2 Honeycomb Nanostructure and Asymmetric Supercapacitor with

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Formation of g-C3N4@Ni(OH)2 honeycomb nanostructure and asymmetric supercapacitor with high energy and power density Bitao Dong, Mingyan Li, Sheng Chen, Dawei Ding, Wei Wei, Guoxin Gao, and Shujiang Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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ACS Applied Materials & Interfaces

Formation of g-C3N4@Ni(OH)2 honeycomb nanostructure and asymmetric supercapacitor with high energy and power density

Bitao Dong, Mingyan Li, Sheng Chen, Dawei Ding, Wei Wei, Guoxin Gao, Shujiang Ding*

Department of Applied Chemistry, School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China.

Corresponding author: [email protected] (Shujiang Ding)

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ABASTRACT: Nickel hydroxide (Ni(OH)2) has been regarded as a potential next generation electrode material for supercapacitor owing to its attractive high theoretical capacitance. However, practical application of Ni(OH)2 is hindered by its lower cycling life. To overcome the inherent defects, herein we demonstrate a unique interconnect honeycomb structure of g-C3N4 and Ni(OH)2 synthesized by an environmental-friendly one-step method. In this work, g-C3N4 own an excellent chemical stability and support a perpendicular charge transporting direction in charge-discharge process, facilitating electron transportation along that direction. The as-prepared composite exhibits higher specific capacities (1768.7 F g-1 at 7 A g-1, 2667 F g-1 at 3 mV s-1, respectively) compared to Ni(OH)2 aggregations (968.9 F g-1 at 7 A g-1) and g-C3N4 (416.5 F g-1 at 7 A g-1), as well as better cycling performance (~84%

retentions

after

4000

cycles).

As

asymmetric

supercapacitor,

g-C3N4@Ni(OH)2//Graphene exhibits high capacitance (51 F g-1) and long cycle life (72% retentions after 8000 cycles). Moreover, high energy density of 43.1 Wh kg-1 and power density of 9126 W kg-1 has been achieved. These attractive performance reveals that g-C3N4@Ni(OH)2 with honeycomb architecture could find potential application as electrode material for high performance supercapacitors.

KEYWORDS:

Asymmetric

supercapacitor;

g-C3N4;

g-C3N4/Ni(OH)2 hybrids; Honeycomb structure.

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Ni(OH)2

nanosheets;

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1. Introduction Supercapacitors are important energy storage devices due to their high power density (>10000 W kg-1), good cycling stability (>105), fast charging and discharging rate (within second) and low maintenance cost.1-4 Pseudocapacitors have great potential as supercapacitor candidates for its high energy density and fast reversible surface redox reactions.5-9 However, currently most pseudocapacitive materials present either poor cycling stability or poor electrical conductivity.10-11 Among those materials, nickel hydroxide (Ni(OH)2) exhibits excellent theoretical specific capacitance ( ~3750 F g−1),12 yet hindered from practical use for low surface area, inferior structural stability and poor performance aforementioned of pseudocapacitive materials.13-19 To address those problems, various strategies including constructing hybrid materials with high electrical conductivity; downsizing hybrid materials to nanoscale; using chemically stable conducting substrates have proved to be effective to improve its performance.5, 20-22

Graphitic carbon nitride (g-C3N4), as a sheet-like crystallite, has attracted intense research interest due to its excellent chemical and thermal stability, appealing electronic structure and low cost.23-25 Traditionally, g-C3N4 is a soft polymer which can be easily grafted by other materials, and well-crystallized materials with lamellar structure favors charge transfer along horizontal direction.26-28 Recently, Christoph et.al reported that electron transport of g-C3N4 is predominantly perpendicular to the sheets of g-C3N4, and as a result, g-C3N4 is considered a promising candidate material for electrochemical applications.29-30

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In this work, the stable feature of g-C3N4 were made full use as ideal conducting scaffold. A honeycomb structure composed of Ni(OH)2 nanosheets were grown on g-C3N4 and hybrid structures of g-C3N4@Ni(OH)2 (CN/Ni) are formed. Meanwhile, an asymmetric supercapacitor device using CN/Ni and reduce graphene oxide (derived from graphene oxide by thermal recovery31) as positive and negative electrode materials in KOH (aq.) electrolyte was assembled (Scheme 1). The asymmetric supercapacitor CN/Ni//Graphene (denoted as CN/Ni//G) shows a superior performance with a high specific capacitance of 51 F g-1 and stable specific capacitance of 22.8 F g-1 for 8000 cycles at a current density of 3 A g-1. Meanwhile, the energy power densities reach to 43.1 Wh kg-1 and 9126 W kg-1, respectively. The asymmetric supercapacitor CN/Ni//G is considered a promising electrode material with superior performance owing to the following considerations. Firstly, the improved conductivity of the CN/Ni hybrid provided by the perpendicular charge transport pathway of g-C3N4 in turn, favors electron diffusion during the fast charge/discharge process; Secondly, Ni(OH)2 nanosheets can support each other to prevent excessive aggregations which is beneficial for contacting with electrolyte (as illustrated in Scheme 2). Finally, more active sites are exposed due to the honeycomb structure of CN/Ni hybrid, so that the electrochemical reactions are promoted. In summary, g-C3N4-Ni(OH)2 are considered as an ideal combination for the electrode of supercapacitors.

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Scheme 1 Schematic illustration of CN/Ni-G asymmetric supercapacitor.

Scheme 2 Schematic illustration of CN/Ni hybrid nanostructure. 2. Experimental Section 2.1 Material Synthesis For the typical fabrication of CN/Ni nanocomposites, 10 mg of g-C3N4 nanosheets (CN) firstly was dispersed in 40 mL deionized (DI) water and sonicated for 1 hour until CN nanosheets dissolved in DI water sufficiently. Then, 0.25 mmol of Ni(NO3)2.6H2O, 0.025 mmol of trisodium citric (TSC) and 0.25 mmol of hexamethylenetetramine (HMT) were added above suspension and stirred violently for about 60 min. Subsequently the above mixed solution was heated in an oil bath with gently stirring at 90 °C for 10h. After slowly cooled to room temperature, the precipitate was harvested by centrifugation, washed thoroughly with DI water and

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ethanol, and further dried at 50 °C for 10 h in vacuum. In comparison, the Ni(OH)2 aggregations were also synthesised use the same procedure without g-C3N4 template. 2.2 Characterization The morphological information of sample was characterized by Field-emission scanning electron microscopy (FESEM, JEOL, JSM-7000F) with an Energy Dispersive X-ray detector (EDX) (JEOL JED-2300) and Transmission electron microscopy (TEM, JEOL, JEM-2100). The crystallographic information of materials were collected by Lab-X-6000 (X-ray diffraction, XRD). The chemical state of products were investigated by X-ray photoelectron spectroscopy (Axis Ultra, Kratos (UK)). The thermogravimetric analysis was recorded by METTLER-TOLEDO TGA 1. To calculate the specific surface area of the final hybrids, N2 sorption measurements were conducted on an Autosorb 6B at liquid N2 temperature and the Brunauer-Emmett-Teller (BET) method was used. The Barrett-Joyner-Halenda (BJH) method was employed to obtain their pore size distributions, which were derived from the adsorption branches. 2.3 Electrochemical measurements In this work, two kinds of methods, three-electrode system and two-electrode system, were used to value the electrochemical performance of CN/Ni nanocomposites on a CHI 660E workstation with aqueous KOH (6M) as electrolyte. The working electrodes were prepared via mixing the electroactive materials (70 wt %), carbon black (20 wt %) and polymer binder (polyvinylidene fluoride; PVDF, 10 wt %). Then the slurry was spread on a piece of nickel foam at 10MPa as current collector and was

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dried at 120 °C for 12 h in vacuum. Notably, 50-100 cyclic voltammetry cycles are necessary to activate the electrode materials in electrolyte before testing. For the three-electrode measurement, Pt foil was used as counter electrode and a standard calomel electrode (SCE) as the reference electrode. The loading mass of CN/Ni electrode was estimated at 0.72mg cm-2. For the two-electrode valuation, a piece of permeable separator (Celgard 3501) was placed between two working electrodes (CN/Ni and graphene) to construct an asymmetric supercapacitor. In two-electrode, the loading mass of CN/Ni//G here was estimated at 3.2 mg cm-2. The electrochemical tests were conducted with a CHI 660E workstation in an aqueous KOH (6 M) electrolyte. 3. Results and discussion As shown in Figure S1, the g-C3N4 nanosheets present a smooth surface with thicknesses range from several atomic layers to micrometers. Ni(OH)2 nanosheets are regularly anchored on the surface of each g-C3N4 nanosheets to form the CN/Ni honeycomb complex through a one-step process (Figure 1A). In-depth SEM characterization proves that the honeycomb configuration is composed of interconnected Ni(OH)2 nanosheets with thicknesses estimated at ~20 nm (Figure 1B). To better illustrate the unique architectural feature, representative TEM image of CN/Ni is shown in Figure 1C. It is observed that the CN/Ni sample consists of well-defined nanosheets with thicknesses of several nanometers. The dark field TEM of CN/Ni is also shown in Figure 1D. EDX elemental mapping is used to characterize the spatial distribution of Ni(OH)2 and g-C3N4 in the CN/Ni hybrids. As shown in

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Figure 1E-H, all C, N, Ni and O elements are detected with dense distribution of Ni and O, suggesting that the Ni(OH)2 nanosheets were covered uniformly on g-C3N4. The element analysis performed by Energy Dispersive X-ray (EDX) was shown in Figure S2. And the atomic percent of C, N, O and Ni are approximately 34.24, 22.27, 28.39 and 15.11, respectively.

Figure 1 (A-D) FESEM and TEM images of CN/Ni composite. E–H) Elemental mapping images of CN/Ni composite. Crystal phase and composition of the hybrid structure are characterized by X-ray diffraction (XRD) with the patterns of CN/Ni, Ni(OH)2 and g-C3N4 shown in Figure 2A (I, II and III, respectively). CN/Ni hybrid apparently reveals that the combined diffraction peaks at 27.4o, 33.4o and 59.7o are corresponding to (002) of g-C3N4, (101) and (110) of Ni(OH)2, respectively. The crystal phases of (101) and (110) fit well with standard diffraction patterns of α-Ni(OH)2 (JCPDS No. 38-0715).32-33 The chemical states of each element on the surface of CN/Ni were characterized by X-ray photoelectron spectroscopy (XPS). As shown in Figure 2B, the Ni 2p peaks are corresponding to the Ni 2p3/2 (852.8–858.7 eV) and Ni 2p1/2 (870.5–876.9 eV), proving that Ni ion exists +2 valence state. The results in Figure 2C exhibit that the C-C peak is estimated at 284.6 eV proves that the defects contain sp2 bonded carbon. 8


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The peak at 284.2 eV, 285.4 eV and 288.4 eV corresponds to the coordination of C=C, C-N and C-O.33 The N 1s XPS spectra of the CN/Ni (Figure 2D) shows that the peaks at binding energies of 398.3 and 399.3 eV are corresponding to Pyridine N and Pyrrolic N.34

Figure 2 (A) XRD patterns of CN/Ni (I), Ni(OH)2 aggregates (II) and g-C3N4 (III); XPS spectra of (B) Ni 2p; (C) C 1s and (D) N 1s.

The weight fraction of g-C3N4 and Ni(OH)2 in the hybrid material is characterized by thermogravimetric analysis (TGA) with the results shown in Figure S3. The TGA curves of g-C3N4, Ni(OH)2 aggregations and CN/Ni exhibit a decrease at around 200 °C, which is attributed to the loss of moisture. In the curve of Ni(OH)2, thermal decomposition of Ni(OH)2 to NiO completed almost at 300 °C. And the curve of g-C3N4 shows decomposition of g-C3N4 at 700 °C. The curve of CN/Ni shows a weight loss at ~300 °C, sugesting the chemical decomposition of Ni(OH)2. With increased temperature, a significant weight loss is observed from 350 °C to 600 °C,

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which implies decomposition of g-C3N4. The residual sample weight is around 44.7 wt% over 600 °C, which could be ascribed to degradation of Ni(OH)2 to NiO. Therefore, the amount of Ni(OH)2 can be calculated as following: Amount of Ni(OH)2 = 44.7% × M(Ni(OH)2) / M(NiO) = 55.5%, where the molecular weights M(Ni(OH)2) and M(NiO) are 92.69 and 74.69 respectively.35 The specific surface area and pore size of CN/Ni materials were investigated by Brunauer-Emmett-Teller (BET). As shown in Figure S4, a type IV isotherm (H4 hysteresis loops) was exhibited in the relative pressure range of 0.25-0.95.) A relative high specific surface area of CN/Ni (62.4 m2 g-1) and proper pore size of 61 nm is helpful to provide sufficient reactive sites to enhance rate performance. As working electrode, the CN/Ni active materials were evaluated using three-electrode where a standard calomel electrode (SCE) acts as reference electrode and Pt foil as the counter electrode in an aqueous KOH (6M) electrolyte. Figure 3A shows the cyclic voltammetry (CV) curve of CN/Ni at various scanning rate from 3~50 mV s-1. Noticeably, a pair of redox peaks appear in the potential range from 0~0.55 V for all sweep rates (revealing the characteristics of pseudocapacitors) is related to the Faradaic reactions of Ni-OH/Ni-O-OH. It is commonly accepted that at low discharge current densities, both surface and bulk play important roles in the redox activities, whereas they are dominated by the surface activity at high discharge current densities.34, 36 According to the following equation: C = ∫ IdV / vmV , where C is the specific capacitance (F g-1), V is the potential (V) during the electrochemical reaction, I is the response current (A cm-1), v is the potential scan rate (mV s-1) and m

10


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is the mass of the electroactive materials (g cm-1).37 Figure 3B presents the dependence of specific capacitances of CN/Ni on CV at different scan rates. The specific capacitances are calculated to be 2667, 1783, 1060, 783 and 550 F g-1 at scan rates of 3, 5, 10, 30 and 50 mV s-1, respectively. With increased scan rate, the cathodic and anodic peaks shift to negative and positive directions, respectively, to bring about the decreased specific capacitance of the CN/Ni electrode materials.37 The specific capacitance of electrode can also be calculated by the equation: C = I × ∆t /(m × ∆V ) , where I is the discharge current, ∆t is the discharge time, ∆V is the discharge voltage range and m is the mass of the active material.38 The specific capacitance values at different discharge current densities are calculated by the above equation and the results are shown in Figure 3C, which proved that with the increase of current densities, the specific capacitances decrease gradually as expected. In Figure 3D, a higher capacitance value of Ni(OH)2 is observed due to the redox reactions,39 and the specific capacitances of CN/Ni are calculated to be 1768.7, 1600, 1368.9, and 1226.7 F g-1, corresponding to the discharge current densities of 7, 10, 20 and 30 A g-1, respectively. In comparison, a low specific capacitance was found for Ni(OH)2, which is 968.9 F g-1 at 7 A g-1 but decreased sharply to 633.3 F g-1 at 30 A g-1. While g-C3N4 presents low and stable specific capacitances vary from 416.5 F g-1 to 315 F g-1. Figure 3E reveals the voltage profiles of charge-discharge curves for the first 20 cycles at current density of 10 A g-1, and the columbic efficiency is approaching 100% for each cycle. In the first cycle, the specific capacitance is about 1600 F g-1 and progressively decreases to 1345.5 F g-1 after 4000 cycles, which means that overall,

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almost 16% capacity losses with an capacity loss of 0.004% each cycle is observed at shown in Figure 3F. The excellent stability as well as high specific capacitance of the composites enables them as promising candidate in supercapacitors.40 In comparison, the capacitances of Ni(OH)2 aggregations and pure g-C3N4 are as low as 926.6 F g-1 and 377.5 F g-1 at 10 A g-1, respectively. The capacitance of Ni(OH)2 decreases to 502.22 F g-1 after 2000 cycles with about 45.7% overall capacity loss. For g-C3N4 electrode, the capacitance is 345 F g-1 after 2000 cycles of discharging and reserved 91.4% capacity for initial cycle, proving the excellent stability of g-C3N4.

Figure 3 (A) CV curves of CN/Ni electrode at different scan rates (range from 3~50 mV s-1); (B) Calculated specific capacitance of CN/Ni electrode based on CV curves; 12


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(C) Galvanostatic discharge curves of CN/Ni electrode at various discharge current densities; (D) Calculated specific capacitance of CN/Ni, Ni(OH)2 and g-C3N4 electrode material based on galvanostatic discharge curves; (E) Galvanostatic charge-discharge curves at a current density of 10 A g−1; (F) Specific capacitance retention versus cycle number of CN/Ni, Ni(OH)2 and g-C3N4 electrode material at a current density of 10 A g−1.

As for a Asymmetric supercapacitors, the charge balance follows the relationship:

q + = q − , and the charge stored by each electrode equals to the multiplication of the specific capacitance (C), the potential range for the charge/discharge process (∆V) and the mass of the electrode active materials (m) follows the equation:41

q = I × ∆t m + / m− = q − / q + Calculated by the analytical results of the C values for CN/Ni and reduce graphene oxide, the mass ratio of two electrodes was estimated to be 0.25 (m (CN/Ni)/ m (graphene)) in the asymmetric supercapacitors. The performance of CN/Ni and reduced graphene oxide-based asymmetric supercapacitor (donated as CN/Ni–G) was tested in a full-cell step. Typical CV curves of the CN/Ni//G asymmetric supercapacitor in 6 M KOH (aq.) electrolytes at various scan rates in the range from 0 to 1.3 V, as shown in Figure 4A. As an example of CV curve at 100 mv.s-1 scan rate. In the charge process, two peaks were observed at around 1.17 V and 0.81 V, which implies the oxidation of Ni

2+

to Ni

3+

. And in the

discharge process, 0.89 V and 0.6 V can be related to the reduction of Ni

3+

to Ni

2+

.

The slight nonlinearities discharge curves between 0 and 1.3 V at different current densities (0.8~3 A g-1, shown in Figure 4B) match the CV results. According to the

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discharge curves, the capacitance of CN/Ni//G asymmetric supercapacitor reaches 51 F g-1 at 0.8 A g-1 as shown in Figure 4C. With increased current density, the specific capacitance decreases gradually since the migration of electrolyte ion was bind at high current densities. It is commonly accepted that a long cycle life is critically important for supercapacitors. Figure 4D shown the cycling performance of a CN/Ni//G based asymmetric supercapacitor at a current density of 3 A g-1 for 8000 cycles. After undergoing 8000 cycles, the capacitance of the cells reserved 72%, which indicates the asymmetric supercapacitor have excellent long term stability. The electrochemical performance of CN/Ni electrode with different mass-loading are also discussed in the Figure S4. One couple of redox peaks in the CV curves of CN/Ni (1.03 mg cm-2) and CN/Ni (1.4 mg cm-2) were attributed to redox reaction of Ni(OH)2 , revealing a pseudocapacitance behavior. The specific capacitance was calculated from the discharge curve according to the equation:. The specific capacitance of CN/Ni (1.03 mg cm-2 ) is 1375.1 F g-1 and CN/Ni (1.4 mg cm-2) is 1155.8 F g-1 at current density of 7 A g-1 which are lower than CN/Ni (0.72 mg cm-2). The Nyquist impedance spectroscopy is applied to investigate electronic conductivity of electrode materials. Nyquist plots of CN/Ni, g-C3N4 and Ni(OH)2 aggregations are shown in Figure 5A. It can be seen that g-C3N4 shows the least diameter of a semicircle and Ni(OH)2 aggregations exhibit the largest one. CN/Ni combine the features of both g-C3N4 and Ni(OH)2 aggregations indicating CN/Ni composite electrode enhance conductive capability relative to Ni(OH)2 aggregations. The simulated electrochemical parameters of CN/Ni, g-C3N4 and Ni(OH)2 using

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equivalent circuits are shown in Figure 5B, in which Rs is solution resistance, CPE is constant phase element, Rct means transfer resistance and Ws is assigned to the finite Nernst diffusion impedance. The fitted value of impedance parameters listed in Table S1, representing a Rct of 169.9 Ω on CN/Ni electrode, shows the fast electron transfer rate on the electrode.42

Figure 4 Electrochemical characterizations of CN/Ni-G asymmetric supercapacitor. (A) CV curves at various scan rates (range from 1~100 mV s-1). (B) Galvanostatic discharge curves at different current densities. (C) Calculated specific capacitance at various current densities. (D) Specific capacitance retention versus cycle number of CN/Ni-G at a current density of 3 A g−1.

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Figure 5 (A) Nyquist plots of CN/Ni, g-C3N4 and Ni(OH)2 aggregations, respectively; (B) Equivalent circuits used to fit the experimental data. Rs is solution resistance, CPE is constant phase element, Rct means transfer resistance and Ws is assigned to the finite Nernst diffusion impedance. Energy density (E) and power density (P) are two major parameters that could reveal the electrochemical performance for a full-cell. E and P can be calculated follow the

1 equation: E = CV 2 and P = E / t , where V is the potential at the end of charge (V) 2 and t is the discharge time (s).43 The Ragone plot of CN/Ni//G asymmetric supercapacitors in the voltage window from 0 to 1.3V at different current density is shown in Figure 6, the results prove that E decreases with increased P. The CN/Ni//G asymmetric supercapacitor shows an E value as high as 43.1 to 19.3 Wh kg-1 and P is 1870 to 9126 W kg-1, which is higher than most reported Ni(OH)2 based supercapacitors, such as Ni(OH)2/graphene–porous graphene,37 AC//β-Ni(OH)2/Ni foam,41 Ni(OH)2–AC43 and Ni(OH)2/graphene.44 Therefore, our asymmetric supercapacitors are promising candidate for practical energy storage applications in the future.

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Figure 6 Ragone plot related to energy and power density of asymmetric supercapacitor at 1.3V(●) with comparsion to other reported: Ni(OH)2/graphene– porous graphene (∆), AC//β-Ni(OH)2/Ni foam (☆), Ni(OH)2–AC (

▽

) and

Ni(OH)2/graphene (○).

4.Conclusions In conclusion, we have successfully assembled a novel asymmetric supercapacitor in which CN/Ni and graphene are used as positive and negative electrodes respectively. The asymmetric supercapacitor exhibits a high specific capacitance and a stable cycling life. Meanwhile, the energy density and power density are as high as 43.1 Wh kg-1 and 9126 W kg-1, respectively. The outstanding performance is attributed to the stability of g-C3N4 and the predominant perpendicular charge transfer facilitates rapid ion exchanges during the charge/discharge cycle even at high current densities. This method offers an effective alternative approach to prepare g-C3N4 based transition metal oxide composites as high-performance electrodes for supercapacitors.

Acknowledgements This research was supported partially by the National Natural Science Foundation of China (Nos. 51273158, 21303131); The fundamental Research Funds for the Central Universities (xjj2015119).

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Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: SEM and TEM images of g-C3N4 nanosheets, EDX images of CN/Ni materials, TGA Curves of CN/Ni materials, N2 adsorption-desorption isotherms of the CN/Ni materials, CV curves and galvanostatic discharge curves of CN/Ni electrode materials at different mass-loading, Impedance parameters of CN/Ni electrode materials.

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Table of Contents (TOC)

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2 Honeycomb Nanostructure and Asymmetric Supercapacitor with

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