Electrical double layer characteristics of nanoporous carbon derived from titanium carbide

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1 Electrochimica Acta 51 (2006) Electrical double layer characteristics of nanoporous carbon derived from titanium carbide L. Permann a,b,m.lätt a,b, J. Leis a,b, M. Arulepp a,b, a Tartu Technologies Ltd., 185 Riia Str., Tartu, Estonia b Department of Chemistry, University of Tartu, 2 Jakobi Str., Tartu, Estonia Received 5 May 2005; received in revised form 16 June 2005; accepted 16 June 2005 Available online 9 August 2005 Abstract Electrochemical parameters of the nanoporous carbide-derived carbon organic electrolyte have been studied by cyclic voltammetry and electrochemical impedance spectroscopy. The gas adsorption measurements have been used for evaluating the specific surface area, pore size distribution and porosity as the essential parameters influencing the double layer performance of carbon. The region of ideal polarizability, values of series and parallel resistance, capacitance and other important electrical double layer parameters were established. It was shown that specific capacitance of typical nanoporous carbon derived from titanium carbide is in the range of F cm 3 or F g 1 and it depends on the synthesis conditions. The influence of the electrolyte solvent to the capacitance was insignificant, although acetonitrile was advantageous with respect of smaller viscosity Elsevier Ltd. All rights reserved. Keywords: Carbide-derived carbon; Nanoporous carbon; Non-aqueous electrolyte solution; Electrical double layer capacitor 1. Introduction The electrical double layer capacitors (EDLC) gain more and more attention as the prospective energy storage devices in various pulse energy generation systems. The electrical double layer characteristics of porous carbon electrodes determine the performance of EDLC, where the electrical charge is stored in the double layer and it is based mainly on electrostatic interactions [1]. Recently, it was shown that the nanoporous carbide-derived carbon offers advanced possibilities as the electrode material in supercapacitors [2]; however, to achieve the best association of energy and power density, the carbon material and its pore characteristics have to be optimised. This paper reports the results of systematic studies of the interface: non-aqueous electrolyte nanoporous carbon derived from TiC, whereby the structure and poros- Corresponding author. Tel.: ; fax: address: matia@park.tartu.ee (M. Arulepp). ity of carbon were varied by changing synthesis conditions. Hereby, the paper continues the series of publications about the EDLC performance of carbide-derived carbon materials [3 8], which synthesis and evaluation is particularly described in Refs. [9 12]. 2. Theoretical background The capacitance of EDLC depends mainly on the surface area of carbon material used for the preparation of the electrodes. Theoretically, the higher is surface area of the activated carbon, the higher specific capacitance could be expected. The specific capacitance should be defined as the specific surface area of carbon multiplied by the double layer capacitance, C dl /F cm 2 [1]. However, the practical situation is more complicated and usually the capacitance measured does not have the linear relationship with the specific surface area of the electrode material. In fact, some activated carbons /$ see front matter 2005 Elsevier Ltd. All rights reserved. doi: /j.electacta

2 L. Permann et al. / Electrochimica Acta 51 (2006) with smaller specific surface area demonstrate higher specific capacitance values than those with larger surface area. There are two main reasons for this phenomenon: (1) the double layer capacitance, C dl, varies with various types of activated carbons that were made from the different types of precursors (through different processes and subsequent treatments); (2) the nanopores having small diameter may be impenetrable to the electrolyte solution, simply because the electrolyte ions, especially big organic ions and ions with the hydration shell, are too big to enter into the nanopores. The surface area of these inaccessible nanopores therefore will not contribute to the total double layer capacitance of the electrode material. It should be noted that the carbonaceous materials show the frequency-dependent capacitance, though the capacitance should be independent of frequency. This abnormal frequency dependence is called a distributed characteristic or frequency dispersion of the electrical properties [1]. A circuit element with the distributed characteristic cannot be exactly expressed as a combination of a finite number of ideal circuit elements, except in certain limiting cases. The distributed characteristic results mainly from the two origins [1,13 22]: (1) It appears non-locally when a dimension of a system under study (electrode thickness or pore length) is longer than the characteristic length (for example, diffusion length or penetration depth), which is a function of frequency. This type of distributed characteristic exists even when all system properties are homogeneous and space-invariant (double layer charging of a porous electrode, diffusion in diffusion-limited systems, adsorption of anions and cations, surface reconstruction and transformation in adlayers). (2) The distributed characteristic is attributed to the various heterogeneities, geometric inhomogeneity, such as the surface roughness or the distribution of pore size as well as the crystallographic anisotropy and the surface disorder of a polycrystalline electrode. Beginning essentially with the work of de Levie [17,19,20], a large number of various models have been developed [1,15 22] to describe theoretically the experimental behaviour of the porous electrodes. A very important direction is the investigation of the influence of the pore geometry on the data of electrical impedance spectroscopy (EIS) [21,22]. Some authors use simple modifications of the classical Randles Frumkin Melik Gaikazyan equivalent circuits [23 25], involving a constant phase element or Warburg diffusion impedance modified according to the boundary conditions [1,15,26,27], as well as the branched transmission line equivalent circuits [1,15,28]. Advanced theory for macroscopically homogeneous porous electrode has been developed by Paasch et al. [18]. This model describes three main processes: (a) ionic conductivity in the pore electrolyte and electronic conductance in the electrode (solid) phase; (b) charging the double layer at the solid liquid interface; (c) the simple charge transfer reaction (CTR) at the interface. Using the usual for electrochemistry conditions [18], the electrode impedance is obtained as [ρ Z(ω) = A ρ2 2 coth(dβ) ρ 1 + ρ 2 β + 2ρ 1ρ 2 1 ρ 1 + ρ 2 β sinh(dβ) + dρ ] 1ρ 2 (1) ρ 1 + ρ 2 where ρ 1 and ρ 2 are resistivity of solid phase (electrode material) and solution and β is expressed as: β = 1 ( ) k + iω 1/2 and ω 1 = K d ω 1 d 2 (2) where ω 1 is a characteristic frequency, related with the finite field diffusion and the parameters k and K are defined as characteristic frequency and field diffusion constant, respectively. k g ctr C 1 = j 0nF CR g T 1 k = (4) CS e (ρ 1 + ρ 2 ) According to Paash theory, the ac penetration length λ is defined as: λ = 1 Reβ ( 1K/k (1 + ω 2 /k 2 ) 1/ Materials and methods (3) ) 1/2 (5) 3.1. Preparation and characteristics of the nanoporous carbon material A series of nanoporous carbon materials were synthesised from TiC (A.C. Starck, FSSS 2.5 m) through the chlorination process as in detail described elsewhere [10,12]. To vary the content and size of nanopores in resultant carbon materials, the synthesis temperature was varied in the range of C. After reaction, i.e. the complete transformation of carbide into the nanoporous carbon, the excess of chlorine and other residues of gaseous by-products were removed with a flow of argon and subsequent heating of the carbon in hydrogen atmosphere at 800 C. In some cases, the adsorption properties of carbon materials were improved by special oxidative treatment in nanopores [12]. A selection of structural characteristics of selected carbon powders, e.g. specific surface area, micropore volume and area obtained from low temperature nitrogen sorption data, are presented in Table 1. The specific surface area (S a ) according to BET and Langmuir theories has been evaluated using the multiple point method at relative pressure values up to P/P 0 = 0.2 and the total pore volume (V tot ) was calculated from the volume of nitrogen adsorbed at near to saturation pressure, P/P 0 = 0.99.

3 1276 L. Permann et al. / Electrochimica Acta 51 (2006) Table 1 The physical properties of carbon derived from TiC Material S a, BET (m 2 g 1 ) S a, Langmuir (m 2 g 1 ) V tot (m 3 g 1 ) V m (m 3 g 1 ) MPD a (Å) SkeletonC SkeletonC SkeletonC a MPD, median pore diameter. The volume of micropores (V m ) was derived from the socalled t-plot dependence of nitrogen sorption isotherm. The pore size distribution (Fig. 1) was derived from adsorption isotherm according to the density functional theory (DFT). Median pore diameter (MPD) was calculated according to the Horvath Kawazoe theory. The common property of all these three carbon materials is that almost 100% of pores are less than 2.0 nm and most of pores are less than 1.0 nm in size. The major difference is the very small but important variation in the average pore size (cf. MPD in Table 1) and the pore size distribution below 1.0 nm as seen in Fig. 1. Said figure presents the comparison of SkeletonC 2 and the same material after oxidation in nanopores, i.e. SkeletonC 3. The TEM image at 200 kv for the typical carbon powder of present study is shown in Fig. 2. It is worth to mention that on TEM images, all nanoporous carbon materials derived from TiC below 900 C look very similar and can be characterized as a solid package of continuous disordered graphene sheets. Though, at 1000 C, the carbon is slightly coarser Preparations and the experimental set-up for electrochemical study The polarizable electrodes from TiC-derived carbon powders were manufactured as follows. The mixture of 94% (wt.) nanoporous carbon and 6% (wt.) PTFE binder (Aldrich, 60% suspension in water) was rolled stepwise into the carbon film with a final thickness of 100 ± 5 m. After drying, the raw electrode sheets were plated from one side Fig. 2. TEM image of carbon syntesised from TiC. with a thin aluminum layer (4 ± 1 m) using the plasmaactivated physical vapor deposition. The electrolytes from pure acetonitrile (AN, Riedel-de Haën H 2 O < 0.003%) and propylene carbonate (PC, anhydrous, Merck Selectipur ) containing 1 M triethyl-methyl ammoniumtetrafluoroborate ((C 2 H 5 ) 3 CH 3 NBF 4, Stella, H 2 O < 0.005%) were prepared in dry atmosphere. Disc-shaped carbon electrodes (0.43 cm 2 ) were investigated using three-electrode set-up. Before experiments, the carbon electrodes were vacuum-dried at 180 C for 2 h. The standard glass cell with large counter electrode (apparent area of 30 cm 2 ) and saturated calomel electrode (SCE) were used. During the experiments, the electrolyte solution was saturated with argon (99.999%) to avoid the contamination by oxygen. The potentiostat galavanostat 1286 Solartron and FRA 1255 Solartron were used to record all spectra. The frequency range from 1 MHz to 1 mhz and the ac modulation 5 mv was used when recording the EIS spectra. 4. Results and discussion 4.1. Cyclic voltammetry (j, E) curves Fig. 1. Pore size distribution of carbon powders according to the density functional theory. The cyclic voltammetry j, E curves were measured in 1 M electrolyte solutions (Fig. 3a and b) at scan rates of 50, 20,

4 L. Permann et al. / Electrochimica Acta 51 (2006) Fig. 3. j, E curves expressed as capacitance vs. potential between 1.4 and +1.4 V vs. SCE for SkeletonC 3 in acetonitrile (a) and in propylene carbonate (b) electrolyte solutions at different scan rates. Fig. 4. j, E curves expressed as capacitance vs. potential between 2 and +1.8 V for SkeletonC 3 in different electrolyte solutions at potential scan rate ν =2mVs 1. 10, 5, 2 and 1 mv s 1. The experimental data show that the shape of j, E curves is independent of the sequential number of a cycle n when n 5. At small scan rates below 10 mv s 1, j, E curves have nearly mirror image symmetry of the current responses about the zero current line in both of electrolyte solutions. The nanoporous carbon electrodes demonstrate a smooth curve in the region of potentials from 1.4 to +1.4 V (versus SCE), indicating that the carbon electrodes are ideally polarisable in this region. Furthermore, it is seen in the Fig. 4, that the region of ideal polarisability ( E) of the nanoporous carbide-derived carbon electrodes safely exceeds 3 V. At higher scan rates, ν 10 mv s 1, the j, E curves recorded in propylene carbonate solution become distorted from the mirror image symmetry. It is caused by the high resistivity of the electrolyte in the porous material. It should be noted that the establishment of the adsorption equilibrium in nanopores is a very slow process, especially when relatively viscous propylene carbonate solution (η 2.5 mm 2 s 1 ) is used. One could expect that the less viscous is electrolyte, the higher should be the effective diffusion coefficient for the ions in nanopores. This would explain, why the shape of j, E curves is close to the ideal in acetonitrile solution (Fig. 3a), which is significantly less viscous (η 0.3 mm 2 s 1 ). The dominating pore size of carbon materials, considered in this paper, was more or less identical, 7 8 Å, according to the data in Table 1 and Fig. 1. The experiments of cyclic voltammetry assured that Et 3 MeN + -ions are too big to enter into the nanopores less than 7 Å. Therefore, the current density values with SkeletonC 1 were unexpectedly small in negative region of potentials (Fig. 5b). Oppositely, there was no big difference in current density values of different carbon materials in positive region of potentials. Fig. 3a and b show that in j, E curves recorded at small scan rates, the current density crosses the minimum in the region of potentials of 0.23 V E 0.31 V (versus SCE in H 2 O), which is explained by the diffused nature of the electric double layer in the region of zero charge [1]. The current minima were noticeably better expressed in acetonitrile solutions. It was observed that the potential value in this minimum is slightly dependent on the direction of potential scan, especially, when higher scan rates of potential, ν 5mVs 1, Fig. 5. j, E curves expressed as capacitance vs. potential between 1.4 and +1.4 V vs. SCE (H 2 O) for different SkeletonC materials at potential scan rate ν =10mVs 1 in acetonitrile (a) and propylene carbonate (b) electrolyte solutions.

5 1278 L. Permann et al. / Electrochimica Acta 51 (2006) Table 2 Specific capacitance C/F cm 3 of carbon electrodes calculated from electrochemical impedance spectroscopy at ac frequency 10 mhz Material ρ (g cm 3 ) C S NEG (F cm 3 ) C S POS (F cm 3 ) AN PC AN PC SkeletonC SkeletonC SkeletonC were applied. This phenomenon has to be caused by diffusion and adsorption/desorption rate of ions that is rather slow in nanopores. The values of differential capacitance were obtained as: C = jν 1 = j(dedt 1 ) (6) The capacitance values calculated from j, E curves reveal that the capacitance is practically independent of the potential scan rate when ν 2mVs 1. In acetonitrile solution, the capacitance C/F cm 2 of carbon electrodes is almost independent of the potential scan rate. The differential capacitance values calculated from j, E curves were compared with the capacitance values calculated from constant current charge/discharge curves and from impedance spectra. Three methods, each is widely used for evaluating specific capacitance, yield different capacitance values. Hence, the constant current discharge and cyclic voltammetry produce rather similar values due to similarity of cycling behaviour in all potential ranges. The capacitance values calculated from different methods are compared in Tables 2 and 3. The average specific capacitance values of investigated carbon materials ranged between 80 and 105 F cm 3 in acetonitrile and between 70 and 90 F cm 3 in propylene carbonate, respectively. The capacitance value per weight of carbon, F g 1, is at almost the same level with widespread activated carbon materials. The capacitance calculated from EIS at 10 mhz is notably different when propylene carbonate electrolyte is used. This could be caused by the non-equilibrium state of the charge when the high-viscosity electrolyte is used. In other words, the frequency of 10 mhz is still too high for establishing the capacitance of nanoporous EDL materials. Table 3 Specific capacitance C/F cm 3 of carbon electrodes calculated from cyclic voltammetry (CV) at 10 mv s 1 and constant current (CC) at 10 ma cm 2 Material ρ (g cm 3 ) C NEG (F cm 3 ) C POS (F cm 3 ) CV CC CV CC AN PC AN PC AN PC AN PC SkeletonC SkeletonC SkeletonC Electrochemical impedance spectroscopy The complex impedance plane plots (so-called Nyquist plots) were measured in 1.0 M TEMA AN and PC electrolyte solutions in the range of ac frequency from 1 MHz to 1 mhz at 1.4 and 1.4 V (versus SCE in H 2 O). The obtained results demonstrate that the shape of the Z, Z -plot depends noticeably on the nanoporous carbon (Fig. 6a and b), electrolyte solution and also on the electrode potential (Fig. 7a and b). The model of series connection of resistance and capacitance was used for the calculation of differential series capacitance C S and the series resistance R S. According to the Fig. 8a d, the C S values reach the limiting value of C S(ω 0) at f 5 mhz, indicating that diffusion is a limiting stage in the adsorption process. The kinetics of diffusion processes differs significantly in different electrolyte solutions, therefore, the series and parallel capacitance values, C S and C P, achieve the limiting value at different ac frequencies (Fig. 9a and b). In acetonitrile, the frequency range, where C S reaches the limiting value, is practically independent on the electrode material. The C S(ω 0) depends remarkably on the carbon material in more viscous PC electrolyte solution. With the carbon SkeletonC 1, which had the highest content of nanopores among the samples studied, the restricted diffusion of electrolyte ions into the pores was observed at negative electrode potentials (Fig. 8b). Obviously, the electrolyte cations were too big to penetrate the nanopores. This phenomenon did not appear in positive region of potentials (Fig. 8d). The differential series capacitance of the electrode material depends Fig. 6. Nyquist plots for different SkeletonC in acetonitrile (a) and propylene carbonate (b) solutions at 1.4 V.

6 L. Permann et al. / Electrochimica Acta 51 (2006) Fig. 7. Nyquist plots for SkeletonC 3 in acetonitrile (a) and propylene carbonate (b) solutions at different potential, noted in figure. Fig. 8. Series capacitance vs. frequency of different SkeletonC in acetonitrile (a and c) and propylene carbonate (b and d) electrolyte solutions at different potentials 1.4 V (a and b) and +1.4 V (c and d). on the electrode potential. However, the dependency was rather small, but still observable in acetonitrile (Fig. 8a and c). It was observed that EDL capacitance of the positively charged nanoporous electrodes is higher compared to the negative electrode (Fig. 8a d). One reason of course is that specific adsorption of the positively and negatively charged ions is different. However, the main reason obviously is that the so-called ion size of Et 3 MeN + cations (positively charged ions) is bigger than that of BF 4 anions, 7 and 5 Å, respectively, according to Ref. [29]. The EDL capacitance of positively charged electrode was independent of solvent nature. This confirms that pore diam- Fig. 9. C S, C P vs. frequency for SkeletonC 3 in acetonitrile (a) and propylene carbonate (b) solutions at 1.4 V (vs. SCE).

7 1280 L. Permann et al. / Electrochimica Acta 51 (2006) Fig. 10. Phase angle vs. frequency of different SkeletonC in acetonitrile (a) and propylene carbonate (b) solutions at 1.4 V (vs. SCE). eters of Å are sufficient to ensure the maximum adsorption of BF 4 ions on the nanoporous SkeletonC. The complex plane plots, measured for porous carbon electrolyte solution interface at constant E (Figs. 6a,b and 7a,b), can be divided into the four main sections. At high ac penetrability (at very low ac frequency f 0.1 Hz), the nanoporous carbon electrode behaves like a plane surface since the penetration depth λ (Eq. (5)) is larger than the length of the pore so that an ac signal detects very large amount of the pore volume. In this region of the Z, Z -plot (called the planar section [1]), the phase angle approaches asymptotically to (Fig. 10a and b). However, according to the data in Figs. 6a,b and 7a,b; the impedance (as well as the phase angle in Fig. 10a and b) at low ac frequency shows the non-ideal constant phase element (CPE) behaviour. At lower penetrability (at higher ac frequency, f 100 Hz), the penetration depth is smaller than the length of the pores, so that the ac signal detects only a part of the pore volume, i.e. the nanopore surface. The region of ac frequency where phase angle approaches to 45 is called the porous section of Z, Z -plot. Furthermore, there is a transition section between the porous and planar sections in the Z, Z -plot. At higher frequencies, there is a well expressed slightly depressed semicircle, with the depression angle β of only somewhat higher than 0, whereby β = 0 would correspond to the purely charge transfer-limited heterogeneous process and β =45 to the diffusion-limited stage [1,8]. Thus, the very fast heterogeneous adsorption step, i.e. the partial charge transfer reaction, is probably possible in the case of the porous carbon electrolyte solution interface [1]. 5. Conclusions A series of nanoporous carbide-derived carbon materials characterized by the surface area of 1400 m 2 g 1, were synthesised from TiC. The carbon materials were investigated in 1MEt 3 MeNBF 4 solutions of propylene carbonate (PC) and acetonitrile (AN), using three-electrode test cell and several electrochemical testing procedures. It was shown that the carbide-derived carbon materials used in this study have electrochemical window of more than 3 V. The EDL capacitance of nanoporous carbon materials was in the range of F g 1 and F cm 3 and it was dependent on the volume and median pore diameter of micropores. Slightly different capacitance values were obtained in AN and PC electrolyte solutions. The phase angle and R S depend on the solvent nature as well as on applied potential. Acknowledgements This work was supported by Skeleton Technologies and Tartu Tehnoloogiad OÜ. The authors wish to thank colleagues from Tartu Tehnoloogiad for the kind assistance in performing the experiments. References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, NY, [2] Y.A. Maletin, N.G. Strizhakova, V.Y. Izotov, A.A. Mironova, S.G. Kozachkov, V.V. Danilin, S.N. Podmogilny, M. Arulepp, J.A. Kukushkina, A.J. Kravchik, V.V. Sokolov, A. Perkson, J. Leis, J. Zheng, S.K. Gordeev, J.Y. Kolotilova, J. Cederström, C.L. Wallace, Patents US 6,602,742, 2003 and US 6,697,249, [3] E. Lust, G. Nurk, A. Jänes, M. Arulepp, L. Permann, P. Nigu, P. Möller, Condens. Matter Phys. 5 (2002) 307. [4] E. Lust, G. Nurk, A. Jänes, M. Arulepp, P. Nigu, P. Möller, S. Kallip, V. Sammelselg, J. Solid State Electrochem. 7 (2003) 91. [5] M. Arulepp, L. Permann, J. Leis, A. Perkson, K. Rumma, A. Jänes, E. Lust, J. Power Sources 133 (2004) 320. [6] A. Jänes, L. Permann, M. Arulepp, E. Lust, Electrochem. Commun. 6 (2004) 313. [7] A. Jänes, L. Permann, M. Arulepp, E. Lust, J. Electroanal. Chem. 569 (2004) 257. [8] A. Jänes, L. Permann, P. Nigu, E. Lust, Surf. Sci. 560 (2004) 145. [9] J. Leis, A. Perkson, M. Arulepp, M. Käärik, G. Svensson, Carbon 39 (2001) [10] J. Leis, A. Perkson, M. Arulepp, P. Nigu, G. Svensson, Carbon 40 (2002) [11] A. Perkson, J. Leis, M. Arulepp, M. Käärik, S. Urbonaite, G. Svensson, Carbon 41 (2003) [12] J. Leis, A. Perkson, M. Arulepp, PCT patent application WO , 2004.

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