Synthesis and Characterization of FLG/Fe 3 O 4 Nanohybrid Supercapacitor

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1 727 A publiction of CHEMICAL ENGINEERING TRANSACTIONS VOL. 43, 215 Chief Editors: Suro Pierucci, Jiří J. Klemeš Copyright 215, AIDIC Servizi S.r.l., ISBN ; ISSN The Itlin Assocition of Chemicl Engineering Online t DOI: 1.333/CET Synthesis nd Chrcteriztion of FLG/Fe 3 O 4 Nnohybrid Supercpcitor Mri Srno, Cludi Cirillo, Eleonor Ponticorvo*, Polo Cimbelli Deprtment of Industril Engineering nd Centre NANO_MATES, University of Slerno Vi Giovnni Polo II, Fiscino (SA), Itly eponticorvo@unis.it Few lyer grphene (FLG)/Fe3O4 nnohybrid hs been successfully synthesized by n one-step chemicl strtegy, consisting in the thermolysis of suitble mgnetite precursor in orgnic solvent in the presence of surfctnts nd FLG. This wy provides experimentl esiness nd potentil low-cost production. Rmn spectroscopy, Trnsmission electron microscopy Energy dispersive X-ry spectroscopy (TEM-EDS), thermogrvimetric nlysis coupled with mss spectromery (TG-DTG-MS) nd X-ry diffrction (XRD) were employed for chrcteriztion. The nnoprticles with size of 5 nm, re monodispersed on FLG. The FLG- Fe3O4 hybrid supercpcitor hs good electrochemicl cpcitnce performnce within potentil rnge from to 1. V. The resulting composite exhibited high specific cpcitnces of 227 F/g nd good cyclbility in three-electrode electrochemicl cell. The excellent electrochemicl performnces cn be ttributed to the high conductivity of FLG s well s to the uniform nno-size of iron oxide dispersed on the support. 1. Introduction Supercpcitors (SC) re considered one of the newest innovtions in the field of electricl energy storge. Their performnces re minly due to their high power density, long lifecycle, nd bridging function for the power/energy gp between trditionl dielectric cpcitors (which hve high power output) nd btteries/fuel cells (which hve high energy storge). SCs cn be divided into two groups: (i) Electricl double lyer cpcitor (EDLC), where the cpcitnce is ttributed to the ccumultion of chrges t the electrodeelectrolyte interfce. The typicl electrode mteril of EDLC is high surfce re crbon, such s ctivted crbon, crbon fiber nd more recently nnocrbons such s crbon nnotubes (Cimbelli et l., 24; Di Brtolomeo et l., 29; Srno et l., 212; Srno et l., 213b), erogel, etc. In prticulr, the successful fbriction of grphene (Srno et l., 213) hs inspired lrge reserch ctivity for the development of new grphene-bsed ultrcpcitors (Stoller et l., 28); (ii) Redox cpcitors, where bttery-type oxidtionreduction rection occurs, leding to the pseudocpcitnce. In generl, metl oxides re suitble to provide higher energy density for SC thn conventionl crbon mterils nd better electrochemicl stbility thn polymer mterils. The generl requirements for metl oxides in SC ppliction re the following: (1) the oxide should be electroniclly conductive, (2) the metl cn exist in two or more oxidtion sttes tht coexist over continuous rnge with no phse chnges involving irreversible modifictions of 3-dimensionl structure. Moreover, rough clcultion hs indicted tht 9% of superconductor s cost comes from the electrode mteril, s n exmple, despite the exceptionl properties of ruthenium oxide, its cost is prcticl impediment. Mny studies hve focused on combining RuO2 with chep metl oxides to reduce the cost of using precious metls (Wng et l., 212). As lterntive pproches, reserchers hve put significnt effort into finding cheper nd environmentlly friendly mterils tht exhibit electrochemicl behviour similr to RuO2, (these lterntive mterils include MnO2, NiO, Fe3O4, nd V2O5), nd to study hybrid cpcitors consisting of composites of crbon mterils (e.g. nnocrbons, crbon nnotubes, grphene) nd metl oxides. The high surfce re nd conductivity of nnocrbons offer support for the metl oxide nnoprticles, voiding their gglomertion nd permitting to chieve better profit of their properties. Fe3O4 is n inexpensive mteril exhibiting pseudocpcitnce. Formerly, low specific cpcitnce (5.3 F/g in 1 M Plese cite this rticle s: Srno M., Cirillo C., Ponticorvo E., Cimbelli P., 215, Synthesis nd chrcteriztion of flg/fe3o4 nnohybrid supercpcitor, Chemicl Engineering Trnsctions, 43, DOI: 1.333/CET

2 728 N2SO4 solution) hs been reported for Fe3O4 (Wng et l., 212). More recently, specific cpcitnce of F/g for Fe3O4 film in 1M N2SO4 solution hs been reported, with cpcitive retention of 88.8% fter 5 chrging/dischrging cycles (Wng et l., 212). The highest specific cpcitnce (185 F/g evluted t 1mA/cm 2 ) for uniform nnosized Fe3O4 nnoprticles in 3M KOH between -1 nd volt hs been recently reported (Mitchell et l., 214). Du et l. (Du et l, 29) prepred n ctivted crbon-mgnetite (AC)/Fe3O4 electrode ble to delivery specific cpcitnce of 37.9 F/g t current density of.5 ma/cm 2 iming to improving the specific energy density nd limiting the voltge of crbonceous mterils, dditionlly their initil cpcity hs been kept t 82% fter 5 cycles. Gun et l. in 213 (Gun et l., 213) reported n improved specific cpcitnce nd n excellent cyclic stbility for their crbon nnotubes-mgnetite (CNT)/Fe3O4 in 6M KOH solution in the potentil windows -1 volt. In prticulr, t 1 ma/cm 2 cpcitnce vlues of 8.1 F/g, 36.1 F/g nd F/g for crbon nnotubes, Fe3O4 nnoprticles nd CNT/Fe3O4 respectively were found, indicting tht the presence of supporting mteril cn increse the cpcitnce of fctor of 3.2. Some ppers reported results of SC behviour of Fe3O4 nnoprticles on grphene oxide (GO), even fter reduction (rgo). In prticulr, mximum specific cpcitnce of 18 F/g in 1M H2SO4 solution nd 65 F/g in 1M N2SO4 t 1mV/s, hs been obtined for Fe3O4-functionlized grphene, in the potentil windows 1 Volt (Mishr et l., 211). They lso found specific cpcitnce of 14 F/g in 1M H2SO4 solution t 1 A/g. Qu et l. (Qu et l., 211) in the window -1 Volt in LiOH solution nd Shi et l. (Shi et l., 211) between in 1M KOH, reported high cpcitnce vlues in the presence of very high mount of Fe3O4 on rgo. Recently (Wng et l., 214), high specific dischrge cpcitnce of 22.1 F/g t.5 A/g, tht remins stble fter 3 chrge/dischrge cycles, hs been reported for Fe3O4/rGO (reduced grphene oxide electrode) in 1M KOH between -1.1 nd volt. Despite severl ppers reporting SC dt for Fe3O4 on GO, even fter reduction (rgo), never results hve been reported in the rnge of 1 volts nd in H2SO4 solution, regrding Fe3O4 nnoprticles dispersed on the more conductive grphene, obtined by physicl exfolition of grphite (Srno et l., 214b). The ltter process is inherently less lborious nd does not require lrge number of chemicl regents nd complex procedures, returning few lyer grphene sheets with n intct bckbone structure, without secondry functionlities, which insertion results in reduced electricl performnces. Nnomterils cn be synthesized by severl different methods, mong them the bottom-up chemicl strtegies re very fruitful, becuse they provide experimentl esiness nd potentil low-cost production (Altvill et l., 213), together with n excellent synthesis control of monodispersed size nnoprticles. In this pper we wnt to point out for the first time novel one-step synthesis strtegy for the preprtion of few lyer grphene nnosheets-mgnetite (FLG)/Fe3O4 nnohybrid. Such strtegy is bsed on the therml decomposition of mgnetite precursor in orgnic solvent in the presence of surfctnts nd FLG. This mteril consists of Fe3O4 nno-sized prticles, coted with orgnic chins, which prevent ggregtion nd increse the wettbility nd therefore the dispersion on FLG, obtined by physicl exfolition of grphite. It is worth noticing: (i) we hve obtined the highest vlue of the cpcity mesured (227 F/g) in presence of very low mount of Fe3O4 compred with other ppers; nd (ii) the lyers rrngement in FLG, which consists of certin frction of monolyers together with more thicker frctions (Srno et l., 214b) could open new perspectives for further improvements. Rmn Spectroscopy, Trnsmission Electron Microscopy (TEM), Thermogrvimetric nlysis coupled with Mss Spectrometry (TG- DTG-MS) nd X-ry diffrction (XRD) were employed for chrcteriztion. The electrochemicl performnces of the FLG/Fe3O4 hybrid supercpcitor were investigted by cyclic voltmmetry (CV) nd glvnosttic chrge/dischrge tests in 1M H2SO4 solution. The results of CV nd the chrge/dischrge mesurements proved tht this kind of hybrid supercpcitor hs good electrochemicl cpcitnce performnce within potentil rnge from to 1 V. 2. Experimentl The preprtion of mgnetite nnoprticles (Srno et l., 214) ws crried out using stndrd irless procedures nd commercilly vilble regents. Absolute ethnol, hexne, nd dichloromethne (99%) were used s received. Benzyl ether (99%), 1,2-hexdecnediol (97%), oleic cid (9%), oleylmine (>7%), Iron(III) cetylcetonte were purchsed from Aldrich Chemicl Co. In prticulr, Fe(cc)3 (2 mmol), 1,2- hexdecnediol (1 mmol), oleic cid (6 mmol), oleylmine (6 mmol), nd phenyl ether (2 ml) were mixed nd mgneticlly stirred under nitrogen flow. The mixture ws heted to 2 C for 3 min nd then, under blnket of nitrogen, furtherly heted to reflux (265 C) for 3 min. The formed blck-brown mixture ws cooled to room temperture by removing the het source. Under mbient conditions, ethnol (4 ml) ws dded to the mixture, nd blck mteril ws precipitted nd seprted vi centrifugtion. The blck product ws then dissolved in hexne. Centrifugtion (6 rpm, 1 min) ws pplied to remove ny undispersed residue. The nnoybrid were then precipitted with ethnol, centrifuged (6 rpm, 1 min) to remove the solvent, nd redispersed into hexne. The chrcteriztion ws obtined by different techniques. Trnsmission electron

729 microscopy (TEM) imges were cquired using FEI Tecni electron microscope, equipped with n EDX probe, nd operted t 2 KV with LB6 filment s source of electrons.

XRD mesurements were performed with Bruker D8 X-ry diffrctometer using CuKα rdition.

3 729 microscopy (TEM) imges were cquired using FEI Tecni electron microscope, equipped with n EDX probe, nd operted t 2 KV with LB6 filment s source of electrons. Rmn spectr were obtined t room temperture with micro-rmn spectrometer Renishw invi with 514 nm excittion wvelength (lser power 3 mw) in the rnge 1-3 cm -1. XRD mesurements were performed with Bruker D8 X-ry diffrctometer using CuKα rdition. Thermogrvimetric nlysis (TG-DTG) t 1 K/min heting rte ws performed in flowing ir with SDTQ 6 Anlyzer (TA Instruments) coupled with mss spectrometer. For the electrochemicl mesurements 4 mg of ctlyst were dispersed in 8 μl of 5 wt% Nfion solution to form homogeneous ink. Then the ctlyst ink ws loded onto 3 mm dimeter glssy crbon electrode. The electrochemicl performnces of the FLG-Fe3O4 hybrid supercpcitor were investigted by cyclic voltmmetry (CV) nd glvnosttic chrge/dischrge tests in 1M H2SO4 solution in the potentil rnge from to 1. V. Sturted clomel, grphite nd lodble glssy crbon electrode were used s the reference electrode, the counter electrode nd the working electrode, respectively. 3. Results nd discussion b 5 nm 2 nm Figure 1: TEM imges of FLG/Fe3O4 (, b). TEM imges, crried out to provide insights into the morphology of FLG/Fe3O4 nnohybrid, re reported in Figure 1 nd 1b. Figure 1 shows typicl low mgnifiction imge of the product, tht consists of 5 nm nnoprticles (stndrd devition σ=1.8 nm), dispersed on FLG preventing gglomertion. The TEM imges revel tht the Fe3O4 nnoprticles re ttched to the grphene sheets, even fter the ultrsoniction used to disperse FLG/Fe3O4 nnohybrid for TEM chrcteriztion. FLG, tht were obtined by physicl exfolition of grphite (Srno et l., 214b) hve: lterl sizes of few micrometres; number frction of monolyer grphene (number of monolyers/totl number of flkes) equl to 22%; more thn 85% of the totl sheets number hve lyers rnging from 1 to 13; nd frction of the totl sheets number with bout 3 lyers. The XRD pttern ws used to identify the nnoprticles structure (Figure 2). All the pek positions t 3.1 (2), 35.4 (311), 43. (4), 53.7 (422) 57.2 (511) nd 62.6 (44) re consistent with the stndrd X-ry dt of the mgnetite phse (Go et l., 211; Sun et l., 29; Zhou et l., 21). The peks re brodened s typicl of very low prticles size. The min prticles size mesured by using the Scherrer eqution (5.1 nm) is close to the TEM results. It mens tht the s synthesized prticles re single crystls. Besides mgnetite peks we did not observe ny other oxide peks in the diffrction pttern. Becuse the XRD pttern of mgnetite nd mghemite re very similr, Rmn spectroscopy ws used to distinguish the different structurl phses of iron oxides. Mgnetite hs the min bnd centred t 668 cm -1 (A1g) (see Figure 2b), no other chrcteristic iron oxide bnds were observed. In the high wvelength rnge the typicl spectrum of FLG is lso reported. The two most intense fetures re the G pek t ~157 cm -1 nd the 2D bnd t ~27 cm -1, which differs from tht typicl for grphite, consisting of two components 2D1 nd 2D2 (the second with higher intensity thn the first), hs quite flt pex nd cn be esily deconvolved with lmost two peks. A brod D- bnd cn be lso seen, likely due to the FLG edges (tking into ccount the lser spot dimension nd the flkes size (Srno et l., 214b)). TG-DTG under flux of O2 (2 v/v %) in N2 is shown in Figure 3. From room temperte to 5 C weight loss due to orgnic chins relese, see the totl ion current (TIC) of the most intense mss frgments pek: m/z = 3 form oleylmine nd oleic cid, hppens together with CO2 relese due to their oxidtion in presence of the few percentge of oxygen. The FLG oxidtion begins t bout 5 C. TG-DTG mesurement in ir (Figure 3b) cn identify the reltive mount of: the coting gents on the

4 73 surfce of the nnoprticles, FLG nd Fe3O4. The TG curve in Figure 3b disply two weight loss processes. The first weight loss, s for the nlysis under nitrogen reched flow (Figure 3), is due to the orgnic chins nd corresponds to bout 5 wt.% of the smple. The second weight loss (DTG pek t 645 C) corresponds to FLG oxidtion, the TG residue t 9 C (bout 21 wt.%) being comptible with the strting mount of FLG nd the formtion of oxidized mgnetite. 2 FLG b G 2D Intensity.u Intensity (.u.) A 1g D Θ Wwelenght (cm -1 ) Figure 2: XRD diffrction ptterns (), nd Rmn spectrum (b) of FLG/Fe3O4 nnohybrid. Weight (%) Intensity.u Temperture ( C) m/z 3 m/z 44 2, 1,5 1, Deriv. Weight (%), - Weight (%) ensity.u Temperture ( C) b Deriv. Weight (%) Figure 3: TG-DTG-MS nlysis in O2 (2 v/v %)/N2 flow () nd TG-DTG in ir (b) of FLG/Fe3O4 nnohybrid. In order to evlute the electrochemicl properties of FLG/Fe3O4 nnohybrid, cycling voltmmetry (CV) nd glvnosttic chrge/dischrge tests were performed. The CV curves t mv/s in the potentil window of 1 V in 1M H2SO4 electrolyte re presented in Figure 4. These curves re not cler rectngulr in shpe nd show step (i.e. pseudocpcitnce) in the rnge from to 1V. They exhibit quite mirror-imge chrcteristics, which indicte good cpcitive behvior. The specific cpcitnce ws clculted from the dischrge curve (Figure 5, b, c) using the following eqution: C = (IΔt)/(ΔVm), where C is the specific cpcitnce, I is the current (A), Δt is the dischrge time, ΔV the voltge pplied nd m is the mss of working electrode. According to the dischrge curve, cpcitnce of 227 F/g, t current density of.23 A/g, ws obtined: (i) with very low mount of Fe3O4, if compred with other ppers; nd (ii) on conductive FLG, which consists of certin frction of monolyers together with more thicker frctions, obtined by simple process tht does not require the use of number of chemicl regents nd complex procedure. As could be expected the specific cpcitnce decreses with the incresing of current density, which originte form the internl resistnce of the electrode, to 191 F/g t.6 A/g nd 18 F/g t 2A/g. The high reported vlue of cpcitnce in the present work compred to other reported vlue my be ttributed to the uniform dispersion of mgnetite nnoprticles over the surfce of FLG. The cpcity retention hs been one of the min topic tht seemed to hinder the use of Fe3O4 for SC (Wng et l., 212). However, the problem ppers overcome by composite mterils in which the nnoprticles re dispersed on nnocrbons (Gun et l., 213;). Moreover, recently encourging results were lso obtined with nnoprticles lone, chrcterized by uniform nno-size (Mitchell et l., 214). Figure 5d shows the long cycle stbility of our nnohybrid s evluted t 2 A/g. The electrode mteril retins 91% of its initil cpcitnce fter 1 cycles, indicting good stbility nd long cycle life. The excellent cycling performnces of the monohybrid cn be ttributed to the fvourble function of FLG for nchoring nnosized

5 731 Fe3O4 prticles, which effectively prevents their ggregtion. The energy density nd power densities cn be further clculted from these results (see Figure 5e) using the following equtions: E =1/2C(ΔV) 2, P=E/t, where E is the energy density, C is the specific cpcitnce, ΔV is the potentil rnge, P is power density nd t is the time to dischrge. The nnohybrid reches n energy density of 63 Wh/kg t power density of 232 W/Kg, nd still remins 5 Wh/kg t power density of 2 W/Kg, exhibiting lrge power rnge tht cn be obtined while mintining reltively high energy density. The results illustrte tht the nnohybrid hve very promising properties for the development of low cost nd environmentl friendly high performnce supercpcitor. Current (A/g) mvs -1 1 mvs -1 2 mvs -1 5 mvs -1 1 mvs -1,,2,4,6,8 1, Figure 4 Cyclic voltmmetry of the nnohybrid. 1, b c 2 A/g 1,,6 A/g 1,,23 A/g Cpcitnce retention (F/g), Time (s) , 2 Time (s) Cycle Number 2 A/g Time (sec) d Power density (W/kg), Time (s) Energy density (Wh/kg) e Figure 5 Glvnosttic chrge/dischrge curves of the nnohybrid t 2 A/g (),.6A/g (b) nd.23 A/g (c). Cpcitnce retention t 2 A/g (d). 4. Conclusions A nnohybrid consisting of Fe3O4 nno-sized prticles, coted with orgnic chins, which prevent ggregtion nd increse the wettbility nd therefore the dispersion on very conductive FLG, obtined by physicl exfolition of grphite, hs been prepred by new nd fcile one pot synthesis process.

6 732 The smple ws wide chrcterized by the combined use of different techniques nd tested in the rnge of 1 volts nd in H2SO4 solution. It is worth noticing: (i) the highest vlue of cpcity (227 F/g) nd good energy nd power density performnces, if compred with literture results, despite the very low mount of Fe3O4. Moreover, the lyered rrngement of FLG, which consists of certin frction of monolyers together with more thicker frctions, cn open new perspectives for further improvements. References Altvill C., Srno M., Cimbelli P., Sentore A., Petrone V., 213, New 'chimie douce' pproch to the synthesis of hybrid nnosheets of MoS2 on CNT nd their nti-friction nd nti-wer properties, Nnotechnology, 24, Cimbelli P., Snnino D., Srno M., Ngy J.B., 24, Chrcteriztion of nnocrbons produced by CVD of ethylene in lumin or lumino-silicte mtrices, Adv. Eng. Mter., 6, Di Brtolomeo A., Srno M., Giubileo F., Altvill C., Iemmo L., Pino S., Bobb F., Longobrdi M., Scrfto A., Snnino D., Cucolo A.M., Cimbelli P., 29, Multiwlled crbon nnotube films s smll-sized temperture sensors, J. Appl. Phys., 15, Du X., Wng C., Chen M., Jio Y., Wng J., 29, Electrochemicl Performnces of Nnoprticle Fe3O4/Activted Crbon Supercpcitor Using KOH Electrolyte Solution, J. Phys. Chem. C, 113, Go M., Li W., Dong J., Zhng Z., Yng B., 211, Synthesis nd Chrcteriztion of Superprmgnetic Fe3O4@SiO2 Core-Shell Composite Nnoprticles, World J. Condens. Mtter Phys., 1, Gun D., Go Z., Yng W., Wng J., Yun Y., Wng B., Zhng M., Liu L., 213, Hydrotherml synthesis of crbon nnotube/cubic Fe3O4 nnocomposite for enhnced performnce supercpcitor electrode mteril, Mter. Sci. Eng. B, 178, Mishr A.K., Rmprbhu S., 211, Functionlized Grphene-Bsed Nnocomposites for Supercpcitor Appliction, J. Phys. Chem. C, 115, Mitchell E., Gupt R.K., Mensh-Drkw K., Kumr D., Rmsmy K., Gupt B. K., Khol P., 214, Fcile synthesis nd morphogenesis of superprmgnetic iron oxide nnoprticles for high-performnce supercpcitor pplictions, New J. Chem., 38, Qu Q., Yng S., Feng X., 211, 2D Sndwich-like Sheets of Iron Oxide Grown on Grphene s High Energy Anode Mteril for Supercpcitors, Adv. Mter., 23, Srno M., Cirillo C., Piscitelli R., Cimbelli P., 213, A study of the key prmeters, including the crucil role of H2 for uniform grphene growth on Ni foil, J. Mol. Ctl. A: Chem., 366, Srno M., Snnino D., Leone C., Cimbelli P., 212, Evluting the effects of operting conditions on the quntity, qulity nd ctlyzed growth mechnisms of CNTs, J. Mol. Ctl. A: Chem. 357, Srno M., Tmburrno A., Arurult L., Fontorbes S., Pntni R., Dts L., Cimbelli P., Srto M.S., 213b, Electricl conductivity of crbon nnotubes grown inside mesoporous nodic luminium oxide membrne, Crbon, 55, Srno M., Cirillo C., Cimbelli P., 214, Selective grphene covering of monodispersed mgnetic nnoprticles, Chem. Eng. J. 246, Srno M., Grmell A., Cirillo C., Cimbelli P., 214b, MoS2/MoO2/Grphene Electroctlyst for HER, Chemicl Engineering Trnsction, 41, DOI: 1.333/CET Shi W., Zhu J., Sim D.H., Ty Y.Y., Lu Z., Zhng X., Shrm Y., Srinivsn M., Zhng H., Hng H.H., Yn Q., 211, Achieving high specific chrge cpcitnces in Fe3O4/reduced grphene oxide nnocomposites, J. Mter. Chem., 21, Stoller M.D., Prk S., Zhu Y., An J., Ruoff R.S., 28, Grphene-Bsed Ultrcpcitors, Nno Lett., 8, Sun X., Zheng C., Zhng F., Yng Y., Wu G., Yu A., Gun N., 29, Size-Controlled Synthesis of Mgnetite (Fe3O4) Nnoprticles Coted with Glucose nd Gluconic Acid from Single Fe(III) Precursor by Sucrose Bifunctionl Hydrotherml Method, J. Phys. Chem. C., 113, Wng G., Zhng L., Zhng J., 212, A review of electrode mterils for electrochemicl supercpcitors, Chem. Soc. Rev., 41, Wng Q., Jio L., Du H., Wng Y., Yun H., 214, Fe3O4 nnoprticles grown on grphene s dvnced electrode mterils for supercpcitors, J. Power Sources, 245, Zhou G., Wng D.-W., Li F., Zhng L., Li N., Wu Z.-S., Wen L., Qing L.G., Cheng H.-M., 21, Grphene- Wrpped Fe3O4 Anode Mteril with Improved Reversible Cpcity nd Cyclic Stbility for Lithium Ion Btteries, Chem. Mter., 22,

Electronic Supplementary Information

Electronic Supplementary Information Electronic Supplementry Informtion Assemling cron-coted hollow α-fe 2 nnohorns on the CNT ckone for superior lithium storge cpility Corresponding uthor: Xiong Wen (Dvid) Lou* School of Chemicl nd Biomedicl