C@KCu₇S₄ microstructure for solid-state supercapacitors

Abstract

Here we report a high-performance all-solid-state C@KCu₇S₄ hybrid supercapacitor based on carbon (C) particles coated on a KCu₇S₄ electrode. The C@KCu₇S₄ hybrid supercapacitor (2 mg C) shows good electrochemical behavior with a large specific capacitance of 352 F g⁻¹ at a scan rate of 10 mV s⁻¹, the highest energy density of 26.2 W h kg⁻¹ and the highest power density of 994.8 W kg⁻¹, still retaining 86% of the capacitance after 2000 cycles. Moreover, a light-emitting diode (LED) can be lit by three supercapacitors in series for about 3.5 min, indicating the good application prospects of using C@KCu₇S₄ hybrid supercapacitors for energy storage.

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Introduction

Electrochemical capacitors (ECs), also called supercapacitors or ultracapacitors, have attracted much attention in the automotive and consumer electronics industry due to their high capacitance, pulse power capability, and long cycle life.^1–4 Recently, nanomaterials with high specific surface area have been widely utilized to improve the charge accumulation and ion transport in ECs.⁵ Carbonaceous materials, such as carbon nanotubes (CNTs) and graphene, can accelerate charging–discharging of electric double-layer capacitors (EDLCs),^6–9 and have been widely used in supercapacitors. Because of the different charge storage mechanism, ECs can be often divided into three types: electric double-layer capacitors (EDLCs), pseudocapacitors, and hybrid electrochemical capacitors. The performance of ECs is mainly determined by the electrochemical activity and kinetics of the electrodes. Therefore, to improve the energy density of ECs at high rates, it is critical to enhance the kinetics of ion and electron transport in the electrodes.¹⁰ The hybrid supercapacitors usually combine one battery-type faradaic electrode (as energy source) with the other capacitive electrode (as power source) in the same cell.^11,12 They reveal more increased capacitance and improved energy density than EDLCs, but the cycle ability of the faradaic electrode is still limited. So it still has a great challenge to develop the most promising structure or architecture that dramatically enhance the capacity while maintaining the excellent rate capability and charge–discharge cycling life.¹³ For practical applications, it is necessary to find appropriate electroactive materials and integrate them into specific device configurations, therefore, the development of new material platforms to fabricate energy storage devices becomes an indispensible and challenging task.^14,15

The KCu₇S₄ microwire is a novel and good material of the supercapacitors and KCu₇S₄ microwire shows a good electrochemical performance. In particular, the Mn/KCu₇S₄ hybrid supercapacitor has been reported and displayed a good electrochemical performance with a large specific capacitance of 1620 F g⁻¹ at a scan rate of 1 mV s⁻¹.¹⁶ The KCu₇S₄ consists of compound crystal lattice structure, and one of which forms quasi-one-dimensional channels and the K⁺ ions occupy the channels.¹⁶ Because of its double larger channel structure, it not only enhances the ionic and electronic transport, but also shortens the ionic diffusion path. When a bias is applied across the electrodes, the K⁺ ions can well exchange with Li⁺ ions.¹⁶ So the KCu₇S₄ microwires exhibit excellent electrochemical properties.

To further improve the electrochemical behavior of the KCu₇S₄ microwire supercapacitors, a suitable material can be used to hybrid with the KCu₇S₄. Here, we display a C@KCu₇S₄ hybrid supercapacitors fabricated by coating carbon particles on the surface of the KCu₇S₄ film. It is showed that the C@KCu₇S₄ hybrid supercapacitor with 2 mg C coating achieves the outstanding electrochemical performances with a large specific capacitance of 352 F g⁻¹ at a scan rate of 10 mV s⁻¹, the highest energy density of 26.2 W h kg⁻¹ and the highest power density of 994.8 W kg⁻¹. Besides, we have connected three hybrid supercapacitors units in series to light one light-emitting diode (LED) for 3.5 min (ESI Fig. S2†). All the experimental results indicate the promising prospect of using C@KCu₇S₄ hybrid nanostructure as supercapacitors.

Experimental

Synthesis of the KCu₇S₄ microwires

Single-crystalline KCu₇S₄ microwires were synthesized by hydrothermal method, which was reported elsewhere.¹⁶

Assembly of the solid-state supercapacitor

First, the KCu₇S₄ microwires electrode was prepared as follows: the KCu₇S₄ microwires were pressed into thin films in 10 MPa with laminator, and then the KCu₇S₄ film was fixed in the copper sheet with silver paste. Second, the C@KCu₇S₄ hybrid electrode was obtained as follows: (1) 1, 2, 4 mg C powder was weighed in electronic balance (accuracy = ±0.0001 g), respectively. And then the weighed C and 2 mL deionized water were put into 3 mL centrifuge tubes, respectively. The diameter of carbon particles in the powder is about 2–5 μm. The mixtures were dispersed uniformly with ultrasonic oscillation. (2) The mixed C was dropped on the KCu₇S₄ microwires film surface by the microsyringe. And then the C@KCu₇S₄ electrode was dried in room temperature naturally. Finally, the C@KCu₇S₄ supercapacitor was assembled by two pieces of the C@KCu₇S₄ electrodes with a separator (Whatman 8 μm filter paper) and a solid electrolyte (polyvinyl alcohol PVA–LiCl gel) sandwiched between. PVA–LiCl gel electrolyte was simply made as follows: 6 g LiCl was mixed with 60 mL deionized water and then 6 g PVA power was added. The whole mixture was heated to 85 °C under vigorous stirring until the solution become clear.¹⁷ Then the solution was keep at 85 °C without stirring. Before the assembling, the KCu₇S₄ microwires electrodes, C@KCu₇S₄ hybrid electrodes were immersed into the PVA–LiCl solution for 5 min. After the PVA–LiCl gel solidified, the solid-state supercapacitors were prepared.

Characterization

The method of characterization was reported elsewhere.¹⁶ In brief, the morphologies, chemical composition, and the structure of the products were characterized by field-emission scanning electron microscopy (Nova 400 Nano SEM) and XRD (BDX3200 China). X-ray photoelectron spectrometer (XPS) analysis was performed on an ESCA Lab MKII using Mg Ka as the exciting source. The electrochemical properties of the electrodes were investigated with CHI 760D electrochemical workstation.

Results and discussion

Fig. 1 shows the XRD patterns of the prepared KCu₇S₄ microwire and C@KCu₇S₄ hybrid structure. From the pattern of A, we know that all the diffraction peaks match the KCu₇S₄ structure (JCPDS: 47-1334), with the lattice constant of a = 10.167 Å, b = 10.167 Å, c = 3.825 Å. With regard to another weaker peak not being presented, the reason is as follows: crystal texture of KCu₇S₄ maybe caused by the lack of weak peak and the angle of the test is rather small (10 to 80°). For the crystals with lower copper content, a characteristic peak splitting occurs at 2θ = ∼35.4° (400 and 301) and ∼39.7° (420 and 321), which is attributed to the cell variation along the c-axis.¹⁸ Superlattice reflection induces double c axes for the KCu₇S₄ phase.¹⁸ All the carbon diffraction peaks match the C structure (JCPDS: 41-1487) as shown in B, with the lattice constant of a = 2.47 Å, b = 2.47 Å, c = 6.724 Å. So we can know that the carbon particles were mixed into the KCu₇S₄ microwires films. To identify the chemical status of Cu element in the samples, XPS analysis of the KCu₇S₄ microwires was carried out. Fig. S1a† exhibits the XPS survey spectrum of the KCu₇S₄ microwires and the peaks of K 2p, Cu 2p, Cu 3p, Cu 3s, S 2p, S 2s, C 1s and O 1s can be clearly observed. The weak peaks of C and O may come from CO₂, H₂O and O₂ adsorbed on the surface of the sample.¹⁹ The high-resolution XPS spectrum of Cu 2p (Fig. S1b†) shows the binding energies of Cu 2p_3/2 and Cu 2p_1/2 peaks at 932.3 and 952.2 eV, respectively, which are in agreement with the reported values of the main Cu⁺ peak (Cu⁺ binding energy = 932.2 eV and 952.4 eV),^20,21 revealing Cu¹⁺ ions were dominant in KCu₇S₄ microwires. Fig. 2a shows SEM images of prepared KCu₇S₄ microwires with the diameter of around 2 μm and the length up to 110 μm. Local energy dispersive X-ray spectroscopy (EDS) analysis was also conducted for the KCu₇S₄ microwires (Fig. 2b). Besides the Si signal coming from the substrate, K, Cu and S were detected from the microwires, which indicate that the main compositions of the product were K, Cu and S. The SEM images of the surface of C@KCu₇S₄ hybrid electrode are presented in Fig. 2c, from which we can know that the C particles were nearly evenly coated on the surface of the KCu₇S₄ microwires film and the sizes of them are about 2–5 μm. Fig. 2d shows the SEM image of the cross section of C@KCu₇S₄ hybrid electrode. The KCu₇S₄ microwires film has been formed porous structure which can significantly absorb electrolyte, acting as electrolyte reservoirs to facilitate ions transport between KCu₇S₄ microwires and electrolyte.

Fig. 1 (a) XRD patterns of KCu₇S₄ microwires, and C@KCu₇S₄ hybrid electrode.

Fig. 2 (a) SEM images of KCu₇S₄ microwires. (b) EDS pattern of KCu₇S₄ microwires. (c) SEM images of the surface of C@KCu₇S₄ hybrid electrode. (d) SEM images of the cross section of C@KCu₇S₄ hybrid electrode.

Fig. 3a shows CV curves of KCu₇S₄ microwires electrodes at various scan rates with potential window ranging from −0.8–0.8 V, and the maximum specific capacitance of 155 F g⁻¹ is achieved at a scan rate of 10 mV s⁻¹. And the galvanostatic charging–discharging curves for the KCu₇S₄ microwires electrodes at different currents are shown in Fig. S1(a),† revealing that the KCu₇S₄ microwires can be used for the supercapacitors.

Fig. 3 (a) CV curves for KCu₇S₄ microwires electrodes at various scan rates. (b) and (c) CV curves for C@KCu₇S₄ hybrid supercapacitors at different scan rates with different C particles coating quality of 2 and 4 mg. (d) Specific capacitances of KCu₇S₄ and C@KCu₇S₄ hybrid electrodes at different scan rates.

To improve the capacitance of the KCu₇S₄ microwires supercapacitors, we coated C particles on the surface of the KCu₇S₄ film. The C@KCu₇S₄ hybrid supercapacitors with 2 and 4 mg C coating display better electrochemical performances as shown in Fig. 3b and c, where CV curves at various scan rates respectively. The CV curves of the C@KCu₇S₄ hybrid supercapacitors with 1 mg C coating at various scan rates are presented in Fig. S2(b) (ESI†). From these CV curves, we can clearly see the capacitance of the KCu₇S₄ electrode is significantly improved by coating C particles. The specific capacitance of the C@KCu₇S₄ hybrid supercapacitors by coating of 1, 2 and 4 mg C particles are about 294, 352 and 191 F g⁻¹ at a scan rate of 10 mV s⁻¹, respectively, and the results are shown in Fig. 3d, which are calculated by the mass of C and KCu₇S₄ microwires. The highest specific capacitance is obtained from the C@KCu₇S₄ hybrid supercapacitors with 2 mg C, which is higher than that of bare KCu₇S₄ microwires (155 F g⁻¹ at a scan rate of 10 mV s⁻¹). The specific capacitance of the C@KCu₇S₄ hybrid supercapacitors with 4 mg C is lower than that with 2 mg C. Because the increase of C content may induce a larger in the film thickness, while only a thin layer of the C@KCu₇S₄ might efficiently be charged and discharged. Such improved performances of the C@KCu₇S₄ hybrid supercapacitors are mainly attributed to the C particles. Because of its large specific surface and good electrical conductivity of the C@KCu₇S₄ hybrid electrodes, it not only expands the penetration area of the electrolyte, but also shortens the ion diffusion path. In addition, it also enhances the ionic and electronic transport through the electrode system. Fig. 4a shows the galvanostatic charging–discharging curves for 2 mg C@KCu₇S₄ hybrid supercapacitors at different currents, which show its good linear voltage–time profiles. The energy and power densities (E and P) are calculated using equation and , where C_s, V, m and t are the total capacitance, cell voltage, mass of the electrode, discharging time.¹⁷ The Ragone plots of the C@KCu₇S₄ hybrid supercapacitor with 2 mg C is shown in Fig. 4b, which exhibit the highest energy and power densities are 26.2 W h kg⁻¹ at a power density of 261.4 W kg⁻¹ and 994.8 W kg⁻¹ at an energy density of 9.5 W h kg⁻¹.

Fig. 4 (a) Galvanostatic charging–discharging curves for C@Cu₇KS₄ hybrid electrode at different current (the effective area of each electrode is about 1.2 cm²). (b) Ragone plot for 2 mg C@KCu₇S₄ hybrid supercapacitors. (C) Photograph of the light-emitting-diode (LED) driven by a device composed of three supercapacitors connected in series, (C1) in the dark and (C2) in the bright. (C3) Photograph of the capacitance region. (d) Cycle performance of C@KCu₇S₄ hybrid electrode over 2000 cycles at a fixed current of 5 mA.

To demonstrate actual application, we connected three supercapacitors in series to light a LED (the power is about 36–48 mW) as shown in Fig. 4 (C1 and C2), indicating that the LED can be lighted for about 3.5 min (the more details see the ESI in Fig. S3†). Fig. 4 (C3) shows the area of the electrode is about 1.2 cm². The long-term cycling stability of the C@KCu₇S₄ hybrid supercapacitors (2 mg C) is also examined by a cyclic charge–discharge process at a fixed current of 5 mA in Fig. 4d. The specific capacitance of the C@KCu₇S₄ hybrid supercapacitors kept almost 86% after 2000 cycles, which reveals it has a good long-term cyclic performance. All these revealing that the C@KCu₇S₄ hybrid supercapacitor has a good electrochemical performance.

Conclusion

In summary, the C@KCu₇S₄ hybrid supercapacitor (based on coating 2 mg C on the KCu₇S₄ electrode) shows outstanding electrochemical performances with the largest specific capacitance of 352 F g⁻¹ at the scan rate of 10 mV s⁻¹, the highest energy density of 26.2 W h kg⁻¹ and the highest power density of 994.8 W kg⁻¹. In addition, it retains 86% of its initial capacitance after 2000 cycles. Furthermore, it also can be used to light a LED for 3.5 min by three hybrid supercapacitors units in series. All these results suggest that the C@KCu₇S₄ hybrid structure has a promising potential for the high-performance supercapacitors.

Acknowledgements

This work has been funded by the NSFC (11204388), the SRFDP (20120191120039), the Fundamental Research Funds for the Central Universities (no. CQDXWL-2014-001, no. CDJZR12225501, no. CQDXWL-2013-012).

References

1. B. E. Conway, Electrochemical supercapacitors: scientific fundamentals and technological applications, Kluwer Academic/Plenum, New York, 1999.
2. B. E. Conway, J. Electrochem. Soc., 1991, 138, 1539.
3. L. Q. Mai, F. Yang, Y. L. Zhao, X. Xu, L. Xu and Y. Z. Luo, Nat. Commun., 2011, 2, 381.
4. H. Wang, H. S. Casalongue, Y. Liang and H. J. Dai, J. Am. Chem. Soc., 2010, 132, 7472.
5. C. Zhou, Y. W. Zhang, Y. Y. Li and J. P. Liu, Nano Lett., 2013, 13, 2078.
6. J. J. Yoo, K. Balakrishnan, J. S. Huang, V. Meunier, B. G. Sumpter, A. Srivastava, M. Conway, A. L. M. Reddy, J. Yu, R. Vajtai and P. M. Ajayan, Nano Lett., 2011, 11, 1423.
7. G. Yu, L. Hu, N. Liu, H. Wang, M. Vosgueritchian, Y. Yang, Y. Cui and Z. Bao, Nano Lett., 2011, 11, 4438.
8. Y. W. Cheng, S. T. Lu, H. B. Zhang, C. V. Varanasi and J. Liu, Nano Lett., 2012, 12, 4206.
9. Y. W. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach and R. S. Ruoff, Science, 2011, 332, 1537.
10. C. Z. Yuan, L. Yang, L. R. Hou, L. F. Shen, X. G. Zhang and X. W. Lou, Energy Environ. Sci., 2012, 5, 7883.
11. G. H. Yu, L. B. Hu, M. Vosgueritchian, H. L. Wang, X. Xie, J. R. McDonough, X. Cui, Y. Cui and Z. N. Bao, Nano Lett., 2011, 11, 2905.
12. H. l. Wang, Z. W. Xu, A. Kohandehghan, Z. Li, K. Cui, X. H. Tan, T. J. Stephenson and D. Mitlin, ACS Nano, 2013, 7, 5131.
13. L. Yang, S. Cheng, Y. Ding, X. B. Zhu, Z. L. Wang and M. L. Liu, Nano Lett., 2012, 6, 1530.
14. J. Feng, X. Sun, C. Z. Wu, L. L. Peng, C. W. Lin, S. G. Hu, J. L. Yang and Y. Xie, J. Am. Chem. Soc., 2011, 133, 17832.
15. C. L. Zhang, H. H. Yin, M. Han, Z. H. Dai, H. Pang, Y. L. Zheng, L. Q. Lan, J. C. Bao and J. M. Zhu, ACS Nano, 2014, 8, 3761.
16. S. G. Dai, Y. Xi, C. G. Hu, J. L. Liu, K. Y. Zhang, X. L. Yue and L. Chen, J. Mater. Chem. A, 2013, 1, 15530.
17. P. H. Yang, X. Xiao, Y. Z. Li, Y. Ding, P. F. Qiang, X. H. Tan, W. J. Mai, Z. Y. Lin, W. Z. Wu, T. Q. Li, H. Y. Jin, P. Y. Liu, J. Zhou, C. P. Wong and Z. L. Wang, ACS Nano, 2013, 7, 2617.
18. L. He, R. Mackay, S. J. Hwu, Y. K. Kuo, M. J. Skove, Y. Yokota and T. Ohtani, Chem. Mater., 1998, 10, 3172.
19. G. J. Yu, Y. Hai and B. Cheng, J. Phys. Chem. C, 2011, 115, 4953.
20. M. Y. Wang, L. Sun, Z. Q. Lin, J. H. Cai, K. P. Xie and C. J. Lin, Energy Environ. Sci., 2013, 6, 1211.
21. M. C. McShane and C. S. Kyoung, Phys. Chem. Chem. Phys., 2012, 14, 6112.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04893h

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