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Design and synthesis of 3D hierarchical NiCo2S4@MnO2 core–shell nanosheet arrays for high-performance pseudocapacitors† Kaibing Xu,‡ab Qilong Ren,‡a Qian Liu,a Wenyao Li,c Rujia Zou*a and Junqing Hu*a We report on the development of 3D hierarchical NiCo2S4@MnO2 core–shell nanosheet arrays on Ni foam for supercapacitors. In our design, the highly conductive NiCo2S4 nanosheets can serve not only as a good pseudocapacitive material, which can be contributed to the capacitance of the whole electrode, but also as
Received 29th March 2015 Accepted 11th May 2015
a 3D conductive scaffold for loading MnO2 materials, which can overcome the limited conductivity of MnO2 itself. Furthermore, the 3D NiCo2S4@MnO2 hybrid electrode can provide efficient and rapid pathways for ion
DOI: 10.1039/c5ra05554g
and electron transport. These merits together with the elegant synergy between NiCo2S4 and MnO2 lead to
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a high areal capacitance of 2.6 F cm2 at 3 mA cm2 and good cyclic stability.
1. Introduction Supercapacitors, also known as ultracapacitors or electrochemical capacitors, hold a great promise in the future portable electronics, hybrid electronic vehicles and a number of microdevices due to their high specic power, long cycling life and fast charge and discharge rates.1–4 Supercapacitors can be oen distinguished into two major types according to the chargestorage mechanism involved in energy storage as well as the active materials used.2,5 One is electrical double-layer capacitors (EDLCs), where electrical energy is stored by electrostatic accumulation of charges in the electric double-layer near electrode–electrolyte interfaces, commonly use carbon-active materials with high surface area as electrodes.6–8 Another type of supercapacitor is the so-called pseudocapacitor, in which electrical energy is mainly stored by fast and reversible faradaic reactions, thus offering much higher specic capacitance than EDLCs.9–11 Typical active pseudocapacitive materials include transition metal oxides and conducting polymers. Among these available electrode materials, MnO2 has received tremendous interest as the most promising electrode material due to its many advantages such as low-cost, natural abundance, environmental friendliness and most importantly, high theoretical specic capacitance (1370 F g1).6,12–14 However, due to its poor a
State Key Laboratory for Modication of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, China. E-mail:
[email protected];
[email protected]
b
Research Center for Analysis and Measurement, Donghua University, Shanghai, 201620, China
c School of Material Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
† Electronic supplementary 10.1039/c5ra05554g
information
(ESI)
available.
See
DOI:
electric conductivity (105–106 S cm1),12,15 the observed specic capacitance values for MnO2 are much lower than its theoretical value. Recently, much attention has been focused on designing and constructing three-dimensional (3D) hybrid nanostructures with large surface area and short diffusion path for electrons and ions. In particular, the additive/binder-free electrode architectures, which can avoid the “dead surface” and tedious process in traditional slurry-coating electrode and signicantly improve the utilization rate of electrode materials even at high rates.3,5,9 For example, Cheng et al.16 synthesized 3D Co3O4@ MnO2 hierarchical porous nanoneedle array with a specic capacitance of 1693.2 F g1. Lou et al.17 developed hierarchical NiCo2O4@MnO2 core–shell heterostructured nanowire arrays with a specic capacitance of 3.31 F cm2. However, the conductivity of these backbone materials should be further improved to support fast electron transport. More recently, transition metal suldes have been extensively studied as novel electrode materials with improved electrochemical performance.18–24 NiCo2S4 material has higher electrochemical activity than its corresponding single component sulphides.18,21,25 Most importantly, NiCo2S4 exhibited an electrical conductivity 100 times as high as that of NiCo2O4.26,27 Of notable examples include the work by Wang et al.,26 who developed NiCo2S4 single crystalline nanotube arrays for loading additional electroactive materials, the as-formed CoxNi1x(OH)2/NiCo2S4 electrode showed the highest areal capacitance of 2.86 F cm2, good rate capability, and cycling stability. Dong et al.28 reported NiCo2S4@MnO2 core–shell heterostructures electrode materials showed a specic capacitance of 1337.8 F g1 and excellent cycling stability, however, the fabrication processes for electrode are complicated. Therefore, NiCo2S4 materials can serve as an ideal scaffold for loading additional electroactive
‡ These authors contributed equally to this work.
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pseudocapacitive materials to improve its electrical conductivity and capacitive performance. Herein, we present the design and fabrication of 3D hierarchical NiCo2S4@MnO2 core–shell nanosheet arrays on Ni foam and their improved capacitive performance. This unique design has the following merits: rstly, highly conductive NiCo2S4 nanosheets will provide electron “superhighways” for charge storage and delivery, which could overcome the limited conductivity of MnO2 itself and improve the utilization rate of electrode materials. Secondly, both of the NiCo2S4 and MnO2 are good pseudocapacitive electrode materials, of which a material-combination will provide synergistic and multifunctional effects. Thirdly, the 3D hierarchical NiCo2S4@MnO2 materials directly grown on Ni foam could avoid “dead” volume caused by the tedious process of mixing active materials with polymer binder/conductive additives. Beneting from the compositional features and the unique electrode structure, the NiCo2S4@MnO2 hybrid electrode exhibits greatly improved electrochemical performance with high areal capacitance (2.6 F cm2 at 3 mA cm2) and good cyclic stability.
2.
Results and discussion
Our strategy for fabricating 3D hierarchical NiCo2S4@MnO2 core–shell nanosheet arrays as pseudocapacitive electrode materials involves three-step, as shown in Fig. 1. Firstly, mesoporous NiCo2O4 nanosheet arrays were grown vertically on Ni foam via a hydrothermal and annealing process. Secondly, a facile anion exchange reaction has been performed to achieve NiCo2S4 nanosheet arrays, which involves treating NiCo2O4 nanosheet arrays in a Na2S aqueous solution under hydrothermal conditions. During this process, S2, HS, and H2S in Na2S solution can serve as the sulfur sources to convert NiCo2O4 to NiCo2S4 homogeneously and keep the original morphology.4 Thirdly, the MnO2 nanosheets were unique coated onto NiCo2S4 nanosheet arrays through an efficient and controllable electrodeposition process. The NiCo2S4@MnO2 core–shell nanosheet arrays directly grown on Ni foam could avoid “dead” volume caused by the tedious process of mixing active materials with polymer binder/conductive additives. What is more, the highly conductive NiCo2S4 nanosheet arrays grown on Ni foam can not only contribute to the capacitance of the whole electrode but also overcome the limited conductivity of MnO2 itself. The X-ray diffraction (XRD) patterns of the NiCo2O4 and NiCo2S4 materials are shown in Fig. 2, demonstrating the overall crystal structure and phase purity of the products. The as-prepared materials are scratched down from Ni foam
XRD patterns of the as-synthesized NiCo2O4 and NiCo2S4 samples and the corresponding standard XRD patterns. Fig. 2
substrate before measurement in order to reduce the impact of the Ni foam. Prior to ion exchange, the diffraction peaks for the NiCo2O4 (black curve) can be well ascribed to the cubic phase NiCo2O4 (JCPDS no. 20-0781). Aer the conversion, all the diffraction peaks for the NiCo2S4 (blue curve) can be indexed to the cubic phase NiCo2S4 (JCPDS no. 20-0782), suggesting successful conversion of NiCo2O4 into NiCo2S4. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are employed to investigate the morphologies and microstructures of the NiCo2O4 and NiCo2S4 nanosheet arrays on Ni foam. It is revealed that high-density NiCo2O4 nanosheets were uniformly grown on the Ni foam with a smooth surface, and they interconnect with each other to form a wall-like structure (Fig. 3a and b). From Fig. S1,† it can be seen that the NiCo2O4 nanosheets possess a highly porous structure, which is favorable for subsequent ion exchange reaction.22,29 The mesoporous NiCo2O4 nanosheets allow sulfur sources to diffuse easily into the core of each NiCo2O4 nanosheet. Aer ion exchange reaction with Na2S, the surface of the obtained NiCo2S4 nanosheets becomes rough, while maintaining the original array architecture, alignment, and physical contact with the substrates (Fig. 3c and d). A high-magnication SEM image, Fig. 3d, shows that numerous pores can be
Low- and high-magnification SEM images of (a and b) NiCo2O4 nanosheet arrays and (c and d) NiCo2S4 nanosheet arrays. (e and f) TEM and HRTEM images of NiCo2S4 nanosheet. Fig. 3
Schematic illustration for the fabrication process of the 3D hierarchical NiCo2S4@MnO2 core–shell nanosheet arrays on Ni foam. Fig. 1
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discerned on the surface of the NiCo2S4 nanosheets. Fig. 3e presents the typical TEM images of an individual NiCo2S4 nanosheet, from which the mesoporous structure with the sizes in the range of a few nanometers can be observed clearly. The high-resolution TEM image (HRTEM) shown in Fig. 3f reveals that the NiCo2S4 has a lattice fringe with interplane spacing of 0.191 nm, corresponding to the {422} plane of NiCo2S4. These results demonstrate that the NiCo2O4 nanosheets were successfully transformed into NiCo2S4 nanosheets. Due to the highly conductive of NiCo2S4 nanosheet, it can serve as a 3D conductive scaffold for loading MnO2 electrode materials so as to enhance the electrochemical capacitor performance. Fig. 4a–c show SEM images of the 3D hierarchical NiCo2S4@MnO2 core–shell nanosheet arrays, demonstrating that the MnO2 were successfully coated on the NiCo2S4 nanosheet arrays. The integration of MnO2 into the array does not deteriorate the ordered structure, and the thickness of the composites nanosheets increased to about 100–150 nm. The MnO2 nanosheets are interconnected with each other to form a highly porous surface morphology. Thus, the electrolyte can transportation not only at the active materials surface but also throughout the bulk due to the presence of convenient diffusion channels. TEM image in Fig. 4d clearly shows that the surface of NiCo2S4 nanosheet is enclosed by a thin and continuous MnO2 layer. Energy dispersive X-ray (EDX) spectrum (Fig. 4e) demonstrates that the Ni, Co, Mn, O and S can be detected in the hybrid composites. The Cu and C signals come from the carbonsupported Cu grid. Moreover, X-ray photoelectron spectroscopy (XPS) studies also determine the chemical composition of the composites as shown in Fig. 4f. The Mn 2p3/2 peak is centered at 642.3 eV and Mn 2p1/2 peak at 654.2 eV, with a binding energy separation of 11.9 eV, which are in good agreement with previous reported peak binding energy separation observed in MnO2.30 We then investigate the electrochemical properties of the NiCo2S4@MnO2 electrode in a three-electrode conguration using 1 M NaOH aqueous solution as electrolyte, with a Pt counter electrode and a saturated calomel electrode (SCE) reference electrode. Fig. 5a shows the cyclic voltammogram (CV) curves of the MnO2, NiCo2O4, NiCo2S4 and NiCo2S4@MnO2
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electrodes at a scan rate of 10 mV s1 in a potential window of 0.1 to 0.5 V. the pristine NiCo2O4 electrode materials showed a pair of redox peaks, which can be attributed to the following faradaic redox reactions:31,32 NiCo2O4 + OH + 2H2O % NiOOH + 2CoOOH + H2O + e, CoOOH + OH % CoO2 + H2O + e. Similarly, a pair of redox peaks can also be observed in the CV curve for NiCo2S4 electrode, which is mainly attributed to the faradaic redox reactions related to MS/MSOH and MSOH/MSO, where M refers to Ni or Co.26,33 It is also demonstrated that the NiCo2S4 electrode materials present pseudocapacitive properties. Moreover, the integrated CV area for the NiCo2S4 electrode is signicantly larger than the primal NiCo2O4 electrode, indicating a substantial improvement of electrochemical capacitance due to higher electric conductivity of NiCo2S4 electrode materials for facilitating electron and electrolyte ion transport. Moreover, the NiCo2S4@MnO2 electrode exhibits much higher current densities than that of MnO2 and NiCo2S4 electrode due to the synergistic effects between MnO2 and NiCo2S4 electrode materials and highly conductive of NiCo2S4 electrode materials: (1) mesoporous NiCo2S4 nanosheets with good electrical conductivity directly grown on Ni foam will provide electron “superhighways” for charge storage and delivery, which could overcome the limited conductivity of MnO2 itself nanosheets. (2) Both of the NiCo2S4 and MnO2 are good pseudocapacitive electrode materials, of which a material-combination will provide synergistic and multifunctional effects. It is worth noting that the CV integrated areas (Fig. S2†) of the pure and treated (underwent hydrothermal in Na2S solution) Ni foam are both negligible as compared with that of the NiCo2S4@MnO2 electrode, revealing the almost no capacitance contribution of the current collector. The galvanostatic charge–discharge (CD) measurements are performed in the voltage range of 0.1 to 0.45 V at a current density of 3 mA cm2, as shown in Fig. 5b. It clearly shows that the NiCo2S4@MnO2 electrode exhibits much longer discharging time than that of the other electrodes, demonstrating enhanced capacitances. The areal capacitance of the electrode materials was calculated based on the discharge curves (Fig. S3†) measured at different current densities using the following equation:10 C ¼ It/SDV, where I (A) is the current
Fig. 4 (a–c) Low- and high-magnification SEM images, (d) TEM image and (e) EDX spectra of the 3D hierarchical NiCo2S4@MnO2 core–shell nanosheet arrays, (f) XPS spectra of Mn 2p for the NiCo2S4@MnO2 composites.
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Fig. 5 (a) CV curves, (b) CD curves and (c) specific capacitances of the MnO2, NiCo2O4, NiCo2S4 and NiCo2S4@MnO2 electrode materials. (d) Cycling stability of NiCo2S4 and NiCo2S4@MnO2 electrode materials.
used for the charge–discharge, t (s) is the discharge time, DV (V) is the voltage interval of the discharge, and S is the geometrical area of the electrode, as shown in Fig. 5c. Signicantly, the areal capacitance of NiCo2S4@MnO2 electrode was much higher than that of the other electrodes in the whole current density range. The highest areal capacitance obtained for the NiCo2S4@MnO2 electrode was 2.6 F cm2 at a current density of 3 mA cm2, which is much higher than the values obtained for the NiCo2S4 electrode (1.33 F cm2), NiCo2O4 electrode (0.44 F cm2) and MnO2 electrode (0.057 F cm2). Even at a current density of 20 mA cm2, the NiCo2S4@MnO2 electrode still yielded a high areal capacitance of 1.17 F cm2. To the best of our knowledge, the areal capacitance performance of the NiCo2S4@MnO2 electrode reported here is also much higher than most of previously reported electrode materials, such as Co3O4@MnO2 (0.56 F cm2 at 11.25 mA cm2),5 NiCo2S4/CFC (1.33 F cm2 at 1 mA cm2),34 (CoxNi1x)9S8/HCNA (1.32 F cm2 at 1 mA cm2),35 Co0.85Se (929.5 mF cm2 at 1 mA cm2).36 Electrochemical impedance spectroscopy (EIS) is also applied to investigate electrochemical behaviors of the electrodes. Fig. S4† shows the EIS spectrum of the NiCo2S4 and NiCo2S4@MnO2 electrode materials. The equivalent series resistance (ESR) of the NiCo2S4@MnO2 electrode materials (0.11 U) is much smaller than that of the bare NiCo2S4 (0.31 U), indicating a lower diffusion resistance. The long-term cycling performance of the NiCo2S4 and NiCo2S4@MnO2 electrodes was evaluated at a scan rate of 50 mV s1. As shown in Fig. 5d, the overall capacitance retention for the NiCo2S4 electrode is 52% aer 3000 cycles. In contrast, the NiCo2S4@MnO2 electrode displayed a remarkable long-term cycling stability with 103.9% capacitance retention aer 5000 cycles. The enhanced pseudocapacitive performances of the NiCo2S4@MnO2 electrode material are mainly attributed to their unique mesoporous core–shell structure and highly conductive of NiCo2S4 electrode materials which provide more
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active sites for efficient electrolyte ion transportation not only at the active materials surface but also throughout the bulk. Aer long-term cycling test, the structural integrity and basic morphology of the NiCo2S4@MnO2 and NiCo2S4 electrode are overall well preserved with little deterioration, which are shown by the SEM image (Fig. S5†). The poor cycling stability of the NiCo2S4 electrode could be attributed to the following reasons: during the fast and reversible faradaic reactions, the capacitance decreases maybe relate to the degradation of the active material in the process of OH.23,37 However, the MnO2 can act as a buffer for the volume change during a cycling test, providing an assurance for better cycling performance of the NiCo2S4@MnO2 electrode.
3.
Conclusions
In summary, we have designed and successfully prepared 3D hierarchical NiCo2S4@MnO2 core–shell nanosheet arrays on Ni foam for high-performance pseudocapacitors. The highly conductive NiCo2S4 materials can serve as an ideal scaffold for loading MnO2 materials, which can overcome the limited conductivity of MnO2 itself. Moreover, The NiCo2S4 materials may also function as active materials for charge storage and make contribution to the capacitance. The 3D hierarchical NiCo2S4@MnO2 electrode materials show outstanding electrochemical performance in supercapacitors such as high areal capacitance (2.6 F cm2 at 3 mA cm2) and good cyclic stability (103.9% capacitance retention aer 5000 cycles).
4. Experimental section Synthesis of NiCo2S4 nanosheet arrays All the reagents here were analytical grade (purchased from Sinopharm) and used without further purication. The NiCo2O4
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nanosheet arrays were synthesized according to our previous work.38 In a typical synthetic procedure, a piece of Ni foam was carefully cleaned with 3 M HCl solution in an ultrasound bath for 30 min to remove the NiO layer on the surface, and then washed by deionized water and absolute ethanol several times. 0.3 g Ni(NO3)2$6H2O, 0.6 g Co(NO3)2$6H2O and 0.8 g hexamethylenetetramine (HMT) were dissolved into a mixed solution of 30 mL deionized water and 20 mL ethanol under vigorous magnetic stirring. Aer stirring for 30 min, asobtained solution was transferred into a 60 mL polytetrauoroethylene (PTFE) (Teon)-lined autoclave with a piece of clean Ni foam immersed into the reaction solution. The autoclave was sealed and maintained at 95 C for 8 h in an electric oven, and then cooled down to room temperature. The products on the Ni foam were carefully washed with deionized water and absolute ethanol, and then dried at 60 C overnight. Aerward, the samples were annealed at 300 C for 2 h at a ramping rate of 1 C min1. For synthesis of NiCo2S4 nanosheet arrays, the NiCo2O4 nanosheet arrays grown on Ni foam was immersed into a 60 mL aqueous solution containing 1 g of Na2S and kept at 95 C for 24 h. Aer cooling down naturally to the room temperature, the NiCo2S4 nanosheet arrays were taken out and washed with deionized water and ethanol, then dried at 60 C for 8 h under vacuum. The mass loading of the NiCo2S4 on Ni foam was calculated to be around 1.5 mg cm2.
Electrochemical deposition of MnO2 nanosheets on the NiCo2S4 nanosheet arrays The Ni foam with NiCo2S4 nanosheet arrays was used as a working electrode for electrodeposition of MnO2 nanosheets, which was deposited at 0.9 V (vs. SCE) in a solution containing 0.346 g manganese ammonium (MnAc2), 0.0308 g ammonium acetate (NH4Ac) and 2 mL dimethyl sulfoxide (DMSO) for 10 min at room temperature. Aer depositing, the as-prepared NiCo2S4@MnO2 electrode materials were washed with deionized water and absolute ethanol, and then dried at 60 C for 2 h under vacuum.
Materials characterization As-synthesized products were characterized with a D/max-2550 PC X-ray diffractometer (XRD; Rigaku, Cu-Ka radiation), a scanning electron microscopy (SEM; S-4800), a transmission electron microscopy (TEM; JEM-2100F) equipped with an energy dispersive X-ray spectrometer (EDX) and X-ray photoelectron spectroscopy (XPS, PHI5000VersaProbe).
Electrochemical measurement Electrochemical performances were performed on an Autolab (PGSTAT302N potentiostat) using a three-electrode mode in a 1 M NaOH solution. The reference electrode and counter electrode were SCE and platinum, respectively. The Ni foam supported electrode materials acted directly as the working electrode.
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Acknowledgements This work was nancially supported by the National Natural Science Foundation of China (Grant nos 21171035, 51472049 and 51302035), the Key Grant Project of Chinese Ministry of Education (Grant no. 313015), the PhD Programs Foundation of the Ministry of Education of China (Grant nos 20110075110008 and 20130075120001), the National 863 Program of China (Grant no. 2013AA031903), the Science and Technology Commission of Shanghai Municipality (Grant no. 13ZR1451200), the Fundamental Research Funds for the Central Universities, the Program Innovative Research Team in University (IRT1221), the Shanghai Leading Academic Discipline Project (Grant no. B603) and the Program of Introducing Talents of Discipline to Universities (no. 111-2-04).
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