An Ideal Electrode Material, 3D Surface-Microporous Graphene for Supercapacitors with Ultrahigh Areal Capacitance

Transcription

1 Supporting Information An Ideal Electrode Material, 3D Surface-Microporous Graphene for Supercapacitors with Ultrahigh Areal Capacitance Liang Chang, 1 Dario J. Stacchiola 2 and Yun Hang Hu 1, * 1. Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI , USA 2. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA * yunhangh@mtu.edu S-1

2 Content 1. Experimental section 2. Characterization of SMG 3. Characterization of activated carbon 4. Calculation of average micropore deepness of SMG 5. Different reaction condition influence on materials characterization and its corresponding electrochemical performance 6. Electrochemical performance of SMG in three-electrode configuration 1. Experimental section 1.1 Synthesis of 3D surface-microporous graphene Metal Na cubes were loaded into a ceramic tube reactor and 50 psi CO 2 was introduced into the reactor at room temperature. Then, the reactor was heated to 520 o C and the temperature was maintained for 12 h. Finally, the obtained products were treated with 37 wt% HCl, followed by deionized water washing for several times until PH=7 and drying at 80 o C oven for overnight. The obtained carbon materials are donated as surface-microporous graphene (SMG). 1.2 Characterization of 3D surface-microporous graphene (SMG) The structure of SMG was evaluated by Field emission scanning electron microscope (FESEM, Hitachi-4700) with energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM, JEOL JEM2010F). X-ray diffraction (XRD) was used to determine the crystal structures of solid products with a Scintag S-2

3 XDS-2000 powder diffract meter (Cu Kα (λ= Å)). A Jobin-Yvon LabRAM HR800 Raman Spectrometer with a helium-neon laser operating at nm in air condition was utilized to get Raman spectrum. Pore size distribution and surface area of SMG were measured by N 2 adsorption/desorption at liquid nitrogen temperature (77 K) with ASAP 2000 instrument. The composition and functional groups of SMG were characterized by Fourier transform infrared (FTIR) spectra (a Perkin Elmer Spectrum One with KBr pellets method) and X-ray photoelectron spectroscopy (XPS, a Kratos Ultra AXIS DLD XPS with a monochromated Al source). 1.3 Fabrication of electrodes and performance test of electric double-layer capacitors The SMG electrode was fabricated as follows: SMG, carbon black (conductive additive), and poly(tetrafluoroethylene) (binder) were fully mixed with 80:10:10 weight ratio in isopropyl alcohol to form a homogeneous paste. Then the paste was rolled to a rectangular strip and pressed on 1x1 cm 2 nickel foam (as current collector). The obtained electrode was dried in 80 o C oven for 24 h to get rid of organic solvent of binder and isopropyl alcohol. For comparison purpose, the same procedure was exploited to fabricate the activated carbon electrode with activated carbon (AC) (Aldrich Chemical Company, Inc). The electric double-layer performance was tested with symmetrical two-electrode configuration, in which the SMG electrodes (or AC electrodes) were used as both cathode and anode, a glassy microfiber filter (Whatman, GF/F) as separator located between the electrodes, and 2 M KOH aqueous solution as electrolyte. The assembled supercapacitor cells were subjected to cyclic voltammetry (CV) tests and S-3

4 galvanostatic charge/discharge measurements in the potential range of 0~1 V with electrochemical workstation (Princeton Potentiostat/Galvanostat Model 273A). The scan rates of CV curves were 30~300 mv/s, and the current density of charge/discharge profiles were 0.5 ~10 A/g. Electrochemical impedance spectroscopy (EIS) was conducted at frequency of 0.01~10 5 Hz with three-electrode configuration, in which working electrode, counter electrode, and reference electrode are SMG electrode (or AC electrode), Pt electrode, and saturated Hg/Hg 2 Cl 2 electrode, respectively. 2. Characterization of SMG Figure S1. Illustration of reaction: graphene formation with Na and CO 2 reaction, and simultaneously pore-creation-reaction between graphene and CO 2. S-4

5 Figure S2. (A) SEM image and (B) TEM image of SMG. Figure S3. (A) Raman spectrum, and (B) pore size distribution of SMG. Figure S4. AFM image of SMG with section analysis. S-5

6 Figure S5. Three galvanostatic charge/discharge cycles at current density of 0.5 A/g. Figure S6. Characterization of 3D SMG: (A) XPS spectra and (B) FTIR spectrum. 3. Characterization of activated carbon Figure S7. Characterization of activated carbon, (A) N 2 adsorption/desorption curves, (B) pore size distribution, (C) and (D) SEM images. S-6

7 Activated carbon (AC, ~100 mesh) was purchased from Aldrich Chemical Company, Inc. Its surface area and pore size distribution were measured with N 2 adsorption at 77 K. In Figure S7A, the N 2 adsorption/desorption curves present type II isotherm with a type H3 hysteresis loop. The surface area calculated from BET model is 853 m 2 /g in which 500 m 2 /g is from micropore areas, indicating AC is a microporous material. The pore size distribution shown in Figure S7B further demonstrated that the pore sizes of the most pores are smaller than 5 nm. Microstructure of AC was shown by FESEM images (Figure S7C and S7D). 4. Calculation of average micropore-deepness of SMG The theoretic surface area (S theory ) of one obtain graphene sheet (consisting of 6 graphene layers) without any pores can be calculated as S theory = = 438(m 2 /g) (S1) When micropores are considered as cylinder pores, the wall surface area (excluding bottom areas) of the micropores in graphene sheets can be calculated as S wall = S total S theory = = 452(m 2 /g) (S2) The external surface area (S ex ) of graphene sheets, which includes the surface area of meso/macro channels, can be calculated as S ex = S total S micropores = = 101(m 2 /g) (S3) The average diameter (D) of micropores is 1.6 nm. Furthermore, we can also express S ex as S-7

8 S ex = S theory nπ( D 2 )2 (S4) where n is the number of micropores in a gram graphene. Therefore n = S theory S ex π( D 2 )2 = (S5) 3.14( ) 2 = (1/g) The introduction of S wall, n, h, and D values into Equation 1 gives the average deepness (h) of micropores as h = 0.54 (nm) (S6) 5. Different reaction condition influence on materials characterization and its corresponding electrochemical performance To determine the optimal reaction condition, we investigated the reaction temperature influence on obtained materials. Except for reaction at 520 o C for 12 h, carbon materials can be obtained at 500 o C and 550 o C for 12 h, which were denoted as 500 o C 12 h sample and 550 o C 12 h sample, respectively. As shown in Figure S8, 500 o C 12 h sample presented a porous carbon nanowalls cluster, and 550 o C 12 sample exhibited a flower-like graphene structure. The surface information and pore size distribution was collected by N 2 adsorption/desorption curves at 77 K (Figure S9). As reaction temperature increased from 500 o C to 520 o C and then 550 o C, surface areas from BET model were increased from m 2 /g to m 2 /g and then 1087 m 2 /g. The micropore volumes were 0.26 cc/g, 0.36 cc/g and 0.44 cc/g for 500 o C 12 h sample, 520 o C 12 h sample and 550 o C 12 h sample, respectively. Those sample s electrochemical performances were conducted with symmetrical two-electrode S-8

9 configuration in 2 M KOH (Figure S10 and S11). 500 o C 12 h sample possessed a rectangular CV shapes at scan rate of 50 and 100 mv/s and suffered a polarization at 200 mv/s. Similarly, its galvanostatic charge/discharge profile with triangle curves experienced IR drop increase along with current densities. Besides, the rate performance (current densities from 0.5 A/g to 10 A/g) and cycling performance (5000 cycles at current density of 10 A/g) exhibited a capacitance drop. In contrast, 550 o C 12 h sample with relative low capacitance but good capacitance retention reflected by ideal CV curves, triangle charge/discharge profile, good rate performance and excellent cycling stability. The difference of electrochemical performance for these two samples can attribute the synergistic effect of morphology, pore distribution, surface area, chemical stability. Figure S8. SEM images of (A) 500 o C 12 h sample and (B) 550 o C 12 h sample. S-9

10 Figure S9. N 2 adsorption/desorption curves at 77 K and it pore size distribution for (A, B) 500 o C 12 h sample and (C, D) 550 o C 12 h sample. S-10

11 Figure S10. Electrochemical performance of 500 o C 12 h sample: (A) CV curves at scan rate of 50~200 mv/s, (B) galvanostatic charge/discharge profiles at current densities of 0.5~10 A/g, (C) current densities vs specific capacitance, (D) cycling performance for 5000 cycles. S-11

12 Figure S11. Electrochemical performance of 550 o C 12 h sample: (A) CV curves at scan rate of 50~200 mv/s, (B) galvanostatic charge/discharge profiles at current densities of 0.5~10 A/g, (C) current densities vs specific capacitance, (D) cycling performance for 5000 cycles. 6. Electrochemical performance of SMG in three-electrode configuration The electrochemical performance of SMG electrode was conducted in three-electrode configuration with a Pt electrode as counter electrode and a Hg/Hg 2 Cl 2 electrode in saturated KCl as a reference electrode. The galvanostatic charge/discharge test was performed in the potential range of -0.2~ -1 V at current density from 0.5 A/g to 20 S-12

13 A/g. As shown in Figure S12, the specific capacitance can be F/g at 0.5 A/g, F/g at 10 A/g and 145 F/g at 20 A/g. Figure S12. Rate performance of SMG electrode in three-electrode configuration S-13