Materials Chemistry and Physics Volume 103 Issue 2-3 2007 [Doi 10.1016_j.matchemphys.2007.02.038] Yan Shan; Lian Gao -- Formation and Characterization of Multi-walled Carbon NanotubesCo3O4 Nanocomposites for s

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  Materials Chemistry and Physics 103 (2007) 206–210 Materials science communication Formation and characterization of multi-walled carbonnanotubes/Co 3 O 4  nanocomposites for supercapacitors Yan Shan, Lian Gao ∗ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China Received 18 July 2005; received in revised form 3 April 2006; accepted 18 February 2007 Abstract A novel type of multi-walled carbon nanotubes (MWNTs)/Co 3 O 4  composite electrode for supercapacitors has been prepared through a facileand effective method, which combined the acid treatment of MWNTs and in situ decomposition of Co(NO 3 ) 2  in  n -hexanol solution at 140 ◦ C.The samples are characterized by TEM, XRD and electrochemical testing, respectively. The MWNTs/Co 3 O 4  composites show high capacitorproperty, and their best specific capacitance is up to 200.98Fg − 1 , which is significantly greater than that of pure MWNTs (90.1Fg − 1 ). Withdifferent MWNTs contents, they show Faradaic redox capacitive behavior to double-layer capacitive behavior, which may be due to the differentmicrostructure of the composites.© 2007 Elsevier B.V. All rights reserved. Keywords:  Fullerenes; Composite materials; Electrochemical properties 1. Introduction Supercapacitors(alsocalledelectrochemicalcapacitors)haveattractedmuchattentionbecauseoftheirhighpowerdensityandlong cycle life (>100,000 cycles), and been widely applied aspower sources for electric vehicles, computers and so on. Gen-erally,supercapacitorscanbeclassifiedintotwotypes[1–2]:(1) the electric double-layer capacitors based on carbon materialswith a high surface area and (2) the faradic pseudo-capacitorsbased on metal oxides and conducting polymers. When hav-ing the same electrode area, the faradic pseudo-capacitors havehigher energy density while the electric double-layer capacitorshave higher power density. In order to apply supercapacitors tovarious practical devices, it is necessary to develop the superca-pacitors with both high power density and high energy density.Carbon nanotubes (CNTs) are attractive material for super-capacitors due to their unique one-dimensional mesoporousstructure, high specific surface area, low resistivity and goodchemical stability [3–5]. Though their power density is high, the energy density is not satisfying. Many methods have beenemployed to improve the capacitance of CNTs [6], includ- ∗ Corresponding author. Tel.: +86 21 52412718; fax: +86 21 52413122.  E-mail address: (L. Gao). ing: (a) treatment with acids or bases for activating the CNTsand/or increasing the micropore volume of the CNTs, and (b)modification of the CNTs with conducting polymers such aspolypyrrole [7,8] or transition-metal oxides [9–13]. Because of the degradation of polymers, the CNTs/conducting poly-mer electrodes do not have long cycle life [1]. Transition-metal oxidesattachedtoCNTsareexpectedtobemoreusefulbecausetheytakeadvantageofbothdouble-layercapacitanceandfaradicpseudo-capacitance. The supercapacitive behavior of severaltransition-metal oxides such as RuO 2  [1,4,6], IrO 2  [9], NiO  x  [10–13] has been evaluated. Among them, RuO 2  has been con-sidered as one of the most promising candidate for electrodes,but the high cost limits its wide application, so it is urgent tolook for a substitute material to replace RuO 2 .SpinelCo 3 O 4  isalsoanimportanttransition-metaloxide,andhas great application in heterogeneous catalysts, anode mate-rials in Li-ion rechargeable batteries, solid-state sensors, solarenergyabsorbers,ceramicpigmentsandelectrochromicdevices[14–18].Suchabroadperspectiveofutilizationmakestheprepa-ration of Co 3 O 4  nanostructure attract much attention, but fewcover the carbon nanotube-Co 3 O 4  composites. Fu et al. [17]synthesized beaded cobalt oxide nanoparticles along CNTs insupercriticalfluid(containingethanolandCO 2 )andstudiedtheirelectrical transport properties as Schottky-junction diode. In theprevious work, we [18] reported a facile and effective method 0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2007.02.038  Y. Shan, L. Gao / Materials Chemistry and Physics 103 (2007) 206–210  207 to modify MWNTs with Co 3 O 4  nanocrystals and investigatedthe electrochemical performance in Li + insertion and extrac-tionreactions,itwasfoundthattheMWNTs/Co 3 O 4  compositeshave larger lithium storage capacities and a better cycle-ability.Herein, we examined their property for surpercapacitors for thefirst time. The results showed that the composites could be apromising candidate for electrochemical supercapacitors. 2. Experimental procedure Multi-walled carbon nanotubes (MWNTs) prepared by the catalytic decom-position of CH 4  were kindly provided by Chengdu Institute of OrganicChemistry, Chinese Academy of Sciences. They were dried at 140 ◦ C for 24hbefore use. A typical experimental procedure was described as follows. Firstly,the dried MWNTs were treated by concentrated nitric acid (Analytical grade) at140 ◦ C for 6h according to the procedure described in literature [19]. Then the treated MWNTs were rinsed with distilled water and dried at 100 ◦ C for 12h.Secondly, 0.5g Co(NO 3 ) 2 · 6H 2 O was dissolved into 40mL  n -hexanol to form ared solution. Seventy milligrams of acid treated MWNTs were dispersed in thered solution by ultrasonication for 2h. After that, this solution was refluxedat 140 ◦ C for 10h. After cooled to ambient temperature, the products werewashed with cyclohexane repeatedly to remove any impurities and dried. Aseries of MWNTs/Co 3 O 4  composites with different content of MWNTs wereprepared,andthesamplesweredenotedasC-Co 3 O 4 -  x  %(  x  istheweightpercentof MWNTs). 2.1. Characterization X-Ray diffraction (XRD) was used to identify the phase of the samplewith a D/MAX 2550V (Rigaku, Japan) diffractometer using Cu K   radia-tion ( λ =1.5406 ˚A). TEM images were obtained on a JEOL 200CX electronmicroscope with an Energy Dispersive Spectrometer (EDS) using an accel-erating voltage of 200kV. The electrochemical cyclic voltammograms (CV)experiments were performed using CHI 660A electrochemical workstation (CHinstruments Inc., USA) in a three-electrode arrangement, including a work-ing electrode (MWNTs electrode or Co 3 O 4  /MWNTs composite electrode), aPt foil counter-electrode and a Hg/Hg 2 SO 4  reference electrode. The workingelectrodes were prepared by pressing MWNTs or Co 3 O 4  /MWNTs composites,acetylene black and PVDF binder (weight ratio of 85:10:5) on 10mm × 10mmnickelgauze(asthecurrentcollector).Theexperimentswereperformedatroomtemperature with 1M KOH as electrolyte. 3. Results and discussion Fig. 1 shows the XRD patterns of the pure MWNTsand MWNTs/Co 3 O 4  composites. The most intense peaks of MWNTs correspond to the (002) reflection, (100) and (101)reflection in Fig. 1(a). In Fig. 1(b), except the peak at 2 θ   =26 ◦ ,which is corresponding to the (002) reflection of the MWNTs,all the other peaks belong to the characteristic peaks of spinelCo 3 O 4 , which are close to the values on JCPDS card (No. 43-1003) with the lattice constant of   a =8.083 ˚A. The broad peaksof Co 3 O 4  in the XRD pattern indicate that the nanoparticlesis small, and the average size of Co 3 O 4  calculated by Scherrerequationisabout8nm.Noobviouspeakscorrespondingtoothercobalt oxide are observed.Fig. 2 shows the typical TEM images of the MWNTs/Co 3 O 4 with different MWNTs content. Fig. 2(a and b) are the low magnificationimagesofC-Co 3 O 4 -4.8wt%.TheyrevealthattheMWNTs are coated with a layer of Co 3 O 4  nanocrystals with asizeoflessthan10nm,whichisconsistentwiththeXRDresults.The pristine MWNTs we used here have lengths ranging from Fig. 1. XRD patterns of pure MWNTs (a) and MWNTs/Co 3 O 4  composites (b). severaltotensofmicrometerswithinnerdiameteraround10nmand outer diameter 40nm [20]. So the coating Co 3 O 4  layer wasestimatedtobearound15–30nminthickness.ThecoatingoftheindividualMWNTsisnotveryuniform,afewsegregatedCo 3 O 4 nanocrystals conglomerate on the Co 3 O 4  coating layer as thearrow shown in Fig. 2(a). The other dissociative Co 3 O 4  aggre-gate to spherical nanoparticles with a diameter of 20–40nm,which distribute among the MWNTs. Fig. 2(c) is an enlarged selected area in Fig. 2(b), it displays that the Co 3 O 4  particlescoverage on the surface of MWNTs present planar spacing of 0.46 or 0.23nm, which is assigned to (111) and (311), respec-tively. It is also found that some neighboring Co 3 O 4  have thesame planar spacing just as the ellipse shown in Fig. 2(c), indi- catingCo 3 O 4  particleshavethetrendoforientedagglomeration,whichresultsinthecoatinglayermoreuniforminsomeregions.Fig. 2(d and e) are the images of C-Co 3 O 4 -50wt%. They showa very different morphology from C-Co 3 O 4 -4.8wt%. MWNTsare not fully coated by Co 3 O 4 , but aggregated Co 3 O 4  balls witha size of ca.150nm attach to the sidewall of MWNTs, mostlyon the tips of MWNTs, where more defects exist than the side-wall [21]. It seems like the aggregated Co 3 O 4  balls are threadedtogether by MWNTs.It is known that after oxidized with nitric acid, the surface of MWNTs possesses a great deal of functional carboxyl groupsandbecomesnegativelycharged[22].Thepositivecobaltionsin the hydrophobic  n -hexanol solution are adsorbed to the surfaceofthenitrictreatedMWNTsthroughelectrostaticattractionandthen in situ decompose into Co 3 O 4  according to the followingprocess:3Co(NO 3 ) 2air ,n -hexnal , 140 – 180 ◦ C −→  Co 3 O 4  ↓+ 6NO 2  ↑ + O 2  ↑  (1)However, cobalt ions may be preferably absorbed on theimperfections of the MWNTs surface and the surface hydrox-yls can lead to the aggregation of metal oxide particles [23], so there are some aggregated Co 3 O 4  balls with a size of ca.150nmattach to the sidewall of MWNTs, just like they are threadedtogether by MWNTs as shown in Fig. 2(e). When the MWNTs  208  Y. Shan, L. Gao / Materials Chemistry and Physics 103 (2007) 206–210 Fig. 2. TEM and HRTEM images of MWNTs/Co 3 O 4  composites with different MWNTs contents: (a and b) C-Co 3 O 4 -4.8wt%, (c) is the HRTEM of selected areain (b), and (d and e) C-Co 3 O 4 -50wt%. content is far below the content of cobalt oxide in the solu-tion, on the one hand, cobalt ions continuously adsorb onthe as-srcinated Co 3 O 4  coating layer and then crystallize toCo 3 O 4  nanoparticles; on the other hand, some Co 3 O 4  agglom-erates in the solution will adsorb on the as-srcinated Co 3 O 4 coating layer, just as the arrow shown in Fig. 2(a). These might be the reasons why MWNTs can be thickly coated withCo 3 O 4 .Itwasfoundthatsurfacehydroxylsarethemainreasonfortheaggregation of metal oxide nanoparticles [23]. Compared with the fast nucleation and aggregation growth in aqueous solution,it is thought that nanocrystals aggregating in the nonaqueoussolution are kinetically slower due to less surface hydroxyls andgreater viscosity, which provides the nanocrystals enough cush-ion to rotate to find the low-energy configuration interface andform perfectly oriented aggregation. So some Co 3 O 4  nanopar-ticles’ oriented aggregations are observed.Fig. 3 shows the cyclic voltammograms (CV) of MWNTselectrodes in 1M KOH at the sweep rate of 100mVs − 1 . It canbe seen that the MWNTs electrodes present ideal double-layerbehaviorsexceptthepeaksobservedat0.1VversusHg/Hg 2 SO 4 for the first cycle, which is attributed to redox reactions of thefunctional groups on MWNTs, such as carboxylic acid on the Fig.3. CyclicvoltammogramsofMWNTselectrodesin1MKOHinthepoten-tial range of  − 0.8 to 0.4V vs. Hg/Hg 2 SO 4  at the sweep rate of 100mVs − 1 .  Y. Shan, L. Gao / Materials Chemistry and Physics 103 (2007) 206–210  209 surfaceofMWNTs.Itisalsofoundthataftercontinuouscycling,the peak diminished due to the irreversibility of the redox reac-tion,andtheCVcurvespresentarectangularshapethatisaclearproof of double-layer capacitance properties.It can be seen clearly from Fig. 3 that MWNTs showthe double-layer capacitance properties, but how about afterMWNTs were modified with Co 3 O 4 ? The comparison of cyclicvoltammograms between MWNTs and MWNTs/Co 3 O 4  com-posite electrodes is shown in Fig. 4. It can be seen that the MWNTs/Co 3 O 4  composite electrodes have a high capacitorperformance, and they have higher specific capacitance thanMWNTs (have larger area of the CV curves), but they showdifferent capacitive behaviors. C-Co 3 O 4 -4.8wt% shows theFaradaic redox capacitive behavior, while C-Co 3 O 4 -50wt% Fig. 4. Cyclic voltammograms (sweep rate: 10mVs − 1 ) of the electrodes in1M KOH in the potential range of   − 0.6 to 0.4V vs. Hg/Hg 2 SO 4 . (a) AcidtreatedMWNTselectrode,(b)MWNTs/Co 3 O 4  compositeelectrodesC-Co 3 O 4 -4.8wt% and (c) C-Co 3 O 4 -50wt%. behaves like a typical electric double-layer supercapacitor,without obvious pseduocapacitance. Though C-Co 3 O 4 -50wt%showsnopseduocapacitanceofCo 3 O 4 ,theareaoftheCVcurvein Fig. 4(c) is much larger than Fig. 4(a), which attributes to the action of Co 3 O 4 . The difference of the two compositesin capacitive behavior may have some relationship with themicrostructuresofthecomposites.Thecapacitanceofanelectro-chemical device depends on the separation between the chargeon the electrode and the counter charge in the electrolyte [6]. In the C-Co 3 O 4 -4.8wt% electrode, the content of Co 3 O 4  is dom-inant and the surfaces of MWNTs are fully coated with a thick layer of Co 3 O 4  just like shown in Fig. 2(a and b), it is easier for the charge to have action with Co 3 O 4 , so the C-Co 3 O 4 -4.8wt% electrode behaves the capacitive behavior of Co 3 O 4 .While in the C-Co 3 O 4 -50wt% electrode, the Co 3 O 4  aggregatedballsonlyattachontheimperfectionsofMWNTs,andmostsur-faces of MWNTs are free of coating, so the capacitive behaviorofC-Co 3 O 4 -50wt%mayattributetothecooperationofMWNTswith Co 3 O 4 .ThespecificcapacitancesoftheMWNTs,C-Co 3 O 4 -4.8wt%and the C-Co 3 O 4 -50wt% electrodes in Fig. 4(a–c) are 90.1, 166.08 and 200.98Fg − 1 , respectively. The specific capacitancewas calculated by: C m  = imv where  m  is the mass of active material,  v  the potentialsweep rate, and  i  is the even current response defined by i =    V  c V  a i ( v )d v/ ( V  c − V  a ),where V  a  and V  c  representthelowestand highest voltage, respectively.  i  is obtained through inte-grating the area of the curves in Fig. 4(a–c). It is found that the capacitances of MWNTs/Co 3 O 4  composite electrodes arehigherthanthoseofMWNTs,whichmaycomefromthepseudo-capacitance of Co 3 O 4  according to the following equation[24]:Co 3 O 4 + H 2 O  +  OH − ↔  3CoOOH  +  e − (2)We can see that C-Co 3 O 4 -50wt% has higher specific capaci-tance than C-Co 3 O 4 -4.8wt%, which maybe due to the differentconductanceintwoelectrodes.Itisreportedthatthecapacitancehas great relationship with conductance of the electrode. Co 3 O 4 is a semiconductor with a lower conductance and the content of Co 3 O 4  in C-Co 3 O 4 -4.8wt% electrode is much too high, whichmaketheconductanceofC-Co 3 O 4 -4.8wt%lowerthanthatofC-Co 3 O 4 -50wt%, so C-Co 3 O 4 -50wt% electrode shows a higherspecific capacitance.When the content of MWNTs increases to 50%, the spe-cific capacitance is reached to 200.98Fg − 1 , which is morethan twice as high as pure MWNTs. RuO 2  is one of the mostpromising candidates for electrodes and often used to modifythe CNTs to improve the capacitance. Compared with RuO 2 ,Co 3 O 4  is much cheaper, and the specific capacitance of theMWNTs/Co 3 O 4 compositesisappreciablycomparablewiththatofRuO 2  /CNTs(295Fg − 1 )[4].Thus,theMWNTs/Co 3 O 4  com-posites could be a prosing candidate for use as electrochemicalsupercapacitors.
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