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Influence of oxygen treatment on electric double-layer capacitance of activated carbon fabrics
Carbon 40 (2002) 667–674
Influence of oxygen treatment on electric double-layer capacitance of activated carbon fabrics Chien-To Hsieh, Hsisheng Teng* Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Received 20 December 2000; accepted 30 April 2001
Abstract Oxygen treatment at 2508C on polyacrylonitrile-based activated carbon fabric was conducted to explore the influence of carbon–oxygen complexes on the performance of capacitors fabricated with the carbon fabric. Surface analysis showed that most of the oxygen functional groups created from the oxygen treatment were the carbonyl or quinone type. The performance of the capacitors was tested in 1 M H 2 SO 4 , using potential sweep cyclic voltammetry and constant current charge–discharge cycling. It was found that the Faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Due to the treatment the overall specific capacitance showed an increase up to 25% (e.g., from 120 to 150 F g 21 at a current density of 0.5 mA cm 22 ). However, the distributed capacitance effect, the inner resistance and the leakage current were found to increase with the extent of oxidation. It is suggested that due to the local changes of charge density and the increase in redox activity the presence of the carbonyl- or quinone-type functional groups may induce double-layer formation, Faradaic current, surface polarity, and electrolyte decomposition. 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; B. Oxidation; C. Electrochemical analysis; D. Electrochemical properties, Functional groups
1. Introduction Carbons of different forms have been extensively used in a variety of electrochemical systems [1], such as batteries and electric double-layer capacitors (EDLCs). Previous studies have pointed out that carbon fibers from polyacrylonitrile (PAN) precursor provide good electrical properties and high strength [1,2]. Recently, interest in PAN-based activated carbon fabrics for the use as porous electrodes in EDLCs has emerged. One of the advantages of carbon fabrics over fibers or powders is that there is no binding material required in the fabrication of electrodes. The cost of binding materials, such as polytetrafluoroethylene or polyvinylidenefluoride, for electrode preparation is usually high, and the binders may block some entrances of the pores in the porous carbons, hence resulting in diminishment of double-layer capacitance. It is well known that the pore structures of carbon electrodes affect the performance of the resulting *Corresponding author. Tel.: 1886-6-2385-371; fax: 1886-62344-496. E-mail address: [email protected] (H. Teng).
capacitors [3–7]. The effects of the specific surface area, as well as the pore size distribution, of the carbon electrodes has been discussed. Apart from the physical structures, the chemical characteristics, such as the distribution of heteroatoms on carbon surface, would also influence the capacitor performance. The carbon–oxygen complexes, i.e., the oxygen functional groups, in carbons have been reported to be the important surface groups affecting the wettability, chemical reactivity, and electrical properties [1,8,9]. How these complexes can affect the performance of the resulting EDLCs was rarely reported in the literature. Oxygen functional groups can be formed on carbons by treatments such as electrochemical oxidation [2], cold plasma treatment [10], chemical oxidation in HNO 3 or H 2 SO 4 solutions [9,11], and gaseous oxidation with oxidizing gases [12]. In examining the effects of oxygen functional groups, employing gaseous oxidation to create the functional groups has the advantages such as convenience and no significant impact on physical characteristics of the carbons. Without the intrusion of pore structure change, the effects of oxygen functional groups can be exclusively discussed.
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00182-8
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Within the above scope the performance of EDLCs fabricated using oxidized PAN-based carbon fabrics was examined. Gaseous oxidation using oxygen as the oxidizing agent was employed in the present work. The chemical composition of the resulting carbon–oxygen complexes was analyzed, in the attempt to elucidate the effects of different oxygen functional groups during the course of double layer formation at the electrolyte–carbon interface.
2. Experimental
2.1. Oxidation of activated carbon fabrics PAN-based activated carbon fabric supplied by Taiwan Carbon Technology, Taiwan, was used to fabricate the electrodes of EDLCs. The fabric has a thickness of 0.4–0.6 mm. The diameter of the carbon fibers in the fabric is ca. 15 mm. Oxidation was carried out with thermal treatment at 2508C under an oxygen atmosphere. Because of the negligible extent of carbon gasification at this treatment temperature, weight gain due to oxygen chemisorption, rather than weight loss, was observed in the oxidation. The treatment was conducted for different lengths of time to prepare samples containing different populations of oxygen functional groups on the surface.
2.2. Characterization of carbon fabrics Physical characteristics of the carbon fabrics were determined by N 2 adsorption at 21968C, using an automated adsorption apparatus (Micromeritics, ASAP 2010). Surface areas and micropore volumes of the samples were evaluated with the application of the BET and Dubinin– Radushkevich (D–R) equations, respectively. The amount of N 2 adsorbed at relative pressure near unity was employed to determine the total pore volume, which corresponds to the sum of the micropore and mesopore volumes [13]. The average pore diameter was estimated according to the surface area and total pore volume, assuming that the pores are parallel and cylindrical. A temperature programmed desorption (TPD) technique was employed to analyze the population of carbon–oxygen complexes on the carbon fabrics. The experiments were
carried out under a helium flow, by heating 0.05 g of sample from room temperature to 9008C with a linear rate of 308C min 21 . The evolution of CO and CO 2 during TPD was continuously monitored with non-dispersive infrared analyzers. The chemical composition of the oxygen functional groups was also studied by X-ray photoelectron spectroscopy (XPS). The XP spectra were recorded with a Fisons VG ESCA210 spectrometer and Mg K a radiation. The spectra were smoothed and a non-linear background was subtracted. The deconvolution of the spectra was performed using a non-linear least-squares fitting program with a symmetric Gaussian function. The surface composition of the samples was calculated with C 1s and O 1s peaks [14] and appropriate sensitivity factors.
2.3. Electrochemical measurements Two electrode cells were used to examine the electrochemical performance of the carbon fabrics. The electrodes consisted of 2 cm 2 carbon fabrics and stainless steel foils serving as the current collector. The cells were constructed with two facing carbon electrodes, sandwiching a piece of filter paper as separator. All electrochemical measurements were performed at ambient temperature, by soaking the cells in an electrolyte solution of 1 M H 2 SO 4 . Cyclic voltammetric measurements of the cells were made in the potential range of 20.6 to 0.6 V applied between the two electrodes. The potential scan rate ranged from 0.5 to 5 mV s 21 . The capacitance of the cells was measured by charging the capacitors to 0.6 V, followed by discharging to 0 V at constant currents of 0.5, 1, 2, and 5 mA. The leakage current test was performed at a floating potential of 0.6 V for more than 12 h.
3. Results and discussion
3.1. Surface characteristics of carbon fabrics The physical characteristics of the carbon fabrics are given in Table 1. The original carbon fabric is designated as CF and the oxidized fabrics are designated as CFO1, CFO2 and CFO3, which were derived from oxidizing CF for 0.5, 1 and 6 h, respectively. The pore size distribution
Table 1 Physical characteristics of the carbon fabrics used as the electrodes for electrochemical measurements Carbon type
Oxidation time (h)
BET surface area (m 2 g 21 )
Pore volume (cm 3 g 21 )
CF CFO1 CFO2 CFO3
0 0.5 1 6
1290 1280 1270 1190
0.62 0.61 0.61 0.57
Pore size distribution Micro (%)
Meso (%)
100 100 100 100
0 0 0 0
Average pore diameter (nm) 1.93 1.92 1.92 1.92
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shows that these carbons are highly microporous. The surface area and pore volume decrease slightly with the extent of oxygen treatment. This indicates that introduction of oxygen functional groups in micropores has hindered the access of N 2 to some of these pores. Although physical characteristics of carbons, including the degree of order [15] and the exposed area [6], have been shown to affect the performance of the resulting capacitors, their influence will be considered to be minor in the present work since the degree of order cannot be altered at 2508C and the changes in surface area and pore size distribution caused by the oxygen treatment are, as a matter of fact, not notable. The population of oxygen functional groups built up during the oxygen treatment has been evaluated using TPD. Past studies have reported that during the course of TPD oxygen functional groups such as carboxyl, anhydride and lactone groups would evolve as CO 2 while hydroxyl, carbonyl and quinone groups would evolve as CO [1,16,17]. The accumulated amounts of CO and CO 2 evolution during TPD are given in Table 2. The results show that the amount of CO 2 evolved from these carbon fabrics increases slightly with the extent of treatment, while the amount of CO evolved was significantly enhanced upon oxidation. The increase in total oxygen evolution from the fabrics with the extent of oxidation can be attributed mainly to the formation of the CO-desorbing complexes, which are essentially basic in chemical nature [8]. XPS was employed for further analysis of the composition of the oxygen functionalities on the carbon fabrics. The C 1s and O 1s peaks of the scan spectra have binding energies of ca. 284.6 and 533.5 eV, respectively. Quantitative analysis was carried out to determine the surface C and O concentrations, and the (O 1s) /(C 1s) atomic ratios are listed in Table 3, showing that the ratio increases with the extent of oxidation. The trend of (O 1s) /(C 1s) varying with the oxidation level is similar to that of total oxygen evolution in TPD shown in Table 2. The broad C 1s peak ranging from 280 to 292 eV in the XP spectra may comprise peaks contributed by several carbonbased functional groups that have different binding energies. These binding energy peaks have been identified as C
Table 2 Accumulated amounts and percentages of CO and CO 2 evolution from the carbon fabrics during temperature programmed desorption Carbon type CF CFO1 CFO2 CFO3
CO evolution mmol g 0.23 0.41 0.58 0.81
21
CO 2 evolution %
mmol g
58 68 74 76
0.17 0.19 0.20 0.25
21
% 42 32 26 24
Total O evolution (mmol g 21 ) 0.57 0.79 0.98 1.31
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Table 3 (O 1s) /(C 1s) atomic ratio and distribution of oxygen functional groups of carbon fabrics determined from deconvolution of XP C 1s spectra Carbon type
(O 1s) /(C 1s)
CF CFO1 CFO2 CFO3
0.064 0.086 0.11 0.13
Functional group distribution C–O (%)
C=O (%)
O–C=O (%)
48 40 30 24
14 28 41 51
38 32 29 25
or C–H at 284.6 eV, C–O at 286.7 eV, C=O at 288.4 eV and O–C=O at 289.7 eV [18–20]. The C 1s peak of each carbon can be deconvoluted using a peak synthesis procedure in which Gaussian peak shape was assumed to fit each component with a fixed binding energy and an average full width half maximum value of 2.0 eV. The distribution of oxygen-containing groups of each carbon was thus determined from deconvolution of XP C 1s spectra, and the results are shown in Table 3. It can be seen that the percentage of O–C=O determined from XPS is similar to that of CO 2 -desorbing complexes shown in Table 2. Both the percentages of C–O and O–C=O decrease with the oxidation level, and the increase in oxygen content upon oxidation is mainly contributed by the formation of C=O, which would evolve as CO upon thermal treatment. The XPS and TPD results of carbon– oxygen complex analysis are qualitatively consistent with each other.
3.2. Electrochemical performance of the resulting capacitors The cyclic voltammetric tests on the two-electrode capacitors were conducted within a potential range of 20.6 to 0.6 V at different potential sweep rates. Figs. 1 and 2 shows the typical voltammograms of the capacitors fabricated with different carbon fabrics. The figures demonstrate that the electrodes are stable in the acid solution within the potential range employed. Faradaic current exhibiting as abrupt current increase was not observed in the voltammogram for the CF capacitor. The intensity of Faradaic current was found to increase with the increasing oxygen extent of the electrodes, as inferred from the gradual increase of anodic and cathodic currents at ca. 0.2 and 20.2 V, respectively, with the increasing extent of oxygen treatment. The voltammograms also exhibit that the induced current is an increasing function of the oxidation level, indicating the increase of capacitance upon oxidation. Rectangular voltammograms, in which current quickly reaches a truly horizontal value after reversal of the potential sweep, were observed for the capacitor made of CF. However, the delay for the current to reach a horizon-
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Fig. 1. Cyclic voltammograms of (a) CF and (b) CFO1 electrode (2 cm 2 ) in 1 M H 2 SO 4 at different potential sweep rates.
Fig. 2. Cyclic voltammograms of (a) CFO2 and (b) CFO3 electrode (2 cm 2 ) in 1 M H 2 SO 4 at different potential sweep rates.
tal value near the reversal of the potential sweep and the increase of slope in the horizontal region were enhanced with the extent of oxidation. It is very likely that the presence of surface oxides may retard the motion of electrolytes and thus increase the ohmic resistance of electrolytes along the axial direction of micropores, which would combine with the existence of the distributed capacitance to cause the delay of current inversion [21– 23]. The retarded electrolytic motion due to oxidation can be partially supported by the observation that the accessibility of the micropores to N 2 was hindered by the introduction of the oxides, which results in the decreased N 2 surface area and pore volume. In addition, surface oxides are strong polar sites that would adsorb water molecules and thus hinder the migration of electrolytes in pores. Apart from the distributed capacitance effects, Faradaic current may also hinder current inversion [22]. Since the voltammograms of the oxidized electrodes show the presence of Faradaic currents, the surface oxides created from the oxygen treatment may be responsible for inducing chemical interactions (such as redox reactions) between the carbon fabric and the electrolyte [1], which will be discussed later in this paper. Figs. 1 and 2 also show that both the anodic and cathodic charging currents, Ia and Ic , respectively, are increasing functions of the potential sweep rate in the cyclic voltammetric measurements. The current plateaus of Ia and Ic are parallel and the difference, DI (5Ia 2Ic ), was found to be relatively invariant over the majority of the potential range employed. The values of DI of different capacitors obtained with different sweep rates are shown in Fig. 3. DI is seen to be an increasing function of the oxidation level, indicating an increase of capacitance upon oxidation. The double-layer capacitance, Cdl , of the electrodes can be estimated from the slope of the linear relationship in Fig. 3 [24]. The calculated data are listed in Table 4, showing that the capacitance increases slightly with the oxidation level. The intercept of the linear plots of DI against potential sweep rate represents the Faradaic current [24], IF . The values of IF of these electrodes are also compared in Table 4. It can be seen that IF was significantly promoted through the introduction of oxygen functional groups (from ca. 0 mA g 21 for CF to ca. 20 mA g 21 for CFO3). Both Cdl and IF contribute to the total capacitance. Faradaic current, the contributor of pseudocapacitance, may derive from some carbon–electrolyte interactions, such as electrosorption and redox reactions that are essentially reversible upon potential change [25]. Data in Table 4 show that the increase in double-layer capacitance (Cdl ) due to oxidation is not obvious, while the chemical interactions at the solid–liquid interface were significantly enhanced with oxidation. It has been claimed that the presence of Faradaic current would make it difficult for the current to reach a truly horizontal value in cyclic voltammetric tests [22]. This is in agreement with the observation that deviation from rectangular shape of
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Fig. 3. Variation of DI (difference between anodic and cathodic charging currents) with potential sweep rate in cyclic voltammetric measurements using CF (n), CFO1 (1), CFO2 (,) and CFO3 (h) electrodes (2 cm 2 ).
voltammograms was enhanced with the extent of carbon oxidation that has been found to induce the Faradaic current. To illustrate the influence of oxidation on the performance of constant-current charge and discharge, Fig. 4 shows the potential against time curves of the CF and CFO3 capacitors charged and discharged at a constant current of 0.5 mA. The CFO3 capacitor has a higher capacitance for both charge and discharge, but possesses a larger ‘IR drop’ at the beginning of discharge. Based on the results of charge–discharge cycling, the specific discharge capacitance of a single electrode in the capacitors can be calculated according to:
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Fig. 4. Comparison of charge–discharge curves of CF and CFO3 electrodes (2 cm 2 ) at 0.5 m.
equivalent single-electrode capacitors in series. The capacitances of different electrodes obtained with different discharge currents are shown in Fig. 5. The capacitance decreases with discharge current, suggesting that the potential difference between the mouth and the bottom of the micropores increases with the current due to ohmic resistance of the electrolyte in the axial direction of micropores. As for the effect of oxidation, the specific capacitance was found to increase with the extent of
C 5 (2It) /(WDE) where I is the discharge current, t the discharge time, W the carbon fabric mass on an electrode and DE the potential difference in discharge, excluding the portion of IR drop. The factor of 2 comes from the fact that the total capacitance measured from the test cells is the sum of two Table 4 Double-layer capacitance and Faradaic current of the carbon electrodes determined from cyclic voltammograms Carbon type
Cdl (F g 21 )
IF (mA g 21 )
CF CFO1 CFO2 CFO3
121 123 123 130
0.082 5.7 7.5 22
Fig. 5. Variation of specific discharge capacitance with discharge current for different electrodes (2 cm 2 ) charged at 0.5 mA to 0.6 V.
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oxidation, which is in agreement with the results from cyclic voltammetric measurements. A previous study has also reported an increase of capacitance resulting from electrochemical oxidation of carbon electrodes [26]. Comparing the capacitances of CF and CFO3 shows that there is ca. 25% capacitance increase achieved through introduction of surface oxides (e.g., an increase from 120 to 150 F g 21 at a current of 1 mA, or a current density of 0.5 mA cm 22 ). In an ideal formation of electric double layer with acids serving as the electrolyte, the equilibrium reaction at the negative electrode during charging is simply presented as: . Cx 1 H 1 5 . Cx / / H 1 where .C x represents the carbon electrode surface, H 1 the proton of the acidic electrolyte, and .C x / / H 1 the double layer where the charges are accumulated on the two sides of the double layer. The process is due to a simple physical adsorption, by electrostatic forces between the carbon surface and the proton. Basically, the numbers of ions involved in building the double layer match the charge density developed on the electrodes. The data of the present work suggest that, as the extent of oxidation increases, surface oxides would provide more available sites for proton adsorption in the micropores of the carbon fabrics. The preceding analysis of oxygen functional groups has shown that oxidized carbons contain a significant amount of carbonyl- or quinone-type groups, i.e., C=O, on the surface. With the presence of these groups, an equilibrium reaction may occur in carbon electrode [1]: . Cx O 1 H 1 5 . Cx O / / H 1
polarity, i.e., a surface electrostatic field, to carbons [1,25,27,28]. The interaction of the electrostatic field at the surface with the dipole moment of water molecules plays an important role in determining the wettability of carbons in aqueous solutions. It is generally recognized that the micropores of carbons cannot be fully wetted in aqueous solutions, and thus are not fully accessible to electrolyte or hydrated molecules in the liquid phase. The increase in oxygen content of the carbon fabric may improve the wettability of the internal structure of micropores in the carbon electrodes, thus resulting in a corresponding increase in specific capacitance. A sudden potential drop at the very beginning of the constant current discharge is usually observed for EDLCs, and this drop has been designated as the IR drop. This potential drop has been attributed to the resistance of electrolytes and the inner resistance of ion migration in carbon micropores [29]. The latter usually has a higher contribution to the whole IR drop. Assuming that the inner resistance is one of the intrinsic characteristics of the carbon electrodes, the potential drop measured should be proportional to the discharge current. Fig. 6 shows the variation of the IR drop with the discharge current for the different electrodes. It can be seen that the potential drop increases linearly with the current, and the slope of this linear relationship was used to estimate the inner resistance of the electrodes. The resistance is an increasing function of the oxidation level, with resistances of 22, 30, 35 and 57 V for CF, CFO1, CFO2 and CFO3, respectively, in agreement with the results using electrochemical oxidation [26]. This result suggests that the imparted polarity may hinder the motion of ionic species. The increase in the resistance does not play a role in affecting the specific
where .C x O / / H 1 represents a proton adsorbed by a carbonyl or quinone-type site, basically induced by iondipole attraction. This specific adsorption process, which is different from the formation of .C x / / H 1 on non-specific sites through dispersion interactions [12,15], may facilitate an excess specific double layer capacitance due to the local changes of electronic charge density. It is thus suggested that the increase of capacitance upon oxidation arises partially from the enhanced dipole affinity towards protons in an acid solution. In charging the negative electrode, a strong bond may also form between quinone-type groups and protons due to electron transfer across the double layer: . C x O 1 H 1 1 e 2 5 . C x OH where .C x OH represents a hydroquinone-type complex and e 2 an electron. The reaction would proceed backwards during discharge. This type of functional groups, which would provide redox activity, may be responsible for IF and thus the pseudocapacitance component of the overall capacitance of these oxidized carbon electrodes [25]. In addition, oxygen functional groups impart a surface
Fig. 6. Variation of IR drop with discharge current for electrodes (2 cm 2 ) charged at 0.5 mA to 0.6 V.
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capacitance during the constant-current charge–discharge cycling, since the IR drop has been excluded in capacitance calculation. However, increasing the resistance would result in a higher potential drop and thus a reduction of specific capacity (in units of mA h g 21 ) of the electrodes, especially at a high current density. The leakage currents of CF and CFO3 capacitors charged at 1 mA were measured at 0.6 V floating as shown in Fig. 7. The current decreased gradually with time from 1 mA to around 1310 22 mA in 12 h. Sample CFO3 has a higher leakage current than CF throughout the entire period of measurement. The steady-state currents determined after 12 h of charge were 1.1310 22 and 9.4310 23 mA for CF and CFO3, respectively. The steady-state current may be ascribed mainly to electronic current that results from irreversible decomposition of electrolyte components. It should be noted that this electronic current is not included in IF that derives from the reversible interactions between carbon and electrolyte. The increase of leakage current suggests that the oxygen functional groups may serve as active sites that catalyze the the electrochemical oxidation or reduction of carbon or the decomposition of electrolyte components [1,30]. The stability of the prepared capacitors can be examined by conducting repeated charge–discharge cycling. A capacitor equipped with CFO3 electrodes was charged and discharged between 0 and 0.6 V at 1 mA to confirm the stability. The coulombic efficiency (h ) of an electric double-layer capacitor can be calculated from the equation [31]:
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Fig. 8. Variation of capacitance stability and coulombic efficiency with cycle number for CFO3 electrode (2 cm 2 ) charged and discharged at 1 mA in 1 M H 2 SO 4 .
where t D and t C are the times required for discharge and charge, respectively. The variations of discharge capacitance and coulombic efficiency with cycle number are shown in Fig. 8. The results exhibit that the capacitor has stable capacitance (about 150 F g 21 ) and coulombic efficiency (about 99.5%) over 100 cycles.
4. Conclusions
h 5 (t D /t C ) 3 100
Fig. 7. Variation of leakage current with time for CF and CFO3 electrodes at a floating potential of 0.6 V.
The present work has demonstrated that the capacitance of PAN-based carbon fabric in H 2 SO 4 can be enhanced with oxygen treatment at 2508C. The results of N 2 adsorption showed that the oxygen treatment has little influence on the physical characteristics of the carbon. Surface analysis using TPD and XPS showed that the amount of surface oxides increased with the extent of oxygen treatment and the increase was contributed mainly by the formation of carbonyl- or quinone-type groups that normally desorb as CO during TPD. Cyclic voltammetric study on the resulting electrodes showed that the Faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Constant current charge–discharge cycling reflected the importance of the distributed capacitance effects of these porous electrodes, which was inferred from the decrease in capacitance with the discharge current. Due to the treatment with oxygen, the overall specific discharge capacitance was found to have an increase up to 25% (e.g., from 120 to 150 F g 21 at a current density of 0.5 mA cm 22 ), while both the resistance determined from the IR drop and the leakage current increased with the extent of oxidation. It is suggested that increasing the population of carbonyl-
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or quinone-type surface groups through oxygen treatment is beneficial for capacitance promotion, but the advantage can be offset with the coupling increases in inner resistance and leakage current. The capacitors prepared in the present work exhibit excellent capacitance stability with a coulombic efficiency of 99.5% over 100 cycles.
Acknowledgements Financial support from the National Science Council of Taiwan is gratefully acknowledged. The project number is NSC 89-2214-E-006-041.
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Influence of oxygen treatment on electric double-layer capacitance of activated carbon fabrics Chien-To Hsieh, Hsisheng Teng* Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Received 20 December 2000; accepted 30 April 2001
Abstract Oxygen treatment at 2508C on polyacrylonitrile-based activated carbon fabric was conducted to explore the influence of carbon–oxygen complexes on the performance of capacitors fabricated with the carbon fabric. Surface analysis showed that most of the oxygen functional groups created from the oxygen treatment were the carbonyl or quinone type. The performance of the capacitors was tested in 1 M H 2 SO 4 , using potential sweep cyclic voltammetry and constant current charge–discharge cycling. It was found that the Faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Due to the treatment the overall specific capacitance showed an increase up to 25% (e.g., from 120 to 150 F g 21 at a current density of 0.5 mA cm 22 ). However, the distributed capacitance effect, the inner resistance and the leakage current were found to increase with the extent of oxidation. It is suggested that due to the local changes of charge density and the increase in redox activity the presence of the carbonyl- or quinone-type functional groups may induce double-layer formation, Faradaic current, surface polarity, and electrolyte decomposition. 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; B. Oxidation; C. Electrochemical analysis; D. Electrochemical properties, Functional groups
1. Introduction Carbons of different forms have been extensively used in a variety of electrochemical systems [1], such as batteries and electric double-layer capacitors (EDLCs). Previous studies have pointed out that carbon fibers from polyacrylonitrile (PAN) precursor provide good electrical properties and high strength [1,2]. Recently, interest in PAN-based activated carbon fabrics for the use as porous electrodes in EDLCs has emerged. One of the advantages of carbon fabrics over fibers or powders is that there is no binding material required in the fabrication of electrodes. The cost of binding materials, such as polytetrafluoroethylene or polyvinylidenefluoride, for electrode preparation is usually high, and the binders may block some entrances of the pores in the porous carbons, hence resulting in diminishment of double-layer capacitance. It is well known that the pore structures of carbon electrodes affect the performance of the resulting *Corresponding author. Tel.: 1886-6-2385-371; fax: 1886-62344-496. E-mail address: [email protected] (H. Teng).
capacitors [3–7]. The effects of the specific surface area, as well as the pore size distribution, of the carbon electrodes has been discussed. Apart from the physical structures, the chemical characteristics, such as the distribution of heteroatoms on carbon surface, would also influence the capacitor performance. The carbon–oxygen complexes, i.e., the oxygen functional groups, in carbons have been reported to be the important surface groups affecting the wettability, chemical reactivity, and electrical properties [1,8,9]. How these complexes can affect the performance of the resulting EDLCs was rarely reported in the literature. Oxygen functional groups can be formed on carbons by treatments such as electrochemical oxidation [2], cold plasma treatment [10], chemical oxidation in HNO 3 or H 2 SO 4 solutions [9,11], and gaseous oxidation with oxidizing gases [12]. In examining the effects of oxygen functional groups, employing gaseous oxidation to create the functional groups has the advantages such as convenience and no significant impact on physical characteristics of the carbons. Without the intrusion of pore structure change, the effects of oxygen functional groups can be exclusively discussed.
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00182-8
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668
Within the above scope the performance of EDLCs fabricated using oxidized PAN-based carbon fabrics was examined. Gaseous oxidation using oxygen as the oxidizing agent was employed in the present work. The chemical composition of the resulting carbon–oxygen complexes was analyzed, in the attempt to elucidate the effects of different oxygen functional groups during the course of double layer formation at the electrolyte–carbon interface.
2. Experimental
2.1. Oxidation of activated carbon fabrics PAN-based activated carbon fabric supplied by Taiwan Carbon Technology, Taiwan, was used to fabricate the electrodes of EDLCs. The fabric has a thickness of 0.4–0.6 mm. The diameter of the carbon fibers in the fabric is ca. 15 mm. Oxidation was carried out with thermal treatment at 2508C under an oxygen atmosphere. Because of the negligible extent of carbon gasification at this treatment temperature, weight gain due to oxygen chemisorption, rather than weight loss, was observed in the oxidation. The treatment was conducted for different lengths of time to prepare samples containing different populations of oxygen functional groups on the surface.
2.2. Characterization of carbon fabrics Physical characteristics of the carbon fabrics were determined by N 2 adsorption at 21968C, using an automated adsorption apparatus (Micromeritics, ASAP 2010). Surface areas and micropore volumes of the samples were evaluated with the application of the BET and Dubinin– Radushkevich (D–R) equations, respectively. The amount of N 2 adsorbed at relative pressure near unity was employed to determine the total pore volume, which corresponds to the sum of the micropore and mesopore volumes [13]. The average pore diameter was estimated according to the surface area and total pore volume, assuming that the pores are parallel and cylindrical. A temperature programmed desorption (TPD) technique was employed to analyze the population of carbon–oxygen complexes on the carbon fabrics. The experiments were
carried out under a helium flow, by heating 0.05 g of sample from room temperature to 9008C with a linear rate of 308C min 21 . The evolution of CO and CO 2 during TPD was continuously monitored with non-dispersive infrared analyzers. The chemical composition of the oxygen functional groups was also studied by X-ray photoelectron spectroscopy (XPS). The XP spectra were recorded with a Fisons VG ESCA210 spectrometer and Mg K a radiation. The spectra were smoothed and a non-linear background was subtracted. The deconvolution of the spectra was performed using a non-linear least-squares fitting program with a symmetric Gaussian function. The surface composition of the samples was calculated with C 1s and O 1s peaks [14] and appropriate sensitivity factors.
2.3. Electrochemical measurements Two electrode cells were used to examine the electrochemical performance of the carbon fabrics. The electrodes consisted of 2 cm 2 carbon fabrics and stainless steel foils serving as the current collector. The cells were constructed with two facing carbon electrodes, sandwiching a piece of filter paper as separator. All electrochemical measurements were performed at ambient temperature, by soaking the cells in an electrolyte solution of 1 M H 2 SO 4 . Cyclic voltammetric measurements of the cells were made in the potential range of 20.6 to 0.6 V applied between the two electrodes. The potential scan rate ranged from 0.5 to 5 mV s 21 . The capacitance of the cells was measured by charging the capacitors to 0.6 V, followed by discharging to 0 V at constant currents of 0.5, 1, 2, and 5 mA. The leakage current test was performed at a floating potential of 0.6 V for more than 12 h.
3. Results and discussion
3.1. Surface characteristics of carbon fabrics The physical characteristics of the carbon fabrics are given in Table 1. The original carbon fabric is designated as CF and the oxidized fabrics are designated as CFO1, CFO2 and CFO3, which were derived from oxidizing CF for 0.5, 1 and 6 h, respectively. The pore size distribution
Table 1 Physical characteristics of the carbon fabrics used as the electrodes for electrochemical measurements Carbon type
Oxidation time (h)
BET surface area (m 2 g 21 )
Pore volume (cm 3 g 21 )
CF CFO1 CFO2 CFO3
0 0.5 1 6
1290 1280 1270 1190
0.62 0.61 0.61 0.57
Pore size distribution Micro (%)
Meso (%)
100 100 100 100
0 0 0 0
Average pore diameter (nm) 1.93 1.92 1.92 1.92
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shows that these carbons are highly microporous. The surface area and pore volume decrease slightly with the extent of oxygen treatment. This indicates that introduction of oxygen functional groups in micropores has hindered the access of N 2 to some of these pores. Although physical characteristics of carbons, including the degree of order [15] and the exposed area [6], have been shown to affect the performance of the resulting capacitors, their influence will be considered to be minor in the present work since the degree of order cannot be altered at 2508C and the changes in surface area and pore size distribution caused by the oxygen treatment are, as a matter of fact, not notable. The population of oxygen functional groups built up during the oxygen treatment has been evaluated using TPD. Past studies have reported that during the course of TPD oxygen functional groups such as carboxyl, anhydride and lactone groups would evolve as CO 2 while hydroxyl, carbonyl and quinone groups would evolve as CO [1,16,17]. The accumulated amounts of CO and CO 2 evolution during TPD are given in Table 2. The results show that the amount of CO 2 evolved from these carbon fabrics increases slightly with the extent of treatment, while the amount of CO evolved was significantly enhanced upon oxidation. The increase in total oxygen evolution from the fabrics with the extent of oxidation can be attributed mainly to the formation of the CO-desorbing complexes, which are essentially basic in chemical nature [8]. XPS was employed for further analysis of the composition of the oxygen functionalities on the carbon fabrics. The C 1s and O 1s peaks of the scan spectra have binding energies of ca. 284.6 and 533.5 eV, respectively. Quantitative analysis was carried out to determine the surface C and O concentrations, and the (O 1s) /(C 1s) atomic ratios are listed in Table 3, showing that the ratio increases with the extent of oxidation. The trend of (O 1s) /(C 1s) varying with the oxidation level is similar to that of total oxygen evolution in TPD shown in Table 2. The broad C 1s peak ranging from 280 to 292 eV in the XP spectra may comprise peaks contributed by several carbonbased functional groups that have different binding energies. These binding energy peaks have been identified as C
Table 2 Accumulated amounts and percentages of CO and CO 2 evolution from the carbon fabrics during temperature programmed desorption Carbon type CF CFO1 CFO2 CFO3
CO evolution mmol g 0.23 0.41 0.58 0.81
21
CO 2 evolution %
mmol g
58 68 74 76
0.17 0.19 0.20 0.25
21
% 42 32 26 24
Total O evolution (mmol g 21 ) 0.57 0.79 0.98 1.31
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Table 3 (O 1s) /(C 1s) atomic ratio and distribution of oxygen functional groups of carbon fabrics determined from deconvolution of XP C 1s spectra Carbon type
(O 1s) /(C 1s)
CF CFO1 CFO2 CFO3
0.064 0.086 0.11 0.13
Functional group distribution C–O (%)
C=O (%)
O–C=O (%)
48 40 30 24
14 28 41 51
38 32 29 25
or C–H at 284.6 eV, C–O at 286.7 eV, C=O at 288.4 eV and O–C=O at 289.7 eV [18–20]. The C 1s peak of each carbon can be deconvoluted using a peak synthesis procedure in which Gaussian peak shape was assumed to fit each component with a fixed binding energy and an average full width half maximum value of 2.0 eV. The distribution of oxygen-containing groups of each carbon was thus determined from deconvolution of XP C 1s spectra, and the results are shown in Table 3. It can be seen that the percentage of O–C=O determined from XPS is similar to that of CO 2 -desorbing complexes shown in Table 2. Both the percentages of C–O and O–C=O decrease with the oxidation level, and the increase in oxygen content upon oxidation is mainly contributed by the formation of C=O, which would evolve as CO upon thermal treatment. The XPS and TPD results of carbon– oxygen complex analysis are qualitatively consistent with each other.
3.2. Electrochemical performance of the resulting capacitors The cyclic voltammetric tests on the two-electrode capacitors were conducted within a potential range of 20.6 to 0.6 V at different potential sweep rates. Figs. 1 and 2 shows the typical voltammograms of the capacitors fabricated with different carbon fabrics. The figures demonstrate that the electrodes are stable in the acid solution within the potential range employed. Faradaic current exhibiting as abrupt current increase was not observed in the voltammogram for the CF capacitor. The intensity of Faradaic current was found to increase with the increasing oxygen extent of the electrodes, as inferred from the gradual increase of anodic and cathodic currents at ca. 0.2 and 20.2 V, respectively, with the increasing extent of oxygen treatment. The voltammograms also exhibit that the induced current is an increasing function of the oxidation level, indicating the increase of capacitance upon oxidation. Rectangular voltammograms, in which current quickly reaches a truly horizontal value after reversal of the potential sweep, were observed for the capacitor made of CF. However, the delay for the current to reach a horizon-
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Fig. 1. Cyclic voltammograms of (a) CF and (b) CFO1 electrode (2 cm 2 ) in 1 M H 2 SO 4 at different potential sweep rates.
Fig. 2. Cyclic voltammograms of (a) CFO2 and (b) CFO3 electrode (2 cm 2 ) in 1 M H 2 SO 4 at different potential sweep rates.
tal value near the reversal of the potential sweep and the increase of slope in the horizontal region were enhanced with the extent of oxidation. It is very likely that the presence of surface oxides may retard the motion of electrolytes and thus increase the ohmic resistance of electrolytes along the axial direction of micropores, which would combine with the existence of the distributed capacitance to cause the delay of current inversion [21– 23]. The retarded electrolytic motion due to oxidation can be partially supported by the observation that the accessibility of the micropores to N 2 was hindered by the introduction of the oxides, which results in the decreased N 2 surface area and pore volume. In addition, surface oxides are strong polar sites that would adsorb water molecules and thus hinder the migration of electrolytes in pores. Apart from the distributed capacitance effects, Faradaic current may also hinder current inversion [22]. Since the voltammograms of the oxidized electrodes show the presence of Faradaic currents, the surface oxides created from the oxygen treatment may be responsible for inducing chemical interactions (such as redox reactions) between the carbon fabric and the electrolyte [1], which will be discussed later in this paper. Figs. 1 and 2 also show that both the anodic and cathodic charging currents, Ia and Ic , respectively, are increasing functions of the potential sweep rate in the cyclic voltammetric measurements. The current plateaus of Ia and Ic are parallel and the difference, DI (5Ia 2Ic ), was found to be relatively invariant over the majority of the potential range employed. The values of DI of different capacitors obtained with different sweep rates are shown in Fig. 3. DI is seen to be an increasing function of the oxidation level, indicating an increase of capacitance upon oxidation. The double-layer capacitance, Cdl , of the electrodes can be estimated from the slope of the linear relationship in Fig. 3 [24]. The calculated data are listed in Table 4, showing that the capacitance increases slightly with the oxidation level. The intercept of the linear plots of DI against potential sweep rate represents the Faradaic current [24], IF . The values of IF of these electrodes are also compared in Table 4. It can be seen that IF was significantly promoted through the introduction of oxygen functional groups (from ca. 0 mA g 21 for CF to ca. 20 mA g 21 for CFO3). Both Cdl and IF contribute to the total capacitance. Faradaic current, the contributor of pseudocapacitance, may derive from some carbon–electrolyte interactions, such as electrosorption and redox reactions that are essentially reversible upon potential change [25]. Data in Table 4 show that the increase in double-layer capacitance (Cdl ) due to oxidation is not obvious, while the chemical interactions at the solid–liquid interface were significantly enhanced with oxidation. It has been claimed that the presence of Faradaic current would make it difficult for the current to reach a truly horizontal value in cyclic voltammetric tests [22]. This is in agreement with the observation that deviation from rectangular shape of
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Fig. 3. Variation of DI (difference between anodic and cathodic charging currents) with potential sweep rate in cyclic voltammetric measurements using CF (n), CFO1 (1), CFO2 (,) and CFO3 (h) electrodes (2 cm 2 ).
voltammograms was enhanced with the extent of carbon oxidation that has been found to induce the Faradaic current. To illustrate the influence of oxidation on the performance of constant-current charge and discharge, Fig. 4 shows the potential against time curves of the CF and CFO3 capacitors charged and discharged at a constant current of 0.5 mA. The CFO3 capacitor has a higher capacitance for both charge and discharge, but possesses a larger ‘IR drop’ at the beginning of discharge. Based on the results of charge–discharge cycling, the specific discharge capacitance of a single electrode in the capacitors can be calculated according to:
671
Fig. 4. Comparison of charge–discharge curves of CF and CFO3 electrodes (2 cm 2 ) at 0.5 m.
equivalent single-electrode capacitors in series. The capacitances of different electrodes obtained with different discharge currents are shown in Fig. 5. The capacitance decreases with discharge current, suggesting that the potential difference between the mouth and the bottom of the micropores increases with the current due to ohmic resistance of the electrolyte in the axial direction of micropores. As for the effect of oxidation, the specific capacitance was found to increase with the extent of
C 5 (2It) /(WDE) where I is the discharge current, t the discharge time, W the carbon fabric mass on an electrode and DE the potential difference in discharge, excluding the portion of IR drop. The factor of 2 comes from the fact that the total capacitance measured from the test cells is the sum of two Table 4 Double-layer capacitance and Faradaic current of the carbon electrodes determined from cyclic voltammograms Carbon type
Cdl (F g 21 )
IF (mA g 21 )
CF CFO1 CFO2 CFO3
121 123 123 130
0.082 5.7 7.5 22
Fig. 5. Variation of specific discharge capacitance with discharge current for different electrodes (2 cm 2 ) charged at 0.5 mA to 0.6 V.
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oxidation, which is in agreement with the results from cyclic voltammetric measurements. A previous study has also reported an increase of capacitance resulting from electrochemical oxidation of carbon electrodes [26]. Comparing the capacitances of CF and CFO3 shows that there is ca. 25% capacitance increase achieved through introduction of surface oxides (e.g., an increase from 120 to 150 F g 21 at a current of 1 mA, or a current density of 0.5 mA cm 22 ). In an ideal formation of electric double layer with acids serving as the electrolyte, the equilibrium reaction at the negative electrode during charging is simply presented as: . Cx 1 H 1 5 . Cx / / H 1 where .C x represents the carbon electrode surface, H 1 the proton of the acidic electrolyte, and .C x / / H 1 the double layer where the charges are accumulated on the two sides of the double layer. The process is due to a simple physical adsorption, by electrostatic forces between the carbon surface and the proton. Basically, the numbers of ions involved in building the double layer match the charge density developed on the electrodes. The data of the present work suggest that, as the extent of oxidation increases, surface oxides would provide more available sites for proton adsorption in the micropores of the carbon fabrics. The preceding analysis of oxygen functional groups has shown that oxidized carbons contain a significant amount of carbonyl- or quinone-type groups, i.e., C=O, on the surface. With the presence of these groups, an equilibrium reaction may occur in carbon electrode [1]: . Cx O 1 H 1 5 . Cx O / / H 1
polarity, i.e., a surface electrostatic field, to carbons [1,25,27,28]. The interaction of the electrostatic field at the surface with the dipole moment of water molecules plays an important role in determining the wettability of carbons in aqueous solutions. It is generally recognized that the micropores of carbons cannot be fully wetted in aqueous solutions, and thus are not fully accessible to electrolyte or hydrated molecules in the liquid phase. The increase in oxygen content of the carbon fabric may improve the wettability of the internal structure of micropores in the carbon electrodes, thus resulting in a corresponding increase in specific capacitance. A sudden potential drop at the very beginning of the constant current discharge is usually observed for EDLCs, and this drop has been designated as the IR drop. This potential drop has been attributed to the resistance of electrolytes and the inner resistance of ion migration in carbon micropores [29]. The latter usually has a higher contribution to the whole IR drop. Assuming that the inner resistance is one of the intrinsic characteristics of the carbon electrodes, the potential drop measured should be proportional to the discharge current. Fig. 6 shows the variation of the IR drop with the discharge current for the different electrodes. It can be seen that the potential drop increases linearly with the current, and the slope of this linear relationship was used to estimate the inner resistance of the electrodes. The resistance is an increasing function of the oxidation level, with resistances of 22, 30, 35 and 57 V for CF, CFO1, CFO2 and CFO3, respectively, in agreement with the results using electrochemical oxidation [26]. This result suggests that the imparted polarity may hinder the motion of ionic species. The increase in the resistance does not play a role in affecting the specific
where .C x O / / H 1 represents a proton adsorbed by a carbonyl or quinone-type site, basically induced by iondipole attraction. This specific adsorption process, which is different from the formation of .C x / / H 1 on non-specific sites through dispersion interactions [12,15], may facilitate an excess specific double layer capacitance due to the local changes of electronic charge density. It is thus suggested that the increase of capacitance upon oxidation arises partially from the enhanced dipole affinity towards protons in an acid solution. In charging the negative electrode, a strong bond may also form between quinone-type groups and protons due to electron transfer across the double layer: . C x O 1 H 1 1 e 2 5 . C x OH where .C x OH represents a hydroquinone-type complex and e 2 an electron. The reaction would proceed backwards during discharge. This type of functional groups, which would provide redox activity, may be responsible for IF and thus the pseudocapacitance component of the overall capacitance of these oxidized carbon electrodes [25]. In addition, oxygen functional groups impart a surface
Fig. 6. Variation of IR drop with discharge current for electrodes (2 cm 2 ) charged at 0.5 mA to 0.6 V.
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capacitance during the constant-current charge–discharge cycling, since the IR drop has been excluded in capacitance calculation. However, increasing the resistance would result in a higher potential drop and thus a reduction of specific capacity (in units of mA h g 21 ) of the electrodes, especially at a high current density. The leakage currents of CF and CFO3 capacitors charged at 1 mA were measured at 0.6 V floating as shown in Fig. 7. The current decreased gradually with time from 1 mA to around 1310 22 mA in 12 h. Sample CFO3 has a higher leakage current than CF throughout the entire period of measurement. The steady-state currents determined after 12 h of charge were 1.1310 22 and 9.4310 23 mA for CF and CFO3, respectively. The steady-state current may be ascribed mainly to electronic current that results from irreversible decomposition of electrolyte components. It should be noted that this electronic current is not included in IF that derives from the reversible interactions between carbon and electrolyte. The increase of leakage current suggests that the oxygen functional groups may serve as active sites that catalyze the the electrochemical oxidation or reduction of carbon or the decomposition of electrolyte components [1,30]. The stability of the prepared capacitors can be examined by conducting repeated charge–discharge cycling. A capacitor equipped with CFO3 electrodes was charged and discharged between 0 and 0.6 V at 1 mA to confirm the stability. The coulombic efficiency (h ) of an electric double-layer capacitor can be calculated from the equation [31]:
673
Fig. 8. Variation of capacitance stability and coulombic efficiency with cycle number for CFO3 electrode (2 cm 2 ) charged and discharged at 1 mA in 1 M H 2 SO 4 .
where t D and t C are the times required for discharge and charge, respectively. The variations of discharge capacitance and coulombic efficiency with cycle number are shown in Fig. 8. The results exhibit that the capacitor has stable capacitance (about 150 F g 21 ) and coulombic efficiency (about 99.5%) over 100 cycles.
4. Conclusions
h 5 (t D /t C ) 3 100
Fig. 7. Variation of leakage current with time for CF and CFO3 electrodes at a floating potential of 0.6 V.
The present work has demonstrated that the capacitance of PAN-based carbon fabric in H 2 SO 4 can be enhanced with oxygen treatment at 2508C. The results of N 2 adsorption showed that the oxygen treatment has little influence on the physical characteristics of the carbon. Surface analysis using TPD and XPS showed that the amount of surface oxides increased with the extent of oxygen treatment and the increase was contributed mainly by the formation of carbonyl- or quinone-type groups that normally desorb as CO during TPD. Cyclic voltammetric study on the resulting electrodes showed that the Faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Constant current charge–discharge cycling reflected the importance of the distributed capacitance effects of these porous electrodes, which was inferred from the decrease in capacitance with the discharge current. Due to the treatment with oxygen, the overall specific discharge capacitance was found to have an increase up to 25% (e.g., from 120 to 150 F g 21 at a current density of 0.5 mA cm 22 ), while both the resistance determined from the IR drop and the leakage current increased with the extent of oxidation. It is suggested that increasing the population of carbonyl-
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or quinone-type surface groups through oxygen treatment is beneficial for capacitance promotion, but the advantage can be offset with the coupling increases in inner resistance and leakage current. The capacitors prepared in the present work exhibit excellent capacitance stability with a coulombic efficiency of 99.5% over 100 cycles.
Acknowledgements Financial support from the National Science Council of Taiwan is gratefully acknowledged. The project number is NSC 89-2214-E-006-041.
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