Precisely Geometry Controlled Microsupercapacitors for Ultrahigh

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Precisely Geometry Controlled Micro-Supercapacitors for Ultrahigh Areal Capacitance, Volumetric Capacitance, and Energy Density Jungjoon Yoo, Segi Byun, Chan-Woo Lee, Chung-Yul Yoo, and Jin Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03786 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Chemistry of Materials

Precisely Geometry Controlled Micro-Supercapacitors for Ultrahigh Areal Capacitance, Volumetric Capacitance, and Energy Density Jungjoon Yoo,†,* Segi Byun,‡ Chan-Woo Lee,§ Chung-Yul Yoo,† and Jin Yu‡ †Separation

and Conversion Materials Laboratory, Energy Efficiency and Materials Research Division, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea ‡Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea § R&D Platform Center, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea ABSTRACT: Micro-supercapacitors are micro-scale rechargeable energy storage devices that can support or replace batteries in ultra-small electronic devices. Although the use of high-capacitive, two-dimensional materials is promising, other methods are needed to reach a high capacitance and energy density, which cannot be achieved by fully utilizing the surface of electrode materials. Here, we introduce an effective strategy to control the geometry of interdigital microelectrodes for achieving an ultrahigh capacitance, utilizing the edge effect of in-plane structured graphene and improving ion transport. Theoretical calculations are employed to investigate the electrochemical enhancement at the edge of reduced graphene oxide in a KOH electrolyte. The presence of edges is predicted to enhance the capacitance by electronic redistribution. We report areal and volumetric stack capacitances (40 mF/cm2 and 98 F/cm3, respectively) and energy densities (5.4 μWh/cm2 and 13.7 mWh/cm3, respectively) that are much higher than those of any other micro-supercapacitors containing micrometer-thick interdigital electrodes. This improvement is attributed to synergistic effects between numerous edge planes fabricated by a high-resolution laser-drilling process and a well-matched electrolyte as well as the in-plane structure of heattreated graphene oxide, which provides minimal channel space for efficient ion transport. Our strategy provides a versatile method for designing high-performance micro-supercapacitors and is promising for the development of micro-energy storage devices for sub-miniature electronics that require a high energy density.

1. Introduction The demand for micro-energy storage systems is increasing because the future evolution of paper-thin, compact microelectronic devices is inevitable. These small electronic devices typically use embedded, miniaturized power sources to maximize performance. Thus, a micro-power source must be long-lived and safe. However, conventional batteries (e.g., Li-ion batteries) are difficult to apply to microelectronic devices; Li-ion batteries are innately dangerous because they contain lithium and can overheat, and they have a limited lifecycle because of their strong redox reactions. Thus, the development of a viable micro-power source is a persistent, serious issue that restricts the advancement of microelectronic devices. Supercapacitors are emerging as rechargeable energy storage devices. Although supercapacitors exhibit good performances, such as fast charging/discharging, a high power density, and semi-permanent lifetimes, they have only one-tenth the gravimetric energy density of conventional batteries.1,2 This is a significant disadvantage for their use as energy storage devices. However, for microelectronic devices, the volumetric and areal energy densities (rather than the gravimetric energy density) may be more important because these devices are already sufficiently lightweight. Many studies have reported supercapacitors with high volumetric and areal energy densities

close to those of Li thin-film batteries.3–9 In this respect, a micro-supercapacitor with high volumetric and areal energy densities, power densities, and a semi-permanent lifetime would be a highly suitable power source for a microelectronic device. Furthermore, micro-supercapacitors are low-cost and environmentally friendly because their electrodes primarily contain carbon. They are also nonexplosive, safe, and semi-permanent because they store energy through ion adsorption/desorption rather than redox reactions. Because a micro-supercapacitor has a planar geometry, a two-dimensionally structured electrode material would be advantageous for configuring highspatial-efficiency electrodes within a device. Graphene is quite suitable as a highly conductive two-dimensional carbon electrode material, and we successfully obtained a high capacitance by taking full advantage of the edge planes and in-plane structure of this material. The edge planes of graphite can physically and chemically store approximately 10 times more charge than the basal planes.1015 This is called the edge effect, which is a well-accepted experimental phenomenon. The edge effect can be achieved by controlling the geometry of two-dimensional graphene electrodes. For example, if the width of the microelectrodes is reduced while maintaining the geometrical area of the devices, the number of microelectrodes and the length of the interspace increase, increasing the area of the edge planes. Some researchers have tried to

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improve the performance of micro-supercapacitors by controlling the geometry of electrodes.16-19 They transformed the electrode geometry into various patterns, resulting in an improved performance. This work indicates that the performance of micro-supercapacitors can be further enhanced if the geometry of the microelectrodes can be precisely controlled. We employed a laser drilling system, which is a high-resolution microetching system that uses a diode-pumped UV Nd:YAG laser with a spot size of approximately 10-25 μm, to precisely control the geometry of the microelectrodes. Many previous studies on micro-supercapacitors have primarily focused on exploiting the high conductivity of carbon nanomaterials. Thus, nanocarbon-based microsupercapacitors have achieved outstanding power performances that are comparable to electrolytic capacitors. The Brunet and Kaner groups obtained power densities of 250 W/cm3 using onion-like carbon and 141 W/cm3 using laser-scribed graphene.3,5 These excellent values are comparable to the power densities of electrolytic capacitors. However, more research is needed on microsupercapacitors with energy densities superior to those of Li thin-film batteries. In this study, we controlled the geometry of interdigital microelectrodes to obtain an ultrahigh capacitance by maximizing the number of edge planes of the in-plane structured graphene along with the realization of an inplane structure for efficient ion transport.4,5,20–24 This was easily and rapidly implemented using a high-resolution laser drill micro-patterning method. To investigate and verify the capacitance enhancement at the edge of reduced graphene oxide (RGO) in KOH electrolyte, we performed density functional theory (DFT) calculations to investigate the electronic redistribution which is expected to be promoted from structural deviation by K adsorption. The electronic redistribution includes various phenomena (e.g., band edge shift, orbital splitting, band filling) that can be directly associated with the capacitance behavior. Thus, elucidating the electronic redistribution is imperative to understand the origin of the capacitance improvement. Electrical double-layer (EDL) capacitance and additional pseudocapacitance are effectively generated from the synergistic effects between numerous edge planes and the well-matched aqueous electrolyte, and the in-plane structure of heat-treated graphene oxide (HRGO), which provides a minimal channel space for efficient ion transport. Hence, we successfully obtained the highest areal and volumetric stack capacitance and energy density yet reported.

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membrane filter (0.22-μm pore size, Millipore) using a vacuum filtering system to produce a GO film. The asprepared GO film was heat-treated at 200 °C for 30 min. During the heat treatment, the GO film was sandwiched between two silicon wafers to prevent curling (Figure S1). This process properly reduced the heat-treated GO films, which were determined to be HRGO, HRGO-D, and HRGOT. The HRGO film thickness was approximately 2 μm (Figure S2). 2.2 1-V micro-supercapacitor fabrication process As illustrated in Figure 1, the HRGO film (2 × 10 mm2), cut using an ultraviolet (UV) laser drilling system (LTS Co.), was pasted onto a cleaned glass substrate with a commercial epoxy resin. Current collectors were produced through chromium (~200 nm) and gold (~700 nm) sputtering and by electroplating gold (~2 μm) on the external edges of the HRGO film. Here, a homemade mask was used to cover the active area of the HRGO film so that no chromium/gold was deposited onto that portion of the device. The entire device area, excluding a small portion of the current collector that was in contact with the lead wires, was thinly coated with commercial epoxy to prevent direct contact between the current collectors and the electrolyte. Finally, the HRGO film was isolated into two electrodes through the physical creation of a micrometer-sized patterned interspace using the UV laser drilling system.

Figure 1. Two-dimensional edge-enhanced-graphene microsupercapacitor fabrication process for a 1-V device. (a) Heattreated GO film (HRGO, 2 × 10 mm2) on a glass substrate. (b) Electrode active area masking using a homemade mask with chromium (adhesion layer) and gold (current collector) sequential sputtering for electrical contacts. (c) Gold electroplating of the external edges of the HRGO film for secure electrical contacts. (d) Thin coating of an entire device with commercial epoxy, excluding a small portion for electric contact, to prevent direct contact between the current collectors and electrolyte. (e) Creation of a micrometer-sized interspace using a UV laser drilling system to divide the film into two electrodes. (f) Fabricated device, including the specifications and digital image of a micro-patterned HRGO film electrode.

2. Experimental Section 2.1 HRGO film preparation Graphite (SP-1, Bay Carbon, Bay City, MI) was used to prepare graphene oxide (GO). GO was synthesized using a modified Hummer’s method and dried under vacuum for 24 h.25 GO (10, 20, or 30 mg) was added to a beaker containing deionized water (10 mL) and sonicated for 30 min to prepare the HRGO, HRGO-D, and HRGO-T films, respectively. The GO solution was filtered through a Durapore

2.3 5-V micro-supercapacitor fabrication process As illustrated in Figure S3, the HRGO film (10 × 10 mm2), cut using a UV laser drilling system (LTS Co.), was pasted on a cleaned glass substrate using commercial epoxy. Current collectors were produced through chromium (~200 nm) and nickel (~450 nm) sputtering on the external edges of the HRGO films. Here, a homemade mask was used to cover the active area of the HRGO film so that no chromi-

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um/nickel was deposited onto that portion of the device. The entire device area, excluding a small portion of the current collector that was in contact with the lead wires, was thinly coated with commercial epoxy to prevent direct contact between the current collectors and the electrolyte. The HRGO film was then separated into five cells using the UV laser drilling system; the five cells were subsequently formed into independent unit cells. Moreover, to electrically connect the five cells in series, connectors were formed as current collectors between the cells. A homemade mask was produced, and chromium (~200 nm) and nickel (~450 nm) were then sequentially sputtered onto the exposed substrate surface between the five cells only, so that the separated cells were electrically connected. To enhance the connection between the unit cells and the current collectors and to reduce the series resistance, nickel was thickly electroplated (~2 μm) onto the Cr/Ni sputtered surface between the unit cells, forming nickel walls. The nickel walls prevented the electrolyte that was placed within the unit cells from penetrating neighboring cells. In the final step, each of the five unit cells was isolated into two electrodes by physically creating a micrometer-sized patterned interspace using the UV laser drilling system. 2.4 Electrochemical measurements A KOH electrolyte (5.5 M) was used to measure the electrochemical properties. Cyclic voltammetry (CV) measurements, charge-discharge (CD) testing, and electrochemical impedance spectroscopy (EIS) were conducted with a Bio-Logics VSP potentiostat using a two-electrode system. The CV measurements were performed at various scan rates from 10 to 100 mV/s at 0−1 V and 0−5 V. Galvanostatic CD curves were obtained using a suitable constant current density for each device. EIS measurements were conducted at a DC bias of 0 V with a 10-mV sinusoidal signal over a frequency range of 500 kHz to 10 mHz. 2.5 Structure and property characterizations Scanning electron microscopy (SEM) images were taken using a field-emission scanning electron microscope (Hitachi S4800), and X-ray photoelectron spectroscopy (XPS) analysis was conducted using a multi-purpose XPS instrument (Sigma Probe, Thermo VG Scientific). The microstructures were analyzed using X-ray diffraction (XRD; Cu Kα, Rigaku, D/MAX-RC (12 kW)). A thermogravimetric analyzer (LABSYS, Setaram) was used to determine the mass variation according to temperature at a heating rate of 5 °C/min up to 1,000 °C under a N2 atmosphere. Finally, a focused ion beam (FEI Helios NanoLab™) was utilized to identify the electrode thickness. 2.6 Electrochemical performance calculations The capacitance was evaluated from the integrated discharge area of the CV curves. The cell capacitance (Ccell) was calculated from Ccell = I/(dV/dt), where I is the average current (A), V is potential (V), and t is time (s). - Areal capacitance: specific capacitance in the area of one electrode; - Areal stack capacitance: specific capacitance in the area of an entire device, including both positive and negative electrodes and the interspace between them;

- Volumetric capacitance: specific capacitance in the volume of one electrode; - Volumetric stack capacitance: specific capacitance in the volume of an entire device, including both positive and negative electrodes and the interspace between them. The areal stack and volumetric stack capacitances were deduced from the relationships CAREAL = Ccell/A and CVOL = Ccell/VT, where A is the entire geometrical area (0.2 cm2 for a 1-V device and 1 cm2 for a 5-V device) including both electrodes and the interspace between them, and VT is the total volume (cm3) that incorporates the thickness of the active substance in A (VT = A × active material thickness (~2 μm)). The areal and volumetric capacitances for one electrode were deduced from the relationships Careal = 2Ccell/a and Cvol = 2Ccell/v, where a is the geometrical area of one electrode (cm2, excluding the interspace area) and v is the volume of one electrode (cm3, excluding the interspace volume). The energy density (E) and the power density (P) were calculated from EAREAL = 0.5CcellΔV2/A and PAREAL = IV/A for the areal performance and EVOL = 0.5CcellΔV2/VT and PVOL = IV/VT for the volumetric performance, respectively, where ΔV is the potential window (V). 2.7 DFT calculations Spin-polarized DFT calculations were performed using VASP26,27 with the generalized gradient approximation (GGA-PBE) for exchange-correlation functionals. Projectoraugmented-wave (PAW) potentials with valence configurations of 2s22p2, 2s22p4, 1s1, and 3s23p64s1 were used for C, O, H, and K atoms, respectively. The basis set kinetic energy cutoff was 500 eV. The convergence thresholds for electronic self-consistent iterations and ionic relaxation were set to 0.0001 eV and 0.001 eV/A, respectively. 2×12×1 Gamma-point centered Monkhorst-Pack meshes were employed for RGO supercells based on 18 hexagons with 5 and 1/2 armchair chains. This half armchair chain embodied the edge region, which was passivated by H atoms, and was not involved in K adsorption. Finally, the RGO sheets were separated by a vacuum region of 20 Å. 3. Results and Discussion Here, we present a new two-dimensional, edge-enhancedgraphene micro-supercapacitor fabrication process by controlling the geometry of interdigital microelectrodes, employing a one-shot patterning method using a laser drilling system that produces many defective edges and efficient ion transport channels on a graphene sheet within a short time (2–5 min). Unlike laser scribing, which is a reduction method, laser drilling is a precise microetching technique to create micropatterns. The fabrication process is shown schematically in Figure 1; the process is described in more detail in the Electronic Supplementary Information (ESI). A GO film obtained through a vacuum filtering system and heat-treated at 200 °C for 30 min was used. This key process (Figure S1) partially reduces the HRGO film at an ultimate reduction temperature of 200 °C to yield both conductance and functional (f)-groups for high capacitance. If highly resistive GO could be effectually

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tuned to RGO with many f-groups and good conductance, the maximum capacitance within a minimum volume could be obtained. The thermogravimetric analysis (TGA) and electrical-resistance temperature-dependence data presented in Figure S4 show that the weight and electrical resistance of GO dramatically decrease near ~200 °C, indicating the removal of f-groups, and the highly resistive GO becomes highly conductive RGO. At that point, the loss of fgroups should be minimized to retain the high capacitance; therefore, the reduction of GO should be conducted at the lowest possible temperature. Previous studies have reported that GO reduced at 200 °C shows a higher capacitance than GO reduced at higher reduction temperatures, such as 700 and 800 °C.28,29 Similarly, we found that the GO film reduced at 200 °C shows a much higher capacitance than the GO films reduced at 700 and 800 °C (Figure S5aS5d). Although lower electrical resistance values are obtained for the films at higher reduction temperatures (Figure S5e), the GO films reduced at 700 and 800 °C show an unusually small capacitance because of GO restacking and the removal of f-groups. Figure S5f shows that the two peaks observed for the GO film reduced at 200 °C merge to one peak when the reduction temperature is increased to 700 or 800 °C. This indicates that GO is restacked and effective ion transport channels are removed during the high-temperature reduction process. The C 1s/O 1s values (atomic ratios) obtained through the XPS survey spectra increase from 3.46 (GO film reduced at 200 °C, HRGO film) to 28.2 (GO film reduced at 800 °C), indicating that the fgroups, which contribute additional capacitance, are mostly removed at high reduction temperatures (Figure S5g). Hence, the GO heat treatment at 200 °C is optimal to produce an HRGO film with both conductance and many fgroups for a high capacitance. From previous reports,12,30– 34 we note that a considerable portion of the unusually high capacitance can be attributed to basal and edge-plane f-groups. The HRGO film (2 × 10 or 10 × 10 mm2) was pasted onto a cleaned glass substrate using commercial epoxy (Figure 1a), and current collectors were produced by sputtering chromium and gold and then electroplating gold onto the external edges of the HRGO film (Figure 1b and 1c). The entire device, excluding a small portion of the current collector in contact with the lead wires, was thinly coated with commercial epoxy to prevent direct contact between the current collectors and the electrolyte (Figure 1d). The film was isolated into two electrodes by physically creating a micrometer-sized patterned interspace using a UV laser drilling system (Figure 1e). The device specifications and a digital image of the micro-patterned HRGO film electrodes are given in Figure 1f. Hence, a two-dimensional edgeenhanced-graphene micro-supercapacitor with an in-plane structure and numerous edge planes was produced using the laser drill micro-patterning method, leading to an unusually high capacitance in a small, confined space. The 5-V micro-supercapacitor fabrication process is also shown in Figure S3 and is almost identical to that of the 1-V microsupercapacitor (discussed in the experimental section).

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Digital images of the 1-V and 5-V fabricated devices are shown in Figure S6. We developed several important techniques to successfully accomplish our strategy: fabrication of an in-plane structured graphene electrode yielding efficient ion transport with a compact volume, an electrolyte that is well-matched to the electrode for additional pseudocapacitance, and the abovementioned laser drill micro-patterning process to produce numerous edge planes and improve ion transport in an in-plane structure. The former two techniques were developed as described in the ESI, while the development of the latter is given below. Many researchers have developed high-performance micro-supercapacitors using various techniques, such as photolithography, laser scribing, ink-jetting, electrophoretic deposition, and layer-by-layer methods.3,4,5,22,23,35,36 In this study, we utilize laser micro-drilling with a high resolution to precisely control the geometry of the microelectrodes. The laser drill micro-patterning method is the optimum technique to readily produce numerous edge planes and improve ion transport in the in-plane structures. The micro-patterning process is conducted within a few minutes using a UV laser drilling system. Laser drilling is a precise microetching technique for the formation of micropatterns; it should not be confused with laser scribing, which is a reduction method. Compared to conventional photolithography, which accounts for 30−40% of the total cost of a flat-panel display, this method has significant advantages, including a considerably simpler, faster, and more precisely controlled process and the capacity to create various patterns using easy software operations. Highpriced equipment, numerous chemicals, and complicated photolithography microfabrication steps are not required. Using this laser micro-patterning process, we rapidly obtained in-plane structures, leading to efficient charge propagation between the graphene layers.4,5,20–24 For more efficient ion transport, we controlled and patterned the width of the interdigital electrodes and laser-spot-sized interspace (~80 and ~25 μm, respectively) using a highresolution laser drilling system; these values are smaller than previously reported values.3–5,21 The miniaturization of the microelectrode width and interspace shortens the ion transport distance, leading to facile charge propagation and the formation of an efficient EDL (details are provided later in the document to elucidate the dependence of the electrochemical properties of the micro-supercapacitor based on the number of interdigital electrodes). Because of the narrow width and interspace, 92 interdigital microelectrodes, significantly more than that used in other reports,3–5,17,37,38 were successfully constructed within 1 × 0.2 cm2, i.e., the 1-V device dimensions (for the 5-V device, a total of 92 × 5 interdigital microelectrodes in 1 × 1 cm2 were used). Then, we attempted to fabricate the maximum number of edge orientation planes (Figure 1e). The minimization of the interdigital microelectrode widths and interspace increases the total area of the edge planes and improves ion transport, leading to micro-supercapacitors with significantly higher capacitances because of the edge effect and shortened ion diffusion length.10–15

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Chemistry of Materials with one broad peak. In contrast, the film-type GO is partially reduced because the film has a relatively confined structure that interrupts the escape of H2O molecules and f-groups, resulting in two broad peaks in the XRD pattern for the HRGO film.39 The interlayer distances of the HRGO film (peak 1: 5.6 Å, peak 2: 3.8 Å) are larger than the K+ and OH− ions (~1.38 and ~1.53 Å, respectively).40,41 Because of the coexistence of more- and less-reduced GO, the HRGO film has sufficient interlayer space to effectively transport the ions. Additionally, the HRGO film retains many f-groups that can contribute to the pseudocapacitance and aid efficient ion transport owing to their hydrophilicity.12,15,42 XPS measurements indicate a dramatic reduction of the oxygen content in the HRGO film compared to the as-prepared GO film (Figure 2c). The C 1s/O 1s values (atomic ratios) obtained through the XPS survey spectra increase from 2.61 (as-prepared GO film) to 3.46 (HRGO film), which is sufficient for its use as a conductive electrode (~340 S/m). These measurements were conducted on the film surface, exposing the basal planes of GO or HRGO. XPS measurements were also conducted on the cross-section of the HRGO film, exposing the edge planes of HRGO (Figure 2c). Note that the C 1s/O 1s value for the cross-section of HRGO is 1.83, which is 1.9 times lower than the value measured on the HRGO film surface. The measurement for the edge plane indicates an oxygen content that is 1.9 times higher than that at the basal plane, indicating that the edge effect contributes to additional pseudocapacitance. The edge effect of graphite is a well-accepted experimental phenomenon, whereby the area-normalized capacitance of the graphite edge plane is approximately 10 times greater than that of the basal plane.10–15 Using a theoretical approach, we tried to elucidate if this trend could also be observed in RGO. Detailed configurations in our RGO model for DFT calculations are adopted from the XPS analysis. To compare the stabilities of oxygen species on the basal plane and at the edge, the desorption energies of oxygen ions of the RGO model are calculated using eq. 1. A more positive value for Edesorb (in eV) indicates a more stable oxygen ion.

Figure 2. Electrode microstructures and material characteristics. (a) Cross-sectional SEM images of a micro-patterned HRGO film electrode at various magnifications. (b) XRD patterns for the as-prepared GO film (top), HRGO film (middle), and HRGO powder (bottom). The HRGO film has a sufficiently large interlayer space to effectively transport ions because of the coexistence of more-reduced GO (peak 2) and lessreduced GO (peak 1). (c) XPS profiles of the surface of asprepared GO (top) and HRGO (middle) films from subsequent thermal reduction of GO films. The C 1s/O 1s ratio increases from 2.61 to 3.46, implying GO film reduction. XPS profiles of a cross-section of the HRGO film (bottom). The C 1s/O 1s ratio is exhibited for the cross-section of the HRGO film.

The HRGO film has a typical layered structure with a thickness of ~2 μm (Figure 2a and Figure S2). As shown in the magnified cross-sectional SEM image, the films are dense with clefts/channels, while water molecules and f-groups in the GO layers were removed during the heat-treatment. These clefts/channels function as electrolyte ion pathways. The HRGO film surface is slightly rough, with partially reduced GO layer boundaries (Figure S7). Figure 2b shows the XRD patterns of the GO and HRGO films, and GO powder heat-treated at 200 °C (HRGO powder). In the asprepared GO film, the typical XRD pattern of the (002) plane is observed, with a strong, sharp peak at 2θ = ~11.2° (peak 1). After heat treatment at 200 °C, the GO (002) peak moves to 2θ = ~15.8°, and another type of (002) peak emerges at 2θ = ~23.6° (peak 2) in the resulting HRGO film. The right shift and broadened full-width-at-halfmaximum (FWHM) of peak 1 is attributed to vaporization of intercalated H2O molecules. During the out-gassing of H2O, the d002 value is reduced and the thicknesses of the single GO clusters (evaluated using Scherrer’s equation) decreases because the stable GO layers are exfoliated into few-layer pieces. The broader peak 2 with a smaller d002 value originates from the dramatic removal of the f-groups and residual H2O molecules from the exfoliated GO layer (cluster). Importantly, this indicates that HRGO films can be produced at 200 °C. This tendency of the GO film is unique when compared to the GO powder. Only one peak with a broad FWHM exists in the XRD patterns of the HRGO powder. This implies that the intercalated H2O molecules and f-groups easily escape the GO layers during the heat treatment, resulting in evenly reduced GO powder

Figure 3. (a and b) Schematic description of the RGO sheet model for the DFT calculations, and (c) desorption energy (Edesorb) of oxygen ions in the RGO model: (a) top view of the RGO model and (b) side view of the RGO model. The index for the oxygen ions in (a) and (b) is consistent with that in (c).

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RGO with O RGO (1)

Figure 3a and Figure 3b show top and side views, respectively, of the RGO model used in our study. In Figure 3b, the graphene model shows bending toward the basal plane of graphene with no oxygen adsorbed. In Figure 3c, Edesorb > 0.0 implies that the oxygen ion is preferably adsorbed on graphene when μ(O2) = E(O2); Edesorb < 0.0 implies that the oxygen ion is unstable on graphene when μ(O2) = E(O2). In Figure 3, oxygen ions are categorized as those on the basal plane (light blue region) and those at the edge (orange region). Oxygen ions at the edge are predicted to be more stable than those on the basal plane despite the high oxygen coverage at the edge. In the model, an oxygen ion with another oxygen ion on an adjacent site (which eventually forms an oxygen molecule of like geometry) has a relatively low stability with respect to the oxygen ion that is not near any neighboring oxygen species; this causes a deviation of Edesorb (Figure 3c). To investigate the contribution of K to the capacitance behavior of RGO, K ions are adsorbed on oxygen ions on the basal plane and at the edge of the RGO model. For all configurations, K ions are initially located above the oxygen ions that are perpendicular to the RGO substrate using the theoretical bond length of the KO molecule (2.31 Å).43 Figure 4b and 4c provide results for K ions adsorbed at the edge of the RGO model; Figure 4d and 4e provide results for K ions adsorbed on the basal plane of the RGO model. Each panel in Figure 4 includes i) a top view of the fully relaxed structure of RGO with K adsorbate (denoted hereafter as K*, where * indicates adsorption sites), ii) the adsorption energy of K* per K-O bond (Ead(K)), and iii) a table containing partial atomic charge data (second column) and structural deviation data (third column). For Ead(K), its definition is employed because the edge of the RGO model is significantly smaller than that of the basal plane because the model is developed based on a single graphene sheet. By this definition, a direct comparison between the adsorption behaviors of K ions on the basal plane and at edge becomes feasible. The partial atomic charge, q, in the table is derived from the Bader charge analysis. Charges calculated from the Bader analysis are not constrained to integer values, as for other similar approaches based on electronic structure calculations. While a direct quantitative comparison between the partial charge of atoms and their oxidation states can be difficult,44 the qualitative trend of oxidation states can be favorably captured by the Bader charges.45,46 △q from Figure 4b–e is the change of the partial atomic charge with respect to q of the RGO model without K* (Figure 4a). This is to evaluate the electronic redistribution in RGO (specifically, oxygen ions) caused by K*. The key observations in Figure 4 are summarized as follows: K RGO with K RGO K !"# (2) First, Ead(K) (in eV/bond in eq. 2) is persistently higher at the edge than on the basal plane, which indicates the relatively higher stability of K* at the edge. This can be interpreted by a large electron redistribution—specifically,

Figure 4. Deviations in the oxygen partial charges and structure of RGO by K adsorption. (a) RGO without K adsorption; (b) and (c) RGO with K adsorption at the edge, and (d) and (e) RGO with K adsorption on its basal plane. Ead is the adsorption energy of the K ion divided by the number of K-O bonds. q (only for (a)) indicates the oxygen partial charge, △q is the deviation in partial charge of oxygen ions with respect to q in (a). All q values are calculated using the Bader topological analysis. |△d| is the absolute value of the atomic displacement.

electron transfer—from graphene to oxygen ions from bond breaking at the edge. The partial charges of oxygen ions in Figure 4a are consistently low (more negative) at the edge, which implies that oxygen ions at the edge are electron-rich. Even though the partial charges are deviated by K*, this deviation does not alter the trend. Second, △q shows different behaviors with respect to the K* sites. Specifically, K* at the edge induces a notable change in the oxygen partial charge (Figure 4b and 4c). However, the trends in △q are rather inconsistent. In Figure 4b, among the oxygen ions at the edge (O6, O7, and O8), O8 loses electrons by 0.36e. Meanwhile, the other two oxygen ions either minimally gain (O6) or lose (O7) electrons. Opposite behavior is observed in Figure 4c: O7 and O8 gain electrons by K* with a maximum of 0.63. These observations can be interpreted within an electrontransfer picture, one of the well-known mechanisms for capacitance. However, a closer look at how the partial charge of the K ion deviates based on its adsorption on the

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RGO substrate can provide a different interpretation. For all four cases in Figure 4, the partial charge of K deviates by only 0.04, which is one order of magnitude smaller than △q for the oxygen ions. These unbalanced deviations of partial charge between K and the oxygen ions are relatively insignificant in Figure 4d and 4e. However, it is still difficult to find clear evidence of electron transfer from K to the oxygen ions or vice versa. Third, the structural deviation, |△d|, (third column in each table in Figure 4) by K*, with the observed trend of △q, indicates that the partial charge deviation of the RGO substrate is strongly correlated to its structural deviation in response to K*. For K* located on the basal plane of RGO (Figure 4d and 4e), |△d| is insignificant; therefore, the corresponding △q is also insignificant. The large value of |△d| at the edge (O6, O7, and O8 of Figure 4d) describes the local distortion at the edge and parallel shift of the oxygen ions. For K* at the edge (Figure 4b and 4c), |△d| is localized at the oxygen ions of the edge (O6, O7, and O8). The RGO edge has a more open structure than the basal plane and the K ion has multiple and strong electro-static interactions with the oxygen ions with large partial charges; therefore, these electrostatic interactions at the edge readily ignite a large structural deviation, which is followed by significant electronic redistribution. Thus, capacitance enhancement by electron redistribution could be expected at the edge of RGO. To obtain additional atomic–level insights into the electron redistribution of RGO by K adsorption, we analyzed the density of states (DOS) for our RGO models with and without K* (Figure 5). Specifically, the figure shows the DOS of three models representing RGO structures without K adsorption (a), with K adsorption at the edge (b), and with K adsorption on the basal plane (c). Here, Figure 5a, 5b, and 5c correspond to the structures in Figure 4a, 4c, and 4d, respectively. When a K ion is adsorbed at the edge of RGO (Figure 5b), additional states of C and O are observed near the Fermi energy (EF); this is different from that observed for RGO without K adsorption (Figure 5a). This clearly indicates electron redistribution in the RGO structures by K adsorption and supports our interpretations based on partial atomic charges. States from interactions between RGO and K* are observed at approximately -1.5 eV and above 3.0 eV. A similar trend is observed for RGO with K* on the basal plane (Figure 5c). In Figure 5c, hybridization between C and O from RGO and K* is observed near 2.0 eV, and the states near -1.5 eV are reduced with reference to Figure 5b. These different energy levels in Figure 5b and 5c are mostly attributed to their different electro-static interaction trends. All states from K* in Figure 5 are located far from EF. Also, despites the significant difference in Ead (K), the changes of the states below EF caused by K* are insignificant. Electronic redistributions of Figure 5b and 5c are compared with their inset figures showing DOS at -1.0 eV < EF < 1.0 eV. They evidently show that Figure 5b has more states than Figure 5c. Specifically, more O states than C states are observed for both inset figures, and this behavior is more

Figure 5. Density of states (DOS) of (a) RGO only, (b) RGO with K adsorption on its edge, and (c) RGO with K adsorption on its basal plane. The inset for each figure shows the DOS at -1.0 eV < EF < 1.0 eV. obvious in Figure 5b than in Figure 5c. Consequently, the DOS analysis strongly supports our explanations on electron redistributions. To complete our data, adsorption behavior of OH from a KOH electrolyte is also analyzed and compared in the ESI. Similar trend with K case is also observed for OH adsorption. In addition, it will be very interesting to investigate the contribution of the solvation effect to adsorption behavior of K* an OH* since actual RGO operates with KOH electrolyte. However, previous study shows that theoretical solvation energy for OH adsorbed on IrO2 surface is minuscule (-0.21 eV).47 Considering that Ead difference between the adsorbates at edge area and those on basal plane is on the order of 1.0 eV, we believe that considering solvation energy will not alter our quantitative conclusions. Therefore, we do not consider solvation effect in this study. Our future research focusing on our findings from

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Figure 6. Dependence of the electrochemical properties of the micro-supercapacitor on the number of interdigital electrodes. (a) Dimensions of 1-V micro-supercapacitors with 92, 46, and 23 interdigital electrodes and 2 electrodes (no pattern). Geometrical area of the electrodes = width × length × total number of interdigital electrodes + 2 × side × length of the device. The geometrical area of the entire device includes the geometrical area of the positive and negative electrodes and the interspace between them. (b) Volumetric stack capacitance of each device for 10−100 mV/s scan rates. The device with 92 interdigital electrodes exhibits a superior performance. (c) Volumetric stack capacitance of each device at a scan rate of 10 mV/s. The capacitance increases proportionally according to the increase in the number of interdigital electrodes. (d) EIS data (symbols) and fitting results (lines) of the devices with different numbers (92, 46, and 23) of interdigital electrodes under an open circuit voltage. (e) Variation of the fitted electrolyte (Relectrolyte), electrode (Relectrode), and charge transfer (Rct) resistances as a function of the number of interdigital electrodes.

theoretical approaches like electronic structure calculations will tackle this issue in depth. Experimentally, we fabricated micro-supercapacitors with 92, 46, and 23 interdigital electrodes by controlling the width of the interdigital electrodes (400 to 80 μm) and two electrodes with no patterning to determine the effect of the edge plane on the supercapacitive performances, as shown in Figure 6 and Figure S8. Although the geometrical area of the electrodes is reduced as more interdigital electrodes are used (19.7 → 15.6 mm2), the capacitance increases (Figure 6b and 6c, 68 → 98 F/cm3). The length of the interspace increases according to the increase in the number of interdigital electrodes when the width of the interdigital electrodes decreases, leading to a greater number of edge planes. Resultantly, we reasonably assume that the increase in edge planes and the decrease in the width of microelectrodes leads to the increase in the capacitance because of the edge effect and efficient ion transport. The edge planes terminating in numerous f-groups can contribute remarkably to the capacitance increases, and the shortened ion diffusion length according to the decrease in width of microelectrodes can allow for efficient utilization of the electrochemical surface area, establishing an efficient EDL. Additionally, two other kinds of patterns are prepared to determine the dependence of the electrochemical properties on the configuration of the electrodes (Figure S9). The electrodes have the same geometrical area of

microelectrodes as for micro-devices with 96 interdigital microelectrodes. However, the capacitance is not comparable to that for the micro-device with 96 interdigital electrodes. The specification and electrochemical properties are described in more detail in the ESI. To identify the separate contributions of physical adsorption of surface ions and surface redox reactions related to f-groups, EIS measurements were carried out as a function of the number of the interdigital electrodes (92, 46, and 23) using 5.5 M KOH electrolyte, as shown in Figure 6d. These data were analyzed using the equivalent circuit to provide an understanding of the electrochemical processes. R1 is the electrolyte resistance, R2 and Q1 are the resistance and constant phase element, respectively, of the HRGO electrode, R3 and Q2 are the resistance and constant phase element, respectively, of the faradic charge transfer reaction, and WO is the finite length Warburg impedance. The values obtained from the equivalent circuit analysis are summarized in Table S1. Figure 6e shows decreases in the electrode and charge transfer resistances as the number of interdigital electrodes increases, while the electrolyte resistance is fairly constant. By increasing the number of interdigital electrodes by a factor of four, from 23 to 92 (while decreasing the width of the interdigital electrodes from 400 to 80 μm), the electrode resistance is about four times lower, indicating that EDL is efficiently more established, and the charge transfer resistance decreases by a

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Figure 7. Electrochemical measurements for a 1-V micro-supercapacitor. (a) A 1-V micro-supercapacitor obtained by laser drill micro-patterning on an “in-plane” structured HRGO film. (b) Cyclic voltammetry curves at different scan rates. (c) Galvanostatic charge-discharge curves at a constant stack current density of 1 A/cm3. (d) Cycling stability of a 1-V micro-supercapacitor with an HRGO film and 5.5 M KOH electrolyte. The device exhibits an effective cycling stability and retains 83.5% of its initial capacitance performance, even after 10,000 cycles. (e) Specific areal capacitance at scan rates of 10−100 mV/s. The areal capacitance for one electrode and areal stack capacitance reached 202 and 40 mF/cm2, respectively, at 10 mV/s. (f) Specific volumetric capacitance at scan rates of 10−100 mV/s. The volumetric capacitance for one electrode and volumetric stack capacitance reached 503 and 98 F/cm3, respectively, at 10 mV/s. Even at a scan rate of 100 mV/s, the volumetric stack capacitance maintained approximately 74 F/cm3. (g) Electrochemical impedance spectroscopy measurements conducted at 0 V dc bias with a sinusoidal signal of 10 mV from 500 kHz to 10 mHz.

factor of 12. The reduction in the charge transfer resistance is much larger than that of the electrode resistance, suggesting that the edge-plane f-groups play a crucial role in enhancing the pseudocapacitive electrochemical properties of the HRGO microelectrodes. Electrochemical tests were conducted to verify whether our strategies for obtaining a high capacitance could be realized in a micro-device. The specific capacitance was derived from CV curves by integrating the discharge area. The CV curves were recorded over multiple scans in the 10–100 mV/s range (Figure 7b, Figure S10). Several drops of 5.5 M KOH were applied to the active area of the device to act as an electrolyte. The CV patterns are almost rectangular (Figure 7b, Figure S10), even at high scan rates, confirming that an efficient EDL is established, and that fast ion transport is feasible for both micro-electrodes. Galvanostatic CD curves (Figure 7c) are obtained at a constant current density of 1 A/cm3. These CD curves are nearly triangular, confirming the formation of an efficient EDL and effective charge propagation across the two microelectrodes. The areal capacitances of the micro-supercapacitors as a function of scan rate are displayed in Figure 7e. The microsupercapacitor with the HRGO film and 5.5 M KOH electrolyte exhibited an areal capacitance (for one electrode) of 101 mF/cm2 at 10 mV/s. When the loading level of the electrode material was three times higher (HRGO-T), an areal capacitance (for one electrode) of 201.5 mF/cm2 at

10 mV/s is achieved. This value is superior to that seen in a recent study, which reported an areal capacitance of 49 mF/cm2 in an interdigital configuration (Table S2), and is comparable to 205 mF/cm2 at a slow scan rate of 1 mV/s in a three-electrode test.48 The areal stack capacitance of the micro-supercapacitor using the HRGO-T film changed from 40 mF/cm2 at 10 mV/s to 30 mF/cm2 at 100 mV/s (Figure 7e). These results are significantly better than those currently reported for EDL micro-supercapacitors, which typically exhibit areal stack capacitance values from 0.4–2.32 mF/cm2.3,5,49,50 The volumetric stack capacitance values of the microsupercapacitors are displayed as a function of scan rate in Figure 7f. The micro-supercapacitor with the HRGO film and the 5.5 M KOH electrolyte exhibit a volumetric stack capacitance of 98 F/cm3 at 10 mV/s. This is very similar to the value determined for ultrathin planar graphene supercapacitors (85.4 F/cm3, multi-layered, Figure S11, Table S3), and is close to the double-layer capacitance limit reported in our previous paper.20 This is the highest reported value for a micro-supercapacitor containing micrometerthick electrodes.3–5,22–24,37,38,48 For most EDL microsupercapacitors with micro-scale thicknesses, levels of a few farads per cubic centimeter have been reported,3–5,24 as shown in Table S2. However, our micro-supercapacitor provides a stack capacitance that is over 10 times greater than this value. Even at a scan rate of 100 mV/s, a volumetric stack capacitance of 74 F/cm3 could be obtained, which is equivalent to 78% of the result at 10 mV/s. With

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respect to the volumetric capacitance for one electrode, our device has a much higher capacitance (503 F/cm3) than that of a recently reported micro-supercapacitor using carbide-derived carbon (350 F/cm3 in interdigital, 410 F/cm3 in three-electrode configuration)48 and a submicron pseudocapacitor using MoS2 (178 F/cm3).21 Additionally, this value is relatively higher than that of commercially available activated carbon, which has a volumetric capacitance of ~80 F/cm3.51 This unusually high capacitance is accomplished using the in-plane structure and numerous edge planes, combined with the synergistic electrolyte, leading to efficient ion penetration and capacitance. A large difference in capacitance exists between the graphite basal and edge planes (2–7 and 70 μF/cm2, respectively).10-15 Therefore, the edge planes contribute more significantly to the capacitance than the basal planes. We show that this trend could be applicable to RGO in the theoretical calculation section described above. In addition, the edge planes terminating in numerous f-groups can contribute remarkably to additional capacitance increases, in concert with the basal planes of the HRGO containing residual fgroups. This phenomenon has been reported previously,12– 15,20,52,53 and we also demonstrate in the above theoretical calculation that the same phenomenon occurs on the basal and edge planes of reduced graphite oxide. We also showed that f-groups in RGO can significantly contribute to the capacitance.30 In our earlier study, we identified the f-group and well-matched electrolyte combination that leads to additional capacitance. In short, the basic KOH electrolyte produces a large capacitance by reacting with acidic f-groups, such as phenol and carboxyl structures. Compared to the neutral Na2SO4 electrolyte, which does not participate in redox reactions with the f-groups, we discovered that large f-groups contribute to the capacitance (e.g., a 2.3-fold increase). To utilize this phenomenon, we attempted to produce as many edge planes as possible using the laser drill micro-patterning method, and the micro-patterned electrodes with numerous edge planes were exposed to the basic KOH electrolyte for redox reactions with plentiful f-groups. Hence, our microsupercapacitor achieved EDL capacitance and additional pseudocapacitance. A five-fold difference in capacitance exists between the 5.5 M KOH and 1M Na2SO4 electrolytes (as shown in Figure 7e and 7f), as opposed to the 2.3-fold difference reported in our previous study.30 The edge effects were also verified by the impedance data. In Figure 7g, we examine the Nyquist plots measured using 5.5 M KOH and 1 M Na2SO4 electrolytes. In the neutral Na2SO4 electrolyte, the redox reaction with the f-groups is weak and the resulting semi-circle pattern at high frequency, which corresponds to a faradic charge-transfer behavior, is indistinct. In contrast, for the KOH-based electrolyte, a clear semi-circular shape is exhibited at high frequency, indicating redox processing with acidic f-groups. Therefore, considerably more pseudocapacitance can be expected for the KOH electrolyte. In the low-frequency range, the KOH electrolyte trend line is shorter and steeper than that of the Na2SO4 electrolyte. This indicates that the KOH

Page 10 of 14

electrolyte contributes to a significantly higher capacitance and behavior close to that of an ideal capacitor. Therefore, we can expect that microelectrodes with in-plane structures and numerous edge planes will obtain additional pseudocapacitance and EDL capacitance when exposed to the KOH electrolyte. Furthermore, the two-dimensional edge-enhanced-graphene micro-supercapacitors using 5.5 M KOH and a KOH-based polymer-gel electrolyte demonstrate effective cycling stabilities and retain 83.5% and 94.7% of their initial capacitance performances, respectively, even after 10,000 cycles (Figure 7d, Figure S12). We also examined 5-V series-connected microsupercapacitors with an area of 1 × 1 cm2 to confirm the operation of the devices at higher voltages (Figure 8). In addition, the 5-V micro-device was fabricated to demonstrate that a micro-supercapacitor with a high operating voltage can be simply and rapidly realized in a small area. The 5-V micro-supercapacitor was constructed by integrating the abovementioned 1-V units, which were positioned in series to achieve an operating voltage of 5 V across a 1 cm2 area. A detailed fabrication method is given in Figure S3. All other conditions were identical to those used for the 1-V micro-supercapacitor. As with the 1-V

Figure 8. Electrochemical measurements for a 5-V microsupercapacitor over a 1 × 1 cm2 area. (a) A 5-V microsupercapacitor obtained through laser drill micro-patterning on an “in-plane”-structured HRGO film. (b) Cyclic voltammetry curves obtained at different scan rates. (c) Galvanostatic charge-discharge curves measured at a constant current density of 25 mA/cm3. (d) Specific volumetric capacitance at scan rates of 10−100 mV/s. Despite the 5-V series connection, the volumetric stack capacitance of the entire device was approximately 1.44 F/cm3 at 10 mV/s. (e) Electrochemical imped-

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ance spectroscopy measurements conducted at 0 V dc bias with a sinusoidal signal of 10 mV from 500 kHz to 10 mHz.

a

mV/s, our device can function at 1.3 F/cm3. In terms of the volumetric capacitance for one electrode, our 5-V device provides a higher value (199 F/cm3) than a sub-micronscale 1-V pseudocapacitor using interdigitated MoS2 electrodes (178 F/cm3).21 Hence, the combination of the laser drill micro-patterning method and the compact HRGO films containing minimal ion transport channels easily and rapidly produce numerous edge planes and in-plane structures; such unique electrode configurations then lead to efficient ion penetration and capacitance increases when matched with an appropriate electrolyte. As a result, we obtained an exceptionally high capacitance. Note that the unit capacitance of the 5-V micro-supercapacitor is, in fact, less than that of the 1-V micro-supercapacitor. To locate five units within 1 × 1 cm2, the lengths of the interdigitated electrodes are shortened from 1.47 to 1.16 mm. Accordingly, the total area of the edge plane per unit device decreases from 0.60 to 0.47 mm2, presumably reducing the unit capacitance. Moreover, this seems attributable to the reduction of the active area per unit (15.6 → 15.4 mm2, Table S4) and the contact resistance that may occur when the five units are connected in series. Further optimization of the fabrication process will improve the unit capacitance of these micro-supercapacitors with high operating voltages. The overall performances of the micro-supercapacitors are summarized in the form of Ragone plots in Figure 9a and 9b. In terms of the areal energy density, our micro-devices show the best performance in the interdigital configuration (Figure 9a). Interestingly, the areal energy density of our system could be simply enhanced by increasing the amount of HRGO. The energy density of our microsupercapacitor with an HRGO-T film and 5.5 M KOH is 5.4 μWh/cm2, which is about 3.5 times higher than that of a recently reported micro-device with carbide-derived carbon interdigital electrodes.48 The volumetric energy and power densities of our devices are also compared with those of a 500-mAh thin-film Li battery, a 2.75-V/44-μF commercial AC electrochemical capacitor (AC-EC), and a 3V/300-mF Al electrolytic capacitor (Figure 9b). The 1-V micro-supercapacitor yields an energy density of up to 13.7 mWh/cm3, which exceeds that of the thin-film Li battery by a factor of 1.7 and is the highest reported value for any micro-supercapacitor containing micrometer-thick electrodes, as shown in Table S2. In addition, the 1-V micro-supercapacitor delivers a power density of up to 7.4 W/cm3. This is three orders of magnitude higher than that of the thin-film Li battery and approximately seven times higher than that of the commercial AC-EC. This value even approaches that of the Al electrolytic capacitor, which is normally categorized as an ultra-high-power device. The 5V series-connected micro-supercapacitor, which can be run at a high voltage, yields an energy density of up to 5 mWh/cm3. This performance is comparable to that of a thin-film Li battery and is approximately seven times higher than that of the commercial AC-EC. Furthermore, the 5V micro-supercapacitor could transfer a power density of up to 643 mW/cm3. This is ~118 times greater than the corresponding value for the thin-film Li battery, and is comparable to that of the commercial AC-EC. The energy densities of the fabricated devices calculated by including

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micro-device, the CV curves are almost rectangular, even at high scan rates. This again confirms that an efficient EDL is established and that fast ion transport is feasible in both microelectrodes (Figure 8b). In addition, a galvanostatic CD curve (Figure 8c) was obtained at a constant current density of 25 mA/cm3; the CD curve is nearly triangular, again verifying an effective EDL and charge propagation across the two electrodes. The volumetric stack capacitance of the 5-V microsupercapacitor as a function of the scan rate is shown in Figure 8d. The 5-V device with the 5.5 M KOH electrolyte yielded a stack capacitance of 1.44 F/cm3 at 10 mV/s. This value seems to be significantly lower than that reported for the 1-V device (~98 F/cm3), but we must consider that the operating voltage of this series-connected device is 5 V. Although the five units of the 1-V device are connected in series, the stack capacitance of this device is still comparable to that of other EDL micro-supercapacitors examined to date,3–5 as shown in Table S2. Even at a scan rate of 100

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the current collectors, electrodes, and interspace filled with the electrolyte are also shown in Table S5. Overall, these performance results indicate that our devices have sufficient energy for use as micro-scale energy storage devices. Additionally, the high-rate performances of the fabricated micro-supercapacitors are described in the ESI and Figure S13, indicating that our devices are high-energy type power source.

ACKNOWLEDGMENT

4. Conclusion We demonstrated an effective strategy for 2D high-energy edge-enhanced-graphene micro-supercapacitors by controlling the geometry of microelectrodes utilizing a 2D inplane design and the graphene edge-plane effect, which yielded high-capacitance energy-storage characteristics. Through a theoretical approach on the edge of RGO, we determined that the capacitance improvement by electron redistribution could occur in the KOH electrolyte. The highest reported areal and volumetric stack capacitances for a micro-supercapacitor, 40 mF/cm2 and 98 F/cm3, respectively, and an energy density of 13.7 mWh/cm3, which exceeded that of a lithium thin-film battery and is the highest reported value for any micro-supercapacitor with interdigital electrodes, were obtained. This unusually high performance was realized through the in-plane structure and numerous edge planes, combined with a synergistically matched electrolyte, yielding efficient ion penetration and capacitance. The in-plane structure and numerous edge planes were fabricated using a laser drill micropatterning process and a compact HRGO film, which provided the minimal channel space for ion transport. From a practical perspective, these devices could be embedded and utilized as micro-energy storage devices for subminiature electronics, and could provide the required areal and volumetric energy density for such applications.

ABBREVIATIONS

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This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B8-2417-02). C.-W. L. acknowledges supports by Grants NRF-2016M3A7B4024138 (Nano-Material Technology Development Program) and NRF2016M3D1A1027667 (Future Materials Discovery Program).

CD, charge-discharge; CV, cyclic voltammetry; DFT, density functional theory; EDL, electrical double-layer; EIS, electrochemical impedance spectroscopy; FWHM, full-width-at-halfmaximum; GO, graphene oxide; HRGO, heat-treated graphene oxide; RGO, reduced graphene oxide; SEM, scanning electron microscopy; TGA, thermogravimetric analysis; UV, ultraviolet; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction.

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ASSOCIATED CONTENT Supporting Information. Two techniques to obtain superb power and energy characteristics, dependence of the electrochemical properties on the configuration of microelectrodes, high-rate performance of the fabricated devices, adsorption behavior of OH on RGO substrate, schematic of HRGO film preparation, SEM images, micro-supercapacitor fabrication and digital images of the devices, TGA data, CV measurements, cycling stability, EIS fitting parameters, comparisons to other reported devices, performance evaluations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

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