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RuO2·xH2O composites for electrochemical capacitors
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Electrochimica Acta 53 (2008) 3296–3304
Soft template synthesis of mesoporous Co3O4/RuO2·xH2O composites for electrochemical capacitors Yang Liu, Weiwei Zhao, Xiaogang Zhang ∗ College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 20 September 2007; received in revised form 6 November 2007; accepted 11 November 2007 Available online 19 November 2007
Abstract Co3 O4 /RuO2 ·xH2 O composites with various Ru content (molar content of Ru = 5%, 10%, 20%, 50%) were synthesized by one-step coprecipitation method. The precursors were prepared via adjusting pH of the mixed aqueous solutions of Co(NO3 )2 ·6H2 O and RuCl3 ·0.5H2 O by using Pluronic123 as a soft template. For the composite with molar ratio of Co:Ru = 1:1 annealed at 200 ◦ C, Brunauer–Emmet–Teller (BET) results indicated that the composite showed mesoporous structure, and the specific surface area of the composite was as high as 107 m2 g−1 . The electrochemical performances of these composites were measured in 1 M KOH electrolyte. Compared with the composite prepared without template, the composite with P123 exhibited a higher specific capacitance. When the molar content of Ru was rising, the specific capacitance of the composites increased significantly. It was also observed that the crystalline structures as well as the electrochemical activities were strongly dependent on the annealing temperature. A capacitance of 642 F/g was obtained for the composite (Co:Ru = 1:1) annealed at 150 ◦ C. Meanwhile, the composites also exhibited good cycle stability. Besides, the morphologies and textural characteristic of the samples were also investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM). © 2007 Elsevier Ltd. All rights reserved. Keywords: Pluronic123; Soft template; Co3 O4 ; RuO2 ·xH2 O; Composite; Co-precipitation; Electrochemical capacitors
1. Introduction Ruthenium oxide, with its perfect electrocatalytic activity [1–4], high electronic and proton conductivities, has been attracted considerable attention and been widely used in hydrocarbon sensors, gas-evolving reactions [5] and catalysts [6]. The highlight application of ruthenium oxide material is the usage as electrode material in energy storage electrochemical capacitors [7–9]. In general, electrochemical capacitors can be categorized into two groups: double-layer capacitors and redox capacitors. The former electrode materials mainly include high surface area activated carbon such as carbon aerogels, nanotubes and nanofibers; the latter consists of metal oxides and conducting polymers. As a redox-type metal oxide, ruthenium oxide is a perfect material for electrochemical capacitors due to its higher energy than that of high surface area carbon materials, and better electrochemical stability than that of conducting polymers.
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Corresponding author. Tel.: +86 25 52112902; fax: +86 25 52112626. E-mail address: [email protected] (X. Zhang).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.11.022
It is well known that the capacitance of RuO2 electrode results from redox pseudocapacitance. However, the current response resembles an ideal capacitor, which is a “rectangular” shape under cyclic voltammatry [10]. Due to the above advantages of RuO2 material, much effort has been devoted to developing new RuO2 materials or RuO2 -based composites. Zheng suggested that the amorphous hydrated ruthenium oxide obtained by sol-gel method showed a high specific capacitance (720 F/g) [11,12], which was known as a large breakthrough of supercapacitors. However, despite the remarkable performance of this material, pure RuO2 material is still too expensive for large-scale commercialization, which have prompted the research interest to focus on alternative cheap electrode materials (such as MnO2 , NiO, Co3 O4 , etc.) or dispersion/alloying of ruthenium oxide in other less costly oxide matrics forming ruthenium-based composites. It has been reported that RuO2 -VOx electrode prepared by a dip-coated method had showed interesting performances [13]; Soudan also investigated nanocrystalline Tix Fey Ruz On compounds prepared by high energy ball milling [14]. Some other metal oxide composites, such as NiO/RuO2 , (Ru–Sn)Ox·nH2 O, WO3 /RuO2 and
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RuO2 /TiO2 had also been discussed [15–18]. All these above work had proved that the ruthenium-based composite materials had good capacitive behavior, which could be attributed to the incorporation of other elements into RuO2 structure. Spinel Co3 O4 is a good material with excellent electrocatalytic activity for oxygen [19] or chlorine evolution reaction [20]. Even if the cycle reversibility is not good, Co3 O4 is also considered as one promising potential candidates for supercapacitor electrode materials due to its environmental friendliness, low cost, and favorable pseudocapacitive characteristics [21]. Some reports about RuO2 + Co3 O4 electrodes with various molar ratio of Co:Ru prepared by thermal decomposition method was also intensively studied either in alkaline solutions or in acid conditions which showed good electrochemical behavior or electrocatalytic properties [22,23]. All above merits make the application of spinel Co3 O4 possible as a partial substitute of the noble metal. In recent years, mesoporous materials (e.g. mesoporous carbon or metal oxide) have been attracted widely attentions which usually show excellent supercapacitive performance. RuO2 based porous electrodes (known as DSA) prepared by thermal decomposition method were intensively investigated in the early reports [24,25]. And also, various approaches have been employed to prepared mesoporous metal oxides by using hard templates (such as AAO, silica or carbon template). It has been reported that the desired three-dimensional mesoporous architecture of RuO2 ·xH2 O nantubular array prepared by use of the membrance-templated synthesis route showed a capacitance of 1300 F/g with high energy and power density [26]. Moreover, surfactant template synthesis of mesoporous anhydrous RuO2 materials with high specific area has also been performed. Since redox reactions mainly occur on the surface layer, metal oxide with a larger surface area, interconnected mesoporous structure with controlled pore size and low resistivity is expected to show higher energy density due to the short path for ion diffusion and transmission in the electrolyte. However, it is reported that the capacitance of anhydrous RuO2 prepared by calcination removing P123 template was only 58 F/g [27], which is far lower than the amorphous hydrated RuO2 [11] fabricated by sol-gel method (720 F/g). The reason is that well-crystalline anhydrous RuO2 prevented the proton permeation in to the bulk material and the capacitance of this anhydrous RuO2 was mainly from the surface reaction [28]. In this present paper, the mesoporous hydrous Co3 O4 / RuO2 ·xH2 O composite materials with high specific surface area are prepared by one-step co-precipitation method with the usage of Pluronic123 as a soft template. This approach is not only simpler and easier to obtain mesoporous metal oxide than nanocasting method using silica template, but also combines the both merits of Co3 O4 and RuO2 ·xH2 O materials in capacitive behavior. Meanwhile, the electrode reactions of Co3 O4 /RuO2 ·xH2 O composite occur not only on the surface of the electrode but also in the bulk of the electrode due to the high water content of amorphous RuO2 ·xH2 O composition [29], the specific capacitance of composites would be higher than that of sample prepared by thermal decomposition method. In this work, various molar ratio of Co:Ru composites ranging from
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19:1 to 1:1 were prepared and investigated because the content of RuO2 played a vital role on the specific capacitance of the composite. In addition, the influence of the heat treatment temperature on the capacitive performance of Co3 O4 /RuO2 ·xH2 O composite are also studied since the particle size, specific surface area and water content of composite are very dependent on the annealing temperature. Especially, nanocrystalline RuO2 ·xH2 O annealed at temperatures close to its crystalline temperatures usually exhibits the highest specific capacitance [11,30,31]. 2. Experimental 2.1. Synthesis of Co3 O4 /RuO2 ·xH2 O composite electrodes The composite materials were synthesized as follows: taking the composite with molar ratio of Co:Ru = 1:1 for the example, 2 g of block copolymer surfactant Pluronic123 (Aldrich, Mw = 5800) was dissolved in 40 ml volume of distilled water to form homogeneous solution, then 0.216 g of Co(NO3 )2 ·6H2 O and 0.209 g of RuCl3 ·0.5H2 O were respectively added into the above solution. In this process, the mixture was stirred vigorously by a magnetic bar. Subsequently, the 0.2 M aqueous solution of NaOH was slowly dropped into the above solution to adjust the pH value to 10 (Ru3+ and Co2+ can nearly turn into precipitation in total at this pH value). When the pH value was approximately equal to 10, the stirring was stopped. After placing for three hours, the precipitate was filtered and washed for several times with ethanol and deionized water, respectively. Then the residue was dried at 60 ◦ C under vacuum for 12 h. Finally, the product was annealed in air at various temperatures ranging from 100 ◦ C to 900 ◦ C for 3 h, and the composites were obtained. According to the molar ratio of Co to Ru (19:1, 9:1, 4:1, 1:1), the other composites were prepared with the same method (only annealed at 200 ◦ C), respectively. The sample without using P123 was also prepared under the same experiment conditions. The working electrodes were prepared with the as-prepared Co3 O4 /RuO2 ·xH2 O composites. Typically, 75 wt.% active material, 15 wt.% acetylene black and 10 wt.% polyvinylidenefluoride (PVDF) were well mixed in an agate mortar, and a small amount of N-methyl-pyrrolidone (NMP) was added to the composite to make more homogeneous mixture, which was then pressed on nickel mesh (1 cm × 3 cm) under the pressure of 15 MPa that served as a current collector to get an approximate thickness of 0.2 mm. Then the above electrodes were dried in vacuum at 60 ◦ C for 24 h to remove the solvent. The mass of the active material was about 8–10 mg. 2.2. Characterization and electrochemical tests The samples were characterized by X-ray diffraction (XRD) technique, nitrogen sorption measurements and so forth. XRD pattern of the sample was conducted on a Bruker D8ADVANCE diffractometer with Cu K␣ radiation of wavelength λ = 0.15418 nm). The pore properties of the samples were investigated using nitrogen adsorption on an automatic volumetric sorption analyzer (Micromeritics Corp., ASAP2010) at liquid
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nitrogen temperature. Prior to the measurements, the samples were degassed at a temperature of 150 ◦ C for 6 h under vacuum. The specific surface areas (SBET) were determined according to the Brunauer–Emmett–Teller (BET) method in the relative pressure range of 0.05–0.25. Pore size distribution (PSD) curves were calculated by the Barrett–Joyner–Halenda (BJH) method from the adsorption branches. The microscopic features and the compositions of the samples were observed with a scanning electron microscope (SEM) (Quanta 2000) operated at 20.0 kV. The transmission electron microscopy (TEM) images were taken on a FEI Tecnai G2 20 S-TWIN microscope operated at 200 kV. Thermal data of the composites were determined by thermogravimetric analyzer (TGA) and different scanning calorimeter (DSC) in air at 10 ◦ C/min (Instrument, SDT Q600). All the electrochemical measurements were performed in a three-electrode beaker-type cell at room temperature. The as-prepared composites were used as working electrodes; a platinum foil as the counter electrode and a SCE electrode as the reference electrode. 1 M KOH aqueous solution was employed as electrolyte. The electrochemical tests were examined by means of CHI660B electrochemical workstation and BT2042 battery system.
3. Results and discussion 3.1. Morphology and textural characteristics of compositions Particle size and distribution of the Co3 O4 /RuO2 ·xH2 O composite plays an important role on the utilization of electrode. Scanning electron microscopy was used to observe the surface morphologies of the composites (molar ratio of Co:Ru = 1:1) annealed at 200 ◦ C. The corresponding images are shown in Fig. 1a and b. It is typically observed that small particles stick together to form porous agglomerates. The particle morphology and size distribution of the samples were examined by TEM and shown in Fig. 1c and d. Mesoporous composite with pore size of ∼20 nm are clearly observed, which can be confirmed by nitrogen adsorption–desorption isotherms (see Fig. 4). It also can be seen that these agglomerates are comprised of smaller nanosized particles with a probably diameter of <3 nm. From the above results and discussion, the mesoporous Co3 O4 /RuO2 ·xH2 O composites can be easily prepared with coprecipitation method by using P123 as soft template. This mesoporous structure not only can reduce the diffusion resistance of electrolyte but also allows a short path of electroactive ions
Fig. 1. SEM (a, b) and TEM (c, d) images of the composites (molar ratio of Co:Ru = 1:1) annealed at 200 ◦ C.
Y. Liu et al. / Electrochimica Acta 53 (2008) 3296–3304 Table 1 The molar content of various Co3 O4 /RuO2 ·xH2 O composites from EDS Co:Ru in composites
Co at.% Ru at.%
19:1
9:1
4:1
1:1
94.7 5.3
89.4 10.6
80.8 19.2
51.6 48.4
(OH− ) transportation from the electrolyte to the composite. In addition, the composition of all Co3 O4 /RuO2 ·xH2 O composites with various molar ratio of Co:Ru were measured by EDS. It can be observed from Table 1 that the molar content of Ru:Co is approximately equal to the ratio of that in precursor solution, which demonstrates that Ru3+ and Co2+ in the precursor solutions can be precipitated in total at pH value of 10. The crystalline structures of the samples were characterized by using X-ray diffraction technology, shown in Fig. 2. Fig. 2A
Fig. 2. (A) XRD patterns of Co3 O4 /RuO2 ·xH2 O composites prepared with different Co:Ru molar ratio (a = 19:1, b = 9:1, c = 4:1, d = 1:1) annealed at 200 ◦ C; (B) XRD patterns of the composites with molar ratio of Co:Ru = 1:1 annealed at different temperatures. The squares mark the peaks of rutile RuO2 and the cycles mark spinel Co3 O4 .
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exhibits the patterns of samples with different molar ratio of Co:Ru annealed at 200 ◦ C in air. For the sample (a), the XRD patterns of spinel Co3 O4 (cycle, 2θ = 19.0◦ , 31.3◦ , 36.8◦ , 44.8◦ , 59.3◦ , 65.2◦ ) are obviously observed. However, the characteristic peaks of RuO2 (square, 2θ = 28.0◦ , 35.0◦ , 54.4◦ ) are weak, which may be attribute to low content of RuO2 . As the Ru molar content increases from 5% to 50%, the characteristic peaks of RuO2 gradually become clear, but the characteristic peaks intensity of Co3 O4 decrease. These results indicated that the precursors (Co(OH)2 /Ru(OH)3 ·xH2 O) can be oxided as crystalline Co3 O4 /RuO2 ·xH2 O composites by annealing the precursors at 200 ◦ C in air. Since the specific capacitance, conductivity and cycle life of RuO2 -based composite electrodes are very dependant on the heat treatment temperature, the investigation of annealing temperature effect to the crystalline structure was also operated. Fig. 2B shows the XRD patterns of the composites with molar ratio of Co:Ru = 1:1 annealed at different temperatures varying from 100 ◦ C to 900 ◦ C. As is displayed in Fig. 2B, no obvious peaks are found when annealing at 100 ◦ C or 150 ◦ C, which ascribes to the amorphous structure of the composite. It can be clearly observed that the peaks intensity gradually increases and the width of these peaks decreases with rising of heat treatment temperature, indicating that the crystalline structure is gradually formed in this process. Notably, when the temperature is raised to 600 ◦ C, the peaks were sharp and matched well to the characteristic patterns of rutile structure of anhydrous RuO2 and spinel structure of Co3 O4. In addition, the composite annealed at 900 ◦ C also is comprised of rutile structure of RuO2 and spinel structure of Co3 O4 . From the reports of reference [5], only the sample of Ru:Co = 4:1 annealed at 400 ◦ C is a single phase, and decomposes to rutile RuO2 and spinel Co3 O4 at temperatures higher than 800 ◦ C. It is reasonable that the XRD patterns of the composites annealed both at 600 ◦ C and at 900 ◦ C show rutile structure of RuO2 and spinel structure of Co3 O4 , because the Ru:Co of composite prepared in this paper is less than 1:1. All above facts suggest that temperature is an important factor for controlling crystalline structure and water content of the composites, which will significantly affect the products’ electrochemical performance (see Fig. 7). The water content has an important role on electrochemical performance of the composite, which can be investigated by TGA/DSC method and the corresponding curves are shown in Fig. 3, The TGA curve indicates the weight continuously decreases from 60 ◦ C to 800 ◦ C and the total weight loss is about 26%. The rapid decrease (the weight loss of 8%) from 60 ◦ C to 95 ◦ C is almost attributable to the evaporation of physically adsorbed water, which can also be clarified by endothermic peaks of DSC in the range of 60 ◦ C to 95 ◦ C. For the precursor is comprised of Co(OH)2 ·nH2 O and Ru(OH)3 ·xH2 O, the exothermic peak centered at 185 ◦ C may be ascribed to the common contribution of the two components for removal of chemically bound water and formation of Co3 O4 or RuO2 ·xH2 O crystallites. This result is corresponding to the XRD patterns (see Fig. 2B). From 200 ◦ C to 600 ◦ C, another 10 wt.% loss occur, indicating a further loss of bound water and formation of larger crystals. From the data of TGA, the water content of
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Fig. 3. TGA (a) and DSC (b) curves of the composites with molar ratio of Co:Ru = 1:1 (precursor, not annealed) from 60 ◦ C to 900 ◦ C.
mesoporous Co3 O4 /RuO2 ·xH2 O composites is higher than that of RuO2 ·xH2 O prepared by sol–gel method [11]. The samples with molar ratio of Co:Ru = 1:1 annealed at various temperatures of 100 ◦ C, 150 ◦ C, 200 ◦ C, 400 ◦ C and 600 ◦ C can be denoted as Co3 O4 /RuO2 ·1.3H2 O,Co3 O4 /RuO2 ·0.9H2 O,Co3 O4 /RuO2 ·0.6H2 O,Co3 O4 /RuO2 ·0.3H2 O and Co3 O4 /RuO2 ·0.05H2 O, respectively. 3.2. The porous structure of the composites Nitrogen adsorption/desorption isotherms and the corresponding Barrett–Joyner– Halenda (BJH) pore size distribution for the composites (molar ratio of Co:Ru = 1:1) prepared with or without P123 are shown in Fig. 4. The BET surface area of sample (a) is 107 m2 g−1 , which is much higher than that (57 m2 g−1 ) of sample (b). As can be seen from isotherms of the two samples, a gradual uptake of nitrogen adsorption volume and one hysteresis loop in the region of P/P0 from 0.8 to 1.0 suggests a sorption characteristic of the porous materials and it can be ascribed to inter-particle porosity among neighboring particles (see Fig. 1). By using the adsorption branch of the isotherm and calculated by BJH method, sample (a) has a narrow pore size distribution (average pore size 18 nm) mainly in the region of mesopore (with diameters ranging between 2 nm and 50 nm)
Fig. 4. Nitrogen sorption isotherms and the corresponding pore size distribution of samples (molar ratio of Co:Ru = 1:1) annealed at 200 ◦ C, (a) with P123 soft template and (b) without P123 soft template.
and centered at about 20 nm, but sample (b) is of a relatively broad pore size distribution. Compared with sample (b), such mesoporous microstructures and high surface area of sample (a) are believed to facilitate larger capacitance and higher power density because ions transfer rate in the pore system and the extent of electrode/electrolyte interfacial area are determined by the porosity of the electrode material. That means that these original mesopores make the penetration of electrolytes into the whole oxide matrix facilely, which further reduce the diffusion resistance [33]. In addition, larger surface area of sample (a) is good to surface reactions, which is important in the enhancement of the electrochemical properties. Various factors influence the chemical reactions which makes it difficult to determine the exact mechanism in the composite formation process. According to our experimental results, it seems to conform to the formation mechanism shown schematically in Fig. 5. Firstly, P123 dissolved in water and assembled to form micelle (critical micelle concentration (CMC) of P123 is about 0.2 g l−1 [32], particle size 15–20 nm and core size 10 nm). When Co(NO3 )2 ·6H2 O and RuCl3 ·0.5H2 O were added into the above solution, we assumed that Co2+ and Ru3+ ions absorbed on the surface of the P123 micelles. With the rising of pH, the precipitates of Co(OH)2 and Ru(OH)3 ·xH2 O
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Fig. 5. The formation mechanism of Co3 O4 /RuO2 ·xH2 O composites prepared by using P123 as a soft template.
gradually formed and coated on the surface of these micelles. When the as-synthesized products were washed with water and ethanol for several times, the P123 was removed and the mesoporous precursor was obtained. Finally, Co3 O4 /RuO2 ·xH2 O composites were obtained by heating the precursor at different temperatures in atmosphere, and the precursor (Co(OH)2 and Ru(OH)3 ·xH2 O composite) was oxidized and dehydrated to turn into Co3 O4 /RuO2 ·xH2 O composites at a higher temperature. In this process, small particles overlapped together and the structure of composite collapsed (see Fig. 1). As is known, low P123 concentrations (0.2 g l−1 or less) showed partly aggregated micelles. In this paper, we employed the P123 concentration (50 g l−1 ) far higher than CMC in order to form more stable micelles with size ranging from 15 nm to 20 nm, which can explain the pore size distribution (average pore size is 18 nm) obtained from BET result (see Fig. 4). 3.3. Electrochemical characterization The obtained electrodes were investigated in three-electrode beaker-type cells by cyclic voltammetry and galvanostatic charge/discharge. 3.3.1. Capacitive performance of Co3 O4 /RuO2 ·xH2 O with various Ru content Fig. 6A shows the typical CVs of Co3 O4 /RuO2 ·xH2 O composites synthesized by one-step co-precipitation method using P123 template (at various molar ratio of Co:Ru) in 1 M KOH electrolyte between −0.85 V and 0.35 V (vs. Hg/HgO) at a scan rate of 10 mV/s. From curve (a), the current response of the sample with molar ratio of Co:Ru = 19:1 annealed at 200 ◦ C exhibits a rectangular shape under cyclic voltammatry from −0.85 V to 0.1 V. The reason may be that the capacitance of this composite mainly results from surface reactions of the Co3 O4 because of the low content of RuO2 . Moreover, we can observe that the currents of all composites increase rapidly at the potential
Fig. 6. (A) Cyclic voltammograms of Co3 O4 /RuO2 ·xH2 O composite (annealed at 200 ◦ C) electrodes in 1 M KOH electrolyte at a scan rate of 10 mV/s. The molar ratio of Co:Ru, (a) 19:1, (b) 9:1, (c) 5:1, (d) 1:1. (B) Cyclic voltammograms of the composite with molar ratio of Co:Ru = 1:1 annealed at 200 ◦ C in 1 M KOH electrolyte at various scan rates of 5 mV/s, 10 mV/s, 20 mV/s and 50 mV/s.
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Table 2 The specific capacitances of Co3 O4 /RuO2 ·xH2 O composites with different molar ratio of Co:Ru annealing at 200 ◦ C in 1 M KOH electrolyte Composites
Specific capacitance (F/g)
Co:Ru = 19:1 Co:Ru = 9:1 Co:Ru = 4:1 Co:Ru = 1:1 Co:Ru = 1:1 (without P123)
92 120 211 462 305
range of 0.1–0.35 V when the potential is on the positive sweep. These irreversible peaks might be ascribed to Co(+2)/Co(+3) solid-state oxidation transition. The dissolution phenomenon of the composites is also observed when the potential is over 0.45 V (vs. Hg/HgO). The similar result was also reported for “RuO2 + Co3 O4 electrodes” used in chlorine and oxygen evolution reactions [22,23]. From a comparison of curves (a)–(d), the current values get larger in the whole potential range, and the broad redox peaks became more distinct in the potential range of −0.4 V to −0.2 V, which might be attributed to the increasing of RuO2 content [12]. 3.3.2. Effects of mesoporous structure on the capacitive performance of the composite with molar ratio of Co:Ru = 1:1 The CVs of sample with Co:Ru = 1:1 annealed at 200 ◦ C were also investigated at various scan rates and shown in Fig. 6B. At a low scan rate of 5 mV/s, the CV shows a pair of redox peaks and poor redox reversibility, which attributes to the redox reactions of Co3 O4 . Moreover, as the increasing of scan rate, though the capacitances decrease, the redox peaks gradually disappear and the CVs also exhibit good capacitive performance. The above phenomena should result from two factors. First, the mesoporous structure and high surface area of the composite will maintain facile electrolyte ions penetration and fast surface reactions. The merits of this mesoporous structure are obvious that the capacitance of composite is far more than capacitance of the sample without using P123 (see Table 2). In addition, since the mesoporous composite is prepared by self-assemble method using P123 as a soft template, the particles and pores are not as regular and ordered as which of samples prepared by hard template [26], the electron pathways are not very smooth in the rapid charge/discharge reactions. The increasing of electronhopping resistance results from the decreasing of capacitance at a high scan rate. Second, the capacitance of the composite is not only from the surface reactions but also from the redox reactions in the bulk, supporting the fact that a high capacitance is obtained at a low scan rate. Due to the electrode reactions are mainly controlled by surface reactions of composite at a high scan rate, the decrease of capacitance is also reasonable. 3.3.3. Influence of annealing temperature on the capacitive behavior The annealing temperature has a significantly effect on the capacitance of Co3 O4 /RuO2 ·xH2 O composites, especially for
Fig. 7. Cyclic voltammograms of Co3 O4 /RuO2 ·xH2 O (Co:Ru = 1:1) composite electrodes in 1 M KOH electrolyte at a scan rate of 10 mV/s under different annealing temperatures: (a) 150 ◦ C, (b) 100 ◦ C, (c) 200 ◦ C, (d) 400 ◦ C, (e) 600 ◦ C, (f) 900 ◦ C.
Ru-enriched oxide, so the typical CVs of Co3 O4 /RuO2 ·xH2 O (Co:Ru = 1:1) composite annealed at various temperatures were measured at a scan rate of 10 mV/s and shown in Fig. 7. According to Fig. 7a and b (annealed at 100 ◦ C, 150 ◦ C), each CV exhibits the two asymmetric board peaks, indicating poor redox reversibility. This phenomenon may result from amorphous structure of the composite which show higher water content than the composite annealed at high temperatures. When the annealing temperature is raised to 900 ◦ C step by step, the voltammetric currents decrease rapidly and the two board peaks gradually reduce and disappear at last. This reason could be that the water content of RuO2 decrease little by little and crystalline RuO2 ·xH2 O is formed with the increasing of annealing temperature, which inhibit the electrolyte ions transportation in the bulk. In addition, we also observe that the curve (d) and curve (e) both display oxidation peaks at 0.15 V (vs. Hg/HgO) when the CVs are on the positive sweep, which can be ascribed to Co(+2)/Co(+3) solid-state oxidation transition. And also, the peaks of curve (d) at −0.45 V and curve (e) at −0.3 V were caused by Co(+3)/Co(+2) solid-state deoxidation transition. Nevertheless, annealing at 900 ◦ C, the curve (f) exhibits a rectangular-like shape, but the current value decrease obviously. In general, the crystallites may be slowly agglomerated into grain annealing at high temperature, which tends to enhance the crystal lattice and reduce the specific area. So, the above result is reasonable, because this well-crystallized structure makes the electrolyte ions difficult to insert into the bulk of the crystal but only react with Co3 O4 /RuO2 ·xH2 O on the surface of the crystal which shows very little capacitance. In a summary of the above results, the capacitive behavior of amorphous hydrous composite is better than that of well-crystallized, dehydrated composite, since the redox reaction of amorphous hydrous composite can occur not only on the surface but also in the bulk of the powder [28]. Galvanostatic charge/discharge measurements of Co3 O4 / RuO2 ·xH2 O composites were carried out in 1 M KOH electrolyte with a current density of 1 A g−1 . Fig. 8A and B exhibit
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Table 3 The specific capacitances of Co3 O4 /RuO2 ·xH2 O composites (Co:Ru = 1:1) annealed at different temperatures in 1 M KOH electrolyte.
Fig. 8. (A) Charge/discharge curves of Co3 O4 /RuO2 ·xH2 O composites, the molar ratio of Co:Ru: (a) 19:1, (b) 9:1, (c) 5:1, (d) 1:1. (B) Charge/discharge curves of the composite with molar ratio of Co:Ru = 1:1 annealed at different temperatures. (a) 900 ◦ C, (b) 600 ◦ C, (c) 400 ◦ C, (d) 200 ◦ C, (e) 100 ◦ C, (f) 150 ◦ C.
the galvanostatic charge/discharge curves of Co3 O4 /RuO2 composite with various molar ratio of Co:Ru and annealed at different temperatures, respectively. The capacitance value was obtained from galvanostatic discharge curves according to the equation: Cp =
I ×t m × V
where I is the current of discharge, t is the discharge time, m is the mass of the active material, V is the potential drop in the discharge progress, and Cp is the specific capacitance of the composite material. In this paper, we employ the current density I/m = 1 A g−1 . It can be seen that these curves exhibit stable charge–discharge behavior in a wide potential range of −0.75 V to 0. 25 V (vs. Hg/HgO), but not all these curves exhibit linear E–t response. It can be observed from Fig. 8A that the composite shows poorer reversibility with increasing of Ru content. However, the reversibility of composite with molar ratio of Co:Ru = 1:1 becomes better as the increasing of annealing temperature and the E–t curves are shown in Fig. 8B. The specific capacitance is higher in the potential range of −0.35 V to −0.2 V than −0.2 V to 0.25 V or −0.75 V to −0.45 V. This phenomena is reasonable since the capacitance of the composite arises from
Annealing temperature (◦ C)
Specific capacitance (F/g)
100 150 200 400 600 900
581 642 462 317 231 57
the common contributions of surface reactions of the composite, Co(+3)/Co(+2) solid-state redox transition and redox reactions of RuO2 ·xH2 O. According to the above equation, the capacitances of composite materials both with different molar ratio of Co:Ru and annealing at different temperatures are calculated and listed in Tables 2 and 3, respectively. From the data of Table 2, It is distinctly observed that the specific capacitance of the as-prepared Co3 O4 /RuO2 ·xH2 O composites increases with Ru content. Compared with the composite (Co:Ru = 1:1) prepared without P123 template, the composite with P123 shows a higher capacitance. This result indicates that such large increase in capacitance mainly ascribes to the mesoporous structure and high specific surface area of composite (see Fig. 4). Table 3 shows capacitances of the composite with molar ratio of Co:Ru = 1:1 annealing at different temperatures in 1 M KOH electrolyte. We observe that the capacitance decrease rapidly with rising of annealing temperature. The sample annealed at 150 ◦ C shows the highest specific capacitance of 642 F/g. However, the specific capacitance is only 57 F/g for the sample annealed at 900 ◦ C, which demonstrates that the capacitances of the as-prepared composites are very dependent on the annealing temperatures and the amorphous composite usually exhibits higher capacitance than crystalline composite [11]. Fig. 9 shows the cycle life of the composite with molar ratio of Co:Ru = 1:1 annealed at different temperatures, and the composite annealed at higher temperature usually shows better recyclability. For the composite annealed at 150 ◦ C, it displays a capacitance retention ratio of 94% after 1000th charge/discharge cycles, revealing
Fig. 9. Cycle life of the composite with molar ratio of Co:Ru = 1:1 annealed at different temperatures: (a) 150 ◦ C, (b) 200 ◦ C, (c) 400 ◦ C, (d) 600 ◦ C, (e) 900 ◦ C.
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good recyclability. The capacitance only decreased about 3% for the composite annealed at 200 ◦ C when the electrode was cycled the same 1000 times, and the capacitances of the samples hardly decreased at a higher temperature (≥400 ◦ C). On the basis of the above results and discussion, increasing annealing temperature, although the capacitance decreases rapidly, the cycle stabilities of the Co3 O4 /RuO2 ·xH2 O composites can be enhanced significantly. 4. Conclusions A series of Co3 O4 /RuO2 ·xH2 O (Co:Ru = 19:1, 9:1, 5:1, 1.1) composites had been successfully prepared by one-step coprecipitation method with the usage of P123 as a soft template. This method employed in our experiment was simpler and easier to prepare mesoporous composites. Moreover, from the results of electrochemical measurements, it can clearly be seen that the specific capacitances and stability of the composites are strongly dependent on the heat treatment temperature. A highest capacitance of 642 F/g was obtained from the composite with molar ratio of Co:Ru = 1:1 at 150 ◦ C and the capacitance gradually decrease with rising of the heat treatment temperature ranging from 150 ◦ C to 900 ◦ C, but the recyclability of the composites is improved. This phenomenon should be attributed to the fact that the well-crystallized and dehydrated composites makes the electrolyte ions difficult insert into the bulk of the crystal but only react with Co3 O4 /RuO2 ·xH2 O on the surface of the crystal. Furthermore, this study provides a common method to develop new type of RuO2 -based material which can be used to prepare other RuO2 -based materials such as RuO2 /NiO, RuO2 /SnO2 and so on. Acknowledgments The present work was supported by National Basic Research Program of China (973 Program) (No. 2007CB209703), National Natural Science Foundation of China (No. 20403014, No. 20633040) and National Natural Science Foundation of Jiangsu Province (No. BK2006196).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
J.F.C. Boodts, S. Trassatti, J. Electrochem. Soc. 173 (1990) 3784. B. Cornell, D. Simonsson, J. Electrochem. Soc. 140 (1993) 3123. C. Baruffaldi, S. Cattarin, M. Musiani, Electrochim. Acta 48 (2003) 3921. S. Cattarin, P. Guerriero, M. Musiani, Electrochim. Acta 46 (2001) 4229. J. Jirkovsk´y, M. Makarova, P. Krtil, Electrochem. Commun. 8 (2006) 1417. L. Zhang, H. Kisch, Angew. Chem. Int. Ed. 39 (2000) 3921. B.J. Lee, S.R. Sivakkumar, J.M. Ko, J.H. Kim, S.M. Jo, D.Y. Kim, J. Power Sources 168 (2007) 546. E.V. Jelenkovic, K.Y. Tong, W.Y. Cheung, S.P. Wong, Microelectron. Reliab. 43 (2003) 49. S. Ardizzone, G. Fregonara, S. Trasatti, Elecrochim. Acta 35 (1990) 263. S. Trasatti, G. Buzzanca, J. Electroanal. Chem. 29 (1971) Ap1. J.P. Zheng, T.R. Jow, J. Electrochem. Soc. 142 (1995) L6. J.P. Zheng, P.J. Cygan, T.R. Jow, J. Electrochem. Soc. 142 (1995) 2699. Y. Takasu, T. Nakamura, H. Ohkawauchi, Y. Murakami, J. Electrochem. Soc. 144 (1997) 2601. P. Soudan, J. Gaudet, D. Guay, D. B´elanger, R. Schulz, Chem. Mater. 14 (2002) 1210. X.M. Liu, X.G. Zhang, Electrochim. Acta 49 (2004) 229. C.-C. Wang, C.-C. Hu, J. Electrochem. Soc. 152 (2005) A370. Y.U. Jeong, A. Manthiram, J. Electrochem. Soc. 148 (2001) A189. Y.G. Wang, X.G. Zhang, Electrochim. Acta 49 (2004) 1957. R.N. Sing, J.-F. Koenig, G. Poillerat, P. Chartier, J. Electrochem. Soc. 137 (1990) 1408. R. Boggio, A. Carugati, G. Lodi, S. Trasatti, J. Appl. Electrochem. 15 (1985) 335. C. Lin, J.A. Ritter, B.N. Popov, J. Electrochem. Soc. 145 (1998) 4097. N. Krstaj´ıc, S. Trasatti, J. Electrochem. Soc. 142 (1995) 2675. L.M. Da Silva, J.F.C. Boodts, L.A. De Faria, Electrochim. Acta 46 (2001) 1369. S. Trasatti, Elecrochim. Acta 32 (1987) 369. S. Trasatti, Elecrochim. Acta 45 (2000) 2377. C.-C. Hu, K.-H. Chang, M.-C. Lin, Y.-T. Wu, Nano Lett. 6 (2006) 2690. V. Subramanian, S.C. Hall, P.H. Smith, B. Rambabu, Solid State Ionics 175 (2004) 511. J.P. Zheng, Y. Xin, J. Power Sources 110 (2002) 86. W. Dmowski, T. Egami, K.E. Swider-Lyons, C.T. Love, D.R. Rolison, J. Phys. Chem. B 106 (2002) 12677. C.-C. Hu, C.-C. Wang, Electrochem. Commum. 4 (2002) 554. C.-C. Hu, W.-C. Chen, Elecrochim. Acta 49 (2004) 3469. P. Alexandridis, J.F. Holzwarth, T.A. Hatton, Macromolecules 27 (1994) 2414. K.-H. Chang, C.-C. Hu, C.-Y. Chou, Chem. Mater. 19 (2007) 2112.
Electrochimica Acta 53 (2008) 3296–3304
Soft template synthesis of mesoporous Co3O4/RuO2·xH2O composites for electrochemical capacitors Yang Liu, Weiwei Zhao, Xiaogang Zhang ∗ College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 20 September 2007; received in revised form 6 November 2007; accepted 11 November 2007 Available online 19 November 2007
Abstract Co3 O4 /RuO2 ·xH2 O composites with various Ru content (molar content of Ru = 5%, 10%, 20%, 50%) were synthesized by one-step coprecipitation method. The precursors were prepared via adjusting pH of the mixed aqueous solutions of Co(NO3 )2 ·6H2 O and RuCl3 ·0.5H2 O by using Pluronic123 as a soft template. For the composite with molar ratio of Co:Ru = 1:1 annealed at 200 ◦ C, Brunauer–Emmet–Teller (BET) results indicated that the composite showed mesoporous structure, and the specific surface area of the composite was as high as 107 m2 g−1 . The electrochemical performances of these composites were measured in 1 M KOH electrolyte. Compared with the composite prepared without template, the composite with P123 exhibited a higher specific capacitance. When the molar content of Ru was rising, the specific capacitance of the composites increased significantly. It was also observed that the crystalline structures as well as the electrochemical activities were strongly dependent on the annealing temperature. A capacitance of 642 F/g was obtained for the composite (Co:Ru = 1:1) annealed at 150 ◦ C. Meanwhile, the composites also exhibited good cycle stability. Besides, the morphologies and textural characteristic of the samples were also investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM). © 2007 Elsevier Ltd. All rights reserved. Keywords: Pluronic123; Soft template; Co3 O4 ; RuO2 ·xH2 O; Composite; Co-precipitation; Electrochemical capacitors
1. Introduction Ruthenium oxide, with its perfect electrocatalytic activity [1–4], high electronic and proton conductivities, has been attracted considerable attention and been widely used in hydrocarbon sensors, gas-evolving reactions [5] and catalysts [6]. The highlight application of ruthenium oxide material is the usage as electrode material in energy storage electrochemical capacitors [7–9]. In general, electrochemical capacitors can be categorized into two groups: double-layer capacitors and redox capacitors. The former electrode materials mainly include high surface area activated carbon such as carbon aerogels, nanotubes and nanofibers; the latter consists of metal oxides and conducting polymers. As a redox-type metal oxide, ruthenium oxide is a perfect material for electrochemical capacitors due to its higher energy than that of high surface area carbon materials, and better electrochemical stability than that of conducting polymers.
∗
Corresponding author. Tel.: +86 25 52112902; fax: +86 25 52112626. E-mail address: [email protected] (X. Zhang).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.11.022
It is well known that the capacitance of RuO2 electrode results from redox pseudocapacitance. However, the current response resembles an ideal capacitor, which is a “rectangular” shape under cyclic voltammatry [10]. Due to the above advantages of RuO2 material, much effort has been devoted to developing new RuO2 materials or RuO2 -based composites. Zheng suggested that the amorphous hydrated ruthenium oxide obtained by sol-gel method showed a high specific capacitance (720 F/g) [11,12], which was known as a large breakthrough of supercapacitors. However, despite the remarkable performance of this material, pure RuO2 material is still too expensive for large-scale commercialization, which have prompted the research interest to focus on alternative cheap electrode materials (such as MnO2 , NiO, Co3 O4 , etc.) or dispersion/alloying of ruthenium oxide in other less costly oxide matrics forming ruthenium-based composites. It has been reported that RuO2 -VOx electrode prepared by a dip-coated method had showed interesting performances [13]; Soudan also investigated nanocrystalline Tix Fey Ruz On compounds prepared by high energy ball milling [14]. Some other metal oxide composites, such as NiO/RuO2 , (Ru–Sn)Ox·nH2 O, WO3 /RuO2 and
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RuO2 /TiO2 had also been discussed [15–18]. All these above work had proved that the ruthenium-based composite materials had good capacitive behavior, which could be attributed to the incorporation of other elements into RuO2 structure. Spinel Co3 O4 is a good material with excellent electrocatalytic activity for oxygen [19] or chlorine evolution reaction [20]. Even if the cycle reversibility is not good, Co3 O4 is also considered as one promising potential candidates for supercapacitor electrode materials due to its environmental friendliness, low cost, and favorable pseudocapacitive characteristics [21]. Some reports about RuO2 + Co3 O4 electrodes with various molar ratio of Co:Ru prepared by thermal decomposition method was also intensively studied either in alkaline solutions or in acid conditions which showed good electrochemical behavior or electrocatalytic properties [22,23]. All above merits make the application of spinel Co3 O4 possible as a partial substitute of the noble metal. In recent years, mesoporous materials (e.g. mesoporous carbon or metal oxide) have been attracted widely attentions which usually show excellent supercapacitive performance. RuO2 based porous electrodes (known as DSA) prepared by thermal decomposition method were intensively investigated in the early reports [24,25]. And also, various approaches have been employed to prepared mesoporous metal oxides by using hard templates (such as AAO, silica or carbon template). It has been reported that the desired three-dimensional mesoporous architecture of RuO2 ·xH2 O nantubular array prepared by use of the membrance-templated synthesis route showed a capacitance of 1300 F/g with high energy and power density [26]. Moreover, surfactant template synthesis of mesoporous anhydrous RuO2 materials with high specific area has also been performed. Since redox reactions mainly occur on the surface layer, metal oxide with a larger surface area, interconnected mesoporous structure with controlled pore size and low resistivity is expected to show higher energy density due to the short path for ion diffusion and transmission in the electrolyte. However, it is reported that the capacitance of anhydrous RuO2 prepared by calcination removing P123 template was only 58 F/g [27], which is far lower than the amorphous hydrated RuO2 [11] fabricated by sol-gel method (720 F/g). The reason is that well-crystalline anhydrous RuO2 prevented the proton permeation in to the bulk material and the capacitance of this anhydrous RuO2 was mainly from the surface reaction [28]. In this present paper, the mesoporous hydrous Co3 O4 / RuO2 ·xH2 O composite materials with high specific surface area are prepared by one-step co-precipitation method with the usage of Pluronic123 as a soft template. This approach is not only simpler and easier to obtain mesoporous metal oxide than nanocasting method using silica template, but also combines the both merits of Co3 O4 and RuO2 ·xH2 O materials in capacitive behavior. Meanwhile, the electrode reactions of Co3 O4 /RuO2 ·xH2 O composite occur not only on the surface of the electrode but also in the bulk of the electrode due to the high water content of amorphous RuO2 ·xH2 O composition [29], the specific capacitance of composites would be higher than that of sample prepared by thermal decomposition method. In this work, various molar ratio of Co:Ru composites ranging from
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19:1 to 1:1 were prepared and investigated because the content of RuO2 played a vital role on the specific capacitance of the composite. In addition, the influence of the heat treatment temperature on the capacitive performance of Co3 O4 /RuO2 ·xH2 O composite are also studied since the particle size, specific surface area and water content of composite are very dependent on the annealing temperature. Especially, nanocrystalline RuO2 ·xH2 O annealed at temperatures close to its crystalline temperatures usually exhibits the highest specific capacitance [11,30,31]. 2. Experimental 2.1. Synthesis of Co3 O4 /RuO2 ·xH2 O composite electrodes The composite materials were synthesized as follows: taking the composite with molar ratio of Co:Ru = 1:1 for the example, 2 g of block copolymer surfactant Pluronic123 (Aldrich, Mw = 5800) was dissolved in 40 ml volume of distilled water to form homogeneous solution, then 0.216 g of Co(NO3 )2 ·6H2 O and 0.209 g of RuCl3 ·0.5H2 O were respectively added into the above solution. In this process, the mixture was stirred vigorously by a magnetic bar. Subsequently, the 0.2 M aqueous solution of NaOH was slowly dropped into the above solution to adjust the pH value to 10 (Ru3+ and Co2+ can nearly turn into precipitation in total at this pH value). When the pH value was approximately equal to 10, the stirring was stopped. After placing for three hours, the precipitate was filtered and washed for several times with ethanol and deionized water, respectively. Then the residue was dried at 60 ◦ C under vacuum for 12 h. Finally, the product was annealed in air at various temperatures ranging from 100 ◦ C to 900 ◦ C for 3 h, and the composites were obtained. According to the molar ratio of Co to Ru (19:1, 9:1, 4:1, 1:1), the other composites were prepared with the same method (only annealed at 200 ◦ C), respectively. The sample without using P123 was also prepared under the same experiment conditions. The working electrodes were prepared with the as-prepared Co3 O4 /RuO2 ·xH2 O composites. Typically, 75 wt.% active material, 15 wt.% acetylene black and 10 wt.% polyvinylidenefluoride (PVDF) were well mixed in an agate mortar, and a small amount of N-methyl-pyrrolidone (NMP) was added to the composite to make more homogeneous mixture, which was then pressed on nickel mesh (1 cm × 3 cm) under the pressure of 15 MPa that served as a current collector to get an approximate thickness of 0.2 mm. Then the above electrodes were dried in vacuum at 60 ◦ C for 24 h to remove the solvent. The mass of the active material was about 8–10 mg. 2.2. Characterization and electrochemical tests The samples were characterized by X-ray diffraction (XRD) technique, nitrogen sorption measurements and so forth. XRD pattern of the sample was conducted on a Bruker D8ADVANCE diffractometer with Cu K␣ radiation of wavelength λ = 0.15418 nm). The pore properties of the samples were investigated using nitrogen adsorption on an automatic volumetric sorption analyzer (Micromeritics Corp., ASAP2010) at liquid
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nitrogen temperature. Prior to the measurements, the samples were degassed at a temperature of 150 ◦ C for 6 h under vacuum. The specific surface areas (SBET) were determined according to the Brunauer–Emmett–Teller (BET) method in the relative pressure range of 0.05–0.25. Pore size distribution (PSD) curves were calculated by the Barrett–Joyner–Halenda (BJH) method from the adsorption branches. The microscopic features and the compositions of the samples were observed with a scanning electron microscope (SEM) (Quanta 2000) operated at 20.0 kV. The transmission electron microscopy (TEM) images were taken on a FEI Tecnai G2 20 S-TWIN microscope operated at 200 kV. Thermal data of the composites were determined by thermogravimetric analyzer (TGA) and different scanning calorimeter (DSC) in air at 10 ◦ C/min (Instrument, SDT Q600). All the electrochemical measurements were performed in a three-electrode beaker-type cell at room temperature. The as-prepared composites were used as working electrodes; a platinum foil as the counter electrode and a SCE electrode as the reference electrode. 1 M KOH aqueous solution was employed as electrolyte. The electrochemical tests were examined by means of CHI660B electrochemical workstation and BT2042 battery system.
3. Results and discussion 3.1. Morphology and textural characteristics of compositions Particle size and distribution of the Co3 O4 /RuO2 ·xH2 O composite plays an important role on the utilization of electrode. Scanning electron microscopy was used to observe the surface morphologies of the composites (molar ratio of Co:Ru = 1:1) annealed at 200 ◦ C. The corresponding images are shown in Fig. 1a and b. It is typically observed that small particles stick together to form porous agglomerates. The particle morphology and size distribution of the samples were examined by TEM and shown in Fig. 1c and d. Mesoporous composite with pore size of ∼20 nm are clearly observed, which can be confirmed by nitrogen adsorption–desorption isotherms (see Fig. 4). It also can be seen that these agglomerates are comprised of smaller nanosized particles with a probably diameter of <3 nm. From the above results and discussion, the mesoporous Co3 O4 /RuO2 ·xH2 O composites can be easily prepared with coprecipitation method by using P123 as soft template. This mesoporous structure not only can reduce the diffusion resistance of electrolyte but also allows a short path of electroactive ions
Fig. 1. SEM (a, b) and TEM (c, d) images of the composites (molar ratio of Co:Ru = 1:1) annealed at 200 ◦ C.
Y. Liu et al. / Electrochimica Acta 53 (2008) 3296–3304 Table 1 The molar content of various Co3 O4 /RuO2 ·xH2 O composites from EDS Co:Ru in composites
Co at.% Ru at.%
19:1
9:1
4:1
1:1
94.7 5.3
89.4 10.6
80.8 19.2
51.6 48.4
(OH− ) transportation from the electrolyte to the composite. In addition, the composition of all Co3 O4 /RuO2 ·xH2 O composites with various molar ratio of Co:Ru were measured by EDS. It can be observed from Table 1 that the molar content of Ru:Co is approximately equal to the ratio of that in precursor solution, which demonstrates that Ru3+ and Co2+ in the precursor solutions can be precipitated in total at pH value of 10. The crystalline structures of the samples were characterized by using X-ray diffraction technology, shown in Fig. 2. Fig. 2A
Fig. 2. (A) XRD patterns of Co3 O4 /RuO2 ·xH2 O composites prepared with different Co:Ru molar ratio (a = 19:1, b = 9:1, c = 4:1, d = 1:1) annealed at 200 ◦ C; (B) XRD patterns of the composites with molar ratio of Co:Ru = 1:1 annealed at different temperatures. The squares mark the peaks of rutile RuO2 and the cycles mark spinel Co3 O4 .
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exhibits the patterns of samples with different molar ratio of Co:Ru annealed at 200 ◦ C in air. For the sample (a), the XRD patterns of spinel Co3 O4 (cycle, 2θ = 19.0◦ , 31.3◦ , 36.8◦ , 44.8◦ , 59.3◦ , 65.2◦ ) are obviously observed. However, the characteristic peaks of RuO2 (square, 2θ = 28.0◦ , 35.0◦ , 54.4◦ ) are weak, which may be attribute to low content of RuO2 . As the Ru molar content increases from 5% to 50%, the characteristic peaks of RuO2 gradually become clear, but the characteristic peaks intensity of Co3 O4 decrease. These results indicated that the precursors (Co(OH)2 /Ru(OH)3 ·xH2 O) can be oxided as crystalline Co3 O4 /RuO2 ·xH2 O composites by annealing the precursors at 200 ◦ C in air. Since the specific capacitance, conductivity and cycle life of RuO2 -based composite electrodes are very dependant on the heat treatment temperature, the investigation of annealing temperature effect to the crystalline structure was also operated. Fig. 2B shows the XRD patterns of the composites with molar ratio of Co:Ru = 1:1 annealed at different temperatures varying from 100 ◦ C to 900 ◦ C. As is displayed in Fig. 2B, no obvious peaks are found when annealing at 100 ◦ C or 150 ◦ C, which ascribes to the amorphous structure of the composite. It can be clearly observed that the peaks intensity gradually increases and the width of these peaks decreases with rising of heat treatment temperature, indicating that the crystalline structure is gradually formed in this process. Notably, when the temperature is raised to 600 ◦ C, the peaks were sharp and matched well to the characteristic patterns of rutile structure of anhydrous RuO2 and spinel structure of Co3 O4. In addition, the composite annealed at 900 ◦ C also is comprised of rutile structure of RuO2 and spinel structure of Co3 O4 . From the reports of reference [5], only the sample of Ru:Co = 4:1 annealed at 400 ◦ C is a single phase, and decomposes to rutile RuO2 and spinel Co3 O4 at temperatures higher than 800 ◦ C. It is reasonable that the XRD patterns of the composites annealed both at 600 ◦ C and at 900 ◦ C show rutile structure of RuO2 and spinel structure of Co3 O4 , because the Ru:Co of composite prepared in this paper is less than 1:1. All above facts suggest that temperature is an important factor for controlling crystalline structure and water content of the composites, which will significantly affect the products’ electrochemical performance (see Fig. 7). The water content has an important role on electrochemical performance of the composite, which can be investigated by TGA/DSC method and the corresponding curves are shown in Fig. 3, The TGA curve indicates the weight continuously decreases from 60 ◦ C to 800 ◦ C and the total weight loss is about 26%. The rapid decrease (the weight loss of 8%) from 60 ◦ C to 95 ◦ C is almost attributable to the evaporation of physically adsorbed water, which can also be clarified by endothermic peaks of DSC in the range of 60 ◦ C to 95 ◦ C. For the precursor is comprised of Co(OH)2 ·nH2 O and Ru(OH)3 ·xH2 O, the exothermic peak centered at 185 ◦ C may be ascribed to the common contribution of the two components for removal of chemically bound water and formation of Co3 O4 or RuO2 ·xH2 O crystallites. This result is corresponding to the XRD patterns (see Fig. 2B). From 200 ◦ C to 600 ◦ C, another 10 wt.% loss occur, indicating a further loss of bound water and formation of larger crystals. From the data of TGA, the water content of
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Fig. 3. TGA (a) and DSC (b) curves of the composites with molar ratio of Co:Ru = 1:1 (precursor, not annealed) from 60 ◦ C to 900 ◦ C.
mesoporous Co3 O4 /RuO2 ·xH2 O composites is higher than that of RuO2 ·xH2 O prepared by sol–gel method [11]. The samples with molar ratio of Co:Ru = 1:1 annealed at various temperatures of 100 ◦ C, 150 ◦ C, 200 ◦ C, 400 ◦ C and 600 ◦ C can be denoted as Co3 O4 /RuO2 ·1.3H2 O,Co3 O4 /RuO2 ·0.9H2 O,Co3 O4 /RuO2 ·0.6H2 O,Co3 O4 /RuO2 ·0.3H2 O and Co3 O4 /RuO2 ·0.05H2 O, respectively. 3.2. The porous structure of the composites Nitrogen adsorption/desorption isotherms and the corresponding Barrett–Joyner– Halenda (BJH) pore size distribution for the composites (molar ratio of Co:Ru = 1:1) prepared with or without P123 are shown in Fig. 4. The BET surface area of sample (a) is 107 m2 g−1 , which is much higher than that (57 m2 g−1 ) of sample (b). As can be seen from isotherms of the two samples, a gradual uptake of nitrogen adsorption volume and one hysteresis loop in the region of P/P0 from 0.8 to 1.0 suggests a sorption characteristic of the porous materials and it can be ascribed to inter-particle porosity among neighboring particles (see Fig. 1). By using the adsorption branch of the isotherm and calculated by BJH method, sample (a) has a narrow pore size distribution (average pore size 18 nm) mainly in the region of mesopore (with diameters ranging between 2 nm and 50 nm)
Fig. 4. Nitrogen sorption isotherms and the corresponding pore size distribution of samples (molar ratio of Co:Ru = 1:1) annealed at 200 ◦ C, (a) with P123 soft template and (b) without P123 soft template.
and centered at about 20 nm, but sample (b) is of a relatively broad pore size distribution. Compared with sample (b), such mesoporous microstructures and high surface area of sample (a) are believed to facilitate larger capacitance and higher power density because ions transfer rate in the pore system and the extent of electrode/electrolyte interfacial area are determined by the porosity of the electrode material. That means that these original mesopores make the penetration of electrolytes into the whole oxide matrix facilely, which further reduce the diffusion resistance [33]. In addition, larger surface area of sample (a) is good to surface reactions, which is important in the enhancement of the electrochemical properties. Various factors influence the chemical reactions which makes it difficult to determine the exact mechanism in the composite formation process. According to our experimental results, it seems to conform to the formation mechanism shown schematically in Fig. 5. Firstly, P123 dissolved in water and assembled to form micelle (critical micelle concentration (CMC) of P123 is about 0.2 g l−1 [32], particle size 15–20 nm and core size 10 nm). When Co(NO3 )2 ·6H2 O and RuCl3 ·0.5H2 O were added into the above solution, we assumed that Co2+ and Ru3+ ions absorbed on the surface of the P123 micelles. With the rising of pH, the precipitates of Co(OH)2 and Ru(OH)3 ·xH2 O
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Fig. 5. The formation mechanism of Co3 O4 /RuO2 ·xH2 O composites prepared by using P123 as a soft template.
gradually formed and coated on the surface of these micelles. When the as-synthesized products were washed with water and ethanol for several times, the P123 was removed and the mesoporous precursor was obtained. Finally, Co3 O4 /RuO2 ·xH2 O composites were obtained by heating the precursor at different temperatures in atmosphere, and the precursor (Co(OH)2 and Ru(OH)3 ·xH2 O composite) was oxidized and dehydrated to turn into Co3 O4 /RuO2 ·xH2 O composites at a higher temperature. In this process, small particles overlapped together and the structure of composite collapsed (see Fig. 1). As is known, low P123 concentrations (0.2 g l−1 or less) showed partly aggregated micelles. In this paper, we employed the P123 concentration (50 g l−1 ) far higher than CMC in order to form more stable micelles with size ranging from 15 nm to 20 nm, which can explain the pore size distribution (average pore size is 18 nm) obtained from BET result (see Fig. 4). 3.3. Electrochemical characterization The obtained electrodes were investigated in three-electrode beaker-type cells by cyclic voltammetry and galvanostatic charge/discharge. 3.3.1. Capacitive performance of Co3 O4 /RuO2 ·xH2 O with various Ru content Fig. 6A shows the typical CVs of Co3 O4 /RuO2 ·xH2 O composites synthesized by one-step co-precipitation method using P123 template (at various molar ratio of Co:Ru) in 1 M KOH electrolyte between −0.85 V and 0.35 V (vs. Hg/HgO) at a scan rate of 10 mV/s. From curve (a), the current response of the sample with molar ratio of Co:Ru = 19:1 annealed at 200 ◦ C exhibits a rectangular shape under cyclic voltammatry from −0.85 V to 0.1 V. The reason may be that the capacitance of this composite mainly results from surface reactions of the Co3 O4 because of the low content of RuO2 . Moreover, we can observe that the currents of all composites increase rapidly at the potential
Fig. 6. (A) Cyclic voltammograms of Co3 O4 /RuO2 ·xH2 O composite (annealed at 200 ◦ C) electrodes in 1 M KOH electrolyte at a scan rate of 10 mV/s. The molar ratio of Co:Ru, (a) 19:1, (b) 9:1, (c) 5:1, (d) 1:1. (B) Cyclic voltammograms of the composite with molar ratio of Co:Ru = 1:1 annealed at 200 ◦ C in 1 M KOH electrolyte at various scan rates of 5 mV/s, 10 mV/s, 20 mV/s and 50 mV/s.
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Table 2 The specific capacitances of Co3 O4 /RuO2 ·xH2 O composites with different molar ratio of Co:Ru annealing at 200 ◦ C in 1 M KOH electrolyte Composites
Specific capacitance (F/g)
Co:Ru = 19:1 Co:Ru = 9:1 Co:Ru = 4:1 Co:Ru = 1:1 Co:Ru = 1:1 (without P123)
92 120 211 462 305
range of 0.1–0.35 V when the potential is on the positive sweep. These irreversible peaks might be ascribed to Co(+2)/Co(+3) solid-state oxidation transition. The dissolution phenomenon of the composites is also observed when the potential is over 0.45 V (vs. Hg/HgO). The similar result was also reported for “RuO2 + Co3 O4 electrodes” used in chlorine and oxygen evolution reactions [22,23]. From a comparison of curves (a)–(d), the current values get larger in the whole potential range, and the broad redox peaks became more distinct in the potential range of −0.4 V to −0.2 V, which might be attributed to the increasing of RuO2 content [12]. 3.3.2. Effects of mesoporous structure on the capacitive performance of the composite with molar ratio of Co:Ru = 1:1 The CVs of sample with Co:Ru = 1:1 annealed at 200 ◦ C were also investigated at various scan rates and shown in Fig. 6B. At a low scan rate of 5 mV/s, the CV shows a pair of redox peaks and poor redox reversibility, which attributes to the redox reactions of Co3 O4 . Moreover, as the increasing of scan rate, though the capacitances decrease, the redox peaks gradually disappear and the CVs also exhibit good capacitive performance. The above phenomena should result from two factors. First, the mesoporous structure and high surface area of the composite will maintain facile electrolyte ions penetration and fast surface reactions. The merits of this mesoporous structure are obvious that the capacitance of composite is far more than capacitance of the sample without using P123 (see Table 2). In addition, since the mesoporous composite is prepared by self-assemble method using P123 as a soft template, the particles and pores are not as regular and ordered as which of samples prepared by hard template [26], the electron pathways are not very smooth in the rapid charge/discharge reactions. The increasing of electronhopping resistance results from the decreasing of capacitance at a high scan rate. Second, the capacitance of the composite is not only from the surface reactions but also from the redox reactions in the bulk, supporting the fact that a high capacitance is obtained at a low scan rate. Due to the electrode reactions are mainly controlled by surface reactions of composite at a high scan rate, the decrease of capacitance is also reasonable. 3.3.3. Influence of annealing temperature on the capacitive behavior The annealing temperature has a significantly effect on the capacitance of Co3 O4 /RuO2 ·xH2 O composites, especially for
Fig. 7. Cyclic voltammograms of Co3 O4 /RuO2 ·xH2 O (Co:Ru = 1:1) composite electrodes in 1 M KOH electrolyte at a scan rate of 10 mV/s under different annealing temperatures: (a) 150 ◦ C, (b) 100 ◦ C, (c) 200 ◦ C, (d) 400 ◦ C, (e) 600 ◦ C, (f) 900 ◦ C.
Ru-enriched oxide, so the typical CVs of Co3 O4 /RuO2 ·xH2 O (Co:Ru = 1:1) composite annealed at various temperatures were measured at a scan rate of 10 mV/s and shown in Fig. 7. According to Fig. 7a and b (annealed at 100 ◦ C, 150 ◦ C), each CV exhibits the two asymmetric board peaks, indicating poor redox reversibility. This phenomenon may result from amorphous structure of the composite which show higher water content than the composite annealed at high temperatures. When the annealing temperature is raised to 900 ◦ C step by step, the voltammetric currents decrease rapidly and the two board peaks gradually reduce and disappear at last. This reason could be that the water content of RuO2 decrease little by little and crystalline RuO2 ·xH2 O is formed with the increasing of annealing temperature, which inhibit the electrolyte ions transportation in the bulk. In addition, we also observe that the curve (d) and curve (e) both display oxidation peaks at 0.15 V (vs. Hg/HgO) when the CVs are on the positive sweep, which can be ascribed to Co(+2)/Co(+3) solid-state oxidation transition. And also, the peaks of curve (d) at −0.45 V and curve (e) at −0.3 V were caused by Co(+3)/Co(+2) solid-state deoxidation transition. Nevertheless, annealing at 900 ◦ C, the curve (f) exhibits a rectangular-like shape, but the current value decrease obviously. In general, the crystallites may be slowly agglomerated into grain annealing at high temperature, which tends to enhance the crystal lattice and reduce the specific area. So, the above result is reasonable, because this well-crystallized structure makes the electrolyte ions difficult to insert into the bulk of the crystal but only react with Co3 O4 /RuO2 ·xH2 O on the surface of the crystal which shows very little capacitance. In a summary of the above results, the capacitive behavior of amorphous hydrous composite is better than that of well-crystallized, dehydrated composite, since the redox reaction of amorphous hydrous composite can occur not only on the surface but also in the bulk of the powder [28]. Galvanostatic charge/discharge measurements of Co3 O4 / RuO2 ·xH2 O composites were carried out in 1 M KOH electrolyte with a current density of 1 A g−1 . Fig. 8A and B exhibit
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3303
Table 3 The specific capacitances of Co3 O4 /RuO2 ·xH2 O composites (Co:Ru = 1:1) annealed at different temperatures in 1 M KOH electrolyte.
Fig. 8. (A) Charge/discharge curves of Co3 O4 /RuO2 ·xH2 O composites, the molar ratio of Co:Ru: (a) 19:1, (b) 9:1, (c) 5:1, (d) 1:1. (B) Charge/discharge curves of the composite with molar ratio of Co:Ru = 1:1 annealed at different temperatures. (a) 900 ◦ C, (b) 600 ◦ C, (c) 400 ◦ C, (d) 200 ◦ C, (e) 100 ◦ C, (f) 150 ◦ C.
the galvanostatic charge/discharge curves of Co3 O4 /RuO2 composite with various molar ratio of Co:Ru and annealed at different temperatures, respectively. The capacitance value was obtained from galvanostatic discharge curves according to the equation: Cp =
I ×t m × V
where I is the current of discharge, t is the discharge time, m is the mass of the active material, V is the potential drop in the discharge progress, and Cp is the specific capacitance of the composite material. In this paper, we employ the current density I/m = 1 A g−1 . It can be seen that these curves exhibit stable charge–discharge behavior in a wide potential range of −0.75 V to 0. 25 V (vs. Hg/HgO), but not all these curves exhibit linear E–t response. It can be observed from Fig. 8A that the composite shows poorer reversibility with increasing of Ru content. However, the reversibility of composite with molar ratio of Co:Ru = 1:1 becomes better as the increasing of annealing temperature and the E–t curves are shown in Fig. 8B. The specific capacitance is higher in the potential range of −0.35 V to −0.2 V than −0.2 V to 0.25 V or −0.75 V to −0.45 V. This phenomena is reasonable since the capacitance of the composite arises from
Annealing temperature (◦ C)
Specific capacitance (F/g)
100 150 200 400 600 900
581 642 462 317 231 57
the common contributions of surface reactions of the composite, Co(+3)/Co(+2) solid-state redox transition and redox reactions of RuO2 ·xH2 O. According to the above equation, the capacitances of composite materials both with different molar ratio of Co:Ru and annealing at different temperatures are calculated and listed in Tables 2 and 3, respectively. From the data of Table 2, It is distinctly observed that the specific capacitance of the as-prepared Co3 O4 /RuO2 ·xH2 O composites increases with Ru content. Compared with the composite (Co:Ru = 1:1) prepared without P123 template, the composite with P123 shows a higher capacitance. This result indicates that such large increase in capacitance mainly ascribes to the mesoporous structure and high specific surface area of composite (see Fig. 4). Table 3 shows capacitances of the composite with molar ratio of Co:Ru = 1:1 annealing at different temperatures in 1 M KOH electrolyte. We observe that the capacitance decrease rapidly with rising of annealing temperature. The sample annealed at 150 ◦ C shows the highest specific capacitance of 642 F/g. However, the specific capacitance is only 57 F/g for the sample annealed at 900 ◦ C, which demonstrates that the capacitances of the as-prepared composites are very dependent on the annealing temperatures and the amorphous composite usually exhibits higher capacitance than crystalline composite [11]. Fig. 9 shows the cycle life of the composite with molar ratio of Co:Ru = 1:1 annealed at different temperatures, and the composite annealed at higher temperature usually shows better recyclability. For the composite annealed at 150 ◦ C, it displays a capacitance retention ratio of 94% after 1000th charge/discharge cycles, revealing
Fig. 9. Cycle life of the composite with molar ratio of Co:Ru = 1:1 annealed at different temperatures: (a) 150 ◦ C, (b) 200 ◦ C, (c) 400 ◦ C, (d) 600 ◦ C, (e) 900 ◦ C.
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good recyclability. The capacitance only decreased about 3% for the composite annealed at 200 ◦ C when the electrode was cycled the same 1000 times, and the capacitances of the samples hardly decreased at a higher temperature (≥400 ◦ C). On the basis of the above results and discussion, increasing annealing temperature, although the capacitance decreases rapidly, the cycle stabilities of the Co3 O4 /RuO2 ·xH2 O composites can be enhanced significantly. 4. Conclusions A series of Co3 O4 /RuO2 ·xH2 O (Co:Ru = 19:1, 9:1, 5:1, 1.1) composites had been successfully prepared by one-step coprecipitation method with the usage of P123 as a soft template. This method employed in our experiment was simpler and easier to prepare mesoporous composites. Moreover, from the results of electrochemical measurements, it can clearly be seen that the specific capacitances and stability of the composites are strongly dependent on the heat treatment temperature. A highest capacitance of 642 F/g was obtained from the composite with molar ratio of Co:Ru = 1:1 at 150 ◦ C and the capacitance gradually decrease with rising of the heat treatment temperature ranging from 150 ◦ C to 900 ◦ C, but the recyclability of the composites is improved. This phenomenon should be attributed to the fact that the well-crystallized and dehydrated composites makes the electrolyte ions difficult insert into the bulk of the crystal but only react with Co3 O4 /RuO2 ·xH2 O on the surface of the crystal. Furthermore, this study provides a common method to develop new type of RuO2 -based material which can be used to prepare other RuO2 -based materials such as RuO2 /NiO, RuO2 /SnO2 and so on. Acknowledgments The present work was supported by National Basic Research Program of China (973 Program) (No. 2007CB209703), National Natural Science Foundation of China (No. 20403014, No. 20633040) and National Natural Science Foundation of Jiangsu Province (No. BK2006196).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
J.F.C. Boodts, S. Trassatti, J. Electrochem. Soc. 173 (1990) 3784. B. Cornell, D. Simonsson, J. Electrochem. Soc. 140 (1993) 3123. C. Baruffaldi, S. Cattarin, M. Musiani, Electrochim. Acta 48 (2003) 3921. S. Cattarin, P. Guerriero, M. Musiani, Electrochim. Acta 46 (2001) 4229. J. Jirkovsk´y, M. Makarova, P. Krtil, Electrochem. Commun. 8 (2006) 1417. L. Zhang, H. Kisch, Angew. Chem. Int. Ed. 39 (2000) 3921. B.J. Lee, S.R. Sivakkumar, J.M. Ko, J.H. Kim, S.M. Jo, D.Y. Kim, J. Power Sources 168 (2007) 546. E.V. Jelenkovic, K.Y. Tong, W.Y. Cheung, S.P. Wong, Microelectron. Reliab. 43 (2003) 49. S. Ardizzone, G. Fregonara, S. Trasatti, Elecrochim. Acta 35 (1990) 263. S. Trasatti, G. Buzzanca, J. Electroanal. Chem. 29 (1971) Ap1. J.P. Zheng, T.R. Jow, J. Electrochem. Soc. 142 (1995) L6. J.P. Zheng, P.J. Cygan, T.R. Jow, J. Electrochem. Soc. 142 (1995) 2699. Y. Takasu, T. Nakamura, H. Ohkawauchi, Y. Murakami, J. Electrochem. Soc. 144 (1997) 2601. P. Soudan, J. Gaudet, D. Guay, D. B´elanger, R. Schulz, Chem. Mater. 14 (2002) 1210. X.M. Liu, X.G. Zhang, Electrochim. Acta 49 (2004) 229. C.-C. Wang, C.-C. Hu, J. Electrochem. Soc. 152 (2005) A370. Y.U. Jeong, A. Manthiram, J. Electrochem. Soc. 148 (2001) A189. Y.G. Wang, X.G. Zhang, Electrochim. Acta 49 (2004) 1957. R.N. Sing, J.-F. Koenig, G. Poillerat, P. Chartier, J. Electrochem. Soc. 137 (1990) 1408. R. Boggio, A. Carugati, G. Lodi, S. Trasatti, J. Appl. Electrochem. 15 (1985) 335. C. Lin, J.A. Ritter, B.N. Popov, J. Electrochem. Soc. 145 (1998) 4097. N. Krstaj´ıc, S. Trasatti, J. Electrochem. Soc. 142 (1995) 2675. L.M. Da Silva, J.F.C. Boodts, L.A. De Faria, Electrochim. Acta 46 (2001) 1369. S. Trasatti, Elecrochim. Acta 32 (1987) 369. S. Trasatti, Elecrochim. Acta 45 (2000) 2377. C.-C. Hu, K.-H. Chang, M.-C. Lin, Y.-T. Wu, Nano Lett. 6 (2006) 2690. V. Subramanian, S.C. Hall, P.H. Smith, B. Rambabu, Solid State Ionics 175 (2004) 511. J.P. Zheng, Y. Xin, J. Power Sources 110 (2002) 86. W. Dmowski, T. Egami, K.E. Swider-Lyons, C.T. Love, D.R. Rolison, J. Phys. Chem. B 106 (2002) 12677. C.-C. Hu, C.-C. Wang, Electrochem. Commum. 4 (2002) 554. C.-C. Hu, W.-C. Chen, Elecrochim. Acta 49 (2004) 3469. P. Alexandridis, J.F. Holzwarth, T.A. Hatton, Macromolecules 27 (1994) 2414. K.-H. Chang, C.-C. Hu, C.-Y. Chou, Chem. Mater. 19 (2007) 2112.