MOF-Derived Hybrid Hollow Submicrospheres of Nitrogen-Doped

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MOF-Derived Hybrid Hollow Submicrospheres of Nitrogen-Doped Carbon-Encapsulated Bimetallic Ni−Co−S Nanoparticles for Supercapacitors and Lithium Ion Batteries Mingjie Yi,† Chaoqi Zhang,‡ Cong Cao,† Chao Xu,§ Baisheng Sa,† Daoping Cai,*,† and Hongbing Zhan*,† †

College of Materials Science and Engineering, Fuzhou University, Fujian 350108, P. R. China Catalonia Institute for Energy Research (IREC), Sant Adrià del Besòs, Barcelona, Spain § Xiamen Talentmats New Materials Science & Technology Co., Ltd., Xiamen, Fujian 361015, China Inorg. Chem. Downloaded from pubs.acs.org by WEBSTER UNIV on 03/01/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: The development of bimetallic transition-metal sulfide and nitrogen-doped carbon composites with unique hollow structure is highly desirable for energy storage applications but is also challenging. In the present work, we demonstrate a facile metal−organic framework engaged strategy for synthesizing bimetallic nickel cobalt sulfide and nitrogen-doped carbon composites with hollow spherical structure (denoted as hollow Ni−Co−S-n/NC composites) and a Ni/Co molar ratio (n value) that can be easily controlled. When evaluated as electrode materials for both supercapacitors and lithium ion batteries, it is found that the hollow Ni−Co−S-0.5/NC composite with a Ni/Co molar ratio of 0.5 exhibits optimal electrochemical performance. The hollow Ni−Co−S-0.5/NC composite exhibits a high specific capacity of 543.9 C g−1 at 1 A g−1 and maintains a capacity retention of 67.3% when the current density is increased to 20 A g−1. An asymmetric supercapacitor based on the hollow Ni−Co−S-0.5/NC composite is fabricated, which shows good electrochemical performance with a high energy density of 39.6 W h kg−1 at a power density of 808 W kg−1. For lithium storage, the hollow Ni−Co−S-0.5/NC composite manifests a high reversible discharge capacity of 755.0 mA h g−1 at 200 mA g−1 for 200 cycles as well as good rate capability. The excellent electrochemical performance could be attributed to the desirable structural, compositional, and component advantages. This work could offer new insight into the rational design and synthesis of highly efficient electrode materials for both supercapacitors and lithium ion batteries.

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INTRODUCTION Nowadays, ever-increasing energy shortage requirements and ecological pollution have stimulated tremendous concern for developing alternative energy conversion and storage systems.1,2 Supercapacitors and lithium ion batteries have been considered to be two kinds of ideal electrochemical energy storage devices owing to their high power/energy density and good cycling stability and being environmentally benign.3−5 Recently, transition-metal sulfides (TMSs) have been investigated as a new type of potential electrode material with improved performance compared to that of well-studied transition-metal oxides (TMOs).6,7 In particular, bimetallic TMSs have drawn extensive attention because of their high specific capacitance/capacity, improved electrical conductivity, and better electrochemical activity.8−15 Although many TMSbased electrode materials have been reported, their rate capability and long-term cycling stability still need further improvement owing to their low intrinsic conductivity and poor mechanical stability. © XXXX American Chemical Society

One attractive strategy for overcoming the above problem is to synthesize carbon-based composites.16−20 Carbon materials can improve the electrical conductivity, buffer the stress induced by volumetric changes, and restrict the aggregation and dissolution of the active materials, thereby improving both the cycling stability and rate capability.20 Among various carbon materials, nitrogen-doped carbon (NC) has attracted particular research interest, and a variety of NC-based composites have been reported to date.21−26 Because a nitrogen atom is more electronegative than a carbon atom, the nitrogen doping in carbon materials can generate defect sites and enhance the electrical conductivity as well as improve the wettability in electrolytes.22 Additionally, structural engineering has been regarded as another effective strategy for promoting the electrochemical performance. A hollow structure possesses a large surface area, abundant active sites, a Received: December 25, 2018

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DOI: 10.1021/acs.inorgchem.8b03594 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis Procedure for the Hollow Ni−Co−S-n/NC Composites

Figure 1. SEM images of the (a) Ni−Co−BTC-0.5 precursor, (b) Ni−Co-0.5/NC composite, and (c and d) Ni−Co−S-0.5/NC composite. (e−g) TEM and (h) HRTEM images and (i) elemental mapping of the hollow Ni−Co−S-0.5/NC composite.

Metal−organic frameworks (MOFs), a type of porous material assembled by metal cations/clusters and organic ligands, have served as versatile templates/precursors for obtaining porous carbon-based composites with diverse morphologies, compositions, and structures.31−33 When annealed in an inert atmosphere, the uniformly distributed metal cations and organic linkers in MOFs can be concurrently converted to metal/metal compounds and carbon, respectively. Also, the surface of the metal/metal compounds is usually well covered with the carbon derived from organic ligands, which can help to suppress their excessive growth to large sizes.34,35 Furthermore, owing to the strong coupling effect between the

shortened charge transfer distance, and reduced aggregation of nanosized subunits, which have been widely studied in energyrelated applications.27−30 For instance, Shen et al. recently synthesized NiCo2S4 ball-in-ball hollow spheres for supercapacitors, which exhibited a specific capacity of 1036 F g−1 at a current density of 1.0 A g−1. On the basis of the above considerations, it could be very meaningful to construct bimetallic TMSs and NC-based composites with hollow structure to boost the electrochemical performance in terms of the specific capacity, rate capability, and long-term cycling stability. B

DOI: 10.1021/acs.inorgchem.8b03594 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a, b) XRD patterns of the Ni−Co−BTC-0.5 precursor, hollow Ni−Co-0.5/NC, Ni−Co−S-0/NC, Ni−Co−S-0.3/NC, Ni−Co−S-0.5/ NC, and Ni−Co−S-1.0/NC composites. (c) TGA curve of the hollow Ni−Co−S-0.5/NC composite. (d) XPS survey spectrum. (e−i) Highresolution Ni 2p, Co 2p, S 2p, C 1s, and N 1s peaks of the hollow Ni−Co−S-0.5/NC composite.

spheres are synthesized by the solvothermal method as a precursor.37 The molar ratios of Ni/Co in the Ni−Co−BTC-n precursor can be manually controlled, and n indicates the molar ratio of Ni/Co. Second, the Ni−Co−BTC-n precursor is converted to hollow Ni−Co-n/NC composites, in which the metallic Ni and Co or Ni−Co-n alloy nanoparticles are coated with NC after carbonation treatment of the Ni−Co−BTC-n precursor with the assistance of melamine. Melamine is a lowcost industrial material that serves as the nitrogen source. During the carbonation treatment process, melamine could generate NH3 gas. In such a nitrogen-rich atmosphere at elevated temperature, the doping of nitrogen in carbon is realized.22,23 Finally, the hollow Ni−Co−S-n/NC composites are obtained by a hydrothermal sulfidation treatment of the hollow Ni−Co-n/NC composites in the presence of thioacetamide (TAA). The as-synthesized products were first examined by scanning electron microscopy (SEM). Figure 1a,b displays the SEM images of the Ni−Co−BTC-0.5 precursor and corresponding hollow Ni−Co-0.5/NC composite with a Ni/Co molar ratio of 0.5. The Ni−Co−BTC-0.5 precursor consists of uniform solid submicrospheres, and the surfaces of these submicrospheres are very smooth. Fortunately, the spherical morphology is well inherited after the calcination treatment. Evidently, the hollow interior can be clearly distinguished from some cracked

MOF-derived metal/metal compounds and carbon, the resultant carbon-based composites commonly exhibit improved electrochemical performance.32 As a typical example, Liu et al. reported that the MOF-derived Co9S8/C nanocages exhibited greatly enhanced lithium storage performance compared to that of normal carbon-coated Co 9 S 8 /C nanocages.36 In this work, we demonstrate a facile MOFengaged strategy for synthesizing the bimetallic nickel cobalt sulfide (Ni−Co−S) nanoparticle and NC composites with hollow spherical structure and different values of the Ni/Co molar ratio (denoted as hollow Ni−Co−S-n/NC composites). Among the resultant composites, the Ni−Co−S nanoparticles are well encapsulated within the NC matrix. The molar ratios of Ni/Co in the hollow Ni−Co−S-n/NC composites can be easily controlled. Benefitting from advanced structural, compositional, and component advantages, the as-synthesized hollow Ni−Co−S-0.5/NC composite exhibits optimal electrochemical performance when evaluated as an electrode material for both supercapacitors and lithium ion batteries.

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RESULTS AND DISCUSSION The synthesis strategy for hollow Ni−Co−S-n/NC composites consists of three steps, as schematically illustrated in Scheme 1. First, homogeneous bimetallic Ni−Co−BTC-n solid submicroC

DOI: 10.1021/acs.inorgchem.8b03594 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) CV curves of the different electrodes at a scan rate of 5 mV s−1. (b) CV curves of the hollow Ni−Co−S-0.5/NC composite recorded at various scan rates. (c) GCD curves of the different electrodes at a current density of 1 A g−1. (d) GCD curves of the hollow Ni−Co−S-0.5/NC composite at different current densities. (e) Calculated capacity as a function of the current density. (f) Cycling performance of the hollow Ni− Co−S-n/NC composites at a current density of 6 A g−1.

observed lattice fringes with a d spacing of 0.34 nm belongs to the (002) planes of graphitic carbon. The catalytic effect of Ni and Co nanoparticles could promote the formation of graphitic carbon during the annealing process. The uniform distribution of C, N, S, Co, and Ni elements in the hollow Ni−Co−S-0.5/ NC composite can be proven by elemental mapping analysis and the high-angle annular dark-field (HAADF) scanning TEM image (Figure 1i). Powder X-ray diffraction (XRD) patterns of the Ni−Co− BTC-0.5 precursor and corresponding Ni−Co-0.5/NC composite are given in Figure 2a. The Ni−Co−BTC-0.5 precursor is amorphous, which is consistent with the literature.37 After calcination, the diffraction peaks of the Ni−Co-0.5/NC composite can be indexed to the metallic Co (JCPDS no. 15-0806) and Ni (JCPDS no. 04-0850) or NiCo alloy.38 Figure 2b shows the XRD patterns of the hollow Ni−Co−S-n/ NC composites with different Ni/Co molar ratios in which the diffraction peaks can be indexed to NiCo2S4 (JCPDS no. 200782) and Co9S8 (JCPDS no. 65-6801), indicating the coexistence of NiCo2S4 and Co9S8 phases after hydrothermal sulfidation. The weight contents of NC in the hollow Ni−Co− S-n/NC composites were measured by thermogravimetric analysis (TGA) and are displayed in Figure 2c and Figure S5. The weight loss of the hollow Ni−Co−S-0.5/NC composite might be attributed to the oxidation of NiCo2S4 and Co9S8 to NiO and Co3O4, respectively, and the combustion of NC.39 Assuming that the remaining products after TGA are pure NiO and Co3O4, the content of NC in the Ni−Co−S-0.5/NC composite is estimated to be about 55.2 wt %, which is higher than for the other three products. X-ray photoelectron spectroscopy (XPS) was further conducted to detect the near-surface chemical composition and chemical state of the hollow Ni−Co−S-0.5/NC composite. Figure 2d displays the survey spectrum, which also confirms the existence of the Ni, Co, S, N, and C elements. Figure 2e,f shows the highresolution Ni 2p and Co 2p spectra in which both bivalent

submicrospheres (Figure S1). Figure 1c,d shows SEM images of the hollow Ni−Co−S-0.5/NC composite at low and high magnifications. No obvious morphological changes are observed after the subsequent sulfidation reaction. The magnified SEM image reveals that the hollow Ni−Co−S-0.5/ NC composite has a rather rough surface compared to the Ni− Co-0.5/NC composite. The energy-dispersive X-ray spectroscopy (EDS) analysis confirms the successful convention of the Ni−Co-0.5/NC composite to the Ni−Co−S-0.5/NC composite (Figure S2). The EDS result also indicates the values of the molar ratio of Ni/Co in the hollow Ni−Co-0.5/NC and Ni− Co−S-0.5/NC composites are about 0.5, along with the existence of a nitrogen element. In addition, other hollow Ni− Co-n/NC and Ni−Co−S-n/NC composites with different Ni/ Co molar ratios are also synthesized, and the corresponding SEM images and EDS analyses are shown in Figures S2 and S3. The detailed geometrical morphology and structure of the hollow Ni−Co−S-0.5/NC composite are further investigated through scanning electron microscopy (TEM). As shown in Figure 1e,f, the distinct contrast between the shell and interior cavity of the submicrospheres evidently indicates the unique hollow structure, which is consistent with the SEM observation. The magnified TEM image (Figure 1g) clearly demonstrates that the tiny Ni−Co−S nanoparticles are well distributed within the carbon matrix. The Brunauer−Emmett− Teller (BET) measurement shows that the specific surface area of the hollow Ni−Co−S-0.5/NC composite is about 10.27 m2 g−1 and that the pore size is mainly around 3.5 nm (Figure S4). In general, such a porous structure can supply more active sites for electrochemical reactions as well as ensure sufficient penetration of the electrolyte.12,30 In the high-resolution TEM (HRTEM) image (Figure 1h), it is observed that the Ni−Co− S nanoparticles are well encapsulated in the NC matrix. The interplanar spacing of 0.299 nm could be assigned to the (311) planes of Co9S8, while the interplanar distance of 0.332 nm could correspond to the (220) planes of NiCo2S4. Besides, the D

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Figure 4. (a) Schematic illustration of the Ni−Co−S-0.5/NC//AC ASC. (b) CV curves of Ni−Co−S-0.5/NC//AC ASC recorded at different scan rates. (c) GCD curves at different current densities. (d) Calculated capacitance as a function of the current density. (e) Cycling performance at a current density of 4 A g−1, with the inset being the Coulombic efficiency of the ASC device during cycling. (f) Ragone plot of the Ni−Co−S-0.5/ NC//AC ASC.

Ni2+/Co2+ and trivalent Ni3+/Co3+ cations are detected in the hollow Ni−Co−S-0.5/NC composite. It is worthy mentioning that such mixed valence of the Ni and Co elements could be beneficial to high electrochemical performance.17,40 In the S 2p spectrum (Figure 2g), the peak centered at 161.5 eV corresponds to sulfur ions in low coordination on the surface, while another peak located at 162.4 eV can be assigned to metal−sulfur bonds.11 Figure 2h gives the high-resolution C 1s spectrum including three subpeaks from C−C, C−N, and C O bonds. The high-resolution N 1s spectrum (Figure 2i) reveals that the nitrogen atoms contain different oxidation states including pyridinic-N (13.5 wt %), pyrrolic-N (53.6 wt %), and graphitic-N (32.9 wt %).41 The percentage of N in the hollow Ni−Co−S-0.5/NC composite is calculated to be about 4.96 wt %. Therefore, the XPS results further confirm the successful doping of MOF-derived carbon with nitrogen with the assistant of melamine. Moreover, the chemical composition of the monometallic hollow Ni−Co−S-0/NC composite was also examined by XPS, and the results can be found in Figure S6. The as-synthesized hollow Ni−Co−S-n/NC composites with different molar ratios of Ni/Co were first investigated as electrode materials for supercapacitors. The typical cyclic voltammetry (CV) curves at a scan rate of 5 mV s−1 within the potential window of 0−0.65 V for the hollow Ni−Co−S-n/NC composites are presented in Figure 3a. Obviously, the welldefined redox peaks are ascribed to reversible Faradaic reactions of MS/MSOH and MSOH/MSO (in which M refers to Ni and Co ions), demonstrating their battery-type pseudocapacitive characteristics. Among them, the hollow Ni− Co−S-0.5/NC composite shows the largest area of the CV curve, indicating that the highest specific capacity is achieved. Besides, the hollow Ni−Co−S-0.5/NC composite is also superior to the corresponding hollow Ni−Co−S-0.5/C composite without nitrogen doping. Figure 3b shows the CV

curves of the hollow Ni−Co−S-0.5/NC composite at various scan rates in the range of 5−100 mV s−1 within the potential window from 0 to 0.65 V. Even at a high scan rate of 100 mV s−1, the shape of the CV curve is still well retained, indicating the ideal rate capability of the hollow Ni−Co−S-0.5/NC composite. In agreement with the CV results, the hollow Ni− Co−S-0.5/NC composite exhibits the maximum discharge time at a current density of 1 A g−1 (Figure 3c). Figure 3d displays the GCD curves of the Ni−Co−S-0.5/NC composite at various current densities. In addition, the CV and GCD curves for the other three electrodes at different rates are shown in Figure S7. According to the GCD curves, the specific capacity of the electrode can be calculated, and the result is shown in Figure 3e. Remarkably, the hollow Ni−Co−S-0.5/ NC composite exhibits the highest specific capacities of 543.9, 519.6, 489.6, 457.8, 436.8, 424.0, 406.8, 382.4, and 366.0 C g−1 at different current densities of 1, 2, 4, 6, 8, 10, 12, 16, and 20 A g−1, respectively. This demonstrates that about 67.3% of the capacity is retained with a 20-fold increase in current density, revealing that the hollow Ni−Co−S-0.5/NC composite has a good rate capability. As shown in Figure 3f, the hollow Ni− Co−S-0.5/NC composite also exhibits excellent cycling stability. Electrochemical impedance spectroscopy (EIS) results further reveal that the hollow Ni−Co−S-0.5/NC composite has the lowest charge-transfer resistance as well as ideal electrolyte ion diffusion behavior (Figure S8).13 Furthermore, an ASC device based on a hollow Ni−Co−S0.5/NC composite as the cathode and activated carbon (AC) as the anode was prepared to confirm its practical application, as schematically illustrated in Figure 4a. Figure 4b shows the CV curves of the Ni−Co−S-0.5/NC//AC ASC at different scan rates. The GCD curves of the Ni−Co−S-0.5/NC//AC ASC at different current densities and the specific capacitance can be calculated accordingly, as shown in Figure 4c. As displayed in Figure 4d, the specific capacitance of the ASC E

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Figure 5. (a) Cycling performance of the hollow Ni−Co−S-n/NC composites at a current density of 200 mA g−1. (b) CV and (c) GCD curves of the hollow Ni−Co−S-0.5/NC composite. (d) Rate performance of the Ni−Co−S-0.5/NC composite at various current densities. (e) Comparison of the rate performance of the hollow Ni−Co−S-0.5/NC composite and other reported metal sulfide and carbon-based composites. (f) SEM image of the Ni−Co−S-0.5/NC composite after cycling.

showed a high level of 111.2 F g−1 at a current density of 1 A g−1, and approximately 68.5% of the initial capacitance is kept at 10 A g−1 (according to the total mass of electrode materials on the anode and cathode electrodes). As shown in Figure 4e, the cycling stability of the Ni−Co−S-0.5/NC//AC ASC device is further tested at a current density of 4 A g−1 for up to 3000 cycles. It is worth pointing out that the ASC device retains 81.5% of its initial capacitance, revealing its excellent electrochemical stability. The Coulombic efficiency remains almost 100% during the cycling process, indicating good electrochemical reversibility. Figure 4f shows the Ragone plots of the energy density and power density for the Ni−Co−S-0.5/ NC//AC ASC. Notably, our ASC can display a high energy density of 39.6 W h kg−1 at a power density of 808 W kg−1 and still retains 27.1 W h kg−1 at a high power density of 7910 W kg−1. The performance of our ASC is better than that of many other recently reported ASCs, such as NiCo2S4/Co9S8//AC (33.5 W h kg−1 at 150 W kg−1),14 rGO/CoNiSx/NC//AC (32.9 W h kg−1 at 229 W kg−1),25 NiCo2S4@Ni(OH)2@PPy// AC (34.7 W h kg−1 at 120 W kg−1),42 C-NiCo2S4//AC (7.1 W h kg−1 at 8000 W kg−1),43 Ni(OH)2/CuCo2S4/Ni//AC (10.5 W h kg−1 at 5480 W kg−1),44 Co9S8//AC (20.0 W h kg−1 at 828 W kg−1),45 NiCo2S4 microdumbbells//AC (35.4 W h kg−1 at 381.2 W kg−1),46 NiCo2S4 nanoboxes//AC (17.1 W h kg−1 at 2250 W kg−1),47 NiCo2S4 ellipsoids//AC (28.9 W h kg−1 at 188 W kg−1),48 Ni7S6/Co3S4 nanoboxes//AC (18.8 W h kg−1 at 2256 W kg−1),49 and is superior to that of other materials shown in Table S1.50−58 The high energy and power density further suggest that the hollow Ni−Co−S-0.5/NC composite is promising for practical applications in supercapacitors. Furthermore, the lithium storage properties of the assynthesized hollow Ni−Co−S-n/NC composites were also investigated. Figure 5a shows the discharge capacity vs cycle number of the hollow Ni−Co−S-n/NC composites at a current density of 200 mA g−1. It can be seen that the hollow Ni−Co−S-0.5/NC composite also exhibits optimal electrochemical performance as an anode material for lithium ion

batteries. The discharge capacity of the hollow Ni−Co−S-0.5/ NC composite is still as high as 755.0 mA h g−1 after 200 cycles, which is much higher than that of the other three electrodes. Figure 5b shows the first three CV curves of the hollow Ni−Co−S-0.5/NC composite at a scan rate of 0.2 mV s−1 within the potential window of 0.01−3.0 V. In the first cathodic sweep, the two reduction peaks located at 1.57 and 1.20 V could be attributed to the Li+ insertion into the lattices of NiCo2S4 and Co9S8, followed by further reduction of ionic Ni and Co to metallic Ni and Co and the formation of Li2S.36 The third peak at 0.64 V could be related to the formation of the solid-electrolyte interphase (SEI) film.21,59 During the anodic process, two peaks at 2.10 and 2.34 V could be ascribed to the oxidation of metallic Ni and Co to NiSx and CoSx, respectively.18,60 Remarkably, the CV curves overlap very well from the second cycle onward, indicating good reversibility of the electrochemical reactions. Figure 5c shows the first three GCD curves of the hollow Ni−Co−S-0.5/NC composite at a current density of 200 mA g−1. The observed plateaus are in good agreement with the CV result. The hollow Ni−Co−S0.5/NC composite exhibits a high discharge capacity of 1032.9 mA h g−1 with an initial Columbic efficiency of 70.8%. The large capacity loss in the first cycle could be ascribed to the generation of the SEI layer and some incomplete conversion reactions. In the second cycle, the reversible discharge capacity of the hollow Ni−Co−S-0.5/NC composite is about 694.4 mA h g−1. The rate capability of the hollow Ni−Co−S-0.5/NC composite is then evaluated at different current densities ranging from 100 to 5000 mA g−1. As showed in Figure 5d, the average discharge capacities are about 718.4, 653.7, 624.9, 560.4, 492.1, and 383.3 mA h g−1 at the current densities of 100, 200, 500, 1000, 2000, and 5000 mA g−1, respectively. While the current density reverses back to 100 mA g−1, the discharge capacity can be turned back to the original value, indicating the good rate capability. It has been noted that the hollow Ni−Co−S-0.5/NC composite possess a better rate performance compared to those of other carbon-based F

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Inorganic Chemistry composites (Figure 5e).18,19,60−63 The morphology and characterization of the hollow Ni−Co−S-0.5/NC composite after cycling were further examined and are shown in Figure 5f and Figure S9. It is impressive that no significant morphological and structural variations were observed and Ni, Co, S, N, and C elements were also detected, indicating good structural integrity and chemical stability. On the basis of the above results, the hollow Ni−Co−S-0.5/ NC composite exhibits excellent electrochemical performance as electrode materials for both supercapacitors and lithium ion batteries, which could be attributed to the structural, compositional, and component advantages. First, bivalent Ni2+/Co2+ and trivalent Ni3+/Co3+ cation contents can provide richer redox reactions.17,40 Second, the NC with high electrical conductivity could facilitate the transport of electrons within the electrode.21,22 Third, the NC can effectively prevent the aggregation and dissolution of the encapsulated Ni−Co−S nanoparticles during the cycling process, resulting in good cycling stability.18,39 Fourth, the hollow and porous structure could accommodate the volumetric change and allow for easy penetration of electrolyte to ensure fast ion diffusion and sufficient redox reactions.6,30,64

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (grant no. 2017YFB0701700), the National Natural Science Foundation of China (grant no. 51872048), the Natural Science Foundation of Fujian Province (grant nos. 2017J01687 and 2018J01677), and the Science Foundation of the Department of Education of Fujian Province (grant no. JAT170093).

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CONCLUSIONS A series of bimetallic Ni−Co−S-n/NC composites with hollow structure have been successfully design and synthesized through a facile MOF-engaged strategy in which the Ni− Co−S nanoparticles are well encapsulated within the MOFderived NC matrix. This strategy includes the carbonation treatment of the bimetallic Ni−Co−BTC precursor with different Ni/Co molar ratios and a subsequent hydrothermal sulfidation process. Electrochemical characterizations show that the hollow Ni−Co−S-0.5/NC composite with a Ni/Co molar ratio of 0.5 exhibits optimal electrochemical performance as electrode materials for both supercapacitors and lithium ion batteries. For practical applications, as-fabricated Ni−Co−S-0.5/NC//AC ASC can achieve a high energy density of 39.6 W h kg−1 at a power density of 808 W kg−1 and still retain 27.1 W h kg−1 at a high power density of 7910 W kg−1. These results indicate that the hollow Ni−Co−S-0.5/ NC composite could be a promising electrode material for supercapacitors and lithium ion batteries. We also anticipate that the bimetallic hollow Ni−Co−S-n/NC composites could find applications in other fields.

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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/acs.inorgchem.8b03594. Experimental methods, additional characterizations, and electrochemical results (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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

Baisheng Sa: 0000-0002-9455-7795 Hongbing Zhan: 0000-0002-8748-6642 G

DOI: 10.1021/acs.inorgchem.8b03594 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b03594 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03594 Inorg. Chem. XXXX, XXX, XXX−XXX