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Effect of reaction time on the synthesis and electrochemical properties of Mn3O4 nanoparticles by microwave assisted reflux method
Applied Surface Science 259 (2012) 624–630
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Effect of reaction time on the synthesis and electrochemical properties of Mn3 O4 nanoparticles by microwave assisted reflux method K. Vijaya Sankar a , S.T. Senthilkumar a , L. John Berchmans b , C. Sanjeeviraja c , R. Kalai Selvan a,∗ a b c
Solid State Ionics and Energy Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641046, India Electropyro Metallurgy Division, Central Electrochemical Research Institute, Karaikudi 630006, India School of Physics, Alagappa University, Karaikudi 630003, India
a r t i c l e
i n f o
Article history: Received 17 May 2012 Received in revised form 16 July 2012 Accepted 17 July 2012 Available online 24 July 2012 Keywords: Microwave assisted reflux synthesis Reaction time Dislocation density Active site Energy density Supercapacitor
a b s t r a c t Spherical Mn3 O4 nanoparticles were synthesized by microwave assisted reflux method at different reaction times (1, 5, 10, 15, and 20 min). The single phase formation of Mn3 O4 nanoparticles was identified through XRD analysis. The FT-IR and Raman spectra revealed the presence of functional groups of Mn3 O4 and further support the XRD results. The spherical morphology of Mn3 O4 was identified via SEM analysis. The cyclic voltammetry analysis implies that 15 min synthesized Mn3 O4 (MN-15) shows the higher specific capacitance of 135 F g−1 among all the prepared Mn3 O4 electrodes. The EIS spectra of MN-15 substantiate the less charge-transfer resistance (Rct ) of 0.553 , when compared with the other samples. The discharge capacitance of MN-15 was 103 F g−1 at 0.5 mA cm−2 in 1 M NaNO3 solution. The cycling stability curve over 100 cycles implies that the discharge capacitance is increased from 47 to 68 F g−1 at 5 mA cm−2 . This capacitance enhancement during cycling is due to the influence of phase or morphological variation of Mn3 O4 electrodes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the electrochemical capacitors or supercapacitors are widely investigated for energy storage applications because of their interesting feature of high power and energy density than batteries and conventional capacitor. In general, the supercapacitors are filling the gap between batteries and conventional capacitor in terms of its power and energy density. It is classified into two types based on their charge storage mechanism, as pseudocapacitor and electric double layer capacitor (EDLC) [1]. In the later case, the carbon based active electrode materials are used because its stores the charge based on the double layer formation at the electrode–electrolyte interfaces [2]. The transition metal oxides and conducting polymers [3] are widely used as active electrode materials for pseudocapacitors. However, the transition metal oxides possess higher capacitance because charges stored based on fast redox reaction [4]. RuO2 is one of the transition metal oxides which possess higher specific capacitance of 720 F g−1 but its usage is limited because of its high cost of the starting materials, requires strong acidic electrolyte (1 M H2 SO4 ), and toxicity [5,6]. In the present scenario, it is believed that the manganese oxides may replace RuO2 for supercapacitor applications because of its
∗ Corresponding author. E-mail address: [email protected] (R.K. Selvan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.087
low cost, nontoxic or environmental friendly nature and multiple valency states. The manganese oxides have different forms such as MnO2 , Mn2 O3 , Mn3 O4 , and MnO due to its different oxidation states. Among the above mentioned structures, Mn3 O4 is one of the stable mixed oxides state (Mn2+ (Mn3+ )2 O4 ) and having spinel structure. Mn3 O4 is widely used in various fields such as electrode material for supercapacitor [7], anode material for lithium batteries [8], cathode material for fuel cells and metal-air batteries [9], and also used as an effective catalyst for the decomposition of waste gas and Mn3 O4 act as a suitable material to control the air pollution [10]. Various synthesis techniques have been employed for the synthesis of Mn3 O4 nanoparticles including chemical bath deposition (CBD) [7], hydrothermal [11,12], solution-combustion [13], reflux [14], successive ionic layer adsorption and reaction (SILAR) [15], oxidation precipitation [16], sonochemical [17], solvothermal, and microwave [18–23] methods. Among the various synthesis methods, microwave assisted reactions possess the added advantage of faster reaction time than the conventional refluxing method. Here, the Mn3 O4 nanoparticles were prepared at a less reaction time compared to the reaction time of conventional refluxing of 4 h [14]. The reason for this being that the microwaves are directly coupled with molecules and faster the reaction speed [18]. Apte et al. have reported the synthesis of Mn3 O4 using microwave with power ranges from 50 to 500 W with various reaction times from
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1 to 5 min [19,20]. Berthelin et al. have prepared Mn3 O4 in flash microwave method with power of 1 kW and the reaction time of 5 min [21]. Malinger et al. has reported the microwave assisted hydrothermal method for the synthesis of Mn3 O4 with power of 100 W and the reaction time was 4 h [22]. Zhou et al. also reported the microwave synthesis of Mn3 O4 /worm-like mesoporous carbon composite with power of 100 W and the reaction time was 1 min [23]. These observations suggested that the microwave assisted synthesis is a suitable method for the preparation of nanoparticles with lesser reaction time. However, in this study, the Mn3 O4 nanoparticles were prepared using microwave assisted reflux synthesis method at different reaction time of 1, 5, 10, 15, and 20 min with low power of 20 W. The characteristics such as crystallinity, presence of functional groups, morphology were analyzed using various techniques and also investigated the effect of reaction time on the electrochemical performance of Mn3 O4 nanoparticles in 1 M NaNO3 electrolyte. 2. Experimental methods and materials All the chemicals were used as analytical grade without any further purification. Manganese chloride tetrahydrate (MnCl2 ·4H2 O) and sodium hydroxide (NaOH) were purchased from Himedia. Sodium nitrate (NaNO3 ) was purchased from Loba. Ethylene glycol (EG) was purchased from Merck. For the typical synthesis of 1 g of Mn3 O4 , the MnCl2 ·4H2 O (2.276 g) and NaOH (0.920 g) were dissolved individually in 40 ml and 10 ml of de-ionized water, respectively. Then the dissolved MnCl2 ·4H2 O was added with EG (50 ml). Further the NaOH solution was added drop wise in to the above mixture under vigorous stirring until the color of the solution was changed in to brown color. Subsequently, the entire solution was placed in the microwave reflux system [24,25]. The microwave was irradiated in different reaction time such as 1, 5, 10, 15, and 20 min with the on/off conditions of 15 s ON and 15 s OFF in order to avoid the overheating with power of 20 W. Finally, the as prepared sample was centrifuged several times in double distilled water, ethanol and dried at 100 ◦ C overnight. Hereafter the as prepared samples are named MN-1 (1 min), MN-5 (5 min), MN-10 (10 min), MN-15 (15 min), and MN-20 (20 min), respectively. The phase purity and compound formation are characterized by an X-ray diffractometer, Bruker D8 Advance with Cu K␣ radiation. The functional groups are identified using FT-IR PerkinElmer make model RXI instrument. The Raman analysis of our samples carried out in the instrument of Laser Raman confocal microprobe (LabRam HR 800) where the He–Ne laser ( = 633 nm) was used as the excitation source with output power of 17 mW which was focused on to a spot of 1 m. The morphology of the as prepared samples was analyzed using scanning electron microscope (Hitachi-S3000 H SEM). The electrochemical performance is investigated using Bio-logic (SP 150). For the electrode preparation the Mn3 O4 active material, carbon black, and PVdF are taken in the weight ratio of 80:15:5. All these are mixed together using the solvent NMP (Nmethylpyrrolidone). The detailed electrode preparation is given elsewhere [3]. The electrochemical analysis were carried out in three-electrode configuration with Mn3 O4 coated graphite sheet, Pt, and SCE as working, counter, and reference electrodes, respectively in 1 M NaNO3 electrolyte.
Fig. 1. XRD pattern of the Mn3 O4 nanosphere samples of (a) MN-1, (b) MN-5, (c) MN-10, (d) MN-15 and (e) MN-20.
different reaction times (1, 5, 10, 15, and 20 min) are shown in Fig. 1. The diffraction peaks of the as prepared Mn3 O4 samples at 2 = 28.85, 32.29, 36.03, 37.97, 44.35, 50.70, 58.46, 59.83, and 64.53◦ corresponds to the Miller indices or lattice planes of (1 1 2), (1 0 3), (2 1 1), (0 0 4), (2 2 0), (1 0 5), (3 2 1), (2 2 4), and (4 0 0), respectively. Therefore, it can be indexed to the tetragonal hausmannite structure (space group I41/amd) of Mn3 O4 . The less intensity of diffracted peaks reveals that the low crystallinity of the as prepared samples. The lattice constants, lattice density, and cell volume of the samples are calculated and are tabulated in Table 1. The obtained ˚ c = 9.47(1) A. ˚ These lattice constant values are a = b= 5.768(2) A, values are in good agreement with the reported values [12,26,27]. However, no other impurity peaks are (any other manganese oxides) found in XRD pattern which indicates the phase purity of the as prepared Mn3 O4 samples. The calculated average crystallite size of the Mn3 O4 samples is 17, 18, 24, 15, and 18 nm corresponding to the samples MN-1, MN-5, MN-10, MN-15, and MN-20, respectively. The relation between the theoretical surface area and dislocation density with reaction time are shown in Fig. 2. It shows that MN-15 has high dislocation density and high surface area
3. Results and discussion 3.1. XRD analysis The phase purity and crystalline structure of the obtained Mn3 O4 nanoparticles are characterized by X-ray diffraction method. The XRD patterns of the Mn3 O4 samples prepared at
Fig. 2. Effect of reaction time on surface area and dislocation density.
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Table 1 Structural parameters of Mn3 O4 samples. Sample
Lattice constant
Grain size (nm)
Lattice density (g/cm3 )
Cell volume
˚ Calculated (A)
˚ Standard (A)
˚ 3 Calculated (A)
˚ 3 Standard (A)
MN-1
a = b = 5.7681 c = 9.4649
a = b = 5.763 c = 9.456
17
314.911
314.05
4.825
MN-5
a = b = 5.7673 c = 9.4852
a = b = 5.763 c = 9.456
18
315.500
314.05
4.816
MN-10
a = b = 5.7699 c = 9.466714
a = b = 5.763 c = 9.456
24
315.159
314.05
4.822
MN-15
a = b = 5.7694 c = 9.4700
a = b = 5.763 c = 9.456
15
315.219
314.05
4.821
MN-20
a = b = 5.7705 c = 9.4768
a = b = 5.763 c = 9.456
18
315.562
314.05
4.8215
among all other samples due to the lower crystallite size [28]. The dislocation density was calculated using the following expression =
1 (lines/m2 ) D2
(1)
where D is crystallite size in nm. 3.2. FT-IR analysis The presence of functional groups in Mn3 O4 samples are identified through FT-IR analysis. Fig. 3 shows the FT-IR spectra of Mn3 O4 samples. The observed different modes and its corresponding wave numbers are given in Table 2. The samples show a broad band around 3397 ± 4 cm−1 indicating the presence of OH group in the as prepared samples. The small bands are observed at approximately 1570 cm−1 and 1380 cm−1 corresponds to the adsorption of molecules from moisture and bending vibration of O H joined with metal (Mn) atoms. The two significant peaks observed at approximately 627 and 509 cm−1 that reveals the coupling between the Mn O stretching modes of tetrahedral and octahedral sites, respectively. That is, the vibration band around 627 cm−1 corresponds to the characteristics of Mn O stretching mode in tetrahedral sites, similarly the vibration band observed around 509 cm−1 is associated with distortion vibration of Mn O in an octahedral site [17,26,29,30].
Fig. 3. FT-IR spectra of the Mn3 O4 nanosphere samples of (a) MN-1, (b) MN-5, (c) MN-10, (d) MN-15 and (e) MN-20.
3.3. Raman analysis The Raman spectra of Mn3 O4 samples are shown in Fig. 4. A single broad peak was observed at 644.89, 639.22, 637.22, 637.22, and 644.11 cm−1 corresponding to MN-1, MN-5, MN-10, MN-15, and MN-20 samples, respectively. The observed broadened peaks may be due to the smaller crystallite size and the less crystallinity. Because of their lower crystallite size, the uncertainty of momentum is high which leads to the broadening of peaks [31]. The observed peak value (∼640 cm−1 ) is comparable with the reported values of ∼658 cm−1 [7,32]. 3.4. Morphological analysis The effects of reaction time on the morphology of the samples are examined using SEM analysis. The SEM images of Mn3 O4 are shown in Fig. 5. It can be seen that the particles are well defined in the size ranging from 60 to 200 nm. However, some particle agglomeration is also observed for all the samples except MN-15 and MN-20. The SEM images of MN-1, MN-5, and MN-10 show a partial aggregation of particle with irregular shape. This aggregation of nanoparticles is due to the effect of microwave heating because it forms the hot surface on primarily formed nanoparticles. When increasing the reaction time above 10 min, the agglomeration of the particle gets reduced and the individual spherical like particle can be visualized for the samples MN-15 and MN-20. From these images, it is clear that the aggregation of particles decreases
Fig. 4. Raman spectra of the Mn3 O4 nanosphere samples of (a) MN-1, (b) MN-5, (c) MN-10, (d) MN-15 and (e) MN-20.
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Table 2 Assignments of FT-IR spectra. Wave numbers (cm−1 )
Modes
MN-1
MN-5
MN-10
MN-15
MN-20
3397.56 1571.67 1374.06 633.002 510.87
3394.88 1558.73 1376.99 634.95 510.41
3401.28 1606.34 1413.36 630.23 510.67
3393.14 1626.66 1401.03 611.324 502.366
3399.01 1572.00 1365.04 622.85 508.06
and smaller nanoparticles are dissolved with increase in the reaction time, finally the individual nanosphere is formed [32]. It infers that the reaction time does not possess the considerable morphological variations in the samples but the agglomeration of the nanoparticles gets decreased at longer reaction time (20 min). The particle size histogram corresponding to MN-15 sample is given in Fig. 5(f). The maximum number of particles are found to be approximately in the range of 60–120 nm and the minimum number of particles are found to be in the range of 120–200 nm. 3.5. Electrochemical analysis 3.5.1. Cyclic voltammetry analysis To demonstrate the electrochemical properties of the Mn3 O4 nanoparticles cyclic voltammetry (CV) analysis was carried in 1 M NaNO3 electrolyte in the potential range of 0–1.2 V. Fig. 6(a) shows the CV curves of all the Mn3 O4 electrodes at 2 mV s−1 and (b) corresponds to the CV curve of MN-15 electrode at different scan rates. It can be seen that the shape of the curve shows the quasi-rectangular like form which confirm the Mn3 O4 electrodes have high reversibility during electrochemical reaction in the electrolyte of 1 M NaNO3 . Moreover, the CV curves do not show any distortion even at higher scan rates which reveals the good electrochemical properties of Mn3 O4 (Fig. 6(b)). It also shows that the area under the CV curve was increased with increasing of scan rates and also similar kind performance observed in other samples. Fig. 6(c) reveals that the capacitance was decreased with increase in scan rate. It is well
OH group Adsorption of water moisture Bending vibration of O H bonds connected with Mn atoms Mn O stretching mode in tetrahedral sites Distortion vibration of Mn O in an octahedral site
known that the voltammetric current is directly proportional to the scan rate [33]. In addition, at low scan rate, the ions from the electrolyte are completely utilizing the available sites in inner and outer surfaces of the electrode material, because the ions from the electrolyte has enough time to utilize all the available sites in the electrode material. Therefore it shows high capacitance value. If the scan rate increases, the ions have a difficulty to access all the available sites in the electrode material due to the restricted rate of moment in the electrolyte i.e. the interaction between the electrode and electrolyte get reduced which leads to the decrease of capacitance at high scan rate [34]. Among all the samples, MN-15 nanoparticles possess higher specific capacitance (Fig. 6(c)) which reveals that the reaction time plays a significant role in electrochemical performance of the electrode materials. The obtained higher specific capacitance (135 F g−1 ) for MN-15 at the low scan rate of 2 mV s−1 may due to the higher surface area. The obtained specific capacitance is higher than the reported values such as 14 F g−1 at 5 mV s−1 in 0.5 M K2 SO4 reported by Ghodbane et al. [35], the ball milled Mn3 O4 provides the specific capacitance of approximately 40 F g−1 at 10 mV s−1 in 0.1 mol dm−3 Na2 SO4 and 50 F g−1 with potential window of −0.1 to 0.9 V at 10 mV s−1 in 0.1 mol dm−3 Na2 SO4 [36]. However, the obtained specific capacitance is lower than the reported values such as 143 F g−1 at 2 mV s−1 for Mn3 O4 /CNT composite [37], 148 F g−1 at 5 mV s−1 for Mn3 O4 hollow-tetrakaidecahedrons [38], 172 F g−1 for Mn3 O4 prepared through oxidation–precipitation method [30] and 237 and 226 F g−1 for Mn3 O4 nanoplate modified carbon paste
Fig. 5. SEM images of Mn3 O4 nanosphere samples of (a) MN-1, (b) MN-5, (c) MN-10, (d) MN-15 and (e) MN-20. (f) Histogram of MN-15 sample.
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Fig. 6. Cyclic voltammograms of (a) Mn3 O4 electrodes at a scan rate of 2 mV s−1 in 1 M NaNO3 electrolyte. (b) CV curves of MN-15 electrode at different scan rate of 2, 5, 10, 20, 30, 40, 50 and 100 mV s−1 . (c) Specific capacitance of Mn3 O4 electrodes as a function of scan rate. Table 3 Electrochemical parameters.
electrode (CPE) and a nanoparticles [39]. Mn3 O4 nano-octahedrons of 322 F g−1 at 5 mV s−1 [40], Mn3 O4 thin film prepared via CBD and SILAR method gives the specific capacitance values as 284 F g−1 [29], 314 F g−1 [26] at scan rate of 5 mV s−1 . From that it can be concluded that the electrochemical performance of Mn3 O4 electrode material is highly depend on the method of preparation [11–15], surface area [30], morphology [38,40], composite material [37], loading of the active material [36], crystallinity of the sample [33], electrolyte concentration, and potential window [36]. Subsequently, EIS analysis was carried out for all the samples at open circuit potential (OCV) in 1 M NaNO3 in the frequency range from 0.01 Hz to 105 Hz. The typical Nyquist plot of Mn3 O4 electrodes are shown in Fig. 7. It can be seen that in the higher frequency region, the intersection made on the real axis corresponds to the solution resistance (R1 ) and in the higher to medium frequency region, one depressed semi circle was found which enumerates the charge transfer resistance at the electrode electrolyte interface. Similarly at low frequency region, the spike was found which indicates the Warburg impedance of the electrode material [34,41]. In all the spectra, the phase angle is greater than the 45◦ which indicates that the Mn3 O4 electrodes are suitable for supercapacitor application. EIS data was fitted and the corresponding equivalent circuit is given in Fig. 7 (inset). The fitting parameters such as charge transfer resistance (Rct ) and the solution resistance (R1 ) are given in Table 3. This typical result implies that the Rct value is lower for MN15 electrode due to the higher ionic conductivity. This low Rct value is mainly due to the high dislocation density which leads the higher ionic conductivity (reactivity) which is confirmed in XRD results [42,43]. This observed result further supports the CV behavior.
3.5.2. Charge–discharge behavior Based on the above results, further the galvanostatic charge–discharge analysis was carried out for MN-15 sample. The galvanostatic charge–discharge curves of MN-15 sample are given in Fig. 8. The typical results are measured from 0 to 1.0 V in 1 M NaNO3 electrolyte at different current densities (0.5, 1.0, 3.0, and 5.0 mA cm−2 ). The charge and discharge curve is almost symmetric which indicates that the MN-15 electrode material has high reversibility and reactivity. At the beginning of discharge curve, a small IR or potential drop was found which is mainly due to the internal resistance of the electrode material. The IR drop value is increased with increasing current density [44,45]. The specific capacitance (SC) of the electrode material is calculated and the specific capacitance values of MN-15 electrode are 103, 68, 56, and 47 F g−1 at different current densities of 0.5, 1.0, 3.0, and 5.0 mA cm−2 , respectively. Fig. 9 shows the variation of capacitance and active site [34] with the current density. It shows that at low current density, the discharge capacitance is high due to the available sites in MN-15 electrode material and is fully contributed to the electrochemical reaction. Similarly, at higher
Fig. 7. Nyquist plot for Mn3 O4 electrodes. The insert is the equivalent circuit.
Fig. 8. Galvanostatic charge/discharge curves for the optimized MN-15 electrode at a current density of (a) 0.5, (b) 1, (c) 3 and (d) 5 mA/cm2 .
Sample
R1 ()
RCT ()
MN-1 MN-5 MN-10 MN-15 MN-20
5.757 5.603 6.104 5.726 5.337
2.084 1.930 2.031 0.553 1.791
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given in Fig. 10 (inset). The capacitance of the materials increased from 47 to 68 F g−1 during cycling, where the capacitance of the material is almost maintained after 60th cycle (66–68 F g−1 ). This result indicates that the cycling process may induce the phase or morphological changes or activation effect in Mn3 O4 electrode material. The similar type of observations was already reported [7,11,38,47,48]. A possible phase change mechanism of Mn3 O4 during cycling was expressed as [15] Mn3 O4 → Naı MnOx · nH2 O
(4)
Naı MnOx · nH2 O+yH+ + zNa+ + (y + z)e− Naı+z MnOx · nH2 O (5)
Fig. 9. Specific capacitance and active site variation as a function of current density.
current density, only limited number of available sites is contributing to the electrochemical reaction in turn lower capacitance. It reveals that the specific capacitance is inversely proportional to the current density. The energy (dE ) and power density (dP ) [34,46] was calculated using the following expressions dE =
1 2 CV (Wh kg−1 ) 2
(2)
dP =
dE (kW kg−1 ) t
(3)
where C is the discharge capacitance of MN-15 electrode material F g−1 , V is the operating potential window in volt, and t is the discharge time in s. The maximum obtained energy and power density is 14.33 Wh kg−1 and 48.7 W kg−1 at a current density of 0.5 mA cm−2 . 3.5.3. Cyclic stability In addition, the cyclic stability of the electrode material is important for supercapacitor applications. In order to find the cyclic stability of MN-15 sample, the sample was cycled up to 100 cycles in 1 M NaNO3 electrolyte at a current density of 5 mA cm−2 and the calculated specific capacitance and the columbic efficiency as a function of cycle number is given in Fig. 10. The representative charge–discharge curves for the first 10 cycles are
The reaction (4) represents the phase change of Mn3 O4 into Mn oxides and is irreversible reaction. The later reaction (5) represents the reversible cycling of Mn oxides. The cyclic stability and enhancing of specific capacitance indicates that the prepared Mn3 O4 electrode material is suitable for supercapacitor electrode. Further, the coulombic efficiency is a measure of the charge transfer in an electrochemical system contributing an electrochemical reaction. During charge–discharge cycles, the increase of coulombic efficiency indicates the higher reversibility of Mn3 O4 nanoparticles. 4. Conclusion In summary, Mn3 O4 nanospheres were prepared by microwave assisted reflux synthesis. The phase formation, purity of sample, and the presence of functional groups are identified through XRD, FT-IR, and Raman analysis. The SEM image shows the formation of nanospheres. The CV and EIS results implies that the sample MN15 has high specific capacitance (135 F g−1 ) and low charge transfer resistance (0.553 ), respectively. The lowering of charge transfer resistance in MN-15 may be due to the high dislocation density. The charge–discharge study was carried for MN-15 electrode and its discharge capacitance was 103 F g−1 at 0.5 mA cm−2 . The obtained energy and power density is 14.33 Wh kg−1 and 48.7 W kg−1 at current rate of 0.5 mA cm−2 . The MN-15 electrode has good cyclic stability at extended cycles and the capacitance of electrode material increased from 47 to 68 F g−1 . The electrochemical performance of Mn3 O4 electrodes reveals that the MN-15 electrode provides a better result for supercapacitor application. References
Fig. 10. Effect of number of cycles on the specific capacitance and coulombic efficiency. The insert figure is the charge/discharge cycles (first ten cycles) of MN-15 electrode.
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Effect of reaction time on the synthesis and electrochemical properties of Mn3 O4 nanoparticles by microwave assisted reflux method K. Vijaya Sankar a , S.T. Senthilkumar a , L. John Berchmans b , C. Sanjeeviraja c , R. Kalai Selvan a,∗ a b c
Solid State Ionics and Energy Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641046, India Electropyro Metallurgy Division, Central Electrochemical Research Institute, Karaikudi 630006, India School of Physics, Alagappa University, Karaikudi 630003, India
a r t i c l e
i n f o
Article history: Received 17 May 2012 Received in revised form 16 July 2012 Accepted 17 July 2012 Available online 24 July 2012 Keywords: Microwave assisted reflux synthesis Reaction time Dislocation density Active site Energy density Supercapacitor
a b s t r a c t Spherical Mn3 O4 nanoparticles were synthesized by microwave assisted reflux method at different reaction times (1, 5, 10, 15, and 20 min). The single phase formation of Mn3 O4 nanoparticles was identified through XRD analysis. The FT-IR and Raman spectra revealed the presence of functional groups of Mn3 O4 and further support the XRD results. The spherical morphology of Mn3 O4 was identified via SEM analysis. The cyclic voltammetry analysis implies that 15 min synthesized Mn3 O4 (MN-15) shows the higher specific capacitance of 135 F g−1 among all the prepared Mn3 O4 electrodes. The EIS spectra of MN-15 substantiate the less charge-transfer resistance (Rct ) of 0.553 , when compared with the other samples. The discharge capacitance of MN-15 was 103 F g−1 at 0.5 mA cm−2 in 1 M NaNO3 solution. The cycling stability curve over 100 cycles implies that the discharge capacitance is increased from 47 to 68 F g−1 at 5 mA cm−2 . This capacitance enhancement during cycling is due to the influence of phase or morphological variation of Mn3 O4 electrodes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the electrochemical capacitors or supercapacitors are widely investigated for energy storage applications because of their interesting feature of high power and energy density than batteries and conventional capacitor. In general, the supercapacitors are filling the gap between batteries and conventional capacitor in terms of its power and energy density. It is classified into two types based on their charge storage mechanism, as pseudocapacitor and electric double layer capacitor (EDLC) [1]. In the later case, the carbon based active electrode materials are used because its stores the charge based on the double layer formation at the electrode–electrolyte interfaces [2]. The transition metal oxides and conducting polymers [3] are widely used as active electrode materials for pseudocapacitors. However, the transition metal oxides possess higher capacitance because charges stored based on fast redox reaction [4]. RuO2 is one of the transition metal oxides which possess higher specific capacitance of 720 F g−1 but its usage is limited because of its high cost of the starting materials, requires strong acidic electrolyte (1 M H2 SO4 ), and toxicity [5,6]. In the present scenario, it is believed that the manganese oxides may replace RuO2 for supercapacitor applications because of its
∗ Corresponding author. E-mail address: [email protected] (R.K. Selvan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.087
low cost, nontoxic or environmental friendly nature and multiple valency states. The manganese oxides have different forms such as MnO2 , Mn2 O3 , Mn3 O4 , and MnO due to its different oxidation states. Among the above mentioned structures, Mn3 O4 is one of the stable mixed oxides state (Mn2+ (Mn3+ )2 O4 ) and having spinel structure. Mn3 O4 is widely used in various fields such as electrode material for supercapacitor [7], anode material for lithium batteries [8], cathode material for fuel cells and metal-air batteries [9], and also used as an effective catalyst for the decomposition of waste gas and Mn3 O4 act as a suitable material to control the air pollution [10]. Various synthesis techniques have been employed for the synthesis of Mn3 O4 nanoparticles including chemical bath deposition (CBD) [7], hydrothermal [11,12], solution-combustion [13], reflux [14], successive ionic layer adsorption and reaction (SILAR) [15], oxidation precipitation [16], sonochemical [17], solvothermal, and microwave [18–23] methods. Among the various synthesis methods, microwave assisted reactions possess the added advantage of faster reaction time than the conventional refluxing method. Here, the Mn3 O4 nanoparticles were prepared at a less reaction time compared to the reaction time of conventional refluxing of 4 h [14]. The reason for this being that the microwaves are directly coupled with molecules and faster the reaction speed [18]. Apte et al. have reported the synthesis of Mn3 O4 using microwave with power ranges from 50 to 500 W with various reaction times from
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1 to 5 min [19,20]. Berthelin et al. have prepared Mn3 O4 in flash microwave method with power of 1 kW and the reaction time of 5 min [21]. Malinger et al. has reported the microwave assisted hydrothermal method for the synthesis of Mn3 O4 with power of 100 W and the reaction time was 4 h [22]. Zhou et al. also reported the microwave synthesis of Mn3 O4 /worm-like mesoporous carbon composite with power of 100 W and the reaction time was 1 min [23]. These observations suggested that the microwave assisted synthesis is a suitable method for the preparation of nanoparticles with lesser reaction time. However, in this study, the Mn3 O4 nanoparticles were prepared using microwave assisted reflux synthesis method at different reaction time of 1, 5, 10, 15, and 20 min with low power of 20 W. The characteristics such as crystallinity, presence of functional groups, morphology were analyzed using various techniques and also investigated the effect of reaction time on the electrochemical performance of Mn3 O4 nanoparticles in 1 M NaNO3 electrolyte. 2. Experimental methods and materials All the chemicals were used as analytical grade without any further purification. Manganese chloride tetrahydrate (MnCl2 ·4H2 O) and sodium hydroxide (NaOH) were purchased from Himedia. Sodium nitrate (NaNO3 ) was purchased from Loba. Ethylene glycol (EG) was purchased from Merck. For the typical synthesis of 1 g of Mn3 O4 , the MnCl2 ·4H2 O (2.276 g) and NaOH (0.920 g) were dissolved individually in 40 ml and 10 ml of de-ionized water, respectively. Then the dissolved MnCl2 ·4H2 O was added with EG (50 ml). Further the NaOH solution was added drop wise in to the above mixture under vigorous stirring until the color of the solution was changed in to brown color. Subsequently, the entire solution was placed in the microwave reflux system [24,25]. The microwave was irradiated in different reaction time such as 1, 5, 10, 15, and 20 min with the on/off conditions of 15 s ON and 15 s OFF in order to avoid the overheating with power of 20 W. Finally, the as prepared sample was centrifuged several times in double distilled water, ethanol and dried at 100 ◦ C overnight. Hereafter the as prepared samples are named MN-1 (1 min), MN-5 (5 min), MN-10 (10 min), MN-15 (15 min), and MN-20 (20 min), respectively. The phase purity and compound formation are characterized by an X-ray diffractometer, Bruker D8 Advance with Cu K␣ radiation. The functional groups are identified using FT-IR PerkinElmer make model RXI instrument. The Raman analysis of our samples carried out in the instrument of Laser Raman confocal microprobe (LabRam HR 800) where the He–Ne laser ( = 633 nm) was used as the excitation source with output power of 17 mW which was focused on to a spot of 1 m. The morphology of the as prepared samples was analyzed using scanning electron microscope (Hitachi-S3000 H SEM). The electrochemical performance is investigated using Bio-logic (SP 150). For the electrode preparation the Mn3 O4 active material, carbon black, and PVdF are taken in the weight ratio of 80:15:5. All these are mixed together using the solvent NMP (Nmethylpyrrolidone). The detailed electrode preparation is given elsewhere [3]. The electrochemical analysis were carried out in three-electrode configuration with Mn3 O4 coated graphite sheet, Pt, and SCE as working, counter, and reference electrodes, respectively in 1 M NaNO3 electrolyte.
Fig. 1. XRD pattern of the Mn3 O4 nanosphere samples of (a) MN-1, (b) MN-5, (c) MN-10, (d) MN-15 and (e) MN-20.
different reaction times (1, 5, 10, 15, and 20 min) are shown in Fig. 1. The diffraction peaks of the as prepared Mn3 O4 samples at 2 = 28.85, 32.29, 36.03, 37.97, 44.35, 50.70, 58.46, 59.83, and 64.53◦ corresponds to the Miller indices or lattice planes of (1 1 2), (1 0 3), (2 1 1), (0 0 4), (2 2 0), (1 0 5), (3 2 1), (2 2 4), and (4 0 0), respectively. Therefore, it can be indexed to the tetragonal hausmannite structure (space group I41/amd) of Mn3 O4 . The less intensity of diffracted peaks reveals that the low crystallinity of the as prepared samples. The lattice constants, lattice density, and cell volume of the samples are calculated and are tabulated in Table 1. The obtained ˚ c = 9.47(1) A. ˚ These lattice constant values are a = b= 5.768(2) A, values are in good agreement with the reported values [12,26,27]. However, no other impurity peaks are (any other manganese oxides) found in XRD pattern which indicates the phase purity of the as prepared Mn3 O4 samples. The calculated average crystallite size of the Mn3 O4 samples is 17, 18, 24, 15, and 18 nm corresponding to the samples MN-1, MN-5, MN-10, MN-15, and MN-20, respectively. The relation between the theoretical surface area and dislocation density with reaction time are shown in Fig. 2. It shows that MN-15 has high dislocation density and high surface area
3. Results and discussion 3.1. XRD analysis The phase purity and crystalline structure of the obtained Mn3 O4 nanoparticles are characterized by X-ray diffraction method. The XRD patterns of the Mn3 O4 samples prepared at
Fig. 2. Effect of reaction time on surface area and dislocation density.
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Table 1 Structural parameters of Mn3 O4 samples. Sample
Lattice constant
Grain size (nm)
Lattice density (g/cm3 )
Cell volume
˚ Calculated (A)
˚ Standard (A)
˚ 3 Calculated (A)
˚ 3 Standard (A)
MN-1
a = b = 5.7681 c = 9.4649
a = b = 5.763 c = 9.456
17
314.911
314.05
4.825
MN-5
a = b = 5.7673 c = 9.4852
a = b = 5.763 c = 9.456
18
315.500
314.05
4.816
MN-10
a = b = 5.7699 c = 9.466714
a = b = 5.763 c = 9.456
24
315.159
314.05
4.822
MN-15
a = b = 5.7694 c = 9.4700
a = b = 5.763 c = 9.456
15
315.219
314.05
4.821
MN-20
a = b = 5.7705 c = 9.4768
a = b = 5.763 c = 9.456
18
315.562
314.05
4.8215
among all other samples due to the lower crystallite size [28]. The dislocation density was calculated using the following expression =
1 (lines/m2 ) D2
(1)
where D is crystallite size in nm. 3.2. FT-IR analysis The presence of functional groups in Mn3 O4 samples are identified through FT-IR analysis. Fig. 3 shows the FT-IR spectra of Mn3 O4 samples. The observed different modes and its corresponding wave numbers are given in Table 2. The samples show a broad band around 3397 ± 4 cm−1 indicating the presence of OH group in the as prepared samples. The small bands are observed at approximately 1570 cm−1 and 1380 cm−1 corresponds to the adsorption of molecules from moisture and bending vibration of O H joined with metal (Mn) atoms. The two significant peaks observed at approximately 627 and 509 cm−1 that reveals the coupling between the Mn O stretching modes of tetrahedral and octahedral sites, respectively. That is, the vibration band around 627 cm−1 corresponds to the characteristics of Mn O stretching mode in tetrahedral sites, similarly the vibration band observed around 509 cm−1 is associated with distortion vibration of Mn O in an octahedral site [17,26,29,30].
Fig. 3. FT-IR spectra of the Mn3 O4 nanosphere samples of (a) MN-1, (b) MN-5, (c) MN-10, (d) MN-15 and (e) MN-20.
3.3. Raman analysis The Raman spectra of Mn3 O4 samples are shown in Fig. 4. A single broad peak was observed at 644.89, 639.22, 637.22, 637.22, and 644.11 cm−1 corresponding to MN-1, MN-5, MN-10, MN-15, and MN-20 samples, respectively. The observed broadened peaks may be due to the smaller crystallite size and the less crystallinity. Because of their lower crystallite size, the uncertainty of momentum is high which leads to the broadening of peaks [31]. The observed peak value (∼640 cm−1 ) is comparable with the reported values of ∼658 cm−1 [7,32]. 3.4. Morphological analysis The effects of reaction time on the morphology of the samples are examined using SEM analysis. The SEM images of Mn3 O4 are shown in Fig. 5. It can be seen that the particles are well defined in the size ranging from 60 to 200 nm. However, some particle agglomeration is also observed for all the samples except MN-15 and MN-20. The SEM images of MN-1, MN-5, and MN-10 show a partial aggregation of particle with irregular shape. This aggregation of nanoparticles is due to the effect of microwave heating because it forms the hot surface on primarily formed nanoparticles. When increasing the reaction time above 10 min, the agglomeration of the particle gets reduced and the individual spherical like particle can be visualized for the samples MN-15 and MN-20. From these images, it is clear that the aggregation of particles decreases
Fig. 4. Raman spectra of the Mn3 O4 nanosphere samples of (a) MN-1, (b) MN-5, (c) MN-10, (d) MN-15 and (e) MN-20.
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Table 2 Assignments of FT-IR spectra. Wave numbers (cm−1 )
Modes
MN-1
MN-5
MN-10
MN-15
MN-20
3397.56 1571.67 1374.06 633.002 510.87
3394.88 1558.73 1376.99 634.95 510.41
3401.28 1606.34 1413.36 630.23 510.67
3393.14 1626.66 1401.03 611.324 502.366
3399.01 1572.00 1365.04 622.85 508.06
and smaller nanoparticles are dissolved with increase in the reaction time, finally the individual nanosphere is formed [32]. It infers that the reaction time does not possess the considerable morphological variations in the samples but the agglomeration of the nanoparticles gets decreased at longer reaction time (20 min). The particle size histogram corresponding to MN-15 sample is given in Fig. 5(f). The maximum number of particles are found to be approximately in the range of 60–120 nm and the minimum number of particles are found to be in the range of 120–200 nm. 3.5. Electrochemical analysis 3.5.1. Cyclic voltammetry analysis To demonstrate the electrochemical properties of the Mn3 O4 nanoparticles cyclic voltammetry (CV) analysis was carried in 1 M NaNO3 electrolyte in the potential range of 0–1.2 V. Fig. 6(a) shows the CV curves of all the Mn3 O4 electrodes at 2 mV s−1 and (b) corresponds to the CV curve of MN-15 electrode at different scan rates. It can be seen that the shape of the curve shows the quasi-rectangular like form which confirm the Mn3 O4 electrodes have high reversibility during electrochemical reaction in the electrolyte of 1 M NaNO3 . Moreover, the CV curves do not show any distortion even at higher scan rates which reveals the good electrochemical properties of Mn3 O4 (Fig. 6(b)). It also shows that the area under the CV curve was increased with increasing of scan rates and also similar kind performance observed in other samples. Fig. 6(c) reveals that the capacitance was decreased with increase in scan rate. It is well
OH group Adsorption of water moisture Bending vibration of O H bonds connected with Mn atoms Mn O stretching mode in tetrahedral sites Distortion vibration of Mn O in an octahedral site
known that the voltammetric current is directly proportional to the scan rate [33]. In addition, at low scan rate, the ions from the electrolyte are completely utilizing the available sites in inner and outer surfaces of the electrode material, because the ions from the electrolyte has enough time to utilize all the available sites in the electrode material. Therefore it shows high capacitance value. If the scan rate increases, the ions have a difficulty to access all the available sites in the electrode material due to the restricted rate of moment in the electrolyte i.e. the interaction between the electrode and electrolyte get reduced which leads to the decrease of capacitance at high scan rate [34]. Among all the samples, MN-15 nanoparticles possess higher specific capacitance (Fig. 6(c)) which reveals that the reaction time plays a significant role in electrochemical performance of the electrode materials. The obtained higher specific capacitance (135 F g−1 ) for MN-15 at the low scan rate of 2 mV s−1 may due to the higher surface area. The obtained specific capacitance is higher than the reported values such as 14 F g−1 at 5 mV s−1 in 0.5 M K2 SO4 reported by Ghodbane et al. [35], the ball milled Mn3 O4 provides the specific capacitance of approximately 40 F g−1 at 10 mV s−1 in 0.1 mol dm−3 Na2 SO4 and 50 F g−1 with potential window of −0.1 to 0.9 V at 10 mV s−1 in 0.1 mol dm−3 Na2 SO4 [36]. However, the obtained specific capacitance is lower than the reported values such as 143 F g−1 at 2 mV s−1 for Mn3 O4 /CNT composite [37], 148 F g−1 at 5 mV s−1 for Mn3 O4 hollow-tetrakaidecahedrons [38], 172 F g−1 for Mn3 O4 prepared through oxidation–precipitation method [30] and 237 and 226 F g−1 for Mn3 O4 nanoplate modified carbon paste
Fig. 5. SEM images of Mn3 O4 nanosphere samples of (a) MN-1, (b) MN-5, (c) MN-10, (d) MN-15 and (e) MN-20. (f) Histogram of MN-15 sample.
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Fig. 6. Cyclic voltammograms of (a) Mn3 O4 electrodes at a scan rate of 2 mV s−1 in 1 M NaNO3 electrolyte. (b) CV curves of MN-15 electrode at different scan rate of 2, 5, 10, 20, 30, 40, 50 and 100 mV s−1 . (c) Specific capacitance of Mn3 O4 electrodes as a function of scan rate. Table 3 Electrochemical parameters.
electrode (CPE) and a nanoparticles [39]. Mn3 O4 nano-octahedrons of 322 F g−1 at 5 mV s−1 [40], Mn3 O4 thin film prepared via CBD and SILAR method gives the specific capacitance values as 284 F g−1 [29], 314 F g−1 [26] at scan rate of 5 mV s−1 . From that it can be concluded that the electrochemical performance of Mn3 O4 electrode material is highly depend on the method of preparation [11–15], surface area [30], morphology [38,40], composite material [37], loading of the active material [36], crystallinity of the sample [33], electrolyte concentration, and potential window [36]. Subsequently, EIS analysis was carried out for all the samples at open circuit potential (OCV) in 1 M NaNO3 in the frequency range from 0.01 Hz to 105 Hz. The typical Nyquist plot of Mn3 O4 electrodes are shown in Fig. 7. It can be seen that in the higher frequency region, the intersection made on the real axis corresponds to the solution resistance (R1 ) and in the higher to medium frequency region, one depressed semi circle was found which enumerates the charge transfer resistance at the electrode electrolyte interface. Similarly at low frequency region, the spike was found which indicates the Warburg impedance of the electrode material [34,41]. In all the spectra, the phase angle is greater than the 45◦ which indicates that the Mn3 O4 electrodes are suitable for supercapacitor application. EIS data was fitted and the corresponding equivalent circuit is given in Fig. 7 (inset). The fitting parameters such as charge transfer resistance (Rct ) and the solution resistance (R1 ) are given in Table 3. This typical result implies that the Rct value is lower for MN15 electrode due to the higher ionic conductivity. This low Rct value is mainly due to the high dislocation density which leads the higher ionic conductivity (reactivity) which is confirmed in XRD results [42,43]. This observed result further supports the CV behavior.
3.5.2. Charge–discharge behavior Based on the above results, further the galvanostatic charge–discharge analysis was carried out for MN-15 sample. The galvanostatic charge–discharge curves of MN-15 sample are given in Fig. 8. The typical results are measured from 0 to 1.0 V in 1 M NaNO3 electrolyte at different current densities (0.5, 1.0, 3.0, and 5.0 mA cm−2 ). The charge and discharge curve is almost symmetric which indicates that the MN-15 electrode material has high reversibility and reactivity. At the beginning of discharge curve, a small IR or potential drop was found which is mainly due to the internal resistance of the electrode material. The IR drop value is increased with increasing current density [44,45]. The specific capacitance (SC) of the electrode material is calculated and the specific capacitance values of MN-15 electrode are 103, 68, 56, and 47 F g−1 at different current densities of 0.5, 1.0, 3.0, and 5.0 mA cm−2 , respectively. Fig. 9 shows the variation of capacitance and active site [34] with the current density. It shows that at low current density, the discharge capacitance is high due to the available sites in MN-15 electrode material and is fully contributed to the electrochemical reaction. Similarly, at higher
Fig. 7. Nyquist plot for Mn3 O4 electrodes. The insert is the equivalent circuit.
Fig. 8. Galvanostatic charge/discharge curves for the optimized MN-15 electrode at a current density of (a) 0.5, (b) 1, (c) 3 and (d) 5 mA/cm2 .
Sample
R1 ()
RCT ()
MN-1 MN-5 MN-10 MN-15 MN-20
5.757 5.603 6.104 5.726 5.337
2.084 1.930 2.031 0.553 1.791
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given in Fig. 10 (inset). The capacitance of the materials increased from 47 to 68 F g−1 during cycling, where the capacitance of the material is almost maintained after 60th cycle (66–68 F g−1 ). This result indicates that the cycling process may induce the phase or morphological changes or activation effect in Mn3 O4 electrode material. The similar type of observations was already reported [7,11,38,47,48]. A possible phase change mechanism of Mn3 O4 during cycling was expressed as [15] Mn3 O4 → Naı MnOx · nH2 O
(4)
Naı MnOx · nH2 O+yH+ + zNa+ + (y + z)e− Naı+z MnOx · nH2 O (5)
Fig. 9. Specific capacitance and active site variation as a function of current density.
current density, only limited number of available sites is contributing to the electrochemical reaction in turn lower capacitance. It reveals that the specific capacitance is inversely proportional to the current density. The energy (dE ) and power density (dP ) [34,46] was calculated using the following expressions dE =
1 2 CV (Wh kg−1 ) 2
(2)
dP =
dE (kW kg−1 ) t
(3)
where C is the discharge capacitance of MN-15 electrode material F g−1 , V is the operating potential window in volt, and t is the discharge time in s. The maximum obtained energy and power density is 14.33 Wh kg−1 and 48.7 W kg−1 at a current density of 0.5 mA cm−2 . 3.5.3. Cyclic stability In addition, the cyclic stability of the electrode material is important for supercapacitor applications. In order to find the cyclic stability of MN-15 sample, the sample was cycled up to 100 cycles in 1 M NaNO3 electrolyte at a current density of 5 mA cm−2 and the calculated specific capacitance and the columbic efficiency as a function of cycle number is given in Fig. 10. The representative charge–discharge curves for the first 10 cycles are
The reaction (4) represents the phase change of Mn3 O4 into Mn oxides and is irreversible reaction. The later reaction (5) represents the reversible cycling of Mn oxides. The cyclic stability and enhancing of specific capacitance indicates that the prepared Mn3 O4 electrode material is suitable for supercapacitor electrode. Further, the coulombic efficiency is a measure of the charge transfer in an electrochemical system contributing an electrochemical reaction. During charge–discharge cycles, the increase of coulombic efficiency indicates the higher reversibility of Mn3 O4 nanoparticles. 4. Conclusion In summary, Mn3 O4 nanospheres were prepared by microwave assisted reflux synthesis. The phase formation, purity of sample, and the presence of functional groups are identified through XRD, FT-IR, and Raman analysis. The SEM image shows the formation of nanospheres. The CV and EIS results implies that the sample MN15 has high specific capacitance (135 F g−1 ) and low charge transfer resistance (0.553 ), respectively. The lowering of charge transfer resistance in MN-15 may be due to the high dislocation density. The charge–discharge study was carried for MN-15 electrode and its discharge capacitance was 103 F g−1 at 0.5 mA cm−2 . The obtained energy and power density is 14.33 Wh kg−1 and 48.7 W kg−1 at current rate of 0.5 mA cm−2 . The MN-15 electrode has good cyclic stability at extended cycles and the capacitance of electrode material increased from 47 to 68 F g−1 . The electrochemical performance of Mn3 O4 electrodes reveals that the MN-15 electrode provides a better result for supercapacitor application. References
Fig. 10. Effect of number of cycles on the specific capacitance and coulombic efficiency. The insert figure is the charge/discharge cycles (first ten cycles) of MN-15 electrode.
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