Textural and Capacitive Characteristics of Hydrothermally Derived

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Textural and Capacitive Characteristics of Hydrothermally Derived RuO2‚xH2O Nanocrystallites: Independent Control of Crystal Size and Water Content Kuo-Hsin Chang, Chi-Chang Hu,* and Chih-Yin Chou Laboratory of Electrochemistry and AdVanced Materials, Department of Chemical Engineering, National Chung Cheng UniVersity, 168 UniVersity Road, Min-Hsiung, Chia-Yi 621, Taiwan ReceiVed December 13, 2006. ReVised Manuscript ReceiVed February 10, 2007

RuO2‚xH2O nanoparticulates in different crystal sizes with various water contents were prepared via a hydrothermal synthesis route, successfully demonstrating the independent control of crystal size and water content of RuO2‚xH2O. The crystalline and hydrous nature of hydrothermally derived RuO2‚xH2O particulates not only reduces the proton diffusion resistance but also enhances the electronic conductivity for the redox transitions of active species. Novel and unique properties of hydrothermally derived RuO2‚ xH2O, i.e., effective inhibition of crystallite coalescence upon annealing, relatively high thermal stability, and maintenance of the original nanostructure, are attributable to the coalescence barrier of RuO2‚xH2O crystallites due to the lattice energy. Maintaining/fine-tuning the original nanostructure of annealed RuO2‚ xH2O crystallites with high mesoporosity favors the penetration of electrolytes into the whole oxide matrix. This effect of not only reducing the proton diffusion resistance but also improving the electron pathways promotes the utilization of RuO2‚xH2O in supercapacitor applications.

Introduction In 1971, Trasatti et al. studied the electrochemical behavior of RuO2-based dimensionally stable anodes (i.e., DSA) for chlorine evolution and proposed that anhydrous RuO2 crystals show capacitive-like i-E responses.1 Furthermore, extremely high redox reversibility of ruthenium oxide was investigated from the studies of hydrous, hyper-extended RuO2 thin film on Ru metal by Conway et al. in 1978.2 The ideal reversibility, desirable to the pseudocapacitor application, had been attributed to the hydrous nature of electrochemically formed RuO2‚xH2O thin film, resulting in the ease of the protonexchange process because the redox transitions of RuO2 involve the double injection/expel of protons and electrons.2-4 RuOa(OH)b + δH+ + δe- T RuOa-δ(OH)b+δ After extensive investigations on the electrochemical characteristics of RuO2,1-5 there was a breakthrough in the development of hydrous RuO2 as an electrode material for pseudocapacitors in 1995 because Zheng et al. showed the ultrahigh specific capacitance (720 F g-1) of sol-gel derived RuO2‚xH2O through careful control of the annealing process.6 After these pioneering studies, many research groups in the world started to synthesize RuO2‚xH2O through various * Corresponding author. E-mail: [email protected].

(1) Trasatti, S.; Buzzanca, G. J. Electroanal. Chem. 1971, 29, A1. (2) Hadzi-Jordanov, S.; Angerstein-Kozlowska, H.; Vukovic, M.; Conway, B. E. J. Electrochem. Soc. 1978, 125, 1471. (3) Michell, D.; Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1978, 89, 11. (4) Trasatti, S. Electrochim. Acta 1991, 36, 225. (5) Conway, B. E. Electrochemical Supercapacitors; Kluwer Academic/ Plenum: New York, 1999. (6) (a) Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6. (b) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699.

methods.7-19 In addition, several important analyses were performed to identify the charge storage/delivery mechanism of RuO2‚xH2O, including X-ray adsorption near-edge structure (XANES) and extended X-ray adsorption fine structure (EXAFS) analyses,20 the X-ray scattering method,21 the solidstate 1H NMR,22 electrochemical impedance spectroscopy (EIS) studies,23 X-ray photoelectron spectra (XPS),16,24 and EQCM methods.8b All the above studies try to get a further (7) Hu, C. C.; Huang, Y. H. J. Electrochem. Soc. 1999, 146, 2465. (8) (a) Kim, I. H.; Kim, K. B. Electrochem. Solid-State Lett. 2001, 4, A62. (b) Kim, I. H.; Kim, K. B. J. Electrochem. Soc. 2004, 151, E7. (9) (a) Lin, C.; Ritter, A.; Popov. B. N. J. Electrochem. Soc. 1999, 146, 3155. (b) Kim, H.; Popov. B. N. J. Power Sources 2002, 104, 52. (10) Suh, D. J.; Park, T. J.; Kim, W. I.; Honh, I. K. J. Power Sources 2003, 117, 1. (11) (a) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y. Angew. Chem., Int. Ed. 2003, 42, 4092. (b) Sugimoto, W.; Yokoshima, K.; Ohuchi, K.; Murakami, Y.; Takasu, Y. J. Electrochem. Soc. 2006, 153, A255. (12) Battaglin, G.; Rigato, V.; Zahdolin, S.; Benedetti, A.; Ferro, S.; Nanni, L.; Battisti, A. D. Chem. Mater. 2004, 16, 946. (13) Hu, C. C.; Chen, W. C.; Chang, K. H. J. Electrochem. Soc. 2004, 151, A281. (14) Swider, K. E.; Love, C. T.; Rolison, D. R. J. Electrochem. Soc. 2005, 152, C158. (15) (a) Jang, J. H.; Kato, A.; Machida, K.; Naoi, K. J. Electrochem. Soc. 2006, 153, A321. (b) Min, M.; Machida, K.; Jang, J. H.; Naoi, K. J. Electrochem. Soc. 2006, 153, A334. (16) Chang, K. H.; Hu, C. C. J. Electrochem. Soc. 2004, 151, A958. (17) (a) Chang, K. H.; Hu, C. C. Electrochem. Solid-State Lett. 2004, 7, A466. (b) Chang, K. H.; Hu, C. C. Appl. Phys. Lett. 2006, 88, 193102. (18) Ke, Y. F.; Tsai, D. S.; Huang, Y. S. J. Mater. Chem. 2005, 15, 2122. (19) Hu, C. C.; Chang, K. H.; Lin, M. C.; Wu, Y. T. Nano Lett. 2006, 6, 2690. (20) McKeown, D. A.; Hagans, P. C.; Carette, L. P. L.; Russell, A. E.; Swider, K. E.; Rolison, D. R. J. Phys. Chem. B 1999, 103, 4825. (21) Dmowski, W.; Egami, T.; Swider, K. E.; Love, C. T.; Rolison, D. R. J. Phys. Chem. B 2002, 106, 12677. (22) Fu, R.; Ma, Z.; Zheng, J. P. J. Phys. Chem. B 2002, 106, 3592. (23) (a) Swider, K. E.; Merzbacher, C. I.; Hagans, P. L.; Rolison, D. R. Chem. Mater. 1997, 9, 1248. (b) Sugimoto, W.; Iwata, H.; Yokoshima, K.; Murakami, Y.; Takasu, Y. J. Phys. Chem. B 2005, 109, 7330.

10.1021/cm0629661 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007

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understanding on the charge storage/delivery mechanism, to improve the utilization of active materials, or to obtain a simple, effective synthesis method for RuO2‚ xH2O. Accordingly, from our recent review on developing RuO2-based electrode materials for pseudocapacitors,25 the key factors determining their performances generally include (1) electrochemical reversibility of electroactive materials, (2) electronic resistance “between” and “within” electroactive particles, (3) contact resistance at the interface between current collector and active materials, (4) proton conductivity and diffusivity between and within particulates, and (5) pore structure and surface area. The above understanding provides the directions/rules in designing RuO2‚xH2O for supercapacitors of next generation.11,16-19,26 For example, Sugimoto et al. developed layered ruthenic acid hydrates with excellent capacitive performances because the crystalline structure and the hydrous nature of interlayers provide very facile pathways for proton and electron transports, respectively.11 Recently, RuO2‚xH2O nanotubular arrayed electrodes were synthesized by means of the anodic deposition technique coupled with the membranetemplated synthesis route in our group. The desired 3D mesoporous architecture of annealed RuO2‚xH2O nanotubular arrayed electrodes simultaneously maintains the facility of electrolyte penetration, the ease of proton exchange/diffusion, and the metallic conductivity of crystalline RuO2, exhibiting unexpectedly ideal capacitive performances, e.g., ultrahigh specific capacitance (∼1300 F g-1 measured at 10 mV s-1), excellent charge/discharge behavior at 1000 mV s-1, and high-frequency (4.0-7.8 kHz) capacitive responses.19 On the basis of these successes, how to design and tailor RuO2based oxides for the next generation supercapacitors from the understanding of key factors is of both academic and practical importance in the supercapacitor technologies. Theoretically, the crystal size of RuO2‚xH2O particles is controllable by varying the annealing time and temperature, which is believed to reduce the electron-hopping resistance between and within particles.6,13,20,21,23a,25 However, if the bridging oxo bonds within and between RuO2‚xH2O particulates due to sintering and crystal growth are formed via annealing, the specific capacitance of RuO2‚xH2O decreases very sharply because of the loss in the electroactive sites.13,20,21,25 In addition, the crystal size and water content of RuO2‚xH2O are inversely and simultaneously changed with varying the annealing time and temperature. Hence, it is very difficult to independently control the crystal size and water content of RuO2‚xH2O. In our preliminary study, crystalline RuO2‚xH2O nanoparticulates with high water content were prepared via a mild hydrothermal process, which has been shown to be a promising electrode material for electrochemical supercapacitors.17 In this article, we report the material (24) Foelske, A.; Barbieri, O.; Hahn, M.; Ko¨tz, R. Electrochem. SolidState Lett. 2006, 9, A268. (25) Chang, K. H.; Wu, Y. T.; Hu, C. C. Key Factors Determining the Performances of RuO2-Based Supercapacitors. In Recent AdVances in Supercapacitors; Gupta, V., Ed.; Transworld Research Network: Kerala, India, 2006; Chapter 3, pp 29-56. (26) (a) Park, J. H.; Ko, J. M.; Park, O. O. J. Electrochem. Soc. 2003, 150, A864. (b) Kim, I. H.; Kim, J. H.; Lee, Y. H.; Kim, K. B. J. Electrochem. Soc. 2005, 152, A2170. (c) Ye, J. S.; Cui, H. F.; Liu, X.; Lim, T. M.; Zhang, W. D.; Sheu, F. S. Small 2005, 1, 560.

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characteristics, electrochemical properties, and annealing effects for hydrothermally derived RuO2‚xH2O nanocrystallites to demonstrate their unique merits for supercapacitor applications. Experimental Section Synthesis of RuO2‚xH2O and Electrode Preparation. The synthesis procedure of RuO2‚xH2O nanocrystallites is completely followed our previous work.17 A 20 mM RuCl3‚xH2O (Alfa Aesar) solution was kept in a Teflon-lined autoclave with a stainless steel shell. This autoclave was heated to 180 °C at 10 °C min-1 in an oven. The autoclave was cooled to room-temperature naturally after being kept at 180 °C for different time periods. RuO2‚xH2O particulates were efficiently obtained by means of a centrifuge and then washed with pure water several times until the particulates reached pH 7. The precipitates were dried in a reduced-pressure oven overnight at room temperature for material analysis. Prior to the dip-coating process, commercial 99% titanium substrates (10′10′0.5 mm) were first mechanically polished by emery, which was blown by a high-pressure air compressor. These substrates were degreased with soap and water and etched for 1.5 h in a 6 M HCl solution at ca. 90 °C. They were then rinsed with water again and pickled for 10 min in a solution consisting of DMF (N,N-dimethyl-formamide, Wako E.P.), water, and HF (Wako E.P.) in a 40:7.5:25 volume ratio. After the pickling pretreatment, the substrates were rinsed with acetone and water and finally dried under a cool airflow. These substrates were dipped in an aqueous solution containing 3 mg cm-3 RuO2‚xH2O without any binder or conductive additives and then dried in a reduced pressure oven at 85 °C for 20 min. The above procedure was repeated several times to reach the desired mass, and finally, the products were further dried in a reduced pressure oven for 8 h at room temperature. The loading mass of RuO2‚xH2O is ca. 0.4-0.5 mg cm-2. The side and back of all electrodes were coated with PTFE films and the exposed area was 1 cm2 for electrochemical characterization. Characterization. The nanostructure and electron diffraction patterns of oxides were examined by means of a transmission electron microscope (TEM, JEM-3010, JEOL). X-ray diffraction patterns were obtained from an X-ray diffractometer (Rigaku Miniflex system) using a Cu target (CuKR ) 1.5418 Å) at an angle speed of 4° 2θ min-1. Raman spectrograms were measured using a 3D nanometer scale Raman PL microspectrometer (Tokyo Instruments, INC) with 632.8 nm radiation of a HeNe laser, which was focused in a circle area less than 1 µm in diameter. Thermal data of oxides were determined by thermogravimetric/differential thermal analyses (Perkin-Elmer Instruments, Diamond TG/DTA), which was performed in an air flow at 5 °C min-1 from room temperature to 600 °C. Electrochemical characteristics of RuO2‚xH2O-coated electrodes were examined by means of an electrochemical analyzer system, CHI 633A (CH Instruments) at 25 °C in a three-compartment cell. An Ag/AgCl electrode (Argenthal, 3 M KCl, 0.207 V vs SHE at 25 °C) was used as the reference and a piece of platinum gauze was employed as the counter electrode. A Luggin capillary was used to minimize errors due to iR drop in the electrolytes. All solutions used in this work were prepared with 18 MΩ cm water produced by a reagent water system (Milli-Q SP, Japan). The electrolytes used for the electrochemical characterization were degassed with purified nitrogen gas before measurements for 25 min, and this nitrogen was passed over the solutions during the measurements.

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Figure 1. TEM images of RuO2‚xH2O nanoparticles with hydrothermal time equal to (a) 1.5, (b) 3, (c) 6, and (d) 24 h.

Results and Discussion Textural Characteristics of As-Prepared RuO2‚xH2O. The particle size and distribution generally influence the utilization of RuO2‚xH2O, which is an essential concern in using noble metal oxides. The morphologies and particle sizes of RuO2‚xH2O with different hydrothermal times were identified by means of a transmission electron microscope (TEM, see Figure 1a-d). Particulate morphologies in the nanometer scale are clearly observed from the TEM images, indicating that RuO2‚xH2O nanoparticles are easily obtained under mild hydrothermal conditions. Note the very narrow particle size distribution of oxides (see Figure S1 in the Supporting Information), indicating the highly uniform size of RuO2‚xH2O particulates. The average particle diameter of RuO2‚xH2O with the hydrothermal time equal to 1.5, 3, 6, 24 h is about 1.55, 1.62, 1.90, and 2.60 nm, respectively. The above result indicates the slight influence of hydrothermal time on the average particle size of RuO2‚xH2O. However, crystal growth is significant for the samples with a longer hydrothermal time (see below for ED and XRD results). The former result implies that most ruthenium precursors were rapidly precipitated to form RuO2‚xH2O particulates during the initial period of hydrothermal synthesis; meanwhile, condensation of hydroxyl groups and growth of crystallites occurred sequentially. Thus, the typical steps in hydrothermal synthesis, such as crystal dissolution, nucleation, and recrystallization,27 should not occur in the posthydrothermal process. Moreover, most surface Ru atoms should be enriched with hydroxyl groups because the nanometer-sized RuO2‚xH2O particulates prepared in this work show the property of high surface/volume ratios (see below). Accordingly, prolonging the hydrothermal time is proposed as “hydrothermal annealing”, which promotes the (27) (a) Walton, R. I. Chem. Soc. ReV. 2002, 31, 230. (b) Xi, G.; Xiong, K.; Zhao, Q.; Zhang, R.; Zhang, H.; Qian, Y. Cryst. Growth Des. 2006, 6, 577.

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Figure 2. Electron diffraction patterns of RuO2‚xH2O nanoparticles with hydrothermal time equal to (a) 1.5, (b) 3, (c) 6, and (d) 24 h.

condensation of hydroxyl groups coupled with the growth of crystallites but remains the hydrous nature for RuO2‚xH2O. The electronic conductivity of RuO2‚xH2O is determined by its crystalline structure, which is very important for electron hopping within the oxide particulates during the charge/discharge tests. Note the absence of clear diffraction rings in Figure 2a for RuO2‚xH2O with the hydrothermal time equal to 1.5 h, although this oxide has been confirmed to be of the rutile structure (see Raman spectra). The above facts indicate the formation of extremely local RuO6 rutile structure for the as-prepared RuO2‚xH2O nanoparticulates with hydrothermal time e1.5 h. This statement is also supported by the voltammetric behavior of RuO2‚xH2O with hydrothermal time e1.5 h (see below). The number and locus of diffraction rings in the electron diffraction (ED) patterns are increased and become clearer with prolonging the hydrothermal time. This evidence reveals that the crystal size of RuO2 in every particulate is controllable by varying the hydrothermal time. The gray of electron diffraction rings are reasonably ascribed to the few-nanometer size and waterenriched nature of RuO2‚xH2O nanoparticles, resulting in the poor constructive diffractions. Effects of the hydrothermal time on the crystal size and local structure of as-prepared RuO2‚xH2O particulates were further examined by X-ray diffraction (XRD), shown in Figure 3a. The XRD patterns show four broad diffraction peaks corresponding to rutile RuO2 (in comparing with the PDF 43-1027 file of rutile RuO2). These diffraction peaks become clearer with prolonging the hydrothermal time, revealing successful control of the RuO2‚xH2O crystal size by varying the hydrothermal time. The Raman spectra shown in Figure 3b were used to identify the microstructure information on the molecular scale of hydrothermally derived RuO2‚xH2O particulates. From the analyses of ED and XRD patterns shown in Figures 2a and 3a, we cannot obtain constructive diffractions for RuO2‚xH2O

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Figure 3. (a) Powder XRD patterns of RuO2‚xH2O with the hydrothermal time equal to (1) 0.75, (2) 1.5, (3) 3, (4) 6, and (5) 24 h. (b) The Raman spectra of RuO2‚xH2O with the hydrothermal time equal to (1) 1.5, (2) 6, and (3) 24 h.

with the hydrothermal time equal to 1.5 h. However, three main Raman peaks corresponding to crystalline RuO2 in the rutile form (i.e., Eg, A1g, and B2g) are clearly found from curve 1 in Figure 3b. The red shift of ca. 20-25 cm-1 in comparison with the RuO2 single crystal is attributed to the nanoscale-size effect.28 Because amorphous RuO2‚xH2O does not exhibit Raman peaks in the investigated wavelength region,9b the three Raman peaks observed on curve 1 does indicate the formation of local RuO6 rutile crystalline structure for the as-prepared RuO2‚xH2O with the hydrothermal time of 1.5 h. Note that the intensity of a Raman peak will increase and the full-width at half-maximum (fwhm) of the peak will decrease with increasing the crystal size, whereas red-shifts in position will be found with a decrease in the crystal size.28 Accordingly, the crystal size of as-prepared RuO2‚xH2O nanoparticles increases with prolonging the hydrothermal time from a comparison of Raman spectra in Figure 3b. This phenomenon, revealing the crystal growth of hydrothermal annealing, is identical to the ED and XRD results. To maintain the rapid charge/discharge rate, high water content is an essential requirement for the facile proton diffusion within RuO2‚xH2O nanocrystallites, which can be measured by thermogravimetric/differential thermal analyses (TG/DTA). Note the total weight loss of about 22, 20 and 17 wt % for RuO2‚xH2O with the hydrothermal time equal to 1.5, 6, and 24 h, respectively (see Figure 4a), which (28) (a) Mar, S. Y.; Chen, C. S.; Huang, Y. S.; Tiong, K. K. Appl. Surf. Sci. 1995, 90, 497. (b) Ryan, J. V.; Berry, A. D.; Anderson, M. L.; Long, J. W.; Stroud, R. M.; Cepak, V. M.; Browning, V. M.; Rolison, D. R.; Merzbacher, C. I. Nature 2000, 406, 169.

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Figure 4. (a) TGA and (b) DTA curves of RuO2‚xH2O with the hydrothermal time equal to (1) 1.5, (2) 6, and (3) 24 h.

corresponds to RuO2‚2.08H2O, RuO2‚1.85H2O, and RuO2‚ 1.51H2O, respectively. The small error, ca. 1.6%, in determining the total weight loss of the samples prepared in different batches at the same condition indicates that the gradual decrease in the water content with prolonging the hydrothermal time is reasonably attributed to condensation of hydroxyl groups and growth of RuO2 crystallites. On the other hand, RuO2‚xH2O with hydrothermal time equal to 24 h does show a crystalline structure, but its water content is still very high. The above properties demonstrate the unique texture of RuO2 prepared by means of a hydrothermal route in the aqueous media. From an examination of all DTA curves in Figure 4b, several features have to be mentioned. First, endothermic peaks in the range from room temperature to 60 °C are presumably due to the evaporation of physically adsorbed water, corresponding to an obvious weight loss in the TGA curves shown in Figure 4a. Second, the first exothermic peaks, centered at ca. 150 °C, are attributable to the formation of bridging oxo bonds coupled with the removal of chemically bound water (e.g., 2Ru-OH f RuO-Ru + H2O) within nanoparticles, resulting in the further growth of RuO2 crystallites.6b,29 The presence of this exothermic peak reveals the hydrous nature of hydrothermally derived RuO2‚xH2O nanocrystallites. Third, very broad exothermic peaks centered at ca. 350 °C are most likely due to the coalescence and growth of RuO2‚xH2O nanocrystallites to form larger crystals. This statement is further confirmed from the effects of annealing temperature on the physicochemical properties of RuO2‚xH2O (see below). (29) Rolison, D. R.; Hagans, P. L.; Swider, K. E.; Long, J. W. Langmuir 1999, 15, 774.

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Figure 5. Scheme for the independent control of crystal size and water content by “hydrothermal annealing” for hydrothermally derived RuO2‚xH2O: (a) free hydroxyl-ruthenium species in the octahedral form; (b) rapid precipitation of hydroxyl-ruthenium species and condensation of hydroxyl groups to form extremely local RuO6 crystalline structure for RuO2‚xH2O primary nanoparticles in a narrow particle size distribution; and (c) further condensation of hydroxyl groups and gradual growth of RuO2 nanocrystallites in every primary particulate with prolonging the hydrothermal time.

On the basis of the above TEM images, ED patterns, XRD patterns, Raman spectra, and TG/DTA analyses, we successfully achieved control of the crystal size but maintaining the high water content of RuO2‚xH2O nanoparticles by using a hydrothermal synthesis route and varying the hydrothermal time. A schematic model is proposed in Figure 5 to explain the formation mechanism, the crystal growth, and the maintenance of high water content of hydrothermal-derived RuO2‚xH2O nanocrystallites. In step 1, the dissolution and hydration of RuCl3‚xH2O in the aqueous solution will cause the formation of chloro-hydroxyl-ruthenium species,30a which should be oxidized by dissolved oxygen molecules and completely transformed into the hydroxyl-ruthenium species in the octahedral form, [Ru(OH)4(H2O)2] or [Ru(OH)6]2,30b under hydrothermal conditions. The rapid and complete precipitation of these hydroxyl-ruthenium species in the initial hydrothermal period (