Engineering three-dimensional hybrid supercapacitorsand microsupercapacitors for high-performanceintegrated energy storageMaher F. El-Kadya,b, Melanie Ihnsa, Mengping Lia, Jee Youn Hwanga, Mir F. Mousavia,c, Lindsay Chaneya,Andrew T. Lecha, and Richard B. Kanera,d,1

aDepartment of Chemistry and Biochemistry and California NanoSystems Institute, University of California, Los Angeles, CA 90095; bDepartment ofChemistry, Faculty of Science, Cairo University, Giza 12613, Egypt; cDepartment of Chemistry, Tarbiat Modares University, Tehran 14115-175, Iran;and dDepartment of Materials Science and Engineering, University of California, Los Angeles, CA 90095

Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved March 4, 2015 (received for review October 30, 2014)

Supercapacitors now play an important role in the progress ofhybrid and electric vehicles, consumer electronics, and military andspace applications. There is a growing demand in developinghybrid supercapacitor systems to overcome the energy densitylimitations of the current generation of carbon-based supercapa-citors. Here, we demonstrate 3D high-performance hybrid super-capacitors and microsupercapacitors based on graphene and MnO2

by rationally designing the electrode microstructure and combin-ing active materials with electrolytes that operate at high vol-tages. This results in hybrid electrodes with ultrahigh volumetriccapacitance of over 1,100 F/cm3. This corresponds to a specific ca-pacitance of the constituent MnO2 of 1,145 F/g, which is close tothe theoretical value of 1,380 F/g. The energy density of the fulldevice varies between 22 and 42 Wh/l depending on the deviceconfiguration, which is superior to those of commercially availabledouble-layer supercapacitors, pseudocapacitors, lithium-ion capaci-tors, and hybrid supercapacitors tested under the same conditionsand is comparable to that of lead acid batteries. These hybridsupercapacitors use aqueous electrolytes and are assembled in airwithout the need for expensive “dry rooms” required for buildingtoday’s supercapacitors. Furthermore, we demonstrate a simpletechnique for the fabrication of supercapacitor arrays for high-volt-age applications. These arrays can be integrated with solar cells forefficient energy harvesting and storage systems.

supercapacitor | microsupercapacitor | graphene | metal oxide

As a result of the rapidly growing energy needs of modern life,the development of high-performance energy storage devi-

ces has gained significant attention. Supercapacitors are promisingenergy storage devices with properties intermediate between thoseof batteries and traditional capacitors, but they are being im-proved more rapidly than either (1). Over the past couple ofdecades, supercapacitors have become key components of every-day products by replacing batteries and capacitors in an increasingnumber of applications. Their high power density and excellentlow-temperature performance have made them the technology ofchoice for backup power, cold starting, flash cameras, regenerativebraking, and hybrid electric vehicles (2, 3). The future growth ofthis technology depends on further improvements in energy den-sity, power density, calendar and cycle life, and production cost.According to their charge storage mechanism, supercapacitors

are classified as either electric double-layer capacitors (EDLCs)or pseudocapacitors (2). In EDLCs, charge is stored throughrapid adsorption–desorption of electrolyte ions on high-surface-area carbon materials, whereas pseudocapacitors store chargevia fast and reversible Faradaic reactions near the surface of metaloxides or conducting polymers. The majority of supercapacitorscurrently available in the market are symmetric EDLCs featuringactivated carbon electrodes and organic electrolytes that providecell voltages as high as 2.7 V. Although these EDLCs exhibit highpower density and excellent cycle life, they suffer from low energy

density because of the limited capacitance of carbon-based elec-trodes. The specific pseudocapacitance of Faradaic electrodes(typically 300–1,000 F/g) exceeds that of carbon-based EDLCs;however, their performance tends to degrade quickly upon cycling.Studies during the past few years have demonstrated an at-

tractive alternative to conventional EDLCs and pseudocapaci-tors by using hybrid systems. Using both Faradaic and non-Faradaic processes to store charge, hybrid capacitors can achieveenergy and power densities greater than EDLCs without sacri-ficing the cycling stability and affordability that have so far lim-ited the success of pseudocapacitors (4). Several combinationsof materials, such as RuO2 (5), Co3O4 (6), NiO (7), V2O5 (8),Ni(OH)2 (9), and MnO2 (10), have been studied for preparinghybrid supercapacitors. Among these, MnO2-based systems areparticularly attractive as MnO2 is an earth-abundant and envi-ronmentally friendly material with a high theoretical specific ca-pacitance of 1,380 F/g (11). However, the poor ionic (10−13 S/cm)and electronic (10−5–10−6 S/cm) conductivity of pristine MnO2often limits its electrochemical performance. Recent reportsshow that some high-performance results can be achieved onlyfrom ultrathin MnO2 films that are a few tens of nanometers inthickness (12). Nevertheless, the thickness and the area-nor-malized capacitance of these electrodes are not adequate formost applications. A promising approach to realize practicalapplications of MnO2 is to incorporate nanostructured MnO2 onhighly conductive support materials with high surface areas suchas nickel nanocones (13), Mn nanotubes (14), activated carbon(15), carbon fabric (16), conducting polymers (17), carbonnanotubes (18, 19), and graphene (20–24). Although promising

Significance

Batteries run just about everything portable in our lives suchas smartphones, tablets, computers, etc. Although we havebecome accustomed to the rapid improvement of portableelectronics, the slow development of batteries is holding backtechnological progress. Thus, it is imperative to develop newenergy storage devices that are compact, reliable, and energydense, charge quickly, and possess both long cycle life andcalendar life. Here, we developed hybrid supercapacitors thatcan store as much charge as a lead acid battery, yet they canbe recharged in seconds compared with hours for conven-tional batteries.

Author contributions: M.F.E.-K. designed research; M.F.E.-K., M.I., M.L., J.Y.H., and L.C.performed research; A.T.L. contributed new reagents/analytic tools; M.F.E.-K., M.I., M.L.,J.Y.H., M.F.M., and R.B.K. analyzed data; and M.F.E.-K. wrote the paper.

Conflict of interest statement: This work was supported by Nanotech Energy, Inc.

This article is a PNAS Direct Submission.1To whom correspondence should be addressed. Email: kaner@chem.ucla.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1420398112/-/DCSupplemental.

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specific capacitances of 148–410 F/g have been achieved, suchvalues were obtained only under slow charge–discharge rates andthey were found to decrease rapidly as the discharge rate wasincreased. Moreover, many of these materials have low packingdensity with large pore volume, meaning that a huge amount ofelectrolyte is needed to build the device, which adds to the massof the device without adding any capacitance (25). Accordingly,the energy density and power density of these systems are verylimited on the device level. To solve these critical problems,we have developed promising hybrid electrodes based on 3Dgraphene doped with MnO2 nanoflowers. By rationally designingthe structure of the graphene substrate to achieve high conduc-tivity, suitable porosity, and high specific surface area, one mayexpect to not only achieve a high gravimetric capacitance, butalso to improve the volumetric capacitance (23). Furthermore,the high surface area of nanostructured MnO2 provides moreactive sites for the Faradaic reactions and shortens the ion dif-fusion pathways that are crucial for realizing its full pseudoca-pacitance. We show that hybrid supercapacitors based on thesematerials can achieve energy densities of up to 42 Wh/l com-pared with 7 Wh/l for state-of-the-art commercially availablecarbon-based supercapacitors. Interestingly, these graphene–MnO2 hybrid supercapacitors use aqueous electrolytes and areassembled in air without the need for the expensive dry roomsrequired for building today’s supercapacitors.Whereas great efforts have been made for the fabrication

of macroscale hybrid supercapacitors, there are only a fewstudies on the design and integration of hybrid materials intomicrosupercapacitors (26). This is likely due to complicatedmicrofabrication techniques that often involve building 3Dmicroelectrodes with micrometer separations. Here, we presenta simple, yet versatile technique for the fabrication of 3D hybridmicrosupercapacitors based on graphene and MnO2. These mi-crodevices enable an ultrahigh capacitance per footprint approach-ing 400 mF/cm2, which is among the highest values achieved forany microsupercapacitor (26). They can also provide an energydensity of up to 22 Wh/l, more than two times that of lithium thin-film batteries. These developments are promising for micro-electronic devices such as biomedical sensors and radiofrequencyidentification tags where high capacity per footprint is crucial.

ResultsIn designing supercapacitor electrodes, special efforts are madeto ensure that they are capable of providing high energy density

and high power density. This requires optimization of thepreparation conditions to facilitate ionic and electronic trans-port within the electrodes (27) as illustrated in Fig. 1 (see alsoSI Appendix, section 1).

Synthesis and Characterization of 3D Macroporous Laser-Scribed-Graphene–MnO2 Electrodes. To experimentally realize energy-denseand high-power supercapacitor electrodes, we integrated a highlyconductive and high-surface-area 3D laser-scribed graphene (LSG)framework with MnO2 as schematically illustrated in Fig. 2A. The3D LSG framework was produced from the laser scribing of GOfilms following our previously reported method (27), upon whichthe color changes from golden brown to black. The LSG frame-work was subsequently coated in situ with MnO2 using an elec-trochemical deposition technique as described in Materials andMethods; Fig. 2B. Note in Fig. 2B that the graphene electrode turnsdarker in color after electrodeposition, a visual indication of theloading of MnO2. It is well accepted that the conductivity and massloading of the active materials have a significant impact on theelectrochemical behavior of supercapacitor electrodes. Here, themass loading of MnO2 is controlled by adjusting the depositioncurrent and deposition time. Fig. 2C shows that the MnO2loading changes almost linearly with the deposition time at anapplied current of 0.25 mA/cm2 and an average deposition rateestimated to be ∼6 μg/min.In addition to interesting electrical properties, the LSG–MnO2

electrodes are monolithic and demonstrate superb mechanicalintegrity under large mechanical deformation. Fig. 2D showsthat an LSG–MnO2 electrode can be bent significantly withoutdamage. We evaluated the foldability of LSG–MnO2 electrodesby measuring their electrical resistance under successive bendingcycles. The resistance varies only slightly up to a bending radiusof 5.0 mm and can be completely recovered after straighteningno matter whether the bending is positive (convex) or negative(concave). Notably, after 1,000 cycles of bending and straight-ening at a concave bend radius of 5.0 mm, the resistance hasincreased by only about 2.8%; Fig. 2E.

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Fig. 1. Rational design of high-energy–high-power hybrid supercapacitorelectrodes. Improving the ionic current (IC) and electronic current (EC) withinthe electrode is a key. Different approaches have been explored including(A) compact thick films of metal oxide (here, MnO2); (B) nanostructuredmetal oxide films; (C) addition of conductive materials to the nanostructuredmetal oxide; and (D) our approach is growing nanostructured metal oxideon 3D interconnected graphene networks with high surface area and highelectronic conductivity.

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electrode.

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The evolution of morphology corresponding to different de-position times was examined by scanning electron microscopy(SEM); Fig. 3 A–D. The SEM micrographs show the generalmorphology and detailed microstructure of a typical sampleprepared by 120 min of deposition. MnO2 has been uniformlycoated onto the surface of graphene throughout the entire film.Moreover, the electrodeposited MnO2 particles show a nano-flower-shaped hierarchical architecture with a clear interface be-tween MnO2 and the graphene substrate which is consistent withprevious studies (20). Closer inspection of the MnO2 nanoflowersshows that they are made up of hundreds of ultrathin nanoflakesthat are 10–20 nm thick (see also SI Appendix, Fig. S3). Thesenanoflakes are interconnected together to form mesoporousMnO2 with a large accessible surface area, thus offering numer-ous electroactive sites available to the electrolyte which promotesfast surface Faradaic reactions.The 3D structure of LSG–MnO2 electrodes was further ana-

lyzed using cross-sectional SEM; Fig. 3D. The 3D porous structureof LSG is preserved after the deposition of MnO2 without anyagglomerations. The graphene surface has been uniformly coatedwith MnO2 over the entire cross-section. In addition, energy-dispersive X-ray spectroscopy (EDS) provides elemental maps of

C, O, and Mn, which confirms that a homogeneous coating ofMnO2 throughout the 3D macroporous framework has been cre-ated. The X-ray photoelectron spectroscopy (XPS) data shownin Fig. 3 F and G further confirm the chemical composition ofthe deposited oxide; see SI Appendix for details.

Assembly and Electrochemical Performance of Symmetric LSG–MnO2

Supercapacitors. To test the electrochemical performance of LSG–

MnO2 macroporous frameworks, a supercapacitor pouch cell wasassembled from two symmetric electrodes separated by a Cel-gard M824 ion porous separator and impregnated with 1.0 MNa2SO4 electrolyte; Fig. 4A. The cells were tested by cyclicvoltammetry (CV) over a wide range of scan rates from 1 to1,000 mV/s. As an example, consider the LSG–MnO2 sample witha deposition time of 3 min: the supercapacitor shows nearly rect-angular CV profiles up to a scan rate as high as 1,000 mV/s, in-dicating excellent charge storage characteristics and ultrafast re-sponse time for the electrodes; Fig. 4B. The capacitances of thedevices made with different deposition times were calculated fromCV profiles and are presented in Fig. 4C. Note that the ca-pacitance was calculated using the total volume of the cellstack, rather than a single electrode. This includes the volumeof the current collector, the active material, the separator, andthe electrolyte.We observe that the capacitance depends strongly on the

loading amount of the pseudocapacitive MnO2 and increasessignificantly with deposition time from 0 to 960 min. For ex-ample, a stack capacitance of up to ∼203 F/cm3 can be achievedwith the sample at a 960-min deposition time. This translates toa volumetric capacitance of 1,136.5 F/cm3 when calculated basedon the volume of the active material per electrode only. Thisvalue is much higher than the capacitance of activated car-bons (60–80 F/cm3), carbide-derived carbons (180 F/cm3), bareLSG (12 F/cm3), activated microwave exfoliated graphite oxide(60 F/cm3), and liquid-mediated chemically converted graphene(CCG) films (263.3 F/cm3), indicating that the volumetric capaci-tance of carbon-based electrodes can be significantly improved byincorporating pseudocapacitive materials (SI Appendix, Table S1).Furthermore, this value is higher than some of the best valuesreported previously for MnO2-based supercapacitors: 16.1 F/cm3

for carbon nanotube-polypyrrole–MnO2 sponge, 130 F/cm3 forgraphene–MnO2–CNT, 246 F/cm3 for CNT–MnO2, 108 F/cm3 for

Binding Energy (eV)7580859095

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Fig. 4. Electrochemical performance of LSG–MnO2 sym-metric supercapacitors. (A) Schematic of the device.(B) CV profiles for an LSG–MnO2 (3 min) supercapacitor atdifferent scan rates. (C) Evolution of the stack capaci-tance of LSG with various mass loadings of MnO2 asa function of scan rate. (D) Specific capacitance due toMnO2 only as a function of the loadings measured ata scan rate of 1 mV/s. (E) Charge–discharge curves of anLSG–MnO2 (3 min) supercapacitor at different currentdensities. (F) Change of stack capacitance of an LSG–MnO2 (120 min) supercapacitor as a function of currentdensity. Data for CCG–MnO2 (120 min) and Au–MnO2

(120 min) supercapacitors are presented for comparison.(G and H) Progression of the real (C’) and imaginary (C’’)parts of the stack capacitance of CCG (G) and LSG (H) asa function of frequency. (I) Comparison of the LSG–MnO2

(120 min) hybrid capacitor with a commercially availableactivated carbon supercapacitor (2.7 V/10 F), pseudoca-pacitor (2.6 V/35 mF), and a lithium-ion hybrid capacitor(2.3 V/220 F).

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mesoporous carbon/MnO2, and 90 F/cm3 for ultraporous carbon–MnO2. In addition, depending on the deposition time, ultrahighareal capacitances of up to ∼0.8 F/cm2 per footprint of the devicecan be achieved. This compares favorably with commercial carbonsupercapacitors that typically provide ∼0.3 F/cm2.

This unprecedented performance can be understood by sep-arating the contribution of the MnO2 nanoflowers from the av-erage capacitance of the LSG–MnO2 electrodes. The specificcapacitance of MnO2 depends on the mass of the active materialreaching a maximum value of 1145 F/g, which is about 83% ofthe theoretical capacitance at a mass loading of 13% of MnO2;Fig. 4D. This remarkable performance can be attributed to theelectrode microstructure that facilitates the transport of ions andelectrons and provides abundant surfaces for charge–transferreactions, ensuring a greater utilization of the active materials.To demonstrate the superior properties of LSG–MnO2 mac-

roporous electrodes, MnO2 was also electrodeposited on bothCCG and gold substrates under the same conditions and theirelectrochemical performance is presented in Fig. 4F. Not onlydoes the CCG–MnO2 exhibit lower capacitance, but its perfor-mance falls off very quickly at higher charge–discharge rates.This can be attributed to the restacking of graphene sheetsduring the fabrication of the CCG electrodes, resulting in a sig-nificant reduction in the surface area and eventually closing offmuch of the porosity (28). In addition, the Au–MnO2 super-capacitor shows extremely low capacitance because of the limitedsurface area and structural properties as can be seen in Fig. 1A.LSG–MnO2, on the other hand, shows a stack capacitance of∼50 F/cm3 that is more than four times higher than CCG–MnO2and about three orders of magnitude higher than Au–MnO2. Theenhanced capacitance and rate capability of the LSG–MnO2further confirms its optimized structure, which synergizes theeffects of both effective ion migration and high electroactivesurface area, thus enabling high and reversible capacitive be-havior even at high charge–discharge rates. The optimized ionicdiffusion of the LSG network was also confirmed from electro-chemical impedance spectroscopy with a response time of 23 msfor LSG compared with 5,952 ms for the CCG electrodes; Fig. 4G and H) (see also SI Appendix, section 2 and Figs. S1 and S2).In fact, the LSG–MnO2 supercapacitor shows superior volumetriccapacitance and rate capability compared with commercially

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Fig. 5. Asymmetric supercapacitor based on graphene–MnO2 as the posi-tive electrode and LSG as the negative electrode. (A) Schematic showing thestructure of the assembled supercapacitor in 1.0 M Na2SO4 electrolyte.(B and C) Electrochemical performance of the asymmetric supercapacitor afterincreasing the potential window from 0.8 to 2.0 V. (D) Change of the stackcapacitance as a function of current density. (E) Electrochemical perfor-mance of the device under different bending angles. (F) Cycling stability ofthe device tested over 10,000 cycles at a scan rate of 1,000 mV/s. Change ofthe ESR during cycling is also shown.

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Fig. 6. Engineering of 3D interdigitated microsupercapacitors with high energy density. (A–C) Illustration of the fabrication process for an asymmetricmicrosupercapacitor based on LSG–MnO2 as the positive electrode and LSG as the negative electrode. (D) Photograph showing the asymmetric micro-supercapacitor. (E) Optical microscope image showing the LSG–GO–LSG-MnO2 interface. (F) SEM image of the interface between GO and LSG shows theselective electrodeposition of MnO2 on LSG only. (Inset) Magnified view of the GO and LSG area. (G–I) Comparison of the stack capacitance of the supercapacitorbetween the sandwich structure and the planar interdigitated structure for (G) asymmetric, MnO2 deposition time 3 min and (H and I) symmetric devices.

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available activated carbon supercapacitors, pseudocapacitors, andlithium-ion hybrid capacitors as can be seen from Fig. 4I.

Construction of Asymmetric Supercapacitors. Considering the highpseudocapacitance of the LSG–MnO2 electrode and the fastcharge–discharge of the double-layer capacitance of the LSGelectrode, an asymmetric supercapacitor was assembled usingLSG–MnO2 as the positive and LSG as the negative electrode, asschematically illustrated in Fig. 5A (see also SI Appendix, section3). Here, a charge balance between the two electrodes wasachieved by controlling the deposition time of MnO2 at thepositive electrode and the thickness of the graphene film at thenegative electrode. The electrochemical performance of an asym-metric cell that uses LSG–MnO2 with 13% MnO2 mass loading(3-min deposition time) for the positive electrode is shown in Fig.5 B and C. The cell exhibits an ideal capacitive behavior with nearlyrectangular CV profiles and highly triangular charge/dischargecurves; Fig. 5 B and C. The CV profiles retain their rectangularshape without apparent distortions with increasing scan rates upto an ultrahigh rate of 10,000 mV/s, indicating the high ratecapability of this asymmetric supercapacitor. Interestingly, theasymmetric cell presents a wide and stable operating potentialwindow up to 2.0 V in aqueous electrolyte that should affordhigh energy density. Furthermore, as the MnO2 deposition time isincreased from 3 to 960 min, the stack capacitance increases sig-nificantly from around 3 to 76 F/cm3, meaning that the storedenergy and power can be greatly improved in the asymmetricstructure; Fig. 5D. These cells can also retain their high capacitywhen faster charge and discharge rates are needed. In addition, theas-fabricated supercapacitor is highly flexible and can be foldedand twisted without affecting the structural integrity of thedevice or its electrochemical performance, holding promise asa practical energy storage system for flexible electronics; Fig. 5E.Long cycle life is another important feature for commercially

viable supercapacitors. Indeed, the asymmetric supercapacitor isvery stable as it maintains over 96% of its original capacity after10,000 charge–discharge cycles tested at a high scan rate of 1,000mV/s; Fig. 5F. The equivalent series resistance (ESR) of thesupercapacitor was monitored during cycling using a Nyquist plot.The device demonstrates a slight increase of ESR in the first 1,000cycles with only subtle changes over the remaining cycles.

Three-Dimensional Interdigitated Microsupercapacitors. The de-velopment of microsupercapacitors with high capacity per footprintarea is crucial for the miniaturization of energy storage devices formodern electronic applications (26). Unfortunately, current state-

of-the-art systems still suffer from low areal capacity (26): <11.6mF/cm2 for carbons (29, 30), <78 mF/cm2 for conducting polymers(31), and <56.3 for metal oxides (32). We designed new hybridmicrosupercapacitors as illustrated in Fig. 6 A–C, in which thepositive and negative electrodes are separated into a 3D inter-digitated structure. This structure was achieved by combining thetechniques of “top-down” LightScribe lithography with “bottom-up” selective electrodeposition. First, 3D interdigitated LSGmicroelectrodes were produced by the direct writing of graphenepatterns on GO films using a consumer-grade LightScribe DVDburner. Subsequently, MnO2 nanoflowers were selectively electro-deposited on one set of the LSG microelectrodes using the cellsetup described in the schematic diagram (Materials and Methods).The width of the microelectrodes is adjusted to match the chargebetween the positive and negative poles of the microdevice. Fig. 6Dshows a digital photograph of an asymmetric microsupercapacitorthat consists of alternating positive and negative electrodes. Thelighter microelectrodes correspond to bare graphene (negativeelectrodes), whereas the other side turns darker in color afterthe electrodeposition of MnO2 (positive electrodes). The op-tical microscope image shows a well-defined pattern and sharpboundaries between the microelectrodes; Fig. 6E. SEM furtherconfirmed the conformal structure of this asymmetric micro-supercapacitor. A magnified view at the interface between GOand graphene shows the selective electrodeposition of MnO2on the graphene area only; Fig. 6F.Electrochemical characterization shows that the asymmetric

microsupercapacitor provides enhanced volumetric capacitanceand rate capability compared with a conventional sandwich-typeasymmetric supercapacitor; Fig. 6G. Symmetric hybrid micro-supercapacitors show a similar behavior (Fig. 6 H and I), withthe areal capacitance approaching 400 mF/cm2. To the best ofour knowledge, this is the highest areal capacitance achieved sofar in an interdigitated microsupercapacitor. The stack capaci-tance significantly improves to ∼250 F/cm3 (volumetric capaci-tance per electrode is 1,197 F/cm3), which is much higher thanvalues previously reported for EDLC, pseudo-, and hybrid mi-crosupercapacitors: 1.3 F/cm3 for carbon onions, 2.35–3.05 F/cm3

for graphene, 1.08 F/cm3 for CNT, 3.1 F/cm3 for graphene–CNT, 180 F/cm3 (electrode) for carbide-derived carbon, 588 F/cm3

for polyaniline nanofibers, 317 F/cm3 (electrode) for vanadium

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2 V/ 2.5 Ah Lead Acid battery4 V/500 μAh Li thin film battery3 V/300 μF Al electro. capacitor2.7 V/50 mF AC supercapacitor2.7 V/10 F AC supercapacitor2.7 V/350 F AC supercapacitor2.6 V/35 mF Pseudo-capacitor2.3 V/220 F Li ion capacitorAu/MnO2 Sym supercapacitorCCG/MnO2 Sym supercapacitorLSG/MnO2 Sym sandwichLSG/MnO2 Sym interdigitatedLSG/MnO2 Asym sandwich

Fig. 7. Ragone plot comparing the energy and power density of LSG–MnO2

supercapacitors with a number of commercially available energy storagedevices: a lead acid battery, a lithium thin-film battery, an aluminum elec-trolytic capacitor, activated carbon supercapacitors of variable sizes, a pseu-docapacitor, and a lithium-ion hybrid capacitor. Performance data for Au–MnO2 and CCG–MnO2 are also included to demonstrate the importance ofthe microstructure of the electrodes.

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Fig. 8. Direct fabrication of supercapacitor arrays for high-voltage appli-cations and integrated energy storage. (A) Direct fabrication of an asym-metric supercapacitor array consisting of 9 cells in a single step. Charge–discharge curves of asymmetric supercapacitor arrays connected in series(3 cells in series, 3S), in parallel (3 cells in parallel, 3P), and in a combination ofseries and parallel (3 series × 3 parallel, 3S × 3P). A single device is shown forcomparison. (B) Integration of a supercapacitor array with solar cells for ef-ficient solar energy harvesting and storage.

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disulfide nanosheets, and 178 F/cm3 for molybdenum disulfidenanosheets (SI Appendix, Table S2).

DiscussionThe energy and power density of the LSG–MnO2-based super-capacitors are presented in Fig. 7. To put these results in per-spective with current technology, we characterized a number ofcommercially available carbon-based supercapacitors, pseudo-capacitors, hybrid supercapacitors, and Li-ion hybrid capacitorsand the results are presented in the same plot. These deviceswere tested under the same dynamic conditions as LSG–MnO2.For all devices, the calculations were made based on the volumeof the full cell that includes the current collector, active material,separator, and electrolyte. The energy density of the hybridLSG–MnO2 varies between 22 and 42 Wh/l depending on theconfiguration (symmetric, asymmetric and sandwich, interdigi-tated) and the mass loading of MnO2. By comparison, the LSG–

MnO2 hybrid supercapacitors store about 6 times the capacityof state-of-the-art commercially available EDLC carbon super-capacitors. They are also superior to pseudocapacitors, hybridsupercapacitors, and supercapacitor–lithium-ion battery hybrid(Li-ion capacitors). Furthermore, LSG–MnO2 supercapacitorscan provide power densities up to ∼10 kW/l, which is 100 timesfaster than high-power lead acid batteries and 1,000 times fasterthan a lithium thin-film battery.To meet the high voltage requirements, supercapacitors are

often put into a bank of cells connected together in series. Thisresults in bulky supercapacitor modules, which are appropriatein some cases but often cause problems in applications where thetotal size of the power source is critical. Here, we proposea different design in which an array of separate electrochemicalcells is directly fabricated in the same plane and in one step asshown in Fig. 8A (see also SI Appendix, sections 4, 5 and Figs.

S4–S8). This configuration shows very good control over thevoltage and current output. In addition, this array can be in-tegrated with solar cells for efficient solar energy harvesting andstorage, as shown in Fig. 8B and SI Appendix, Movie S1.In summary, we have developed a simple and scalable approach

for the fabrication of hybrid LSG–MnO2 3D supercapacitors andmicrosupercapacitors that are compact, reliable, and energydense, charge quickly, and possess long lifetime. Given that MnO2is widely used in alkaline batteries [selling ∼10 billion units peryear (33)] and the scalability of graphene-based materials, webelieve that graphene–MnO2 hybrid electrodes offer promise forreal-world applications.

Materials and MethodsSynthesis of LSG–MnO2, Au–MnO2, and CCG–MnO2 Electrodes. The LSGframework was prepared by focusing a laser beam from a LightScribe DVDburner on a DVD disk coated with graphite oxide. First, the DVD disk iscovered by a film of gold-coated polyimide (Astral Technology Unlimited,Inc.) or a sheet of polyethylene terephthalate. This was coated with a 2% GOdispersion in water using the doctor blade technique and left for drying for5 h under ambient conditions. A computer-designed image is printed ontographite oxide to make the appropriate LSG pattern (27). This was followedby the electrodeposition of MnO2 from 0.02 M Mn(NO3)2 in 0.1 M NaNO3

aqueous solution using a standard three-electrode setup, where a piece ofLSG (1 cm2) is used as the working electrode, Ag–AgCl as the referenceelectrode (BASi), and a platinum foil (2 cm2, Sigma-Aldrich) as the counter-electrode. The deposition was achieved by applying a constant current of250 μA/cm2 for different time periods between 3 and 960 min. For moreinformation on the assembly of supercapacitors, microsupercapacitors, andcharacterization methods, please see SI Appendix, Materials and Methods.

ACKNOWLEDGMENTS. We thank Matthew Kowal for making graphite oxidedispersions, and Michael Yeung for help with energy dispersive X-ray analysis.This work was supported by Nanotech Energy, Inc.

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