High performance supercapacitor electrodes based on deoxygenated graphite oxide by ball milling

Electrochimica Acta 109 (2013) 874–880

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

High performance supercapacitor electrodes based on deoxygenated graphite oxide by ball milling Dacheng Zhang, Xiong Zhang, Xianzhong Sun, Haitao Zhang, Changhui Wang, Yanwei Ma ∗ Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 3 June 2013 Received in revised form 5 July 2013 Accepted 25 July 2013 Available online xxx Keywords: Graphene Supercapacitors Ball milling

a b s t r a c t Many applications such as supercapacitors require high quality graphene as well as gram-scale production. In the present work, we report a gram-scale, environment-friendly method to produce reduced graphene oxide by short time ball milling. The results of X-ray diffraction, X-ray photoelectron spectroscopy, Raman shift, scanning electron microscopy and transmission electron microscopy indicate that graphene oxide was reduced after ball milling and exfoliated into graphene. Meanwhile, the electrochemical properties of such graphene as electrode materials were obviously enhanced. The maximum capacitance is as high as 212 F g−1 at 0.2 A g−1 in 1 M H2 SO4. Enhanced performance of supercapacitor electrode based on this graphene is mainly attributed to better conductivity which will promote the establishment of both double-layer and pseudocapacitance due to their rich edge functionality after ball milling. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, research on the electrochemical capacitors (ECs) has attracted growing attention for their use in high power energy storage devices. The mechanisms of ECs include two ways: (i) double-layer capacitance arising from the charge separation at an electrode/electrolyte interface and (ii) pseudocapacitance arising from fast, reversible faradaic reactions occurring at or near a solid electrode surface [1]. Commonly, porous carbon materials are the electrodes of double-layer capacitors [2], while transition metal oxides [3] and conducting polymers [4] are corresponding to pseudocapacitors. More recently, graphene has attracted unprecedented scientific and technological interest due to its unique electrical and mechanical properties in a short span of time, since its discovery in 2004 [5,6]. As a new 2-dimensional carbon material, graphene has a great potential in the application of ECs due to its superior electrical conductivity, high specific surface area, and chemical stability [7–10]. Many of these applications require high quality graphene as well as gram-scale production. Nowadays, several methods have been developed for producing graphene, such as micromechanical exfoliation [11], chemical vapor deposition epitaxial growth [12] and chemical route via reduction of graphene oxide (GO) [13–17]. Among the existing methods, solution-based chemical reduction of GO (rGO) offers the possibility to prepare

∗ Corresponding author. Fax: +86 10 82547137. E-mail address: [email protected] (Y. Ma). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.07.184

high-quality graphene with oxygen functional groups. However, this process needs long time ultrasonic to get monolayer GO and filtrate process to treat highly stable graphene dispersion. Actually, it is difficult to achieve gram scale production because of the low concentration product (about 1 g L−1 ) to prevent the graphitic sheets from aggregating in solution. In order to prevent the restacking of rGO, nano-spacers (e.g., metallic ions and CNTs) have been used to obtain stable rGO-based solutions [18,19]. However, the production capacity of graphene cannot fulfill current demands for the above reasons. Consequently, it is of great interest to develop non-liquid routes to prepare large scale graphene-based materials with high quality. For instance, Chakrabarti et al. provided an innovative route for producing fewer layer graphene by burning magnesium metal in dry ice [20]. Yang et al. reported that low-temperature exfoliated graphene by vacuum-promoted exfoliation of GO [21]. Generally, the graphene from GO has better electrochemical performance than that directly from graphite because the reduced GO exhibits much pseudocapacitance contributed by oxygen functional groups [22]. To obtain high quality and large-scale graphene, we developed a new method to reduce GO by directly ball milling. Ball milling is a widely used method to improve properties for preparation and after-treatment of several materials such as activated carbon (AC) [23] and carbon nanotubes (CNTs) [24]. Recently, some scientists obtained graphene with graphite by ball milling [25–28]. However, these graphenes have no excellent electrochemical performance due to actually intrinsic of graphite and it always needs long-term milling time (>50 h) to exfoliate graphite due to strong force between interlayer. Overview the liquid reduction of GO, GO

D. Zhang et al. / Electrochimica Acta 109 (2013) 874–880

has poor chemical stability because it can be easily reduced by both strong and weak reducing agent even by hydrothermal without any reductant. Therefore, we thought the oxygen functional groups on graphene also can be reduced by mechanical friction during ball milling. During the ball milling process, the GO were readily exfoliated into graphene sheets along with loss of unstable oxygen functional groups under the shear forces applied by the milling impact due to weak Van der Waals force between GO layers. As a result, this process can not only keep GO layer structure but also make the GO be reduced and nanosized. Consequently, the electrochemical performance of GO would be enhanced due to both contribution of pseudocapacitance from residual stable oxygen groups, like carboxyl and a hydroxyl group on the edge of graphene, and good conductivity after removing epoxyethers groups by ball milling. In the present work, the maximum capacitance of ball milling graphite oxide (mGO) is as high as 212 F g−1 at 0.2 A g−1 in 1 M H2 SO4 . Although high specific capacitances have been reported in several sorts of graphene as supercapacitor electrodes, it is worthwhile pointing out that both gram-scale and high electrochemical performance graphene has not been well studied. 2. Experimental 2.1. Sample preparation The exfoliated GO was prepared using a modified Hummers method [29]. With a ball-to-GO weight ratio of 60:1, 1.4 g of the GO and 84 g of steel balls were introduced to a milling tank. 1 mL of acetone as milling medium was poured into the tank for preventing the agglomeration of the powers. Ball-milling was carried out in a QM-1SP4 high-energy mill (planetary-type, Nanjing University Instrument Factory, China) under air atmosphere with a rotation speed of 300 rpm from 5 to 50 h. The product was dried to remove moisture at 60 ◦ C for 24 h. The samples are signed as mGOtime, where time is the ball milling time (hour). 2.2. Characterization X-ray diffraction (XRD) analyses were performed using a X’Pert ˚ operated at 40 kV Pro system with Cu K␣ radiation ( = 1.54060 A) and 40 mA. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI Quantear SXM (ULVAC-PH INC) which used Al as anode probe in 6.7 × 10−8 Pa. Raman spectra were obtained on a RM 2000 microscopic confocal Raman spectrometer (Renishaw in Via Plus, England) employing a 514 nm laser beam. The BET surface areas of the samples were measured by nitrogen gas adsorption in the dry state at 77 K. The microstructure and surface morphology of the samples were investigated using a scanning electron microscope (Hitachi S-4800, SEM) operating at 5.0 kV. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were taken using a FEI Tecnai G2 F20 microscope with an accelerating voltage of 200 kV. Electronic conductivity was measured on compacted pellets by standard four-probe resistance using Physical Property Measurement System (PPMS). 2.3. Electrochemical measurements The working electrode was prepared by pressing a mixture of polyvinylidene fluoride (PVDF), acetylene black and active materials at a weight ratio of 1:2:7 onto a stainless steel with a spatula, and dried at 100 ◦ C for 12 h. The counter and the reference electrodes were Pt plate and saturated calomel electrode (SCE), respectively. All electrochemical tests were carried out using by a CHI 660C electrochemical workstation in 1 M H2 SO4 and 6 M KOH at room

875

Fig. 1. XRD patterns of GO and mGO; inset: enlarged image XRD patterns of GO10.

temperature. As for the H2 SO4 electrolyte systemcyclic voltammetry (CV) was carried out at scan rates ranging from 10 to 500 mV s−1 within the potential range of 0–0.9 V (vs. SCE). Galvanostatic charge/discharge was carried out at current densities ranging from 0.1 to 5 A g−1 within the potential range of 0–0.9 V (vs. SCE). Electrochemical impedance spectrometry (EIS) was recorded under the conditions: AC voltage amplitude of 5 mV, frequency range from 105 to 0.1 Hz. Cycle-life test was carried out through CV at the scan rate of 50 mV s−1 for 4500 cycles. As for the KOH electrolyte system: CV was carried out at scan rates ranging from 10 to 50 mV s−1 within the potential range of −0.9 to 0 V (vs. Hg/HgO). Galvanostatic charge/discharge was carried out at current densities ranging from 1 to 5 A g−1 within the potential range of −0.9 to 0 V (vs. Hg/HgO). 3. Results and discussion Fig. 1 shows the XRD patterns of the GO and mGO at various milling times. It can be seen that the evolution of the (0 0 2) diffraction peak during ball-milling is affected by the milling time. The intensity of (0 0 2) is excessively strong for GO because of its good crystal. After ball-milling from 5 to 10 h, the peak shifts from 2 = 11.4◦ to 23.8◦ , implying the decrease of interlayer spacing due to the elimination of the oxygen-containing groups on the in-plane graphene layers. However, the peak (0 0 2) has been re-shift to the near that of GO after 20 h of ball-milling. These results confirm that GO would be reduced when ball-milling time is less than 20 h and re-oxidized when more than 20 h. As shown in the inset image, the (0 0 2) peak is obviously broaden and the (1 0 0) peak at 42.8◦ also appears, indicating less layers than that of GO and restore of graphite structure. The layers of mGO can be calculated by Bragg equation and Scherrer formula [30]. D-spacing of the (0 0 2) peak corresponds to the interlayer spacing and obeys Bragg’s equation: 2d sin  = 

(1)

The mean size of crystal in the vertical of (0 0 2) faces can be calculated by Scherrer formula: L=

K B2 cos 

(2)

where  is the wave length of X-rays, B2␪ is the full widths and K, a constant which was 0.91. The average number of layers per stack for Scherrer’s approach, n was obtained from: n=

2K tan  L = B2 d

(3)

The value n of mGO10 is about 1.57 by calculation, indicating the formation of fewer-layer graphene. It is well known that Raman scattering is very sensitive to the microstructure of carbon materials [31–34]. Fig. 2 shows the Raman

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Fig. 2. Raman spectra of GO (a) and mGO (b).

spectra of GO after different ball-milling times. The fluorescence background for the GO spectra was subtracted using the Wire 2.0 Renishaw software. The intensity of D band is therefore often used as an indicator for the degree of disorder in carbon-based materials. As shown in Table 1, the D peak has shifted from 1359 to 1334 cm−1 after ball milling and then rebounded to 1336 cm−1 . In general, the D-peak position will decrease with increasing disorder [35]. The intensity of ID /IG also increases, indicating higher concentration of defects, with increasing ball milling time from 5 to 10 h and then re-decreased. It is interesting that the position of 2D peaks for all samples is at nearly 2700 cm−1 , which indirectly suggests 2–3 layers graphene [36]. Stable 2D peaks also demonstrate no re-stack obviously appears during the ball milling process. The keeping of fewer layer structures and decreasing of oxygen content will enhance the properties of GO. Moreover, the maximum Brunauer–Emmett–Teller (BET) surface area of mGO is about 55.5 m2 g−1 and its conductivity is 2.7 × 10−2 S m−1 which is several orders of magnitude larger than that of GO. It is worth noting that the optimal value is from the sample mGO10 for both BET surface area and conductivity, which would support highest electrochemical properties among all samples. Furthermore, The adsorption–desorption isotherm (Fig. S1) was of type-IV with a H3 hysteresis loop at P/Po ∼ 0.45–0.8, indicating a mesoporous material with a mean pore size of 6.08 nm as can seen from Table 1. X-ray photoelectron spectroscopy (XPS) has become an increasingly available and powerful tool for understanding the nature of many different types of functional groups on the surface. Fig. 3 shows the C1s XPS spectra of mG10. From the C1s XPS spectrum of GO (Fig. 3a), six different peaks centered at about Table 1 Comparison of the Raman shift data, conductivity. BET surface and mean pore size, oxygen content and its species, and max capacitance of GO and mGO. The oxygen species percentage was obtained from the area of corresponding peaks for fitted C1s XPS spectra. Samples

D peak ID /IG 2D peak Conductivity (S m−1 ) BET surface (m2 g−1 ) Mean pore size (nm) Oxygen content (at.%) O C O C O C O C C OH C C C C Max capacitance (F g−1 )

GO

mGO5

mGO10

mGO20

mGO50

1359 0.84 2705 10−6 – – 33.3% 8.30% 8.50% 33% 10.40% 12.50% 27.10% 94

1341 0.85 2705 1.4 × 10−4 0.1557 – – – – – – – – 125

1334 1.05 2704 2.7 × 10−2 55.4875 6.08 21.7% 5.40% 6.50% 8.60% 5.40% 31.20% 43% 212

1336 0.95 2703 0.9 × 10−2 9.6414 3.78 29.7% 11.8 4.10% 9.90% 4.80% 24.30% 44.90% 199

– – 6 × 10−2 2.2296 6.3 33.5% 14.3% 4.9% 15.1% 6.7% 19.7% 39.3% 23

Fig. 3. C1s XPS spectra of GO (a) and mGO10 (b).

284.2, 284.8, 286.1, 286.6, 287.4, and 288.4 eV, are corresponding to C C, C C, C OH (alkoxy), C O C (epoxy), C O, and O C O (carboxyl) groups, respectively [19]. After ball milling, the intensities of all C1s peaks of the carbon binding to oxygen, especially the peak of C O C (epoxy), decrease remarkably (Fig. 3b), suggesting that most oxygen functional groups are successfully removed. This result shows that the as-synthesized mGO was partially repaired from sp3 hybridized carbon atoms dominated in GO to graphite structure of sp2 hybridized carbon atoms [37]. Moreover, the C/O atomic ratio of mGO10 (3.6) is higher than that of GO (2.0) by semi-quantitative analysis of XPS. It is noted that the intensity of O C O and C O C is re-increasing when ball milling time more than 10 h as shown in Fig. S2 and Table 1. It demonstrates that mGO will be re-oxided under long ball milling time which is in good agreement with XRD results. It is widely accepted that the oxygen-containing functional groups were removed as CO2 and CO molecules (>200 ◦ C) [38]. However, it is difficult to reach high temperature for ball milling. It is noted that the product just taken from milling tank was a little moist, indicating the removal of oxygen as form of water. It is possible that C O in-plane of GO and OH will be integrated as water molecules. As for the effect of electrochemical properties, on the one hand, the residual stable oxygen groups on mGO will make a positive contribution of pseudocapacitance for supercapacitor electrodes. On the other hand, the electronic conductivity will be significantly improved because C O (epoxy and alkoxy) on the plane of graphene was removed. As these two sorts of contribution, mGO would have higher electrochemical properties after ball milling. Fig. 4a and b shows the SEM images of GO and mGO10. The morphology of GO is typically sandwiched packed on the side and wrinkle in the field of vision whereas that of mGO10 has been exfoliated into several fragments, suggesting the grain refinement after ball milling. As shown in Fig. 4c, TEM images of mGO10 revealed monolayer graphene with a size of several micro-meters on the

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Fig. 4. SEM images of GO (a) and mGO10 (b); TEM images (c) and HRTEM images (d) of mGO10.

top of the carbon membrane. HRTEM images (Fig. 4d), conducted on the area surrounded by the grid, show no clear lattice at the edge of graphene, suggesting the presence of single layer structures. Furthermore, there are little black spots on the graphene surface, indicating weak graphitization during the ball milling. The electrochemical properties of the GO and mGO were studied in three electrode systems by cyclic voltammetry (CV) galvanostatic charge/discharge and EIS. Fig. 5a shows the CV measurement data of GO and mGO10 at a scan rate of 50 mV s−1 in 1 M H2 SO4 . It can be found that the area surrounded by CV curves for mGO10 electrode is apparently larger than that of the GO. Different from two pairs of peaks for modification of multi-walled carbon nanotubes [39], in the present work, only one oxidation peaks are observed at 0.43 V which is indicative of pseudocapacitive for reduced graphene materials [40]. The oxygen containing groups, such as pyrone-like functionalities (part of C bond and C O) in the surface of carbon materials can make effects on the electrochemical behaviors [41,42]. The possible redox reactions as followed [17,43]: C O + H+ + e−  C(OH)–

(4)

+

−

C–O–C

+ H + e → C–C(OH)

(5)

C–O–C

+ H+ + e− → C C

(6)

+ H2 O

As we know, the first quinine–hydroquinone conversion is always reversible. However, it will be irreversible for Eqs. (5) and (6) because of the instability of ethylene oxide derivative which readily reacts with nucleophile (H+ ) via the SN2 mechanism in

acidic. Here, the intensity of redox peaks for mGO10 is obviously stronger than that of GO, implying easier electrochemical reaction due to enhanced conductivity. Therefore, it can be inferred that suitable oxygen composition is also beneficial for the electrochemical properties of mGO in supercapacitor electrodes through pseudofaradic reactions of these oxygen active groups. From Fig. 5b, the CVs of mGO10 are nearly rectangular in shape at scan rates from 100 to 500 mV s−1 , which indicates the capacitive behavior over a wide range of voltage scan rates. To further investigate the kinetics in CV process, we employed an analysis based on the work of Chen et al. [44]. The total voltammetric charge (qt ) of the electrode materials is separated into two parts: surface capacitive charge (qs ) and diffusion-controlled charge (qd ). Assuming semi-infinite linear diffusion, within a reasonable range of sweep rates, qs can be derived by plotting the total voltammetric charge qt against the reciprocal of the square root of the potential sweep rate (v) and extrapolating v to ∞, according to the following equation: qt = qs + c v−0.5

(7)

As shown in Fig. 5c, the total charge stored versus (scan rate)0.5 is fitted as a line very well. The intercept of the line at Y axis is 84.66 C g−1 , which is corresponded to qs . And qd can be calculated by deviation between qt and qs . The correlation coefficient (R) of fitted line is nearly to 1, meaning easier ion access and charge transfer in the active material even at high scan rate. The capacitive and diffusion-controlled contributions to total capacity are displayed in Fig. 5d. The capacitive charge contribution is as high as 92.6% at 200 mV s−1 , and still stays 77.5% at 10 mV s−1 , strongly suggesting

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Fig. 5. (a) Cyclic voltammograms of GO and mGO10 electrodes at a scan rate of 50 mV s−1 ; (b) cyclic voltammograms of mGO10 electrode at different scan rates; (c) the total voltammetric charge qt against the reciprocal of the square root of the potential sweep rate (v) and extrapolating v to ∞; (d) surface (qs ) and diffusion (qd ) charge contribution of mGO10 from 10 to 200 mV s−1 ; (e) galvanostatic charge–discharge profile of GO and mGO10 electrodes at a current density of 0.2 A g−1 ; (f) the specific capacitance of GO and mGO electrodes at different current density; all tests were measured in 1 M H2 SO4 at a potential window from 0 to 0.9 V vs. SCE. Active material mass for single electrode is about 0.5 mg cm−2 .

high rate capability of mGO electrodes which is mainly attributed to enhanced conductivity of mGO after ball milling. As can be seen from Fig. 5e, the charge–discharge curve of GO and mGO10 electrode, exhibits almost linear, indicating good capacitive behavior of capacitors. The discharge time of mGO10 electrode is much higher than that of GO, which proves a useful method of obtaining enhanced performance of graphene-based supercapacitor electrodes by ball milling. The specific capacitance can be evaluated from the slope of the discharge curves according to the formula below: C=

I

−

 U  t

m

(8)

where I is the discharge current, (U/t) is the average slope of the discharge curve after the IR drop, m is the mass of active material in a single electrode. As shown in Fig. 5f and Table 1, the mGO10 electrode achieves the highest specific capacitance (212 F g−1 ) among all of samples at 0.2 A g−1 and has a good rate capability because of keeping capacitance stable with the increase in scan rates. Although the capacitance is not high compared to other reports, it demonstrates that it is possible to prepare both gram-scale and high

electrochemical performance graphene to meet practical applications. However, too long ball milling time is not beneficial for the electrochemical properties of mGO. When the ball milling time reaches 50 h, the capacitance sharply decreases to only 23 F g−1 due to re-stack and re-oxidation of mGO which is in good agreement with XRD results. Additionally, the electrochemical performances in 6 M KOH were also investigated. The mGO10 electrodes obtained the capacitance of 183 F g−1 at 2 A g−1 . As shown in Fig. S3, the charge–discharge and CV curves can keep good capacitive behavior indicating that the electrochemical performances in KOH are comparable to those in H2 SO4 . Fig. 6 displays the Nyquist diagrams of electrodes for different ball milling time. A semi-circle arc and a straight line have been observed. The high-frequency arc is ascribed to the doublelayer capacitance in parallel with the charge transfer resistance at the contact interface between electrode and electrolyte solution [45]. At lower frequencies, the impedance plot should theoretically be a vertical line for ideal capacitive behavior [46]. The bigger slope angle at the low frequency the better capacitive it is. As can be seen from the impedance plot, the mGO10 and mGO20 electrodes have better capacitive behavior of the supercapacitors than that of the mGO5 and mGO10, further confirming the importance

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Z111100056011007), the National Natural Science Foundation of China (Nos. 21001103 and 51025726). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2013.07.184. References

Fig. 6. Nyquist impedance plots of GO and mGO in the frequency range of 100 kHz to 0.1 Hz; inset: Nyquist impedance plots in high frequency area.

Fig. 7. Retention of specific capacitance versus the cycle number measured at a scan rate of 50 mV s−1 .

of milling time. The equivalent series resistance (ESR) is 0.91  (mGO5), 0.72  (mGO10), 0.98  (mGO20), 1.1  (mGO50), respectively, which is be consistent with conductivity results. To further investigate the stability of mGO electrodes, long cycle life of supercapacitors is a crucial parameter for their practical applications. The long-term cycle stability of the mGO10 was also tested in this study by repeating the 4500 cycles between 0 and 0.9 V (vs. SCE) at a scan rate of 50 mV s−1 . The capacitance retention as a function of cycle number is presented in Fig. 7. There is a small decrease of capacitance in the first 1000 cycles and it keeps 95.3% of initial value after 4500 cycles, indicating that it has good cycle stability. 4. Conclusions In summary, a gram-scale, environment-friendly method to produce reduced graphene oxide by ball milling has been developed. The electrochemical properties of the resulted product as electrode materials have been obviously enhanced. Studies of various samples for different milling time demonstrate that it is important for controlling milling time to obtain high quality graphene material. The capacitance enhancement can be attributed that better conductivity which will promote the establishment of both double-layer and pseudocapacitance generated by oxygen functional groups after ball milling. Optimized results in our work show mGO10 has the highest capacitance (212 F g−1 ), long cycle life and high rate capability, which is comparable to that of most graphene. Acknowledgements This work was partially supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KJCX2-YW-W26), Beijing Municipal Science and Technology Commission (No.

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