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Ultrafine nano zirconia as electrochemical pseudocapacitor material
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Short communication
Ultrafine nano zirconia as electrochemical pseudocapacitor material Wei Zhang, Yueyue Tan, Yilong Gao, Jianxiang Wu, Bohejin Tangn College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, PR China Received 6 July 2014; received in revised form 5 October 2014; accepted 8 October 2014
Abstract Ultrafine nano zirconia was prepared by a one-step sol–gel method and investigated by powder X-ray diffractions (PXRD) and transmission electron microscope (TEM). The results showed that its shape is spherical with the particle size from 2 to 3 nm. Electrochemical properties of nano zirconium oxide electrodes were measured by cyclic voltammetry, charge/discharge test and EIS in 6 M KOH electrolyte. The results obtained show that the ultrafine nano zirconia has a specific capacitance as high as 95 F g 1 at scan rate of 5 mV s 1. The capacitance is more than twice of the conventional zirconia, which was prepared from precipitation. In addition, the pseudocapacitor material exhibits excellent cycling performance, retaining 77% of the initial capacity after 500 cycles. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: E. Capacitors; E. Electrodes; D. ZrO2; A. Sol–gel processes
1. Introduction Supercapacitors combine the advantages of both high power density and high energy density. Supercapacitors are important for use in hybrid electrical vehicles to provide peak power during acceleration in combination with batteries [1]. The electrode materials are the most important factor in determining the properties of supercapacitors [2,3]. So a dramatic improvement in storage performance of a supercapacitor can be achieved by the development of new nano-structured materials [4]. However, the high cost of some metal oxides and poor cycling performance of conductive polymers limits their application. Zirconia has several properties that make it a useful material. These properties include high density, hardness, electrical conductivity, wear resistance, biocompatibility, mechanical strength and low cost, which have been widely used in practical application fields [5,6]. In the previous reports, we rarely found this materials was applied in the supercapacitors. The main reason is that the pure zirconia does not provide a high specific capacity. So many reports researched the zirconia loaded on the carriers. For examples, Mahdi Nasibi et al. [7] evaluated nano n
Corresponding author. E-mail address: [email protected] (B. Tang).
ZrO2/carbon black as a possible electrode candidate for electrochemical capacitors. The results show that the 30:60:10 (carbon black: ZrO2:polytetrafluoroethylene) type electrode has a specific capacitanceas high as 43.20 F g 1 at scan rate of 10 mV s 1. Soumen Giri et al. [8] applied the method of in-situ hydrothermal to synthesize graphene/zirconium oxide composite from respective precursors graphene oxide and zirconium oxy-nitrate. The results show that specific capacitanceas can reach 1359.99 F g 1 at scan rate of 1 mV s 1. The compounds added with ZrO2 were used as electrode materials. For examples, the objective study of YoungSang Jang et al.’s [9] study was to develop a new class of one-dimensional Sn–ZrO2 nanocrystal decorated CNFs. The electrochemical studies proved that the addition of ZrO2 in carbon nanofibrous matrix, showed higher capacitance and better stability than pristine and monometallic samples. Layered structures of cubic yttria-stabilized zirconia (YSZ) sandwiched between two layers of platinum/YSZ composites with the platinum concentration varying between 20 and 55 vol% were prepared for solid state supercapacitor materials. From the research, they found that large double-layer capacitances were obtained due to the presence of large three-dimensional electrode surface areas in the composite layers [10]. In this paper, ultrafine nano zirconia was prepared by a onestep sol–gel method. This zirconia was evaluated as a possible candidate electrode material for pseudocapacitor using different
http://dx.doi.org/10.1016/j.ceramint.2014.10.047 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
W. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
2
techniques including cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge and discharge.
3. Results and discussion 3.1. Material characterization
2. Experimental 2.1. Material synthesis Sol–gel synthesis: A solution of 128.9 mg (0.4 mmol) zirconium oxychloride octahydrate and 10 ml N,N-dimethylformamide (DMF) was mixed for 10 min and sealed in an 25 ml Teflon-lined autoclave (pH ¼ 4.5). The autoclave was heated at 383 K for 120 h. The resulting sol was washed with degassed distilled water for four times and dried in vacuum at 353 K (denoted as sample 1). Precipitation synthesis [11]: Zirconia was prepared by the precipitation method from zirconium oxychloride. A desired quantity of zirconium oxychloride was dissolved in degassed distilled water. To this clear solution with pH ¼ 2 a required amount of hydrolyzing agent (NH4OH) was added slowly until the solution pH reached 9. The obtained precipitate was aged for 12 h at room temperature and washed several times with degassed distilled water to make it free from chloride ions. Then the obtained hydroxide was dried in vacuum at 353 K (denoted as sample 2) All the reagents were of analytical grade and used as received without further purification (Shanghai Chemical Reagent Company).
In the sol–gel process, the zirconia nanoparticles are generally synthesized in the presence of HPC polymer, which creates steric hindrance to particle agglomeration. In our work, zirconium dioxide was prepared by a one-step sol–gel method with DMF. The main reason of formation sol is that enol exists in the DMF, which provide hydroxyl for zirconia. But the process of providing hydroxyl is very tardiness. So the size of as-prepared nano zirconia particles is super fine, approximately 2 nm to 3 nm (Fig. 1). In addition, the method of sol–gel also causes preferential growth orientation of zirconia. As shown in Fig. 2, their preferred orientations are (1 0 0), (1 0 2) and ( 1 2 1). The particle size of zirconium was estimated by the Scherrer’s equation using the ( 1 1 1) reflection: d ¼ 0:9λ=β cos θ
2.2. Electrochemical measurements Electrodes for electrochemical performance were constructed by mixing active material, carbon black and poly tetrafluoroethylene (PTFE) binder with the weight percent ratio of 75:20:5, then it was dispersed in alcohol and the mixtures was pressed onto nickel foams at a pressure of 12 MPa. The geometric surface area of the prepared working electrode is 1 cm2, and then the electrodes were dried in a vacuum at 90 1C for 1 h to remove the solvent. Nickel foam (1.6 mm thick, 95% purity, Goodfellow) was used as a current collector. The electro-chemical measurements were carried out using CHI660D electrochemical workstation (ShanghaiChen Hua, Inc.). The conventional three-electrode cell was equipped with a Pt plate as the counter electrode and an Ag electrode as the reference electrode [12]. CV tests were performed within the range of 0.1 and 0.6 V, using scan rates of 5, 10, 20, 30 and 50 mV s 1. EIS measurements were also carried out in the frequency range of 100 kHz to 0.01 Hz. All experiments were carried out at a room temperature and in 6 M KOH solution.
Fig. 1. The TEM micrograph of nano zirconia.
2.3. Compositional and structural characterization The powder X-ray diffractions (PXRD) of the samples were performed on a diffraction meter (D/Max-RB) with Cu-Kα radiation (λ ¼ 1.54056 Å) and a graphite monochromator at 50 kV, 100 mA. The size and morphological view of the alloy powders were observed by a transmission electron microscope (TEM) on Hitachi H-800 microscope.
Fig. 2. XRD patterns of the sample 1 and 2.
Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
W. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
where λ is the wavelength of the Cu Kα radiation, β is the fullwidth at half-maximum (FWHM) of the diffraction peak and θ is the Bragg’s diffraction angle of the peak. The particle crystalline size of zirconia was calculated to be 1.2 nm. Additionally, from Fig. 1 it seems that the particle size of zirconia bigger than the value calculated from the XRD pattern. This may result from the actually aggregated zirconia nanoparticles. 3.2. Electrochemical characterization Electrochemical performances of samples were tested in aqueous of 6 M KOH solution at the scan rate of 5, 10, 20, 30, 40 and 50 mV s 1. Fig. 3(a) and (b) shows the typical CV (at various scan rates) of sample 1 and 2 between 0.1 and 0.6 V. The cyclic voltammetry (CV) curves have two typical redox peaks, indicating it is pseudocapacitance. All the CV curves are almost symmetric, and the peak current increases with increasing scan rate with a little shift in both cathodic and anodic peak potentials with respect to scan rate, suggesting their good reversibility of fast charge–discharge response. From the CV, the specific capacitance can be estimated as follows [13]: Z 1 C¼ IdV 2 v Δm ΔV
3
where C is the specific capacitance (F g 1), v is the scan rate (V s 1), Δm is the mass of the active material (g), ΔV is the potential window (V) and IdV represents the area under CV curve (Q). The specific capacitance values calculated are shown in Fig. 3(c), as a function of scan rate of CV. It is noteworthy that the ultrafine nano zirconia as high as 95 F g 1 has been obtained at a scan rate of 5 mV s 1 and the ultrafine nano zirconia of 62 F g 1 was obtained even at a high scan rate of 50 mV s 1. These values of ultrafine nano zirconia are much higher than the values obtained from a conventional zirconia. A main reason is that the activity of zirconia has improved with the particle size decreased. Fig. 4(a) shows the galvanostatic charge and discharge curves of sample 1 and 2 in the voltage range of 0.1 V and 0.5 V. The nonlinear charging/discharging profiles indicate a significant contribution of pseudocapacitance from zirconia. The specific capacitance can be calculated from the galvanostatic charge–discharge curve according to equation [14]: I Δt C¼ ΔV Δm where C is the specific capacitance (F g 1), I is the current (A), Δt is the discharge time (s), ΔV is the potential window (V) and Δm is mass of the electroactive material (g). From Fig. 4(a), the sample 1 pseudocapacitance shows the longer
Fig. 3. (a) and (b) The cyclic voltammetry curves of sample 1 (2.7 mg) and 2 (9.9 mg) at an increasing voltage scanning rate of 5, 10, 20,30,40, and 50 mV s 1; (c) The specific capacitance of sample 1 and 2 at different scan rates. Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
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W. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
Fig. 4. (a) The charge–discharge curves of sample 1(10.0 mg) and 2(9.9 mg) at a current density of 0.5 A g 1; (b) Impedance spectra of sample 1 and 2; (c) Cycling performance of sample 1 at a scan rate of 10 m V 1. Z0 is real impedance. Z″ is imaginary impedance.
charge time of 60 s and the higher specific capacitance of 75 F g 1 at a charge and discharge current density of 0.5 A g 1. The capacitance is more than twice of the sample 2, which has the specific capacitance only 34 F g 1. We have also used EIS to characterize the capacitive behavior of nano zirconia and conventional zirconia (Fig. 4(b)). It shows the electrochemical impedance spectrum in the form of Nyquist plots for different zirconia electrodes, where Z0 and Z″ are the real and imaginary parts of the impedance, respectively. The plot contains a semi-circle at high frequencies, which is due to Faradic charge transfer resistance, and a nearly straight line along the imaginary axis at a low-frequency component due to the mass transport limit. The solution resistance Rs of 0.8 and 0.5 ohm were obtained from the x-intercept of the impedance spectra in Fig. 4(b). The electrolyte diffusion resistances of different zirconia are slightly different, but the charge transfer resistance of nano zirconia is smaller than the conventional zirconia, according from the size of radius in Fig. 4(b). So it enhanced the conductivity of nano zirconia. The stability was evaluated by 500 charging–discharging cycles at a scan rate of 10 mV s 1, as shown in Fig. 4(c). There is a little increase in the value of specific capacitance in the first 25 cycles and then the electrode decreases until the steady trend. The specific capacitances drop by 23% from
78 to 60 F g 1 during the 500 cycles, indicating nano zirconia as well as long term electrochemical and mechanical cycling stabilities. 4. Conclusions In summary, ultrafine nano zirconia oxide was prepared by special sol–gel method. The particle size of nano zirconia is from 2 to 3 nm. The electrochemical tests show that the pseudo supercapacitor exhibits good electrochemical capacitance performance, excellent discharge rate and good stability. The specific capacitance value of nano zirconia is 95 F g 1 in 6 M KOH at the scan rate of 5 mv s 1. This value is about 2.1-times higher than that of the conventional zirconia. The above process used by us is cost–effective to expand the application of zirconia for storage purposes. In addition, the sol–gel method for the preparation of zirconia is also expected to be applied to other systems. Acknowledgment The authors would like to thank Dr. Zhi Yan for analyse of XRD.
Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
W. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
References [1] L. Lam, R. Louey, Development of ultra-battery for hybrid-electric vehicle applications, J. Power Sources 158 (2) (2006) 1140–1148. [2] S.-L. Kuo, N.-L. Wu, Composite supercapacitor containing tin oxide and electroplated ruthenium oxide,, Electrochem. Solid-State Lett. 6 (5) (2003) A85–A87. [3] T. Tao, Q. Chen, H. Hu, Y. Chen, MoOosub4 3o/sub4 nanoparticles distributed uniformly in carbon matrix for supercapacitor applications, Mater. Lett. 66 (1) (2012) 102–105. [4] C. Yuan, X. Zhang, L. Su, B. Gao, L. Shen, Facile synthesis and selfassembly of hierarchical porous NiO nano/micro spherical superstructures for high performance supercapacitors, J. Mater. Chem. 19 (32) (2009) 5772–5777. [5] K. Tomishige, Y. Furusawa, Y. Ikeda, M. Asadullah, K. Fujimoto, CeO2– ZrO2 solid solution catalyst for selective synthesis of dimethyl carbonate from methanol and carbon dioxide, Catal. Lett. 76 (1-2) (2001) 71–74. [6] Z. Lu, Z. Zhu, X. Zheng, Y. Qiao, J. Guo, C.M. Li, Biocompatible fluorescence-enhanced ZrO2–CdTe quantum dot nanocomposite for in vitro cell imaging, Nanotechnology 22 (15) (2011) 155604. [7] M. Nasibi, M.A. Golozar, G. Rashed, Nano zirconium oxide/carbon black as a new electrode material for electrochemical double layer capacitors, J. Power Sources 206 (2012) 108–110. [8] S. Giri, D. Ghosh, C.K. Das, Growth of vertically aligned tunable polyaniline on graphene/ZrO2 nanocomposites for supercapacitor energy‐ storage application, Adv. Funct. Mater. (2013).
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[9] Y.-S. Jang, T. Amna, M.S. Hassan, J.-L. Gu, I.-S. Kim, H.-C. Kim, J.-H. Kim, S.-H. Baik, M.-S. Khil, Improved supercapacitor potential and antibacterial activity of bimetallic CNFs–Sn–ZrO 2 nanofibers: fabrication and characterization, RSC Adv. 4 (33) (2014) 17268–17273. [10] M. Hendriks, M. Heijman, W. Van Zyl, J. Ten Elshof, H. Verweij, Solid state supercapacitor materials: layered structures of yttria-stabilized zirconia sandwiched between platinum/yttria-stabilized zirconia composites, J. Appl. Phys. 90 (10) (2001) 5303–5307. [11] V.R. Reddy, D.W. Hwang, J.S. Lee, Photocatalytic water splitting over ZrO2 prepared by precipitation method, Korean J. Chem. Eng. 20 (6) (2003) 1026–1029. [12] Y. Gao, J. Wu, W. Zhang, Y. Tan, T. Tang, S. Wang, B. Tang, J. Zhao, SO42/SnO osub4 2o /sub4 as a new electrode for electrochemical supercapacitors, Ceram. Int. 40 (6) (2014) 8925–8929. [13] V. Nithya, R. Kalai Selvan, D. Kalpana, L. Vasylechko, C. Sanjeeviraja, Synthesis of Bio sub4 2 o/sub4 WO osub4 6o/sub4 nanoparticles and its electrochemical properties in different electrolytes for pseudocapacitor electrodes, Electrochim. Acta 109 (2013) 720–731. [14] B. Hu, X. Qin, A.M. Asiri, K.A. Alamry, A.O. Al-Youbi, X. Sun, Fabrication of Ni (OH)o sub4 2o /sub4 nanoflakes array on Ni foam as a binder-free electrode material for high performance supercapacitors, Electrochim. Acta 107 (2013) 339–342.
Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Short communication
Ultrafine nano zirconia as electrochemical pseudocapacitor material Wei Zhang, Yueyue Tan, Yilong Gao, Jianxiang Wu, Bohejin Tangn College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, PR China Received 6 July 2014; received in revised form 5 October 2014; accepted 8 October 2014
Abstract Ultrafine nano zirconia was prepared by a one-step sol–gel method and investigated by powder X-ray diffractions (PXRD) and transmission electron microscope (TEM). The results showed that its shape is spherical with the particle size from 2 to 3 nm. Electrochemical properties of nano zirconium oxide electrodes were measured by cyclic voltammetry, charge/discharge test and EIS in 6 M KOH electrolyte. The results obtained show that the ultrafine nano zirconia has a specific capacitance as high as 95 F g 1 at scan rate of 5 mV s 1. The capacitance is more than twice of the conventional zirconia, which was prepared from precipitation. In addition, the pseudocapacitor material exhibits excellent cycling performance, retaining 77% of the initial capacity after 500 cycles. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: E. Capacitors; E. Electrodes; D. ZrO2; A. Sol–gel processes
1. Introduction Supercapacitors combine the advantages of both high power density and high energy density. Supercapacitors are important for use in hybrid electrical vehicles to provide peak power during acceleration in combination with batteries [1]. The electrode materials are the most important factor in determining the properties of supercapacitors [2,3]. So a dramatic improvement in storage performance of a supercapacitor can be achieved by the development of new nano-structured materials [4]. However, the high cost of some metal oxides and poor cycling performance of conductive polymers limits their application. Zirconia has several properties that make it a useful material. These properties include high density, hardness, electrical conductivity, wear resistance, biocompatibility, mechanical strength and low cost, which have been widely used in practical application fields [5,6]. In the previous reports, we rarely found this materials was applied in the supercapacitors. The main reason is that the pure zirconia does not provide a high specific capacity. So many reports researched the zirconia loaded on the carriers. For examples, Mahdi Nasibi et al. [7] evaluated nano n
Corresponding author. E-mail address: [email protected] (B. Tang).
ZrO2/carbon black as a possible electrode candidate for electrochemical capacitors. The results show that the 30:60:10 (carbon black: ZrO2:polytetrafluoroethylene) type electrode has a specific capacitanceas high as 43.20 F g 1 at scan rate of 10 mV s 1. Soumen Giri et al. [8] applied the method of in-situ hydrothermal to synthesize graphene/zirconium oxide composite from respective precursors graphene oxide and zirconium oxy-nitrate. The results show that specific capacitanceas can reach 1359.99 F g 1 at scan rate of 1 mV s 1. The compounds added with ZrO2 were used as electrode materials. For examples, the objective study of YoungSang Jang et al.’s [9] study was to develop a new class of one-dimensional Sn–ZrO2 nanocrystal decorated CNFs. The electrochemical studies proved that the addition of ZrO2 in carbon nanofibrous matrix, showed higher capacitance and better stability than pristine and monometallic samples. Layered structures of cubic yttria-stabilized zirconia (YSZ) sandwiched between two layers of platinum/YSZ composites with the platinum concentration varying between 20 and 55 vol% were prepared for solid state supercapacitor materials. From the research, they found that large double-layer capacitances were obtained due to the presence of large three-dimensional electrode surface areas in the composite layers [10]. In this paper, ultrafine nano zirconia was prepared by a onestep sol–gel method. This zirconia was evaluated as a possible candidate electrode material for pseudocapacitor using different
http://dx.doi.org/10.1016/j.ceramint.2014.10.047 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
W. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
2
techniques including cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge and discharge.
3. Results and discussion 3.1. Material characterization
2. Experimental 2.1. Material synthesis Sol–gel synthesis: A solution of 128.9 mg (0.4 mmol) zirconium oxychloride octahydrate and 10 ml N,N-dimethylformamide (DMF) was mixed for 10 min and sealed in an 25 ml Teflon-lined autoclave (pH ¼ 4.5). The autoclave was heated at 383 K for 120 h. The resulting sol was washed with degassed distilled water for four times and dried in vacuum at 353 K (denoted as sample 1). Precipitation synthesis [11]: Zirconia was prepared by the precipitation method from zirconium oxychloride. A desired quantity of zirconium oxychloride was dissolved in degassed distilled water. To this clear solution with pH ¼ 2 a required amount of hydrolyzing agent (NH4OH) was added slowly until the solution pH reached 9. The obtained precipitate was aged for 12 h at room temperature and washed several times with degassed distilled water to make it free from chloride ions. Then the obtained hydroxide was dried in vacuum at 353 K (denoted as sample 2) All the reagents were of analytical grade and used as received without further purification (Shanghai Chemical Reagent Company).
In the sol–gel process, the zirconia nanoparticles are generally synthesized in the presence of HPC polymer, which creates steric hindrance to particle agglomeration. In our work, zirconium dioxide was prepared by a one-step sol–gel method with DMF. The main reason of formation sol is that enol exists in the DMF, which provide hydroxyl for zirconia. But the process of providing hydroxyl is very tardiness. So the size of as-prepared nano zirconia particles is super fine, approximately 2 nm to 3 nm (Fig. 1). In addition, the method of sol–gel also causes preferential growth orientation of zirconia. As shown in Fig. 2, their preferred orientations are (1 0 0), (1 0 2) and ( 1 2 1). The particle size of zirconium was estimated by the Scherrer’s equation using the ( 1 1 1) reflection: d ¼ 0:9λ=β cos θ
2.2. Electrochemical measurements Electrodes for electrochemical performance were constructed by mixing active material, carbon black and poly tetrafluoroethylene (PTFE) binder with the weight percent ratio of 75:20:5, then it was dispersed in alcohol and the mixtures was pressed onto nickel foams at a pressure of 12 MPa. The geometric surface area of the prepared working electrode is 1 cm2, and then the electrodes were dried in a vacuum at 90 1C for 1 h to remove the solvent. Nickel foam (1.6 mm thick, 95% purity, Goodfellow) was used as a current collector. The electro-chemical measurements were carried out using CHI660D electrochemical workstation (ShanghaiChen Hua, Inc.). The conventional three-electrode cell was equipped with a Pt plate as the counter electrode and an Ag electrode as the reference electrode [12]. CV tests were performed within the range of 0.1 and 0.6 V, using scan rates of 5, 10, 20, 30 and 50 mV s 1. EIS measurements were also carried out in the frequency range of 100 kHz to 0.01 Hz. All experiments were carried out at a room temperature and in 6 M KOH solution.
Fig. 1. The TEM micrograph of nano zirconia.
2.3. Compositional and structural characterization The powder X-ray diffractions (PXRD) of the samples were performed on a diffraction meter (D/Max-RB) with Cu-Kα radiation (λ ¼ 1.54056 Å) and a graphite monochromator at 50 kV, 100 mA. The size and morphological view of the alloy powders were observed by a transmission electron microscope (TEM) on Hitachi H-800 microscope.
Fig. 2. XRD patterns of the sample 1 and 2.
Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
W. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
where λ is the wavelength of the Cu Kα radiation, β is the fullwidth at half-maximum (FWHM) of the diffraction peak and θ is the Bragg’s diffraction angle of the peak. The particle crystalline size of zirconia was calculated to be 1.2 nm. Additionally, from Fig. 1 it seems that the particle size of zirconia bigger than the value calculated from the XRD pattern. This may result from the actually aggregated zirconia nanoparticles. 3.2. Electrochemical characterization Electrochemical performances of samples were tested in aqueous of 6 M KOH solution at the scan rate of 5, 10, 20, 30, 40 and 50 mV s 1. Fig. 3(a) and (b) shows the typical CV (at various scan rates) of sample 1 and 2 between 0.1 and 0.6 V. The cyclic voltammetry (CV) curves have two typical redox peaks, indicating it is pseudocapacitance. All the CV curves are almost symmetric, and the peak current increases with increasing scan rate with a little shift in both cathodic and anodic peak potentials with respect to scan rate, suggesting their good reversibility of fast charge–discharge response. From the CV, the specific capacitance can be estimated as follows [13]: Z 1 C¼ IdV 2 v Δm ΔV
3
where C is the specific capacitance (F g 1), v is the scan rate (V s 1), Δm is the mass of the active material (g), ΔV is the potential window (V) and IdV represents the area under CV curve (Q). The specific capacitance values calculated are shown in Fig. 3(c), as a function of scan rate of CV. It is noteworthy that the ultrafine nano zirconia as high as 95 F g 1 has been obtained at a scan rate of 5 mV s 1 and the ultrafine nano zirconia of 62 F g 1 was obtained even at a high scan rate of 50 mV s 1. These values of ultrafine nano zirconia are much higher than the values obtained from a conventional zirconia. A main reason is that the activity of zirconia has improved with the particle size decreased. Fig. 4(a) shows the galvanostatic charge and discharge curves of sample 1 and 2 in the voltage range of 0.1 V and 0.5 V. The nonlinear charging/discharging profiles indicate a significant contribution of pseudocapacitance from zirconia. The specific capacitance can be calculated from the galvanostatic charge–discharge curve according to equation [14]: I Δt C¼ ΔV Δm where C is the specific capacitance (F g 1), I is the current (A), Δt is the discharge time (s), ΔV is the potential window (V) and Δm is mass of the electroactive material (g). From Fig. 4(a), the sample 1 pseudocapacitance shows the longer
Fig. 3. (a) and (b) The cyclic voltammetry curves of sample 1 (2.7 mg) and 2 (9.9 mg) at an increasing voltage scanning rate of 5, 10, 20,30,40, and 50 mV s 1; (c) The specific capacitance of sample 1 and 2 at different scan rates. Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
4
W. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
Fig. 4. (a) The charge–discharge curves of sample 1(10.0 mg) and 2(9.9 mg) at a current density of 0.5 A g 1; (b) Impedance spectra of sample 1 and 2; (c) Cycling performance of sample 1 at a scan rate of 10 m V 1. Z0 is real impedance. Z″ is imaginary impedance.
charge time of 60 s and the higher specific capacitance of 75 F g 1 at a charge and discharge current density of 0.5 A g 1. The capacitance is more than twice of the sample 2, which has the specific capacitance only 34 F g 1. We have also used EIS to characterize the capacitive behavior of nano zirconia and conventional zirconia (Fig. 4(b)). It shows the electrochemical impedance spectrum in the form of Nyquist plots for different zirconia electrodes, where Z0 and Z″ are the real and imaginary parts of the impedance, respectively. The plot contains a semi-circle at high frequencies, which is due to Faradic charge transfer resistance, and a nearly straight line along the imaginary axis at a low-frequency component due to the mass transport limit. The solution resistance Rs of 0.8 and 0.5 ohm were obtained from the x-intercept of the impedance spectra in Fig. 4(b). The electrolyte diffusion resistances of different zirconia are slightly different, but the charge transfer resistance of nano zirconia is smaller than the conventional zirconia, according from the size of radius in Fig. 4(b). So it enhanced the conductivity of nano zirconia. The stability was evaluated by 500 charging–discharging cycles at a scan rate of 10 mV s 1, as shown in Fig. 4(c). There is a little increase in the value of specific capacitance in the first 25 cycles and then the electrode decreases until the steady trend. The specific capacitances drop by 23% from
78 to 60 F g 1 during the 500 cycles, indicating nano zirconia as well as long term electrochemical and mechanical cycling stabilities. 4. Conclusions In summary, ultrafine nano zirconia oxide was prepared by special sol–gel method. The particle size of nano zirconia is from 2 to 3 nm. The electrochemical tests show that the pseudo supercapacitor exhibits good electrochemical capacitance performance, excellent discharge rate and good stability. The specific capacitance value of nano zirconia is 95 F g 1 in 6 M KOH at the scan rate of 5 mv s 1. This value is about 2.1-times higher than that of the conventional zirconia. The above process used by us is cost–effective to expand the application of zirconia for storage purposes. In addition, the sol–gel method for the preparation of zirconia is also expected to be applied to other systems. Acknowledgment The authors would like to thank Dr. Zhi Yan for analyse of XRD.
Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047
W. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
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Please cite this article as: W. Zhang, et al., Ultrafine nano zirconia as electrochemical pseudocapacitor material, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.10.047