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Dry-Processed, Binder-Free Holey Graphene Electrodes for Supercapacitors with Ultrahigh Areal Loadings Evan D Walsh, Xiaogang Han, Steven D. Lacey, Jae-Woo Kim, John W. Connell, Liangbing Hu, and Yi Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09951 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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ACS Applied Materials & Interfaces

Dry-Processed, Binder-Free Holey Graphene Electrodes for Supercapacitors with Ultrahigh Areal Loadings Evan D. Walsh,1 Xiaogang Han,2 Steven D. Lacey,1,2 Jae-Woo Kim,3 John W. Connell,4 Liangbing Hu,2,* and Yi Lin3,5,* 1

NASA Interns, Fellows, and Scholars (NIFS) Program, NASA Langley Research Center,

Hampton, Virginia 23681; 2Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742; 3National Institute of Aerospace, 100 Exploration Way, Hampton, Virginia 23666-6147; 4Mail Stop 226, Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, Virginia 23681-2199; and 5Department of Applied Science, The College of William and Mary, Williamsburg, VA 23185

*To

whom

correspondence

should

be

addressed:

Y.L.:

[email protected];

L.H.:

[email protected]

Keywords: Supercapacitors, graphene, holey graphene, dry processing, areal capacitance

ACS Paragon Plus Environment

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Abstract For commercial applications, the need for smaller footprint energy storage devices requires more energy to be stored per unit area. Carbon nanomaterials, especially graphene, have been studied as supercapacitor electrodes and can achieve high gravimetric capacities affording high gravimetric energy densities. However, most nanocarbon-based electrodes usually exhibit a significant decrease in their areal capacitances when scaled to the high mass loadings typically used in commercially available cells (~10 mg/cm2). One of the reasons for this behavior is that the additional surface area in thick electrodes is not readily accessible by electrolyte ions due to the large tortuosity. Furthermore, the fabrication of such electrodes often involves complicated processes that limit the potential for mass production. Here, holey graphene electrodes for supercapacitors that are scalable in both production and areal capacitance are presented. The lateral surface porosity on the graphene sheets were created using a facile single-step air oxidation method, and the resultant holey graphene was compacted under ambient conditions into mechanically robust monolithic shapes that can be directly used as binder-free electrodes. In comparison, pristine graphene discs under similar binder-free compression molding conditions were extremely brittle and thus not deemed useful for electrode applications. The coin cell supercapacitors, based on these holey graphene electrodes, exhibited small variations in gravimetric capacitance over a wide range of areal mass loadings of ~1 – 30 mg/cm2 at current densities as high as 30 mA/cm2, resulting in the near-linear increase of the areal capacitance (F/cm2) with the mass loading. The prospects of the presented method for facile binder-free ultrathick graphene electrode fabrication are discussed.


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Introduction Supercapacitors, also known as electrochemical capacitors, have been gaining increased attention for use as versatile energy storage devices because of their high power densities, and long cycling lifetimes.1,2 Most improvements in supercapacitor performance, in terms of their energy densities, come through engineering the electrode material and architectures to increase the amount of surface area in contact with the electrolyte, thereby improving the ability of the ions to diffuse throughout the cell. Graphene, a two-dimensional hexagonal lattice of carbon atoms, has become a popular choice for supercapacitor electrodes due to its high theoretical specific surface area (up to 2675 m2/g), good electrical conductivity, and excellent chemical stability. 3-10 However, the tendency of individual graphene sheets to restack due to strong intersheet interactions significantly decreases its effective surface area. To combat such negative effects from restacking, many schemes have been employed to introduce extra spacing between the individual sheets to form a class of graphene materials or architectures generally referred to as “porous graphene”.11-14 As a sub-category of porous graphene, graphene with through thickness holes, also called “holey graphene” (hG, or “graphene nanomesh”), has recently been drawing attention for enhanced supercapacitor performance. 15-18 The presence of holes allows the facile ion transport through the graphene plane, resulting in significantly reduced tortuosity.15-17,19 In addition to the benefit on the electrode architecture, these holes often feature abundant edge functional groups, providing intrinsically enhanced electrochemical performance at the material level.18,20,21 For example, our team demonstrated that hG from a single-step air oxidation approach exhibited almost twice the capacitance in aqueous electrolytes, in comparison to pristine graphene from thermal exfoliation.18 It was demonstrated that hole edge-attached functional groups, graphitic plane integrity, and available mesopores in the assembly were the key parameters for the observed improvements. Binder-free hG


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supercapacitor electrodes, made with a commonly adopted wet filtration method by passing a hG dispersion through a filtration membrane, exhibited high packing density (1.2 g/cm3) but with retained through-thickness ion transport due to the presence of holes on the graphene sheets.15 As a result, the volumetric capacitances (i.e., capacitance measured by volume) of these electrodes were found much higher than those of pristine graphene with low density (0.2 g/cm3). Performance of supercapacitors, generally in combined consideration of their capacitances, power densities, and energy densities, is usually benchmarked by a device’s gravimetric or volumetric data.22-24 However, the highest reported values for these figures of merit are often based on devices with very small mass loadings that either cannot be scaled up due to fabrication limitations, or whose performance is reduced significantly upon increasing the amount of material, especially in terms of electrode thickness.25 Another measure of cell performance that avoids these problems is the areal specific capacitance. This parameter, by combining the gravimetric capacitance and mass areal density of the electrode, gives an idea of how large a lateral footprint a device requires for practical use. High areal mass loading electrodes with retained gravimetric performance are desirable in practical applications compared to the low loading counterparts. Commercial electrodes based on activated carbon typically have an areal mass loading of 10 mg/cm2.3,25 Therefore, the areal loadings of advanced electrodes demonstrated in research laboratories should be equivalent or even exceed this benchmark for the results to be practically meaningful and competitive. However, such task has been challenging because increasing electrode thickness with mass loading often leads to significant reduction in device performance.3,25 The conventional method to prepare a nanocarbon-based supercapacitor electrode is typically a multi-step “slurry” process involving the use of a polymer binder with an organic solvent.26,27 To make electrodes without the non-conductive binders eliminates the binder


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deadweight and the extra mixing process, and improves electrode fabrication efficiency, conductivity and overall device performance. Among the recent efforts to make binder-free electrodes from graphene materials,27 wet filtration methods have been commonly adopted and have been also applied to hG.15,17,19 However, such processes are usually time consuming because of the need to effectively disperse the graphene in a solvent and subsequent complete solvent removal after filtration. In addition, if an organic solvent is used, the process can be costly and may pose environmental concerns, especially during electrode scale-up. In this article, it is reported that hG prepared from the aforementioned single-step air oxidation method is readily compressible, without the necessity of solvents or binders, into monolithic discs that were directly used as supercapacitor electrodes. The dry-pressing process is highly facile and can be readily scaled up. Binder-free hG-based supercapacitor electrodes of a variety of thicknesses and areal mass loadings were thus made. The areal mass loading for commercial activated carbon electrodes and most literature studies typically do not exceed 10 mg/cm2, whereas increasing the loading or thickness causes the electrode performance to drop significantly due to much increased tortuosity.3 In contrast, the dry-pressed hG electrodes in this work exhibited stable gravimetric capacitance up to an areal density as high as 30 mg/cm2, suggesting consistently low tortuosity despite the increase of the electrode thickness. As a result, ultrahigh areal performances were achieved with these high mass loading dry-pressed hG electrodes.

Results and Discussion Preparation of hG and Fabrication of Dry-Pressed Electrodes One of the most attractive features of the current approach is that both the synthesis of hG and the fabrication of electrodes were carried out through facile solventless techniques that can


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be readily scaled up. hG powder was prepared through a previously described one-step, catalystfree thermal oxidation method.17,18 The as-obtained hG powder was subsequently pressed into monolithic round discs using a hydraulic press at room temperature without any solvents or additives (Scheme 1). These discs were then directly used as supercapacitor electrodes. Without the use of any solvent or binder, the entire process is environmental friendly with gained efficiencies in both time and cost.

Scheme 1. Preparation of hG electrodes using hydraulic dry compression. Dry-pressing G powder results into weak, loosely bound G discs, while hG discs thus prepared are dense and robust. The hG discs can be used as electrodes for supercapacitor applications after removing one or both separation layers. The use of separation layers is necessary to prevent the adhesion of the hG pellet onto the pressing die surface.

In the hG synthesis, commercially available pristine graphene powder (G, prepared from thermal exfoliation of graphene oxide,28 now annually produced in multi-ton quantities in the United States29) was heated in ambient air in an open-ended tube furnace for a set duration. The


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hG product, a lightweight powder, was immediately obtained after cooling the reaction. This material is comprised of graphene sheets with surface holes in the range of 5 – 15 nm in diameter that penetrated through the nanosheet thickness, as shown in a typical scanning electron microscopy (SEM) image in Figure 1a. As reported previously,18 the formation mechanism of hG is the selective carbon oxidation/gasification at the intrinsic surface defects of pristine graphene sheets. In comparison to G, the hG materials had slightly increased specific surface area values (~600 – 700 m2/g for hG vs. ~500 m2/g for G) with enhanced mesopore content. They were also enriched in oxygen (~4% vs. ~1% for hG vs. G, respectively) in the form of hole edge-attached functional groups, such as carboxyl, ketone, and hydroxyl. The heating temperature and duration could be varied during the hG preparation, while still yielding hG products, but with different weight loss values (or “hole density”). The general finding was that the long duration oxidation (10 – 15 h) at the lower temperature domain of the graphene thermal decomposition event (~400 – 450°C in air) often resulted in an optimal combination of mesopore formation, and the enrichment of oxygen functional groups. These merits significantly benefited the intrinsic electrochemical capacitive performance of the hG products.18 Therefore, the hG materials prepared under the above conditions with a hole density of ~20% were selected and used throughout the current work, unless otherwise specified.


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Figure 1. (a) A typical SEM image of a hG nanosheet from preparation at 430°C/10 h. (b) Dependence of hG electrode thicknesses on the areal mass loadings (mA) from dry pressing at 60 and 200 MPa. Dashed lines are linear regressions passing through the origin. (c) and (d) are optical micrographs of cross-sections of hG electrodes both from dry pressing at 60 MPa weighed at ~21 and ~52 mg (or ~12 and ~30 mg/cm2), respectively. The shiny layer at the bottom of each cross-section was the Al foil piece that was left attached. (e) and (f) are photographs of a freestanding 20 mg (or 11 mg/cm2) hG disc from dry-pressing of a hG sample at 200 MPa. Both Celgard separation films that were peeled off from the disc are also shown in (e).

With dry hydraulic pressing at room temperature, the hG powder was highly compressible and formed robust monolithic shapes. In comparison, G pellets prepared under similar conditions, or even greater pressure and under longer duration, were typically loosely bound together and very brittle. Mechanistically, the holes on the hG nanosheets, under the dry pressing conditions, allowed the escape of entrapped air within the powdery sample upon spatial


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confinement of the laterally extended nanosheets. The presence of holes also improved intersheet adhesion in the compressed specimen, with the increased population of the hole edge atoms. When the compacting pressure was released, these inter-sheet interactions also helped retain the as molded disc’s shape with minimal elastic rebound. As a result, the stacked hG sheets retained the compressed and folded form, forming dense discs that were easily handled with tweezers. In order to fabricate electrodes for use in supercapacitors, the hG powder was loaded in between two precisely cut aluminum (Al) foil discs (~1.5 cm in diameter) into a pressing die also with ~1.5 cm in inner diameter, and compressed using a hydraulic press for ~15 minutes. As expected, the thickness of the hG pellet linearly increased with the areal mass loading (mA) at a given compressing load, as shown in Figure 1b. This provided a constant packing density (ρ), which can be calculated from the linear regression slope. For example, at 60 MPa, the thickness values for the hG pellets with masses of 2.2, 11.2, and 51.9 mg (equivalent to mA of 1.3, 6.5, and 30 mg/cm2) were measured to be 18.6 ± 4.6, 110.6 ± 10.0, 467.6 ± 13.7 µm, respectively. The packing density ρ calculated from the linear regression slope was 0.64 g/cm3. The electrode packing density could be increased by further increasing the compressing load. For example, the average ρ value at 200 MPa increased to 0.79 g/cm3. After dry pressing, the Al foil disc on one side of the resulting hG pellet was carefully peeled off using sharp tweezers with the attached opposite foil serving as the electrode support and current collector (Scheme1, Figure 1c and d). For pellets with low mass loadings (< 5 mg), it was often found challenging to separate the second foil piece simply because the pellet was too thin to be effectively handled as a free-standing piece. For those with higher loadings (> 10 mg), both foil pieces were readily peeled off to form a free-standing electrode. However, if directly pressed without any Al foil or other separation layers/films, the hG powder was often found


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partially stuck to the die pieces after compressing, making it difficult to isolate the pellets without affecting their integrity. Other than Al foil, other precisely cut films with flat surfaces, such as porous polypropylene membrane (Celgard), could also be used as the separation layer when preparing larger-mass electrodes, and subsequently peeled off to form free-standing electrodes (Scheme 1, Figure 1e and f). Overall, the solventless and binder-free pressing process for electrode preparation is highly scalable in the amount of hG powder used and the resulting size (both thickness and lateral area) of the final electrodes. It is also worth re-emphasizing that the dry compressibility is unique for hG, while a G electrode prepared using the similar process could not hold their shape when the areal mass loading was larger than 1 mg/cm2. It has already been demonstrated that hG has intrinsically higher capacitive performance than G, due to higher mesopore fraction and enriched oxygen functional groups, thus, the results reported here focus on hG only.18

Supercapacitor Properties of Dry-Pressed hG Electrodes A key advantage for the dry-pressing-based process is its excellent scalability because there is no theoretical limit for the amount of hG material to be placed into the pressing die. This enables convenient tuning of electrode thickness to reach ultrahigh areal mass loadings for supercapacitor electrodes. However, the challenging question is how well such thick electrodes would perform electrochemically. To address this question, hG electrodes with the same diameter but varying masses were prepared under a pressure of 60 MPa as discussed in the previous section. The electrode masses ranged from ~2 mg to as high as ~50 mg, corresponding to a rather wide areal mass loading range of ~1 – 30 mg/cm2 per electrode. Electrochemical properties of symmetric supercapacitor devices, each with two identical dry-pressed hG electrodes, were evaluated in the form of CR2032 coin cells. In each cell, a


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common organic electrolyte (1 M tetraethylammonium tetrafluoroborate, or TEABF4, in acetonitrile) and a Celgard membrane separator were used. Although some volumetric expansion of the dry-pressed electrodes was observed after addition of electrolyte solution, the assembled coin cell devices were robust and exhibited excellent performance, as discussed below. The cyclic voltammetry (CV) curves at scan rates of 10 and 100 mV/s for devices with 5 different areal mass loadings (1, 3, 6, 12, and 30 mg/cm2 per electrode) are shown in Figure 2a and b, respectively. Rectangular-shaped CV curves are typical indicators for domination of electrical double-layer charge storage. All curves at 10 mV/s exhibited such rectangular shapes (Figure 2a). At 100 mV/s (Figure 2b), only the CV curve for the device with the highest loading (30 mg/cm2) exhibited slight shape distortion indicating somewhat lower yet still reasonable rate performance. In addition, the overall current response and the enclosed area by the curve also increased with the increase of areal mass loading. In fact, the current values at 2.7 V were almost linearly dependent on the areal mass loading at both scan rates (Figure 2c), indicating excellent total capacitance scaling in the entire mass loading range for these devices.


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Figure 2. Scalability of symmetric supercapacitors with dry-pressed hG electrodes: (a) and (b) are CV curves at scan rates of 10 and 100 mV/s, respectively, for devices with per electrode areal mass loadings (mA) of (from inner to outer) 1 (black), 3 (green), 6 (blue), 12 (dark red), and 30 (red) mg/cm2, respectively; (c) the dependence of current values at 2.7 V vs. mA at both scan rates; (d) and (e) are the GCD curves at current densities of 0.25 and 1 A/g, respectively for the same supercapacitors (with the same color designations); (f) the relationship of gravimetric (Cm, black) and areal (CA, red) capacitances, respectively, vs. mA at both current densities.

The findings from CV curves were further corroborated with galvanostatic chargedischarge (GCD) measurements. In these experiments, gravimetric capacitance (Cm) of the supercapacitor electrodes were calculated according to the discharge time (t): Cm = 4×I×t/(V×m), where I is the applied current, V is the actual working voltage window of the discharge event after considering the internal resistance (iR) drop (V = 2.7 – ViR), and m is the total mass of applied hG in the device including both electrodes.30 GCD data at current densities of 0.25 and 1 A/g for the same above-mentioned devices are compared in Figures 2d and e, respectively. The large iR drops at 1 A/g for high mass loading devices were due to very high absolute current


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applied, e.g. up to ~100 mA for the device with 30 mg/cm2 electrodes. The iR (iR = ViR/I) was calculated to be only ~3.6 Ω, among the lowest for electrodes with various mass loadings, suggesting the excellent charge transfer properties in these electrodes. Nevertheless, without considering the iR drops, all GCD curves were quite symmetric, suggesting that the charge and discharge processes were highly reversible. Also, the slopes for all discharge curves at the same current densities were similar, suggesting similar Cm values despite the wide mass loading range. For example, all Cm values at 0.25 A/g fell in the range of ~ 37 – 45 F/g (Figure 2f). This behavior was quite similar at 1 A/g; even the device with 30 mg/cm2 mass loading only exhibited a < 25% decrease in Cm when compared to that with 12 mg/cm2 mass loading. Capacitance values calculated from integration of CV curves followed a similar trend (Supporting Information Figure S1). Such capacitance retention strongly suggests the rather negligible tortuosity increase in the mass loading range in the current study, which could be readily attributed to the presence of through-thickness holes of hG sheets and the sufficient inter-sheet ion transport pathways for the dry-pressed electrodes. The high electrode mass loadings, owing to the unique compressibility of hG powders, resulted in ultrahigh areal capacitance (CA = Cm × mA). As also shown in Figure 2f, CA increased almost linearly with mA because Cm remained fairly independent of mass loading at a discharge current of 0.25 A/g. The linear trend was similar at 1 A/g (and at 10 and 100 mV/s; see Figure S1). With areal mass loading of 30 mg/cm2, ultrahigh areal capacitances of 1.25 and 0.86 F/cm2 were achieved at 0.25 and 1 A/g, respectively. These were among the best performing neat carbon-based supercapacitor electrodes in the literature (Supporting Information Table S1), despite the relatively low intrinsic Cm resulting from the multi-layered nature (5 – 7 atomic layers on average17,18) of the starting graphene used in this work.


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For nanomaterial-based supercapacitor studies, it is common practice to acquire and interpret capacitance data in terms of gravimetric current density (Im) in order to directly compare the intrinsic electrochemical performance of various materials. However, for actual device development, especially for the currently discussed devices with high areal capacitances, it is also informative to discuss results in terms of areal current density (IA). In Figure 3a, the Cm values obtained at various Im (0.25, 0.5, 1, and 2 A/g) for all five devices with different mass loadings are plotted against IA. It appears that all Cm values exhibited an inverse linear relationship with IA. The electrode capacitance with the fixed electrode area (~1.73 cm2) was largely independent of its total mass, but only decreased with higher applied current. This result strongly suggests that the whole mass of each electrode in the entire mass loading range investigated (1 – 30 mg/cm2) was effectively utilized in the electrical double-layer charge storage. At a lower applied current region (< 30 mA/cm2), the Cm can be considered as an approximate constant (40 ± 5 F/g) despite the mass loading. The decrease in performance with higher IA was due to ineffective pore access during ultrafast ion exchange, which was likely the intrinsic kinetic limitation of the current material.


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Figure 3. Dependence of (a) gravimetric (Cm) and (b) areal capacitance (CA) values on the areal current density (IA) for hG electrodes with various per electrode areal mass loadings (mA) of 1 (black), 3 (green), 6 (blue), 12 (dark red), and 30 (red) mg/cm2, respectively. The blue-banded region in (a) is a highlight indicating a near-linear relationship of Cm vs. IA. The black-colored dashed lines in (b) are guidelines indicating the trends at each different denoted gravimetric current density (Im). The red-colored dashed line indicates the data for the device with a mA of 30 mg/cm2.

Plotting CA against IA provided a visual viewpoint of the scalability for the current system. As shown in Figure 3b, at a lower Im (such as 0.25 and 0.5 A/g), the CA values almost linearly increased with the mass loading, similar to what was portrayed in Figure 2f. This trend only leveled off when a higher IA was applied (such as the devices with mass loadings of 12 and 30 mg/cm2 at 2 A/g). The extrapolation of the data indicate that, as long as IA is kept below the same threshold of ~30 mA/cm2 as where Cm remained relatively constant, the electrode in the device can be furthered scaled up, in a near-linear fashion, to achieve even higher CA. Supercapacitors with dry-pressed hG electrodes generally exhibited good cyclability. For example, as shown in Figure 4a, the capacitance of the device with 12 mg/cm2 electrodes (from 1 ton) still retained ~80% with near 100% Coulombic efficiency after 10,000 cycles at Im of 1 A/g (or IA ~ 24 mA/cm2). Cyclability of the device with 30 mg/cm2 electrodes (from 1 ton) decreased at the same Im, but still retained ~65% after 2,000 cycles (>99% Columbic efficiency) (Figure


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4b). However, it should be noted that the device was cycled at a much higher areal current density (IA ~ 60 mA/cm2).

Figure 4. Cycling performance, including percentage capacitance retention (red, open circles) and the corresponding Coulombic efficiency (black, solid squares) of hG supercapacitor devices with (a) mA of 12 mg/cm2 operated at 1 A/g (or 24 mA/cm2) and (b) mA of 30 mg/cm2 operated at 1 A/g (or 60 mA/cm2).

The excellent electrochemical performance for devices with dry-pressed hG electrodes were further studied with electrochemical impedance spectroscopy (EIS) measurements. As shown in Figure 5a, the Nyquist plots for devices with 1, 6, and 30 mg/cm2 mass loadings exhibited vertical lines in the low frequency region, indicating excellent capacitive behavior for all samples.31 These lines shared similar slopes, indicating similar capacitance values. Equivalent series resistance (ESR) values were estimated by extending this line to the real impedance (Z') axis, and the results (on the order of ~3 – 10 Ω) were consistent with the calculations from the iR drops in GCD measurements. At the high frequency region, devices with higher mass loadings exhibited smaller semicircles, indicating lower charge-transfer resistance. Higher mass loading devices typically exhibited slightly lower resistance in the high frequency region, likely due to the better contacts from the larger thicknesses within the coin cell assembly. Increasing mass


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loading did result in reduced ion transport, as evident at the mid-frequency region (indicated by two green-colored dots on each curve) where the slopes were nearly 45°, which appeared to be the more distinctive for the device with the higher mass loading. The higher frequency ends of the ion transport event were similar (~120 Hz) for all devices, but the higher mass loading devices extended into lower frequencies (low frequency ends were approximately 50, 2, and 0.1 Hz for devices with 1, 6, 30 mg/cm2 mass loadings, respectively).

Figure 5. EIS measurement data. (a) Nyquist plots, (b) imaginary gravimetric capacitances [Cm″(ω)] and (c) total areal capacitance [CA(ω)] for hG supercapacitor devices with various mA of 1 (black), 6 (blue), and 30 mg/cm2 (red). The green-colored dots in (a) highlight the approximate mid-frequency region dominated by ion transport events (approximately 120 – 50, 120 – 2, and 120 – 0.1 Hz for devices with 1, 6, and 30 mg/cm2, respectively).

The imaginary capacitance Cm"(ω) and the total areal capacitance CA(ω) were plotted against the frequency (f), as shown in Figure 5b and c, respectively. The gravimetric capacitance values were similar for all three devices, and therefore their areal performance scaled with the mass loading, consistent with the findings from CV and GCD data. Also, there was an increase in relaxation time constant (τ0 = 1/f0, where f0 is the peak frequency value from Cm"(ω))32 with the mass loading (τ0 = 1.9, 7.7, and 39 s for 1, 6, and 30 mg/cm2 devices, respectively), again consistent with the somewhat reduced ion transport efficiency, and thus rate performance. In general, the relaxation time values for the medium mass loading devices (5 - 15 mg/cm2) were


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comparable with commercial activated carbon-based device (~10 s).23 Interestingly, while higher pressing load increased the packing density of the electrodes, as discussed in the previous section, the further τ0 increase was rather subtle at the same high mass loading (39 and 50 s for 30 mg/cm2 electrodes prepared at 200 MPa and 580 GPa, respectively), indicating that the mesopores still remained largely accessible to the ions.32 This result confirmed that the holey nature of the hG sheets in the dry-pressed electrodes remained functional in the facilitation of the ion transport through the electrode thickness, despite the compressing pressure, which is important to further improve the device volumetric performance. The gravimetric (Em) and areal energy density (EA) values followed similar trends as Cm and CA values, due to their near-proportional relationship: Em or A = (1/8) Cm or A V2, where V is the actual working voltage window after subtracting the iR drop. As shown in the gravimetric Ragone plot in Figure 6a, the Em values remained rather constant (~8 – 10 Wh/kg) at lower Im (< 0.5 A/g) for all electrodes. In comparison, the EA values increased almost linearly with mass loadings in the same current region (Figure 6b). The highest value was ~2.7 Wh/m2 obtained at 0.25 A/g for the device with two 30 mg/cm2 electrodes. The working voltage window (V) did decrease when higher mass loading and higher Im were used (e.g., Figure 2e), which contributed to the less EA improvement under those conditions. The evolution of gravimetric (Pm) and areal power density (PA) values (Pm or A = Em or A/∆t, where ∆t is the discharge time) are also shown in the Ragone plots in Figure 6. Although Pm values slightly decreased with mass loadings at each Im used (e.g. 336 vs. 310 W/kg at 0.25 A/g and 1313 vs. 974 W/kg at 1 A/g for mass loadings of 1 vs. 30 mg/cm2), PA values significantly increased even at higher current densities. For example, the PA values at 1 A/g were 14.7 W/m2 for the device with 1 mg/cm2 mass loading, but increased to a value of 287 W/m2 for that with 30 mg/cm2 mass loading (i.e., a 1.5-cm diameter coin cell provided a total power of 49 mW). Overall, the areal power performance of the devices based on


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the dry pressed electrodes with ultrahigh areal loadings exceeded most devices reported in the literature, while the areal energy performances of these devices were on par, despite the relatively low intrinsic capacitance values (Table S1).

Figure 6. (a) Gravimetric- and (b) areal-based Ragone plots for hG supercapacitor devices with various mA of 1 (black), 3 (green), 6 (blue), 12 (dark red), and 30 (red) mg/cm2, respectively. Dashed lines are guides to the eye for the various current densities used in the measurements.

Conclusions A facile processing technique was demonstrated to fabricate hG-based electrodes for supercapacitors that have exceptionally high areal specific capacitances at high areal mass loading. This was enabled by the facile scalable preparation of hG and the unique dry compressibility of this material. Despite a wide variation in the electrode areal mass loading (1 – 30 mg/cm2), the gravimetric capacitances of the electrodes were consistent with the varying areal current densities (up to ~30 mA/cm2 measured in this work), which led to proportionally increased areal capacitances with the increase of mass loading but retained low tortuosity. The results suggested that the device areal performance might be further scaled at even higher hG mass loadings as long as the areal current density does not exceed the above threshold. The ease


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of hG preparation and its ability to maintain its electrochemical properties at high mass loadings demonstrates the potential of this material to be a superb candidate for practical supercapacitor applications. It is highly expected that further optimization of the nanosheet layer number, the physical and chemical characteristics of the holes of the hG materials, and the electrode packing density integrated with the molecular size and chemistry of the electrolyte, will result in scalable devices fabricated with this fast, low-cost, and environmentally friendly methodology with supercapacitor performances superior to the current state-of-the-art. Furthermore, these drypressed monolithic ultrathick hG electrodes with high through-thickness mass transport properties are also being explored for other high performance energy storage devices in our laboratories and will be reported separately.

Methods Materials. Graphene was provided by Vorbeck Materials (Vor-X reduced 070; lot: BK77x). Tetraethylammonium tetrafluoroborate (TEABF4, electrochemistry grade, ≥ 99.0%) and acetonitrile (HPLC grade, ≥ 99.93%) were purchased from Aldrich. All chemicals were used as received. Al foil (15 µm in thickness), Celgard membranes (25 µm in thickness), CR2032 coin cell cases, and other accessories, such as stainless steel spacers and springs, were all purchased from MTI Corporation. Preparation of hG. The starting graphene powder (0.3 – 1.5 g) was placed in an alumina or quartz crucible and heated in static air with an open-ended tube furnace (MTI Corporation; Model OTF-1200X-80-II) at a ramp rate of 10 °C/min and held isothermally at 430 °C for 10 h. After cooling to room temperature, hG was obtained as a black powder. Fabrication of hG Electrodes. Separation layer discs (Al foil or Celgard, 1.5 cm in diameter) were first cut using a Compact Precision Disc and Ring Cutter (MTI Corporation;


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Model MSK-T-08) equipped with interchangeable 15 and 19 mm cutting dies. One piece of a separation layer disc was first loaded in a stainless steel forming die with an inner diameter of 14.85 mm (MTI Corporation; Model EQ-Die-15D), followed by a measured amount of hG powder (~2 – 50 mg), and then another separation layer. The die was then placed in a hydraulic press (Carver Hydraulic Unit Model #3925) and a predetermined load was applied. In this work, ~10 and ~35 kN were initially applied (equivalent to compressing pressure values of ~60 and ~200 MPa, respectively, for the 14.85-mm diameter die). After a predetermined duration (typically 15 minutes), the die was unloaded, and the pellet was removed from the die. One or both separation layers were then gently peeled off using tweezers. Most electrodes used in this work were hG pellets with one side still attached with an Al foil disc as a current collector. Supercapacitor Assembly. Symmetric supercapacitor devices were assembled in the format of CR2032 coin cells in an Ar-filled glove box. Typically, one electrode was first placed in the cathode case, followed by the addition of a few drops (~100 µL) of electrolyte (1 M TEABF4 in acetonitrile). Two 19-mm Celgard separation layers soaked with the same electrolyte were then placed on top of the electrode, followed by placing another identical electrode with a few more drops of electrolyte. Stainless steel spacers and a wave spring were then placed on the top, and then the anode case. The entire assembly was placed in a hydraulic crimping device (MTI Model MSK-110) and crimped at a pressure of ~5.5 MPa affording a functional device. Measurements. Scanning electron microscopy (SEM) images were acquired using a Hitachi S-5200 field emission SEM (FE-SEM) system at an acceleration voltage of 30 kV. X-ray photoelectron spectroscopy (XPS) data were collected on a Kratos Axis 165 X-ray photoelectron spectrometer operating in hybrid mode using monochromatic Al Kα x-rays (1486.7 eV). Brunauer-Emmett-Teller (BET) surface area values and Barrett-Joyner-Halenda (BJH) pore


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characteristics were obtained from nitrogen adsorption-desorption isotherms collected using a Quantachrome Nova 2200e Surface Area and Pore Size Analyzer system. All electrochemical measurements were conducted using a Bio-Logic VMP3 electrochemical station. For cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements, a voltage window of 2.7 V was used with 1 M TEABF4 in acetonitrile as the electrolyte. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 1 MHz to 0.01 Hz with an amplitude of 10 mV and 10 points measured per decade. Calculations of the real [C'(ω)], imaginary [C"(ω)], and total [C(ω)] capacitances were based on the following formula:15

Cm'(ω) = 4 × |Z"(Ω)|/[m × 2πf × (|Z'(Ω)|2 + |Z"(Ω)|2] Cm"(ω) = 4 × |Z'(Ω)|/[m × 2πf × (|Z'(Ω)|2 + |Z"(Ω)|2] Cm (ω) = [(Cm'(ω))2 + (Cm"(ω))2]½ CA(ω) = Cm(ω) × mA where m is the total mass of the electrodes, f is the frequency, Z'(Ω) and Z"(Ω) are the real and imaginary parts of the impedance, respectively, and mA is the areal mass loading of a single electrode.

Acknowledgements. Y.L. gratefully acknowledges the financial support from the NASA Langley Internal Research and Development (IRAD) Program and the Leading Edge Aeronautics Research for NASA (LEARN) program (Grant number NNX13AB88A). E.D.W. and S.D.L. (partially) were IRAD-supported interns under the supervisions of Y.L. and J.W.C. via NASA Interns, Fellows, and Scholars (NIFS) Program.


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Supporting Information Available: A plot on capacitance values calculated using CV data and a table comparing the gravimetric and areal performances of dry-pressed holey graphene supercapacitor electrodes reported in this work to other graphene/holey graphene-based electrodes in the recent literature. This material is available free of charge via the Internet at http://pubs.acs.org.


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Dry-Processed, Binder-Free Holey Graphene ... - ACS Publications

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