Ionics (2016) 22:1253–1258 DOI 10.1007/s11581-016-1741-y
SHORT COMMUNICATION
Improved performance of supercapacitors constructed with activated carbon papers as electrodes and vanadyl sulfate as redox electrolyte Zhihui Liang 1 & Jinshu Wen 1 & Bing Guo 1 & Zhiyu Cheng 1 & Yongfu Qiu 1 & Pingru Xu 1 & Hongbo Fan 1 & Chunyong He 2,3
Received: 17 March 2016 / Revised: 14 April 2016 / Accepted: 12 May 2016 / Published online: 6 June 2016 # Springer-Verlag Berlin Heidelberg 2016
Abstract In this report, the supercapacitor was constructed with activated carbon paper as electrodes and 3 M H2SO4 solution bearing 1 M VOSO 4 as electrolyte. This supercapacitor showed high areal capacitance of 2146 mF/ cm2 with around 16 times higher than that of 134 mF/cm2 in pristine electrolyte of 3 M H2SO4 and good cycling performance retaining 92 % after 5000 cycles. The full-scale and enlarged Nyquist-type impedance spectra further indicate that the supercapacitor has low diffusion resistance and exhibits good power capability. Constructed by this way, the performances of supercapacitors were improved for the synergistic effect of the absorbance and catalysis of the activated carbon paper as well as redox properties of the couples VO2+/VO2+.
Keywords Supercapacitor . Carbon paper . Vanadyl sulfate . Electrolyte
Introduction In recent times, the electric double layer capacitors (EDLCs) have drawn great research interests owing to their outstanding electrochemical behaviors, such as high power density, excellent stability, long cycle life, light-weight, and good safety [1–4]. However, the EDLCs usually have low energy density. Recently, an innovative approach of adding redox species such as VO2+/VO2+ into the electrolytes was used [5]. This technique is very simple and cost effective to improve the performance of EDLCs via electron transfer on the electrode-electrolyte interface through reversible redox reactions. To enhance the VO2+/VO2+ redox reaction, the catalytic processes with hydroxyl or carbonyl functionalized carbon materials were introduced [6]. In this paper, the surface of carbon paper with hydroxyl and carbonyl species was functionalized by a novel effective strategy, and VOSO4 was added into electrolyte. The supercapacitor constructed by this way showed that the areal capacitance of 2146 mF/cm2 for activated carbon paper (ACP)-120 was about 16 times higher than that of 134 mF/cm2 in pristine electrolyte of 3 M H2SO4 and the good cycling performance retained 92 % after 5000 cycles.
* Zhiyu Cheng
[email protected] * Yongfu Qiu
[email protected]
Experimental Preparation of activated carbon paper
1
College of Chemistry and Environmental Engineering, Guangdong Engineering and Technology Research Center for Advanced Nanomaterials, Dongguan University of Technology, Guangdong 523808, People’s Republic of China
2
Institute of High Energy Physics, Chinese Academy of Sciences (CAS), Beijing 100049, China
3
Dongguan Neutron Science Center, Dongguan 523803, China
The activated carbon papers were prepared as follows: the carbon papers (CPs) were purchased from Jixing Sheng’an company with the thickness of 0.30 mm. Their surfaces were oxidized by a potassium dichromate lotion containing 20.0 mL of concentrated H 2 SO 4 (98 %) and 2.0 g of K2Cr2O7 at room temperature for controlled periods of time
1254
Ionics (2016) 22:1253–1258
(30, 60, and 120 s). It should be noted that a longer treatment time than 120 s will lead to deterioration of the mechanical properties of carbon paper greatly. The samples were taken out and washed with distilled water for three times and then heated at 150 °C for 2 h. The as-prepared samples were denoted as ACP-30, ACP-60, and ACP-120, respectively. Characterization Scanning electron microscope (SEM) micrographs of treated and untreated carbon papers were acquired by using a JEOL 6701F microscope with accelerating voltage of 5 kV. The powder X-ray diffraction (XRD) patterns of the samples were obtained on a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ = 0.154 nm) operated at a scanning speed of 0.025°/s over the 2θ range of 10–80°. Nitrogen adsorption experiments were performed to determine specific surface areas of the samples with a JWGB Sci. & Tech BK132F automatic adsorption apparatus. In order to further understand chemical bonding status of the samples over the treatment, attenuated total reflectance Fourier transform infrared spectroscopy (ATRFTIR) was measured on a Thermo Nicolet 6700 spectrometer. Contact angles were tested from a surface tension meter (Dataphysics OCA20, Germany). XPS spectra were obtained by a Kratos Axis Ultra Dldxps instrument equipped with a monochromatic Al Ka (hν = 1486.6 eV) X-ray source. All of the XPS spectra were collected at a takeoff angle of 90°, with the path energy of the photoelectron analyzer at 40 keV and a step size of 0.1 eV. The electrochemical properties of supercapacitors were studied by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements on a CHI 440a electrochemical work station in 3 M H2SO4 solution bearing 1 M VOSO4 as the electrolyte using three-electrode system. Since the solubility of VOSO4 in 3 M H2SO4 solution is 1.786 M at room temperature, so the higher VOSO4 concentration will lead to easy precipitation of crystal, so 3 M H2SO4 solution bearing 1 M VOSO4 as the electrolyte was used [7]. Carbon papers with an area of 1.0 cm2 were used as the working electrode, Ag/AgCl (3.0 M KCl) and a Pt wire as the reference and counter electrode, respectively. The electrochemical impedance spectroscopy (EIS) was obtained by a M2273 potentiostat (EG&G), with the frequency range from 0.1 to 100 Hz and the AC amplitude of 10 mV.
Fig. 1 SEM images of CP (a), ACP-30 (b), ACP-60 (c), and ACP-120 (d)
modified by potassium dichromate lotion. The crystal structures of treated carbon papers were not changed, and the typical XRD pattern for ACP is shown in Fig. 2a, which is ascribed to carbon (PDF#12-0212) with lattice constants of a = 0.246 nm and c = 0.674 nm. The specific surface areas of samples were measured by nitrogen adsorption–desorption method. In Fig. 2b, the typical adsorption isotherms for CP and ACP-120 are shown, and a sharp increase near the relative pressure P/Po of 1 indicates that micropores or mesopores in the samples could be ignored. Analysis based on the Brunauer–Emmett–Teller (BET) equation is as follows [8]: 1 c1 p 1 h . i¼ þ c ν p ν m mc 0 ν p0 p 1 where p and p0 are the equilibrium and the saturation pressure of the adsorbates at the temperature of adsorption, ν is the adsorbed gas quantity, νm is the maximum monolayer adsorption quantity, c is the BET constant, and the specific surface areas for CP and ACP are calculated as 8.4 and 22.2 m2/g, respectively. It is clear that the specific surface area for ACP is about 2.6 times higher than that of CP, which is ascribed to the fact that the potassium dichromate lotion corroded the surface and accordingly led to a larger specific surface area. The surface element analysis was performed by the X-ray photoelectron spectroscopy (XPS) for ACP-120 in a wide energy range. In Fig. 2c, d, the binding energy was determined by reference to C 1s line at 284.8 eV. Generally, elements C and O can be observed in ACP. The O concentration is calculated to IO =SO Ii
Results and discussion
be 18.3 atom% based on the equation, CO ¼
SEM, XRD, nitrogen adsorption, and XPS
CO is the oxygen concentration, IO and Ii are the peak intensities of oxygen and other elements, and SO and Si are the relative sensitivity factors of oxygen and other elements [9]. In order to measure the chemical states of the element C, core level XPS spectra of C 1s in ACP have been observed and
∑ =Si
, where
i
The surface morphologies of activated carbon papers turned rougher with the increased treated time, as shown in Fig. 1. This indicated that the surfaces of carbon papers were
Ionics (2016) 22:1253–1258
1255
Fig. 2 a XRD pattern of ACP at room temperature. b N2 adsorption–desorption isotherms for CP and ACP-120. c XPS spectrum in the whole energy range for ACP-120. d C 1s XPS spectra of ACP-120
are shown in Fig. 2d. Through decomposition, five binding energies 284.8, 285.9, 287.9, 289.6, and 291.6 eV can be observed, respectively. They are assigned to the carbons in functional groups of C–C, C–O–C, –C–OH, C=O, and O–C=O [10]. This result indicates that the surface of the carbon paper has been oxidized and functionalized after the potassium dichromate lotion treatment. ATR-FTIR and wetting property ATR-FTIR was used to determine molecular-scale information on the surfaces of samples. Before the measurement, the physical adsorbed water in the samples was removed through heating at 40 °C under vacuum for 24 h. In Fig. 3a, the samples ACP-30, ACP-60, and ACP-120 obviously show the peaks at 1054, 1214, 1722, and 3426 cm−1; however, for the CP, no corresponding peaks could be found, implying that groups of C–O, C–H, C=O, and –OH were generated after oxidization treatment [11–13]. The coexistence of –OH and C=O groups as well as the out-of-plane vibration at about 900 cm−1 for carboxyl (–COOH) groups suggests that the – COOH groups have been successfully introduced on the surface of treated carbon papers [11]. In order to characterize the wetting property of the surface, the water contact angle measurement was carried out. The
Fig. 3 a ATR-FTIR spectra recorded for CP, ACP-30, ACP-60, and ACP-120. b Contact angle of samples as function of oxidation time: (a) CP, (b) ACP-30, (c) ACP-60, (d) ACP-120
1256
contact angle is the angle measured through the liquid, where a liquid–vapor interface meets a solid surface, which quantifies the wettability of a solid surface by a liquid via the Young equation: γSG γSL γLG cosθC ¼ 0, where γSG , γSL , and γLG are the solid–vapor interfacial energy, the solid–liquid interfacial energy, and the liquid–vapor interfacial energy, and θC is the equilibrium contact angle [14]. In Fig. 3b, the descending contact angles for CP, ACP-30, ACP-60, and ACP-120 could be observed. This suggests that their surface hydrophilicity gradually enhanced via the potassium dichromate lotion treatment for longer time. The enhancement of surface hydrophilicity is ascribed to the intake of hydroxyl (–OH), carbonyl (>C=O), and carboxyl (–COOH) groups on ACP after oxidation treatment, and these groups are deemed to serve as strong polar sites to absorb water molecules [15]. Cyclic voltammetry, galvanostatic charge/discharge, and EIS The typical CV curves for CP, ACP-30, ACP-60, and ACP120 at a scan rate of 5 mV s−1 in 3 M H2SO4 solution bearing Fig. 4 a Cyclic voltammograms for CP, ACP-30, ACP-60, and ACP-120 at a scan rate of 5 mV s−1 in 3 M H2SO4 solution bearing 1 M VOSO4. b The corresponding calculated areal capacitances of the samples from a. c CV curves for ACP-120 at a scan rate of 5 mV s−1 in different electrolytes (a–e are the solutions with 0, 0.1, 0.2, 0.5, and 1 M VOSO4 added into 3 M H2SO4, respectively), and inset is the corresponding enlarge image. d The corresponding calculated areal capacitances of ACP-120 in different electrolytes from c. e CV diagrams for ACP-120 collected at different scan rates in 3 M H2SO4 solution bearing 1 M VOSO4. f The corresponding calculated areal capacitances from e
Ionics (2016) 22:1253–1258
1 M VOSO4 are shown in Fig. 4a, and their calculated areal capacitances were 938, 1217, 1687, and 2146 mF/cm2 shown Uc
in Fig. 4b using the equation, C ¼ SυðU1c Ua Þ ∫Ua IðUÞdU [16], where C is the areal capacitance (F/cm2), S is the projected surface area of the carbon paper (cm2), u is the potential scan rate (V/s), Uc Ua is the sweep potential range during (U) discharging branch, and I(U) denotes the response current density (A/cm2). This implies that carbon papers treated by the potassium dichromate lotion for 120 s own excellent capacitive characteristics. Once the VOSO4 was added, the areal capacitance of 2146 mF/cm2 for ACP-120 was about 16 times higher than that of 134 mF/cm2 in pristine electrolyte of 3 M H2SO4. In Fig. 4c, d, with the concentration of VOSO4 increasing, the surrounded areas of CV curves increase, and the corresponding areal capacitances are 134, 430, 719, 1140, and 2146 mF/cm2, indicating that the additive VOSO4 greatly affected on the areal capacitances. The inset of Fig. 4c shows that a couple redox peaks at 0.4–0.5 V for the ACP samples in the pristine electrolyte of 3 M H2SO4 could be observed and which could be ascribed to the redox of function groups of C– O–C, –C–OH, C=O, and O–C=O on their surfaces [17]. The
Ionics (2016) 22:1253–1258
typical CV curves and the calculated areal capacitances for ACP-120 at different potential scan rates are given in Fig. 4e, f. It is obvious that the areal capacitances decrease fast with the potential scan rates increasing from 5 to 100 mV/ s, suggesting that the CV processes were controlled by the relatively slow diffusion of ions in electrolyte such as VO2+, SO42−, H+, and OH− [18]. Supercapacitors with ACP-120 as electrodes and 3 M H2SO4 solution bearing 1 M VOSO4 as electrolyte exhibit high performance, as further confirmed using galvanostatic charge/discharge measurements in the three-electrode cell. In Fig. 5a, the charge–discharge curves with asymmetrical and nonlinear for ACP-120 at various current densities indicate the redox properties of the VO2+/VO2+. According to the equation IΔt C single ¼ ΔVS ðÞ, where I, Δt, ΔV, and S are the constant current, the discharge time, the voltage change during the discharge process, and the plane area, the calculated areal capacitances for ACP-120 are shown in Fig. 5b. The corresponding values are 2067, 1785, 1550, 1190, and 845 mF/cm2 under discharging at varied specific current densities 30, 40, 50, 70, and 100 mA/cm2. These values suggest excellent capacitive response for ACP-120. In order to design high-performance supercapacitors, the electrode materials and electrolytes are the two important components playing a fundamental role during the charge– discharge processes. In our cases, the additive VOSO4 was added into electrolyte, which shall help the energy storage
Fig. 5 a Charge–discharge curves at various current densities for ACP120 in 3 M H2SO4 solution bearing 1 M VOSO4. b Areal capacitance calculated based on charge–discharge curves from plot a as a function of current density
1257
by using the redox properties of the couples VO2+/VO2+, and then the activated carbon papers were used as good electrodes because their surfaces have hydroxyl (–OH), carbonyl (>C=O), and carboxyl (–COOH) groups; these groups act as strong polar sites to absorb vanadium ions and catalytic activity points to catalyze the redox reaction of VO2+/VO2+ [19]. With the synergistic effect of the absorbance and catalysis of the activated carbon paper as well as redox properties of the couples VO2+/VO2+, the performance of supercapacitors was improved. In addition to the areal capacitance, the cycle life is another important factor for supercapacitors in particular application [20]. In Fig. 6a, the capacitance decay for ACP-120 over 5000 cycles is as low as 8 %, implying its long cycle life. The resistance plays a key role for power capability of supercapacitors according to the equation Psc ¼ 9ð1 E F ÞV 2o =16Rsc , where V o is the rated voltage of the supercapacitor, Rsc is the resistance of the supercapacitor, and EF is the efficiency of the supercapacitor [21]. In Fig. 6b, the resistances of the supercapacitors on the CP and ACP-120 electrodes were collected using electrochemical impedance spectroscopy (EIS). The full-scale and enlarged Nyquist-type impedance spectra contain nearly upward sloping lines, indicating that both electrodes have
Fig. 6 a The capacitance retention test over 5000 cycles at a current density of 100 mA cm−2 for ACP-120 in 3 M H2SO4 solution bearing 1 M VOSO4. b Nyquist electrochemical impedance spectra of CP and ACP-120 in 3 M H2SO4 solution bearing 1 M VOSO4. The inset shows a magnified view of the high-frequency region of the impedance spectra
1258
the low diffusion resistance and exhibit the good power capability [22].
Ionics (2016) 22:1253–1258
References 1.
Conclusion In summary, in this report, the activated carbon paper was prepared firstly and its surface hydrophilicity was enhanced due to the intake of hydrophilic groups such as hydroxyl (– OH), carbonyl (>C=O), as well as carboxyl (–COOH) after treatment by potassium dichromate lotion. Then, the supercapacitor was constructed with activated carbon paper as electrodes and 3 M H2SO4 solution bearing 1 M VOSO4 as electrolyte. This supercapacitor showed the high areal capacitance of 2146 mF/cm2 with around 16 times higher than that of 134 mF/cm2 in pristine electrolyte of 3 M H2SO4, and the good cycling performance retaining 92 % after 5000 cycles. The full-scale and enlarged Nyquist-type impedance spectra further indicate that the supercapacitor has the low diffusion resistance and exhibits the good power capability. The performances of supercapacitors constructed by this way were improved because of the synergistic effect of the absorbance and catalysis of the activated carbon paper as well as redox properties of the couples VO2+/VO2+. Acknowledgments The work described in this paper was supported by the characteristic innovation project for natural science in Guangdong provincial key platform and major scientific research projects for colleges and universities (2015KTSCX139), the Natural Science Foundation of Guangdong Province (No. DG15313035), the Foundation for the Excellent Young Teachers in Higher Education Institutions of Guangdong (Qiu Yongfu, 2014), and the Natural Science Foundation of Guangdong Province (No. 2015A030313651). We thank Qiu Yan (
[email protected]) for her linguistic assistance during the preparation of this manuscript.
2. 3. 4. 5. 6. 7. 8. 9. 10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Zhong C, Deng YD, Hu WB, Qiao JL, Zhang L, Zhang JJ (2015) Chem Soc Rev 44:7484–7539 Peng X, Peng LL, Wu CZ, Xie Y (2014) Chem Soc Rev 43:3303– 3323 Wang GP, Zhang L, Zhang JJ (2012) Chem Soc Rev 41:797–828 Tian XC, Shi MZ, Xu X, Yan MY, Xu L, Khan AM, Han CH, He L, Mai LQ (2015) Adv Mater 27:7476–7482 Senthilkumar ST, Selvan RK, Ponpandian N, Melo JS, Lee YS (2013) J Mater Chem A 1:7913–7919 Li W, Liu J, Yan C (2013) Carbon 55:313–320 Rahman F, Skyllas-Kazacos M (1998) J Power Sources 72:105– 110 Brunauer S, Emmett PH, Teller E (1938) J Am Chem Soc 60:309– 319 Qiu YF, Wang L, Leung CF, Liu GJ, Yang SH, Lau TC (2011) Appl Catal A Gen 402:23–30 Yang DG, Velamakanni A, Bozoklu G, Park S, Stoller M, Piner RD, Stankovich S, Jung I, Field DA, Ventrice CA, Ruoff RS (2009) Carbon 47:145–152 Sun B, Skyllas-Kazacos M (1992) Electrochim Acta 37:2459–2465 Osorio AG, Silveira ICL, Bueno VL, Bergmann CP (2008) Appl Surf Sci 255:2485–2489 Hsieh CT, Teng H, Chen WY, Cheng YS (2010) Carbon 48:4219– 4229 Chow TS (1998) J Phys Condens Matter 10:L445–L451 Giovambattista N, Debenedetti PG, Rossky PJ (2007) J Phys Chem B 111:9581–9587 Yan J, Fan ZJ, Wei T, Cheng J, Shao B, Wang K, Song LP, Zhang ML (2009) J Power Sources 194:1202–1207 Gui Z, Zhu HL, Gillette E, Han XG, Rubloff GW, Hu LB, Lee SB (2013) ACS Nano 7:6037–6046 Zhu SM, Zhou HS, Hibino M, Honma I, Ichinara M (2005) Adv Funct Mater 15:381–386 Yue L, Li W, Sun F, Zhao L, Xing L (2010) Carbon 48:3079–3090 Cheng Z, Tan G, Qiu Y, Guo B, Cheng F, Fan H (2015) J Mater Chem C 3:6166–6171 Burke A (2000) J Power Sources 91:37–50 Qiu Y, Xu P, Guo B, Cheng Z, Fan H, Yang M, Yang X, Li J (2014) RSC Adv 4:64187–64192
