carbon composites with high specific surface area

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Carbon 45 (2007) 2759–2767 www.elsevier.com/locate/carbon

Low-temperature preparation and electrochemical capacitance of WC/carbon composites with high specific surface area Wei Liu, Yasushi Soneda *, Masaya Kodama, Junya Yamashita, Hiroaki Hatori National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 9 May 2007; accepted 14 September 2007 Available online 21 September 2007

Abstract WC/carbon composites (WCCs) with high specific surface area were synthesized by the direct carbonization of a mixture of hydroxylpropyl cellulose, polyvinyl alcohol, K2WO4 and K2CO3 at 900 °C in flowing N2. The resultant material was characterized using X-ray diffraction, thermogravimetric analysis, nitrogen sorption and scanning electron microscopy. The electrode performance of this material for use as a capacitor was studied using cyclic voltammetry and galvanostatic charge–discharge measurements. The BET specific surface area of the WCCs varied from 300 to 1000 m2/g depending on the amount of K2CO3 added during the preparation. Samples prepared with small amounts of K2CO3 contained a large amount of mesoporosity. Electrochemical characterization revealed that WC was slowly oxidized to tungsten oxy-hydroxides, and pseudocapacitance due to the redox reactions of tungsten oxy-hydroxides was superimposed on the double-layer capacitance of the carbon support. Consequently large specific capacitance was observed. Galvanostatic charge–discharge measurements of a WCC (ca. 5 wt% WC) resulted in total specific capacitances as high as 477 and 184 F/g at current densities of 20 and 1000 mA/g, respectively. The long-term cycle stability of WCC was also verified by a 5000 cycle charge–discharge test at 1 A/g. Ó 2007 Elsevier Ltd. All rights reserved.

1. Introduction Early transition metal carbides with unique physical and chemical properties have found wide application in material science. In particular, tungsten and molybdenum carbides utilized as heterogeneous catalysts have attracted much attention, because they exhibit catalytic properties similar to those of the noble metals [1,2]. With the goal of applying carbides in ceramic science, catalysis and adsorption, the preparation of carbides with small particle size and high surface area is under intensive investigation. Generally, solution state reactions [3,4] and gas–solid reactions [5–18] are employed to prepare high surface area transition metal carbides. The temperature-programmed reaction of carboncontaining gaseous reagents (methane, ethane, butane, etc.) with solid state metal compounds is one representative

*

Corresponding author. Fax: +81 29 861 8936. E-mail address: [email protected] (Y. Soneda).

0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.09.032

method, and has proven to be effective in the preparation of high surface area transition metal carbides [5–9]. Moreover, the carbothermal hydrogen reduction method was explored to prepare transition metal carbides or activated carbon supported carbides with high surface area, whereby solid carbon was used as a carbon source for reaction with vaporized metal oxides or supported metal oxides in flowing H2. This method has been used to prepare vanadium carbides [10], silicon carbide [11,12], molybdenum carbides [13–16], and tungsten carbides [17,18] with high surface area. And the catalytic activities of these carbides have been examined for hydrotreating processes, such as hydrodenitrogenation, hydrogenation reactions, and hydrocarbonreforming reactions. It is well known that the most important tungsten carbides are W2C and WC. W2C is not thermally stable, while WC is a stable compound. Besides the applicability for hydrotreating processes, WC has been studied as an inexpensive alternative electrocatalyst to the noble metals [19– 25]. Superior electrocatalytic activity of WC is expected,

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because it has the advantage of resistance to acidic solutions and catalytic poisons including carbon monoxide, hydrocarbons, and hydrogen sulfide. Usually, WC is prepared by a two-step process, wherein the tungsten oxide is firstly reduced to tungsten in a hydrogen atmosphere, and the obtained tungsten metal is then reacted with carbon at 1400–1600 °C to produce the carbides. WC obtained by this process has a very low surface area. Therefore, the carbothermal hydrogen reduction method described above has been utilized for the preparation of tungsten carbides with high surface area [17,18]. However, the main components of the tungsten compounds resulting from this preparation are W2C together with tungsten oxides. High surface area carbon supported WC has been not successfully prepared at relatively low-temperatures, because of the difficulty of forming WC. Today, the production of WC with high surface area at low-temperatures is still subject to investigation. In previous works [26,27], we found that simple carbonization of a mixture of hydroxylpropyl cellulose (HPC) and K2WO4 in flowing N2 resulted in the simultaneous formation of nanostructured WC and porous carbon. The striking result is that WC can be easily produced at 800 °C even without the use of H2 as the reducing gas. Furthermore, electrochemical characterization of the resultant WC/carbon composite (WCC) showed that the composite had potential application as an electrode material for electrochemical capacitors. In this work, the low-temperature preparation of WCC materials was studied further, with the goal of obtaining adjustable porosity as well as higher specific surface area. The electrode performance of these materials for use in a capacitor was also investigated. 2. Experimental 2.1. Preparation and characterization The chemicals used in this study were potassium tungstate (VI) K2WO4 (Wako), potassium carbonate K2CO3 (Wako), HPC (MW = 1,000,000, Aldrich), 99+% hydrolyzed poly(vinyl alcohol) (PVA, MW = 89,000–98,000, Aldrich) and ethanol (99.5 vol%, Wako). The detailed preparation process for the WCCs is shown in Fig. 1. Firstly, 1.0 g of K2WO4 and a specified amount of K2CO3 were dissolved in 20 mL of H2O, followed by the addition of 80 mL of ethanol. Then, a mixture of 2.0 g HPC and 2.0 g PVA was slowly added under vigorous stirring. A gel was obtained after 1–2 h. Subsequently, a rotary evaporator was used to remove the solvents at 70 °C. After drying at 100 °C for one day, the obtained mixture of HPC, PVA, K2WO4 and K2CO3 was moved into a ceramic boat and placed in a tubular resistance furnace controlled by a temperature programmer. Carbonization was performed in a N2 flow (200 mL/min) by heating to 900 °C at a heating rate of 5 °C/min and with a soaking time of 3 h. The black solids obtained were ground and then added to a 0.5 M KOH solution to remove unreacted K2WO4. After washing several times with distilled water, followed by filtration and drying, the WCCs were finally obtained. Samples were prepared with various amounts of K2CO3 and are referred to as WCCn, where n denotes the numbers 0–5, as defined in Table 1. X-ray diffraction (XRD) patterns were recorded on a diffractometer (RU-300, Rigaku. Co. Ltd.) using Cu Ka radiation with a scanning range (2h) from 10° to 90°. Nitrogen sorption isotherms were measured at 196 °C with a Belsorp 18A analyser (Bel Japan Inc.). Specific surface

Solution(20 mL): 1.0 g K2WO4 + (0 0.5) g K2CO3 addition of 80 mL EtOH addition of 2.0 g HPC + 2.0 g PVA under vigorous stirring 1 2 h stirring Gel solvent removal Solid Mixture: K2WO4 + HPC + PVA + K 2CO3 carbonization: 900 oC, 3 h, N2 washing, drying WC/Carbon composites Fig. 1. Preparation process for the WC/carbon composites.

areas were calculated using the Brunauer–Emmett–Teller (BET) method. Investigation of the WCCs was carried out using field-emission-type scanning electron microscopy (FESEM; Hitachi S-4700). The WC content in the materials was determined from the ignition loss at 450 °C, measured using a thermogravimeter (TG; Rigaku ThermoPlus 8120). The content of O in the carbon structure was determined by standard elemental analysis.

2.2. Electrode performance for capacitor The electrode for electrical double layer capacitor (EDLC) measurements was prepared by mixing 10 mg of the sample powders with acetylene black as an electrical conductor and PTFE (polytetrafluoroethylene) as a binder in a mass ratio of 80:10:10, respectively. The mixture was blended using acetone as a solvent to achieve a homogeneous mixture, and was then pressed into a pellet and subsequently dried at 100 °C for 1 h under vacuum. The pellet was then compressed onto a platinum mesh, which was used as the working electrode. A three-electrode test cell was used for the measurement of EDLC performance, equipped with the prepared working electrode, a Ag/AgCl reference electrode, and platinum plate as the counter electrode. 1 mol/L H2SO4 was used as the electrolyte. The performance of the sample powder as an electrode was determined from cyclic voltammetry with a scanning rate of 5 mV/s in the potential window from –0.1 to 0.9 V, and also from charge–discharge measurements with various current densities from 20 to 1000 mA/g. The capacitance values of the WCCs are expressed in the unit of F/g. In particular, the specific capacitance obtained from galvanostatic charge–discharge experiments was carefully calculated so as to avoid the additional current flow due to oxygen generation and also the IR drop in the electrode.

3. Results and discussion 3.1. Structure of WCCs Fig. 2 shows the XRD patterns of WCCs prepared with various amounts of K2CO3. The diffraction peaks of WC are clearly resolved in addition to the weak peaks of W and W2C, with no other peaks due to other tungsten oxides. In accordance with the literature values (JCPDS 25– 1047), the diffraction peaks were attributed to hexagonal

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Table 1 BET specific surface areas, as analyses, WC content, and capacitance of the WC/carbon composites Sample

WCC0 WCC1 WCC2 WCC3 WCC4 WCC5 a b c d e f g h

K2CO3 (g)a 0 0.1 0.2 0.3 0.4 0.5

SBET (m2/g)b 310 452 594 747 897 975

Vtotal (mL/g)c 0.19 0.21 0.27 0.31 0.39 0.40

as analysis

C (F/g)h

Content

Smicro/Smeso (m2/g)d

Vmicro/Vmeso (mL/g)e

WC (wt%)

O (wt%)g

235/74 397/57 540/64 674/62 –f –f

0.10/0.09 0.17/0.04 0.24/0.03 0.30/0.01 –f –f

44 40 22 17 12 5

10.5 10.1 17.7 13.7 8.6 14.8

72 108 167 232 258 296

Amount of K2CO3. SBET, specific surface area calculated by the BET method. Vtotal, total pore volume at P/P0 = 0.99. Smicro and Smeso, surface areas of micropores and mesopores, respectively. Vmicro and Vmeso, pore volumes of micropores and mesopores, respectively. WCC4 and WCC5 are mainly microporous. The content of O in the carbon structure was determined by standard elemental analysis. Specific capacitance, calculated from charge–discharge cycling measurements at a current density 50 mA/g in a 1 M H2SO4 aqueous electrolyte.

Fig. 2. XRD patterns of the WCCs: (a) WCC0, (b) WCC1, (c) WCC2, (d) WCC3, (e) WCC4, and (f) WCC5.

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WC. The intensity of the WC diffraction peaks gradually decrease with the increase in the amount of K2CO3 used. When the K2CO3 addition reaches 0.5 g, the diffraction peaks of WC become very weak. Correspondingly, the WC content in the composites gradually decreases as the amount of K2CO3 is increased (Table 1). Fig. 3 shows the nitrogen adsorption and desorption isotherms for the WCCs. Except for WCC5, all the samples display a type-IV isotherm. A H4-type hysteresis loop with delayed capillary evaporation was observed in the isotherms at a relative pressure (P/P0) of approximately 0.5. The WCC5 sample shows a reversible type-I adsorption isotherm, suggesting microporosity. The textural parameters are listed in Table 1 together with the results of the as analyses of nitrogen adsorption/desorption isotherms. The BET specific surface areas of the WCCs are strongly dependent on the amount of K2CO3 added during the preparation of the materials. It is well known that K2CO3 is an effective activator in the preparation of activated carbons. Thus, increased addition of K2CO3 resulted in larger BET specific surface areas. The results of the as analyses indicate that the increase in the BET specific surface areas with the increased addition of K2CO3, is due to the increase in the micropore surface area. However, the mesopore surface areas are hardly affected by the addition of K2CO3. Therefore, it was concluded that the addition of K2CO3 mainly causes an increase in microporosity.

Fig. 3. Nitrogen adsorption and desorption isotherms: (a) WCC0, (b) WCC1, (c) WCC2, (d) WCC3, (e) WCC4, and (f) WCC5.

The morphology of the WC particles in the composites was studied using FESEM with a detector for backscattered electrons (YAGBSE), in order to distinguish WC particles inside the carbon. SEM images of WCC0, WCC2 and WCC4 are shown in Fig. 4. For all the samples, the WC particles have irregular shape with particle sizes ranging from several tens of nanometers to ca. 2 lm. In addition, the WC particles show a slight tendency to increase in size with increased addition of K2CO3. When the amount of K2CO3 used is below 0.1 g, most of the WC particles are less than 300 nm, except for a few larger particles (300 nm–1 lm). Higher resolution images of the WC particles are shown in the right half of the figure. It seems that parts of the WC particles are composed of many small primary WC particles for those cases where more K2CO3 has been added. In previous works [26,27], we reported the preparation of WCCs by the one-step carbonization of a mixture of HPC and K2WO4 in flowing N2. The samples obtained had BET specific surface areas ranging from 200 to 500 m2/g. In order to prepare WCCs with adjustable porosity, as well as higher specific surface area, K2CO3 was introduced into the mixture of HPC and K2WO4 during the preparation. However, the pyrolysis of HPC became vigorous and produced less carbon in the presence of K2CO3. Therefore PVA was used together with HPC as carbon precursors in the preparation. It was found that WC was produced by the simple carbonization of a mixture of K2WO4, K2CO3, HPC and PVA without the need of external addition of H2 as the reducing gas. The reason for the reduction of K2WO4 to WC was explained by the large amount of strongly reducing reactants that are provided during the pyrolysis of the carbon precursors, HPC and PVA. The carbonization of K2WO4 was then investigated with only PVA. It was shown that with PVA as the only carbon precursor, WC could not be produced at 900 °C. Thus, the reduction of tungsten was ascribed to the pyrolysis of HPC. Cellulose is one of the basic components of wood and has been used as raw material to produce activated carbon; therefore, the pyrolysis of cellulose has been well studied. When heated at moderate temperatures (300–450 °C) under an inert atmosphere, cellulose undergoes various dehydration, fragmentation, elimination, and condensation reactions to yield gaseous products (CO + H2O + CO2), tar, and char. However, the presence of alkali-metal ions, the temperature, and the heating rate strongly affect the decomposition pathways. Above 500 °C, mainly CO and H2 are released. A reasonable deduction was made; that K2WO4 is firstly reduced to W metal by CO or H2 due to the pyrolysis of HPC, and then the W metal is carburized to form WC. Recently, the preparation of tungsten carbide/oxide particles and tungsten carbide nanorods and nanoplatelets was reported by utilizing the pyrolysis of organic reagents [28,29]. In this work, we present a new approach for the preparation of WCCs, whereby porous carbon and WC

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Fig. 4. FESEM images of WCC0 (a), WCC2 (b), and WCC4 (c). The scale bars of the images on the left and right sides are 5 lm and 1 lm, respectively.

particles are produced at the same time to construct a composite. The difference with the carbothermal hydrogen reduction method, where activated carbon was used as carbon source, is that the carbon and reducing gases required for the preparation of the WCCs are supplied by the pyrolysis of HPC and PVA polymers. The resultant WCCs have high specific surface area with mesoporosity and embedded WC particles. Therefore, WCC is expected to be applicable as an alternative electrocatalyst to Pt, as well as a heterogeneous catalyst for hydrotreating processes. 3.2. Electrochemical properties of the composite electrodes Electrochemical capacitors are broadly classified into electrical double layer capacitors (EDLCs) and pseudocapacitors [30,31]. EDLCs store energy by physical charge separation, while pseudocapacitors utilize a redox reaction at the interface. Activated carbons are attractive electrode materials for EDLCs. It is generally considered that the micropores of active carbons are not easily wetted by the electrolyte and part loss of capacitance is resulted. Therefore, mesoporous carbons have become popular as an alternative EDLC material for overcoming this disadvan-

tage. However, the long-held issue has been questioned by the works of Chmiola et al. on titanium carbide-derived carbons [32,33], wherein an anomalous increase in capacitance was observed when the pore sizes are less than 1 nm. The improved capacitance is attributed to the distortion of solvation shells in small pores of carbon which enhances the accessibility of the carbon surface to the electrolyte. Furthermore, deposition of transition metal oxides, such as Ru, Ir, W, Mo, Mn, Ni, Co, etc., on the carbon surface is widely utilized in order to increase the capacitance values [34–41]. In the previous section, we have shown that the prepared WCCs have large BET specific surface areas and embedded WC particles. Therefore, the applicability of WCCs as electrical capacitors was investigated. Typical cyclic voltammograms of the WCCs, recorded at a scan rate of 5 mV/s, are shown in Fig. 5. The areas of the charge and discharge curves of the CV plots clearly increase with increasing scan cycle number up to 150, suggesting enhancement of capacitance. After 150 cycles, the CV curves are reversible and the charge/discharge areas remain constant. As for the apparent increase in specific capacitance, the reasons for this are considered to be two-fold. One cause may be due to the rich oxygen functionality in

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Fig. 5. Cyclic voltammograms of WCCs recorded at a scan rate of 5 mV/s in an electrolyte of 1 M H2SO4 aqueous solution: (a) WCC0, (b) WCC1, (c) WCC2, (d) WCC3, (e) WCC4, and (f) WCC5.

the prepared carbon which enhances the pseudocapacitative character of the material. The carbon precursors, HPC and PVA, are both known to be the oxygen-rich polymers, and the content of oxygen in the carbons prepared from these polymers is strongly affected by the conditions of carbonization. The elemental analyses of the WCCs (Table 1) show that the content of oxygen in the carbon structures is in the range of 8–15 wt%. Thus, the contribution of oxygen functionality to the increase in specific capacitance has to be taken into account. The other is the slow oxidation of WC particles during the process of CV scanning within

150 cycles. In the previous work, we reported the XRD results and electrochemical properties of pure WC and WC/carbon composites [27]. After CV measurement, the diffraction peaks of WC disappeared with the appearance of the peaks of tungsten oxy-hydroxides. This reasonably pointed to that WC was gradually oxidized to tungsten oxy-hydroxides, WOx(OH)y, during the charge and discharge processes. And the resultant tungsten oxy-hydroxides underwent the following redox reactions: WOx ðOHÞy þ dHþ þ de () WOxd ðOHÞyþd :

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WOx(OH)y was reversibly oxidized and reduced with a simultaneous exchange of protons from the H2SO4 aqueous solution during the charge–discharge process. Consequently the specific capacitance was increased through the superimposition of the pseudocapacitance of tungsten oxy-hydroxides on the double-layer capacitance of the carbon substrate. Considering the shape of the CV curves after 150 scans, it can be seen that the CV curves of WCC0 and WCC1 are obviously different from those of the other samples. The redox peaks associated with pseudofaradiac reactions are remarkable in Fig. 5a and b. It is well known that an ideal electrical double layer capacitance behavior of an electrode material is expressed in the form of a rectangular shape of the CV curve. The CV curves recorded for samples WCC0 and WCC1 clearly show a deviation from such a rectangular shape, and the charge accumulation in the capacitor electrodes is strongly dependent on the electrode potential. This is interpreted to indicate that the electrode materials have pseudocapacitance properties. Samples WCC2–5 show rectangular-like i–E profiles, which are similar to typical CV curves for activated carbon. This indicates that the voltammetric currents of samples WCC2–5 come mainly from the electric double layer charge/discharge process. The cycling capability was galvanostatically studied for all of the WCCs at a constant current ranging from 20– 1000 mA/g. Fig. 6 shows the typical charge–discharge curves that are demonstrated for WCC5. The charge/discharge curves are not exactly linear, because of the overlap of redox reactions from the presence of tungsten oxyhydroxides. The specific capacitance was calculated from the equation, C ¼ iDt=mDV ; where i is the current density, Dt is the discharge time, m is the mass of the WCC, and DV is the potential range of the charge–discharge cycle. The capacitance measured is the sum of the electric double layer capacitance and the pseudocapacitance due to the redox reactions of the

Fig. 6. Typical charge–discharge cycling recorded at a constant current of 50, 100 and 200 mA/g for WCC5.

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tungsten oxy-hydroxides. The specific capacitances of WCC5, estimated from the discharge curves in Fig. 6, are 296, 253, and 234 F/g at 50, 100, and 200 mA/g, respectively. The ratio of discharging time (thus discharge capacitance) divided by charging time (charging capacitance) from the curves in Fig. 6 exceeded 100%, for instance, 105% is obtained for the case of 200 mA/g of current density. This observation is corresponding to the evolution in CV curve (Fig. 5). Since WC nano-particles in the electrode materials converted to tungsten oxy-hydroxides during charge–discharge cycle, the capacitance increased up to certain cycles. Fig. 7 illustrates the calculated specific capacitance for all the samples against the current density. The specific capacitance of the WCCs rapidly decreases with increasing current density in the range of 20–200 mA/g. The reason for this may be due to the slow rate of the redox reactions of the tungsten oxy-hydroxides. WCC4 and WCC5 show rather large capacitance in the low current density range. In particular, an extremely large capacitance of 477 F/g was observed for WCC5 at a current density of 20 mA/g. In addition to the double layer capacitance, the large measured capacitance can be mainly attributed to pseudocapacitance due to surface redox reactions of the tungsten oxy-hydroxides. Fig. 8 shows the dependence of the specific capacitance on the cycle number for galvanostatic charge and discharge at a current density of 1 A/g in 1 M H2SO4 electrolyte for sample WCC5. The measured specific capacitance was as high as ca. 180 F/g and remained even after 5000 cycles. No loss of capacitance was found during the galvanostatic charge–discharge process. The sample exhibited long-term cycle stability and good electrochemical reproducibility at high current density. It is well known that the specific capacitance of a carbon electrode can be significantly

Fig. 7. Specific capacitance as a function of current density for the WCCs.

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trode system. For the CV measurements, an apparent increase in specific capacitance was observed with increasing scan cycle up to 150 cycles. This was partly interpreted that WC was gradually oxidized to tungsten oxy-hydroxides, which in turn underwent redox reactions and contributed to the increase in the specific capacitance. The WCC5 sample (5 wt% WC) exhibited high specific capacitance (ca. 180 F/g at 1 A/g) as well as long-term cycle stability at high current density. The capacitance was considered to be due to a combination of the double layer capacitance and pseudocapacitance associated with surface redox reactions of tungsten oxy-hydroxides. References

Fig. 8. Specific capacitance of WCC5 as a function of the cycle number of galvanostatic charge and discharge at a current density of 1 A/g.

increased by depositing oxides of noble and transition metals, such as Ru, Mn, Ni, Co, etc., on the carbon surface. For a carbon electrode material with pseudocapacitance due to hydrated noble and transition metal oxides, the electrode performance is restricted by the electrolyte. Although a highly acidic electrolyte such as sulfuric acid has the advantage of fast chemisorption of H+ in an acidichydrated oxide electrode system, neutral electrolytes such as Na2SO4, KCl, etc. are generally chosen for the replacement of highly acidic electrolytes. The reason for this is that noble and transition metal oxides dissolve in highly acidic electrolytes over the period of cycling time, which leads to a rapid reduction in capacitance with respect to cycling. Therefore, it was deduced that the good cyclic performance exhibited by WCC5 in 1 M H2SO4 is due to the non-dissolution of tungsten oxy-hydroxides under the repetition of redox reactions during long-term charge–discharge cycling. The lack of tungsten oxy-hydroxide particle agglomeration was also taken into account. 4. Conclusion We have demonstrated a simple and effective approach for the low temperature preparation of WCCs with large specific surface area. WC particles were produced in a size range from several tens of nanometers to ca. 2 lm without the need of external addition of H2 as a reducing agent. SEM images revealed that the WC particles were composed of many small primary particles. The BET specific surface area of the composites increased and the WC content decreased with the increase in the amount of K2CO3 added during preparation of the materials. Mesoporosity was obtained when only a small amount of K2CO3 was used. The performance of WCCs as capacitor electrodes was tested using CV and galvanostatic charge–discharge methods in a 1 M H2SO4 aqueous solution with a three-elec-

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