Research Article www.acsami.org
Assembling Hollow Cobalt Sulfide Nanocages Array on Graphenelike Manganese Dioxide Nanosheets for Superior Electrochemical Capacitors Hao Chen, Min Qiang Wang, Yanan Yu, Heng Liu, Shi-Yu Lu, Shu-Juan Bao,* and Maowen Xu Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing 400715, P. R. China S Supporting Information *
ABSTRACT: Metal-organic framework (MOF)-derived hollow cobalt sulfides have attracted extensive attention due to their porous shell that provides rich redox reactions for energy storage. However, their ultradispersed structure and the large size of MOF precursors result in relatively low conductivity, stability, and tap density. Therefore, the construction of an array of continuous hollow cages and tailoring of the inner cavity of MOF-derived materials is very effective for enhancing the electrochemical performance. Herein, we in situ assembled small Co-based zeolitic imidazolate framework (ZIF-67) on the both sides of negatively charged MnO2 nanosheets to fabricate a hierarchical sandwich-type composite with hollow cobalt sulfide nanocages/graphene-like MnO2. The graphene-like MnO2 nanosheets acted not only as a structure-directing agent to grow a ZIF-67 array but also as a promising electroactive material of electrochemical capacitors to provide capacitance. As an electrode material of supercapacitors, the as-prepared composites exhibit high specific capacitance (1635 F g−1 at 1 A g−1), great rate performance (reaching 1160 F g−1 at 10 A g−1), and excellent cycling stability (80% retention after 5000 cycles). The outstanding electrochemical properties of our designed materials can be attributed to the unique nanostructure that improved electrical conductivity, created more reactive active sites, and increased the diffusion pathway for electrolyte ions. KEYWORDS: metal−organic frameworks, manganese dioxide nanosheets, cobalt sulfide, hollow nanocages array, electrochemical capacitors
1. INTRODUCTION The fast depletion of fossil fuels and environmental crises from global warming have stimulated tremendous research efforts to exploit sustainable and renewable energy sources, energystorage systems, and relevant advanced technologies. Electrochemical capacitors (ECs), as a type of energy-storage device, have aroused widespread attention due to their fast charging/ discharging feature, high power capacity, long cycle life, and excellent safety. These properties may fill the power-density gap between conventional capacity and Li-ion batteries, making them promising candidates to power mobile electronics and hybrid electric vehicles, among other devices.1−4 The composition and microstructure of electrode materials play a pivotal role in determining the electrochemical performance of advanced energy-storage devices. The latest research and development in this area have focused on exploring novel electrode materials for next-generation ECs. Various carbon materials, transition metal oxides/hydroxides/sulfides, and conducting polymers have been the three most commonly reported major electrode materials.5−11 However, as is wellknown, each material has its own obvious advantages and disadvantages. Generally, carbon-based materials, a typical © 2017 American Chemical Society
electrode material, store capacitance from the reversible adsorption−desorption of ions on their surfaces, leading to a high power density and long cycle life but low energy density. In contrast, the capacitance of metal oxides/hydroxides/sulfides and conducting polymers is derived from a redox-type process similar to the one in batteries that produces high energy density but is kinetically unfavorable and has poor cycling stability.12 To bridge the performance gap between these materials, plentiful studies have designed various nanostructures to construct effective nanoarchitectures as a vital prerequisite for boosting electrochemical performance.13 Particularly, hollow nanostructures offer great benefits for the construction of advanced electrodes because they combine advantages of both hollow cavity and porous thin wall. Cavities in the hollow structure are beneficial for increasing specific capacitance due to the large contact area of electrolyte materials and higher content of active species. The porous walls consist of nanosized subunits that ensure a larger specific surface area and greatly Received: August 12, 2017 Accepted: September 18, 2017 Published: September 18, 2017 35040
DOI: 10.1021/acsami.7b12069 ACS Appl. Mater. Interfaces 2017, 9, 35040−35047
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a, b) FESEM and TEM images of the as-prepared ultrathin MnO2 nanosheets, respectively. (c) FT-IR spectra of MnO2. (d) Zeta potential of the pristine MnO2 and MnO2/Co2+ dispersed in deionized (DI) water.
Figure 2. (a) Schematic synthesis of CoSx@MnO2. (b) FESEM images of ZIF-67@MnO2. (c) TEM image of sandwich-type ZIF-67@MnO2. (d) FESEM image of ZIF-67@MnO2. (e) FESEM image of CoSx@MnO2. (f, g) TEM images of CoSx@MnO2 with different magnifications.
example, Chen’s group reported hollow nanopolyhedra-layered double hydroxides using ZIF-67 as a template for electrode material of supercapacitors with a specific capacitance of 1203 F g−1 at 1 A g−1.17 Lou’s group reported an onion-like hollow NiCo2S4 derived from Co-based coordination polymer spheres, which delivered a specific capacitance of 1016 A g−1 at 2 A g−1
shorten the transport distance of ions, making it possible to charge and discharge more rapidly.14−16 Recently, metal− organic frameworks (MOFs), formed by metal ions and rigid organic molecules in a periodically porous architecture, have been proven to be an appealing precursor, or a sacrificial template, for the construction of hollow structures. For 35041
DOI: 10.1021/acsami.7b12069 ACS Appl. Mater. Interfaces 2017, 9, 35040−35047
Research Article
ACS Applied Materials & Interfaces and was used as an electrode material for supercapacitors.18 However, common drawbacks of MOF-derived hollow structures include dispersible structures and a large cavity resulting from their dispersible structure and large-sized MOF materials, which cause relatively low conductivity, stability, and tap density due to the independent particles structure and excess empty space within the particles. To enhance the specific capacitance, stability, and power densities of ECs fabricated from these hollow materials, the independent hollow particles should form a continuous conductive network and the inner cavity of the hollow structure should be confined on a nanoscale.16,19−21 More recently, direct assembly of lowdimensional electroactive materials on substrates is particularly attractive due to the formation of a 3D interconnected porous structure, which is significant for providing a sufficient porous channel, large surface area, and well-defined electrolyte diffusion pathways. Furthermore, electroactive nanostructure materials directly adhered on the substrates form a continuous conductive network that can facilitate charge transport, reducing internal resistance and boosting the cycling stability of ECs.13 For example, Fan’s group reported a 3D Co3O4@ MnO2 core/shell array as the electrode materials of supercapacitors, which possessed higher capacitance and maintained a better cyclic stability than pristine Co3O4.12 Yin’s group prepared a 3D structured, graphene-wrapped, MOF-derived, Co-doped porous carbon, which exhibited higher specific capacitance and better cycling stability than single MOFderived, Co-doped porous carbon when used as an electrode material in a battery.22 On the basis of the above analysis and examples, we effectively designed and synthesized an integrated smart architecture that holds great potential to achieve superior electrochemical performance, in which the advantages of the architecture and electroactive materials of each component were fully manifested. In our work, cobalt sulfide with rich redox reactions and manganese dioxide with high theoretical specific capacitance (1300 F g−1) were selected as the promising electroactive materials to construct an electrode of ECs.9,23 As shown in Figure 1a and b, very slack MnO2, consisting of ultrathin layer sheets with rich folds similar to graphene, was prepared in our experiment. Interestingly, we found that the surface of the asprepared MnO2 nanosheets possessed rich functional groups (Figure 1c) and negative charges (Figure 1d), which inspired use of the MnO2 nanosheets as a structure-directing agent to assemble other nanoelectroactive materials on the sheets as a method to construct a 3D porous architecture. As shown in Figure 2a, cobalt ions were absorbed on the surface of negatively charged MnO2 ultrathin nanosheets by electrostatic interactions, providing nucleation sites for the formation of ZIF-67. After the addition of 2-methylimidazole during the hydrothermal process, MnO2 nanosheets covered by Co2+ could restrain the growth of ZIF-67. A uniform network of small ZIF-67 nanocubes continuously assembled on the surface of MnO2 nanosheets to form a sandwich-type structure (denoted as ZIF-67@MnO2). Via a simple sulfuration process, these ZIF-67 nanocubes transformed into hollow cobalt sulfides nanocages (denoted as CoSx@MnO2) with a small size (∼50 nm) and ultrathin shell (∼10 nm) and still firmly anchored on both sides of the MnO2 nanosheets. In this approach, the negatively charged ultrathin MnO2 nanosheets acted as a structure-directing agent to synthesize the special nanostructure and also as a promising electroactive material of ECs to provide redox capacitance. As an EC electrode material, our designed
3D CoSx@MnO2 composites delivered a high specific capacity (1635 F g−1 at 1 A g−1), outstanding rate capability (remained 71% from 1 to 10 A g−1), and excellent cycling stability (81% retention after 5000 cycles at 5 A g−1), which is far better than those of single CoSx (1013 F g−1 at 1 A g−1, remained 65% from 1 to 10 A g−1, and 45% retention after 5000 cycles at 5 A g−1) and MnO2 (229 F g−1 at 1 A g−1). The excellent electrochemical property of CoSx@MnO2 is attributed to the following reasons: (1) The hierarchical 3D porous nanostructure of our designed materials was advantageous for the fast diffusion of electrolytes. (2) Continuous hollow nanostructures formed a continuous microcosmic conductive network and greatly improved the electrical conductivity. (3) Small-sized CoSx nanocages with ultrathin shells assembled on the MnO2 nanosheets, offering a bigger surface area and more reactive active sites. Furthermore, a hybrid supercapacitor, which obtained a high-energy density of 49.56 Wh kg−1 at the power density of 800 W kg−1, was also fabricated in our work. Our proposed synthetic methods and designed novel nanostructured hybrids offer an effective opportunity to assemble various continuous hollow-structure materials on the ultrathin negatively charged surface of MnO2 or on the surface of other negatively charged electroactive materials.
2. EXPERIMENTAL SECTION Synthesis. All chemicals were of analytical grade and were purchased from commercial sources without further purification. Preparation of MnO2. Manganese acetate acid (0.50 g) and ethylenediaminetetraacetic acid disodium salt (EDTA, 1.5 g) were dissolved into 50 mL of deionized water at 30 °C under magnetic stirring. Subsequently, 50 mL of 0.25 M NaOH aqueous solution was added dropwise to the above solution. Then, 50 mL of 0.12 M K2S2O8 aqueous solution was added dropwise to initiate the chemical reaction. The mixed solution was kept at 40 °C for 12 h, and then the obtained precipitates were filtered, washed with deionized water several times, and dried at 80 °C for 12 h to obtain the ultrathin MnO2 nanosheets. Preparation of ZIF-67@MnO2. MnO2 nanosheets (0.02 g) were first uniformly dispersed in 10 mL of methanol under ultrasonication for 2 min, and then 0.16 g of Co(NO3)2·6H2O was dissolved into the above solution under stirring for 30 min at room temperature. Subsequently, 6 mL of CH3OH solution containing 0.33 g of 2methylimidazole was added dropwise to the above solution. With continuous stirring for 2 h, the obtained precipitate was collected by centrifugation, washed with methanol several times, and then dried at 60 °C for 12 h. For comparison, ZIF-67 was also synthesized following the same procedure without adding MnO2 nanosheets. Preparation of CoSx@MnO2. The as-prepared ZIF-67@MnO2 (0.02 g) and thioacetamide (0.060 g) were dissolved into 40 mL of ethanol with stirring. The resultant solution was transferred into a sealed glass bottle and heated at 80 °C for 3 h under continuous stirring. Subsequently, the products were collected by centrifugation, washed with ethanol several times, and then dried at 60 °C for 12 h. For comparison, the CoSx was also synthesized following the same procedure, where ZIF-67@MnO2 was replaced with ZIF-67. Materials Characterization. The surface morphology and structure of the as-prepared samples were characterized using scanning electron microscopy (FESEM, JSM-7800F) and transmission electron microscopy (TEM, JEOL 2100). The XRD patterns of the as-prepared samples were recorded using powder X-ray diffraction (Shimadzu XRD-7000) with Cu Kα radiation. The composition and surface chemical states of the as-prepared samples were characterized by energy-dispersive X-ray spectroscopy (EDS, INCA X-Max 250) and Xray photoelectron spectroscopy (XPS, Escalab 250xi). Nitrogen adsorption−desorption isotherms were recorded on Autosorb-1 (Quantachrome Instruments). Inductively coupled plasma mass spectrometry (ICP-MS) analysis was performed on PerkinElmer PE Optima 8000. The specific surface area was obtained using a 35042
DOI: 10.1021/acsami.7b12069 ACS Appl. Mater. Interfaces 2017, 9, 35040−35047
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) XRD patterns. (b) High-resolution S 2p XPS spectra of CoSx@MnO2. (c) High-resolution Co 2p XPS spectra of CoSx@MnO2 and CoSx. (d) High-resolution Mn 2p XPS spectra of CoSx@MnO2 and MnO2.
respectively. The absorption peaks at 1380 and 1070 cm−1 may be assigned to the interaction of Mn with the layered inorganic host species, such as Na+ and H+, while the peak at 460 cm−1 can be attributed to Mn−O stretching vibrations.26 To examine the electrostatic interaction between Co2+ and MnO 2 , the zeta potentials of MnO 2 and MnO 2 /Co 2+ suspension were measured. As shown in Figure 1d, the zeta potential values of MnO2 and MnO2/Co2+ were determined to be −40.7 and +28.1 mV, respectively. It is clear that the asprepared MnO2 displayed a markedly negatively charged surface, which may be attributed to the numerous functional groups on the surface of MnO2 and the ultrathin layer structure. After mixing with Co2+ solution, the MnO2 surface was fully covered by Co2+ via electrostatic interaction.27,28 The in situ assembling and confined growing process of ZIF67 on the surface of ultrathin MnO2 nanosheets, which further transferred to hollow structured CoSx and firmly anchored on the surface of MnO2, were observed by FESEM and TEM. As seen in Figure 2b−d, ZIF-67 nanocubes around 50 nm grew on both sides of MnO2 to construct a sandwich-structured composite, and the graphene-like ultrathin MnO2 acted as a template and binder to connect the ZIF-67 and form a continuous network. After sulfuration, the sandwich-type structure of ZIF-67@MnO2 was maintained perfectly. The high-resolution FESEM image (Figure 2e) of CoSx@MnO2 reveals that the hierarchical network of continuous hollow nanocages uniformly formed on the surface of MnO2. More details of the as-prepared composites were further observed by TEM (Figure 2f), which reveals the uniform distribution of CoSx hollow nanocages on the graphene-like ultrathin MnO2 to form a continuous hollow CoSx nanocage network. The inner cavity can be observed by contrast of the hollow interiors and shells, which were constructed of compactly arranged nanoparticles with a thickness of ∼10 nm (Figure 2g). The ultrathin
multipoint Brunauer−Emmett−Teller (BET) model, and the pore-size distribution was obtained by the Barrett−Joyner−Halenda (BJH) model. The infrared (IR) spectra were characterized using an IR device (Nicolet Model Nexus 470), and zeta potential was obtained using the Nano ZS90 (Malvern Instruments). Electrochemical Measurements. The electrochemical performances of the as-prepared samples were estimated in a three-electrode cell using 2 M KOH as the electrolyte. The working electrode was prepared by mixing the active material (75 wt %), acetylene black (15 wt %), and polytetrafluoroethylene (PTFE) (10 wt %). The slurry was coated onto the nickel foam (1 × 1 cm and 1 mg cm−2) and was then dried at 60 °C for 12 h. The platinum foil was used as a counter electrode, and Hg/HgO electrode was used as the reference electrode. The hybrid supercapacitors (HSCs) were prepared using as-prepared CoSx@MnO2 as the positive electrode materials and bioderived active carbon (BAC)24 as the negative electrode material. The preparation method of the negative electrode was the same as that for the working electrode. Electrochemical impedance spectroscopy (EIS) was recorded within 0.01−100 000 Hz at open-circuit potential. Cyclic voltammetry (CV) and EIS were performed on an electrochemical workstation (CHI 760E Shanghai Chenhua, China). Charge and discharge measurements were performed on Land CT2001A test system (Wuhan LAND electronics Co., Ltd., China).
3. RESULTS AND DISCUSSION The microstructure and morphology of the MnO2 are shown in parts a and b of Figure 1, respectively. It is clear that the asprepared MnO2 was very slack and consisted of ultrathin layer sheets with rich folds liked that of graphene reported in previous literature.25 TEM images further display its ultrathin two-dimensional (2D) graphene-like structure. Fourier tranform infrared (FT-IR) was used to characterize the functional groups of MnO2 nanosheets, and the results are shown in Figure 1c. The bands around 3460 and 1640 cm−1 are respective of the stretching vibrations of interlayer water molecules and bending vibrations of hydroxyl groups, 35043
DOI: 10.1021/acsami.7b12069 ACS Appl. Mater. Interfaces 2017, 9, 35040−35047
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) CV curves at a scan rate of 50 mv s−1; (b) Nyquist plots; (c) galvanostatic charge/discharge curves at 0.5 A g−1; (d) specific capacitance of the as-prepared samples at different current densities; and (e) cyclic stability of sandwich-structured CoSx@MnO2 at 5 A g−1.
Furthermore, from Figure S4, the XRD pattern of ZIF-67@ MnO2 was well-indexed to delta-phase MnO2 and simulated ZIF-67. The XRD patterns of the CoSx and CoSx@MnO2 show a broad peak around 23°, indicating the amorphous nature of these samples, which is similar to those previously reported.29 However, no MnO2 peaks were observed in the XRD patterns of the CoSx@MnO2 sample, which may be accountable to the following reasons: (1) the surface of MnO2 was covered by amorphous CoSx; (2) during the sulfuration process, a portion of surface MnO2 was sulfureted to form amorphous manganese sulfide, so that the diffraction peaks of MnO2 were flooded by amorphous peaks. The EDS were further used to analyze the elemental compositions and distribution of the CoSx@MnO2 (Figure S7a) and CoSx (Figure S7b). As displayed in Figure S4a, the atomic ratio was ∼1:1.28 for Co/S, which is similar to that of CoSx (∼1:1.30 for Co/S, Figure S4b), and the atomic ratio of Co/Mn in CoSx@MnO2 was ∼3:1. The EDS elemental mapping analysis also suggests that the various elements were homogeneously distributed throughout the whole sample (inset of Figure S7a and b). The atomic ratio of Co and Mn in CoSx@ MnO2 was also measured by ICP (Co/Mn = 2.85:1), and it is very similar to the value (3:1) obtained from EDS analysis. XPS was used to characterize the chemical states of elements on the surface of MnO2, CoSx, and CoSx@MnO2. The S 2p spectrum of CoSx@MnO2 is shown in Figure 2b and displays two main peaks assigned to S2− (162.1 and 162.2 eV) and SOx
shell thickness can well-facilitate the diffusion of the electrolyte throughout the whole structure and significantly enhance the rate capability of supercapacitors. To investigate the role of MnO2 in the formation of the sandwich-type composite structure, pure ZIF-67 crystals were also synthesized and sulphureted under the same conditions. As shown in Figure S2a and b, the ZIF-67 particles were dispersible, and the size of ZIF67 particles was ∼500 nm, which is far bigger than those grown on the surface of MnO2. To further investigate the microstructure of the as-prepared samples, N2 adsorption−desorption isotherms and corresponding pore-size distribution of CoSx@MnO2, CoSx, and MnO2 were obtained, as shown in Figure S3a and b. The II-type adsorption−desorption isotherm and H3-type hysteresis loops indicate the mesoporous characterization of the three asprepared samples. The surface area of the hierarchical CoSx@ MnO2 nanostructures was calculated to be 100.5 m2 g−1 using the multipoint BET method, which is greater than that of the CoSx hollow cages (42.1 m 2 g−1) and ultrathin MnO 2 nanosheets (80.0 m2 g−1). The higher specific surface area of the CoSx@MnO2 should ascribe to small size (∼50 nm) and ultrathin shell (∼10 nm) of the CoSx nanocages array. The crystal structures of the as-prepared products were characterized by XRD. As seen in Figure 3a, the XRD patterns of ultrathin MnO2 nanosheets were well-indexed to delta-phase MnO2 (JCPDS card no. 43-1456) without any impurity peaks. 35044
DOI: 10.1021/acsami.7b12069 ACS Appl. Mater. Interfaces 2017, 9, 35040−35047
Research Article
ACS Applied Materials & Interfaces
Figure 5. (a) Charge-storage mechanism of the CoSx@MnO2 cathode and the activated carbon anode in the hybrid supercapacitors system. (b) CV curves at different scan rates and (c) charging−discharging curves at different current densities of the hybrid supercapacitors. (d) Cyclic stability of the hybrid system at 5 A g−1. (e) Ragone plot of the CoSx@MnO2//AC hybrid supercapacitors and related reported elsewhere. (f) Digital image of the hybrid supercapacitors cell voltage and the inset of a red-light-emitting diode lighted by the hybrid supercapacitor device.
resulting CoSx@MnO2 displayed redox peaks at about 0.26 and 0.54 V, which are attributed to the synergistic redox contribution from the redox reaction of CoSx nanocages and partially vulcanized MnO2 .Compared to hollow CoSx hollow cages and ultrathin MnO2 nanosheets, the peak current and area of the CV curve of CoSx@MnO2 are obviously larger, indicating that a larger capacitance of CoSx@MnO2 was obtained. The area and peak current of the CV curve of the nickel foam were very small, which suggests that the capacitance of Ni foam is negligible compared to that of the electrode materials. The ion and charge-transfer characteristics of the as-prepared samples were investigated using EIS measurement. The corresponding Nyquist plot (Figure 4b) was obtained at open-circuit potential. Generally, the diameter of the semicircle at the high-frequency region (inset of Figure 4b) represents the charge-transfer resistance, and the straight line in the lowfrequency region (Figure 4b) refers to the diffusive resistance of the electrolyte in the electrode and host materials. It can be determined from Figure 4b that CoSx@MnO2 had the smallest electrochemical reaction resistance and ion-diffusion resistance among the three samples, which is attributed to its hierarchical 3D continuous hollow nanostructured network. It is well-accepted that galvanostatic charge−discharge examination can be used to estimate the specific capacitance of electrode materials. As shown in Figure 4c, two different steps appeared during the charge and discharge of all samples. The parallel to vertical direction of charge and discharge curves on the first steps indicates double-layer capacitance and pseudocapacitance behavior, whereas the parallel to horizontal direction of the potential vs time (0.5−0.3 V) reveals batterylike behavior.4 The capacitance of as-prepared CoSx@MnO2, CoSx cages, and MnO2 nanosheets were calculated to be 1720, 1013, and 230 F g−1, respectively, at 0.5 A g−1. Obviously,
(168.6 and 170.0 eV). The SOx peaks may have been caused by adsorbed oxygen on the active surface of CoSx@MnO2. The spectrum of Co 2p in CoSx@MnO2 (Figure 3c) may be best described by two spin−orbit doublets and two satellite peaks (identified as “Sat.”). The first doublets at 779.0 and 794.0 eV and the second doublets at 781.3 and 797.4 eV are ascribed to Co3+ and Co2+, respectively.30 The existence of Co3+ indicates that some Co2+ on the surface of sample was oxidized. Furthermore, as shown in Figure 3e, the binding energies and intensity of Co 2p in CoSx@MnO2 are very similar to those in CoSx, which indicates the similar chemical state of Co on the surface of CoSx@MnO2 and CoSx. The Mn 2p spectra of MnO2 decomposed into four peaks (Figure 3d), where the two peaks ∼642.0 and ∼652.9 eV are ascribed to Mn 2p3/2 and Mn 2p1/2 of the Mn3+ species, respectively, and the two peaks ∼642.7 and ∼654.2 eV are ascribed to the Mn4+.31 It should be noted that the binding energies of Mn3+ 2p in CoSx@MnO2 were negatively shifted compared to those of Mn3+ 2p in MnO2, and the intensity of Mn3+ 2p in CoSx@MnO2 was stronger than Mn3+ 2p in MnO2. The latter result is good proof that a portion of Mn4+ on the surface of MnO2 was reduced to Mn3+ and formed amorphous sulfide during the sulfuration process. This is consistent with the XRD results that do not show any diffraction peaks of MnO2 in the CoSx@MnO2. The electrochemical performance of as-prepared samples as the active electrode material of supercapacitors was first measured by CV. It is apparent that all CV curves show a couple of redox peaks within the measured potential range. For CoSx, a pair of oxidation/reduction peaks around 0.31 and 0.57 V originated from the faradaic redox reaction related to Co2+/ Co3+. A couple of redox peaks appeared around 0.34 and 0.46 V for ultrathin MnO2 nanosheets, which are ascribed to the insertion/extraction of K+ on the ultrathin MnO2 nanosheets.12,25 After combining the above two components, the 35045
DOI: 10.1021/acsami.7b12069 ACS Appl. Mater. Interfaces 2017, 9, 35040−35047
Research Article
ACS Applied Materials & Interfaces hierarchical 3D sandwich-type continuous hollow CoSx@MnO2 possessed the highest specific capacitance, which is in good agreement with observations based on the CV curves. The rate behavior of hierarchical sandwich-type CoSx@MnO2 can be seen in Figure S8b, and the analogous shape of the galvanostatic charge/discharge curves at different current densities is suggestive of good charge-transport behaviors in the hierarchical 3D sheetlike, continuous hollow-structure CoSx@MnO2 electrode. Figure 4d illustrates the current-density dependence on the specific capacitance of the three as-prepared samples. Within the current-density test range, the CoSx@MnO2 always delivered a higher specific capacitance than the other samples. Moreover, after increasing the current density from 1 to 10 A g−1, the specific capacitance of CoSx@MnO2 still was maintained up to 71%, while hollow CoSx was only maintained at 65%. Durability is an important parameter for the practical application of electrode materials. The cycling stabilities of CoSx@MnO2 and hollow CoSx electrode were evaluated based on the charging/discharging cycling measurements at 5 A g−1 for 5000 cycles, as presented in Figure 4e. Only 20% capacitance loss was observed for CoSx@MnO2, which is more stable than that of hollow CoSx with 46% capacitance retention. The above results indicate that the hierarchical sandwichtype CoSx@MnO2 is a highly promising candidate for SCs. Nevertheless, the energy density of SCs is inferior to that of batteries, which limits their widespread application in the future. Hence, it is imperative to boost the energy density of SCs without sacrificing their power capability and cycling ability to fulfill the growing demand for energy. A promising approach to address these issues is to fabricate hybrid supercapacitors (HSCs) that combine the advantages of both batteries and SCs and can deliver high energy density and power capability simultaneously. Hence, a HSC device was fabricated in our work by employing CoSx@MnO2 as the positive electrode material and bioderived active carbon (BAC) as the negative electrode. According to measurements and calculations of the three-electrode device (Figure S9), the specific capacitance of BAC was determined to be 250 F g−1 at 1 A g−1 with 1.0 V potential window, and the optimized mass ratio between the negative and positive materials was ∼0.15.32 Figure 5b shows the CV curves of the CoSx@MnO2//BAC HSC device at different scan rates, where a potential window of 1.6 V was operated without significant polarization. The galvanostatic charge−discharge curve (Figure 5c) of the CoSx@MnO2// BAC device shows good symmetry and a linear slope, demonstrating good capacitive behavior. Meanwhile, the CoSx@MnO2//BAC device delivered a high specific capacitance (145 F g−1) at 1A g−1. Figure 5d exhibits the excellent cycling stability of the CoSx@MnO2//BAC device with its capacitance retention reaching 92% even after 5000 cycles. The power and energy properties, as critical evaluating parameters of HSCs, were calculated and used in a Ragone plot (Figure 5e). The CoSx@MnO2//BAC hybrid supercapacitors exhibited an energy density of 49.56 Wh kg−1 at a power density of 800 W kg−1 and a power density of 8000 W kg−1 at an energy density of 33.75 Wh kg−1, which is superior to many previously reported HSCs.10,29,33−36
as a sacrifice template. We discovered that the ultrathin MnO2 nanosheets with a positively charged surface are a promising electroactive material that not only provides redox capacitance in ECs but also acts as a structure-directing agent to design special nanostructures. The as-prepared CoSx@MnO2 as the electrode material for supercapacitors delivered a desirably high specific capacitance (1720 F g−1 at 0.5 A g−1) with excellent rate performance and cycling stability (maintained up to 71% from 1 to 10 A g−1 and only 20% capacitance loss at 5 A g−1 for 5000 cycles). These advantages render CoSx@MnO2 attractive for practical applications in hybrid supercapacitors, and the asassembled CoSx@MnO2//BAC hybrid supercapacitor shows higher energy density and power density than many previously reported HSCs. The excellent electrochemical properties of CoSx@MnO2 are attributed to several structural properties, which are described as follows: (1) The hierarchical 3D porous structure reduces the diffusion path length of electrolyte ions. (2) The continuous hollow nanocages form a continuous microcosmic conductive network, which greatly improves the electrical conductivity of the materials. (3) The small cavity and ultrathin shell offer a large surface area and more active sites. Thus, the as-prepared novel 3D sandwich-type, continuous, hollow nanostructure materials are promising for energy storage, while our proposed method could be a universal “in situ assembling” approach to synthesize various continuous hollow structure materials on the ultrathin negatively charged surface of MnO2 or the surface of other negatively charged electroactive materials.
4. CONCLUSIONS In summary, a 3D hierarchical sandwich-typed hollow CoSx@ MnO2 composite has been successfully fabricated using ZIF-67
(1) Winter, M.; Brodd, R.-J. What are Batteries, Fuel Cells, and Supercapacitors. Chem. Rev. 2004, 104, 4245−4270. (2) Miller, J.-R.; Simon, P. Electrochemical Capacitors for Energy Management. Science 2008, 321, 651−652.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12069. FESEM images, N2 adsorption/desorption isotherms, pore-size distribution, XRD patterns, EDS element composition diagrams, elemental mapping images, CV curves, galvanostatic charge/discharge curves, and specific capacitance at different current densities (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shu-Juan Bao: 0000-0002-2052-2178 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (no. 21773188), Fundamental Research Funds for the Central Universities (XDJK2017D003 and XDJK2017B055), Program for Excellent Talents in Chongqing (102060-20600218), and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011) and Chongqing Key Laboratory for Advanced Materials and Technologies.
■ 35046
REFERENCES
DOI: 10.1021/acsami.7b12069 ACS Appl. Mater. Interfaces 2017, 9, 35040−35047
Research Article
ACS Applied Materials & Interfaces
High Performance Lithium Sulfur Battery. Nano Energy 2016, 23, 15− 26. (23) Wang, J.-G.; Kang, F.; Wei, B. Engineering of MnO2-Based Nanocomposites for High-Performance Supercapacitors. Prog. Mater. Sci. 2015, 74, 51−124. (24) Jia, M.; Niu, Y.; Mao, C.; Liu, S.; Zhang, Y.; Bao, S.-j.; Xu, M. Porous Carbon Derived from Sunflower as A Host Matrix for UltraStable Lithium−Selenium Battery. J. Colloid Interface Sci. 2017, 490, 747−753. (25) Ma, Z.; Shao, G.; Fan, Y.; Wang, G.; Song, J.; Shen, D. Construction of Hierarchical α-MnO2 Nanowires@ Ultrathin δ-MnO2 Nanosheets Core-Shell Nanostructure With Excellent Cycling Stability for High-Power Asymmetric Supercapacitor Electrodes. ACS Appl. Mater. Interfaces 2016, 8, 9050−9058. (26) Huang, M.; Zhang, Y.; Li, F.; Zhang, L.; Ruoff, R.-S.; Wen, Z.; Liu, Q. Self-Assembly of Mesoporous Nanotubes Assembled from Interwoven Ultrathin Birnessite-type MnO2 Nanosheets for Asymmetric Supercapacitors. Sci. Rep. 2015, 4, 3878. (27) Ong, W.-J.; Tan, L.-L.; Chai, S.-P.; Yong, S.-T.; Mohamed, A.-R. Surface Charge Modification via Protonation of Graphitic Carbon Nitride (g-C3N4) for Electrostatic Self-Assembly Construction of 2D/ 2D Reduced Graphene Oxide (rGO)/g-C3N4 Nanostructures Toward Enhanced Photocatalytic Reduction of Carbon Dioxide to Methane. Nano Energy 2015, 13, 757−770. (28) Song, C.; Wu, D.; Zhang, F.; Liu, P.; Lu, Q.; Feng, X. Gemini Surfactant Assisted Synthesis of Two-Dimensional Metal Nanoparticles/Graphene Composites. Chem. Commun. 2012, 48, 2119− 2121. (29) Jiang, Z.; Lu, W.; Li, Z.; Ho, K.-H.; Li, X.; Jiao, X.; Chen, D. Synthesis of Amorphous Cobalt Sulfide Polyhedral Nanocages for High Performance Supercapacitors. J. Mater. Chem. A 2014, 2, 8603− 8606. (30) Cui, X.; Xie, Z.; Wang, Y. Novel CoS2 Embedded Carbon Nanocages by Direct Sulfurizing Metal-Organic Frameworks for DyeSensitized Solar Cells. Nanoscale 2016, 8, 11984−11992. (31) Zhang, X.; Yu, P.; Zhang, H.; Zhang, D.; Sun, X.; Ma, Y. Rapid Hydrothermal Synthesis of Hierarchical Nanostructures Assembled from Ultrathin Birnessite-Type MnO2 Nanosheets for Supercapacitor Applications. Electrochim. Acta 2013, 89, 523−529. (32) Godillot, G.; Taberna, P.-L.; Daffos, B.; Simon, P.; Delmas, C.; Guerlou-Demourgues, L. High Power Density Aqueous Hybrid Supercapacitor Combining Activated Carbon and Highly Conductive Spinel Cobalt Oxide. J. Power Sources 2016, 331, 277−284. (33) Li, H.; Yu, M.; Wang, F.; Liu, P.; Liang, Y.; Xiao, J.; Wang, C.; Tong, Y.; Yang, G. Amorphous Nickel Hydroxide Nanospheres with Ultrahigh Capacitance and Energy Density as Electrochemical Pseudocapacitor Materials. Nat. Commun. 2013, 4, 1894−1900. (34) Rakhi, R.; Alhebshi, N.-A.; Anjum, D.-H.; Alshareef, H.-N. Nanostructured Cobalt Sulfide-on-Fiber with Tunable Morphology as Electrodes for Asymmetric Hybrid Supercapacitors. J. Mater. Chem. A 2014, 2, 16190−16198. (35) Salunkhe, R.-R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J.-H.; Yamauchi, Y. Asymmetric Supercapacitors Using 3D Nanoporous Carbon and Cobalt Oxide Electrodes Synthesized from a Single Metal−Organic Framework. ACS Nano 2015, 9, 6288−6296. (36) Chen, W.; Xia, C.; Alshareef, H.-N. One-Step Electrodeposited Nickel Cobalt Sulfide Nanosheet Arrays for High-Performance Asymmetric Supercapacitors. ACS Nano 2014, 8, 9531−9541.
(3) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854. (4) Wang, Y.; Song, Y.; Xia, Y. Electrochemical Capacitors: Mechanism, Materials, Systems, Characterization and Applications. Chem. Soc. Rev. 2016, 45, 5925−5950. (5) Li, B.; Dai, F.; Xiao, Q.; Yang, L.; Shen, J.; Zhang, C.; Cai, M. Nitrogen-Doped Activated Carbon for A High Energy Hybrid Supercapacitor. Energy Environ. Sci. 2016, 9, 102−106. (6) Zhang, Y.-Z.; Wang, Y.; Xie, Y.-L.; Cheng, T.; Lai, W.-Y.; Pang, H.; Huang, W. Porous Hollow Co3O4 With Rhombic Dodecahedral Structures for High-Performance Supercapacitors. Nanoscale 2014, 6, 14354−14359. (7) Hu, C.-C.; Chang, K.-H.; Lin, M.-C.; Wu, Y.-T. Design and Tailoring of The Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors. Nano Lett. 2006, 6, 2690−2695. (8) Zhao, M.-Q.; Zhang, Q.; Huang, J.-Q.; Wei, F. Hierarchical Nanocomposites Derived from Nanocarbons and Layered Double Hydroxides-Properties, Synthesis, and Applications. Adv. Funct. Mater. 2012, 22, 675−694. (9) Bao, S.-J.; Li, C.-M.; Guo, C.-X.; Qiao, Y. Biomolecule-Assisted Synthesis of Cobalt Sulfide Nanowires for Application in Supercapacitors. J. Power Sources 2008, 180, 676−681. (10) Hu, H.; Guan, B. Y.; Lou, X.-W.-D. Construction of Complex CoS Hollow Structures With Enhanced Electrochemical Properties for Hybrid Supercapacitors. Chem. 2016, 1, 102−113. (11) Qu, G.; Cheng, J.; Li, X.; Yuan, D.; Chen, P.; Chen, X.; Wang, B.; Peng, H. A Fiber Supercapacitor with High Energy Density Based on Hollow Graphene/Conducting Polymer Fiber Electrode. Adv. Mater. 2016, 28, 3646−3652. (12) Liu, J.; Jiang, J.; Cheng, C.; Li, H.; Zhang, J.; Gong, H.; Fan, H.J. Co3O4 Nanowire@MnO2 Ultrathin Nanosheet Core/Shell Arrays: A New Class of High-Performance Pseudocapacitive Materials. Adv. Mater. 2011, 23, 2076−2081. (13) Yu, Z.; Tetard, L.; Zhai, L.; Thomas, J. Supercapacitor Electrode Materials: Nanostructures from 0 to 3 Dimensions. Energy Environ. Sci. 2015, 8, 702−730. (14) Li, Y.; Shi, J. Hollow-Structured Mesoporous Materials: Chemical Synthesis, Functionalization and Applications. Adv. Mater. 2014, 26, 3176−3205. (15) Lou, X.-W.-D.; Archer, L.-A.; Yang, Z. Hollow Micro-/ Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987−4019. (16) Yu, X.-Y.; Yu, L.; Lou, X.-W.-D. Metal Sulfide Hollow Nanostructures for Electrochemical Energy Storage. Adv. Energy Mater. 2016, 6, 1501333. (17) Yilmaz, G.; Yam, K.-M.; Zhang, C.; Fan, H.-J.; Ho, G.-W. In Situ Transformation of MOFs into Layered Double Hydroxide Embedded Metal Sulfides for Improved Electrocatalytic and Supercapacitive Performance. Adv. Mater. 2017, 29, 1606814. (18) Guan, B.-Y.; Yu, L.; Wang, X.; Song, S.; Lou, X.-W.-D. Formation of Onion-Like NiCo2S4 Particles via Sequential IonExchange for Hybrid Supercapacitors. Adv. Mater. 2017, 29, 1605051. (19) Jiao, L.; Zhou, Y.-X.; Jiang, H.-L. Metal−Organic FrameworkBased CoP/Reduced Graphene Oxide: High-Performance Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Sci. 2016, 7, 1690− 1695. (20) Zhong, H.x.; Wang, J.; Zhang, Y.w.; Xu, W.l.; Xing, W.; Xu, D.; Zhang, Y.f.; Zhang, X.b. ZIF-8 Derived Graphene-Based NitrogenDoped Porous Carbon Sheets as Highly Efficient and Durable Oxygen Reduction Electrocatalysts. Angew. Chem., Int. Ed. 2014, 53, 14235− 14239. (21) Wei, J.; Hu, Y.; Wu, Z.; Liang, Y.; Leong, S.; Kong, B.; Zhang, X.; Zhao, D.; Simon, G.-P.; Wang, H. A Graphene-Directed Administratorssembly Route to Hierarchically Porous Co−Nx/C Catalysts for High-Performance Oxygen Reduction. J. Mater. Chem. A 2015, 3, 16867−16873. (22) Li, Z.; Li, C.; Ge, X.; Ma, J.; Zhang, Z.; Li, Q.; Wang, C.; Yin, L. Reduced Graphene Oxide Wrapped MOFs-Derived Cobalt-Doped Porous Carbon Polyhedrons as Sulfur Immobilizers as Cathodes for 35047
DOI: 10.1021/acsami.7b12069 ACS Appl. Mater. Interfaces 2017, 9, 35040−35047
