Keunsik Leeab,
Doyoung Kimac,
Yeoheung Yoona,
Junghee Yangb,
Ho-Gyeong Yund,
In-Kyu Youd and
Hyoyoung Lee*abc
aCentre for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon 440-746, Republic of Korea. E-mail: hyoyoung@skku.edu
bDepartment of Chemistry, Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon 440-746, Republic of Korea
cDepartment of Energy Science, Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon 440-746, Republic of Korea
dInformation & Communications Core Technology Research Laboratory, Electronics and Telecommunication Research Institute (ETRI), Daejeon 305-700, Republic of Korea
First published on 6th July 2015
In order to obtain a high performance supercapacitor, there are several factors that must be achieved including a high specific surface area (SSA), high electrical conductivity, and a high diffusion rate of the electrolyte due to an appropriate pore volume. Herein, we report a high performance supercapacitor using activated non-stacked reduced graphene oxide (a-NSrGO) that has a high SSA (up to 999.75 m2 g−1) with intrinsic high graphene conductivity (1202 S m−1) and fast diffusion of the electrolyte. Due to a high total pore volume (5.03 cm3 g−1) and a wide pore size distribution from macro- to micropores (main pore width: 0.61 – 0.71 nm) in the a-NSrGO sheets, the as-prepared a-NSrGO electrode shows high specific capacitance (105.26 F g−1) and a short relaxation time (τ0 = 1.5 s) in a propylene carbonate (PC)-based organic electrolyte. A maximum energy density of 91.13 W h kg−1 and a power density of 66684.73 W kg−1 were estimated in a fully packaged coin cell. The high performance of the a-NSrGO supercapacitors is attributed to their specific appearance and enlarged pore distribution with high SSA.
Supercapacitors (or ultracapacitors) that form a double layer of electrolyte ions on the surface of conductive electrodes are considered to be a main contender for energy storage due to their high power performance, long life cycles, and low maintenance costs.7,8 Graphene is an obvious material choice for this application due its high surface area and high conductivity. Currently, graphene-based electrochemical double layer capacitor (EDLC) prototypes lead the field in capacitance, energy density, and power density because of their high electrical conductivity, accessible and defined pore structure, and low equivalent series resistance (ESR).4 However, although numerous studies have detailed the use of graphene nanosheets in supercapacitors, real device applications9 that are scalable are still in progress; many obstacles remain that still need to be overcome. To enhance the capacitive effect for a supercapacitor, several factors, including the electrical conductivity and specific surface area (SSA), have been considered. The use of graphene flakes is contrary to the use of the bulk amorphous sp3 carbon materials (e.g., activated carbons (ACs)), even when these materials have a high SSA.10 Therefore, a solution is required to provide materials with both high SSA and high electrical conductivity. In spite of efforts to increase the SSA of graphene flakes, however, there are still problems such as re-stacking or aggregation of the graphene flakes. To avoid these problems, composite materials with graphene flakes have been used to generate pseudo-capacitance causing redox reactions. However, these materials lack capacitance retention and have a low rate performance due to their demand for a redox reaction.
Recently, controlling the pore volume of graphene supercapacitor electrodes has been one of the most effective and efficient factors for enabling a fast charging/discharging diffusion performance in supercapacitors, which utilizes narrowly controlled sub-nanopores.11–13 For example, materials with only a high SSA and no control over the pore distribution are not suitable for EDLC supercapacitors, which require fast charging and discharging via fast diffusion pathways for the electrolyte. However, until now, although graphene flakes activated with KOH have been known to have high SSAs, the failure to control the pore volume of the 3D graphene electrodes has yielded materials that are insufficient, especially for EDLC supercapacitors. Therefore, in order to obtain good EDLC supercapacitors, a material must have a high SSA and electrical conductivity, while also maintaining a high pore volume in the 3D graphene flake electrodes in order to allow the electrolyte to diffuse easily.
To solve the critical problems facing EDLC supercapacitors, we report on activated non-stacked reduced graphene oxide (a-NSrGO) electrodes that have high conductivity and a high SSA, with a wide pore distribution (from macro- to micro-pores) for a best-fit energy storage performance. These materials have been carefully designed to introduce the non-stacked rGO 3D nanostructure (to maintain a high pore volume and high SSA) and also to introduce KOH-etching of the graphene flakes (to give a high SSA and many diffusion pathways for the electrolyte). The strategy for the synthesis of a-NSrGO is displayed in Fig. 1. First, we synthesized graphene oxide (GO) by a modified Hummer’s method. Then, the non-stacked graphene oxide (NSGO) was prepared via an anti-solvent method using ethanol and hexane, which we developed previously.14 Briefly, the GO sheets were dispersed in ethanol (1 mg mL−1) to allow the GO to be fully exfoliated through strong homogenization. After pouring the non-polar hexane solvent, the resulting precipitates of the non-stacked GO (NSGO) sheets were fully dried using a rotary evaporator. The NSGO was then reduced to make NSrGO through a conventional thermal reduction process. To make a-NSrGO, the NSrGO was then placed in a 7 M KOH solution, followed by filtration and drying. The resulting mixture was put in a tube furnace under the flow of an inert gas at 100 sccm and heated to 800 °C for 1 h. Finally, a-NSrGO was obtained after washing the unreacted KOH, and many types of salt and metal ions, with pure water until the pH reached 7. The chemical reaction of the activation process with KOH has already been suggested: 6KOH + C ↔ 2K + 3H2 + 2K2CO3.15,16
The evolution of the changes in the real (C′) and imaginary (C′′) parts of the capacitance versus frequency were plotted from the following equations: C′(ω) = −Z′′(ω)/ω|Z (ω)|2 and C′′(ω) = Z′(ω)/ω|Z (ω)|2, where, ω is the pulsation, C′(ω) is the real part of the capacitance, C′′(ω) is the imaginary part of the capacitance, Z(ω) is the electrochemical impedance, and Z′(ω) and Z′′(ω) are the real and imaginary parts of the impedance, respectively, defined as Z′2 + Z′′2 = |Z(ω)|2.
The energy density and power density of the electrodes were calculated using the following equations: E = (1/2 × 1000/3600) × Cs (ΔV)2 and P = (Vmax − Vdrop)2/4(RESR)m, where Cs (F g−1) is the specific capacitance of the electrode, ΔV (V) is the potential range, E (W h kg−1) is the energy density, P (W kg−1) is the power density, m (g) is the weight of the electrode materials (two electrodes), and the effective series resistance (ESR) is estimated by using the voltage drop at the beginning of the discharge (Vdrop) at a certain constant current Icons (A) with following formula: RESR = Vdrop/(2Icons).
Raman spectroscopy confirmed the synthesis and structural characterization of the a-NSrGO, as shown in Fig. 3a. The D bands of the a-NSrGO appeared at 1349 cm−1. The D band is due to the A1g symmetry mode caused by breathing vibrations of six-member sp2 carbon rings, and only becomes active in the presence of disorder.18 Additionally, the G band of the a-NSrGO was observed at 1583 cm−1. The G band has E2g symmetry, which involves the in-plane bond-stretching motion of pairs of carbon sp2 atoms. This band does not require the presence of six-member rings; it occurs at all sp2 sites as opposed to only those in the rings.19 The integrated D band to G band ratio (ID/IG) slightly increases from 1.02 in NSrGO to 1.20 in a-NSrGO, due to the etching during the activation process (Fig. S3†).
The a-NSrGO was also characterized with X-ray photoelectron spectroscopy in order to determine its structure (Fig. 3b). Primary C1s peaks appear at binding energies of 283.1 eV and 284.5 eV, which are assigned to sp2 carbon bonds (C–C/CC). There are other weak peaks at 285.7 eV, 287.1 eV, and 288.3 eV, which can be identified as the sp3 carbon bonds of the hydroxyl carbon peak (C–O), carbonyl peak (C
O), and carboxyl peak (C(O)O), respectively. This indicates that the a-NSrGO is dominated by sp2 conjugated carbon. Furthermore, the C/O ratio was 15.11, which means that the obtained a-NSrGO is mainly composed of carbon atoms. The XPS data of GO and NSrGO are shown in Fig. S4.† Due to the presence of oxygen functional groups, GO has a relatively strong peak at 287.1 eV compared to the carbonyl (C
O) sp3 carbon bond. However, the NSrGO shows a strong peak (similar to the a-NSrGO), indicating that the NSrGO is composed of carbon atoms and that the a-NSrGO also maintains a carbon-dominant composition despite the KOH activation process.
The porosity was analyzed by N2 gas adsorption–desorption measurements with advanced methods based on the Brunauer–Emmett–Teller (BET) theory for specific surface area, the Barrett–Joyner–Halenda (BJH) method for wide pore distribution, and the Horvath–Kawazoe (HK) method for sub-nanopores (pore width < 1 nm). The HK method is capable of generating model isotherms more efficiently than density functional theory (DFT) to characterize the pore size distribution of microporous materials.20 Fig. 3c shows N2 sorption isotherms with a pronounced hysteresis in the P/P0 range (0.45 to 1.0), indicating the presence of a large number of mesopores the in a-NSrGO.21 The BET calculation implied that the specific surface area of the a-NSrGO is 999.75 m2 g−1, which is higher than that of the KOH-untreated NSrGO (670.65 m2 g−1). In addition, the total pore volume of the a-NSrGO (5.03 cm3 g−1) is also much higher than that of the untreated NSrGO (4.11 cm3 g−1).14 Although the a-NSrGO has a higher total pore volume, it shows a lower volume than the NSrGO at the same weights (Fig. S5†). Therefore the a-NSrGO has a high packing density, which means that it can show a higher performance aspect of volumetric capacitance. The plot of the BJH method (blue line in Fig. 3d) shows the wide pore size distribution of the a-NSrGO (from mesopore to macropore), verifying that the maintained crumpled structure of the a-NSrGO can provide effective diffusion pathways for the electrolyte ions to enter the micropores. Furthermore, the HK plot (red line in Fig. 3d) shows the sub-nanopores (micropores) of the a-NSrGO, where the main pore width is between 0.61 and 0.71 nm, and a wide range of pores from mesopores to macropores. The electrical conductivity of the NSrGO and a-NSrGO were also characterized to clearly compare the aspect of material performance. Although the NSrGO and a-NSrGO did not include sub-conductive materials such as carbon black, the NSrGO and a-NSrGO show high electrical conductivities of 1209 S m−1 and 1202 S m−1, respectively. More importantly, the a-NSrGO still maintained its high electrical conductivity after the KOH activation process (Table S1†). As a result, the crumpled and etched pores of the a-NSrGO allow for the easy accessibility of electrolyte ions into the macropores, mesopores, and sub-nanopores.
To further confirm the supercapacitor behavior, electrochemical impedance spectroscopy (EIS) measurements were performed in a frequency range from 100 kHz to 10 mHz with an amplitude of 10 mV AC. Fig. S6† shows the Nyquist plot of a-NSrGO. The plot features a vertical curve, indicating the nearly-capacitive performance of the cell. The inset of Fig. S6† is a magnified view of the high frequency range. A Nyquist plot for an electric double layer capacitor is analyzed by looking at the high frequency semicircle, which is caused by the effective series resistance (RS). This is attributed to the ionic conductivity between the electrode materials and electrolyte.22 The a-NSrGO showed very low RS (1.76 Ω), representing the low intrinsic internal resistance of the electrode materials and electrolyte of the supercapacitor cells.23–25 A transition between the RC semicircle and the migration of the electrolyte was observed at a frequency of 146 Hz, which corresponds to a charge transfer resistance (RCT) of 14.83 Ω. The KOH-untreated NSrGO, used as a control, was also characterized by electrochemical measurements for comparison with the a-NSrGO. The electrochemical information of the NSrGO electrode is also shown in Fig. S7 and S8.† Although the NSrGO showed a rectangular shape in its CV and a symmetric triangle shape in its galvanostatic charge–discharge curves, similar to a-NSrGO, the a-NSrGO (in comparison with the NSrGO) showed a larger area in its CV curve and a longer discharge time (124.7 s at a current density of 0.5 A g−1) in its galvanostatic charge–discharge curves, indicating that the a-NSrGO exhibited a superior energy storage performance.
To consider the supercapacitor as a whole by using the impedance data, Fig. 4d presents the real part of the capacitance (C′) change (filled circle, black line) and the imaginary part of the capacitance (C′′) change (open circle, red line) versus frequency for a-NSrGO. When the frequency decreases, the C′ significantly increases and then tends to be less frequency-dependent. This is related to the electrode structure and the interface between the electrode and electrolyte.26,27 However, the C′′ curve shows a peak at frequency f0, describing the dielectric relaxation time constant as τ0 = 1/f0 = 1.5 s. τ0 is the minimum time needed to discharge all of the energy from the cell with an efficiency greater than 50%.28 Surprisingly, the a-NSrGO shows a short relaxation time in spite of using a PC-based organic electrolyte, which has a higher viscosity and dielectric constant than an acetonitrile (ACN) electrolyte (activated carbon supercapacitor using PC for τ0 = 48 s and ACN for τ0 = 10 s).26 Commonly, PC electrolyte-based cells are more attractive compared to ACN-based cells due to their stable, non-toxic, and non-volatile properties, especially for commercial applications. However, PC-based cells typically exhibit a slower charging–discharging performance than the ACN-based cells.29 Finally, in Fig. 5a, the rectangular CV area of the a-NSrGO is two times larger than that of the NSrGO. This verifies the fact that the specific capacitance of a-NSrGO is two times higher than NSrGO for energy storage performance in PC-based cells and operates two times faster up to the same capacitance. Furthermore, due to its short relaxation time (1.5 s), the a-NSrGO also demonstrates high power performance.
In the Ragone plot (Fig. 5b), a maximum energy density of 91.13 W h kg−1 for a-NSrGO was estimated in the fully packaged coin cell, which is much higher than the maximum energy density of 49.31 W h kg−1 for NSrGO. The maximum power density of a-NSrGO (66684.73 W kg−1) was also a higher value than that of NSrGO (46
512.73 W kg−1). Supercapacitors contain current collectors, electrolytes, separators, binders, and packing. Because the carbon weight accounts for about 30% of the total mass (30 wt%) of packaged commercial supercapacitors, a factor of 1/3 to 1/4 is frequently used to extrapolate the energy density and power density of the device from the performance of materials.30 To extrapolate the energy density and power density values in the fully packaged cell, we assumed that the carbon material weight accounts for about 30 wt% of the total mass of the packaged commercial cell.21,30 The normalized maximum energy density and power density of a-NSrGO were 30.38 W h kg−1 and 22
228.24 W kg−1, respectively. These are much higher than those of the current state-of-the-art commercial supercapacitors.31 As a result, the superior performances of the a-NSrGO, including a high pore volume, SSA, and electrical conductivity, can synthetically provide high power performance as well as high energy storage effects, which were higher than previously reported energy and power density values of crumpled structure carbon materials (Table S2†).
Footnote |
† Electronic supplementary information (ESI) available: Detailed experimental methods, figures relating to the characterization of used GO, NSrGO, and a-NSrGO with electrochemical properties. See DOI: 10.1039/c5ra10246d |
This journal is © The Royal Society of Chemistry 2015 |