INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 17 (2006) 2865–2869
doi:10.1088/0957-4484/17/12/007
Electrochemical capacitors based on electrodeposited ruthenium oxide on nanofibre substrates Young Rack Ahn1,2 , Mi Yeon Song1 , Seong Mu Jo1 , Chong Rae Park2 and Dong Young Kim1,3 1 Optoelectronic Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea 2 School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea
E-mail:
[email protected]
Received 3 March 2006, in final form 5 April 2006 Published 26 May 2006 Online at stacks.iop.org/Nano/17/2865 Abstract Electrodeposition of RuO2 on electrospun TiO2 nanorods using cyclic voltammetry is shown to increase the capacitance of RuO2 . This phenomenon can be attributed to the large surface areas of the nanorods. Among several ranges of deposition, the range from 0.25 to 1.45 V with respect to Ag/AgCl was effective. The electrode deposited with this range exhibited a specific capacitance of 534 F g−1 after deposition for 10 cycles with a scan rate of 50 mV s−1 . The structural water content in RuO2 was quite different depending on the deposition potential range. Higher amounts of structural water increased the charge storage capability. The stability of the electrode was tested using cyclic voltammetry over 300 cycles.
(crystalline form) RuO2 is a metallic electron conductor [3].
1. Introduction Electrochemical capacitors (ECs) have generated significant interest due to their high power density. ECs can be used as the peak-power source in electric vehicles, memory back-up devices, or in turn-on power sources for fuel cells [1]. Electrochemical capacitors generally have one of two forms: electric double layer capacitors (EDLCs) or pseudocapacitors [2]. The energy storage mechanism of EDLCs relies on the separation of charges (non-faradaic process) at the interface between an electrode and electrolyte. In pseudocapacitors, by contrast, the faradaic process occurs in addition to the simple charge separation. Therefore, the charge storage capability of pseudocapacitors is typically larger than that of EDLCs. One of the more promising materials for use in pseudocapacitors is ruthenium dioxide (RuO2 ), which is stable in acidic solution and shows a metallic conductivity in the crystalline form. Hydrous RuO2 , on the other hand, is a mixed conductor that conducts protons and electrons in acidic solution (as shown in equation (1)), while anhydrous 3 Author to whom any correspondence should be addressed.
0957-4484/06/122865+05$30.00
RuOx (OH) y + δ H+ + δ e− RuOx−δ (OH) y+δ .
(1)
During the charging–discharging process, the protons and electrons are transferred between RuO2 and the electrolyte solution. Thus, it is desirable to have high electronic conductivity as well as high proton conductivity. These properties can be controlled through high temperature treatment. Specifically, increasing the temperature increases the electronic conductivity of RuO2 but decreases the proton conductivity, so that an optimum state exists [3]. RuO2 prepared by the sol–gel method has shown extraordinary capacitance [4, 5]. However, the high cost of the precursor materials restricts their widespread use in the bulk form. Instead, the RuO2 composites with various materials such as nanostructured carbon aerogel [6] and fibrous carbon paper [7] or thinly deposited layers of RuO2 [8, 9] have been investigated. The manganese oxide nanowires were also used for the active materials of electrochemical capacitors [10]. For example, one study demonstrated that thin layers of RuO2 can be electrodeposited on certain substrates using cyclic voltammetry [1, 11].
© 2006 IOP Publishing Ltd Printed in the UK
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The aim of the present study was to enhance the charge storage capability (specific capacitance) of RuO2 by depositing a thin layer of this material onto substrates having a large specific surface area. For this purpose, cyclic voltammetry was used to electrodeposit RuO2 onto TiO2 nanofibres from an aqueous solution of RuCl3 . The as-produced electrode was studied for pseudocapacitor applications in aqueous 0.5 M H2 SO4 electrolyte. The effects of varying the deposition potential range and the stability of the electrode were investigated.
2. Experimental details Titanium dioxide (TiO2 ) nanofibres were prepared using an electrospinning process, the detailed procedures of which are described elsewhere [12]. Briefly, the precursor solution was obtained by dissolving 6 g of titanium(IV) propoxide (Aldrich), 2.4 g of acetic acid, and 3 g of poly(vinyl acetate) (MW = 850 000 g mol−1 ) in N , N -dimethylformamide (DMF). The resulting solution was electrospun onto a titanium plate (99.7%, Aldrich) in an electric field of 1.4 kV cm−1 (205B-50R, Bertan), at a flow rate of 50 µl min−1 . The spinning rate was controlled using a syringe pump (KD Scientific 220). The as-spun TiO2 web was then pressed using preheated plates at 120 ◦ C for 10 min. The pressed electrode was then calcined (to remove the polymer and to develop the anatase TiO2 structure) at 450 ◦ C for 30 min in air. Electrodeposition of ruthenium dioxide (RuO2 ) was carried out on electrospun TiO2 using a 0.05 M aqueous ruthenium chloride (RuCl3 ·n H2 O, Aldrich) solution. The precursor solution was stirred during the deposition, and the temperature was maintained at 50 ◦ C. Cyclic voltammetry was used in the deposition of RuO2 ·n H2 O using a three-electrode system (EG&G PAR, model 273A), comprising a Pt counter electrode and a saturated Ag/AgCl reference electrode. The deposited RuO2 electrodes were heat treated at 175 ◦ C. The amount of RuO2 ·n H2 O used was obtained from the weight difference before and after deposition. A titanium plate was also used as a substrate for comparison. The Ti-plate was polished using emery paper, degreased in soap and water, and then rinsed with acetone. The same procedure for RuO2 electrodeposition was carried out on the roughened Ti-plate surface. The RuO2 deposited electrodes were examined using cyclic voltammetry in aqueous 0.5 M sulfuric acid solution. The counter and reference electrodes used were Pt and saturated Ag/AgCl, respectively. X-ray photoelectron spectroscopy was performed using a PHI 5800 ESCA system equipped with an Al Kα monochromatic source. The background pressure of the analysis chamber was 2 × 10−10 Torr. Multipak (PHI) was used to curve fit the XPS core level spectrum. The morphologies of the RuO2 deposited electrodes were observed using FESEM (JSM-6330F, JEOL), while the specific surface area was measured by the nitrogen adsorption–desorption method (Sorptomatic 1990, ThermoFinnigan).
3. Results and discussion Electrospinning is an efficient technique for producing polymeric or inorganic nanofibres within a range of diameters from several micrometres down to tens of nanometres. In 2866
Figure 1. Scanning electron micrographs of (a) as-spun TiO2 web and (b) TiO2 nanorod electrode which was pressed at 120 ◦ C for 10 min, followed by calcination (inset, TEM).
electrospinning, a high voltage is applied between a grounded plate and the thin tip of a nozzle connected to a syringe containing a viscous solution. When the voltage is high enough to overcome the surface tension of the solution, a droplet of solution produced at the tip is transformed into a Taylor cone. One or more jets of the solution are then ejected from the end of the Taylor cone, after which they travel towards the grounded plate, eventually resulting in a nanofibre web. The Ti-plate was used as a current collector, because other metal plates, such as stainless steel, nickel, and aluminum, are unstable in acidic solution. Here, a TiO2 nanofibre web was electrospun onto the Ti-plate from a solution of sol–gel precursor with poly(vinyl acetate) (PVAc) in DMF. The PVAc polymer binder increases the viscosity of the solution, enabling the formation of fibres during electrospinning. The as-formed electrospun web was first pressed between preheated plates at 120 ◦ C for 10 min, and then calcined at 450 ◦ C in air to remove the polymer binder and to develop the anatase structure of TiO2 . After calcination, the morphology of the hot-pressed RuO2 electrode was found to be different from that of the TiO2 web electrode. In this respect, the original web structure of the as-spun fibres was retained, but each of the fibres was composed of TiO2 nanorods, as shown in figures 1(a) and (b). We previously reported on the formation of fibrils in electrospun TiO2 nanofibres, and the production of TiO2 nanorods [12, 13]. As-spun TiO2 nanofibres are formed with a bundle structure comprising several fibrils within the core. The fibrils in the TiO2 nanofibre were separated out due to high pressure. As a result, TiO2 nanorods were formed, in which the original web structure of the as-spun fibres was retained, resulting in a large increase in the specific surface area. The Brunauer–Emmet–Teller surface area of the TiO2 web after calcination (without pressing) was 31 m2 g−1 , while that of the pressed TiO2 web after calcination was 89 m2 g−1 .
Electrochemical capacitors based on electrodeposited ruthenium oxide on nanofibre substrates
Figure 2. Cyclic voltammograms of RuO2 electrodes deposited on (a) surface-roughened Ti and (b) TiO2 web electrode.
Cyclic voltammetry was used to deposit RuO2 onto electrospun TiO2 nanofibres. A surface-roughened Ti-plate was also used as a substrate to compare the effect of TiO2 nanofibres on the capacitance of RuO2 . Here, the deposition scan rate was 50 mV s−1 and the scan range was 0.25–1.45 V with respect to the Ag/AgCl electrode. It has been suggested that, during the cathodic sweep of the cyclic voltammetric deposition, the cathodic current generates OH− ions in the aqueous media according to the electrochemical reactions below (equations (2) or (3)) [14]. In this step, the ruthenium species precipitates as either hydrated ruthenium oxide RuO2 ·n H2 O or ruthenium hydroxide.
or
2H2 O + 2e− H2 + 4OH−
(2)
O2 + 2H2 O + 4e− 4OH− .
(3)
During the anodic sweep, the deposited ruthenium species is oxidized to either Ru(IV) or Ru(VI). These procedures are repeated in each deposition cycle. Cyclic voltammetry is widely used among several methods (e.g., galvanostatic charge–discharge, self-discharge, and ac impedance spectroscopy) to measure capacitance [15, 16]. A three-electrode system, comprising a Pt counter electrode and a KCl-saturated Ag/AgCl reference electrode, was used. Here, the cyclic voltammetry study was conducted in an aqueous 0.5 M sulfuric acid solution electrolyte within the range from 0 to 1 V with respect to the Ag/AgCl reference electrode, at a scan rate of 10 mV s−1 . The average specific capacitance was calculated according to the equation:
Csp =
i v×w
(4)
where i, v , and w represent the average current, scan rate, and weight of deposited RuO2 , respectively. The average current was calculated from the corresponding cyclic voltammograms. Negligibly small capacitances were observed when Ti or TiO2 electrodes were used without RuO2 deposition. Therefore, the specific capacitance was calculated with only the weight of RuO2 . The cyclic voltammograms produced by the deposited RuO2 electrodes are shown in figure 2. Here, the
Figure 3. Scanning electron micrographs of RuO2 electrodes deposited onto TiO2 nanorods. After (a) 25 cycles, (b) 50 cycles, and (c) 100 cycles of deposition with the scan range of 50 mV s−1 .
capacitance of RuO2 deposited on the surface-roughened Tiplate is 4.4 mF cm−2 , whereas that of RuO2 deposited on TiO2 nanorods is significantly higher, at 74.8 mF cm−2 . This difference in capacitance is still high even after normalizing it with respect to the weight of the RuO2 deposit. The specific capacitance of RuO2 deposited on the surface-roughened Tiplate was 109 F g−1 , whereas that of RuO2 deposited on the TiO2 web was 534 F g−1 . The charge storage mechanism of RuO2 is essentially a protonation process, as shown in equation (1), where protons and electrons participate in the charge–discharge reaction of RuO2 . As such, the capacitance of RuO2 is limited by the proton conductivity [17], and thinner films are more favourable for application as pseudocapacitors. In this study, TiO2 nanofibre substrates provide a large specific surface area in which to obtain thinner RuO2 films. Scanning electron micrographs of the deposited RuO2 electrodes are shown in figure 3. In these micrographs, the contour of the TiO2 nanofibres is still discernible after 25 2867
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Figure 4. The capacitance of the electrode as a function of deposited weight of RuO2 .
Figure 5. Cyclic voltammograms of RuO2 electrodes deposited with various potential ranges.
Table 1. Summary of specific capacitances and ratios of XPS spectra as a function of deposition scan ranges. Deposition scan Specific capacitance (F g−1 ) O 1s(H2 O)/Ru 3d(RuO2 ) rangea (V)
−0.2–1.0 0.2–1.2 0.2–1.4 0.25–1.45 0.5–1.5 −0.2–1.5 a
10 348 457 534 309 78
— 0.19 0.23 0.45 — —
Reference electrode was a saturated Ag/AgCl electrode.
and 50 deposition cycles (figures 3(a) and (b) respectively). However the surface of the TiO2 nanofibres was completely covered with RuO2 after 100 cycles (figure 3(c)). As the thickness of the deposited RuO2 increased, the interstitial pores of the TiO2 nanofibres became increasingly filled with the deposited material, such that the electrode was completely covered with RuO2 . Thus, the nanofibre surface tends to become flatter, with a concomitant reduction in surface area. The capacitance of the electrode as a function of the deposited RuO2 weight is shown in figure 4, where the deposition scan rate was 50 mV s−1 and the scan range was from 0.25 to 1.45 V with respect to Ag/AgCl. As the number of deposition cycles was increased, the amount of deposited RuO2 increased accordingly; however, the average specific capacitance was observed to decrease. This decrease in capacitance is due to the increasing thickness and decreasing surface area of RuO2 . The oxidation state of ruthenium typically changes from Ru(II) at 0 V to Ru(IV) at 1.4 V with respect to a standard hydrogen electrode [4]. Among several deposition potential ranges tested, the range from 0.2 to 1.5 V with respect to Ag/AgCl was selected. As shown in figure 5 and table 1, slight changes to the potential range deeply affected the resulting specific capacitance. As the lower potential limit was lowered, the weight of the deposited material became increasingly less, while the specific capacitance became steadily poorer. When the upper potential limit was increased, however, the weight of the RuO2 deposit increased, but the specific capacitance was observed to fall. 2868
Figure 6. XPS spectra of (a) Ru(3d) and (b) O(1s) regions.
An XPS analysis was carried out on the RuO2 electrode to investigate the chemical state of the ruthenium and oxygen in the deposits. Prior to analysis, the deposited electrodes were treated at 175 ◦ C for 2 h. The high-resolution XPS spectra of Ru(3d) and O(1s) are shown in figure 6. Here, the Ru 3d core level spectra are attributed to the 5/2 and 3/2 spin–orbit components located at 281.2 and 285.3 eV, respectively. The peak position of the Ru(3d) level was maintained even after
Electrochemical capacitors based on electrodeposited ruthenium oxide on nanofibre substrates
only a slight change in the current after 50 cycles. Figure 7(b) shows the variation in the capacitance as a function of the number of cycles. The capacitance decays 5% during the first 20 cycles, and then maintains almost the same value thereafter. The capacitance of RuO2 on TiO2 nanorods was maintained at about 93% after 300 cycles compared to the initial cycle.
4. Conclusions Cyclic voltammetry was used to electrodeposit thin films of RuO2 onto electrospun porous TiO2 nanorods. The as-formed thin RuO2 layers exhibited high specific capacitance. Cyclic voltammograms of the electrodes were rectangular shaped and featureless, which is characteristic of RuO2 . Among several ranges of deposition, the range from 0.25 to 1.45 V with respect to Ag/AgCl was effective. The specific capacitance was 534 F g−1 after deposition for 10 cycles with a scan rate of 50 mV s−1 . Between 0.25 and 1.45 V, the deposited RuO2 had higher structural water content than in films deposited at lower potential ranges. From XPS measurements of the deposited RuO2 electrode films, the area ratio of O 1s to Ru 3d increased from 0.19 to 0.45 when the deposition range was changed from 0.2–1.2 V to 0.25–1.45 V with respect to Ag/AgCl. The presence of sufficient structural water in the RuO2 was found to increase the charge storage capability. After 300 cycles, the capacitance of RuO2 on TiO2 nanorods was maintained at about 93% of the value for the initial cycle. Figure 7. (a) Cyclic voltammograms of RuO2 and (b) the capacitance as a function of number of cycles.
References
deposition at different potential ranges; however, the O(1s) peak position was shifted to a higher energy (from 529.5 to 530.5 eV) as the deposition potential range was moved to a higher potential. A curve fitting of the high-resolution XPS spectra, carried out using Multipak software (PHI), showed that the area ratios of O(1s) from water (533.1 eV) with respect to Ru(3d) were quite different according to the deposition potential ranges applied. As the deposition potential range was changed from 0.2–1.2 V to 0.25–1.45 V, the area ratio of O(1s) to Ru(3d) increased from 0.19 to 0.45. As such, the water content was found to be higher after deposition at a higher potential range. This increase in water content facilitates proton transfer. Since RuO2 is expected to transfer protons and electrons during the charge–discharge process, facile transport paths for both charge carriers must exist. Generally, oxygenion vacancies or structural water are involved in the transfer of protons [3]. Therefore, a higher content of water in RuO2 would lead to a more facile transport of protons in the material, resulting in a higher specific capacitance. The stability of the RuO2 electrode was also studied using cyclic voltammetry, where the electrode was cycled 300 times at a scan rate of 50 mV s−1 . As shown in figure 7(a), the current was found to decrease during the first 20 cycles, with
[1] Hu C C and Huang Y H 1999 J. Electrochem. Soc. 146 2465–71 [2] Conway B E 1999 Electrochemical Supercapacitors (Dordrecht: Kluwer–Academic) pp 13–5 [3] McKeown D A, Hagans P L, Carette L P L, Russell A E, Swider K E and Rolison D R 1999 J. Phys. Chem. 103 4825–32 [4] Zheng J P, Cygan P J and Jow T R 1995 J. Electrochem. Soc. 142 2699–703 [5] Long J W, Swider K E, Merzbacher C I and Rolison D R 1999 Langmuir 15 780–5 [6] Miller J M and Dunn B 1999 Langmuir 15 799–806 [7] Park J H and Park O O 2002 J. Power Sources 109 121–6 [8] Wang Y G and Zhang X G 2004 Electrochim. Acta 49 1957–62 [9] Hu C C and Chen W C 2004 Electrochim. Acta 49 3469–77 [10] Wu M-S 2005 Appl. Phys. Lett. 87 153102 [11] Park B O, Lokhande C D, Park H S, Jung K D and Joo O S 2004 Mater. Chem. Phys. 87 59–66 [12] Song M Y, Ahn Y R, Jo S M, Kim D Y and Ahn J P 2005 Appl. Phys. Lett. 87 113113 [13] Song M Y, Ahn Y R, Jo S M and Kim D Y 2005 Mater. Res. Soc. Symp. Proc. 836 107–12 [14] Zhitomirsky I 1999 J. Mater. Sci. 34 2441–7 [15] Sugimoto W, Kizaki T, Yokoshima K, Murakami Y and Takasu Y 2004 Electrochim. Acta 49 313–20 [16] Sato Y, Yomogida K, Nanaumi T, Kobayakawa K, Ohsawa Y and Kawai M 2000 Electrochem. Solid-State Lett. 3 113–6 [17] Zheng J P 1999 Electrochem. Solid-State Lett. 2 359–61
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