carbon paper composites

Electrochimica Acta 55 (2010) 5294–5300

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Influence of oxidation level on capacitance of electrochemical capacitors fabricated with carbon nanotube/carbon paper composites Chien-Te Hsieh ∗ , Wei-Yu Chen, Yu-Shun Cheng Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 320, Taiwan

a r t i c l e

i n f o

Article history: Received 6 January 2010 Received in revised form 23 April 2010 Accepted 23 April 2010 Available online 2 May 2010 Keywords: Carbon paper Carbon nanotubes Electric double-layer capacitance Gaseous oxidation Electrochemical capacitors

a b s t r a c t Gaseous oxidation of carbon papers (CPs) decorated with carbon nanotubes (CNTs) with varying degrees of oxidation was conducted to investigate the influence of surface oxides on the performance of electrochemical capacitors fabricated with oxidized CNT/CP composites. The oxidation period was found to significantly enhance the O/C atomic ratio on the composites, and the increase in oxygen content upon oxidation is mainly contributed by the formation of C O and C–O groups. The electrochemical behavior of the capacitors was tested in 1 M H2 SO4 within a potential of 0 and 1 V vs. Ag/AgCl. Both superhydrophilicity and specific capacitance of the oxidized CNT/CP composites were found to increase upon oxidation treatment. A linearity increase of capacitance with O/C ratio can be attributed to the increase of the population of surface oxides on CNTs, which imparts excess sites for redox reaction (pseudocapacitance) and for the formation of double-layer (double-layer capacitance). The technique of ac impedance combined with equivalent circuit clearly showed that oxidized CNT/CP capacitor imparts not only enhanced capacitance but also a low equivalent series resistance. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Carbon materials are finding an increasing number of applications in energy storage, such as supports for electrocatalysts for fuel cells [1–5] and electrode materials for electric double-layer capacitor (EDLC) [6–8]. Recently, EDLCs using carbon electrodes have received considerable attention because they are promising candidates for secondary power sources employed in electric vehicles that need pulse-current supply [9]. Owing to their vast pore infrastructures, porous carbons (e.g., active carbon) are appropriate electrode materials in EDLCs that display higher electrochemical surface coverage for the formation of electric double layer. Recently, the carbon nanotube (CNTs)-based capacitor has emerged because of higher electric conductivity (graphite-like structure) and mesoporosity (2 nm < pore size < 50 nm) [10], accessible for high charge/discharge capability. Our previous study reported an efficient chemical vapor deposition (CVD) technique to grow CNTs on different carbon matrixes, including polyacrylonitrile-based carbon fabric [11], CNT with large tubular size of 100 nm [12], and active carbon [8]. The grown CNT composites not only offer an increase of specific capacitance (i.e., additional mesoporous surface area) but also provide an excellent electric contact resistance between CNTs and current collector, showing a potential for elec-

∗ Corresponding author. Tel.: +886 3 4638800x2577; fax: +886 3 4559373. E-mail address: [email protected] (C.-T. Hsieh). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.04.085

trode materials in EDLCs. Except the above carbons, carbon paper (CP) provides alternative selection for use of carbon electrode in EDLCs, which are rarely discussed in literature. The CP is basically composed of irregularly microscaled carbon fibers and its thickness was approximately 100–200 ␮m. The use of CP as carbon substrate for EDLCs possesses two advantages: (i) no binder required and (ii) easy-to-fit electrode area, in the electrode fabrication. Accordingly, this present work aims to grow CNTs on CP, forming a nano-/micro scaled carbon composites. The CNT junction on microscale fibers is adjusted by the density of metallic catalysts, and CVD technique allows the growth of CNT forest upon CP substrate. It is well known that the pore structures of carbon electrodes affect the performance of the resulting capacitors [13–17]. The effects of the specific surface area, as well as the pore size distribution, of the carbon electrodes have been discussed. Surface oxidation of carbon electrodes is the other key approach in affecting the specific capacitance of the EDLCs. The presence of oxygen surface complexes on carbon electrodes of an EDLC has been shown to affect the performance of the capacitor [18]. This capacitance enhancement is attributed to the fact that in addition to the charge accumulation mechanism that forms the double layer on the carbon surface, there are possible contributions from hetero-oxygen atom surface complexes that would provide sites for reversible chemisorption of a working ion, thus giving rise to pseudocapacitance [19–21]. Oxygen functional groups can be formed on carbons by treatments such as electrochemical oxidation [22], cold plasma treatment [23], chemical oxidation in HNO3 or H2 SO4 solutions

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Fig. 1. FE-SEM images for (a) fresh CP and CNT/CP composites with different oxidation levels: (b) CC-0, (c) CC-1, and (d) CC-3 samples.

[18], and gaseous oxidation with oxidizing gases [24]. Ye et al. have pointed out that the electro-oxidation not only implants surface oxides (e.g., carbonyl, carboxyl, hydroxyl) but also etches carbon surface or opens the caps of nanotubes [25]. Among these oxidation treatments, gaseous oxidation shows an effective effect for improving the performance of the EDLCs without any structural damage. This delivers a better understanding on the influence of oxidation level on carbon-based capacitors. As a result, this study aims to investigate (i) the applicability of using the CNT/CP composite as an electrode material for EDLCs, and (ii) the influence of oxidation level on specific capacitance of the prepared capacitors. Different oxidation levels of carbon composites have been compared with respect to their double-layer capacitances and high-rate capability, analyzed by cyclic voltammetry (CV), charge–discharge cycling, AC impedance combined with equivalent circuit. 2. Experimental The CP samples (TGP-H, Toray Composites Inc., Japan) used here served as carbon substrates, which is decorated with CNT forest. This paper is basically suitable for carbon electrodes or gas diffusion layer for fuel cells because of its high electrical and thermal conductivity. The paper with a 190-␮m thickness consists of irregular carbon fiber that has a diameter of approximately 8–10 ␮m. A procedure for the fabrication of CNT/CP carbon composite is similar to our previous study, which has been reported elsewhere [11]. A brief description for the fabrication of CNT/CP composites was described as follows: first, a 2 cm × 1 cm CP was chemically oxidized by using nitric acid (0.1N HNO3 ) oxidation at 90 ◦ C. This chemical oxidation allowed the implantation of surface oxides, such as carbonyl, carboxyl, and hydroxyl groups, on the CP surface. The oxidized CP was then impregnated with 0.5 M Ni nitrate at ambient temperature. This ionic adsorption process enables Ni2+ ions to interact with each surface acidic group. After that, a thermal reduction process was performed at 350 ◦ C under an H2 atmosphere, ensuring the deposition of metallic Ni and other nickel oxide on the CP surface. A CVD technique was used to grow CNT forest on the Ni-coated

CP under a carbon-containing atmosphere with the chemical composition of Ar:H2 :C2 H2 = 94:1:5 (v/v/v). The vapor-growth process was carried out in a vertical furnace at 900 ◦ C for a growth period of 1 h, thus giving CNT/CP composites. A surface cleaning process at 700 ◦ C under Ar atmosphere was first employed to remove surface oxides or impurity from the resulting CNT/CP carbon composites. After cooling down, gaseous oxidation was carried out with thermal treatment at 200 ◦ C under an extra purification of oxygen atmosphere. The treatment was conducted for different lengths of time to prepare the carbon samples containing different populations of oxygen functional groups on the surfaces. Surface characteristics of these carbon composites were determined by gas adsorption. An automated adsorption apparatus (Micromeritics, ASAP 2020) was employed for these measurements. Specific surface areas and mesopore volumes of the carbon samples were evaluated with the application of BET and Barrett–Joyner–Halenda (BJH) equations, respectively. The pore structures of the CNT/CP composites were characterized by fieldemission scanning electron spectroscope (FE-SEM, JEOL JSM-5600). To identify chemical composition on the carbon composites, the surface oxides of the carbons were studied using X-ray photoelectron spectroscopy (XPS). The XP spectra were recorded with a Fison VG ESCA210 spectrometer and Mg K␣ radiation. The spectra were smoothed and a nonlinear background was subtracted. The deconvolution of the spectra was performed using a nonlinear least squares fitting program with a symmetric Gaussian function. The surface composition of the samples was calculated with C 1s and O 1s peaks [26] and appropriate sensitivity factors. Electrochemical measurement of CNT/CP electrodes was carried out at ambient temperature using 1 M H2 SO4 as the electrolyte solution. A Pt wire was used as the counter electrode, and an Ag/AgCl was utilized as the reference. The working electrodes were constructed by pressing the CPs onto stainless steel foil as current collector. The electrode area and the mass of active materials are ca. 2 cm2 and 0.02 g respectively. CV measurements of the electrodes were made in the potential range of −0.2 to 0.8 V vs. Ag/AgCl. The potential scan rate was set at 10 mV/s. The capacitance of the

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Table 1 Surface characteristics of fresh and oxidized CNT/CP composites determined from nitrogen physisorption at −196 ◦ C. Carbon type

CC CC-1 CC-2 CC-3 a b c d

Oxidation time (h)

0 1 2 3

SBET a (m2 /g)

Vt b (cm3 /g)

115.4 120.5 122.3 117.2

Pore size distribution

0.69 0.70 0.75 0.79

Vmicro c (%)

Vmeso d (%).

0.8 0.7 0.7 0.6

99.2 99.3 99.3 99.4

Specific surface area computed using BET equation. Total pore volume estimated at a relative pressure of 0.98. Micropore volume determined from the DR equation. Mesopore volume determined from the subtraction of micropore volume from total pore volume.

CNT/CP capacitors was measured by charging the capacitors to 1.0 V, followed by discharging them to 0 V at constant currents of 1, 3, and 5 mA. An electrochemical impedance spectrum analyzer (CH Instrument, Inc., CHI 608) was employed to measure and analyze the ac electrochemical impedance spectra. In the present work, the potential amplitude of ac was equal to 5 mV, and the frequency was from 100 kHz to 10 mHz.

3. Results and discussion Fig. 1(a) and (b) illustrates typical FE-SEM images of fresh CP and CNT/CP composites, respectively. The images clearly show that each carbon fiber has a smooth surface, whereas a large amount of CNTs are decorated on the fiber surface after the CVD technique. The resulting CNT/CP composite exhibit unique carbon–carbon junction, contributed from the growth of nanotubes on metallic Ni-based catalytic sites. The entangled nanotubes have an average diameter of 20–40 nm and a length of several micrometers. Fig. 1(c) and (d) shows the FE-SEM images for CNT/CP composites with different oxidation periods, 1 and 3 h, respectively. It can be seen that the influence of oxidation level on the nanotube shape seems to be minor because the oxidized CNT/CP composites still maintain a similar morphology. This observation differs with the previous study, using an electrochemical oxidation on CNTs [25,27]. Their approach was able to electrochemically etch the surface of CNTs, generating roughened nanotubes. N2 physisorption at −196 ◦ C was conducted as a primary measurement to compare the surface structures of the carbon composites. The physical characteristics of the carbon composites determined from the N2 adsorption are given in Table 1. These carbon samples were labeled using the nomenclature of the CNT/CP composites, CC, followed by the oxidation time. This table indicates that fresh CNT/CP composites (i.e., CC-0 sample) are mainly mesoporous, and after surface oxidation, the mesopores still contribute a major part to total pore volumes of oxidized carbons, that is, CC-1, CC-2, and CC-3 samples (mesopore fraction > 99%). Moreover, all CNT/CP samples appear an identical porous structure, that is, specific surface area: 115.4–122.3 m2 /g, and total pore volume: 0.69–0.79 cm3 /g. Because a very slight porosity change of the oxidized samples is observed, this proves that the gaseous oxidation treatment does not affect the surface morphology and the pore size distribution of CNT/CP composites, as observed in Fig. 1. Accordingly, the influence of physical characteristics on the performance of the resulting capacitors can be considered to be ignored. XPS was used to analyze the composition of the oxygen functionalities on the carbon composites. The C 1s and O 1s peaks of the scan spectra have binding energies of ca. 284.6 and 533.5 eV, respectively [26]. The survey scans for CC-0 and CC-3 samples are shown in Fig. 2. Quantitative analysis was performed to evaluate the O/C atomic ratios listed in Table 2, showing an increasing trend of O/C ratio with the oxidation level. The broad C 1s peak ranging from 280 to 292 eV in the XP spectra may contain peaks contributed

Fig. 2. XP spectra of CNT/CP composites with different oxidation levels: (a) CC-0 and (b) CC-3 samples. The inset of this figure is the XP spectrum of C 1s deconvoluted by multi-Gaussian function.

by several carbon-based functional groups that have different binding energies. These binding energy peaks have been identified as C 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 [28–31]. 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. As shown in the inset of Fig. 2, 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 2. Two types of surface oxide groups, C–O and C O, are chemically attached to Table 2 Chemical composition and oxygen group distribution of CNT/CP composites with different oxidation levels. Carbon type

CC CC-1 CC-2 CC-3 a

O/C ratioa

0.06 1.70 2.09 2.68

Functional group distributionb C–O (%)

C O (%)

– 61.5 60.4 62.7

– 38.5 39.6 37.3

O/C atomic ratio determined by XP spectra, i.e., O 1s and C 1s peaks. Functional group distribution determined by deconvolution of XP C 1s peaks using a multiple Gaussian function.

b

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Fig. 3. Cross-sectional views of water droplet on different CNT/CP surfaces: (a) CC-0 and (b) CC-3.

the surface of CNTs, for example, imperfect sites such as broken bonds or curved tubes. The ratio of C–O to C O group remains at approximately 3:2, despite an increase in the oxidation level. This result reflects that the increase in oxygen content upon oxidation is mainly contributed by the formation of C O groups. The wetting property of carbon composites before and after gaseous oxidation was also investigated. The static contact angles of water droplets on the carbon composites are measured to be 151.8 ± 2.1◦ (CC surface) and 134.6 ± 1.1◦ (CC-3 surface), as shown in Fig. 3. After the oxidation treatment, water drop rapidly passes through the CNT forest and then completely wets the whole CP substrate. This result confirms that the wettability of CNT/CP surfaces has changed from hydrophobicity to hydrophilicity because of the appearance of surface oxide groups on CNTs. This obvious transformation is presumably due to the fact that surface oxides are strong polar sites that would adsorb water molecules [28], inducing the wetting behavior. To illustrate the influence of oxidation on capacitive behavior, the CV tests of fresh CP, oxidized CP and CNT/CP electrodes were carried out within a potential range of −0.2 to 0.8 V at a sweep rate of 10 mV/s. The typical voltammograms of the capacitors fabricated with different carbon samples is illustrated in Fig. 4. Apparently, fresh CP and oxidized CP electrodes provide relatively tiny capacitance to one decorated with CNTs. That is the reason why we could ignore the contribution of capacitance from fresh CP and oxidized CP electrodes. The voltammograms exhibit that the induced cur-

Fig. 4. Cyclic voltammograms for different types of CNT/CP electrodes in 1 M H2 SO4 at 10 mV/s.

Fig. 5. Charge–discharge curves of CNT–CP capacitor charged at different currents of 1 and 5 mA to 1.0 V in 1 M H2 SO4 . The area of the carbon electrodes is 2 cm2 .

rent is an increasing function of the oxidation level, indicating the increase of capacitance upon oxidation. Faradaic current exhibiting as abrupt current increase was not observed in the voltammogram for the CNT/CP capacitors. It is worth noting that the intensity of Faradaic current was found to increase with the increasing O/C ratio of the electrodes. This can be inferred from the gradual increase of anodic and cathodic currents within the potential range of 0.2–0.5 V vs. Ag/AgCl, with the increasing extent of oxygen treatment. Rectangular voltammograms, in which the current quickly reaches a truly horizontal value after reversal of the potential sweep, were observed for the capacitor made of CNT/CP composites. The delay for the current to reach a horizontal value near the reversal of the potential sweep at high sweep rate of 10 mV/s is not obvious with the extent of oxidation. This finding reflects that the influence of distributed capacitance on the oxidized CNT/CP capacitors seems to be minor. The phenomenon can be ascribed to the fact that (i) tubular CNTs offer a large number of active sites for rapid adsorption of ions, and (ii) ionic transportation in the mesoporous tubes seems to be a small diffusion resistance, even at high oxidation extent. In addition to the enhanced capacitance from Faradaic current within 0.2–0.5 V vs. Ag/AgCl, the capacitive current shows an increasing function of the oxidation level of the capacitors. This situation means that the increase in double-layer capacitance due to oxidation becomes obvious, which can be attributed to the enhanced accessibility to hydrophilic surface coverage in aqueous electrolyte. This is in agreement with the observation that the rectangular shape of voltammograms was enhanced with the extent of CNT oxidation. Fig. 5 shows the potential against time curves of the CNT/CP capacitor charge at a constant current of 5 mA. Compared with CC-0 capacitor, CC-3 capacitor appears to have a higher specific capacitance for both charge and discharge but exhibits a slightly larger “IR drop” at the beginning of discharge. As to the result, the oxidation extent shows a great influence on the performance of the resulting capacitor. From the results of charge–discharge cycling, the specific discharge capacitance of a single electrode in the capacitors can be calculated. The capacitances of different CNT/CP capacitors obtained with different discharge currents are shown in Fig. 6. As observed from Fig. 6, the capacitance was found to decrease with discharge current. This possibly originates from one reason that the

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Fig. 6. Variation of capacitance with discharge current density for CNT/CP capacitors charged and discharged in 1 M H2 SO4 .

Fig. 8. Variation of capacitance stability with cycle number for CC-3 capacitors charged and discharged at 3 mA in 1 M H2 SO4 . The area of the electrodes is 2 cm2 .

potential difference between the mouth and the bottom of the CNTs increases with the current because of ohmic resistance of the electrolyte in the axial direction of tubes. As for the effect of oxidation, the capacitance increases with the oxidation level at the same discharge current, that is, CC-3 capacitor reaches the maximal value of 165 F/g at 0.5 mA/cm2 . Comparing the capacitance of CC, CC-3 shows that there is ca. 79% capacitance increase achieved through the introduction of surface oxides on CNTs, for example, an increase from 92.5 to 165 F/g at a current density of 0.5 mA/cm2 . The specific discharge capacitance of different CNT/CP capacitors, according to Fig. 6, varying with the O/C atomic ratio (as shown in Table 2) are plotted in Fig. 7. Because the effect of physical structure has been ruled out, the influence of chemical characteristics such as the

functionalities of the carbons on the capacitance, should be taken into account. A linearity relationship between the capacitance and the oxidation extent delivers to one message that surface oxidation plays a crucial role in improving the performance of CNT/CP capacitors. Previous study has pointed out that the extent of oxidation increases surface oxides that would provide more available sites for proton adsorption in the micropores of the carbon fabrics [28]. The preceding analysis of oxygen functional groups has shown that oxidized carbons contain a significant amount of carbonyl- or quinone-type groups, that is, C O, on the CNT surface. With the presence of C O groups, an equilibrium reaction would take place in carbon electrode [33]: > Cx O + H+ ↔ > Cx O//H+

Fig. 7. Relationship between specific capacitance and O/C atomic ratio for CNT/CP capacitors charged and discharged in 1 M H2 SO4 .

(1)

where >Cx O//H+ represents a proton adsorbed by a carbonyl or quinone-type site, basically derived from an ion-dipole attraction. This specific adsorption process, which is different from the formation of >Cx //H+ on nonspecific sites through dispersion interactions [33], generate an excess specific double-layer capacitance due to the local changes of electronic charge density. Additionally, oxygen functional groups impart a surface polarity, that is, a surface electrostatic field, to carbons [32,34]. The interaction of the electrostatic field at carbon surface with the dipole moment of water molecules plays an important role in determining the wettability of carbons in aqueous solutions, demonstrated by water contact angle measurement as shown in Fig. 3. It is generally recognized that the pores of carbons cannot be fully wetted in aqueous solutions and thus are not fully accessible to electrolyte or hydrated molecules in the liquid phase [34]. The increase in oxygen content of the CNT/CP composites improve the wettability of the internal structure in the carbon electrodes, showing a corresponding increase in specific capacitance. The stability of the prepared capacitors can be examined by conducting repeated charge–discharge cycling. A capacitor equipped with CNT-ACF electrodes was charged and discharged between 0 and 1 V to confirm the stability, as shown in Fig. 8. The variation of discharge capacitance with cycle number exhibits that the capacitor has stable capacitance over 500 cycles.

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Table 3 Components and related parameters of the equivalent circuit (Fig. 9(b)) for the CNT/CP capacitors in 1 M H2 SO4 (deviation for each component < 9.1%).

Fig. 9. (a) Nyquist impedance plots of different CNT/CP electrodes in 1 M H2 SO4 with frequency ranging from 100 kHz to 1 mHz. The inset is high-frequency Nyquist plots with frequency ranging from 100 kHz to 0.12 Hz. (b) Equivalent circuit for the impedance behavior of CNT/CP electrodes.

The technique of ac impedance spectroscopy was employed to analyze the electrochemical behavior of CNT/CP capacitors. The impedance spectra of different capacitors are shown as Nyquist plots in Fig. 9(a). There is an intersection in the real axis in the high-frequency region, followed by a single quasi semicircle in the frequency region between 100 kHz and 1 Hz. This semicircle in the high-frequency region can be attributed to the presence of (i) an interface between the CNT/CP composites and the current collector, and (ii) an RC loop involving a double-layer capacitance in parallel with a resistance [35]. Afterward, the plot transforms to an almost vertical line with decreasing frequency. The almost vertical lines (ca. 70–75◦ ) at low frequencies correspond to the capacitive response of the porous carbons [32,34]. An equivalent circuit for describing the capacitive behavior of CNT/CP capacitors can be considered as follows: the bulk solution resistance (RS ), the double-layer capacitance (Cdl ), the Faradaic resistance (RF ), corresponding to the reciprocal of the potential-dependent charge transfer rate in redox reactions, the pseudocapacitance (Cp ), associated with the potential dependence of redox-site concentration, and Cb and Rb due to the impedance between the CNT/CP composites and the backing plate for the electrical connection. The combination of the circuit elements is proposed and shown in Fig. 9(b). Accordingly, the overall impedance, Z, of the equivalent circuit can be expressed as Z = RS +

1 Rb + jωRb Cb + 1 jωCdl + (jωCp /(jωRF Cp + 1))

(2)

Eq. (2) can be divided into two limiting cases: low- and high-frequency regions. The impedance in low-frequency region suggests the pure capacitive behavior [32], where the overall capacitance, Cdl and Cp , of CNT/CP capacitors can thus be estimated. At sufficiently high frequencies, Eq. (2) corresponds to a locus show-

Electrode

RS ()

Rc ()

Cc (␮F/cm2 )

RF ()

Cdl + Cp (F/g)

CC-0 CC-1 CC-2 CC-3

0.94 0.95 0.97 0.98

6.21 6.26 6.01 6.34

3.85 4.50 4.31 4.52

0.69 0.64 0.61 0.60

105.2 130.2 142.8 160.6

ing solution, electrical connection, and charge transfer resistances, that is, RS , Rb , and RF . A computer software (Z-view), incorporated with the equivalent circuit in Fig. 9(b), was able to resolve each component of the circuit within a small error (<9.1%). These calculated values for all CNT/CP capacitors are collected in Table 3. Both the slight variation of the bulk solution and the electrical connection between the CNT/CP composites and the current collector resistance with different capacitors can be observed. The overall resistance determined is the so-called equivalent series resistance, Res (=RS + Rb + RF ), which is composed of bulk solution resistance, connection resistance, and charge transfer resistance [9,12]. The value of Res for all CNT/CP electrodes ranges from 7.59 to 7.92 , which is much lower than pure CP and active carbon-based electrodes of ∼150  and 20–50 , respectively. The introduction of CNTs upon CP substrate is credited to significantly reduce the connection resistance and charge transfer resistance. In other words, the crystalline CNTs may effectively reduce the resistance of interfacial interface between the carbons and the back plate. The interfacial resistance thus tends to be a small value as a result of the existence of CNTs. This is presumably due to one reason that the voids and interspaces originated from microscaled fiber aggregates would be filled with CNTs, inducing a better interfacial contact. With increasing the extent of oxidation, the Res values of oxidized capacitors seems to be identical with fresh one, showing an insignificant effect of the oxidation extent. Additionally, it can be seen that the value of Cb can be negligibly small compared with the overall capacitance. The overall capacitances, Cdl + Cp , obtained from impedance analysis show that the oxidation treatment obviously enhances the performance of the CNT/CP capacitors, which is analogous to that observed from CV and charge–discharge cycling measurements. The total capacitance corresponds to a measure of electrochemical activity or energystorage capability. Accordingly, CNT/CP electrodes with different oxidation level have an order as follows: CC-3 (160.6 F/g) > CC2 (142.8 F/g) > CC-1 (130.2 F/g) > CC-0 (105.2 F/g). The order of the magnitude is in agreement with charge–discharge cycling at constant current density. On the basis of the above results, oxidized CNT/CP capacitor, for example, CC-3, is capable of imparting not only a better specific capacitance but also a lower equivalent series resistance. However, future work for examining (i) the appropriate oxidation level and (ii) the efficient approach of surface oxidation is required, to improve the performance of the CNT-based capacitors. 4. Conclusions We have demonstrated that gaseous oxidation is an efficient approach to improve the electrochemical capacitance of CNT/CP capacitors in H2 SO4 solution. The CVD technique was capable of decorating a large number of CNTs onto CP substrates, using acetylene and Ni nanoparticles as the carbon precursor and catalyst, respectively. The resulting CNTs on CP display a coiled shape with an average diameter of 20–40 nm and a length of several micrometers. The period of gaseous oxidation was found to significantly enhance the oxidation level of the composites, and the ratio of C–O to C O group remains at approximately 1:3, despite an increase of

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the oxidation level. This result shows that the increase in oxygen content upon oxidation is mainly contributed by the formation of C O groups, according to the analysis of oxygen group distribution. This oxidation treatment produces oxidized CNT/CP electrodes with higher capacitance. The overall specific capacitance was found to increase with the extent of oxidation, that is, there is ca. 79% capacitance increase achieved through the introduction of surface oxides on CNTs. For example, an increase from 92.5 to 165 F/g at a current density of 0.5 mA/cm2 . The CV, charge–discharge cycling, ac impedance measurements showed an identical result. This capacitance enhancement can be attributed to two reasons as follows: (i) an excess specific double-layer capacitance contributed from protons adsorbed by carbonyl or quinine-type sites, and (ii) surface accessibility to aqueous electrolyte enhanced due to the superhydrophilicity from oxidized CNT/CP surface. The analysis of ac impedance combined with equivalent circuit also clearly showed that oxidized CNT/CP capacitor imparts not only enhanced capacitance but also a low equivalent series resistance.

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