Journal of Power Sources 201 (2012) 368–375
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Journal of Power
Sources
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j p o w s o u
r
High power supercapacitor electrodes bad on flexible TiC-CDC nano-felts
Yu Gao a ,b ,Volker Presr b ,Lifeng Zhang c ,Jun J.Niu b ,John K.McDonough b ,Carlos R.Pérez b ,Haibo Lin a ,Hao Fong c ,∗,Yury Gogotsi b ,∗∗
a
College of Chemistry,Jilin University,Changchun 130012,PR China
b
Department of Materials Science and Engineering,A.J.Drexel Nanotechnology Institute,Drexel University,Philadelphia,PA 19104,USA c
Department of Chemistry,South Dakota School of Mines and Technology,Rapid City,SD 57701,USA
a r t i c l e
i n f o
Article history:
Received 6October 2011
Received in revid form 29October 2011Accepted 31October 2011
Available online 6 November 2011
Keywords:
Carbide-derived carbon Electrospinning Nano-felt在线中译英
Supercapacitor Titanium carbide
a b s t r a c t
Flexible electrospun titanium carbide (TiC)nano-felts were converted into carbide-derived carbon (C
DC)by dry chlorination at temperatures between 200and 1000◦C and ud as binder-free supercapac-itor electrodes.In the carbide nano-felt,TiC nano-crystals (20–30nm)were embedded in a matrix of disordered carbon.After chlorination,the porous CDC nano-fibers/felts maintain their size,shape,and flexibility.With the increa of synthesis/chlorination temperature,the degree of carbon ordering incread.Electrochemical characterizations in 1M H 2SO 4and 1.5M tetraethylammonium tetrafluo-roborate in acetonitrile were carried out on binder-free electrodes with galvanostatic cycling,cyclic voltammetry,and electrochemical impedance spectroscopy.The highest gravimetric capacitance was identified for the CDC nano-felt synthesized at the highest temperature of 1000◦C,reaching 135F g −1in aqueous and 120F g −1in organic electrolytes.In contrast to powder or monolithic supercapacitor electrodes made of conventional activated,templated,or carbide-derived carbons,this material demon-strated excellent high-power handling ability;and ∼50%of the low-rate capacitance was maintained at a very high scan rate of 5V s −1.
© 2011 Elvier B.V. All rights rerved.
1.Introduction
Electrodes ud in electrochemical double layer capacitors (EDLC;also known as supercapacitors)ar
英语三级作文万能模板
e commonly bad on a mixture of activated carbon (∼85–90wt.%)with polymeric binder (∼5–10wt.%)and conductive additives (mostly carbon black;∼5wt.%)[1,2].While the choice of activated carbons is motivated by high specific surface area and electrical conductivity,the poly-meric binder is a processing-related necessity,ensuring mechanical integrity while significantly lowering the conductivity and rate handling ability of the electrodes.The latter is only partially com-pensated by the addition of carbon black particles which makes another processing step necessary and increas the cost of the device without contributing to capacitance [3].When compared to activated carbons,carbide-derived carbons (CDC)with their tun-able porosity and conductivity have been shown to posss a higher gravimetric and volumetric capacitance [4].
CDCs have been derived from a large number of crystalline (micro-and/or nano-sized)binary and ternary carbides (e.g.,SiC,
∗Corresponding author.Tel.:+16053941229;fax:+16053941232.∗∗Corresponding author.Tel.:+12158956446;fax:+12158951934.
E-mail address:hao.fong@sdsmt.edu (H.Fong),gogotsi@drexel.edu (Y.Gogotsi).
ltpsTiC,Ti 2AlC)[4]and polymer-derived carbides (PDC),such as Si–C [5],Si–C–N [6],Si–O–C [7],and Ti–
C [8].PDC–CDCs offer a high level of control over the pore size and surface area.With the prence of micro-and meso-pores,many of them have networks of hierar-chical pores which are known to facilitate ion transport required for achieving high power [9–11].While small pores account for large surface area and,hence,high specific capacitance;larger pores facilitate ion mobility throughout CDC particles.The conformal character of the CDC synthesis ensures that complex shapes and geometries of the PDC are maintained and,thus,can be utilized for advanced device ,manufacturing 3-dimensional textures or conformal energy storage devices.
Supercapacitor electrodes with improved electronic conduc-tivity and rate performance can be produced via elimination of polymer binders and development of binder-free porous electrodes showing improved ion accessibility.Binder-free micro-supercapacitors produced by following either the top-down approach (carbide thin films were transformed into carbon and subquently patterned)[12]or the bottom-up approach (elec-trophoretic deposition of carbon particles on a patterned current collector [13]or growth of CNT arrays [14])have demon-strated improved rate handling ability compared to conventional polymer-bonded electrodes.Previous rearch has indicated that electrospinning can be ud to prepare carbon-bad electrodes for supercapacitors [15]or lithium-ion batteries [16].A high power
0378-7753/$–e front matter © 2011 Elvier B.V. All rights rerved.doi:10.1016/j.jpowsour.2011.10.128
Y.Gao et al./Journal of Power Sources201 (2012) 368–375369
goaperformance has been demonstrated for electrospun carbonfibers [17]and very recently[8],we developed a microporous carbon nano-felt from a pre-ceramic polymeric precursor that main-tained mechanicalflexibility and showed stable electrochemical performance over tens of thousands of charge-discharge cycles. CDCfiber/felt electrodes derived from a pre-ceramic precursor have three levels of porosity:large inter-fiber pores(ensuring ion accessibility across the entire thickness of the electrode;tens of nanometers),mesopores(facilitating ion transport through the cross-ction of individualfibers),and micropores(providing large surface area and,thus,high capacitance).As the inter-particle resistance is eliminated in continuousfiber networks,electron transport is improved and the rate handling ability increas accordingly.coat是什么意思
Our previous work[8]illustrated the proof-of-concept of CDC nano-felt electrodes derived from electrospun TiC nano-fiber. In this study,we have systematically investigated paramet-ric correlations among the synthesis conditions(temperature), the CDC nano-felt structures(carbon ordering,pore siz
e,pore size distribution,and surface area),and the resulting properties (conductivity,ries resistance,capacitance,and rate handling ability).
2.Materials and methods
2.1.Material synthesis
The TiC nano-fibrous mats(nano-felts)were prepared through following the procedure reported in Ref.[18].Titanium(IV)n-butoxide(TiBO)and furfuryl alcohol(FA)were lected as the titanium and carbon sources,respectively.Polyvinylpyrrolidone (PVP)was ud as the carrying polymer for electrospinning,N,N-dimethylformamide(DMF)was ud as the solvent for making a spin dope,and acetic acid(HAc)was added into the spin dope as the catalyst for hydrolysis of TiBO and polymerization of FA.The optimum spin dope contained10wt.%TiBO,10wt.%FA,10wt.% PVP,and2.5wt.%HAc.An18-gauge stainless-steel needle with a 90◦blunt tip was adopted as the spinneret,and a positive volt-age of15kV was applied to the spinneret during electrospinning. A25cm diameter roller covered with aluminum foil was ud as the nanofiber collector;the roller speed was t at100rpm,and the needle-roller distance was t at25cm.Four days after elec-trospinning,the200m thick nanofibrous mat was removed from the aluminum foil,and subquently pyrolyzedfirst at325◦C in air and then at1400◦C in argon.
TiC nano-felts were placed on a quartz boat and inrted into a horizontal tube furnace(quartz glass),purged in argonflow and heated at20◦C min−1to the desired temperature(200–1200◦C), and then expod to theflow of dry chlorine gas(10–15cm3min−1) for3h.After chlorination,the nano-felts were annealed at a t tem-perature(200–600◦C)for2h underflowing hydrogen,to remove residual chlorine and chlorides trapped in pores.The hydrogen treatment was carried out either at the synthesis temperature for samples chlorinated at/or below600◦C,and at600◦C for samples chlorinated at higher temperatures.
2.2.Structural characterization
Microscopic analys were carried out via scanning electron microscopy(SEM;Zeiss Supra50VP,operating at5kV)and transmission electron microscopy(TEM;JEOL,2010F,operating at 200kV).TEM specimens were prepared by dispersing the samples in isopropanol followed by placing the suspensions over cop-per grids with lacey carbonfilm.SEM samples were mounted on a carbon stub and analyzed without conductive coatings.Raman spectroscopy was carried out with an inVia confocal Raman microspectrometer(Renishaw)using an Ar ion lar(488nm,∼1m lateral spot size)for excitation.A Lorentzian function was assumed for peakfitting and spectral deconvolution.X-ray diffrac-tion(XRD)was carried out using a Siemens D500diffractometer with Cu-K␣radiation( =1.54
06˚A)operating at30mA and40kV. The XRD patterns were collected using step scans with the step of 2Âbeing0.01◦and the count time of2s per step between the2Âof 10◦and80◦.
2.3.Pore characterization
Prior to gas sorption analysis,all samples were degasd at 200◦C in low vacuum(0.1Torr)for24h to remove the adsorbed species.Gas adsorption analysis using N2(at−196◦C)as the adsor-bate was conducted with the Quadrasorb surface area and pore size analyzer(Quantachrome Instruments).With the information from N2sorption(covering the range of0.6–50nm),the pore size dis-tribution(PSD)and specific surface area(SSA)were calculated and the experimental error was approximately10–15%.Assuming slit-shaped pores,the quenched solid density functional theory(QSDFT) was ud[19].The BET SSA[20]was calculated in the linear regime between0.05and0.30P/P0[21].With a type IV isotherm and a hys-teresis loop,only the adsorption branch was ud for calculation of the PSD from N2sorption.The average pore size was calculated as the volume-weighted average pore diameter.
2.4.Electrochemical characterization
Symmetrical two-electrode cells were asmbled with stainless-steel(aqueous electrolyte)or aluminu
m(organic electrolyte)current collectors and Gore PTFE parator.The current collectors were coated with a carbon-bad conductive paint to minimize the contact resistance with the electrode.Elec-trochemical characterization in1M H2SO4(10×5mm2cell)and 1.5M TEA-BF4(10×10mm2cell)in acetonitrile was carried out using galvanostatic cycling,cyclic voltammetry,and impedance spectroscopy.Nano-felts were ud as produced without the addition of any polymeric binder and/or conductive additive and directly placed on the current collector.Time constants and frequency dispersion were analyzed via impedance spectroscopy. Cyclic voltammetry was performed at scanning rates between 0.01V s−1and100V s−1within a voltage window of−0.5to0.6V. Specific capacitance‘C’was calculated from galvanostatic cycling using Eq.(1):
C=
2i
m(dV/dt)
(1)
where m is the carbon mass of one electrode,i is the discharge current and dV/dt is the slope of the discharge curve.
From cyclic voltammetry,the capacitance can be derived through the Eq.(2):
C=
2(1/ E)
idV/v
m
(2)
where E is the voltage window,i is the discharge current,V is the voltage,and v is the scan rate.
The EDLCs were tested at constant current charge/discharge regimes(between10and100mA cm−2)within the voltage range from−0.5to0.6V in1M H2SO4.The impedance complex plane(Z ,−Z )plots(Nyquist plots)for EDLCs have been measured within the range of AC frequency f from10−2to105Hz and atfixed cell voltage of0V(Z is the real part and Z is the imaginary part of the impedance,respectively).
370Y.Gao et al./Journal of Power Sources 201 (2012) 368–
375
Fig.1.XRD pattern (a)and Raman spectra (b)of TiC-CDC nano-felts and their precursor of electrospun TiC nano-fibrous felt.The dependence of FWHW of D-band and ID/IG ration on the synthesis temperature (c).Pore size distributions (d).SSA and average pore diameter as a function of chlorination temperature (e).
3.Results and discussion
infection3.1.Structure of TiC-CDC nano-felts
XRD was carried out to investigate the structural changes in the TiC-CDC nano-felts under different chlorination conditions.Fig.1a shows that chlorination lectively extracts Ti from the electrospun TiC nano-fibers/felts,with trace amounts of un-reacted nano-TiC (<5wt.%)found only in the TiC-CDC nano-felt sample chlorinated at 200◦C.The two very broad peaks centered at the 2Âangles of ∼24◦and ∼44◦were attributed to diffu scattering from disor-dered carbon.The abnce of sharp peaks corresponding to graphite indicates the disordered nature of TiC-CDC.
Raman spectra of the electrospun TiC precursor and TiC-CDC nano-felts are shown in Fig.1b.The spectra show the two charac-teristic peaks for carbon:(i)the D-mode at 1342–1353cm −1that is characteristic for disordered carbon with finite sizes of crystallites,and (ii)the graphite G-mode at 159
0–1601cm −1(Fig.1b).In the spectra from the TiC precursor,only Raman peaks of carbon can be
hobbyisten related to the D-and G-modes [22,23]of graphitic and disor-dered carbon prent in the precursor fibers.All Raman modes of TiC-CDC nano-felt prepared by chlorination at 200◦C have a larger full-width at half maximum (FWHW)compared to the precursor and the FWHM further decreas with the increa of the chlorina-tion temperature (Fig.1c).Before chlorination,the carbon related signal corresponds to 25–30wt%of amorphous carbon surrounding the nanocrystalline TiC grains.This carbon pha is more ordered (i.e.,it shows more narrow Raman modes)than the carbon derived from low temperature chlorination of TiC.After chlorination,the initially prent carbon pha still contributes to the Raman signal;the latter,however,is dominated by the signal from the TiC-CDC which shows the typical dependency of CDC for of the carbon order-ing on the chlorination temperature,and the fraction of graphitic carbon increas with temperature [4].
Nitrogen sorption was ud to characterize the PSD,the SSA,and the pore volume of TiC-CDC nano-felts and the precursor (Fig.1d and e,and Table 1).Table 1shows that raising the chlorination tem-perature increas the SSA and volume of pores.The precursor has a
Y.Gao et al./Journal of Power Sources201 (2012) 368–375
371
Fig.2.SEM(a,c,e,and g)and TEM(b,d,f,and h)images of the as-received TiC-CDC nano-felts(a and b)and samples after chlorination at200(c and d),800(e and f),and 1000◦C(g and h).
SSA value of384m2g−1,an average pore size of2.55nm,and a pore volume of0.33cm3g−1.The BET SSA increas from935m2g−1at 400◦C to1468m2g−1at800◦C and the total pore volume increas from0.98to1.43cm3g−1,respectively(Fig.1e).After chlorination at800◦C,the average pore size is3.46nm(Table1)with the more narrow PSD of the studied samples,while the smallest average pore size has been found after chlorination at200◦C(1.75nm).The com-bination of nano-scale pores,partially graphitic carbon(which is
372Y.Gao et al./Journal of Power Sources 201 (2012) 368–375
Table 1
Values of SSA,pore size,and pore volume acquired from N 2gas sorption for TiC-CDC nano-felts and the precursor at −196◦C.oneandonly
BET SSA a (m 2g −1)
DFT SSA b (m 2g −1)
Total pore volume (cm 3g −1)
Average pore size c (nm)
Precursor
4093840.33 2.55Nano-felt (200◦C)8627950.56 1.75Nano-felt (400◦C)9358430.98 3.55Nano-felt (600◦C)13741430 1.27 3.25Nano-felt (800◦C)14681352 1.43 3.46Nano-felt (1000◦C)
1188
834
0.95
2.56
a The BET SSA [20]was calculated in the linear regime between 0.05and 0.30P /P 0[21].
b DFT SSA was calculated assuming slit-pore geometry using QSDFT deconvolution [19].c
The average pore size is the volume-weighted average diameter bad on QSDFT data.
electrically conductive),and continuous fibers makes the TiC-CDC nano-felts attractive for the application of supercapacitor electrode.
As en from electron micrographs (Fig.2),the TiC precursor is compod of TiC nanocrystals embedded in a carbon matrix with 1–3layers of graphitic carbon surrounding the TiC crystals (Fig.2b).SEM and TEM images (Fig.2)show a gradual structural evolution with increasing chlorination temperature.At 200◦C (Fig.2d),the TiC-CDC nano-felts consist predominantly of amorphous carbon with the graphitic contributions being only the residual graphite layers around former TiC nanocrystals that have been completely transformed into CDC (Fig.2c and d).Higher chlorination temper-atures correspond to higher carbon mobility (Fig.2e–h),which is consistent with Raman and XRD analys.At the highest synthesis temperature (1000◦C),the fibers in the TiC-CDC nano-felt consist of a pore network in which pores are parated by one or two carbon layers only.
3.2.Electrochemical measurements
Cyclic voltammograms (CVs)of the nano-felt precursor and the TiC-CDC nano-felts at 0.01V s −1scan rate in 1M H 2SO 4(Fig.3a)show that the capacitance increas with the chlorination tem-per
ature.Minor electrochemical reactions occur as evidenced by small peaks in the CVs for the 800◦C and 1000◦C samples as related to surface functional groups.Contrary to the obrvations for microparticulate TiC-CDC powder [24],the rectangular shape of the CV curves indicates more purely capacitive behavior mean-ing that at this scan rate,the equivalent ries resistance (ESR)of the electrode related to the hindrance of ion motion in the pores is low.In aqueous electrolyte,a maximum capacitance of 135F g −1from cyclic voltammetry was obtained from the felt synthesized at the highest chlorination temperature (Fig.3b and Table 2)which is in agreement with galvanostatic charge/discharge
measurements
Fig.3.Cyclic voltammograms of the TiC-CDC nano-felts at 0.01V s −1(a)and the gravimetric capacitance of TiC-CDC nano-felts and precursor at different scan rate in 1M H 2SO 4(b).The power-handling ability of TiC-CDC nano-felts and precursor were revealed when plotting the scan rate vs.the normalized capacitance C /C 0(c).Gravimetric capacitance obtained via galvanostatic charge/discharge measurements for the TiC-CDC nano-felts and precursor (d).
Y.Gao et al./Journal of Power Sources201 (2012) 368–375373 Table2
Electrochemical results for TiC-CDC nano-felts and the precursor(CV,cyclic voltam-
metry;GC,galvanostatic charge/discharge)tested in1M H2SO4.
Gravimetric capacitance(F g−1)Time
constant(s)
北京市翻译公司
Resistance
( cm2)
CV GC
Precursor37300.455 1.1
Nano-felt(200◦C)65420.420 1.7
Nano-felt(400◦C)94900.337 1.5
Nano-felt(600◦C)1071030.322 1.2
Nano-felt(800◦C)1181030.303 1.2
托福词汇下载Nano-felt(1000◦C)1351200.088 1.0
(120F g−1);the TiC-CDC nano-felt chlorinated at1000◦C has a high, but not the highest,SSA but the smallest average pore diameter compared to other nano-felts.
All samples showed a high rate handling ability,as en from Fig.3c.The best performance at high charge/discharge rates was en for either TiC-CDC synthesized at high temperatures(≥600◦C) or the TiC nano-felt precursor,with TiC precursor having an over-all small gravimetric capacitance(37F g−1).At a sweep rate of 0.01V s−1,the TiC-CDC nano-felt made from chlorination at1000◦C yielded the highest capacitance of135F g−1that dropped to74%of the initial value at∼1V s−1and to40%with
high rate of5V s−1 (Fig.3c and Table2).The TiC-CDC nano-felts(400,600,and800◦C) maintained more than80%of the initial value at∼1V s−1and 50%at5V s−1.The loss of capacitance is caud by an increa of the ion transport related resistance and caus conventional activated carbon or carbide-derived carbon powder electrodes to lo their capacitance quickly at scan rates above0.1V s−1;thus, their maximum charge/discharge rates are substantially limited. Bleda-Martinez ported the decrea of capacitance to below 5%of the initial value at the sweep rate of0.1V s−1for activated car-bonfibers(ACF)in1M Na2SO4[25].At this scan rate,except for the felt made from chlorination at200◦C(maintained86%),other nano-felts maintained more than90%capacitance of the initial value.
The time constant and resistance of the TiC-CDC nano-felts and the precursor are plotted in Fig.4a and Table2.It is apparent that the time constant and resistance for1000◦C felt is lower compared to other felts.It is considered that the advantageous microstruc-ture of TiC-CDC nano-felts,such as a3-D network,the prence of hierarchic mesopores,and the amorphous free carbon,would jointly contribute to the high power performance.In addition,the fiber structure minimizes the electrolyte diffusion distance to the interior surface area.
To study the effect of frequency on the carbon electrodes,EIS was performed.Nyquist plots for TiC-C
DC nano-felts and precursor (Fig.4b and Table2)show a moderate equivalent ries resistance, calculated as the intercept that the linear region makes with the real impedance axis,for all samples.Also evident from Fig.4b is the mi-circle that occurs at moderate frequencies which is attributed to the interface between the carbonfilm and current collector inter-facial resistance.In the low frequency regime,capacitors asmbled from CDCs synthesized at all temperatures show ideal capacitive behavior,with a near vertical line parallel to the imaginary axis.
Fig.4c shows the evolution of real capacitance C versus frequency and Fig.4d shows the evolution of the imaginary capac-itance C with frequency from which the time constant can be derived.For most nano-felts,there is little change in the time con-stant.The corresponding time constant of most TiC-CDC nano-felts was around0.350s(except for the felt synthesized at1000◦C, which was0.088s)compared to200s for conventional TiC-CDC synthesized at600◦C[26],0.700s for MWCNT[27],and0.026s for carbon-onion micro-supercapacitors[17].As expected,both
ries
Fig.4.The time constant and resistance in1M H2SO4in comparison with the precursor(a).Nyquist plots for TiC-CDC nano-felts and precursor(b).Progression of the real capacitance,C ,for TiC-CDC nano-felts and precursor with frequency(c),and the imaginary capacitance,C ,for TiC-CDC nano-felts and precursor as a function of frequency(d).
