Zhihong
Ai
,
Zhonghua
Hu
*,
Yafei
Liu
,
Mengxuan
Fan
and
Peipei
Liu
Department of Chemistry, Tongji University, 1239 Siping Road, Shanghai 200092, China. E-mail: huzh@tongji.edu.cn; Fax: +86-21-65982287; Tel: +86-21-65982594
First published on 27th October 2015
Novel 3D flower-like CoNi2S4/carbon nanotube composites with a porous structure were designed through a facile precursor transformation approach as electrode materials for supercapacitors. The resulting samples are characterized by X-ray diffraction, Raman spectra, energy dispersive spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption–desorption. Their electrochemical performance was investigated by means of cyclic voltammetry, galvanostatic charge–discharge, impedance spectra and cycle life. By selecting carbon nanotubes as the conductive support for the growth of CoNi2S4, the as-obtained CoNi2S4/carbon nanotube composites displayed an ultrahigh specific capacitance of 2094 F g−1 at 1 A g−1 and a good rate capability (72% capacity retention at 10 A g−1). These results above suggest the great potential of the unique flower-like CoNi2S4/carbon nanotube composites in the development of high-performance electrode materials for supercapacitors.
More recently, NiCo2S4 has been reported to exhibit higher conductivity than NiCo2O4 owing to the lower optical band gap energy than that of NiCo2O4.14 Similar with NiCo2O4, CoNi2S4 is also a binary metallic species and possesses richer valence states than the corresponding one-component metallic sulfides nickel sulfide (NiS) and cobalt sulfide (Co9S8).15 Chen14 and his co-workers synthesized an urchin-like NiCo2S4 electrode material by a precursor transformation method and the as-obtained product exhibits a high specific capacitance of 1025 F g−1 at 1 A g−1. More recently, Yu et al. synthesized uniform NixCo3−xS4 hollow nanoprisms16 with thioacetamide (TAA) as the sulfur source. Zhang et al.17 synthesized urchin-, tube-, flower-, and cubic-like NiCo2S4 microstructures through controlling the composition of the solvents and found that the solvent polarity and solubility have a significant effect on the process of nucleation and crystal growth. In order to increase the efficient utilization of electrodes, electroactive materials supported by conductive substrates, such as Ni foam,18 graphene19,20 and carbon cloth,21,22 were also reported. Their electrochemical properties were greatly improved due to the high conductivity, large surface area and three-dimensional network of these substrates. Besides them, carbon nanotubes are also highly conductive and have a large surface area, in favor of electron transportation and material loading. However, there are not very many reports on the study of the combination of CNTs and nickel cobalt sulfides, so more research needs to be done in this area, especially in the area of constructing a novel structure using them.
In this work, a new composite electrode material CoNi2S4/CNT was designed by a two-step approach, including the deposition of the Co–Ni precursor on CNTs and the transformation of the Co–Ni precursor via anion exchange. Before the reaction, CNTs were pretreated with acid to increase the amount of oxygen-containing functional groups on the CNT surface in order to improve the deposition of the Co–Ni precursor on the CNT substrate. In the synthesis process, urea, a weak base, was used as a precipitant to control the crystal growth of the nickel–cobalt precursor in the first step and Na2S was taken as the sulfur source for the anion exchange reaction in the second step. The as-obtained CoNi2S4/CNT composites were characterized by XRD, Raman spectra, EDS, FESEM, BET and electrochemical tests. The relation between their unique structural properties and capacitive performance was investigated.
A facile hydrothermal method was used for the synthesis of the Ni–Co sulfide/CNTs composite. Firstly, 4 mmol NiSO4·6H2O, 2 mmol CoSO4·7H2O and 24 mmol CO(NH2)2 were added into 40 mL DI water and stirred for 30 minutes to form a homogeneous solution. Meanwhile, 40 mg of acid-treated CNTs and 15 mg of polyvinyl pyrrolidone (PVP) were put in 40 mL of ethanol and underwent ultrasonic treatment for 30 min. Then the two solutions were mixed and underwent ultrasonic treatment for 30 minutes. Secondly, the mixed solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 120 °C for 6 h. After cooling down to room temperature, the precursor was separated by centrifugation, washed with DI water and ethanol several times, and dried at 80 °C for 12 h.
0.15 g of precursor and 0.54 g of Na2S·9H2O were added into 80 mL of DI water with vigorous stirring for 30 minutes. Then the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 12 h. After cooling down, the product was collected and washed with DI water and ethanol several times, and finally dried at 80 °C.
To study the influence of the CNT content on the composite, different CNT amounts, 0, 30, 40, 50 and 60 mg, were added in the synthesis. The obtained composite samples are denoted as M-0, M-30, M-40, M-50 and M-60, respectively.
(1) |
Fig. 1b shows the XRD patterns of the as-prepared CoNi2S4/CNT sample. The characteristic peaks at 16.3°, 26.7°, 31.5°, 38.2°, 47.4°, 50.3° and 55.0°, respectively, correspond to (111), (220), (311), (400), (511) and (440) planes of the cubic CoNi2S4 phase (JCPDS Card No. 24-0334). Besides, the two weak peaks in the range between 20° and 25° are believed to be caused by the acid-treated CNTs, which always have a typical peak at 26°.23,24
Fig. 2 presents the Raman spectra of the pure acid-treated carbon CNTs and the CoNi2S4/CNT composites in the range from 800 to 4000 cm−1. Three typical graphic peaks can be seen in both the Raman spectra of the pure acid-treated carbon CNTs and the CoNi2S4/CNT composites: the peak at 1348 cm−1, the D band, is attributed to the breathing modes of the rings or κ-point photons of A1g symmetry, the symbol of the defects on the CNT surface;25 the peak at 1585 cm−1, the G band, is attributed to the splitting E2g stretching mode of graphite; the peak at 2086 cm−1, the 2D band, results from a harmonic of the D band.26 These two Raman spectroscopies give a good demonstration about the existence of CNTs in the composites.
The EDS spectrum proves the existence of Co, Ni, S, C and O elements in the CoNi2S4/CNT sample as shown in Fig. 3. The elemental molar ratio of Co and Ni is 1:2.03, which is very close to the ratio of Ni and Co atoms in the formula of CoNi2S4. Noticeably, the content of C is 37.87%, including both the small amount of C in the substrate and other C in the CNTs. The O element is supposed to come from oxygen functional groups on the surface of the CNTs or residual oxygen atoms, because of incomplete sulfur treatment.
The morphological features of the CoNi2S4/CNT samples were studied by FESEM, as shown in Fig. 4. Fig. 4(a and b) displays the FESEM image of the precursor, which appears to be a 3D flower-like structure with a diameter of approximately 2 μm and is composed of lots of interconnected nanosheets of about 10 nm in thickness. Besides, it can be found that the MWCNTs (short and tiny rods) are partly enwrapped in the nanosheet and partly lie between nanosheets, as shown in the red rectangle in the high-magnification image of Fig. 4b, in favor of forming the basic 3D flower texture of the flower structure. And the shape was well preserved after sulfidation as can be seen in Fig. 5a.
Moreover, it can be observed that the surface of the nanosheets became rather rough and porous, as shown in the high-magnification images in Fig. 5b. Here, it is worthwhile to point out that in the process of crystal growth, PVP, a non-ionic surfactant, could induce faces toward different directions to grow with different speed and lead to the final flower-like structure.
Besides, the TEM characterization of CoNi2S4 (a) and CoNi2S4/CNT (b) was also performed, as can be seen in Fig. 6. The edges of their nanosheets are presented and compared. There seem to be many slim rods in the nanosheet of CoNi2S4/CNT, which is a symbol of the existence of CNTs in the sample. But the lattice spacing was not given due to its weak crystallinity, which is in accordance with the XRD characterization results.
Fig. 7a shows that the oxidation and reduction peaks can be clearly observed in the potential range of −0.3–0.6 V, suggesting good pseudocapacitive characteristics. With an increase in the scan rate from 5 mV s−1 to 30 mV s−1, the anodic peak shifts to a higher potential and the cathodic peak shifts to a lower potential. And more peaks appear at around 0.2 V, which may result from the abundant Faradic redox processes of Ni2+/Ni3+ and Co2+/Co3+/Co4+ transitions associated with the anion OH−. The redox reactions in the alkaline electrolyte are based on the following mechanism:27
NiS + OH− → NiSOH + e− | (2) |
CoS + OH− → CoSOH + e− | (3) |
CoSOH + OH− → CoSO + H2O + e− | (4) |
The distortion of the CV curves becomes more and more serious as the scan rate increases from 5 mV s−1 to 30 mV s−1. It is because the ionic diffusion rate in the case of high scan rate was not fast enough to satisfy electronic neutralization during the redox reactions.
Fig. 7b shows the galvanostatic charge–discharge curves of the CoNi2S4/CNTs electrodes within the potential window between 0 and 0.35 V at the discharge current density of 1, 2, 5, 7 and 10 A g−1, respectively. The shape of the charge–discharge curves exhibits a typical pseudocapacitive behavior. Encouragingly, for M-40, the specific capacitance values are as high as 2094, 2022, 1855, 1694 and 1510 F g−1 at current densities of 1, 2, 5, 7 and 10 A g−1, respectively, as shown in Fig. 7c, which are much higher than those of other NiCo2S4 and CoNi2S4 materials reported in the literature as listed in Table 1. The higher capacitance of the CoNi2S4/CNTs electrode probably stemmed from its novel 3D flower-like structure with many pores formed between interconnected nanosheets, which has a high efficiency of the space and provided enough active sites for electrochemical reactions with electrolytes.
Sample | Specific capacitance (F g−1/1 A g−1) | Rate retention (%) | Current range (A g−1) | BET (m2 g−1) | Ref. |
---|---|---|---|---|---|
Urchin-like NiCo2S4 | 1025 | 77.3 | 1–20 | — | 14 |
CoNi2S4/graphene | 2009 | 49.8 | 1–20 | 48.4 | 28 |
Nanostructured NiCo2S4 | 1050 | 66.2 | 1–50 | 20.3 | 18 |
NiCo2S4 hollow | 895 | 65.4 | 1–20 | 30.0 | 16 |
Nanoprisms | |||||
NiCo2S4 nanotubes | 933 | 58.9 | 1–5 | 32.0 | 29 |
NiCo2S4 nanosheets on graphene | 1300 (3 A g−1) | 52.3 | 3–20 | 79.1 | 20 |
Porous NiCo2S4 hexagonal nanoplates | 966 | 95.5 | 1–10 | 17.4 | 30 |
CoNi2S4 nanoparticles | 1169 | 60.1 | 1–5 | 21.6 | 18 |
Mesoporous NiCo2S4 nanoparticles | 972 (2 A g−1) | 86.8 | 2–20 | 42.8 | 31 |
Ball-in-ball hollow NiCo2S4 spheres | 1036 | 68.1 | 1–20 | — | 32 |
Hollow tubular NiCo2S4 | 1046 (2 A g−1) | 68.0 | 2–20 | 108.4 | 33 |
3D flower-like CoNi2S4 | 2094 | 72.0 | 1–10 | 31.5 | Our work |
Cycle stability is also an important factor determining the electrochemical performance of the electrode materials. In the present work, cycle stability experiments were carried out at a charge–discharge current density of 10 A g−1 in the potential range of 0–0.35 V for 1000 repetitive cycles, as shown in Fig. 7(d). The specific capacitance loss after 1000 cycles was 16%, suggesting that the capacity retention was as high as 84%. The last 10 cycles are presented as an inset. It was found that the shapes of the charge–discharge curves were nearly unchanged, indicating the good reversibility and cyclic life of CoNi2S4/CNT electrodes.
Fig. 8(a) shows the typical CV curves of M-0, M-30, M-40, M-50, and M-60 at a scanning rate of 5 mV s−1 in the potential range from −0.3 V to 0.6 V. Evidently, M-40 has the largest CV area, meaning that the M-40 electrode has the highest specific capacitance. The specific capacitance of the composite electrodes increases with increasing CNT addition from 0 mg to 40 mg, but decreases from 40 mg to 60 mg. Thus the addition of 40 mg of CNTs to the composite is the optimal value. The galvanostatic charge–discharge at 1 A g−1 in a potential window of 0–0.35 V is shown in Fig. 8(b). The sample M-40 exhibits the longest discharge time, suggesting that it has the highest specific capacitance. Reasonably, it is because of the synergistic effect of the pseudocapacitance of CoNi2S4 and the double-layer capacitance of the CNTs.34 But when the content of CNTs exceeds 40 mg, the double-layer capacitance of the CNTs has a greater effect on the electrodes compared with the pseudocapacitance of CoNi2S4, so that the capacitance is decreased instead.
Fig. 8 (a) CVs; (b) charge–discharge curves; (c) impedance spectra; (d) specific change with different current density of the samples M-0, M-30, M-40, M-50, and M-60. |
The test result of electrochemical impedance spectroscopy (EIS) is shown in Fig. 8(c); all EIS curves have a similar shape consisting of a semicircle on the left and an inclined line on the right. Among them, the inclined line of M-40 in the low-frequency region is almost parallel to the vertical axis, which means that the Warburg resistance is rather small and exhibits a fast ion-diffusion behavior in the electrolyte. The change in capacitance with the increasing current density, also called rate capacity, is another important factor other than the specific capacitance and cycle life. As shown in Fig. 8(d), when the current density changes from 1 A g−1 to 10 A g−1, the capacitance retention of M-0, M-30, M-40, M-50, and M-60 are 56%, 57%, 72%, 73%, and 83%, respectively. Interestingly, the rate capacity was improved with the increasing amount of CNTs, which may due to the increase in the conductivity that facilitates access for electron transfer and benefits the fast charging and discharging.14
BET characterization was carried out to investigate the specific surface area and pore-size distribution of the as-prepared samples. The adsorption/desorption isotherms are of the combination of Type I and Type V isotherms as shown in Fig. 9(a), and with a typical H3 hysteresis loop, suggesting the presence of mesopores for CoNi2S4/CNTs composite samples M-0 and M-40. Their pore size distributions are shown in Fig. 9(b). It is clear that most pores are in the range of 4–15 nm. The mesoporous structure possibly came from the slot pores35 between the interconnected nanosheets in the flower structure.
Fig. 9 (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding pore size distributions of M-0 and M-40. |
The specific surface area, pore volume and average pore size details are listed in Table 2.
Samples | S BET (m2 g−1) | V tot (cm3 g−1) | PS (nm) |
---|---|---|---|
S BET, BET specific surface area; Vtot, total pore volume; PS, average pore size. | |||
M-0 | 16.2 | 0.081 | 19.9 |
M-40 | 31.5 | 0.108 | 13.7 |
It is noteworthy to point out that the specific area of M-40 was much larger and the average pore size became smaller than that of M-0. The improved specific surface area can be attributed to its hierarchical structure with CNTs embedded in and between the sheets consisting of the flower structure, which is beneficial to the mass and charge transport during the electrode reaction. Besides, the average pore size of M-40 shifted to a smaller size than that of M-0, leading to higher specific surface area and more active sites for electrochemical reactions. Thus both the improved specific area and reasonable mesopore size distribution are responsible for the enhanced electrochemical properties.
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