CMK-5-Based High Energy Density Electrical Double Layer Capacitor for AC Line Filtering.

Compact electrical double layer capacitors (EDLCs) can be applied to the AC line filtering process and potentially replace conventional bulky aluminum electrolytic capacitors. However, to realize the AC line filtering application, the energy density of the EDLCs needs to be significantly increased while high power density is maintained. In this work, we demonstrate the EDLCs fabricated with ordered mesoporous carbon, CMK-5, and small amounts of single walled carbon nanotubes (SWNTs), which exhibit the highest areal energy density among various EDLCs reported so far and power performance sufficient for AC line filtering. High energy density of CMK-5/SWNT EDLCs can be attributed to high capacitance arising from the bimodal mesoporosity of CMK-5 and high operation voltage owing to the pores compatible with ion sizes in organic electrolytes. High power density is ascribed to the high electrical conductivity and straight ordered pore structure of CMK-5, enabling facile ion movements. Therefore, the carbon pore structure is one of the critical factors to be controlled for the enhancement of the overall performance of EDLCs for AC line filtering. Our demonstration would greatly contribute to the scientific and technological advancement of AC line filtering EDLCs.

Introduction

Electrical double layer capacitors (EDLCs) or supercapacitors have applications in a variety of technological areas such as consumer electronics, transportation vehicles, regenerative braking systems, medical devices, and microgrids.[1−3] The performance of supercapacitors bridges the gap between those of batteries and electrolytic capacitors. Supercapacitors typically show 10 to 100 times higher power density than batteries, while the energy density is lower by a similar magnitude.[4,5] Hence, the enhancement of the energy density of supercapacitors has traditionally been the primary focus to complement or replace batteries.[6−8] More recently, demands for the enhancement of power density have increased because of new potential applications such as substitution of bulky electrolytic capacitors with compact supercapacitors, which contribute to the miniaturization of electronic devices.[9,10] One of the important applications of electrolytic capacitors is alternating current (AC) line filtering, which requires high power performance. However, the power performance of supercapacitors based on conventional activated carbon significantly falls behind that of electrolytic capacitors.[9,11]

Recently, the power performance has been greatly improved with new carbon electrode materials, and it has been demonstrated that supercapacitors can indeed be applied for AC line filtering. For example, supercapacitors with vertically oriented graphene nanosheets, electrochemically reduced graphene oxides, and vertically aligned carbon nanotubes have shown sufficiently fast responses and maintained their capacitive behaviors against 120 Hz AC signals.[9,12−16] Their high power performances are attributed to their open pore structures allowing facile access of ions; however, their macropores are unnecessarily large, which lowers the density of electrode materials and hence the capacitance. Subsequently, it was demonstrated that supercapacitors with a compact form of mesoporous carbon such as carbon nanotubes, mesoporous CMK-3, and Ketjen black (KB) can also respond sufficiently quickly to 120 Hz signals.[17−20] However, despite significant improvement in the power performance of supercapacitors, they are applicable only to low voltage AC line filtering. To extend their application to high voltage AC line filtering, which has a much larger market than low voltage AC line filtering, the energy density needs to be significantly improved while high power density is maintained. Therefore, it is indispensable to explore carbon materials with various pore sizes, structures, and distributions, which is one of the most critical factors influencing the capacitance and energy density of supercapacitors.[21,22]

In this work, we demonstrate that supercapacitors with ordered mesoporous carbon, CMK-5, exhibit high areal energy density with high power performance sufficient for AC line filtering. CMK-5 is chosen because of its high specific surface area, electrical conductivity, bimodal mesoporosity, and straight pore structure.[23−25] A small amount of single walled carbon nanotubes (SWNTs) (10 wt %) acts as both the conductive additive and binder. Compared to CMK-3, which has been previously demonstrated to be an excellent electrode material for high power applications, CMK-5 has an additional straight pore structure.[25−27] This provides increased surface area for charge storage while maintaining facile ion accessibility. Moreover, the mesopores of CMK-5 exhibit high compatibility with organic electrolyte ions leading to an operation voltage of 2.8 V. The unique pore structure of CMK-5 enables high capacitance and high operation voltage and simultaneously allows a sufficiently fast response speed for AC line filtering.

Results and Discussion

SBA-15 is mesoporous silica with a hexagonal array of straight pores and used as a sacrificial template for CMK-5 (Figure a).[28] The straight mesopores are connected side-to-side by micropores. The pitch and diameter of the mesopores are 9.9 ± 0.4 and 6.1 ± 0.6 nm, respectively. Transmission electron microscopy (TEM) images, a Brunauer–Emmett–Teller (BET) isotherm, and a low-angle X-ray diffraction (XRD) pattern of the SBA-15 are presented in Figure S1.[28−30] Furfuryl alcohol (FA) is a liquid organic compound, which easily infiltrates into the mesopores and micropores of the silica template. Once the template is filled with FA, a multistep heat treatment polymerizes and carbonizes the FA (Figure b). As a result, carbon tubes are formed inside the mesopores of SBA-15.[23,26,31] The correct amount of FA is required to form hollow carbon tubes instead of solid carbon rods.[27] Finally, SBA-15 is removed by diluted aqueous hydrofluoric acid (HF) leading to hexagonally arrayed carbon tubes (Figure c). Ions in electrolytes have facile access to both the inside and outside of the straight carbon tubes arranged hexagonally (Figure d).[32,33]

Figure 1

(a) SBA-15 silica template with hexagonally arranged straight pores. (b) Infiltration and carbonization of furfuryl alcohol in the SBA-15 template. (c) Carbon inverse replica (CMK-5) remains after SBA-15 removal. (d) Facile ion accessibility to the entire surface of CMK-5.

Electron microscopy reveals the morphology of a CMK-5/SWNT thin film and the detailed pore structure of a CMK-5 particle. Scanning electron microscopy (SEM) images show a uniform CMK-5/SWNT thin film prepared on a current collector (Figure a,b). The CMK-5 particles have rod shapes with average width and length of 0.43 ± 0.08 and 1.20 ± 0.21 μm, respectively (the number of samples = 100). SWNTs are well-dispersed and form a solid electrical network in CMK-5/SWNT electrode materials (Figure b). SWNTs are ideal to connect individual CMK-5 particles because they are highly conductive and mechanically flexible.[34,35] They also provide high specific surface area for charge storage and play the role of a binder.[19,20] Mass loading of a CMK-5/SWNT thin film is determined by the amount of CMK-5/SWNT used and hence can easily be controlled. TEM images clearly show the straight tubular structure of CMK-5. The top view of a CMK-5 particle shows the hexagonal array of hollow carbon tubes with pitch and inner diameter of 9.6 ± 0.4 and 2.8 ± 0.6 nm, respectively (Figure c). The wall thickness of the carbon tubes is estimated to be 2.6 ± 0.4 nm. A lateral view consistently shows both the straight inner pores of the tubes (indicated by white arrows) and the pores between the tubes (Figure d). Low-angle XRD confirms the ordered structure of the CMK-5 (Figure S2).[23]

Figure 2

(a) Low- and (b) high-magnification scanning electron microscope (SEM) images of a 30 μg/cm2 CMK-5/single walled carbon nanotube (SWNT) film (CMK-5:SWNT = 9:1) on a Au/Ti/Al foil. Transmission electron microscope (TEM) images of the (c) front (pitch = 9.6 ± 0.4 nm, inner diameter = 2.8 ± 0.6 nm, and wall thickness of the nanotubes = 2.6 ± 0.4 nm) and (d) lateral side of CMK-5.

The nitrogen adsorption–desorption isotherm of CMK-5 powders is of type-IV with a hysteresis loop at relative pressures of 0.4–0.6, indicating the mesoporosity of CMK-5 (Figure a).[36,37] Specific surface area of CMK-5 is estimated to be 1483 m2/g, which is approximately 1.5 times higher than that of CMK-3.[19] CMK-5 has bimodal pore size distribution with two peaks at 3.2 and 4.2 nm corresponding to the inner pores and spacing between the carbon tubes, respectively (Figure b). The XRD peaks at around 26 and 44° and Raman spectrum with the G-band and D-band reflect the graphiticity of CMK-5 (Figure c,d).[38,39] X-ray photoelectron spectroscopy (XPS) indicates that the ratio of carbon and oxygen is approximately 96:4 (Figure e). The deconvolution of the C 1s peak shows the presence of sp2 carbon in C=C, sp3 carbon in C–C, carbonyl groups, carboxyl groups, and carbonates (Figure f).

Figure 3

(a) N2 adsorption/desorption isotherms and (b) Barrett–Joyner–Halenda (BJH) pore size distribution of CMK-5 (average pore size = 3.3 nm, peak positions = 3.2 and 4.2 nm). (c) X-ray diffraction (XRD) pattern, (d) Raman spectrum, (e) X-ray photoelectron spectroscopy (XPS) survey spectrum, and (f) C 1s peak of CMK-5 powder.

CMK-5/SWNT supercapacitors exhibit excellent performance in terms of both energy and power characteristics. Cyclic voltammograms measured at a scan rate of 100 V/s are rectangular indicating ideal EDLC type behaviors (Figure a). There exists a linear relationship between the current density and scan rate over a wide range of scan rates indicating the excellent power performance (Figure b). The supercapacitors with 20 and 30 μg of CMK-5/SWNT show linear relationships up to 500 V/s. As the mass loading increases, the behavior slightly deviates from ideal linear relations. For instance, the supercapacitor with 50 μg of CMK-5/SWNT starts to deviate at a scan rate of approximately 350 V/s. Galvanostatic charge–discharge (GCD) curves also indicate that CMK-5/SWNT supercapacitors behave as ideal EDLCs (Figure c). GCD curves measured at 1 mA/cm2 are triangular without any noticeable IR drop. The capacitance increases from 910 to 2065 μF/cm2 as the mass loading increases from 20 to 50 μg. Furthermore, the high capacitance values are well maintained even at high current densities (Figure d). For example, the capacitance of the 30 μg supercapacitor reduces only by 9.5% from 1236 to 1119 μF/cm2, while the current density increases by an order of magnitude from 1 to 10 mA/cm2.

Figure 4

Electrochemical properties of a CMK-5/SWNT supercapacitor (mass loading = 20, 30, 40, and 50 μg/cm2). (a) Cyclic voltammograms (CVs). (b) A relationship between current density and CV scan rate. (c) Galvanostatic charge–discharge (GCD) curves measured at 1 mA/cm2. (d) Areal capacitance (Careal) estimated from the GCD curves at various current densities.

The CMK-5/SWNT supercapacitor exhibits 30% higher capacitance than the previously reported CMK-3/SWNT supercapacitor with similar mass loading (1236 vs 963 μF/cm2 at 1 mA/cm2).[19] The enhanced capacitance can be attributed to the higher specific surface area of CMK-5. While CMK-3 is composed of carbon rods, CMK-5 consists of carbon tubes. The inner pores of CMK-5 provide additional surface area for charge accumulation. Considering that CMK-5 has approximately 50% higher surface area than CMK-3 according to BET analysis, the surface area of pores accessible by ions in the electrolyte is less than that of pores accessible by N2 gas. Some portions of the pores may be approached only by smaller N2 molecules but not by the larger electrolyte ions, probably due to some irregularities in the CMK-5 pore structure.

The response of the CMK-5/SWNT supercapacitor is faster with lower mass loading of the electrode materials. The negative phase angle of the 20 μg CMK-5/SWNT supercapacitor is 77.3° at 120 Hz enabling AC line filtering applications (Figure a). As the mass loading increases from 20 to 50 μg, the phase angle reduces from 77.3 to 70.2°. Consistently, the supercapacitor shows smaller time constants for lower mass loadings as indicated by the peak positions in Figure b. On the other hand, the capacitance increases from 512 to 1331 μF/cm2 at 120 Hz as the mass loading increases from 20 to 50 μg (Figure c). The increase in the capacitance with respect to the mass loading is due to the increased surface area, while the reduction in the response speed and the magnitude of phase angle is due to the increased distance that the electrolyte ions have to travel during charging and discharging processes.[20] The results show that there is a tradeoff relationship between power performance and capacitance.

Figure 5

Frequency responses of CMK-5/SWNT supercapacitors in a two-electrode configuration with different mass loadings (20, 30, 40, and 50 μg/cm2). (a) Bode-phase plots. (b) Plots of the imaginary part (C″) of capacitance vs frequency. (c) Plots of areal capacitance vs frequency (Careal at 120 Hz = 713 μF/cm2 for 30 μg/cm2). (d) Nyquist plots. The inset shows equivalent series resistances (ESRs).

Equivalent series resistances (ESRs) of the supercapacitors are all near 0.3 Ω as shown in the Nyquist plots (Figure d). Overall, the CMK-5/SWNT supercapacitor exhibits comparable power performance to the CMK-3/SWNT supercapacitor yet with much higher capacitance. The inner pores of CMK-5 are straight, allowing sufficiently fast movement of the electrolyte ions into the pores. The properties of the supercapacitors with various mass loadings are summarized in Table . Electrochemical impedance spectra of the supercapacitor are well described with the equivalent circuit in Figure S3a. Fitting results exhibit the capacitance and resistance values that are consistent with those obtained experimentally. In addition, we note that self-discharge is not a critical factor because the timescale of the self-discharging process is much greater than that of 120 Hz AC line filtering (minutes vs milliseconds) (Figure S3b).

Table 1

Properties of CMK-5/SWNT Electrical Double Layer Capacitors (EDLCs) with Different Mass Loadings

mass loading (μg/cm2)	Careal,0.1Hz (μF/cm2)	Careal,120Hz (μF/cm2)	–Φ120Hz (°)	ESR (Ω cm2)	Eareal,120Hz (μW h/cm2)	τRC,120Hz (ms)	τ0 (ms)
20	812	512	77.3	0.34	0.44	0.301	0.173
30	1096	713	77.0	0.28	0.62	0.307	0.199
40	1397	909	74.4	0.26	0.79	0.371	0.240
50	1852	1331	70.2	0.31	1.16	0.478	0.422

AC line filtering has been successfully demonstrated with an electrical circuit equipped with a CMK-5/SWNT supercapacitor (Figure a). First, an AC input signal (±3.25 V, 60 Hz), generated using an oscilloscope, is applied to the circuit, as shown in Figure b. Subsequently, the AC signal is rectified by a full bridge rectifier composed of four diodes into a pulsating DC signal ranging from 0 to 2.48 V at 120 Hz (Figure c). Finally, the pulsating DC signal is smoothed into a constant DC of 2.25 V by the supercapacitor in parallel with a resistor (Figure d). The resulting output DC signal is constant without ripples demonstrating the excellent AC line filtering function. In addition, the CMK-5/SWNT supercapacitor exhibited excellent cyclability. The capacitance of the supercapacitor decreased by 10.7% over 10000 cycles (Figure a). Most of the reduction occurred over the first couple of thousand cycles and then gradually decreased. GCD curves at the 1st, 5000th, and 10000th cycles are presented in Figure b.

Figure 6

(a) Electrical circuit diagram of alternating current (AC) line filtering. (b) An AC input signal (60 Hz, Vpeak = ±3.25 V). (c) A rectified pulsating direct current signal (V = 0–2.48 V, 120 Hz). (d) A constant DC output (∼2.25 V, 1 kΩ).

Figure 7

(a) Cycling stability of a CMK-5/SWNT supercapacitor and (b) GCD curves at different cycles (constant current = 10 mA/cm2).

Finally, the operation voltage window of CMK-5/SWNT supercapacitors can be extended to 2.8 V by implementing asymmetric current collectors. The CMK-5/SWNT deposited on a Au/Ti/Al current collector has a stability window between −1.9 and 0.8 V (vs Ag/Ag+), while the CMK-5/SWNT deposited on a C/Al current collector is stable between −1.6 and 1.0 V (vs Ag/Ag+) as previously demonstrated with KB.[20] With this asymmetric configuration, the operation voltage window of the supercapacitor is extended to 2.8 V, which is greater by 0.3 V than that of the supercapacitor with symmetric current collectors. Rectangular cyclic voltammograms measured at high scan rates over 2.8 V are displayed in Figure a (Ccell = 1247 μF/cm2 at 50 V/s). Triangular galvanostatic charge–discharge curves with negligible IR drops are shown in Figure b. Nyquist plot indicates that the EDLC is highly capacitive and has low ESR (∼0.28 Ω) (Figure c). The Bode-phase plot shows a high negative phase angle at 120 Hz (∼76.1°) (Figure d). Both the high capacitance (782 μF/cm2 at 120 Hz) and wide operation voltage window (2.8 V) impart CMK-5/SWNT supercapacitors with the highest areal energy density (0.85 μW h/cm2 at 120 Hz) among EDLCs developed to date, which have been fabricated with various carbon electrode materials such as CMK-3, KB, carbon nanofibers, carbon nanotubes, and graphenes, for AC line filtering applications.[17,19,20,40−44] Various EDLC properties including areal energy density are listed in Table . Our demonstration would be an important cornerstone for the development of high-power EDLCs and extension of their applications.

Figure 8

Electrochemical properties of a CMK-5/SWNT supercapacitor with asymmetric current collectors (mass loading = 30 μg/cm2). (a) CVs measured at several scan rates. (b) GCD curves measured at various current densities. (c) A Nyquist plot. (d) A Bode-phase plot.

Table 2

Performance Comparison of Various EDLCs for the AC Line Filtering Applicationa

electrode material	Careal,120Hz (μF/cm2)	–Φ120Hz (°)	ESR (Ω cm2)	Eareal,120Hz (μW h/cm2)	voltage window (V)	τRC,120Hz (ms)	ref
CMK-5/SWNT	782	76.1	0.28	0.852	2.8	0.322	this work
KB/CNT	574	80.0	0.28	0.625	2.8	0.240	(20)
CBC (6 M KOH)	4500	∼80.0	0.009	0.506	0.9	N/A	(40)
CBC (1 M TEABF4 in AN)	510	80.0	∼0.10	0.443	2.5	N/A
CMK-3/CNT	559	80.3	0.25	0.485	2.5	0.228	(19)
EOG/CMF	490	80.4	0.45	0.425	2.5	N/A	(41)
pristine carbon nanostructure	444	80.0	∼1.5	0.385	2.5	N/A	(42)
B-doped carbon nanostructure	386	80.0	∼1.5	0.335	2.5	N/A
SWNT	282	82.2	0.26	0.245	2.5	0.181	(17)
EOG/CCP	1500	83.0	0.04	0.169	0.9	N/A	(43)
EOG/CNF	160	80.0	0.28	0.139	2.5	N/A	(44)

KB/CNT: Ketjen black/carbon nanotube; CBC: carbonized cross-linked carbon nanofiber aerogel derived from pyrolysis of bacterial cellulose; TEABF4: tetraethylammonium tetrafluoroborate; AN: acetonitrile; CMK-3/CNT: carbon mesostructured by KAIST-3/carbon nanotube; EOG: edge-oriented graphene; CMF: carbonized cellulose microfiber; SWNT: single walled carbon nanotubes; CCP: carbonized cellulose paper; CNF: encircling carbon nanofiber.

Conclusions

We demonstrated that CMK-5/SWNT EDLCs exhibit excellent performance for AC line filtering. CMK-5 has bimodal mesopores among and inside the carbon tubes, which are hexagonally arranged with pitch and inner diameter of 9.6 and 2.8 nm, respectively. The bimodal mesopores lead to high energy density by providing high specific surface area for high capacitance and compatibility with organic electrolytes for a wide operational voltage window. Moreover, the straight pore structure enables fast ion movement during charging and discharging processes. CMK-5/SWNT thin films are prepared via a vacuum filtration technique and used as electrodes. As the mass loading increases (20 to 50 μg per electrode), the cell capacitance measured at 120 Hz increases from 512 to 1331 μF/cm2 while the magnitude of the phase angle decreases from 77.3 to 70.2°. AC line filtering has been successfully demonstrated with CMK-5/SWNT EDLCs. Finally, by implementing different current collectors, the operational voltage window can be extended to 2.8 V. Combined with high capacitance, the wide voltage window of CMK-5/SWNT supercapacitors leads to the highest areal energy density (0.85 μW h/cm2) at 120 Hz among various EDLCs.

Experimental Section

Preparation of SBA-15 Template

The P123 triblock copolymer (4.0 g, Sigma-Aldrich) was added into 1.6 M hydrochloric acid (152 mL, Sigma-Aldrich) solution. The solution was magnetically stirred at 38 °C overnight. Subsequently, 8.8 g of tetraethyl orthosilicate (99%, Sigma-Aldrich) was added into the solution and stirred for 6 min at 38 °C. The white milky solution was transferred to a Teflon-lined autoclave for a hydrothermal reaction carried out at 38 and 100 °C for 24 h. The product was collected by vacuum filtration and dried at 80 °C. Subsequently, it was calcined at 550 °C for 5 h in air.[29,30]

Preparation and Characterization of CMK-5

CMK-5 powder was synthesized by using furfuryl alcohol (FA, 98%, Sigma-Aldrich) dissolved in trimethylbenzene (TMB, 98%, Sigma-Aldrich) as a carbon source and oxalic acid (98%, Sigma-Aldrich) as a polymerization catalyst. The SBA-15 template (0.2 g) was added to the FA/TMB solution (volume ratio of FA/TMB = 1/3) stirred at 60 and 80 °C for 3 h each. The resulting composite was heat-treated under Ar flow in a tube furnace at 150 °C for 3 h. Subsequently, it was heated up to 300 °C at a heating rate of 1 °C/min and up to 850 °C at 5 °C/min, where it was held for 4 h. CMK-5 was obtained by etching the silica template with 5% hydrofluoric acid (HF, Sigma-Aldrich) solution.[23,25]

The morphology and structure of the CMK-5 powder were characterized by transmission electron microscopy (TEM) (FEI, Tecnai 20) and X-ray diffraction (XRD) (Rigaku, D/max-2500 V/PC) using Cu Kα radiation (λ = 0.154 nm). Chemical properties were analyzed by X-ray photoelectron spectroscopy (XPS) using Al Kα radiation (=1486.6 eV) (ULVAC-PHI Inc., PHI X-Tool) and Raman spectroscopy (Horiba, LabRAM ARAMIS IR2) with a 532 nm laser. Specific surface area and pore size distribution were characterized by nitrogen adsorption–desorption isotherms (Micromeritics, ASAP2020).

Preparation of a CMK-5/SWNT Film

CMK-5 (2 mg) and SWNTs (carbon >90%, Sigma-Aldrich, 2 mg) were dispersed in propylene carbonate (PC, 99.7%, Sigma-Aldrich, 20 mL) by bar sonication (Sonics & Materials Inc., VC 750) for 20 min. A small portion of the CMK-5 and SWNT solution (weight ratio = 9:1) was taken, added into methanol (CH3OH, 99.9%, JT-Baker, 300 mL), and bar-sonicated for 10 min. The CMK-5/SWNT solution was poured onto an anodic aluminum oxide (AAO, Whatman, diameter ≈ 47 mm, pore size ≈ 0.2 μm) membrane. A CMK-5/SWNT film was formed on the AAO membrane by vacuum filtration. Subsequently, the AAO membrane was dissolved in a 3 M sodium hydroxide (NaOH, 97%, Sigma-Aldrich) aqueous solution, which was eventually exchanged with fresh deionized water. The CMK-5/SWNT film floating on water can be deposited onto a current collector by gently removing the water with a syringe. CMK-5/SWNT materials were deposited on a Au/Ti/Al foil or a carbon-deposited Al (C/Al) foil current collector. The C/Al current collector was prepared according to the procedure described in a previous report.[20] The morphology of CMK-5/SWNT electrodes was characterized by field-emission scanning electron microscopy (Hitachi, S-4700).

Fabrication and Electrochemical Characterization of CMK-5/SWNT Supercapacitor

A supercapacitor cell was assembled with two CMK-5/SWNT electrodes, two current collectors, a separator, and an electrolyte in a glove box. We employed a polytetrafluoroethylene (PTFE, Millipore, thickness = 30 μm, pore size = 0.1 μm) membrane as a separator. 1 M tetraethylammonium tetrafluoroborate (TEABF4, ≥99.0%, Sigma-Aldrich) dissolved in acetonitrile (AN, 99.8%, Sigma-Aldrich) was used as an organic electrolyte. Cyclic voltammetry (CV), GCD, and electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical analyzer (Bio-Logic, VSP-300). EIS was performed using an input signal of 10 mV sine wave AC with a 0 V direct current bias. The AC line filtering circuit was composed of a full bridge rectifier made of four diodes (1N4004), a resistor (NTREX, 1 kΩ), and the CMK-5/SWNT supercapacitor on a breadboard (E-Call, EIC-108). The sinusoidal AC input signal was generated by a function generator (Keysight Technologies Inc., 33210A), and the output signal was measured by using an oscilloscope (Keysight Technologies Inc., DSOX2004A).