Doped Sites at Basal-Planes

Transcription

1 SUPPORTING INFORMATION Nitrogen-Doped Graphene for High Performance Ultracapacitors and the Importance of Nitrogen- Doped Sites at Basal-Planes Hyung Mo Jeong, Jung Woo Lee, Weon Ho Shin, Yoon Jeong Choi, Hyun Joon Shin, Jeung Ku Kang*,, and Jang Wook Choi*, Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 373-1, Guseong Dong, Yuseong Gu, Daejeon , Republic of Korea Pohang Accelerator Laboratory and Department of Physics Pohang University of Science and Technology San 31, Hyojadong, Pohang , Republic of Korea NanoCentury KAIST Institute and Graduate School of EEWS (WCU), Korea Advanced Institute of Science & Technology, 373-1, Guseong Dong, Yuseong Gu, Daejeon , Republic of Korea * 1

2 N-doped graphene synthesis Graphite was first oxidized to graphene oxide (GO) by Hummer s method g of graphite was dipped in an acid solution where 1 g of NaNO 3 and 6.3 g of KMnO 4 were dissolved in 92 ml of sulfuric acid. All chemicals used in this study were purchased from Sigma Aldrich. The dried GO was reduced by a plasma enhanced chemical vapour deposition (PECVD) process. In this process, the GO is first reduced by a hydrogen plasma process (500 W in power, 4 torr of H 2 gas with 100 sccm in flow rate). Subsequently, after the reduction step, nitrogen plasma (500 W in power, 14 torr of N 2 gas with 91 sccm in flow rate) was introduced onto the graphene surfaces to complete NG (Figure 1a). Finally, the samples were annealed at 300 o C for 3 hrs to remove residual functional groups on the NG surface. Scanning photoemission microscopy SPEM was performed at the U7 beam line of the Pohang Light Source (PLS). The X-ray was focused by a diffractive X-ray lens (Fresnel zone plate), and the photoelectrons from the sample produced by the focused X-rays were analyzed by an electron energy analyzer. The focused X-ray size (space resolution) was 0.5 µm in diameter. The photoelectrons were collected using photon energy is 630 ev. The binding energies were corrected for specimen charging by referencing the Pt 4f 7/2 peak to 70.9 ev. The experiment was performed in an ultrahigh vacuum (UHV) chamber with a base pressure Torr. The energy of the photoelectrons emitted from the NG was analyzed with an electron energy analyzer (Physical Electronics: model PHI 3057 with a 16-channel detector). The analyzer was located at an angle of 55 from the surface normal. 2

3 Ultracapacitor cell preparation A slurry was first prepared by dissolving NG and polyvinylidene Fluoride (pvdf) (w:w=9:1) in N-Methylpyrrolidone (NMP). The slurry was casted on nickel foil and dried in a vacuum oven overnight. For the paper and textile UCs, the same slurries but with no binder were prepared and sprayed onto the substrates. For aqueous electrolyte devices, two pieces of NG casted onto the substrates were first attached on glass slides. Platinum leads were in contact with the back sides of the substrates in order to connect to an electrochemical tester (Biologic VMP3). Both glass slides were assembled with a separator (Whatman 8 µm filter paper) sandwiched in between. The cell assemblies were wrapped with parafilm, and then dipped in the electrolyte solutions. For organic electrolyte devices, cells were assembled in the same way but 1.0 M of tetraethylammoniumtetrafluoroborate ((C 2 H 5 ) 4 NBF 4 ) in acetonitrile was used. For all the types of cell, the active areas overlapped by both NG films were 1.53 cm 2. Typical mass loadings for data shown in Figure. 2 were ~1 mg/cm 2. Galvanostatic Measurements for Ultracapacitor Testing Capacitance, energy density, and power density are all characterized by galvanostatic measurements. Various current densities shown in Figure 2c and d were applied to cells while potentials between both electrodes are sweeping between cut-off values (0 V 0.8 V in aqueous phase, 0 V 2.5 V in organic phase). Voltages were recorded every 0.01 ~ 0.2 second. 3

4 Data Analysis In the galvanostatic data, the IR drop at the upper cut-off potential and slope in the discharge curve are used to obtain the power and energy density, respectively. The power is calculated by using P = V 2 / [4RM] in which V is the cut-off potential, R is the internal resistance, and M is the total mass of NG electrodes on both sides. The internal resistance is evaluated from the voltage drop at the beginning of each discharge: R = ΔV ir / 2i in which ΔV ir is the voltage drop between the first two points in the voltage drop at its top cut-off. This voltage drop is also referred to as the ir drop. i is the current applied. The specific capacitance (C s ) is calculated by using C s = i/-[δv/δt]m = i/-(slope m) in which i is the current applied, the slope is that of the discharge curve after the ir drop, and m is the mass of NG on one electrode. Similarly, energy density (E) is calculated using E = 0.5CV 2 M in which V and M are the same notations as the power calculation, and C is the measured capacitance. All of the calculations herein are consistent with those in reference 3 in the main text. 4

5 X-ray diffraction pattern Figure S1. An X-ray diffraction pattern of NG 500W 1min. Raman spectra 5

6 Figure S2. Raman spectra of the NG and reduced GO by hydrazine. The red and black lines correspond to those of the NG 500W 1min and the reduced GO, respectively. Ultracapacitor cyclovoltammetric measurements Figure S3. Cyclovoltammetric data for various graphene based ultracapacitors measured at a scanning rate of 20 mv s -1. The data indicates the capacitance of NG UC increases significantly compared to those of pristine graphene UCs. 6

7 Cycle life test (full version) Figure S4. Cycle life data for the NG UC on paper. The data were measured up to 230,000 cycles over a time period of ~ 3 months. Ragone plot 7

8 Figure S5. Ragone plot for the NG UC. Both energy and power densities were calculated based on the NG mass only. The data from reference 2 in the main text are also presented for comparison (black). These reference data were also calculated based on the electrode mass only. Surface area measurement of graphene The surface area of graphene electrode plays a very important role in determining the specific capacitance of UC. However, the exact measurement of the graphene BET surface area is a challenging task because graphene layers are easily agglomerated when they were prepared as powders for the BET measurements. The difficulty of BET measurement has been supported by a wide range of reported values (184 ~ 705 m 2 /g) 3-5 even for the samples prepared in the exactly same procedures. Also, most reported values are by far smaller than its theoretical BET surface area of ~ 2600 m 2 /g, 6 again suggesting the agglomerating nature of the sample. Despite the experimental difficulty in the BET measurement, we believe that the BET surface area shouldn t change much upon the plasma treatment because the plasma treatment alters only atomic structure of graphene, not the original single layer character of graphene. 8

9 AFM scanning Figure S6. AFM scanning to ensure the single atomic layer thickness. This AFM image corresponds to the 3 min plasma-processed graphene whose C1s image is shown in Figure 3c. The right-hand side height profile confirms that the graphene samples we probed using SPEM are a single atom layer thick. DFT calculations First-principles DFT calculations were performed using the Vienna Ab-initio Simulations Package (VASP) with the projected augmented wave method, periodic boundary conditions and generalized gradient approximation considering the Perdew-Burke-Ernzerhof exchange- 9

10 correlation functionals. To model the basal-plane graphene, we used a single-layer graphene slab using 96 carbon atoms (C 96 ). The two most common graphene edge types, zigzag and armchair edges were modeled using the 6 zigzag chains (C 72 H 12 ) and 9 dimers with 72 carbon atoms (C 72 H 16 ), respectively (Figure S7). We also optimized stable N-configurations structures at basal-planes and edges using first-principles DFT calculations (Figure S8 ~ S10). All the calculations were performed with the gamma k-point set of The cutoff energies for plane waves were chosen to be 500 ev, and the supercell was large enough with lattice parameters of a=20.00 Å, b=14.74 Å for graphene and zigzag edges and Å for armchair edges, and c=17.02 Å for graphene and armchair edges and Å for zigzag edges. The equilibrium atomic structures were obtained by fully relaxing the atomic coordinates with fixed lattice parameters and force cut-off of ev/å. To obtain the binding energies between K + ions and each graphene configuration, we also optimized the K + -bound to graphene structures. The preferred low-energy structures are illustrated in Figure S11 ~ S13. We define a binding energy (E b ) of K + ion as E b = E(K + -graphene) (E(K + ) + E(graphene)), where E(K + -graphene), E(K + ), and E(graphene) correspond to the total energies of the K + -bound to graphene structure, the K + ion, and the graphene structure, respectively. 10

11 Figure S7. Structural graphene models of a) the basal plane, b) the zigzag-edge, and c) the armchair-edge. (Brown and white represent C and H atoms, respectively) Figure S8. Calculated nitrogen configurations at basal plane graphene: a) N-Q, b) three N-6, c) N-6 with H-terminated dangling bonds, d) N-5 with H-terminated dangling bonds, e) N-5 anion with H-terminated dangling bonds, and f) N-5 anion with two N-6. (Brown, blue, and white represent C, N, and H atoms, respectively) 11

12 Figure S9. Calculated nitrogen configurations at zigzag-edge graphene: a) N-Q, b) N-6, c) N-5 and d) N-5 anion types. (Brown, blue, and white represent C, N, and H atoms, respectively) Figure S10. Calculated nitrogen configurations at armchair-edge graphene: a) N-Q, b) N-6, c) N-5 and d) N-5 anion types. (Brown, blue, and white represent C, N, and H atoms, respectively) 12

13 Figure S11. K + -bound structures and binding energies of basal plane graphene: a) pristine, b) N-Q, c) three N-6, d) N-6 with H-terminated dangling bonds, e) N-5 with H-terminated dangling bonds, f) N-5 anion with H-terminated dangling bonds, and g) N-5 anion with two N-6. (Brown, blue, and white represent C, N, and H atoms, respectively) 13

14 Figure S12. K + -bound structures and binding energies of zigzag-edge graphene: a) pristine, b) N-Q, c) N-6, d) N-5, and e) N-5 anion. (Brown, blue, violet and white represent C, N, K, and H atoms, respectively) 14

15 Figure S13. K + -bound structures and binding energies of armchair-edge graphene: a) pristine, b) N-Q, c) N-6, d) N-5, and e) N-5 anion. (Brown, blue, violet and white represent C, N, K, and H atoms, respectively) 15

16 XPS data Figure S14. XPS data for bulk-scale samples measured at each plasma duration. 16

17 Table S1. Elements contents data for NG samples at each plasma duration. Reference (Supplementary) (1) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, (2) Hu, L. B.; Choi, J. W.; Yang, Y.; Jeong, S.; La Mantia, F.; Cui, L. F.; Cui, Y. Proc. Nat. Acad. Sci. USA 2009, 106, (3) Pan, D.; Wang, S.; Zhao, B.; Wu, M.; Zhang, H.; Wang, Y.; Jiao, Z. Chem. Mater. 2009, 21, (4) Lian, P.; Zhu, X.; Liang, S.; Li, Z.; Yang, W.; Wang, H. Electrochimica Acta 2010, 55, (5) Stoller, M. D.; Park, S. J.; Zhu, Y. W.; An, J. H.; Ruoff, R. S. Nano Lett. 2008, 8, (6) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442,