Application of graphene and graphene-based materials in clean energy-related devices

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2009; 33:1161–1170Published online 14 August 2009 in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/er.1598

Application of graphene and graphene-based materialsin clean energy-related devices

Minghui Liang, Bin Luo and Linjie Zhi�,y

National Center for Nanoscience and Technology, Beiyitiao 11, Zhongguancun, Beijing 100190, People’s Republic of China

SUMMARY

The unique properties of graphene render it as a versatile material applying in various energy-related devices. Solar cellswith transparent and conductive graphene film as window electrode have exhibited considerable power conversionefficiency. The graphene-based materials used as anode in lithium ion secondary batteries displayed excellent cyclingperformance and high capacities. Supercapacitors with great potential for practical application have been fabricated aswell using graphene-based materials as electrode. Graphene has emerged as a promising two-dimensional nanomaterialfor developing economic and efficient energy-related devices. In this review, the recent progress concerning theapplication of graphene and graphene-based materials in clean energy-related devices has been summarized briefly.Copyright r 2009 John Wiley & Sons, Ltd.

KEY WORDS: graphene; solar cell; lithium ion batteries; supercapacitors

1. INTRODUCTION

Developing energy-related materials and devices isof primary importance to meet the globe energydemand. Solar cells, rechargeable lithium ionbatteries (RLBs), and electrochemical double layercapacitors (EDLCs) are believed to provide cleanenergy with almost zero waste emission before thedisposal of these devices. Solar cells generateelectrical energy by photo-voltaic conversion[1–4], while RLBs [5] and EDLCs [6] are the

devices of storing electricity. Electrode materialsare the essential components of these devices, andhave close relation to the performance, efficiency,and cost of the devices.

Graphene, a two-dimensional and single atomicthick plane, is the building unit of graphite. Sincethe discovery of free-standing graphene [7], itsproperties have been investigated intensively, in-cluding room temperature quantum Hall effect [8],optical properties [9], mechanical properties [10],thermoelectrical transport properties [11], etc.

*Correspondence to: Linjie Zhi, National Center for Nanoscience and Technology, Beiyitiao 11, Zhongguancun, Beijing 100190,People’s Republic of China.yE-mail: [email protected]

Contract/grant sponsor: National Center for Nanoscience and Technology of ChinaContract/grant sponsor: Chinese Academy of Sciences; contract/grant number: KJCX2-YW-M11Contract/grant sponsor: Ministry of Science and Technology of China; contract/grant numbers: 2009AA03Z328, 2009DPA41220

Received 14 April 2009

Revised 6 July 2009

Accepted 6 July 2009Copyright r 2009 John Wiley & Sons, Ltd.

Owing to its unique properties, graphene hasbeen attracted more attention for applicationin various areas such as field effect transistors[12], sensors [13, 14], and electromechanicalresonators [15]. Recently, graphene and graphene-based materials have been selected as electrodematerials of energy-related devices, and quitepromising results have been obtained. Herein, wewill try to summarize briefly some of the recentprogresses concerning the application of grapheneand graphene-based materials in solar cells, RLBsand EDLCs.

2.1 Graphenes applied in solar cells

Window electrode is usually one of the key parts inmost of the solar cells that fulfills the standards ofgood conductance, transparency, and suitablework function [16]. Presently, indium tin oxide(ITO) is the frequently selected window electrodesin solar cells [17]. However, their drawbackshinder the application, which include (1) limitedcontent of indium on the earth; (2) the iondiffusion from ITO to polymer layers in organicsolar cell; (3) the poor transparency in near-infrared region; (4) instability of ITO at acid orbase conditions [16, 18]. In addition, the brittlenessof the ITO limits its use in flexible organicphotovoltaics [19]. Searching for alternative win-dow electrode with low cost, easy availability andgood performance is a promising way to solvethese problems. Transparent film of carbonnanotubes (CNTs) has been used as electrode inorganic solar cells, and the power conversionefficiency (PCE) can reach 2.5% [20], which iscomparable to the reference device with ITO aselectrode. However, the high surface roughness ofthe CNT film may hinder its further application incommercial products. On the other hand, theprocess of CNT preparation is highly energyconsuming whatever the preparation method ofCVD (chemical vapor deposition) or arc dischargeis used. In comparison, the graphene can beprepared at low termperature (less than 1001C)from graphite, for example, by reducing ofgraphene oxide with hydrazine [21].

Compared to CNTs, graphene is an ideal two-dimensional material with excellent electron

transport properties [7], which can be assembledinto film electrode with lower roughness [16]. Mostinterestingly, the transparency of graphene couldbe very high because of its atomic thickness. Sinceits conductivity behavior, graphene has been in-corporated with polymers [22] or silica [23] toobtain conductive composites. However, the con-ductivity of these materials is relatively low, whichis not good enough to be used as electrode in op-toelectronics. Recently, we used exfoliated gra-phite oxide as starting material, followed with filmdeposition and thermal reduction, to obtain agraphene film with a thickness of 10 nm (Atomicforce microscopy (AFM) characterization), atransparency of higher than 70% and a con-ductivity of several hundreds S cm�1 [16]. For thefirst time, dye-sensitized solar cells have beenfabricated successfully using graphene film aswindow electrode (Figure 1) The experimental re-sults opened a new door for searching alternativewindow electrode of ITO, though the PCE of thisgraphene-based solar cell was not so good as thatof the FTO-based reference device. Obviously, as aresult of the non-optimized cell conditions, there isstill large room left for further improvement of thecells. For instance, the property of graphene filmhas been further modified by a bottom-up ap-proach using a monodispersed molecular nano-graphene as building units [18]. The preparedgraphene film showed stronger interactions withthe substrate than that of graphite oxide-inducedfilm. The film of 4 nm thickness holds a transpar-ency of 90% at a wavelength of 500 nm. Organicsolar cells have been successfully fabricated usingsuch a window electrode of graphene film with85% transparency. The highest external quantumefficiency (EQE) of the cell with graphene filmwindow electrode was observed as 43% at theillumination of 520 nm monochromatic light; andat the same condition, that of the organic solar cellwith ITO as electrode was found to be 47%(Figure 2). Under the monochromatic light with awavelength of 510 nm, the PCE of this graphene-based cell was 1.53%, which was similar to thatof ITO-based cell (1.5%). However, under theillumination of the simulated solar light, thecell with graphene film and ITO as electrodematerials had an overall efficiency of 0.29 and

M. LIANG, B. LUO AND L. ZHI1162

Copyright r 2009 John Wiley & Sons, Ltd. Int. J. Energy Res. 2009; 33:1161–1170

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1.17%, respectively. The output voltage ofgraphene-based cell is comparable to that of ITO-based cells, suggesting that the graphene has anappropriate work function for this type of organicsolar cells. And the relative low Isc, FF, and EQEof the cell may result from the high resistance ofthe as prepared graphene films.

Obviously, graphene films with excellent con-ductance, good transparency in both the visibleand near-infrared regions, ultrasmooth surfacewith tunable wettability, high chemical and ther-mal stabilities and flexibility for transfer betweenalternative substrates, can be used not only used insolar cells as electrode but also in many otheroptoelectronic devices [24].

Graphene film prepared by spin coating gra-phite oxide on quartz slice and subsequent reduc-tion has been used as window electrode for thefabrication of organic solar cell as well by Chenet al. [19,25]. AFM characterization revealed thatthe film thickness could be less than 10 nm, andsurface roughness of the film was ca. 3 nm. Furtherinvestigation demonstrated that the transparency

of the film was better than 80% when the filmthickness was less than 20 nm, but the resistance ofthe film ranged from 5 to 1MO. The organic solarcell fabricated using this graphene film as electrodeheld a PCE of 0.4%, still lower than that of thecell with ITO as electrode (0.84%), which wasprobably due to the larger resistance of thegraphene film.

Besides window electrode, graphene film can beused as counter electrode as well in dye-ensitizedsolar cells. Usually, platinum film on transparentconducting oxide (TCO) glass is frequentlyselected as counter electrode material of dye-ensitized solar cells as a consequence of its highactivity. As the cost of DSSCs production mainlycomes from TCO and dyes [26], it is moreimportant to replace the TCO by the graphenefilm than to replace platinum by graphene film inreducing the cost of DSSCs.

The possibility of graphene film as hole col-lecting materials was explored recently [27]. Edaet al. reported the preparation of transparent andconductive graphene film first by vacuum filtration

Figure 1. Illustration and performance of dye-sensitized solar cell based on graphene electrodes. (a) Illustration of dye-sensitized solar cell using graphene film as electrode, the four layers from bottom to top are Au, dye-sensitizedheterojunction, compact TiO2, and graphene film. (b) The energy level diagram of graphene/TiO2/dye/spiro-OMeTAD/Au device. (c) I–V curve of graphene-based cell (solid line) and the FTO-based cell (dashed line), illuminated under AM

solar light (1 sun) [16].

APPLICATION OF GRAPHENE AND GRAPHENE-BASED MATERIALS 1163


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of graphite oxide to form a film, followed withhydrazine vapor reduction and low temperatureannealing under nitrogen to reduce the resistanceof the film. The authors found that treating thefilm with SOCl2 can reduce the resistance by afactor of 5. The obtained graphene film was fab-ricated into organic solar cells with PEDOT:PSS,poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester as active material, andaluminum as top electrode. The PCE of the asfabricated cells was around 0.1%. The resultsof reference experiment without graphene film insolar cell revealed that the reduced graphite oxide

film rather than PEDOT:PSS acted as hole col-lecting electrode in the cell.

Recently, Shi et al. [28] reported a new counterelectrode for dye-ensitized solar cell, in which the1-pyrenebutyrate functionalized graphene wasincorporated with FTO (G-FTO) to form theelectrode. The PCE (2.2%) of the cell with G-FTOas counter electrode is significantly higher thanthat (0.048%) of the cell with only FTO as counterelectrode, but is lower than that with platinum ascounter electrode (3.98%). Considering thatPEDOT: PSS holds good transparency and mod-erate conductance as counter electrode, Graphene

Figure 2. (a) Illustration of the organic solar cell; the four layers from bottom to top are Ag, a blend of P3HT andPCBM, TGF, and quartz, respectively. (b) A schematic representation of charge transfer and transport as an energylevel diagram; the work function has nost been measured for TGFs, but it was presumed to be 4.5 eV. (c) EQE ofa TGF-based cell (solid curve) and an ITO-based cell (dashed curve). (d) I–V curve of a TGF-based cell (center) andan ITO-based cell (inset) illuminated under simulated solar light (dashed curve) and with monochromatic light ofwavelength 510 nm (solid curve). The calibrated light intensity of the simulated solar light is 167Wm�2 for ITO and

119Wm�2 for TGF as anode [18].



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doping of PEDOT:PSS was investigated as elec-trode in DSSCs [29]. The cell with graphene/PEDOT:PSS as counter electrode showed a PCEof 4.5%, which was comparable to that of cellswith Pt as counter electrode (6.3%), and washigher than that of cells with PEDOT:PSS ascounter electrode (2.3%).

In addition to its application as electrode ma-terials, the graphene has been investigated as ac-tive acceptor material in organic solar cells [30].Using solution-processed, functionalized grapheneas electron acceptor, and poly(3-octylthiophene)(P3OT) and P3HT as electron donor, solar cellwith a PCE of 1.4% was obtained under simulatedlight at optimized conditions. The functionaliza-tion of graphene was conducted by the reaction ofexfoliated graphite oxide with phenyl isocynate.

In summary, graphene has been emerged as oneof the interesting nanomaterials for application insolar cells, especially as alternative of conventionalelectrode materials. The relatively low PCE of theas fabricated cell is probably due to the relativelyhigh resistance of the presently prepared graphenefilms. To figure out the gap of the conductancebetween commercial ITO glass and graphene-basedfilms (graphene films or CNT films), the ratio oftransmittance to resistance (T%/R) of the electrodeis used as a criterion to evaluate the difference.Obviously, the value of T/R of ITO (about 0.06)[16] is much higher than that of graphene films(about 0.0004) [16] or CNTs films (about 0.0035) [17].Further research work will be therefore focusedmainly on the improvement of the electron transferbehavior of the graphene films.

2.2. Graphene and graphene-based materials appliedin RLBs and EDLCs

RLBs and EDLCs are frequently used devices forelectricity storage. Generally, the energy density ofRLBs is much higher than that of EDLCs, whilethe power density and rate performance of RLB isnot so excellent as that of EDLCs. Owing to theirdifferent functions, the supplementary combina-tion of both RLB and EDLC may increase theoverall efficiency of the two devices [6]. Of course,searching for various electrode materials withnovel structures and enhanced functions is always

one of the most important topics for both RLBsand EDLCs. Graphene, holding excellent electrontransfer behavior and unique two-dimensionalsurface, has been selected as a potential electrodematerial for improving the performance of RLBsand EDLCs.

RLB, composed of anode, electrolyte, andcathode, is a lithium ion-induced device for elec-tricity supply [5]. On charging, the lithium ions,extracted from cathode material, pass through theelectrolyte and insert into the anode material.Discharge reverses the procedure. Graphite isusually selected as the anode material of RLBsbecause of its good cycling performance. Gra-phene can be considered as the building unit ofgraphite. Theoretic calculation suggests that everysix carbon atoms can host one lithium ion to forma LiC6 structure when lithium ions are intercalat-ing into the graphene layers of anode graphite,resulting in a theoretical capacity of 372mAhg�1

for graphite. Compared to the relatively low ca-pacity of graphite in which the graphene layers arestacked orderly, disordered carbon, with graphenesheets arranged as a ‘house of cards’, affords muchhigher capacity as anode in RLBs, though its ir-reversible capacitance for the first charge-dis-charge cycle is high [31]. Variation of thealignment of graphenes in the material influencessignificantly the electrochemical performance ofthe anode. Theoretical calculation [32] suggestedthat a distance of 0.77–0.83 nm between every twographene sheets would be ideal to improve theanode performance, in that space a double layerlithium ions could be intercalated in-between theneighboring graphene sheets, and in the meantime, the electrolyte could not penetrate inside.Obviously, controllable alignment of graphenesheets in the material is not so easy. However, it isclear that an enhanced carbon anode should befabricated by graphene sheets in a controlledfashion.

The graphite, by oxidation, can be readily ex-foliated into thin graphene oxide sheets dispersiblein aqueous solution [21]. After chemical reductionof the graphene oxide by hydrazine, graphenesheets with a thickness of 3–7 nm has beenobtained [33]. As anode material in RLBs,these graphene sheets exhibited a lithium storage



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capacitance of 540mAhg�1 after 20 charge–discharge cycles, significantly higher than that ofgraphite. The incorporation of reduced grapheneoxide with CNT or C60 improved further the ca-pacitance to 730 and 784mAh g�1, respectively.An increase of the d-space between graphenesheets for reduced graphene oxide, graphene-CNT,and graphene-C60 was detected as 0.365, 0.40, and0.42 nm, respectively, which might be the reason ofdifferent capacity for these materials.

As alternative anode materials to graphite,some metals or their oxides, such as Sn and SnO2,have attracted the attention due to their quite hightheoretical lithium ion storage capacitance [5].However, the major drawback of these materialsis the large volume expansion and shrinkage duringthe charge–discharge cycling, which is caused bythe reversible chemical reaction or alloying be-tween lithium ion and metal (oxides). Strategies[34,35] for improving their cycling performancehave been proposed. Reducing the particle size canimprove their cycling performance in certain de-gree. [36] Unfortunately, the capacity decaying ofthe material is unavoidable. An efficient strategy isto encapsulate the metal (oxide) particles withother materials [37]. Graphene or graphene-basedcarbonaceous materials have been proofed to beone of the promising covers for this purpose.

Graphene-containing carbon-covered metals ormetal oxides have been studied as anode in RLBs.The cycling performance of the modified anodematerial was found to be enhanced significantly[37–40]. The graphene-containing covers were be-lieved to play a role of not only mechanicallystable buffer but a good media for electron and iontransportation. For instance, a composite of gra-phene and SnO2 has been used as anode for RLBs.Its reversible capacity reached 810mAh g�1, andmaintained at 570mAhg�1 even after 30 cycles,which is about 70% retention of the initial capa-city [41]. The naked SnO2 showed an initial capa-city of 550mAh g�1, and only 60mAhg�1 leftafter 15 cycles.

We have developed an approach using one-steppyrolysis of carbon-rich organic-tin precursor to-wards the formation of hollow tin nanoparticlesencapsulated with carbon [42]. Tin hollow nano-particles with an average diameter of less than

20 nm were covered by a disordered carbon layerof 2–3 nm thick. The material with a tin content ofca. 43wt% exhibited highly stable and reversiblecapacity of 550mAhg�1 after 20 cycles (Figure 3),which was suggested to be closely related to: (1)the small size of the material (less than 20 nm), sothat diffusion distance reduced to only a fewnanometer; (2) the nanosized and hollow struc-ture, restricting the volume variation of tin nano-particles; (3) the graphene-containing carboncapsule that can effectively relief the expansionand shrinkage of tin particles during the cycling.

Pyrolyzing structurely defined metal-organiccompounds affords higher possibility of resultingin carbon/metal(oxide) composite in a controlledfashion [43]. Graphene-encapsulated Co3O4 na-noparticles has been prepared by this method [44].The graphene-covered Co3O4 nanoparticles, asanode material for RLBs (Figure 4), showed aninitial reversible capacity of 920mAhg�1, and theapacity of 940mAh g�1 after 20 cylces, with thecoulombic efficiency of 99%. Compared to theexcellent cycling performance of graphene-coveredCo3O4, the capacity decay of naked Co3O4 nano-particles was quite fast, with a specific capacity ofonly 120mAhg�1 left after 20 cycles.

EDLCs (also called supercapacitors or ultra-capacitors), composed of a separator, two elec-trodes and electrolyte, store electric energy byforming double electrical layers at the interface

Figure 3. Galvanostatic discharge–charge curves of thetin/carbon nanocomposite cycled at a rate of C/5 betweenvoltage limits of 0.01 and 3V. Inset shows the cycling-

performance of the tin/carbon nanocomposite [42].



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between electrode and electrolyte. The charge anddischarge of EDLC can be completed in seconds.Therefore, EDLC has much higher power deliveryor uptake than that of batteries, but its energydensity is relatively low. On the other hand,EDLCs have higher-specific energy density thanthat of conventional capacitors as a result of thehigh specific surface area (SSA) of EDLCs elec-trodes. EDLCs are therefore considered as devicesbetween batteries and conventional capacitors.However, further improving the energy and powerdensity of EDLCs was regarded as an importanttask to supply energy more efficiently whenEDLCs are coupled with batteries [45].

As the capacity of EDLC is proportional to theinterface surface between electrolyte and electrode,porous materials with high surface area is expectedas electrode. Active carbon is so far the commonlyused electrode material because of its good

conductance and huge SSA. However, experi-mental studies suggested that micro-pores withdiameter smaller than 1 nm might have little con-tribution to the capacitance as the pores are toosmall to be accessible by electrolyte [6,46].

Various carbons, including activated carbons,carbon fibers, nanotubes, inions, nanohorns, andboron doped carbons, have been tested as elec-trodes in EDLCs. SSA is always one of the mostimportant parameters. For carbon materials, thetheoretical calculation suggested that their max-imum SSA could be higher than 2600m2 g�1 [47],which is similar to that of free-standing graphene.However, even if the SSA of active carbon reached2598m2 g�1 (BET), its capacity can only reach87.8 F g�1, which is less than that of carbon withthe SSA of 1459m2 g�1 (93.8 F g�1). The experi-mental results were attributed to that the porewall in the porous carbon with larger SSA cannot

Figure 4. (a,b) SEM images and (c,d) TEM images of the obtained carbon/cobalt nanocomposite [44].



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accommodate the same amount of charge underthe same electrode potential [47]. Holding bothexcellent electron transfer behavior and ultrahighSSA, graphene is emerged as a promising non-porous electrode material for EDLCs.

Long time ball milling of graphite can exfoliatethe graphene sheets in certain degree. After 70 hball milling, the graphite with increased SSA hasbeen obtained, resulting in a significantly im-proved capacitance of up to 205F g�1, and a cy-cling performance with a capacitance retention of90% after 5000 cycles [48]. The improvement ofcapacitance of graphite may be caused by forminglarge amount of graphenes with functional edgesand increasing the roughness of basal plane con-taining many defects. The capacitance of graphiteafter ball milling for 30 h held a capacitance of170F g�1.

Graphene materials prepared by thermal treat-ment of graphite oxide has been reported as elec-trode for EDLCs [49]. The capacitance of thismaterial was 117F g�1 at a scan rate of 100mV s�1,and 100Fg�1 at a scan rate of 1000mV s�1, withH2SO4 aqueous solution as electrolyte. The highestpower density of this material reached 32 kWkg�1.Using ionic liquid as electrolyte, a capacitanceof 75Fg�1 was obtained. In compare, Grapheneincorporated with nitrogen has been prepared bychemical reduction of exfoliated graphite oxideusing hydrazine as reductive agent [45], and its SSA

was 705m2 g�1. The electrochemical analysisrevealed that, in KOH aqueous solution, thespecific capacitance was 102F g�1 at a scan rate of100mV s�1, and the capacitance in ionic liquid wasslight lower.

Alternatively, graphene-based materials can beobtained by a bottom-up approach in a bettercontrollable fashion. Highly porous carbonaceousmaterials with one-dimensional nanostructures hasbeen prepared using carbon-rich polyphenyleneas precursor with subsequent pyrolysis at hightemperatures [50]. Materials obtained at 8001Cand with a SSA as high as 1140m2 g�1 has beenselected as electrode in EDLCs. A superior specificcapacitance of up to 304F g�1 was obtained at ascan rate of 5mV s�1, and the capacitance was243F g�1 at a scan rate of 100mV s�1 (Figure 5).The enhanced capacitive behavior of this materialmight be due to the properly alignment ofgraphene sheets as well as the interconnected na-noscale channels distributed in a one-dimensionalarchitecture.

In general, graphene and graphene-basedmaterial have exhibited excellent performance aselectrode mateials in both RLBs and EDLCs.However, these studies are still at the primarystage, especially in the view point of graphene.Further studies are necessary with a focus ongraphene structures and functions, particularly theexfoliation of graphene, the defects formation andfunction, the alignment of graphene in the mate-rial, and the controllable incorporation of gra-phene with other functional materials.

3. CONCLUDING REMARKS

Graphene, as a rapidly raised star in nanomaterialscience, has attracted great attention all over theworld. In this paper, some of the recent progresseson the application of graphene as electrodematerials for clean energy-related devices havebeen briefly summarized. The excellent electrontransfer behavior and the unique two-dimensionalnature of graphene render it versatile as electrodein various devices. Graphene films with goodtransparency, high conductance, and properwork function are promising alternative electrode

Figure 5. Cyclic voltammograms of one dimensionalmesoporous nanocarbon at sweep rates of 5, 10, 25, 50,

100mV s�1 in 1M H2SO4 electrolyte [50].



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material of ITO and FTO in solar cells and otheroptoelectronics. Graphene-based electrode materi-als with appropriate defects and alignment patternof graphene sheets displayed enhanced capacitiveactivity and good cycling performance than that ofgraphite in RLBs. The encapsulation of metal ormetal oxide nanoparticles with graphene canimprove significantly the cycling performance ofthe particles as anode in RLBs. In addition,graphene-constructed materials with mesoporousstructures and large accessible surface area dis-played dramatically increased capacitance inEDLCs. Enhanced power density of EDLC canbe achieved via the incorporation of graphene intothe electrode. Although it is still at an early stage,graphene has emerged as a promising electrodematerial in clean energy-related devices.

ACKNOWLEDGEMENTS

Financial support from National Center forNanoscience and Technology of China, Chinese Academyof Sciences (KJCX2-YW-M11), and the Ministry ofScience and Technology of China (2009AA03Z328,2009DPA41220) is acknowledged.

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DOI: 10.1002/er

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