Kinetics of Double-Layer Formation: Influence of Porous Structure

Bleda-Martinez , M. J.; Macia-Agullo , J. A.; Lozano-Castello , D.; Morallon , E.; Cazorla-Amoros , D.; Linares-Solano , A. Carbon 2005, 43, 2677– 2...
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Energy Fuels 2010, 24, 3378–3384 Published on Web 03/04/2010

: DOI:10.1021/ef901521g

Kinetics of Double-Layer Formation: Influence of Porous Structure and Pore Size Distribution† M. J. Bleda-Martı´ nez,‡ D. Lozano-Castell o,‡ D. Cazorla-Amor os,*,‡ and E. Morall on§ ‡

Departamento de Quı´mica Inorg anica and §Departamento de Quı´mica Fı´sica and Instituto Universitario de Materiales, Universidad de Alicante, Ap. 99, E-03080, Alicante, Spain Received December 13, 2009. Revised Manuscript Received February 19, 2010

Porous carbon materials have been prepared from different precursors (anthracite and carbon fibers) by different activation methods (KOH and CO2), obtaining a series of samples with very different porous texture and porous structure. Electrochemical impedance spectroscopy has been used to analyze the kinetics of the charge-discharge process during the behavior of these materials as supercapacitors. Capacitance reduction percentages (CRPs) from the starting value (at the lowest frequency of the measurements) to the value at around 0.02 Hz frequency were calculated for all of the samples. The results pointed out that, from a kinetic point of view, porous materials with wider micropore size distribution (MPSD) have a better performance, because porosity is accessible to the electrolyte even at high charge-discharge rates. In addition, quite different behavior between activated carbons (ACs) and activated carbon fibers (ACFs) has been observed when Na2SO4 is used as the electrolyte; a widening of the porosity produces a lower CRP (better kinetic behavior) in ACF than in AC, which is most likely due to the existence of a smaller tortuosity of networks of porosity in ACF than in AC. This different behavior is much less important when H2SO4 is used as the electrolyte, which is due to the higher mobility of proton in solution compared to any other ion. Thus, kinetics of the double-layer formation is improved using (i) samples with wide MPSD, (ii) samples with a porous texture with low tortuosity, and (iii) acidic medium as the electrolyte.

activated carbons (ACs) are the most widely used electrode materials for EDLCs.2-10 Since many years ago, it is well-known that, in general, the larger the surface area that an EDLC can provide for adsorption of ions on electrodes, the more energy can be stored in the EDLC.2-4 However, detailed analyses using porous carbon with tailored porosity have demonstrated that not all of the pores are effective in the charge accumulation. For example, it has been shown that the very narrow micropores do not contribute to the total double-layer capacitance because of a molecular sieving effect.6-11 Moreover, it has been seen that the efficiency of pore filling, i.e., of double-layer formation, is optimal when the pore size is around 0.7 nm in aqueous media and 0.8 nm in organic electrolyte.12,13 Most of these studies have been performed using low currents or low scan rates. However, from an application point of view, it is very interesting to know how the materials behave at high charge/ discharge rates, because the accessibility of the porosity will also depend upon the process rate. It has been seen that, for a porous electrode, the ohmic dissipation of energy varies down the pore (there is a continuous increase of electrolytic resistance down the pore from its entrance).14 Thus, it is very important to study the kinetics of the double-layer formation using porous materials to guarantee that they can be used in

1. Introduction The electric double-layer capacitor (EDLC) is an increasingly popular energy-storage device because of its energy and power density bridging the gap between batteries and classical electric capacitors.1 One of the applications of the EDLC is as a second power system in electric vehicles. This application requires EDLC to have a small energy loss and a high energy density to improve the efficiency of electric vehicles. The energy-storage mechanism in EDLCs is based on an electrostatic attraction between charges along the double layer formed at the electrode/electrolyte interface. Because this phenomenon is controlled by the surface area of the interface, † This paper has been designated for the special section Carbon for Energy Storage and Environment Protection. *To whom correspondence should be addressed. Fax: (þ34) 965903454. E-mail: [email protected]. (1) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Springer Publishing: New York, 1999. (2) Liu, X.; Osaka, T. J. J. Electrochem. Soc. 1996, 143, 3982–3986. (3) Liu, X.; Osaka, T. J. J. Electrochem. Soc. 1997, 144, 3066–3071. (4) Shi, H. Electrochim. Acta 1996, 41, 1633–1639. (5) Qu, D.; Shi, H. J Power Sources 1998, 74, 99–107. (6) Lozano-Castello, D.; Cazorla-Amor os, D.; Linares-Solano, A.; Shiraishi, S.; Kurihara, H.; Oya, A. Carbon 2003, 41, 1765–1775. (7) Kierzek, K.; Frackowiak, E.; Lota, G.; Gryglewicz, G.; Machnikowski, J. Electrochim. Acta 2004, 49, 515–523. (8) Guo, Y.; Qi, J.; Jiang, Y.; Yang, S.; Wang, Z.; Xu, H. Mater. Chem. Phys. 2003, 80, 704–709. (9) Shiraishi, S.; Kurihara, H.; Tsubota, H.; Oya, A.; Soneda, Y.; Yamada, Y. Electrochem. Solid-State Lett. 2001, 4, A5–8. (10) Bleda-Martinez, M. J.; Macia-Agullo, J. A.; Lozano-Castello, D.; Morallon, E.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2005, 43, 2677–2684.

r 2010 American Chemical Society

(11) Eliad, L.; Salitra, G.; Soffer, A.; Aurbach, D. J. Phys. Chem. B 2001, 105, 6880–6887. (12) Raymundo-Pi~ nero, E.; Kierzek, K.; Machnikowski, J.; Beguin, F. Carbon 2006, 44, 2498–2507. (13) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845–854. (14) Conway, B. E.; Pell, W. G. J. Power Sources 2002, 105, 169–181.

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Table 1. Activation Conditions Used for the Preparation of ACs and ACFs from Different Precursors and Using Different Activation Processes

sample

precursor

activating agent

AK1 AK2 AK3 AK4 DC1 DC2 DK HK1 HK2

anthracite anthracite anthracite anthracite CF, Donacarbo CF, Donacarbo CF, Donacarbo CF, Hexcel CF, Hexcel

KOH KOH KOH KOH CO2 CO2 KOH KOH KOH

activating agent/carbon ratio (weight)

activation temperature (°C)

activation time (h)

N2 flow rate (mL/min)

1:1 2:1 3:1 4:1

750 750 750 750 890 890 750 750 750

1 1 1 1 3 9 1 1 1

800 800 800 500 100 100 500 800 800

4:1 2:1 4:1

high-power applications. No previous systematic studies have been performed about the influence of pore size distribution on capacitance when the process rate is changed. Only few works have been performed in that sense; for example, Qu et al.15 obtained electrochemical-accessible time to the pore of different sizes from the fitting of AC impedance data. Thus, the objective of the present work is to carry out a systematic study about the influence of the porous structure and pore size distribution on the kinetics of the electric double-layer charge-discharge process. This study has been performed using electrochemical impedance spectroscopy, which is a useful tool to investigate the distributed characteristics of porous electrodes. To cover a wide a range of specific surface area, pore size distribution, and pore structure, a series of ACs and activated carbon fibers (ACFs) were prepared from an anthracite and two different carbon fibers by different activation methods (KOH and CO2). In addition, two commercial ACs from Maxsorb were also studied.

Donacarbo fibers were placed into a horizontal under flowing nitrogen. The furnace was purged with nitrogen for 30-60 min, and then the temperature was increased at 5 °C/min under flowing nitrogen until the activation temperature. Once the activation temperature was reached, nitrogen flow was switched to CO2 (100 mL/min). The furnace temperature and CO2 flow were kept constant for the desired period of activation. At the end of the activation period, the sample was cooled under nitrogen (100 mL/min). Table 1 collects the activation conditions and the nomenclature of the samples. 2.3. Characterization of Materials. Porosity characterization of all samples has been carried out by physical adsorption (N2 at 77 K and CO2 at 273 K) using an automatic adsorption system (Autosorb-6, Quantachrome). The total micropore volume [VDR(N2)] (pore size smaller than 2 nm) has been calculated from the application of the Dubinin-Radushkevich (DR) equation to the N2 adsorption at 77 K. The narrow micropore volume [VDR(CO2)] (pore size smaller than around 0.7 nm) has been assessed from CO2 adsorption at 273 K using the DR equation.18-20 The adsorbate densities used have been 0.808 and 1.023 g/mL, for N2 at 77 K and CO2 at 273 K, respectively.18-20 The apparent specific surface area has been calculated by using the Brunauer-Emmett-Teller (BET) equation. Moreover, the mesopore volume has been calculated as the difference between the volume adsorbed at a relative pressure of 0.99 and the total micropore volume [VDR(N2)]. Composite electrodes were prepared from porous carbon material, acetylene black (Strem Chemicals), and binder [poly(vinylidene dichloride) (PVDC) co-polymer, aqueous dispersion (55% solids), Waterlink Sutcliffe Carbons], in a ratio of 77:10:13 wt %, respectively. The materials were mixed and pressed up to 100 bar for 10 min. After that, the composite electrode was placed in a stainless-steel mesh as a current collector and pressed up to 75 bar for 2 min. The standard three-electrode cell configuration was employed. A reversible hydrogen electrode (RHE) was used as the reference, and a platinum wire was employed as a counter electrode. Electrochemical impedance spectroscopy measurements were performed using a frequency response detector FRD-100 connected to a potentiostat EG&G (model 273). The potential was fixed at 0.3 V (RHE), where the electrochemical contribution of the material is mainly capacitive (redox reactions of the surface oxygen groups in acidic medium take place at higher potentials).21 The sinusoidal signal amplitude was 50 mV. The frequency interval between 10.0 kHz and 500 μHz (taking 20 points logarithmically distributed) was analyzed in two different electrolytes: 1 M H2SO4 and 1 M Na2SO4 solutions.

2. Experimental Section 2.1. Preparation of ACs. A series of four powdered ACs have been prepared by chemical activation of a Spanish anthracite using KOH as the activating agent following the procedure published elsewhere.16 The raw material was impregnated in a KOH solution (hydroxide/carbon ratio, wt/wt) varying from 1:1 to 4:1. Chemical activations were carried out in a horizontal furnace under a N2 atmosphere with a heating rate of 20 °C/min up to 750 °C. The holding time at the maximum temperature was 1 h. After the heat treatment, samples were washed with 5 M HCl, vacuum-filtered 3 times, then washed with hot distilled water (80 °C), and filtered until the filtrate, which was free of chloride ions. The cleaned samples were dried in an oven at 120 °C for 24 h. Table 1 includes the nomenclature and the preparation conditions for each sample of the present study. In addition to these samples, two commercial ACs, Maxsorb-3000 (sample MX3) and Maxsorb SP (sample MX2), have also been included. 2.2. Preparation of ACFs. Two different precursors were employed for the preparation of ACF: commercial isotropic coal tar pitch-based carbon fibers (Donacarbo S-241, Osaka Gas Co., Ltd.) (mean fiber diameter of 13 μm) and highperformance commercial PAN-based carbon fibers (Hexcel) (mean fiber diameter of 6 μm). To develop porosity, these carbon fibers (CFs) were chemically activated using KOH as the activating agent, following the procedure described above (see preparation conditions and nomenclature in Table 1). Moreover, one of these precursors (Donacarbo S-241) was also activated using a different activation method, the so-called physical activation process, which is described elsewhere.17

(18) Rodriguez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1988; Vol. 21, pp 1-146. (19) Cazorla Amor os, D.; Alca~ niz Monge, J.; Linares Solano, A. Langmuir 1996, 12, 2820–2824. (20) Cazorla Amor os, D.; Alca~ niz Monge, J.; de la Casa Lillo, M. A.; Linares Solano, A. Langmuir 1998, 14, 4589–4596. (21) Bleda-Martınez, M. J.; Lozano-Castello, D.; Morallon, E.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2006, 44, 2642–2651.

(15) Qu, D.; Shi, H. J. Power Sources 1998, 74, 99–107. (16) Lozano-Castello, D.; Lillo-Rodenas, M. A.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2002, 39, 741–749. (17) Maci a-Agull o, J. A.; Moore, B. C.; Cazorla-Amor os, D.; Linares-Solano, A. Carbon 2004, 42, 1367–1370.

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Table 2. Porous Texture Characterization Results and Maximum Double-Layer Capacitance (Cmax) Corresponding to the ACs and ACFs Prepared, with Commercial ACs Also Included sample

SBET (m2/g)

Vmeso (cm3/g)

VDR(N2) (cm3/g)

VDR(CO2) (cm3/g)

Vtotal (cm3/g)

percentage of narrow porosity

Cmax(Na2SO4) (F/g)

Cmax(H2SO4) (F/g)

AK1 AK2 AK3 AK4 MX2 MX3 DC1 DC2 DK HK1 HK2

1036 1949 2584 2817 2400 3054 479 659 1602 262 599

0.03 0.02 0.03 0.05 0.00 0.24 0.01 0.01 0.02 0.02 0.03

0.46 0.87 1.20 1.31 1.10 1.46 0.22 0.30 0.72 0.12 0.27

0.38 0.56 0.70 0.57 0.54 0.49 0.22 0.30 0.59 0.18 0.22

0.49 0.89 1.23 1.36 1.10 1.69 0.24 0.30 0.73 0.21 0.30

77 63 57 42 49 29 93 100 81 86 74

123 164 177 198 140 196 28 40 154 39 83

122 217 260 259 222 237 11 96 154 49 118

Figure 1. Nyquist plots corresponding to two ACs (AK1 and AK2) and an ACF (DK) in 1 M Na2SO4 solution.

0.8 nm18-20 have been prepared. Thus, this series includes samples with different MPSDs, containing from 29% of narrow porosity (sample MX3) up to 100% of narrow porosity (sample DC2). Moreover, sample MX3 also presents an important contribution of the mesopore volume. Thus, these materials are suitable to study how the porous texture and pore size distributions affect the kinetics of the double layer charge-discharge process. It should also be remarked that the pore structure of ACs and ACFs have significant differences, as evidenced by the faster masstransfer rates for the ACFs in gas- and liquid-phase processes.22 3.2. Impedance Spectroscopy Results. Impedance spectroscopy, which distinguishes the resistance and capacitance of devices, was employed to analyze the performance of the different porous carbon materials prepared in this study. Figure 1 shows, as an example, Nyquist curves corresponding to two ACs (AK1 and AK2) and an ACF (DK) in 1 M Na2SO4 solution, with the shape of the curves being similar for the rest of the samples. The shape of the curves obtained in this study is concordant with that proposed by Keiser et al.,23 who examined the variation of the pore impedance with the frequency for various geometries of a single pore. In the low-frequency region, where nearly complete penetration of

3. Results and Discussion 3.1. Porous Texture Characterization Results. Table 2 contains the BET surface areas, mesopore volume (Vmeso), and micropore volumes calculated from N2 adsorption data at 77 K [VDR(N2)] and CO2 adsorption data at 273 K [VDR(CO2)] corresponding to all of the samples prepared in this study and also the two commercial ACs. The total pore volume (Vtotal) has also been estimated [Vtotal = Vmeso þ VDR(N2)]. For the sample HK1, which presents a VDR(CO2) higher than VDR(N2), the total pore volume has been calculated as Vtotal = Vmeso þ VDR(CO2). Finally, the percentage of narrow porosity has been assessed as [VDR(CO2)/Vtotal  100]. From the results presented in Table 2, it is seen that this series of samples covers a very wide range of apparent surface areas (BET surface areas from 260 to more than 3000 m2/g). The comparison between the total micropore volume [VDR(N2)] and the narrow micropore volume [VDR(CO2)] gives an idea of the micropore size distribution (MPSD). It must be remarked that, in most of the cases, VDR(N2) > VDR(CO2), which indicates that the mean pore size of these samples is higher than 0.8 nm approximately.18-20 However, the sample HK1 has a VDR(N2) < VDR(CO2) because of the presence of a very narrow microporosity (mean pore size lower than 0.5 nm approximately), where the N2 adsorption at 77 K presents diffusional problems.18-20 In addition, two samples (DC1 and DC2) with a very homogeneous microporosity [VDR(N2) ≈ VDR(CO2)] and mean pore size around

(22) Suzuki, M. Carbon 1994, 32, 577–586. (23) Keiser, H.; Beccu, K. D.; Gutjahr, M. A. Electrochim. Acta 1976, 21, 539–543.

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Figure 2. Maximum double-layer capacitance (Cmax) values versus the BET surface area for all of the samples of the present study in (a) 1 M Na2SO4 solution and (b) 1 M H2SO4 solution.

ions into pores would be allowed, the vertical line exhibits the domination of the capacitive behavior at the electrolyte/ carbon interface. It can be observed that, at the lower frequency range, the curve of DK is nearly vertical to the real axis, which indicates that the diffusion resistance of electrolyte ions in the pores of DK is small, having a route for the ions to move. The capacitance (C) can be calculated according to eq 1 -Z 00 ð1Þ C ¼ 2πf jZj2

carbon materials with very different porosities and pore structure have been studied. In our previous research, it has been demonstrated that the porous structure depends upon the activation process, observing that, even when the same type of activation process is used, i.e., physical activation, just by changing the activating agent (i.e., from CO2 to steam), the resulting porous structure is different, as deduced from mechanical property measurements25 and microSAXS measurements.26 Thus, the changes of the porous structure because of different activation processes could affect the capacitance values. The capacitance results obtained in the present work agree with those obtained with a similar type of sample in a previous study, where the capacitance values were measured by the galvanostatic method (at 2 mA) in H2SO4.10 In that study, the relationship between capacitance and porosity was shown, but it was also remarked that CO-type oxygen groups have a positive contribution to the capacitance. In the present study, the effect of the surface chemistry is minimized because the measurements have been preformed at 0.3 V potential, in which the redox reaction corresponding to

where f is the frequency and |Z|2 = Z0 2 þ Z00 2, with Z0 and Z00 being the real and imaginary parts of the complex impedance, respectively.24 For the lowest frequency of the measurements (500 μHz), the maximum double layer capacitance (Cmax) value can be estimated from eq 1. Figure 2 contains the estimated Cmax values for all of the samples of the present study in both electrolytes versus their apparent specific surface area. It is observed that, in both electrolytes, the capacitance increases with the surface area, with that increase being lower at the very high surface area region. Some deviations from this general trend can be seen, which is reasonable, taking into account the fact that

(25) Alca~ niz-Monge, J.; Cazorla-Amor os, D.; Linares-Solano, A.; Yoshida, S.; Oya, A. Carbon 1994, 32, 1277–1283. (26) Lozano-Castell o, D.; Raymundo-Pi~ nero, E.; Cazorla-Amor os, D.; Linares-Solano, A.; Muller, M.; Riekel, C. Carbon 2002, 40, 2727– 2735.

(24) Taberna, P. L.; Simon, P.; Fauvarque, J. F. J. Electrochem. Soc. 2003, 150, A292–A300.

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Figure 3. Capacitance loss versus frequency corresponding to two ACs (AK1 and AK2) and an ACF (DK) in 1 M Na2SO4 solution.

Figure 4. CRPs (from the starting value to the value at around 0.02 Hz) versus the percentage of narrow microporosity for all of the samples in 1 M Na2SO4 solution.

surface oxygen groups does not take place and does not contribute to capacitance.21 When the values of Cmax obtained in both media are compared, it is seen that, for the same sample, Cmax is higher in H2SO4 solution (Figure 2b) than in Na2SO4 electrolyte (Figure 2a). In the literature, the importance of having an adequate pore size to obtain high values of capacitance has been pointed out, concluding that the efficiency of pore filling, i.e., double-layer formation, depends upon the electrolyte used, with optimal being when the pore size is around 0.7 nm in aqueous media and 0.8 nm in organic electrolyte.12,13 Thus, the fact that Cmax is higher in H2SO4 medium than in Na2SO4 medium can be most likely due to the smaller proton size. When eq 1 is applied to each frequency in the interval where the behavior is almost purely capacitive, the capacitance frequency dependence is obtained. Figure 3 presents, as an example, the curves corresponding to the same samples included in Figure 1, in Na2SO4 electrolyte. As expected, a decrease of capacitance is obtained when the frequency increases for all of the samples, although the reduction of capacitance values depends upon the material, being lower for the ACF (DK sample). It can be seen that, in the case of the ACF, capacitance values remain more or less stable until

2 mHz. Considering that the loss of capacitance with frequency is related to the kinetics of the processes, these results seem to indicate that micropores existing in ACF can be more easily accessible by an alternating current than that in AC. From an application point of view, these results are very interesting for applications where high charge/discharge rates are demanded. To corroborate this statement, i.e., to confirm if a relationship exists between kinetics and pore structure, a comparison between all of the samples prepared in this study, which constitute a wide series of highly porous carbon materials with different porosities and structure, has been made. It is important to note that this type of comparison using a large number of materials is rarely found in the literature. To quantify the level of reduction of capacitance, capacitance reduction percentages (CRPs) from the starting value (at the lowest frequency, where there is enough time for the formation of the double layer in the porosity) to the value at around 0.02 Hz frequency were calculated for all of the samples. Figure 4 plots the CRPs versus the percentage of narrow microporosity (see Table 2) for all of the samples in Na2SO4 medium. A clear linear relationship is observed; that is, the higher the percentage of narrow microporosity, the higher the CRP. That means that, from a kinetic point of view, 3382

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Figure 5. CRPs (from the starting value to the value at around 0.02 Hz) versus the percentage of narrow microporosity for all of the samples in 1 M H2SO4 solution.

porous carbon materials with a wide mean pore size have better performance; i.e., the porosity is more easily accessible for the electrolyte even at high charge-discharge rates. It must be remarked that quite different trends are obtained for the ACs and ACFs. In the case of the ACs, the widening of the porosity (i.e., decrease of the percentage of narrow microporosity) does not produce a considerable improvement of the kinetic behavior (i.e., after a significant widening of the porosity obtained for the sample MX3 (29% of narrow porosity); still quite high values of CRP are obtained (92%). However, in the case of the ACF, the CRP decreases importantly when the porosity becomes wider, obtaining the lowest loss of capacitance (77%) for the ACF HK2 (74% of narrow porosity). Thus, for Na2SO4 electrolyte, ACF seem to be more useful in high-power applications than ACs. The fact that a widening of the porosity produces better kinetic behavior in ACF than in AC in Na2SO4 medium is most likely due to the existence of a smaller tortuosity of networks of porosity in ACF than in AC, which makes the ACF have a shorter path length for the ions to move than in the case of AC.22 The important effect of the different path lengths of two carbon materials was also observed by Shiraishi et al.27 In that case, they studied the reversibility of an ion-adsorbing/desorbing process and compared an activated carbon nanofiber (ACNF) with a 100-200 nm fiber diameter with a conventional ACF with a fiber diameter of ∼10 μm. They observed that the ACF showed a significant irreversible adsorption of the 1-ethyl-3methylimidazolium (EMImþ) cation in EMImBF4. However, the ACNF effectively suppressed the irreversibility, suggesting the important effect of the shorter path length existing on the ACNF on the ion-adsorbing/desorbing process. An analogous analysis has been performed in H2SO4 medium, and the results of CRPs versus the percentage of narrow microporosity have been included in Figure 5. Similar to the results obtained in Na2SO4 electrolyte, a widening of the porosity (lower percent of narrow porosity) produces an improvement of the kinetic behavior (lower percent loss of capacitance). However, in this electrolyte, the improve-

ment is much more important than in Na2SO4 medium (see higher slopes of both linear trends compared to Figure 4). Thus, for the AC with the lowest percentage of narrow porosity (29%) (sample MX3), the percentage loss of capacitance obtained is 92% in Na2SO4 medium and only 76% in H2SO4 medium. Moreover, when H2SO4 medium is used, slopes in the same order of magnitude are obtained for both ACs and ACFs. These facts can be due to the higher mobility of proton in aqueous solution compared to any other ion because of its different charge-transfer mechanism. In the case of proton, one of the mechanisms considers that the charge transfer takes place through the H2O molecules transforming to H3Oþ and with the active participation of the hydrogen bonds. Thus, if water can reach inside the porosity, the proton will also. However, for the rest of the ions, as in the case of Na2SO4 medium, ions have to move through the porosity, so that the process is less favorable kinetically. It should be remarked that, even in H2SO4 medium, the kinetic behavior of the ACs is worse than that for ACFs with similar porosity. For example, for samples with a similar percentage of narrow porosity [samples HK2 (74%) and AK1 (77%)], the percentage of loss of capacitance is much lower in the case of the ACF HK2 (80%) than in the case of the AC AK1 (99%). The best result in H2SO4 medium is obtained for the ACF HK1, with 86% of narrow porosity. The value of percentage of loss of capacitance for this sample in H2SO4 medium is only 59%, with the lowest loss of capacitance being obtained in the present study. In the case of ACs, the best result in H2SO4 medium corresponds to the AC MX3, with 76% of loss of capacitance. Thus, when porous carbon materials are required for high rates of charge/discharge processes, acidic medium as the electrolyte and porous carbons with wide MPSD, containing some mesoporosity and small tortuosity of networks of porosity, are desired. The high mobility of proton together with the existence of porosity where the electrolyte can access even at high charge-discharge rates makes porous carbon materials useful in high-power applications. 4. Conclusions Porous carbon materials have been prepared from different precursors (anthracite and carbon fibers) by different activation methods (KOH and CO2) obtaining a series of samples

(27) Shiraishi, S.; Miyauchi, T.; Sasaki, R.; Nishina, N.; Oya, A.; Hagiwara, R. Electrochemistry 2007, 75, 619–621.

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with very different porous texture and porous structure. Electrochemical impedance spectroscopy has been used to analyze the kinetic behavior of these materials as supercapacitors. Results point out that pore size distribution is a key parameter in the kinetics of the double-layer process. Materials with a high narrow micropore volume show a high CRP from the starting capacitance value (at the lowest frequency of the measurements) to the value at around 0.02 Hz frequency. However, those materials with wider MPSD have lower CRP. Thus, from a kinetic point of view, porous materials with wider MPSD have a better performance, because porosity is accessible to the electrolyte even at high charge-discharge rates. In addition, quite different behavior between AC and ACF has been observed, especially when Na2SO4 is used as the electrolyte; a widening of the porosity produces a lower CRP (better kinetic behavior) in ACFs than in ACs, which is most

likely due to the existence of a smaller tortuosity of networks of porosity in ACFs than in ACs. This different behavior between both types of materials is much less important when H2SO4 is used as the electrolyte, which is due to the higher mobility of proton in aqueous solution compared to any other ion. It can be concluded that kinetics of the double-layer formation in porous carbons is improved using (i) samples with wide MPSD, (ii) samples with a porous texture with low tortuosity (ACF), especially in Na2SO4 medium, and (iii) acidic medium as the electrolyte, because of the higher mobility of the proton compared to any other ion. Acknowledgment. The authors thank the Spanish Ministry of Science and Innovation, FEDER, and Plan E (Projects CTQ2006-08958/PPQ, MAT2007-60621, and CTQ2009-10813) and GV (ACOMP/2009/174 and PROMETEO/2009/047).

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