Synthesis of Capsule-like Porous Hollow Nanonickel Cobalt

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Synthesis of Capsule-like Porous Hollow Nano-nickel Cobalt Sulfides via Cation Exchange Based on the Kirkendall effect for High-Performance Supercapacitors Yongfu Tang, Shunji Chen, Shichun Mu, Teng Chen, Yuqing Qiao, Shengxue Yu, and Faming Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01268 • Publication Date (Web): 31 Mar 2016 Downloaded from http://pubs.acs.org on March 31, 2016

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ACS Applied Materials & Interfaces

Synthesis of Capsule-like Porous Hollow Nano-nickel Cobalt Sulfides via Cation Exchange Based on the Kirkendall Effect for High-performance Supercapacitors Yongfu Tang a,*, Shunji Chen a, Shichun Mu b, Teng Chen a, Yuqing Qiao a, Shengxue Yu a, Faming Gao a a

Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, Hebei, 066004, China

b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China

Abstract To construct a suitable three-dimensional structure for ionic transport on the surface of the active materials for a supercapacitor, porous hollow nickel cobalt sulfides are successfully synthesized via a facile and efficient cation-exchange reaction in a hydrothermal process involving the Kirkendall effect with γ-MnS nanorods as a sacrificial template. The formation mechanism of the hollow nickel cobalt sulfides is carefully illustrated via the tuning reaction time and reaction temperature during the cation-exchange process. Due to the ingenious porous hollow structure that offers a high surface area for electrochemical reaction and suitable paths for ionic transport, porous hollow nickel cobalt sulfide electrodes exhibit high electrochemical performance. The Ni1.77Co1.23S4 electrode delivers a high specific

*Corresponding author. Tel.: +86 13780351724 E-mail address: [email protected] 1

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capacity of 224.5 mAh g-1 at a current density of 0.25 A g-1 and a high capacity retention of 87.0 % at 10 A g-1. An all-solid-state asymmetric supercapacitor, assembled with a Ni1.77Co1.23S4 electrode as the positive electrode and a home-made activated carbon electrode as the negative electrode (denoted as NCS//HMC), exhibits a high energy density of 42.7 Wh kg-1 at a power density of 190.8 W kg-1 and even 29.4 Wh kg-1 at 3.6 kW kg-1. The fully charged as-prepared asymmetric supercapacitor can light up a light emitting diode (LED) indicator for more than 1 hour, indicating promising practical applications of the hollow nickel cobalt sulfides and the NCS//HMC asymmetric supercapacitor.

Keywords: Hollow structure, Nickel cobalt sulfide, Kirkendall effect, Supercapacitor, Cation-exchange reaction, Hydrothermal synthesis

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INTRODUCTION Clean and renewable energy resources are urgently required to deal with the exhaustion of fossil fuels and environmental pollution, which affects the worldwide population.1-6 Energy storage is one of the key challenges for the practical application of some types of renewable energy such as solar energy and wind energy. Supercapacitors, as electrochemical energy storage devices with high power density, low cost, fast charge-discharge rate and long cycling life, have been attracting widespread interest.7-11 Based on the charge-storage mechanism, these supercapacitors can be divided into two categories: (1) electric double-layer capacitors (EDLCs) storing charges via the electrolyte ionic adsorption/desorption process between the active materials of the electrode and the electrolytes and (2) pseudocapacitors delivering energy through fast Faradaic redox reactions on the electrode surface.12-17 Carbon-based electrode materials suffer from low specific capacitance and low energy density because they are restricted by limits on increasing their effective surface area and the charge storage mechanism. This hinders their practical applications.18-21 In contrast, the pseudocapacitance materials, storing more charges via Faradaic reactions, possess much higher specific capacitance than carbon-based materials. Among the pseudocapacitance materials, the transition metal sulfides have been attracting much attention in recent years due to their higher electrical conductivity and electrochemical activity than the corresponding transition metal oxides and hydroxides.22-26 The micro-morphology of electrode materials is one of the key factors

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determining the electrochemical performances of pseudocapacitors. Hollow nanostructures, offering more active sites for electrochemical reactions and short ionic transport paths, which provide the enhanced rate capability of supercapacitors, are the promising nanostructures for supercapacitor materials.27-29 An ion-exchange process in a hydrothermal system is an efficient strategy to obtain hollow nanoarchitecture materials based on the nano-scale Kirkendall effect due to the fast ion-exchange rate in the hydrothermal process.30-35 However, most of the ion-exchange processes widely used in the synthesis of the hollow metal sulfide are anion-exchange processes.10, 31-36 As reported, NiCo2S4 nanotubes were synthesized via anion-exchange with NiCo2O4 as the sacrificial template.32 Pu et al. prepared hollow hexagonal NiCo2S4 nanoplates with Ni(OH)2-Co(OH)2 as precursors via substituting a sulfide ion by a hydroxide ion based on Kirkendall diffusion.10 Hollow nickel sulfide spheres were obtained via a facile anion-exchange with nickel silicate as the template.35 In contrast, the synthesis of hollow metal sulfides based on cation-exchange processes is rarely reported. Nevertheless, due to the various morphologies of metal sulfides (MnS, ZnS and so on),29,

37-39

which can be used as templates, the success of the cation-exchange

synthesis for hollow nickel cobalt sulfides will markedly enrich their morphologies and nanostructures, as well as boost their widespread applications in electrochemical devices. In this work, capsule-like porous hollow nickel cobalt sulfides were successfully synthesized via a cation-exchange process using γ-MnS as the sacrificial template in a hydrothermal system. The formation mechanism of the porous hollow nickel cobalt

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sulfides is illustrated in detail by tuning the synthesis conditions including reaction time, reaction temperature and the MnS template. The fast cation-exchange process based on the Kirkendall effect plays important roles in the formation of the porous hollow structure, although the dissolution-crystallization process coexists in the hydrothermal system. Due to the fact that the unique porous hollow structure can offer a greater number of active sites for reversible redox reactions and suitable paths for ionic transport, the hollow nickel cobalt sulfide electrodes with battery-type characteristics exhibit not only high specific capacity (224.5 mAh g-1 at 0.25 A g-1) but also high rate-capacity (87.0 % capacitance retention at 10 A g-1). The all-solid-state asymmetric supercapacitor, assembled with hollow nickel cobalt sulfide as the positive electrode, exhibits high performance and promising practical applications. EXPERIMENTAL SECTION Preparation of porous hollow nickel cobalt sulfide materials. The nickel cobalt sulfides with porous hollow structure were synthesized by a facile hydrothermal process based on cation-exchange reaction with γ-MnS nanocrystals as the sacrificial template. The synthesis of the γ-MnS template was performed as follows: 0.15 g MnCl2·4H2O and 0.15 g sodium citrate were dissolved in 90 ml deionized water. Subsequently, 30 ml of 0.1 M Na2S·9H2O aqueous solution was added dropwise to the mixed solutions with vigorous stirring. Then, the mixture thus obtained was transferred into a 200 ml Teflon®-lined stainless-steel autoclave and was heated at 120 °C for 12 h. After being allowed to cool naturally to room temperature, the as-obtained pink precipitate was washed several times with deionized water.

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The as-prepared γ-MnS precursors were then ultrasonically dispersed directly into 100 ml of deionized water. A Ni(NO3)2·6H2O solution with a concentration of 10 g L-1 and a Co(NO3)2·6H2O solution with the same concentration (10 g L-1) were uniformly added to the dispersion with the designed Ni/Co ratios (1:0, 3:1, 1:1, 1:3 and 0:1) with magnetic stirring for 10 min. Then, the obtained mixture was transferred into a 200 ml Teflon®-lined stainless-steel autoclave and was heated at 120 °C for 24 h. Finally, the nickel cobalt sulfides were obtained after centrifuging, washing and vacuum drying. To investigate the formation mechanism of the porous hollow nickel cobalt sulfides, the samples with a Ni/Co ratio of 1:1 were synthesized at 120 °C by a similar process with various reaction times (1 h, 2 h, 4 h, 8 h, 12 h and 24 h). Moreover, the nickel cobalt sulfides were also obtained using different hydrothermal temperatures (80 °C, 100 °C, 120 °C, 140 °C and 160 °C) for 24 h to further investigate the effect of temperature on the morphology of nickel cobalt sulfides. To investigate the effect of the templates on the cation-exchange reaction, the cation-exchange process using the α-MnS precursor as the sacrificial template was also conducted. The α-MnS precursor was synthesized in the following steps: 0.245 g MnCl2·4H2O was dissolved in 30 ml of deionized water, to which was added 10 ml of ethylenediamine with stirring. Subsequently, 0.6 g of thiourea dispersed into 30 ml of deionized water was added dropwise to the above solutions with stirring for 10 min. The brown suspension thus obtained was transferred into a 100 ml Teflon®-lined stainless-steel autoclave to be heated at 140 °C for 15 h. After the products were

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washed several times using distilled water and ethyl alcohol, the α-MnS powders were collected by drying under vacuum at 70 °C for 12 h. The cation-exchange process was the same as that used for the preparation of the porous hollow Ni-Co sulfides. Physiochemical characterizations. The real Ni/Co ratios in the porous hollow nickel cobalt sulfides were measured using a AA-6800F/G atomic absorption spectrophotometer (AAS, Shimadzu Corp., Japan). The crystal structure and phase composition of the samples were analyzed using X-ray diffraction (XRD) patterns (Bruker AXS D8 diffractometer with Cu Kα radiation) with the detected diffraction angle (2 theta) ranging from 10° to 90° with a step size of 0.06°. The pore-size distribution and surface area of the nickel cobalt sulfide samples were measured by their N2 adsorption-desorption isotherms (BET system, ASAP 2020 V3.01 G). The morphology of these samples was characterized by transmission electron microscopy (TEM, HT7700, 100 kV) and scanning electron microscopy (SEM, KYKY-2800B, 15 kV) with an energy-dispersive X-ray spectroscopy (EDS) detector. Fabrication

of

nickel

cobalt

sulfide

electrodes

and

electrochemical

measurements. The electrochemical properties of the porous hollow nickel cobalt sulfide electrodes were evaluated by cycle voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on a CHI 604E workstation, and galvanostatic charging-discharging (GC) measurements on a Land CT 2001A. All electrochemical measurements for the active materials were carried out by a three-electrode system in 2.0 M KOH electrolyte. Therein, Hg/HgO and Pt film (1×1cm2) were used as reference electrode and counter electrode, respectively. The working electrodes were

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prepared by mixing the nickel cobalt sulfide, acetylene black and polyvinylidene difluoride (PVDF, 1% wt) with the mass ratio of 80:15:5 as the active materials coated on a nickel foam (1×1 cm2) in a vacuum drying oven at 70 °C for 12 h. Under z pressure of 10 MPa for 1 minute, the working electrodes were obtained. To decrease the test deviation during the electrochemical measurements, the mass loading of all the nickel cobalt sulfide electrodes was approximately 4.0 mg cm-2 . To investigate the performances of the different porous hollow nickel cobalt sulfides in a practical application, an all-solid-state asymmetric supercapacitor, known as an NCS//HMC supercapacitor, was assembled with the as-prepared nickel cobalt sulfide as the positive material and a home-made activated carbon as the negative material. The preparation of the home-made carbon electrode was similar to that of the nickel cobalt sulfide electrode, which was described above. Ultimately, the mass loading of the active materials in the positive and negative electrodes was 4.56 and 8.32 mg cm-2, respectively. The solid-state electrolyte was prepared as follows: 1.0 g of agar gel was placed in culture dish, and a reasonable amount of saturated potassium hydroxide solution was added slowly with continuous stirring for 20 minutes. The mixture was heated at 70 °C until it was converted to a jelly. Then, the jelly was allowed to cool in naturally and was used as solid-state electrolyte. The supercapacitor was assembled by pressing the positive and negative electrodes into the solid-state electrolyte and encapsulating them in a valve bag. RESULT AND DISCUSSION The capsule-like porous hollow nickel cobalt sulfides are synthesized via a facile

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hydrothermal cation-exchange reaction process with manganese sulfide nanorods as the hard sacrificial template described as follows. As shown in Scheme 1, the solid olive-like nano-MnS nanocrystals are obtained via a hydrothermal method with Mn2+ ions and sodium sulfide as the precursor. Sodium citrate was used as structure-directing agent to control the morphology, as well as a weak reductant to prevent form the oxidation of MnS during the synthesis process. Subsequently, in the cation-exchange process, the Ni2+ and Co2+ ions in the Ni-Co nitrate solutions were substituted for the Mn2+ ions in the γ-MnS precursors. The capsule-like porous hollow nickel cobalt sulfides were obtained in this manner based on the Kirkendall effect during the cation-exchange process. Figure 1A shows the SEM image of the γ-MnS template. Uniform olive-like γ-MnS nanorods with a size of ~ 100 nm are clearly observed (inset of Figure 1A). The TEM images of the γ-MnS template demonstrate a solid structure rather than a hollow structure (Figures S1A and S1B). After the cation-exchange process, capsule-like porous hollow Co1.77Ni1.23S4 nanocrystals with a wall thickness of 10 nm are clearly observed in the SEM images of Co1.77Ni1.23S4 sample (Figure 1B), indicating the successful synthesis of the hollow cobalt nickel sulfides. Moreover, the size of the hollow Co1.77Ni1.23S4 nano-capsules is similar to that of the γ-MnS template, verifying that the capsule-like hollow Co1.77Ni1.23S4 is derived from the olive-like γ-MnS template. The TEM image of the Co1.77Ni1.23S4 sample (Figure 1C) confirms the morphology of the porous hollow structure. The magnified TEM images show that some small pores are located in the wall of the hollow structure (Figure

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S1C), verifying the existence of the longitudinal Kirkendall effect during the cation-exchange process. The XRD patterns of the γ-MnS template and the as-prepared hollow Co1.77Ni1.23S4 sample are given in Figure 1D. As shown, a pure γ-MnS phase is observed in the MnS template (a line). After the cation-exchange process, spinel cobalt nickel sulfides are successfully obtained. The peaks located at 16.3°, 26.7°, 31.5°, 38.2°, 50.3° and 55.0° are ascribed to the (1 1 1), (2 0 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) facets of spinel nickel cobalt sulfide (JCPDS No. 43-1477), respectively. However, the residual γ-MnS, metal hydroxide and spinel metal oxides are also observed in the XRD patterns. The ring-like plots in the selected area electron diffraction pattern (Figure S1D) confirm the poly-crystalline structure of the porous hollow Co1.77Ni1.23S4 sample. Figure 2A shows the high-resolution TEM (HRTEM) image and fast Fourier transform (FFT, inset) of the wall in the porous hollow nickel cobalt sulfide. As shown, the facets with dimensions of 0.544 nm and 0.236 nm, corresponding to the (1 1 1) and (4 0 0) facets of spinel sulfide, respectively, are clearly observed in the HRTEM image. The FFT plots confirm the diffraction of the (1 1 1), (3 1 1) and (4 0 0) facets of the spinel sulfide. The N2 adsorption-desorption isotherm and the pore-size distribution of the Co1.77Ni1.23S4 sample were conducted to investigate its porous structure. As shown in Figure 2B, the IV-type hysteresis curve and the most probable pore size of 4 nm (inset of Figure 2B) demonstrate that many mesopores are located in the wall of the hollow cobalt nickel sulfides. This result is in good agreement with the TEM images. The mesopores in the wall of the hollow structure offer the path for

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a penetration of electrolytes into the inside of the hollow structure to enhance the surface utilization of the nickel cobalt sulfides. The specific surface area of Co1.77Ni1.23S4 sample is as high as 31.6 m2 g-1. Figures 2C and 2D show the mapping images of the elements S, Ni, Co and Mn in the Co1.77Ni1.23S4 sample. The S, Ni and Co elements are uniformly distributed in the Co1.77Ni1.23S4 sample, indicating the successful cation-exchange process. The existence of the element Mn in the obtained Co1.77Ni1.23S4 sample implies a residue of MnS, which is consistent with the diffraction peak of γ-MnS in the XRD pattern (Figure 1D) of the Co1.77Ni1.23S4 sample. To explore the formation mechanism of the porous hollow nickel cobalt sulfides, the effect of the hydrothermal reaction time and the reaction temperature of cation-exchange process are investigated in detail. As first, the hydrothermal cation-exchange time play an extremely important role in the formation of the porous hollow nickel cobalt sulfide nano-capsules. The TEM images of the nickel cobalt sulfides obtained by 1, 2, 4, 8, 12 and 24 h at 120 °C with designed Ni/Co ratio of 1:1 in the precursor are given in Figure 3. As shown, the hollow structure is gradually formed with the increasing of the reaction time, verifying the success of the cation-exchange process as well as the Kirkendall effect during this process. However, graphene-like nanosheets coated on the nanoparticles are also observed (Figures 3A-3C) when the cation-exchange time is short (1 h, 2 h and 4 h), which should be ascribed to the fact that the dissolution of MnS nanocrystals coexists with the cation-exchange process. The sulfide ions from the dissolution of MnS may be

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precipitated with the nickel and cobalt ions to form these metal sulfide nanosheets. Moreover, the hydrolysis of sulfide ions (Equations 1-2), which can produce hydroxide ions, can also cause the precipitation of the nickel and cobalt ions as metal hydroxides. These assumptions can be demonstrated by the EDX elemental measurements on the different zones of the nanosheets and the nanoparticles, respectively. As shown in Figure S2, only the elements Ni and O are detected in the nanosheets (Figures S2A-S2C), whereas all of the elements S, Mn, Co and Ni are observed in the analysis of the nanoparticles (Figures S2D-S2F). This result implies that the nanosheets may be composed of nickel hydroxide or oxide, whereas the nanoparticles are composed of metal sulfides. S 2− + H 2O ⇔ HS − + OH −

(1)

HS − + H 2O ⇔ H 2 S (aq) + OH −

(2)

The XRD patterns of the cation-exchanged products using different hydrothermal exposure times shown in Figure 4 can also confirm the different composition of the nanosheets and the nanoparticles, which can illustrate the formation process of the porous hollow cobalt nickel sulfides. The diffraction peaks of Ni(OH)2, together with a large amount of γ-MnS and little of the NixCo3-xS4 phase, are observed in the products resulting from short reaction times (a-c lines in XRD patterns for the products obtained at 1 h, 2 h and 4 h), demonstrating that the nanosheets in the samples are Ni(OH)2 phases, consistent with the EDX element analysis results (only the elements Ni and O are present in the nanosheets). When the reaction time is longer than 8 h, spinel cobalt nickel sulfide phases are present in the

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products (d-f lines in XRD patterns for the products obtained at 8 h, 12 h and 24 h). With increasing reaction time, the content of spinel cobalt nickel sulfide gradually increases, accompanied by a decrease in the γ-MnS phase. Meanwhile, the nanosheets disappear when the reaction time is longer than 8 h, accompanied by the appearance of the porous hollow structure, which is shown in the TEM images (Figure 3). These results confirm that the cation-exchange process and the Kirkendall effect play important roles in the formation of the porous hollow structure, although the dissolution-recrystallization of the metal sulfides/hydroxides also occurs during this process. The hydrothermal reaction temperature during the cation-exchange process also affects the morphology and structure of the nickel cobalt sulfides. Figure 5 displays the TEM images of the obtained nickel cobalt sulfides under different hydrothermal reaction temperatures at a fixed time of 24 h with the designed Ni/Co ratio of 1:1 in the precursors. The nanosheets, which have been characterized as Ni(OH)2 phase in the EDX (Figure S2) and XRD patterns (Figure 4), are homogeneously present around the nanoparticles when the hydrothermal reaction temperature is as low as 70 °C (Figure 5B). If the hydrothermal reaction temperature is increased to 100 °C, partial hollow-structured nickel cobalt sulfides are formed, although the floss-like Ni(OH)2 nanosheets are still observed (Figure 5C). Similar hollow structures are obtained when the reaction temperature is raised to 120, 140 and 160 °C (Figures 5D-5F). However, the crystal structure and phase composition of the hollow structural products are different, as demonstrated by the XRD patterns. As shown in Figure 6,

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the characteristic diffraction peaks of spinel NixCo3-xS4 are clearly observed in the samples obtained at 120 and 140 °C. The peaks located at approximately 19°, 27°, 32°, 38°, 50° and 55° are corresponding to the (111), (220), (311), (400), (511) and (440) planes of spinel sulfides, respectively. However, when the reaction time is as high as 160 °C, the peaks of the spinel NixCo3-xS4 are mainly converted into a spinel oxide phase, indicating that the high temperature cause the phase transformation from sulfide to oxide. The clear diffraction peaks of Ni(OH)2 in the XRD patterns of the products obtained at the lower temperature (70 and 100 °C) confirm the dissolution-recrystallization process coexisting with the cation-exchange process. Therefore, maintaining a higher temperature to achieve the dominant effect of the cation-exchange process rather than the dissolution-recrystallization process is another key factor in obtaining porous hollow nickel cobalt sulfides. However, an excessively high temperature will cause the phase transformation of the spinel nickel cobalt sulfides into oxides. The effect of the phase structure of the MnS template on the cation-exchange reaction is also examined to illustrate the formation of the porous hollow structure. The α-MnS template was prepared by the hydrothermal method at 140 °C for 15 h, which was clearly described in the experimental section. The characterizations (including SEM images, XRD pattern and element mapping images) of the α-MnS template and the nickel cobalt sulfide obtained by the cation-exchange process are shown in Figure S3. As shown in Figure S3A, octahedral α-MnS nanocrystals are obtained for use as the template. After the cation-exchange process, the octahedral

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nanocrystals are still observed in the product (Figure S3B), indicating that it is difficult to obtain the hollow structure using α-MnS nanocrystals the template. However, the XRD patterns (Figure S3C) and EDX element analysis (Figures S3D and S3E) reveal that the α-MnS nanocrystals have been transformed to nickel cobalt sulfides with a phase structure similar to α-MnS, indicating to the success of the cation-exchange process between the Mn ions and Ni/Co ions, although the hollow structure is not present. The failure of the formation of the hollow structure using the α-MnS as template should be ascribed to the different structures of α-MnS, which has a rock salt structure, and γ-MnS, which has a lamellar structure. As shown in Scheme 1 and Scheme S1, based on the Kirkendall effect, the ionic transport rate from the crystals to the solution should be faster than that from the solution to the crystals. γ-MnS with the lamellar structure (Scheme S1a) is more suitable for ionic transport than α-MnS with a rock salt structure (Scheme S1b). The layered Mn ions in γ-MnS are easily exchanged at a fast rate (Scheme S1a), whereas the alternating Mn ions (with sulfide ions) in α-MnS are exchanged with greater difficulty at a low rate (Scheme S1b). Therefore, γ-MnS is a more ideal sacrificial template than α-MnS for obtaining a hollow structure via the cation-exchange process. Moreover, capsule-like porous hollow nickel cobalt sulfides with various Co/Ni ratios, as well as pure nickel sulfide and cobalt sulfide, are also successfully synthesized via this efficient cation-exchange process based on the Kirkendall effect. The SEM images of the porous hollow nickel cobalt sulfides with the designed Ni/Co ratios of 1:0, 3:1, 1:1, 1:3 and 0:1 are shown in Figure 7. Capsule-like hollow

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nanocrystals with uniform size of ~ 100 nm are clearly observed in all of these samples. The TEM images of these samples confirm the porous hollow structure of these sulfides (Figure S4). To investigate the actual composition of these sulfides, the Ni/Co in these sulfides are measured by AAS and are collected in Table S1. As shown, the actual Ni/Co ratios in the nickel cobalt sulfides with the designed Ni/Co ratios of 3:1, 1:1 and 1:3, are 3.53:1, 1.44:1 and 1:1.45, respectively. Therefore, these nickel cobalt sulfides are denoted as the Ni2.34Co0.66S4, Ni1.77Co1.23S4 and Ni1.23Co1.77S4 samples. This result demonstrates that the nickel ion is more easily ion-exchanged than the cobalt ion. The spinel phase structures detected in the XRD patterns confirm the success of the cation-exchange process for these cobalt nickel sulfides with various Ni/Co ratios (Figure S4F). In Co3S4 and Ni1.23Co1.77S4, the sharp peaks are found at approximately 26°, 28°, 29°, 46°, 50°, and 54°, corresponding to the (100), (002), (101), (110), (103) and (112) planes of the hexagonal phase of γ-MnS (JCPDS 40-1289), respectively. This also confirms that with the high cobalt content it is not easy to exchange Mn2+ from the γ-MnS templates. In contrast, a small amount of the nanosheets are observed in the Ni3S4 and Co3S4, whereas uniform porous hollow structures are observed in the Ni2.34Co0.66S4, Ni1.77Co1.23S4 and Ni1.23Co1.77S4 samples. This may be attributed to the existence of a synergistic effect between Ni2+ and Co2+, enhancing the cation-exchange process. As shown in Scheme 2, the porous hollow structure of the nickel cobalt sulfides is superior to the solid structure, not only providing a high surface area for the electrochemical reactions but also a suitable ionic transport path for electrolytes,

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which is favorable for the fast and reversible redox reactions required in the pseudocapacitors. Therefore, the nickel cobalt sulfides with a porous hollow structure are expected to exhibit high supercapacitive performance. As shown in Figure 8, the electrochemical properties of the five different metal sulfide materials are evaluated by a three-electrode system in 2 M KOH electrolyte solution. The typical redox peaks in the CV curves (Figure 8A) of all of these electrodes, except the Co3S4 electrode at a scan rate of 1 mV s-1, indicate that these active materials are battery-type active materials,4,

40-41

corresponding to the redox reactions involved that are given in

Equations S1-S3.29 With an increasing in the cobalt content, both the anodic and cathodic peaks of these electrodes gradually shift to a negative potential, which should be attributed to the fact that the redox potential of the transformation between CoS and CoSOH is lower than that of the transformation between NiS and NiSOH. The larger integrated area of the CV curves for the nickel cobalt sulfide electrodes compared with either the cobalt sulfide or nickel sulfide electrodes corresponds to their higher specific capacities, indicating a synergistic effect between the cobalt species and nickel species on the electrochemical performance of the binary sulfide electrodes. In contrast to other electrodes, sharp peaks are observed in the Ni3S4 electrode, which should be assigned to the incomplete discharge of the pure nickel sulfide electrode due to its low electronic conductivity.42-43 The addition of a cobalt species can markedly improve the conductivity of the nickel cobalt binary sulfide electrodes, which can enhance the rate-performance and cycle life of these binary sulfide electrodes. Figure S5A shows the CV curves of the Ni1.77Co1.23S4 electrode at

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different potential scan rates of 1 to 100 mV s-1. Even at 50 and 100 mV s-1, the redox peaks are clearly observed, indicating the good redox rate performance of the Ni1.77Co1.23S4 electrode. The high rate performance of the porous hollow structure nickel cobalt sulfide electrodes should be attributed to a suitable ionic transport path (Scheme 2) and their high electronic conductivity. These results can be confirmed by the EIS spectra. As shown in Figures S5B and S5C, all EIS spectra of these electrodes consist of a semicircle loop in the high-frequency region representing the charge transfer resistance (Rct) and a straight line in the low-frequency region, representing the double-layer capacitance (CPE) and Warburg impedance (W). The intersection of the EIS plots and the real axis represents the ohmic resistance (RΩ), which strongly relies on the contact resistances between each of the active materials and the current collector, as well as the electrolyte solution resistance.44 Based on the fitting of an equivalent circuit (inset of Figure S5B), the corresponding resistance values are listed in Table S2. The low RΩ and Rct value of the Ni1.77Co1.23S4 electrode demonstrates its higher electronic conductivity and faster charge transfer rate than those of other electrodes. The higher CPE values of the Co3S4 and Ni1.77Co1.23S4 electrodes indicate their capacitive characteristics. To further investigate the electrochemical properties of the hollow nickel cobalt sulfide electrodes for supercapacitor applications, the GC measurements of different sulfide electrodes are performed in 2 M KOH electrolyte solution at a current density of 0.25 A g-1. As shown in Figure 8B, except for the Co3S4 electrode, the porous hollow nickel cobalt binary sulfide electrodes show sloped voltage plateaus, reflecting

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their pseudocapacitive characteristics. The specific capacities of the Ni3S4, Ni2.34Co0.66S4, Ni1.77Co1.23S4, Ni1.23Co1.77S4 and Co3S4 electrodes at 0.25 A g-1 are 163.1, 224.5, 182.0, 141.0 and 19.7 mAh g-1, respectively. The higher capacities of the nickel cobalt binary sulfide electrodes than those of the pure nickel sulfide and cobalt sulfide electrodes should be attributed to the uniform porous hollow structure offering a higher surface area for electrochemical reactions, as well as the synergistic effect between the nickel species and the cobalt species in the binary metal sulfides.1, 29 The specific capacities of porous hollow nickel cobalt sulfide electrodes at different current densities are shown in Figure 8C to display the rate-performance of these electrodes. Even at a high current density of 10 A g-1, the Ni1.77Co1.23S4 electrode delivers a high specific capacity of 158.3 mAh g-1, exhibiting a high capacitance retention of 87.0 % compared with 182.0 mAh g-1 at 0.25 A g-1. The capacity retention rates of the Ni3S4, Ni2.34Co0.66S4, Ni1.23Co1.77S4 and Co3S4 electrodes at 10 A g-1 are 22.0 %, 63.0 %, 61.9 % and 100 %, respectively. The Ni1.77Co1.23S4 electrode, exhibiting a lower specific capacitance than the Ni2.34Co0.66S4 electrode at a low current density, shows a higher specific capacitance at higher current density, corresponding to the higher rate performance of the Ni1.77Co1.23S4 electrode. The superior rate performance of the Ni1.77Co1.23S4 electrode should be attributed to not only the unique porous hollow structure, which possesses more active sites for charge transfer and a suitable path for ionic transport, but also the higher electrical conductivity of the binary metal sulfide electrodes demonstrated in Table S2. A similar phenomenon, observed between the Ni3S4 and the Ni1.23Co1.77S4 electrodes

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should be attributed to a similar reason. Moreover, the Ni1.77Co1.23S4 electrode with suitable cobalt content exhibits a high cycling performance. As shown in Figure 8D, after 1000 cycles, the specific capacitance of the Ni1.77Co1.23S4 electrode is still 96.3 mAh g-1 with a capacitance retention of 55.7 %, which is higher than the other sulfide electrodes (Figure S5D). The Coulombic efficiency of Ni1.77Co1.23S4 electrode is continuously higher than 95 % in these 1000 cycles. The cycling performances of the five nickel cobalt sulfide electrodes are shown in Figure S5D to investigate the effect of cobalt content on the cycling performance of nickel cobalt sulfide electrode. With the addition of cobalt, the Co-rich cobalt nickel sulfide electrodes (Ni1.77Co1.23S4 and Ni1.23Co1.77S4 electrodes) exhibit a higher cycle life than that of either the pure nickel sulfide or the Ni-rich Ni1.77Co1.23S4 electrode. To evaluate the practical application of the porous nickel cobalt sulfide electrodes, an all-solid-state asymmetric supercapacitor, denoted as an NCS//HMC supercapacitor, is assembled with the as-prepared Ni1.77Co1.23S4 as the positive electrode material and a home-made carbon (HMC) as the negative electrode material. The XRD pattern, TEM images and electrochemical properties are given in Figure S6. As shown, the HMC electrode is composed of uniform mesoporous activated carbon (Figures S6A-S6B) with a graphite structure (Figure S6C). Due to the suitable structure (discussed in Supporting Information), the HMC electrode exhibits high capacitive performance (Figures S6D-S6F). The electrochemical properties of the as-prepared NCS//HMC supercapacitor are shown in Figure 9. As shown, the quasi-rectangular shapes in the CV curves (Figure 9A), as well as the quasi-triangular shapes in the GC

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curves (Figure 9B), indicate the typical capacitive characteristic of the NCS//HMC supercapacitor. The quasi-rectangular shape of the CV curve is still observed even at a high scan rate (100 mV s-1), demonstrating the high rate capacity of the as-prepared NCS//HMC supercapacitor (Figure 9A), which is also confirmed by the GC curves. The good symmetry of the GC curves at different current densities indicates that the Ni1.77Co1.23S4 electrode possesses good electrochemical reaction reversibility (Figure 9B). The specific capacitances of the NCS//HMC supercapacitor at different current densities (based on the total mass of both the positive and negative materials) are given in Figure 9C. The as-evaluated specific capacitances at 0.1, 0.25, 0.5, 1, 2 and 5 A g-1 are 146.55, 129.83, 119.38, 110.79, 100.57 and 94.99 F g-1, respectively. The high capacitance retention of 64.82 % at 5 A g-1 further confirms the superior rate performance of the electrode, which may be attributed to the hollow structure and the high electronic conductivity of the Ni1.77Co1.23S4 materials. Moreover, the NCS//HMC supercapacitor exhibits high coulombic efficiencies in all of these current densities. The lower coulombic efficiency (slightly higher than 95%) at a low current density than at high current density should be attributed to the fact that the ratio of the side reactions at a low current density is higher than that at a high current density. Figure 9D shows Ragone plots of the energy density versus the power density of the NCS//HMC supercapacitor. The energy density is calculated by the equation E = 1 / 2CV 2 where C is the capacitance, V is the cell voltage and E is the energy density. As shown, the energy density is 42.7 Wh kg-1 at a power density of 190.8 W kg-1 and 29.4 Wh kg-1 even at 3.6 kW kg-1. These values are higher than those of the

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Ni3S2/MWCNT-NC//AC

(19.8

Wh

kg-1

at

798

W

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kg-1),45 the

CoNi2S4

nanosheet//activated carbon (27.2 Wh kg-1 at 2.46 kW kg-1)31 and NiO//AC (~ 20 Wh kg-1)46 asymmetric supercapacitor. Based on the equation E = 1 / 2CV 2 , the excellent performance of the NCS//HMC supercapacitor should be ascribed to the high cell voltage of 1.55 V and the high specific capacities of the high-power battery-type Ni1.77Co1.23S4 electrode. The NCS//HMC supercapacitor also exhibits a high cycling performance. As shown in Figure S7, the capacitance retention rates at the 2000th and 5000th cycle are 72.5 % and 57.5 %, respectively. Furthermore, the series assembled with two single NCS//HMC supercapacitors can light up a red LED indicator for more than 1 h after being fully charged (Figures 9E and 9F). Based on its excellent performance and practical application, the NCS//HMC supercapacitor and the porous hollow cobalt nickel sulfides are a promising device and active materials for energy storage. CONCLUSIONS In summary, the capsule-like porous hollow nickel cobalt binary sulfides are successfully synthesized via a cation-exchange reaction in hydrothermal system with

γ-MnS as the sacrificial template. The formation mechanism of the capsule-like porous hollow nanostructure of the nickel cobalt sulfides is illustrated in detail by tuning the reaction time, reaction temperature and the sacrificial template. The fast cation-exchange process based on the Kirkendall effect plays an important role in the formation of the porous hollow structure, although the dissolution-crystallization process coexists in the hydrothermal system. The porous hollow structure with the

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high surface area can not only enormously increase the active sites for Faradaic redox reaction on the electrode surface but also efficiently offer short channels for electrolyte ionic transport. For these reasons, the Ni1.77Co1.23S4 electrode exhibits a high specific capacity of 224.5 mAh g-1 and a high capacitance retention of 87.0 % at a current density of 10 A g-1. Moreover, an all-solid-state asymmetric supercapacitor, assembled using the Ni1.77Co1.23S4 as the positive material and denoted as the NCS//HMC supercapacitor, delivers a high energy density of 42.7 Wh kg-1 at a power density of 190.8 W kg-1. The series assembled with two as-prepared NCS//HMC supercapacitors lights up a red LED indicator for more than 1 h, indicating the promising practical application of the NCS//HMC supercapacitor. Based on these exciting results, the low-cost porous hollow nickel cobalt sulfides and the high performance NCS//HMC supercapacitor will boost the practical application of these materials and supercapacitor devices in the storage of energy. ASSOCIATED CONTENT

Supporting Information Supplementary Figures: S1. High-magnification TEM images of samples, S2. Elemental analysis of γ-MnS template, S3. Physicochemical characterization of the cation-exchange process with α-MnS nanocrystals as a template, S4. XRD pattern and TEM images of porous hollow nickel cobalt sulfide samples with different Ni/Co ratios. S5. Supplementary electrochemical properties of the obtained nickel cobalt sulfide electrodes, S6. Physicochemical characterization and electrochemical properties of the home-made activated carbon, S7. Cycling performance of the

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NCS//HMC supercapacitor. Supplementary Equations, Supplementary Tables: S1. Elemental analysis of samples via AAS, S2. Electrochemical parameters of the nickel cobalt sulfide electrodes. AUTHOR INFORMATION

Corresponding author * E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Yongfu Tang received funding from National Natural Science Foundation of China (No. 21406191), China Postdoctoral Science Foundation (No. 2012M520597 & 2015T80232), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20131333120011) and Youth Top-notch Talent Support Program of Higher Education in Hebei Province (No. BJ2016053). Wenfeng Guo received funding from National Natural Science Foundation of China (No. 61275100). Shengxue Yu received funding from Natural Science Foundation of Hebei Province (B2015203406). REFERENCES (1) Xiao, J.; Wan, L.; Yang, S.; Xiao, F.; Wang, S. Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for High-performance Pseudocapacitors. Nano Lett. 2014, 14, 831-838. (2) Subramanian, V.; Zhu, H.; Vajtai, R.; Ajayan, P. M.; Wei, B. Hydrothermal

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Figure Captions Scheme 1. Scheme for the formation of porous hollow cobalt nickel sulfides. Scheme 2. Scheme for the suitability of the porous hollow structure for ionic transport.

Figure 1. SEM image and magnified SEM image (inset) of (A) the γ-MnS template and (B) the porous hollow Co1.77Ni1.23S4 sample, (C) TEM image and magnified TEM image (inset) of the porous hollow Co1.77Ni1.23S4 sample, (D) XRD patterns of the

γ-MnS template and the porous hollow Co1.77Ni1.23S4 sample. Figure 2. (A) High-resolution TEM image and the FFT plots (inset) of the porous hollow Co1.77Ni1.23S4 sample, (B) Nitrogen adsorption-desorption isotherm and pore size distribution (inset) of the Co1.77Ni1.23S4 sample, (C) SEM image for the element-mapping image and (D) S, Ni, Co and Mn element-mapping images based on the (C) image.

Figure 3. TEM images of nickel cobalt sulfides obtained from cation-exchange process at 120 oC in hydrothermal system for (A) 1 h, (B) 2 h, (C) 4 h, (D) 8 h, (E) 12 h and (F) 24 h, respectively.

Figure 4. XRD patterns of nickel cobalt sulfides obtained from the cation-exchange process at 120 °C in a hydrothermal system for 1 h, 2 h, 4 h, 8 h, 12 h and 24 h, respectively.

Figure 5. TEM images of (A) the γ-MnS template and nickel cobalt sulfides obtained from cation-exchange process in a hydrothermal system for 24 h at (B) 70 °C, (C) 100 °C, (D) 120 °C, (E) 140 °C and (F) 160 °C, respectively.

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Figure 6. XRD patterns of the γ-MnS template and nickel cobalt sulfides obtained from the cation-exchange process in a hydrothermal system for 24 h at 70 °C, 100 °C, 120 °C, 140 °C and 160 °C, respectively.

Figure 7. SEM images of (A) the γ-MnS template and nickel cobalt sulfides obtained from the cation-exchange process in a hydrothermal system at 120 °C for 24 h with the Ni/Co ratios of (B) 1:0, (C) 3:1, (D) 1:1, (E) 1:3 and (F) 0:1, respectively.

Figure 8. (A) CV curves and (B) charge-discharge curves of the various nickel cobalt sulfide electrodes with different Ni/Co ratios, (C) Specific capacities of the various nickel cobalt sulfide electrodes with different Ni/Co ratios based on the charge-discharge curves, (D) Cycle life performance and Coulombic efficiencies of the Ni1.77Co1.23S4 electrode.

Figure 9. (A) CV curves and (B) charge-discharge curves of the as-prepared NCS//HMC all-solid-state supercapacitor, (C) Specific capacitances and Coulombic efficiencies of the NCS//HMC all-solid-state supercapacitor, (D) Ragone plots (power density vs. energy density) of the NCS//HMC all-solid-state supercapacitor. (E) Photograph of the LED indicator lit by the fully charged NCS//HMC all-solid-state supercapacitor and (F) the luminances of the LED indicator after being lit for different time periods.

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Table of Contents Graphic

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Scheme 1 460x400mm (96 x 96 DPI)


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Figure 1 905x789mm (50 x 50 DPI)



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Figure 4 297x210mm (300 x 300 DPI)


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Figure 6 297x210mm (150 x 150 DPI)


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Figure 8 1080x762mm (50 x 50 DPI)


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Figure 9 1092x1130mm (45 x 45 DPI)


Synthesis of Capsule-like Porous Hollow Nanonickel Cobalt

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