ARTICLE pubs.acs.org/JPCC
Photocatalytically Reduced Graphite Oxide Electrode for Electrochemical Capacitors Hsin-Chieh Huang,† Cheng-Wei Huang,† Chien-Te Hsieh,‡ Ping-Lin Kuo,† Jyh-Ming Ting,§ and Hsisheng Teng*,†,|| †
)
Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan ‡ Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32023, Taiwan § Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan
bS Supporting Information ABSTRACT: Graphene sheets are an ideal carbon material with the highest area available for electrolyte interaction and can be obtained by reducing graphite oxide (GO). This study presents the photocatalytic reduction of GO in water with mercury-lamp irradiation. The specific capacitance of the reduced GO in an H2SO4 aqueous solution reached levels as high as 220 F g1. This is because of the double layer formation and the reversible pseudocapacitive processes caused by oxygen functionalities at the sheet periphery. The rate capability for charge storage increases with irradiation time due to the continued reduction of oxygenated sites on the graphene basal plane. Alternating current impedance analysis shows that prolonged light irradiation promotes electronic percolation in the electrode, significantly reducing the capacitive relaxation time. With a potential widow of 1 V, the resulting symmetric cells can deliver an energy level of 5 Wh kg1 at a high power of 1000 W kg1. These cells show superior stability, with 92% retention of specific capacitance after 20 000 cycles of galvanostatic chargedischarge.
1. INTRODUCTION Electrochemical capacitors (ECs) are able to store a large amount of charge, and can deliver a much higher power rating than rechargeable batteries or fuel cells.13 The mechanism for storing energy in ECs is based on the surface interaction of electrolyte ions.4 Therefore, EC electrodes consist of porous carbon materials with a high surface area and acceptable conductivity.59 Slit-shaped micropores of activated carbon, composed of graphitic crystallites, impose a space constriction on the electrolyte ions.10 An exfoliated graphite structure is an ideal form of carbon because space constriction is minimal.1115 Appropriately reducing exfoliated graphite oxide (GO) is a major step in synthesizing graphene-like carbon suitable for ECs. Graphite oxide is the intermediate state between graphene and graphite,16,17 but it easily exfoliates and disperses in aqueous solution like wrinkled paper because the oxygen functionalities are hydrophilic. Fully oxidized GO is an insulator, in contrast to partially oxidized GO and graphene that are semiconductors and conductors, respectively.18 Therefore, lowly oxidized GO and graphene can serve as EC electrodes. GO reduction uses chemical methods,19 with reducing agents such hydrazine,20,21 dimethylhydrazine,17 hydroquinone,22 and NaBH4,23,24 thermal treatments,25 or light assisted reduction.26 However, these methods are unfavorable practices due to environmental or r 2011 American Chemical Society
process convenience concerns. Mutual photocatalytic reduction between GO sheets occurs when a GO suspension is irradiated with UV.16 This is because light irradiation can excite electrons from the valence band of GO (the O 2p orbital) to the conduction band (the antibonding π* orbital). Thus, the degree of irradiation varies the degree of reduction, and therefore the electrical conductivity of GO. With this understanding, this study synthesizes carbon electrodes for ECs from mutual photocatalytic reduction of GO. The oxygen functionality composition of the reduced GO varies with the degree of light irradiation and thus affects the performance of the resulting ECs. This study demonstrates that this irradiated GO (irr-GO) represents a promising capacitive material, which stores energy based on both double-layer formations and pseudocapacitive interaction mechanisms.
2. EXPERIMENTAL SECTION Graphite oxide was prepared from a natural graphite powder (Bay Carbon, SP-1) using a modification of Hummers’ method.27 Graphite powder (5 g) and NaNO3 (2.5 g; Merck, Received: June 1, 2011 Revised: August 19, 2011 Published: September 19, 2011 20689
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The Journal of Physical Chemistry C Germany) were introduced to concentrated H2SO4 (18 M, 115 mL; Wako, Japan) in an ice-bath and stirred for 10 min. KMnO4 (15 g; J. T. Baker) was added gradually, while stirring, so the temperature of the mixture would not exceed 20 °C. The mixture was then stirred at 35 °C for 24 h. Deionized water (230 mL) was then slowly added to the mixture, and the mixture was stirred at 98 °C for 15 min. The suspension was further diluted to 1000 mL and stirred for 30 min. The reaction was concluded by adding H2O2 (12 mL, 35 wt %; Shimakyu, Japan) and stirring at room temperature. After multiple washings with deionized water (3 1000 mL), the GO specimen was obtained by drying the precipitate of the final slurry at 40 °C for 24 h. The irr-GO specimens were obtained from photocatalytic reduction of GO at approximately 25 °C in a system with inner irradiation. The light source was a 450 W high-pressure mercury lamp (UM452, Ushio, Japan). A magnetic stirrer mixed 0.2 g of the GO specimen suspended in 450 mL of pure water in a quartz inner irradiation cell. A jacket between the mercury lamp and the reaction chamber was filled with flowing thermostatted cooling water. Because GO is capable of reducing water under irradiation,16 observations revealed steady H2 evolution during GO reduction. To obtain irr-GO specimens with different reduction levels, the photocatalytic reduction was performed for varying lengths of time (28 h). The Fourier transform infrared spectroscopy (FTIR) spectrum, in diffuse reflectance mode, was used to analyze the oxygen functionalities using a Nicolet 5700 spectrometer. X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra DLD, U.K.) with Al Kα radiation was also used to quantitatively analyze the chemical composition of the GO specimens. This study used a symmetrical, two-electrode capacitor cell to examine the electrochemical performance of the irr-GO electrodes. These electrodes consisted of a 1 cm2 carbon film (2 mg, fixed under stress without using any binder and conducting agent) and titanium foil as the current collector. The symmetrical cell comprises two facing carbon electrodes, which sandwich a cellulose filter paper used as the separator. The cells were assembled under stress to ensure close contact in the carbon carbon and carbonTi foil interface. Using 2 M H2SO4 as the electrolyte solution, all electrochemical measurements were performed at approximately 25 °C. An electrochemical analyzer (Solartron Analytical, Model 1470E, U.K.) recorded the charge storage behavior by making cyclic voltammetric measurements within a potential range of 1 to 1 V applied across the two electrodes. An ac impedance spectrum analyzer (Zahner-Elektrik IM6e, Germany) with computer software measured the impedance behavior of the capacitor cells. The measurements were conducted at 0 V with an ac potential amplitude of 5 mV and a frequency range 2 mHz1 MHz. The electrochemical analyzer recorded cell performance using galvanostatic chargedischarge measurements for the symmetric cells between 0 and 1 V, with a current density range 0.1100 A g1.
3. RESULTS AND DISCUSSION 3.1. Chemical Characteristics of GO Sheets. Figure 1 shows the FTIR spectra of the as-synthesized GO and the irr-GO after 4 h of photocatalytic reduction (irr-GO4). The number following the symbol irr-GO represents the irradiation time in hours. In the FTIR spectrum of GO, the epoxy (970 cm1) and tertiary alcohol (1370 cm1) groups are present. These groups increase the basal interlayer distance and make GO hydrophilic.28,29 The
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Figure 1. FTIR transmission spectra: (a) graphite oxides, GO; (b) photocatalytically reduced GO with 4 h irradiation, irr-GO4. The symbols Ph and Al represent phenolic and tertiary alcoholic, respectively.
Figure 2. Schematic of the structural model of the irr-GO sheets with oxygen functionalities preferentially attached to the sheet periphery.
appearance of phenolic C—O (1220 cm1) and ketone CdO (1720 cm1) signals indicates the presence of phenol and carboxylic acid in the periphery of GO sheets.29 The broad absorption peak at 30003600 cm1 for O—H stretching is a result of the hydroxyl functionality and water which also signals an H—O—H bending at 1620 cm1.28 The photocatalytic reduction significantly reduced the absorption intensities of the oxygen functionalities. The irr-GO4 spectrum reveals bands of C—O stretching (1060 cm1), phenolic C—O stretching, water H—O—H bending, CdO stretching, and O—H stretching, while those of epoxy and tertiary alcohol disappear. This indicates the presence of phenolic, carbonyl, and carboxyl groups in the periphery of the irr-GO sheets,29 but the epoxy and tertiary alcohol, on the basal plane, are lost after reduction with light irradiation. On the basis of the above FTIR analysis, Figure 2 shows a schematic of the structural model of the irr-GO sheets with oxygen functionalities preferentially attached to the sheet periphery. This study further uses XPS to analyze the composition of the oxygen functionalities of the graphene sheets.3036 The peaks of the scan spectra have binding energies of approximately 284.6 (C 1s) and 533.5 (O 1s) eV, Figure 3. The O 1s peak intensity decreases with the irradiation time. Quantitative analysis determined the surface O and C concentrations, and Table 1 shows the atomic ratios (O 1s)/(C 1s) for the GO specimens of varying irradiation degrees. The (O 1s)/(C 1s) ratio is a decreasing function of the irradiation time, which agrees with the results of 20690
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Figure 3. Full-range XPS spectra: (a) GO; (b) irr-GO4; (c) irr-GO8.
Table 1. (O 1s)/(C 1s) Atomic Ratio Determined from the Full-Range XPS Spectra (Figure 3) and Carbon Bonding Composition Determined from the C 1s XPS (Figure 4) for the GO and irr-GO Specimens carbon bonding composition (%) carbon type
(O 1s)/(C 1s)
C—C
C—O
CdO
GO
0.34
40
40
20
irr-GO4
0.20
52
30
18
irr-GO8
0.17
59
20
20
FTIR analysis. The broad C 1s peak, ranging from 280 to 292 eV in the XPS spectra, comprises peaks contributed by several oxygen functionalities that have different binding energies. Figure 4 shows the C 1s spectra (—) of GO, irr-GO4, and irrGO8 as examples. These spectra were decomposed into three peaks (---) and fitted using a symmetric Gaussian function. These three peaks are due to C—C (284.6 eV), C—O (286.3 eV), and CdO (288.1 eV). Table 1 lists the composition of the functionalities obtained from the C 1s spectra. The proportion of the C—O group shows a constant decrease with irradiation time, indicating a continuing removal of epoxy and tertiary alcohol on
Figure 4. C 1s XPS spectra (—): (a) GO; (b) irr-GO4; (c) irr-GO8. These spectra were decomposed into three peaks (---) that were fitted using a Gaussian function.
the basal plane caused by irradiation. This is supported by analysis with Raman spectra (see the Supporting Information). The lasting C—O group in irr-GO8 should correspond to the peripheral phenolic and carboxyl functionalities. The electrical conductivity of the GO sheets would therefore increase due to oxygen removal and formation of an sp2 network on the basal plane. The proportion of the CdO group remained constant after irradiation, indicating that the CdO group resisted the photocatalytic reduction. These XPS results confirm the presence of phenolic, carbonyl, and carboxyl groups in the periphery of the irr-GO sheets (see Figure 2). 3.2. Charge Storage Behavior of Resulting Electrodes. Potential scan cyclic voltammetric analysis provides the basic charge storage information of an electrode. Figure 5 shows the potential scan cyclic voltammograms of the EC cells assembled with the GO and irr-GO specimens. The induced current is negligibly low for the as-synthesized GO, but increases drastically after GO is photocatalytically reduced. This indicates that the conductivity of the as-synthesized GO is too low to store a charge and the photocatalytic reaction effectively reduced GO for 20691
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Figure 7. Nyquist impedance plots of the GO, irr-GO2, irr-GO4, and irr-GO8 cells in 2 M H2SO4 at an applied potential of 0 V, an ac amplitude of 5 mV, and frequencies 2 mHz1 MHz. The inset shows the magnification of the high-frequency region of the spectra for the irrGO cells.
Figure 5. Cyclic voltammograms of the symmetric two-electrode capacitors assembled with GO and irr-GO specimens: (a) GO; (b) irr-GO2; (c) irr-GO4; (d) irr-GO8. The electrodes consisted of a 1 cm2 carbon film (2 mg) and titanium foil current collector.
Table 2. Resistance Components and Relaxation Times of the Symmetric GO and irr-GO Cells in 2 M H2SO4 Determined by Alternating Current Impedance Spectroscopy at an Applied Potential of 0 V and by Galvanostatic Charge Discharge Measurements
a
Figure 6. Specific capacitance of carbon electrodes derived from GO with varying times of photocatalytic irradiation.
electrical conduction in the electrodes.16 The appearance of the faradaic peaks, superimposed on the current plateau in the voltammogram, reflects that the double layer formation and the pseudocapacitive processes of oxygen functionalities, especially the carbonyl- or quinine-type groups,37,38 contribute to the capacitance. Figure 5 illustrates the separation in the anodic and cathodic peak potentials. These separations are small, reflecting the high reversibility of the pseudocapactive charge storage. The reversibility can be attributed to the fact that the oxygen functionalities remaining after reduction are located at the periphery of the irr-GO sheets, and are therefore highly accessible to the electrolyte. Figure 6 shows the capacitance variation with the irradiation reduction time for the irr-GO cells. The capacitance values were obtained by integrating the voltammograms. The capacitance increased with the irradiation time and reached an asymptotic value of 220 F g1 (scanned at 1 mV s1) after 4 h of irradiation. The influence of the potential scan rate on the capacitance value, similar to that on the faradaic peak potential (Figure 5), is not
carbon type
Ri (Ω)
Rc (Ω)
Rp (Ω)
Rt (Ω)
τ (s)
GO irr-GO2
0.54 0.35
a 1.8
b 0.33
b 2.5
b 0.72
irr-GO4
0.19
1.7
0.21
2.1
0.59
irr-GO8
0.21
0.69
0.45
1.4
0.39
A large value. b An unavailable value.
obvious. This indicates that the irr-GO electrodes are suitable for high-rate operations due to the relatively lower resistance for charge transport in the electrodes and charge interaction at the functionality sites of GO sheets. The performance of ECs, especially in terms of power density, is closely related to the resistance element of the cells. Alternating current impedance spectroscopy, which distinguishes the resistance and capacitance of devices, revealed the resistance of the capacitor cells in the energy storage process. Figure 7 shows the Nyquist plots of the ac impedance spectra of the GO and irr-GO cells. All three irr-GO cells had better capacitive performance than that of the GO cell, as the full-scale spectra mainly contains vertical lines. The inset of Figure 7 shows the high-frequency region of the impedance spectra for the irr-GO cells. The internal resistance (i.e., the intercept at the Re(Z) axis) was 0.54 Ω (110 kHz), 0.35 Ω (64 kHz), 0.19 Ω (64 kHz), and 0.21 Ω (50 kHz) for the GO, irr-GO2, irr-GO4, and irr-GO8 cells, respectively. Table 2 lists these values for comparison with other resistance values. Internal resistance (Ri), or equivalent series resistance, represents the electrical resistance associated with the contacts and the bulk ionic resistance associated with the migration in electrolyte solution. The chemical composition and internal resistance values (Tables 1 and 2) indicate that irradiation reduces the Ri value by removing oxygen functionalities. However, irradiation longer than 4 h may reduce the wettability of irrGO for electrolyte penetration, therefore resulting in a slight increase in the Ri value. In the high frequency region of the impedance spectra, an arc appears before the plot transforms completely to a vertical line. 20692
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Figure 9. Complex plane plots of capacitance for the irr-GO2, irr-GO4, and irr-GO8 cells in 2 M H2SO4 at an applied potential of 0 V.
Figure 8. (a) Typical galvanostatic chargedischarge curves of the symmetric irr-GO4 cell charged at 0.5 mA and discharged at varying rates. (b) Variation of IR drop with discharge current for the symmetric irr-GO cells. The electrodes consisted of a 1 cm2 carbon film (2 mg) and titanium foil current collector.
This arc is an impedance characteristic of electrical contact in an electrode.39 The distance between the intercepts of the arc at the Re(Z) axis signifies contact resistance (Rc), which is associated with the contact between the neighboring irr-GO sheets or between the sheets and current collector.40,41 Table 2 shows the Rc values, which exhibit a decreasing trend with the irradiation time. These are 1.8 Ω for irr-GO2, 1.7 Ω for irr-GO4, and 0.69 Ω for irr-GO8. Prolonged irradiation enhances the electrical conductivity between the neighboring irr-GO sheets or between the carbon and current collector. This indicates reconstruction of the sp2 network via reduction of sp3 oxygenated sites. Therefore, irr-GO specimens with a longer irradiation time are expected to have superior high-rate performance. The IR drop, a sudden drop at the very beginning of discharge, is associated with the overall resistance (Rt) of a cell. The IR drop is proportional to the discharge current because Rt is an intrinsic characteristic of a cell. This study conducted galvanostatic chargedischarge measurements to evaluate the IR drop and therefore the Rt value. Figure 8a shows an example of galvanostatic chargedischarge results of irr-GO cells, exhibiting a typical capacitive charge-storage mechanism. Figure 8b summarizes the variation of IR drop with discharge current for irr-GO cells. The IR drop increases linearly with discharge current for each cell, and the slope of this linear relationship corresponds to the Rt value. The Rt value thus determined comprises Ri, Rc, and electrolyte resistance in pores (Rp).42 The Rp values were calculated by subtracting Ri and Rc from Rt. Table 2 summarizes the resistance components of the cells. The Rt value decreases steadily with the irradiation time. The resistance in pores makes a minor contribution to the overall resistance for irr-GO2 and irrGO4, indicating the pores formed by congregation of the GO sheets are readily accessible to electrolyte. The contact resistance
accounts for a majority of the overall resistance. However, the irrGO8 cell has a large Rp. This indicates that significant oxygen removal must have narrowed interstitial spacing between stacked GO sheets and reduced the wettability to hinder electrolyte penetration. This analysis on resistance components reveals that the removal of oxygen functionalities on the graphene basal plane for suppressing Rc is responsible for the significant decrease in Rt with the irradiation time. The ac impedance spectra (Figure 7) provide a focused analysis of resistive behavior at high frequencies. To have a better perspective on the capacitive response, Figure 9 plots capacitance in the complex plane. The following formula provides such capacitance43,44 C¼
1 1 ¼ jωZ jω½ReðZÞ þ j ImðZÞ
ð1Þ
where Z is the overall √ impedance of the capacitor cells, j is the imaginary unit (j = 1), and ω is the angular frequency. This format is advantageous for visualizing the capacitive charging process at low frequencies. We discovered the data in the capacitance complex plane by incorporating eq 1 with the impedance data in Figure 7. The Re(C) value of Figure 9 corresponds to the in-phase response of a capacitor element to potential variation, while the Im(C) value is the out-of-phase response of a resistance. The magnitude of the overall capacitance is (Re2(C) + Im2(C))1/2. Figure 9 shows incomplete semicircles in the complex plane. By extrapolation, the value at which the semicircles intersect the Re(C) axis represents the ultimate capacitance of the electrodes. The irr-GO electrodes have similar ultimate capacitance values of approximately 250 F g1. The results of Figure 3 also show similar capacitance values for the irr-GO electrodes at relatively lower scan rates. This is because the irradiation time has little influence on the population of the peripheral oxygen functionalities (see Table 1), and these functionalities are responsible for pseudocapacitive charging. The complex plane of the capacitive response (Figure 9) makes it possible to estimate the average relaxation time (τ) for charge storage in the electrode: τ = (ωmax)1, where ωmax is the frequency at the maximum Im(C) observed in the capacitance complex plane.42 Table 2 lists the τ values of the irr-GO cells, revealing that an increase in light irradiation time improves the rate of charge storage in the GO specimens. The τ value was 0.72 s for irr-GO2 and decreased substantially to 0.39 s for irr-GO8. The data in Table 2 demonstrates that the reduced Rc is the primary factor for the improved chargestorage rate. 20693
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Figure 10. Ragone plots of the symmetric irr-GO cells in 2 M H2SO4 with a potential widow of 1 V.
the variation of capacitance with the cycle number. We detected a small degradation of 6% in capacitance after 4000 cycles. Even if the cycling number achieves 20 000, the capacitance retains about 92% of its origin. The minor capacitance decay may be due to gradual chemical structure change of the oxygen functionalities on the graphene sheets. The high stability of this irrGO material can be attributed to the fact that the unstable functionalities on the basal plane are almost entirely removed by irradiation. The lasting functionalities are also stable and reversible to the chargedischarge process. This demonstrates that the photocatalytic reduction of GO creates a carbon framework containing oxygen functionalities capable of swift surface reactions. A recent study activated graphene oxide with KOH at 800 °C to obtain a porous carbon, which exhibited high energy density with organic and ionic liquid electrolyte.45 In comparison with this previous study, the irradiation method developed in the present work has the advantage of being easy and convenient, and the resulting irr-GO electrodes are suitable for aqueous solutions that induce pseudocapacitance. The specific electronic structure of GO offers a facile way of photosynthesizing carbonrelated materials for capacitor applications.
P ¼ ðI V Þ=2m
ð2Þ
4. CONCLUSIONS This study shows that facile photocatalytic reduction of GO can synthesize irr-GO materials with capacitance as high as 220 F g1 in an aqueous electrolyte. The irr-GO specimens provide oxygen-removed basal planes for double-layer capacitance and oxygen functionalities located in the sheet periphery for highly reversible pseodocapacitance. The Ragone plots of the resulting symmetric cells show excellent power and energy densities, with performance increasing with irradiation time and reaching an optimum at 8 h. Prolonged irradiation enhances the electrical conductivity of electrodes to give superior high-rate performance. However, photocatalytic oxygen removed narrows the spacing between stacked GO sheets and hinders electrolyte penetration. Galvanostatic cycling measurements show that cells can have 92% retention of specific capacitance after 20 000 cycles. This photocatalytic reduction preferentially reduces oxygen functionalities on the basal plane for electronic conduction, while leaving the peripheral functionality unchanged for pseudocapacitance.
E ¼ Pt
ð3Þ
’ ASSOCIATED CONTENT
Figure 11. Variation of specific capacitance with cycle number for the irr-GO8 electrode in a symmetrical cell that was charged and discharged between 0 and 1 V at 10 A g1.
3.3. Electrochemical Capacitor Performance. To have a comprehensive perspective on the capacitor performance, this study uses the galvanostatic chargedischarge data for the symmetric cells to correlate the power and energy densities of the cells based on
where P is the power density, I is the applied constant current, V is the potential window, m is the total weight of the symmetric electrodes, and t is the time for complete discharge. Figure 10 summarizes the P and E results in the Ragone plots, showing that cell performance improves by increasing the time for irradiation reduction. This is due to increased conductivity. Figure 10 shows that the energy density can be as high as 5 Wh kg1 at a power density of 1000 W kg1 for applications within a small potential window of 1 V. The present study found that the optimal irradiation time is 8 h as further photocatalytic removal of oxygen functionalities reduces the pseudocapacitance and carbon wettability. The cell performance deteriorates with further increasing the irradiation time to, for example 10 h (see the Supporting Information), primarily due to decreased capacitance and increased Rp. The irr-GO8 electrode in a symmetrical cell was charged and discharged between 0 and 1 V at 10 A g1 for 20 000 cycles to confirm the stability of capacitance with cycling. Figure 11 shows
bS
Supporting Information. Raman spectra of the GO, irrGO4, irr-GO8 specimens. Ragone plots showing the performance of symmetric cells assembled with irr-GO specimens from 2 to 10 h light irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: 886-6-2385371. Fax: 886-6-2344496.
’ ACKNOWLEDGMENT This research is supported by the National Science Council of Taiwan (98-2221-E-006-110-MY3, 100-3113-E-006-001, and 100-3113-E-006-012), and the Bureau of Energy, Ministry of Economic Affairs (100-D0204-2). 20694
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dx.doi.org/10.1021/jp205133g |J. Phys. Chem. C 2011, 115, 20689–20695
