NiCo2S4 nanotubes anchored 3D nitrogen-doped graphene

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NiCo2S4 nanotubes anchored 3D nitrogen-doped graphene framework as electrode material with enhanced performance for asymmetric supercapacitors Yiying Chen, Tao Liu, Liuyang Zhang, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00284 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Sustainable Chemistry & Engineering

NiCo2S4 nanotubes anchored 3D nitrogen-doped graphene framework as electrode material with enhanced performance for asymmetric supercapacitors

Yiying Chen a, Tao Liu a, Liuyang Zhang a, * and Jiaguo Yu a, b, *

a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,

Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China b

Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

*Corresponding Author. Email Address: [email protected] (Liuyang Zhang); [email protected] (Jiaguo Yu)

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Abstract NiCo2S4 is considered as a promising candidate for high-performance supercapacitors as it stores energy through fast surface redox reactions. However, it suffers from severe aggregation which deteriorates their stability. Herein, uniformly-distributed NiCo2S4 nanotubes anchored on 3D ultrathin nitrogen (N)-doped graphene framework (NGF) are obtained via facile hydrothermal method followed by sulfurization process. NGF composed of conductive three-dimensional graphene scaffolds cannot only ensure even distribution of NiCo2S4, but also facilitate electron transport. Meanwhile, NiCo2S4 nanotubes with hollow structures can shorten the diffusion pathway of electrolyte ions. The strong synergistic effect between NiCo2S4 nanotubes and NGF substrate helps to obtain satisfactory electrochemical properties. Accordingly, the as-prepared NiCo2S4/NGF composite exhibits a specific capacitance of 1240 F g–1 (558 C g–1) at 1 A g–1, almost two-fold of its counterpart NiCo2S4 (688.9 F g–1/310 C g–1). Moreover, the corresponding hybrid energy device (NiCo2S4/NGF\\AC) demonstrates superb energy density of 36.8 Wh kg–1 and outstanding cycling stability. This admirable design of NiCo2S4/NGF composite holds great promise as an electrode material in supercapacitor application. Keywords: NiCo2S4, nanotubes, 3D nitrogen-doped graphene framework, melamine sponge, asymmetric supercapacitor

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Introduction With the depletion of fossil fuels,1-3 scientists have never stop developing renewable and eco-friendly energy sources that can replace the conventional polluting energy sources.4-5 Electrochemical capacitors (also known as supercapacitors), which can accommodate well with these intermittent energy sources, have attracted considerable attention among researchers.6-7 Compared with traditional energy devices, supercapacitors possess prominent merits including fast recharging ability and high power density, which can satisfy the ever developing demands for electric vehicles. Theoretically, one trait of supercapacitors lies in that they stabilize over extremely long lifespan even at high charging/discharging rate with negligible volume change.8 While supercapacitors are endowed with numerous inherent advantages, their practical application can be severely hampered by their low energy densities when compared to other energy storage devices such as batteries and fuel cells. Therefore, considerable attempts have been conducted to elevate the energy density without sacrificing their high power density and long cycling stability. Transition-metal materials such as MnO2,9-10 NiS11 and Co3O412 have been intensively explored as pseudocapacitive electrodes and their capacitance can be easily tailored by experimental design.13-14 Among them, NiCo2S4, as one of the binary metal sulfides, has been deemed as a candidate with great promise owing to its higher conductivity (~100 times) and higher electro-activity in comparison with its oxide counterpart NiCo2O4.15 However, serious aggregation of bare NiCo2S4 will result in blocked electrochemical active sites and untimely electron transfer, and consequently 3



lead to capacitance decay. To overcome this issue, extensive studies have focused on downsizing NiCo2S4 to expose more electroactive sites and thereby enhancing its electrochemical performance.16 Hence, NiCo2S4 with diversified morphologies such as nanoparticles,17 nanoplates,18 nanoflakes19 and nanotubes20 are desired. For instance, Jiang’s group21 synthesized NiCo2S4 macrospheres composed of numerous porous nanotubes. Accordingly, the macrospheres exhibited specific capacitance as high as 933 F g–1 at 1 A g–1 with extremely low capacitance retention (only 66% of its initial capacitance was retained after 1000 cycles). The agglomerated structure of the macrospheres can be easily degraded during the redox reactions, thus resulting in the poor cycling performance. Another effective and commonly adopted approach is to bring in a conductive substrate so as to ameliorate the aggregation of metal sulfides and further enhance the conductivity of the active materials. As far as it is concerned, diversified matrices such as Ni foam,22 carbon sphere,23 graphene24-25 and carbonized polymer sponges26 are preferred. Particularly, graphene materials with 3D architectures have kindled numerous interest due to their hierarchical pore structures and highly conductive networks. This kind of architecture, as an appealing backbone, not only offer plenty of surface sites for the homogenous growth of active metal sulfides, but also enhance the conductivity of the overall composite. In addition, the strong coupling effect derived from the intimate contact between 3D graphene scaffolds and metal sulfides can effectively prevent the nanostructured metal sulfides from detaching from the graphene surface. To further improve the electrochemical activity of the graphene-supported 4


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composites, research on matrix materials has been directed towards N doping.27 Considering the porous structure, conductive and lightweight nature, considerable attempts have been conducted to fabricate hybrid electrode based on 3D graphene framework and ternary metal sulfides. For example, Yu prepared graphene decorated Ni3S2 on Ni foam for overall water splitting.28 However, preparations of 3D graphene framework by typical CVD methods or high-temperature reactions and subsequent metal sulfide growth have several shortcomings such as tedious, time-consuming and environmentally hostile procedures. However, preparations of 3D graphene framework by typical CVD methods or high-temperature reactions and subsequent metal sulfide growth have several shortcomings such as tedious, time-consuming and environmentally hostile procedures. Herein, we fabricated uniformly distributed NiCo2S4 nanotubes anchored on 3D N-doped graphene framework (NGF) via a facile two-step method, namely hydrothermal reaction and subsequent sulfurization. Melamine sponge is chosen as a raw material to prepare NGF due to its low price and mechanical-stable 3D network. After coating melamine sponge with GO sheets, the NGF can be easily fabricated by fast flame reduction with in-situ N-doping simultaneously. NGF with a hollow interior and ultrathin wall can either facilitate the migration of the electrolyte ions to NiCo2S4 nanotubes or expedite electron transfer to achieve a fast charging-discharging process. NiCo2S4 nanotubes with small diameters of approximately 50 nm feature abundant electrochemical active sites, thus resulting in favorable electrochemical performance. NiCo2S4/NGF composite (denoted as NCS/NGF) shares the concerted merits of the 5



respective two materials. The potential application of this composite in asymmetric supercapacitors has been thoroughly investigated. Therefore, we aim to optimize the electrochemical performance of the NCS/NGF composite and testify their promising prospect in the energy storage field.

Experimental Sections Synthesis of N-doped graphene framework (NGF) Graphene oxide was synthesized by a modified Hummers’ method through oxidation of natural graphite powder as reported elsewhere.24 As-prepared GO solution was diluted to the concentration of 1 mg mL-1 in ethanol for further use. Monolith melamine sponges are suggested to be pretreated before use. In a typical procedure, they were cut into small pieces and cleaned by deionized (DI) water and ethanol assisted with ultrasonication. After drying at 80℃ overnight, the melamine sponges were then immersed into the as-prepared GO solution and pressed with tweezers for several times to ensure that the melamine sponges were completely saturated by GO solution. Finally, the melamine sponges were calcinated on the ethanol flame for 1 min until their color become deep gray with ~10% volume shrinkage to ensure the fully removal of the polymer template.

Synthesis of NCS/NGF composite Typically, 2 mmol of Co(NO3)2·6H2O, 1 mmol of NiCl2·6H2O, 3.6 mmol of urea and 0.8 mmol of hexadecyl trimethyl ammonium bromide (CTAB) were dissolved into 20 6


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mL of DI water. The mixture was stirred for 1 h to obtain a transparent pink solution before being transferred into a 50 mL Teflon-lined stainless steel autoclave. After 5 mg of the as-obtained NGF immersed into the precursor solution, the autoclave was sealed and heated at 120℃ for 3 h. The NiCo-precursor/NGF obtained by hydrothermal growth was then carefully collected and rinsed with DI water and ethanol for a few times, followed by drying at 80℃ for 12 h. The NCS/NGF was synthesized via a facile sulfurization treatment based on Kirkendall effect. Typically, 0.1 mmol Na2S·9H2O was dispersed into 20 mL of DI water by ultrasonication for 20 min. Afterwards, the as-prepared NiCo-precursor/NGF composite was immersed into the Na2S solution. The hydrothermal reaction was performed at 160℃ for 6 h. After cooling down naturally to room temperature, the as-obtained NCS/NGF was taken out and carefully rinsed with DI water and ethanol, followed by vacuum-drying under 80℃ overnight. For comparison, bare NiCo2S4 (denoted as NCS) was synthesized using the same process without adding NGF. Material characterization, electrode preparation and calculation of different parameters can be found in the supporting information.

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Results and discussion

Figure 1. Schematic diagram for the preparation of NiCo2S4 (NCS)/NGF composite.

Figure 1 schematically illustrates the procedure to synthesize NiCo2S4/NGF composite. To elaborate, ultrathin and hollow N-doped graphene framework can be obtained by heating a melamine sponge, which is pre-soaked in graphene oxide suspension, on an ethanol flame. The flame provides sufficient energy to decompose melamine sponge and produce N-containing radicals. These radicals can participate in the reduction process of GO and reduced GO cover the sponge template.29 During the calcination treatment, melamine sponge saturated with GO decomposes gradually with slight volume shrinkage and undergoes an apparent color change from dark brown to black. Meanwhile, GO is reduced during the calcination process, resulting in the diminishment of the oxygen functional groups. Thenceforth, N-doped NGF is achieved successfully. Afterwards, NGF served as a backbone is homogenously coated with 8


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NiCo-precursors after the one-pot hydrothermal growth. Ultimately, NiCo2S4 nanotubes were obtained via a continuous conversion of the precursors by S2– ion exchanging reaction.

Figure 2. SEM images of (a) NGF, (b) NiCo-precursor/NGF, (c) NCS/NGF and (d) NCS.

The surface morphologies of the NGF, NiCo-precursor/NGF, NCS/NGF and bare NCS were examined by SEM (Figure 2). The bare NCS was prepared in the absence of NGF. The NGF (Figure 2a) presents a 3D interconnected network composed of hollow graphene scaffolds with diameters of approximately 5 μm, which derives from melamine sponge. It testifies that the integrity of the 3D honeycomb-like microstructure of melamine sponge (Figure S1, Supporting Information) keeps intact even it goes 9



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through calcination to remove the polymer template. The AFM result of a NGF fragment shown in Figure S2 illustrates that its thickness is about 2.5 nm, confirming its ultrathin feature. Since the thickness of graphene monolayer is about 0.35 nm, the scaffold of NGF is roughly composed of 7-8 graphene monolayers.30 Since NGF is composed of hollow and ultrathin struts, the inner-pore is accessible which reduces diffusion distance and lowers the ion-transport resistance.

Interestingly, both NiCo-

precursor and NCS display the nanoneedle structure in the presence of NGF, whereas it exhibits hedgehog structure without the presence of NGF (Figure 2d). Noticeably, by comparing Figure 2a, 2b and 2c, the skeleton of NGF is well preserved. This can be ascribed to the strong attachment between NGF matrix and NCS nanoneedles. The strong coupling further enhances the efficiency of the electron-transport and the stability during electrochemical measurements as well. Therefore, it can be inferred that NGF with open structure is conducive to the even growth of metal sulfides. Figure S3 displays the FE-SEM image of the GO/NCS composite. Stacked graphene as well as few-layer graphene can be observed simultaneously. Moreover, it is difficult for NCS nanotubes to evenly anchor on GO when compared with NCS/NGF. Therefore, we speculate that without a mechanical-robust skeleton, graphene sheets are prone to aggregate and stack together and thus leading to declined sites for the growth of NCS. In other words, NGF with 3D interconnected architecture can provide sufficient active sites for the even growth of NCS nanotubes. Overall, we can conclude that NCS nanotubes are inclined to distribute uniformly in the presence of NGF, and the coupling effect of the NCS/NGF composite is competent to expedite the proliferation of 10


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electrolyte ions and ameliorate the cycling stability of the supercapacitors.

Figure 3. XRD patterns of GO, NGF, NCS/NGF and NCS.

The X-ray diffraction peaks of both NCS/NGF and NCS centered at 16.3°, 26.8°, 31.5°, 38.1°, 50.4° and 55.2° can be readily assigned to (111), (220), (311), (400), (511) and (440) planes of cubic-phase NiCo2S4 (JCPDS Card No. 43-1477),21,

31-32

as

illustrated in Figure 3. The NGF exhibits a broad peak located at around 25.6°, which is associated with the (002) plane of typical graphite-like structures caused by low degree of crystallization. In contrast to GO, NGF does not show the peak at 10°, i.e. (001) reflection of GO, corroborating the successful reduction of GO during the calcination process.33 The existence of the (002) peak and disappearance of the (001) peak indicate that the struts of NGF are composed of few-layer GO sheets, in agreement 11



with the abovementioned AFM result.34 In the diffraction pattern of NCS/NGF, the original peak of NGF disappears, which can be accredited to its low content in the NCS/NGF composite. No diffraction peaks belonging to other sulfides such as Co9S8 or Ni7S6 are observed in NCS/NGF composite. Other than XRD, Raman spectra of these four abovementioned samples are also displayed and compared in Figure S4. Except for bare NCS, all the rest samples declare two representative Raman peaks of graphitic materials. The graphite band (G) centered at 1590 cm–1 is characteristic of graphite due to the in-plane vibration of sp2 carbon atoms. The disorder band (D) centered at 1320 cm–1 arises from the breathing mode of sp2 carbon atoms.35 Therefore, the intensity ratio of D band to G band (ID/IG) for NGF is 1.08, while this value for original GO is 0.91, signifying the effectual reduction of GO during the calcination process.36 The rise of the ID/IG ratio indicates an increase in the defect sites after N-doping.37 The Raman peak at 650 cm–1 can be ascribed to Raman active A1g mode.38 Noting that the Raman shift of NCS/NGF witnesses a deviation of 20 cm–1 from bare NGF according to the literature, this becomes a modest illustration of the intimate contact between NCS nanotubes and NGF substrate.

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Figure 4. (a) EDS mapping of NCS/NGF. (b,c) TEM images of NiCo-precursor/NGF and NCS/NGF. (d) HR-TEM images of NCS/NGF composite.

The EDS mapping images in Figure 4a confirm the uniform distribution of S, Ni and Co as the main elements in NCS/NGF composite. N element from NGF has not been detected by EDS which is due to the full coverage of NCS nanotubes on the NGF surface. The EDX spectrum of the NCS/NGF composite (Figure S5) further confirms this statement. The atomic ratio of N/C in NCS/NGF is mere 0.0085 based on the EDX result. Typical TEM images of NCS/NGF composite identify that the nanoneedles in SEM are actually hollow (Figure 4c), in comparison with the TEM image of NiCoprecursor/NGF in Figure 4b. The diameter of a single NCS nanotube is estimated to be 50 nm. The transformation from the nanoneedle structure of NiCo-precursors/NGF into 13



the nanotube structure of NCS can be ascribed to Kirkendall effect during sulfurization treatment. Namely, sulfide ions (S2–) react with metal cations on the surface of every nanoneedle at high temperature. Differences in diffusion rates between inward diffused S2– and outward diffused metal ions promote the emergence of voids inside the nanoneedles. Eventually, the NCS nanotubes are acquired. The HR-TEM image (Figure 4d) presents a well-defined lattice fringe with d-spacing of 0.235 nm, which is indexed to the (220) plane of cubic-phase NiCo2S4. It is noteworthy that the relatively loose internal microstructure of the nanotubes can avail the use of inner active sites for redox reactions and thereby boost its electrochemical performance.21 The specific surface area of the samples and their corresponding pore properties were measured by N2 adsorption-desorption isotherms. The isotherm of NCS/NGF composite can be classified into Type Ⅳ with Type H3 hysteresis loops (Figure S6). The BET surface area of NCS/NGF was calculated to be 19 m2 g–1 (Table S1), with most of the pores distributed at 20-30 nm.

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Figure 5. (a) XPS survey scan for NCS/NGF. High resolution XPS spectra of (b) Ni, (c) Co and (d) S 2p spin-orbits for NCS/NGF composite.

X-ray photoelectron spectroscopy (XPS) results of NGF are given in Figure S7. The survey spectrum of NGF further confirms the existence of C, N and O elements. The high-resolution N 1s spectrum can be deconvoluted into three peaks. The peaks centered at 398.6, 400.6 eV are corresponding to pydridinic-N and pyrrolic-N, respectively. Another peak centered at 402 eV is associated with graphitic-N, which helps to ameliorate the conductivity of graphene. In addition, according to the analysis of XPS results, nitrogen accounts for 10.6 at% in NGF. Further investigations into the wettability of NGF were performed by the contact angle measurements where de15



ionized (DI) water was selected as contact medium, as displayed in Figure S8. The commercial melamine sponge as a template exhibits super-hydrophilic property with a contact angle of 0o. GO shows a contact angle of 75.3o. In contrast, the contact angle for NGF is measured to be 40.9o owing to the possession of numerous N-containing groups. The improved wettability of NGF can be interpreted by the fact that the rearrangement of electrons in carbon-based materials caused by the incorporation of pydridinic-N and pyrrolic-N atoms facilitates the penetration of electrolyte into smaller pore structures.39-40 Predominant peaks of S, C, N, O, Co and Ni elements in NCS/NGF are presented in Figure 5a. In Figure 5b, two satellites (Sat.) with two pairs of peaks centered at 852.9 and 870.3 eV for Ni2+ and 856.3 and 873.8 eV for Ni3+ are presented in Ni 2p spectrum.41 Similarly, in Figure 5c, the existence of Co3+ (778.4 and 793.7 eV) and Co2+ (780.6 and 796.2 eV) can be ascertained.42 Two major peaks centered at 161.6 eV and 162.77 eV in S 2p spectrum (Figure 5d) correspond to S3/2 and S1/2, respectively, which can be ascribed to metal-sulfur bonds and sulfur vacancies.43-44 Moreover, significant peak shifts of NCS/NGF can be clearly observed in Figure S9 underlying the strong interconnection between NCS nanotubes and NGF substrate.

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Figure 6. (a) CV curves of NGF, NCS and NCS/NGF measured at 10 mV s–1 ；(b) GCD curves measured；(c) Comparison of specific capacitance measured at different current densities and (d) Cycling stability of NCS/NGF and NCS

To evaluate the electrochemical performance of the NCS/NGF and bare NCS materials as electrodes, electrochemical measurements including CV and GCD tests were performed. Figure S10a and S10c show typical CV curves of NCS/NGF and bare NCS at various scan rates. Apparently, both of them exhibit a pair of redox peaks, which comes from the reversible faradic processes of Ni2+/Ni3+ and Co2+/Co3+/Co4+ occurred within the active materials.45 The corresponding redox reactions can be itemized as follows:

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CoS + 𝑂𝐻 ― ↔CoSOH + 𝑒 ―

(1)

CoSOH + 𝑂𝐻 ― ↔𝐶o𝑆𝑂 + 𝐻2𝑂 + e ―

(2)

NiS + 𝑂𝐻 ― ↔𝑁𝑖𝑆𝑂𝐻 + e ―

(3)

With increasing sweeping rate, the anodic peaks shift toward higher potential and simultaneously the cathodic peaks shift towards lower potentials, indicating that the diffusion of OH– cannot compensate with its consumption under the high sweeping rate. In Figure 6a, the NCS/NGF composite possesses larger integral area than bare NCS and NGF in the CV curve, suggesting that the capacitance of bare NCS is dramatically enlarged by the addition of NGF. This can also be proved by their GCD curves in Figure 6b. With a stable potential window of 0-0.45 V, their GCD curves are nearly symmetrical at various current densities (ranging from 1 to 20 A g–1) in Figure S10b and S10d. The electrochemical performance of NGF can be found in Figure S11. In addition, Figure 6c plots the calculated specific capacitances of both samples. Encouragingly, NCS/NGF delivers almost twice the discharging time of the bare NCS, further confirming its superior charge-storing ability. The NCS/NGF electrode exhibits specific capacitance as high as 1240 F g–1 (558 C g–1) at 1 A g–1, almost two-fold of its counterpart NCS (688.9 F g–1/310 C g–1). The electrochemical performances of recent reported Ni-Co related materials are summarized in Table S2. Moreover, it yields specific capacitance of 1208, 1110 and 1005 F g–1 (543.6, 499.5, 452.3 C g–1) at 2, 5 and 10 A g–1, respectively. Noticeably, even when the current density increases 20 times (20 A g–1), the NCS/NGF delivers the specific capacitance as high as 861 F g–1 (387.5 C g–1, rate capability retains 69.4%). The insufficient effective interaction between NCS 18


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and OH– ions under high current densities can interpret the drop of its specific capacitance with increasing current density. Figure S12 compares CV and GCD results of NCS/NGF and bare nickel foam. Since the bare nickel foam exhibits much smaller capacitance than the composite, the contribution of nickel foam to the overall capacitance can be neglected. The long-term cycling performance is also taken into account for assessing the stability of materials. Hence, two electrode materials were conducted up to 5000 times via GCD measurements and the tested potential window was 0-0.45 V, which can be clearly observed from Figure 6d. As the cycle number increases, the NCS/NGF electrode exhibits better durability against long-term cycling (80% of the original specific capacitance was kept) than its counterpart NCS (capacitance retention of 69%). The incorporation of NGF provides sufficient growth sites for the uniform distribution of NCS, preventing the aggregation of the nanotubes. Therefore, the NCS/NGF electrode can effectively ensure the contact between active material and electrolyte in the long-term cycling process, resulting in a better cycling durability. By selecting highly conductive NGF as substrate, the electron transport kinetics of NCS/NGF can be elevated, thereby enhancing its electrochemical performance. Figure S13 presents the Nyquist plots of NCS/NGF and bare NCS. The Nyquist plots of these two materials are both composed of one small semicircle at higher frequency region which represents the charge-transfer resistance, and a straight oblique line at lower frequency region which demonstrates the diffusion-controlled Warburg capacitive behavior. Owing to the fact that the exposed interior sites of hollow NGF struts speed up electrolyte-ion transfer, steeper slope in the low frequency region 19



can be clearly observed for NCS/NGF. The aforementioned performance undoubtedly corroborates the appealing overall performance of the NCS/NGF composite. The reasons for the enhancement are proposed. Firstly, the incorporation of N into graphene framework not only provide plenty of effective active sites in redox reactions, but also provide faradaic pseudocapacitance by functional groups, contributing to the improvement of the specific capacitance of electrode material. Secondly, the conductivity of the composite is enhanced after the addition of NGF, which can be ascribed to the enrichment of surface defect sites derived from the substitution of C atom by N atom. Finally, strong coupling effect between the NGF substrate and each NCS individual nanotube ensures their intimate contact and facilitates the effective electron transfer.

Figure 7. (a) Schematic illustration of the asymmetric supercapacitor based on the NCS/NGF and AC electrodes. (b) CV curves of AC and NCS/NGF measured at 10 mV

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s–1. (c) CV curves of NCS/NGF\\AC cell with various working potential windows.

In view of the superb electrochemical performance of NCS/NGF, an asymmetric supercapacitor was assembled so as to further value the NCS/NGF electrode for realistic energy storage application. Namely, the NCS/NGF composite positive electrode and active carbon (AC) were used as negative electrode (denoted as NCS/NGF\\AC; Figure 7a), respectively. As displayed in Figure 7b, the stable operating potential window of each electrode was evaluated prior to electrochemical tests to guarantee against damaging. The stabilized potential windows for the NCS/NGF and AC electrode are 0~0.5 and –1~0 V, respectively. Owing to the different voltage range of two kinds of electrode, the operating potential may be further extended after their suitable match. The potential window of the as-assembled energy device was determined by a sequence of CV operated at 10 mV s–1, as can be found in Figure 7c. Apparently, there appears a bump in the CV curve when the operating voltage window is extended to 1.6 V, which can be interpreted by the irreversible hydrolytic reaction. Therefore, a broad working potential of 1.5V is affirmed. The mass ratio of NCS/NGF to AC was optimized at 2.5:1 based on their specific capacitances. Figure S14 reveals the detailed electrochemical performance of active carbon.

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Figure 8. (a) CV curves, (b) GCD curves, (c) capacitance retention of NCS/NGF and bare NCS. (d) Cycling stability of NCS/NGF measured at 3 A g–1.

For comparison, another NCS\\AC cell was also assembled with an optimized mass ratio of 3:1. Similarly, the NCS\\AC cell can withstand a working voltage of 01.5 V (Figure S15). Figure S16 displays the detailed electrochemical performance of these two ASC cells. Figure 8a compares the electrochemical performance of these two asymmetric supercapacitors. Obviously, the NCS/NGF\\AC possesses larger integrated area under CV curve than NCS\\AC, underlying the superiority of NCS/NGF composite. Equivalently, the GCD curve of NCS/NGF\\AC measured at 1 A g–1 (Figure 8b) displays a nearly triangle-like shape with longer discharging time, indicating its higher 22


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specific capacitance and better electrochemical activity. The specific capacitances of two cells calculated on the basis of the discharging time in the GCD curves are displayed in Figure 8c. The NCS/NGF\\AC ASC delivers maximum specific capacitance of 117.6 F g–1 (176.4 C g–1 ) at 0.5 A g–1, almost two-fold than that of NCS\\AC ASC (70.2 F g–1/ 105.3 C g–1). In addition to the prominent specific capacitance, the outstanding cycling stability of the NCS/NGF\\AC cell also should be taken into account. The stability of NCS/NGF\\AC ASC was tested at 3 A g–1 by charge-discharge measurement for 3000 cycles (Figure 8d). Impressively, there appears no obvious decay for the specific capacitance of the NCS/NGF\\AC ASC during the first 2000 cycles before a slight drop arises. Notwithstanding, the NCS/NGF\\AC asymmetric supercapacitor still preserves 92% of its original capacitance (65.4 F g–1/ 98.1 C g–1) after 3000 cycles, indicating its superior cyclic durability. The outstanding cycling stability of the NCS/NGF\\AC ASC favors its practical application as energy storage apparatus.

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Figure 9. Ragone plots of NCS/NGF\\AC and NCS\\AC in comparison to the value reported in the literature with similar nickel cobalt sulfide material as electrode material. The insert shows 22 LEDs lit by two NCS/NGF\\AC devices in series.

Table 1. Comparison of energy densities based on NiCo2S4 material in the present work. Sample

Energy density

Ref.

NiCo2S4 nanotubes/NGF\\AC

36.8 Wh kg–1 at 375 W kg–1

This work

Core-shell NiCo2S4 on Ni

22.8 Wh kg–1 at 160 W kg–1

foam\\porous carbon

24


22

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Mesoporous NiCo2S4 NPs\\AC

28.3 Wh kg–1 at 245 W kg–1

46

NiCo2S4 NPs on reduced graphene

27.5 Wh kg–1 at 446.5 W kg–1

47

31.5 Wh kg–1 at 156.6 W kg–1

48

NiCo2S4/CFP\\AC

17.3 Wh kg–1 at 180 W kg–1

49

Ni–Co–S@N-pCNF\\AC@ N-pCNF

21.6 Wh kg–1 at 134.9 W kg–1

50

oxide\\AC NiCo2S4 nanotube arrays on Ni foam\\rGO

Figure 9 displays comparative Ragone plots of the corresponding power and energy densities for ASCs to evaluate their performance in real application. The NCS\\AC ASC possesses 21.9 Wh kg–1 at power density of 375 W kg–1. In contrast, the NCS/NGF\\AC ASC reaches a much higher energy density of 36.8 Wh kg–1 at 375 W kg–1 and an energy density of 20.17 Wh kg–1 can still be maintained at 3000 W kg–1. When compared to those of relevant materials reported previously, the NCS/NGF\\AC ASC exhibits an alluring performance, as shown in Table 1. The application of NCS/NGF\\AC ASC was evaluated by illuminating 22 LED bulbs with working voltage of 2.2 V each. The inset photo demonstrates that the 22 LEDs connected in parallel can be lit by two NCS/NGF\\AC devices connected in series after each device being charged for 90 s. The LEDs dimmed after five minutes. Moreover, one piece of NCS/NGF\\AC device with the working potential of 1.5 V can drive a small fan effectively for about 1 minute (Figure S16). The prominent electrochemical performance of NCS/NGF composite can be 25



mainly interpreted by the following aspects: (a) the heteroatom doping of nitrogen into the 3D graphene framework cannot only enhance the conductivity of the substrate but also significantly elevate surface wettability; (b) NGF with ultrathin and hollow struts are favorable for providing enough space for electrolyte diffusion, lowering the ion transport resistance and offering additional electroactive sites; (c) NGF with 3D architecture can offer plenty of surface sites for the growth of NCS nanotubes, preventing the nanotubes from aggregation.

Conclusions In summary, NiCo2S4 nanotubes are well-anchored on the surface of 3D N-doped graphene framework via facile hydrothermal method. Benefiting from plentiful merits provided by the 3D conductive NGF and NCS nanotubes, the NCS/NGF composite possesses almost double the original capacitance of bare NCS and superior long-cycle lifespan. Moreover, the NCS/NGF\\AC asymmetric supercapacitor delivers a decent energy density of 36.8 Wh kg–1 at 375 W kg–1 together with excellent cycling stability. Therefore, this work reveals the promising potential of the NCS/NGF composite as positive electrode material in asymmetric supercapacitors.

Supporting Information The experimental section including synthesis and characterization of materials, as well as electrode preparation and calculation; SEM and FESEM images; AFM images; Raman spectra; EDX spectrum; N2 adsorption/desorption isotherm; XPS spectra; 26


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contact angle measurement results; CV curves and GCD curves; capacitive performance comparison; Nyquist plots; digital image powering a small fan.

Acknowledgements The research was supported by National Natural Science Foundation of China (U1705251, 21801200 and 51872220) and the Fundamental Research Funds for the Central Universities (WUT: 2018IVA089 and 182459018).

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35



For Table of Contents Use Only

The hierarchical NiCo2S4/3D N-doped graphene framework was fabricated by a facile, sustainable and cost-effective method, and it is testified to be a promising contender in asymmetric supercapacitor.

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NiCo2S4 nanotubes anchored 3D nitrogen-doped graphene

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