- Email: [email protected]
USY composite and activated carbon electrodes
Materials Science and Engineering A 473 (2008) 317–322
A novel asymmetric capacitor based on Co(OH)2/USY composite and activated carbon electrodes Yan-Yu Liang a , Hu-Lin Li a,b , Xiao-Gang Zhang a,∗ a
College of Materials Science and Technology of Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China b College of Chemistry and Chemical Engineering of Lanzhou University, Lanzhou 730000, PR China Received 14 December 2006; received in revised form 21 March 2007; accepted 23 March 2007
Abstract A new hybrid electrochemical capacitor has been successfully designed by using Co(OH)2 /USY composite and active carbon (AC) as positive and negative electrode, respectively. The Co(OH)2 /USY composite material is cobalt hydroxide loaded on ultra-stable Y zeolite (designated Co(OH)2 /USY) through self-directed growth process. The Co(OH)2 /USY and AC electrodes were individually tested in 1 M KOH aqueous electrolyte in order to define the adequate balance of active materials in the capacitor as well as the working voltage. To fabricate the hybrid cell systems, it achieved the maximum capacitive performance of 110 F/g, which is the highest value compared with the other similar metal oxide in AC hybrid capacitor systems in the same medium. Also it was found that the asymmetric cell worked in the larger voltage range and exhibited higher energy and power characteristics. The notable improved electrochemical performances strongly suggest that the hybrid cell can be a promising capacitor devices applied for the energy storage. © 2007 Published by Elsevier B.V. Keywords: Hybrid capacitors; Co(OH)2 /USY; AC; Energy density; Power density; Potential window
1. Introduction Growing environmental concerns and increasing depletion of fossil fuels have created great interest in alternative energy technologies. Electrochemical redox capacitors often called supercapacitors or ultra-capacitors are widely recognized as promising devices exhibiting much higher capacity and higher power characteristics in comparison with conventional dielectric capacitors and rechargeable batteries systems, respectively. The high capacity supercapacitors mainly come from the faradic reaction within the electroactive materials [1–4], or the high surface area of electrode materials (i.e. double-layer capacitance) [3–5]. The high power performance of supercapacitors is attributed to the fast charge/discharge characteristics of a double-layer process and/or the high electrochemical reversibility of redox transitions within electrode materials [6]. Thus, supercapacitors are becoming attractive energy storage systems particularly for applications involving high power requirements.
∗
Corresponding author. Tel.: +86 25 52112626; fax: +86 25 52112626. E-mail address: [email protected] (X.-G. Zhang).
0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.03.087
However, compared to rechargeable battery systems, which are the most common electrical energy storage devices, supercapacitors still present very important drawback (e.g. the amount of energy density is relatively low) and precludes the extensive industrial utilization in energy storage. The amount of energy accumulated in supercapacitor is proportional to the capacitance (C) and voltage (V) according to the formula: E = 21 CV 2
(1)
The capacitance depends essentially on the electrode materials used, whereas the operating voltage is determined by the stability window of the electrolyte. Two obvious approaches enhance the electrochemical energy characterizations. The former can be achieved to use larger capacitance materials. Previously, we have reported a new strategy to prepare a nanocomposite of cobalt and nickel hydroxide on ultra-stable Y zeolite (USY) molecular sieves, and demonstrated their novel applications as electrochemical capacitors materials [7–9]. Unfortunately, the composite material has a narrow operation potential (on account of the real galvanostatic discharge window is about 0.45 V), it is thereby limited to practical applications for the reason of energy density.
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Y.-Y. Liang et al. / Materials Science and Engineering A 473 (2008) 317–322
Recently, asymmetric or hybrid supercapacaitor regarded as the new trend in electrochemical capacitors has been reported greatly [10–17]. As far as the conformation of the devices, such new type asymmetric supercapacitors are different from either the dielectric capacitors or the conventional batteries, which are based on one electrode stores charge through a reversible nonfaradic process of ionic movement on the surface of an actived carbon or the hole of a nano-pore carbon material, and another one is to utilize a reversible faradic reaction of metal oxides, conducting polymers and intercalated compounds. It is possible to reach the high working voltage and high energy density by choosing a proper electrode material, contributed to a significant increasing of the overall energy density of the supercapacitor devices [11]. In the present work, we had firstly introduced Co(OH)2 /USY as a positive electrode to fabricate an asymmetric supercapacitor in combination with actived carbon as the negative electrode in 1 M KOH electrolyte solution. The primary electrochemical characterization was investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge test. Our objective consists in optimizing the hybrid capacitors, working in an aqueous medium, which are able to provide power and energy densities comparable to the symmetric capacitor system. 2. Experimental 2.1. Preparation of the electrode materials Ultra-stable Y zeolite was prepared by repeated proton exchange of zeolite NaY (LZPCC, Lanet-Y30, Si/Al = 2.3, BET surface area = 800 m2 /g) with aqueous solution of NH4 NO3 at 35 ◦ C for 15 h, followed by steaming at 650 ◦ C for 3 h. Thirty milliliter aqueous solution of NH3 ·H2 O (5 ml, 25–28 wt%) was added dropwise to the aqueous solution of CoCl2 ·6H2 O (0.0126 M, 24.79 wt%), and the same weight of USY with vigorous magnetic stirring until a green precipitation of Co(OH)2 /USY composite was formed. The final pH was slowly adjusted to be around 8.5. The resulting precipitate was filtered using a centrifugal filtration method and washed with distilled water and ethanol several times. The final product was heated at 100 ◦ C in air for 12 h (note that in this work, all the Co(OH)2 /USY material is referred to Co(OH)2 phase with 50% in weight percentage). The morphologies of Co(OH)2 /USY composite were characterized by scanning electron microscopy (SEM) in a FEI Quanta 200 scanning microscope and transmission electron microscopy (TEM, FEI, Tecani 20). Commercial activate carbon with a specific area of 2390 cm2 /g was used as the negative electrode material without further treatment. 2.2. Electrochemical tests The electrode of Co(OH)2 /USY nanocomposite was prepared according to the following steps. Eighty weight percent of active powder was mixed with 7.5 wt% of acetylene black (>99.9%) and 7.5 wt% of conducting graphite in an agate mortar until a homogeneous black powder was obtained. To this mixture,
5 wt% of poly(tetrafluoroethylene) dried powder (PTFE) was added with a few drops of ethanol. After brief evaporation drying, the resulting paste was pressed at 5 MPa to nickel gauze with a nickel wire for electric connection. The electrode assembly was dried for 16 h at 70 ◦ C in air. Each electrode contained 10 mg redox active material and has a geometric surface area of about 1 cm2 . The active carbon (AC) electrode was prepared by the same method as the positive electrode described above, it consisted of 80 wt% AC, 7.5 wt% of acetylene black, 7.5 wt% of conducting graphite and 5 wt% PTFE. The electrochemical behavior of the resulting compounds was characterized by cyclic voltammetry and galvanostatic charge/discharge test in 1 M KOH electrolyte. The experiments were carried out in a two- or three-electrode cell. A platinum gauze electrode and a saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. CV and charge/discharge measurements were performed on a CHI 660 instrument (Chenhua, Shanghai). 3. Results and discussion 3.1. The characterizations of Co(OH)2 /USY nanocomposite The morphology of pure USY and the Co(OH)2 /USY composite are examined by scanning electron microscopy and transmission electron microscopy. As shown in Fig. 1a, pure USY particles, with the average diameter about 500 nm, show regular morphology with good dispersion. Comparing Fig. 1a with Fig. 1b, reveals a fundamental morphology change taking place on the outer surface of patent USY. The Co(OH)2 /USY composite appears fuzzy in the SEM image. Higher-magnification TEM image further illustrates that a significant amount of network-like phase covers the exterior of the patent USY particles. It is noteworthy that the network-like structure shows anisotropic morphology characteristics forming a loosely packed microstructure in the nanometer scale, which allows a good access of electroactive ions in the electrolyte to the Co(OH)2 /USY material. There is no doubt that such a texture of the capacitor electrodes is optimal for the fast ion diffusion and migration, so that the capacitive performance of the Co(OH)2 /USY electrode should be remarkable. 3.2. Electrochemical tests 3.2.1. The electrochemical characterizations of Co(OH)2 /USY nanocomposite and AC To evaluate the electrochemical properties of the prepared Co(OH)2 /USY composite and the commercial AC, we directly use these two materials to fabricate electrodes for electrochemical capacitors. The applicability of the electrodes was evaluated by cyclic voltammetry and galvanostatic charge/discharge test. Fig. 2 shows typical CV curves of Co(OH)2 /USY nanocomposite within a potential window of 0 to 0.5 V and AC within a potential window of −1 to 0 V at a scan rate of 10 mV/s, respectively. The positive polarization of Co(OH)2 /USY could be limited by O2 evolution because of water decomposition from the electrolyte. Similarly, the negative polarization of AC could
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319
Fig. 2. Cyclic voltammograms of (a) Co(OH)2 /USY composite electrode at a scan rate of 10 mV/s in 1 M KOH within a potential window of 0 to 0.5 V and (b) AC electrode at a scan rate of 10 mV/s in 1 M KOH within a potential window of −1 to 0 V.
Fig. 1. (a) SEM image of pure USY and (b) SEM and TEM images of Co(OH)2 /USY.
also be limited by H2 evolution. In the previous studies [7–9], it was demonstrated the pure USY had a negligible integral area under the current–potential response, suggesting USY alone with very small specific capacitance properties. In this case, it is not shown individually in Fig. 2. According to these observations, the Co(OH)2 phases of the composite are responsible for the main capacitance sources. Furthermore, the shape of the CV reveals that the capacitance characteristics of Co(OH)2 is distinct from that of the electric double-layer systems, which is due to a faradic reaction of Co(OH)2 , shown as follows: Co(OH)2 + OH− ↔ CoOOH + H2 O + e−
(2)
CoOOH + OH− ↔ CoO2 + H2 O + e−
(3)
A CV curve was recorded in the same conditions with an AC electrode (Fig. 2, curve b). The potential range of CV was shifted toward more negative region compared to Co(OH)2 /USY, and
has a rectangular shape based on the adsorbed charges on the surface to form electric double-layer capacitance. On account of the absorbed charges on the electrode surface increase linearly with increasing potential, the current intensity of AC shown in CV curve is potential independent, which proves the charge storage mechanism was mainly attributed to double-layer capacitance. Fig. 3 shows the typical galvanostatic charge/discharge curves (at a current density of 5 mA/cm2 ) of Co(OH)2 /USY electrode within a potential window of 0 to 0.45 V and AC electrode within a potential window of −1 to 0 V, respectively. The linear charge/discharge curves are observed in AC electrode, which is attributed to linear correlation of the absorbed charge on the interface with the applied potential. That means the specific capacitance is independent on the applied potential nature, distinctly accompanied with a nonfaradic process on the interface. The charge/discharge curves of the Co(OH)2 /USY is not perfectly linear as for AC capacitors, the slope of charge/discharge curves indicating the potential dependent nature of faradic reaction. Nonetheless, the specific capacitance of both Co(OH)2 /USY and AC case can be calculated from C = I/(dV/dt), where dV/dt is the slope of the linear
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Fig. 3. Charge/discharge behavior of (a) Co(OH)2 /USY composite electrode and (b) AC electrode at a current density of 5 mA/cm2 .
part of the discharge curve and I is the absolute value of the current density. From the result of Fig. 3 the specific capacitance of Co(OH)2 /USY is 745 F/g (note that in this work, if no specific explanation, all the capacitance is referred to Co(OH)2 /USY material without correcting for the weight percentage of the Co(OH)2 ) at 5 mA/cm2 in the potential range of 0 to 0.45 V, and AC capacitors is 287 F/g at the same current density in the potential range of −1 to 0 V. Therefore, the optimal positive/negative mass ration of Co(OH)2 /USY and AC was set almost 1:2.6.
Co(OH)2 /USY positive electrode and AC negative electrode at the voltage scan rate of 5, 10 and 20 mV/s. The potential range of the unit cell can be predicted by the individual CV curves, as shown in Fig. 2. At a fully oxidized state, Co(OH)2 /USY gives about 0.5 V but the AC-based electrode gives about −1 V at a fully reduced state, whereas the discharge process proceeds until the potential of both electrodes is the same. Thus, the potential of the asymmetric unit cell can reach 1.5 V in the fully charged state. Fig. 5 shows the typical galvanostatic charge/discharge curves of the asymmetric capacitors in the potential window of 0 to 1.5 V at a function of current density. During the charge and discharge process, non-linear behavior of the unit cell is observed, as expected from the corresponding CV curves (Fig. 4). The results shown in Fig. 5 demonstrate that this unit cell exhibits the typical response of hybrid-type EC capacitors [18]. The specific capacitance of the capacitor (Cm ) was calculated as follows: Cm =
C It = m Vm
(4)
3.2.2. The electrochemical characterizations of the asymmetric capacitors Based on the above electrochemical characteristics investigation of the individual capacitor, it is possible to combine AC and Co(OH)2 /USY to obtain an asymmetric capacitor with a potential window close to 1.5 V without a significant decomposition of the aqueous solvent during the charge/discharge cycle. A similar conclusion was reached by Wang et al. for CoAl double hydroxide but their operation capacitor voltage was restricted to 1.2 V [11]. Fig. 4 shows CV curves of the hybrid capacitor with
where I is the current density of discharge, t the total time of discharge and V is the potential range. The m is the mass of active materials in the asymmetric capacitor (including positive and negative electrode). The specific capacitance of the hybrid capacitor at current densities of 2, 5, 10, and 25 mA/cm2 were 110, 105, 102, 98 F/g, respectively. Even though under the large current density of 25 mA/cm2 , nearly 90% of the initial amount can be reached. To our best knowledge, it is the highest value compared with the other similar metal oxide in AC hybrid capacitor systems in the same medium. In our previous studies [7–8], we have successfully reported that the supporting USY material offers a nanometer template for the formation of network-like structure Co(OH)2 . It shows anisotropic morphology characteristics forming a loosely packed microstructure, and all this was provided an important morphological basis for the reversibility and bulk accessibility of fast faradic reactions. Consequentially, we have achieved the novel capacitive performance Co(OH)2 /USY composite as the
Fig. 4. Cyclic voltammograms of at different scan rates in 1 M KOH within a potential window of 0 to 1.5 V.
Fig. 5. Charge/discharge behavior of the hybrid capacitor at different current densities in 1 M KOH within a potential window of 0 to 1.5 V.
Y.-Y. Liang et al. / Materials Science and Engineering A 473 (2008) 317–322
Fig. 6. Reagone plot relating power density to achievable energy density of the supercapacitor: (a) asymmetric supercapacitor based on Co(OH)2 /USY and AC electrode; (b) symmetric supercapacitor based on Co(OH)2 /USY electrode.
electrode materials applied for supercapacitors. Unfortunately, the Co(OH)2 /USY composite material has a narrow operation potential (on account of the real galvanostatic discharge window is about 0.45 V), it is thereby limited to practical applications for the reason of energy density. In the present work, the active carbon is used to fabricate the hybrid capacitor, which stores charge through a double-layer absorption mechanism in the negative potential range. It ensures the novel Co(OH)2 /USY composite to proceed faradic reaction with a constant utilization in the positive potential range. Therefore, not only Co(OH)2 /USY electrode but also the AC electrode has fully exerted their capacitive roles in the corresponding potential range. We speculate that this is one of the important reasons responsible for the excellent capacitive characteristics of the hybrid unit cell. Fig. 6 presents Reagone plots of the hybrid capacitors and the symmetric capacitors (using two Co(OH)2 /USY electrodes). The energy density (E) of a supercapacitor can be calculated by using the following equation: E = V dq C=
q V
E=C
(5) V dV = 21 C(V )2
where C is the capacitance of the hybrid capacitor, V is the operating potential window, and q is the total accumulated charge on the surface. The specific power density (P) of the supercapacitor can be calculated according to the following equation: P=
I V m
(6)
where I is the current density of discharge, V the potential range of a supercapacitor and m is the mass of active materials in the asymmetric supercapacitor including positive and negative electrode.
321
Fig. 7. Cycle life of the hybrid capacitor at the current densities of 2 mA/cm2 .
Because all the components of the cell are not yet optimized, these Reagone plots only provide an estimation of the performance of the hybrid-type cell. From Fig. 6, it is clear that both the energy density and power density greatly increased compared with the symmetric type. For example, the real energy density of the hybrid capacitor was 30.625 W h/kg at a power density of 520.8 W/kg. In the symmetric capacitor cell, the related energy density is 18.646 W h/kg and the power density is 281.25 W/kg. This can be explained by the energy density and power density which critically depend on the real working voltage as described in Eqs. (5) and (6). The notable improved electrochemical performance is rooted on the practical electrochemical stability window greatly increasing from 0.45 to 1.5 V by assembling the hybrid devices. Although, in previous paper [7,8], we have synthesized the novel capacitance characteristics material of Co(OH)2 /USY, it is not preferable to the classical hydrous RuO2 in terms of the limited working potential in practical applications. In the hybrid case, combination of AC with Co(OH)2 /USY to fabricate the unit cell, it could not only deliver a wide working potential, but also enhance the performance of energy density especially under large current density. On the other hand, high-rate discharge ability of the hybrid cell was also superior to that of the symmetric cell. The cycling stability was tested over 500 cycles illustrated in Fig. 7. It exhibited a loss of 19% in capacitance during the first 100 cycles. After this stage, a nearly constant value was observed and remained still 95% of the 100th cycle. The capacitance fading may originate from the slow oxidation of Co(OH)2 to CoOOH owning to the Co3+ is more stable than Co2+ under alkali environment. 4. Conclusion In this work, novel capacitive characteristics of Co(OH)2 /USY nanocomposite was prepared by self-directed growth process. On account of the limited potential window of the Co(OH)2 /USY composite, an asymmetric superca-
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pacitor based on Co(OH)2 /USY composite as the positive electrode and AC as the negative electrode were fabricated in 1 M KOH electrolyte. The results of cyclic voltammetry and galvanostatic charge/discharge test exhibit good electrochemical capacitive performance of 110 F/g within the potential range from 0 to 1.5 V, which is the highest value compared with other similar metal oxide in AC hybrid capacitor systems. It also shows a higher working voltage than the symmetric capacitor, leading to a high energy density especially under large current density. The notable improved electrochemical performance is rooted on the practical electrochemical stability window greatly increasing from 0.45 to 1.5 V by assembling the hybrid devices. Future research to optimize the components of the hybrid devices, particularly to enhance the power and energy capability, is currently in progress. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 20403014; 20633040) and the National Natural Science Foundation of Jiangsu Province (No. BK2006196).
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A novel asymmetric capacitor based on Co(OH)2/USY composite and activated carbon electrodes Yan-Yu Liang a , Hu-Lin Li a,b , Xiao-Gang Zhang a,∗ a
College of Materials Science and Technology of Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China b College of Chemistry and Chemical Engineering of Lanzhou University, Lanzhou 730000, PR China Received 14 December 2006; received in revised form 21 March 2007; accepted 23 March 2007
Abstract A new hybrid electrochemical capacitor has been successfully designed by using Co(OH)2 /USY composite and active carbon (AC) as positive and negative electrode, respectively. The Co(OH)2 /USY composite material is cobalt hydroxide loaded on ultra-stable Y zeolite (designated Co(OH)2 /USY) through self-directed growth process. The Co(OH)2 /USY and AC electrodes were individually tested in 1 M KOH aqueous electrolyte in order to define the adequate balance of active materials in the capacitor as well as the working voltage. To fabricate the hybrid cell systems, it achieved the maximum capacitive performance of 110 F/g, which is the highest value compared with the other similar metal oxide in AC hybrid capacitor systems in the same medium. Also it was found that the asymmetric cell worked in the larger voltage range and exhibited higher energy and power characteristics. The notable improved electrochemical performances strongly suggest that the hybrid cell can be a promising capacitor devices applied for the energy storage. © 2007 Published by Elsevier B.V. Keywords: Hybrid capacitors; Co(OH)2 /USY; AC; Energy density; Power density; Potential window
1. Introduction Growing environmental concerns and increasing depletion of fossil fuels have created great interest in alternative energy technologies. Electrochemical redox capacitors often called supercapacitors or ultra-capacitors are widely recognized as promising devices exhibiting much higher capacity and higher power characteristics in comparison with conventional dielectric capacitors and rechargeable batteries systems, respectively. The high capacity supercapacitors mainly come from the faradic reaction within the electroactive materials [1–4], or the high surface area of electrode materials (i.e. double-layer capacitance) [3–5]. The high power performance of supercapacitors is attributed to the fast charge/discharge characteristics of a double-layer process and/or the high electrochemical reversibility of redox transitions within electrode materials [6]. Thus, supercapacitors are becoming attractive energy storage systems particularly for applications involving high power requirements.
∗
Corresponding author. Tel.: +86 25 52112626; fax: +86 25 52112626. E-mail address: [email protected] (X.-G. Zhang).
0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.03.087
However, compared to rechargeable battery systems, which are the most common electrical energy storage devices, supercapacitors still present very important drawback (e.g. the amount of energy density is relatively low) and precludes the extensive industrial utilization in energy storage. The amount of energy accumulated in supercapacitor is proportional to the capacitance (C) and voltage (V) according to the formula: E = 21 CV 2
(1)
The capacitance depends essentially on the electrode materials used, whereas the operating voltage is determined by the stability window of the electrolyte. Two obvious approaches enhance the electrochemical energy characterizations. The former can be achieved to use larger capacitance materials. Previously, we have reported a new strategy to prepare a nanocomposite of cobalt and nickel hydroxide on ultra-stable Y zeolite (USY) molecular sieves, and demonstrated their novel applications as electrochemical capacitors materials [7–9]. Unfortunately, the composite material has a narrow operation potential (on account of the real galvanostatic discharge window is about 0.45 V), it is thereby limited to practical applications for the reason of energy density.
318
Y.-Y. Liang et al. / Materials Science and Engineering A 473 (2008) 317–322
Recently, asymmetric or hybrid supercapacaitor regarded as the new trend in electrochemical capacitors has been reported greatly [10–17]. As far as the conformation of the devices, such new type asymmetric supercapacitors are different from either the dielectric capacitors or the conventional batteries, which are based on one electrode stores charge through a reversible nonfaradic process of ionic movement on the surface of an actived carbon or the hole of a nano-pore carbon material, and another one is to utilize a reversible faradic reaction of metal oxides, conducting polymers and intercalated compounds. It is possible to reach the high working voltage and high energy density by choosing a proper electrode material, contributed to a significant increasing of the overall energy density of the supercapacitor devices [11]. In the present work, we had firstly introduced Co(OH)2 /USY as a positive electrode to fabricate an asymmetric supercapacitor in combination with actived carbon as the negative electrode in 1 M KOH electrolyte solution. The primary electrochemical characterization was investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge test. Our objective consists in optimizing the hybrid capacitors, working in an aqueous medium, which are able to provide power and energy densities comparable to the symmetric capacitor system. 2. Experimental 2.1. Preparation of the electrode materials Ultra-stable Y zeolite was prepared by repeated proton exchange of zeolite NaY (LZPCC, Lanet-Y30, Si/Al = 2.3, BET surface area = 800 m2 /g) with aqueous solution of NH4 NO3 at 35 ◦ C for 15 h, followed by steaming at 650 ◦ C for 3 h. Thirty milliliter aqueous solution of NH3 ·H2 O (5 ml, 25–28 wt%) was added dropwise to the aqueous solution of CoCl2 ·6H2 O (0.0126 M, 24.79 wt%), and the same weight of USY with vigorous magnetic stirring until a green precipitation of Co(OH)2 /USY composite was formed. The final pH was slowly adjusted to be around 8.5. The resulting precipitate was filtered using a centrifugal filtration method and washed with distilled water and ethanol several times. The final product was heated at 100 ◦ C in air for 12 h (note that in this work, all the Co(OH)2 /USY material is referred to Co(OH)2 phase with 50% in weight percentage). The morphologies of Co(OH)2 /USY composite were characterized by scanning electron microscopy (SEM) in a FEI Quanta 200 scanning microscope and transmission electron microscopy (TEM, FEI, Tecani 20). Commercial activate carbon with a specific area of 2390 cm2 /g was used as the negative electrode material without further treatment. 2.2. Electrochemical tests The electrode of Co(OH)2 /USY nanocomposite was prepared according to the following steps. Eighty weight percent of active powder was mixed with 7.5 wt% of acetylene black (>99.9%) and 7.5 wt% of conducting graphite in an agate mortar until a homogeneous black powder was obtained. To this mixture,
5 wt% of poly(tetrafluoroethylene) dried powder (PTFE) was added with a few drops of ethanol. After brief evaporation drying, the resulting paste was pressed at 5 MPa to nickel gauze with a nickel wire for electric connection. The electrode assembly was dried for 16 h at 70 ◦ C in air. Each electrode contained 10 mg redox active material and has a geometric surface area of about 1 cm2 . The active carbon (AC) electrode was prepared by the same method as the positive electrode described above, it consisted of 80 wt% AC, 7.5 wt% of acetylene black, 7.5 wt% of conducting graphite and 5 wt% PTFE. The electrochemical behavior of the resulting compounds was characterized by cyclic voltammetry and galvanostatic charge/discharge test in 1 M KOH electrolyte. The experiments were carried out in a two- or three-electrode cell. A platinum gauze electrode and a saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. CV and charge/discharge measurements were performed on a CHI 660 instrument (Chenhua, Shanghai). 3. Results and discussion 3.1. The characterizations of Co(OH)2 /USY nanocomposite The morphology of pure USY and the Co(OH)2 /USY composite are examined by scanning electron microscopy and transmission electron microscopy. As shown in Fig. 1a, pure USY particles, with the average diameter about 500 nm, show regular morphology with good dispersion. Comparing Fig. 1a with Fig. 1b, reveals a fundamental morphology change taking place on the outer surface of patent USY. The Co(OH)2 /USY composite appears fuzzy in the SEM image. Higher-magnification TEM image further illustrates that a significant amount of network-like phase covers the exterior of the patent USY particles. It is noteworthy that the network-like structure shows anisotropic morphology characteristics forming a loosely packed microstructure in the nanometer scale, which allows a good access of electroactive ions in the electrolyte to the Co(OH)2 /USY material. There is no doubt that such a texture of the capacitor electrodes is optimal for the fast ion diffusion and migration, so that the capacitive performance of the Co(OH)2 /USY electrode should be remarkable. 3.2. Electrochemical tests 3.2.1. The electrochemical characterizations of Co(OH)2 /USY nanocomposite and AC To evaluate the electrochemical properties of the prepared Co(OH)2 /USY composite and the commercial AC, we directly use these two materials to fabricate electrodes for electrochemical capacitors. The applicability of the electrodes was evaluated by cyclic voltammetry and galvanostatic charge/discharge test. Fig. 2 shows typical CV curves of Co(OH)2 /USY nanocomposite within a potential window of 0 to 0.5 V and AC within a potential window of −1 to 0 V at a scan rate of 10 mV/s, respectively. The positive polarization of Co(OH)2 /USY could be limited by O2 evolution because of water decomposition from the electrolyte. Similarly, the negative polarization of AC could
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319
Fig. 2. Cyclic voltammograms of (a) Co(OH)2 /USY composite electrode at a scan rate of 10 mV/s in 1 M KOH within a potential window of 0 to 0.5 V and (b) AC electrode at a scan rate of 10 mV/s in 1 M KOH within a potential window of −1 to 0 V.
Fig. 1. (a) SEM image of pure USY and (b) SEM and TEM images of Co(OH)2 /USY.
also be limited by H2 evolution. In the previous studies [7–9], it was demonstrated the pure USY had a negligible integral area under the current–potential response, suggesting USY alone with very small specific capacitance properties. In this case, it is not shown individually in Fig. 2. According to these observations, the Co(OH)2 phases of the composite are responsible for the main capacitance sources. Furthermore, the shape of the CV reveals that the capacitance characteristics of Co(OH)2 is distinct from that of the electric double-layer systems, which is due to a faradic reaction of Co(OH)2 , shown as follows: Co(OH)2 + OH− ↔ CoOOH + H2 O + e−
(2)
CoOOH + OH− ↔ CoO2 + H2 O + e−
(3)
A CV curve was recorded in the same conditions with an AC electrode (Fig. 2, curve b). The potential range of CV was shifted toward more negative region compared to Co(OH)2 /USY, and
has a rectangular shape based on the adsorbed charges on the surface to form electric double-layer capacitance. On account of the absorbed charges on the electrode surface increase linearly with increasing potential, the current intensity of AC shown in CV curve is potential independent, which proves the charge storage mechanism was mainly attributed to double-layer capacitance. Fig. 3 shows the typical galvanostatic charge/discharge curves (at a current density of 5 mA/cm2 ) of Co(OH)2 /USY electrode within a potential window of 0 to 0.45 V and AC electrode within a potential window of −1 to 0 V, respectively. The linear charge/discharge curves are observed in AC electrode, which is attributed to linear correlation of the absorbed charge on the interface with the applied potential. That means the specific capacitance is independent on the applied potential nature, distinctly accompanied with a nonfaradic process on the interface. The charge/discharge curves of the Co(OH)2 /USY is not perfectly linear as for AC capacitors, the slope of charge/discharge curves indicating the potential dependent nature of faradic reaction. Nonetheless, the specific capacitance of both Co(OH)2 /USY and AC case can be calculated from C = I/(dV/dt), where dV/dt is the slope of the linear
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Fig. 3. Charge/discharge behavior of (a) Co(OH)2 /USY composite electrode and (b) AC electrode at a current density of 5 mA/cm2 .
part of the discharge curve and I is the absolute value of the current density. From the result of Fig. 3 the specific capacitance of Co(OH)2 /USY is 745 F/g (note that in this work, if no specific explanation, all the capacitance is referred to Co(OH)2 /USY material without correcting for the weight percentage of the Co(OH)2 ) at 5 mA/cm2 in the potential range of 0 to 0.45 V, and AC capacitors is 287 F/g at the same current density in the potential range of −1 to 0 V. Therefore, the optimal positive/negative mass ration of Co(OH)2 /USY and AC was set almost 1:2.6.
Co(OH)2 /USY positive electrode and AC negative electrode at the voltage scan rate of 5, 10 and 20 mV/s. The potential range of the unit cell can be predicted by the individual CV curves, as shown in Fig. 2. At a fully oxidized state, Co(OH)2 /USY gives about 0.5 V but the AC-based electrode gives about −1 V at a fully reduced state, whereas the discharge process proceeds until the potential of both electrodes is the same. Thus, the potential of the asymmetric unit cell can reach 1.5 V in the fully charged state. Fig. 5 shows the typical galvanostatic charge/discharge curves of the asymmetric capacitors in the potential window of 0 to 1.5 V at a function of current density. During the charge and discharge process, non-linear behavior of the unit cell is observed, as expected from the corresponding CV curves (Fig. 4). The results shown in Fig. 5 demonstrate that this unit cell exhibits the typical response of hybrid-type EC capacitors [18]. The specific capacitance of the capacitor (Cm ) was calculated as follows: Cm =
C It = m Vm
(4)
3.2.2. The electrochemical characterizations of the asymmetric capacitors Based on the above electrochemical characteristics investigation of the individual capacitor, it is possible to combine AC and Co(OH)2 /USY to obtain an asymmetric capacitor with a potential window close to 1.5 V without a significant decomposition of the aqueous solvent during the charge/discharge cycle. A similar conclusion was reached by Wang et al. for CoAl double hydroxide but their operation capacitor voltage was restricted to 1.2 V [11]. Fig. 4 shows CV curves of the hybrid capacitor with
where I is the current density of discharge, t the total time of discharge and V is the potential range. The m is the mass of active materials in the asymmetric capacitor (including positive and negative electrode). The specific capacitance of the hybrid capacitor at current densities of 2, 5, 10, and 25 mA/cm2 were 110, 105, 102, 98 F/g, respectively. Even though under the large current density of 25 mA/cm2 , nearly 90% of the initial amount can be reached. To our best knowledge, it is the highest value compared with the other similar metal oxide in AC hybrid capacitor systems in the same medium. In our previous studies [7–8], we have successfully reported that the supporting USY material offers a nanometer template for the formation of network-like structure Co(OH)2 . It shows anisotropic morphology characteristics forming a loosely packed microstructure, and all this was provided an important morphological basis for the reversibility and bulk accessibility of fast faradic reactions. Consequentially, we have achieved the novel capacitive performance Co(OH)2 /USY composite as the
Fig. 4. Cyclic voltammograms of at different scan rates in 1 M KOH within a potential window of 0 to 1.5 V.
Fig. 5. Charge/discharge behavior of the hybrid capacitor at different current densities in 1 M KOH within a potential window of 0 to 1.5 V.
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Fig. 6. Reagone plot relating power density to achievable energy density of the supercapacitor: (a) asymmetric supercapacitor based on Co(OH)2 /USY and AC electrode; (b) symmetric supercapacitor based on Co(OH)2 /USY electrode.
electrode materials applied for supercapacitors. Unfortunately, the Co(OH)2 /USY composite material has a narrow operation potential (on account of the real galvanostatic discharge window is about 0.45 V), it is thereby limited to practical applications for the reason of energy density. In the present work, the active carbon is used to fabricate the hybrid capacitor, which stores charge through a double-layer absorption mechanism in the negative potential range. It ensures the novel Co(OH)2 /USY composite to proceed faradic reaction with a constant utilization in the positive potential range. Therefore, not only Co(OH)2 /USY electrode but also the AC electrode has fully exerted their capacitive roles in the corresponding potential range. We speculate that this is one of the important reasons responsible for the excellent capacitive characteristics of the hybrid unit cell. Fig. 6 presents Reagone plots of the hybrid capacitors and the symmetric capacitors (using two Co(OH)2 /USY electrodes). The energy density (E) of a supercapacitor can be calculated by using the following equation: E = V dq C=
q V
E=C
(5) V dV = 21 C(V )2
where C is the capacitance of the hybrid capacitor, V is the operating potential window, and q is the total accumulated charge on the surface. The specific power density (P) of the supercapacitor can be calculated according to the following equation: P=
I V m
(6)
where I is the current density of discharge, V the potential range of a supercapacitor and m is the mass of active materials in the asymmetric supercapacitor including positive and negative electrode.
321
Fig. 7. Cycle life of the hybrid capacitor at the current densities of 2 mA/cm2 .
Because all the components of the cell are not yet optimized, these Reagone plots only provide an estimation of the performance of the hybrid-type cell. From Fig. 6, it is clear that both the energy density and power density greatly increased compared with the symmetric type. For example, the real energy density of the hybrid capacitor was 30.625 W h/kg at a power density of 520.8 W/kg. In the symmetric capacitor cell, the related energy density is 18.646 W h/kg and the power density is 281.25 W/kg. This can be explained by the energy density and power density which critically depend on the real working voltage as described in Eqs. (5) and (6). The notable improved electrochemical performance is rooted on the practical electrochemical stability window greatly increasing from 0.45 to 1.5 V by assembling the hybrid devices. Although, in previous paper [7,8], we have synthesized the novel capacitance characteristics material of Co(OH)2 /USY, it is not preferable to the classical hydrous RuO2 in terms of the limited working potential in practical applications. In the hybrid case, combination of AC with Co(OH)2 /USY to fabricate the unit cell, it could not only deliver a wide working potential, but also enhance the performance of energy density especially under large current density. On the other hand, high-rate discharge ability of the hybrid cell was also superior to that of the symmetric cell. The cycling stability was tested over 500 cycles illustrated in Fig. 7. It exhibited a loss of 19% in capacitance during the first 100 cycles. After this stage, a nearly constant value was observed and remained still 95% of the 100th cycle. The capacitance fading may originate from the slow oxidation of Co(OH)2 to CoOOH owning to the Co3+ is more stable than Co2+ under alkali environment. 4. Conclusion In this work, novel capacitive characteristics of Co(OH)2 /USY nanocomposite was prepared by self-directed growth process. On account of the limited potential window of the Co(OH)2 /USY composite, an asymmetric superca-
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pacitor based on Co(OH)2 /USY composite as the positive electrode and AC as the negative electrode were fabricated in 1 M KOH electrolyte. The results of cyclic voltammetry and galvanostatic charge/discharge test exhibit good electrochemical capacitive performance of 110 F/g within the potential range from 0 to 1.5 V, which is the highest value compared with other similar metal oxide in AC hybrid capacitor systems. It also shows a higher working voltage than the symmetric capacitor, leading to a high energy density especially under large current density. The notable improved electrochemical performance is rooted on the practical electrochemical stability window greatly increasing from 0.45 to 1.5 V by assembling the hybrid devices. Future research to optimize the components of the hybrid devices, particularly to enhance the power and energy capability, is currently in progress. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 20403014; 20633040) and the National Natural Science Foundation of Jiangsu Province (No. BK2006196).
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