Supporting Information for

Transcription

1 Supporting Information for Solution Processable Holey Graphene Oxide and Its Derived Macrostructures for High-Performance Supercapacitors Yuxi Xu,*, Chih-Yen Chen, Zipeng Zhao, Zhaoyang Lin, Chain Lee, Xu Xu, Chen, Wang, Yu Huang, Muhammad Imran Shakir, and Xiangfeng Duan*, Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA Sustainable Energy Technologies Center, College of Engineering, King Saud University, Riyadh 11421, Kingdom of Saudi Arabia Experimental Methods 1. Graphene oxide (GO) synthesis and purification GO was prepared by oxidation of natural graphite powder according to the modified Hummers method. S1 Briefly, graphite (3.0 g) was added to concentrated sulfuric acid (70 ml) under stirring at room temperature, then sodium nitrate (1.5 g) was added, and the mixture was cooled to 0 C. Under vigorous agitation, potassium permanganate (9.0 g) was added slowly to keep the temperature of the suspension lower than 20 C. Successively, the reaction system was transferred to a C water bath for about 0.5 h, forming a thick paste. Then, 140 ml of water was added, and the solution was stirred for another 15 min. An additional 500 ml of water was added followed by a slow addition of 20 ml of H 2 O 2 (30%), turning the color of the solution from brown to yellow. The mixture was filtered and washed with 1:10 HCl aqueous solution (250 ml) to remove metal ions followed by repeated washing with water and centrifugation to remove the acid. The resulting solid was dispersed in water by ultrasonication for 1 h to make a GO aqueous dispersion (0.5 wt %). The obtained brown dispersion was then subjected to 30 min of centrifugation at 4000 rpm to remove any aggregates. Finally, it was purified by dialysis for one week to remove the remaining salt impurities for the following experiments.

2 2. Preparation of solution processable holey graphene oxide (HGO) HGO was prepared according to the following procedure: Typically, 5 ml 30% H 2 O 2 aqueous solution was mixed with 50 ml 2 mg/ml GO aqueous dispersion and then heated at 100 C for 4 h under stirring. The as-prepared HGO was purified by centrifuging and washing the above mixture to remove the residual H 2 O 2 and then re-dispersed in water by vibration or ultrasonication for a few tens of seconds to produce a homogeneous HGO aqueous dispersion with a concentration of 2 mg/ml. The preparation of HGO can be easily scaled up. Control GO (cgo) was prepared by the similar method without adding H 2 O Preparation of reduced holey graphene oxide hydrogels (HGHs) HGHs were prepared according to the following procedure: 0.5 ml 1 M sodium ascorbate aqueous solution was added into 10 ml 2 mg/ml HGO aqueous dispersion and then the homogeneous mixture was heated at 100 C for 2 h without stirring. The as-prepared HGHs were taken out of the vial with a tweezer and immersed in pure water to remove any impurities for the following experiments. The size and shape of HGH can be easily controlled by changing the type of reactors. Reduced graphene oxide hydrogels (GHs) were also prepared under the same condition with GO as the starting material for comparison. 3. Preparation of solution processable reduced holey graphene oxide (HG) and its frees-standing paper (HGP) HG aqueous dispersion was prepared according to Li's method. S2 Briefly, 175 µl ammonia solution (28 wt% in water) and 25 µl hydrazine solution (35 wt% in water) were mixed with 50 ml 0.25 mg/ml HGO aqueous dispersion and then heated at 95 C for 1 h without stirring to produce a homogeneous black HG dispersion. The HGP was made by vacuum filtration of the HG aqueous dispersion through an Anodisc membrane filter (47 mm in diameter, 0.2 mm pore size; Whatman) followed by vacuum drying at room temperature. The typical thickness of the HGP was ~9 µm. Reduced graphene oxide dispersion and reduced graphene oxide paper (GP) were also prepared under the same condition with GO as the starting material for comparison. 4. Fabrication of GH- and HGH-based supercapacitors with aqueous and organic electrolyte Slices of HGH with a thickness of ~1 mm were first cut from the purified cylindrical HGHs. For assembly of supercapacitors with aqueous electrolyte: the HGH slices were immersed in 1.0 M H 2 SO 4 aqueous electrolyte for 12 h under stirring to exchange their interior water with electrolyte.

3 For assembly of supercapacitors with organic electrolyte: the HGH slices were first immersed in pure ionic liquid 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4 ) under vacuum at 100 C for 12 h to exchange their interior water with EMIMBF 4 and then transferred to a 2.0 M EMIMBF 4 in acetonitrile (AN) solution for another 12 h. Subsequently, the HGH slices solvated with aqueous and organic electrolytes were placed on the platinum or aluminum foils, and compressed using hydraulic press at a rate of ~0.5 cm min -1 during which the squeezed electrolytes were removed by filter papers. The samples were kept under a 100 MPa pressure for 1 min to form well-adhered films with a packing density of 0.66 g/cm 3 on the metal foils. Next, two same HGH films (both with a net-weight of ~1 mg and an areal mass of ~1 mg/cm 2 ) on separate metal foils were directly used as electrodes without any other additives or further treatments such as drying and thermal annealing, and separated by an ion-porous separator (Celgard 3501) soaked with electrolytes. All the components were assembled into a layered structure and tightly sealed by parafilm for electrochemical measurements. The assembly of supercapacitors with EMIMBF 4 /AN electrolyte was done in a glove box filled with Ar. The GH-based supercapacitors were fabricated under the same condition for comparison. 5. Fabrication of GP- and HGP-based supercapacitors with aqueous and organic electrolyte Rectangular HGP pieces with size of ~1 cm 2 and an areal mass of ~1 mg/cm 2 were first cut from the pristine HGP. For assembly of supercapacitors with aqueous electrolyte: the HGP pieces were immersed in 1.0 M H 2 SO 4 aqueous electrolyte for 12 h under stirring. For assembly of supercapacitors with organic electrolyte: the HGP pieces were immersed 2.0 M EMIMBF 4 in AN solution under stirring for 12 h. Then, the HGP pieces were attached onto metal foils and separated by an ion-porous separator (Celgard 3501) soaked with electrolytes. All the components were assembled into a layered structure by sandwiching the entire supercapacitor device between two rigid glass slides using clips and tightly sealed by parafilm for electrochemical measurements. The GP-based supercapacitors were fabricated under the same condition for comparison. 6. Fabrication of HGP-based flexible solid-state supercapacitors First, the H 2 SO 4 -polyvinyl alcohol (PVA) gel electrolyte was prepared as follows: 1 g of H 2 SO 4 was added into 10 ml of de-ionized water, and then 1 g of PVA power was added. The whole mixture was heated to 85 C under stirring until the solution became clear. Second, two HGP strips were immersed in the hot solution for 15 min with a small part kept out for electrical connection and

4 picked out for air-drying at room temperature for 12 h to evaporate excess water. Then the two electrodes were pressed together under a pressure of ~1 MPa for 10 min, which allowed the polymer gel electrolyte on each electrode to combine into one thin separating layer to form an integrated device. The typical thickness of the whole device was around 30 µm determined by the screw micrometer. 7. Structural characterization and analysis. The morphologies of HGO, HGH and HGP were characterized by SEM (JEOL 6700) and TEM (FEI CM120). Raman spectra were recorded on a RM 2000 Microscopic confocal Raman spectrometer (Renishaw) using a 514 nm laser beam. Methylene blue (MB) dye adsorption method was employed to measure the specific surface areas. MB adsorption is a standard method for measuring the specific surface area of graphitic materials, with 1 mg of adsorbed MB molecules covering 2.54 m 2 of surface area. S3 The specific surface areas were calculated by adding a piece of GH, HGH, GP or HGP into a standard concentration of MB in DI water for a total of 24 h to reach adsorption equilibrium. The MB concentration was determined by analyzing the supernatant through UV-vis spectroscopy at a wavelength of 665 nm and compared to the initial standard concentration of MB prior to interacting with the material. 8. Electrochemical characterization and analysis. All the electrochemical experiments were carried out using VersaSTAT 4 from Princeton Applied Research. The electrochemical impedance spectroscopy measurements were performed at open circuit potential with a sinusoidal signal over a frequency range from 100 khz to 10 mhz at an amplitude of 10 mv. The cycle life tests were conducted by galvanostatic charge/discharge measurements. The specific capacitances (C wt ) derived from galvanostatic discharge curves were calculated based on the following formula: C wt = 2(I t)/(m V), where I is the constant discharge current, t is the time for a full discharge, m is the net mass of one electrode, and V represents voltage drop upon discharging (excluding the IR drop). The corresponding volumetric capacitances (C vol ) were calculated using: C vol = C wt ρ, where ρ is the packing density of graphene in electrode films. The energy density against two electrodes in device was calculated using the following formula: E wt = C wt V 2 /8 and E vol = C vol V 2 /8, respectively, where V is the operating voltage. For the leakage current test, the device was first charged to 1.0 V at 2 ma and then the potential was kept at 1.0 V for 2 h while acquiring the current data. For the self-discharge test, the device was first

5 charged to 1.0 V at 2 ma and kept at 1.0 V for 15 min, and then the open potential of the device was recorded as a function of time. Supplementary Figures and Tables Figure S1. (a) Nitrogen adsorption and desorption isotherms and (b) BJH pore size distribution of freeze-dried HGO and GO. Figure S2. Deconvoluted C1s XPS profiles of GO, cgo and HGO.

6 Figure S3. (a) Photographs of the resulting solutions under different reaction times during the preparation of HGO. TEM images of the products under the reaction time of 8 h (b) and 16 h (c). Figure S4. C1s XPS profiles of HGO, freeze-dried HGH, and HG. Figure S5. SEM images of holey graphene hydrogels before (a) and after (b) mechanical pressing for preparation of supercapacitor electrodes. Although the apparent morphology of holey graphene hydrogel changed from a porous network to a compact structure upon mechanical pressing, our

7 previous studies have confirmed that the robust interlock of graphene sheets in the 3D network can allow mechanical pressing to reduce the pore size of the network while largely maintaining the original stacking characteristics of graphene and its interconnected solvated porous structure (Ref. 28,29 in the article), which is highly favorable for supercapacitor application. Figure S6. (a) Leakage current curve of the HGP-based solid-state supercapacitor charged at 2 ma to 1.0 V and kept at 1.0 V for 2 h. The device showed a low leakage current of ~4 µa. (b) Self-discharge curve of the device after charged at 1.0 V for 15 min. Normally, most supercapacitors are operated in the range of V max (the voltage at the beginning of discharge) to approximately 1/2 V max. Thus, the time required for the voltage across the device to change from V max to 1/2 V max was measured to be 12.5 h, which is comparable to those of commercial supercapacitors with self-discharge rates of 8 to 20 h. S4 Table S1. Capacitive performance of representative porous carbon nanomaterials in aqueous and organic electrolytes based on an electrical double-layer mechanism. Materials Single-walled CNTs arrays Commercial activated carbon Aqueous electrolyte Packing C wt / F g -1 C vol / Electrolyte density / (current F cm -3 (voltage) g cm -3 density) 0.5 NA NA Et 4NBF 4/PC (4.0 V) 0.5~ ~200 80~110 Et 4NBF 4/AN (2.5~3.0 V) Organic electrolyte C wt / F g -1 (current density) 160 C vol / E wt / E vol / F cm -3 Wh kg -1 Wh L -1 Ref S5 80~120 48~84 20~30 12~21 S6 Carbide derived 0.53 NA NA EMIMTFSI S7 carbon (3.0 V) (0.3 A/g) Chemically Et 4NBF 4/AN S8 modified graphene (1.33 A/g) (2.5 V) (1.33 A/g)

8 Laser scribed graphene (1 A/g) 9.7 EMIMBF 4 (3.5 V) Curved graphene 0.3 NA NA EMIMBF 4 (4.0 V) Activated graphene EMIMBF 4 (1 A/g) (3.5 V) 276 (5.0 A/g) S S S11 a-mego 0.36 NA NA BMIMBF 4/AN S12 (3.5 V) (1.4 A/g) Compressed 0.75 NA NA BMIMBF 4/AN S13 a-mego (3.5 V) (1.2 A/g) asmego 0.59 NA NA EMIMTFSI/AN S14 (3.5 V) (2.1 A/g) Electrolyte-mediate 1.25~ EMIMBF 4/AN S15 d graphene (1 A/g) (3.5 V) N-doped graphene NA 280 NA Et 4NBF 4/AN 220 NA 48 NA S16 (2.5 V) Reduced graphene NA 205 NA NA NA NA NA NA S17 oxide (0.1 A/g) Thermal expanded NA 264 NA Et 4NBF 4/AN 120 NA 12.8 NA S18 graphene oxide (0.1 A/g) (1.75 V) (0.1 A/g) HGH EMIMBF 4/AN (3.5 V) This work HGP EMIMBF 4/AN (3.5 V) Note: The specific capacitance values shown in the Tables S1 are typically based on the mass of active electrode materials. In contrast to our additive-free HGH and HGP electrodes, polymer binder (usually polytetrafluoroethylene) and/or conductive additive (usually carbon black) are used to mix with active materials to make supercapacitor electrodes. These additives account for wt.% of the overall electrode materials, which will further decrease the specific capacitances when normalized to the total mass of electrodes. Table S2. Capacitive performance of representative flexible solid-state supercapacitors based on carbon nanotubes (CNTs) and graphene films with an electrical double-layer mechanism. Materials Substrate Mass Electrode loading thickness Packing density C wt / F g / g cm -3 (current density) Single-walled PET 33.3 µg/cm 2 ~0.6 µm CNTs -1 C vol / Device Ref. F cm -3 thickness 61.6 NA S19

9 CNTs Paper 0.23 mg/cm 2 NA NA 80 Single-walled Paper NA NA NA CNTs Laser scribed PET or Al 36.3 µg/cm 2 ~7.6 µm graphene foil Graphene Paper 0.68 mg/cm 2 NA NA 68.1 Graphene PET NA ~5 µm NA Graphene None NA ~1 µm NA 85 Graphene PI 2 mg/cm 2 ~120 µm Doped None NA NA NA 124 graphene (5 mv/s) HGP None 1 mg/cm 2 ~9 µm NA 720 µm S20 NA 1.3 mm S µm S9 NA NA S22 NA NA S23 NA NA S24 32 NA S25 NA NA S ~30 µm This work Note: Most of previous flexible solid-state supercapacitors use substrates for loading the electrode materials. Meanwhile, the mass loading and/or the packing density of the electrode materials are usually low. All of these will greatly decrease the ratio of electrode materials in the entire device and at the same time increase the total weight and volume of the entire device, which results in low specific capacitances when normalized by the total weight or volume of the entire device. Supplementary References S1. Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, S2. Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101. S3. McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Alonso, M. H.; Milius, D. L.; Car, R.; Prud'homme, R. K.; Aksay, I. A. Chem. Mater. 2007, 19, S4. El-Kady, M. F.; Kaner, R. B. Nat. Commun. 2013, 4, S5. Najafabadi, A. L.; Yasuda, S.; Kobashi, K.; Yamada, T.; Futaba, D. N.; Hatori, H.; Yumura, M.; Iijima, S.; Hata, K. Adv. Mater. 2010, 22, E235. S6. Burke, A. Electrochim. Acta 2007, 53, S7. Largeot, C.; Portet, C.; Chmiola, J.; Taberna, P. L.; Gogotsi, Y.; Simon, P. J. Am. Chem. Soc. 2008, 130, 2730.

10 S8. Stoller, M. D.; Park, S. J.; Zhu, Y. W.; An, J. H.; Ruoff, R. S. Nano Lett. 2008, 8, S9. El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Science 2012, 335, S10. Liu, C. G.; Yu, Z.; Neff, D.; Zhamu, A.; Jang, B. Z. Nano Lett. 2010, 10, S11. Zhang, L.; Zhang, F.; Yang, X.; Long, G.; Wu, Y.; Zhang, T.; Leng, K.; Huang, Y.; Ma, Y.; Yu, B.; Chen, Y. S. Sci. Rep. 2013, 3, S12. Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; Su, D.; Stach, E. A.; Ruoff, R. S. Science 2011, 332, S13. Murali, S.; Quarles, N.; Zhang, L. L.; Potts, J. R.; Tan, Z.; Lu, Y.; Zhu, Y.; Ruoff, R. S. Nano Energy 2013, 2, 764. S14. Kim, T. Y.; Jung, G.; Yoo, S.; Suh, K. S.; Ruoff, R. S. ACS Nano 2013, 7, S15. Yang, X. W.; Cheng, C.; Wang, Y. F.; Qiu, L.; Li, D. Science 2013, 341, 534. S16. Jeong, H. Mo.; Lee, J. W.; Shin, W. H.; Choi, Y. J.; Shin, H. J.; Kang, J. K.; Choi, J. W. Nano Lett. 2011, 11, S17. Wang, Y.; Shi, Z. Q.; Huang, Y.; Ma, Y. F.; Wang, C. Y.; Chen, M. M.; Chen, Y. S. J. Phys. Chem. C 2009, 113, S18. Lv, W.; Tang, D. M.; He, Y. B.; You, C. H.; Shi, Z. Q.; Chen, X. C.; Chen, C. M.; Hou, P. X.; Liu, C.; Yang, Q. H. ACS Nano 2009, 3, S19. Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G. Nano Lett. 2009, 9, S20. Kang, Y. J.; Chung, H.; Han, C. H.; Kim, W. Nanotechnology 2012, 23, S21. Hu, S.; Rajamani, R.; Yu, X. Appl. Phys. Lett. 2012, 100, S22. Weng, Z.; Su, Y.; Wang, D.-W.; Li, F.; Du, J.; Cheng, H.-M. Adv. Energ. Mater. 2011, 1, 917. S23. Choi, B. G.; Hong, J.; Hong, W. H.; Hammond, P. T.; Park, H. ACS Nano 2011, 5, S24. Choi, B. G.; Chang, S. J.; Kang, H. W.; Park, C. P.; Kim, H. J.; Hong, W. H.; Lee, S.; Huh, Y, S. Nanoscale 2012, 4, S25. Xu, Y. X.; Lin, Z. Y.; Huang, X. Q.; Liu, Y.; Huang, Y.; Duan, X. F. ACS Nano 2013, 7, S26. Wu, Z. S.; Winter, A.; Chen, L.; Sun, Y.; Turchanin, A.; Feng, X. L.; Müllen, K. Adv. Mater. 2012, 24, 5130.