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Textural and pseudocapacitive characteristics of sol–gel derived RuO2·xH2O: Hydrothermal annealing vs. annealing in air
Electrochimica Acta 54 (2009) 978–983
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Textural and pseudocapacitive characteristics of sol–gel derived RuO2 ·xH2 O: Hydrothermal annealing vs. annealing in air Kuo-Hsin Chang a,b , Chi-Chang Hu a,∗,1 , Chih-Yin Chou b a b
Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 22 August 2008 Accepted 24 August 2008 Available online 30 August 2008 Keywords: Supercapacitors Hydrothermal annealing Sol–gel RuO2 ·xH2 O
a b s t r a c t Hydrous ruthenium dioxide (RuO2 ·xH2 O) prepared in a modified sol–gel process was subjected to annealing in air and water at various temperatures for supercapacitor applications. The textural and pseudocapacitive characteristics of RuO2 ·xH2 O annealed in air and water were systematically compared to show the benefits of annealing in water (denoted as hydrothermal annealing). An important concept that hydrothermal annealing effectively restricts condensation of hydroxyl groups within nanoparticles, inhibits crystal growth, and maintains high water content of RuO2 ·xH2 O is demonstrated in this work. The unique textural characteristics of hydrothermally annealed RuO2 ·xH2 O are attributable to the highpressured, water-enriched surroundings which restrain coalescence of RuO2 ·xH2 O nanocrystallites. The crystalline, hydrous nature of hydrothermally annealed RuO2 ·xH2 O favors the utilization of active species in addition to a merit of minor dependence of specific capacitance on the scan rate of CV for pseudocapacitors. As a result, RuO2 ·xH2 O with hydrothermal annealing at 225 ◦ C for 24 h exhibits 16 wt.% water, an average particle size of about 7 nm, and specific capacitance of ca. 390 F g−1 . © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Due to perfect electrochemical reversibility [1,2], excellent cycle life in H2 SO4 [2], and high specific capacitance [3–5], RuO2 in both amorphous and crystalline forms has been widely recognized as the most promising electrode material essential to supercapacitor applications in the past 10 or more years [3–12]. In order to promote the energy density of miniaturized supercapacitors for modern electronics, using pure RuO2 electrodes is preferred (ca. 150–260 F cm−2 ) [13]. This high specific capacitance is attributed to pseudocapacitance from the redox transitions of superficial RuO2 involving the double injection/expel of protons and electrons [1,3]: RuOa (OH)b + ıH+ + ıe− ↔ RuOa−ı (OH)b+ı
(1)
which has been confirmed by means of electrochemical quartz crystal microbalance studies recently [14,15]. For practical usage, crystalline RuO2 with excellent electronic conductivity, high chemical stability, and long cycling life is the prior consideration. Hence, sol–gel derived RuO2 ·xH2 O with an
∗ Corresponding author at: Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsin-Chu 30013, Taiwan. Tel.: +886 3 573 6027; fax: +886 3 573 6027. E-mail address: [email protected] (C.-C. Hu). 1 ISE member. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.08.041
amorphous structure is always annealed in air at elevated temperatures to obtain the above properties [3–5,16–20]. However, based on the mechanism of double injection/expel of protons and electrons, pseudocapacitance of RuO2 ·xH2 O has been optimized when the electronic and protonic transports are balanced [5,7,16,17]. Upon annealing in air, more anhydrous RuO2 regions will form when water is irreversibly lost from structure, resulting in a decrease in the volume of protonic transport paths but an increase in the metallic conduction paths [16,17]. In addition, the original finetuned nanostructure will be destroyed meanwhile reduction in specific surface area of RuO2 ·xH2 O with an increase in proton diffusion barrier occurs at the same time [3,5,16–19]. Therefore, how to effectively control electron and proton conducting pathways is the key optimizing the pseudocapacitive performance of RuO2 ·xH2 O [7,16,17]. Based on the above viewpoints, independent control of electron and proton conducting pathways of RuO2 particulates is an ideal concept for optimizing the pseudocapacitive performance of RuO2 ·xH2 O [21]. In our previous studies [21–23], hydrous, crystalline RuO2 ·xH2 O nanoparticles can be effectively obtained by using a hydrothermal synthesis process. The crystalline quality of RuO2 ·xH2 O can be improved by prolonging the hydrothermal time. In addition, well-crystalline RuO2 ·xH2 O shows the relatively thermal stability, obviously inhibiting coalescence of nanoparticles and restricting crystal growth of RuO2 . In this work, an idea that annealing in a water-enriched environment (denoted
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as hydrothermal annealing) will restrict condensation of hydroxyl groups between particulates, inhibit crystallite growth, and maintains high water content of sol–gel derived RuO2 ·xH2 O is proposed, which is different from hydrothermal synthesis of RuO2 ·xH2 O converted from RuCl3 ·xH2 O. To prove the benefits of hydrothermal annealing, the textural and electrochemical characteristics of RuO2 ·xH2 O annealed in air and water will be systematically compared.
where m, V, qa and qc indicates the mass of RuO2 ·xH2 O, the potential window of CVs, the voltammetric charges integrated from the positive and negative sweeps of CV, respectively. All solutions used in this work were prepared with 18 M cm water produced by a reagent water system (Milli-Q SP, Japan). The electrolyte, 0.5 M H2 SO4 , was degassed with purified nitrogen gas for 25 min before measurements and this nitrogen was passed over the solutions during the measurements.
2. Experimental
3. Results and discussion
2.1. Synthesis of RuO2 ·xH2 O
3.1. Textural characteristics
Hydrous ruthenium dioxide (RuO2 ·xH2 O) was prepared via a modified sol–gel process according to our previous work [3] while the re-dissolving and shaking step of cleaned oxide precipitates in an aqueous NH3 solution was not carried out. These sol–gel derived RuO2 ·xH2 O powders were further annealed in air or water at various temperatures for 24 h. The hydrothermally annealed RuO2 ·xH2 O powders were efficiently obtained by means of a centrifuge. These powders were dried in a reduced-pressure oven overnight (>8 h) at room temperature for material and electrochemical characterization.
The significant inhibition of particulate coalescence and crystal growth by means of hydrothermal annealing is clearly demonstrated from several comparisons of the textural characteristics of sol–gel derived RuO2 ·xH2 O annealed in air and water. Typical XRD patterns of sol–gel derived RuO2 ·xH2 O annealed at different temperatures in air and water are shown as Fig. 1(a and b), respectively. In Fig. 1(a), pristine RuO2 ·xH2 O powders transferred from an amorphous phase into a crystalline structure when they were subjected to annealing in air at 150 ◦ C since diffraction peaks corresponding to RuO2 crystals in the rutile form are obviously observed on curve 2. Note the increase in the diffraction intensity and reduction in the full-width at half-maximum (FWHM) of diffraction peaks with increasing the annealing temperature. These evidences reveal the typical condensation of bulk hydroxyl groups and growth of RuO2 crystals for pristine RuO2 ·xH2 O upon air-annealing at elevated temperatures [3,5,16–19]. In Fig. 1(b), on the other hand, all XRD patterns show very broad diffraction peaks, indicating the formation of tiny RuO2 ·xH2 O nanocrystallites. Thus, particle coales-
2.2. Electrode preparation Both normally and hydrothermally annealed RuO2 ·xH2 O powders were mixed with 5 wt.% polyvinylidenfluoride binder. N-Methyl-2-pyrrolidone was dropped into the above mixtures and ground to form the coating slurry. This slurry was smeared onto the pretreated graphite substrates and dried in a vacuum oven at 50 ◦ C overnight. The amount of RuO2 ·xH2 O coated onto the substrate is about 1.0 mg/cm2 . The pretreatment procedure of the 10 mm × 10 mm × 3 mm graphite substrates (Nippon Carbon EGNPL, N.C.K.) completely followed our previous work [3]. The above RuO2 ·xH2 O/graphite electrodes were employed as the working electrode in the electrochemical tests. 2.3. Textural analyses The nanostructure of oxides were examined by means of a transmission electron microscope (TEM, JEM-2010, JEOL). X-ray diffraction patterns were obtained from an X-ray diffractometer (Rigaku Miniflex system) using a Cu target (Cu K␣ = 1.5418 Å) at an angle speed of 1◦ (2) min−1 . Thermal data of oxides were determined by thermogravimetric/differential thermal analyses (PerkinElmer Instruments, Diamond TG/DTA), which was performed in an air flow at 10 ◦ C min−1 from room temperature to 600 ◦ C. 2.4. Electrochemical measurements Electrochemical characteristics of various RuO2 ·xH2 O/graphite electrodes were obtained by means of an electrochemical analyzer system, CHI 633A (CH Instruments) at 25 ◦ C in a threecompartment 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. The specific capacitance (CS ) of oxides is estimated from cyclic voltammograms (CVs) measured at various scan rates according to the following equation: CS =
qa + |qc | 2m × V
(2)
Fig. 1. XRD patterns of (1) pristine and (2–5) annealed RuO2 ·xH2 O in (a) air and (b) water at (2) 150 ◦ C, (3) 175 ◦ C, (4) 200 ◦ C, and (5) 225 ◦ C for 24 h.
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Fig. 2. TGA curves of (1) pristine and (2–5) annealed RuO2 ·xH2 O in (a) air and (b) water at (2) 150 ◦ C, (3) 175 ◦ C, (4) 200 ◦ C, and (5) 225 ◦ C for 24 h.
cence and crystal growth of RuO2 ·xH2 O are concluded to effectively inhibit under the hydrothermal annealing environment. This statement is confirmed from the average crystal size of sol–gel derived RuO2 ·xH2 O annealed at 225 ◦ C in air and water for 24 h, equal to 19.1 and 2.3 nm, respectively. Because proton diffusion ability within RuO2 particles is one of the key factors affecting the performances of Ru-based supercapacitors [7,14,15], thermogravimetric analyses (TGA) were used to determine the water content of RuO2 ·xH2 O annealed in air and water at various temperatures. The water content is defined as the weight loss between room temperature and 500 ◦ C. In Fig. 2(a), the water content of RuO2 ·xH2 O rapidly reduces from ca. 25 to ca. 5 wt.% when oxide has been annealed in air at or above 150 ◦ C for 24 h. This effect is reasonably attributed to the absence of physically adsorbed water and condensation of bulk hydroxyl groups in a normal annealing process. In Fig. 2(b), however, the water content of RuO2 ·xH2 O with hydrothermal annealing at all temperatures is very high; the lowest one containing ca. 16 wt.% is the oxide annealed at 225 ◦ C. The above results indicate that hydrothermal annealing restricts crystallization of RuO2 within the primary RuO2 ·xH2 O nanoparticles without significant condensation of superficial hydroxyl groups between particulates because water molecules in the hydrothermal bath serve as a barrier for particulate aggregation. Results of differential thermal analyses (DTA) are shown in Fig. 3 to gain an understanding on the thermal behavior of sol–gel derived RuO2 ·xH2 O before and after annealing treatments. From an examination of all DTA curves in Fig. 3, several features have to be mentioned. First, the pristine and hydrothermally annealed RuO2 ·xH2 O samples show an endothermic peak below 100 ◦ C, attributable to the evaporation of physically adsorbed water. This
Fig. 3. DTA curves of (1) pristine and (2–5) annealed RuO2 ·xH2 O in (a) air and (b) water at (2) 150 ◦ C, (3) 175 ◦ C, (4) 200 ◦ C, and (5) 225 ◦ C for 24 h.
is supported by the obvious weight loss in the same temperature range shown in Fig. 2(b). In addition, high content of physically adsorbed water can be ascribed to an aggregated, mesoporous structure of RuO2 ·xH2 O primary nanoparticles resulting from the inhibition of particle coalescence under hydrothermal annealing. Second, the first exothermic peaks in the range from 120 to 200 ◦ C (see the DTA curves of pristine and hydrothermally annealed RuO2 ·xH2 O samples) are attributable to the formation of bridging oxo bonds due to condensation of hydroxyl groups (e.g., 2Ru OH → Ru O Ru + H2 O) coupled with the removal of chemically bound water within particulates [21]. This result reveals that RuO2 ·xH2 O annealed in water at all temperatures remains its hydrous nature. Therefore, the absence of the first exothermic peak in Fig. 3(a) for air-annealed RuO2 ·xH2 O indicates significant crystallization of RuO2 during annealing in air. Third, the second, broad, exothermic peak between 250 and 500 ◦ C is attributed to the coalescence and growth of RuO2 crystallites to form larger crystals [21], which is very obvious for pristine RuO2 ·xH2 O. Accordingly, the hydrothermally annealed RuO2 ·xH2 O crystallites are more thermally stable than pristine RuO2 ·xH2 O. The effects of annealing in air and water on the morphologies of RuO2 ·xH2 O can be directly observed from the TEM images. From Fig. 4(a and b), the as-prepared RuO2 ·xH2 O mainly consists of randomly dispersed, spherical primary nanoparticles with an average particle size of ca. 3 nm. The multilateral morphology of much larger particles observed in Fig. 4(c and d) reveals coalescence of primary nanoparticles and growth of RuO2 crystals when RuO2 ·xH2 O was annealed in air at temperatures ≥150 ◦ C. In addition, the particle obtained at 200 ◦ C is obviously larger than that treated at 150 ◦ C. The above sintering phenomenon is invisible for
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Fig. 4. TEM images of (a and b) pristine RuO2 ·xH2 O, and (c–f) annealed RuO2 ·xH2 O in (c and d) air and (e and f) water at (c and e) 150 ◦ C, (d) 200 ◦ C, and (f) 225 ◦ C for 24 h.
the hydrothermally annealed RuO2 ·xH2 O when the temperature is kept at 150 ◦ C (see Fig. 4(e)). However, a slight increase of the particle size (ca. 7 nm) is found for RuO2 ·xH2 O hydrothermally annealed at 225 ◦ C for 24 h (Fig. 4(f)). Fortunately, the resultant oxide aggregates remain a mesoporous structure, indicating minor sintering of primary nanoparticles.
From the above XRD patterns, TG/DTA results, and TEM images, the great loss in water content for RuO2 ·xH2 O upon air-annealing at various temperatures can be effectively inhibited by annealing in a water-enriched environment. Also, particle coalescence/sintering and crystal growth are significantly prevented, remaining a mesoporous structure. The crystallization of RuO2 within primary
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Fig. 6. Dependence of the specific capacitance on the annealing temperature for RuO2 ·xH2 O annealed in (1) air and (2) water for 24 h.
Fig. 5. CVs of (1) pristine and (2–5) annealed RuO2 ·xH2 O in (a) air and (b) water at (2) 150 ◦ C, (3) 175 ◦ C, (4) 200 ◦ C, and (5) 225 ◦ C for 24 h.
nanoparticles as well as the high water content and mesoporous structure of resultant oxides proves the unique merits of a hydrothermal annealing process studied in this work. The crystalline, hydrous, and mesoporous structure of RuO2 ·xH2 O will provide very smooth pathways for electron hopping, proton diffusion, and electrolyte penetration during the rapid charge/discharge process (see below). 3.2. Pseudocapacitive characteristics The crystalline and hydrous nature of hydrothermally annealed RuO2 ·xH2 O provides several benefits for the supercapacitor application in comparing with the sample annealed in air. Fig. 5 shows the typical capacitive behavior of pristine and annealed RuO2 ·xH2 O measured at 25 mV s−1 in 0.5 M H2 SO4 . Curve 1 in Fig. 5(a and 5b) represents the i–E response of amorphous RuO2 ·xH2 O prepared by means of a modified sol–gel method; lower capacitive current densities in the less positive potential region and relatively irreversible behavior were found although the scan rate of CV is only 25 mV s−1 . This relatively irreversible behavior, having been improved by annealing to obtain crystalline RuO2 [3,5,16], by adding conductive carbons [3,11], by introducing crystalline RuO2 [24,25], is reasonably attributed to the poor electronic conductivity of amorphous RuO2 ·xH2 O. The rectangular i–E responses of curves 2–5 in Fig. 5(a) also confirm the merit of annealing since the positive sweeps of these curves show mirror responses of their corresponding negative sweeps and obvious pseudocapacitive currents are found on both positive and negative sweeps from 0 to 0.5 V. On the other hand, voltammetric currents monotonously decay with the annealing temperature, indicating the gradual loss in active sites and the increase in proton diffusion barrier, due to particulate sin-
tering and crystallite growth in the normal annealing process. In Fig. 5(b), improvement in electronic conductivity of RuO2 ·xH2 O by crystallization through means of hydrothermal annealing is also visible. Moreover, the decrease in voltammetric current densities with increasing the hydrothermal annealing temperature is relatively minor, demonstrating a higher utilization of active species in this case. The high utilization of active species for RuO2 ·xH2 O with hydrothermal annealing is quantitatively demonstrated by the dependence of specific capacitance on the annealing temperature, shown in Fig. 6. As mentioned in Introduction, pseudocapacitance of RuO2 ·xH2 O will be optimized when the electronic and protonic transports are balanced [5,7,16,17]. Hence, in comparison with the as-prepared sample, the increase in specific capacitance for RuO2 ·xH2 O annealed at 150 ◦ C in air (curve 1 in Fig. 6) or at 150–175 ◦ C in water (curve 2 in Fig. 6) is reasonably attributed to the improvement of electronic conductivity of amorphous RuO2 ·xH2 O. On the other hand, annealing in air accelerates the condensation of hydroxyl groups to form the crystalline, bridging oxo bonds which reduce the active sites and increase the proton diffusion barrier for redox transitions [3,5,16–19]. Consequently, more anhydrous RuO2 regions are formed, resulting in a decrease in the volume of protonic transport paths although an increase in the metallic conduction paths is obtained. Hence, this anhydrous, crystalline RuO2 generally shows low specific capacitance [3,5,16–19]. From the XRD, TG/DTA and TEM analyses, particle coalescence/sintering and crystal growth of RuO2 ·xH2 O unavoidably occur during annealing in air at/above 150 ◦ C. The specific capacitance of RuO2 ·xH2 O therefore decreases obviously with increasing the annealing temperature (see curve 1 in Fig. 6). On the other hand, the particle-sintering phenomenon of RuO2 ·xH2 O is effectively inhibited by means of hydrothermal annealing. Accordingly, the specific capacitance of RuO2 ·xH2 O with hydrothermal annealing at 225 ◦ C reaches about 390 F g−1 (see curve 2 in Fig. 6) which is much higher than that of RuO2 ·xH2 O annealed in air at the same temperature (ca. 170 F g−1 ). The most attractive merit of anhydrous crystalline RuO2 is the weak dependence of specific capacitance on the scan rate of CV because most active species are formed at the superficial region of RuO2 crystals [1,18,26]. Thus, redox transitions occurring at the superficial region of RuO2 particles show a minor effect of proton diffusion. On the other hand, the specific capacitance of amorphous RuO2 ·xH2 O generally shows strong dependence on the scan rate of CV because this oxide can utilize active species within the particle. Thus, most oxy-hydroxyl-Ru species involve in the energy storage/delivery while electron hopping becomes the bottle-neck step for bulk redox transitions [18,26]. Clearly, curve 1 in Fig. 7 reflects
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has been found in a normal annealing process at/above 150 ◦ C. This effect destroys the original nanostructure, reduces the water content, increases the proton diffusion barrier, and makes a great loss in the active sites and specific capacitance for the supercapacitor application. Hydrothermal annealing, on the other hand, restricts crystallization of RuO2 within the primary RuO2 ·xH2 O nanoparticles without significant condensation of superficial hydroxyl groups between particulates because water molecules in the hydrothermal bath serve as a barrier for particulate aggregation. As a result, the crystalline, hydrous, mesoporous structure of hydrothermal-annealed RuO2 ·xH2 O favors the utilization of active species in addition to a merit that the specific capacitance is weakly dependent on the scan rate of CV. Acknowledgement Fig. 7. Dependence of the specific capacitance on the scan rate of CV for (1) pristine RuO2 ·xH2 O with annealing in (2 and 3) air and (4 and 5) water at (2 and 4) 175 and (3 and 5) 225 ◦ C for 24 h.
this phenomenon which can be reduced by annealing (see curves 2 and 3 for RuO2 ·xH2 O annealed in air). Unfortunately, simultaneous reduction in the specific capacitance is an unavoidable problem in this normal annealing process. From a comparison of curves 2–5, the dependence of specific capacitance on the scan rate of CV for RuO2 ·xH2 O with hydrothermal annealing is very similar to that annealed in air. Under the same annealing temperature and scan rate, the specific capacitance of RuO2 ·xH2 O annealed in water is always much higher than that of the anhydrous oxide. Therefore, we can conclude that the crystalline, hydrous structure of RuO2 ·xH2 O annealed in water favors the utilization of active species in addition to a merit that the specific capacitance is weakly dependent on the scan rate of CV. There may be a doubt that the cycling stability of RuO2 ·xH2 O annealed in water is lower than that of its anhydrous counterpart annealed in air. This is truly possible because the crystal size of air-annealed RuO2 ·xH2 O is obviously larger than that of hydrothermal-annealed samples. During the charge/discharge process, the continuous proton inserting/expelling will destroy the oxo structure gradually, resulting in the slow but continuous dissolution of superficial oxy-hydroxyl-ruthenium species. On the other hand, the rate of capacitance loss for hydrothermally synthesized RuO2 ·xH2 O (very similar to the hydrothermal-annealed oxide) is extremely slow [22], which is acceptable for practical usage. 4. Conclusions The textural and capacitive properties of sol–gel derived RuO2 ·xH2 O annealed in air and water were systematically compared in this work. From XRD, TG/DTA and TEM analyses, condensation of hydroxyl groups accompanied with particle coalescence and crystal growth of amorphous RuO2 ·xH2 O particulates
The financial support of this work, by the National Science Council of ROC, under contract no. NSC 96-2214-E-194-001, is gratefully acknowledged. References [1] S. Hadzi-Jordanov, H. Angerstein-Kozlowska, M. Vukovic, B.E. Conway, J. Electrochem. Soc. 125 (1978) 1471. [2] B.E. Conway, Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum, New York, 1999. [3] C.C. Hu, W.C. Chen, K.H. Chang, J. Electrochem. Soc. 151 (2004) A281. [4] C.C. Hu, K.H. Chang, M.C. Lin, Y.T. Wu, Nano Lett. 6 (2006) 2690. [5] J.P. Zheng, P.J. Cygan, T.R. Row, J. Electrochem. Soc. 142 (1995) 2699. [6] J.P. Zheng, Electrochem. Solid-State Lett. 2 (1999) 359. [7] K.H. Chang, Y.T. Wu, C.C. Hu, in: V. Gupta (Ed.), Recent Advances in the Electrochemical Supercapacitors, Research Signpost, Kerala, India, 2006, (Chapter 3). [8] I.H. Kim, K.B. Kim, Electrochem. Solid-State Lett. 4 (2001) A62. [9] Y.G. Wang, X.G. Zhang, Electrochim. Acta 49 (2004) 1957. [10] C.C. Hu, W.C. Chen, Electrochim. Acta 49 (2004) 3469. [11] M. Min, K. Machida, J.H. Jang, K. Naoi, J. Electrochem. Soc. 153 (2006) A334. [12] C.C. Hu, M.J. Liu, K.H. Chang, Electrochim. Acta 53 (2008) 2679. [13] I.D. Raistrick, in: J. Mchardy, F. Ludwig (Eds.), The Electrochemistry of Semiconductors and Electronics Processes and Devices, Noyes, Park Ridge, NJ, 1992. [14] M.C. Santos, A.J. Terezo, V.C. Fernandes, E.C. Pereira, L.O.S. Bulhoes, J. Solid State Electrochem. 9 (2005) 91. [15] M.C. Santos, L. Cogo, S.T. Tanimoto, M.L. Calegaro, L.O.S. Bulhoes, Appl. Surf. Sci. 253 (2006) 1817. [16] W. Dmowski, T. Egami, K.E. Swider-Lyons, C.T. Love, D.R. Rolison, J. Phys. Chem. B 106 (2002) 12677. [17] D.A. McKeown, P.L. Hagans, L.P.L. Carette, A.E. Russell, K.E. Swider, D.R. Rolison, J. Phys. Chem. B 103 (1999) 4825. [18] W. Sugimoto, H. Iwata, K. Yokoshima, Y. Murakami, Y. Takasu, J. Phys. Chem. B 109 (2005) 7330. [19] H. Kim, B.N. Popov, J. Power Sources 104 (2002) 52. [20] K.H. Chang, C.C. Hu, J. Electrochem. Soc. 151 (2004) A958. [21] K.H. Chang, C.C. Hu, C.Y. Chou, Chem. Mater. 19 (2007) 2112. [22] K.H. Chang, C.C. Hu, Appl. Phys. Lett. 88 (2006) 193102. [23] K.H. Chang, C.C. Hu, Electrochem. Solid-State Lett. 7 (2004) A466. [24] O. Barbieri, M. Hahn, A. Foelske, R. Kotz, J. Electrochem. Soc. 153 (2006) A2049. [25] D. Susanti, D.S. Tsai, Y.S. Huang, A. Korotcov, W.H. Chung, J. Phys. Chem. C 111 (2007) 9530. [26] W. Sugimoto, K. Yokoshima, Y. Murakami, Y. Takasu, Electrochim. Acta 52 (2006) 1742.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Textural and pseudocapacitive characteristics of sol–gel derived RuO2 ·xH2 O: Hydrothermal annealing vs. annealing in air Kuo-Hsin Chang a,b , Chi-Chang Hu a,∗,1 , Chih-Yin Chou b a b
Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 22 August 2008 Accepted 24 August 2008 Available online 30 August 2008 Keywords: Supercapacitors Hydrothermal annealing Sol–gel RuO2 ·xH2 O
a b s t r a c t Hydrous ruthenium dioxide (RuO2 ·xH2 O) prepared in a modified sol–gel process was subjected to annealing in air and water at various temperatures for supercapacitor applications. The textural and pseudocapacitive characteristics of RuO2 ·xH2 O annealed in air and water were systematically compared to show the benefits of annealing in water (denoted as hydrothermal annealing). An important concept that hydrothermal annealing effectively restricts condensation of hydroxyl groups within nanoparticles, inhibits crystal growth, and maintains high water content of RuO2 ·xH2 O is demonstrated in this work. The unique textural characteristics of hydrothermally annealed RuO2 ·xH2 O are attributable to the highpressured, water-enriched surroundings which restrain coalescence of RuO2 ·xH2 O nanocrystallites. The crystalline, hydrous nature of hydrothermally annealed RuO2 ·xH2 O favors the utilization of active species in addition to a merit of minor dependence of specific capacitance on the scan rate of CV for pseudocapacitors. As a result, RuO2 ·xH2 O with hydrothermal annealing at 225 ◦ C for 24 h exhibits 16 wt.% water, an average particle size of about 7 nm, and specific capacitance of ca. 390 F g−1 . © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Due to perfect electrochemical reversibility [1,2], excellent cycle life in H2 SO4 [2], and high specific capacitance [3–5], RuO2 in both amorphous and crystalline forms has been widely recognized as the most promising electrode material essential to supercapacitor applications in the past 10 or more years [3–12]. In order to promote the energy density of miniaturized supercapacitors for modern electronics, using pure RuO2 electrodes is preferred (ca. 150–260 F cm−2 ) [13]. This high specific capacitance is attributed to pseudocapacitance from the redox transitions of superficial RuO2 involving the double injection/expel of protons and electrons [1,3]: RuOa (OH)b + ıH+ + ıe− ↔ RuOa−ı (OH)b+ı
(1)
which has been confirmed by means of electrochemical quartz crystal microbalance studies recently [14,15]. For practical usage, crystalline RuO2 with excellent electronic conductivity, high chemical stability, and long cycling life is the prior consideration. Hence, sol–gel derived RuO2 ·xH2 O with an
∗ Corresponding author at: Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsin-Chu 30013, Taiwan. Tel.: +886 3 573 6027; fax: +886 3 573 6027. E-mail address: [email protected] (C.-C. Hu). 1 ISE member. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.08.041
amorphous structure is always annealed in air at elevated temperatures to obtain the above properties [3–5,16–20]. However, based on the mechanism of double injection/expel of protons and electrons, pseudocapacitance of RuO2 ·xH2 O has been optimized when the electronic and protonic transports are balanced [5,7,16,17]. Upon annealing in air, more anhydrous RuO2 regions will form when water is irreversibly lost from structure, resulting in a decrease in the volume of protonic transport paths but an increase in the metallic conduction paths [16,17]. In addition, the original finetuned nanostructure will be destroyed meanwhile reduction in specific surface area of RuO2 ·xH2 O with an increase in proton diffusion barrier occurs at the same time [3,5,16–19]. Therefore, how to effectively control electron and proton conducting pathways is the key optimizing the pseudocapacitive performance of RuO2 ·xH2 O [7,16,17]. Based on the above viewpoints, independent control of electron and proton conducting pathways of RuO2 particulates is an ideal concept for optimizing the pseudocapacitive performance of RuO2 ·xH2 O [21]. In our previous studies [21–23], hydrous, crystalline RuO2 ·xH2 O nanoparticles can be effectively obtained by using a hydrothermal synthesis process. The crystalline quality of RuO2 ·xH2 O can be improved by prolonging the hydrothermal time. In addition, well-crystalline RuO2 ·xH2 O shows the relatively thermal stability, obviously inhibiting coalescence of nanoparticles and restricting crystal growth of RuO2 . In this work, an idea that annealing in a water-enriched environment (denoted
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as hydrothermal annealing) will restrict condensation of hydroxyl groups between particulates, inhibit crystallite growth, and maintains high water content of sol–gel derived RuO2 ·xH2 O is proposed, which is different from hydrothermal synthesis of RuO2 ·xH2 O converted from RuCl3 ·xH2 O. To prove the benefits of hydrothermal annealing, the textural and electrochemical characteristics of RuO2 ·xH2 O annealed in air and water will be systematically compared.
where m, V, qa and qc indicates the mass of RuO2 ·xH2 O, the potential window of CVs, the voltammetric charges integrated from the positive and negative sweeps of CV, respectively. All solutions used in this work were prepared with 18 M cm water produced by a reagent water system (Milli-Q SP, Japan). The electrolyte, 0.5 M H2 SO4 , was degassed with purified nitrogen gas for 25 min before measurements and this nitrogen was passed over the solutions during the measurements.
2. Experimental
3. Results and discussion
2.1. Synthesis of RuO2 ·xH2 O
3.1. Textural characteristics
Hydrous ruthenium dioxide (RuO2 ·xH2 O) was prepared via a modified sol–gel process according to our previous work [3] while the re-dissolving and shaking step of cleaned oxide precipitates in an aqueous NH3 solution was not carried out. These sol–gel derived RuO2 ·xH2 O powders were further annealed in air or water at various temperatures for 24 h. The hydrothermally annealed RuO2 ·xH2 O powders were efficiently obtained by means of a centrifuge. These powders were dried in a reduced-pressure oven overnight (>8 h) at room temperature for material and electrochemical characterization.
The significant inhibition of particulate coalescence and crystal growth by means of hydrothermal annealing is clearly demonstrated from several comparisons of the textural characteristics of sol–gel derived RuO2 ·xH2 O annealed in air and water. Typical XRD patterns of sol–gel derived RuO2 ·xH2 O annealed at different temperatures in air and water are shown as Fig. 1(a and b), respectively. In Fig. 1(a), pristine RuO2 ·xH2 O powders transferred from an amorphous phase into a crystalline structure when they were subjected to annealing in air at 150 ◦ C since diffraction peaks corresponding to RuO2 crystals in the rutile form are obviously observed on curve 2. Note the increase in the diffraction intensity and reduction in the full-width at half-maximum (FWHM) of diffraction peaks with increasing the annealing temperature. These evidences reveal the typical condensation of bulk hydroxyl groups and growth of RuO2 crystals for pristine RuO2 ·xH2 O upon air-annealing at elevated temperatures [3,5,16–19]. In Fig. 1(b), on the other hand, all XRD patterns show very broad diffraction peaks, indicating the formation of tiny RuO2 ·xH2 O nanocrystallites. Thus, particle coales-
2.2. Electrode preparation Both normally and hydrothermally annealed RuO2 ·xH2 O powders were mixed with 5 wt.% polyvinylidenfluoride binder. N-Methyl-2-pyrrolidone was dropped into the above mixtures and ground to form the coating slurry. This slurry was smeared onto the pretreated graphite substrates and dried in a vacuum oven at 50 ◦ C overnight. The amount of RuO2 ·xH2 O coated onto the substrate is about 1.0 mg/cm2 . The pretreatment procedure of the 10 mm × 10 mm × 3 mm graphite substrates (Nippon Carbon EGNPL, N.C.K.) completely followed our previous work [3]. The above RuO2 ·xH2 O/graphite electrodes were employed as the working electrode in the electrochemical tests. 2.3. Textural analyses The nanostructure of oxides were examined by means of a transmission electron microscope (TEM, JEM-2010, JEOL). X-ray diffraction patterns were obtained from an X-ray diffractometer (Rigaku Miniflex system) using a Cu target (Cu K␣ = 1.5418 Å) at an angle speed of 1◦ (2) min−1 . Thermal data of oxides were determined by thermogravimetric/differential thermal analyses (PerkinElmer Instruments, Diamond TG/DTA), which was performed in an air flow at 10 ◦ C min−1 from room temperature to 600 ◦ C. 2.4. Electrochemical measurements Electrochemical characteristics of various RuO2 ·xH2 O/graphite electrodes were obtained by means of an electrochemical analyzer system, CHI 633A (CH Instruments) at 25 ◦ C in a threecompartment 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. The specific capacitance (CS ) of oxides is estimated from cyclic voltammograms (CVs) measured at various scan rates according to the following equation: CS =
qa + |qc | 2m × V
(2)
Fig. 1. XRD patterns of (1) pristine and (2–5) annealed RuO2 ·xH2 O in (a) air and (b) water at (2) 150 ◦ C, (3) 175 ◦ C, (4) 200 ◦ C, and (5) 225 ◦ C for 24 h.
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Fig. 2. TGA curves of (1) pristine and (2–5) annealed RuO2 ·xH2 O in (a) air and (b) water at (2) 150 ◦ C, (3) 175 ◦ C, (4) 200 ◦ C, and (5) 225 ◦ C for 24 h.
cence and crystal growth of RuO2 ·xH2 O are concluded to effectively inhibit under the hydrothermal annealing environment. This statement is confirmed from the average crystal size of sol–gel derived RuO2 ·xH2 O annealed at 225 ◦ C in air and water for 24 h, equal to 19.1 and 2.3 nm, respectively. Because proton diffusion ability within RuO2 particles is one of the key factors affecting the performances of Ru-based supercapacitors [7,14,15], thermogravimetric analyses (TGA) were used to determine the water content of RuO2 ·xH2 O annealed in air and water at various temperatures. The water content is defined as the weight loss between room temperature and 500 ◦ C. In Fig. 2(a), the water content of RuO2 ·xH2 O rapidly reduces from ca. 25 to ca. 5 wt.% when oxide has been annealed in air at or above 150 ◦ C for 24 h. This effect is reasonably attributed to the absence of physically adsorbed water and condensation of bulk hydroxyl groups in a normal annealing process. In Fig. 2(b), however, the water content of RuO2 ·xH2 O with hydrothermal annealing at all temperatures is very high; the lowest one containing ca. 16 wt.% is the oxide annealed at 225 ◦ C. The above results indicate that hydrothermal annealing restricts crystallization of RuO2 within the primary RuO2 ·xH2 O nanoparticles without significant condensation of superficial hydroxyl groups between particulates because water molecules in the hydrothermal bath serve as a barrier for particulate aggregation. Results of differential thermal analyses (DTA) are shown in Fig. 3 to gain an understanding on the thermal behavior of sol–gel derived RuO2 ·xH2 O before and after annealing treatments. From an examination of all DTA curves in Fig. 3, several features have to be mentioned. First, the pristine and hydrothermally annealed RuO2 ·xH2 O samples show an endothermic peak below 100 ◦ C, attributable to the evaporation of physically adsorbed water. This
Fig. 3. DTA curves of (1) pristine and (2–5) annealed RuO2 ·xH2 O in (a) air and (b) water at (2) 150 ◦ C, (3) 175 ◦ C, (4) 200 ◦ C, and (5) 225 ◦ C for 24 h.
is supported by the obvious weight loss in the same temperature range shown in Fig. 2(b). In addition, high content of physically adsorbed water can be ascribed to an aggregated, mesoporous structure of RuO2 ·xH2 O primary nanoparticles resulting from the inhibition of particle coalescence under hydrothermal annealing. Second, the first exothermic peaks in the range from 120 to 200 ◦ C (see the DTA curves of pristine and hydrothermally annealed RuO2 ·xH2 O samples) are attributable to the formation of bridging oxo bonds due to condensation of hydroxyl groups (e.g., 2Ru OH → Ru O Ru + H2 O) coupled with the removal of chemically bound water within particulates [21]. This result reveals that RuO2 ·xH2 O annealed in water at all temperatures remains its hydrous nature. Therefore, the absence of the first exothermic peak in Fig. 3(a) for air-annealed RuO2 ·xH2 O indicates significant crystallization of RuO2 during annealing in air. Third, the second, broad, exothermic peak between 250 and 500 ◦ C is attributed to the coalescence and growth of RuO2 crystallites to form larger crystals [21], which is very obvious for pristine RuO2 ·xH2 O. Accordingly, the hydrothermally annealed RuO2 ·xH2 O crystallites are more thermally stable than pristine RuO2 ·xH2 O. The effects of annealing in air and water on the morphologies of RuO2 ·xH2 O can be directly observed from the TEM images. From Fig. 4(a and b), the as-prepared RuO2 ·xH2 O mainly consists of randomly dispersed, spherical primary nanoparticles with an average particle size of ca. 3 nm. The multilateral morphology of much larger particles observed in Fig. 4(c and d) reveals coalescence of primary nanoparticles and growth of RuO2 crystals when RuO2 ·xH2 O was annealed in air at temperatures ≥150 ◦ C. In addition, the particle obtained at 200 ◦ C is obviously larger than that treated at 150 ◦ C. The above sintering phenomenon is invisible for
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Fig. 4. TEM images of (a and b) pristine RuO2 ·xH2 O, and (c–f) annealed RuO2 ·xH2 O in (c and d) air and (e and f) water at (c and e) 150 ◦ C, (d) 200 ◦ C, and (f) 225 ◦ C for 24 h.
the hydrothermally annealed RuO2 ·xH2 O when the temperature is kept at 150 ◦ C (see Fig. 4(e)). However, a slight increase of the particle size (ca. 7 nm) is found for RuO2 ·xH2 O hydrothermally annealed at 225 ◦ C for 24 h (Fig. 4(f)). Fortunately, the resultant oxide aggregates remain a mesoporous structure, indicating minor sintering of primary nanoparticles.
From the above XRD patterns, TG/DTA results, and TEM images, the great loss in water content for RuO2 ·xH2 O upon air-annealing at various temperatures can be effectively inhibited by annealing in a water-enriched environment. Also, particle coalescence/sintering and crystal growth are significantly prevented, remaining a mesoporous structure. The crystallization of RuO2 within primary
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Fig. 6. Dependence of the specific capacitance on the annealing temperature for RuO2 ·xH2 O annealed in (1) air and (2) water for 24 h.
Fig. 5. CVs of (1) pristine and (2–5) annealed RuO2 ·xH2 O in (a) air and (b) water at (2) 150 ◦ C, (3) 175 ◦ C, (4) 200 ◦ C, and (5) 225 ◦ C for 24 h.
nanoparticles as well as the high water content and mesoporous structure of resultant oxides proves the unique merits of a hydrothermal annealing process studied in this work. The crystalline, hydrous, and mesoporous structure of RuO2 ·xH2 O will provide very smooth pathways for electron hopping, proton diffusion, and electrolyte penetration during the rapid charge/discharge process (see below). 3.2. Pseudocapacitive characteristics The crystalline and hydrous nature of hydrothermally annealed RuO2 ·xH2 O provides several benefits for the supercapacitor application in comparing with the sample annealed in air. Fig. 5 shows the typical capacitive behavior of pristine and annealed RuO2 ·xH2 O measured at 25 mV s−1 in 0.5 M H2 SO4 . Curve 1 in Fig. 5(a and 5b) represents the i–E response of amorphous RuO2 ·xH2 O prepared by means of a modified sol–gel method; lower capacitive current densities in the less positive potential region and relatively irreversible behavior were found although the scan rate of CV is only 25 mV s−1 . This relatively irreversible behavior, having been improved by annealing to obtain crystalline RuO2 [3,5,16], by adding conductive carbons [3,11], by introducing crystalline RuO2 [24,25], is reasonably attributed to the poor electronic conductivity of amorphous RuO2 ·xH2 O. The rectangular i–E responses of curves 2–5 in Fig. 5(a) also confirm the merit of annealing since the positive sweeps of these curves show mirror responses of their corresponding negative sweeps and obvious pseudocapacitive currents are found on both positive and negative sweeps from 0 to 0.5 V. On the other hand, voltammetric currents monotonously decay with the annealing temperature, indicating the gradual loss in active sites and the increase in proton diffusion barrier, due to particulate sin-
tering and crystallite growth in the normal annealing process. In Fig. 5(b), improvement in electronic conductivity of RuO2 ·xH2 O by crystallization through means of hydrothermal annealing is also visible. Moreover, the decrease in voltammetric current densities with increasing the hydrothermal annealing temperature is relatively minor, demonstrating a higher utilization of active species in this case. The high utilization of active species for RuO2 ·xH2 O with hydrothermal annealing is quantitatively demonstrated by the dependence of specific capacitance on the annealing temperature, shown in Fig. 6. As mentioned in Introduction, pseudocapacitance of RuO2 ·xH2 O will be optimized when the electronic and protonic transports are balanced [5,7,16,17]. Hence, in comparison with the as-prepared sample, the increase in specific capacitance for RuO2 ·xH2 O annealed at 150 ◦ C in air (curve 1 in Fig. 6) or at 150–175 ◦ C in water (curve 2 in Fig. 6) is reasonably attributed to the improvement of electronic conductivity of amorphous RuO2 ·xH2 O. On the other hand, annealing in air accelerates the condensation of hydroxyl groups to form the crystalline, bridging oxo bonds which reduce the active sites and increase the proton diffusion barrier for redox transitions [3,5,16–19]. Consequently, more anhydrous RuO2 regions are formed, resulting in a decrease in the volume of protonic transport paths although an increase in the metallic conduction paths is obtained. Hence, this anhydrous, crystalline RuO2 generally shows low specific capacitance [3,5,16–19]. From the XRD, TG/DTA and TEM analyses, particle coalescence/sintering and crystal growth of RuO2 ·xH2 O unavoidably occur during annealing in air at/above 150 ◦ C. The specific capacitance of RuO2 ·xH2 O therefore decreases obviously with increasing the annealing temperature (see curve 1 in Fig. 6). On the other hand, the particle-sintering phenomenon of RuO2 ·xH2 O is effectively inhibited by means of hydrothermal annealing. Accordingly, the specific capacitance of RuO2 ·xH2 O with hydrothermal annealing at 225 ◦ C reaches about 390 F g−1 (see curve 2 in Fig. 6) which is much higher than that of RuO2 ·xH2 O annealed in air at the same temperature (ca. 170 F g−1 ). The most attractive merit of anhydrous crystalline RuO2 is the weak dependence of specific capacitance on the scan rate of CV because most active species are formed at the superficial region of RuO2 crystals [1,18,26]. Thus, redox transitions occurring at the superficial region of RuO2 particles show a minor effect of proton diffusion. On the other hand, the specific capacitance of amorphous RuO2 ·xH2 O generally shows strong dependence on the scan rate of CV because this oxide can utilize active species within the particle. Thus, most oxy-hydroxyl-Ru species involve in the energy storage/delivery while electron hopping becomes the bottle-neck step for bulk redox transitions [18,26]. Clearly, curve 1 in Fig. 7 reflects
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has been found in a normal annealing process at/above 150 ◦ C. This effect destroys the original nanostructure, reduces the water content, increases the proton diffusion barrier, and makes a great loss in the active sites and specific capacitance for the supercapacitor application. Hydrothermal annealing, on the other hand, restricts crystallization of RuO2 within the primary RuO2 ·xH2 O nanoparticles without significant condensation of superficial hydroxyl groups between particulates because water molecules in the hydrothermal bath serve as a barrier for particulate aggregation. As a result, the crystalline, hydrous, mesoporous structure of hydrothermal-annealed RuO2 ·xH2 O favors the utilization of active species in addition to a merit that the specific capacitance is weakly dependent on the scan rate of CV. Acknowledgement Fig. 7. Dependence of the specific capacitance on the scan rate of CV for (1) pristine RuO2 ·xH2 O with annealing in (2 and 3) air and (4 and 5) water at (2 and 4) 175 and (3 and 5) 225 ◦ C for 24 h.
this phenomenon which can be reduced by annealing (see curves 2 and 3 for RuO2 ·xH2 O annealed in air). Unfortunately, simultaneous reduction in the specific capacitance is an unavoidable problem in this normal annealing process. From a comparison of curves 2–5, the dependence of specific capacitance on the scan rate of CV for RuO2 ·xH2 O with hydrothermal annealing is very similar to that annealed in air. Under the same annealing temperature and scan rate, the specific capacitance of RuO2 ·xH2 O annealed in water is always much higher than that of the anhydrous oxide. Therefore, we can conclude that the crystalline, hydrous structure of RuO2 ·xH2 O annealed in water favors the utilization of active species in addition to a merit that the specific capacitance is weakly dependent on the scan rate of CV. There may be a doubt that the cycling stability of RuO2 ·xH2 O annealed in water is lower than that of its anhydrous counterpart annealed in air. This is truly possible because the crystal size of air-annealed RuO2 ·xH2 O is obviously larger than that of hydrothermal-annealed samples. During the charge/discharge process, the continuous proton inserting/expelling will destroy the oxo structure gradually, resulting in the slow but continuous dissolution of superficial oxy-hydroxyl-ruthenium species. On the other hand, the rate of capacitance loss for hydrothermally synthesized RuO2 ·xH2 O (very similar to the hydrothermal-annealed oxide) is extremely slow [22], which is acceptable for practical usage. 4. Conclusions The textural and capacitive properties of sol–gel derived RuO2 ·xH2 O annealed in air and water were systematically compared in this work. From XRD, TG/DTA and TEM analyses, condensation of hydroxyl groups accompanied with particle coalescence and crystal growth of amorphous RuO2 ·xH2 O particulates
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