Highly Regulated Electrodeposition of Needle-Like Manganese Oxide

Nov 15, 2010 - High performance wire-shaped supercapacitive electrodes based .... electrospun carbon nanofiber/MnO2 coaxial nano-cables for high energ...
1 downloads 0 Views 2MB Size
Download PDF
J. Phys. Chem. C 2010, 114, 21861–21867

21861

Highly Regulated Electrodeposition of Needle-Like Manganese Oxide Nanofibers on Carbon Fiber Fabric for Electrochemical Capacitors Mao-Sung Wu,* Zong-Sin Guo, and Jiin-Jiang Jow Department of Chemical and Materials Engineering, National Kaohsiung UniVersity of Applied Sciences, Kaohsiung 807, Taiwan ReceiVed: September 9, 2010; ReVised Manuscript ReceiVed: October 28, 2010

The needle-like manganese oxide (R-MnO2) nanofibers were anodically deposited onto the conductive carbon fiber fabric for electrochemical capacitor application. The charge storage mechanism in the manganese oxide nanofiber electrode was investigated using cyclic voltammetry in aqueous and nonaqueous electrolytes containing various alkali metal salts. The results revealed that the protons intercalate into the interior region of the manganese oxide bulk in aqueous electrolytes, whereas the alkali metal cations reach only the nearsurface region. The specific capacitance of the manganese oxide nanofibers coated on carbon fabric could reach as high as 432 F g-1, which is much higher than that of manganese oxide nanofibers coated on stainless steel (177 F g-1) at a charge/discharge current density of 5 A g-1 in 1 M Na2SO4 aqueous electrolyte. The improved capacitance could be attributed to the combination of spaced manganese oxide nanofibers and the conductive carbon fibers, which facilitates the electron conduction and electrolyte transport through the film electrode. 1. Introduction The manganese oxide-based electrochemical capacitor has attracted much research attention due to its low cost, natural abundance, high theoretical capacitance, and good environmental compatibility. On the basis of the Faraday’s law, the theoretical specific capacitance of the stoichiometric reduction of MnO2 to MnOOH can be calculated to be approximately 1110 F g-1 in a potential window of 1 V.1 However, the practical specific capacitance values of manganese oxides reported in the literature are much less than the theoretical value. There are many factors affecting the specific capacitance of manganese oxide film electrodes. One of which is the microstructure of the film that considerably affects the penetration of electrolyte into micropores, electronic conductivity of the film, and proton transport through the manganese oxide lattice.1,2 A high specific capacitance of about 700 F g-1 in a potential window of 0.9 V can be obtained in the sol-gel derived manganese oxide ultrathin film,1 but the ultrathin film limits its practical application. An increase in film thickness decreases the specific capacitance of the manganese oxide film.3,4 The thick film is detrimental to the electron conduction from the current collector to the outer layer of the manganese oxide electrode due to the inherently poor electronic conductivity of manganese oxide. Moreover, the electrolyte is more difficult to penetrate into the inner layer of thick film; only the outer layer of the electrode is exposed to the electrolyte. To improve the electronic conductivity of manganese oxide film, conductive additives, such as carbon nanotubes,5-9 exfoliated graphite materials,10,11 graphene oxide,12 and carbon powder,13 have been incorporated into the MnO2 films. Manganese oxides with particular nanostructures, such as nanofibers,14 nanoflakes,15 nanorods,16-18 and nanowires,19-23 have been demonstrated to be the promising candidates for electrochemical capacitor applications. The wettability between elec* To whom correspondence should be addressed. E-mail: ms_wu@ url.com.tw. Fax: 886-9-45614423.

trode materials and the electrolyte also is a crucial factor in improving the electrochemical performance of manganese oxide electrodes.11 Therefore, it is interesting to deposit the spaced manganese oxide nanofibers on the high electronic conductive materials. This combination facilitates the electron conduction and electrolyte transport through the film electrode, consequently improving the capacitive behavior of manganese oxide. The methods used for the fabrication of manganese oxide film electrodes are mostly based on the coating manganese oxide powder mixed with binder and conductive additive on the conductive substrates. The electrochemical deposition technique has some advantages over the others: morphology, weight, and thickness of the film may be easily controlled by tuning the current, bath composition, and bath temperature.24,25 In this work, the needle-like manganese oxide nanofibers were anodically deposited onto the conductive carbon fabric of spaced carbon fibers for electrochemical capacitor application. Moreover, the charge storage mechanism in manganese oxide/carbon fabric was investigated. 2. Experimental Section Manganese oxide nanofibers were deposited onto the carbon fabric (CF) substrate (2 cm × 2 cm) made up of individual carbon fibers about 8 µm in diameter (Beam Associate Co. Ltd., Taiwan) by applying an anodic current of 1 mA in a solution bath of 0.1 M manganous acetate and 0.1 M sodium sulfate mixture for about 10 min at room temperature.26 Similarly, the manganese oxide film was also deposited on a stainless steel (SS) substrate for comparison. Prior to anodic deposition, the substrates (CF and SS) were washed in acetone and deionized water, respectively. The plating solution was stirred with a Teflon stir bar at a rotational speed of 100 rpm on a magnetic hot plate during the deposition process. After deposition, the film electrodes were rinsed several times in deionized water and dried at 300 °C for 1 h in air. All electrochemical experiments were carried out in a three-compartment cell in

10.1021/jp108598q  2010 American Chemical Society Published on Web 11/15/2010

21862

J. Phys. Chem. C, Vol. 114, No. 49, 2010

Wu et al.

Figure 1. (a) SEM image of the manganese oxide coated on CF. (b) High-magnification image showing the manganese oxide coated on CF. (c) SEM image of the manganese oxide coated on SS. (d) TEM images of the manganese oxide coated on CF.

which a saturated calomel electrode (SCE) was used as the reference electrode and a platinum foil with dimensions of 2 cm × 2 cm was the counter electrode. The surface morphology of the electrochemically deposited manganese oxide electrodes was examined with a scanning electron microscope (FE-SEM, JEOL, JEOL-6330) with an accelerating voltage of 15 keV. The crystal structure of manganese oxide coated on CF was identified by a glance angle X-ray diffractometer (GAXRD, Rigaku, D/MAX2500) with a Cu KR target (wavelength ) 1.54056 Å) and an incidence angle of 2°. Diffraction data were collected for 1 s at each 0.04° step width over 2θ, ranging from 10° to 90°. The nanostructure of the manganese oxide coated on CF was characterized by a transmission electron microscope (FE-TEM, JEOL JEM-1400) with an accelerating voltage of 200 kV. TEM specimens were prepared by the following procedure: the manganese oxide nanofibers coated on CF were stripped off and suspended in ethanol with ultrasonic vibrations for 5 min; a drop of the manganese oxide supernatant was then transferred onto a standard holey carbon-covered copper TEM grid. The capacitive behavior of the manganese oxide electrodes was determined by cyclic voltammetry and galvanostatic charge/ discharge in a three-electrode cell with 1 M Na2SO4 electrolyte at room temperature. To investigate the charge storage mechanism in manganese oxide, the cyclic voltammograms (CVs) of manganese oxide were carried out in various electrolytes of alkali metal salts at room temperature. The electrolyte was prepared using either the deionized distilled water or propylene carbonate (PC) as a solvent. Four electrolytes, 1 M Na2SO4 in H2O, 1 M Li2SO4 in H2O, 0.7 M K2SO4 in H2O, and 1 M LiClO4 in PC, were used for CV tests. The potential was cycled at various scan rates using a potentiostat (CH Instruments, CHI 608) in a potential range of 0-1.2 V versus SCE. Galvanostatic charge/discharge and cycle-life stability were performed by a source meter (Keithley, 2400 source meter) in a potential range of 0-1.2 V versus SCE. AC impedance measurements were

performed by means of a potentiostat (CH Instruments, CHI 608) coupled to a frequency response analyzer under an opencircuit condition. An ac perturbation amplitude of 10 mV versus the open-circuit potential was applied in a frequency range from 50 kHz to 0.1 Hz. 3. Results and Discussion Figure 1 shows the SEM and TEM images of manganese oxides coated on CF and SS. Samples were annealed at 300 °C for 1 h in air before observations. Clearly, the manganese oxide with spaced radial nanofibers could be uniformly coated on the individual carbon fibers of CF by use of the anodic electrodeposition technique. The average diameter and length of manganese nanofibers coated on CF were measured to be approximately 20 nm and 1 µm, respectively. The spacing between manganese oxide nanofibers is necessary to accommodate the electrolyte during the charging/discharging test. As shown in Figure 1a, some manganese oxide nanofibers were arranged roughly perpendicular to the surface of carbon fibers, whereas the manganese oxide nanofibers deposited on SS tended to aggregate to form agglomerates (Figure 1c). The nucleation depends on the overpotential during electrochemical deposition. The potential variation with time during anodic deposition was measured, and the related graph is not shown here. At the beginning of the galvanostatic deposition, the corresponding potential decreased rapidly and then stabilized in the ongoing deposition. The manganese oxide coated on CF (0.32 V) showed a much lower depositing potential (overpotential) than that coated on SS (0.51 V) at an anodic current of 1 mA due to the high surface area of CF compared with SS. At higher overpotentials, the nucleation rate increases, which results in the formation and growth of smaller nuclei. The overlapping effect may cause smaller nanofibers to grow into agglomerates. Therefore, when the manganese oxide film was deposited on CF (low overpotential), the nanofibers tended to form spaced

Electrodeposition of Needle-Like R-MnO2 Nanofibers

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21863

Figure 2. Schematic illustration of the proposed manganese oxide architecture for electrochemical capacitor application.

radial nanofibers; a film with highly aggregated nanofibers was deposited on the SS (high overpotential). The manganese oxide film with needle-like nanofibers is expected to have a positive effect on the capacitive behavior probably due to its high surface area and short ion diffusion path for electrochemical reaction. As mentioned above, the electronic conductivity is a crucial factor in enhancing the capacitive behavior of electrodes. It is interesting to combine the nanosized manganese oxide and high conductive CF to enhance the capacitive behavior of manganese oxide electrodes. Figure 2 shows a schematic illustration of the proposed manganese oxide architecture for electrochemical capacitor application. The CF is highly porous in nature with interconnected carbon fibers, allowing fast electron conduction and high accessibility of the electrolyte within the fabric. The spaced radial manganese oxide nanofibers aligned on carbon fibers facilitate the electrolyte transport through the film. Electrochemical reactions between manganese oxide and the electrolyte are expected to be improved significantly by this tailored architecture. The specific capacitance of manganese oxide depends on its crystal structure, which is susceptible to the annealing temperature.27 The as-prepared manganese oxides usually contain water in their structures. The water content enhances the transportation of active ionic species and consequently increases the specific capacitance of manganese oxides.28 Reddy et al.29 pointed out that MnO2 dried at 400 °C is left with no water content, leading to the loss of capacitance. They also concluded that the chemically bound water is the only factor that may play an important role in enhancing the capacitive behavior of MnO2 electrodes. Pang et al.1 have shown that the specific capacitance value of electrodeposited manganese oxide film annealed in air at 300 °C is higher than that of film dried at room temperature.1 Therefore, the annealing temperature of the deposited manganese oxide electrodes was set at 300 °C in this work. Figure 3 shows the XRD patterns of CF and manganese oxide coated on CF after annealing at 300 °C for 1 h in air. In addition to the diffraction peaks of CF, the XRD pattern of manganese oxide could be assigned to R-MnO2 (JCPDS 44-0141). The diffraction peak at about 2θ ) 37.5° was broad, which indicates a poor crystallinity and a small grain size of the deposited manganese

Figure 3. XRD patterns of CF and manganese oxide-coated CF after annealing at 300 °C for 1 h in air.

oxide nanofibers after annealing at 300 °C. The average grain size of the manganese oxide coated on CF was determined to be approximately 5 nm using Scherrer’s equation with a diffraction peak at 2θ ) 37.5°: D ) 0.9λ/(β · cos θ), where λ is the X-ray wavelength, β is the full width at half-maximum (fwhm), and θ is the Bragg angle. A smaller grain size can provide a larger specific surface area for electrolyte access. To obtain high current output from the capacitor, the surface area of the electrodes in contact with the electrolyte must be as large as possible. There are two proposed mechanisms for the charge storage in manganese oxide materials. The first one is based on the surface adsorption/desorption of alkali metal cations (C+) from the neutral electrolyte30-32 + (MnO2)surface + C+ + e- T (MnO2 C )surface

(1)

where C+ ) Na+, K+, and Li+. The adsorption and desorption occur only on the surface layer of manganese oxide. Therefore, eq 1 is likely to be predominant in amorphous manganese oxide with a high specific surface area. The study by Toupin et al.32 has shown that only the Mn surface atoms are involved in the pseudocapacitive redox processes in 0.1 M Na2SO4 aqueous electrolyte. Thus, the high capacitance seems to be related to

21864

J. Phys. Chem. C, Vol. 114, No. 49, 2010

Wu et al.

the high surface area of the MnO2 powder rather than the intercalation of Na+ ions and/or protons in the bulk of R-MnO2.32 On the basis of their findings, the pseudocapacitance is conspicuously dependent on the effective surface area, which is associated with the particle size and pore distribution. Fine tuning of the pore structure and structural morphology using a surfactant template has been demonstrated to increase the effective surface area and the capacitance.4,23 The second mechanism involves the intercalation/deintercalation of protons (H+) or alkali metal cations in the bulk of manganese oxide as follows1,31

MnO2 + H+ + e- T MnOOH

(2)

MnO2 + C+ + e- T MnOOC

(3)

The second mechanism is expected to be predominant in crystalline manganese oxide.33,34 There are many different crystal forms of manganese dioxide, such as R-MnO2, β-MnO2, and γ-MnO2, for electrode materials. The specific capacitance of the crystallized manganese oxides has been shown to be dependent on the crystalline structure, especially with the size of the tunnels able to provide limited intercalation and deintercalation of cations during electrochemical reaction.35 R-MnO2 has been demonstrated to exhibit high capacitance because it contains larger (2 × 2) tunnels within its structure for facilitating intercalation and deintercalation of cations.36 The β-MnO2 structure is more stable in the manganese dioxide family because its structure consists of narrow (1 × 1) tunnels. However, this stable structure hinders the intercalation and deintercalation of cations, therefore resulting in a very low capacitance.34 The CVs were performed to identify the charge storage mechanism in manganese oxide-coated CF. Figure 4a shows the CVs of manganese oxide film electrodes carried out in various aqueous electrolytes of alkali metal salts at a scan rate of 25 mV s-1. Obviously, in addition to the oxygen evolution reaction at around 1.2 V, a couple of redox peaks appeared in aqueous electrolytes during cyclic voltammetry measurements, indicating that the manganese oxide in aqueous electrolytes exhibits Faradiac capacitance. The CV shapes of the manganese oxide-coated CF electrodes in aqueous electrolytes were almost the same, regardless of the alkali metal salt used. This finding suggested that the effect of alkali cations on the capacitive behavior of manganese oxide can be neglected. Wen et al.37 indicated that the capacitance of the MnO2 electrode shows little variation with changing the alkali cations in aqueous solutions. The redox peaks may arise from the intercalation and deintercalation of cations or protons into the bulk of manganese oxide due to the diffusion-limited reaction according to eq 2 or 3. Interestingly, the redox peaks of the manganese oxide electrode disappeared at all CV scan rates in a nonaqueous electrolyte with organic solvent (PC) and 1 M LiClO4 in the same potential window of 0-1.2 V (Figure 4b). The proton intercalation could be considerably mitigated by the limited proton amount in the nonaqueous electrolyte. Therefore, the redox reactions of manganese oxide in aqueous electrolytes came mainly from the intercalation and deintercalation of protons via eq 2 rather than those of alkali cations via eq 3. In aqueous electrolytes, it is reasonable that the intercalation/deintercalation and adsorption/ desorption of alkali metal cations may reach only the nearsurface region; protons can reach the interior region of the manganese oxide matrix due the relatively small size of protons compared with the size of cations. Therefore, the contribution

Figure 4. (a) CVs of manganese oxide carried out in various aqueous electrolytes of alkali metal salts at a scan rate of 25 mV s-1. (b) CVs of manganese oxide carried out in a PC-based electrolyte containing 1 M LiClO4 at various scan rates.

Figure 5. Nyquist plots of the manganese oxide coated on CF carried out in aqueous electrolyte (1 M Na2SO4 in H2O) and nonaqueous electrolyte (1 M LiClO4 in PC).

of protons to the pseudocapacitive process cannot be negligible in aqueous solution.37 Kuo et al.38 also pointed out that the bulk intercalation/deintercalation of protons into/from the structure of ε-MnO2 plays the predominant role in several aqueous alkali salt solutions. Figure 5 displays the Nyquist plots of manganese oxide film coated on CF carried out in aqueous and nonaqueous electrolytes. The semicircle in the high-frequency range corresponds to the charge-transfer resistance caused by the Faradaic reaction and the double-layer capacitance on the manganese oxide surface. The charge-transfer resistance of manganese oxide film in nonaqueous electrolyte was much higher than that of film in aqueous electrolyte. The charge-transfer resistance was correlated with the intercalation and deintercalation of cations. In

Electrodeposition of Needle-Like R-MnO2 Nanofibers

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21865

Figure 6. CVs of the CF substrate and manganese oxides coated on CF and SS substrates at a scan rate of 25 mV s-1.

aqueous electrolyte, the protons could intercalate into the interior region of the manganese oxide matrix, resulting in a low chargetransfer resistance. The manganese oxide film in nonaqueous electrolyte showed a relatively high charge-transfer resistance, reflecting that the specific capacitance primarily originates from the EDL capacitance, rather than the Faradaic capacitance. Figure 6 shows the CVs of manganese oxides coated on CF and SS substrates at a scan rate of 25 mV s-1 in 1 M Na2SO4 aqueous solution. Both electrodes exhibited redox peaks, indicating that, in addition to the electric double-layer capacitance, the manganese oxides also exhibit Faradiac capacitance resulting from the intercalation and deintercalation of protons. The responding current density of manganese oxide coated on CF was much higher than that of manganese oxide coated on the SS substrate. The higher the responding current density, the larger is the specific capacitance of the manganese oxide coated on CF. Moreover, the CV curve of the CF substrate (after heating at 100 °C for 1 h in air) in 1 M Na2SO4 aqueous solution, which is also shown in Figure 6, was scanned for comparison; the capacitance value was very low (less than 10 F g-1) compared with the main MnO2 material. Conclusively, capacitance came mainly from the MnO2 rather than the CF substrate. The specific capacitance (C) of manganese oxide electrodes during the cathodic process can be calculated according to the following equation

C)

1 ν · ∆V

1.2

∫ i · dV

(4)

0

where i is the instantaneous cathodic current density (A g-1) at a given potential, ∆V is the potential window (V), V is the applied potential (V), and ν is the scan rate (V s-1). The specific capacitance of MnO2-coated CF in 1 M Na2SO4 aqueous solution at a scan rate of 25 mV s-1 was calculated to be approximately 410 F g-1, which is much higher than that of MnO2 deposited on SS (160 F g-1). It is generally believed that the chargetransfer resistance, Ohmic resistance (electrolyte and electrode resistances), and mass-transfer resistance can affect the capacitive behavior of electrodes. By use of nanosized materials, some of the above factors may be negligible because they can provide a high specific surface area and fast redox reactions for reducing the charge-transfer resistance. In addition, the diffusion resistance of protons within the bulk of materials can be mitigated by the short diffusion path in nanosized materials. Hence, the

Figure 7. (a) Galvanostatic charge/discharge curves of the manganese oxide films at a current density of 10 A g-1 for three cycles. (b) Specific capacitance variation of manganese oxide films with various charge/ discharge current densities. The manganese oxides were coated on CF and SS substrates.

major limiting factors for determining the capacitive behavior of electrodes are the electronic conductivity (electron transport in the bulk of materials) and electrolyte penetration (proton transport within pores). In this work, the manganese oxide electrode of spaced radial nanorods was electrodeposited on the electronic conductive scaffolds of carbon fiber fabric. This unique architecture facilitates both the electrolyte percolation and the electron conduction, thus leading to higher surface area utilization. Figure 7a displays the galvanostatic charge/discharge curves of manganese oxide films coated on CF and SS at a current density of 10 A g-1. The MnO2 film coated on SS showed a sloping potential profile during charging and discharging, resulting from the EDLC behavior. A small sloping potential profile with a potential plateau was observed in the MnO2-coated CF electrode. The potential plateau arose from the Faradaic reaction. The charging time and discharging time of the MnO2coated SS electrode in each cycle were much shorter than that of the MnO2-coated CF electrode. The specific capacitance of MnO2 films during the galvanostatic test could be calculated according to the following equation

C)

i · ∆t ∆V

(5)

where ∆V (V) is the potential window, and i is the discharge current density (A g-1) applied for time ∆t (s). The specific capacitances of MnO2 films coated on CF and SS substrates were calculated to be approximately 392 and 161 F g-1, respectively. The values calculated from the galvanostatic

21866

J. Phys. Chem. C, Vol. 114, No. 49, 2010

Figure 8. Relationships between the peak current and the square root of the scan rate in cathodic and anodic processes.

discharging curves at a current density of 10 A g-1 were similar to those calculated from the CV curves at a scan rate of 25 mV s-1. The capacitance response of an electrode during high-rate charging and discharging (or high-rate CV scan) has become a crucial factor in electrochemical capacitor applications owing to the high power demand from portable electronic power tools and electric vehicles. Figure 7b shows the specific capacitance variation of manganese oxides with various charge/discharge current densities. The specific capacitance of manganese oxide coated on CF at each current density was much higher than that of manganese oxide coated on SS. The capacitance value of manganese oxide coated on CF reached as high as 432 F g-1 at a low current density of 5 A g-1 and reduced to 305 F g-1 at a high current density of 40 A g-1. On the other hand, the manganese oxide coated on SS only reached 177 F g-1 at a low current density of 5 A g-1 and reduced to 137 F g-1 at a high current density of 40 A g-1. As revealed in Figure 7b, the capacitance value was slightly decreased with increasing the current density, indicating that the electrochemical reaction on manganese oxide surface is fast enough. A decrease in capacitance value with increasing the current density can be explained by the slow diffusion of H+ ions into the 2 × 2 tunnels of R-MnO2. At higher current densities, diffusion limits the movement of H+ ions due to the time limit, and only the outer active surface is utilized for the charge storage. However, at lower current densities, most of the active sites can be utilized for charge storage. We believed that the improved capacitance may result from the unique architecture of MnO2 nanofibercoated CF. This architecture is basically associated with two significant advantages. First, the nanofiber configuration can guarantee that each nanofiber participates in the electrochemical reaction because each nanofiber is in electric contact with a carbon fiber and also interfaced with the electrolyte solution. The high surface contact between manganese oxide nanofibers and the carbon fibers results in a small contact resistance, which favors the high rate performance. Second, the open space between nanofibers allows for easy diffusion and migration of the electrolyte. This architecture is very helpful for high-power applications when the capacitor is charged or discharged at high current density. Figure 8 shows the linear relationships between the peak current and the square root of the scan rate in the cathodic and anodic processes. The linearity suggests a diffusion-limited reaction (semiinfinite diffusion). The peak current density ip may

Wu et al.

Figure 9. Relationship between capacitance retention and cycle number of both deposited manganese oxide films.

be expressed by the classical Randles-Sevick equation at 25 °C39

ip ) (2.69 × 105)n3/2CoAsD1/2ν1/2

(6)

where n is the number of electrons transferred, ip is the current density (A g-1), D (cm2 s-1) is the diffusion coefficient of the rate-limiting species (proton), As is the apparent surface area (cm2 g-1), ν is the scan rate (V s-1), and Co (mol cm-3) is the maximum proton concentration. By fitting the slope of ip versus V1/2 in Figure 8 into eq 6, the proton diffusion coefficient in the manganese oxide during cathodic and anodic processes can be estimated. Clearly, the slope of eq 6 in the anodic process was slightly higher than that of the cathodic process, reflecting that the diffusion coefficient of proton deintercalation is higher than that of intercalation. The cycle-life stability of manganese oxide electrodes was carried out by galvanostatic charge/discharge at a current density of 10 A g-1. Figure 9 shows the relationship between capacitance retention and cycle number of both deposited films. The capacitance retention of MnO2-coated CF decreased at the beginning of the 100 cycles and then stabilized for the ongoing cycles. The manganese oxide-coated SS showed a pronounced decay in capacitance retention after 2000 cycles. A decrease in specific capacitance could result from the disintegration of the MnO2 electrode during repeated charge and discharge cycles. A dark brown deposit was observed at the bottom of the cells after the cycling test. The loss of MnO2 active material from the MnO2-coated SS electrode surface was found to be greater than that from the MnO2-coated CF electrode after the cycling test. A better cycle-life stability of the manganese oxide nanofiber coated on CF could be attributed the good contact between the manganese oxide and carbon fibers. In addition to the electronic conductivity and electrolyte accessibility, the adhesion between oxide particles and the particles and the substrate also plays an important role in enhancing the capacitive behavior of the manganese oxide electrode. It was reported that the use of polyethylenimine (PEI) as an additive enables the formation of adherent manganese oxide films and reduces cracking during drying.40 In this work, the spaced radial manganese oxide nanofibers coated on carbon fibers could provide a better adhesion than the big aggregates of manganese oxide coated on SS. The high capacitance retention reflected a

Electrodeposition of Needle-Like R-MnO2 Nanofibers high durability of the manganese oxide-coated CF for electrochemical capacitor application in neutral electrolytes. 4. Conclusions The manganese oxide film with spaced radial nanofibers was uniformly coated on the individual carbon fibers of CF by use of the anodic electrodeposition technique. The carbon fibers could provide more electronic conductive paths for fast electron conduction. The spaced manganese oxide nanofibers were arranged roughly perpendicular to the surface of the carbon fibers, allowing high accessibility of electrolyte within the CF. XRD results indicated that the manganese oxide nanofiber film after annealing at 300 °C can be assigned to R-MnO2 with poor crystallinity and a small grain size. The redox peaks appeared in the aqueous electrolytes containing various alkali metal salts during cyclic voltammetry, whereas that disappeared in the PC (organic solvent) electrolyte containing 1 M LiClO4. The proton intercalation could be considerably mitigated by the limited proton amount in the organic electrolyte. Therefore, the redox reactions of manganese oxide in aqueous electrolytes came mainly from the intercalation and deintercalation of protons rather than those of alkali cations. The manganese oxide coated on CF also exhibited high capacitance and high cycle-life stability compared with that coated on SS during the galvanostatic charge/discharge test. The improved capacitive behavior resulted from the unique architecture and spaced radial manganese oxide nanofibers coated on carbon fibers, which is easier for electrolyte penetration and electron conduction. References and Notes (1) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444. (2) Pang, S. C.; Anderson, M. A. J. Mater. Res. 2000, 15, 2096. (3) Wei, J.; Nagarajan, N.; Zhitomirsky, I. J. Mater. Process. Technol. 2007, 186, 356. (4) Devaraj, S.; Munichandraiah, N. Electrochem. Solid-State Lett. 2005, 8, A373. (5) Raymundo-Pinero, E.; Khomenko, V.; Frackowiak, E.; Beguin, F. J. Electrochem. Soc. 2005, 152, A229. (6) Lee, C. Y.; Tsai, H. M.; Chuang, H. J.; Li, S. Y.; Lin, P.; Tseng, T. Y. J. Electrochem. Soc. 2005, 152, A716. (7) Bordjiba, T.; Be´langer, D. J. Electrochem. Soc. 2009, 156, A378. (8) Zhang, H.; Cao, G.; Wang, Z.; Yang, Y.; Shi, Z.; Gu, Z. Nano Lett. 2008, 8, 2664.

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21867 (9) Reddy, A. L. M.; Shaijumon, M. M.; Gowda, S. R.; Ajayan, P. M. J. Phys. Chem. C 2010, 114, 658. (10) Wan, C.; Azumi, K.; Konno, H. Electrochim. Acta 2007, 52, 3061. (11) Yan, J.; Fan, Z.; Wei, T.; Zhang, M. J. Mater. Sci.: Mater. Electron. 2010, 21, 619. (12) Chen, S.; Zhu, J.; Wu, X.; Han, Q.; Wang, X. ACS Nano 2010, 4, 2822. (13) Chang, J. K.; Lin, C. T.; Tsai, W. T. Electrochem. Commun. 2004, 6, 666. (14) Li, J.; Zhitomirsky, I. Mater. Chem. Phys. 2008, 112, 525. (15) Chou, S.; Cheng, F.; Chen, J. J. Power Sources 2006, 162, 727. (16) Wu, M. S.; Chiang, P. C. J. Electrochem. Solid-State Lett. 2004, 7, A123. (17) Subramanian, V.; Zhu, H.; Wei, B. J. Power Sources 2006, 159, 361. (18) Babakhani, B.; Ivey, D. G. J. Power Sources 2010, 195, 2110. (19) Wu, M. S. Appl. Phys. Lett. 2005, 87, 153102. (20) Xu, C. L.; Bao, S. J.; Kong, L. B.; Li, H.; Li, H. L. J. Solid State Chem. 2006, 179, 1351. (21) Xu, M.; Kong, L.; Zhou, W.; Li, H. J. Phys. Chem. C 2007, 111, 19141. (22) Xu, C.; Zhao, Y.; Yang, G.; Li, F.; Li, H. Chem. Commun. 2009, 7575. (23) Jiang, R.; Huang, T.; Liu, J.; Zhuang, J.; Yu, A. Electrochim. Acta 2009, 54, 3047. (24) Hu, C. C.; Tsou, T. W. Electrochim. Acta 2002, 47, 3523. (25) Hu, C. C.; Tsou, T. W. Electrochem. Commun. 2002, 4, 105. (26) Tench, D.; Warren, L. F. J. Electrochem. Soc. 1983, 130, 869. (27) Devaraj, S.; Munichandraiah, N. J. Electrochem. Soc. 2007, 154, A80. (28) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2669. (29) Reddy, R. N.; Reddy, R. G. J. Power Sources 2004, 132, 315. (30) Lee, H. Y.; Goodenough, J. B. J. Solid State Chem. 1999, 144, 220. (31) Toupin, M.; Brousse, T.; Be´langer, D. Chem. Mater. 2004, 16, 3184. (32) Toupin, M.; Brousse, T.; Be´langer, D. Chem. Mater. 2002, 14, 3946. (33) Ragupathy, P.; Vasan, H. N.; Munichandraiah, N. J. Electrochem. Soc. 2008, 155, A34. (34) Devaraj, S.; Munichandraiah, N. J. Phys. Chem. C 2008, 112, 4406. (35) Brousse, T.; Toupin, M.; Dugs, R.; Athoue¨l, L.; Crosnier, O.; Be´langer, D. J. Electrochem. Soc. 2006, 153, A2171. (36) Wu, M.; Snook, G. A.; Chen, G. Z.; Fray, D. J. Electrochem. Commun. 2004, 6, 499. (37) Wen, S.; Lee, J. W.; Yeo, I. H.; Park, J.; Mho, S. Electrochim. Acta 2004, 50, 849. (38) Kuo, S. L.; Wu, N. L. J. Electrochem. Soc. 2006, 153, A1317. (39) Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D.; Robinson, J. Instrumental Methods In Electrochemistry; Ellis Horwood Series in Physical Chemistry; Ellis Horwood: New York, 1985. (40) Nagarajan, N.; Humadi, H.; Zhitomirsky, I. Electrochim. Acta 2006, 51, 3039.

JP108598Q