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Synthesis of MnO2 particles under slow cooling process and their capacitive performances
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of MnO2 particles under slow cooling process and their capacitive performances Rusi Rusi, S.R. Majid n Centre for Ionics University Malaya, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 21 March 2013 Accepted 21 June 2013
The focus of this work is to investigate the capacitive cycle behaviour of MnO2 particles deposited on stainless steel foil after slow and fast cooling processes as electrode candidate material in redox supercapacitors using charge/discharge and cyclic voltammetry at different scan rates. XRD results have proven the formation of MnO2 particles. SEM and TEM are also employed to characterise the nature of produced particles, which confirm that a spherical and flower-like nanostructure of MnO2 was obtained after fast and slow cooling processes. A specific capacitance of 180 F g−1 is obtained at a scan rate of 1 mV s−1 in 0.5 M KOH aqueous electrolyte. The proposed electrode shows excellent long-term cyclic stability while maintaining a small internal resistance. & 2013 Published by Elsevier B.V.
Keywords: Electrodeposition Amorphous materials Energy storage and conversion
1. Introduction Electrochemical supercapacitors are being investigated in a wide range of energy storage applications due to their potential for use in next generation power devices, and they have been categorised as double-layer capacitance and pseudocapacitance [1,2]. To date, great efforts have been devoted to develop more practical pseudocapacitive materials, including NiO, V2O5, RuO2, MnO2, etc. [3–5]. Most researchers have focused on ruthenium oxide (RuO2), which displays a fairly high specific capacitance (∼700 F g−1), but its use is severely limited by its high cost, which derives from the scarcity of Ru. Conversely, MnO2 is cheaper and less toxic. In fact, it has a wide window of operating potential in mild electrolyte and higher theoretical specific capacitance (∼1370 F g−1). For these reasons, MnO2 has gained much attention as a pseudocapacitive electrode material. Past research has indicated that several MnO2 preparation methods can be employed, such as sol–gel, chemical co-precipitation, chemical vapour deposition (CVD) and electrophoretic deposition (EPD) [6–8]. The properties of MnO2 are determined by crystalline structure, particle size, shape and surface area. According to Babakhani and Ivey [1], rod-like structures of MnO2 electrodes synthesised via anodic deposition show a specific capacitance of 185 F g−1. In the work of Yousefi et al. [9], an electrode from MnO2 nanowire prepared using cathodic electrodeposition exhibited specific capacitance of 237 F g−1 at a scan rate of 10 mV s−1. This paper reports on a simple preparation of spherical and flower-like
MnO2 particles deposited on stainless steel (SS) substrate from Mn (CH3COO)2 4H2O electrolyte solution at different concentrations as an electrode material for redox supercapacitors. The deposited MnO2 particles were evaluated as electrodes for redox capacitors by cyclic voltammetry and galvanostatic charge/discharge cycling in a potential range from 0 to 0.65 V in 0.5 M KOH electrolyte solution. 2. Experimental procedure Prior to the electrodeposition process, different molarities of Mn(CH3COO)2 4H2O were prepared in distilled water and stirred for 30 min to form a homogeneous solution at room temperature. The electrodeposition on 4 cm2 stainless steel was performed using Autolab PGSTAT12, the current density and deposition time were fixed at 2 mA cm−2 and 10 min respectively. The stainless steel foil was placed at 1 cm distance from a carbon rod and AgCl was used as a reference electrode. The deposits were rinsed with distilled water before being annealed at 300 1C for 6 h. X-ray powder diffraction (XRD) analysis was performed on a Siemens D5005 Diffractometer with CuKα radiation (λ ¼1.5418 Å). Scanning electron microscopy (SEM) and Transmission electron microscopic (TEM) images were obtained using Quanta FEG250 and JEM-2100F microscopes. Cyclic voltammetry and charge/discharge cycling were conducted using CHI600D and Neware instruments. 3. Results and discussion
n
Corresponding author. Tel.: +60 379674238; fax: +60 379674146. E-mail address: [email protected] (S.R. Majid).
The structure of manganese dioxide coating can be influenced by parameters such as complex agents, deposition current density,
0167-577X/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2013.06.069
Please cite this article as: Rusi R, Majid SR. Synthesis of MnO2 particles under slow cooling process and their capacitive performances. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.06.069i
R. Rusi, S.R. Majid / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
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voltage and pH [10]. In this work, the effect of the cooling process can be determined from the morphologies of as-prepared MnO2 samples after slow and fast cooling processes, as characterised by SEM, shown in Fig. 1. The as-prepared MnO2 after fast cooling for 0.0025 M and 0.01 M Mn(CH3COO)2 4H2O, Fig. 1(a–b) has produced uniform spherical particles with diameters of about 250 to 380 nm. The surface morphology of MnO2 electrode prepared from a higher concentration of Mn(CH3COO)2 4H2O is more compact and cracked, Fig. 1(c). This is attributable to a higher deposited mass load, which leads to poor contact with the stainless steel, thus resulting in the increased resistance of the electrode surface during the cycling test. As can be seen from Fig. 1(d), a comparatively different structure of deposited MnO2 from the 0.01 M Mn (CH3COO)2 4H2O sample is obtained after the slow cooling process, namely the flower-like structure [11]. This structure is obvious from the TEM image, Fig. 1(e), as is the presence of radially aligned spikes with 1.0 to 1.5 nm diameter, on average, and 30 to 40 nm in length, on average, resulting in the formation of flowerlike morphology. The crystalline state of the studied sample was analysed by XRD. The XRD pattern of deposited MnO2 thin films on SS substrates is shown in Fig. 2. No distinct diffraction peak other than for the SS substrate can be detected in the XRD pattern, which probably indicates that the film consisted of MnO2 colloidal particles in amorphous phase [12] and the thinness of the asprepared film [13]. In the XRD pattern of scraped MnO2, the presence of peaks at 2θ ¼29.1, 37.3, 42.5 and 56.6 confirms the formation of α-MnO2, indexed by the circles (JCPDS NO. 44-0141) [14]. The broad and low intensity of the peaks implies the amorphous nature of MnO2 [15], which is feasible for supercapacitor application due to easy penetration of ions through the bulk of the active material [16]. Comparative electrochemical characterisation was conducted to investigate the electrode performance of as-prepared manganese dioxide produced after fast and slow cooling processes in 0.5 M KOH aqueous electrolyte solution at a fixed scan rate of 5 mV s−1, as depicted in Fig. 3 (a– b). In the potential range from −0.35to 0.65 V, the CV characteristics of electrodes produced after fast and slow cooling processes are a box-like shape indicating the pseudocapacitive behaviour of the electrodes. The electrolyte, KOH, can work at a voltage up to
1µm
∼0.6 V, which is agreeable with the potential window of the CV curves. Above this voltage, faradaic process starts to take place due to oxygen evolution reaction [17]. The specific capacitance (CS) value can be calculated using the equation C¼ i/(sm), where i is the current, s is the slope of the discharge curve and m is the mass of the electrode. When the concentration of electrolyte deposition are 0.025 M, 0.005 M and 0.01 M the deposited mass loads are 4.3 10−5 g, 1.4 10−4 g and 8.2 10−4 g at fast cooling while 4 10−5 g, 1.28 10−4 g, 9.4 10−4 g at slow cooling, respectively. The CS of as-prepared MnO2 electrodes produced after fast cooling is lower than that of electrodes produced after slow cooling. The optimised CS, 143 Fg−1, is obtained when a 0.01 M sample is used to prepare MnO2 electrode after undergoing slow cooling and is 18% higher than the capacitance of an electrode produced after fast cooling, 133 Fg−1. The CS values for lower concentration samples, i.e. 0.005 and 0.025 M, are 128 and 116 Fg−1 and 115 and 108 Fg−1 for slow and fast cooling, respectively. The best MnO2 sample, produced after slow cooling from 0.01 M, was tested at different scan rates, as illustrated in Fig. 3(c), and the CS values were calculated, as shown in Fig. 3(d). The CS of the MnO2 electrode produced after slow cooling was 180 Fg−1 at a scan rate of 1 mV s−1 and 142 Fg−1 at a higher scan rate of 7 mV s−1. To evaluate the stability of the MnO2 electrode produced after a slow cooling process, constant-current charge/discharge curves of the as-prepared electrode at 1 mA constant current are shown in Fig. 3 (e). The charge–discharge curves exhibit a symmetric shape, 211 310
301
600 α-MnO2 SS
0.0025M 0.005M 0.01M
Fig. 2. XRD pattern of deposited MnO2 on SS.
1µm
1µm
5µm Fig. 1. SEM of deposited MnO2 produced after fast cooling (a) 0.0025 M, (b) 0.01 M and (c) 0.02 M, after slow cooling (d) 0.01 M (e) TEM image of 0.01 M.
Please cite this article as: Rusi R, Majid SR. Synthesis of MnO2 particles under slow cooling process and their capacitive performances. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.06.069i
R. Rusi, S.R. Majid / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
3
Fig. 3. CV of MnO2 electrode (a) fast cooling (b) slow cooling (c) 0.01 M (slow cooling), (d) CS of MnO2 prepared from 0.01 M (slow cooling) at different scan rates (e) charge–discharge curve of MnO2 at different concentration samples at 1 mA constant current and (f) CS versus cycle number.
implying that the flower-like structure has a good electrochemical capacitive characteristic. As the cycle lifetime is one of the limits in supercapacitor applications, a cyclic stability test for 1000 cycles for the flower-like MnO2/stainless steel electrode was carried out in a potential window ranging from 0 to 0.65 V, and the values of CS with respect to the cycle numbers are shown in Fig. 3(f). The CS increases slightly until the 300th cycle and is consistent until the 1000th cycle, which can be attributed to the activation effect of electrochemical cycling. The MnO2 exhibits a good long cycle life with about 98% initial CS retained after the 1000th cycle. There is no indication of detachment deposited MnO2 from electrode surfaces and, moreover, during the cycling process, the coulombic efficiency (discharge capacitance/charge capacitance) remains at 98%. This good electrochemical stability reveals that a highly reversible redox reaction took place between the electrolyte and MnO2 electrode.
Acknowledgements This project was funded by the University of Malaya (HIRGJ21002-73852) and Rusi thanks the Skim Bright Sparks University Malaya (SBSUM) for the scholarship awarded. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusion
[11] [12]
In summary, using a simple electrodeposition method, spherical and flower-like MnO2 was successfully prepared after fast and slow cooling processes. Owing to its unique structure, the CS of the flower-like MnO2 electrode is as high as 180 F g−1 at a scan rate of 1 mV s−1 in 0.5 M KOH solution. Our work gives a new insight into synthesising electrode materials for supercapacitors and other energy-storage devices.
[13] [14] [15] [16] [17]
Babakhani B, Douglas GI. J Power Source 2010;195:2110–7. Wei WF, Cui XW, Chen WX, Ivey DG. Chem Soc Rev 2011;40:1697–721. Lia J, Yang QM, Zhitomirskya I. J Power Sources 2008;185:1569–74. Salunkhe RR, Jang K, Yu H, Yu S, Ganesh T, Han SH, et al. J Alloys Compd 2011;509:6677–82. Zhang Y, Feng H, Wu XB, Wang LZ, Zhang AQ, Xia TC, et al. Int J Hydrogen Energy 2009;34:4889–99. Reddy RN, Reddy RG. J Power Sources 2003;124:330–7. Lee HY, Manivannan V, Goodenough JB. C R Chim 1999;2:565–77. Kang YJ, Kim B, Chung H, Kim W. Synth Met 2010;160:2510–4. Yousefi T, Golikand AN, Mashhadizadeh MH, Aghazadeh M. Curr Appl Phys 2012;12:193–8. Lin HY, Sun YP, Weng BJ, Yang CT, Suen NT, Liao KH, et al. Electrochim Acta 2007;52:6548–53. Shi F, Wang F, Dai H, Dai J, Deng J, Liu Y, et al. Appl Catal A Gen 2012;433–434:206–13. Dubal DP, Dhawale DS, Gujar TP, Lokhande CD. Appl Surf Sci 2011;257:3378–82. Takahashi K, Wang Y, Cao GZ. J Phys Chem B 2005;109:48–51. Jiang RR, Huang T, Liu JL, Zhuang JH, Yu AS. Electrochim Acta 2009;54:3047–52. Wang JG, Yang Y, Huang ZH, Kang F. J Power Sources 2013;224:86–92. Gujar TP, Shinde VR, Lokhande CD, Kim WY, Jung KD, Joo OS. Electrochem Commun 2007;9:504–10. Iwakura C, Nohara S, Furukawa N, Inoue H. Solid State Ionics 2002;148:487–92.
Please cite this article as: Rusi R, Majid SR. Synthesis of MnO2 particles under slow cooling process and their capacitive performances. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.06.069i
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Q1 16 17Q3 Q2 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of MnO2 particles under slow cooling process and their capacitive performances Rusi Rusi, S.R. Majid n Centre for Ionics University Malaya, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 21 March 2013 Accepted 21 June 2013
The focus of this work is to investigate the capacitive cycle behaviour of MnO2 particles deposited on stainless steel foil after slow and fast cooling processes as electrode candidate material in redox supercapacitors using charge/discharge and cyclic voltammetry at different scan rates. XRD results have proven the formation of MnO2 particles. SEM and TEM are also employed to characterise the nature of produced particles, which confirm that a spherical and flower-like nanostructure of MnO2 was obtained after fast and slow cooling processes. A specific capacitance of 180 F g−1 is obtained at a scan rate of 1 mV s−1 in 0.5 M KOH aqueous electrolyte. The proposed electrode shows excellent long-term cyclic stability while maintaining a small internal resistance. & 2013 Published by Elsevier B.V.
Keywords: Electrodeposition Amorphous materials Energy storage and conversion
1. Introduction Electrochemical supercapacitors are being investigated in a wide range of energy storage applications due to their potential for use in next generation power devices, and they have been categorised as double-layer capacitance and pseudocapacitance [1,2]. To date, great efforts have been devoted to develop more practical pseudocapacitive materials, including NiO, V2O5, RuO2, MnO2, etc. [3–5]. Most researchers have focused on ruthenium oxide (RuO2), which displays a fairly high specific capacitance (∼700 F g−1), but its use is severely limited by its high cost, which derives from the scarcity of Ru. Conversely, MnO2 is cheaper and less toxic. In fact, it has a wide window of operating potential in mild electrolyte and higher theoretical specific capacitance (∼1370 F g−1). For these reasons, MnO2 has gained much attention as a pseudocapacitive electrode material. Past research has indicated that several MnO2 preparation methods can be employed, such as sol–gel, chemical co-precipitation, chemical vapour deposition (CVD) and electrophoretic deposition (EPD) [6–8]. The properties of MnO2 are determined by crystalline structure, particle size, shape and surface area. According to Babakhani and Ivey [1], rod-like structures of MnO2 electrodes synthesised via anodic deposition show a specific capacitance of 185 F g−1. In the work of Yousefi et al. [9], an electrode from MnO2 nanowire prepared using cathodic electrodeposition exhibited specific capacitance of 237 F g−1 at a scan rate of 10 mV s−1. This paper reports on a simple preparation of spherical and flower-like
MnO2 particles deposited on stainless steel (SS) substrate from Mn (CH3COO)2 4H2O electrolyte solution at different concentrations as an electrode material for redox supercapacitors. The deposited MnO2 particles were evaluated as electrodes for redox capacitors by cyclic voltammetry and galvanostatic charge/discharge cycling in a potential range from 0 to 0.65 V in 0.5 M KOH electrolyte solution. 2. Experimental procedure Prior to the electrodeposition process, different molarities of Mn(CH3COO)2 4H2O were prepared in distilled water and stirred for 30 min to form a homogeneous solution at room temperature. The electrodeposition on 4 cm2 stainless steel was performed using Autolab PGSTAT12, the current density and deposition time were fixed at 2 mA cm−2 and 10 min respectively. The stainless steel foil was placed at 1 cm distance from a carbon rod and AgCl was used as a reference electrode. The deposits were rinsed with distilled water before being annealed at 300 1C for 6 h. X-ray powder diffraction (XRD) analysis was performed on a Siemens D5005 Diffractometer with CuKα radiation (λ ¼1.5418 Å). Scanning electron microscopy (SEM) and Transmission electron microscopic (TEM) images were obtained using Quanta FEG250 and JEM-2100F microscopes. Cyclic voltammetry and charge/discharge cycling were conducted using CHI600D and Neware instruments. 3. Results and discussion
n
Corresponding author. Tel.: +60 379674238; fax: +60 379674146. E-mail address: [email protected] (S.R. Majid).
The structure of manganese dioxide coating can be influenced by parameters such as complex agents, deposition current density,
0167-577X/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2013.06.069
Please cite this article as: Rusi R, Majid SR. Synthesis of MnO2 particles under slow cooling process and their capacitive performances. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.06.069i
R. Rusi, S.R. Majid / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
voltage and pH [10]. In this work, the effect of the cooling process can be determined from the morphologies of as-prepared MnO2 samples after slow and fast cooling processes, as characterised by SEM, shown in Fig. 1. The as-prepared MnO2 after fast cooling for 0.0025 M and 0.01 M Mn(CH3COO)2 4H2O, Fig. 1(a–b) has produced uniform spherical particles with diameters of about 250 to 380 nm. The surface morphology of MnO2 electrode prepared from a higher concentration of Mn(CH3COO)2 4H2O is more compact and cracked, Fig. 1(c). This is attributable to a higher deposited mass load, which leads to poor contact with the stainless steel, thus resulting in the increased resistance of the electrode surface during the cycling test. As can be seen from Fig. 1(d), a comparatively different structure of deposited MnO2 from the 0.01 M Mn (CH3COO)2 4H2O sample is obtained after the slow cooling process, namely the flower-like structure [11]. This structure is obvious from the TEM image, Fig. 1(e), as is the presence of radially aligned spikes with 1.0 to 1.5 nm diameter, on average, and 30 to 40 nm in length, on average, resulting in the formation of flowerlike morphology. The crystalline state of the studied sample was analysed by XRD. The XRD pattern of deposited MnO2 thin films on SS substrates is shown in Fig. 2. No distinct diffraction peak other than for the SS substrate can be detected in the XRD pattern, which probably indicates that the film consisted of MnO2 colloidal particles in amorphous phase [12] and the thinness of the asprepared film [13]. In the XRD pattern of scraped MnO2, the presence of peaks at 2θ ¼29.1, 37.3, 42.5 and 56.6 confirms the formation of α-MnO2, indexed by the circles (JCPDS NO. 44-0141) [14]. The broad and low intensity of the peaks implies the amorphous nature of MnO2 [15], which is feasible for supercapacitor application due to easy penetration of ions through the bulk of the active material [16]. Comparative electrochemical characterisation was conducted to investigate the electrode performance of as-prepared manganese dioxide produced after fast and slow cooling processes in 0.5 M KOH aqueous electrolyte solution at a fixed scan rate of 5 mV s−1, as depicted in Fig. 3 (a– b). In the potential range from −0.35to 0.65 V, the CV characteristics of electrodes produced after fast and slow cooling processes are a box-like shape indicating the pseudocapacitive behaviour of the electrodes. The electrolyte, KOH, can work at a voltage up to
1µm
∼0.6 V, which is agreeable with the potential window of the CV curves. Above this voltage, faradaic process starts to take place due to oxygen evolution reaction [17]. The specific capacitance (CS) value can be calculated using the equation C¼ i/(sm), where i is the current, s is the slope of the discharge curve and m is the mass of the electrode. When the concentration of electrolyte deposition are 0.025 M, 0.005 M and 0.01 M the deposited mass loads are 4.3 10−5 g, 1.4 10−4 g and 8.2 10−4 g at fast cooling while 4 10−5 g, 1.28 10−4 g, 9.4 10−4 g at slow cooling, respectively. The CS of as-prepared MnO2 electrodes produced after fast cooling is lower than that of electrodes produced after slow cooling. The optimised CS, 143 Fg−1, is obtained when a 0.01 M sample is used to prepare MnO2 electrode after undergoing slow cooling and is 18% higher than the capacitance of an electrode produced after fast cooling, 133 Fg−1. The CS values for lower concentration samples, i.e. 0.005 and 0.025 M, are 128 and 116 Fg−1 and 115 and 108 Fg−1 for slow and fast cooling, respectively. The best MnO2 sample, produced after slow cooling from 0.01 M, was tested at different scan rates, as illustrated in Fig. 3(c), and the CS values were calculated, as shown in Fig. 3(d). The CS of the MnO2 electrode produced after slow cooling was 180 Fg−1 at a scan rate of 1 mV s−1 and 142 Fg−1 at a higher scan rate of 7 mV s−1. To evaluate the stability of the MnO2 electrode produced after a slow cooling process, constant-current charge/discharge curves of the as-prepared electrode at 1 mA constant current are shown in Fig. 3 (e). The charge–discharge curves exhibit a symmetric shape, 211 310
301
600 α-MnO2 SS
0.0025M 0.005M 0.01M
Fig. 2. XRD pattern of deposited MnO2 on SS.
1µm
1µm
5µm Fig. 1. SEM of deposited MnO2 produced after fast cooling (a) 0.0025 M, (b) 0.01 M and (c) 0.02 M, after slow cooling (d) 0.01 M (e) TEM image of 0.01 M.
Please cite this article as: Rusi R, Majid SR. Synthesis of MnO2 particles under slow cooling process and their capacitive performances. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.06.069i
R. Rusi, S.R. Majid / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
3
Fig. 3. CV of MnO2 electrode (a) fast cooling (b) slow cooling (c) 0.01 M (slow cooling), (d) CS of MnO2 prepared from 0.01 M (slow cooling) at different scan rates (e) charge–discharge curve of MnO2 at different concentration samples at 1 mA constant current and (f) CS versus cycle number.
implying that the flower-like structure has a good electrochemical capacitive characteristic. As the cycle lifetime is one of the limits in supercapacitor applications, a cyclic stability test for 1000 cycles for the flower-like MnO2/stainless steel electrode was carried out in a potential window ranging from 0 to 0.65 V, and the values of CS with respect to the cycle numbers are shown in Fig. 3(f). The CS increases slightly until the 300th cycle and is consistent until the 1000th cycle, which can be attributed to the activation effect of electrochemical cycling. The MnO2 exhibits a good long cycle life with about 98% initial CS retained after the 1000th cycle. There is no indication of detachment deposited MnO2 from electrode surfaces and, moreover, during the cycling process, the coulombic efficiency (discharge capacitance/charge capacitance) remains at 98%. This good electrochemical stability reveals that a highly reversible redox reaction took place between the electrolyte and MnO2 electrode.
Acknowledgements This project was funded by the University of Malaya (HIRGJ21002-73852) and Rusi thanks the Skim Bright Sparks University Malaya (SBSUM) for the scholarship awarded. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusion
[11] [12]
In summary, using a simple electrodeposition method, spherical and flower-like MnO2 was successfully prepared after fast and slow cooling processes. Owing to its unique structure, the CS of the flower-like MnO2 electrode is as high as 180 F g−1 at a scan rate of 1 mV s−1 in 0.5 M KOH solution. Our work gives a new insight into synthesising electrode materials for supercapacitors and other energy-storage devices.
[13] [14] [15] [16] [17]
Babakhani B, Douglas GI. J Power Source 2010;195:2110–7. Wei WF, Cui XW, Chen WX, Ivey DG. Chem Soc Rev 2011;40:1697–721. Lia J, Yang QM, Zhitomirskya I. J Power Sources 2008;185:1569–74. Salunkhe RR, Jang K, Yu H, Yu S, Ganesh T, Han SH, et al. J Alloys Compd 2011;509:6677–82. Zhang Y, Feng H, Wu XB, Wang LZ, Zhang AQ, Xia TC, et al. Int J Hydrogen Energy 2009;34:4889–99. Reddy RN, Reddy RG. J Power Sources 2003;124:330–7. Lee HY, Manivannan V, Goodenough JB. C R Chim 1999;2:565–77. Kang YJ, Kim B, Chung H, Kim W. Synth Met 2010;160:2510–4. Yousefi T, Golikand AN, Mashhadizadeh MH, Aghazadeh M. Curr Appl Phys 2012;12:193–8. Lin HY, Sun YP, Weng BJ, Yang CT, Suen NT, Liao KH, et al. Electrochim Acta 2007;52:6548–53. Shi F, Wang F, Dai H, Dai J, Deng J, Liu Y, et al. Appl Catal A Gen 2012;433–434:206–13. Dubal DP, Dhawale DS, Gujar TP, Lokhande CD. Appl Surf Sci 2011;257:3378–82. Takahashi K, Wang Y, Cao GZ. J Phys Chem B 2005;109:48–51. Jiang RR, Huang T, Liu JL, Zhuang JH, Yu AS. Electrochim Acta 2009;54:3047–52. Wang JG, Yang Y, Huang ZH, Kang F. J Power Sources 2013;224:86–92. Gujar TP, Shinde VR, Lokhande CD, Kim WY, Jung KD, Joo OS. Electrochem Commun 2007;9:504–10. Iwakura C, Nohara S, Furukawa N, Inoue H. Solid State Ionics 2002;148:487–92.
Please cite this article as: Rusi R, Majid SR. Synthesis of MnO2 particles under slow cooling process and their capacitive performances. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.06.069i