Electrochemical capacitors based on highly porous carbons prepared by KOH activation

Electrochimica Acta 49 (2004) 515–523

Electrochemical capacitors based on highly porous carbons prepared by KOH activation K. Kierzek a , E. Frackowiak b,∗,1 , G. Lota b , G. Gryglewicz a , J. Machnikowski a a

Institute of Chemistry and Technology of Petroleum and Coal, Wrocław University of Technology, Gda´nska 7/9, 50-344 Wrocław, Poland b Institute of Chemistry and Technical Electrochemistry, Pozna´ n University of Technology, Piotrowo 3, 60-965 Pozna´n, Poland Received 13 May 2003; received in revised form 12 August 2003; accepted 27 August 2003

Abstract Various coal and pitch-derived carbonaceous materials were activated for 5 h at 800 ◦ C using potassium hydroxide and 1:4 component ratio. Porosity development of the resultant activated carbons (ACs) was assessed by N2 sorption at 77 K and their capability of the charge accumulation in electric double layer was determined using galvanostatic, voltammetric and impedance spectroscopy techniques. ACs produced from different precursors are all microporous in character but differ in terms of the total pore volume (from 1.05 to 1.61 cm3 g−1 ), BET surface area (from 1900 to 3200 m2 g−1 ) and pore size distribution. Very promising capacitance values, ranging from 200 to 320 F g−1 , have been found for these materials operating in acidic 1 mol l−1 H2 SO4 electrolytic solution. The variations in the electrochemical behavior (charge propagation, self-discharge, frequency response) are considered in relation to the porous texture characteristics, elemental composition but also possible effect of structural ordering due to various precursor materials used. Cycling investigation of all the capacitors has been also performed to compare ability of the charge accumulation for different carbon materials during subsequent cycles. © 2003 Elsevier Ltd. All rights reserved. Keywords: Electrochemical capacitor; Charge/discharge; Activated carbon; Coal tar pitch; KOH activation; Microporosity

1. Introduction The understanding of the complex relationship between porous structure characteristics and the electrochemical behavior is very meaningful for the development of electric double layer capacitors (EDLC), where the charge is accumulated at the electrode/electrolyte interface, mainly by the electrostatic attraction forces. Though the high total surface area is a primary requirement for the electrode material, when a huge charge is to be stored in a reasonably small device, some other aspects of the surface physics and surface chemistry can be critical for the electrode performance [1,2]. Firstly, this is a pore size distribution and a surface wettability that determine the accessibility of the porous system to a given electrolyte. On the other hand, the structural features of the solid phase are responsible for the value of the specific capacitance, i.e. corresponding to the unit surface, ∗ Corresponding author. Tel.: +48-61-665-3632; fax: +48-61-665-2571. E-mail address: [email protected] (E. Frackowiak). 1 ISE Member.

0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.08.026

which is reported to be ca. 20 ␮F cm−2 . Specific capacitance of an activated carbon (AC) is also strongly dependent on the electrolyte characteristics and properties, such as the ions dimensions, the dielectric constant, the solvating power and the viscosity. Capacitance of a carbon-based capacitor results also from conductivity of the carbon-binder composite, blockage of carbon pores by the binder, thickness of the composite, suitable separator, . . . etc. As a result, the charge accumulated in a mass (or volume) unit of a practical porous material varies widely from one activated carbon to another [3]. The proper selection of carbonaceous precursor and activation variable seems to be therefore of primary importance for further EDLC development. The activation of green petroleum cokes with potassium hydroxide was recognized several years ago as an efficient way for producing of microporous activated carbons with the extremely high surface area [4]. More recent studies revealed [5] a distinct effect of the carbonaceous precursor nature and activation variables on the porous texture of KOH-treated solids, which can vary from strongly microporous with a narrow pores to those showing rather wide pore size distribution.

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Recently, a series of porous carbons prepared by activation of mesophase pitch with KOH at various reagent ratio was evaluated as electrode material in electric double layer capacitors [6]. It is reported that the capacitance of the electrodes increases to a maximum of 130 F g−1 with the BET specific surface area, which varies in the range of 1300–2860 m2 g−1 as the KOH to pitch ratio increases from 1.5 to 4.5. An opposite trend is observed when the capacitance is calculated per BET surface area unit. Authors relate this decrease to the enhanced electrode resistance due to deepening of the pores. The present work is focused on the understanding the relationship between the parent material nature and the electrochemical behavior of resultant activated carbons. A series of carbonaceous materials of different origin and thermal history is treated with KOH under relatively severe conditions. The resultant materials are carefully evaluated in terms of porosity development and charge accumulation in the electric double layer.

2. Experimental 2.1. Materials Carbonaceous materials, which are used in this study, include coal, coal semi-coke, pitch mesophase, pitch semicoke and activated carbon. The coal sample (C) was high volatile bituminous coal (hvBb) from Szczyglowice (Poland) mine. The QI-free coal-tar pitch used in the study was prepared on a laboratory scale from the industrial coke oven tar (Makoszowy cokery). The coal semi-coke (CS) and pitch semi-coke (PS) were produced by the heat treatment of corresponding parent materials at 520 ◦ C with heating rate of 5 ◦ C min−1 and 2 h soaking time. The preparation of mesophase sample (PM) comprised the treatment at 450 ◦ C for 7 h with a continuous stirring. All the treatments were performed under argon in a vertical Pyrex retort of 45 mm diameter. The activated carbon used was a commercial A 400 Chemviron product. 2.2. Preparation of activated carbons Carbonaceous precursors were ground to the particle size less than 630 ␮m. A physical mixture of a given precursor and the anhydrous KOH at 1:4 weight ratio was activated at 800 ◦ C for 5 h under argon flow of 15 dm3 h−1 . The treatment was carried out in a nickel boat placed in a 36 mm diameter quartz tube. The heating rate was 60 ◦ C min−1 to 200 ◦ C and then 10 ◦ C min−1 to the final temperature. The resultant materials were washed repeatedly with 10% solution of HCl and distilled water to remove chloride ions and next were dried at 110 ◦ C for 6 h. The activated carbons produced from the listed precursors are designed A-C, A-CS, A-PM, A-PS and A-AC, respectively.

2.3. Chemical and structural characteristics Ash and volatile matter contents were determined according to the respective standard procedures. The optical texture was evaluated using polarized light optical microscopy of polished surface of samples. Elemental composition (CHNS) was analyzed using VarioEl elemental analyzer. Oxygen content was calculated by difference. The particle size distribution of activated carbons produced was determined using Mastersizer 2000 (Malvern Instruments Ltd.). The porous texture was characterized by adsorption/desorption of nitrogen at 77 K using an automatic adsorption unit NOVA2200 (Quantachrome). The pores were classified according to IUPAC recommendation into micropores (<2 nm width), mesopores (2–50 nm width) and macropores (>50 nm width) [7]. The contribution of micropore volume and the mesopore volume distribution according to size was evaluated using Kelvin equation with the Halsey pore diameter correction [8]. The mesopores were divided with respect to width into three categories 2–3 nm (small), 3–5 nm (medium) and 5–50 nm (large). The mean micropore width (LD ) was calculated by applying the Dubinin equation [9] up to relative pressure p/p0 ≤ 0.015. 2.4. Electrochemical characteristics Two-electrode capacitors were built from the KOH activated carbon samples and 1 mol l−1 H2 SO4 solution was an electrolytic solution. The electrodes consist of 85% of carbon, 10% of polyvinylidene fluoride PVDF and 5% of acetylene black. Diameter of electrode is 10 mm, hence, its geometric surface area is 0.785 cm2 . Thickness of electrode is 0.22 mm (±0.02 mm). The mass of electrodes ranges between 10 and 13 mg. The two activated carbon pellets separated by the glassy fibrous paper were directly placed inside the teflon Swagelok® cell without lamination on metallic net. The Swagelok® system is commonly used for testing electrode materials in electrochemical capacitors. It consists of two tightly screwed cylinders that play the role of current collectors for two pellet electrodes. For acidic medium cylinders with golden disks (diameter of 12 mm) were used as current collectors to eliminate some parasitic reactions. Such construction allows good electrical contacts for capacitor assembly and a high reproducibility of results. For electrochemical measurements MacPile-Biologic (France) and ARBIN Instruments (USA) have been used. Voltammetry experiments at the scan rate from 1 to 20 mV s−1 , galvanostatic charge/discharge characteristics in the voltage range from 0 to 0.8 V and impedance spectroscopy from 100 kHz to 1 mHz (AUTOLAB-ECOCHEMIE BV) were used for the estimation of capacitance values. The values were expressed in farads (F) per mass of carbon material.

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3. Results and discussion 3.1. Basic characteristics of precursors and activation products The characteristics of the carbonaceous materials used in the activation are shown in Tables 1 and 2. In terms of activation behavior, the relevant differences among various precursors should be in the mineral matter content, the carbon content (as related to both the chemical composition of parent material and heat treatment temperature) and the structural ordering. The higher carbon content in the precursor the lower burn-off when comparable activation conditions are used (Table 3). The only exception from the trend is the activated carbon. The origin of the enhanced burn-off on the AC activation seems to be of different nature than in the case of “hydrocarbon type” precursors (C, CS, PM, PS). In the latter materials, a considerable part of KOH is spent on the dehydration of parent material which precedes the etching. The high-temperature etching is believed to occur in the latter class of materials to a limited extent due to reduced concentration of active agent (KOH + K2 CO3 ) at high temperature stage of the treatment. High degree of carbonization of AC implies that most of KOH is used for the high-temperature etching reaction. In addition, the developed surface area creates a large interface for the reaction, so contributing to the large burn-off. Table 1 Volatile matter and the optical texture of the precursors Sample

Ash (wt.%)

Volatile matter (wt.%)

Optical texture

C CS PM PS AC

3.1 3.8 0.5 0.3 7.1

31.9 15.7 17.4 6.6 n.d.

Isotropic Isotropic Anisotropic Anisotropic Isotropic

Table 2 Elemental analysis (wt.%, daf basis) of the precursors used Sample

Carbon

Hydrogen

Nitrogen

Sulfur

Oxygen

C CS PM PS AC

86.00 87.88 93.35 93.90 94.00

5.12 3.35 3.74 3.29 0.37

1.65 1.71 1.07 0.94 0.46

0.38 0.32 0.21 0.20 0.90

6.86 6.74 1.63 1.67 4.27

Table 3 Burn-off and ash content of the KOH activated carbons Sample

Burn off (wt.%)

Ash (wt.%)

A-C A-CS A-PM A-PS A-AC

60.2 44.5 39.5 33.6 56.7

2.8 1.1 1.8 2.3 2.4

517

The treatment with KOH of coal and derived materials (including AC) seems to be associated with demineralization, however, a secondary mineral matter is introduced as evidenced by the presence of ash after burning of all activation products. Anisotropic appearance, with predominating flow type texture, of pitch-derived materials proves the superior extent of structural ordering. The produced activated carbons show a wide particle size distribution from 50 to 200 ␮m with a maximum located at about 120 ␮m. This means that a partial disintegration of precursor particles occurs during activation under severe conditions of the study. 3.2. Porosity characterization The isotherms of N2 adsorption at 77 K for the KOH activated carbons are presented in Fig. 1. All the samples give Type I isotherm according to the IUPAC classification [7]. The small slope of the isotherms indicates that the activated carbons are essentially microporous solids. There is a lack of adsorption–desorption hysteresis. The parameters of the porous texture of activated carbons calculated from the presented isotherms are given in Table 4. Fig. 2 shows the mesopore size distribution. The activated carbons from coal (A-C) and coal semi-coke (A-CS) are discriminated among the studied materials by the largest BET surface area. A higher total pore volume VT in the case of A-C (1.61 cm3 g−1 versus 1.45 cm3 g−1 ) correlates with a wider pore size distribution. One should notice both a more developed mesoporosity (Fig. 2) and a bigger size of micropores as indicated by the mean micropore width LD (1.39 nm versus 1.36 nm). The explanation of the most developed porosity in the A-C is a specific constitution of raw coal compared to the heat-treated materials. Small aromatic lamellae, which in fact should be considered as a precursor of the defected graphene layer of activated carbon in that kind of organic material, are very poorly packed due to a number of peripheric groups. The substituents in form of aliphatic and heteroatom functional groups are believed to be preferentially burnt-off during KOH activation. Both the pitch-derived activated carbons (A-PM and A-PS) show quite similar porous texture characteristics in terms of pore volume, surface area and mesopore size distribution. All the parameters are somewhat lower compared to previous materials. The only meaningful difference in the porosity characteristics seems to be the occurring of wider micropores in the mesophase-derived material as suggested by a higher value of LD (1.37 nm versus 1.34 nm). A-AC shows a quite reasonable both the VT and SBET values (1.05 cm3 g−1 and 1900 m2 g−1 , respectively), however, the porosity development is distinctly inferior to the other materials. This is mainly due to a lower micropore volume while that of mesopores is in the similar range as in the A-CS, A-PM and A-PS. The pore size distribution of A-AC can be described as the bimodal, with relatively big

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1200

Volume of N2 (cm3 g-1)

1000

800

600

A-C A-CS A-PM A-PS A-AC

400

200

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-1 0

pp

Fig. 1. Nitrogen adsorption isotherms (77 K) of the KOH activated carbons prepared using different precursors. Table 4 Porosity parameters of the KOH activated carbons

A-C A-CS A-PM A-PS A-AC

SBET (m2 g−1 )

VT (cm3 g−1 )

3150 3190 2660 2750 1900

1.612 1.445 1.209 1.227 1.051

Pore volume distribution (cm3 g−1 ) V<2 nm

V2–3 nm

V3–5 nm

V5–50 nm

1.189 1.184 1.011 1.048 0.721

0.242 0.200 0.132 0.116 0.129

0.099 0.038 0.040 0.033 0.087

0.082 0.023 0.026 0.030 0.114

Micropore size (LD ) (nm)

Average pore size (nm)

1.39 1.36 1.37 1.34 1.29

2.04 1.82 1.82 1.79 2.21

„ST03-212 1st Resubmitted Version”

100

<2 nm 2-3 nm 3-5 nm 5-50 nm

80

Contribution (%)

Sample

60

40

20

0 A-C

A-CS

A-PM

A-PS

A-AC

Sample Fig. 2. Pore size distribution of the KOH activated carbons prepared using different precursors.

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Table 5 Capacitance values (F g−1 ) of the KOH activated carbons (A-C; A-CS; A-PM; A-PS; A-AC) estimated by galvanostatic discharge at 2 mA, cyclic voltammetry at 2 mV s−1 scan rate and impedance spectroscopy at 1 mHz Sample

Galvanostatic discharge (F−1 g−1 )

Cyclic voltammetry (F−1 g−1 )

Impedance spectroscopy (F−1 g−1 )

Specific capacitance (␮F cm−2 )

A-C A-CS A-PM A-PS A-AC

312 223 294 261 198

317 235 299 241 198

282 206 273 227 193

9.9 7.0 11.0 9.5 10.4

Specific capacitance ␮F cm−2 calculated per surface area of carbon.

contribution of narrow micropores (low LD ) and large mesopores (see Fig. 2). This characteristic seems to result from the overlapping the original pore system with that created during KOH activation. One can expect that due to a higher temperature of the preceding heat treatment the AC is more resistive to the KOH activation so leaving narrower micropores, however, this view is in contrast with large burn-off. As we can see, total pore volume, mean micropore width and mesopore size distribution are different, in most cases, and they are dependent on the precursor used, but all produced active carbons are essentially microporous. KOH activation can develop highly microporous materials with pore volume about 1.6 cm3 g−1 and BET surface area over 3000 m2 g−1 . 3.3. Electrochemical properties The reproducible results of capacitance measurements obtained from three methods (galvanostatic discharge, cyclic voltammetry, impedance spectroscopy at 1 mHz) and the calculated values of the specific capacitance per surface area are given in Table 5. Capacitance value expressed in Farads (F) is presented by the following formula: C=

dq dE

or

C=I

dt dE

(1)

For the calculation of specific capacitance (F g−1 ) from cyclic voltammetry measurements, the average capacitive current (mA) is taken and divided by a scan rate (mV s−1 ). Taking into account the two-electrode system a capacitance of single electrode is C1 = 2 Ccell /mass of one electrode, i.e., electrode with a smaller mass. Capacitance can be related to the mass of electrode or to the mass of active material that is 85 wt.% of the total pellet. In our case, the capacitance is always expressed in Farad per active carbon material. In galvanostatic experiments, the values of imposed current (mA) are multiplied by the discharge time (s). Such obtained charge (q) is divided by the range of voltage (V) in which capacitor is discharged. Due to the two-electrode system the result is treated as above, i.e. multiplied by two and divided by a mass of smaller electrode. For impedance measurements, the capacitance value inserted in Table 5 are calculated for frequency equal to

1 mHz using the following formula: 1 C= 2πfZ

(2)

All the prepared carbons show very satisfactory capability of charge accumulation in the electric double layer. The extremely high capacitance (ca. 300 F g−1 ) is measured for coal and mesophase-derived activated carbons (A-C and A-PM). The excellent performance of A-C correlates with the highest total pore volume (1.6 cm3 g−1 ) and surface area (3150 m2 g−1 ) as well as with a relatively large size of pores. When taking into account a noticeably less developed porosity of A-PM (2660 m2 g−1 ), the reason of a comparable to A-C capacitance can be the intrinsic surface properties. The anisotropic texture of mesophase suggests a higher structural ordering of the solid phase of resultant activated carbon, which results in an improved electric conductivity. This view is consistent with the highest specific capacitance (11.0 ␮F cm−2 ) of the sample. The present study does not supply a reasonable explanation for the clearly inferior performance of A-PS compared to A-PM even if these two carbons are almost identical from microtextural point of view. The only difference which should be noticed is a lower average micropore size LD of A-PS. Such analysis suggests that not only porous parameters decide about values of capacitance but also conducting properties, affinity to electrolytic solution, wettability, hydrophobic/hydrophilic character and other intrinsic properties of carbon. In the case of A-AC, the lowest value of the capacitance well correlates with a less developed surface area. It should be however noticed a relatively high specific capacitance (10.4 ␮F cm−2 ) as a result of more suitable, in terms of the double layer formation, pore size distribution. It fits well with the fact that only electrochemically available electrode/electrolyte interface forms an electrical double layer (EDL), hence the close micropores or pores not adapted to the size of solvated anions and cations will not take part in EDL charging. Our results confirm a well-known statement that there is no direct proportional dependence between surface area of carbon and capacitance measured [10,11]. Optimal pore size distribution of carbon, taking into account the type of electrolyte used, can assure a good capacitor performance. Very careful correlation of physicochemical characteristics of carbon materials and capacitance values obtained from a few electrochemical techniques can

K. Kierzek et al. / Electrochimica Acta 49 (2004) 515–523

give helpful information about a suitable choice of material for capacitor electrode. The activated carbons of our study show clearly superior performance as electrode material compared to the mesophase pitch-derived carbon of similar porosity characteristics (pore volume 1.63 cm3 g−1 , surface area 2860 m2 g−1 ) reported in [6]. The reasons of the discrepancy seem to lie in different both the size of carbon particles and the construction of electrode. One can speculate that using a fine powder (<20 ␮m) [6] leads to an inferior conductivity of electrode due to more frequent interparticle boundaries and larger consumption of non-conducting binder. Moreover, the binder can contribute to the pore entrance blockage to a greater extent, thus reducing the porosity of electrode composite. In our case, greater particle sizes of carbon (average 120 ␮m) and moderate amount of binder (10 wt.%) are at the origin of better conductivity of electrode and higher capacitance values than at the previous work [6]. We also proved that composition of our electrodes preserves a high porosity of electrode material (only little blockage of pores) because after making pellets a porosity of carbons decreases only ca. 20%. The highly developed surface area of KOH activated carbons with a suitable particle size seems to be a primary reason of the exceptionally high capability for charge accumulation in electric double layer of such carbons. The capacitance of ca. 300 F per gram of active material can be reached in this type of carbon despite limited accessibility of the surface area, as indicated by relatively low specific capacitance calculated per unit of the BET surface area. For comparison, the conventional steam activation seems to give carbons of inferior capacitance values. Using exactly the same technique of capacitance measurements for a series of steam activated carbons, the maximum capacitance we could obtain was about 160 F g−1 at a specific capacitance of about 16 ␮F cm−2 [12]. The sample used was an activated carbon of relatively high surface area (1200 m2 g−1 ) and mesopore contribution of about 37%.

160 140 120

Z'' / ohm

520

100 80 60 40 20 0 -20 0

40

80

120

160

Z' / ohm Fig. 4. Impedance spectroscopy measurements for the carbon sample A-PM. Mass of electrodes: 12.2/12.8 mg. C = 273 F g−1 (at 1 mHz).

Galvanostatic discharge is the most accurate method for capacitance evaluation. Fig. 3 shows a representative example of the galvanostatic charge/discharge characteristics (I = 2 mA) for capacitor built from activated carbon A-PM. Typical triangular shape confirms good capacitive properties of this material, however, a meaningful ohmic drop (0.25 cm−2 ) with some moderate diffusion polarization in the micropores is observed during switching time. Similar characteristics differing only by the discharge time were observed for all the investigated carbons. A good capacitive behavior has been also confirmed by the impedance spectroscopy measurements presented in the form of Nyquist plot (Fig. 4). Very correct perpendicular dependence of the imaginary part of impedance versus the real part was found for all the investigated carbons. The value of capacitance at 1 mHz was calculated and compared with the results from other electrochemical techniques (Table 5). Generally, the impedance measurements supply slightly lower values due to the fact that alternating current penetrates into the electrode bulk with some hindrance. Very useful information can be also obtained from the dependence of capacitance versus frequency f (Hz). In this case, the different values of

0.9 0.8 0.7 0.6

E/V

0.5 0.4 0.3 0.2 0.1 0 -0.1 0

1000

2000

3000

4000

5000

6000

7000

t/s Fig. 3. Galvanostatic charge/discharge characteristics of capacitor built from KOH activated carbon A-PM (mass of electrodes 12.2/12.8 mg) I = 2 mA.

K. Kierzek et al. / Electrochimica Acta 49 (2004) 515–523

521

300 250

A-PM A-C A-AC A-PS A-CS

C/Fg

-1

200 150 100 50 0

-50 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

f / Hz Fig. 5. Capacitance values of the KOH activated carbons as a function of frequency during impedance spectroscopy measurements. Electrolyte: 1 mol l−1 H2 SO4 .

frequencies from 1 mHz to 100 kHz are taken for calculation using Eq. (2). It indicates until which frequency the energy of capacitor can be withdrawn (Fig. 5). It looks that for carbons A-PM and A-C the capacitance values at 100 mHz still maintain 200 F g−1 and at 1 Hz these carbons can supply up to 100 F g−1 that is interesting from practical point of view. Another parameter that decides about capacitor utility is the time constant (RC). These values were calculated from the low frequency value of C and the high frequency value of the equivalent series resistance (ESR). Estimated values of RC for all the capacitors were ranging from 2 to 3 s. It proves that quick charge propagation takes place during performance of capacitors built from such highly porous carbons. The results of voltammetry experiments at scan rates from 1 to 20 mV s−1 , presented for better legibility as a function of C = f(E), confirm it as well and an example of such behavior for capacitor built from A-PM carbon is

shown in Fig. 6. The observed decrease of the capacitance from 300 to 200 F g−1 that is connected with some diffusion limitation in the highly microporous carbons proves that for high current loads or quick scan rates still the mesoporosity of carbon materials is not sufficient. Durability of capacitor is one of the most important feature of capacitor for practical application, hence, all the capacitors were cycled in galvanostatic regime of 165 mA g−1 . The results are shown in Fig. 7, and in the conclusion, the carbons A-AC, A-PS and A-PM present a good cycleability. On the other hand, carbons A-CS and A-C pointed out a capacitance loss of 100 F g−1 after 2500 cycles. The loss of capacity during cycling is essentially connected with the values of self-discharge and leakage currents for capacitor. Such parameters for carbon A-PM are presented in Fig. 8. The values of leakage current for this carbon is only 4.0 mA g −1 whereas self-discharge is ca. 50% after 20 h. Contrarily,

600 500 400

C / F g-1

300

1 mV/s 2 mV/s 5 mV/s 10 mV/s 20 mV/s

200 100 0 -100 -200 -300 -400 0

0.2

0.4

0.6

0.8

E /V Fig. 6. Voltammetry characteristics of a capacitor built from the KOH activated carbon (A-PM) at different scan rates of voltage. Electrolytic solution: 1 mol l−1 H2 SO4 .

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K. Kierzek et al. / Electrochimica Acta 49 (2004) 515–523 350 300

C / F g -1

250 200 A-PM A-AC A-PS A-C S A-C

150 100 50 0 0

250

500

750

1000 1250 1500 1750 2000 2250 2500

cycle Fig. 7. Cycleability of capacitors built from the KOH activated carbons at current load of 165 mA/g.

1.2

2.5 E/V

1.0

2 I / mA 1.5

0.6 1

I / mA

E/V

0.8

0.4 0.5

0.2 0.0

0 0

5

10

15

20

25

t/h Fig. 8. Leakage current and self-discharge of the capacitor built from A-PM carbon.

two examples of carbons A-CS and A-C that are not optimal for durability have higher leakage currents of 8.2 and 12.8 mA g−1 , respectively. Self-discharge reaches 80% after 20 h for these carbons. It is clear that not only the high values of capacitance are sufficient for practical use but also stable capacitance with cycling. The inferior performance of A-C and A-CS in terms of enhanced capacity loss, leakage current and self-discharge can result from the presence of numerous oxygen groups created on activation. This type of functionalities can be slowly reduced during cycling in a non-reversible way, via redox reactions. In conclusion, the prepared KOH activated carbons give the excellent capacitance values over 300 F g−1 despite rather moderate specific capacitance per surface area, being in the range of 7–11 ␮F cm−2 . Apparently, this can be considered as an effect of the restrictions in the accessibility of the micropore surface area in the case of the KOH-activated materials with extremely developed porosity. It is noteworthy to underline that a world-wide known microporous carbon PX21 investigated in the same conditions, gave only

ca. 240 F g−1 with specific capacitance of 8 ␮F cm−2 . It seems that carbon A-PM is a definitively better candidate for capacitor electrode. The detailed analysis proved that wide micropores and narrow mesopores play a crucial role for ions transportation during charging of electrical double layer of carbon with extremely developed surface area. Apart from the pore structure, specific capacitance of carbons has been also affected by conductivity parameters of electrodes determined by the particle size and composition of electrodes. For a long-term capacitor performance, the carbons with a low leakage current, i.e. without surface functionality should be selected to fulfill a practical demand. Additionally, a low cost of carbons based on natural precursors makes them extremely attractive materials for capacitor application.

Acknowledgements This work was supported by the State Committee for the Scientific Research in Poland (Project No. T 09B 01619).

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