Hierarchitectures of mesoporous flowerlike NiCo2S4 with excellent pseudocapacitive properties

Hierarchitectures of mesoporous flowerlike NiCo2S4 with excellent pseudocapacitive properties

Materials Letters 187 (2017) 24–27 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet Hie...

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Materials Letters 187 (2017) 24–27

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Hierarchitectures of mesoporous flowerlike NiCo2S4 with excellent pseudocapacitive properties

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Xianbin Liu , Ziping Wu School of Materials Science and Engineering, Jiangxi University of Science and Technology, Hong Qi Road, Ganzhou 341000, China

A R T I C L E I N F O

A BS T RAC T

Keywords: NiCo2S4 Mesoporous Flowerlike Porous nanosheets Pseudocapacitive properties

Micro-/nanostructures with unique structural features could be endowed with intriguing properties. A mesoporous flowerlike NiCo2S4 was synthesized via hydrothermal method followed by thermal annealing treatment. The resultant NiCo2S4 presented flowerlike structure constructed with rich porous nanosheets, and then delivered a high specific surface area. Because of the 3D hierarchical structure, the NiCo2S4 electrode revealed amazing pseudocapacitive properties with high specific capacitance (1516 F/g at 2 A/g) and a favorable rate capability (89.3% from 2 to 16 A/g). Moreover, the specific capacitance remained 89.6% of its initial value after 1000 cycles. The superior results indicated that mesoporous flowerlike NiCo2S4 could be a promising material for supercapacitor electrodes.

1. Introduction With the exhaustion of petroleum energy and increasing environmental pollution, the development of clean and renewable alternative energy is extremely urgent [1]. Supercapacitors, especially the pseudocapacitors, have attracted much attention because of their fast charge/discharge process, long life time and high power density [2– 4]. However, the Faradaic reaction mainly takes place on the atoms near the surface of used electrode materials, limiting their performance. One straightforward strategy that has been extensively investigated for maximizing the performance is constructing micro-/nanostructures [5–7]. Until now, many types of micro-/nanostructural metal oxides including microspheres [7], nanowires [8], nanorods [9], nanotubes [10] and nanosheets [11] have been successfully synthesized. However, the configurations of micro-/nanostructures appear to be relatively simple, not leading to the best performance. It is significant to design and fabricate multiple architectures made up of various micro-/nanostructures, meanwhile not limited to the metal oxides. Due to the higher electric conductivity and capacity compared with metal oxide counterparts, ternary metallic sulfide (NiCo2S4) has been regarded as promising materials for energy storage applications [12,13]. Recently, nanostructured NiCo2S4, such as nanoprisms, [14] hollow nanospheres [15] and hollow hexagonal nanoplates [16] et al., have been reported as supercapacitor electrodes with considerable properties. In addition, hierarchical structures of NiCo2S4 grown on conductive collectors have shown certain advantages, which promotes



electronic transmission [17,18]. With these exciting advances, to construct multiple architectures with porous building blocks is of significance. In this work, we reported a hierarchically structured NiCo2S4 prepared by hydrothermal method followed by thermal annealing treatment. The NiCo2S4 featured with flowerlike morphology and porous petals, facilitating the contact and transport of electrolyte, and thus delivered excellent pseudocapacitive properties. 2. Experimental section Materials synthesis: the mesoporous flowerlike NiCo2S4 was prepared by hydrothermal method following by thermal annealing treatment. Briefly, Ni(NO3)2·6H2O, Co(NO3)2·6H2O and CS(NH2)2 with a molar ratio of 1:2:4 were dissolved in 90 mL ultrapure water by magnetic stirring, and 20 mmol hexamethylenetetramine (HMT) was added. The obtained homogenous solution was transferred into a 100 mL Teflon reaction autoclave and heated at 200 °C for 12 h. After cooling to room temperature, the product was washed with ethanol and water repeatedly, and then dried at 60 °C for 24 h. Finally, the product was annealed at 350 °C in an argon atmosphere for 6 h to obtain the flowerlike NiCo2S4. Characterization: the microstructural characteristics were obtained using a field-emission scanning electron microscope (FESEM, ZEISS SUPRA 55, Germany) with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscope (TEM, FEI Tecnai G2F20, USA). The crystalline structure, pore structure and elemental composition were analyzed by X-ray diffractometer (XRD, Rigaku Ultima III,

Corresponding author. E-mail address: [email protected] (X. Liu).

http://dx.doi.org/10.1016/j.matlet.2016.10.059 Received 31 August 2016; Received in revised form 13 October 2016; Accepted 14 October 2016 Available online 15 October 2016 0167-577X/ © 2016 Elsevier B.V. All rights reserved.

Materials Letters 187 (2017) 24–27

X. Liu, Z. Wu

Fig. 1. (a) SEM, (b) EDS element mappings and (c) nitrogen adsorption-desorption isotherms of the flowerlike NiCo2S4; (d) TEM, (e) HRTEM and (f) SAED pattern of the NiCo2S4 nanosheets. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

apparent hysteresis loop and belongs to type IV, indicating the mesoporous feature of the as-prepared sample. The Brunauer– Emmett–Teller (BET) specific surface area is calculated to be 81.3 m2/g, which is higher than most of other structures for NiCo2S4 [14–16]. The pore diameter from Barrett–Joyner–Halenda (BJH) analysis is approximately 5.0 nm, which is favorable for ion diffusion. The microstructure of NiCo2S4 was further analyzed using TEM. Fig. 1d clearly reveals the ultrathin structure of the NiCo2S4 nanosheets. And in Fig. 1e, the nanosheets show porous structure with rich fine pores (red marks). The pore diameter is ~5.0 nm, which is consistent with the BJH value. Additionally, the selected area electronic diffraction (SAED) pattern (Fig. 1f) presenting several diffraction rings, demonstrates the polycrystallinity of NiCo2S4. The phase and crystallinity were further confirmed by XRD analysis. As shown in Fig. 2a, the NiCo2S4 displayed obvious diffraction peaks at 16.3°, 26.8°, 31.5°, 38.1°, 50.4°, 55.2° and 65.1°, which are corresponded to the (111), (220), (311), (400), (511), (440) and (533) planes of cubic crystal NiCo2S4 (JCPDS 43-1477), respectively [16]. The chemical composition of NiCo2S4 was further analyzed by XPS. The high-resolution XPS spectra were fitted by using a Gaussian method. As shown in Fig. 2b, the Ni 2p core-level spectrum was well fitted with two spin-orbit doublets, characteristic of Ni2+ and Ni3+, and two shake-up satellites (Sat.). Similarly, the Co 2p spectrum was fitted with two spin-orbit doublets (Fig. 2c), characteristic of Co2+ and Co3+, and two shake-up satellites. Fig. 2d shows the spectrum of the S 2p region, in which two resolved peaks of binding energies at approximately 162.0 and 163.1 eV were attributed to S 2p1/2 and S 2p3/2. The binding energy at 163.1 eV is ascribed to the metal-sulfur bonds. These results demonstrate that the NiCo2S4 contain Ni2+, Ni3+, Co2+, Co3+ and S2− [15,18]. The pseudocapacitive properties of the mesoporous flowerlike

Japan), surface area and pore size analyzer (NOVA 2000e, USA) and Xray photoelectron spectroscopy (XPS, Thermal Scientific ESCALAB 250, USA), respectively. Electrochemical measurement: the electrochemical properties of the flowerlike NiCo2S4 were evaluated by using a three-electrode system with Pt sheet counter electrode and standard Hg/HgO reference electrode in 6 M KOH electrolyte solution. The working electrode was prepared by mixing active material, acetylene black and polytetrafluoroethylene with a weight ratio of 80:15:5 in N-methyl-2-pyrrolidinone. Then the mixed slurries were coated onto foam nickel (1×1 cm2), pressed and dried at 60 °C under a vacuum. Cyclic voltammetric (CV) and galvanostatic charge-discharge behaviors were tested at a potential range of 0–0.6 V. Electrochemical impedance spectroscopy (EIS) measurements were conducted in the frequency range of 10−2 to 105 Hz with an amplitude of 10 mV. 3. Results and discussion The size and morphology of NiCo2S4 were elucidated by SEM. Fig. 1a shows that NiCo2S4 displayed a flowerlike structure composed of many ultrathin petals, extremely similar to a rose (the inset photograph). The size of flowerlike NiCo2S4 is close to 4 µm and that of the petals is in the range of 1–2 µm; these ultrathin petals interweave together forming a huge open space. Fig. 1b displays the element mapping of NiCo2S4 with a geometrical distribution. It is obvious that the NiCo2S4 sample is basically made up of Ni, Co and S, and the three elements are uniformly and continuously dispersed. The Ni/Co/S atom ratio is about 1:1.92:3.96, which is very close to the initial stoichiometric ratio of 1:2:4. The specific surface area and pore structure were calculated from the nitrogen adsorption-desorption isotherms. As shown in Fig. 1c, the isotherm of NiCo2S4 has an 25

Materials Letters 187 (2017) 24–27

X. Liu, Z. Wu

Fig. 2. (a) XRD pattern, high-resolution XPS spectra of (b) Ni 2p, (c) Co 2p and (d) S 2p of the flowerlike NiCo2S4.

electrolyte. Second, the petals with fine pores provide inner space for fast accessibility of ion to the reaction surface. Overall, the distinctive and stable architecture of the mesoporous flowerlike NiCo2S4 enables a fast electron-ion transfer to achieve better pseudocapacitive characteristic compared with other structures reported in the previous literature, as shown in Table 1. Electrochemical impedance spectroscopy (EIS) measurement was used to analyze the capacitive behavior of NiCo2S4. The Nyquist plots are plotted in Fig. 3d, and the equivalent circuit diagram is shown in the inset. It is clear that the NiCo2S4 electrode exhibited a near-vertical curve in the low frequency region, indicating an ideal capacitive behavior. Moreover, the electrode shows low solution resistance (Rs, 1.46 Ω) and charge-transfer resistance (Rct, 0.50 Ω).

NiCo2S4 were analyzed by the cyclic voltammetric (CV) and galvanostatic charge-discharge tests. Fig. 3a shows the CV curves of the NiCo2S4 electrode at various scan rates ranged from 10 to 100 mV/s. Obviously, all the curves exhibit two pair of well-define redox peaks, indicating the fast and reversible processes. The legible peaks can be attributed to the Faradic redox based on the following reactions [18]: CoS+OH−↔CoSOH+e− CoSOH+OH−↔CoSO+H2O+e− NiS+OH−↔NiSOH+e− The galvanostatic charge-discharge measurements were conducted at different current densities from 2 to 16 A/g. As shown in Fig. 3b, the curves present distinct voltage plateaus at approximately 0.35 V, demonstrating the pseudocapacitive characteristics. The specific capacitance is calculated according to the discharge curves and plotted in Fig. 3c. The NiCo2S4 electrode exhibited a marvelous specific capacitance of 1516 F/g at current density of 2 A/g. With the current density increasing to 16 A/g, the specific capacitance still achieved a high value of 1354 F/g (89.3% retention). The cyclic performance was tested at 2 A/g. As shown in Fig. 3c, the specific capacitance still retained 89.6% of the initial capacitance after cycling 1000 cycles, indicating outstanding cycling stability. These results indicate that the NiCo2S4 electrode possessed impressive pseudocapacitive properties of relatively high specific capacitance, superb rate capability and outstanding cycling stability. The excellent properties were attributed to the advantage of the mesoporous flowerlike structure of NiCo2S4: First, the microflower structure constructed with interlaced petals accelerates the electron transport and increases the diffusion paths for the

4. Conclusions In summary, 3D mesoporous flowerlike NiCo2S4 was successfully prepared using a hydrothermal method followed by thermal annealing treatment. The resulting NiCo2S4 possessed rich mesopores in the petals with a size of 5 nm, accompanied by a high surface area of 81.3 m2/g. When employed as electrodes, the NiCo2S4 exhibited excellent pseudocapacitive properties: high specific capacitance (1516 F/g at 2 A/g), good rate capability (89.3% from 2 A/g to 16 A/ g) and outstanding cycling stability (89.6% after 1000 cycles). These results are ascribed to the distinctive architecture of the mesoporous microflowers. We hope this study will excite the imagination to construct innovative structures and new materials for high-performance supercapacitor electrodes. 26

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Fig. 3. (a) CV, (b) galvanostatic charge-discharge curves, (c) rate capability and cyclic performance, (d) Nyquist plots of the flowerlike NiCo2S4. [2] P. Simon, Y. Gogotsi, B. Dunn, Science 343 (2014) 1210–1211. [3] H.Y. Lee, J.B. Goodenough, J. Solid State Chem. 144 (1999) 220–223. [4] N. Goubard Bretesché, O. Crosnier, C. Payen, F. Favier, T. Brousse, Electrochem. Commun. 57 (2015) 61–64. [5] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. Van Schalkwijk, Nat. Mater. 4 (2005) 366–377. [6] Y. Yuan, X. Xia, J. Wu, X. Huang, Y. Pei, J. Yang, S. Guo, Electrochem. Commun. 13 (2011) 1123–1126. [7] V. Subramanian, H. Zhu, B. Wei, J. Power Sources 159 (2006) 361–364. [8] S.L. Chou, J.Z. Wang, S.Y. Chew, H.K. Liu, S.X. Dou, Electrochem. Commun. 10 (2008) 1724–1727. [9] Q. Qu, P. Zhang, B. Wang, Y. Chen, S. Tian, Y. Wu, R. Holze, J. Phys. Chem. C 113 (2009) 14020–14027. [10] D.N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, S. Iijima, Nat. Mater. 5 (2006) 987–994. [11] S. Biswas, L.T. Drzal, Chem. Mater. 22 (2010) 5667–5671. [12] J. Xiao, X. Zeng, W. Chen, F. Xiao, S. Wang, Chem. Commun. 49 (2013) 11734–11736. [13] H. Wan, J. Jiang, J. Yu, K. Xu, L. Miao, L. Zhang, H. Chen, Y. Ruan, CrystEngComm 15 (2013) 7649–7651. [14] L. Yu, L. Zhang, H.B. Wu, X.W.D. Lou, Angew. Chem. Int. Ed. 126 (2014) 3785–3788. [15] L. Shen, L. Yu, H.B. Wu, X.Y. Yu, X. Zhang, X.W.D. Lou, Nat. Commun. 6 (2015) 6694. [16] J. Pu, F. Cui, S. Chu, T. Wang, E. Sheng, Z. Wang, ACS Sustain. Chem. Eng. 2 (2013) 809–815. [17] H. Chen, J. Jiang, L. Zhang, D. Xia, Y. Zhao, D. Guo, T. Qi, H. Wan, J. Power Sources 254 (2014) 249–257. [18] L. Shen, J. Wang, G. Xu, H. Li, H. Dou, X. Zhang, Adv. Energy Mater. 5 (2015) 140097. [19] Y. Zhang, M. Ma, J. Yang, C. Sun, H. Su, W. Huang, X. Dong, Nanoscale 6 (2014) 9824–9830. [20] Y. Zhu, Z. Wu, M. Jing, X. Yang, W. Song, X. Ji, J. Power Sources 273 (2015) 584–590. [21] H. Chen, J. Jiang, L. Zhang, H. Wan, T. Qi, D. Xia, Nanoscale 5 (2013) 8879–8883.

Table 1 Comparison of the flowerlike NiCo2S4 with other structural NiCo2S4 reported in the literature. Materials and morphology

Specific capacitance

Rate capability

Preparation method

BET

NiCo2S4 hollow hexagon [16] NiCo2S4 nanoprisms [14] Tube-like NiCo2S4 [19] NiCo2S4 nanoparticles [20] NixCo9−xS8 urchins [21] NiCo2S4 hollow spheres [15] Flowerlike NiCo2S4

437 F/g (1 A/ g) 895.2 F/g (1 A/g)

53.2% (20 A/g) 65.4% (20 A/g)

Template

/

Template

30 m2/g

1048 F/g (3 A/g) 1440 F/g (2 A/g)

50.1% (20 A/g) 75.1% (50 A/g)

Hydrothermal

34.7 m2/g

Solvothermal

42.8 m2/g

1404 F/g (2 A/g) 1036 F/g (1 A/g) 1516 F/g (2 A/g)

41.3% (8 A/g) 68.1% (20 A/g) 89.3% (16 A/g)

Solvothermal

18.0 m2/g

Solvothermal

53.9 m2/g

Hydrothermal

81.3 m2/g

Acknowledgements The authors greatly acknowledge the financial supports by the Department of Science & Technology of Jiangxi Province (Grant No. 20153BCB23011). References [1] S.F. Tie, C.W. Tan, Renew. Sustain. Energy Rev. 20 (2013) 82–102.

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