Free-standing, well-aligned ordered mesoporous carbon nanofibers on current collectors for high-power microsupercapacitor

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1 Free-standing, well-aligned ordered mesoporous carbon nanofibers on current collectors for high-power microsupercapacitor applications Eunae Kang, a Gumhye jeon a and Jin Kon Kim* a A National Creative Research Center for Block Copolymer Self-Assembly, Department of Chemical Engineering, Pohang University of Science and Technology, Gyungbuk , Korea, jkkim@postech.ac.kr <Electronic Supplementary Information> 1

2 (a) (b) 500nm 200nm Fig. S1 Side-view TEM (a) and SEM (b) images of OMC-NA 2

3 Raman spectrum for OMC-NA is shown in Fig. S2. The G-band (graphitic carbon) and D- band (disordered carbon) are clearly observed at 1590 cm 1 and 1350 cm 1, respectively. The G band at 1590 cm -1 is associated with an E 2g mode of graphite, corresponding to the sp 2 carbon atoms in graphite layer. 1 The D band at 1340 cm 1 corresponds to vibration of sp 3 - carbon atoms, and it is related to the defective sites in carbon materials. 1 The relative intensity ratio of the G and D bands (I G /I D ratio) is proportional to the graphitizing degree. I G and I D are determined by the area after de-convolution of peaks. The I G /I D ratio is estimated to be 0.723, indicating defect sites and well defined graphitic domains coexist on the surface of OMC-NA. Fig. S2 Raman spectrum of OMC-NA and de-convolution of the curve into two peaks 3

4 Experimental 1. 1 Fabrication of a film electrode with 3D mesoporous carbon fibers. For this synthesis, phenolic resol, employed as a carbon source, was prepared following the procedure reported in reference. 2 The solution for the electrode was prepared by dissolving 0.05 g of PS-b-PEO (Mn = 26,000; weight fraction of PEO block = 0.19) and 0.2 g of phenolic resol in 3 ml of THF. A porous anodic aluminum oxide membrane (AAO) (Whatman: Cat. No ) was used as a template for a film electrode with 3D mesoporous carbon fibers. An AAO template was placed onto a silicon wafer for easy handling. 0.1 ml of the above solution was casted onto the AAO template, penetrating into the AAO pores, before the composite was dried for 2 h at room temperature. The residual sol on the AAO was washed with pure THF, resulting in a sol layer a few hundred nanometers thick. This resol/block copolymer in the AAO template was cross-linked at 100 C overnight. This sample was then heat-treated at 700 C for 2 h at a rate of 1 C min 1. A gold layer with a thickness of 200 nm was deposited onto the back side of the electrode sample. This electrode was connected with copper wire and the back side of the electrode sealed with insoluble additives to avoid exposure of the gold. The AAO template in the electrode was removed using 5 wt% NaOH solution. To prepare ordered mesoporous carbon bulk-wires (OMC-BW), the same solution was dropped onto the AAO surface following the same procedure as for the ordered mesoporous carbon nanofiber arrays (OMC-NA). But the residual composite sol on AAO was completely removed to form each carbon bulk wires. (Fig S3). After heat-treatment and removal of the AAO template, the nanowire-shaped powder was obtained. For fabrication of ordered mesoporous carbon bulk powders (OMC-BP), the same solution was simply casted into a glass chalet and the resultant film heat-treated up to 700 C, followed by grinding into fine powder with a mortar and pestle (Fig S4). 4

5 Fig. S3. Upper panel: Synthetic scheme of ordered mesoporous carbon bulk-wires (OMC-BW). (a) TEM and (b) SEM images, (c) nitrogen adsorption-desorption isotherm, and (d) pore size distribution of OMC-BW. From TEM and SEM images of OMC-BW, it shows the ordered mesoporous carbon nanofibers with 1D wire structure. The porosity of OMC-BW was investigated by the N 2 adsorption/desorption isotherm in Fig. S3c. A sharp capillary condensation at p/p 0 of ~0.9 is observed, suggesting that large and uniform pores are well-developed. The pore size (d) is calculated to be 26.2 nm, based on the Barrett-Joyner-Halenda (BJH) method (Fig.S3d): 3 5

6 ( ) in which is surface tension of liquid nitrogen, VL is liquid molar volume of nitrogen, R is gas constant, T is absolute temperature, P 0 /P is relative pressure, and t is thickness. From Fig. S3c, the pore volume and surface area are cm 3 /g and m 2 /g, respectively. 6

7 Fig. S4. Upper panel: Synthetic scheme of ordered mesoporous carbon bulk powders (OMC-BP). (a) TEM and (b) SEM images, (c) nitrogen adsorption-desorption isotherm, and (d) pore size distribution of OMC-BP. From TEM and SEM images of OMC-BW, it shows the well-ordered 2 dimensional mesoporous carbon. The porosity of OMC-BP was investigated by the N 2 adsorption/desorption isotherm in Fig. S4c. A sharp capillary condensation at p/p 0 of ~0.9 is observed, suggesting that large and uniform pores are well-developed. The pore size is calculated to be 24 nm, based on the BJH method (Fig.S4 d). The pore volume and surface area are cm 3 /g and m 2 /g, respectively. 7

8 1. 2 Characterization and electrochemical measurements Transmission electron microscopy (TEM) characterization was carried out using a Jeol EM-2010 microscope. Scanning electron microscopy (SEM) characterization was carried out using a Hitachi S4800. The XRD study was performed with a PANalytical X Pert diffractometer (Cu-Kα radiation) with an X Celerator detector. Nitrogen adsorption-desorption isotherms were tested at 77 K on a Micromeritics Tristar II 3020 system. Small-angle X-ray scattering (SAXS) data were measured with an apparatus consisting of an 18 kw rotating anode X-ray generator (Rigaku Co., Cu-Kα, λ = Å) and a one-dimensional (1D) position-sensitive detector (M. Braun Co.). Electrical characterization was performed with a three-electrode configuration in aqueous 2 M H 2 SO 4 using a platinum mesh and Ag/AgCl as the counter electrode and reference electrode, respectively. The film electrode described in main manuscript was directly used as the working electrode. OMC-BW and OMC-BP were also examined as counter references. The active materials were mixed with polytetrafluoroethylene (PTFE) binder and carbon additive (Ketjenblack ECP-600JD, KB) in a weight ratio of 10:1:1 and coated onto 1 cm 1 cm stainless steel Exmet as the current collector. Cyclic voltammetry (CV) were conducted with a Gamry Reference 600 potentiostat (Gamry Instruments, Warminster, PA, USA). The CV was performed in the potential rage of 0.2 to 0.8 V (vs. Ag/AgCl) with a scan rate of mv s 1. Galvanostatic charge-discharge analysis was conducted with a WBCS 3000 battery cycler (Wanatech Inc., Republic of Korea). The charge-discharge curves were collected in the cutoff voltage range of V (vs. Ag/AgCl) with various current densities. 8

9 Current(A g -1 ) Current(A g -1 ) Capacity(F g -1 )) Current(A g -1 ) mv/s scan rate 25 OMC-BP (c) OMC-BW ONC-NA (a) 160 OMC-BP OMC-BW OMC-NA Scan rate(mvs -1 ) Voltage(Ag/AgCl) mV/s scan rate OMC-BP OMC-BW OMC-NA (b) Voltage(Ag/AgCl) 1000 mv/s scan rate 150 ONC-BP OMC-BW 100 OMC-NA (d) Voltage(Ag/AgCl) Fig. S5 (a) Capacity profiles at different scan rate of mvs -1 calculated from CV. Cyclic voltammograms of OMC-NA (ordered mesoporous carbon nanofibers arrays) compared with OMC- BW (ordered mesoporous carbon bulk-wires) and OMC-BP(ordered mesoporous carbon bulk powders) in 2M H 2 SO 4 at different scan rate of (b) 10 mvs -1, (c) 100 mvs -1 and (d) 1000 mvs -1. 9

10 2. The performance comparison of OMC-NA with activated carbon, CMK-3 and CNT Supercapacitor can be classified into electrical double layer capacitor (EDLC) and pseudo capacitor depending on energy storage mechanism. We compare electrochemical performances of our device with those of other carbon-based materials (activated carbon, the state-of-the art nanostructured carbons such as CNTs and template OMCs) as EDLC electrode materials. Because carbon-based materials accumulate the electrostatic charge at electrode/electrolyte interface, the specific capacitance is strongly influenced by specific surface areas (SSA). Commercially available activated carbons have ~100 F g -1 capacitance depending on the surface areas (<1000 m 2 g -1 ). 4 However, they have disordered pores and large particle size compared with OMC-NA (or OMC-BW), which results in poor rate capabilities (i.e., low power at high current densities). Representative template carbon (for instance, CMK-3 synthesized from SBA-15 hard template) has high surface area (~1100 m 2 g -1 ), ordered and narrow pore size and interconnected pores, resulting in the excellent capacitance of ~132 F g Although this exhibited high performance as an EDLC electrode material, the synthesis needs multi-steps and tedious hard template method, which is not good for the commercialization. Moreover, it cannot be aligned directly on the current collectors to achieve high power microsupercapacitor. CNTs can be employed to construct aligned nanostructures directly on current collectors, 6 similar to OMC-NA in our work But, the specific capacitance is only 13 F g -1. In addition, supercapacitor electrodes made of CNT powder with surface area of 430 m 2 g -1 exhibited specific capacitances of ~100 F g -1. 7,8 A relatively low surface area of CNTs limits the electrochemical performance. OMC-NA in this work exhibited capacitance of ~160 F g -1, even though it is synthesized through simple and facile soft-templating method, which might be very useful for commercialization. Also, soft-templating method allows us to construct aligned nanostructures directly on current collectors for high power microsupercapacitors. Fig. S6 gives Ragone plot for various carbon materials (commercial activated carbon (Norit Super 50), 9 commercial carbon nanotube, 10 and CMK Although the energy density of CMK-3 is higher than that of OMC-NA, the synthesis of CMK-3 needs multi-steps and tedious hard template method. 10

11 Energy density (Wh/kg) 1 OMC-NA AC CMK-3 CNT Power density (W/kg) Fig. S6 Ragone plot of various carbon materials including OMC-NA (AC: commercial activated carbon (Norit Super 50), 9 CNT: commercial carbon nanotube, 10 and CMK ) 11

12 Micropores in Mesoporous Mesopores in fibers nanofibers Electronic conductive paths - Electronic conductive paths Micropores in Mesoporous fibers Macropores between CNFs : Macropores between carbon ionic buffer reservoirs nanofibers :ionic buffer reservoirs : anion of electrolyte Macropores between CNFs : ionic buffer reservoirs : anion of electrolyte : cation of electrolyte - : electron : anion of electrolyte : cation of electrolyte :electron : cation of electrolyte - : electron Fig. S7. Schematic representation illustrating advantages of OMC-NA. This structure can provide enough space between mesoporous carbon fibers for electrolyte storage and direct electronic path ways to the current collectors. 12

13 Specific capacitance(fg -1 ) Current(A g -1 ) Capacity(F g -1 ) 3. The reproducibility of device performance and cyclability at higher current density for OMC- NA In order to confirm the reproducibility of device performance, we again performed the cyclic voltammograms by using different batch of OMC-NA, and the results are shown in Fig. S8. Comparing Fig. S5, we found that the capacity differences at various scan rates are within 10%. (a) 200 (b) Voltage(Ag/AgCl) Scan rate(mvs -1 ) Fig. S8 (a) Cyclic voltammograms of reproduced OMC-NA at different scan rate of mvs -1 (b) plot of capacity obtained by CV versus scan rate We also test the cyclibility of OMC-NA at a high current density of 11.3 A g -1 and the result is shown in Fig. S9. The capacitance is not stable compared with that performed at lower current density. This is due to the rapid voltage change during charge-discharge for short period of time Cycle number Fig S9. Cyclibility of OMC-NA at high current density of 11.3 A g -1 13

14 4. The density of carbon nanofibers We used a commercially available AAO (Whatman: Cat. No ) having irregular pore size distribution, as shown in Fig. S10. A A-1 Fig S10. (A) SEM image of AAO template and (A-1) an image by using the image J program. By using the image J program, the pore volume was estimated to Because the conversion of the resol precursor and PS in PS-b-PEO to carbon was ~ 0.5, the volume of carbon nanofibers is estimated to Finally, the pore sizes can easily be controlled when one uses two step anodization. 12,13 Reference 1. V. Georgakilas, D. Voulgaris, E. Vázquez, M. Prato, D. M. Guldi, A. Kukovecz and H. Kuzmany J. Am. Chem. Soc., 2002, 124, R. Liu, Y. Shi, Y. Wan, Y. Meng, F. Zhang, D. Gu, Z. Chen, B. Tu, D. Zhao, J. Am. Chem. Soc. 2006, 128, E. P. Barrett, L. G. Joyner and P. P. Halenda J. Am. Chem. Soc., 1951, 73, J. Gamby, P. L. Taberna, P. Simon, J. F. Fauvarque and M. Chesneau J. power sources 2001, 101, F. Lufrano and P. Staiti, Int. J. Electrochem. Sci. 2010, 5, B. Kim, H. Chung, B. K. Min, H. Kim and W. Kim Bull. Korean chem. Soc. 2010, 31, C. Niu, E. K. Sichel, R. Hoch, D. Moy and H. Tennent, Appl. Phys. Lett., 1997, 70, H. Pan, J. Li and Y. Feng nanoscale Res. Lett. 2010, 5, E.J. Ra, E. Raymundo-Pin ero, Y.H. ee and F. Be guina, Carbon 2009, 47, M. Skunik, M. Chojak, I. A. Rutkowska and P. J. Kulesza Electrochimica Acta 2008, 53, W. Xing, S.Z. Qiao, R.G. Ding, F. Li, G.Q. Lu, Z.F. Yan and H.M. Cheng Carbon 2006, 44, H. Masuda, K. Yada, and A. Osaka, Jpn. J. Appl. Phys. Part , 37, L A.-P. Li, F. Muller, A. Birner, N K.ielsch, and U. Gosele, Adv. Mater. 1999, 11,