Honeycomb-like composite structure for advanced solid state asymmetric supercapacitors

Accepted Manuscript Honeycomb-like composite structure for advanced solid state asymmetric supercapacitors Huailin Fan, Wei Liu, Wenzhong Shen PII: DOI: Reference:

S1385-8947(17)30872-0 http://dx.doi.org/10.1016/j.cej.2017.05.121 CEJ 17020

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

20 March 2017 18 May 2017 18 May 2017

Please cite this article as: H. Fan, W. Liu, W. Shen, Honeycomb-like composite structure for advanced solid state asymmetric supercapacitors, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej. 2017.05.121

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Honeycomb-like composite structure for advanced solid state asymmetric supercapacitors Huailin Fan, a,b Wei Liu a,band Wenzhong Shena,* Dr. Huailin Fana,b a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

Academy of Sciences, Taiyuan, 030001, PR China b

University of Chinese Academy of Sciences, Beijing, 100049, PR China

E-mail:[email protected] Dr. Wei Liua,b a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

Academy of Sciences, Taiyuan, 030001, PR China b

University of Chinese Academy of Sciences, Beijing, 100049, PR China

E-mail:[email protected] Prof. Wenzhong Shen a,* a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

Academy of Sciences, Taiyuan, 030001, PR China * E-mail: [email protected] Fax: +86-351-4041153

Keywords: honeycomb-like carbon, NiCo2S4 nanosheets, supercapacitors, energy storage

Abstract Three dimension honeycomb-like porous hybrid material (3DHPC-NiCo2S4), which was assembled from interconnection carbon nanosheets covering vertical NiCo2S4 nanosheets, was fabricated using one-step carbonization cellulose followed by hydrothermal process and sulfidation. As supercapacitor electrode, the composites show excellent capacitance performance resulting from interconnection carbon nanosheets as power component and multivalent metal sulfides NiCo2S4 nanosheets as energy component. The synergetic effect leads to 1168 and 684 F/g at current densities of 1 and 20 A/g at three-electrode system, respectively. The composite of Fe2O3 nanorods and honeycomb-like carbon materials was prepared (3DHPC-Fe2O3) with similar method. Solid state asymmetric supercapacitor fabricated with 3DHPCFe2O3 as negative and 3DHPC-NiCo2S4 as positive electrode (3DHPCNiCo2S4//3DHPC-Fe2O3) can delivered a superior gravimetric energy density of 44.4 Wh/kg (volumetric energy density of 1.71 mWh/cm3) at 1.62 KW/kg (volumetric power density 62.94 mW/cm3) and outstanding cycling stability (88.5% retention after 10000 cycles).

1.

Introduction

The ever-growing demand of green transportation and portable electronic instrument stimulate the mushroom growth of energy storage equipment with low-cost, high energy density, long cycle life and excellent rate performance [1-4]. Supercapacitors have received considerable attention during the past decades, which are considered as one of the propitious energy storage devices and bridge the gap between batteries and conventional capacitors due to their high power delivery, long cycle life and quick charging capacity [5-7]. Various morphology carbon materials have been reported to investigate electrode applications due to their high specific surface area, benign electrical conductivity, low cost and excellent chemical stability. However, the energy density of carbon-based supercapacitor is still relatively low, which has impeded their potential applications. Therefore, the improvement of

energy density is crucial for supercapacitor to meet future energy demands without sacrificing power density and rate performance. Generally speaking, increasing specific surface area of carbon materials is favor of storage more ions to the enhanced energy density of supercapacitor, but the electron transmission route will be impaired for increasing surface area. Besides, suitable carbon morphology by rational design will be needed, which can efficiently utilize the pore channel at high charging and discharging current especially. Thus, novel nanostructure carbon materials including carbon nanotubes [8], carbon nanofibers [9, 10], carbon nanosheets [11, 12], carbon sphere [13], flower-like carbon [14], carbon aerogel [15] and their composites [16] have been synthesized for supercapacitor application. Carbon nanosheets with nanoscale thickness could shorten ions diffusion paths, which show outstanding application performances in energy storage/conversion [6, 17]. However, carbon nanosheets easily re-aggregate and stack with high surface energy leading to a deteriorated availability of nanosheets nanostructure [18]. Honeycomb-like porous carbon assembled by uniformed planar carbon materials is considered a satisfactory assembly since honeycomb-like carbon networks with fully structural interconnectivity could avoid nanosheets re-aggregating during shortening ion/electron pathways. Honeycomb-like carbon have been obtained from various sustainable biomass such as pomelo peel [19], silk cocoon [20], plane tree fluff [21] etc; however, their capacitance haven’t satisfied practical requirement for supercapacitor. Moreover, cheaper and more easily acquired carbon source comparing to mentioned above will be need to prepare honeycomb-like carbon. Combining electrical double-layer capacitance from the ion adsorption between carbon materials and electrolyte with pseudocapacitance from metallic compound or conductive polymers redox reaction is another effective strategy to enhance energy density [22-24]. Transition metal sulfides such as cobalt sulfides [25] and nickel sulfides [5] have been considered as pseudocapacitive electrode material owing to their great electrochemical performance. The preparation methods of carbon-metal sulfides composites and their synergistic effect have been investigated [1, 23, 26, 27]. The carbon materials serve as the frames to support metal sulfides preventing agglomeration, buffering the large volume

expansion and assuring efficient reaction of metal sulfides; moreover, they also provide the conductive nets to connect metal sulfides [28, 29]. The metal sulfides offer high energy density by storing the charge of redox reactions. NiCo2S4 is outstanding bimetal sulfides owing to its higher reversible capacity and more sensitive electrical conductivity comparing with mono-metal sulfides [30-34]. However, the disadvantage of single NiCo2S4 electrode is low rate performance and short cycle life. Thus, various composites of NiCo2S4 with various morphology carbon materials have been explored to improve its rate performance and cycle life, such as, 1D Ni-Co oxide and sulfide nanoarray/carbon aerogel [35], NiCo2S4@reduced graphene oxide@carbon nanotube[36], NiCo2S4 nanosheets@carbon foams [37], CNTs/NiCo2S4 [38] composites and so forth. It is necessary to the rational design of multicomponent electrode materials with energy component and power component for supercapacitor. Herein, three dimension honeycomblike porous carbon materials consisting of interconnection carbon nanosheets (denoting as 3DHPC) was prepared by cellulose and NiCo2S4 nanosheets were erectly grown on the surface of 3DHPC by hydrothermal process and sulfidation (denoting as 3DHPC-NiCo2S4). Honeycomb-like carbon materials possess abundant macropore, which can supply space for NiCo2S4 nanosheets growing and shorten diffusion paths for the electrolyte ion. The assemble carbon nanosheets connected the isolated NiCo2S4 nanosheets to form conductive network, which enable electron to fast transport. These structure properties facilitate the intimate contact of electrode and electrolyte offering a large interfacial area for reaction and develop sufficiently superiority of carbon materials as power component and multivalent metal sulfides NiCo2S4 nanosheets as energy component. The combined effects led to outstanding specific capacitance of 1169 and 684 F/g at current densities of 1 and 20 A/g for the composites in 6 mol/L KOH aqueous solution at three-electrode system, respectively. The composites of Fe2O3 nanorods and honeycomb-like carbon materials were prepared (3DHPCFe2O3). Solid state asymmetric supercapacitor fabricated with 3DHPC-Fe2O3 as negative and 3DHPC-NiCo2S4 as positive electrode (3DHPC-NiCo2S4//3DHPC-Fe2O3) could displayed a specific capacitance of 126.8 F/g at 2 A/g in PVA-KOH gel electrolyte. In addition, the solid state device showed a superior gravimetric energy density of 44.4 Wh/kg (volumetric energy

density of 1.71 mWh/cm3) at 1.62 KW/kg (volumetric power density 62.94 mW/cm3) and outstanding cycling stability (88.5% retention after 10000 cycles).

2. Experimental Section 2.1 Syntheses of 3DHPC In a typical experiment, cellulose (10 g) was dispersed into deionized water (100 ml). Then KOH (8 g) and urea (2 g) was added into the above mixture with vigorous stirring for 5h at 80 °C and dried at 80 °C, the obtained mixture were carbonized by heating to 800 °C at the heating rate of 5 °C/min for 1 h under N2 flow. Finally, the resultant was washed with deionized water thoroughly; and the sample was denoted as 3DHPC after drying at 80 °C. 2.2 Preparation of 3DHPC-NiCo2S4 and 3DHPC-Fe2O3 The 3DHPC-NiCo2S4 was prepared through hydrothermal route followed by sulfidation. 0.565g Ni(NO3)2·6H2O, 1.16g Co(NO3)2·6H2O, 1.24g hexamethylenetetramine and 0.15g 3DHPC were put into 50 mL of water and 20 mL ethanol to form a dark dispersion under stiring. The solution was then transferred to a Teflon-lined stainless steel autoclave and kept at 90 °C for 6 h. After hydrothermal growth, the 3DHPC covered with Ni, Co-precursors was taken out and carefully filtered and rinsed several times with deionized water. Then, the solid product with 1.2g Na2S·9H2O were immersed in 70 ml deionized water and kept at 160 °C for 8 h. The 3DHPC-NiCo2S4 was obtained by filtering and drying. The similar structure composite 3DHPC-NiCo2S4-1 was prepared with 0.283g Ni(NO3)2·6H2O, 0.58g Co(NO3)2·6H2O, 0.62g hexamethylenetetramine, and 3DHPC-NiCo2S4-2 was prepared with 1.13g Ni(NO3)2·6H2O, 2.32g Co(NO3)2·6H2O, 2.48g hexamethylenetetramine at the case of the same other conditions, respectively. While 3DHPC-Fe2O3 was synthesized using hydrothermal route followed by thermal process. 0.946g FeCl3·6H2O, 0.497g Na2SO4 and 0.3g 3DHPC were put into 70 mL of water and the solution was transferred to a Teflon-lined stainless steel autoclave and kept at 160 °C for 8 h. The filtered product was heated at 300°C for 3 h and the 3DHPC-Fe2O3 was obtained. NiCo2S4 nanoflower and Fe2O3 nanorods were prepared using the same methods without 3DHPC.

2.3 Materials Characterization Nitrogen adsorption-desorption isotherms were measured by Micromeritics ASAP 2020 adsorption apparatus. The microstructure of the 3DHPC, 3DHPC-NiCo2S4 and 3DHPC-Fe2O3 were investigated with the aid of scanning electron microscope (SEM, S-4800 Field Emission-Scanning Electron Microscope) and transmission electron microscope (TEM, JEM2000 FX) with a field emission gun operating at 200 kV. TG was carried out under air from room temperature to 700 °C at 10 °C/min (Rigaku, TG-MS 8120). The XPS analysis were performed on a Perkin-Elmer PHI pectrometer with monochromatic Al Kα (1486.6 eV) source by ESCALAB 250 (Thermo Electron). XRD patterns were characterized on a Rigaku 167 MiniFlexII X-ray diffractometer with Cu Kα radiation at a scanning speed of 4°/min. Rounded carbon flake (≈diameter of 12 mm, thickness of 0.2mm) were obtained by pressing the carbon powders applying a pressure of 5 MPa. The electrical conductivity was measured on a carbon flake by a 4-point probe resistivity measurement instrument. (Suzhou Tongchuang, SZT-2A).

2.4 Electrochemical Measurements The electrochemical properties of the samples were investigated in 6 mol/L KOH solution (CHI660E, Shanghai Chenhua, China). In three-electrode system platinum acted as the counter electrode and Hg/HgO electrode acted as the reference electrode. The working electrode was prepared by blending the samples (80 wt%), carbon black (10 wt%), and polytetrafluoroethylene binder (10 wt%) in 5 mL ethyl alcohol, then ultrasonically processed for 10 min. The solution was casted onto nickel foam current collector (1 x 1cm) followed by drying at 100 °C overnight. The mass of active materials coated on each work electrode was approximately 2mg/cm2. Cyclic voltamogram (CV) and galvanostatic charge-discharge (GCD) curves were obtained. Specific capacitance (C, F/g) of the single electrode was calculated from the discharge curve based on the formula as follows:

C 

2im  Vdt V2

Vf Vi

(1)

where im (A/g) is the current density, V is the potential with an initial and final value of V i and Vf, respectively. Electrochemical impedance spectroscopy (EIS) spectra were collected in the frequency range of 0.01–100,000 Hz with a signal amplitude of 5 mV at the open circuit potential. The asymmetric supercapacitor was fabricated by using 3DHPC-NiCo2S4 as positive electrode and 3DHPC-Fe2O3 as negative electrode and using PVA-KOH gel or 6 mol/L KOH as electrolyte. The work electrode solution was obtained by the above method. The solution was casted onto nickel foam current collector (1×2 cm) followed by drying at 100 °C overnight in a vacuum oven to obtain the electrode. The mass coating on each work electrode was approximate 2 mg/cm2 for 3DHPC-NiCo2S4 or 2.4 mg/cm2 for 3DHPC-Fe2O3. The PVAKOH gel electrolyte was prepared as follows: 6 g of KOH and 6 g of PVA powder were added into 60 mL of deionized water with stirring at 85 °C for 4 h. The specific capacitance, gravimetric energy density, gravimetric power density, volume energy density and volume power density were calculated from the following equations:

Ccell 

Eg 

2im  Vdt V2

(2)

Ccell  V 2  1000 (3) 2  3600

Pg 

E g  3600 (4) t

Ev 

E g  m total (5) V

Pv 

E v  3600 (6) t

where im (A/g) is the current density, V is the potential with an initial and final value, respectively, Eg (W h/kg) is the gravimetric energy density based on active materials mass, Pg (W/kg) is the gravimetric power density based on active materials mass, Ev (mW h/cm3) is the

volume energy density, Pv (mW/cm3) is the volume power density and V (cm3) is the volume of the whole device (0.3mm×15.09mm×25.03mm). The Ev and Pv are based on device volume including current collector, solid electrolyte, active materials and packaging material.

3.

Result and discussion

Scheme 1. Schematic of the synthesis of 3DHPC-NiCo2S4 and 3DHPC-Fe2O3 electrodes and the assembly of solid state device with 3DHPC-NiCo2S4 as the positive electrode and 3DHPC-Fe2O3 as the negative electrode.

The synthesis route of 3DHPC-NiCo2S4 was illustrated in Scheme 1. At first, a mixture of urea, KOH and cellulose into water at 80 °C was stirred. The cellulose adequately adsorbed urea and OH- by hydrogen bond or electrostatic attraction [39]. After drying at 80 °C, plentiful KOH and urea were evenly embedded in the dried production. During carbonization, KOH and urea particles played vital function in the formation of 3D honeycomb-like structure. KOH not only acted as the activated reagent to etch carbon atoms during the carbonization process to generate numerous micropores and mesopores for charge accommodation, but also served as template to produce macropores, which could be as the channel for electrolyte ions rapid transport and diffusion and provide space for NiCo2S4 nanosheets growing. At high temperature, gas produced by decomposing urea could be used as expansive action reagent to reduce the thickness of carbon framework and to promote the 3D honeycomb-structured porous carbon forming. 2D NiCo2S4 nanosheets were uniformly distributed on 3DHPC through hydrothermal method followed by sulfidation process. 1D Fe2O3 nanorods were evenly grown on 3DHPC using hydrothermal method followed by thermal treatment.

a)

intensity/ a.u.

3DHPC

b)

3DHPC-NiCo2S4 JCPDS:43-1477

c)

3DHPC-Fe2O3

JCPDS:33-0664

10

20

30

40

50

60

70

2 Theta / degree Figure 1. XRD patterns of 3DHPC, 3DHPC-NiCo2S4 and 3DHPC-Fe2O3.

Figure 1 showed the XRD patterns of 3DHPC, 3DHPC-NiCo2S4 and 3DHPC-Fe2O3. Two broader diffraction peaks at 2θ = 26.6 and 43.5° appeared in Figure 1a, which could be attributed to the (002) and (100) reflections of the disordered carbon layer, respectively. After the deposition of Ni-Co precursor and sulfidation process, the XRD pattern of composites displayed six peaks at 2θ = 16.3°, 26.7°,31.5°, 38.2°, 50.3° and 55.1° besides two broader diffraction peaks in Figure 1b, which were readily indexed to NiCo2S4 diffraction peaks as (111), (220), (311), (400), (511) and (440) (JCPDS No. 43-1477); this indicated that NiCo2S4 was successfully introduced on the 3DHPC surface. Ferric oxide was deposited on the surface of 3DHPC by hydrothermal process and thermal annealing, and the composites product show two distinct peaks at 2θ = 33.3° and 35.8° (Figure 1c), which could be indexed as the (104) and (110) planes of α-Fe2O3 (JCPDS 33-0664). Other diffraction peaks appeared 2θ = 24.1°, 40.8°, 49.5°, 54.1°, 57.6°, 62.5° and 64.0° could be ascribed to (012), (113), (024), (116), (511), (018), (214) and (300) crystal planes. As shown in Raman spectra (Figure S1), the intensity ratio of the band corresponded to the in-plane vibration of sp2 carbon atoms (G band) at 1576 cm-1 to the band associated with structural defects and partially disordered structures in carbon materials (D band) at 1340 cm-1 in 3DHPC was 1.17. In addition, a 2D band around 2650 cm−1 was observed; this information implied 3DHPC with certain graphitization degree was potential application for electrical conductivity support.

Figure 2. (a) SEM image of 3DHPC, the inset is the photograph of a honeycomb, (b) High resolution SEM images of 3DHPC, (c) SEM image of 3DHPC-NiCo2S4 (d) High resolution SEM images of 3DHPC-NiCo2S4 (e) TEM image of 3DHPC, the inset is high resolution TEM images of 3DHPC (f) TEM image of 3DHPC-NiCo2S4, the inset is high resolution TEM images of 3DHPC-NiCo2S4 (g) High resolution TEM images of NiCo2S4 in 3DHPC-NiCo2S4, the inset is the selected area electron diffraction.

The morphologies and nanostructures of the obtained 3DHPC and hybrid nanostructures materials were investigated by SEM and TEM. The images (Figure 2a and b) showed that 3DHPC exhibited open honeycomb-like microstructure, which consisted of numerous macropores with micron diameters and interconnected carbon nanosheets with the thickness of 100 to150 nm (inserted in Figure 2b). Carbon network structure was observed by TEM (Figure 2e) and the pore channel on the surface of carbon nanosheets was appeared at high resolution TEM images (inserted in Figure 2e). 3D architecture derived from carbon

nanosheets is advantageous to shortening the ion diffusion pathways, and minimizing diffusion resistance as the electrode material or supporting materials for supercapacitor. The elements of N, C and O were uniformly distributed in 3DHPC from mapping images (Figure S2). For comparison, the SEM image of porous carbon prepared using cellulose and KOH without urea was shown in Figure S3a, it was irregularity structure with several microns pores, this suggested that urea was necessary to synthesize honeycomb-like carbon. Meanwhile honeycomb-like microstructure was not formed without KOH yet (Figure S3b). The electrical conductivity for 3DHPC was 1.86 S/cm, this was consistent with its certain graphitization degree by Raman spectrum. So, 3DHPC could be served as supporting for loading pseudocapacitance materials to enhance the specific capacitance. 3DHPC-NiCo2S4 composites was prepared with by hydrothermal method and sulfidation. These oxygencontaining groups of hydroxyl groups and carbonyl groups on 3DHPC could interact with Ni2+ and Co2+ metal ions during hydrothermal process [40]. The complex of metal with oxygen-containing groups coated on 3DHPC surface in a shape of nanosheets (inserted in Figure S4), NiCo2S4 nanosheets were produced on 3DHPC surface during sulfidation process. The macropores channel and of 3DHPC wasn’t blocked off (Figure 2c, d) and the original honeycomb-like microstructure of 3DHPC was still kept after loading NiCo2S4. The thickness of the NiCo2S4 was about 15 nm from high magnification SEM image (inserted in Figure 2d). From the TEM images, the transmittance of 3DHPC-NiCo2S4 was weaker than that of 3DHPC due to the loading of NiCo2S4 (Figure 2f). The surface availability of 3DHPC was related the loading amount of NiCo2S4, more surface of 3DHPC was exposed at low loading amount of NiCo2S4, (Figure S5a) while the macropore was blocked at high loading amount of NiCo2S4 (Figure S5b) The interplanar spacing of NiCo2S4 nanosheets was measured about 0.54 nm in Figure 2g, which matched well with the (111) lattice planes of NiCo2S4 phase. The elements of C, N, O, Co, Ni and S were well dispersed in 3DHPC-NiCo2S4 proved by mapping images (Figure S6). As for 3DHPC-Fe2O3, Fe2O3 nanorods with a diameter of about 10-20 nm and a length of about 200 nm uniformly covered the whole surface of 3DHPC after calcination at 300°C. The 3DHPC-Fe2O3 composites still maintained excellent honeycomb-like microstructure (Figure

S7). The elements of C, N, O, Co, Ni and S were well distributed in 3DHPC-Fe2O3 from mapping images (Figure S8). In comparison, only nanosheets-assembled flower like NiCo2S4 and Fe2O3 nanorods were formed under similar synthesis conditions without the addition of 3DHPC substrates (Figure S9). Thermal behaviors of all samples were shown under air atmosphere from room temperature to 700 °C (Figure S10). The 3DHPC was rapidly decomposed from 450 °C and the weight residual was about 2 wt% at 600 °C. There was almost no weight loss for pure NiCo2S4 and Fe2O3. The weight loss of 3DHPC-NiCo2S4 and 3DHPC-Fe2O3 were 39.5% and 63.6% at 600 °C, respectively, it could be ascribed to the weigh loss of 3DHPC [28]. Therefore, the mass percents of NiCo2S4 in 3DHPC-NiCo2S4 and Fe2O3 in 3DHPC-Fe2O3 were 60.5% and 36.4%, respectively. N2 adsorption/desorption isotherms of 3DHPC, 3DHPC-NiCo2S4 and 3DHPC-Fe2O3 were shown in Figure S11. The BET specific surface areas and pore volumes of 3DHPC, 3DHPCNiCo2S4 and 3DHPC-Fe2O3 were 1003.6, 540.6, 410.2 m2/g, and 0.557, 0.373, 0.317 cm3/g, respectively.

850

Ni 2p3/2

Ni 2p

Ni 2p1/2

b

3+

Ni Satellite peak 2+ 874.6eV 879.5 eV Ni Satellite peak 870.5 eV 3+

Ni 856.7eV

861.5 eV

860

3+

870

Binding energy /eV

880

Co 778.8 eV 3+

Co 793.7eV

2+

Co 781.4 eV Satellite peak 785.5 eV

780

790

Co 2p 1/2 2+

Co 798.0eV

800

Binding energy /eV

S 2p

C

Co 2p

Co 2p 3/2

S 2p 1/2 161.7 eV

Intensity /a.u.

2+

Ni 853.3eV

Intensity /a.u.

Intensity /a.u.

a

Satellite peak 803.0 eV

810

Satellite peak 169.3 eV

S 2p 3/2 162.9 eV

160

162

164

166

168

170

172

Binding energy /eV

Figure 3. High-resolution XPS spectra of (a) Ni 2p, (b) Co 2p, and (c) S 2p for the NiCo2S4 in 3DHPC-NiCo2S4.

XPS were measured to obtain more information on the structure and composition of 3DHPC, 3DHPC-NiCo2S4 and 3DHPC-Fe2O3 composites. The 3DHPC survey spectrum (Figure S9a) indicated the coexistence of N, O, and C elements. The doping N species was from urea pyrolysis and the C1s at 284.5 eV was graphitized carbon in 3DHPC. The 3DHPCNiCo2S4 survey spectrum (Figure S9b) indicated the introduction of Ni, Co, and S elements and the results were consistent with the conclusion of XRD discussion. The Ni 2p spectrum

was fitted into spin-orbit doublets (Ni2+ and Ni3+) and two shakeup satellites using a Gaussian fitting method (Figure 3a). The binding energy at 853.3 eV in Ni 2p3/2 and 870.5 eV in Ni 2p1/2 conformed with the spin-orbit feature of divalent states, while the binding energy at 856.7 eV in Ni 2p3/2 and 874.6 eV in Ni 2p1/2 were characteristic of trivalent states [37, 4142]. As for the Co 2p XPS spectrum of 3DHPC-NiCo2S4 composites, it showed spin-orbit doublets including a low energy band (Co 2p3/2) and a high energy band (Co 2p1/2) (Figure 3b). The spin-orbit splitting value of Co 2p1/2 and Co 2p3/2 is over 15 eV, suggesting the coexistence of Co2+ and Co3+ and the fitted peaks at 781.4 eV and 798.0 eV are indexed to Co2+, whereas the other two peaks at 778.8 eV and 793.7 eV belong to Co3+ [7, 36, 43]. These results manifested that the chemical composition of NiCo2S4 in the composites contains Ni2+, Ni3+, Co2+ and Co3+ [7, 28, 30, 34]. Based on the Ni2+ and Ni3+ peak area of Ni 2p2/3, the ration of Ni2+ and Ni3+ was 1:2.7 and based the Co2+ and Co3+ peak area of Co 2p2/3, the ration of Co2+ and Co3+ was 1:1.2. The existence of Co3+/Co2+ and Ni3+/Ni2+ polyvalent metal in the 3DHPC-NiCo2S4 composite supplies numerous active sites for capacitance. Figure 3c showed the S 2p spectrum region, the peaks at 162.9 eV and 161.7 eV were corresponded to the S 2p3/2 and S 2p1/2 and the binding energy at 169.3 eV could be assigned to the satellite [29]. According to elements peak area and corresponding sensitivity factor, the ratio of Ni, Co and S elements in 3DHPC-NiCo2S4 was 1:1.79:4.09, which was well matched with the result of NiCo2S4 inferred by XRD. Compared with 3DHPC, Fe signals (Fe 2p, Fe 3p) emerged in 3DHPC-Fe2O3 XPS survey spectra after introducing Fe2O3 nanorods on the 3DHPC surface (Figure S12c). As for Fe 2p spectrum (Figure S12d), two binding energy of 711.0 and 724.8 eV were associated with Fe 2p3/2 and Fe 2p1/2 spin orbit peaks, which were coupled with a broad satellite peak at 718.8 eV and 732.5 eV, respectively. This indicated that the Fe was existed as Fe3+ and no other Fe2+ state [44, 45]. a

150

100 50 0 -50

10mV/s 50mV/s 100mV/s 300mV/s 500mV/s

-100 -150 -200 -1.0

-0.8

-0.6

-0.4

-0.2

Potential / (V vs. Hg/HgO)

0.0

Current density / A/g

Current density / A/g

150

b 5mV/s 10mV/s 20mV/s 50mV/s 100mV/s

100

50

0

-50

-100 -0.2

0.0

0.2

0.4

0.6

Potential / (V vs. Hg/HgO)

Figure 4. CV curves of (a) 3DHPC, (b) 3DHPC-NiCo2S4.

Figure 4a showed the CV curves of 3DHPC recorded at different scan rates from 5 to 500 mV/s in three-electrode system. All curves exhibited a typical quasi-rectangular shape with faradaic humps stemming from redox reactions associated with the heteroatoms of N and O [39, 46]. Meanwhile, excellent rate performance demonstrated that 3D honeycomb-nanosheet networks facilitated faster ion mobility. Figure 4b showed the CV curves of 3DHPC-NiCo2S4 electrode at different scan rates of 5, 10, 20, 50 and 100 mV/s within a potential window of 0.2 to 0.7 V (vs Hg/HgO). Notably, compared with CV curves of 3DHPC, a couple of redox peaks obviously appeared in CV curves of 3DHPC-NiCo2S4, which could be ascribed to the reversible redox reactions of Co2+/Co3+ and Ni2+/Ni3+ in KOH solution according with the following equation [33]: CoS + HO- ↔CoSOH + eCoSOH + HO- ↔ CoSO + H2O+ eNiS + HO- ↔ NiSOH + eHowever, at the same scan rate, the 3DHPC-NiCo2S4 possessed a larger enclosed area than those of NiCo2S4 and 3DHPC electrodes, which indicated that the capacity of 3DHPCNiCo2S4 electrode was improved. This results mainly benefited from the specific structure of the 3DHPC-NiCo2S4, namely the NiCo2S4 nanosheets could intersect coatings onto 3DHPC contributing to the redox reaction, and the honeycomb 3DHPC as a forceful supporting possess abundant carbon nanosheets surface. The typical CV curves of 3DHPC-Fe2O3 with a potential window from -1.15 to -0.20 V versus Hg/HgO at different scan rates were shown in Figure S13b, a pair of pseudocapacitive redox peaks appeared due to the reversible conversion between Fe2+ and Fe3+ [16, 43, 47]. Even though the scan rate up to 100 mV/s, the CV curves still retained a similar shape, suggesting an excellent rate capability. Figure S13b also showed pure the CV curves of Fe2O3 nanorods. The composites CV curves obvious displayed outstanding redox performance and rate behavior compared with pure metallic

compound, indicating that this unique hybrid structure is beneficial to electrochemical

a 20, 15,

10, 5, 2,

1 A/g

-0.2

0.0

100, 80, 60, 50, 40, 30 A/g

0.5

b 30, 20, 15, 10, 5, 2, 1A/g

-0.2

Potential / V vs Hg/HgO

Potential / V vs Hg/HgO

0.0

Potential / V vs Hg/HgO

properties.

-0.4 -0.6 -0.8 -1.0 0

-0.4

2

4

6

8

10 12 14

Time / s

-0.6

-0.8

0.4

0.3

0.2

0.1

0.0

-1.0 0

100

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400

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0

600

200

400

600

800

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Time / s

Time / s

Current / A 35

25

20

15

10

5

0

c

-100 100

d

3DHPC/NiCo2S4 3DHPC NiCo2S4

80

-80

1600

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Figure 5. GCD curves of (a) 3DHPC and (b) 3DHPC-NiCo2S4, (c) rate capability of 3DHPC, NiCo2S4 and 3DHPC-NiCo2S4, (d) Nyquist plots of 3DHPC, NiCo2S4 and 3DHPC-NiCo2S4 (Inset magnifies the data in the high-frequency range).

The electrochemical performances of the samples were further examined by GCD technique to obtain capacitance values. The GCD curves of the 3DHPC electrodes ranging from 1 to 100 A /g in 6 mol/L KOH electrolyte were shown in Figure 5a. Approximately perfect symmetric triangle curves from excellent EDLC capacitive behavior were observed at different current densities, implying less energy loss during the charge-discharge process. A slight distortion maybe resulted from pseudocapacitance generated by nitrogen and oxygen groups. At the current density of 1A/g, the capacitance of 3DHPC was 254 F/g, and capacitance value slightly decreased with the increasing current density as the result of electrolyte diffusion resistance. At a high current density of 30A/g, the capacitance value of 3DHPC was 207 F/g, which remained 82% of that at 1A/g. it was superior to other carbon nanosheets previous reported [11, 48-51]. The excellent rate capability of 3DHPC benefited from its nanosheets structure diffusion pathway and less diffusion resistance from honeycomb-like morphology. Figure 5b demonstrated typically metallic compound GCD curves in the potential range from 0 to 0.5 V at current densities of 1-30 A/g. These nonlinear

curves at different rates reconfirmed the pseudocapacitive speciality stemming from multivalence cobalt and nickel. The specific capacity of the 3DHPC-NiCo2S4 electrode obtained from the GCD curves was 1169, 1140 and 1009 F/g at current densities of 1, 2 and 5A/g, respectively (Figure 5c). When the current density increased 10, 20 and 30 times, the capacitance still, retained 80%, 58% and 41% of that at 1 A/g, suggesting outstanding rate property of 3DHPC-NiCo2S4 electrode (Figure 5c). For comparison, the flower like NiCo2S4 electrode illustrated 252 F/g at 30 A/g keeping about 23% of that at 1 A/g. The 3DHPCNiCo2S4-1 exhibited 874 F/g at 1A/g, which is less than that of 3DHPC-NiCo2S4 (1160F/g) at low loading NiCo2S4. While the capacitance of 3DHPC-NiCo2S4-2 was 1194 F/g at 1A/g, which was near to that of 3DHPC-NiCo2S4 because overloaded NiCo2S4 could not be sufficiently utilized. The deduced results proved that the 3DHPC-NiCo2S4 electrode performance was obviously superior to that of the other NiCo2S4 composite materials, as seen in Table S1. The excellent electrochemical performance might be attributed to the following facts. Firstly, the honeycomb structure consisting of carbon nanosheets established excellent conductive network shortening the electrolyte ion and electron diffusion distance; secondly, the macropore structure of 3DHPC supplied abundant electrolyte/electrode interfaces for charge accommodation; thirdly, the NiCo2S4 nanosheets in situ grown on carbon nanosheets ensured valid electronic transmission between 3DHPC and each NiCo2S4 nanosheets. The discharge behaviors of 3DHPC-Fe2O3 and Fe2O3 were investigated by GCD in the potential range from -1.05 to -0.2 V at current densities of 1-10 A/g. The specific capacitance of 3DHPC-Fe2O3 and Fe2O3 calculated on discharge curves and rate capabilities based on capacitance at 1 A/g were shown in Figure S14d. The capacitance retention was 44% (317 F/g) at the scan rate increased as high as 10 times from 1 to 10 A/g (28% for the Fe2O3 electrode). EIS was also performed to compare the electrochemical properties of the samples, and the results were shown in Figure 5d. The curves of the low frequency data exhibited Warburg impedance representing by the slope of the plot at the low frequency range, which was mostly caused by the diffusion rate of electrolyte ions in the electrode materials. Both curves of 3DHPC-NiCo2S4 and 3DHPC displayed similar slope and the slope of NiCo2S4 was gentler than that of 3DHPC-NiCo2S4 and 3DHPC, which suggested electrolyte diffusion impedance was improved by carbon supporting. The 3DHPC showed the smallest equivalent

series resistance calculated based on the value of the horizontal axis intercept in the high frequency region, which was related to Ohmic resistance derived from electrolyte, active electrode materials. a 1.6

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Figure 6. (a) CV curves of 3DHPC-NiCo2S4//3DHPC-Fe2O3 with PVA-KOH electrolyte, (b) GCD curves of 3DHPCNiCo2S4//3DHPC-Fe2O3 with PVA-KOH electrolyte, (c) Nyquist plots for 3DHPC-NiCo2S4//3DHPC-Fe2O3 with KOH and PVA-KOH electrolyte (Inset magnifies the data in the high-frequency range), (d) cycling stability of 3DHPC-NiCo2S4//3DHPC-Fe2O3 solid-state asymmetric supercapacitor (the left inset shows the CV curves of 1st and 10000th and the right inset show Nyquist plots curves of the 1st and 10000th).

In view of the high capacitance and redox characteristics of composites electrode from the synergistic effect of 3DHPC and NiCo2S4 nanosheets, an all solid asymmetric supercapacitor device with PVA-KOH as electrolyte was assembled with 3DHPC-Fe2O3 as negative electrode materials and 3DHPC-NiCo2S4 as positive electrode materials (denoted as 3DHPCNiCo2S4//3DHPC-Fe2O3). In addition, 3DHPC-NiCo2S4//3DHPC-Fe2O3 cell with 6 mol/L KOH electrolyte was also packed to compare the electrochemical behavior of solid electrolyte-based devices. When the voltage window was 1 V, the observed near-rectangular curve indicated the charge propagation in EDLC. When the potential window was increased to 1.4 V, oxidation and reduction peaks appeared in the CV results, indicating the incomplete pseudocapacitive character. Furthermore, when the operation potential window increased up

to 1.6 V, clear redox peaks appeared in the CV curve (Figure S15) [16]. The voltage window of 0-1.6V was chosen to explore the electrochemical property of the assembled cells. Based on charge balance theory (m+/m- = C-V-/C+V+, C-=548 F/g, V-= 0.85 V, C+=1140 F/g, V+=0.5V) and their respective specific capacitances at 2A/g in the above three-electrode setup [3], the mass ratio of the positive electrode (3DHPC-NiCo2S4) with the negative electrode (3DHPC-Fe2O3) was fixed as 1:1.2 A series of CV curves of the all solid asymmetric supercapacitor devices were shown in Figure 6a. Apparently, a combination of characteristics of both electrical double layer capacitances and pseudocapacitive appeared in the overall CV curves. At low potential, it demonstrated weak current response associated with the electric double layer. The current density increased in respond to a faradic reaction with potential increasing. Similar redox characteristics were observed in all of the CV curves demonstrating high rate capability of the solid devices. The aqueous electrolyte device showed more obvious redox peaks comparing with the solid asymmetric supercapacitor at high scan rates especially, because the solid electrolyte was unfavorable for ion diffusion restricting the vigorous redox reaction of NiCo2S4 and Fe2O3 during charge-discharge processes (Figure S16a). Figures 6b further showed the GCD for the solid state 3DHPC-NiCo2S4//3DHPC-Fe2O3 ranging from 2 to 10 A/g. The specific capacitance calculated from GCD was 133.5 F/g at a scan rate of 2 A/g (based on the mass of the 3DHPC-NiCo2S4 and 3DHPC-Fe2O3). At current density of 10 A/g, the specific capacitance was 42.5 F/g. In addition, the aqueous cell was measured by GCD in Figure S16b. The aqueous cell exhibited a high specific capacitance of 145.7 F/g at a current density of 2 A/g. At a higher current density of 10 A/g, the specific capacitance was 72.5 F/g. The nonlinear curves further confirmed the contribution of pseudocapacitance. Moreover, the inclined section in the PVA-KOH cell (Figure 6b) transformed to approximate plateau regions (Figure S16b), indicating the more effective redox reaction in aqueous eletrolyte. For the sake of explore the reason and deeper understanding of the electrochemical performance, EIS measurement was performed and Nyquist plots of the both cells were shown in Figure 6c. In the high-frequency part (inserted in Figure 6c), the intersection of the curve with the Z’ axis (representing the equivalent series resistance Rs) of the PVA-KOH device (0.82 Ω) was higher than that of KOH device (0.65 Ω), which overall indicated the solid electrolyte exhibited tedious ionic resistance, inferior wettability and compatibility. In

addition, the PVA-KOH device exhibited a higher Rct value representing the charge transfer resistance (4.18 Ω) than that of KOH device (2.55 Ω), indicating the lower ionic conductivity of the PVA-KOH gel electrolyte. At low frequency zones, the both cells reached full capacitance due to the slow movement of electrolyte ions promoting the accessing more of the electrode surface, so the slopes of the straight lines for both cells were almost same. The life spans of the both cells were measured with 4 A/g for 10000 cycles (Figure 6d, S16d) and the CV and EIS before and after long term (10000 cycles) were inserted. The solid 3DHPCNiCo2S4//3DHPC-Fe2O3 cell still remained at 88.5% of its initial capacitance (101.2 F/g) revealing a good cycling performance. The SEM images (Figure S17) of 3DHPC-NiCo2S4 after 10000 cycles showed the thickness of NiCo2S4 nanosheets increased obviously after 10000 cycles and even transformed into particle blocking partly the honeycomb-like structure. The both CV curves at 100 mV/s (Figure 6d left inserted) indicated redox psedocapacitance was weakened after circulation and the both EIS curves (Figure 6d right inserted) imply the diffusion resistance was enlarged after long-term running due to the consumption of the gel electrolyte [16]. The cycling performance of the aqueous cell was superior to that of the solid device. The above conclusions implied that the transmit efficiency of charge and ion diffusion rate should be relatively dull using the solid state electrolyte, and the interface contacting between electrode materials and solid electrolyte was no so tight because the solid electrolyte was difficult to diffuse into the inner layer of the electrodes. However, solid-state energy storage device offered a number of non-fungible advantages such as non-volatility, non-toxicity, non-corrosiveness, non-flammability and so forth comparing

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Ref. 45 MnS//activated carbon Ref. 46 Co3O4@Ni(OH)2// activated carbon Ref. 47 capsule-like Ni1.77Co1.23S4// activated carbon Ref. 48 NiCo2S4@NiO//active carbon Ref. 26 MnO2/carbon fiber//graphene

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Figure 7. (a) Gravimetric Ragone plots of the solid state 3DHPC-NiCo2S4//3DHPC-Fe2O3 device compared with the selected previous solid-state supercapacitor (b) Volume Ragone plots of the solid state 3DHPC-NiCo2S4//3DHPC-Fe2O3 device compared with the selected previous solid-state supercapacitor (the insets is the measuring image of thickness, width and length of solid state device.)

The gravimetric energy and power densities of the solid system were calculated based on the date of GCD curves and the Ragone plot were shown in Figure 7a. The solid asymmetric device reached a superior gravimetric energy density of 44.4 and 15.1 Wh/kg at a power density of 1.62 and 6.9 KW/kg, respectively. Those results of the solid cell were equivalent or superior to those reported the all-solid-state supercapacitor [26, 52-59], or other metal sulfide (listed in Table S2). The 3DHPC-NiCo2S4//3DHPC-Fe2O3 supercapacitor had the highest volumetric energy density of 1.71 mWh/cm3 volumetric power density 62.94 mW/cm3, and it was compared with other solid state supercapacitor (Figure 7b) [40, 60-65].

Figure 8. (a) Digital photo of single device supplying a domestic quartz clock (b) Digital photo of two devices in series lighting up a red light-emitting diode indicator.

After charging from 0 to 1.6V by 20s, single device supplied a domestic quartz clock (rated voltage: 1.5V) to work for 52 min showing its potential application in energy storage. Two solid devices in series were fabricated using 3DHPC-NiCo2S4//3DHPC-Fe2O3 to get a high output voltage. Compared with the single cell, the discharge voltage window was up to 3.2 V (Figure S19). The tandem devices could light up a red light-emitting diode indicator (rated voltage: 2 V) for about 20 min. The honeycomb-like structure composites with NiCo2S4 nanosheets in order grown on the carbon nanosheets surface were rationally designed and synthesized by one-step carbonization followed by hydrothermal process and sulfidation. The composites electrode

shows outstanding application prospect from energy component NiCo2S4 nanosheets intimately contacting carbon nanosheets and from power component honeycomb-like carbon framework.

4.

Conclusion

In conclusion, the three-dimensional honeycomb-like porous carbon consisting of carbon nanosheets was obtained by a low cost one-step carbonization process using cellulose as carbon precursor; afterwards, Ni-Co precursor nanosheets vertically deposited on the carbon nanosheets surface by hydrothermal growth and NiCo2S4 nanosheets were obtained by further sulfidation. The 3DHPC-NiCo2S4 as electrode materials exhibited desirable electrochemical performance with a high specific capacitance of 1169 F/g at 1 A/g and 684 F/g capacitance retention at 20 A/g. The fabricated all solid-state asymmetric supercapacitor 3DHPCNiCo2S4//3DHPC-Fe2O3 had a high gravimetric energy density of 44.4 Wh/kg (volumetric energy density: 1.71 mWh/cm3) at power density of 1.62 KW/kg (volumetric power density: 62.94 mW/cm3) and stable cycle performance with 88.5% capacitance retention after 10000 cycles. The macropore structure from honeycomb-like hybrids not only provided space for NiCo2S4 vertical growth benefiting to sufficient redox but also ensured electrolyte ion diffusion into the whole electrode for high efficient charge storage in three-dimensional directions. Moreover, nanosize interconnected carbon nanosheets shortened transportation distance for the electrons to other parts. Furthermore, abundant micropore provided electrical double-layer capacitance and doping with N heteroatoms assists in the adsorption of the electrolyte ions onto the carbon materials by improving the wettability of the electrode. The all solid device excellent electrochemical performances should be ascribed to the synergistic effect of NiCo2S4 nanosheets, Fe2O3 nanorods and carbon frame. This outstanding properties are very encouraged to synthesize more novel honeycomb-like framework composite, which not only supply reactive room for active ingredient but also possess low mass transfer resistance. The hybrid materials also have potential applications in catalysis, environmental fields, and so on.

Acknowledgment The authors gratefully acknowledge the financial support by National Science Foundation of China (No. 21276266 and U1510122).

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Figure captions Scheme 1. Schematic of the synthesis of 3DHPC-NiCo2S4 and 3DHPC-Fe2O3 electrodes and the assembly of solid state device with 3DHPC-NiCo2S4 as the positive electrode and 3DHPC-Fe2O3 as the negative electrode.

Figure 1. XRD patterns of 3DHPC, 3DHPC-NiCo2S4 and 3DHPC-Fe2O3.

Figure 2. (a) SEM image of 3DHPC, the inset is the photograph of a honeycomb, (b) High resolution SEM images of 3DHPC, (c) SEM image of 3DHPC-NiCo2S4 (d) High resolution SEM images of 3DHPC-NiCo2S4 (e) TEM image of 3DHPC, the inset is high resolution TEM images of 3DHPC (f) TEM image of 3DHPC-NiCo2S4, the inset is high resolution TEM images of 3DHPC-NiCo2S4 (g) High resolution TEM images of NiCo2S4 in 3DHPCNiCo2S4, the inset is the selected area electron diffraction.

Figure 3. High-resolution XPS spectra of (a) Ni 2p, (b) Co 2p, and (c) S 2p for the NiCo 2S4 in 3DHPC-NiCo2S4.

Figure 4. CV curves of (a) 3DHPC, (b) 3DHPC-NiCo2S4. Figure 5. GCD curves of (a) 3DHPC and (b) 3DHPC-NiCo2S4, (c) rate capability of 3DHPC, NiCo2S4 and 3DHPC-NiCo2S4, (d) Nyquist plots of 3DHPC, NiCo2S4 and 3DHPC-NiCo2S4 (Inset magnifies the data in the high-frequency range).

Figure 6. (a) CV curves of 3DHPC-NiCo2S4//3DHPC-Fe2O3 with PVA-KOH electrolyte, (b) GCD curves of 3DHPC-NiCo2S4//3DHPC-Fe2O3 with PVA-KOH electrolyte, (c) Nyquist plots for 3DHPC-NiCo2S4//3DHPCFe2O3 with KOH and PVA-KOH electrolyte (Inset magnifies the data in the high-frequency range), (d) cycling stability of 3DHPC-NiCo2S4//3DHPC-Fe2O3 solid-state asymmetric supercapacitor (the left inset shows the CV curves of 1st and 10000th and the right inset show Nyquist plots curves of the 1st and 10000th).

Figure 7. (a) Gravimetric Ragone plots of the solid state 3DHPC-NiCo2S4//3DHPC-Fe2O3 device compared with the selected previous solid-state supercapacitors (b) Volume Ragone plots of the solid state 3DHPCNiCo2S4//3DHPC-Fe2O3 device compared with the selected previous solid-state supercapacitors (the insets is the measuring image of thickness, width and length of solid state device.)

Graphical abstract

Highlights

Honeycomb-like porous material assemble from interconnection carbon nanosheets.

Carbon nanosheet are covered by erect NiCo2S4 nanosheets.

Carbon nanosheets are power component and NiCo2S4 nanosheets are energy component.

The solid device shows excellent volumetric energy and power densities.