1、文章编号: 1007- 8827( 2014) 06- 0426- 06一步水热法制备氢氧化镍-石墨烯复合材料及其电化学性能袁博1， 郑晓雨1， 张辰1， 吕伟2， 李宝华2， 杨全红1， 2( 1 天津大学 化工学院， 天津 300072;2 清华大学深圳研究生院 炭功能材料工程实验室， 广东 深圳 518055)摘要: 通过一步水热法制备出具有三维网络结构的氢氧化镍- 石墨烯复合材料( Ni( OH)2- GS) 。这一独特的结构可以提供良好的离子传输通道， 同时可以有效地提高氢氧化镍与电解液的接触面积和材料的导电性。结果表明， Ni( OH)2的质量分数为 84%时， 复合材料具有最佳的
2、电化学性能， 在 5 mV s1的扫速下比电容为 1 461 F g1， 在 100 mV s1的扫速下比电容为 682 F g1( 容量保持率为 47%) ， 并且具有良好的循环稳定性。关键词: 超级电容器; 氢氧化镍; 石墨烯; 三维网络结构中图分类号:TB332文献标识码:A基金项目: 国家重点基础研究发展计划( 2014CB932403) ; 国家自然科学基金( 51302146) ; NSAF ( U1330123) ; 深圳市科技计划项目( JCYJ20130402145002430) ; 博士后科学基金( 2012M520012， 2013T60111) ; 广东省创新研发团队计
3、划( 2009010025) ; 深圳市战略新兴产业发展专项资金( 重点实验室) 项目( ZDSYS20140509172959981) 通讯作者: 吕伟， 博士， 讲师 E- mail:lv wei sz tsinghua edu cn作者简介: 袁博， 硕士研究生 E- mail:tjuyuanbo126 comAssembly of Ni( OH)2- graphene hybrids with ahigh electrochemical performance by a one- pot hydrothermal methodYUAN Bo1， ZHENG Xiao- yu1， ZHA
4、NG Chen1， LU Wei2， LI Bao- hua2， YANG Quan- hong1， 2( 1 School of Chemical Engineering and Technology，Tianjin University，Tianjin300072，China;2 Engineering Laboratory for Functionalized Carbon Materials，Graduate School at Shenzhen，Tsinghua University，Shenzhen518055，China)Abstract:A Ni( OH)2- graphene
5、hybrid with a three dimensional ( 3D)interconnected graphene network was prepared by a simpleone-pot hydrothermal method The 3D structure constructed of flexible and planar graphene sheets ( GS)forms an effective electrontransfer network and provides a continuous pore structure for ion transport Mor
6、eover，this structure avoids the aggregation of GSand Ni( OH)2，resulting in a high utilization rate of Ni( OH)2at high contents This study shows the hybrid has a high rate capabili-ty and cyclic stability and exhibits a high specific capacitance of 1 461 F g1at a scan rate of 5 mV s1with a Ni( OH)2ma
7、ssloading of 84 mass%Keywords: Supercapacitor;Ni( OH)2;Graphene sheets; 3D networkeceived date: 2014- 07- 28; evised date: 2014- 12- 05Foundation item: National Basic esearch ProgramofChina ( 2014CB932403 ) ; National Science Foundation ofChina( 51302146) ;NSAF ( U1330123) ;Shenzhen Basic esearch Pr
8、oject ( JCYJ20130402145002430) ;China Post-doctoral Science Foundation ( 2012M520012，2013T60111) ;Guangdong Province Innovation D Team Plan( 2009010025) ;ZDSYS( 20140509172959981) Corresponding author:LU Wei，Ph D ，Lecturer E- mail:lv wei sz tsinghua edu cnAuthor introduction:YUAN Bo，Master Candidate
9、 E- mail:tjuyuanbo126 comEnglish edition available online ScienceDirect (http: www sciencedirect comsciencejournal18725805 ) DOI: 101016/S1872- 5805( 14) 60147- 51IntroductionSupercapacitor is considered as an important de-vice for the next generation energy storage for itshigher power density than
10、batteries and higher energydensity than conventional dielectric capacitors1- 4 There are two types of supercapacitors based on dif-ferent energy storage behaviors，which are electricaldouble layer capacitors ( EDLCs)based on ion ad-sorption on surface and pseudocapacitors based on第 29 卷第 6 期2014 年 12
11、 月新型炭材料NEW CABON MATEIALSVol 29No 6Dec 2014electrochemical redox reactions of the electrode mate-rials1， 5- 9 Pseudocapacitive materials such as conduc-ting polymers7， 10 and metal oxides ( or hydroxides)exhibit much higher specific capacitance than carbonmaterials which are always used for EDLCs si
12、ncemuch more energy is stored via the electrochemicalreactionsthantheionadsorptiononsurfaceNi( OH)2is one of the most promising active materi-als for pseudocapacitors because of its high theoreticalspecific capacitance ( 2 082 Fg1)and low costHowever，pseudocapacitive materials have a low elec-trical
13、 conductivity，a poor cycling stability and a slowkinetics11 Numerous efforts have been devoted topreparing Ni( OH)2nanostructures with different mor-phologies toimprove the electrochemical kineticsthrough enhancing the active surface area and lower-ing the ion diffusion resistance，such as platelet-
14、likesheet，flower- like structure12， 13 ，nanoparticles14 ，microspheres15 ， nanotubes16 ， and nanorods17 Another way is to uniformly load it on the carbonframework to avoid its aggregation and improve its u-tilization rate while the conductivity of the Ni( OH)2is enhanced by forming the hybrids Many k
15、inds ofcarbon，including graphene sheets ( GS ) ，carbonnanotube and porous carbon，have been investiga-ted11， 17- 19 Although above approaches efficiently a-melioratetheelectrochemicalperformanceofNi( OH)2， theelectrochemicalperformancestillneeds to be improved due to the far lower theoreticalcapacita
16、nce and relatively inferior rate capabilityGS have been studied extensively as new carbonframework in electrochemical energy storage devicesowing to its high surface area， good electrical conduc-tivity and chemical stability Graphene- based materi-als with a three- dimensional ( 3D)porous networkhav
17、e a unique hierarchical architecture which not onlyprevents GS from stacking，but also allows fast iondiffusion20- 22 A combination of pseudocapacitivematerials with 3D graphene framework has the poten-tial to take advantages of the two components to a-chieve a high electrochemical performanceIn ourp
18、revious work，we have found that the 3D grapheneskeleton can be a fast electron and ion transport net-work for metal oxide particles in electrochemical ap-plications23 Besides，interactions between the ox-ides and GS also help control the morphology of ox-ides and increase its stability during cycling
19、18， 24- 27 Herein，we demonstrate a simple one- pot hydro-thermal method to directly assemble a Ni( OH)2nano-plate- GS hybrid with a 3D microstructure ( Ni( OH)2-GS) The Ni( OH)2nanoplates are uniformly dispers-ed on the surface of GS which construct a 3D networkleaving unhindered ion diffusion paths
20、 and forming acontinuous conductive network This structure realizesthe high loading and utilization rate of Ni ( OH)2Thus，the Ni( OH)2- GS hybrid shows a high specificcapacitance of 1 461 F g1and good rate capability2ExperimentalPreparation of Ni( OH)2- GS hybrids:Graphiteoxide was first prepared fr
21、om natural graphite by amodified Hummers method In a typical preparationfor the hybrids， 160 mg graphite oxide was ultrasoni-cated in 80 mL de- ionized water for 2 h to obtain agraphene oxide ( GO)suspension Then，2 51 g Ni( NO3)26H2O and 0 69 g NaOH were added intothe above GO suspension under sonic
22、ation for 1 hThe obtained mixed suspension was transferred into a100 mLTeflonautoclave， whichwasheatedto180 and held for 12 h，and the obtained hydrogelwas washed with de- ionized water several times andfreeze- dried to produce the Ni( OH)2- GS hybrid Inorder to investigate the influence of the Ni (
23、OH)2fraction on the structure and electrochemical perform-ance，the amount of Ni( NO3)2was controlled to tunethe fraction of Ni( OH)2in the hybrids，and the ob-tained samples were denoted as Ni ( OH)2- GS- X%( X is referred as the weight percentages of Ni( OH)2in the hybrids) For a reference，pure Ni(
24、OH)2wasprepared with the same procedure without the additionof GOStructuralcharacterization: X- raydiffraction( XD)measurements were conducted at room tem-perature using a reflection mode ( BrukerD- 8，Germa-ny) Thermogravimetric analysis ( TG，igaku，Ja-pan)was performed at 800 with a heating rate of1
25、0 min1in air atmosphere Scanning electron mi-croscopic ( SEM )and transmission electron micro-scopic ( TEM )observations were performed with aHitachi S- 4800 ( Hitachi，Japan)and a JEOL 2100F( JEOL，Japan) ，respectivelyElectrochemical performance measurement:Forthe preparation of the electrode，a mixtu
26、re of activematerials and PTFE with a weight ratio of ( 90 10)was ultrasonicated for 20 min in an ethanol solutionand dropped on a Ni foam For comparison，the elec-trode of Ni( OH)2with conductive additive is pre-pared by a mixture of Ni( OH)2，conductive additiveof Super- P ( abbreviated as SP)and PT
27、FE with aweight ratio of 74 16 10 The electrodes were testedat room temperature using an electrochemistry work-station ( Metrohm，Switzerland) The electrochemis-try workstation was used to measure the cyclic volta-mmery ( CV ) and the electrochemical impedancespectra ( EIS) The CV tests were conducte
28、d in a724第 6 期YUAN Bo et al:Assembly of Ni( OH)2- graphene hybrids with a high electrochemical performance voltage window of 0 05 0 5 V with different scanrates3esults and discussionThe proposed preparation strategy is shown inFig 1a The GO and Ni( NO3)2mixture dispersionwas subjected to a hydrother
29、mal process at 180 for12 h to obtain a 3D hydrogel，then a freeze- dryingmethod was used to fix the 3D structure to obtain theNi( OH)2- GS hybrids The formed 3D structure canbe observed from the SEM image shown in Fig 1b，which is similar to the pure GS foam as we reportedpreviously23 ，with the only d
30、ifference of a muchthickerporewallduetotheincorporationofNi( OH)2 The distribution of the Ni( OH)2nano-plates on the GS can be seen from the TEM image inFig 1c，where the nanoplates of tens of nanometersare uniformly dispersed without aggregation becausethe graphene sheet acts as a substrate to make
31、Ni2 +disperse on it uniformly then these ions grew intoNi( OH)2nanoplates under the hydrothermal condi-tion as described in our previous study18， 28 Duringthe hydrothermal process， the Ni2 +- anchored GOsheets were reduced into GS and they interconnectedwith each other forming the 3D porous structur
32、e29 Fig 1( a)Scheme of the preparation process of Ni( OH)2- GS，( b)SEM and ( c)TEM images of Ni( OH)2- GS- 84%In order to investigate the influence of the con-tents of Ni ( OH)2on the electrochemical perform-ance，a series of samples with different Ni ( OH)2contents were prepared According to the mas
33、s con-tent of NiO measured by the TG analysis in air atmos-phere ( Fig 2a) ，the mass ratios of Ni( OH)2of dif-ferent hybrids prepared were calculated to be 31%，63%， 84% and 91% All these hybrids show a set ofcharacteristic diffraction peaks of - Ni( OH)2and nopeak of other phases of NiO is detected
34、in the XDresults，indicating that the hybrids are of high purity( Fig 2b) The four characterized peaks at 19，33，39，and 52 correspond to the ( 001) ，( 100) ，( 101)and ( 102)diffraction planes，respectively，and the relative intensity of the corresponding diffrac-tion peaks for the Ni( OH)2- GS is signif
35、icantly de-creased with the decrease of the Ni( OH)2contentThe absence of the typical ( 002)peak representingthe layered structure of graphite suggests no aggrega-tionofthegraphenelayersoccursbecausetheNi( OH)2nanoplates avoid the layer- by- layer stackingof GSThe microstructure of the hybrids with
36、differentNi( OH)2contents are found to be of great differencefrom each other as demonstrated by the SEM imagesin Fig 2c- f It is noted that the Ni( OH)2nanoplatescontained in all samples has a hexagonal structure，which has a better electrochemical activity than theparticular case as it has a higher
37、active surface areaand a faster electrochemical reactive kinetic due to theplanar structure The pure Ni( OH)2and Ni( OH)2-GS- 91% show the similar structure and the nanoplatesaggregate heavily as shown in Fig 2b，c and no 3Dstructure can be observed Although the aggregationof Ni( OH)2does not appear
38、in Ni( OH)2- GS- 31%，the 3D structure cannot be observed either This canbe ascribed to the possibility that Ni2 +promotes theassembly of GS since the metal ions can act as the in-teraction joint between the GS29 The 3D structure isformed when more Ni2 +ions are introduced in the re-action，and such a
39、 structure can be maintained whenthe Ni( OH)2content increases to 84% ( Fig 2e) Inthis situation，the specific surface area of 98 m2 g1is still reached，guaranteeing the ample ion diffusionchannels and high accessible electrochemical activesurface Ni( OH)2nanoplates are anchored on the GS824新型炭材料第 29
40、卷surface formingan efficient face- to- face binding，which benefits the electron transfer between the twocomponents The above factors ensure the good elec-trochemical performance of the hybridsFig 2( a)TG curves and ( b)XD patterns of Ni( OH)2- GS with different contents of Ni( OH)2( c- f)SEM images
41、of Ni( OH)2- GS hybrids with different contents of Ni( OH)2( 100%， 91%， 84% and 31%，respectively) The specific capacitances of Ni( OH)2- GS con-taining 31%，63%，84%，91% Ni( OH)2and pureNi( OH)2are 323， 797， 1 461，1 540，1 190 F g1at a scan rate of 5 mVs1，respectively When thescan rate increases to 100
42、 mV s1，the correspondingspecific capacitances decreases to 163，383，682，570， 320 F g1respectively，which has a retention of50%， 48%，47%，37% and 26% compared to thatat 5 mV s1( Fig 3a，b) Note that these results arecalculated based on the whole electrode materials butwithout PTFE，so the capacitances of
43、Ni( OH)2- GS-31% and Ni( OH)2- GS- 63% with low Ni( OH)2con-tents are much lower than the pure Ni( OH)2 Wehave found that when the mass ratio of Ni( OH)2islower than 84%，the capacitance retentions of the hy-brids are similar to each other，suggesting continuousion diffusion channels，while too much GS
44、 are notnecessary for the improvement of energy densityGalvanostatic discharge curves of Ni( OH)2- GS- 84%at different current densities are presented in Fig 3cNi( OH)2- GS- 84% also demonstrates a great electro-chemical stability and only a slight decrease in capaci-tance (6%)occurs over 1 000 cycl
45、es at a high cur-rent density of 2 A g1( Fig 3d) ，indicating a goodstability of the 3D structureFig 4a shows the CV curves of Ni( OH)2- GS-84% The anodic peak ( positive current density)oc-curred around 0 35 V ( vs Ag /AgCl)indicates anoxidation process related to an oxidation of - Ni( OH)2to - NiOO
46、H，whereas the cathodic peak( negative current density)observed around 0 1 V( vs Ag /AgCl)corresponds to a reduction processfollowing the Faradic reactions of Ni( OH)2:- Ni( OH)2+ OH - NiOOH + H2O + eThe symmetric characteristic of the anodic andcathodic peaks indicates an excellent reversibility oft
47、he Ni( OH)2- GS electrode In addition，it can beseen that the shapes of these CV curves and the peakseparation that refers to the internal ion diffusion andelectron transfer resistance show almost no significantchanges with the increase of scan rates from 5 to100 mV s1，implying a fast mass transfer，a
48、 fast re-action kinetics and a little equivalent series resistanceHowever，the potentials of the redox peaks of thepure Ni( OH)2electrode shift obviously and seriouspolarization occurs due to a large reaction internal re-sistance of the electrode and a poor conductivity Inorder to demonstrate the adv
49、antages of the hybridsstructure，conductive additive ( SP，which has a verygood conductivity)is added in the pure Ni( OH)2electrode with the similar content as that of GS in thehybrids Fig 4c shows the specific capacitance of theNi (OH )2- GS- 84%， Ni (OH )2withSPandNi( OH)2without SP at various scan
50、rates Althoughthe capacitance and rate capability of Ni( OH)2areenhanced after adding SP because of the improvedconductivity，it still shows a much lower capacitanceand capacitance retention ( 38%)than Ni( OH)2- GS( capacitance retention:47%) ，which should be as-cribed to a decreased continuity of el
