Rational Design of Self-Supported Ni3S2 Nanosheets Array for

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A rational design of self-supported Ni3S2 nanosheets array for advanced asymmetric supercapacitor with a superior energy density Jun Song Chen, Cao Guan, Yang Gui, and Daniel John Blackwood ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14746 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

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ACS Applied Materials & Interfaces

A rational design of self-supported Ni3S2 nanosheets array for advanced asymmetric supercapacitor with a superior energy density Jun Song Chen,ab* Cao Guan,b Yang Gui,b Daniel John Blackwoodb* a

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-state Electronics, University of Electronic Science and Technology of People’s Republic of China, 610054 Chengdu, People’s Republic of China. Email: [email protected].

b

Department of Materials Science and Engineering, National University of Singapore, Singapore 117574. Email: [email protected].

Abstract We report a rationally designed two-step method to fabricate self-supported Ni3S2 nanosheet arrays. We first used 2-methylimidazole (2-MI), an organic molecule commonly served as organic linkers in metal-organic frameworks (MOFs), to synthesize an α-Ni(OH)2 nanosheet array as a precursor, followed by its hydrothermal sulfidization into Ni3S2. The resulting Ni3S2 nanosheet array demonstrated superior supercapacitance properties, with a very high capacitance of about 1,000 F g-1 being delivered at a high current density of 50 A g-1 for 20,000 charge-discharge cycles. This performance is unparalleled by other reported nickel sulfide-based supercapacitors, and is also advantageous compared to other nickel-based materials such as NiO, Ni(OH)2. An asymmetric supercapacitor was then established, exhibiting a very stable capacitance of about 200 F g-1 at a high current density of 10 A g-1 for 10,000 cycles and a surprisingly high energy density of 202 Wh kg-1. This value is comparable to that of the lithium-ion batteries, i.e., 180 Wh kg-1. The potential of the material for practical applications was evaluated by building a quasi-solid-state asymmetric supercapacitor which showed good flexibility and power output, and two of these devices connected in series were able to power up 18 green light-emitting diodes. Keywords: Ni3S2, nanosheets, self-supported, asymmetric supercapacitor, high energy density

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1 Introduction Supercapacitors play a very important role in the battle against the global energy crisis.1-5 Based on how the electrons are stored, supercapacitors can be categorized into two types.4, 6 The first type consists of electric double-layer capacitors (EDLCs), which utilize high-surface-area conducting materials, e.g., carbonaceous materials with a high porosity,6-7 as electrodes to store electrons by electrostatic interactions. The second category is the pseudocapacitors, where electrochemically active materials, e.g., metal oxides/hydroxides/sulphides8-14 or conducting polymers,15-16 are employed to store electrons via fast redox reactions. In order for large-scale practical applications, enhancing the energy density to match that of battery systems is one of the challenging tasks in improving the performance of supercapacitors.1, 4-5 The energy density is directly related to the capacitance by the following equation:12 E = ½CV2, where E is the energy density (Wh kg-1), C is the specific capacitance (F g-1), and V is the working potential (V) of the device. Since pseudocapacitors rely on rapid faradaic reactions for the storage and release of the electrons, they usually display higher capacitance than EDLCs.1 However, due to the semiconducting or insulating nature of most metal oxides and hydroxides,17-18 these metal-based electrode materials in pseudocapacitors have to be ingeniously designed to achieve highly efficient electron/ion diffusion kinetics19-20 for an optimal energy output. Recent developments in nanotechnology and material chemistry have provided us with some promising solutions to address this issue. From the nanostructure engineering perspective, two-dimensional (2D) nanosheets have demonstrated their advantages in various applications8, 21 over other types of nanostructures due to their anisotropic structure granting shorter diffusion path (thus, faster diffusion kinetics) for electrons and ions. Moreover, building a binder-free self-supported nanoarray directly on the current collector will create a highly efficient electron conducting pathway from the active material to the substrate.1920, 22-24

From the chemistry perspective, because sulfur has a lower electronegativity than oxygen,


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metal sulfides possess a better structural stability than oxides, thus allowing faster electron transport.8 Yet, previous examples of practicing these technologies as an independent tactics for electrode material design resulted in energy densities still inferior to those of batteries.1, 4-5 To overcome such a limitation, we rationally designed an integrated approach that utilizes the abovementioned technologies of both nanostructure engineering and material chemistry modification, to construct a self-supported Ni3S2 nanosheet array directly on the Ni foam substrate. As illustrated in Scheme 1, we first used 2-methylimidazole (2-MI) as an important structure directing agent to synthesize an α-Ni(OH)2 nanosheet array as a precursor, by a simple wet chemical method, followed by the hydrothermal sulfidization of the Ni(OH)2 into phase-pure Ni3S2. The supercapacitive properties of the as-prepared Ni3S2 nanosheet array exhibited an almost unparalleled specific capacitance of about 1000 F g-1 at a very high current rate of 50 A g-1 for 20,000 cycles. Such a performance not only cannot be matched by nickel sulphide-based electrodes, but also is superior compared to other nickel-based materials such as NiO or Ni(OH)2.8 As pointed out earlier, Ni3S2 can also be used for photocatalytic application in addition to electrode material in energy storage devices.25 An asymmetric supercapacitor (ASC) was then assembled with a high capacitance of 200 F g-1 at a current rate of 10 A g-1 for 10,000 cycles, and a surprisingly high energy density of 202 Wh kg-1, which is even comparable to that of Li-ion batteries, i.e., 180 Wh kg-1.1, 4-5 A quasisolid-state ASC was also developed, and the device displayed good structural flexibility and energy output, where two of these devices connected in series were able to power up 18 green lightemitting-diodes (LEDs), demonstrating great application potential for wearable electronic devices.

2 Results and Discussion 2.1. Material fabrication and characterization Even though 2-MI has previously been used for the synthesis of zeolitic imidazolate frameworks (ZIFs) like ZIF-67 and ZIF-8,26-28 the structure of the α-Ni(OH)2 nanosheet precursor synthesized in



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the current system differs dramatically from these MOFs (Figure S1). As depicted in Figure 1A, the surface of the Ni foam substrate is uniformly covered by the vertically aligned Ni(OH)2 nanosheets (denoted as Ni(OH)2@Ni). With a closer look (Figure 1B), these nanosheets are shown to have a high structural flexibility and are arranged in an orderly manner with a fairly dense packing. Under an even higher magnification (Figure 1B, inset), these nanosheets present a smooth surface, with each sheet having a thickness about 20 nm. The cross-sectional view (Figure 1C) shows that these nanosheets form a uniform film with a thickness about 1.4 µm on the surface of the substrate. The sheet-like structure was also viewed using transmission electron microscopy (TEM; Figure 1D), and the colloidal suspension of the nanosheets exhibited a Tyndall effect (Figure 1D, inset), which is due to the light scattering caused by the α-Ni(OH)2 nanosheets in the solution.29-30 Figure 1E shows a portion of the nanosheets with some visible lattice fringes, which exhibit an interplanar distance of 0.23 nm corresponding to the (015) plane of α-Ni(OH)2. It is important to point out that no product was formed in the absence of 2-MI, suggesting the crucial roles of 2-MI in the current synthesis system: it facilitates the crystal growth of the Ni(OH)2 nanosheets and also promotes the formation of the array structure on the Ni foam substrate. The crystallographic information of these nanosheets was gathered by X-ray diffraction (XRD), and all the identified peaks in the XRD pattern (Figure 1F) can be assigned to rhombohedral αNi(OH)2 (PDF card number: 38-0715, space group: R, a = b = 3.08 Å, c = 23.41 Å). The absence of peaks due to other chemical phases implies that the as-prepared sample is of high phase purity. The valence state of the sample was verified by X-ray photoelectron spectroscopy (XPS; Figure S2), confirming the Ni2+ oxidation state of Ni(OH)2. The analysis of the N1s and C1s spectra has evidenced the presence of 2-MI in the sample. Consistently, the thermogravimetric analysis (TGA; Figure S3) also suggests the presence of certain amount of organic compound as well as moisture in the Ni(OH)2 crystal structure, while the nitrogen adsorption-desorption isotherm (Figure S4) indicates that the sample is highly porous with a surface area of about 105 m2 g-1.


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The versatility of the current system is highlighted by growing these α-Ni(OH)2 nanosheets on other types of substrates (Figure S5), such as Cu foam, stainless-steel mesh, and even non-metallic carbon paper, with a little variation in the apparent structure of the product. Furthermore, this system can also be extended to grow phase-pure Co(OH)2 nanosheets on Ni foam (Co(OH)2@Ni; Figure S6), and the samples demonstrated almost identical structures to their Ni(OH)2 counterpart. These data suggest that 2-MI is in fact a powerful agent which could probably be utilized for the synthesis of a wide range of nanomaterials for different applications. Another important reason for us to use 2-MI in our system is the inspiration from ZIFs such as ZIF-67,26-28, 31-32 which is a cobalt-containing metal-organic framework (MOF). ZIF-67 utilized 2-MI as the organic linker, and has demonstrated excellent chemical stability as its structure was well retained after various processes including calcination or sulfidization.26-27, 31 As shown in Figure 2A, the 2D nanostructure of the original Ni(OH)2 can be retained in the Ni3S2 nanosheet array (denoted as Ni3S2@Ni) after the sulfidization, even though the orientation of these nanosheets becomes more random (Figure 2B). Under a higher magnification, these nanosheets, with a slightly larger thickness of about 70 nm compared to the parent hydroxide, are covered with a wrinkle-like structure on the surface (Figure 2C). These nanosheets also form a uniform film with a thickness of about 700 nm which still remains in excellent contact with the substrate (Figure 2D). The TEM analysis suggests that the wrinkle-like structures are also nanosheets with fine dimensions (Figure 2E), and the HRTEM image in Figure 2F depicts the edge region of one nanosheet containing several randomly oriented crystallites. Some lattice fringes can be clearly observed with an interplanar distance of about 0.2 nm, corresponding to the (202) plane of Ni3S2. Based on the above experimental observations, we can conclude that there are two important roles for 2-MI in our system: firstly, it directed the formation of the precursor Ni(OH)2 nanosheets on the Ni foam substrate, forming a self-supported nanoarray with a robust pathway for electron transfer; secondly, it granted these nanosheets with a high structural stability, and both the two-dimensional



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structure and the electron diffusion passage can be well retained after the hydroxide-to-sulphide conversion. The chemical composition of the Ni3S2@Ni sample was investigated by XRD (Figure 3A), where all the identified peaks can be unambiguously assigned to rhombohedral Ni3S2 (JCPDS No.: 44-1418, S. G.: R32, ao = bo = 5.7454 Å, co = 7.1350 Å),22 and the absence of any peak due to the parent Ni(OH)2 phase confirms the complete transformation to Ni3S2. The oxidation state of Ni in Ni3S2 was verified using XPS (Figure 3B), and two main peaks at binding energies of 855.7 and 873.4 eV can be clearly observed, with each followed by a smaller satellite peak (marked by sate. in the figure). These satellite peaks are usually due to the excitation of electrons in the valence band to higher energy levels.22, 33 Consistent with previous report,22 the two main peaks can be attributed to Ni 2p3/2 and Ni 2p1/2, respectively. From the Raman spectrum (Figure 3C), 6 peaks at Raman shifts of 187 (A1), 201 (E), 222 (E), 303 (E), 324 (A1), and 350 (E) cm-1 can be clearly identified, and they can be assigned to the two A1 and four E vibration modes of Ni3S2.22 As reported previously,34 the Ni3S2 crystal has 15-dimensional representations in total, and the irreducible representation ᴦ of the phonon modes based on symmetry can be expressed as:34-35 ,

ᴦ

= 2 + 3 + 5 , (1)

where the three transitional modes A2 + E are silent, the two A1 modes are infrared active, the remaining two A2 modes are Raman active and the four E bands are both Raman and infrared active, making a total of six Raman-active bands are present. The locations of the Raman modes in the present work are within 3 cm-1 of those reported by Chen and Liu,36 and thus confirm the rhombohedral structure of the Ni3S2.

2.2. Electrochemical performance in a three-electrode system The as-prepared Ni3S2@Ni was first applied as the electrode material for a supercapacitor in a threeelectrode system. Cyclic voltammograms (CVs) at different scan rates were conducted with the 6 ACS Paragon Plus Environment

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results shown in Figure 4A, where a pair of redox couple can be clearly identified at 0.45 V (positive sweep) and 0.3 V (negative sweep) vs. SCE at the scan rate of 2 mV s-1. These peaks, which are the origin of the pseudocapacitive capacitance of Ni3S2, can be attributed to the following redox reaction:22, 37 Ni3S2 + 3OH− ↔ Ni3S2(OH)3 + 3e−

(2)

The capacitance C (F g-1) can then be calculated by using the following equation:38-39 =

! # $(%)&% ( ! )

(3)

where m is the mass of the active material, v is the scan rate, and (Va-Vc) is the voltage window of the scan. As a result, specific capacitances of 1325, 1170, 1020, 985 and 765 F g-1 can be calaculated at the scan rates of 2, 5, 10, 25, and 50 mV s-1, respectively (Figure 4B). The precursor Ni(OH)2@Ni and the free-standing Ni3S2 prepared without the Ni foam substrate demonstrate much lower capacitance values at these scan rates. Figure 4C shows the discharge curves at different current densities, and the corresponding capacitance obtained by using the following equation:22 C = I × ∆t ÷ ∆V

(4)

where C is the capacitance in F g-1, I is the current density in A g-1, ∆t is the discharge time in s, and ∆V is the discharge voltage window in V. The specific capacitance can then be calculated to be 2885, 2490, 2290, and 1680 F g-1 at current rates of 2, 5, 10, and 25 A g-1, respectively (Figure 4D). Remarkably, even at a very high current rate of 50 A g-1, the Ni3S2@Ni is still able to deliver a high capacitance of 1232 F g-1. As summarized in a very recent review article,8 such a performance not only cannot be matched by previously reported nickel sulfide-based materials (detailed values are compared in Table S1), but also is superior to other nickel-based electrode materials, e.g., NiO, Ni(OH)2, and Ni2P.8, 10 As expected, the specific capacitances of the Ni(OH)2@Ni and free-standing Ni3S2 at these current rates are much lower compared to the Ni3S2@Ni sample. Considering that both samples 7 ACS Paragon Plus Environment


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contain similar self-support nanoarrays with sheet-like subunits, the greatly enhanced performance cannot be achieved merely by constructing nanomaterials with a unique structure, and should be attributed to the current rationally designed approach that integrated both nanostructure engineering and material chemistry modification, i.e., sulfidization. Such a chemical transformation leads to the Ni3S2 phase that possesses a low resistivity of only 0.001 Ohm cm,40 while that of the parent Ni(OH)2 is as high as 0.1 Ohm cm.41 As a result, Ni3S2 is intrinsically a more efficient electronic conductor than its hydroxide counterpart, giving rise to the much better performance of the former than the latter. Such a difference in the conductivity was also reflected in the electrochemical impedance spectroscopy (EIS) results (Figure S7). Both samples have a semicircle in the high frequency range, which can be assigned to the Faradaic charge transfer process at the electrode-electrolyte interface. The semicircle of Ni3S2@Ni has a significantly smaller diameter compared to Ni(OH)2@Ni, suggesting that the charge transfer resistance of the former is much smaller (i.e., more efficient charge transfer) compared to the latter before sulfidization. It is worth mentioning that, the capacitance values displayed in Figure 4D appear to be significantly higher than those shown in Figure 4B. Such a difference could be due to the different current rates applied for these two measurements: In Figure 4A, for the reduction peak at 0.3 V at the lowest scan rate of 2 mV s-1, the current density can be nearly 20 A g-1 (the loading of the active material is 1.08 mg). This implies that the current applied during the CV test is not comparable to that applied during the constant current discharge (Figure 4D), thus, to directly compare these two sets of data might be difficult. We then evaluated the long-term stability of the sample at the highest current rate of 50 A g-1. From Figure 4E, it is apparent that the capacitance exhibits an initial drop from about 1300 to 1000 F g-1 over the first 2000 cycles, and then stays quite stable with no noticeable decay for the remaining 18000 cycles. Post-mortem analysis (Figures S8) showed that the sheet-like subunits can be largely retained after the long-term test. More importantly, the Ni3S2 film remains in good contact with the


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substrate and no detachment from the Ni foam substrate can be identified, suggesting that the conducting pathway between the two components is still intact, maintaining a highly efficient electron transport. Since the pseudocapactors store electrons by redox reactions taking place at the near-surface region of the electrode material, these nanosheets with a two-dimensional structure provide a short diffusion length for the electrons. This leads to a performance which is significantly better than self-supported Ni3S2 with other morphologies, such as nanorods42 or hierarchical dendrites,43 confirming the beneficial effect of the as-prepared nanosheet array. It is important to mention that treating the bare Ni foam substrate by the present sulfidization process does create a layer of featureless Ni3S2 on the substrate surface with a mass loading of about 1.48 mg cm-2 (Figure S9). This material demonstrated very low electrochemical activity with a negligible capacitance delivered even at a low current rate of 6.75 A g-1 (Figure S10A). Moreover, if we consider the areal capacitance, i.e., mF cm-2, of the as-prepared Ni3S2@Ni and the sulfidized bare nickel foam to exclude the effect of the loading mass of the active material, the former still demonstrates a much higher capacitance than the latter (Figure S10B), confirming that the very high capacitance of the Ni3S2@Ni sample essentially originated from the Ni3S2 nanosheet array. Nevertheless, because it has an identical chemical composition with the upper sulfide nanoarray while is physically a part of the conducting current collector, this Ni3S2 layer could serve as an important transition medium that bridges the active Ni3S2 nanosheets and the metallic Ni foam. Thus, the electronic conducting pathway between these two components, which was initially established by constructing a self-supported nanosheet array using 2-MI, is now reinforced by having such a Ni3S2 layer. As illustrated by the electrochemical data above, Ni3S2@Ni show significantly better performance compared to the free-standing Ni3S2 without this architecture. Based on the above experimental observations, we conclude that the enhanced supercapacitive properties of Ni3S2@Ni synthesized can be attributed to:



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(1) a highly efficient electron transport pathway that was established by 2-MI between the active Ni3S2 and the Ni foam substrate, which was also strengthened by the sulfidization creating a Ni3S2 layer on the surface of the substrate connecting these two components; (2) the unique sheet-like structure of Ni3S2 array providing a short diffusion path and a high surface area for the electrochemical reactions.

2.3. Full cell performance We subsequently assembled an asymmetric supercapacitor (ASC) using the as-prepared Ni3S2@Ni as the cathode and activated carbon (AC) as the anode (Ni3S2||AC).22 After cycling the device with different voltage windows (Figure S11), we determined that the possible working voltage range for the current Ni3S2||AC ASC is 0 – 1.8 V. The CVs at different scan rates (Figure 5A) show a pair of current peaks at about 0.6 V and 1.3 V vs. SCE (at 50 mV s-1), and the corresponding specific capacitance is calculated to be 880, 730, 620, 480, and 380 F g-1 at the scan rates of 2, 5, 10, 25, and 50 mV s-1, respectively (Figure 5B). The capacitive contribution can be quantified by the following equation:44-45 i (V) = k1·v + k2·v1/2

(5)

where i (V) is the current at a specific voltage, and it can be separated into the capacitive effects i

(k1·v) and diffusion-controlled reactions (k2·v1/2). Plotting /ν versus ν-½ allows the constants k1 and k2 to be determined, which uncovers the ratio between the capacitive and the diffusion-controlled capacitances. As shown in Figure S12, at slow sweep rates the diffusion-controlled insertion / extraction of OH− ions46 processes dominate the capacitance. Because of the unique 2D nanosheet structure with largely exposed active sites for the surface-dependent redox reactions,46 such an ion exchange process was greatly facilitated. This is also consistent with the EIS spectra displayed in Figure S7 as discussed above. With the increasing scan rate the surface capacitive contribution starts to dominant and it reaches about 66% at the highest scan rate of 50 mV s-1.


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Figure 5C displays the galvanostatic discharge curves at different current rates, and specific capacitances of 510, 430, 320, and 180 F g-1 can be delivered at current rate of 2, 5, 10, and 25 A g-1, respectively (Figure 5D). Even at the highest current density of 50 A g-1, a high capacitance of 100 F g-1 can still be obtained. When cycled at a constant current rate of 10 A g-1 for a prolonged chargedischarge test (Figure 5E), the ASC device demonstrated a high reversible capacitance of 200 F g-1 for 10,000 cycles. It can be observed that the capacitance drops from 325 F g-1 to 200 F g-1 during the first 2000 cycles. This has been commonly reported in ASCs,39, 47 and we suspect that it may be due to the discrepancies in the storage capabilities between the two electrode materials. This is most likely a process to reach the capacitance balance between these two electrodes. As such, the capacitance of the ASC quickly reaches a stable value after the initial decrease. We then plotted the energy density vs. power density curves in a Ragone plot (Figure 5F), and at a power density of 1.7 kW kg-1, the Ni3S2||AC ASC shows a very high energy density of 202 Wh kg1

. Considering for lithium-ion batteries at high energy densities of more than 100 Wh/kg, the

corresponding power densities are usually lower than 0.1 kW/kg,48 our results compare very favorably to those reported data.1, 4-5 Even at a very high power density of 17 kW kg-1, an energy density of 62 Wh kg-1 can still be delivered. As compared in Table S1, such values are significantly higher than those reported in the previous works for nickel sulfide-based ASCs,47,

49-59

further

confirming the advantage of the Ni3S2 nanosheet array prepared using 2-MI in the current system. We believe that even higher energy and power densities can be achieved by matching the current Ni3S2@Ni with an anode material with a higher capacitance than the activated carbon presently used. Inspired by such impressively high energy and power densities, we further investigated the practical use of this ASC device by developing a quasi-solid-state (qSS) ASC with a poly(vinyl alcohol)-NaOH gel electrolyte and a activated carbon anode. As shown in Figure 6A, the dimension of the actual working qSS ASC (marked by the blue dotted rectangle) is only 1 × 2.5 × 0.12 cm3. This device demonstrates excellent mechanical flexibility (not elasticity) and robustness (Figure 6B),



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and its electrochemical properties are not significantly influenced at different bending states (Figure S12). To confirm the potential application of our device in portable electronics, two connected qSS ASCs were charged for 10 s and then connected to, and lighted up 18 paralleled green LEDs (Figure 6C), demonstrating remarkable energy output and great application potential for wearable electronic devices.

3 Conclusion In this work, we rationally designed a two-step method to synthesize self-supported Ni3S2 nanosheet array using 2-MI. In the first step, 2-MI assisted the growth of precursor Ni(OH)2 nanosheets into self-supported arrays on different substrates such as Ni foam, and also granted them with good structural stability and allowed the sheet-like structure be perfectly retained after the subsequent transformation from Ni(OH)2 to Ni3S2. The resulting Ni3S2 nanosheet array demonstrated greatly enhanced capacitive properties with a high reversible capacitance of about 1000 F g-1 being delivered at a very high current rate of 50 A g-1 for 20000 cycles. Such a performance is unmatched by many reported nickel sulphide-based electrode materials, and it can be attributed to the Ni3S2 with a unique 2D nanosheet structure which greatly facilitates the Faradaic redox reactions at the surface. An asymmetric capacitor based on this material achieved a superior energy density of 202 Wh kg-1 at a power density of 1.7 kW kg-1, which is comparable to that of lithium-ion batteries. A quasi-solidstate ASC was subsequently assembled, and the device showed excellent mechanical stability and flexibility with great potential for practical application by lighting up 18 green LEDs. By setting the current study as an example, we wish to shed light on the future design of high-performance electrode materials for energy storage applications.


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4 Experimental Section Materials synthesis. The α-Ni(OH)2 nanosheets were synthesized by a facile wet chemical method. Briefly, a 25 ml methanol solution containing 3 mmol 2-methylimidazole was added into another 25 ml methanol solution with 10 mmol Ni(NO4)2. The mixture was transferred into a sealed plastic bottle, which was then heated at 60 oC in an electrical oven for 24 h. The resulting light green powder was collected and washed thoroughly with ethanol by centrifugation before it was dried at 60 o

C overnight. The self-supported α-Ni(OH)2 nanosheets array on Ni foam (Ni(OH)2@Ni) was

synthesized following the same method with a piece of 2 cm × 4 cm Ni foam placed in the reaction solution. The loading of α-Ni(OH)2 on the substrate was about 0.9 mg·cm-2. For the sulfidization process, Ni(OH)2@Ni was first gently pressed into an approximately 0.7 mm-thick film, and then placed into a 30 ml aqueous solution containing 0.1 g of thioacetamide. The entire reaction medium was transferred into a polytetrafluorethylene-lined stainless steel autoclave, and kept at 100 oC for 6 h before it was cooled down naturally to room temperature. The product was taken out and flushed with copious amount of DI water, then dried at 60oC overnight. The loading of the active material in Ni3S2@Ni was about 1.08 mg cm-2, and this includes the Ni3S2 nanosheet array and Ni3S2 formed from the sulfidization of the Ni foam substrate. The free-standing Ni3S2 was synthesized following the same protocol without adding the Ni foam substrate. Material Characterization. Sample morphology was analyzed with the aid of a Field Emission Scanning Electron Microscopes (FESEM; Zeiss) to which an energy dispersive spectroscope was attached and a transmission electron microscope (TEM; JEM2010F). Crystallographic structures were identified using a Brucker D8 Advancer (Cu Kα, λ = 1.54 Å). A Micromeritics ASAP 2020 Surface Area and Porosity Analyzer were used to measure the Brunauer-Emmett-Teller (BET) surface area. The chemical state of the elements in the samples were determined using a Kratos Axis UltraDLD X-ray photoelectron spectrometer equipped with an Al Kα X-ray source (1486.6 eV) and all peaks were referenced to the C1s at 284.5 eV. The Raman spectroscopy measurements were



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conducted on a LabRam HR Evolution Raman microscope using an Ar ion laser at 514 nm as the excitation source. Electrochemical measurements. The electrochemical tests were conducted in 1 M NaOH aqueous solution with the aid of a Bio-Logic SP-200 electrochemical workstation, using a Pt gauze counter electrode and saturated calomel reference electrode (SCE). Cyclic voltammetry was preformed over a potential window of 0 V to 0.8 V (vs. SCE) at different scan rates. For the ASC, the anode was a 70:20:10 wt% mixture of activated carbon to carbon black Super-P-Li to polyvinylidene (PVDF), and the mixture was subsequently pasted on to a Ni foam substrate. The energy density (E) and power density (P) were calculated using the following equations:

= ∆% )

( = ∆*

(6) (7)

where C is the specific capacitance in F·g-1, ∆V is the voltage window in volts and ∆t is the time for the discharge in seconds. To construct the full cell with quasi-solid state electrolyte, firstly 6 g Poly(vinyl alcohol) (PVA) was dissolved in 30 ml H2O plus 30 ml of 1 M NaOH at 85 °C to form a solid-state electrolyte. Secondly, the two electrodes were held in the electrolyte for 2 minutes at ambient conditions before being assembled face-to-face; note that the polymer electrolyte also plays the role of the ion-porous separator. Once the electrolyte solidified the full-cell is sufficiently tough to be packed and tested. Each qSS ASC contains about 2.5 mg active material. Each of the lighted green LEDs has a power requirement of approximately 40 mW.

Acknowledgement This work is financially supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Programme (CRP Award No. NRF-CRP 10-2012-6).


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Supporting Information SEM images, EDX results, XPS spectrum, TGA data, N2 adsorption-desorption isotherm, EIS spectra, cyclic voltammograms, long term charge-discharge test, and the comparison between the performance of the current device and from other literature. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Table of Content



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Figures and caption

Scheme 1. Illustration of the current two-step approach. Step I is a low-temperature wet-chemical process at 60 oC, which uses 2-methylimidazole (2-MI) as a structural directing agent to grow αNi(OH)2 nanosheets on Ni foam substrate. Step II is a hydrothermal sulfidization treatment to convert Ni(OH)2 into phase-pure N3S2. This process will also sulfidize the surface layer of the Ni foam substrate into Ni3S2, which serves as an important connection between the Ni3S2 nanosheets and the Ni foam substrate, creating a highly efficient electron diffusion path.

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Figure 1. Characterization results of the α-Ni(OH)2 nanosheets grown on the Ni foam substrate (Ni(OH)2@Ni): scanning electron microcopy (SEM) images (A − C); transmission electron microscopy (TEM; D) and high-resolution TEM (HRTEM) image (E). X-ray diffraction (XRD) pattern of the free-standing α-Ni(OH)2 nanosheets (F). The inset in B shows a magnified image of the α-Ni(OH)2 nanosheets. The inset in D shows the Tyndall effect of a colloidal suspension containing the α-Ni(OH)2 nanosheets.



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Figure 2. SEM (A – D) images of Ni3S2@Ni. TEM (E) and HRTEM (F) images of the Ni3S2 nanosheets isolated from the Ni foam support.

Figure 3. Characterization results of Ni3S2@Ni: XRD pattern (A), X-ray photoelectron spectrum of Ni 2p (B), and Raman spectrum (C).


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Figure 4. Supercapacitor performance of Ni3S2@Ni in a three-electrode system: cyclic voltammetry (CV) at different scan rates and the corresponding capacitance calculated from A (B). Galvanostatic discharge curves at different current rates (C) and the corresponding capacitance calculated from C (D). Long term charge-discharge performance at a current rate of 50 A g-1 (E). The performance of Ni(OH)2@Ni and free-standing Ni3S2 were added in B and D as comparison.



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Figure 5. The performance of the Ni3S2@Ni||AC ASC: CVs at different scan rates (A) and the corresponding capacitance calculated from A (B). Galvanostatic discharge curves at different current rates (C) and the corresponding capacitance calculated from C (D). Long term charge-discharge performance at a current rate of 10 A g-1 (E). Comparison of the energy density vs. power density curves of the current work and other literatures (F).


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Figure 6. Digital photographs of the Ni3S2@Ni||AC qSS ASC at an unbent (A) and bent state (B). The blue dotted rectangle in A marks the portion of the actual working device. (C) shows the photograph of two ASCs connected in series powering up 18 green LEDs arranged in the pattern of “NUS”.


Rational Design of Self-Supported Ni3S2 Nanosheets Array for

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