Hierarchical Pore-Patterned Carbon Electrodes for High Volumetric

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Hierarchical Pore-Patterned Carbon Electrodes for High Volumetric Energy Density Microsupercapacitors Cheolho Kim, and Jun Hyuk Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03958 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Materials & Interfaces

Hierarchical Pore-Patterned Carbon Electrodes for High Volumetric Energy Density Microsupercapacitors Cheolho Kim, and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University Sinsu-dong 1, Seoul, 04107, Republic of Korea Corresponding author, E-mail: [email protected]

KEYWORDS: Microsupercapacitors; Volumetric energy densities; Hierarchical pores; Carbonization; Interference lithography

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ABSTRACT Microsupercapacitors (MSCs) are attractive for applications in next-generation mobile and wearable devices and have the potential to complement or even replace lithium batteries. However, many previous MSCs have often exhibited a low volumetric energy density with high-loading electrodes due to the non-uniform pore structure of the electrodes. To address this issue, we introduced a uniform-pore carbon electrode fabricated by 3D interference lithography. Furthermore, a hierarchical pore-patterned carbon (hPC) electrode was formed by introducing a micropore by chemical etching into the macropore carbon skeleton. The hPC electrodes were applied to solid-state MSCs. We achieved constant volumetric capacitance and a corresponding volumetric energy density for electrodes of various thicknesses. The hPC MSC reached a volumetric energy density of approximately 1.43 mWh/cm3. The power density of the hPC MSC was 1.69 W/cm3. We could control the capacitance and voltage additionally by connecting the unit MSC cells in series or parallel, and we confirmed the operation of the LED. We believe that our pore-patterned electrodes will provide a new platform for compact but high-performance energy storage devices.


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INTRODUCTION The concept of smart design, as part of the so-called Industrial Revolution 4.0, is driving the demand for next-generation high-efficiency energy devices. In particular, the development of ultra-small, high-performance energy storage devices is required to produce small nextgeneration microelectronic devices, such as wearable/wireless devices, biosensors, and smart cards.1 Microsupercapacitors (MSCs) have been proposed as an energy storage device for this purpose. Unlike a supercapacitor, MSC is typically fabricated with an electrode structure, including an in-plane positive/negative electrode, so that the device can be manufactured compactly.2 Nonetheless, the advantages of supercapacitors, such as high power density, fast charge/discharge rates and long cycle stability, are inherited.2-4 In particular, recently reported MSCs have approximately 105 times greater power densities than current lithium thin-film batteries (1 - 5 mW/cm3).4 As electrodes for MSCs, carbon-based materials that store charge with electric double layers, such as onion-like carbon particles,5 carbon nanotubes,6 graphene,7 and carbides

and

nitrides,8

have

been

widely applied,

and

their composites

with

pseudocapacitance materials (e.g., RuO2,9 NiO,10 Co3O4,11 and conducting polymers12) have also been utilized. It should be noted that MSC performance is often more dependent on the collective properties of the particles than on the properties of the particles themselves. In practice, MSC electrodes are formed by stacking or assembling these particles.7, 12-13 Many reports have noted the serious deterioration or even loss of the superior properties of each of these particles. For example, graphene, the most commonly applied material, has a large theoretical gravimetric capacitance (~500 F/g) due to its large theoretical surface area (~2000 m2/g)14-15, but the gravimetric capacitance of one actual graphene-assembled electrode only ranges from 100 - 200 F/g.16-21

This could be explained by a significant reduction in the



surface area accessible to electrolyte ions due to the large attraction between the graphene layers.16-17 Thus, the fabrication of electrodes by assembling or stacking carbon particles is inherently limited. Recently, novel processes such as freeze-casting have been proposed to maintain the stacking interval.22 Additionally, methods for including poly-L-lysine,23 silver nanowires,24 etc. between the layers in stacked graphene films has been proposed. However, these methods require a delicate process or multiple processes. Here, we present the fabrication of a monolithic electrode structure rather than the conventional method of stacking or assembly. Pore-patterned multilayer carbon patterns are fabricated by the direct carbonization of a 3D polymer pattern prepared by interference lithography. Unlike conventional approaches, our method can produce well-defined pore patterns with high fidelity. In particular, we create micropores in the carbon skeleton to produce a hierarchical-pore-structure carbon. The formation of micropores can drastically improve the specific surface area, thereby improving the electrochemical double layer capacitance. When this hierarchical pore-patterned carbon (hPC) electrode is applied to the MSC, we confirm that the volumetric capacitance is maintained with the increased electrode thickness. As a result, we achieve constant volumetric energy density for electrodes of various thicknesses. Our results confirm that electrodes of uniform pore structure are essential for high-energy storage.

EXPERIMENTAL PROCEDURES Fabrication of hierarchical pore-patterned carbon: A 3D SU8 pattern was obtained via fourbeam interference lithography. Further details of the interference lithography are described in the supplement note. We directly carbonized the 3D polymer patterns at 900°C at a heating rate of 5°C/min under Ar atmosphere.25 To prepare the microporous carbon, a KOH aqueous


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solution was spin-coated onto the 3D pore-patterned carbon, and then dried in an oven at 90°C. Finally, the sample was heat-treated at 600°C for 30 min at a rate of 5°C/min under Ar atmosphere. The product was then washed with HCl and distilled water to remove KOH or potassium oxide residues.

Fabrication of microsupercapacitors: Au film was thermally evaporated onto the hierarchical pore-patterned carbon film through a physical mask where the open window provided the interdigital electrode design. Typically, we deposited an approximately 10 nm-thick Au film at a rate of ~1.0 Å/s and a chamber pressure of ~1 × 10-6 Torr. Then, oxidative etching was performed to obtain the interdigital electrode structure of positive/negative electrodes. Reactive ion etching (RIE) was conducted with a flux of 40 sccm O2 and 200 W RF power.

Characterization: The surface morphologies were measured using a scanning electron microscope (SEM, JEOl, JSM-6010LA). X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCALAB 250 XPS System) was performed for elemental analysis of the patterned carbon. Raman spectra were recorded using a Raman microscope (Horiba JobinYuan) with an 514 nm excitation. In order to measure electrochemical properties, a threeelectrode system was assembled with a carbon film-attached working electrode, Ag/AgCl reference electrode, and a Pt counter electrode. The working electrode was then immersed in the electrolyte solution, 1.0 M H2SO4 (Aldrich). For the MSCs, PVA/H3PO4 gel electrolyte was prepared by stirring 10 ml of DI water, 1 ml of H3PO4 (80%, Sigma-Aldrich), and 1 g of poly(vinyl alcohol) (Mw = 85,000, Sigma-Aldrich) at 80°C overnight. Then, 0.2 ml of electrolyte was dropped onto the carbon electrode and was solidified overnight by the evaporation of excess water. CV and GCD curves were measured using a VersaSTAT 3 (AMETEK). Calculation of specific capacitance, energy density and power density is



described in detail in the supplement note. RESULTS AND DISCUSSION The fabrication of hPC electrodes for MSCs is described in Scheme 1. The hPC was obtained via the direct carbonization of a 3D macroporous polymer pattern prepared by interference lithography and subsequent micropore generation by a chemical activation process. Briefly, a four-beam interference pattern whereby the iso-intensity surface exhibits face-centered cubic (FCC) symmetry was exposed to SU8 photoresist (Miller-Stephenson) to obtain the polymer pattern. The SU8 pattern was carbonized at 900°C under an Ar environment. Micropores were then formed in the carbon matrix via a high-temperature chemical etching process with KOH pretreatment, thereby forming the 3D hPC. The high-temperature treatment of KOHtreated carbon induces the following reaction: 6KOH + 2C → 2K + 3H2 + 2K2CO3. The resulting carbonate is decomposed into volatile CO and CO2 at higher temperatures, such as 600°C, leaving behind micropores in the carbon matrix.26-27 Further details of this KOHassisted activation reaction are described in the supplementary note. We performed KOH activation in the range of 500 - 700 °C and observed the highest electrochemical properties at 600 °C (see Figure S1). At temperatures above 700 °C, cracks were formed in the pore pattern due to the severe etching of the carbon. The interdigital hPC electrode was fabricated via selective etching through a physical mask on the carbon pattern. The hPC has an FCC lattice structure and was produced as a uniform film without defects with an area of approximately 16 mm2, as observed in Figure 1a. The (111) arrangement of the FCC lattice on the surface is shown in Figure 1b and has a hexagonally arrayed pore structure with a diameter of approximately 1 µm. The fraction of macropores was approximately 50%, and the pore volume was calculated to be approximately 0.864 cm3/g. The cross-sectional SEM image in Figure 1c shows the stacked layers of the (111) plane. Compared with the size of the macropores in the 3D polymer pattern (Figure S2a) and ACS Paragon Plus Environment

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the carbon pattern (Figure S2b), the macropores in the hPC were approximately 35% and 15% larger, respectively, due to the volume of the skeleton being reduced during pyrolytic carbonization and to etching via KOH activation, respectively. In the magnified TEM image of the carbon skeleton, microporous structures with sizes of approximately 1 nm can be confirmed. The SEM/EDX images of the interdigital electrode formed through etching of the hPC shows that the positive and negative electrodes are well separated, as observed in Figure 1e. Our method can be used to create many sample patterns in desired locations on a substrate in a “step-and-repeat” method, as observed in Figure 1h. We examined how the KOH etching process affects the physical properties of carbon and the formation of micropores. First, we investigated micropore formation by tuning the concentration of KOH from 3 M to 9 M. As shown in Figure S3, the various KOH pretreatment concentrations had little effect on the morphology of the carbon pattern. To confirm the increase in surface area due to the formation of micropores, the electrochemically active area was measured by cyclic voltammetry (CV).28 This area denotes the area over which the redox reaction of the electrolyte ions occurs and is therefore directly related to the electrochemical properties, in contrast to the surface area measured by BET analysis. This active area was calculated using the following Randles-Sevcik equation.29-30 Ip = 268,600 n3/2 A D1/2 C v1/2

(1)

where Ip is the peak current (A), A is the electroactive area (cm2), C is the concentration of the electroactive species (mol/cm3), n is the number of exchanged electrons, D is the diffusion coefficient (cm2/s) and v is the scan rate (V/s). This equation calculates the active area through the slope in relation to Ip and v in the electron transfer-controlled process. For the hPC electrode under various activation conditions, the slopes of Ip versus v1/2 over the measured range are linear, indicating that the measured active area is valid, as observed in Figure 2a. As the KOH concentration increases, the active area increases. As the KOH



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concentration used in the activation process increases, the evolution of the micropores can be confirmed, as observed in Figure S4. In particular, the hPC that underwent 9 M KOH treatment exhibited a 13.3-fold higher active area compared with the untreated sample. Previously, in the case of activated carbon, the electroactive surface area tended to saturate when the number of micropores increased, whereas in our case, it increased linearly. 31-32 This is probably due to the ease of ion diffusion through the macropore skeletons of the hPC structure. The BET isotherm of the 9 M KOH-treated hPC reveals a steep adsorption curve over a low relative pressure range, which indicates a type I isotherm corresponding to a microporous material, and the material exhibits a very high specific surface area of approximately 1097 m2/g. (Figure 2b) The total micropore volume was calculated to be 0.479 cm3 / g. From Barrett-Joyner-Halenda (BJH) analysis, micropores of 2 nm or less are dominant, which is similar to the micropores observed in the TEM image. The effect of micropore generation on the carbon characteristics of hPC was analyzed by Raman spectroscopy (Figure 3a). Two typical peaks appeared at 1590 cm−1 and 1340 cm−1; the former represented E2g symmetry (the G mode), and the latter derived from the A1g symmetry of disordered graphite (the D mode).33-34 D and G peaks of similar intensities are often seen in polymer-derived carbon and exhibit vitreous or microcrystalline graphite properties. The intensity ratio between the D and G bands (ID/IG) is inversely proportional to the domain size of the in-plane graphitic crystallites.35-36 As the degree of micropore generation increases, the intensity of the D peak decreases, thereby decreasing ID/IG, as observed in Figure 3a. The spectrum displays two shoulder peaks at approximately 1180 cm-1 and 1500 cm-1, which have been assigned as contributions from sp3 carbon.35

36

This result reveals that more micropores are formed, resulting in more defective carbon sites. Nevertheless, the resistance of the electrode did not increase significantly with an increased number of defects accompanying micropore formation, as observed in Table S1.


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Moreover, XPS was used to characterize the oxygenated groups during KOH-assisted micropore generation. The C 1s band was deconvoluted into the following bands: aromatic carbon (i.e., C−C) at 284.5 eV; carbon singly bound to oxygen in epoxy or alkoxy groups (i.e., C−O) at 285.5 eV; carbon doubly bound to oxygen in carbonyl or carboxylic groups (i.e., C=O) at 287.0 eV; carbon bound to two oxygen atoms in carboxyl, carboxylic anhydride, or ester groups (i.e., −COO) at 288.6 eV; and the characteristic aromatic π−π* transition at 289.9 eV (see Figure 3c).37-38 Peak deconvolution analysis shows that the number of COH/C-O-C and COOH groups in the hPC was significantly increased as the pretreatment concentration of KOH increased, as observed in Figure 3d. The increase in the number of oxygenated groups from KOH-assisted etching may be due to the conversion of -OK groups formed during the etching process into oxygenated groups through an ion-exchange reaction.39 The oxygenated groups improve the electrochemical capacitance by increasing the electrolyte wettability of the carbon surface.40 In particular, C-OH and COOH groups improve the pseudocapacitance properties via a quinone/hydroquinone reaction.41-42 The electrochemical capacitance of the hPC was measured using CV in a threeelectrode cell and 1.0 M H2SO4 electrolyte solution. The CV curves at various scan rates of bare electrodes and of electrodes formed by activation at various KOH concentrations are shown in Figure S5. The CV curves of these samples at 5 mV / s are compared in Figure 3e. The more micropores are formed, the higher the current density in the measured voltage range, which is due to the increase in specific surface area. The calculated areal capacitance was 12 mF / cm2 for the 3 M KOH electrode and approximately 104 mF / cm2 for the 9 M KOH electrode. We also observed an increase in the broad hump in the 0.4 - 0.6 V range because the oxygen group increased with increasing KOH concentration during activation.4243

The galvanostatic charge/discharge (GCD) curves of bare PC and the hPC electrodes

activated at various concentrations of KOH are shown in Figure S6, and the GCD curves at a



current density of 1 mA/cm2 are compared in Figure 3f. Similar to the CV results, the GCD results show an increase in specific capacitance as more micropores are formed, as observed in Figure 3f. The specific capacitance of hPC at 9 M KOH was approximately 10 times higher than that of PC without micropores. In a previous report, increasing the specific area by creating micro- or mesopores resulted in a 40 - 70% improvement in the specific capacitance compared with the increase in surface area.31, 44-46 Considering the 13.3-fold improvement in the electrochemically active surface area in the above analysis, this improvement in the capacitance can be attributed to the increase in the specific surface area, as well as the enhanced pseudocapacitance due to the increase of oxygenated groups. We fabricated solid-state MSCs using hPC electrodes pretreated with 9 M KOH for micropore generation and a PVA-H3PO4 polymer gel electrolyte. First, the effect of the interdigital electrode thickness on the electrochemical performance of hPC was investigated. To accomplish this, hPC electrodes of various thicknesses (1, 2, 3 and 6 µm) were fabricated, as shown in Figure S7. In Figure 4a and 4b, the CV curves of the hPC-electrode MSCs with different thicknesses reveal quasi-rectangular voltammograms, i.e., the current rapidly responds as the scan direction reverses. When comparing the scan rates of 100 mV/s and 1000 mV/s, the CV curves of each electrode show no significant difference in shape. In particular, the rectangular shape is maintained even at high scan rates. These results reveal fast electrolyte ion diffusion in the hPC structure.47 In Figure 4c, the GCD curves of the hPC MSCs are symmetric and triangular, indicating good electric double-layer capacitive behavior. Notably, the areal capacitance calculated from the GCD curves is directly proportional to the thickness of the hPC electrode, as observed in Figure 4d. The electrodes with 1 µm, 2 µm, 3 µm, and 6 µm thickness exhibit an areal capacitance of 1.1 mF / cm2, 1.9 mF / cm2, 3.6 mF / cm2, and 5.9 mF / cm2, respectively. The volumetric capacitance is then 11.0 mF / cm3, 9.3 mF / cm3, 11.0 mF / cm3, and 9.9 mF / cm3 for 1 µm, 2 µm, 3 µm, and 6


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µm thick electrodes, respectively, resulting in a constant value with varying thickness, as observed in Figure 4e. These results also indicate that the volumetric energy density remains constant as the electrode thickness increases. Previous results have often shown that volumetric capacitance decreases with increasing electrode thickness. For example, for an MSC with a graphene-stacked electrode, the volumetric capacitance was decreased by 60% when the film thickness increased from 4.5 µm to 9.2 µm.20 The previous results have shown that as the thickness increases, the ion or electron transport resistance increases due to the non-uniform pore structure of the graphene-stacked electrode, and thus the electrochemical active area does not increase in proportion to the thickness, resulting in a decrease in volumetric capacitance. Therefore, our results show the distinct advantages of hPC electrodes, that is, a well-defined pore network allows rapid charge diffusion, and thus, volumetric capacitance is maintained even with increased electrode thickness. The Nyquist plot shows a nearly vertical line in the lower frequency range for all samples. The electrochemical impedance of the hPC electrode with various thicknesses is observed in Figure 4f. This result confirms facile charge diffusion in the pore structure of the hPC electrode. Figure 4g shows that this capacitance remains at 95% after 30,000 cycles. We measured the stability under higher current density conditions, which also shows that the capacitance remains stable (Figure S8). For various scan rates, the volumetric capacitance is shown in Figure S9, showing a retention of approximately 50% at a 10-fold increase from 10 mV / s to 100 mV / s. The current and potential range can be tuned by connecting MSCs in parallel and/or in series.48 The GCD curves for two units connected in series and in parallel circuits are shown in Figure 5a. In series, the charge potential increases to 2.0 V, which is 2 times higher than that of a single MSC unit. The parallel circuit exhibits a 2-fold increase in discharge time while maintaining the same potential. The three serially connected MSCs integrated on the circuit board presented a sufficiently high applied voltage of 3.0 V, which can power an LED



light (see the inset in Figure 5a). To evaluate the performance of the hPC MSCs, a Ragone plot is presented for our electrode, lithium thin film battery, and electrolytic capacitor, as observed in Figure 5b. The hPC MSC exhibits a maximum energy density of 1.43 mWh/cm3. This energy density is approximately 103 times larger than that of conventional electrolyte capacitors. We also compared the volumetric energy density of the hPC MSC with the previous results, as observed in Figure 5c. For a fair comparison, we excluded the results of sub-micron thin electrodes such as graphene films and doped graphene films.19, 49 In thin-film electrodes, the ion diffusion resistance is small and is usually correlated with a high energy density. However, it should be noted that the actual energy storage (in units of mWh) is not high in this case. We also compared only the results of applying an aqueous electrolyte. Some studies have considered the application of ionic liquids,18 in which case there is an increase in energy density due to the application of high voltages. The volumetric energy density of hPC MSCs is higher than that of previously reported MSCs based on template-assisted graphene nanosheets,50 suspended wavy structured graphene ribbons,51 laser-scribed graphene (LSG),7 SWCNTs,52 micropatterned multi-walled carbon nanotubes (MWCNTs),53 reduced graphene oxide-carbonized cotton fabric electrodes (rGO-CCFs),54 vertically aligned MWCNTs (vMWCNTs),55 3D graphene/CNT carpets (G/CNTCs-MCs),56 rGO/CNTs,57 laser-induced graphene/polyaniline (LIG/PANI),58 boron-doped laser-induced graphene (B-doped LIG),59 RGO/MnO2/AgNWs (silver nanowires),24 MnO2 nanoflakes on silicon nanowires (MnO2@SiNWs)60 and MXene61. Moreover, the maximum power density of the hPC MSC was 1.69 W/cm3. Generally, the power density tends to be smaller as the energy density value increases. Generally, highenergy-density electrodes exhibit relatively low power densities. This is because electrodes with a large energy density usually have a high surface area or high microporosity, and thus,


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there is a large delay in ion diffusion during charging and discharging. The power densities of graphene nanosheets50 and LIG/PANI58 with an energy density greater than 1 mWh/cm3 were 0.3 W/cm3 and 0.8 W/cm3, respectively. Thus, our power density performance is superior to previous results with similar energy densities.

CONCLUSIONS Carbon electrodes with a volumetric energy density that is maintained with increasing thickness are crucial for high-energy MSCs. Previous results have often shown low volumetric capacitance with high-loading electrodes due to non-uniform pores. Here, we present a pore-patterned carbon electrode with uniform pore structure produced using 3D interference lithography. In particular, we fabricated an hPC electrode that formed micropores by chemical activation of a carbon skeleton. Chemical activation also formed surface oxygenate groups with pseudocapacitance properties. The hPC electrode showed 10 times higher capacitance than the bare carbon pattern due to the micropores and oxygenate groups. We fabricated solid-state MSCs with hPC electrodes and a controlled electrode thickness. The hPC MSC showed an areal capacitance in proportion to the increase in the electrode thickness, and as a result, the volumetric capacitance was maintained. This was due to the fast electrolyte ion diffusion through the uniform pore network of the hPC. The hPC MSC reached a volumetric energy density of approximately 1.43 mWh/cm3. The power density of the hPC MSC was 1.69 W/cm3. We could additionally control the capacitance and voltage by connecting the unit MSC cells in series or parallel, and we confirmed the operation of an LED. We believe that our pore-patterned electrodes will provide a new platform for compact but high-performance energy storage devices.

ACKNOWLEDGMENTS



This work was supported by grants from National Research Foundation of Korea (20110030253, 2017M1A3A3A02016667). The authors acknowledge the support from Sogang University (201610050). The Korea Basic Science Institute is also acknowledged for SEM and XPS measurement.

Supporting Information Available: Additional supporting data is available free of charge via the Internet at http://pubs.acs.org


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Scheme 1. Schematic illustration of the fabrication of the hPC MSCs.

Figure 1. a) Surface SEM image of hPC. Scale bar, 10 µm. b) High-resolution image of the surface. Scale bar, 1 µm. c) Cross-sectional SEM image of hPC. Scale bar, 10 µm. The inset shows a high-resolution image of the cross section. Scale bar, 2 µm. d) High-resolution TEM image of hPC. Scale bar, 5 nm. e) EDX mapping of the interdigital electrode. Scale bar, 50 µm. f) Digital image of the interdigital electrode. Scale bar, 1 cm. g) Surface SEM images of the interdigital electrode. Scale bar, 1 µm. h) Digital image of the prepared interdigital electrode. Scale bar, 1 cm.



Figure 2. a) The relationship between the peak current and the square root of the scan rate fitted by the Randles-Sevcik equation for PC and hPCs prepared with 3 M, 5 M, 7 M and 9 M KOH. b) BET isotherm of hPC prepared with 9 M KOH. The inset shows the BJH pore distributions of hPC prepared with 9 M KOH.


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Figure 3. a) Raman spectra of PC and hPCs prepared with 3 M, 5 M, 7 M and 9 M KOH. b) XPS spectra of PC and hPC prepared with 9 M KOH. c) High-resolution O1s XPS spectra of hPC prepared with 9 M KOH. d) Corresponding oxygen configuration ratio of PC and hPCs prepared with 3 M, 5 M, 7 M and 9 M KOH. e) Cyclic voltammetry curves for PC and hPCs prepared with 3 M, 5 M, 7 M and 9 M KOH at a scan rate of 5 mV/s. f) Galvanostatic charge/discharge curves for PC and hPCs prepared with 3 M, 5 M, 7 M and 9 M KOH at a current density of 1 mA/cm2



Figure 4. Cyclic voltammetry curves for the hPC MSCs with thicknesses of 1, 2 and 3 µm at a scan rate of a) 100 mV/s and b) 1000 mV/s. c) Galvanostatic charge/discharge curves of hPC MSCs with thicknesses of 1, 2, 3 and 6 µm at a current density of 0.1 mA/cm2. d) Areal capacitance and e) volumetric capacitance of the hPC MSCs with thicknesses of 1, 2, 3 and 6 µm at a scan rate of 100 mV/s. f) Nyquist plot of the EIS spectra from hPC MSCs with thickness of 1, 2, 3 and 6 µm. g) Cycling performance of hPC MSC at 1mA/cm2.


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Figure 5. a) Galvanostatic charge/discharge curves at current density of 0.1 mA/cm2 of a single hPC MSC device and two MSC devices connected in series and in parallel. The inset is an image of the LED turned on through the MSC integrated into the circuit board. b) Ragone plots of hPC MSC, lithium-thin film battery (4 V/500 µAh) and electrolytic capacitor (3V/300 µF). c) Comparison of energy density of hPC MSC with previously reported carbon electrode MSCs.



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