Amorphous Carbon Nanoparticles as Electrode

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Synthesis of Mn3O4/Amorphous Carbon Nanoparticles as Electrode Material for High Performance Supercapacitor Applications. Nagamuthu Sadayappan, Vijayakumar Subbukalai, and Muralidharan Gopalan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef400212b • Publication Date (Web): 07 May 2013 Downloaded from http://pubs.acs.org on May 19, 2013

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Synthesis of Mn3O4/Amorphous Carbon Nanoparticles as Electrode Material for High Performance Supercapacitor Applications Sadayappan Nagamuthu, Subbukalai Vijayakumar and Gopalan Muralidharan* Department of Physics, Gandhigram Rural Institute - Deemed University, Dindigul, Tamilnadu, India. Fax: +91 451 2454466; Tel: +91 451 2452371; E-mail: [email protected] KEYWORDS: “Green chemistry, Supercapacitors, Mn3O4 nanoparticles”. ABSTRACT: Mn3O4/Amorphous carbon nanoparticles have been synthesized via green chemistry route. Dextrose was used as the reducing agent and starch was used as the capping agent. The x–ray diffraction patterns reveal the Hausmannite tetragonal structure of the synthesized Mn3O4 particles. EDAX analysis confirms the presence of carbon and stoichiometry of Mn3O4. Morphological studies reveal the nanospherical nature of the synthesized particles. The FTIR spectra confirm the presence of Mn–O bonds.

Mn3O4/AC 500 exhibits highest specific

capacitance of 522 F g-1 at a specific current of 1A g-1, when measured from the chargedischarge process. This value is superior to previous reports on Mn3O4 nanoparticles as an electrode for supercapacitors. Higher energy density of 58.72 W h kg-1 could be observed for Mn3O4/AC 500 which is higher than Lead acid batteries and comparable to those for the Nickel hydride batteries. These results indicate that Mn3O4/AC 500 is a promising electrode for supercapacitor applications.

ACS Paragon Plus Environment

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1. Introduction

The diminishing fossil reserves and rapid growth of population have stimulated the urge to develop new energy storage devices. Supercapacitors occupy a place between batteries and conventional capacitors in the Ragone plot (energy density Vs power density). Batteries deliver high energy density and low power density, whereas conventional capacitors deliver high power density and low energy density. This has triggered the need to develop the supercapacitors as alternates to batteries and conventional capacitors. Some of the salient features of the supercapacitors are: long term cycling stability, operating safety, environment benignity and high power uptake and delivery. Based on the energy storage mechanism, the supercapacitors are classified into two types, namely EDLC (Electric double layer capacitors) and pseudocapacitors. For EDLC, carbon materials (activated carbon, carbon nano tube, and graphene) are used as the electrode material. The charge storage is by ion adsorption/desorption at the electrode/electrolyte interface

1-3

. In the case of pseudocapacitors, transition metal oxides and conducting polymers

are used as the electrode material. These undergo Faradic redox reactions

4-8

. EDLCs exhibit

high stability but yield lower capacitance. The lower capacitance has necessitated the need to identify suitable pseudocapacitor materials. The electrochromic behaviour of transition metal oxides indicates their ability to store charges for long period without appreciable leakage 9. This property makes them suitable for pseudocapacitor applications. RuO2 is known to exhibit high electrochemical performance

10

. However its toxic and

expensive nature limits the large scale production for commercial exploitation. Hence large numbers of studies are focused at finding low cost metal oxides as a replacement to RuO2. Oxides of manganese are suitable for supercapacitor application owing to their natural abundance, environment friendly nature and ability of manganese to exist in various oxidation


2

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states. The various oxidation states of manganese result in the formation of MnO2, Mn2O3 and Mn3O4

11-14

. Among these, Mn3O4 plays an important role in supercapacitors application. It is

known to exhibit high specific capacitance and long term cycling stability 15-16. Manganese oxide nanostructures have been synthesized using microwave emulsion method, hydrothermal method, precipitation method and sol gel method etc. Unfortunately most of these methods are hazardous and highly toxic due to the solvents and reducing agents that are used for the synthesis. Due to increased awareness on environmental pollution, the current emphasis is on identifying alternate pathways of synthesis through environment benign green chemistry methods. The green chemistry methods facilitate preparation of nanoscale materials through environment benign, less toxic routes and help to minimize the hazardous waste generated during the process. The same is based on the 12 fundamental principles of green chemistry

17

. The application of these principles for preparation of nanoscale materials offers

products and processes which are inherently safe. Zehra et al

18

have reported preparation of

Mn3O4 nanoparticles via green chemistry approach. They have used natural but expensive ionic liquid medium for the synthesis. In this work we have concentrated on cost effective green chemistry route to synthesize Mn3O4 as an electrode material. Recently Lee et al.,

16

have reported supercapacitor behavior of Mn3O4/graphene.

They

estimated the specific capacitance and cycle life of the electrode material using both CV and chronopotentiometry analysis and could obtain specific capacitance of 114 and 121 Fg-1 respectively from CV and chronopotentiometry analysis. They compared the various works on pure Mn3O4 and carbon based Mn3O4 nanocomposites for supercapacitor applications. From the report of Lee et al., it is noted that the maximum specific capacitance achieved for Mn3O4 and Mn3O4 nanocomposites was 322 Fg-1.

To the best of our knowledge, for the first time we are

reporting high specific capacitance, high energy density and stability against cycling of


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Mn3O4/AC

nanoparticles.

Amorphous

carbon

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based

nanoparticles

enhance

the

specificcapacitance, cycle life of the electrode material and also prevent the aggregation of nanoparticles. Encouraged by good supercapacitor behavior exhibited by carbon based Mn3O4, we have synthesized Mn3O4/AC nanoparticles via green chemistry route. Here we have used dextrose as the reducing agent. Starch has been used as the capping agent. This has resulted in the formation of Mn3O4/amorphous carbon (Mn3O4/AC) nanoparticles. These have been calcined at different temperatures (400 oC, 500 oC, and 600 oC). The particles were prepared by adjusting the temperature and pH of the solution. The supercapacitor behavior of Mn3O4/AC has been studied and the results are presented and discussed in this paper. 2. Experimental All the reagents used in the present work were of analytical grade. Manganese nitrate (98%) was purchased from Sigma Aldrich, Dextrose and starches were purchased from Merck and NaOH was purchased from CDH. These chemicals were used to synthesize Mn3O4/AC nanoparticles. The typical procedure adopted for preparation of Mn3O4/AC is as follows: 0.3423 g of soluble starch was dissolved in 200 mL of distilled water. 20 mL of 0.1 M manganese nitrate and 30 mL of 0.1 M dextrose were added to the prepared starch solution. To this, 1 M NaOH was added drop by drop to adjust the pH of the solution to 10 and the solution was stirred continuously for 3 hours. Finally the precipitate formed in the process was dried at 70 oC for 96 hours. The samples were calcined at various temperatures (400 oC, 500 oC, and 600 oC) for an hour. The calcined samples are named as Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600, the numbers indicating calcining temperature.


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The x-ray diffraction patterns of the samples were recorded using Panalytical XPERTPRO x-ray diffractometer with CuKα radiation.

Scanning electron microscopy (SEM) and

EDAX analysis were made using JEOL – JSM and transmission electron microscopy (TEM) imges were obtained with HITACHI H -7100 KVA to study the morphology of the samples Mn3O4/AC. Multipoint N2 adsorption-desorption experiment was carried out on Quadrasorb Station 1 analyzer using the BET gas adsorption method, at 77 K. The electrochemical properties of prepared samples were studied using CHI-660D electrochemical workstation. The working electrodes were prepared by mixing of 70% Mn3O4/AC as the active material, 20% of activated carbon, 10% PTFE (poly tetra fluoro ethylene) as the binder and few drops of ethanol was used as the solvent. 1 mg of the active material was pasted on a graphite sheet substrate (1cm x 1cm). The substrate was dried at 70 oC for 8 hours. This served as the working electrode, a saturated calomel electrode (SCE) was used as the reference electrode and a platinum wire was used as the counter electrode. 1.0 M aqueous Na2SO4 was used as the electrolyte.

3. Result and discussion: 3.1. Formation of Mn3 O4 This mechanism is similar to the classical reaction of glucose with Tollens reagent. A schematic representation of the mechanism of formation of Mn3O4 /amorphous carbon nanoparticles is presented in Figure 1. The formation can be explained through the following reactions 19-21 as well. Mn(NO3)2+ CH2OH (CHOH)4 CHO + Starch

NaOH

CH2OH (CHOH) 4 COOH + Mn2+ (starch) + OH-1 Mn(OH)2(starch)

partial oxidation

Mn (II, III) (starch) ions

[MnOOH + Mn(OH)2] (starch)

nucleation

calcination

dissolution

hausmannite Mn3O4/ (AC)


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Fig 1 Schematic representation of Mn3O4 / AC formation

The addition of NaOH to the solution converts dextrose to gluconic acid due to opening up of the glucose ring by abstraction of α – proton of the sugar ring oxygen. Hence initially the pH of the solution decreases. Addition of NaOH increases the pH of the solution. The Mn2+ ions react with OH- ions to form the Mn(OH)2 and partially oxidize MnOOH. But Mn(OH)2 and MnOOH are not stable in the reaction time. During the course of the reaction Mn(OH)2 and MnOOH dissolve in the solvent to form Mn (II, III) ions. These ions nucleate and enable the growth of Mn3O4 (starch) (due to the high surface energy of metal ions) leading to the formation of nanoparticles. The samples were treated at 70 ºC for 96 hours and calcined at (400,500 and 600 ºC) for an hour.

The particles formed using this methodology is named as Mn3O4/AC

nanoparticles. 3.2. Structural studies X-ray diffraction patterns of the Mn3O4/AC particles are shown in figure 2. The xray diffraction patterns reveal the formation of crystalline trimanganese tetra oxide (Mn3O4) nanoparticles. The crystalline nature of Mn3O4/AC nanoparticles increase due to the increase of calcination temperature from 400 oC to 600 oC.

Tetragonal Hausmannite Mn3O4 crystal

structure has been observed from the XRD pattern and the peak positions agree with the JCPDS


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card no 24–0734 with the peak at 2θ values of 18.05o, 29.07o, 32.47o, 36.07o, 44.72o, 50.97o, 58.840o, 60.02o, 64.67o, 74.42o which correspond to the (101), (112), (103), (211), (220), (105), (224), (321), (400), (413) planes of body centered tetragonal manganese oxide respectively for the Mn3O4/AC 600. With the increase of calcination temperatures the new peaks at 58.84°, 64.67° and 74.42° appear indicating the increase in the crystalline nature of Mn3O4/AC 600 sample. The poor crystalline nature of Mn3O4/AC 400 and Mn3O4/AC 500 samples offers high electron transport at electrode /electrolyte interface. Fourier transform infrared spectrum (FTIR) of the synthesized nano particles is shown in figure 3. FTIR spectra of all samples exhibit bands in the range of 400 cm-1 to 700 cm-1 attributable to Mn3O4 octahedral sites (MnO-Mn2O3 with Mn2+, Mn3+). Mn3O4 vibration frequencies at 489 cm-1 and 600 cm-1 correspond to Mn-O stretching mode in tetrahedral site and vibration of Mn-O in an octahedral environment 22. A broad band at 1026 cm-1 is assigned to the stretching mode of C-O 23. The absorption bands at 2353cm-1 is related to carbon dioxide 23. The

c Transmitance (a.u)

(413)

(400)

(105)

(220)

(321) (215)

Mn3O4/AC 400 Mn3O4/AC 500 Mn3O4/AC 600

(112) (103) (211)

(101)

absorption bands at 3381 cm-1 and 1625 cm-1 are attributable to the O-H vibrations.

Intensity (a.u)

b

3400 10

20

30

40

50

60

70

2400

1400

600 489

1026

1625

2353

a 3381

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400

80

Wavenum ber(cm -1)

2 theeta (degree)

Figure 2: XRD pattern of Mn3O4/AC samples

Figure 3: FTIR spectra of the Mn3O4/AC samples a) 400 °C, b) 500 °C and c) 600 °C


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3.3 Morphological Studies The morphology of the Mn3O4/AC samples was characterized by SEM and TEM. Fig 4 (a, b and c) shows the SEM images of Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600. These images reveal the formation of Mn3O4 as spherical nanoparticles. The image corresponding to Mn3O4/AC 400 indicates a partially attached particle structure. This attachment of the spherical particles is probably due to the template effect of macromolecule starch, which provides fixed template and it may be converted to carbon on heat treatment. Mn3O4/AC 500 shows well dispersed and almost uniform spherical shaped nanoparticles.

This is possible due to the

elimination of the carbon when the particles are treated at temperatures higher than 400 oC, the same is supported by EDAX measurements, discussed later in this section. Mn3O4/AC 600 reveals formation of larger particles. Fig 4 (d and e) show the TEM images of (Mn3O4/AC 500). The prepared sample has slack structure. From these images we could observe the formation of uniform spherical nanoparticles. This uniform structure helps in efficient electron transport from the electrode/electrolyte interface. It also can be noted that the TEM image of the Mn3O4/AC 500 is not very clear, which may be a result of amorphous carbon coating on the surface of the Mn3O4 particles24. Fig 5 shows the EDAX patterns of Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600 samples. It revealed the presence of carbon in addition to manganese and oxygen, indicating that the capping agent (starch) was converted to carbon by the heat treatment. Thus it appears that the heat treated samples are capped by carbon (obtained by the charring of starch). It could be observed that at higher calcination temperatures the percentage of carbon decreases, possibly due to the carbon burn off. Samples calcined at 400 ºC contain 11% of carbon while the carbon content in the samples calcined at 500 ºC and 600 ºC are 10% and 9% respectively.


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a

Mn3O4/AC 400

Mn3O4/AC 500

b

e

Mn3O4/AC 500

Mn3O4/AC 600

c

d

Mn3O4/AC 500

Figure 4 (a,b and c) SEM images of the Mn3O4/AC samples (d and e) TEM image of Mn3O4/AC 500 sample

O

O

Mn

Mn

C Mn

1

3

5

KeV

7

Mn3O4/AC 600

Mn

Mn

C

-1

O

Mn3O4/AC 500

Mn3O4/AC 400

Mn

C 9

-1

1

3

5

7

9

-1

0

KeV

1

2

3

4

5

6

7

8

9

KeV

Figure 5: EDAX pattern of Mn3O4/AC samples


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120

a

0.75

b

100

Mn3O4/AC 500

-1

0.70

3

Pore volume/ cm g

80

-1

(Mn3O4/AC 400) (Mn3O4/AC 500) (Mn3O4/AC 600)

3

Adsorbed volume/cm g

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

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60 40 20

0.65

0.60

0.55

0 0.50

0.0

0.2

0.4

0.6

0.8

1.0

0

2

Relative Pressure(P/Po)

4

6

8

10

12

14

Pore size/nm

Figure 6. (a) N2 adsorption/desorption isotherms of Mn3O4/AC samples (b) BJH pore size distributions of Mn3O4/AC -500 sample The pore size distribution and surface area of Mn3O4/AC samples are examined by nitrogen adsorption – desorption method. The isotherm profiles are shown figure 6(a). All the Mn3O4/AC samples exhibits type IV hysteresis loop according to the IUPAC classification25. The isotherm profiles illustrate the N2 uptake in the P/Po region of up to 0.8 in the mesoporous region. From these results evidenced by all the Mn3O4/AC samples have the mesoporous structure but Mn3O4/AC 500 has high adsorption volume due to the high pore volume. The pore size distribution of Mn3O4/AC 500 is shown in figure 6(b) and S1 (Supporting information) shows the pore size distribution of Mn3O4/AC 400 and Mn3O4/AC 600 which was estimated from the N2 adoption – desorption branches by the BJH method. The peaks are centered at 2.1 and 2.7 nm for Mn3O4/AC 400, 2.7 and 8.5 nm for Mn3O4/AC 500 and 1.8, 2.7 and 5.7 nm for Mn3O4/AC 600 samples. The pore radius of supercapacitor electrode material size grater than 1 nm is more suitable for aqueous neutral electrolytes26. Among these, ore size distribution of

Mn3O4/AC

500 has the narrow pore size and higher pore volume (0.67 cm3 g-1) compare than Mn3O4/AC 400 and Mn3O4/AC 600. The narrow pore size and higher pore volume offers to the effective electron transport at the electrode / electrolyte interface.


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Energy & Fuels

The BET surface areas of Mn3O4/AC samples, Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600 are measured to be 28.547, 38.494 and 8.240 m2 g-1 respectively. The calcination temperature strongly affects the morphology and elimination of carbon. Mn3O4/AC 600 has the lowest surface area due to the morphology of the sample is damaged and elimination of the carbon at higher temperature. BET analysis demonstrates the Mn3O4/AC 500 has the better pore volume and high surface area is suitable for supercapacitor electrode materials. 3.4. Electrochemical studies

Cyclicvoltammetry, galvanostatic charge-discharge and electrochemical impedance analysis were carried out on the Mn3O4/AC electrode materials. Figure 7(a) shows the cyclicvoltammetric curves of the Mn3O4/AC electrodes recorded at a constant scan rate of 5 mV s-1. The quasi rectangular shape of the CV curves indicates the ideal capacitance nature along with pseudocapacitance of the electrodes. The charge storage mechanism proposed for the oxidation reduction process is given below 27

MnOx (OH)y + δH+ + δe-

↔

MnOx - δ (OH)y + δ

Where MnOx (OH)y and MnOx - δ (OH)y + δ stand for the oxymanganese species under the higher and lower oxidation states respectively. Figure 7(b) and S2 (supporting information) shows the CV curves recorded at various scan rates for Mn3O4/AC 500, Mn3O4/AC 400 and Mn3O4/AC 600. It is observed that the increase of scan rate leads to a decrease in the specific capacitance, due to the electrode/electrolyte interface. At higher scan rates, the ions were intercalated probably only on the surface of the electrode whereas at lower scan rate the ions could diffuse into the inner active sites as well, since lower scan rates provide larger lengths of time permitting better intercalation of the ions with the active sites.

Figure 7(c) gives the variation of specific


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capacitance with scan rate of Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600. From this curve we could come to the conclusion that Mn3O4/AC 500 is a better electrode for the supercapacitor application. Figure 7(d) shows the linear relationship between ν1/2 Vs I (square root of the scan rate Vs average current).

This is indicative of the electrochemical reactions

being controlled by diffusion limited reactions. The ion transport seems to be exclusively through diffusion process in the diffusion layer driven by the difference in the concentration of the electrolyte between the edge of the diffusion layer and the surface of the electrode. As the electrode potential is increased, the concentration of the electrolyte on the electrode depletes leading to an increase in the diffusion and provides the required flux of the electrolyte. However, the surface concentration of electrolyte obviously cannot decrease below zero, there by a situation is reached where any further change of the electrode potential does not necessarily alter the electrolyte flux near the surface of the electrode. The further change of potential is valid for absence of supporting electrolyte. In the present work we did not use any supporting electrolyte hence the reaction is controlled by diffusion limited reactions. Recently Jiang et al.

15

have

reported enhanced specific capacitance of Mn3O4 nano octahedrons (322 F g-1). In the present work we have obtained a specific capacitance of

504 F g-1 with Mn3O4/AC 500 electrode.

Reason for this drastic change may be that the surface to volume ratio of nanoparticles is better than that of nano octahedrons and the presence of 10% amorphous carbon. Using the CV curves, specific capacitance was calculated using the following formula 28

C=

Q -------------------------- (1) m∆V


12

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4

20

a

15

b

-1

Specific current (Ag )

10

-1

Specific current (Ag )

2

0


-2

5 0 -5 -1

5 mV s -1 10 mV s -1 25 mV s -1 50 mV s

-10 -15

-4

-20

0.0

0.2

0.4

0.6

0.8

0.0

0.2

Potential (V)

0.4

0.6

0.8

Potential (V)

550

11 Mn3O4/AC 400 Mn3O4/AC 500 Mn3O4/AC 600

c

500

10

Mn3O4/AC 500

d

9

-1

450 400

Average current

Specific capacitance Fg

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350 300 250 200

8 7 6 5 4 3

150 2 100 0

10

20

30

40

50

2

Scan rate (mV)

3

4

5

6

7

square root of the scan rate

Figure 7(a) CV curves of of Mn3O4/AC electrodes at 5 mVs-1, (b) CV cure of Mn3O4/AC 500 at different Scan rate, (c) Scan rate versus specific capacitance for all electrodes, (d) ν1/2 Vs current for the Mn3O4/AC 500

Where C is the specific capacitance (F g-1), Q is the average charge during anodic and cathodic scan; m is the mass of the active material (g) and ∆ V is the potential window (V). The specific capacitance values are 360, 504 and 340 F g-1, corresponding to the electrodes Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600 at a scan rate of 5 mV s-1 within the potential range of -0.1 to 0.8 V. These results suggest that the electrode Mn3O4/AC 500 has better electrochemical energy storage property compared to the other two, namely Mn3O4/AC 400 and Mn3O4/AC 600. The uniform morphological feature (nanoparticles) and high surface area of Mn3O4/AC500 and 10%


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amorphous carbon probably facilitate the better ion intercalation/deintercalation involving the active sites of the electrode material. Galvanostatic charge-discharge curves of Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600 electrodes at constant current of 1 A g-1 are presented in figure 8(a). The chargedischarge curves exhibit the pseudocapacitance nature of the Mn3O4/AC electrodes. The increase in the charging-discharging time represents a higher specific capacitance of Mn3O4/AC 500 electrodes. Figure 8(b) and S3 (supporting information) shows the charge discharge curves of Mn3O4/AC 500, Mn3O4/AC 400 and Mn3O4/AC 600 at different specific current of (1A g-1, 2.5 A g-1, 5 A g-1 and 10 A g-1). From these curves, it is seen that, at higher specific current the specific capacitance values decreases due to the intercalation of ions at the surface of the active materials in the electrode/electrolyte interface, and 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. The specific capacitance of the electrodes are calculated using the following equation 29

C=

i∆t m∆V

--------------------------- (2)

Where C is the specific capacitance (Fg-1), i is the specific current (A), ‘ ∆t ’ the discharge time, ‘m’ the mass of the active material and ‘ ∆V ’ the potential window (V). The estimated specific capacitance values of Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600 are 333, 522 and 411 Fg-1 at a constant specific current 1 Ag-1 within the potential range -0.1 V to +0.8 V. Mn3O4/AC 500 which exhibits uniform nanoparticle in the TEM offers the highest specific capacitance amongst the three electrodes. Figure 8(c) represents specific capacitance as a function of charging current for particles treated at different temperature. These results are


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similar to those obtained from CV curves, indicating Mn3O4/AC 500 to yield the best electrochemical performance for supercapacitor electrode material.


a

-1

0.6 Potential (V)

Potential (V)

0.6

1 Ag -1 2.5 Ag -1 5Ag -1 10 Ag

b

0.8

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0.0 0

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Time (s)

Time (s)

500 -1

c

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S p e c if ic c a p a c it a n c e ( F g - 1 )

2 50 Mn3O4/AC 400 Mn3O4/AC 500 Mn3O4/AC 600

0

d 20

200

40 150 60 10 0 80 50

10 0

0 0

2

4

6

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12 0 0

50 0

10 0 0

cycles

-1

Specific current Ag

Figure 8(a) Charge-discharge curve of Mn3O4/AC at 1Ag-1, (b) Charge-discharge curve of Mn3O4/AC 500 electrode at different specific current, (c) Current versus specific capacitance curve, (d) Cycling stability and columbic efficiency of Mn3O4/AC 500 electrode


15

C o lu m b ic e f f ic ie n c y ( % )

0.8

Specific capacitancce Fg

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An important requirement of supercapacitor for practical applications is the stability of the electrode with cycling at high rates. The cyclic stability test has been carried out with galvanostatic charge-discharge technique at a constant specific current

of 10 A g-1 in the

potential range -0.1 V to +0.8 V for 1400 continuous cycles. Figure 8(d) and S4 (supporting information) shows the cycling stability and Columbic efficiency of Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600. From the stability curve, it is observed that all the three Mn3O4/AC electrodes have excellent cycling stability over 1,400 cycles without degradation of capacitance. Notably, the specific capacitance values of Mn3O4/AC-500 electrode increases over 1,400 charge-discharge cycles from 183 to 223 F g-1. All the three electrode materials exhibit good results for long term cycling stability but Mn3O4/AC 500 exhibits superior result compared to Mn3O4/AC 400 and Mn3O4/AC 600. The increase of specific capacitance and cyclic stability are probably due to (i) increasing active sites due to continuous intercalation- deintercalation of the ions in the electrode. (ii) the low resistance of the current collector (graphite) and uniform nanoparticle morphology which facilitates the

effective electron transport from the

electrode/electrolyte interface and (iii) Amorphous carbon enhancing the cycling stability of the electrodes 30. The Columbic efficiency was calculated from the following relation 31

η=

td × 100 % ------------------------ (3) tc

Where η is the columbic efficiency, td is the discharging time, tc is the charging time. The columbic efficiency was calculated after every 50 cycles and is shown in the figure 8(d). The columbic efficiency of the Mn3O4/AC 500 exhibits 100% after 1400 cycles32.


16

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Energy & Fuels

Other key factors in energy storage applications of supercapacitors are energy density (ED) and power density (PD). ED and PD Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600 electrodes were calculated using the following equations 32

E=

1 CV 2---------------------- (4) 2

P=

E --------------------------- (5) t

Where E is the energy density (W h kg-1), C is the specific capacitance (F g-1), ‘V’ the potential (V), ‘P’ is the power density (W kg-1) and ‘t’ the discharging time (s). The energy density and power density of the Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600 electrodes were presented in the Ragone plot of figure 9. Mn3O4/AC 500 reveals the high energy density and power density of 58.72 WhKg-1 and 451.6 Wkg-1 respectively. In the present work we have achieved the higher energy density which is higher than Lead acid battery and comparable to the Nickel hydride batteries 2. Another fundamental study regarding supercapacitors is examining the electrodes through electrochemical impedance measurements. Fig 10 shows the Nyquist plot of the electrodes Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600, measured in the frequency range of 0.01Hz to 100 kHz at an open circuit potential of 0.5 V. The spectrum were fitted to an equivalent circuit comprising of an electrolyte resistance Rs, double layer capacitance behavior Cdl, charge transfer resistance Rct, Warburg element Zw and pseudocapacitance Cp 1. The semicircles intercepts the real axis is combination of ionic resistance of electrolyte Rs and charge transfer resistance Rct is obtained from the Nyquist plot. The Rct values are 1.326 Ω, 0.604 Ω and 0.975 Ω corresponding to the electrodes Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600. It


17

Energy & Fuels

controlled the ion diffusion in the electrode/ electrolyte interface. In the lower frequency region the tail is called the Warburg resistance or diffusion resistance (Zw). From this study we could observe that Mn3O4/AC 500 has the lowest Rct value. Hence Mn3O4/AC 500 is the better candidate for supercapacitor application.

10

5000 Mn3O4/AC 400 Mn3O4/AC 500 Mn3O4/AC 600

4000


8

6

3000

Z''/Ώ

-1

Power density (WKg )

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

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4

2000

2

1000

0 0 5

10

15

20

25

30

35

40

45

50

55

60

0

Energy density (WhKg )

Figure 9 Ragone plot of Mn3O4/AC electrodes

2

4

6

8

10

Z'/Ώ

-1

Figure 10 Nyquist plot of Mn3O4/AC 400, Mn3O4/AC 500 and Mn3O4/AC 600

4. Conclusion In conclusion, Mn3O4 nanoparticles have been synthesized by green chemistry method. Through the x-ray diffraction studies the Hausmannite tetragonal structure of Mn3O4 has been observed. FTIR spectra confirm the Mn-O vibrations. SEM and TEM images show the Mn3O4/AC nanoparticles. From the current work, it has been identified that Mn3O4/AC 500 is a better electrode material for supercapacitor applications. Higher energy density of 58.72 WhKg-1 was observed for Mn3O4/AC 500 which was better than Lead acid batteries and comparable to the Nickel hydride batteries.


18

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Energy & Fuels

ACKNOWLEDGMENT The authors thank to the Ministry of New and Renewable Energy (MNRE) –India for providing the

financial

support

through

National

Renewable

Energy

Fellowship

Scheme

(NREF/TU/2011/17). ASSOCIATED CONTENT

Supporting Information. Pore size distribution of Mn3O4/AC 400 and Mn3O4/AC 600. Various scan rate of Mn3O4/AC 400 and Mn3O4/AC 600 electrode spectra. Various specific current for the Mn3O4/AC 400 and Mn3O4/AC 600 electrode spectra. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION

Corresponding Author Dr. G. Muralidharan, Professor of Physics, Gandhigram Rural Institute- Deemed University, Dindidgul, Tamilnadu, India. Fax: +91 451 2454466; Tel: +91 451 2452371; E-mail: [email protected] . REFERENCES 1. Conway,.B.E.; Electrochemical supercapacitors, ed Kluwer-Plenum,1999. 2. Chengguang Liu, Zhenning yu, David neff, Aruna Zhan and Jang Bor Z. Graphene – based supercapacitor with ultra high energy density. Nano Lett. 2010, 10, 4863-4868. 3. Caran Masarapu, Hai feng Zeng, Kai Hasun Haung, and bingqing wei. Effect of temperature on the capacitance of carbon nanotube supercapacitors. ACS NANO. 2009 3, 2199-2206.


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SYNOPSIS TOC


24

Amorphous Carbon Nanoparticles as Electrode

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