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MnO2 composite electrode for electrochemical supercapacitors
Materials Research Bulletin 44 (2009) 1122–1126
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Preparation and characterization of nanostructured NiO/MnO2 composite electrode for electrochemical supercapacitors En-Hui Liu *, Wen Li, Jian Li, Xiang-Yun Meng, Rui Ding, Song-Ting Tan College of Chemistry, Xiangtan University, Hunan 411105, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 June 2008 Received in revised form 18 September 2008 Accepted 7 October 2008 Available online 15 October 2008
Nanostructured nickel–manganese oxides composite was prepared by the sol–gel and the chemistry deposition combination new route. The surface morphology and structure of the composite were characterized by scanning electron microscope and X-ray diffraction. The as-synthesized NiO/MnO2 samples exhibit higher surface area of 130–190 m2 g1. Cyclic voltammetry and galvanostatic charge/ discharge measurements were applied to investigate the electrochemical performance of the composite electrodes with different ratios of NiO/MnO2. When the mass ratio of MnO2 and NiO in composite material is 80:20, the specific capacitance value of NiO/MnO2 calculated from the cyclic voltammetry curves is 453 F g1, for pure NiO and MnO2 are 209, 330 F g1 in 6 mol L1 KOH electrolyte and at scan rate of 10 mV s1, respectively. The specific capacitance of NiO/MnO2 electrode is much larger than that of each pristine component. Moreover, the composite electrodes showed high power density and stable electrochemical properties. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Sol–gel chemistry C. Electrochemical measurements D. Electrochemical properties
1. Introduction Electrochemical capacitors (ECs) have drawn much attention as charge-storage devices for electrical energy owing to their ability to deliver high power and survive high cycle counts. Based on charge-storage mechanism, two major types of ECs can be categorized [1,2]: (i) electrical double-layer capacitors (EDLCs), which store energy by utilizing the double-layer capacitance arising from the charge separation at the electrode–electrolyte interface, mainly focusing on carbon materials [3,4], (ii) pseudocapacitors, which store energy by utilizing the pseudocapacitance arising from the fast and reversible Faradic reactions in the electrode surface formed with electroactive materials, mainly focusing on transition metal oxides and conducting polymers [5– 9]. Normally, pseudocapacitors are potentially able to store more energy than double-layer capacitors. The characteristics required for using one certain material as supercapacitor electrode are a capacitive behavior, a large surface area, a high conductivity, and electrochemical stability. The best example is the amorphous and hydrated ruthenium oxide (aRuO2nH2O), which yields remarkably high specific capacitance (720–780 F g1) [5]. However, the high cost of such metal oxides
* Corresponding author. Tel.: +86 732 8292229; fax: +86 732 8292447. E-mail address: [email protected] (E.-H. Liu). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.10.003
has stimulated researcher to identify other cheap materials that exhibit similar behavior. In this sense, manganese oxides are promising electrochemical capacitor materials due to their low cost, abundance and less harmful nature. Manganese oxides can be synthesized by chemical and electrochemical methods. Most of the manganese oxides reported in the literature showed specific capacitance as high as 600 F g1 for thin films [10,11] and 150– 300 F g1 for powder-based electrodes [12] within a potential window of 0.9–1.2 V in aqueous electrolytes containing KCl, K2SO4, or Na2SO4, KOH. These values are far from the theoretical SC of 1370 F g1 [13]. The capacitance of MnO2 electrode is believed to be predominant due to pseudocapacitance, which is attributed to reversible redox transitions involving exchange of protons and/or cations with the electrolyte [13–14]. However, the resistivity and the equivalent series resistance (ESR) of MnO2 electrode are very large. Therefore, its capacity is limited. In order to overcome this disadvantage, the composite electrode materials of the manganese oxide were prepared with a conducting additive such as carbon material (graphite, carbon nanotube, porous carbon, activated carbon, and carbon aerogel, etc.) [15–20], conducting polymers [5– 9,21–22], metal oxides [23–25], etc. Prasad and Miura [25] reported that the thin films of nickel–manganese oxides synthesized by electrochemical method had fairly high specific capacitance (621 F g1), excellent stability and long cycle life. Such thin films could have shown higher SC values, but they would be suffering from poor energy density values. Higher energy density is
E.-H. Liu et al. / Materials Research Bulletin 44 (2009) 1122–1126
also anticipated for the supercapacitors, which is seemingly difficult with powder-based electrodes materials. So, the challenge with this composite lies in maximizing its electrochemical utilization. The aim of this study is to disperse the NiO/MnO2 effectively and to maximize its electrochemical utilization for charge-storage. For this purpose, nanostructured nickel–manganese oxides composite was prepared by the sol–gel and the chemistry deposition combination new route. Material characterization was carried out by XRD, SEM, BET, cyclic voltammetry, and charge/discharge cycling. The results showed that higher the specific capacitance values, higher power density, and stable electrochemical properties of the prepared NiO/ MnO2 composite were demonstrated. To our knowledge, using this method synthesized NiO/MnO2 composite as electrode for electrochemical supercapacitors has not yet been reported.
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hydrogel without further treatment, was used as reactant in the next step experiment. As-prepared Ni(OH)2 hydrogel was added to 0.2 mol L1 KMnO4 solution which was pretreated with pH 10.3 NaHCO3–Na2CO3 buffer, the mixture was treated in an ultrasonic bath for 30 min to obtain a homogeneous suspension. Subsequently, predesigned Mn(CH3COO)2 solution was slowly dropped into the above mixed suspension solution. The mixed solution was stirred at 70 8C for 10 h. The precipitate, a dark powder, was filtered and washed thoroughly with distilled water till the pH of the filtrate reached 7.0, and dried at the temperature of 80 8C for 24 h in air condition, they were last annealed at the temperature of 300 8C for 2 h. Thus NiO/MnO2 was obtained. According to the loading ratio of nickel oxide, these samples were referred to as the mass ratio of 0–25%. Pure manganese oxide and pure nickel oxide were also prepared for a comparison.
2. Experimental 2.2. Structural characterization 2.1. Synthesis of Ni(OH)2 hydrogel Reagent grade chemicals were obtained commercially and used without further purification unless noted otherwise. The Ni(OH)2 hydrogel was synthesized according to the method reported by Wu et al. [26]. For a typical procedure, NiSO4, NaOC2H5 (in a mole ratio of 1:2) and dehydrated ethanol (EtOH) were added to a threenecked flask equipped with a nitrogen purge, a cooling water condenser, and a magnetic stirrer. The mixture was heated at 75 8C, kept refluxed for 4 h, and the precipitate was filtered out. The resulting transparent solution was cooled to room temperature. While vigorously stirring, a 0.5 mol L1 solution of NH3H2O was added dropwise to get a sol, and the sol was then left 72 h to the formation of a Ni(OH)2 hydrogel eventually. Prepared Ni(OH)2
The X-ray diffraction (XRD) patterns of samples were recorded on a diffractometer (D/MAX-3C) with Cu Ka radiation (l = 1.5406 A˚) and a graphite monochromator at 40 kV and 30 mA. The scanning electron microscope (SEM) images of samples were performed on a LEO1525 microscopy. The surface area of the as-prepared samples was characterized by the adsorption/desorption of nitrogen at 77 K using a NOVA2200 (Quantachrome) automatic adsorption unit. 2.3. Electrode preparation and electrochemical characterization The mass ratio of active materials/acetylene black (AB)/ polyvinylidene fluoride (PVDF) was 77:15:8. First, the active
Fig. 1. SEM images for (a) the precursor of nickel oxides–manganese oxides; (b) NiO/MnO2 composite (with 20 wt.% NiO); (c) MnO2; and (d) NiO.
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materials and acetylene black were mixed and grinded thoroughly, and then the powder mixed well with PVDF which was dissolved in N-methyl-2-pyrrolidone (NMP) to form a slurry. The mixed slurry was pressed onto nickel foam current collectors (F1.0 cm) and dried at 90 8C for 10 h to fabricate electrodes. Mass of the active materials was about 10 mg. The electrochemical experiments were carried out in a threecompartment cell. The prepared active material was used as the working electrode. The Pt sheet was used as the counter electrode. The saturated calomel electrode (SCE) was used as the reference electrode. All the measurements were carried out in a 6 mol L1 KOH electrolyte. Cyclic voltammetry(CV) and galvanostatic charge/discharge tests were performed using a CHI660A electrochemical workstation (CH Instrument, USA).
characteristic peaks at 37.1o and 66.3o endorse the presence of MnO2. It is clearly seen that these peaks of MnO2 are broad and unclear which indicates the amorphous nature of the products. The XRD patterns of MnO2 correspond to crystalline a-MnO2. The result is in accordance with the standard spectrum (JCPDS, Card No. 44-0141). In Fig. 2b, the XRD patterns exhibit the characteristic peaks of rock salt NiO at 2u = 37.09 (1 1 1), 43.10 (2 0 0) and 62.58o (2 2 0), the result is in accordance with the standard spectrum (JCPDS Card No. 4-0835). In NiO/MnO2 composite, the characteristic peaks of NiO and MnO2 are demonstrated in Fig. 2c. Broad peaks suggest the poorly crystallized structure of the as-prepared materials.
3. Results and discussion
The CV is considered to be a suitable tool to indicate the capacitive behavior of any material. A large magnitude of the current and a rectangular type of voltammogram curves, symmetric in anodic and cathodic directions, are the indications of ideal capacitive nature of electrode. The current response rate at the potential reverse point, the so-called rectangularity of CV, is reflected by the kinetic reversibility in electrochemical system. It depends on the reaction constant of the electrochemical Faraday action [27]. Fig. 3 showed the CVs of as-prepared material with different NiO contents at a scan rate of 10 mV s1. In Fig. 3, all CVs showed almost a mirror image with respect to the zero-current line and a rapid current response on voltage reversal at each end potential; namely, the nearly rectangular-like and symmetric I–E responses were observed, indicating ideal capacitive behavior of the materials. The specific capacitance of as-prepared electrodes were estimated from the following equation [13]:
3.1. Structure characterization Fig. 1 shows SEM images of the pure MnO2, the pure NiO, the precursor of NiO/MnO2 and NiO/MnO2 composites. All SEM images were obtained at 100,000 magnification. The average diameter of agglomerate particles was about 10–50 nm. The particles are uniform in size which are well connected yet provide porous structure which is much required for supercapacitors. As was shown in Fig. 1(b), the morphology of NiO/MnO2 remained their original appearances. The change of surface area with increase of NiO in the NiO/ MnO2 composite is shown in Table 1. With increase of NiO in the NiO/MnO2 composite leads to an increase in the BET surface area from 130 to 190 m2 g1. It is important to note that surface area of the electrode material affect the SC. The redox reaction of the electrochemical supercapacitor is primarily surface mechanism and is hence highly dependent on the surface area of the electrode material. As-prepared samples exhibited higher surface area, nanostructured and microporous characteristic (Fig. 1 and Table 1), which favors the anion/cation diffusion and adsorption/desorption during the charge/discharge process. The XRD patterns of pure NiO, pure MnO2 and NiO/MnO2 with 20 wt.% NiO were shown in Fig. 2, respectively. In Fig. 2a, the two Table 1 The surface area of the as-prepared samples. NiO content (%) Surface area (m2 g1)
0 130
100 160
5 148
10 164
15 179
20 190
3.2. Electrochemical characterization
C¼
I
ym
(1)
where I, m and y are the current of middle potential of the CV curves (A), mass of the active material (g), and the scan rate (V s1). Table 2 tabulated the specific capacitances of the NiO/MnO2 composite with different NiO contents, which were measured in 6 mol L1 KOH solution and calculated according to Eq. (1). The increase in the specific capacitance of composite electrodes loading with NiO was obviously observed. When the ratio was less than 10%, the specific capacitance of the composite electrodes
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Fig. 2. XRD patterns for: (a) MnO2; (b) NiO; and (c) NiO/MnO2 with 20 wt.% NiO.
Fig. 3. CV of different composite electrodes, scan rate: 10 mV s1. (a) MnO2; (b) NiO; (c) 5%NiO; (d) 10%NiO; (e) 25%NiO; (f) 20%NiO; and (g) 15%NiO.
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Table 2 The capacitances of the NiO/MnO2 composite electrodes, scan rate: 10 mV s1. NiO content (%) Capacitance (F g1)
0 330
100 209
5 347
10 360
15 382
20 453
25 401
increased very slightly (347 ! 360 F g1), perhaps the electrochemical properties of the composite electrodes had not yet been in a best situation under such a ratio. When up to 20%, the specific capacitance of the composite electrodes increased to the maximum (453 F g1) at a scan rate of 10 mV s1, which is 37.0% higher than pure MnO2 (330 F g1), the result is better than that reported by Kim and Popov [28]. The enhanced specific capacitance may be attributed to synergic effects from each pristine component. On the other hand, may be attributed to the higher specific surface area, and electrochemical stability all reached a best situation, which more available active site could be obtained and more energy could be extracted from the composite. The specific capacitance of the composite electrodes, however, decreased a lot (453 ! 401 F g1) when the NiO ratio increased to 25% compared to that of 20%, this may be attributed to the increase in particle size of NiO, aggregation of NiO, decrease of the path of ions, etc. Fig. 4 showed the variation of the specific capacitance with the scan rate of CV for pure MnO2, pure NiO and NiO/MnO2 with 20 wt.% NiO composite electrodes. We could see from the figure that maximum SC values of 358, 238 and 480 F g1 were obtained at a scan rate of 5 mV s1, respectively. Interestingly, SC value of 282 F g1 was obtained even at a high CV scan rate of 100 mV s1 with the composite electrodes, while only 176 F g1 capacitance was obtained from pure MnO2 electrode. Here the role of the added NiO was proved. Fig. 5 presented charge/discharge curves for pure MnO2, pure NiO and NiO/MnO2 with 20 wt.% NiO composite electrodes, in the potential range between 0 and 0.8 V at a current density of 2 A g1 in 6 mol L1 KOH electrolyte. It could be found from the curves that the voltage varies nearly linearly with time, which indicates good capacitive behavior. The average specific capacitance of composite electrodes can be calculated on the basis of Eq. (2) [13]: C¼
I ðdV=dtÞm
(2)
where I (mA) and dV/dt (mV s1), respectively, denote the applied galvanostatic current and the slope of these chronopotentionmetric curve when the curve is approximately linear and
Fig. 5. Charge/discharge curves of (a) MnO2, (b) NiO and (c) 20%NiO composite electrodes at current density of 2 A g1 in 6 mol L1 KOH electrolyte.
symmetric, and m (g) represents the mass of electroactive materials. The average specific capacitances for pure MnO2, pure NiO and NiO/MnO2 with 20 wt.% NiO composite electrodes, which were obtained from charge/discharge curves on the basis of Eq. (2) are 338, 218 and 460 F g1, respectively. The result is identical with that estimated from the CV curves. It is most likely that the electrochemical characteristics of the composite materials were improved as the result of the addition of NiO to manganese oxide. 4. Conclusions It was found that introducing a proper proportion of the nickel oxide into manganese oxide to form an electro-active material is an effective method to achieve high performance electrochemical capacitor. The enhanced specific capacitance may be attributed to synergic effects from each pristine component. On the other hand, may be attributed to the higher specific surface area, and electrochemical stability, all reached a best situation, which more available active site could be obtained and more energy could be extracted from the composite. XRD, SEM, BET, CV, and galvanostatic charge/discharge were employed to characterize composite materials. When the ratio of NiO was 20 wt.%. the composite electrodes exhibited the maximum specific capacitance of 453 F g1 in 6 mol L1 KOH electrolyte at the scan rate 10 mV s1, 1, which is 37.0% higher than pure manganese oxide electrode. Moreover, the composite electrodes showed high power density and stable electrochemical properties, which indicates that it is a promising material for electrochemical capacitors. Acknowledgements The authors are grateful for Project supported by Hunan Provincial Natural Science Foundation of China (07JJ6015) and by the postdoctoral fund of Xiangtan University. References
Fig. 4. The specific capacitance of composite electrode against the scan rate of CV.
[1] B.E. Conway, Electrochemical Supercapacitors, Kluwer Academic Publisher, New York, 1999, pp. 1–625. [2] B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539. [3] A.G. Pandolfo, A.F. Hollenkamp, J. Power Sources 157 (2006) 11. [4] C. Portet, P.L. Taberna, P. Simon, E. Flahaut, C. Laberty-Robert, Electrochim. Acta 50 (2005) 4174. [5] J.G. Wen, Z.T. Zhou, Mater. Chem. Phys. 98 (2006) 442. [6] X. Zhang, L.Y. Ji, S.C. Zhang, W.S. Yang, J. Power Sources 173 (2007) 1017. [7] S.R. Sivakkumar, J.M. Ko, D.Y. Kim, B.C. Kim, G.G. Wallace, Electrochim. Acta 52 (2007) 7377.
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L.J. Sun, X.X. Liu, K.K.T. Lau, L. Chen, W.M. Gu, Electrochim. Acta 53 (2008) 3036. E.C. Rios, A.V. Rosario, R.M.Q. Mello, L. Micaroni, J. Power Sources 163 (2007) 1137. S.F. Chin, S.C. Pang, M.A. Anderson, J. Electrochem. Soc. 149 (2002) A379. C.C. Hu, C.C. Wang, J. Electrochem. Soc. 150 (2003) A1079. C. Ye, Z.M. Lin, S.Z. Hui, J. Electrochem. Soc. 152 (2005) A1272. S. Devaraj, N. Munichandraiah, Electrochem. Solid-State Lett. 8 (2005) A373. E. Raymundo-Pinero, V. Khomenko, E. Frackowiak, F. Beguin, J. Electrochem. Soc. 152 (2005) A229. [15] G.X. Wang, B.L. Zhang, Z.L. Yu, M.Z. Qu, Solid State Ionics 176 (2005) 1169. [16] S.B. Ma, K.W. Nam, W.S. Yoon, X.Q. Yang, K.Y. Ahn, K.H. Oh, K.B. Kim, J. Power Sources 178 (2008) 483. [17] S.B. Ma, Y.H. Lee, K.Y. Ahn, C.M. Kim, K.H. Oh, K.B. Kim, J. Electrochem. Soc. 153 (2006) C27.
[18] H. Kawaoka, M. Hibino, H.S. Zhou, I. Honma, Electrochem. Solid-State Lett. 8 (2005) A253. [19] C.Y. Lee, H.M. Tsai, H.J. Chuang, S.Y. Li, P. Lin, T.Y. Tseng, J. Electrochem. Soc. 152 (2005) A716. [20] E.H. Liu, X.Y. Meng, R. Ding, J.C. Zhou, S.T. Tan, Mater. Lett. 61 (2007) 3486. [21] K.R. Prasad, N. Munichandraiah, J. Electrochem. Soc. 149 (2002) A1393. [22] F. Fusalba, P. Gouerec, D. Villers, D. Belanger, J. Electrochem. Soc. 148 (2001) A1. [23] G.Y. Zhao, C.L. Xu, H.L. Li, J. Power Sources 163 (2007) 1132. [24] Y.S. Chen, C.C. Hu, Electrochem. Solid-State Lett. 6 (2003) A210. [25] K.R. Prasad, N. Miura, Electrochem. Commun. 6 (2004) 1004. [26] M.Q. Wu, J.H. Gao, S.R. Zhang, A. Chen, J. Porous Mater. 13 (2006) 407. [27] H.Y. Lee, S.W. Kim, H.Y. Lee, Electrochem. Solid-State Lett. 4 (2001) A19. [28] H.S. Kim, B.N. Popov, J. Electrochem. Soc. 150 (2003) D56.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation and characterization of nanostructured NiO/MnO2 composite electrode for electrochemical supercapacitors En-Hui Liu *, Wen Li, Jian Li, Xiang-Yun Meng, Rui Ding, Song-Ting Tan College of Chemistry, Xiangtan University, Hunan 411105, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 June 2008 Received in revised form 18 September 2008 Accepted 7 October 2008 Available online 15 October 2008
Nanostructured nickel–manganese oxides composite was prepared by the sol–gel and the chemistry deposition combination new route. The surface morphology and structure of the composite were characterized by scanning electron microscope and X-ray diffraction. The as-synthesized NiO/MnO2 samples exhibit higher surface area of 130–190 m2 g1. Cyclic voltammetry and galvanostatic charge/ discharge measurements were applied to investigate the electrochemical performance of the composite electrodes with different ratios of NiO/MnO2. When the mass ratio of MnO2 and NiO in composite material is 80:20, the specific capacitance value of NiO/MnO2 calculated from the cyclic voltammetry curves is 453 F g1, for pure NiO and MnO2 are 209, 330 F g1 in 6 mol L1 KOH electrolyte and at scan rate of 10 mV s1, respectively. The specific capacitance of NiO/MnO2 electrode is much larger than that of each pristine component. Moreover, the composite electrodes showed high power density and stable electrochemical properties. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Sol–gel chemistry C. Electrochemical measurements D. Electrochemical properties
1. Introduction Electrochemical capacitors (ECs) have drawn much attention as charge-storage devices for electrical energy owing to their ability to deliver high power and survive high cycle counts. Based on charge-storage mechanism, two major types of ECs can be categorized [1,2]: (i) electrical double-layer capacitors (EDLCs), which store energy by utilizing the double-layer capacitance arising from the charge separation at the electrode–electrolyte interface, mainly focusing on carbon materials [3,4], (ii) pseudocapacitors, which store energy by utilizing the pseudocapacitance arising from the fast and reversible Faradic reactions in the electrode surface formed with electroactive materials, mainly focusing on transition metal oxides and conducting polymers [5– 9]. Normally, pseudocapacitors are potentially able to store more energy than double-layer capacitors. The characteristics required for using one certain material as supercapacitor electrode are a capacitive behavior, a large surface area, a high conductivity, and electrochemical stability. The best example is the amorphous and hydrated ruthenium oxide (aRuO2nH2O), which yields remarkably high specific capacitance (720–780 F g1) [5]. However, the high cost of such metal oxides
* Corresponding author. Tel.: +86 732 8292229; fax: +86 732 8292447. E-mail address: [email protected] (E.-H. Liu). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.10.003
has stimulated researcher to identify other cheap materials that exhibit similar behavior. In this sense, manganese oxides are promising electrochemical capacitor materials due to their low cost, abundance and less harmful nature. Manganese oxides can be synthesized by chemical and electrochemical methods. Most of the manganese oxides reported in the literature showed specific capacitance as high as 600 F g1 for thin films [10,11] and 150– 300 F g1 for powder-based electrodes [12] within a potential window of 0.9–1.2 V in aqueous electrolytes containing KCl, K2SO4, or Na2SO4, KOH. These values are far from the theoretical SC of 1370 F g1 [13]. The capacitance of MnO2 electrode is believed to be predominant due to pseudocapacitance, which is attributed to reversible redox transitions involving exchange of protons and/or cations with the electrolyte [13–14]. However, the resistivity and the equivalent series resistance (ESR) of MnO2 electrode are very large. Therefore, its capacity is limited. In order to overcome this disadvantage, the composite electrode materials of the manganese oxide were prepared with a conducting additive such as carbon material (graphite, carbon nanotube, porous carbon, activated carbon, and carbon aerogel, etc.) [15–20], conducting polymers [5– 9,21–22], metal oxides [23–25], etc. Prasad and Miura [25] reported that the thin films of nickel–manganese oxides synthesized by electrochemical method had fairly high specific capacitance (621 F g1), excellent stability and long cycle life. Such thin films could have shown higher SC values, but they would be suffering from poor energy density values. Higher energy density is
E.-H. Liu et al. / Materials Research Bulletin 44 (2009) 1122–1126
also anticipated for the supercapacitors, which is seemingly difficult with powder-based electrodes materials. So, the challenge with this composite lies in maximizing its electrochemical utilization. The aim of this study is to disperse the NiO/MnO2 effectively and to maximize its electrochemical utilization for charge-storage. For this purpose, nanostructured nickel–manganese oxides composite was prepared by the sol–gel and the chemistry deposition combination new route. Material characterization was carried out by XRD, SEM, BET, cyclic voltammetry, and charge/discharge cycling. The results showed that higher the specific capacitance values, higher power density, and stable electrochemical properties of the prepared NiO/ MnO2 composite were demonstrated. To our knowledge, using this method synthesized NiO/MnO2 composite as electrode for electrochemical supercapacitors has not yet been reported.
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hydrogel without further treatment, was used as reactant in the next step experiment. As-prepared Ni(OH)2 hydrogel was added to 0.2 mol L1 KMnO4 solution which was pretreated with pH 10.3 NaHCO3–Na2CO3 buffer, the mixture was treated in an ultrasonic bath for 30 min to obtain a homogeneous suspension. Subsequently, predesigned Mn(CH3COO)2 solution was slowly dropped into the above mixed suspension solution. The mixed solution was stirred at 70 8C for 10 h. The precipitate, a dark powder, was filtered and washed thoroughly with distilled water till the pH of the filtrate reached 7.0, and dried at the temperature of 80 8C for 24 h in air condition, they were last annealed at the temperature of 300 8C for 2 h. Thus NiO/MnO2 was obtained. According to the loading ratio of nickel oxide, these samples were referred to as the mass ratio of 0–25%. Pure manganese oxide and pure nickel oxide were also prepared for a comparison.
2. Experimental 2.2. Structural characterization 2.1. Synthesis of Ni(OH)2 hydrogel Reagent grade chemicals were obtained commercially and used without further purification unless noted otherwise. The Ni(OH)2 hydrogel was synthesized according to the method reported by Wu et al. [26]. For a typical procedure, NiSO4, NaOC2H5 (in a mole ratio of 1:2) and dehydrated ethanol (EtOH) were added to a threenecked flask equipped with a nitrogen purge, a cooling water condenser, and a magnetic stirrer. The mixture was heated at 75 8C, kept refluxed for 4 h, and the precipitate was filtered out. The resulting transparent solution was cooled to room temperature. While vigorously stirring, a 0.5 mol L1 solution of NH3H2O was added dropwise to get a sol, and the sol was then left 72 h to the formation of a Ni(OH)2 hydrogel eventually. Prepared Ni(OH)2
The X-ray diffraction (XRD) patterns of samples were recorded on a diffractometer (D/MAX-3C) with Cu Ka radiation (l = 1.5406 A˚) and a graphite monochromator at 40 kV and 30 mA. The scanning electron microscope (SEM) images of samples were performed on a LEO1525 microscopy. The surface area of the as-prepared samples was characterized by the adsorption/desorption of nitrogen at 77 K using a NOVA2200 (Quantachrome) automatic adsorption unit. 2.3. Electrode preparation and electrochemical characterization The mass ratio of active materials/acetylene black (AB)/ polyvinylidene fluoride (PVDF) was 77:15:8. First, the active
Fig. 1. SEM images for (a) the precursor of nickel oxides–manganese oxides; (b) NiO/MnO2 composite (with 20 wt.% NiO); (c) MnO2; and (d) NiO.
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materials and acetylene black were mixed and grinded thoroughly, and then the powder mixed well with PVDF which was dissolved in N-methyl-2-pyrrolidone (NMP) to form a slurry. The mixed slurry was pressed onto nickel foam current collectors (F1.0 cm) and dried at 90 8C for 10 h to fabricate electrodes. Mass of the active materials was about 10 mg. The electrochemical experiments were carried out in a threecompartment cell. The prepared active material was used as the working electrode. The Pt sheet was used as the counter electrode. The saturated calomel electrode (SCE) was used as the reference electrode. All the measurements were carried out in a 6 mol L1 KOH electrolyte. Cyclic voltammetry(CV) and galvanostatic charge/discharge tests were performed using a CHI660A electrochemical workstation (CH Instrument, USA).
characteristic peaks at 37.1o and 66.3o endorse the presence of MnO2. It is clearly seen that these peaks of MnO2 are broad and unclear which indicates the amorphous nature of the products. The XRD patterns of MnO2 correspond to crystalline a-MnO2. The result is in accordance with the standard spectrum (JCPDS, Card No. 44-0141). In Fig. 2b, the XRD patterns exhibit the characteristic peaks of rock salt NiO at 2u = 37.09 (1 1 1), 43.10 (2 0 0) and 62.58o (2 2 0), the result is in accordance with the standard spectrum (JCPDS Card No. 4-0835). In NiO/MnO2 composite, the characteristic peaks of NiO and MnO2 are demonstrated in Fig. 2c. Broad peaks suggest the poorly crystallized structure of the as-prepared materials.
3. Results and discussion
The CV is considered to be a suitable tool to indicate the capacitive behavior of any material. A large magnitude of the current and a rectangular type of voltammogram curves, symmetric in anodic and cathodic directions, are the indications of ideal capacitive nature of electrode. The current response rate at the potential reverse point, the so-called rectangularity of CV, is reflected by the kinetic reversibility in electrochemical system. It depends on the reaction constant of the electrochemical Faraday action [27]. Fig. 3 showed the CVs of as-prepared material with different NiO contents at a scan rate of 10 mV s1. In Fig. 3, all CVs showed almost a mirror image with respect to the zero-current line and a rapid current response on voltage reversal at each end potential; namely, the nearly rectangular-like and symmetric I–E responses were observed, indicating ideal capacitive behavior of the materials. The specific capacitance of as-prepared electrodes were estimated from the following equation [13]:
3.1. Structure characterization Fig. 1 shows SEM images of the pure MnO2, the pure NiO, the precursor of NiO/MnO2 and NiO/MnO2 composites. All SEM images were obtained at 100,000 magnification. The average diameter of agglomerate particles was about 10–50 nm. The particles are uniform in size which are well connected yet provide porous structure which is much required for supercapacitors. As was shown in Fig. 1(b), the morphology of NiO/MnO2 remained their original appearances. The change of surface area with increase of NiO in the NiO/ MnO2 composite is shown in Table 1. With increase of NiO in the NiO/MnO2 composite leads to an increase in the BET surface area from 130 to 190 m2 g1. It is important to note that surface area of the electrode material affect the SC. The redox reaction of the electrochemical supercapacitor is primarily surface mechanism and is hence highly dependent on the surface area of the electrode material. As-prepared samples exhibited higher surface area, nanostructured and microporous characteristic (Fig. 1 and Table 1), which favors the anion/cation diffusion and adsorption/desorption during the charge/discharge process. The XRD patterns of pure NiO, pure MnO2 and NiO/MnO2 with 20 wt.% NiO were shown in Fig. 2, respectively. In Fig. 2a, the two Table 1 The surface area of the as-prepared samples. NiO content (%) Surface area (m2 g1)
0 130
100 160
5 148
10 164
15 179
20 190
3.2. Electrochemical characterization
C¼
I
ym
(1)
where I, m and y are the current of middle potential of the CV curves (A), mass of the active material (g), and the scan rate (V s1). Table 2 tabulated the specific capacitances of the NiO/MnO2 composite with different NiO contents, which were measured in 6 mol L1 KOH solution and calculated according to Eq. (1). The increase in the specific capacitance of composite electrodes loading with NiO was obviously observed. When the ratio was less than 10%, the specific capacitance of the composite electrodes
25 186
Fig. 2. XRD patterns for: (a) MnO2; (b) NiO; and (c) NiO/MnO2 with 20 wt.% NiO.
Fig. 3. CV of different composite electrodes, scan rate: 10 mV s1. (a) MnO2; (b) NiO; (c) 5%NiO; (d) 10%NiO; (e) 25%NiO; (f) 20%NiO; and (g) 15%NiO.
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1125
Table 2 The capacitances of the NiO/MnO2 composite electrodes, scan rate: 10 mV s1. NiO content (%) Capacitance (F g1)
0 330
100 209
5 347
10 360
15 382
20 453
25 401
increased very slightly (347 ! 360 F g1), perhaps the electrochemical properties of the composite electrodes had not yet been in a best situation under such a ratio. When up to 20%, the specific capacitance of the composite electrodes increased to the maximum (453 F g1) at a scan rate of 10 mV s1, which is 37.0% higher than pure MnO2 (330 F g1), the result is better than that reported by Kim and Popov [28]. The enhanced specific capacitance may be attributed to synergic effects from each pristine component. On the other hand, may be attributed to the higher specific surface area, and electrochemical stability all reached a best situation, which more available active site could be obtained and more energy could be extracted from the composite. The specific capacitance of the composite electrodes, however, decreased a lot (453 ! 401 F g1) when the NiO ratio increased to 25% compared to that of 20%, this may be attributed to the increase in particle size of NiO, aggregation of NiO, decrease of the path of ions, etc. Fig. 4 showed the variation of the specific capacitance with the scan rate of CV for pure MnO2, pure NiO and NiO/MnO2 with 20 wt.% NiO composite electrodes. We could see from the figure that maximum SC values of 358, 238 and 480 F g1 were obtained at a scan rate of 5 mV s1, respectively. Interestingly, SC value of 282 F g1 was obtained even at a high CV scan rate of 100 mV s1 with the composite electrodes, while only 176 F g1 capacitance was obtained from pure MnO2 electrode. Here the role of the added NiO was proved. Fig. 5 presented charge/discharge curves for pure MnO2, pure NiO and NiO/MnO2 with 20 wt.% NiO composite electrodes, in the potential range between 0 and 0.8 V at a current density of 2 A g1 in 6 mol L1 KOH electrolyte. It could be found from the curves that the voltage varies nearly linearly with time, which indicates good capacitive behavior. The average specific capacitance of composite electrodes can be calculated on the basis of Eq. (2) [13]: C¼
I ðdV=dtÞm
(2)
where I (mA) and dV/dt (mV s1), respectively, denote the applied galvanostatic current and the slope of these chronopotentionmetric curve when the curve is approximately linear and
Fig. 5. Charge/discharge curves of (a) MnO2, (b) NiO and (c) 20%NiO composite electrodes at current density of 2 A g1 in 6 mol L1 KOH electrolyte.
symmetric, and m (g) represents the mass of electroactive materials. The average specific capacitances for pure MnO2, pure NiO and NiO/MnO2 with 20 wt.% NiO composite electrodes, which were obtained from charge/discharge curves on the basis of Eq. (2) are 338, 218 and 460 F g1, respectively. The result is identical with that estimated from the CV curves. It is most likely that the electrochemical characteristics of the composite materials were improved as the result of the addition of NiO to manganese oxide. 4. Conclusions It was found that introducing a proper proportion of the nickel oxide into manganese oxide to form an electro-active material is an effective method to achieve high performance electrochemical capacitor. The enhanced specific capacitance may be attributed to synergic effects from each pristine component. On the other hand, may be attributed to the higher specific surface area, and electrochemical stability, all reached a best situation, which more available active site could be obtained and more energy could be extracted from the composite. XRD, SEM, BET, CV, and galvanostatic charge/discharge were employed to characterize composite materials. When the ratio of NiO was 20 wt.%. the composite electrodes exhibited the maximum specific capacitance of 453 F g1 in 6 mol L1 KOH electrolyte at the scan rate 10 mV s1, 1, which is 37.0% higher than pure manganese oxide electrode. Moreover, the composite electrodes showed high power density and stable electrochemical properties, which indicates that it is a promising material for electrochemical capacitors. Acknowledgements The authors are grateful for Project supported by Hunan Provincial Natural Science Foundation of China (07JJ6015) and by the postdoctoral fund of Xiangtan University. References
Fig. 4. The specific capacitance of composite electrode against the scan rate of CV.
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