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Enhancement in electrochemical behavior of copper doped MnO2 electrode
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Enhancement in electrochemical behavior of copper doped MnO2 electrode Ramaswamy Poonguzhali a, Raju Gobi a,n, Nadana Shanmugam a, Annamalai Senthil Kumar b, Govintha Viruthagiri a, Natesan Kannadasan a a b
Department of Physics, Annamalai University, Annamalai Nagar, Chidambaram 608002, Tamilnadu, India Environmental and Analytical Chemistry Division, Vellore Institute of Technology University, Vellore 632014, India
art ic l e i nf o
a b s t r a c t
Article history: Received 2 November 2014 Received in revised form 24 February 2015 Accepted 16 May 2015
In the present work, we report a simple chemical precipitation method for the preparation of different levels of Cu doped MnO2 nanocrystals. X-ray diffraction, field emission transmission electron microscope and X-ray photoelectron spectroscopy were used to study the material properties. To demonstrate the suitability of the doped products for electrode applications electrochemical properties were evaluated by Cyclic Voltammetry (CV), galvanostatic charge–discharge studies and impedance spectroscopy. The results indicate that the MnO2 electrode modified with 0.1 M of Cu has a better electrode property with a specific capacitance of 583 F/g and an energy density of 80 W h kg 1. & 2015 Published by Elsevier B.V.
Keywords: Electrochemical capacitor Specific capacitance Tetragonal phase Nanoparticles X-ray techniques
1. Introduction
2. Experimental section
Supercapacitors which are also named as electrochemical capacitors have attracted considerable attention due to their high energy density and high efficiency [1]. Among the available transition metal oxides, MnO2 has attracted a great deal of attention for its role as an electrode material for supercapacitors [2]. An efficient approach to enhance the specific capacity of metal oxide is incorporating one or two metal ions into them to form multimetal compounds, such as Mn–Ni [3] oxide, Mn–Co [4] Mn–Fe [5], and Ni–Zn–Co oxides [6]. Several techniques have been employed so far to prepare MnO2 with controlled morphologies and structures, including, thermal decomposition method, sol–gel route, co precipitation method, and electrochemical deposition method [7]. In the present work, we report the simple precipitation method for the preparation of various levels of (0.025–0.125 M) Cu doped MnO2 nanocrystals for supercapacitor applications. The synthesized products were analyzed for X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and field emission transmission electron microscope (FE-TEM) analysis. The electro chemical performance was examined by cyclic voltammetry, charge–discharge and impedance analysis.
For the synthesis of Cu doped MnO2, 6.12 g of manganese acetate tetra hydrate was dissolved in 30 ml of deionized water and stirred vigorously by magnetic stirrer. Then Copper acetate of preferred mole (0.00, 0.025, 0.05, 0.075, 0.1 and 0.125 M) prepared in 20 ml aqueous was mixed drop wise. Further, 5.92 g of KMnO4 in 50 ml of deionized water was added drop by drop to the above mixture. After 5 h of stirring a dark brown colored precipitate of precursor MnO2 was obtained. Finally, the obtained product was annealed in a muffle furnace at 400 °C for 3 h. The crystalline phase of the products was analyzed by XRD measurement using X’PERT-PRO diffractometer. The valence state of the doped product was analyzed by the X-ray photoelectron spectroscopy (HP 5950A Hewlett-Packard). The morphology was observed by FE-TEM (JSM2100F JEOL). The electrochemical characterizations were carried out as mentioned in the previous work [8].
n
Corresponding author. E-mail address: alankoffi[email protected] (R. Gobi).
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of undoped and different levels of Cu doped MnO2 products. All the diffraction peaks correspond to tetragonal phase of α-MnO2 (JCPDS no. 440141) [9]. No peaks related to Cu or other impurities are noted up
http://dx.doi.org/10.1016/j.matlet.2015.05.086 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
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R. Poonguzhali et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
*CuO2
(521)
(211) (301)
(220) (310)
(200)
MnO2: Cu (0.125 M)
MnO2: Cu (0.1 M)
Intensity (a.u)
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(110)
2
MnO2: Cu (0.075 M) MnO2: Cu (0.05 M)
MnO2: Cu (0.025 M)
MnO2 10
20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. X-ray diffraction patterns of undoped and different levels of Cu doped MnO2 nanocrystals.
to 0.075 M of Cu incorporation which indicates the solubility limit of the dopant. However, a small peak related to CuO2 begins to appear in 0.1 M of Cu doping and the peak intensity increases with the increase of Cu content. The chemical composition of cu doped MnO2 has been analyzed by XPS and the results are shown in Fig. 2. The surface scan XPS survey of Cu doped MnO2 reveals the sharp signal of Cu 2p, Mn 2p, O 1s and K 2p as shown in Fig. 2a. Fig. 2b reveals the XPS of Mn 2p, where the presence of peaks at 642.2 eV and 653.9 eV can be indexed to Mn 2p3/2 and Mn 2p1/2 levels, respectively [10]. Especially the Cu 2p spectrum exhibits two distinguished peaks at 933.6 eV and 953.6 eV corresponding to Cu 2p3/2 and Cu 2p1/2 levels respectively (Fig. 2c). Additionally, we predict the satellite peaks of CuO2 at 946 eV and 962 eV, suggesting that the incorporated Cu is in a bivalence state (Cu2 þ ). In the O1s spectrum (Fig. 2d), the sharp peak at 529.65 eV can be attributed to the presence of oxygen in oxide form (Fig. 2d) [11]. The results of XPS are in good agreement with the results of XRD. The FE-TEM image of MnO2 exhibits complete rod formation (Fig. 3a), whereas, on doping the particles are showing both spherical and rod like morphologies with little agglomeration (Fig. 3b). The cyclic voltammograms (CV) of unmodified and modified electrodes of MnO2 recorded in 0.5 M KCl electrolyte at the scan rate of 10 mV s 1 are shown in Fig. 4a. The value of specific capacitance (Cs, F g 1) was calculated from the CV curves according to Eq. (1) [12].
Cs =
Q mΔv
(1)
At the scan rate of 10 mV s 1, the specific capacitance of the MnO2 is 210 F g 1, whereas the modified electrodes of MnO2 exhibit the specific capacitance of 177, 80, 75, 410, 40 F g 1, respectively for 0.025, 0.050, 0.075, 0.1 and 0.125 M of Cu doping. The electrode modified with 0.1 M of Cu exhibits the highest specific capacitance value of 410 F g 1 which is double that of the MnO2 electrode. Fig. 4b shows the variation in specific capacitance of prepared electrode versus the Cu content in the binary oxide. Fig. 4c shows the CV curves of MnO2: Cu (0.1 M) recorded at various scan rates (10, 20, 30, 40, 50, 70, 90 and 100 mV s 1) in 0.5 M KCl solution. By using Eq. (1), the specific capacitance values of the modified electrodes for all the scan rates have been calculated, and a graph is drawn between specific capacitance and scan rate (Fig. 4d). It is obvious that with an increase in the scan rate, the specific capacitance of all the electrode material decreases [13]. The decrease in the specific capacitance at higher scan rate is due to the diffusion effect of proton within the electrode, while at low scan rates, it is due to the occurrence of inner active sites, which undergo complete redox transitions and enhance the specific capacitance [14]. Fig. 5a shows the charge–discharge profile of MnO2 and modified electrodes of MnO2 recorded at constant discharge current density of 10 A g 1. Using the curves, the specific capacitance (Cs), can be calculated using the formula,
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
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Fig. 2. (a) XPS spectra of Cu (0.1 M) doped MnO2 nanocrystals (a) survey spectrum, (b) Mn 2p, (c) Cu 2p and (d) O 1s.
Cs =
iΔt mΔV
(2)
The specific capacitance value obtained from discharge curve for MnO2 is 200 F g 1, whereas, it is 300, 60, 20, 583, and 16 F g 1 respectively, for various levels of Cu doped MnO2 electrodes. The electrode of MnO2 modified with 0.1 M of Cu recorded the highest
specific capacitance of 583 F g 1 at a current density of 10 A g 1. Further, to calculate the rate capability, the charge–discharge curves were drawn for the MnO2 doped with 0.1 M of Cu at a current density ranging from 10 to 50 A g 1 (Fig. 5b). The specific capacitance of the composite electrode, calculated from discharge time, according to the Eq. (2), are 583, 213, 117, 78, and 55 F g 1
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
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Fig. 3. (a, b) FE-TEM images of pristine and copper (0.1 M) doped MnO2.
corresponding to the discharge current densities of 10, 20, 30, 40, and 50 A g 1 respectively. Fig. 5c shows the relation between capacitance and current, at low specific current the specific capacitance increases, due to the intercalation/deintercalation of ions at surface and inner porous of the active materials in the electrode/ electrolyte interface [15]. Even at a relatively high current density of 50 A g 1, it still remains at 55 F g 1 there by exhibiting better rate capability and higher electrochemical capacitance. The plot of energy density against power density (Ragone plot) is given in Fig. 5d. The values of power and energy densities can be estimated from the following equations [16].
E=
1 Cs ΔV 2 2
P = E/t
(
)
(3) (4)
where P, Cs, Q, Δt and E are indicating the power density (W kg 1), specific capacitance of the electroactive material (F g 1), total charge delivered (C), discharge time (s) and average energy density (W h kg 1) respectively. From the Ragone plot, it is observed that the MnO2 modified with 0.1 M of Cu exhibits the highest energy density (80 W h kg 1) as well as power density (164 W kg 1) than the unmodified MnO2. To understand the performance of modified electrode of MnO2, electrochemical impedance spectroscopy (EIS) measurements were performed. Nyquist plots of unmodified and modified electrodes are shown in Fig. 6(a, b). The semicircle at high frequency region belongs to the charge transfer resistance (Rct) of the electrode. This Rct is related to the electroactive surface area of the electrode, due to the Faradaic redox processes of the MnO2 electrodes involving in the exchange of OH ions [17]. Here, the Rct value of MnO2 is 140, whereas, it is reduced to 101 Ω for the
modified electrode. The frequency at which there is a deviation from the semicircle is known as Knee frequency, which reflects the maximum lower frequency at which capacitive behavior is dominant [18]. This confirms the high capacitance of MnO2 on doping.
4. Conclusion MnO2 and copper doped MnO2 nanocrystals have been synthesized by a chemical precipitation method. The tetragonal crystal structure of α-MnO2 was confirmed through XRD analysis. The incorporation of Cu has been confirmed from XPS patterns. The influence of copper doping on electrochemical capacitor performance of MnO2 was investigated by cyclic voltammetry and galvanostatic charge–discharge studies. Among the various concentrations of doping, the 0.1 M of Cu doped MnO2 shows a very high specific capacitance of 583 F g 1 at a constant current density of 10 A g 1. Also it has a specific energy density of 80 W h kg 1 with a power density of 164 W kg 1.
Acknowledgments The authors wish to thank Dr. S. Barathan, Professor and Head, Department of Physics, Annamalai University, for providing necessary facilities to carry out this work. We sincerely thank Mr. R. Shanmugam, Research Associate, Environmental & Analytical Chemistry Division, School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, India for the help they rendered in recording electrochemical measurements.
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
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5
Fig. 4. Electrochemical performance of MnO2 and different concentration of Cu doped MnO2 electrodes (a) CV curves at 10 mV s 1, (b) Specific capacitance of the sample as a function of the Cu content, (c) CV curves at different scan rate (10–100 mV s 1) and (d) the value of specific capacitance at different scan rate.
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
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R. Poonguzhali et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 5. Galvanostatic charge–discharge curves of the MnO2 and different concentration of Cu doped MnO2 electrodes at current density 10 A g 1 (b) Charge–discharge Curves of MnO2: Cu (0.1 M) electrode at different specific current, (c) current versus specific capacitance curve and (d) Ragone plots of energy and power densities of the MnO2 and (0.1 M) MnO2: Cu electrodes.
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
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-Zim''(ohm)
R. Poonguzhali et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Knee Frequency
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0 0
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Knee Frequency
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Zre'(ohm) Fig. 6. (a, b) Nyqusit plot obtained for MnO2 and Cu (0.1 M) doped MnO2 electrodes at 0.4 V over the frequency range 0.01 Hz to 10 KHz in 0.5 M KCl.
References [1] R. Kotz, M. Carlen, Electrochim. Acta 45 (2000) 2483–2498. [2] Songzhan Li, Jian Wen, Xiaoming Mo, Hao Long, Haoning Wang, Jianbo Wang, Guojia Fang, J. Power Sources 256 (2014) 206–211. [3] D. Galizzoli, F. Tantardini, S. Trasatti, J. Appl. Electrochem. 5 (1975) 20–214. [4] A.M. Couper, D. Pletcher, F.C. Walsh, Chem. Rev. 90 (1990) 837–865. [5] E.H. Liu, W. Li, J. Li, X.Y. Meng, R. Ding, S.T. Tan, Mater. Res. Bull. 44 (2009) 1122–1126. [6] D.P. Dubal, W.B. Kim, C.D. Lokhande, J. Phys. Chem. Solids 73 (2012) 18–24. [7] X.J. Zhang, W.H. Shi, J.X. Zhu, W.Y. Zhao, J. Ma, S. Mhaisalkar, T.L. Maria, Y. H. Yang, H. Zhang, H.H. Hng, Q.Y. Yan, Nano Res. 3 (2010) 643–652. [8] Ramaswamy Poonguzhali, Nadana Shanmugam, Raju Gobi, Natesan Kannadasan, Govintha Viruthagiri, Mater. Sci. Semicond. Process. 27 (2014) 553–561. [9] S. Hufner, in: M. Cardona, P. Fulde, K. Von Klitzing, H.J. Queisser, H.K.V. Lotsch (Eds.), Photoelectron Spectroscopy Principle and Applications, Springer-Verlag, Berlin, Heidelberg, 1995, p. p126.
[10] Qun Li, Longwei Yin, Zhaoqiang Li, Xuekum Wang, Yongxin Qi, Jingyum Ma, ACS Appl. Mater. Interface 5 (2013) 10975–10984. [11] L. Chow, O. Lupan, G. Chai, H. Khallaf, L.K. Ono, B. Roldan Cuenya, I. M. Tiginyanu, V.V. Ursaki, V. Sontea, A. Schulte, , Sens. Actuators A 189 (2013) 399–408. [12] Z.B. Lei, F.H. Shi, L. Lu, ACS Appl. Mater. Interface 4 (2012) 1058–1064. [13] O. Lupan, L. Chow, L.K. Ono, B. Roldan Cuenya, G. Chai, H. Khallaf, S. Park, A. Schulte, J. Phys. Chem. C 114 (2010) 12401–12408. [14] B.E. Goodby, J.E. Pemberton, Appl. Spectrosc. 42 (1988) 754–760. [15] P. Lazcano, M. Batzill, U. Diebold, P. Haberle, Phys. Rev. B 77 (2008) 035435. [16] Wijekoon WMKP, Lyktey MYM, P.N. Prasad, J.F. Garvey, J. Phys. D: Appl. Phys. 27 (1994) 1548–1555. [17] J.F. Moulder, W.F. Stickle, K.D. Bompen, Handbook of Photoelectron Spectroscopy, Perkin Elmer Eden Praire, 1991. [18] Montree Sawangphruk, Supree Pinitsoontorn, Jumras Limtr Akul, J.Solid State Electrochem. 16 (2012) 2623–2629.
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhancement in electrochemical behavior of copper doped MnO2 electrode Ramaswamy Poonguzhali a, Raju Gobi a,n, Nadana Shanmugam a, Annamalai Senthil Kumar b, Govintha Viruthagiri a, Natesan Kannadasan a a b
Department of Physics, Annamalai University, Annamalai Nagar, Chidambaram 608002, Tamilnadu, India Environmental and Analytical Chemistry Division, Vellore Institute of Technology University, Vellore 632014, India
art ic l e i nf o
a b s t r a c t
Article history: Received 2 November 2014 Received in revised form 24 February 2015 Accepted 16 May 2015
In the present work, we report a simple chemical precipitation method for the preparation of different levels of Cu doped MnO2 nanocrystals. X-ray diffraction, field emission transmission electron microscope and X-ray photoelectron spectroscopy were used to study the material properties. To demonstrate the suitability of the doped products for electrode applications electrochemical properties were evaluated by Cyclic Voltammetry (CV), galvanostatic charge–discharge studies and impedance spectroscopy. The results indicate that the MnO2 electrode modified with 0.1 M of Cu has a better electrode property with a specific capacitance of 583 F/g and an energy density of 80 W h kg 1. & 2015 Published by Elsevier B.V.
Keywords: Electrochemical capacitor Specific capacitance Tetragonal phase Nanoparticles X-ray techniques
1. Introduction
2. Experimental section
Supercapacitors which are also named as electrochemical capacitors have attracted considerable attention due to their high energy density and high efficiency [1]. Among the available transition metal oxides, MnO2 has attracted a great deal of attention for its role as an electrode material for supercapacitors [2]. An efficient approach to enhance the specific capacity of metal oxide is incorporating one or two metal ions into them to form multimetal compounds, such as Mn–Ni [3] oxide, Mn–Co [4] Mn–Fe [5], and Ni–Zn–Co oxides [6]. Several techniques have been employed so far to prepare MnO2 with controlled morphologies and structures, including, thermal decomposition method, sol–gel route, co precipitation method, and electrochemical deposition method [7]. In the present work, we report the simple precipitation method for the preparation of various levels of (0.025–0.125 M) Cu doped MnO2 nanocrystals for supercapacitor applications. The synthesized products were analyzed for X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and field emission transmission electron microscope (FE-TEM) analysis. The electro chemical performance was examined by cyclic voltammetry, charge–discharge and impedance analysis.
For the synthesis of Cu doped MnO2, 6.12 g of manganese acetate tetra hydrate was dissolved in 30 ml of deionized water and stirred vigorously by magnetic stirrer. Then Copper acetate of preferred mole (0.00, 0.025, 0.05, 0.075, 0.1 and 0.125 M) prepared in 20 ml aqueous was mixed drop wise. Further, 5.92 g of KMnO4 in 50 ml of deionized water was added drop by drop to the above mixture. After 5 h of stirring a dark brown colored precipitate of precursor MnO2 was obtained. Finally, the obtained product was annealed in a muffle furnace at 400 °C for 3 h. The crystalline phase of the products was analyzed by XRD measurement using X’PERT-PRO diffractometer. The valence state of the doped product was analyzed by the X-ray photoelectron spectroscopy (HP 5950A Hewlett-Packard). The morphology was observed by FE-TEM (JSM2100F JEOL). The electrochemical characterizations were carried out as mentioned in the previous work [8].
n
Corresponding author. E-mail address: alankoffi[email protected] (R. Gobi).
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of undoped and different levels of Cu doped MnO2 products. All the diffraction peaks correspond to tetragonal phase of α-MnO2 (JCPDS no. 440141) [9]. No peaks related to Cu or other impurities are noted up
http://dx.doi.org/10.1016/j.matlet.2015.05.086 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98
R. Poonguzhali et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
*CuO2
(521)
(211) (301)
(220) (310)
(200)
MnO2: Cu (0.125 M)
MnO2: Cu (0.1 M)
Intensity (a.u)
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
(110)
2
MnO2: Cu (0.075 M) MnO2: Cu (0.05 M)
MnO2: Cu (0.025 M)
MnO2 10
20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. X-ray diffraction patterns of undoped and different levels of Cu doped MnO2 nanocrystals.
to 0.075 M of Cu incorporation which indicates the solubility limit of the dopant. However, a small peak related to CuO2 begins to appear in 0.1 M of Cu doping and the peak intensity increases with the increase of Cu content. The chemical composition of cu doped MnO2 has been analyzed by XPS and the results are shown in Fig. 2. The surface scan XPS survey of Cu doped MnO2 reveals the sharp signal of Cu 2p, Mn 2p, O 1s and K 2p as shown in Fig. 2a. Fig. 2b reveals the XPS of Mn 2p, where the presence of peaks at 642.2 eV and 653.9 eV can be indexed to Mn 2p3/2 and Mn 2p1/2 levels, respectively [10]. Especially the Cu 2p spectrum exhibits two distinguished peaks at 933.6 eV and 953.6 eV corresponding to Cu 2p3/2 and Cu 2p1/2 levels respectively (Fig. 2c). Additionally, we predict the satellite peaks of CuO2 at 946 eV and 962 eV, suggesting that the incorporated Cu is in a bivalence state (Cu2 þ ). In the O1s spectrum (Fig. 2d), the sharp peak at 529.65 eV can be attributed to the presence of oxygen in oxide form (Fig. 2d) [11]. The results of XPS are in good agreement with the results of XRD. The FE-TEM image of MnO2 exhibits complete rod formation (Fig. 3a), whereas, on doping the particles are showing both spherical and rod like morphologies with little agglomeration (Fig. 3b). The cyclic voltammograms (CV) of unmodified and modified electrodes of MnO2 recorded in 0.5 M KCl electrolyte at the scan rate of 10 mV s 1 are shown in Fig. 4a. The value of specific capacitance (Cs, F g 1) was calculated from the CV curves according to Eq. (1) [12].
Cs =
Q mΔv
(1)
At the scan rate of 10 mV s 1, the specific capacitance of the MnO2 is 210 F g 1, whereas the modified electrodes of MnO2 exhibit the specific capacitance of 177, 80, 75, 410, 40 F g 1, respectively for 0.025, 0.050, 0.075, 0.1 and 0.125 M of Cu doping. The electrode modified with 0.1 M of Cu exhibits the highest specific capacitance value of 410 F g 1 which is double that of the MnO2 electrode. Fig. 4b shows the variation in specific capacitance of prepared electrode versus the Cu content in the binary oxide. Fig. 4c shows the CV curves of MnO2: Cu (0.1 M) recorded at various scan rates (10, 20, 30, 40, 50, 70, 90 and 100 mV s 1) in 0.5 M KCl solution. By using Eq. (1), the specific capacitance values of the modified electrodes for all the scan rates have been calculated, and a graph is drawn between specific capacitance and scan rate (Fig. 4d). It is obvious that with an increase in the scan rate, the specific capacitance of all the electrode material decreases [13]. The decrease in the specific capacitance at higher scan rate is due to the diffusion effect of proton within the electrode, while at low scan rates, it is due to the occurrence of inner active sites, which undergo complete redox transitions and enhance the specific capacitance [14]. Fig. 5a shows the charge–discharge profile of MnO2 and modified electrodes of MnO2 recorded at constant discharge current density of 10 A g 1. Using the curves, the specific capacitance (Cs), can be calculated using the formula,
Please cite this article as: R. Poonguzhali, et al., Enhancement in electrochemical behavior of copper doped MnO2 electrode, Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.05.086i
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Fig. 2. (a) XPS spectra of Cu (0.1 M) doped MnO2 nanocrystals (a) survey spectrum, (b) Mn 2p, (c) Cu 2p and (d) O 1s.
Cs =
iΔt mΔV
(2)
The specific capacitance value obtained from discharge curve for MnO2 is 200 F g 1, whereas, it is 300, 60, 20, 583, and 16 F g 1 respectively, for various levels of Cu doped MnO2 electrodes. The electrode of MnO2 modified with 0.1 M of Cu recorded the highest
specific capacitance of 583 F g 1 at a current density of 10 A g 1. Further, to calculate the rate capability, the charge–discharge curves were drawn for the MnO2 doped with 0.1 M of Cu at a current density ranging from 10 to 50 A g 1 (Fig. 5b). The specific capacitance of the composite electrode, calculated from discharge time, according to the Eq. (2), are 583, 213, 117, 78, and 55 F g 1
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Fig. 3. (a, b) FE-TEM images of pristine and copper (0.1 M) doped MnO2.
corresponding to the discharge current densities of 10, 20, 30, 40, and 50 A g 1 respectively. Fig. 5c shows the relation between capacitance and current, at low specific current the specific capacitance increases, due to the intercalation/deintercalation of ions at surface and inner porous of the active materials in the electrode/ electrolyte interface [15]. Even at a relatively high current density of 50 A g 1, it still remains at 55 F g 1 there by exhibiting better rate capability and higher electrochemical capacitance. The plot of energy density against power density (Ragone plot) is given in Fig. 5d. The values of power and energy densities can be estimated from the following equations [16].
E=
1 Cs ΔV 2 2
P = E/t
(
)
(3) (4)
where P, Cs, Q, Δt and E are indicating the power density (W kg 1), specific capacitance of the electroactive material (F g 1), total charge delivered (C), discharge time (s) and average energy density (W h kg 1) respectively. From the Ragone plot, it is observed that the MnO2 modified with 0.1 M of Cu exhibits the highest energy density (80 W h kg 1) as well as power density (164 W kg 1) than the unmodified MnO2. To understand the performance of modified electrode of MnO2, electrochemical impedance spectroscopy (EIS) measurements were performed. Nyquist plots of unmodified and modified electrodes are shown in Fig. 6(a, b). The semicircle at high frequency region belongs to the charge transfer resistance (Rct) of the electrode. This Rct is related to the electroactive surface area of the electrode, due to the Faradaic redox processes of the MnO2 electrodes involving in the exchange of OH ions [17]. Here, the Rct value of MnO2 is 140, whereas, it is reduced to 101 Ω for the
modified electrode. The frequency at which there is a deviation from the semicircle is known as Knee frequency, which reflects the maximum lower frequency at which capacitive behavior is dominant [18]. This confirms the high capacitance of MnO2 on doping.
4. Conclusion MnO2 and copper doped MnO2 nanocrystals have been synthesized by a chemical precipitation method. The tetragonal crystal structure of α-MnO2 was confirmed through XRD analysis. The incorporation of Cu has been confirmed from XPS patterns. The influence of copper doping on electrochemical capacitor performance of MnO2 was investigated by cyclic voltammetry and galvanostatic charge–discharge studies. Among the various concentrations of doping, the 0.1 M of Cu doped MnO2 shows a very high specific capacitance of 583 F g 1 at a constant current density of 10 A g 1. Also it has a specific energy density of 80 W h kg 1 with a power density of 164 W kg 1.
Acknowledgments The authors wish to thank Dr. S. Barathan, Professor and Head, Department of Physics, Annamalai University, for providing necessary facilities to carry out this work. We sincerely thank Mr. R. Shanmugam, Research Associate, Environmental & Analytical Chemistry Division, School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, India for the help they rendered in recording electrochemical measurements.
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Fig. 4. Electrochemical performance of MnO2 and different concentration of Cu doped MnO2 electrodes (a) CV curves at 10 mV s 1, (b) Specific capacitance of the sample as a function of the Cu content, (c) CV curves at different scan rate (10–100 mV s 1) and (d) the value of specific capacitance at different scan rate.
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Fig. 5. Galvanostatic charge–discharge curves of the MnO2 and different concentration of Cu doped MnO2 electrodes at current density 10 A g 1 (b) Charge–discharge Curves of MnO2: Cu (0.1 M) electrode at different specific current, (c) current versus specific capacitance curve and (d) Ragone plots of energy and power densities of the MnO2 and (0.1 M) MnO2: Cu electrodes.
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-Zim''(ohm)
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0 0
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Zre'(ohm) Fig. 6. (a, b) Nyqusit plot obtained for MnO2 and Cu (0.1 M) doped MnO2 electrodes at 0.4 V over the frequency range 0.01 Hz to 10 KHz in 0.5 M KCl.
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