Ni0.85Se@MoSe2 Nanosheet Arrays as the ... - ACS Publications

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Ni0.85Se@MoSe2 Nanosheet Arrays as the Electrode for HighPerformance Supercapacitors Hui Peng,†,‡ Chunding Wei,† Kai Wang,† Tianyu Meng,† Guofu Ma,‡ Ziqiang Lei,*,‡ and Xiong Gong*,† †

Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States ‡ Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, P. R. China S Supporting Information *

ABSTRACT: In this study, we report novel Ni0.85Se@MoSe2 nanosheet arrays prepared by a facile one-step hydrothermal method through nickel (Ni) foam as Ni precursor and the framework of MoSe2. Owing to the unique interconnection and hierarchical porous nanosheet array architecture, the Ni0.85Se@MoSe2 nanosheet arrays exhibit a high specific capacitance of 774 F g−1 at the current density of 1 A g−1, which is almost 2 times higher than that (401 F g−1) of the Ni0.85Se matrix and about 7 times greater than that (113 F g−1) of the MoSe2 nanoparticles. Moreover, we report an asymmetric supercapacitor (ASC), which is fabricated by using the Ni0.85Se@MoSe2 nanosheet arrays as the positive electrode and the graphene nanosheets (GNS) as the negative electrode, with aqueous KOH as the electrolyte. The Ni0.85Se@MoSe2// GNS ASC possesses an output voltage of 1.6 V, an energy density of 25.5 Wh kg−1 at a power density of 420 W kg−1, and a cycling stability of 88% capacitance retention after 5000 cycles. These results indicate that the Ni0.85Se@MoSe2 nanosheet arrays are a good electrode for supercapacitors. KEYWORDS: supercapacitor, nickel selenide, molybdenum selenide, nanosheet arrays, heterostructure

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INTRODUCTION

conductivity, multiple oxidation states, and special geometric structures.13−22 In this study, we report novel Ni0.85Se@MoSe2 nanosheet arrays prepared by a facile one-step hydrothermal method using nickel (Ni) foam as Ni precursor and the framework for growing MoSe2. It is found that the Ni0.85Se@MoSe2 nanosheet arrays as the electrodes for supercapacitors exhibit the specific capacitance of 774 F g−1 at the current density of 1 A g−1, which is almost 2 times higher than that (401 F g−1) of the Ni0.85Se matrix, and about 7 times higher than that (113 F g−1) of the MoSe2 nanoparticles. Moreover, an asymmetric supercapacitor (ASC) fabricated by using the Ni0.85Se@MoSe2 nanosheet arrays as the positive electrode and the graphene nanosheets (GNS) as the negative electrode, with aqueous KOH as the electrolyte, is developed. The Ni0.85Se@MoSe2// GNS ASC possesses an output voltage of 1.6 V, an energy density of 25.5 Wh kg−1 at a power density of 420 W kg−1, and the cycling stability of 88% capacitance retention after 5000 cycles. All these results demonstrate that the Ni0.85Se@MoSe2 nanosheet arrays are good electrodes for supercapacitors

As energy storage devices, supercapacitors have attracted great attention due to their desirable characteristics which include high power density, fast charge and discharge rates, and excellent cycling stability.1,2 The device performance of supercapacitors greatly relies on the electrochemical properties of associated materials; among them, the intrinsic physical properties and nano/microstructures of electrode materials play critical roles.3 It has been demonstrated that nanostructured electrodes can boost specific capacitance and output power density as well.2 Many efforts, including development of graphene nanosheets,4 conducting polymers,5,6 and porous metal oxides/hydroxides,7 have been devoted to construct nanostructured electrodes. However, low energy densities were observed from the graphene nanosheets.4 Conductive polymers and metal oxides have been extensively utilized in the fabrication of pseudocapacitor electrodes, but their relatively low electrical conductivities have restricted their further applications.5−9 To realize supercapacitors with both high power density and energy density, other novel nanostructured materials working as the electrodes have also been explored.10−12 Among them, two-dimensional (2D) metal chalcogenides stood out due to their unique metallic © 2017 American Chemical Society

Received: February 24, 2017 Accepted: May 9, 2017 Published: May 9, 2017 17067

DOI: 10.1021/acsami.7b02776 ACS Appl. Mater. Interfaces 2017, 9, 17067−17075

ACS Applied Materials & Interfaces

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RESULTS AND DISCUSSION Figure 1 shows the scanning electron microscopy (SEM) images of MoSe2, Ni0.85Se, and Ni0.85Se@MoSe2, respectively. It

It was also reported that the heterointerface can not only prevent the aggregation of 2D MoSe2 nanosheets,16−20,22−24 but also serve as “ion-buffering reservoirs” to facilitate the transportation of electrolyte ions and to buffer the volume change during the charge/discharge process.26 For the realization of the heterointerface in the Ni0.85Se@MoSe2 nanosheet arrays, a hierarchical structure of the Ni0.85Se@ MoSe2 nanosheet arrays constructed from a Ni framework is developed. The Ni foam is selected because it can be used as a precursor to form a Ni0.85Se matrix, and also as a support framework allowing the MoSe2 nanosheets to vertically grow to form a unique architecture. By this method, the aggregation of MoSe2 nanosheets can be limited, and an efficient and fast interfacial charge transfer can be facilitated as well.21 As a result, high-performance supercapacitors are realized. Moreover, nickel selenide (Ni0.85Se), a promising pseudocapacitor electrode,15,23 is selected as the framework because the crystallographic structure of hexagonal Ni0.85Se possesses the space group (P63/mmc) which matches very well with that of the hexagonal MoSe2.22 On the other hand, in the Ni0.85Se@ MoSe2 heterostructures, the Ni0.85Se matrix has good electrical conductivity which endows the rapid transport of electrons along Ni0.85Se to the MoSe2 nanosheets,15 resulting in high rate capability and excellent cycle stability. Thus, the novel Ni0.85Se@MoSe2 nanosheet arrays are prepared by a facile one-step hydrothermal and in situ growth method. The detailed procedure for preparation of the Ni0.85Se@MoSe2 nanosheet arrays is displayed in Figure S1 [see Supporting Information (SI)].

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Figure 1. SEM images of (a) the MoSe2 nanoparticles, (b) the Ni0.85Se matrix, (c) the Ni0.85Se@MoSe2 nanosheet arrays, and (d) the crosssection of the Ni0.85Se@MoSe2 nanosheet arrays.

is clear that aggregated nanoparticles are observed in bare MoSe2 (Figure 1a). The Ni0.85Se matrix (Figure 1b) possesses an uneven surface with many irregular trigonal pyramidal blocks, whereas the Ni0.85Se@MoSe2 (Figure 1c) samples exhibit uniform aligned ultrathin nanosheets with an intertwined structure. Moreover, a homogeneous specific vertical nanosheet array is indeed observed from the Ni0.85Se@MoSe2, as visualized in the cross-sectional SEM image (Figure 1d). It is also found that MoSe 2 is interconnected and nearly covers the entire Ni0.85Se matrix. All these results demonstrate that the nickel framework can effectively and directly induce the growth of the MoSe2 nanosheets, and then form the Ni0.85Se@MoSe2 nanosheet arrays ultimately. The Ni0.85Se and MoSe2 can be grown simultaneously under the one-step hydrothermal crystal growth system. Meanwhile, the MoSe2 ultrathin nanosheets can be vertically grown onto the NiSe matrix to form the novel Ni0.85Se@MoSe2 heterostructure owing to their exhibiting the same hexagonal symmetry crystallographic structure and inducing the orientation growth of crystals.22,25 Such unique architecture is further confirmed by transmission electron microscopy (TEM) measurments. The results are shown in Figure 2a,b and Figure S2 in SI. The observed folded nanosheet layers in TEM images indicate that the ultrathin as-grown MoSe2 nanosheets are developed. Moreover, two sets of parallel lattice fringes are clearly presented in the high-resolution TEM (HR-TEM) image (Figure 2b). One interplanar distance is 0.28 nm, which is ascribed to the (100) plane of hexagonal MoSe2. Another interplanar distance is 0.52 nm, which is ascribed to the (001) plane of Ni0.85Se. These results demonstrate that the Ni0.85Se@MoSe2 nanosheet arrays with well-defined heterostructure are successfully developed. Figure 2c displays the crystal structure and the phase of the Ni0.85Se matrix and the Ni0.85Se@MoSe2 nanosheet arrays. The strong diffraction peaks at 2θ of 33.6°, 45.2°, 51.0°, 60.3°, and 62.1°, corresponding to the (101), (102), (110), (103), and

EXPERIMENTAL SECTION

Materials. Sodium molybdate (Na2MoO4·2H2O), Se powder (99.5%), hydrazine hydrate (N2H4·H2O, 85%), ethanol (99.5%), hydroiodic acid (HCl, 99.99%), and potassium hydroxide (KOH) were purchased from Sigma-Aldrich and used as received without further purification. Nickel foam (1.6 mm thickness) was purchased from MTI Corporation and further purified via treatment with a 5% HCl solution and ethanol sequentially to remove the oxide layer on the surface before the material is used to fabricate electrode materials. Synthesis of the Ni0.85Se@MoSe2 Nanosheet Arrays. The Ni0.85Se@MoSe2 nanosheet arrays were prepared by a facile one-step hydrothermal method without any surfactants. Certain amounts of Na2MoO4·2H2O (30, 60, 90, and 120 mg) were dissolved into 10 mL of deionized water at room temperature. In a separate flask, 5 mmol of Se powder was dissolved in 2.5 mL of hydrazine hydrate (N2H4·H2O, 85%). Then, the Se solution was slowly added into the sodium molybdate solution at room temperature, followed with stirring for another 30 min to obtain a homogeneous solution. Finally, a piece of pretreated nickel foam (2 × 2 cm2) was immersed in the above prepared aqueous solution and transferred into a 25 mL Teflon-lined stainless-steel autoclave. Then, the autoclave was sealed and maintained at 200 °C for 24 h in an oven. After it cooled to room temperature naturally, the samples were rinsed with deionized water and ethanol and then dried at 60 °C. The samples were prepared with 30, 60, 90, and 120 mg of Na2MoO4·2H2O and represented as Ni0.85Se@MoSe 2-1, Ni0.85Se@MoSe 2-2, Ni0.85Se@MoSe 2-3, and Ni0.85Se@MoSe2-4, respectively. For comparison studies, Ni0.85Se and MoSe2 were also prepared under the same processes but without the addition of sodium molybdate or nickel foam, respectively. The mass of Ni0.85Se is about 54.2 mg/cm2, while the mass of Ni0.85Se@ MoSe2 prepared from different amounts of Na2MoO4·2H2O is about 58.9−60.8 mg/cm2. The mole ratios of Ni/Mo in the Ni0.85Se@MoSe2 series calculated from the mass difference between Ni0.85Se@MoSe2 (Ni0.85Se@MoSe2-1, Ni0.85Se@MoSe2-2, Ni0.85Se@MoSe2-3, and Ni0.85Se@MoSe2-4) and Ni0.85Se are 22.81, 20.14, 18.07, and 16.18, respectively. 17068


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Figure 2. (a) TEM image and (b) HR-TEM (with SAED insert) image of the Ni0.85Se@MoSe2 nanosheet arrays; (c) XRD patterns of the MoSe2 nanoparticles, the Ni0.85Se matrix, and the Ni0.85Se@MoSe2 nanosheet arrays; (d) EDS spectrum of the Ni0.85Se@MoSe2 nanosheet arrays; (e−h) element mapping images of the Ni0.85Se@MoSe2 nanosheet arrays (selected from part e).

Ni0.85Se@MoSe2 nanosheet arrays are further characterized by X-ray photoelectron spectroscopy (XPS). The results are shown in Figure 3. The high-resolution spectra of Mo 3d, Ni 2p, and Se 3d are used to determine the oxidation states of these three elements. Two peaks located at 226.7 and 230.1 eV, corresponding to the Mo 3d5/2 and Mo 3d3/2 of Mo (IV), arise from Mo−Se structures (Figure 3b). However, a peak located at 236 eV, which originates from the Mo 3d3/2 of Mo(VI) components, is probably related to the defects in MoSe2, such as the Mo−O phase.27 The peak located at 852.0 eV with its satellite peaks at 854.5 and 860.1 eV, respectively, can be assigned to Ni 2p3/2, while the peak located at 869.3 eV with its satellite peak at 872.4 eV, can be assigned to Ni 2p1/2 (Figure 3c).22 The Ni 2p spectrum, characteristic of Ni2+ and Ni3+ with two shake-up satellites, can be best fit by two spin−orbit doublets.28 For the Se 3d features (Figure 3d), the peaks located at 52.2 and 53.2 eV correspond to Se 3d5/2 and Se 3d3/2, respectively, indicating a −2 valence of Se, while the peak at 57.1 eV suggests the surface oxidation state of Se species.15 In order to control the Ni0.85Se@MoSe2 nanosheet arrays with ideal morphology to approach high-performance supercapacitors, different concentrations of sodium molybdate solutions are used to tune the MoSe2 nanosheets grown in the Ni0.85Se substrate. Figure 4 shows the microstructure

(201) planes of Ni0.85Se (JCPDS 18-0888), are observed from both the Ni0.85Se matrix and the Ni0.85Se@MoSe2 nanosheet arrays. The peaks at 2θ of 12.7°, 31.9°, 38.2°, and 56.4°, corresponding to the (002), (100), (103), and (110) planes of the hexagonal MoSe2 (JCPDS 87-2419), respectively, are observed from the bare MoSe2 nanoparticles (Figure 2c). The broadened diffraction peaks [especially the (002) plane] observed from bare MoSe2 nanoparticles reveal that the MoSe2 possesses relatively low crystallinity and reduced size of the crystallites in different dimensions.24 Thus, the diffraction peaks of the MoSe2 nanosheets are hard to observe in the Ni0.85Se@MoSe2 nanosheet arrays. The composition of the Ni0.85Se@MoSe2 nanosheet arrays is further confirmed by the energy-dispersive X-ray spectroscopy (EDS, Figure 2d) and the elemental maps of Ni, Mo, and Se (Figure 2e−h). The EDS results confirm that the main components of the Ni0.85Se@MoSe2 nanosheet arrays are Ni, Se, and Mo elements. The cross-section SEM image and the corresponding EDS elemental mapping (selected from the square region) analyses also reveal that Ni and Se are homogeneously distributed in the matrix of the Ni0.85Se@ MoSe2 nanosheet arrays, whereas Mo and Se elements are mainly distributed in the outer shell of the Ni0.85Se@MoSe2 nanosheet arrays. The binding energies of elements in the 17069


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Figure 3. XPS spectra of the Ni0.85Se@MoSe2 nanohybrids: (a) survey spectrum, (b) Mo 3d spectrum, (c) Ni 2p spectrum, and (d) Se 3d spectrum.

Figure 4a). As the concentrations of sodium molybdate are increased to 60 mg (Ni0.85Se@MoSe2-2, Figure 4b) and then to 90 mg (Ni0.85Se@MoSe2-3, Figure 4c), the uniform and intertwined ultrathin MoSe2 nanosheets with specific perpendicularly oriented structure are developed. Moreover, the entire Ni0.85Se substrate is fully covered by the MoSe2 nanosheets. However, with the further increase of the concentration of sodium molybdate, nonuniform and collapsed MoSe2 nanosheets without vertical aligned structure are observed (Ni0.85Se@MoSe2-4, Figure 4d). Thus, the Ni0.85Se@MoSe2 nanosheet arrays with vertically aligned MoSe2 nanosheets can be developed by using the proper concentration of sodium molybdate. The electrochemical properties of the Ni0.85Se matrix, bare MoSe2 nanoparticles, and the Ni0.85Se@MoSe2 nanosheet arrays are first evaluated by using them as the workingelectrodes in the supercapacitors with a three-electrode system through cycling voltage (CV) and galvanostatic charge− discharge (GCD) measurements. Figure 5a shows the CV curves of these electrodes at a scan rate of 30 mV s−1 in the potential window −0.2 to ∼0.8 V, with the Hg/HgO electrode as the reference. The Ni0.85Se electrode exhibits a pair of redox peaks, indicating typical Faradaic pseudocapacitance behavior. The redox peaks corresponding to the reversible redox reaction could be expressed as15 Ni0.85Se + OH− ↔ Ni0.85SeOH + e−. One pair of redox peaks observed from the MoSe2 electrode indicates it possesses pseudocapacitive characteristics. The pseudocapacitive reaction could involve electrochemical charge transfer which combines with the insertion and deinsertion of alkaline ions (could be expressed as20 MoSe2 + K+ + e− ↔ MoSe2−K+) and is faradically stored in the electroactive sites of

Figure 4. SEM images of the Ni0.85Se@MoSe2 nanosheet arrays prepared from different concentrations of sodium molybdate: (a) Ni0.85Se@MoSe2-1, (b) Ni0.85Se@MoSe2-2, (c) Ni0.85Se@MoSe2-3, and (d) Ni0.85Se@MoSe2-4 nanosheet arrays.

morphologies of the Ni0.85Se@MoSe2 nanosheet arrays. The Ni0.85Se@MoSe2 nanosheet arrays prepared from sodium molybdate with the concentrations of 30, 60, 90, and 120 mg are represented as Ni0.85Se@MoSe2-1, Ni0.85Se@MoSe2-2, Ni0.85Se@MoSe2-3, and Ni0.85Se@MoSe2-4, respectively. The SEM images shown in Figure 4 indicate that only a few scattered MoSe2 nanosheets are grown on the bare Ni0.85Se substrate as sodium molybdate is 30 mg (Ni0.85Se@MoSe2-1, 17070


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Figure 5. (a) CV curves of the Ni0.85Se, MoSe2, and Ni0.85Se@MoSe2 electrodes in 2 M KOH electrolyte. (b) The CV curves of the Ni0.85Se@ MoSe2-2 electrode at different scan rates. (c) The GCD curves of the Ni0.85Se@MoSe2-2 electrode at various current densities. (d) Specific capacitances of different electrodes at various current densities.

MoSe2.29,30 However, the peak potential and CV curve area of the MoSe2 electrode are smaller than those of the Ni0.85Se electrode and the Ni0.85Se@MoSe2 electrodes. In addition, as the scanning voltage is greater than 0.6 V, the current increased dramatically; this probably originated from the oxygen evolution. All these results demonstrate that the MoSe2 electrode has a relatively low working voltage window and specific capacitance in the KOH electrolyte, whereas the shapes of the CV curves of the Ni0.85Se@MoSe2 electrodes prepared from different concentrations of sodium molybdate are similar to those observed from the Ni0.85Se electrode. However, the oxidation peak potential of the Ni0.85Se@MoSe2 nanosheet array electrode is shifted toward positive, while the reduction peak potential of the Ni0.85Se@MoSe2 nanosheet array electrode is shifted toward negative, as compared with those of the Ni0.85Se matrix electrode. Such shifts are mainly due to the lower conductivity of the Ni0.85Se@MoSe2 nanosheet arrays compared with that of the Ni0.85Se matrix electrode, resulting in the lower reversibility of redox processes of the Ni0.85Se@ MoSe2 nanosheet arrays. Besides, it is observed that the areas of the CV curves of the Ni0.85Se@MoSe2 electrodes are significantly greater than those of the Ni0.85Se electrode and are larger than that of the MoSe2 electrode as well. Among the Ni0.85Se@MoSe2-1, Ni0.85Se@MoSe2-2, Ni0.85Se@MoSe2-3, and Ni0.85Se@MoSe2-4 nanosheet arrays, the Ni0.85Se@MoSe2-2 nanosheet arrays exhibit the largest CV curve area, which is due to its uniform perpendicularly oriented structure as shown in Figure 4. Figure 5b presents the CV curves of the Ni0.85Se@ MoSe2 electrode at different scan rates. Both oxidation and

reduction peaks are present at the high scan rate of 50 mV/s, which indicates that the Ni0.85Se@MoSe2 nanosheet arrays possess very good electrochemical properties, owing to the unique open paths for ion diffusion provided by the perpendicularly oriented structure and increased interface contact between the electrode and the electrolyte. GCD measurement is carried out to further evaluate the electrochemical capacitance of the Ni0.85Se@MoSe2 nanosheet arrays. The electrochemical capacitances of the Ni0.85Se@ MoSe2 (Ni0.85Se@MoSe2-1, Ni0.85Se@MoSe2-2, Ni0.85Se@ MoSe2-3, Ni0.85Se@MoSe2-4) nanosheet arrays and the Ni0.85Se matrix are shown in Figure S3. Multiple charge and discharge platforms, corresponding to the redox reactions, indicate both the Ni0.85Se@MoSe2 nanosheet arrays and the Ni0.85Se matrix possess excellent pseudocapacitive characteristics. However, the Ni0.85Se@MoSe2-2 nanosheet arrays have the longest charge and discharge time, which is consistent with the CV measurements (Figure 5a). These results demonstrate that the Ni0.85Se@MoSe2-2 electrode has the largest specific capacitance value. It is also found that the discharge curves and the corresponding charge curves of above electrodes are nearly symmetric at different current densities (Figure 5c), which indicates that the Ni0.85Se@MoSe2 nanosheet arrays possess excellent reversible redox reaction behavior. Figure 5d presents the specific capacitances at different current densities for these different electrodes. The specific capacitance is estimated on the basis of GCD measurements (the details are described in SI). As expected, the Ni0.85Se@MoSe2-2 nanosheet arrays have the highest specific capacitance among all electrodes. The 17071


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Figure 6. (a) Schematic illustration of the Ni0.85Se@MoSe2//GNS ASC. (b) Comparative CV curves of the Ni0.85Se@MoSe2 nanosheets and GNS, tested in a three-electrode configuration at 30 mV s−1. (c) The CV curves of Ni0.85Se@MoSe2//GNS ASC with various potential ranges at 30 mV s−1. (d) The CV curves of the Ni0.85Se@MoSe2//GNS ASC at different scan rates. (e) The galvanostatic charge/discharge curves of the Ni0.85Se@ MoSe2//GNS ASC at various current densities. (f) The Ragone plots related to energy and power densities of the Ni0.85Se@MoSe2//GNS ASC.

only act as the framework for growing MoSe2 nanosheets, but also improve the electrical conductivities of the Ni0.85Se@ MoSe2 nanosheet arrays, resulting in fast electron transport from the Ni0.85Se matrix to the MoSe2 nanosheets. In addition, the Ni0.85Se@MoSe2-2 nanosheet array electrode possesses about 95% retention of the initial specific capacitance after 1000 cycles (Figure S5). It is also found that the Ni0.85Se@MoSe2 nanosheet array electrode possesses better cycle stability than that of the Ni0.85Se matrix (Figure S5), which further confirms that the heterostructure formed in the Ni0.85Se@MoSe2 nanosheet arrays can buffer the volume change of the hierarchical materials during the charge/discharge processes. Table S1 in SI summarizes the electrochemical performance of the Ni0.85Se@MoSe2 nanosheet arrays and other metal chalcogenides as the electrodes for supercapacitors. On the basis of the electrochemical performance parameters listed in Table S1, we conclude that the Ni0.85Se@MoSe2 nanosheet arrays with hierarchical structure are good positive electrode for supercapacitors.

specific capacitances of the Ni0.85Se@MoSe2-1, Ni0.85Se@ MoSe2-2, Ni0.85Se@MoSe2-3, and Ni0.85Se@MoSe2-4 nanosheet arrays; the Ni0.85Se matrix; and the MoSe2 nanoparticles are 578, 774, 684, 627, 401, and 113 F g−1 at a current density of 1 A g−1, respectively. Moreover, the specific capacitance of the Ni0.85Se@MoSe2-2 nanosheet arrays can remain 489 F g−1 (about 63% capacitance retention) at a high current density of 15 A g−1. The Nyquist plots of the Ni0.85Se, MoSe2, and Ni0.85Se@ MoSe2 electrodes are depicted in Figure S4. The Ni0.85Se has the lowest charge transfer resistance (RCT) among these electrodes. The RCT values of the Ni0.85Se@MoSe2 (Ni0.85Se@ MoSe 2 -1, Ni 0.85 Se@MoSe 2 -2, Ni 0.85 Se@MoSe 2 -3, and Ni0.85Se@MoSe2-4) electrodes are higher than that of Ni0.85Se electrode, but lower than that of MoSe2 electrodes. The RCT values of the Ni0.85Se@MoSe2 (Ni0.85Se@MoSe2-1, Ni0.85Se@ MoSe2-2, Ni0.85Se@MoSe2-3, Ni0.85Se@MoSe2-4) electrodes increase along with increased sodium molybdate precursor contents. These results further indicate that the Ni0.85Se can not 17072


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high-frequency region and a sloping line in the low-frequency region are clearly observed from the Nyquist plot, indicating a low RCT (1.31 Ω) and an ideal capacitive behavior, respectively. The internal series resistance (RS) of 1.34 Ω, which is a combination resistance that includes that of the electrolyte, intrinsic resistance of the electrode materials, and the contact resistance between the current collectors and the electrodes, is obtained from the intercept of the real axis (Z′).40 The obtained low resistance values of RS and RCT reveal that the Ni0.85Se@MoSe2//GNS ASC possesses fast ion diffusion ability and low internal resistance, which is mainly responsible for the improved capacitance performance. The transition from the semicircle to the long tail of the vertical curve in the middle frequencies is the Warburg impedance (ZW), which is a result of the frequency dependence of ion diffusion/transport in the electrolyte to the electrode surface. Moreover, the limit capacitance (CL) and constant phase element (CPE) in an equivalent circuit are usually applied to describe an imperfect capacitor and an equivalent electrical circuit component, respectively.41 The long-term cycling performance of the Ni0.85Se@MoSe2//GNS ASC is evaluated in the 1.6 V voltage windows at a current density of 10 A g−1, as shown in Figure S10. It is found that there is no obvious loss for the Ni0.85Se@ MoSe2//GNS ASC from 500 to 5000 cycles, and the Ni0.85Se@ MoSe2//GNS ASC can retain about 88% of the initial capacitance after 5000 charge−discharge cycles. All of these results demonstrate that the Ni0.85Se@MoSe2//GNS ASC possesses good stability.

Fabrication of asymmetric supercapacitors (ASCs) is one of the effective ways to obtain large operating voltage (V),31 leading to the high energy density (E),1 which is used to evaluate device performance of supercapacitors.31 Toward the end, we fabricate an ASC by using the Ni0.85Se@MoSe2 nanosheet arrays as the positive electrode and highly conductive graphene nanosheets (GNS, Figures S6 and S7) as the negative electrode. The schematic layout of Ni0.85Se@ MoSe2//GNS ASC is shown in Figure 6a. Figure 6b presents comparative CV curves of the Ni0.85Se@MoSe2 nanosheet arrays and GNS nanosheets, tested in a three-electrode configuration at a scan rate of 30 mV s−1. It is observed that the Ni0.85Se@MoSe2 electrode exhibits a potential window of −0.2 to ∼0.8 V, and the GNS electrode possesses a potential window of −1.0 to ∼0 V, respectively. Thus, an operating voltage of 1.8 V is expected from an ASC with the Ni0.85Se@ MoSe2 nanosheet arrays as the positive electrode and the GNS as the negative electrode, since the total two-electrode cell voltage is the sum of the potentials of the positive electrode and the negative electrode.32 Figure 6c shows the CV curves of the Ni0.85Se@MoSe2// GNS ASC in 2 M KOH electrolyte, measured in different voltage windows. It is clear that the ASC exhibits stable electrochemical windows at a voltage less than 1.6 V, indicating that ASC possesses ideal capacitive behavior and good reversibility. However, as the operating voltage extends larger than 1.7 V, the current increases sharply at high potentials, which probably originated from the electrolyte decomposition by oxygen evolution.33 Therefore, the operating voltage range of 0−1.6 V is selected to evaluate the electrochemical performance of the Ni0.85Se@MoSe2//GNS ASC. The CV curves of the Ni0.85Se@MoSe2//GNS ASC at different scan rates from 5 to 50 mV s−1 are displayed in Figure 6d. The CV curve areas increase along with the increasing scan rates, maintaining the original shape and redox peaks even at a high scan rate of 50 mV s−1, which indicates rapid transportation of electrolyte ions and typical reversible Faradaic pseudocapacitive behavior in the system. However, the CV curves of Ni0.85Se@ MoSe2//GNS ASC showed little capacitive behavior in the low potential region (0−0.7 V), which may be due to the low capacitive behavior of the Ni0.85Se@MoSe2 material at low potential, and the asymmetric electrode polarization behavior occurred from different positive and negative electrode materials.33 The nearly symmetric galvanostatic charge/ discharge curves of the Ni0.85Se@MoSe2//GNS ASC at various current densities (Figure 6e) demonstrate that the Ni0.85Se@ MoSe2//GNS ASC has a highly reversible electrochemical response. The corresponding specific capacitances of the Ni0.85Se@MoSe2//GNS ASC at various current densities are displayed in Figure S8. Figure 6f shows a Ragone plot (the energy density versus the power density) of the Ni0.85Se@MoSe2//GNS ASC, which is obtained from GCD data (Figure 6e). The Ni0.85Se@MoSe2// GNS ASC exhibits an energy density of 25.5 Wh kg−1 at a power density of 420 W kg−1 and an energy density of 19.3 Wh kg−1 at a power density of 3900 W kg−1. Such energy density is higher than those of recently reported aqueous ASCs based on metal compounds and carbon material electrodes (Table S2 in SI).31,34−39 The electrochemical impedance spectroscopy (EIS) is carried out to further understand the Ni0.85Se@MoSe2//GNS ASC. Figure S9 presents the Nyquist plot and the corresponding equivalent circuit from the EIS data. The small semicircle in the

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CONCLUSIONS In summary, we reported novel Ni0.85Se@MoSe2 nanosheet arrays prepared by a facile one-step hydrothermal method using nickel foam as both Ni precursor and the framework to grow MoSe2 from the Se powder in the presence of sodium molybdate solution. In light of the unique interconnected aligned ultrathin nanosheet array architecture, the Ni0.85Se@ MoSe2 nanosheet arrays exhibit a high specific capacitance of 774 F g−1 at a current density of 1 A g−1 and a high rate ability. Moreover, an asymmetric supercapacitor (ASC) was further successfully fabricated by using the Ni0.85Se@MoSe2 nanosheet arrays as the positive electrode and the graphene nanosheets (GNS) as the negative electrode in a 2 M KOH electrolyte. The novel Ni0.85Se@MoSe2//GNS ASC exhibits a wide operating voltage of 1.6 V and high energy density (25.5 Wh kg−1 at 420 W kg−1 and 19.3 Wh kg−1 at 3900 W kg−1), as well as good cycling stability. These results indicate that the Ni0.85Se@ MoSe2 nanosheet arrays are good candidate electrodes for supercapacitor applications.

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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/acsami.7b02776. Experimental details, electrode fabrication, electrochemical measurements, Figures S1−S10, and Tables S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 17073


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ACS Applied Materials & Interfaces ORCID

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Ziqiang Lei: 0000-0001-9195-4472 Xiong Gong: 0000-0001-6525-3824 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors acknowledge the National Science Foundation (EECS 1351785) and Air Force Scientific Research (FA955015-1-0292) for financial support. H.P. would like to acknowledge the China Scholarship Council for the Joint Ph. D. program (201508620141).

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