High-Performance Asymmetric Supercapacitor Designed with a Novel

Jun 2, 2017 - Herein, a novel asymmetric supercapacitor (ASC) with high energy density is fabricated based on the NiSe@MoSe2 nanosheet arrays and the ...
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Research Article pubs.acs.org/journal/ascecg

High-Performance Asymmetric Supercapacitor Designed with a Novel NiSe@MoSe2 Nanosheet Array and Nitrogen-Doped Carbon Nanosheet Hui Peng,† Jiezi Zhou,† Kanjun Sun,‡ Guofu Ma,*,† Zhiguo Zhang,† Enke Feng,† and Ziqiang Lei*,† †

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, 967 Anning East Road, Lanzhou 730070, China ‡ College of Chemistry and Environmental Science, Lanzhou City University, 11 Jiefang Road, Lanzhou, 730070, China S Supporting Information *

ABSTRACT: Herein, a novel asymmetric supercapacitor (ASC) with high energy density is fabricated based on the NiSe@ MoSe2 nanosheet arrays and the nitrogen-doped pomelo mesocarps-based carbon nanosheet (N-PMCN) as a positive electrode and a negative electrode, respectively. The novel NiSe@MoSe2 nanosheet arrays are designed and prepared by a facile one-step hydrothermal growth method from nickel foam as a nickel precursor and nucleation framework. The N-PMCN is prepared using simultaneous CaCl2 activation and urea nitrogen-doped processes from thepomelo mesocarps as a biomass-based carbon precursor. Because of the unique nanosheet array architecture of NiSe@MoSe2 and interconnected sheet-like porous morphology with high nitrogen content (∼9 wt %) of N-PMCN, they exhibit a maximum specific capacity of 128.2 mAh g−1 and high specific capacitance of 223 F g−1 at a current density of 1 A g−1. Moreover, the assembled novel NiSe@MoSe2//N-PMCN ASC device with a maximum operating voltage of 1.65 V has demonstrated a high energy density of 32.6 Wh kg−1 at a power density of 415 W kg−1 and outstanding cycling stability with 91.4% capacitance retention after 5000 cycles in aqueous electrolyte. KEYWORDS: Asymmetric supercapacitors, Molybdenum selenide, Nickel selenide, Carbon nanosheet

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(for transition metal compounds).5,7−9 In general, a large operating voltage can be achieved by selecting organic electrolytes since their stability window is up to 2.5−3.0 V.10 Moreover, developing aqueous asymmetric supercapacitors (ASCs) is an effective alternative approach to boost the operating voltage of supercapacitors, because the organic electrolytes are usually lower conductivity, more expensive, and more flammable than aqueous electrolytes.5,11 ASCs are usually combined with pseudocapacitive (faradaic battery-type) positive materials and porous carbon (capacitive-type) negative materials, which could utilize their different potential windows of positive and negative electrodes in the same aqueous electrolytes.12−15 As a

INTRODUCTION

Supercapacitors have attracted great interest as one of the most promising energy storage devices to alleviate environmental and energy challenges due to their unique properties including high power density, long cycling lifetime, and high safety performance.1−3 Unfortunately, they suffer from a lower specific energy than that of lithium-ion batteries and fuel cells, which has greatly restricted their industrial applications.4,5 The energy density (E) usually relies on device capacitance (C) and operating voltage (V) according to the general formula E = 1/2CV2. Therefore, to enhance the E of supercapacitors while retaining their intrinsic high power density, it should be crucial to improve both C and V.6 As we all know, the C mainly relies on the inherent properties of electrode materials, including superior electrical conductivity, high chemical stability, unique nanostructure architecture, and large specific surface area, and possess multiple oxidation states © 2017 American Chemical Society

Received: March 8, 2017 Revised: May 31, 2017 Published: June 2, 2017 5951

DOI: 10.1021/acssuschemeng.7b00729 ACS Sustainable Chem. Eng. 2017, 5, 5951−5963

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nanosheet array structures can provide a large number of exposed reaction active sites and utilize continuous transport pathways in a uniform vertical direction to facilitate electrolyte ion diffusion, thus obtaining a large capacitance and high rate capability. For the negative electrode, the N-PMCN with interconnected 2D nanosheet architectures and a large specific surface area as well as high nitrogen doping level would facilitate a fast charge transport and lead to remarkable electrochemical energy storage characteristics. On the basis of the complementary operating potential window between NiSe@MoSe2 and N-PMCN, the as-fabricated ASC device (NiSe@MoSe2//NPMCN) can operate stably at 1.65 V with a high energy density of 32.6 Wh kg−1 at a power density of 415 W kg−1. Moreover, the NiSe@MoSe2//N-PMCN ASC device also possesses an outstanding cycling stability of 91.4% capacity retention after 5000 cycles.

consequence, ASCs not only have a wide operating voltage but also possess the advantages of both advanced batteries (high energy density) and supercapacitors (high power density and long cycling lifetime). In order to improve the electrochemical performance of ASCs, considerable efforts are devoted to the design of novel positive and negative electrode materials with considerable electrochemical performances.5 Recently, two-dimensional (2D) transition metal chalcogenides have emerged as a new promising class of faradaic battery-type electrode materials due to their earth-abundant nature and unique physicochemical properties, including metallic-like conductivity, multiply available oxidation states, and special sheet-like architectures.16−18 Some metal chalcogenides, including MoS2, NiS, GeSe2, and Co0.85Se with unique 2D hierarchical nanostructures have been investigated as novel positive electrode materials for supercapacitors.16,19−21 Similarly, MoSe2 is a typical transition metal selenide, and its properties are also similar to those of MoS2, both possessing graphene-like nanosheet structures. The Mo ions deliver an alteration of oxidation states from +2 to +6, rendering additional pseudocapacitance ability.18 However, studies are rare of the application of isolated MoSe2 as an active material individually for supercapacitors since they are easily agglomerated because of van der Waals forces.22 Theoretically, fabrication of a new hybrid system via vertical growth of MoSe2 nanosheets onto the conductive matrix to form composite materials with unique heterointerfaces is a feasible way to prevent the aggregation of nanosheets and facilitate a fast and efficient interfacial charge transfer, which is critical to improving the supercapacitive performance.23 Another typical transition metal selenide, nickel selenide, is also an especially promising pseudocapacitor electrode material candidate, owing to the good electrical conductivity, variety of oxidation states, and high theoretical specific capacitance.24,25 In fact, the crystallographic structure of hexagonal nickel selenide matches well with that of the hexagonal MoSe2 because these two transition metal selenides have the same space group (P63/mmc); thus they can provide superior candidates for the construction of a new hybrid system.23 Therefore, constructing the nickel selenide/MoSe2 nanosheetbased hybrid material is promising for the creation of highperformance positive electrode material for supercapacitors. Correspondingly, carbonaceous materials with special architecture (e.g., 2D nanosheets and 3D nanoporous network) are investigated extensively as desirable negative electrode materials due to their high electrical conductivity, large specific surface area, and superior electrochemical stability.26,27 Recently, 2D carbon-based nanomaterials, especially nitrogen-doped biomassbased porous carbon nanosheets, have attracted attention as active materials for high-performance supercapacitors, because they not only provide most features of conventional carbon nanomaterials but also possess a unique 2D interconnection sheet-like porous morphology and high nitrogen content to increase the surface wettability and electrical conductivity of carbon electrode materials.27−29 In this work, we aim at designing advanced electrode materials with high energy storage properties to develop novel aqueous asymmetric supercapacitors (ASCs) with large output voltage and the capability to deliver high energy density. A novel ASC device is designed and fabricated by using a NiSe@MoSe2 nanosheet array as a positive electrode and a nitrogen-doped pomelo mesocarps-based carbon nanosheet (N-PMCN) as a negative electrode in a safe KOH aqueous electrolyte. For the positive electrode, NiSe@MoSe2 with unique hierarchical

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EXPERIMENTAL SECTION

Materials. Sodium molybdate (Na2MoO4·2H2O) was purchased from Tianjing Chemical Co., Ltd., China. Selenium powder (Se, 99.9%), hydrazine hydrate (N2H4·H2O, 85%), and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Calcium chloride (CaCl2) and urea ((NH2)2CO) were purchased from Aladdin Ltd., Shanghai, China. Pomelo mesocarps (the pomelo was grown in the Guangdong province, south China region) were treated with dilute HCl (0.2 M) solution and washed with deionized water subsequently and then dried and smashed before the experiment. Nickel foam was purchased from MTI Corporation and further purification treated with a 5% HCl solution and ethanol sequentially to remove the oxide layer on the surface before used. Synthesis of NiSe@MoSe2 Nanosheet Arrays. The NiSe@ MoSe2 nanosheet arrays were prepared using a facile hydrothermal method without any surfactants. In a typical synthesis, 0.4 g of Se powder was dissolved in 2 mL of N2H4·H2O under violent stirring. In a separate flask, 0.06 g of Na2MoO4·2H2O was dissolved into 10 mL of deionized water at room temperature. Then, the Se solution was slowly added dropwise into the Na2MoO4·2H2O solution at room temperature, and the resulting homogeneous solution was then transferred into a 25 mL Teflon-lined stainless-steel autoclave. After that, a piece of pretreated Ni foam (2 cm × 2 cm, ∼ 0.16 g) was immersed into the mixed solution. The autoclave was sealed and maintained at 200 °C for 24 h. After cooling to room temperature naturally, the sample was thoroughly rinsed with deionized water and ethanol and finally dried in a vacuum oven at 60 °C. For comparison purposes, NiSe and MoSe2 were also prepared under the same process with an equivalent of Se precursor but without adding Na2MoO4·2H2O or Ni foam, respectively. The other preparation conditions are the same as those of the NiSe@MoSe2 nanosheet arrays. Preparation of Nitrogen-Doped Pomelo Mesocarp-Based Carbon Nanosheet (N-PMCN). The N-PMCN was synthesized according to our previous reported work with slight modifications.29 Briefly, 2.0 g of the pretreated pomelo mesocarps, 2.0 g of urea, and 2.0 g of CaCl2 were mixed in 30 mL of ethanol solution, and then the solvent was evaporated in an air-dried oven at 80 °C to form solid block carbon precursor mixtures. Finally, the as-obtained carbon precursor was activated and carbonized at 800 °C for 2 h at a heating rate of 5 °C min−1 in a slow N2 flow. The resulting sample was then repeatedly stirred and washed with 2 M HCl to remove the inorganic salts, and then the sample was filtered and thoroughly washed by distilled water and dried in an oven at 60 °C.

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RESULTS AND DISCUSSION Characterizations of NiSe@MoSe2 Nanosheet Array Positive Electrode Materials. The novel NiSe@MoSe2 nanosheet arrays are designed and prepared via a simultaneous one-step hydrothermal growth method using nickel foam as nickel precursor and nucleation framework and selenium powder 5952

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Figure 1. Schematic illustration of the formation of NiSe@MoSe2 nanosheet arrays.

Figure 2. (a) XRD patterns of the NiSe, MoSe2, and NiSe@MoSe2 samples. FE-SEM images of (b) NiSe, (c) MoSe2, and (d,e) NiSe@MoSe2 nanosheet arrays. (f) Cross-section image of NiSe@MoSe2 nanosheet arrays.

as a Se precursor in the presence of sodium molybdate solution. The fabrication process of the NiSe@MoSe2 heterostructures is schematically illustrated in Figure 1. The NiSe@MoSe 2

heterostructures are designed based on the following considerations: (1) The nickel foam can work as a nickel precursor for the synthesis of NiSe and as a support nucleation framework for 5953

DOI: 10.1021/acssuschemeng.7b00729 ACS Sustainable Chem. Eng. 2017, 5, 5951−5963

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Figure 3. (a,b) TEM images of the MoSe2 nanosheet from the NiSe@MoSe2 nanosheet arrays. (c) Element mapping images of NiSe@MoSe2 nanosheet arrays. (d) EDS spectrum of NiSe@MoSe2 nanosheet arrays. (e−g) The distribution images of various elements of NiSe@MoSe2 nanosheet arrays (selected from the square region of part c).

MoSe2 heterostructures are only composed of NiSe and MoSe2 nanomaterials. The morphologies of the samples were characterized by field emission scanning electron microscopy (FE-SEM). As shown in Figure 2b, the as-grown NiSe shows a lumpy surface with many irregular trigonal pyramidal blocks. One can see that the assynthesized MoSe2 (Figure 2c) exhibits a flowerlike architecture with intertwined nanosheet-like subunits. More interestingly, it is obvious that the NiSe@MoSe2 heterostructures (Figure 2d,e) show large-area and uniform aligned ultrathin nanosheets with an intertwined structure. Furthermore, the cross-sectional SEM image of as-grown NiSe@MoSe2 further displayed the homogeneous vertical nanosheet array structure (Figure 2f). In particular, the uniform vertical MoSe2 nanosheet arrays are almost fully covered and anchored on the NiSe support matrix to form integrated NiSe@MoSe2 heterostructures. Even upon repeated washing and ultrasound processing, the MoSe2 nanosheet arrays are well-anchored on the NiSe support matrix, suggesting the strong combination between MoSe2 and NiSe, which may be due to the good compatibility and matchability between the crystal structure of hexagonal NiSe and MoSe2 since they have the same space group.23 The results also reveal that the nickel foam or nickel compound framework can directly and effectively induce the growth of MoSe2 nanosheet arrays and ultimately form a unique core−shell structure of NiSe@MoSe2 nanosheet arrays. Such novel heterostructure architecture is a

the crystal growth of MoSe2 nanosheets. (2) NiSe and MoSe2 can be simultaneously formed and grown in the same reaction system, promoting the formation of stable and integrated heterostructures. (3) The MoSe2 nanosheets grew and integrated on the surface of the NiSe support matrix to form the unique NiSe@MoSe2 heterostructures. The crystalline phases of the as-prepared NiSe, MoSe2, and NiSe@MoSe2 nanosheet arrays were first investigated by X-ray diffraction (XRD), as shown in Figure 2a. By comparing the XRD pattern of NiSe and standard pattern reference data, all of the diffraction peaks in the NiSe matrix can be indexed to the hexagonal NiSe (JCPDS card no. 65-9451). For MoSe2 material, the diffraction peaks at 2θ = 12.7°, 31.9°, 38.2°, and 56.4°, corresponding to the hexagonal MoSe2 (JCPDS card no. 872419), for the (002), (100), (103), and (110) planes, respectively. For the characteristic reflections from NiSe@ MoSe2, the strong diffraction peaks of NiSe at 2θ = 32.7°, 43.9°, 49.5°, 58.9°, and 60.6°, corresponding to the (101), (102), (110), (103), and (201) planes, respectively, also can be observed. However, we could just observe the weak diffraction peak reflections of MoSe2 in the NiSe@MoSe2 heterostructures, which may be due to the relatively low crystallinity and the reduced size of the crystallites in different dimensions of MoSe2.30 In addition to the diffraction peak reflections of NiSe and MoSe2 in the NiSe@MoSe2, there is no extra peak of impurity that can be detected, which confirms that the NiSe@ 5954

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Figure 4. (a) CV curves of MoSe2, NiSe, and NiSe@MoSe2 in 2 M KOH electrolyte. (b) CV curves of NiSe@MoSe2 at different scan rates. (c) GCD curves of NiSe@MoSe2 at different current densities. (d) Specific capacities of different electrodes at different current densities. (e) Nyquist plots of the NiSe, MoSe2, and NiSe@MoSe2 electrodes. (f) Cycling stability of the NiSe, MoSe2, and NiSe@MoSe2 electrodes at 5 A g−1.

order to further understand the heterostructure composition of NiSe@MoSe2 nanosheet arrays, energy-dispersive X-ray spectroscopy (EDS) elemental mapping was conducted to reveal the actual elements’ distribution between the MoSe2 nanosheets and NiSe matrix (Figure 3c−g). The corresponding EDS color map results confirm that the composition of NiSe@MoSe2 nanosheet arrays mainly consists of Ni, Se, and Mo elements (Figure 3d). The cross-section SEM image (Figure 3c) and corresponding EDS elemental mapping (in the square region of Figure 3c) analyses also reveal the homogeneous distribution of Ni and Se elements in the matrix of NiSe@MoSe2, while Mo and Se elements are mainly distributed in the matrix surface of NiSe@ MoSe2 (Figure 3e−g). The results further confirmed that the MoSe2 nanosheets and NiSe matrix are grown simultaneously from a nickel foam framework.

feasible way to prevent the aggregation of MoSe2 nanosheets and potentially increase the electronic or ion transport path to facilitate the electron transfer between the nanosheet edges and the conductive substrate. Transmission electron microscopy (TEM) was further employed to investigate the structure of NiSe@MoSe2 nanosheet arrays; the results are shown in Figure 3a,b. The TEM images clearly show the intertwined and highly interconnected network morphology of the MoSe2 nanosheet from the NiSe@MoSe2 nanosheet arrays. The obvious folded nanosheet layers indicate the ultrathin nature of the as-grown MoSe2 nanosheets. Some parallel folded textures of MoSe2 can be observed in the highresolution TEM (HRTEM) image (Figure 3b), corresponding to the layers of MoSe2 nanosheets. Furthermore, an interplanar distance of 0.68 nm is distinctly observed (inset in Figure 3b), which agrees well with the (002) plane of hexagonal MoSe2. In 5955

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The GCD curves of the NiSe@MoSe2 nanosheet array electrode were collected between 0 and 0.6 V (vs Hg/HgO) at various specific current densities from 1 to 15 A g−1 (Figure 4c). The charge curves are almost symmetrical to their discharge curves and possess a couple of charging and discharging platforms which correspond to the reflection of oxidation and reduction processes, respectively, indicating typical redox reactions of the material, and these results were consistent with the electrochemical behavior of CV curves. The corresponding discharge capacities at various current densities were calculated from GCD curves (Figure 4c), and the results are shown in Figure 4d. Encouragingly, the NiSe@MoSe2 nanosheet array electrode exhibits an excellent specific capacity of 128.2 mA h g−1 at current densities of 1 A g−1, which is much higher than those of the NiSe (85.2 mAh g−1) and MoSe2 (20.7 mAh g−1) counterparts under the same current density. With the increase of current densities from 1 to 15 A g−1, the NiSe@MoSe2 electrode can still maintain a specific capacity as high as 97.8 mAh g−1, which is about 76% capacity retention, and its value is also comparable to the NiSe matrix electrode (59%). The electrochemical impedance spectroscopy (EIS) spectra of the NiSe, MoSe2, and NiSe@MoSe2 electrodes are depicted in Figure 4e. It could be observed obviously that the NiSe electrode exhibited a smaller charge transfer resistance (Rct) than those of the MoSe2 and NiSe@MoSe2 electrodes. However, the Rct of the NiSe@MoSe2 electrode is obviously lower than that of the MoSe2 electrodes. These results further indicate that the NiSe not only acts as the framework for growing MoSe2 nanosheets but also improves the electrical conductivities of the NiSe@ MoSe2 nanosheet arrays, resulting in fast electron transport from the NiSe matrix to the MoSe2 nanosheets. Moreover, the NiSe@ MoSe2 nanosheet array electrode possesses about 93.7% specific capacity retention after 1000 cycles at a current density of 5 A g−1 (Figure 4f); this cycling stability is significantly better than that of the NiSe electrode (72.9%). The result further confirms that the NiSe@MoSe2 nanosheet arrays with the aligned ultrathin nanosheets can facilitate electrolyte ion diffusion and effectively buffer the volume change during the charge/discharge processes. Table S1 summarizes the electrochemical performance of the NiSe@MoSe2 nanosheet arrays and the typical metal chalcogenides electrodes for supercapacitors. The above results reveal that NiSe@MoSe2 nanosheet arrays exhibit high specific capacity, high rate capability, and excellent cycling stability. The much enhanced electrochemical properties of as-synthesized NiSe@MoSe2 material could be attributed largely to the following factors: (i) The unique vertical nanosheet array structure can prevent the aggregation of MoSe2 nanosheets and provide a path for the ions’ insertion/extraction within the NiSe matrix, which may offer a large number of exposed reaction active sites and transportation channels, improving the specific capacity significantly.22 (ii) The rich void space between interlaced MoSe2 nanosheet arrays provides an open space for easy electrolyte diffusion and can also serve as an “electrolyte reservoir” to promote the transport of electrolyte ions during the charge/discharge process, resulting in high rate capability. Therefore, constructing the NiSe@MoSe2 heterostructures is promising for the creation of alternative positive electrode material for supercapacitors. Characterizations of N-PMCN Negative Electrode Materials. The nitrogen-doped pomelo mesocarp-based carbon nanosheet (N-PMCN) is prepared by simultaneous CaCl2 activation and urea nitrogen-doped treatment using green biowaste (pomelo mesocarps) as a carbon precursor. The N-

The surface chemical bonding state and compositions of the NiSe@MoSe2 nanosheet arrays are further characterized by Xray photoelectron spectroscopy (XPS), as shown in Figure S1 in the Supporting Information (SI). The characteristic peaks of Ni, Mo, and Se elements were observed in the survey scan spectrum (Figure S1a). The high resolution spectra of Mo 3d, Ni 2p, and Se 3d are used to determine their oxidation states. The high resolution XPS spectra of Ni 2p (Figure S1b) showed two major peaks at binding energy values of 852.4 and 869.8 eV which belong to the spin−orbit splitting of Ni 2p3/2 and Ni 2p1/2 levels, respectively. These two spin−orbit splittings near Ni 2p3/2 and Ni 2p1/2, along with both shakeup satellites, designate that the Ni species were in the +2 oxidation state.31 The high resolution XPS spectra of Mo 3d (Figure S1c) showed two peaks located at 226.5 and 229.8 eV arising from Mo−Se structures, which can be assigned to the Mo 3d5/2 and Mo 3d3/2 of Mo(IV). In addition, the binding energy value of 234.1 eV is originated from the Mo 3d3/2 of Mo(VI) components, which may be related with the defects in MoSe2, for example, the Mo−O phase.32 Furthermore, the binding energy values of 52.1 and 53.2 eV are assigned to Se 3d5/2 and Se 3d3/2, respectively, while the binding energy value of 56.9 eV is ascribed to the oxidized Se (Figure S1d).32 The electrochemical behaviors of NiSe@MoSe2 nanosheet arrays were first performed under a three-electrode system by cyclic voltammogram (CV) and galvanostatic charge/discharge (GCD) in a 2 M KOH aqueous electrolyte. A hydrophilic carbon cloth was used as the current collector to prepare the working electrodes (Figure S2b, Supporting Information). Figure 4a shows the typical CV curves of as-synthesized MoSe2, NiSe, and NiSe@MoSe2 nanosheet arrays at a scan rate of 30 mV s−1 in the operating voltage of −0.1−0.65 V (vs Hg/HgO). The CV curve of the carbon cloth shows a horizontal straight line, indicating it possesses a negligible capacitance. By comparison, the CV curve of the pure MoSe2 shows a near-rectangular curve shape with a small electrochemical active area and without any obvious redox peaks at a fast scan rate, which indicates that the pristine MoSe2 possesses a limited capacity. The result is similar to the electrochemical behavior of MoS2, because both MoSe2 and MoS2 belong to the transition metal chalcogenides, and S and Se elements are in the same primary family, thus they have similar properties.19,32 The NiSe electrode exhibits a pair of redox peaks, indicating typical battery-type Faradaic behavior. The corresponding reversible redox reaction could be expressed as follows:25 NiSe + OH− ↔ NiSeOH + e−

(1)

Compared to that of pristine NiSe, it can be found that the CV curve of the NiSe@MoSe2 nanosheet array electrode is similar to that of the NiSe electrode but shows a greater electrochemically active area, indicating the much higher Faradaic redox reactions and larger capacity in the NiSe@MoSe2 nanosheet arrays. In addition, the oxidized and reductive peaks of NiSe@MoSe2 nanosheet arrays are shifted slightly to higher and lower potentials relative to that of the NiSe electrode, respectively, which is likely due to the lower conductivity of the NiSe@MoSe2 nanosheet arrays than that of the pristine NiSe matrix, leading to the low redox reversible processes.33 Figure 4b shows the CV curves of the NiSe@MoSe2 electrode at different scan rates. With an increase of the scan rates from 10 mV s−1 to 50 mV s−1, the CV curve area and the peak current obviously increase, and the pair of redox peaks is still observed clearly. These results suggest that the NiSe@MoSe2 electrode exhibits rapid and reversible Faradaic behavior and high rate ability. 5956

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Figure 5. (a,b) FE-SEM images of N-PMCN. (c,d) TEM images of N-PMCN. (e) Nitrogen adsorption−desorption isotherms of the N-PMCN. (f) Pore size distribution of N-PMCN.

PMCN was designed according to the following considerations: (i) The pomelo mesocarp is mainly composed of lignin and cellulose, the highly cross-linked cellulosic fibrils network being a desirable precursor to forming interconnected porous carbon nanomaterials. (ii) CaCl2 has strong ammonia adsorption ability,40 which can cause volume expansion during the adsorption process. Therefore, CaCl2 can be used to immobilize ammonia which was generated through urea pyrolysis in the activated carbon precursor process, leading to high nitrogen doping content in carbon skeletons. (iii) The urea also can play the dual role of nitrogen source and expansion agent in the whole system,35 thus inducing the formation of a porous carbon nanosheet structure. Therefore, we expect the as-prepared N-

PMCN material to show favorable features, such as large surface area, porous architecture, and high nitrogen content, as the potential negative electrode for supercapacitors. SEM and TEM were employed to investigate the morphology and microstructures of the N-PMCN; the results are shown in Figure 5. The N-PMCN exhibits an interconnected sheet-like structure with many irregular voids, which is revealed by typical SEM images (Figure 5a). Moreover, a lot of soft wrinkles in the edges of N-PMCN nanosheets are clearly visible by high magnification SEM (Figure 5b), which ought to be derived from the ultrathin carbon walls consisting of several layers of graphene nanosheets.36 The highly interconnected and corrugated ultrathin carbon nanosheet was further confirmed by TEM (Figure 5957

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Figure 6. (a) XRD pattern and (b) Raman spectrum of N-PMCN material. (c) XPS spectrum of N-PMCN. (d) N 1s XPS spectra of N-PMCN.

The typical XRD pattern of N-PMCN in Figure 6a shows a typical broad (002) diffraction peak at 2θ = 21.8°, and the corresponding interlayer spacing (0.40 nm) is slightly larger than the natural graphite value (0.34 nm). The results indicated that there are regions of expanded stacking, for example, through corrugated or disordered carbon layers.38 The Raman spectrum (Figure 6b) of N-PMCN shows the typical D-band (about 1348 cm−1) and G-band (about 1601 cm−1) peaks, which are closely related to the disordered graphitic carbon and the graphite sp2 inplane carbon phase, respectively.39 One can see that the NPMCN exhibits a relatively stronger G-band signal than that of the D-band, and their intensity ratio (ID/IG) is calculated to be 0.94, demonstrating that interconnected N-PMCN is mainly made of a pyrolytic turbostratic carbon structure with some ordered graphitic feature.34 The surface element of N-PMCN was further verified by X-ray photoelectron spectroscopy (XPS), as shown in Figure 6c. The peaks located at around 284.4, 400.1, and 532.1 eV are corresponding to the C 1s, N 1s, and O 1s spectra, respectively. Specifically, four types of nitrogen bonding configurations are confirmed by high-resolution XPS (Figure 6d). The peaks at 398.4, 400.6, 401.5, and 402.7 eV can be identified to correspond to pyridinic-N (N-6), pyrrolic-N (N-5), quaternary-N (N-Q), and pyridine-N-oxide (N-X), respectively.29 In addition, the test results of elemental analysis display the nitrogen content in N-PMCN as 9.07 wt %. These results demonstrate that the incorporation with CaCl2 activation and urea gasification expansion methods can obtain carbon materials with a unique ultrathin nanosheet structure and large surface area as well as effective nitrogen doping in carbon skeletons. Thus, it

5c). No lattice fringe can be observed in the HRTEM image (Figure 5d), revealing the disordered amorphous carbon structure of the N-PMCN material. These results also demonstrate that the carbon material with a crumpled and interconnected nanosheet structure can be fabricated by the combination of chemical activation and a gas expansion method.37 The pore-size characterization of the N-PMCN was performed by measuring the nitrogen adsorption/desorption techniques. The isotherm and corresponding pore size distribution curve of the N-PMCN are shown in Figure 5e and f, respectively. They show a type-IV isotherm with obvious hysteresis loops in the relative pressure of ca. 0.50−0.99 P/P0, especially at high relative pressure (P/P0 > 0.90), suggesting the presence of mesoporous and macroporous structures. The noteworthy macroporous structure in N-PMCN may be a reflection of the irregular voids caused by intertwined carbon nanosheets. Besides, the adsorbed volume of N-PMCN is a steep increase at low pressures (P/P0 < 0.40), implying the presence of a certain microporosity. The specific surface area and pore volume of the N-PMCN material measured by the Brunauer−Emmett−Teller (BET) method are 1163 m2 g−1 and 0.82 cm3 g−1, respectively. The corresponding pore size distribution curve of N-PMCN is calculated by the Barrett−Joyner−Halenda (BJH) method, which shows a high N2 sorption pore volume in the small pore width range and the pore size distribution mainly centered in the range of 2−4 nm, signifying the presence of superior mesoporous structures in NPMCN material. 5958

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Figure 7. (a) CV curves of AC and N-PMCN electrodes in 2 M KOH electrolyte. (b) CV curves of N-PMCN at different scan rates. (c) GCD curves of N-PMCN at different current densities. (d) Specific capacitances of AC and N-PMCN at different current densities.

can be assumed that the as-prepared N-PMCN as electrode material for supercapacitors can not only increase the surface wettability and provide an additional capacity of carbon electrode material owing to high nitrogen doping content28 but also shorten the ion diffusion paths and improve the ion buffer storage spaces due to the interconnected porous nanosheet structure. Therefore, the as-prepared N-PMCN can be used as a potential negative electrode for supercapacitor applications. The electrochemical tests of the N-PMCN electrode material were also performed in a three-electrode system using 2 M KOH aqueous electrolyte. For the purpose of comparison, the commercial activated carbon (AC) was also evaluated as an electrode material under the same test conditions. Figure 7a shows the typical CV curves of the N-PMCN and AC electrode materials at a scan rate of 50 mV s−1 within a potential range of −1.0 to 0 V (vs Hg/HgO). The two curves exhibit a similar rectangular shape, which implies that the electrode materials show an ideal capacitive behavior. Obviously, the area surrounded by the CV curve of N-PMCN is larger than that of the studied AC sample, suggesting high capacitance. In addition, the CV curves of N-PMCN still maintain a stable rectangular-like shape at scan rates as high as 150 mV s−1 (Figure 7b), indicating the N-PMCN electrode exhibits a remarkable rate capability. The GCD measurements of the N-PMCN electrode material were carried out at various current densities (0.5 to 15 A g−1) in the operating voltage from −1.0 to 0 V, as shown in Figure 7c. All the charging and discharging curves are highly symmetric under different current densities, implying the electrode possesses highly reversible charge and discharge behaviors. The gravimetric specific capacitance of the materials was calculated from the

discharge portion of the GCD profiles according to the equation (Electrochemical Measurements section, Supporting Information) and the corresponding results as shown in the Figure 7d. As can be noticed, the N-PMCN electrode exhibits a very high specific capacitance of 223 F g−1 at 1 A g−1, which is 1.6-fold larger than the AC electrode under the same current density. With the increase of the discharge current densities from 0.5 to 15 A g−1, the N-PMCN electrode can still preserve a high specific capacitance of 178 F g−1, which is about 77% capacitance retention, and the value is significantly higher than that of the typical electric double-layer capacitive AC electrode material (66%). Therefore, the above results clearly reveal that the high specific capacitance and excellent rate capability of as-prepared N-PMCN are attributed to the unique porous nanosheet structure and high nitrogen doping, which will shorten the ion diffusion paths, improve the ion buffer storage spaces, and provide effective capacitance. Electrochemical Behavior of the As-Assembled Asymmetric Supercapacitor. Considering the excellent electrochemical performances of NiSe@MoSe2 and N-PMCN electrodes with complementary operating voltage in the same KOH aqueous electrolyte, a novel asymmetric supercapacitor (ASC) was assembled based on these materials, which were used as the positive and negative electrodes, respectively. The schematic illustration of the NiSe@MoSe2//N-PMCN ASC device is shown in Figure 8a. To further investigate the stable potential windows of NiSe@MoSe2 and N-PMCN prior to the evaluation of the ASC device, these two electrode materials were first CVtested in a three-electrode cell. As shown in Figure 8b, the NPMCN electrode and NiSe@MoSe2 electrode were measured 5959

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Figure 8. (a) Schematic illustration of the NiSe@MoSe2//N-PMCN ASC. (b) Comparative CV curves of NiSe@MoSe2 nanosheet arrays and NPMCN at 30 mV s−1 in three-electrode cell. (c) CV curves of NiSe@MoSe2//N-PMCN ASC at different scan rates. (d) GCD curves of NiSe@MoSe2// N-PMCN ASC at various current densities. (e) Ragone plots of the NiSe@MoSe2//N-PMCN and NiSe@MoSe2//AC ASC devices and previously reported metal chalcogenide-based ASC devices.42−46 (f) Cycling performance of the NiSe@MoSe2//N-PMCN ASC device.

within a potential window of −1.0 to 0 V and −0.1 to 0.65 V (vs Hg/HgO) at a scan rate of 30 mV s−1 in 2 M KOH solution, respectively. Therefore, the total operating device voltage can be expressed as the sum of the potential ranges of the positive and negative electrodes, and a working voltage of 1.65 V can be achieved when they are assembled into the ASC device. To construct the ASC, the loading mass ratio of electroactive materials (NiSe@MoSe2/N-PMCN) was estimated to be 0.58 according to the charge balance relationship (see Two-Electrode Fabrication section, Supporting Information). In order to ensure electrochemical stability and the practical available operating voltage of the as-fabricated ASC device, it was first evaluated at various voltage windows from 1.2 to 1.8 V (Figure S3, Supporting Information). As expected, the NiSe@ MoSe2//N-PMCN ASC device exhibits an ideal capacitive

behavior with a stable CV curve even at a voltage window up to 1.65 V. However, the current value increased sharply at high potential when the potential window continued to extend to 1.8 V, which probably originated from the electrolyte decomposition by oxygen evolution.41 Therefore, to ensure the stability and electrochemical reversibility of the ASC device, the operating voltage was determined to be 1.65 V for subsequent electrochemical performance tests. In Figure 8c, the CV curves of NiSe@MoSe2//N-PMCN ASC device measured at different scan rates from 10 to 100 mV s−1 with an operating voltage of 1.65 V exhibit distorted semirectangular shapes, which may be attributed to the contribution from the typical Faradaic positive electrode (Figure 8b). Moreover, the original rectangular-like shapes and typical redox peaks are also observed from the CV curves even at high 5960

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min easily when the devices were charged to 3.3 V within only 5 s (inset in Figure 8f) demonstrated a great potential for high power and high energy storage applications.

scan rates, indicating outstanding reversible battery-type Faradaic behavior. The GCD curves of the NiSe@MoSe2//NPMCN ASC device were recorded at different current densities with an operating voltage of 0−1.65 V. As shown in Figure 8d, the ASC device shows good capacitive properties based on its reasonably symmetric charge and discharge curves. For the purpose of comparison, we also fabricated the NiSe@MoSe2// AC ASC device based on NiSe@MoSe2 and AC as positive and negative electrodes, respectively, and evaluated its electrochemical performance (Figure S4, Supporting Information). The Ragone plots of the NiSe@MoSe2//N-PMCN and NiSe@ MoSe2//AC ASC devices are obtained based on their GCD data, as shown in Figure 8e. The NiSe@MoSe2//N-PMCN ASC device exhibits a high energy density of 32.6 Wh kg−1 at a power density of 415 W kg−1 and can be maintained at 19.6 Wh kg−1 at a high power density of 8592 W kg−1. However, the NiSe@ MoSe2//AC ASC device exhibits a maximum energy density of 24.5 Wh kg−1 at a power density of 400 W kg−1 and just retains about 11.1 Wh kg−1 at a power density of 7986 W kg−1. The energy density of the NiSe@MoSe2//N-PMCN ASC device is much higher than that of the NiSe@MoSe2//AC ASC device, th results of which could be attributed to the excellent electrochemical performances of NiSe@MoSe2 and N-PMCN electrodes. Meanwhile, the results also demonstrate that the use of the N-PMCN as a negative electrode can significantly improve the performance of the ACS device. Furthermore, the maximum energy density of the NiSe@MoSe2//N-PMCN ASC device is also comparable to the many previously reported aqueous ASC devices based on metal compounds as positive electrodes and carbon materials as negative electrodes (Table S2, Supporting Information). The EIS test was also conducted to gain further insight into the advantages of the NiSe@MoSe2//N-PMCN ASC device. The EIS spectra and the corresponding simulated equivalent circuit of the as-fabricated ASC device are shown in Figure S5 (Supporting Information). In the low frequency region, the near-vertical slopes indicate the device possesses a typical capacitive behavior and fast electrolyte ions diffusion process,47 which should be attributed to the interconnected hierarchical porous structures of the electrodes as in our above deductive results. In the high frequency area, the as-fabricated ASC device exhibited low bulk resistance (Rs = 0.72 Ω) and low charge-transfer resistance (Rct = 1.36 Ω), the resistance values being obtained and calculated from the intercept of the real axis Z′ and the diameter of the resistorcapacitor loop, respectively. The cycling durability of the ACS device is further evaluated at a current density of 5 A g−1 and shows a high capacitance retention of 91.4% achieved after 5000 cycles, revealing its excellent long-term cyclic stability (Figure 8f). It can be found that the specific capacitance of the ASC device increases distinctly in the initial 200 cycles, which may be caused by the improved wettability of electrode materials during the charge/discharge process. The small charge-transfer resistance and excellent cycling stability of the NiSe@MoSe2// N-PMCN ASC device could be attributed to the original vertical nanosheet array architecture of NiSe@MoSe2 and the porous nanosheet structure of N-PMCN. Their interconnected hierarchical porous structures provide rich open voids for fast electrolyte ion diffusion and effective electron transfer and also serve as an “electrolyte reservoir” to buffer the volume change of the core/shell heterostructure of NiSe@MoSe2 during the charge/discharge process. The demo experiment of two tandem NiSe@MoSe2//N-PMCN ASC devices lighting up the red lightemitting diode (LED, 5 mm diameter) and lasting for about 6

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CONCLUSIONS In summary, a novel NiSe@MoSe2 positive electrode with uniform vertical nanosheet array architecture and considerable specific capacity is designed and prepared by a facile one-step hydrothermal growth method, in which nickel foam was used as a nickel precursor and nucleation framework. Additionally, the porous interconnected N-PMCN negative electrode with a high nitrogen doping level (∼9 wt %) and high specific capacitance is prepared via simultaneous chemical activation and a urea nitrogen-doped method. On the basis of the unique architecture and outstanding electrochemical performance of NiSe@MoSe2 nanosheet arrays and N-PMCN electrodes, a novel NiSe@ MoSe2//N-PMCN ASC device with a maximum operating voltage of 1.65 V was constructed and showed a high energy density and power density (32.6 Wh kg−1 at a power density of 415 W kg−1) as well as outstanding cycling stability. These results indicate their potential to work as promising candidate materials for high-performance energy storage applications. The work presented here not only suggests the possibility to construct stable heterostructures into promising electrode materials but also presents a way to fabricate novel asymmetric supercapacitor devices based on high-performance Faradaic electrode materials and low cost biomass-based carbon materials.

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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/acssuschemeng.7b00729. Electrode fabrication, electrochemical measurements, calculation methods, Figures S1−S5, and Tables S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Ziqiang Lei: 0000-0001-9195-4472 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21664012, 51462032), the program for Changjiang Scholars and Innovative Research Team in University (IRT15R56), the Innovation Team Basic Scientific Research Project of Gansu Province (1606RJIA324). H.P. would like to acknowledge the China Scholarship Council for the Joint Ph. D. program.

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