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1、Nitrogen-doped porous carbons by conversion of azo dyes especiallyin the case of tartrazineZhong Jie Zhanga,b,Chong Chenb,Peng Cuib,*,Xiang Ying Chenb,*aCollege of Chemistry&Chemical Engineering,Anhui Province Key Laboratory of Environment-friendly Polymer Materials,Anhui University,Hefei 230039,Anh
2、ui,PR ChinabSchool of Chemical Engineering,Anhui Key Laboratory of Controllable Chemistry Reaction&Material Chemical Engineering,Hefei University of Technology,Hefei 230009,Anhui,PR Chinah i g h l i g h t sg r a p h i c a la b s t r a c t?A direct carbonization method wasdeveloped for nitrogen-doped
3、 porouscarbon.?Tartrazine can serve as not only car-bonsourcebutalsoasnitrogensource.?High surface areas and pore volumesare achieved with Ca(OAc)2$H2O.?The carbon samples exhibit excellentcapacitive behaviors.a r t i c l e i n f oArticle history:Received 15 March 2013Received in revised form22 Apri
4、l 2013Accepted 1 May 2013Available online 28 May 2013Keywords:TartrazinePorous carbonTemplateSupercapacitorsa b s t r a c tNitrogen-doped porous carbons possessing high surface areas and large pore volumes have been pre-pared by directly heating the mixture of tartrazine and Ca(OAc)2$H2O at 800?C es
5、pecially withoutfurther physical or chemical activation,where Ca(OAc)2$H2O serves as the hard template to regulate thesurface area and pore structures.It reveals that the addition of Ca(OAc)2$H2O can remarkably improvethe surface area and total pore volume.The T-Ca-800-3:1 sample displays the highes
6、t BET surface area as1669 m2g?1and largest total pore volume 0.85 cm3g?1,which is much larger than those without addingCa(OAc)2$H2O.Furthermore,it exhibits excellent capacitive performances,including high specificcapacitance(ca.224.3 F g?1at 0.5 A g?1),good rate capability(the retention of 42.6%at 6
7、0 A g?1)andgood cycling stability(the retention of 92.3%within 5000 cycles).?2013 Elsevier B.V.All rights reserved.1.IntroductionAlong with the increasing power demands of energy storagesystems in the 21st century,supercapacitors have attracted muchattention due to their pulse power supply,long cycl
8、e life(100 000cycles),and high dynamic of charge propagation 1e3.Among thereportedelectrodematerials,carbonmaterialshavegainedconsiderable interest because of high electrical conductivity,lowcost and multiple forms and allotropes,as well as broad chemicalstability in acidic or basic solutions 4,5.Mo
9、re importantly,Simonet al.found that electrochemical double layer capacitors(EDLCs)are able to operate from?50e100?C on the base of the combina-tion of exohedral nanostructured carbon(nanotubes and onions)electrode and a eutectic mixture of ionic liquids 6.Consequently,using carbon as the active mat
10、erial,EDLCs represent more than 80%of the commercially manufactured ECs today 7.*Corresponding authors.Tel./fax:86 551 2901450.E-mail addresses:(Z.J.Zhang),(P.Cui),(X.Y.Chen).Contents lists available at SciVerse ScienceDirectJournal of Power Sourcesjournal homepage: see front matter?2013 Elsevier B.
11、V.All rights reserved.http:/dx.doi.org/10.1016/j.jpowsour.2013.05.010Journal of Power Sources 242(2013)41e49Among the carbon electrodes,porous carbons are of greatimportance especially due to their high surface area and large porevolume,providing an extensively large electrode/electrolyte inter-face
12、 for charge storage 8.Xu et al.prepared porous carbons bydirect carbonization of poly(vinylidene chloride)(PVDC)withoutadding any templates,having BET surface area about 1200 m2g?19.However,in most cases,hard/soft templates are commonlyindispensable for preparing porous carbons.For example,tem-plati
13、ng with zeolites,silica or silicates canyield micro/meso/macro-pores with a narrow pore size distribution,but such ex-situ tem-plate methods are both expensive and inefficient,requiringrigorous etching with HF or KOH to remove the templates 10.Alternatively,other kinds of hard templates,including Mg
14、O 11,Mg(OH)212,Ni(OH)213,CaCO314,have been developed tocater for the template removal utilizing relatively gentle acid suchas HCl.Azo compounds have the functional group ReNNeR0,inwhich R and R0can be either aryl or alkyl.As a consequence ofp-delocalization,aryl azo compounds have vivid colors,espec
15、iallyreds,oranges,and yellows.Therefore,they are used as dyes,commonly known as azo dyes.The development of azo dyes was animportant step in chemical industry.Taking tartrazine as anexample,it is a synthetic lemon yellow azo dye primarily used as afood coloring,but also can be used with Brilliant Bl
16、ue FCF or GreenS to produce various green shades 15.Considering the particularstructure containing phenyls and especially high content nitrogen,tartrazine is for the first time expected to prepare nitrogen-dopedporous carbon at elevated carbonization temperatures.Herein,we present a straightforward
17、carbonization method topreparenitrogen-doped porouscarbons without further physical orchemical activation,using tartrazine as carbon source and calciumacetate as hard template.The mass ratio of tartrazine and calciumacetate was emphatically investigated.The correlative capacitiveperformances were me
18、asured by cyclic voltammetry and galvano-static chargeedischarge techniques.2.ExperimentalAll the analytical chemicals were purchased from SinopharmChemical Reagent(Shanghai)Co.Ltd.and used as received withoutfurther treatment.In present experiment,pure tartrazine was heated at 800?C for2 h under Ar
19、 flowtoobtainT-800-blank sample.On the otherhand,tartrazine and Ca(OAc)2$H2O with mass ratios of 1:1 and 3:1 at800?C for 2 h under Ar flow to obtain T-Ca-800-1:1/3:1 samples.The schematic routes and the unit structure of tartrazine aredepicted in Fig.1.2.1.Typical synthetic procedure for T-Ca-800-3:
20、1 sampleTartrazine and Ca(OAc)2$H2O powder(mass ratio of 3:1)wereground adequately and then placed in a porcelain boat,flushingwith Ar flow for 30 min,and further heated in a horizontal tubefurnace up to 800?C at a rate of 5?C min?1and maintained at800?C for 2 h under Ar flow.The resultant product w
21、as immersedwith dilute HCl solution to remove soluble/insoluble substances,which was further washed with adequate deionized water untilpH 7.Finally,the sample was dried under vacuum at 120?C for12 h to obtain the T-Ca-800-3:1 sample.Regarding the synthetic procedure for T-Ca-800-1:1 sample,it issimi
22、lar to that of T-Ca-800-3:1 sample detailedly depicted aboveexpect for the mass ratio of tartrazine and Ca(OAc)2$H2O powderas 1:1.Regarding the synthetic procedure for T-Ca-blank sample,it issimilartothatofT-Ca-800-3:1samplewithoutusingCa(OAc)2$H2O.2.2.CharacterizationX-ray diffraction(XRD)patterns
23、were obtained on a Rigaku D/MAX2500V with Cu Karadiation.Field emission scanning electronmicroscopy(FESEM)images were taken with a Hitachi S-4800scanning electron microscope.X-ray photoelectron spectra(XPS)were obtained on a VG ESCALAB MK II X-ray photoelectron spec-trometer with an exciting source
24、of Mg Ka(1253.6 eV).The specificsurface area and pore structure of the carbon samples weredetermined by N2adsorptionedesorption isotherms at 77 K(Quantachrome Autosorb-iQ)after being vacuum-dried at 150?Covernight.The specific surface areas were calculated by a BET(BrunauereEmmetteTeller)method.Cumu
25、lative pore volume andpore-size distribution were calculated by using a slit/cylindricalnonlocal density functional theory(NLDFT)model.2.3.Electrochemical measurementsIn order to evaluate the capacitive performances of the as-prepared carbon samples(ca.4 mg)in electrochemical capacitors,a mixture of
26、 80 wt%the carbon powder,15 wt%acetylene black andFig.1.Schematic illustration of the production of nitrogen-doped porous carbon by heating tartrazine with or without Ca(OAc)2$H2O at a designated carbonization temperature of800?C for 2 h under Ar flow,in which Ca(OAc)2$H2O serves as hard template.Z.
27、J.Zhang et al./Journal of Power Sources 242(2013)41e49425 wt%polytetrafluoroethylene(PTFE)binder were fabricated usingethanol as a solvent.Slurry of the above mixture was subsequentlypressed onto nickel foam under the pressure of 20 MPa,serving as acurrent collector.The prepared electrode was placed
28、 in a vacuumdryingoven at 120?C for 24 h.Athree electrode experimental setuptaking a 6 mol L?1KOH aqueous solution as electrolyte was used incyclic voltammetry and galvanostatic chargeedischarge measure-ments on an electrochemical working station(CHI660D,ChenHuaInstruments Co.Ltd.,Shanghai).Here,the
29、 prepared electrode,platinum foil(6 cm2)and saturated calomel electrode(SCE)wereused as the working,counter and reference electrodes,respectively.Specific capacitances derived from galvanostatic tests can becalculated from the equation:C IDtmDVwhere C(F g?1)is the specific capacitance;I(A)is the dis
30、chargecurrent;Dt(s)is the discharge time;DV(V)is the potential win-dow;and m(mg)is the mass of active materials loaded in workingelectrode.Specific energy density(E)and specific power density(P)derived from galvanostatic tests can be calculated from theequations:E 12CDV2P EDtwhere E(Wh kg?1)is the a
31、verage energy density;C(F g?1)is thespecific capacitance;DV(V)is the potential window;P(W kg?1)isthe average power density andDt(s)is the discharge time.3.Results and discussionThe component,crystallinity and purity of the carbon sampleswere studied by XRD technique.By direct carbonization of pureta
32、rtrazine at 800?C for 2 h under Ar flow,black product appearsand the corresponding XRD pattern is shown in Fig.2a,mainlyconsisting of carbon,orthorhombic Na2SO4(JCPDS Card No.37-0808)and other unidentified substances.The formation of Na2SO4originates from the reaction of sulfonate and sodium ions wi
33、thinthe structure of tartrazine,and similar phenomenon also occurs incase of production of porous carbon from calcium lignosulfonate16.The product was subsequently washed with aqueous HCl so-lution and deionized water to remove any soluble/insolubleimpurities,resulting in the formation of relatively
34、 pure carbon,dominated as T-800-blank.The XRD pattern in Fig.2b has onebroad diffraction peak centering at ca.24.9?,indicative feature ofamorphous carbon and close to that of(002)plane of crystalgraphite.Furthermore,one minor and indistinct diffraction peaklocates at ca.24.9?,approximately indexed a
35、s(10)plane 17.Onthe other hand,when heating the mixture of tartrazine andCa(OAc)2$H2O powder as 1:1/3:1 at 800?C,the product iscomposed of amorphous carbon and CaCO3,CaO etc.18,whichwas washed with aqueous HCl solution and deionized water,eventually resulting in T-Ca-800-1:1/3:1.Their XRD patterns a
36、reshown in Fig.2dee,incredibly close to that of T-800-blank.The shapes and sizes of the carbon samples wereinvestigated byFESEM technique.Fig.3a shows the representative FESEM image ofthe T-800-blank sample,which consists of a large number ofmicrometer block particles with irregular shapes.With resp
37、ect toT-Ca-800-1:1/3:1 samples,their FESEM images depicted in Fig.3bed are somewhat similar to that of T-800-blank.Taking into accountthe self-carbonization/decomposition of tartrazine as well as thetemplating effect of calcium acetate at elevated temperatures,nanoscale porous structures are anticip
38、ated to happen within thecarbonproducts,which will further be detected by the following N2adsorption/desorption analysis.The empirical composition,functional groups on the surfaces,chemical state and electronic state of the elements toward thepresent carbons were determined by XPS technique,giving i
39、nfor-mation concerning the outermost 3e4 nm surface layer carbonsurface.Fig.4a indicates the overall XPS survey spectra of the T-800-blank and T-Ca-800-1:1/3:1 samples from0 to1400 eV.All thecarbon samples consisting of C/N/O elements apparently revealtheir purities after washing process.In addition
40、,the relative in-tensities of C/N/O elements differentiate from each other,implyingtheir different content in the products.The high resolution XPSspectra toward C1s are given in Fig.4b,ranging from 280.5 to297.5 eV,which can be closely fitted into three main peaks locatingat ca.284.9,285.8 and 288.3
41、 eV,respectively.In details,the peak atca.284.9 eV is attributed to sp2CC bond,evincing the graphiticnature 19.The peak at ca.285.8 eV might be due to sp3CeC bond20 and/or CeN bond 21.As for the one at ca.288.3 eV,it can beincurred by eOeCO bond 22.Fig.4c gives us the high resolution XPS spectra of
42、O1s with thebinding energy scope from 526.5 to 541.0 eV.All the O1s spectra arealmost the same and can be approximately fitted into three mainpeaks at ca.531.4,532.5 and 533.4 eV,respectively.Hereinto,thepeak at ca.531.4 eV owes to eCO bond;the peak at ca.532.5 eV isdue to CeOeC/CeOH bond;and the on
43、e at ca.533.4 eV can beascribed to OeCO bond 23.Besides,the high resolution N1s1020304050607043.7O24.9O#(b)(a)Intensity(a.u.)2 Theta(deg.)1020304050607043.7o24.9o23.5oIntensity(a.u.)2 Theta(deg.)(e)(d)(c)Fig.2.XRD patterns of the T-800-blank sample before(a)and after(b)being washed with aqueous HCl
44、solution and deionized water to remove any unwanted impurities;as wellas the comparative XRD patterns of the(c)T-800-blank;(d)T-Ca-800-1:1 and(e)T-Ca-800-3:1 samples.Notes:#orthorhombic Na2SO4(JCPDS Card No.37-0808).The inset inthe right XRD pattern is the approximate unit structure of amorphous car
45、bon.Z.J.Zhang et al./Journal of Power Sources 242(2013)41e4943spectra(393.0e410.0 eV)of the carbon samples are depicted inFig.4d,which are also fitted into three main peaks locating at ca.398.6,400.3 and 401.4 eV,respectively.Furthermore,as is indicatedin Fig.4e,there exists four types of nitrogen s
46、pecies withinnitrogen-dopedcarbon,includingpyridinicnitrogen(398.6?0.3 eV),pyrrolic nitrogen(400.5?0.3 eV),graphitic ni-trogen(also as quaternary nitrogen,401.3?0.3 eV),oxidized pyr-idinic nitrogen(402e405 eV)16e18.Therefore,the peaks atFig.4d can orderly be attributed to pyridinic nitrogen,pyrrolic
47、 ni-trogen and graphitic nitrogen.The total XPS peak analysis of thecarbon samples are summarized inTable 1.It is clearly seen that theT-800-blank sample displays higher content of nitrogen speciesthan that of the T-Ca-800-1:1/3:1 samples.The contents of C/N/Oelements listed in Table 1 are to some e
48、xtent consistent with therelative intensities shown in Fig.4a.The surface areas and pore structures of the carbon sampleswere investigated by N2adsorptionedesorption isotherms andpore size distributions calculated by using a slit/cylindrical NLDFTmodel.Fig.5a represent the typical N2adsorptionedesor
49、ptionisotherms of T-800-blank and T-Ca-800-1:1/3:1 samples,demon-strating the similar trends in shapes with relative pressure(P/P0)ranging from 0 to 1.0.According to the classification of adsorptione0200400600800100012001400321Relative Intensity(a.u.)Binding Energy(eV)O1sN1sC1ssurvey(a)1T-800-blank2
50、 T-Ca-800-1:13 T-Ca-800-3:12822852882912942973211T-800-blank2 T-Ca-800-1:13 T-Ca-800-3:1288.3285.8284.9C1s(b)Relative Intensity(a.u.)Binding Energy(eV)5285305325345365385403211T-800-blank2 T-Ca-800-1:13 T-Ca-800-3:1533.4532.5531.4Relative Intensity(a.u.)Binding Energy(eV)O1s(c)3943963984004024044064