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High-Energy All-Solid-State Symmetric Supercapacitor Based on Ni3S2 Mesoporous Nanosheet-Decorated Three-Dimensional Reduced Graphene Oxide Downloaded via UNIV OF SOUTH DAKOTA on August 24, 2018 at 12:07:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Cheng Zhang,†,‡ Ye Huang,† Shaolong Tang,*,† Mingsen Deng,*,¶ and Youwei Du† †

Jiangsu Key Laboratory for Nanotechnology, Collaborative Innovation Center of Advanced Microstructures, Nanjing National Laboratory of Microstructuresand and Department of Physics, Nanjing University, Nanjing 210093, PR China ‡ School of Physics and Electronic Science, Guizhou Normal University, Guiyang 550001, PR China ¶ Guizhou Provincial Key Laboratory of Computational Nano-material Science, Guizhou Synergetic Innovation Center of Scientific Big Data for Advanced Manufacturing Technology, Guizhou Education University, Guiyang 550018, PR China S Supporting Information *

ABSTRACT: The hybrid approach offers opportunities to simultaneously exploit the features of capacitive (especially carbon) and faradaic (redox electroactive) materials to increase energy density and power density of supercapacitors. To achieve an optimized overall electrochemical performance, we have synthesized a hybrid supercapacitor electrode consisting of vertically aligned Ni3S2 mesoporous nanosheets on three-dimensional reduced graphene oxide (Ni3S2/3DrGO) supported by Ni foam with a controllable composition and morphological structure, which thus improve the electrical conductivity as well as provide more chemical reaction sites and shorten the migration path for electrons and ions. By taking advantage of the rational structural features and excellent electrical conductance ability, the Ni3S2/ 3DrGO hybrid nanostructure shows greatly improved electrochemical capacitive performance, including high specific capacitance of 1886 F g−1 (1621 F g−1) at current density of 1.0 A g−1 (20.0 A g−1) and excellent rate capability and cycling stability. Remarkably, an all-solid-state symmetric supercapacitor fabricated by using our pseudocapacitive hybrid nanostructures delivers a high energy density (58.9 Wh kg−1), high power density (3.7 kW kg−1 at 45.8 Wh kg−1), and excellent cycling stability (92% capacitance retention after 30 000 charge−discharge cycles at a constant current density of 10 A g−1). These electrochemical performances are superior to those of the previously reported symmetric supercapacitors, suggesting that these hybrid nanostructures have a huge potential for high-performance energy conversion and storage devices.

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reactions on or near the surface of the electrodes, which is a faradic process.4 Additionally, the EDLCs store electrical energy through charge accumulation at the electrode−electrolyte interfaces, which is a purely physical process. It should be emphasized that these electrochemical and electrophysical energy storage mechanisms can work separately or can be combined in a single device, which is based on the electrode materials used in the supercapacitors.5 In general, the EDLCs with carbonaceous materials can offer higher power density (10.0 kW kg−1) and excellent cycling ability (10 000 cycles).

wing to the increasing energy demand together with the deteriorating environment and decreasing fossil fuels resources, the development of highly efficient energy conversion and storage devices is one of the key challenges of both fundamental and applied research in energy technology.1 Supercapacitors, which act as a bridge between high specific energy batteries and the high specific power electrolytic capacitors, have attracted increasing interest because of their high specific capacitance, fast recharge capability, and cycle efficiency.2,3 In general, supercapacitors can be divided into two types based on the energy storage mechanism and the electrode materials: pseudocapacitors and electrical double-layer capacitors (EDLCs). The pseudocapacitors store electrical energy through fast and reversible redox © 2017 American Chemical Society

Received: January 24, 2017 Accepted: March 3, 2017 Published: March 3, 2017 759

DOI: 10.1021/acsenergylett.7b00078 ACS Energy Lett. 2017, 2, 759−768

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters

Ni3S2 mesoporous nanosheets are successfully grown on 3DrGO scaffold without any polymeric binder and conductive additive. The good electrochemical capacitive activity of Ni3S2 and superior conductivity of 3DrGO synergistically lead to the remarkable electrochemical performances when utilized as electrodes for supercapacitors. In the liquid-electrolyte-based three-electrode tests, Ni3S2/3DrGO delivers a high specific capacitance, good rate capability, and long-term cycling stability. In addition, an all-solid-state symmetric supercapacitor device is prepared based on the Ni3S2/3DrGO as electrodes and a filter paper in-between as separator. The fabricated supercapacitor device possesses remarkable electrochemical capacitor performance with specific energy of 59 Wh kg−1 at a specific power of 0.525 kW kg−1 and specific power of 3.675 kW kg−1 at a specific energy of 46 Wh kg−1. Remarkably, 92% of the capacitance can be retained after 30 000 cycles at 10 A g−1. The rational design concept of effectively decorating metal sulfides on highly conductive 3DrGO scaffold can also be applied to the combination of other electroactive materials and carbon materials for next-generation supercapacitors. The schematic illustration for the fabrication of Ni3S2/ 3DrGO hybrid nanostructures on Ni foam is shown in Figure 1a. First, GO is deposited on the Ni foam substrate via a simple

However, the pseudocapacitors based on faradaic materials can deliver much higher specific capacitance and specific energy (100 Wh kg−1). In order to simultaneously take advantage of EDLCs’ and pseudocapacitors’ energy storage mechanisms, thus leading to faster kinetics and enhanced electrochemical capacitive performance, the hybrid materials combining conducting components (fullerene, carbon nanotube, graphene, porous carbon, and conducting polymers) and electroactive materials (transition-metal oxides, transition-metal hydroxides, and transition-metal sulfides) are the natural target.6−10 Among various carbon-based materials, graphene and related materials have been considered as one of the most promising electrode materials because of their flexibility, large surface area, chemical stability, and excellent electrical and thermal conductivity.11−13 However, there are still two main challenges for the commercialization of graphene-based supercapacitors: (1) Because of the strong interlayer π−π stacking and van der Waals interactions, graphene sheets tend to form irreversible agglomerates, which can significantly reduce the accessible surface area and ion diffusion rate. (2) Whether through liquidphase exfoliation, chemical vapor deposition, or chemical synthesis, the high-cost and time-consuming aspectes of mass production of high-quality graphene nanosheets hinder widespread application in large-scale energy conversion and storage systems. Therefore, three-dimensional (3D) reduced graphene oxide (rGO) has attracted increasing attention because its unique nanoporous structures and 3D properties not only possess high specific surface area (about 3353 m2 g−1) but also allow electrolyte ions to freely propagate in the interconnected networks structures of 3DrGO.14−16 Despite the advantages of the 3D structural features and good electrical conductivity, the specific capacitance and specific energy of 3DrGO are still relatively low because of its intrinsically limited electrochemical activities, thus hindering its practical application in highperformance energy storage devices.17 With the above considerations, a promising route is to develop appropriate composite electrodes integrating 3DrGO and faradaic materials with high electrochemical activity and efficiency to further improve the performances of 3DrGO-based supercapacitors. On the other hand, transition-metal sulfides act as versatile multifunctional materials and have been used in various electrochemical applications, such as electrocatalysts for hydrogen evolution reactions18 and photocatalytic H2 production,19 electrode materials for lithium ion batteries,20 supercapacitors,21−24 and dye-sensitized solar cells.25 Recently, various nanostructures of transition-metal sulfides with high surface area and pore volume have been successfully designed and synthesized via various approaches to tailor the properties for different applications. In addition, transition-metal sulfides exhibit electrical conductivity that is higher than that of transition-metal oxides, thus endowing them with an enormous potential to be used as electrode materials.26 In this study, we design and synthesize a novel hybrid nanostructure composed of interconnected Ni3S2 mesoporous nanosheets and 3D reduced graphene oxide (3DrGO) supported by Ni foam through an electrodeposited method combined with a high-temperature postannealing processes. To achieve an optimized overall electrochemical properties, the high electrochemical activity of Ni3S2 nanosheets and large surface area 3D graphene oxide (GO) are combined in a proper ratio by controlling electrodeposition cycles, followed by calcination under argon atmosphere to get 3DrGO and crystallized Ni3S2 mesoporous nanosheets. The interconnected

Figure 1. (a) Schematic diagram of the fabricating process of Ni3S2/ 3DrGO on Ni foam; (b−e) SEM images of Ni3S2/3DrGO on Ni foam after annealing at 600 °C under different magnifications.

“dipping and drying” process. Second, Ni−S precursor nanosheets are electrodeposited on 3D GO scaffold supported by Ni foam in a three-electrode electrochemical system. Third, an annealing treatment is utilized to get 3D reduction graphene oxide (3DrGO) and crystallized Ni3S2 mesoporous nanosheets (for the details of the experiment, please see Experimental Section). To optimize the overall electrochemical performance of Ni3S2/3DrGO hybrid nanostructures, the average mass 760

DOI: 10.1021/acsenergylett.7b00078 ACS Energy Lett. 2017, 2, 759−768

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Figure 2. TEM images at different magnifications of Ni−S precursor nanosheets and Ni3S2 mesoporous nanosheets after annealing at 600 °C scratched from Ni foam: (a−c) TEM images of Ni−S precursor nanosheets; (d) TEM image of Ni3S2 mesoporous nanosheets at a low magnification; (e) high-resolution TEM (HRTEM) image of Ni3S2 mesoporous nanosheets with the corresponding fast-Fourier transfer image in the inset; and (f) HRTEM image of Ni3S2 mesoporous nanosheets with its selected area electron diffraction pattern in the inset.

Figure 3. (a) XRD patterns of 3DrGO and Ni3S2/3DrGO on Ni foam; (b) EDS spectrum of Ni3S2/3DrGO on Ni foam with the corresponding SEM image in the inset; C 1s XPS spectra for (c) GO and (d) Ni3S2/3DrGO on Ni foam; XPS spectra of Ni3S2/3DrGO for (e) Ni 2p and (f) S 2p.

the Ni3S2/3DrGO hybrid nanostructures after annealing at 600 °C under different magnifications. As can be seen in Figure 1b, a large amount of vertically aligned Ni3S2 nanosheets are uniformly grown on 3DrGO scaffold supported by Ni foam. Figure 1d,e shows the typical high-resolution SEM images of Ni3S2/3DrGO hybrid nanostructures on Ni foam. Clearly, the ultrathin nanosheets are interconnected and form a network structure, resulting in a significant improvement of mechanical

loadings of Ni3S2 can be changed by controlling the electrodeposited cycle numbers. When the Ni3S2 nanosheets are electrodeposited by the cyclic voltammetry (CV) method, where the potential ranges from −1.2 to 0.2 V and the sweep rate is 5 mV per second, the number of electrodeposited cycles is increased from 1 to 5, the mass of Ni3S2 in Ni3S2/3DrGO hybrid nanostructures are about 1.4, 2.7, 3.9, 4.9, and 5.8 mg cm−2, respectively. Figure 1b−e shows the SEM images of 761

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Figure 4. CV curves of (a) 3DrGO and (b) Ni3S2/3DrGO electrodeposited with four cycles at different scan rates; (c) GCD tests of Ni3S2/ 3DrGO electrodeposited with four cycles at various current densities; (d) GCD tests of 3DrGO and Ni3S2/3DrGO electrodeposited with different cycles at 1.0 A g−1; (e) average mass loadings of Ni3S2 and areal capacitances of Ni3S2/3DrGO as a function of electrodeposition cycles at 1.0 A g−1; (f) specific capacitances versus discharge current density for 3DrGO and Ni3S2/3DrGO electrodeposited with different cycles.

nm (Figure 3b in the Supporting Information). Evidently, the interconnected Ni3S2/3DrGO mesoporous nanostructures with high specific surface area offer short transport path lengths for electrons and electrolyte ions, thus increasing the efficient faradaic reactions of Ni3S2/3DrGO. Moreover, the mesopores can server as a robust reservoir for electrolyte and significantly improve the cycling stability by partly alleviating the volume change induced by faradaic reactions during the charge− discharge processes. A HRTEM image as shown in Figure 2f demonstrates that there is one lattice spacing about 0.41 nm, corresponding to the (101) surface of the heazlewoodite phase Ni3S2. Figure 3a gives the XRD patterns of the 3DrGO and Ni3S2/ 3DrGO hybrid nanostructures on Ni foam. For the XRD pattern of 3DrGO, instead of the featured peak of GO (2θ = 10.4°), there is a broad diffraction peak located at 24.6° of the graphite (002) plane, which indicates that GO has been effectively reduced after the annealing treatment. All of the peaks of the composites coincide with those from the heazlewoodite phase Ni3S2 (JCPDS Card No. 44-1418, space group R32(155); a = b = 5.745 Å, c = 7.135 Å, α = β = 90°, γ = 120°) and rGO in position, in addition to two typical peaks from the Ni substrate (JCPDS Card No. 04-0850, space group: Fm3̅m(225); a = b = c = 3.524 Å, α = β = γ = 90°), which further confirms the formation of crystalline Ni3S2/3DrGO hybrid nanostructures. Energy dispersive spectroscopy (EDS) micromicroanalysis of the Ni3S2/3DrGO hybrid nanostructures on Ni foam is shown in Figure 3b. The even distribution of the elements including C, Ni, and S elements confirms the uniform deposition of Ni3S2/3DrGO hybrid nanostructures. The more detailed elemental composition of the hybrid nanostructures is characterized by X-ray photoelectron (XPS) measurements. The high-resolution C 1s XPS spectra of GO and Ni3S2/

stability of the nanostructures. It is observed that the overall morphology of the sample before annealing treatment (Figure 1 in the Supporting Information) has not been changed compared with that of Ni3S2/3DrGO composites after annealing at 600 °C under argon atmosphere. This means that the high-temperature postannealing processes are an extremely effective way to obtain the highly crystalline Ni3S2/ 3DrGO while keeping its morphology. As can be seen in the transmission electron microscopy (TEM) images of the Ni−S precursor nanosheets (Figure 2a−c), the highly porous feature of the Ni−S precursor nanosheets is clearly revealed by showing notable contrast difference between the hollow and solid parts. The highly mesopores nanostructures can be attributed to the gas release during the electrodeposition. Interestingly, the Ni3S2 nanosheet is built layer-by-layer and shows a regular boundary after the thermal transformation (Figure 2d), indicating the good crystallinity of these nanosheets. It is worth noting that the circles in high-resolution TEM (HRTEM) images (Figure 2e,f) indicate the mesopores feature can be perfectly retained after the post heat treatment. Over the whole Ni3S2 nanosheets, the mesopores are distributed uniformly, where the mesopore sizes range from 2 to 10 nm. By employing the liquid nitrogen cryosorption, the surface area of the as-prepared Ni3S2/3DrGO is determined by Brunauer−Emmett−Teller (BET) measurements (Figure 3a in the Supporting Information). The corresponding specific surface area of the Ni3S2/3DrGO can reach 118 m2 g−1. As a result, the vary large specific surface area can enhance the electrochemical performance of a supercapacitor by increasing the contact area between electrode and electrolyte. Furthermore, according to the pore-size distribution analysis, the interconnected Ni3S2/3DrGO mesoporous nanostructures have a broad pore size of 3−52 nm and average diameters of ∼9.9 762

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than that of 3DrGO at 1 A g−1 (the maximum discharge time of Ni3S2/3DrGO electrode electrodeposited with four cycles is 756 s, while the discharge time of 3DrGO is only 48 s), suggesting that the pseudocapacitive Ni3S2 coating can greatly improve the specific capacitance. Because the pseudocapacitors store energy through redox reactions predominately on or near the surface of electrode materials and the Ni3S2 has poor electronic conductivity, it is necessity to investigate the relationship between the amount of electrodeposited Ni3S2 mesoporous nanosheets and the overall electrochemical performance. It is possible to provide a new approach for achieving optimized supercapacitor performance by controlling the composition of Ni3S2 and 3DrGO. As shown in Figure 4e, the average mass loadings of Ni3S2 and areal capacitances of Ni3S2/3DrGO electrodes as a function of electrodeposition cycles at 1.0 A g−1 are also studied. Obviously, the areal capacitances of the Ni3S2/3DrGO electrodes are much higher than that of 3DrGO electrode. With the increase of the amount of electrodeposition cycles, the areal capacitances first increase and then decrease. The highest value is achieved for the Ni3S2/3DrGO electrode electrodeposited with four cycles. For instance, Ni3S2/3DrGO electrode electrodeposited with four cycles has an areal capacitance of 11.31 F cm−2 (about 1886 F g−1), while 3DrGO has only 0.15 F cm−2 (about 136 F g−1). The decreases areal capacitance of Ni3S2/3DrGO with more Ni3S2 coating can be attributed to the weak intrinsic conductance properties of the Ni3S2, and the thicker Ni3S2 hampers the mass transport of electrolyte and charge.30 The gravimetric specific capacitances of 3DrGO electrode and Ni3S2/3DrGO electrodes with different electrodeposited cycles as a function of charge− discharge current densities are summarized in Figure 4f. It is shown that the Ni3S2/3DrGO electrode electrodeposited with four cycles shows a high capacitance of 1886, 1842, 1748, 1674, and 1621 F g−1 at different current densities of 1, 5, 10, 15, and 20 A g−1, respectively. Obviously, the Ni3S2/3DrGO electrode electrodeposited with four cycles possesses a good rate capability characteristic and superior reversible redox reaction with 86.0% of capacitance retention when the current density ranged from 1 to 20 A g−1. In contrast, the gravimetric specific capacitances of 3DrGO are only 133 F g−1 and 125 F g−1 when the current density increases by a factor of 20 (from 1 to 20 A g−1) compared to 9.2% loss for 3DrGO electrode. Additionally, the Ni3S2/3DrGO electrodes electrodeposited with 1, 2, 3, and 5 cycles also illustrate high specific capacitance and excellent capacity retention when the charge−discharge current density increased from 1 to 20 A g−1, corresponding to about 88.4%, 87.3%, 86.1%, and 82.1% capacitance retention, respectively. However, the gravimetric specific capacitances of Ni3S2 are only 641, 544, 510, and 432 F g−1 at different current densities of 1, 5, 10, 15, and 20 A g−1, respectively, and only 67.4% of the initial capacitance can be retained when current density increases from 1 to 20 A g−1 (Figure 6 in the Supporting Information). All the above results suggest that the Ni3S2/3DrGO electrodes not only exhibit high specific capacitance but also show excellent rate capability. Furthermore, the cycling stability of the Ni3S2/3DrGO electrode electrodeposited with four cycles is tested at a constant current density of 2 A g−1 and 10 A g−1 for 30 000 charge−discharge cycles (Figure 5 in the Supporting Information). It is shown that no significant morphology changes can be observed after 30 000 charge−discharge cycles, except the increased roughness on the nanosheets surface due to the high-rate faradaic

3DrGO hybrid nanostructures on Ni foam are shown in Figure 3c,d. The XPS spectrum of GO for C 1s is fitted to two major peaks at 286.2 and 284.8 eV corresponding to C−O and C−C with Gaussian fitting method, respectively. The one minor peak at 287.8 eV corresponds to CO (Figure 3c). By comparison, in the case of Ni3S2/3DrGO, the one major peak of C 1s is located at 284.5 eV (C−C), and two minor peaks are located at around 286.5 eV (C−O) and 287.8 eV (C = O), which further demonstrates that GO has been effectively reduced by a simple postheat treatment. This is consistent with reports elsewhere.27 The XPS spectrum of Ni3S2 for Ni 2p (Figure 3e) is best fitted with Ni 2p1/2 (872.8 eV) and Ni 2p3/2 (855.2 eV) spin−orbit peaks and two shakeup satellite peaks at 879.6 and 861.0 eV (labeled as “Sat.”). The S 2p (Figure 3f) is fitted with one single peak at around 162.6 eV, which confirms that the deposited film is composed of S2−. The XPS measurements are in good agreement with the X-ray diffraction (XRD) and TEM analysis. The electrochemical behavior of 3DrGO and Ni3S2/3DrGO is studied using a three-electrode electrochemical system with KOH aqueous solution (6 M) as the electrolyte. Panels a and b of Figure 4 show the representative CV cures of bare 3DrGO electrode and Ni3S2/3DrGO electrode electrodeposited with four cycles at different scan rates (5−50 mV s−1). As can be seen, the cyclic voltammogram exhibits an analogous shape, and the current density grows with the increasing of scan rate, which indicates a low internal resistance and a high rate capability performance. The 3DrGO exhibits typical quasirectangular shapes, typical for electric double-layer capacitance, while the CV curves of the Ni3S2/3DrGO clearly reveal the pseudocapacitive characters derived from faradaic redox reactions. As expected, the enclosed area of Ni3S2/3DrGO is also much larger than that of 3DrGO, showing the capacitance is significantly increased after the Ni3S2 mesoporous nanosheet decorating. In Figure 4b, it is clear that two redox peaks can be observed around 0.07 and 0.30 V at a scan rate of 5 mV s−1, which should be attributed to the faradic oxidation of Ni3S2 with the alkaline electrolyte: Ni3S2 + 3OH− ↔ Ni3S2(OH)3 + 3e−.28 In addition, the reduction peaks and oxidation peaks move to positive and negative potential with the increasing scan rate, remaining in a symmetrical position at the same time. This illustrates that the Ni3S2/3DrGO can undergo high current charge−discharge cycles and this redox reaction is well reversible. Figure 4c gives the galvanostatic charge−discharge (GCD) tests of Ni3S2/3DrGO electrode electrodeposited with four cycles at different current densities. Clearly, well-defined voltage plateaus can be found during the charge−discharge process, which also indicates that the Ni3S2/3DrGO electrode charged and discharged mainly through faradaic redox reactions.29 Furthermore, it can be seen that voltage plateaus shift to positive potential with the increasing charge current density in the charge process. When the discharge current density changes from 1 to 8 A g−1, the discharge time of Ni3S2/ 3DrGO reaches 756 s and it gradually decreases to 88 s, suggesting a little change in specific capacitance (∼5.9% loss in specific capacitance). As can be seen from Figure 4d, the GCD measurements at 1.0 A g−1 for the 3DrGO electrode and Ni3S2/ 3DrGO electrode electrodeposited with different cycles also suggest that the capacitance is significantly increased after the Ni3S2 mesoporous nanosheet coating. Compared with the voltage plateaus in Ni3S2/3DrGO, the GCD curves of 3DrGO are close to “linear” and nearly symmetric, which indicates a good charge diffusion in the interconnected pores of 3DrGO. The discharge time of Ni3S2/3DrGO electrodes is much larger 763

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Figure 5. Electrochemical behavior of Ni3S2/3DrGO electrodes in full cell. (a) Assembly of an all-solid-state supercapacitor; (b) CV plots of the device at various scan rates; (c) GCD curves, (d) specific capacitance of the device at different current densities; (e) cycling stability at 10 A g−1 and the electrochemical impedance spectra (EIS) before and after 30 000 charge−discharge cycles (inset); (f) Ragone plots compared with values reported in the literatrue.

constant current density of 10 A g−1 (Figure 5e). Furthermore, the Nyquist plots of the all-solid-state symmetric supercapacitor before and after charge−discharge cycles are shown in the inset of Figure 5e. The semicircle at high frequencies corresponds to the faradaic charge-transfer resistance (Rct). The intercept at the part (Z′) is the equivalent series resistance (Rs). Only a small increase of the Rct and Rs can be observed after 30 000 charge−discharge cycles, indicating an excellent cycling stability, which may attribute to the advantageous structure of the as-made electrodes and the all-solid-state device. Specifically, the interconnected and mesoporous nanosheets directly grown on flexible 3DrGO scaffold can partly alleviate the volume change caused by high-rate redox reactions. Moreover, the solid electrolyte can effectually restrain the dissolution of hybrid electrodes. As a key parameter of supercapacitor performance, the Ragone plots show the specific energy and specific power of our all-solid-state device (Figure 5f). When the power density is 0.5 kW/kg, the maximum energy density is achieved (58.9 Wh kg−1). The highest power density is 3.7 kW kg−1 when energy density is 45.8 Wh kg−1. Our results demonstrate clearly that our devices are superior to the previously symmetric supercapacitors, such as NiCo2S4 (31.5 Wh kg−1 at 0.156 kW kg−1),34 NiCo2O4 (34.9 Wh kg−1 at 0.875 kW kg−1),35 NiO//carbon (10.2 Wh kg−1 at 0.025 kW kg−1),36 Co3O4@MnO2 (10 Wh kg−1 at 0.6 kW kg−1),37 Ni(OH)@ 3DrGO (33 Wh kg−1 at 0.65 kW kg−1),38 and Co9S8 (29 Wh kg−1 at 1.0 kW kg−1).39 To elucidate why the all-solid-state symmetric supercapacitor based on an interconnected hybrid nanostructure consisting of ultrathin Ni3S2 mesoporous nanosheets decorated on 3DrGO shows such high specific capacitance, we employ density functional theory to calculate the electronic structure of Ni3S2/ 3DrGO. Figure 6a,b shows the density-of-states (DOS) for

reactions. The Ni3S2/3DrGO electrode electrodeposited with four cycles demonstrates an excellent stability, remaining at 91.0% and 85.3% of its initial capacitance after 30 000 charge− discharge cycles at the constant current density of 2 A g−1 and 10 A g−1, respectively. To evaluate the capacitive performance of Ni3S2/3DrGO electrodes in full cell, an all-solid-state symmetric supercapacitor was assembled, in which two Ni3S2/3DrGO electrodes electrodeposited with four cycles are separated with a filter paper as separator and packed between two PET films. The schematic diagram for the full cell is shown in Figure 5a. CV measurements are conducted at various scan rates within a voltage range from 0 to 0.8 V (Figure 5b). Interestingly, the CV plots exhibit quasi-rectangle shape without distinct redox peaks. It is often observed that the CV curves of the electrode show redox peaks, whereas the corresponding symmetric supercapacitor devices’ curves exhibit a quasi-rectangle shape.31−33 Additionally, the CV quasi-rectangle shape is well-maintained in the charge−discharge processes over a wide range of scan rates (5−80 mV s−1), suggesting the low internal resistance and high rate capability performance. GCD tests of the full cell are shown in Figure 5c. Apparently, discharge curves are nearly symmetrical to the corresponding charge curves, suggesting the excellent capacitive performance for the all-solid-state device. Figure 5d presents the specific capacitance calculated from the discharge curves at different current densities. The solid-state symmetric supercapacitor can achieve a high specific capacitance of 865 F g−1 at 1.5 A g−1. Furthermore, the device still has specific capacitance of 560 F g−1 (64.7% of the initial capacitance) when the current density is up to 15 A g−1. The solid-state symmetric supercapacitor exhibits stable cycling performance, which can maintain 92.0% of the initial capacitance even after 30 000 charge−discharge cycles at a 764

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the interaction between OH and Ni3S2 is substantial. In the case of full-coverage OH on Ni3S2(101) surface sample, the adsorption energy of each OH on Ni3S2(101) is −2.04 eV. Therefore, the vertically aligned Ni3S2 mesoporous nanosheets not only shorten the transport path length for ions but also introduce more exposed Ni atoms to serve as the electrochemically active sites, thus improving faradic redox reactions and fast rate capability.41 The above discussions about the geometrical structure and electronic properties of the Ni3S2/ 3DrGO suggest that the perfect specific capacitance of the interconnected Ni3S2/3DrGO hybrid nanostructures can be assigned to its ultrathin and mesoporous structural features leading to an improved electronic conductivity and increased exposed Ni atoms with lower coordination number on the surface of the Ni3S2 nanosheets and the edges of the mesoporous. In summary, an interconnected hybrid nanostructure consisting of ultrathin Ni3S2 mesoporous nanosheets decorated on 3DrGO is successfully prepared through an electrodeposited method followed by a simple postheat treatment. By taking advantage of the geometrical structure (interconnected porous structure) and electronic properties (the interfacial polarization effect which greatly enhances the adsorption of OH−1 onto the exposed Ni sites and the good electrical conductivity of rGO networks), the tunable composition of the Ni3S2/3DrGO provides the possibility of simultaneously exploiting the features of capacitive and faradaic materials. Using this unique interconnected porous structure and the outstanding electrical properties of Ni3S2/3DrGO hybrid nanostructure, we demonstrate the great potential of electrode materials for high-energy supercapacitors. Moreover, this rational design concept of effectively decorating transition-metal sulfides on carbon materials is versatile and scalable and can be a general strategy for fabricating a broad class of hybrid nanostructures for highperformance energy storage and conversion systems.

Figure 6. Density-of-states of (a) Ni3S2(101) on graphene and (b) Ni3S2(101). The energy is relative to EF. (c) Theoretically optimized models and differential charge density of Ni3S2(101) by first-principles simulations. The blue and red areas represent increase and decrease in electron density, respectively.

Ni3S2(101) on graphene and the DOS for Ni3S2(101), for both spin up and spin down. It reveals that the Ni3S2(101) on graphene possess much increased DOS near the Fermi level as compared to the Ni3S2(101), which could increase its electrical conductivity.40,42 The increased DOS near the Fermi level has one main contribution coming from the C 2p states, which further confirms that the 3DrGO scaffold can act as a conductive network that helps to improve the electric conductivity. The capacitive behavior of Ni3S2 can be ascribed to the OH− quasi-adsorption−desorption process on the surface Ni atoms, which is accompanied by the fast faradic redox reactions on or near the surface of the electrode, indicating that it is especially important to induce more exposed Ni atoms and effectively utilize the exposed Ni atoms. Obviously, the ultrathin and mesoporous structural features of the Ni3S2 give more exposed Ni atoms. In addition, the Ni sites on the surface of the Ni3S2 nanosheets and the edges of the mesoporous have a lower coordination number and a less crowded local environment, thus inducing the charge spatial distribution. As indicated by the differential charge density of Ni3S2(101) surface, electrons are mostly accumulated on S atoms while holes are accumulated on Ni atoms on the Ni3S2 surfaces, as shown in Figure 6c. Such charge spatial separation has been verified by the Bader charge analysis based on firstprinciples simulations. Our results suggest that each bare Ni atom on the surface of Ni3S2(101) can accommodate about 0.05 h+. Such charge spatial separation greatly enhances the adsorption of OH− onto the Ni sites on the surface of the Ni3S2 nanosheets and the edges of the mesoporous, thus enabling fast redox reaction at high rates. In addition, we employ theoretical simulations to study the adsorption of the OH on Ni3S2(101) surface (Supporting Information). The calculations show that

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EXPERIMENTAL SECTION Fabrication of Samples. For the preparation of GO on Ni foam, the compact graphite oxide sheets are deposited on Ni foam substrate by a “dipping and drying” process. GO is prepared by a modified Hummers’s method. A cleared Ni foam sheet (1.0 × 1.0 × 0.1 cm) is immersed into a GO dispersion (2 mg mL−1) for 5 min. Then the Ni foam is taken out and dried for 5 h at room temperature. The dipping and drying process is repeated three times to increase the mass of GO. For the preparation of a hybrid nanostructure consisting of Ni3S2 mesoporous nanosheets decorated on a 3D reduced graphene oxide (Ni3S2/3DrGO) scaffold supported by Ni foam, Ni3S2 precursor nanosheets are electrodeposited on GO scaffold supported by Ni foam in a three-electrode electrochemical system, where the GO on Ni foam is the working electrode and a Pt sheet (1.0 × 1.0 × 0.1 cm) and a saturated Ag/AgCl are the counter electrode and reference electrode, respectively. An aqueous solution with 50 mM NiCl2·6H2O and 1 M thiourea (CH4N2S) is prepared as the electrolyte. Ni3S2 precursor nanosheets are electrodeposited by the cyclic voltammetry method, where the potential ranges from −1.2 to 0.2 V and the sweep rate is 5 mV per second. The mass of Ni3S2 can be well-controlled by adjusting the number of electrodeposition cycles. Then the sample is washed and dried in a vacuum oven for 12 h at 60 °C. To get 3D reduction graphene oxide (3DrGO) and crystallized Ni3S2, the sample is 765

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ACS Energy Letters annealed at 600 °C in an argon gas atmosphere for 1 h with a heating rate of 5 °C min−1. For comparison, the bare 3DrGO on Ni foam is also prepared. Briefly, a cleared Ni foam sheet is immersed into a GO dispersion (2 mg mL−1) for 5 min. Then the Ni foam is taken out and dried for 5 h at room temperature. The dipping and drying process is repeated three times to increase the mass of GO. Finally, the sample is annealed at 600 °C in an argon gas atmosphere for 1 h with a heating rate of 5 °C min−1. Characterization. The morphology of samples is characterized by scanning electron microscopy (SEM, Hi-tachiS3400, Japan) and transmission electron microscopy (TEM, JEOL-2100F, Japan). X-ray diffraction (XRD) patterns are measured on an Xray diffraction instrument (X’TRA, Thermo ARL X’TRA, Switzerland). X-ray photoelectron spectroscopy (XPS) analysis is carried out using a Phi 5000 VersaProbe Scanning ESCA Microprobe (Ulvac-Phi, Inc., Japan). The specific surface area is tested by the Brunauer−Emmett−Teller (ASAP 2020) method. Electrochemical performance measurements are measured on an electrochemical workstation (CHI 660D, Chenhua Instruments, China). Density Functional Theory Calculations. All calculations are performed based on spin-polarized density functional theory (DFT) using the VASP program package.43 The general gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerhof (PBE)44 is used as the exchange− correlation functional, and the projector augmented-wave (PAW)45,46 method is used to describe the ion-electro interaction. A kinetic energy cutoff of 500 eV is used for the plane-wave basis, and the Brillouin zone is sampled in k-space using a Monkhorst−Pack scheme of 3 × 9 × 1. The force and energy convergence criterion are set to 0.01 eV/Å and 10−5 eV, respectively. A system with six layers is employed to model the Ni3S2 surfaces, with three bottom layers fixed to the bulk positions during the relaxation simulations. For the sake of simplicity, 3DrGO is replaced by graphene in all calculations. The adsorption of Ni3S2(101) on graphene has been investigated. The three top layers of Ni3S2 are fixed to the bulk positions at the experiment lattice parameters, and the three bottom layers of Ni3S2 and graphene are fully relaxed during atomic structure relaxation calculations. Electrochemical Measurement. Electrochemical measurements are performed using an electrochemical workstation (CHI 660D) at room temperature. For three-electrode systems tests, Ni3S2/3DrGO or 3DrGO is directly used as the working electrode. A Pt foil (1.0 × 1.0 × 0.1 cm) and a standard calomel electrode (SCE) are used as the counter electrode and the reference electrode, respectively. A KOH aqueous solution (6 M) is used as the electrolyte. In the two-electrode all-solid-state tests, the electrolyte (PVA/KOH gel) is prepared as follows: 3.0 g of KOH is added into 50 mL of deionized water, then 6.0 g of poly(vinyl alcohol) (PVA) powder is added. The solution is then stirred at 90 °C for 30 min. After that two pieces of the Ni3S2/3DrGO electrodes (1.0 × 1.0 cm) are immersed into the PVA/KOH gel for 10 min, they are removed and assembled into all-solid-state symmetric supercapacitor with a filter paper (NKK TF45, 40 μm) as the separator until the water evaporates. The gravimetric specific capacitance in the three-electrode system is obtained from the discharge process by Cg = It/ (ΔVm), and areal capacitance is obtained from the discharge process by Ca = It/(ΔVS), where I is the discharge current, t the discharge time, ΔV the voltage drop upon discharging, m

the total mass of the Ni3S2/3DrGO, and S the geometrical area of the electrode. The energy density (E) and power density (P) in the Ragone plots are calculated by E = CcellΔV2/2 and P = E/ t, where Ccell is the cell capacitance calculated by Ccell = It/ (ΔVm), where I is the discharge current, t the discharge time, ΔV the voltage drop upon discharging, and m the total Ni3S2/ 3DrGO mass of the two electrodes. The amount of hybrid materials is determined by measuring the masses of hybrid materials before and after rGO and Ni3S2 growth using a microbalance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00078. SEM images of the sample before the heat treatment, TEM images of the sample before and after the heat treatment, BET and TGA results of Ni3S2/3DrGO, electrochemical behavior of Ni3S2 and Ni3S2/3DrGO electrode, electrochemical behavior of Ni3S2/3DrGO electrodes in full cell, and DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shaolong Tang: 0000-0002-6705-3209 Mingsen Deng: 0000-0002-3331-3850 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Key Project of Fundamental Research of China (Grant No. 2012CB932304), the National Natural Science Foundation of China (No. 21203037, 11264005) and the Excellent Youth Scientific and Technological Talents of Guizhou Province (No. QKHRZ[2013]01). All the calculations are performed at Guizhou Provincial High-Performance Computing Center of Condensed Materials and Molecular Simulation in GZEU.

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REFERENCES

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