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Energy storage of thermally reduced graphene oxide
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Available online at www.sciencedirect.com
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Energy storage of thermally reduced graphene oxide Jung Min Kim a,1, Won G. Hong a,1, Sang Moon Lee a, Sung Jin Chang a, Yongseok Jun b, Byung Hoon Kim c,**, Hae Jin Kim a,* a
Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Republic of Korea Department of Materials Chemistry and Engineering, Konkuk University, Seoul 143-701, Republic of Korea c Department of Physics, Incheon National University, Incheon 406-772, Republic of Korea b
article info
abstract
Article history:
The energy-storage capacity of reduced graphene oxide (rGO) is investigated in this study.
Received 7 October 2013
The rGO used here was prepared by thermal annealing under a nitrogen atmosphere at
Received in revised form
various temperatures (300, 400, 500 and 600 C). We measured high-pressure H2 isotherms
29 November 2013
at 77 K and the electrochemical performance of four rGO samples as anode materials in Li-
Accepted 23 December 2013
ion batteries (LIBs). A maximum H2 storage capacity of w5.0 wt% and a reversible charge/
Available online xxx
discharge capacity of 1220 mAh/g at a current density of 30 mA/g were achieved with rGO A. Thus, an optimal pore size annealed at 400 C with a pore size of approximately 6.7
Keywords: Graphene oxide
exists for hydrogen and lithium storage, which is similar to the optimum interlayer distance (6.5 A) of graphene oxide for hydrogen storage applications.
Thermal annealing
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Optimal pore size
reserved.
Hydrogen storage Li-ion batteries
1.
Introduction
Graphene oxide (GO) has been intensively studied as an energy-storage material in applications such as rechargeable Li-ion batteries (LIBs), hydrogen-storage components, and supercapacitors. The specific capacitance of reduced GO (rGO) using hydrazine monohydrate [1] and vacuum-promoted exfoliated graphene [2] reaches about 150 F/g, while that of activated GO after a microwave treatment is 200 F/g [3]. For application in a Li-ion battery, an increase in the reversible capacity and good cyclic stability have been achieved using
reduced GO (rGO) [4e7] and rGO/metal oxide nanocomposites [8e13]. Capacity retention values for rGO itself and rGO/metal oxide hybrid have reached w850 mAh/g [5] and w1000 mAh/g, respectively [13]. Regarding hydrogen storage, systematic modulation of the interlayer distance of multilayered GO and pillared-GO by thermal annealing has demonstrated the existence of an optimum interlayer distance of GO at approximately 6.5 A, at which the maximum H2 uptake in multilayered GO was 4.8 wt % at 77 K [14]. With Birch reduction of exfoliated GO, it was reported that hydrogen is chemically stored in only a few layers of graphene up to 5.0 wt% [15].
* Corresponding author. Tel.: þ82 42 865 3953. ** Corresponding author. Tel.: þ82 32 835 8229. E-mail addresses: [email protected] (B.H. Kim), [email protected] (H.J. Kim). 1 These authors contributed equally. 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Fig. 1 e (a) XRD patterns of rGOs show that the interlayer distance of rGOs decreases with an increase of thermal annealing temperature. (b) Raman spectra of rGOs. The prominent D (w1370 cm-1) and G (w1600 cm-1) peaks are well observed in all rGO. (c) The D and G peaks are red-shifted upon increasing Ta.
Despite these experimental accomplishments, few investigations of the factors and mechanisms pertaining to energy storage in carbonaceous materials have been reported. Ionic molecular Li2 was reportedly stored on poly(pphenylene) with nuclear magnetic resonance [16]. The intercalation and adsorption of Li into layered carbon and on the surface, respectively, and reversible binding near hydrogen atoms have been suggested [17]. In addition, at the edge sites of nano-domains and the interspace of between the carbon layers were suggested as adsorption sites for thermally reduced graphene oxide (rGO) [5]. For hydrogen storage, the existence of an optimum interlayer distance between graphene layers at 6.0e7.0 A was proposed by means of theoretical calculations [18e20] and experimental results [14]. In this study, we show that the optimum pore size of GO for both lithium and hydrogen storage exists at w6.7 A. This was achieved after a thermal treatment at temperatures ranging
from 300 to 600 C. The electrochemical performance of an anode material in rGO treated at 400 C showed a maximum stable discharge capacity of 1220 mAh/g at a constant current density of 30 mA/g. The results of high-pressure H2 isotherms at 77 K demonstrate that rGO treated at 400 C stores a maximum H2 uptake of w5.0 wt%. This study indicates the importance of the pore size in the design of materials used for energy storage.
2.
Experimental
2.1.
Sample preparation and characterization
GO was synthesized by a modified Hummers method starting from graphite powder 450 nm in size. The graphite was treated with concentrated H2SO4, K2S2O8 and P2O5. After
Fig. 2 e The C 1s XPS spectra of (a) rGO300, (b) rGO400, (c) rGO500, and (d) rGO600. The sp2 bonding of carbon increases with an increase of Ta. Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Table 1 e Peak areas obtained from C 1s XPS spectra and C/O atomic ratios calculated from XPS spectra. Peak area (%) rGO300 rGO400 rGO500 rGO600
C]C 60.5 60.7 65.3 70.2
Atomic (%) CeOH 16.2 15.9 15.4 11.2
CeOeC 12.9 12.0 11.4 11.2
C]O 3.54 5.72 4.61 4.05
O]CeO 6.85 5.56 3.19 3.34
C/O ratio 5.45 6.25 8.25 8.96
filtering, washing and drying, the product was re-suspended in concentrated H2SO4 and oxidized further with KMnO4 and H2O2. The result was a thick, brownish yellow GO suspension. The GO suspension was centrifuged and washed with 10% HCl and DI water, and the suspension was then dried at 50 C for three days to obtain GO. A thermal treatment at temperatures of 300 Ce600 C was performed for 2 h in an N2 atmosphere. The heating rate was 1 C/min. X-ray diffraction (XRD) patterns were measured with a Bruker D8 Advance diffractometer with a Lynxeye detector using Cu Ka radiation. The surface states were analyzed by x-ray photoelectron spectroscopy (XPS, AXIS-NOVA, Kratos Inc.) using monochromatic Al Ka radiation (1486.6 eV). The binding energy was corrected using the C 1s peak at 284.6 eV. The Raman spectra were recorded by means of laser excitation of 472.8 nm, with a spot size limited by diffraction using a 100 objective lens with an incident power of 23 mW to avoid sample damage or laserinducing heating.
2.2.
Electrochemical performance
The electrochemical performance levels of the rGO samples were evaluated with a Wonatec automatic battery cycler in a
3
CR2016 type coin cell. The electrodes were fabricated by creating a slurry of 70 wt% active material, 7 wt% conducting carbon black, and 23 wt% polyvinylidene fluoride binder in Nmethyl-2-pyrrolidone as a solvent. Using a doctor-blade method, the slurry was applied onto a copper foil current collector and dried at 120 C for 12 h in a vacuum oven. The coated anode foil was then pressed to form a uniform layer and was punched into the form of a disk. The coin cells were assembled by employing a composite electrode with metallic lithium foil as the counter electrode and 1 M LiPF6 (Aldrich 99.99%) dissolved in a solution of ethylene carbonate/ dimethyl carbonate/diethyl carbonate (1:2:1 v/v) as an electrolyte in a glove box filled with argon. The cell was galvanostatically cycled between 0.01 and 3.0 V vs. Li/Liþ at various current densities.
2.3.
High-pressure H2 adsorption
The pores and surface areas of the rGO samples were characterized by an analysis of the N2 adsorptionedesorption isotherms at 77 K (ASAP 2010, Micromeritics). Hydrogen adsorption with high pressure (up to 80 bar) was measured volumetrically with a computer-controlled commercial pressure-composition isotherm (Belsorp HP). 99.9999% hydrogen gas was used in all H2-sorption measurements. For the PCT measurements, the system was calibrated with LaNi5 at room temperature with activated carbon (surface area w3000 m2/g) at 77 K. During the PCT measurement at 77 K, the system was immersed in a liquid-nitrogen Dewar vessel in order to keep the temperature constant at 77 K. Before all measurements, the samples were degassed to a pressure of 4e8 107 mbar at characteristic temperatures.
Fig. 3 e Charge/discharge curves of (a) rGO300, (b) rGO400, (c) rGO500, and (d) rGO600 between 0.01 and 3.0 V vs. Li/LiD at the current density of 30 mA/g. Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
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Fig. 4 e (a) The rate capability of rGO400 with versus current density for five cycles. (b) The cyclability of rGO400 at current density of 1000 mA/g. The anode of rGO400 retains a reversible capacity of 750 mAh/g at current density of 1000 mA/g with coulombic efficiency of 97% after 50 cycles.
3.
Results and discussion
Fig. 1(a) shows the X-ray diffraction patterns of the rGO. The GO interlayer distance decreases from 7.60 A for pristine GO to 3.43 A for rGO600 (the number after rGO represents the annealing temperature) as thermal annealing temperature (Ta) increases. Two peaks for rGO300 and rGO400 indicate that there exist two different types of rGO in the interlayer distance. The Raman spectra for all rGO samples were obtained to identify the variation of the GO due to thermal annealing. D (w1370 cm1) and G (w1600 cm1) peaks were clearly observed in all rGO samples (Fig. 1(b)). Interestingly, the D and G peaks do not vary significantly in terms of their intensities with respect to Ta. However, with an increase in Ta, the G and D peaks are red-shifted compared with those of pristine GO. This demonstrates that the reduction of GO occurs due to thermal annealing. To corroborate this observation, X-ray photoelectron spectroscopy (XPS) was used to study all of the
rGO samples (Fig. 2). In the C 1s spectra, variations in the amount of C]C (284.6 eV), CeOH (285.45 eV), CeOeC (286.35 eV), C]O (287.35 eV), and of O]CeO (288.35 eV) bonds were observed as a result of the thermal annealing. As expected, the sp2 bonding of carbon increases from 60.5% for GO300 to 70.2% for GO600, whereas the other oxygen functional groups decrease with Ta. Furthermore, the C/O atomic ratio calculated from XPS analysis shows an increase in the rGOs as Ta increases (see Table 1). The XPS results indicate that the oxygen functional groups in GO are effectively removed (but not completely) by thermal treatment. Presumably, pyrolysis of the oxygen functional groups at a suitable temperature, yielding CO and CO2, is related to formation of the structural defects on the surface of the rGO for energy storage. To characterize the electrochemical performance of the rGO samples as an anode material, we measured the specific capacities and rate capabilities. Fig. 3 displays the first five charge/discharge curves of rGO300 (Fig. 3(a)), rGO400 (Fig. 3(b)), rGO500 (Fig. 3(c)), and rGO600 (Fig. 3(d)) at a current density of 30 mA/g. The first discharge profile of all rGO samples is different from the other curves, similar to previous results [5]. After the fifth cycle, the capacity decreases to 780 mAh/g for rGO300, 1220 mAh/g for rGO400, 615 mAh/g for rGO500, and 529 mAh/g for rGO600. The maximum capacity was observed in rGO400. Note that the capacity of all rGO samples except for rGO400 decreases continuously as the charge/discharge cycle is repeated. For GO400, however, the capacity is reduced until the second charge/discharge curve but becomes mostly saturated from the third cycle, indicating that only rGO400 shows highly reversible behavior. Hence, we measured the rate capability of rGO400 at various current densities (50e1000 mA/g) for five cycles (Fig. 4(a)). A stable specific capacity of w950 mAh/g was obtained at a low current density of 50 mA/g. Although the capacity is reduced as the current density increases, it is important to note that the high specific capacity of 770 mAh/g is maintained even at a high current density of 1000 mA/g. The anode of rGO400 retains a reversible capacity of 750 mAh/g at a current density of 1000 mA/g with a coulombic efficiency of 97% after 50 cycles (Fig. 4b). Because the bond length of the hydrogen molecules is similar to the size of the lithium ion, it is expected that the maximum H2 adsorption capacity occurs in rGO400. Hence the highpressure H2 isotherms of the rGO samples were measured at 77 K (Fig. 5(a)).
Fig. 5 e (a) High pressure H2 uptake of rGOs at 77 K. The maximum H2 uptake of w5.0 wt% was achieved using rGO400. (b) The maximum value of specific capacity for LIB and hydrogen storage capacity were observed at rGO400. Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
For rGO300, the H2 uptake increases moderately as the pressure increases and then reaches approximately 0.9 wt% at 80 bar. Interestingly, however, a maximum H2 uptake of w5.0 wt% was achieved using rGO400. This value is comparable to that of activated carbon (Maxsorb, surface area w3000 m2/g) measured with the same apparatus (Belsorp HP; see Supplementary Fig. S1). The H2 uptake increases from 0.9 wt% for rGO300 to 5.0 wt% for rGO400, whereas it decreases to 2.3 wt% for rGO500 and to 1.7 wt% for rGO600 with an increase in the annealing temperature. It is worth noting that the trends of the specific capacity for Li-ion batteries and the hydrogen storage capacity were similar and that the maximum values of both were observed with the rGO treated at 400 C (Fig. 5(b)). First, we considered the surface area because a large surface area generally results in a large storage capacity. The BET surface areas are 234 m2/g for rGO300, 403 m2/g for rGO400, and 213 m2/g for rGO600, i.e., proportional to the maximum specific capacity and H2 uptake (see Supplementary Fig. 2). However, the surface area of rGO500 is quite small (62 m2/g) compared to that of the other rGO samples (as confirmed with two separate experiments). Although the surface area is related to the large energy storage capacity in rGO400, this finding suggests that other factors are involved in the enhancement of the energy storage capabilities. Second, we took the interaction between oxygen functional groups and energy sources (H2 and Li ion) into account because the oxygen groups can affect the energy storage capacity of rGO. If it is correct, the energy storage capacity is to depend linearly on the C/O ratio. However, the energy storage capacity has the maximum at rGO400 while C/O ratio increases with the increase of Ta. Finally, we investigated the effect of defects on the capacity. It is well known that there are numerous defects formed by pentagons and heptagons in rGO [21]. We focused on an evolution of the structural defects on the surface of the rGO. In this light, we analyzed the pore widths of the rGO samples with the HorvatheKawazoe adsorption model (HeK method) because HorvatheKawazoe approach has been frequently used in adsorption studies for the extracting of the pore size distribution information of microporous materials (0.35e2 nm) from experimental adsorption isotherms and any shape of pore including slit-like carbon pore [22]. The average micropore width increases from 0.52 nm for rGO300 to 0.80 nm for rGO600 (Table 2). For rGO400, the pore width of 0.67 nm is quite similar to the optimum interlayer distance of 0.6e0.7 nm in graphene [18e20] and multilayer rGO [14]. This reveals that a distance between carbon atoms of w0.67 nm is an important factor related to energy storage in carbon materials. The existence of the same pore size for Li and H2 storage may come from their similar sizes; Li is approximately 152 pm and H2 is approximately 148 pm.
Table 2 e Surface area (SBET) obtained from BET method and the pore width from HeK method. Sample name SBET (m2/g) Pore width ( A) rGO300 rGO400 rGO500 rGO600
234.7 403.7 62.2 213.7
5.2 6.7 7.3 8.0
4.
5
Conclusions
In summary, we demonstrated that 6.7 A is the optimum pore size on the surface of thermally rGO for both lithium and hydrogen storage. Optimization of the pore size using GO was accomplished by thermal reduction in a temperature range of 300e600 C. The rGO400 electrodes exhibit a high reversible specific capacity of 1220 mAh/g at 30 mA/g and 97% capacity retention after 50 cycles (750 mAh/g at 1000 mA/g). The results of high-pressure H2 isotherms at 77 K demonstrate that the maximum H2 uptake of w5.0 wt% is also obtained with the rGO400. This study provides experimental proof of the importance of the optimal pore size and can serve as a guideline for those who design new energy storage materials.
Acknowledgement Byung Hoon Kim was supported by the Incheon National University Research Grant in 20130400.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
references
[1] Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Graphene-based ultracapacitors. Nano Lett 2008;8:3498e502. [2] Lv W, Tang D-M, He Y-B, You C-H, Shi Z-Q, Chen X-C, et al. Low-temperature exfoliated graphenes: vacuum-promoted exfoliation and electrochemical energy storage. ACS Nano 2009;3:3730e6. [3] Zhu Y, Murali S, Stoller MD, Ganesh KJ, Cai W, Ferreira PJ, et al. Carbon-based supercapacitors produced by activation of graphene. Science 2011;332:1537e41. [4] Yoo E, Kim J, Hosono E, Zhou H, Kudo T, Honma L. Large reversible Li storage of graphene nanosheet families for use in rechargeable lithium ion batteries. Nano Lett 2008;8:2277e82. [5] Pan D, Wang S, Zhao B, Wu M, Zhang H, Wang Y, et al. Li storage properties of disordered graphene nanosheets. Chem Mater 2009;21:3136e42. [6] Lian P, Zhu X, Liang S, Li Z, Yang W, Wang H. Large reversible capacity of high quality graphene sheets as an anode material for lithium-ion batteries. Electrochim Acta 2010;55:3909e14. [7] Guo P, Song H, Chen X. Hollow graphene oxide spheres selfassembled by W/O emulsion. J Mater Chem 2010;20:4867e74. [8] Wang D, Choi D, Li J, Yang Z, Nie Z, Kou R, et al. Selfassembled TiO2-graphene hybrid nanostructures for enhanced Li-ion insertion. ACS Nano 2009;3:907e14. [9] Wang H, Cui L-F, Yang Y, Casalongue HS, Robinson JT, Liang Y, et al. Mn3O4-graphene hybrid as a high-capacity anode material for lithium ion batteries. J Am Chem Soc 2010;132:13978e80.
Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
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[10] Zhou G, Wang D-W, Li F, Zhang L, Li N, Wu Z-S, et al. Graphene-wrapped Fe3O4 anode material with improved reversible capacity and cyclic stability for lithium ion batteries. Chem Mater 2010;22:5306e13. [11] Zhu X, Zhu Y, Murali S, Stoller MD, Ruoff RS. Reduced graphene oxide/tin oxide composite as an enhanced anode material for lithium ion batteries prepared by homogenous coprecipitation. J Power Sources 2011;196:6473e7. [12] Kim H, Seo D-H, Kim S-W, Kim J, Kang K. Highly reversible Co3O4/graphene hybrid anode for lithium rechargeable batteries. Carbon 2011;49:326e32. [13] Zhu X, Zhu Y, Murali S, Stoller MD, Ruoff RS. Nanostructured reduced graphene oxide/Fe2O3 composite as a highperformance anode material for lithium ion batteries. ACS Nano 2011;5:3333e8. [14] Kim BH, Hong WG, Yu HY, Han Y-K, Lee SM, Chang SJ, et al. Thermally modulated multilayered graphene oxide for hydrogen storage. Phys Chem Chem Phys 2012;14:1480e4. [15] Subrahmanyam KS, Kumar P, Maitra U, Govindaraj A, Hembram KPSS, Waghmare UV, et al. Chemical storage of hydrogen in few-layer graphene. Proc Natl Acad Sci U S A 2011;108:2674e7.
[16] Sato K, Noguchi M, Demachi A, Oki N, Endo M. A mechanism of lithium storage in disordered carbons. Science 1994;264:556e8. [17] Dahn JR, Zheng T, Liu Y, Xue JS. Mechanisms for lithium insertion in carbonaceous materials. Science 1995;270:590e3. [18] Rzepka M, Lamp P, de la Casa-Lillo MA. Physisorption of hydrogen on microporous carbon and carbon nanotubes. Phys Chem B 1998;102:10894e8. [19] Patchkovskii S, Tse JS, Yurchenko SN, Zhechkov L, Helne T, Seifert G. Graphene nanostructures as tunable storage media for molecular hydrogen. Proc Natl Acad Sci U S A 2005;102:10439e44. mar M, Morris JR. Theoretical investigation [20] Aga RS, Fu CL, Krc of the effect of graphite interlayer spacing on hydrogen absorption. Phys Rev B 2007;76:165404. [21] Go´mez-Navarro C, Meyer JC, Sundaram RS, Chuvilin A, Kurasch S, Burghard M, et al. Atomic structure of reduced graphene oxide. Nano Lett 2010;10:1144e8. [22] Horvath G, Kawazoe K. Method for the calculation of effective pore size distribution in molecular sieve carbon. J Chem Eng Jpn 1983;16:470e5.
Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Energy storage of thermally reduced graphene oxide Jung Min Kim a,1, Won G. Hong a,1, Sang Moon Lee a, Sung Jin Chang a, Yongseok Jun b, Byung Hoon Kim c,**, Hae Jin Kim a,* a
Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Republic of Korea Department of Materials Chemistry and Engineering, Konkuk University, Seoul 143-701, Republic of Korea c Department of Physics, Incheon National University, Incheon 406-772, Republic of Korea b
article info
abstract
Article history:
The energy-storage capacity of reduced graphene oxide (rGO) is investigated in this study.
Received 7 October 2013
The rGO used here was prepared by thermal annealing under a nitrogen atmosphere at
Received in revised form
various temperatures (300, 400, 500 and 600 C). We measured high-pressure H2 isotherms
29 November 2013
at 77 K and the electrochemical performance of four rGO samples as anode materials in Li-
Accepted 23 December 2013
ion batteries (LIBs). A maximum H2 storage capacity of w5.0 wt% and a reversible charge/
Available online xxx
discharge capacity of 1220 mAh/g at a current density of 30 mA/g were achieved with rGO A. Thus, an optimal pore size annealed at 400 C with a pore size of approximately 6.7
Keywords: Graphene oxide
exists for hydrogen and lithium storage, which is similar to the optimum interlayer distance (6.5 A) of graphene oxide for hydrogen storage applications.
Thermal annealing
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Optimal pore size
reserved.
Hydrogen storage Li-ion batteries
1.
Introduction
Graphene oxide (GO) has been intensively studied as an energy-storage material in applications such as rechargeable Li-ion batteries (LIBs), hydrogen-storage components, and supercapacitors. The specific capacitance of reduced GO (rGO) using hydrazine monohydrate [1] and vacuum-promoted exfoliated graphene [2] reaches about 150 F/g, while that of activated GO after a microwave treatment is 200 F/g [3]. For application in a Li-ion battery, an increase in the reversible capacity and good cyclic stability have been achieved using
reduced GO (rGO) [4e7] and rGO/metal oxide nanocomposites [8e13]. Capacity retention values for rGO itself and rGO/metal oxide hybrid have reached w850 mAh/g [5] and w1000 mAh/g, respectively [13]. Regarding hydrogen storage, systematic modulation of the interlayer distance of multilayered GO and pillared-GO by thermal annealing has demonstrated the existence of an optimum interlayer distance of GO at approximately 6.5 A, at which the maximum H2 uptake in multilayered GO was 4.8 wt % at 77 K [14]. With Birch reduction of exfoliated GO, it was reported that hydrogen is chemically stored in only a few layers of graphene up to 5.0 wt% [15].
* Corresponding author. Tel.: þ82 42 865 3953. ** Corresponding author. Tel.: þ82 32 835 8229. E-mail addresses: [email protected] (B.H. Kim), [email protected] (H.J. Kim). 1 These authors contributed equally. 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Fig. 1 e (a) XRD patterns of rGOs show that the interlayer distance of rGOs decreases with an increase of thermal annealing temperature. (b) Raman spectra of rGOs. The prominent D (w1370 cm-1) and G (w1600 cm-1) peaks are well observed in all rGO. (c) The D and G peaks are red-shifted upon increasing Ta.
Despite these experimental accomplishments, few investigations of the factors and mechanisms pertaining to energy storage in carbonaceous materials have been reported. Ionic molecular Li2 was reportedly stored on poly(pphenylene) with nuclear magnetic resonance [16]. The intercalation and adsorption of Li into layered carbon and on the surface, respectively, and reversible binding near hydrogen atoms have been suggested [17]. In addition, at the edge sites of nano-domains and the interspace of between the carbon layers were suggested as adsorption sites for thermally reduced graphene oxide (rGO) [5]. For hydrogen storage, the existence of an optimum interlayer distance between graphene layers at 6.0e7.0 A was proposed by means of theoretical calculations [18e20] and experimental results [14]. In this study, we show that the optimum pore size of GO for both lithium and hydrogen storage exists at w6.7 A. This was achieved after a thermal treatment at temperatures ranging
from 300 to 600 C. The electrochemical performance of an anode material in rGO treated at 400 C showed a maximum stable discharge capacity of 1220 mAh/g at a constant current density of 30 mA/g. The results of high-pressure H2 isotherms at 77 K demonstrate that rGO treated at 400 C stores a maximum H2 uptake of w5.0 wt%. This study indicates the importance of the pore size in the design of materials used for energy storage.
2.
Experimental
2.1.
Sample preparation and characterization
GO was synthesized by a modified Hummers method starting from graphite powder 450 nm in size. The graphite was treated with concentrated H2SO4, K2S2O8 and P2O5. After
Fig. 2 e The C 1s XPS spectra of (a) rGO300, (b) rGO400, (c) rGO500, and (d) rGO600. The sp2 bonding of carbon increases with an increase of Ta. Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Table 1 e Peak areas obtained from C 1s XPS spectra and C/O atomic ratios calculated from XPS spectra. Peak area (%) rGO300 rGO400 rGO500 rGO600
C]C 60.5 60.7 65.3 70.2
Atomic (%) CeOH 16.2 15.9 15.4 11.2
CeOeC 12.9 12.0 11.4 11.2
C]O 3.54 5.72 4.61 4.05
O]CeO 6.85 5.56 3.19 3.34
C/O ratio 5.45 6.25 8.25 8.96
filtering, washing and drying, the product was re-suspended in concentrated H2SO4 and oxidized further with KMnO4 and H2O2. The result was a thick, brownish yellow GO suspension. The GO suspension was centrifuged and washed with 10% HCl and DI water, and the suspension was then dried at 50 C for three days to obtain GO. A thermal treatment at temperatures of 300 Ce600 C was performed for 2 h in an N2 atmosphere. The heating rate was 1 C/min. X-ray diffraction (XRD) patterns were measured with a Bruker D8 Advance diffractometer with a Lynxeye detector using Cu Ka radiation. The surface states were analyzed by x-ray photoelectron spectroscopy (XPS, AXIS-NOVA, Kratos Inc.) using monochromatic Al Ka radiation (1486.6 eV). The binding energy was corrected using the C 1s peak at 284.6 eV. The Raman spectra were recorded by means of laser excitation of 472.8 nm, with a spot size limited by diffraction using a 100 objective lens with an incident power of 23 mW to avoid sample damage or laserinducing heating.
2.2.
Electrochemical performance
The electrochemical performance levels of the rGO samples were evaluated with a Wonatec automatic battery cycler in a
3
CR2016 type coin cell. The electrodes were fabricated by creating a slurry of 70 wt% active material, 7 wt% conducting carbon black, and 23 wt% polyvinylidene fluoride binder in Nmethyl-2-pyrrolidone as a solvent. Using a doctor-blade method, the slurry was applied onto a copper foil current collector and dried at 120 C for 12 h in a vacuum oven. The coated anode foil was then pressed to form a uniform layer and was punched into the form of a disk. The coin cells were assembled by employing a composite electrode with metallic lithium foil as the counter electrode and 1 M LiPF6 (Aldrich 99.99%) dissolved in a solution of ethylene carbonate/ dimethyl carbonate/diethyl carbonate (1:2:1 v/v) as an electrolyte in a glove box filled with argon. The cell was galvanostatically cycled between 0.01 and 3.0 V vs. Li/Liþ at various current densities.
2.3.
High-pressure H2 adsorption
The pores and surface areas of the rGO samples were characterized by an analysis of the N2 adsorptionedesorption isotherms at 77 K (ASAP 2010, Micromeritics). Hydrogen adsorption with high pressure (up to 80 bar) was measured volumetrically with a computer-controlled commercial pressure-composition isotherm (Belsorp HP). 99.9999% hydrogen gas was used in all H2-sorption measurements. For the PCT measurements, the system was calibrated with LaNi5 at room temperature with activated carbon (surface area w3000 m2/g) at 77 K. During the PCT measurement at 77 K, the system was immersed in a liquid-nitrogen Dewar vessel in order to keep the temperature constant at 77 K. Before all measurements, the samples were degassed to a pressure of 4e8 107 mbar at characteristic temperatures.
Fig. 3 e Charge/discharge curves of (a) rGO300, (b) rGO400, (c) rGO500, and (d) rGO600 between 0.01 and 3.0 V vs. Li/LiD at the current density of 30 mA/g. Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
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Fig. 4 e (a) The rate capability of rGO400 with versus current density for five cycles. (b) The cyclability of rGO400 at current density of 1000 mA/g. The anode of rGO400 retains a reversible capacity of 750 mAh/g at current density of 1000 mA/g with coulombic efficiency of 97% after 50 cycles.
3.
Results and discussion
Fig. 1(a) shows the X-ray diffraction patterns of the rGO. The GO interlayer distance decreases from 7.60 A for pristine GO to 3.43 A for rGO600 (the number after rGO represents the annealing temperature) as thermal annealing temperature (Ta) increases. Two peaks for rGO300 and rGO400 indicate that there exist two different types of rGO in the interlayer distance. The Raman spectra for all rGO samples were obtained to identify the variation of the GO due to thermal annealing. D (w1370 cm1) and G (w1600 cm1) peaks were clearly observed in all rGO samples (Fig. 1(b)). Interestingly, the D and G peaks do not vary significantly in terms of their intensities with respect to Ta. However, with an increase in Ta, the G and D peaks are red-shifted compared with those of pristine GO. This demonstrates that the reduction of GO occurs due to thermal annealing. To corroborate this observation, X-ray photoelectron spectroscopy (XPS) was used to study all of the
rGO samples (Fig. 2). In the C 1s spectra, variations in the amount of C]C (284.6 eV), CeOH (285.45 eV), CeOeC (286.35 eV), C]O (287.35 eV), and of O]CeO (288.35 eV) bonds were observed as a result of the thermal annealing. As expected, the sp2 bonding of carbon increases from 60.5% for GO300 to 70.2% for GO600, whereas the other oxygen functional groups decrease with Ta. Furthermore, the C/O atomic ratio calculated from XPS analysis shows an increase in the rGOs as Ta increases (see Table 1). The XPS results indicate that the oxygen functional groups in GO are effectively removed (but not completely) by thermal treatment. Presumably, pyrolysis of the oxygen functional groups at a suitable temperature, yielding CO and CO2, is related to formation of the structural defects on the surface of the rGO for energy storage. To characterize the electrochemical performance of the rGO samples as an anode material, we measured the specific capacities and rate capabilities. Fig. 3 displays the first five charge/discharge curves of rGO300 (Fig. 3(a)), rGO400 (Fig. 3(b)), rGO500 (Fig. 3(c)), and rGO600 (Fig. 3(d)) at a current density of 30 mA/g. The first discharge profile of all rGO samples is different from the other curves, similar to previous results [5]. After the fifth cycle, the capacity decreases to 780 mAh/g for rGO300, 1220 mAh/g for rGO400, 615 mAh/g for rGO500, and 529 mAh/g for rGO600. The maximum capacity was observed in rGO400. Note that the capacity of all rGO samples except for rGO400 decreases continuously as the charge/discharge cycle is repeated. For GO400, however, the capacity is reduced until the second charge/discharge curve but becomes mostly saturated from the third cycle, indicating that only rGO400 shows highly reversible behavior. Hence, we measured the rate capability of rGO400 at various current densities (50e1000 mA/g) for five cycles (Fig. 4(a)). A stable specific capacity of w950 mAh/g was obtained at a low current density of 50 mA/g. Although the capacity is reduced as the current density increases, it is important to note that the high specific capacity of 770 mAh/g is maintained even at a high current density of 1000 mA/g. The anode of rGO400 retains a reversible capacity of 750 mAh/g at a current density of 1000 mA/g with a coulombic efficiency of 97% after 50 cycles (Fig. 4b). Because the bond length of the hydrogen molecules is similar to the size of the lithium ion, it is expected that the maximum H2 adsorption capacity occurs in rGO400. Hence the highpressure H2 isotherms of the rGO samples were measured at 77 K (Fig. 5(a)).
Fig. 5 e (a) High pressure H2 uptake of rGOs at 77 K. The maximum H2 uptake of w5.0 wt% was achieved using rGO400. (b) The maximum value of specific capacity for LIB and hydrogen storage capacity were observed at rGO400. Please cite this article in press as: Kim JM, et al., Energy storage of thermally reduced graphene oxide, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
For rGO300, the H2 uptake increases moderately as the pressure increases and then reaches approximately 0.9 wt% at 80 bar. Interestingly, however, a maximum H2 uptake of w5.0 wt% was achieved using rGO400. This value is comparable to that of activated carbon (Maxsorb, surface area w3000 m2/g) measured with the same apparatus (Belsorp HP; see Supplementary Fig. S1). The H2 uptake increases from 0.9 wt% for rGO300 to 5.0 wt% for rGO400, whereas it decreases to 2.3 wt% for rGO500 and to 1.7 wt% for rGO600 with an increase in the annealing temperature. It is worth noting that the trends of the specific capacity for Li-ion batteries and the hydrogen storage capacity were similar and that the maximum values of both were observed with the rGO treated at 400 C (Fig. 5(b)). First, we considered the surface area because a large surface area generally results in a large storage capacity. The BET surface areas are 234 m2/g for rGO300, 403 m2/g for rGO400, and 213 m2/g for rGO600, i.e., proportional to the maximum specific capacity and H2 uptake (see Supplementary Fig. 2). However, the surface area of rGO500 is quite small (62 m2/g) compared to that of the other rGO samples (as confirmed with two separate experiments). Although the surface area is related to the large energy storage capacity in rGO400, this finding suggests that other factors are involved in the enhancement of the energy storage capabilities. Second, we took the interaction between oxygen functional groups and energy sources (H2 and Li ion) into account because the oxygen groups can affect the energy storage capacity of rGO. If it is correct, the energy storage capacity is to depend linearly on the C/O ratio. However, the energy storage capacity has the maximum at rGO400 while C/O ratio increases with the increase of Ta. Finally, we investigated the effect of defects on the capacity. It is well known that there are numerous defects formed by pentagons and heptagons in rGO [21]. We focused on an evolution of the structural defects on the surface of the rGO. In this light, we analyzed the pore widths of the rGO samples with the HorvatheKawazoe adsorption model (HeK method) because HorvatheKawazoe approach has been frequently used in adsorption studies for the extracting of the pore size distribution information of microporous materials (0.35e2 nm) from experimental adsorption isotherms and any shape of pore including slit-like carbon pore [22]. The average micropore width increases from 0.52 nm for rGO300 to 0.80 nm for rGO600 (Table 2). For rGO400, the pore width of 0.67 nm is quite similar to the optimum interlayer distance of 0.6e0.7 nm in graphene [18e20] and multilayer rGO [14]. This reveals that a distance between carbon atoms of w0.67 nm is an important factor related to energy storage in carbon materials. The existence of the same pore size for Li and H2 storage may come from their similar sizes; Li is approximately 152 pm and H2 is approximately 148 pm.
Table 2 e Surface area (SBET) obtained from BET method and the pore width from HeK method. Sample name SBET (m2/g) Pore width ( A) rGO300 rGO400 rGO500 rGO600
234.7 403.7 62.2 213.7
5.2 6.7 7.3 8.0
4.
5
Conclusions
In summary, we demonstrated that 6.7 A is the optimum pore size on the surface of thermally rGO for both lithium and hydrogen storage. Optimization of the pore size using GO was accomplished by thermal reduction in a temperature range of 300e600 C. The rGO400 electrodes exhibit a high reversible specific capacity of 1220 mAh/g at 30 mA/g and 97% capacity retention after 50 cycles (750 mAh/g at 1000 mA/g). The results of high-pressure H2 isotherms at 77 K demonstrate that the maximum H2 uptake of w5.0 wt% is also obtained with the rGO400. This study provides experimental proof of the importance of the optimal pore size and can serve as a guideline for those who design new energy storage materials.
Acknowledgement Byung Hoon Kim was supported by the Incheon National University Research Grant in 20130400.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2013.12.144
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