Supercapacitors based on carbide-derived carbons synthesised using HCl and Cl2 as reactants

ORIGINAL PAPER

Supercapacitors based on carbide-derived carbonssynthesised using HCl and Cl2 as reactants

I. Tallo & T. Thomberg & H. Kurig & A. Jänes & K. Kontturi & E. Lust

Received: 25 June 2012 /Accepted: 13 August 2012 /Published online: 1 September 2012# Springer-Verlag 2012

Abstract Micro- and mesoporous carbide-derived carbons(CDCs) were synthesised from TiC powder via a gas-phasereaction using HCl and Cl2 within the temperature range of700–1,100 °C. Analysis of X-ray diffraction results show thatTiC-CDCs consist mainly of graphitic crystallites. The first-order Raman spectra showed the graphite-like absorption peaksat ~1,577 cm−1 and the disorder-induced peaks at ~1,338 cm−1.The low-temperature N2 sorption experiments were performed,and specific surface areas up to 1,214 and 1,544 m2g−1 wereobtained for TiC-CDC (HCl) synthesised at T0800 °C andTiC-CDC (Cl2) synthesised at T0900 °C, respectively. Forthe TiC-CDC powders synthesised, a bimodal pore size distri-bution has been established with the first maximum in theregion up to 1.5 nm and the second maximum from 2 to4 nm. The energy-related properties of supercapacitors basedon 1 M (C2H5)3CH3NBF4 in acetonitrile and TiC-CDC (Cl2)and TiC-CDC (HCl) as electrode materials were also investi-gated by cyclic voltammetry, impedance spectroscopy, galva-nostatic charge/discharge and constant power methods. Thespecific energy, calculated at U03.0 V, are maximal for TiC-CDC (Cl2 800 °C) and TiC-CDC (HCl 900 °C), which are 43.1and 31.1 Whkg−1, respectively. The specific power, calculatedat cell potential U03.0 V, are maximal for TiC-CDC (Cl21,000 °C) and TiC-CDC (HCl 1,000 °C), which are 805.2and 847.5 kWkg−1, respectively. The Ragone plots for CDCsprepared by using Cl2 or HCl are quite similar, and at high

power loads, the TiC-CDC material synthesised using Cl2 at900 °C, i.e. the material with optimal pore structure, deliversthe highest power at constant energy.

Keywords Titanium carbide . Hydrogen chloride .

Chlorine . Carbide-derived carbon . Supercapacitor

Introduction

Selective etching of carbides is an attractive technique forcontrolled synthesis of various carbon nanostructures at theatomic level [1–5]. Metal atoms can be extracted fromcarbides in different ways, most commonly by high-temperature chlorination using molecular chlorine and otherhalogens as well as halogen-containing gases or by thermaldecomposition in a vacuum [6]. Development of carbide-derived carbon (CDC) with unique micro- and mesoporousstructures, narrow pore size distribution and the possibilityto fine-tune pore sizes, confirmed previously [1–3, 7–9], hasnoticeably forced the development of applications requiringthe CDC materials [2, 3, 7–12]. Previously, it was found thatthe micropore formation is strongly influenced by initial car-bide density while the formation of mesopores is influencedmainly by the carbide structure and chemical properties, butnot raw carbide density [5]. In the present study, extraction ofTi atoms from TiC was performed by high-temperature treat-ment with either Cl2 or HCl gases. The structure of CDC wascharacterised in detail by X-ray diffraction (XRD), Ramanspectroscopy and high-resolution transmission electron mi-croscopy (HRTEM) methods. The energy-related propertiesof supercapacitors were also investigated by cyclic voltamme-try, electrochemical impedance spectroscopy, galvanostaticcharge/discharge and constant power methods.

I. Tallo : T. Thomberg :H. Kurig :A. Jänes (*) : E. LustInstitute of Chemistry, University of Tartu,14a Ravila Str.,50411 Tartu, Estoniae-mail: [email protected]

K. KontturiDepartment of Chemistry, Aalto University,P.O. Box 16100, 00076 Aalto, Finland

J Solid State Electrochem (2013) 17:19–28DOI 10.1007/s10008-012-1850-0

Experimental results and physical characterisation

Preparation of CDC materials

Titanium carbide (TiC, 99.5 % purity, −325 mesh powder,Sigma-Aldrich) was placed onto a quartz stationary bed reac-tor and reacted with Cl2 or HCl (AGA, 99.99 %), respectively,at chosen fixed reaction temperatures from 700 to 1,100 °C.Details of the etching technique have been reported previously[1, 8, 9], and in our study, the flow rate of Cl2 and HCl wasfixed at 50 or 100 ml/min, respectively. The by-product TiCl4was led away by the stream of excess Cl2 or HCl, and after thereaction, the reactor was flushed with a slow stream of argonto remove the excess residues of gaseous by-products fromcarbon. During heating and cooling, the reactor was flushedwith argon (400 ml/min). The resulting carbon powder wasthereafter treated with H2 at 800 °C during 1.5 h to dechlori-nate thoroughly the TiC-CDC powder as well as to remove theresidual chlorides and oxygen-containing functional groupsfrom the surface of porous carbon under study.

Powder XRD and Raman spectroscopy analysisof TiC-CDC materials

XRD analysis [13] of the powder samples was carried out toinvestigate the structural changes in TiC-CDC that occurredwith different etching agents at different reaction tempera-tures. The XRD measurements were performed using CuKαradiation (45 kV, 35 mA, λ00.154056 nm) with a step size of0.02° of glancing angle θ and with a holding time of 5 s atfixed θ on Bruker D8 Advance diffractometer (Bruker Corpo-ration). The diffraction spectra were recorded at 25 °C andtreated by the AXES 3.0 computer software. The XRD pat-terns (Fig. 1) of synthesised CDCs showed reflectionscorresponding to the graphite (002) and (100)/(101) planesat 2θ~26° and ~43°, respectively. The 002 diffraction peak at

2θ~26° corresponding to parallel graphene layers can beobserved in Fig. 1. The (100)/(101) diffraction peaks at 2θ~43° characterise the 2D in-plane symmetry along the graphenelayers. As the reaction temperature of TiC increased from 700to 1,100 °C, the (002) peak becomes narrower (Fig. 1), indi-cating the ordering processes in the mainly amorphous carbon.Comparison of the XRD patterns of TiC-CDC (Cl2) and TiC-CDC (HCl) materials shows that TiC-CDC (HCl) materialsare more ordered based on more intense (002) and (100)/(101)peaks. However, TiC-CDC (HCl) materials synthesised atlower temperatures contain some traces of raw unreactedtitanium carbide.

The Raman spectra were recorded using a RenishawinVia micro-Raman spectrometer with Ar laser excitation(λL0514 nm). The first-order Raman spectra of perfectlyordered monocrystalline graphite show only one peak at~1,577 cm−1, whereas disordered amorphous carbon gener-ally demonstrates two peaks [14–18]: the so-called graphite(G) peak at ~1,577 cm−1 and the disorder-induced (D) peakat ~1,338 cm−1 (Fig. 2a). The G-peak corresponds to graph-ite in-plane bond-stretching motion of pairs of C atoms insp2 configuration with E2g symmetry. Thus, this mode doesnot require the presence of sixfold C rings, and it occurs atall sp2 sites, not only for those atoms located in hexagonalstructure. The D-peak is a breathing mode with A1g sym-metry which is forbidden in perfect graphite and onlybecomes active in the presence of a disorder in graphitestructure [19]. The spectrum also shows the second-orderpeak of the D-band (2D) at ~2,670 cm−1. The increase of the2D peak is related with the crystallographic ordering of thegraphitic structure [18]. In accordance with the data inFig. 2a, as the reaction temperature for TiC-CDC synthesisincreased from 700 to 1,100 °C, amorphous TiC-CDC be-came more ordered as evident from a decrease in full widthat half maximum (FWHM) values of the D- and G-bands(Table 1), obtained by the fitting of the two Lorentzian

Fig. 1 Characteristic XRDpatterns for porous TiC-CDCprepared by using differentreactants (dashed line denotesCl2, solid line denotes HCl)at different reactiontemperatures (given inthe figure). Diamonds denotecrystallographic reflectionsof residual TiC

20 J Solid State Electrochem (2013) 17:19–28

curves to the G- and D-bands, respectively, using the OriginPro 8 software. According to the data in Fig. 2a, very smallnarrowing of the G-band takes place, indicating the begin-ning of graphitization of porous CDCs. For highly disor-dered carbons, the relation between apparent crystallite sizeLa and ratio of D- and G-band intensities (ID/IG) can bedescribed by the Ferrari–Robertson relation [16, 17]

IDIG

¼ C0L2a ð1Þ

where C′(λ) is the wavelength-dependent parameter and C′(at fixed λ0514 nm)00.55. For small La, the intensity of theD-peak is proportional to the probability of finding a sixfoldring in the cluster area. Thus, in amorphous carbons, thedevelopment of a D-peak indicates ordering, exactly the

opposite to the case of graphite [16, 17]. According to thedata in Table 1, as the chlorination temperature increases,the crystallite size also slightly increases as evidence ofordering of amorphous TiC-CDC with temperature. Fittingdata of the Raman spectra for TiC-CDC (Cl2) and TiC-CDC(HCl) materials show that TiC-CDC (HCl) materials aremore ordered as evidence of slightly smaller FWHM valuesof D- and G-peaks (Table 1), which is in a good accordancewith XRD data.

HRTEM analysis of synthesised TiC-CDC materials

For the morphological studies, the TiC-CDCs were exam-ined by HRTEM on a Tecnai 12 instrument operated at a120-kV accelerating voltage [20]. TEM specimens were

Fig. 2 Raman spectra (a)and HRTEM images (b)for porous TiC-CDC preparedby using Cl2 and HCl asreactants at differentreaction temperatures(given in the figure)

J Solid State Electrochem (2013) 17:19–28 21

prepared from ultrasonic dispersions of the correspondingsamples in 1-butanol. One drop of each suspension was de-posited on a copper grid covered with a holey carbon film.

HRTEM studies of TiC-CDCs formed at various reactiontemperatures revealed the microstructure at the atomic scale(Fig. 2b). Reaction with Cl2 and HCl at 700 °C results in theformation of completely amorphous carbon. Ordering of car-bon and formation of graphitic structures start at ~900 °C.However, only at 900 °C and above, both Raman spectroscopyand XRD detected noticeably increased ordering of mainlyamorphous carbon. A HRTEM micrograph of a sample pro-cessed at 1,100 °C shows a network of well-ordered graphiticsheets within amorphous carbon (Fig. 2b).

Surface area and pore size distribution measurements

The porous structure of TiC-CDC was characterised by low-temperature nitrogen sorption method. The N2 sorptionexperiments were performed at the boiling temperature ofliquid nitrogen (−195.8 °C) using the ASAP 2020 system(Micromeritics). The specific surface area SBET and otherparameters for porous TiC-CDC materials were calculatedaccording to Brunauer–Emmett–Teller (BET) [21, 22] anddensity functional theories (DFT) [23, 24] up to a nitrogenrelative pressure (p/p0) of 0.2. The total volume of pores (Vtot)was obtained at the conditions near to saturation pressurep/p000.995, the micropore area (Smicro) and micropore vol-ume (Vmicro) were calculated using the t-plot method, the poresize distributions were determined using a non-local DFT bythe so-called slit-shaped pore model [25, 26], and thecorresponding data are given in Fig. 3 and Table 2.

According to the data in Table 2, materials synthesisedhave a micro/mesoporous structure, and a maximal specific

surface area up to 1,214 m2g−1 for TiC-CDC (HCl), syn-thesised at T0800 °C, and SBET01,544 m2g−1 for TiC-CDC(Cl2), synthesised at T0900 °C, were obtained. The totalamount of nitrogen adsorbed increases with the reactiontemperature, having a maximal value of 1.01 cm3g−1 forTiC-CDC (Cl2) and 0.70 cm3g−1 for TiC-CDC (HCl), syn-thesised at T01,100 °C. However, the SBET do not change inthe same direction. This is probably caused by increase ofpercentage of larger mesopores in the porous TiC-CDCmaterial with increasing synthesis temperature.

Results and discussion

Electrochemical measurements

The supercapacitor electrodes were composed of an alumini-um current collector and a mixture of the active TiC-CDC(Cl2) or TiC-CDC (HCl) layer and a binder (PTFE, 60 %dispersion in H2O). This mixture was laminated and roll-pressed (HS–160N, Hohsen Corporation, Japan) together toform a flexible layer of the active electrode material with athickness of 100±3 μm. After drying under vacuum, the pureAl layer (2 μm) was deposited onto one side of the CDCs bymagnetron sputtering method.

The electrolyte used was prepared from very pure aceto-nitrile (AN, H2O<20 ppm) and from dry (C2H5)3CH3NBF4(Stella Chemifa). The two-electrode standard Al test cell(HS Test Cell, Hohsen Corporation) with two identicalelectrodes (geometric area ~2.0 cm2) was completed insidea glove box (Labmaster sp, MBraun; O2 and H2O concen-trations lower than 0.1 ppm), and all electrochemical experi-ments were carried out at T020±1 °C. A carefully dried 25-μm-thick TF4425 (Nippon Kodoshi) separator sheet wasused for mechanical separation of the working TiC-CDCelectrodes.

Electrochemical characteristics of the supercapacitors, con-sisting of the TiC-CDC (Cl2) or TiC-CDC (HCl) electrodes and1 M (C2H5)3CH3NBF4 in AN, have been studied by cyclicvoltammetry, constant current charge/discharge, electrochemi-cal impedance spectroscopy methods using a SI1287 Solartronpotentiostat and 1252A frequency response analyser over acfrequency f range of 1 mHz–300 kHz at 5-mV modulation.Constant power method using a BT2000 testing system (ArbinInstruments, USA) was used as well. Three different experi-ments with electrodes prepared from TiC-CDC (Cl2) or TiC-CDC (HCl), synthesised at fixed temperatures, were testedelectrochemically at the same conditions.

CV data

The cyclic voltammetry (CV) curves for supercapacitorsbased on TiC-CDC (Cl2) or TiC-CDC (HCl) electrodes in

Table 1 Raman analysis of TiC-CDCs

Reactant Synthesistemperature(°C)

D-peakFWHM(cm−1)

G-peakFWHM(cm−1)

ID/IG La (nm)

HCl 700 205 76 0.96 1.32

800 180 75 1.03 1.37

900 154 74 1.08 1.40

1,000 97 75 1.34 1.56

1,100 73 72 1.32 1.55

Cl2 700 240 89 1.08 1.40

800 205 80 0.99 1.34

900 185 79 1.03 1.37

1,000 109 78 1.24 1.50

1,100 87 75 1.22 1.49

D-peak FWHM full width at half maximum of the D-peak, G-peakFWHM full width at half maximum of the G-peak, ID/IG ratio of D- andG-peak intensities, La the estimated average crystallite size alonga-direction of the graphite structure


1 M (C2H5)3CH3NBF4 acetonitrile solution, presented inFig. 4a, b, have nearly mirror-image symmetry of the currentresponses about the zero current line, obtained at potentialscan rates ν≤50 mVs−1. The current density j (obtained usingthe flat–cross section geometric surface area) measured at afixed scan rate has been used for calculation of the mediumcapacitance values according to Eq. 1:

C ¼ jv�1: ð2Þ

Equation 1 is correct if the capacitance C is constant (C≠f(U)) and the series resistance Rs→0, if j→0. Thus, Eq. 1 canbe used to obtain the capacitance values only if the currentvalues are very small, as the IR drop is negligible only underthese conditions and the current response is essentially equal tothat of a pure capacitor [27–29]. In a symmetrical two-electrode system, the specific capacitance Cm (Fg−1) for oneactivated carbon electrode can be obtained as follows:

Cm ¼ 2C

m; ð3Þ

where m is the weight (gcm−2) per one activated carbonelectrode, assuming that the positively and negatively chargedelectrodes have the same capacitance at fixed U.

There is moderate difference of specific capacitanceCm (Fig. 4) calculated from CV curves at U03.0 V forTiC-CDC (Cl2) or TiC-CDC (HCl), synthesised from900 to 1,100 °C with a value of 136 Fg−1 for TiC-CDC (Cl2 900 °C) and 97 Fg−1 for TiC-CDC (HCl900 °C). The decrease in Cm can probably be explainedby the lower Vmicro and Vtot values for TiC-CDC (HCl)(Table 2).

The values ofCm start to increase atU≥3.2 V, being causedby electroreduction of O2 and H2O traces at a negativelycharged electrode and by oxidation of surface functionalitiesat a positively charged electrode [2, 29–31]. However, electro-reduction of complex BF4

− anions is possible as well. Withincreasing ν over 50 mVs−1, the cyclic voltammograms, i.e.specific capacitance vs. cell potential dependencies, becomedistorted in the region of potential switchover for all materialsstudied [29–32].

Fig. 3 Incremental porevolume vs. pore width plotsfor TiC-CDC prepared byusing different reactants(dashed line denotes Cl2,solid line denotes HCl) atdifferent reaction temperatures(given in the figure)

Table 2 Results of N2

sorption measurementsof TiC-CDC materials

SBET surface area calculated bythe Brunauer–Emmett–Tellertheory (p/p000.05, …, 0.2),Smicro micropore surface area byt-plot method, Vmicro microporevolume by t-plot method, Vtot

total pore volume at p/p000.999

Reactant Synthesistemperature (°C)

SBET (m2g−1) Smicro (m2g−1) Vmicro (cm

3g−1) Vtot (cm3g−1)

HCl 700 1,085 1,073 0.50 0.54

800 1,214 1,180 0.57 0.66

900 1,170 1,131 0.61 0.73

1,000 1,087 1,009 0.68 0.88

1,100 874 765 0.70 0.96

Cl2 700 1,320 1,300 0.59 0.69

800 1,443 1,418 0.64 0.77

900 1,544 1,503 0.71 0.87

1,000 1,538 1,489 0.71 0.88

1,100 1,448 1,377 0.81 1.01


Constant current charge/discharge data

The supercapacitors were tested at constant current (CC)charge/discharge regimes (from 1 to 50 mAcm−2) within cellpotentials from 0 to 2.7 V. The discharge and charge capaci-tances were calculated from the data of the third cycle. Thecapacitance of the cell C (Fcm−2) was obtained from the slopeof the discharge (or charge) curve according to Eq. 3:

C ¼ jdt

dðΔUÞ ; ð4Þ

where dt/d(U) is the reciprocal value of the slope of thedischarge or charge curve with corresponding currentdensity j. For simplicity, the discharging curve wasapproximated by a linear function; therefore, the medi-um integral capacitance C values were calculated. Thelongest charge/discharge cycle has been observed for TiC-

CDC (Cl2) synthesised at 700 °C (Fig. 5) corresponding tothe highest specific capacitance value calculated from CCcurves, Cm0129 Fg

−1. This value is in a good agreement withthe Cm value calculated from CV curves at ν≤50 mVs−1

(Fig. 4b).The cycling efficiency, i.e. the so-called round-trip efficien-

cy (RTE), has been calculated as the ratio of capacitancesmeasured during discharging and charging of supercapacitors.For all systems analysed, the calculated RTE value remained inthe range of 98–99 %, showing that TiC-CDC (Cl2) and TiC-CDC (HCl) materials are good materials for energy storageapplications.

Based on constant current charge/discharge curves, thevalue of internal resistance, Rint, has been calculated fromthe initial IR drop after changing the current direction (Rint0

dU1/2j, where dU1 is the value of cell potential for 10 ms).The lowest Rint values have been calculated for TiC-CDC(Cl2 1,100 °C) and TiC-CDC (HCl 1,100 °C) which is in

Fig. 4 Specific capacitancevs. cell potential curvescalculated from CV curvesat potential scan rates ν02 (a)and 50 mVs−1 (b) for thesupercapacitors, completedusing TiC-CDC electrodes,prepared at different synthesistemperatures by usingdifferent reactants


Fig. 5 Constant currentcharge/discharge cycles atcurrent density j02 mAcm−2

for supercapacitors,completed using TiC-CDCelectrodes, prepared atdifferent synthesis temperatures(given in the figure)by using different reactants.Magnification of constantcurrent charge/discharge cyclenear the IR-drop region atU = 2.7 V

Fig. 6 Nyquist plots (a)Magnification of the Nyquistplots at higher and mediumfrequency values and specificgravimetric series capacitanceCm vs. ac frequencydependencies (b) at cellpotential U03.0 V forsupercapacitors, completedusing TiC-CDC electrodes,prepared at different synthesistemperatures (given in the fig-ure) by using different reactantsfor synthesis of TiC-CDC(noted in the figure)


agreement with the analysis of impedance data (given inFig. 6 and analysed below).

Nyquist plots

The Nyquist (Z” vs. Z’) plots (Fig. 6a) for supercapacitors havebeen measured within the ac frequency f range of 1 mHz–300 kHz at fixed cell potentials U from 0 to 3.2 V. The shapeof Z” vs. Z’ plots depends noticeably on the CDC materialcharacteristics [33] but is practically independent of U applied.The Nyquist plots consist mainly of three parts: (1) the smallsemicircle, (2) the very well-expressed so-called “porous” re-gion with a slope of α≈−45°, in ac frequency region of 1<f<300Hz, characteristic of themass transfer-limited process in themicro/mesoporous carbon electrode matrix [2, 7, 34, 35] and(3) the double-layer capacitance region with a slope of α≈−90°(“knee” at low frequencies f<1 Hz), obtained by the finite

length effect [2, 7, 34, 36]. The semicircle shape depends onthe adsorption kinetics of ions at the micro/mesoporous TiC-CDC electrode, the series resistance of the material and thecharge transfer resistance inside the macro/mesoporous carbonstructure, as well as on the mass transfer resistance inthe microporous carbon electrode (RCE) at higher ac frequencyf>300 Hz [35].

Extrapolation of the high-frequency part of the Nyquistplot to the condition Z”(ω)00 gives equivalent series resis-tance of the cell (ESR 0 Z’(ω) → ∞). According to the datain Fig. 6a (inset), ESR depends noticeably on the CDCsynthesis conditions and temperature, i.e. on the porosityand graphitic sp2 carbon amount in TiC-CDC. The increaseof ESR for TiC-CDC (Cl2) prepared at lower temperatures(T≤900 °C) is probably mainly caused by the bigger frac-tion of micropores and less curved pores, where the masstransfer rate of ions is limited [37]. Extrapolation of the low-

Fig. 7 Maximal specificenergy and power vs. synthesistemperature plots for thesupercapacitors, completedusing TiC-CDC electrodes,prepared at different synthesistemperatures (given in thefigure) by using differentreactants (open marks denoteCl2, filled marks denote HCl)

Fig. 8 Ragone plotsfor the supercapacitorscompleted usingTiC-CDC electrodes


frequency (f<1 Hz) linear part of the Z” vs. Z’ plots to thecondition Z”(ω)00 gives the sum of (ESR + RCE) and theinternal distribution of the electrolyte resistance values in thepore matrix (RIER ≡ Rpore) of the carbon electrodes. Thus, thetotal polarisation resistance RΣ 0 ESR + RCE + Rpore.

The specific series capacitance Cm values, calculated fromthe Z” vs. Z’ plots, at ac frequency f01 mHz (Fig. 6b) are in agood agreement with the values obtained by CV and CCmethods and data, obtained for other CDC materials studied[1–3, 6–12, 29, 33, 36, 37, 38].

Specific energy and power, Ragone plots

The maximal specific energy Emax (Whkg−1) and power Pmax

(kWkg−1) for the supercapacitors studied can be estimatedusing Eq. 5a and Eq. 5b:

Emax ¼ CΔU2

2mð5aÞ

Pmax ¼ ΔU 2

4ESRmð5bÞ

where C is the capacitance of the cell (Fcm−2), ESR is theequivalent series resistance (Ωcm2), obtained from theNyquist plots at f→∞, and m is the total active materialweight of two electrodes (gcm−2). The maximal specificenergy, calculated at cell potential U03.0 V, for TiC-CDC(Cl2 800 °C) and TiC-CDC (HCl 900 °C) are 43.1 and31.1 Whkg−1, respectively. The maximal specific power,calculated at cell potential U03.0 V, for TiC-CDC (Cl21,000 °C) and TiC-CDC (HCl 1,000 °C) (Fig. 7) are 805.2and 847.5 kWkg−1, respectively.

The Ragone plots (calculated taking into account the totalactive material weight of two electrodes) for the supercapaci-tors, based on TiC-CDC (Cl2) and TiC-CDC (HCl) electrodes,have been obtained from constant power tests within the cellpotential range of 3.0–1.5 V and are shown in Fig. 8. TheRagone plots for TiC-CDC, prepared by using Cl2 or HCl asreactants, are quite similar, and at high power loads, the TiC-CDC (Cl2) material synthesised at 900 °C, i.e. the material withoptimal micro- and mesopore fractions, delivers the highestpower at constant energy.

Conclusions

Micro- and mesoporous carbide-derived carbons were synthes-ised from titanium carbide (TiC) powder via a gas-phase reac-tion using Cl2 or HCl as reactants within the temperature rangeof 700–1,100 °C. Analysis of XRD results shows that TiC-derived carbons (TiC-CDCs) consist mainly of graphitic crys-tallites. The first-order Raman spectra showed the graphite-like

absorption peaks at ~1,577 cm−1 and the disorder-inducedpeaks at ~1,338 cm−1. The low-temperature N2 sorption experi-ments were performed, and specific surface areas up to 1,214and 1,544 m2g−1 were obtained for TiC-CDC (HCl), synthes-ised at T0800 °C, and for TiC-CDC (Cl2), synthesised at T0900 °C, respectively. For the TiC-CDC powders synthesised, abimodal pore size distribution function has been establishedwith the first maximum in the region up to 1.5 nm and thesecondmaximum from 2 to 4 nm. The energy-related propertiesof supercapacitors based on 1 M (C2H5)3CH3NBF4 in acetoni-trile and TiC-CDC (Cl2) and TiC-CDC (HCl) as electrodes werealso investigated by cyclic voltammetry, electrochemical imped-ance spectroscopy, galvanostatic charge/discharge and constantpower methods. The maximal specific energy, calculated at cellpotential U03.0 V, for TiC-CDC (Cl2 800 °C) and TiC-CDC(HCl 900 °C) are 43.1 and 31.1 Whkg−1, respectively. Themaximal specific power, calculated at cell potential U03.0 V,for TiC-CDC (Cl2 1,000 °C) and TiC-CDC (HCl 1,000 °C) are805.2 and 847.5 kWkg−1, respectively.

The Ragone plots (calculated taking into account the totalactive material weight of two electrodes) for the supercapaci-tors, based on TiC-CDC (Cl2) and TiC-CDC (HCl) electrodesand obtained from constant power tests within the potentialrange of 3.0–1.5 V, are quite similar. However, at high powerloads, the TiC-CDC (Cl2) material synthesised at 900 °C, i.e.the material with optimal pore structure, delivers the highestpower at constant energy.

Acknowledgments This work was supported in part by the EstonianMinistry of Education and Research (project SF0180002s08), by theEstonian Centre of Excellence in Science: High Technology Materialsfor Sustainable Development, by the graduate school “Functional Materi-als and Technologies”, receiving funding from the European Social Fundunder project 1.2.0401.09-0079 in Estonia and by the Estonian ScienceFoundation under project no. 8172. Prof. K. Kirsimäe from the Institute ofEcology and Geography and Dr. I. Sildos from the Institute of Physics atthe University of Tartu are thanked for the help with the XRD and Ramanstudies of carbon samples, respectively.

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Supercapacitors based on carbide-derived carbons synthesised using HCl and Cl2 as reactants