; Geng, W.; Wan, J. Facile SelfTemplating Large Scale Preparation of Biomass-Derived 3D Hierarchical Porous Carbon for Advanced Supercapacitors. J. Mater. Chem. A 2015, 3, 18154−18162. (18) Hao, P.; Tian, J.; Sang, Y.; Tuan, C. C.; Cui, G.; Shi, X.; Wong, C. P.; Tang, B.; Liu, H. 1D Ni-Co Oxide and Sulfide Nanoarray/ Carbon Aerogel Hybrid Nanostructures for Asymmetric Supercapacitors with High Energy Density and Excellent Cycling Stability. Nanoscale 2016, 8, 16292−16301. (19) Kim, M.; Yoo, J.; Kim, J. Quasi-Solid-State Flexible Asymmetric Supercapacitor Based on Ferroferric Oxide Nanoparticles on Porous Silicon Carbide with Redox-Active p-Nitroaniline Gel Electrolyte. Chem. Eng. J. 2017, 324, 93−103. (20) Yu, M.; Han, Y.; Li, J.; Wang, L. One-Step Synthesis of Sodium Carboxymethyl Cellulose-Derived Carbon Aerogel/Nickel Oxide Composites for Energy Storage. Chem. Eng. J. 2017, 324, 287−295. (21) Yu, X.; Wang, M.; Gagnoud, A.; Fautrelle, Y.; Moreau, R.; Li, X. Fabrication and Electrochemical Properties of a Graphene-Enhanced Hierarchical Porous Network of Fe3O4 /Carbon Nanobelts. Electrochim. Acta 2017, 248, 150−159. (22) Ran, J.; Xie, L.; Sun, G.; Hu, J.; Chen, S.; Jiang, P.; Shen, X.; Tong, H. A Facile Method for the Preparation of Chitosan-based Scaffolds with Anisotropic Pores for Tissue Engineering Applications. Carbohydr. Polym. 2016, 152, 615−623. (23) Zhang, F.; Liu, T.; Hou, G.; Kou, T.; Yue, L.; Guan, R.; Li, Y. Hierarchically Porous Carbon Foams for Electric Double Layer Capacitors. Nano Res. 2016, 9, 2875−2888. (24) Hao, P.; Zhao, Z.; Leng, Y.; Tian, J.; Sang, Y.; Boughton, R. I.; Wong, C. P.; Liu, H.; Yang, B. Graphene-based Nitrogen Self-doped Hierarchical Porous Carbon Aerogels Derived from Chitosan for High Performance Supercapacitors. Nano Energy 2015, 15, 9−23. (25) Sk, M. M.; Yue, C. Y.; Ghosh, K.; Jena, R. K. Review on Advances in Porous Nanostructured Nickel Oxides and their Composite Electrodes for High-Performance Supercapacitors. J. Power Sources 2016, 308, 121−140. (26) Feng, Y.; Zhang, H.; Zhang, Y.; Bai, Y.; Wang, Y. Novel Peapod NiO Nanoparticles Encapsulated in Carbon Fibers for High-Efficiency Supercapacitors and Lithium-Ion Batteries. J. Mater. Chem. A 2016, 4, 3267−3277. (27) Kim, S. Y.; Jeong, H. M.; Kwon, J. H.; Ock, I. W.; Suh, W. H.; Stucky, G. D.; Kang, J. K. Nickel Oxide Encapsulated Nitrogen-Rich Carbon Hollow Spheres with Multiporosity for High-Performance Pseudocapacitors Having Extremely Robust Cycle Life. Energy Environ. Sci. 2015, 8, 188−194. (28) Qu, G.; Jia, S.; Wang, H.; Cao, F.; Li, L.; Qing, C.; Sun, D.; Wang, B.; Tang, Y.; Wang, J. Asymmetric Supercapacitor Based on Porous N-doped Carbon Derived from Pomelo Peel and NiO Arrays. ACS Appl. Mater. Interfaces 2016, 8, 20822−20830. (29) Wang, M.; Jin, F.; Zhang, X.; Wang, J.; Huang, S.; Zhang, X.; Mu, S.; Zhao, Y.; Zhao, Y. Multihierarchical Structure of Hybridized Phosphates Anchored on Reduced Graphene Oxide for High Power Hybrid Energy Storage Devices. ACS Sustainable Chem. Eng. 2017, 5, 5679−5685. (30) Shao, J.; Song, M.; Wu, G.; Zhou, Y.; Wan, J.; Ren, X.; Ma, F. 3D Carbon Nanocage Networks with Multiscale Pores for High-Rate Supercapacitors by Flower-like Template and in-Situ Coating. Energy Storage Mater. 2018, 13, 57−65. (31) Sun, G.; Ma, L.; Ran, J.; Shen, X.; Tong, H. Incorporation of Homogeneous Co3O4 into A Nitrogen-doped Carbon Aerogel via A Facile in Situ Synthesis Method: Implications for High Performance Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 9542−9554.

(32) Selvam, S.; Balamuralitharan, B.; Jegatheeswaran, S.; Kim, M.Y.; Karthick, S. N.; Anandha Raj, J.; Boomi, P.; Sundrarajan, M.; Prabakar, K.; Kim, H. J. Electrolyte-Imprinted Graphene OxideChitosan Chelate with Copper Crosslinked Composite Electrodes for Intense Cyclic-Stable, Flexible Supercapacitors. J. Mater. Chem. A 2017, 5, 1380−1386. (33) Sun, G.; Li, B.; Ran, J.; Shen, X.; Tong, H. Three-Dimensional Hierarchical Porous Carbon/Graphene Composites Derived from Graphene Oxide-Chitosan Hydrogels for High Performance Supercapacitors. Electrochim. Acta 2015, 171, 13−22. (34) Wu, S.; Hui, K. S.; Hui, K. N.; Kim, K. H. Ultrathin Porous NiO Nanoflake Arrays on Nickel Foam as an Advanced Electrode for High Performance Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 9113−9123. (35) Zhang, L.; Zheng, W.; Jiu, H.; Ni, C.; Chang, J.; Qi, G. The Synthesis of NiO and NiCo2O4 Nanosheets by a New Method and their Excellent Capacitive Performance for Asymmetric Supercapacitor. Electrochim. Acta 2016, 215, 212−222. (36) Liu, T.; Jiang, C.; Cheng, B.; You, W.; Yu, J. Hierarchical Fower-like C/NiO Composite Hollow Microspheres and its Excellent Supercapacitor Performance. J. Power Sources 2017, 359, 371−378. (37) Meng, G.; Yang, Q.; Wu, X.; Wan, P.; Li, Y.; Lei, X.; Sun, X.; Liu, J. Hierarchical Mesoporous NiO Nanoarrays with Ultrahigh Capacitance for Aqueous Hybrid Supercapacitor. Nano Energy 2016, 30, 831−839. (38) Yi, H.; Wang, H.; Jing, Y.; Peng, T.; Wang, X. Asymmetric Supercapacitors Based on Carbon Nanotubes@NiO Ultrathin Nanosheets Core-Shell Composites and MOF-Derived Porous Carbon Polyhedrons with Super-Long Cycle Life. J. Power Sources 2015, 285, 281−290. (39) Elizabeth, I.; Nair, A. K.; Singh, B. P.; Gopukumar, S. Multifunctional Ni-NiO-CNT Composite as High Performing Free Standing Anode for Li Ion Batteries and Advanced Electro Catalyst for Oxygen Evolution Reaction. Electrochim. Acta 2017, 230, 98−105. (40) Li, P.; Jin, Z.; Peng, L.; Zhao, F.; Xiao, D.; Jin, Y.; Yu, G. Stretchable All-Gel-State Fiber-Shaped Supercapacitors Enabled by Macromolecularly Interconnected 3D Graphene/Nanostructured Conductive Polymer Hydrogels. Adv. Mater. 2018, No. e1800124. (41) Salunkhe, R. R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J. H.; Yamauchi, Y. Asymmetric Supercapacitors Using 3D Nanoporous Carbon and Cobalt Oxide Electrodes Synthesized from a Single Metal-Organic Framework. ACS Nano 2015, 9, 6288−6296. (42) Wang, L.; Zhang, G.; Liu, Y.; Li, W.; Lu, W.; Huang, H. Facile Synthesis of a Mechanically Robust and Highly Porous NiO Film with Excellent Electrocatalytic Activity towards Methanol Oxidation. Nanoscale 2016, 8, 11256−11263. (43) Liu, W.; Lu, C.; Wang, X.; Liang, K.; Tay, B. K. In Situ Fabrication of Three-Dimensional, Ultrathin Graphite/Carbon Nanotube/NiO Composite as Binder-Free Electrode for High-Performance Energy Storage. J. Mater. Chem. A 2015, 3, 624−633. (44) You, B.; Kang, F.; Yin, P.; Zhang, Q. Hydrogel-Derived Heteroatom-doped Porous Carbon Networks for Supercapacitor and Electrocatalytic Oxygen Reduction. Carbon 2016, 103, 9−15. (45) He, W.; Wang, C.; Li, H.; Deng, X.; Xu, X.; Zhai, T. Ultrathin and Porous Ni3S2/CoNi2S4 3D-Network Structure for Superhigh Energy Density Asymmetric Supercapacitors. Adv. Energy Mater. 2017, 7, No. 1700983. (46) Liu, M.; Shi, M.; Lu, W.; Zhu, D.; Li, L.; Gan, L. Core-Shell Reduced Graphene Oxide/MnOx@Carbon Hollow Nanospheres for High Performance Supercapacitor Electrodes. Chem. Eng. J. 2017, 313, 518−526. (47) Abouali, S.; Garakani, M. A.; Zhang, B.; Xu, Z. L.; Heidari, E. K.; Huang, J. Q.; Huang, J.; Kim, J. K. Electrospun Carbon Nanofibers with in Situ Encapsulated Co3O4 Nanoparticles as Electrodes for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 13503−13511. (48) Yu, J.; Lu, W.; Smith, J. P.; Booksh, K. S.; Meng, L.; Huang, Y.; Li, Q.; Byun, J.-H.; Oh, Y.; Yan, Y.; Chou, T.-W. A High Performance L

DOI: 10.1021/acsami.8b05967 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Stretchable Asymmetric Fiber-Shaped Supercapacitor with a CoreSheath Helical Structure. Adv. Energy Mater. 2017, 7, No. 1600976. (49) Lai, H.; Wu, Q.; Zhao, J.; Shang, L.; Li, H.; Che, R.; Lyu, Z.; Xiong, J.; Yang, L.; Wang, X.; Hu, Z. Mesostructured NiO/Ni Composites for High-Performance Electrochemical Energy Storage. Energy Environ. Sci. 2016, 9, 2053−2060. (50) Guan, C.; Wang, Y.; Hu, Y.; Liu, J.; Ho, K. H.; Zhao, W.; Fan, Z.; Shen, Z.; Zhang, H.; Wang, J. Conformally Deposited NiO on a Hierarchical Carbon Support for High-Power and Durable Asymmetric Supercapacitors. J. Mater. Chem. A 2015, 3, 23283−23288. (51) Wang, H.; Yi, H.; Chen, X.; Wang, X. Asymmetric Supercapacitors Based on Nano-Architectured Nickel Oxide/ Graphene Foam and Hierarchical Porous Nitrogen-doped Carbon Nanotubes with Ultrahigh-Rate Performance. J. Mater. Chem. A 2014, 2, 3223−3230. (52) Ren, X.; Guo, C.; Xu, L.; Li, T.; Hou, L.; Wei, Y. Facile Synthesis of Hierarchical Mesoporous Honeycomb-like NiO for Aqueous Asymmetric Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 19930−19940. (53) Zheng, M.; Dong, H.; Xiao, Y.; Hu, H.; He, C.; Liang, Y.; Lei, B.; Sun, L.; Liu, Y. Hierarchical NiO Mesocrystals with Tuneable High-Energy Facets for Pseudocapacitive Charge Storage. J. Mater. Chem. A 2017, 5, 6921−6927. (54) Liu, T.; Zhang, L.; Cheng, B.; You, W.; Yu, J. Fabrication of a Hierarchical NiO/C Hollow Sphere Composite and its Enhanced Supercapacitor Performance. Chem. Commun. 2018, 54, 3731−3734. (55) Chen, Z.; Xiong, D. B.; Zhang, X.; Ma, H.; Xia, M.; Zhao, Y. Construction of a Novel Hierarchical Structured NH4-Co-Ni Phosphate toward an Ultrastable Aqueous Hybrid Capacitor. Nanoscale 2016, 8, 6636−6645. (56) Zhao, Y.; Ma, H.; Huang, S.; Zhang, X.; Xia, M.; Tang, Y.; Ma, Z. F. Monolayer Nickel Cobalt Hydroxyl Carbonate for High Performance All-Solid-State Asymmetric Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 22997−23005.

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