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Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance
Materials Chemistry and Physics xxx (2016) 1e8
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Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance X.F. Gong a, J.P. Cheng a, *, K.Y. Ma a, F. Liu a, Li Zhang b, c, XiaoBin Zhang a a
State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Zhejiang University, Hangzhou 310027, PR China b Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China c Shun Hing Institute of Advanced Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China
h i g h l i g h t s NieCo sulfide arrays are directly grown on Ni foam as supercapacitor electrodes. The capacitance increases to 314% of the initial value in the first 5000 cycles. Enlarging the potential range, the areal capacitance can further increase.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 July 2015 Received in revised form 30 November 2015 Accepted 6 February 2016 Available online xxx
The synthesis of NiCo2S4/Co9S8 nanorod arrays directly grown on Ni foam and its application as a supercapacitor electrode were investigated. The electrode demonstrated a superior electrochemical performance and its capacitance could increase to 314% of the initial value after 5000 cycles due to a progressive activation. The highest areal specific capacitance could be 8.08 F cm2 and the corresponding mass specific capacitance was 2068 F g1 at the current density of 5 mA cm2. Even after 10,000 cycles, its areal specific capacitance could be still 5.53 F cm2, 190% of the initial capacitance. When the current density was increased, the areal capacitance would decrease usually. Through increasing the anode voltage, the areal specific capacitance could increase. The combination of high electrical conductivity, porous structure and the synergic effects of nickel and cobalt, played significant roles to obtain supercapacitor electrodes with excellent performances. © 2016 Elsevier B.V. All rights reserved.
Keywords: Chalcogenides Inorganic compounds Chemical synthesis Electrochemical properties
1. Introduction With the concerns about the environmental issues and the depletion of fossil fuels, supercapacitors, also called electrochemical capacitors, are attracting more and more attention due to their high power density and long cycle-life among various energy conversion/storage systems. Considerable efforts have been devoted to explore high-performance supercapacitors and electrode materials. A variety of transition metal oxides and hydroxides [1e4] with variable valence, such as NiO [5,6], Ni(OH)2 [7,8], Co3O4 [9,10], Co(OH)2 [11,12] et al., have been widely investigated as electrode materials because of the rich redox reactions which can offer a high specific capacitance. However, the experimental results
* Corresponding author. E-mail address: [email protected] (J.P. Cheng).
are usually less satisfactory for practical application. Thus, a lot of work was devoted to the fabrication of electrode materials with a high specific capacitance in order to develop a capacitor with a high energy density [13e15]. In addition, it is also essential for the electrode materials to maintain a good rate capability under high chargeedischarge current densities. However, in fact the increase of the current density will lead to a decrease of the specific capacitance, which mainly results from the inaccessibility of electrolyte to the entire electrochemical active materials at the high current density [16]. Thus, it will also lead to a low areal specific capacitance and greatly limit the practical application [17]. Concerning above issues, it is important to facilitate the ions diffusion and electrons transfer, and to improve the utilization efficiency of electrode materials, in order to develop an excellent supercapacitor. Therefore, we should seek for electrodes with the following requirements, (1) rich red-ox reactions, which can exhibit a high specific capacitance; (2) highly electrical conductivity,
http://dx.doi.org/10.1016/j.matchemphys.2016.02.018 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: X.F. Gong, et al., Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.018
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facilitating a fast electron transport; (3) porous and highlyaccessible open structure, to guarantee large amounts of active sites for redox reactions and ion diffusion; (4) free binder and additives. Recently, metal sulfides, such as cobalt sulfides, nickel sulfides and NiCo2S4, have been intensively investigated as electrode materials of supercapacitors, because they are semiconductors and exhibit higher electrical conductivities than the metal oxides [18e26]. Particularly for NiCo2S4, it has aroused much more interest because it shows rich Faradaic redox reactions from simultaneous effects from nickel and cobalt atoms, thus enhancing the capacitive performance [27,28]. We think that the traditional paste method which involves the polymer binder and conductive agent leads to their limited performances. Thus an electrode film based on metal sulfides directly grown on the current collector is a better choice to make an enhanced property [29e31]. However, due to its gradually decay in capacitance during a long time cycling, it is necessary for us to find the underlying reasons and give a feasible technique to overcome this. In this work, we synthesized nickel cobalt sulfides on nickel foam directly through a facial two-step hydrothermal method. They (NiCo2S4/Co9S8/Ni, Co9S8/Ni and Ni3S2/Ni) displayed various morphologies but similar cycling performance when being used as electrodes. The specific capacitance of the metal sulfide electrodes increased at first and then decreased during the cycling tests. Through broadening the potential range, their specific capacitance could be improved to some extent. For the NiCo2S4/Co9S8/Ni electrode, it exhibited a superior capacitive performance, including high areal and mass specific capacitances. Cycling tests demonstrated that its capacitance increased to 314% of the initial value after the first 5000 cycles due to the progressive activation and could maintain 190% of the initial value even after 10,000 cycles at the current density of 5 mA cm2. 2. Experimental 2.1. Materials All the chemicals, including Ni(NO3)2$6H2O, Co(NO3)2$6H2O, urea, Na2S$9H2O and NH4F, were purchased from Aladdin Industrial Corporation, which were of analytical purity and used without further purification. 2.2. Preparation of hydroxide precursor In a typical synthesis, the hydroxide precursor was firstly prepared through a hydrothermal method. Co(NO3)2$6H2O (0.58 g, 2 mmol), Ni(NO3)2$6H2O (0.29 g, 1 mmol), urea (0.6 g, 10 mmol) and NH4F (0.3 g, 8 mmol) were dissolved in 60 mL of water and stirred to form a pink solution. A piece of Ni foam (4 3 cm) was carefully cleaned with 3 M HCl solution in an ultrasound bath for 3 min to remove NiO layer, and then washed with DI water. Above metal-containing solution and Ni foam were then transferred into a Teflon-lined stainless-steel autoclave (100 mL) and maintained at 120 C for 8 h. After the reaction for hydroxide deposition, the sample of hydroxides@Ni foam was ultrasonically cleaned for 2 min in water and ethanol, and dried at 60 C overnight. 2.3. Synthesis of nickel cobalt sulfides In a typical procedure, 1.17 g Na2S$9H2O was dissolved into 60 mL water under magnetic stirring. Then the precursor coated Ni foam and Na2S solution were sealed in a Teflon-lined stainless steel autoclave and maintained at 160 C for 12 h. The Ni foam changed to be black, and was washed with water and ethanol for several
times, then dried in vacuum at 60 C for 12 h. The mass loading of nickel-cobalt sulfides was approximately 3.82 mg cm2 by weighing the Ni foam before and after reactions. In a control experiment, pure cobalt sulfide and nickel sulfide array films on Ni foam were also been prepared through above twostep methods by using 3 mmol Co(NO3)2$6H2O and Ni(NO3)2$6H2O, respectively. Similarly, the mass loading of Co9S8 and Ni3S2 was determined to be approximately 4.4 and 3.37 mg cm2, respectively. 2.4. Morphological and structural characterization The crystalline structure of as-prepared products was analyzed by powder X-ray diffraction (XRD, Shimadzu XRD-6000, Cu Ka). Xray photoelectron spectroscopy (XPS) was measured on the instrument of ESCALAB 250Xi. The morphological investigations of the as-prepared materials were carried out using a scanning electron microscope (SEM, SU-70) and a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F). 2.5. Electrochemical measurements The electrochemical measurements were performed by a standard three-electrode system in a solution of 2 M KOH as electrolyte on a CHI660D electrochemical station. With a Pt plate as the counter-electrode and saturated calomel electrode (SCE) as the reference electrode, a piece of Ni foam (1.5 1.5 cm2) coated with sulfide nanostructures was directly used as a binder-free working electrode. The cyclic voltammetry (CV) and galvanostatic chargeedischarge techniques were employed to investigate the electrochemical properties of the electrodes. The electrochemical impedance spectroscopy (EIS) was obtained in a frequency range between 100 kHz and 0.01 Hz with a perturbation amplitude of 5 mV versus the open-circuit potential. The areal specific capacitances and mass specific capacitance were calculated from the discharge curve based on following equations (1) and (2), respectively.
C¼
IDt F cm2 ADV
(1)
C¼
IDt 1 Fg mDV
(2)
where, I is the current density (A), Dt is the discharge time (s), DV is the potential window of the discharging (V), A is the geometric surface of substrate (cm2), m is the mass loading of active material (g). 3. Results and discussion Fig. 1 shows the XRD patterns of the metal sulfide samples deposited on Ni foam. For Ni3S2/Ni sample, several distinct diffraction peaks at 21.7, 31.1, 37.7, 38.2 , 49.5 , 50.1, 54.5 and 55.0 can be observed, corresponding to the (101), (110), (003), (111), (120), (211), (121) and (300) planes of crystalline Ni3S2 (JCPDF 30-0863), respectively. The other peaks at 44.2 , 51.4 and 76.0 can be indexed to metallic nickel from Ni foam (JCPDF 01-1258). However, for NiCo2S4/Co9S8/Ni and Co9S8/Ni, because of their low crystallinity and fluorescence, only the diffraction peaks at 31.5 and 38.1 can be seen, which are assigned to the (311) and (400) planes of crystalline NiCo2S4 (JCPDF 20-0782), respectively. The diffraction peak at 30.0 can be indexed to Co9S8 (JCPDF 65-6801). However, the relatively weak diffraction peaks of metallic Ni imply that these sulfide films are thickly coated on nickel foam.
Please cite this article in press as: X.F. Gong, et al., Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.018
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Fig. 1. XRD patterns of (a) Ni3S2/Ni, (b) NiCo2S4/Co9S8/Ni and (c) Co9S8/Ni.
To further evaluate the elemental composition and chemical state of the as-prepared sulfide samples, XPS measurements were conducted. The results are presented in Fig. 2. The survey spectrum in Fig. 2a shows the presence of elements including Ni, Co, S, O and C. In the Ni 2p spectrum, the deconvolution of the Ni 2p peaks shows the atoms in 2p3/2 electronic configuration at 856.8 eV
3
(Fig. 2b), suggesting the co-existence of divalent and trivalent states. The peak at 873.4 eV corresponds to the Ni 2p1/2 band. For the Co 2p spectrum in Fig. 2c, it shows a low energy band (Co 2p3/2) at 780.8 eV and a high energy band (Co 2p1/2) at 797.2 eV, consistent with the results reported elsewhere [19]. The spin-orbit splitting value of Co 2p1/2 and Co 2p3/2 is over 15 eV, suggesting the co-existence of Co2þ and Co3þ [32e34]. The S 2p spectrum in Fig. 2d can be divided into a main peak and a shake-up satellite. The component at 163.8 eV is a typical metal-sulphur bond. The binding energy at 168.3 eV is attributed to the shake-up satellite. According to the XPS analysis, the surface of the three samples has a composition of Co2þ, Co3þ, Ni2þ, Ni3þ, and S2, which is in good agreement with the phases of NiCo2S4/Co9S8, Ni3S2 and Co9S8. The morphologies of the as-prepared metal sulfide samples were characterized by SEM and TEM measurements. Fig. 3a, S1a and S2a show the morphologies of nickel-cobalt, cobalt and nickel hydroxide precursors, respectively. These hydroxides were directly grown on the surface of Ni foam to form an array film. From Fig. 3b, S1b and S2b, we can see that the metal sulfides maintain the original morphologies of their hydroxide precursors. From a close observation of the enlarged SEM images in Fig. 3c, S1c and S2c for these metal sulfides, it can be seen that 3-dimensional crossed sheets are formed on the surfaces of nanorods and nanosheets (Ni3S2), which can be also confirmed by the TEM images, as shown in Fig. 3d, S1d and S2d. These TEM images display that the nanorods (with a diameter about 100 nm) and nanosheets of the metal sulfides are hollow and porous (some pores being marked by black
Fig. 2. XPS spectra of (a) full spectrum, (b) Ni 2p, (c) Co 2p and (d) S 2p for NiCo2S4/Co9S8.
Please cite this article in press as: X.F. Gong, et al., Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.018
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Fig. 3. (a) SEM images of NieCo hydroxide precursor, (b,c) SEM and (d) TEM images of NiCo2S4/Co9S8.
arrows), which results from the anions-exchange reaction between S2 and OH or CO2 3 in the hydroxide precursors, indicating a complete conversion from metal hydroxides to metal sulfides. From above results, it is apparent that all the metal sulfide crystals possess a porous structure, which is of great importance to guarantee large amounts of active sites for redox reactions and fast ion diffusion to achieve an excellent electrochemical performance. Fig. 4 shows the digital photographs of bare Ni foam, hydroxides/Ni, and NiCo2S4/Co9S8/Ni from left to right. It can be seen that the shape of Ni foam is well maintained and the color of them is changed due to the different coatings. Metal hydroxides film exhibits light-pink color, while metal sulfide film is black. To evaluate the capacitive performance of the electrodes containing different metal sulfides, their electrochemical properties were investigated. Fig. 5a reveals that the areal specific capacitance of NiCo2S4/Co9S8/Ni electrode is much higher than those of Co9S8/ Ni and Ni3S2/Ni electrodes under the same current densities. It can be assigned to its rich Faradaic redox reactions simultaneously from
both nickel and cobalt ions, which is consistent with the redox peaks in Fig. 5b. Co9S8/Ni and Ni3S2/Ni electrodes displayed one pair of redox peaks, while for NiCo2S4/Co9S8/Ni electrode, two pairs of redox peaks can be clearly observed, which are resulted from the following redox processes of Co2þ/Co3þ/Co4þ and Ni2þ/Ni3þ [35]:
CoS þ OH 4CoSOH þ e
(3)
CoSOH þ OH 4CoSO þ H2 O þ e
(4)
NiS þ OH 4NiSOH þ e
(5)
The rate capability is also a key factor for evaluating the potential applications of supercapacitors. When the current density increased from 5 to 50 mA cm2 (in Fig. 5a), the areal capacitance retentions are 44.7%, 66.5%, and 73.5% for NiCo2S4/Co9S8/Ni, Co9S8/ Ni, and Ni3S2/Ni electrodes, respectively. The comparison of rate capability among the three electrodes can be related to the bulk
Fig. 4. Photograph of bare Ni foam, metal hydroxide/Ni, NiCo2S4/Co9S8/Ni from left to right.
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Fig. 5. (a) Areal capacitance at various current densities from 5 to 50 mA cm2, (b) CV curves at the scan rate of 5 mV s1, (c) EIS Nyquist plots and (d) the cycling performance at the current density of 20 mA cm2 of NiCo2S4/Co9S8/Ni, Co9S8/Ni and Ni3S2/Ni electrodes.
solution and charge-transfer resistance. Fig. 5c shows that Ni3S2/Ni electrode displays a relatively lower bulk solution resistance and charge-transfer resistance than both NiCo2S4/Co9S8/Ni and Co9S8/ Ni electrodes, thus exhibiting a high retention under high current density. It should be mentioned that the three electrodes showed lower resistances when compared with conventional metal hydroxides and oxide due to the high conductivity of metal sulfides [5e12]. Their cycling performances were measured by repetitive charging/discharging tests at the constant current density of 20 mA cm2. As shown in Fig. 5d, the specific capacitances of the three electrodes gradually increase because of the progressive activation in the initial stage. NiCo2S4/Co9S8/Ni, Co9S8/Ni and Ni3S2/ Ni electrodes can increase to 314%, 179%, and 172% of their original values after the first 5000, 1000 and 3800 cycles, respectively. However, their specific capacitances would then decrease to some low values after 10,000 cycles. Compared with the CoeNi hydroxide precursor (In Fig. S3 in supplementary material), the specific capacitance of NiCo2S4/Co9S8/Ni electrode is much higher. We think that the hydrotalcite-like CoeNi hydroxides tend to form betaformed metal hydroxides in the alkaline electrolyte soon [36]. To analyze the increasing areal capacitance and the reasons that lead to the decline of the capacitance, their electrochemical properties at different stages in the 10,000 charging-discharging cycles were further investigated. Fig. 6a shows the CV curves of the 1st cycle, the 5000th cycle and 10000th cycle of the NiCo2S4/Co9S8/Ni electrode at the scan rate of 5 mV s1. After a progressive activation for 5000 cycles, the integrated area within the currentepotential
curves reaches the largest, which indicates the highest specific capacitance. It could be also verified by the galvanostatic chargeedischarge measurements (in Fig. 6c). Fig. 6b shows that the areal capacitance can be as high as 8.08 F cm2 and the corresponding mass specific capacitance is 2068 F g1 at the current density of 5 mA cm2. Even at a high current density of 50 mA cm2, the areal and mass specific capacitance can still be 5 F cm2 and 1250 F g1, respectively. In comparison with previous reports, the measured areal and mass specific capacitance were much higher than other electrode materials directly grown on conductive substrates, including NiCo2O4 arrays [37e39] and NiO arrays [14,40]. 61.8% of the capacitance could be maintained with the current density rising from 5 to 50 mA cm2 due to the high conductivity of metal sulfides and the highly-accessible porous structure, and it could be verified by the following EIS measurements. Fig. 6d shows that in the high frequency area, the bulk solution resistance decreases from 0.34 U to 0.28 U during the first 5000 cycles and the negligible semicircle indicates a low charge transfer resistance, which can guarantee easy electrolyte diffusion and a fast electron transport. Regarding to the dramatic decline of capacitance from the 5000th cycle to the 10000th cycle (in Fig. 5d), several factors should be researched. Firstly, the shifting of anodic peak to anodic direction leads to an incomplete redox reaction and the decline of capacitance, which can be verified from the CV curves (Fig. 6a) and charging-discharging curves (in Fig. 6c). The CV curves in Fig. 6a reveal that with the cycling tests processing, the anodic peak shifts
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Fig. 6. (a) CV curves at the scan rate of 5 mV s1, (b) areal capacitance at various current densities from 5 to 50 mA cm2, (c) charge and discharge curves at the current density of 20 mA cm2, (d) EIS Nyquist plots of NiCo2S4/Co9S8/Ni electrode at the 1st, 5000th and 10000th cycle.
from 0.3 V to 0.48 V, and then to 0.55 V. In Fig. 6c, the nonlinear part in the charging curves rises to around 0.45 V during the cycling test. Above data indicate that the charging process has not been fully completed at the original potential of 0.45 V. Meanwhile, with the increase of the current density, the anodic peak shifts to anodic direction because of electrode polarization. Therefore, the abovementioned factors can account for the drastic decrease of the areal capacitance at high current density (5.53 F cm2 at 5 mA cm2, 2.35 F cm2 at 20 mA cm2, 0.177 F cm2 at 50 mA cm2, retaining 69%, 35%, 3.5% of the corresponding values at the 5000th cycle, respectively, in Fig. 6b). In addition, the capacitive decay may also result from the conversion from metal sulfide to metal hydroxide [24,25], thus leading to the increase in bulk solution resistance of approximately 0.98 U (in Fig. 6d) and inhibiting the ions and electrolytes penetrating into the inner nanostructures of the active materials. Similarly, the shift of redox peaks to the anodic direction, polarization and the increase of resistance could account for the drastic decline of the capacitance for Co9S8/Ni and Ni3S2/Ni electrodes at the 10000th cycle. For Co9S8/Ni electrode, its anodic peak shifts from 0.35 V to 0.5 V (in Fig. S4a), the nonlinear parts of the charging process also move to high voltage direction (Fig. S4c), and there is a clear semicircle indicating the increased charge-transfer resistance (Fig. S4d). Ni3S2/Ni electrode also displays the shifting anodic peak in the CV curves from 0.38 V to 0.55 V (Fig. S5a), the rising nonlinear parts in the charging curves (Fig. S5c) and the
significantly increasing charge transfer resistance to 7 U (Fig. S5d). Based on above analysis, it is expected that we can expand the potential range to allow the redox reaction to finish completely. As shown in Fig. 7a, after expanding the potential range from 0.45 V to 0.55 V at the current density of 20 mA cm2, the areal specific capacitance of NiCo2S4/Co9S8/Ni electrode significantly increases from 2.35 F cm2 (Fig. 6b) to 4.58 F cm2 (Fig. 7b). Even at the current density of 50 mA cm2, the areal capacitance can still reach 2.83 F cm2 (Fig. 7b) when the potential range is from 0 to 0.6 V. Similarly, through enlarging the potential range, the areal capacitance of Co9S8/Ni and Ni3S2/Ni electrodes can be 2.16 and 1.41 F cm2, respectively, even at the high current density of 50 mA cm2 (in Fig. 7b and Fig. S6). To further evaluate the practical application of the NiCo2S4/ Co9S8/Ni electrode, an asymmetric capacitor was fabricated with it as positive electrode and activated carbon as negative electrode. The mass ratio between two electrodes are determined by the capacitance and the potential with the equation of CþmþDUþ ¼ CmDU [41]. The electrochemical properties of the hybrid capacitor were measured with a two-electrode system by the workstation. The CV curves of the capacitor measured at 20 mV s1 under different potential windows from 1.0 to 1.6 V are show in Fig. S7b, in which the redox peaks on the CV profiles can be observed. The galvanostatic chargeedischarge curves for the hybrid capacitor at 1 A g1 are exhibited in Fig. S7c, showing a good capacitive behavior from its symmetric and linear
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on Ni foam. The as-prepared electrode exhibited an excellent electrochemical performance, including high specific capacitance in area and mass. The capacitance could increase to 314% of the initial value in the first 5000 cycles due to the progressive activation. The highest specific capacitance could reach 8.08 F cm2 and 2068 F g1 at the current density of 5 mA cm2. With the continuous charging-discharging cycles, the redox peaks shifted toward the anodic direction. Through broadening the potential range, the areal capacitance could further reach 4.58 and 2.83 F cm2 even at the high current densities of 20 and 50 mA cm2, respectively. Acknowledgement The authors thank the Micro and Nano Fabrication Laboratory of The Chinese University of Hong Kong (CUHK) for technical support. This work was supported by Zhejiang Provincial Natural Science foundation of China under grant No. LY13E020002, Public Projects (analysis and test) of Zhejiang Province (No. 2015C37027) and the Shun Hing Institute of Advanced Engineering (SHIAE) with the Project No. 8115045 at CUHK. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.02.018. References
Fig. 7. (a) Charge and discharge curves at various current density and potential widow of NiCo2S4/Co9S8/Ni electrode, (b) areal capacitance at various current density and potential widow of NiCo2S4/Co9S8/Ni electrode.
chargeedischarge curves. The corresponding specific capacitance based on total mass under various current densities in Fig. S7d shows that it can have the maximum value of 84 F g1 at 0.5 A g1 in the potential window of 1.6 V. It is also indicated that the hybrid capacitor will deliver a higher specific capacitance in a larger potential range under the same current density. The above electrochemical investigation suggests that the binder-free NiCo2S4/Co9S8/Ni electrode can exhibit an excellent capacitive performance and it is a potential candidate for practical application. We can attribute its outstanding electrochemical performances to the following three factors. (I) The electrode can make fully use of rich Faradaic redox reactions from the simultaneous contribution from both nickel and cobalt. (II) The porous structure and the high conductivity of metal sulfide can effectively guarantee facile electrolyte diffusion, a fast electron transport and large amounts of active sites for redox reactions [42,43]. (III) The binderfree electrode can avoid the use of polymer binders and conductive additives, thus reduce the bulk solution and charge-transfer resistance. 4. Conclusions In summary, a binder-free NiCo2S4/Co9S8/Ni electrode with promising electrochemical performances was successfully fabricated through a two-step hydrothermal method. The NiCo2S4/Co9S8 nanorod arrays with a diameter about 100 nm were directly grown
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Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance X.F. Gong a, J.P. Cheng a, *, K.Y. Ma a, F. Liu a, Li Zhang b, c, XiaoBin Zhang a a
State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Zhejiang University, Hangzhou 310027, PR China b Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China c Shun Hing Institute of Advanced Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China
h i g h l i g h t s NieCo sulfide arrays are directly grown on Ni foam as supercapacitor electrodes. The capacitance increases to 314% of the initial value in the first 5000 cycles. Enlarging the potential range, the areal capacitance can further increase.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 July 2015 Received in revised form 30 November 2015 Accepted 6 February 2016 Available online xxx
The synthesis of NiCo2S4/Co9S8 nanorod arrays directly grown on Ni foam and its application as a supercapacitor electrode were investigated. The electrode demonstrated a superior electrochemical performance and its capacitance could increase to 314% of the initial value after 5000 cycles due to a progressive activation. The highest areal specific capacitance could be 8.08 F cm2 and the corresponding mass specific capacitance was 2068 F g1 at the current density of 5 mA cm2. Even after 10,000 cycles, its areal specific capacitance could be still 5.53 F cm2, 190% of the initial capacitance. When the current density was increased, the areal capacitance would decrease usually. Through increasing the anode voltage, the areal specific capacitance could increase. The combination of high electrical conductivity, porous structure and the synergic effects of nickel and cobalt, played significant roles to obtain supercapacitor electrodes with excellent performances. © 2016 Elsevier B.V. All rights reserved.
Keywords: Chalcogenides Inorganic compounds Chemical synthesis Electrochemical properties
1. Introduction With the concerns about the environmental issues and the depletion of fossil fuels, supercapacitors, also called electrochemical capacitors, are attracting more and more attention due to their high power density and long cycle-life among various energy conversion/storage systems. Considerable efforts have been devoted to explore high-performance supercapacitors and electrode materials. A variety of transition metal oxides and hydroxides [1e4] with variable valence, such as NiO [5,6], Ni(OH)2 [7,8], Co3O4 [9,10], Co(OH)2 [11,12] et al., have been widely investigated as electrode materials because of the rich redox reactions which can offer a high specific capacitance. However, the experimental results
* Corresponding author. E-mail address: [email protected] (J.P. Cheng).
are usually less satisfactory for practical application. Thus, a lot of work was devoted to the fabrication of electrode materials with a high specific capacitance in order to develop a capacitor with a high energy density [13e15]. In addition, it is also essential for the electrode materials to maintain a good rate capability under high chargeedischarge current densities. However, in fact the increase of the current density will lead to a decrease of the specific capacitance, which mainly results from the inaccessibility of electrolyte to the entire electrochemical active materials at the high current density [16]. Thus, it will also lead to a low areal specific capacitance and greatly limit the practical application [17]. Concerning above issues, it is important to facilitate the ions diffusion and electrons transfer, and to improve the utilization efficiency of electrode materials, in order to develop an excellent supercapacitor. Therefore, we should seek for electrodes with the following requirements, (1) rich red-ox reactions, which can exhibit a high specific capacitance; (2) highly electrical conductivity,
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facilitating a fast electron transport; (3) porous and highlyaccessible open structure, to guarantee large amounts of active sites for redox reactions and ion diffusion; (4) free binder and additives. Recently, metal sulfides, such as cobalt sulfides, nickel sulfides and NiCo2S4, have been intensively investigated as electrode materials of supercapacitors, because they are semiconductors and exhibit higher electrical conductivities than the metal oxides [18e26]. Particularly for NiCo2S4, it has aroused much more interest because it shows rich Faradaic redox reactions from simultaneous effects from nickel and cobalt atoms, thus enhancing the capacitive performance [27,28]. We think that the traditional paste method which involves the polymer binder and conductive agent leads to their limited performances. Thus an electrode film based on metal sulfides directly grown on the current collector is a better choice to make an enhanced property [29e31]. However, due to its gradually decay in capacitance during a long time cycling, it is necessary for us to find the underlying reasons and give a feasible technique to overcome this. In this work, we synthesized nickel cobalt sulfides on nickel foam directly through a facial two-step hydrothermal method. They (NiCo2S4/Co9S8/Ni, Co9S8/Ni and Ni3S2/Ni) displayed various morphologies but similar cycling performance when being used as electrodes. The specific capacitance of the metal sulfide electrodes increased at first and then decreased during the cycling tests. Through broadening the potential range, their specific capacitance could be improved to some extent. For the NiCo2S4/Co9S8/Ni electrode, it exhibited a superior capacitive performance, including high areal and mass specific capacitances. Cycling tests demonstrated that its capacitance increased to 314% of the initial value after the first 5000 cycles due to the progressive activation and could maintain 190% of the initial value even after 10,000 cycles at the current density of 5 mA cm2. 2. Experimental 2.1. Materials All the chemicals, including Ni(NO3)2$6H2O, Co(NO3)2$6H2O, urea, Na2S$9H2O and NH4F, were purchased from Aladdin Industrial Corporation, which were of analytical purity and used without further purification. 2.2. Preparation of hydroxide precursor In a typical synthesis, the hydroxide precursor was firstly prepared through a hydrothermal method. Co(NO3)2$6H2O (0.58 g, 2 mmol), Ni(NO3)2$6H2O (0.29 g, 1 mmol), urea (0.6 g, 10 mmol) and NH4F (0.3 g, 8 mmol) were dissolved in 60 mL of water and stirred to form a pink solution. A piece of Ni foam (4 3 cm) was carefully cleaned with 3 M HCl solution in an ultrasound bath for 3 min to remove NiO layer, and then washed with DI water. Above metal-containing solution and Ni foam were then transferred into a Teflon-lined stainless-steel autoclave (100 mL) and maintained at 120 C for 8 h. After the reaction for hydroxide deposition, the sample of hydroxides@Ni foam was ultrasonically cleaned for 2 min in water and ethanol, and dried at 60 C overnight. 2.3. Synthesis of nickel cobalt sulfides In a typical procedure, 1.17 g Na2S$9H2O was dissolved into 60 mL water under magnetic stirring. Then the precursor coated Ni foam and Na2S solution were sealed in a Teflon-lined stainless steel autoclave and maintained at 160 C for 12 h. The Ni foam changed to be black, and was washed with water and ethanol for several
times, then dried in vacuum at 60 C for 12 h. The mass loading of nickel-cobalt sulfides was approximately 3.82 mg cm2 by weighing the Ni foam before and after reactions. In a control experiment, pure cobalt sulfide and nickel sulfide array films on Ni foam were also been prepared through above twostep methods by using 3 mmol Co(NO3)2$6H2O and Ni(NO3)2$6H2O, respectively. Similarly, the mass loading of Co9S8 and Ni3S2 was determined to be approximately 4.4 and 3.37 mg cm2, respectively. 2.4. Morphological and structural characterization The crystalline structure of as-prepared products was analyzed by powder X-ray diffraction (XRD, Shimadzu XRD-6000, Cu Ka). Xray photoelectron spectroscopy (XPS) was measured on the instrument of ESCALAB 250Xi. The morphological investigations of the as-prepared materials were carried out using a scanning electron microscope (SEM, SU-70) and a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F). 2.5. Electrochemical measurements The electrochemical measurements were performed by a standard three-electrode system in a solution of 2 M KOH as electrolyte on a CHI660D electrochemical station. With a Pt plate as the counter-electrode and saturated calomel electrode (SCE) as the reference electrode, a piece of Ni foam (1.5 1.5 cm2) coated with sulfide nanostructures was directly used as a binder-free working electrode. The cyclic voltammetry (CV) and galvanostatic chargeedischarge techniques were employed to investigate the electrochemical properties of the electrodes. The electrochemical impedance spectroscopy (EIS) was obtained in a frequency range between 100 kHz and 0.01 Hz with a perturbation amplitude of 5 mV versus the open-circuit potential. The areal specific capacitances and mass specific capacitance were calculated from the discharge curve based on following equations (1) and (2), respectively.
C¼
IDt F cm2 ADV
(1)
C¼
IDt 1 Fg mDV
(2)
where, I is the current density (A), Dt is the discharge time (s), DV is the potential window of the discharging (V), A is the geometric surface of substrate (cm2), m is the mass loading of active material (g). 3. Results and discussion Fig. 1 shows the XRD patterns of the metal sulfide samples deposited on Ni foam. For Ni3S2/Ni sample, several distinct diffraction peaks at 21.7, 31.1, 37.7, 38.2 , 49.5 , 50.1, 54.5 and 55.0 can be observed, corresponding to the (101), (110), (003), (111), (120), (211), (121) and (300) planes of crystalline Ni3S2 (JCPDF 30-0863), respectively. The other peaks at 44.2 , 51.4 and 76.0 can be indexed to metallic nickel from Ni foam (JCPDF 01-1258). However, for NiCo2S4/Co9S8/Ni and Co9S8/Ni, because of their low crystallinity and fluorescence, only the diffraction peaks at 31.5 and 38.1 can be seen, which are assigned to the (311) and (400) planes of crystalline NiCo2S4 (JCPDF 20-0782), respectively. The diffraction peak at 30.0 can be indexed to Co9S8 (JCPDF 65-6801). However, the relatively weak diffraction peaks of metallic Ni imply that these sulfide films are thickly coated on nickel foam.
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Fig. 1. XRD patterns of (a) Ni3S2/Ni, (b) NiCo2S4/Co9S8/Ni and (c) Co9S8/Ni.
To further evaluate the elemental composition and chemical state of the as-prepared sulfide samples, XPS measurements were conducted. The results are presented in Fig. 2. The survey spectrum in Fig. 2a shows the presence of elements including Ni, Co, S, O and C. In the Ni 2p spectrum, the deconvolution of the Ni 2p peaks shows the atoms in 2p3/2 electronic configuration at 856.8 eV
3
(Fig. 2b), suggesting the co-existence of divalent and trivalent states. The peak at 873.4 eV corresponds to the Ni 2p1/2 band. For the Co 2p spectrum in Fig. 2c, it shows a low energy band (Co 2p3/2) at 780.8 eV and a high energy band (Co 2p1/2) at 797.2 eV, consistent with the results reported elsewhere [19]. The spin-orbit splitting value of Co 2p1/2 and Co 2p3/2 is over 15 eV, suggesting the co-existence of Co2þ and Co3þ [32e34]. The S 2p spectrum in Fig. 2d can be divided into a main peak and a shake-up satellite. The component at 163.8 eV is a typical metal-sulphur bond. The binding energy at 168.3 eV is attributed to the shake-up satellite. According to the XPS analysis, the surface of the three samples has a composition of Co2þ, Co3þ, Ni2þ, Ni3þ, and S2, which is in good agreement with the phases of NiCo2S4/Co9S8, Ni3S2 and Co9S8. The morphologies of the as-prepared metal sulfide samples were characterized by SEM and TEM measurements. Fig. 3a, S1a and S2a show the morphologies of nickel-cobalt, cobalt and nickel hydroxide precursors, respectively. These hydroxides were directly grown on the surface of Ni foam to form an array film. From Fig. 3b, S1b and S2b, we can see that the metal sulfides maintain the original morphologies of their hydroxide precursors. From a close observation of the enlarged SEM images in Fig. 3c, S1c and S2c for these metal sulfides, it can be seen that 3-dimensional crossed sheets are formed on the surfaces of nanorods and nanosheets (Ni3S2), which can be also confirmed by the TEM images, as shown in Fig. 3d, S1d and S2d. These TEM images display that the nanorods (with a diameter about 100 nm) and nanosheets of the metal sulfides are hollow and porous (some pores being marked by black
Fig. 2. XPS spectra of (a) full spectrum, (b) Ni 2p, (c) Co 2p and (d) S 2p for NiCo2S4/Co9S8.
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Fig. 3. (a) SEM images of NieCo hydroxide precursor, (b,c) SEM and (d) TEM images of NiCo2S4/Co9S8.
arrows), which results from the anions-exchange reaction between S2 and OH or CO2 3 in the hydroxide precursors, indicating a complete conversion from metal hydroxides to metal sulfides. From above results, it is apparent that all the metal sulfide crystals possess a porous structure, which is of great importance to guarantee large amounts of active sites for redox reactions and fast ion diffusion to achieve an excellent electrochemical performance. Fig. 4 shows the digital photographs of bare Ni foam, hydroxides/Ni, and NiCo2S4/Co9S8/Ni from left to right. It can be seen that the shape of Ni foam is well maintained and the color of them is changed due to the different coatings. Metal hydroxides film exhibits light-pink color, while metal sulfide film is black. To evaluate the capacitive performance of the electrodes containing different metal sulfides, their electrochemical properties were investigated. Fig. 5a reveals that the areal specific capacitance of NiCo2S4/Co9S8/Ni electrode is much higher than those of Co9S8/ Ni and Ni3S2/Ni electrodes under the same current densities. It can be assigned to its rich Faradaic redox reactions simultaneously from
both nickel and cobalt ions, which is consistent with the redox peaks in Fig. 5b. Co9S8/Ni and Ni3S2/Ni electrodes displayed one pair of redox peaks, while for NiCo2S4/Co9S8/Ni electrode, two pairs of redox peaks can be clearly observed, which are resulted from the following redox processes of Co2þ/Co3þ/Co4þ and Ni2þ/Ni3þ [35]:
CoS þ OH 4CoSOH þ e
(3)
CoSOH þ OH 4CoSO þ H2 O þ e
(4)
NiS þ OH 4NiSOH þ e
(5)
The rate capability is also a key factor for evaluating the potential applications of supercapacitors. When the current density increased from 5 to 50 mA cm2 (in Fig. 5a), the areal capacitance retentions are 44.7%, 66.5%, and 73.5% for NiCo2S4/Co9S8/Ni, Co9S8/ Ni, and Ni3S2/Ni electrodes, respectively. The comparison of rate capability among the three electrodes can be related to the bulk
Fig. 4. Photograph of bare Ni foam, metal hydroxide/Ni, NiCo2S4/Co9S8/Ni from left to right.
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Fig. 5. (a) Areal capacitance at various current densities from 5 to 50 mA cm2, (b) CV curves at the scan rate of 5 mV s1, (c) EIS Nyquist plots and (d) the cycling performance at the current density of 20 mA cm2 of NiCo2S4/Co9S8/Ni, Co9S8/Ni and Ni3S2/Ni electrodes.
solution and charge-transfer resistance. Fig. 5c shows that Ni3S2/Ni electrode displays a relatively lower bulk solution resistance and charge-transfer resistance than both NiCo2S4/Co9S8/Ni and Co9S8/ Ni electrodes, thus exhibiting a high retention under high current density. It should be mentioned that the three electrodes showed lower resistances when compared with conventional metal hydroxides and oxide due to the high conductivity of metal sulfides [5e12]. Their cycling performances were measured by repetitive charging/discharging tests at the constant current density of 20 mA cm2. As shown in Fig. 5d, the specific capacitances of the three electrodes gradually increase because of the progressive activation in the initial stage. NiCo2S4/Co9S8/Ni, Co9S8/Ni and Ni3S2/ Ni electrodes can increase to 314%, 179%, and 172% of their original values after the first 5000, 1000 and 3800 cycles, respectively. However, their specific capacitances would then decrease to some low values after 10,000 cycles. Compared with the CoeNi hydroxide precursor (In Fig. S3 in supplementary material), the specific capacitance of NiCo2S4/Co9S8/Ni electrode is much higher. We think that the hydrotalcite-like CoeNi hydroxides tend to form betaformed metal hydroxides in the alkaline electrolyte soon [36]. To analyze the increasing areal capacitance and the reasons that lead to the decline of the capacitance, their electrochemical properties at different stages in the 10,000 charging-discharging cycles were further investigated. Fig. 6a shows the CV curves of the 1st cycle, the 5000th cycle and 10000th cycle of the NiCo2S4/Co9S8/Ni electrode at the scan rate of 5 mV s1. After a progressive activation for 5000 cycles, the integrated area within the currentepotential
curves reaches the largest, which indicates the highest specific capacitance. It could be also verified by the galvanostatic chargeedischarge measurements (in Fig. 6c). Fig. 6b shows that the areal capacitance can be as high as 8.08 F cm2 and the corresponding mass specific capacitance is 2068 F g1 at the current density of 5 mA cm2. Even at a high current density of 50 mA cm2, the areal and mass specific capacitance can still be 5 F cm2 and 1250 F g1, respectively. In comparison with previous reports, the measured areal and mass specific capacitance were much higher than other electrode materials directly grown on conductive substrates, including NiCo2O4 arrays [37e39] and NiO arrays [14,40]. 61.8% of the capacitance could be maintained with the current density rising from 5 to 50 mA cm2 due to the high conductivity of metal sulfides and the highly-accessible porous structure, and it could be verified by the following EIS measurements. Fig. 6d shows that in the high frequency area, the bulk solution resistance decreases from 0.34 U to 0.28 U during the first 5000 cycles and the negligible semicircle indicates a low charge transfer resistance, which can guarantee easy electrolyte diffusion and a fast electron transport. Regarding to the dramatic decline of capacitance from the 5000th cycle to the 10000th cycle (in Fig. 5d), several factors should be researched. Firstly, the shifting of anodic peak to anodic direction leads to an incomplete redox reaction and the decline of capacitance, which can be verified from the CV curves (Fig. 6a) and charging-discharging curves (in Fig. 6c). The CV curves in Fig. 6a reveal that with the cycling tests processing, the anodic peak shifts
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Fig. 6. (a) CV curves at the scan rate of 5 mV s1, (b) areal capacitance at various current densities from 5 to 50 mA cm2, (c) charge and discharge curves at the current density of 20 mA cm2, (d) EIS Nyquist plots of NiCo2S4/Co9S8/Ni electrode at the 1st, 5000th and 10000th cycle.
from 0.3 V to 0.48 V, and then to 0.55 V. In Fig. 6c, the nonlinear part in the charging curves rises to around 0.45 V during the cycling test. Above data indicate that the charging process has not been fully completed at the original potential of 0.45 V. Meanwhile, with the increase of the current density, the anodic peak shifts to anodic direction because of electrode polarization. Therefore, the abovementioned factors can account for the drastic decrease of the areal capacitance at high current density (5.53 F cm2 at 5 mA cm2, 2.35 F cm2 at 20 mA cm2, 0.177 F cm2 at 50 mA cm2, retaining 69%, 35%, 3.5% of the corresponding values at the 5000th cycle, respectively, in Fig. 6b). In addition, the capacitive decay may also result from the conversion from metal sulfide to metal hydroxide [24,25], thus leading to the increase in bulk solution resistance of approximately 0.98 U (in Fig. 6d) and inhibiting the ions and electrolytes penetrating into the inner nanostructures of the active materials. Similarly, the shift of redox peaks to the anodic direction, polarization and the increase of resistance could account for the drastic decline of the capacitance for Co9S8/Ni and Ni3S2/Ni electrodes at the 10000th cycle. For Co9S8/Ni electrode, its anodic peak shifts from 0.35 V to 0.5 V (in Fig. S4a), the nonlinear parts of the charging process also move to high voltage direction (Fig. S4c), and there is a clear semicircle indicating the increased charge-transfer resistance (Fig. S4d). Ni3S2/Ni electrode also displays the shifting anodic peak in the CV curves from 0.38 V to 0.55 V (Fig. S5a), the rising nonlinear parts in the charging curves (Fig. S5c) and the
significantly increasing charge transfer resistance to 7 U (Fig. S5d). Based on above analysis, it is expected that we can expand the potential range to allow the redox reaction to finish completely. As shown in Fig. 7a, after expanding the potential range from 0.45 V to 0.55 V at the current density of 20 mA cm2, the areal specific capacitance of NiCo2S4/Co9S8/Ni electrode significantly increases from 2.35 F cm2 (Fig. 6b) to 4.58 F cm2 (Fig. 7b). Even at the current density of 50 mA cm2, the areal capacitance can still reach 2.83 F cm2 (Fig. 7b) when the potential range is from 0 to 0.6 V. Similarly, through enlarging the potential range, the areal capacitance of Co9S8/Ni and Ni3S2/Ni electrodes can be 2.16 and 1.41 F cm2, respectively, even at the high current density of 50 mA cm2 (in Fig. 7b and Fig. S6). To further evaluate the practical application of the NiCo2S4/ Co9S8/Ni electrode, an asymmetric capacitor was fabricated with it as positive electrode and activated carbon as negative electrode. The mass ratio between two electrodes are determined by the capacitance and the potential with the equation of CþmþDUþ ¼ CmDU [41]. The electrochemical properties of the hybrid capacitor were measured with a two-electrode system by the workstation. The CV curves of the capacitor measured at 20 mV s1 under different potential windows from 1.0 to 1.6 V are show in Fig. S7b, in which the redox peaks on the CV profiles can be observed. The galvanostatic chargeedischarge curves for the hybrid capacitor at 1 A g1 are exhibited in Fig. S7c, showing a good capacitive behavior from its symmetric and linear
Please cite this article in press as: X.F. Gong, et al., Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.018
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on Ni foam. The as-prepared electrode exhibited an excellent electrochemical performance, including high specific capacitance in area and mass. The capacitance could increase to 314% of the initial value in the first 5000 cycles due to the progressive activation. The highest specific capacitance could reach 8.08 F cm2 and 2068 F g1 at the current density of 5 mA cm2. With the continuous charging-discharging cycles, the redox peaks shifted toward the anodic direction. Through broadening the potential range, the areal capacitance could further reach 4.58 and 2.83 F cm2 even at the high current densities of 20 and 50 mA cm2, respectively. Acknowledgement The authors thank the Micro and Nano Fabrication Laboratory of The Chinese University of Hong Kong (CUHK) for technical support. This work was supported by Zhejiang Provincial Natural Science foundation of China under grant No. LY13E020002, Public Projects (analysis and test) of Zhejiang Province (No. 2015C37027) and the Shun Hing Institute of Advanced Engineering (SHIAE) with the Project No. 8115045 at CUHK. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.02.018. References
Fig. 7. (a) Charge and discharge curves at various current density and potential widow of NiCo2S4/Co9S8/Ni electrode, (b) areal capacitance at various current density and potential widow of NiCo2S4/Co9S8/Ni electrode.
chargeedischarge curves. The corresponding specific capacitance based on total mass under various current densities in Fig. S7d shows that it can have the maximum value of 84 F g1 at 0.5 A g1 in the potential window of 1.6 V. It is also indicated that the hybrid capacitor will deliver a higher specific capacitance in a larger potential range under the same current density. The above electrochemical investigation suggests that the binder-free NiCo2S4/Co9S8/Ni electrode can exhibit an excellent capacitive performance and it is a potential candidate for practical application. We can attribute its outstanding electrochemical performances to the following three factors. (I) The electrode can make fully use of rich Faradaic redox reactions from the simultaneous contribution from both nickel and cobalt. (II) The porous structure and the high conductivity of metal sulfide can effectively guarantee facile electrolyte diffusion, a fast electron transport and large amounts of active sites for redox reactions [42,43]. (III) The binderfree electrode can avoid the use of polymer binders and conductive additives, thus reduce the bulk solution and charge-transfer resistance. 4. Conclusions In summary, a binder-free NiCo2S4/Co9S8/Ni electrode with promising electrochemical performances was successfully fabricated through a two-step hydrothermal method. The NiCo2S4/Co9S8 nanorod arrays with a diameter about 100 nm were directly grown
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Please cite this article in press as: X.F. Gong, et al., Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.018