Cathodic Potentiostatic Electrodeposition and Capacitance Characterization of Manganese Dioxide Film

Huo Yuqiu, Zhou Hongcheng School of Science, Northeastern University, Shenyang 110819, China

huoyuqiu@sina.com

Abstract—MnO2 film was obtained on stainless steel surface by cathodic potentiostatic electrodeposition method in KMnO4 aqueous solution in a three-electrode electrochemical cell. The specific capacitance of MnO2 film was studied by cyclic voltammetry method in 0.1M Na2SO4. The results indicated that the film electrolyzed 1.5h under 0.55V voltage in 0.4M KMnO4 solution with pH value of 12 had the largest specific capacitance of 232.94 F.g-1.

Keywords-electrodeposition; KMnO4; MnO2; cyclic voltammetry; capacitance

I. INTRODUCTION Electrochemical capacitors (ECs) are becoming attractive

energy storage systems for their potential applications such as power sources for camera flash equipment, cellular phones, batteries and fuel cells. The energy stored in the ECs is either capacitive or pseudocapacitive in nature. The capacitive (nonfaradaic) process is based on charge separation at the electrode/ electrolyte interface, whereas the pseudocapacitive (faradaic) process relies on redox reactions that occur in the electrode materials [1]. Transition-metal oxides have been identified as possible electrode materials for ECs. Among the many transition-metal oxide materials, the amorphous phase of RuO2·xH2O formed by the sol-gel method at low temperatures shows excellent pseudocapacitive behavior with a large specific capacitance of as high as 720 F.g-1 and excellent reversibility [2]. However, the high cost of ruthenium and the environmental problems associated with the use of strong acidic electrolytes have limited its commercial use. Therefore, it is urgent to investigate alternate transition-metal oxides, which are cheap, available in abundance, nontoxic, and environmentally friendly. Manganese dioxide has attracted much attention because it has these favorable properties and manganese oxide electrode materials have been prepared by various synthetic methods [3-6]. Manganese dioxide was in various structures prepared with different methods or conditions. Different structure and morphology had different affect on their capacitive property [7, 8]. In this article the manganese dioxide film electrodes were obtained by cathodic electrodeposition method with the potassium permanganate as precursor material. The capacitance of the manganese oxide films were investigated by cyclic voltammetry (CV) in 0.1 M Na2SO4 alkaline solution. The articles related to the preparation of manganese dioxide with the potassium permanganate as original material were focus on several methods such as hydrothermal synthesis method [9-11],

redox reaction method [12-14] or decomposition method [15,16]. Cathodic electrodeposition method has not been reported as far as we know.

II. EXPERIMENTAL

A. The pretreatment of stainless steel electrode The stainless steel slice was cut into the size of 1cm×1cm

and then taken as working electrode. First, it was burnished with coarse sand paper, Second, ultrasonic oscillation for 5 min in 0.1M NaOH aqueous solution, 0.1M oxalic acid aqueous solution and acetone respectively. Third, it was dried in infrared fast drying oven. Finally it was weighed.

B. The preparation and characterization of manganese dioxide film electrode The electrochemical measurements were carried out

using a CHI660B in a three electrode electrochemical cell, in which the stainless steel slice electrode was used as the working electrode, a platinum plate as the counter, and a saturated calomel electrode (SCE) as the reference electrode. The manganese dioxide films were obtained by cathodic potentiostatic electrodeposition method under 0.55V voltage with potassium permanganate aqueous solution as precursor material. The working electrode was weighed before and after the deposition, in order to calculate the weight of the deposit. Cyclic voltammetry was performed to determine the electrochemical properties and specific capacitance of the manganese oxide film electrodes in 0.1M Na2SO4 (pH =6) at room temperature. The average specific capacitance (C) was calculated from the following equation

2qCVM

=Δ

(1)

where q is the voltammetric charge, M is the mass of manganese oxide, and ΔV is the potential range. The XRD patterns were determined by D/Max-IIIC (Rigaku, Japan) at room temperature with Cu Kα radiation. Morphology and Energy Dispersive Spectrometry (EDX) were obtained using a JEM-2000EX (JEOL, Japan). Infrared spectra of the samples were recorded on a FT-IR (Nicolet Impact 410, USA) by KBr disk method.

III. RESULTS AND DISCUSSION

A. The comparison experiment Cyclic voltammetry was performed to compare the

electrochemical properties of the stainless steel electrode and

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2010 International Conference on Nanotechnology and Biosensors IPCBEE vol.2 (2011) © (2011) IACSIT Press, Singapore

MnO2 film electrode in 0.1M Na2SO4 at a scan rate of 5mV.s-1. The results are shown in Fig. 1. It is obvious that the capacitance of MnO2 film electrode is far larger than that of stainless steel electrodes, so the influence error of stainless steel can be ignored.

B. Capacitance characterization of the manganese oxide film electrodes The cyclic voltammograms and specific capacitance of

MnO2 film electrodes prepared at pH values of 8.44, 11.93, 12.31, 13.71 were obtained in Fig. 2. The specific capacitance of MnO2 film electrodes was largest at the pH values of 11.93 and 12.31. It maybe because that the MnO2 film electrode that prepared in the strong alkali solution contain some OH- group among the lattice units, the following reaction will occur during the charging discharging process:

MnO2 + H2O + e- = MnOOH + OH- (2)

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4-0.003

-0.002

-0.001

0.000

0.001

0.002

0.003

12

curr

ent(A

)

potential(V vs.SCE)

Figure 1. Cyclic voltammograms of stainless steel electrode (1) and MnO2 film electrode (2).

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.002

-0.001

0.000

0.001

0.002

0.003(a)

4

3 21

1 2 3 4

curr

ent(A

)

potential(V vs.SCE)

8 9 10 11 12 13 14

60

80

100

120

140

160(b)

spec

ific

capa

cita

nc(F

.g-1

)

pH

Figure 2. Cyclic voltammograms (a) and specific capacitance (b) of electrodes prepared with different values of pH at a scan rate of 5 mV.s-1 in

0.1 M Na2SO4 aqueous electrolyte. 1-8.44; 2-11.93; 3-12.31; 4-13.71.

Thus it would aggregate lots of Mn3+ in the lattices and produce cationic spaces, as a result, the transfer rate of proton and alkali metal ion among these cationic spaces will be increased. But if the solution is in a more strong alkali condition, the number of the Mn3+ in the lattices will be reduced and the cationic spaces decreased either. So the specific capacitance decreased. In Fig. 3 the relation of electrolyze time and specific capacitance were represented.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4-0.006-0.005-0.004-0.003-0.002-0.0010.0000.0010.0020.0030.0040.005

543

21

1 2 3 4 5

(a)

curr

ent(A

)

potential(V vs.SCE)

12

345

1.0 1.5 2.0 2.5 3.0 3.5 4.0

160

170

180

190

200

210

220

230 (b)

spec

ific

capa

cita

nce(

F.g-1

)

time(h)

Figure 3. Cyclic voltammograms(a) and specific capacitance(b) of electrodes prepared with different electrolyze time at a scan rate of 5 mV

s-1 in 0.1 M Na2SO4 (aq). 1-1h; 2-1.5h; 3-2h; 4- 3h; 5- 4h.

For the film that prepared at 0.55V voltage, pH value equals to 12, the concentration of KMnO4 solution equals to 0.1M, there is a maximum on the curve of specific capacitance on the electrolyze time, this mean that there are two contrast factors determine the specific capacitance. At the beginning of the electrodeposition, both the specific capacitance and the amount of active material (MnO2) increased with the time. With time extension the amount of MnO2 increased but the thickness of the electrode also increased, the latter one would increase the resistance of the electrode. So the specific capacitance decreased after electrolyze 1.5h. In Fig. 4 the cyclic voltammograms of MnO2 film electrode and the relation of concentrations of KMnO4 on the specific capacitance were obtained. On the conditions of voltage equal to 0.55V, pH value is 12，and electrolyze time is 1.5h, the specific capacitance increased with the concentration of KMnO4 solution. This may because the amount of active MnO2 increase with the concentration of KMnO4 solution. The exact reason needs the further investigation.

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-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4-0.003

-0.002

-0.001

0.000

0.001

0.002

0.003(a)

543

21

1 2 3 4 5

curr

ent(A

)

potential(V vs.SCE)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

170

180

190

200

210

220

230

240(b)

spec

ific

capa

cita

nce (

F.g-1)

concentration(mol.L-1)

Figure 4. Cyclic voltammograms(a) and specific capacitances(b) of electrodes prepared with different KMnO4 concentrations at a scan rate of 5 mV.s-1 in 0.1 M Na2SO4(aq). 1-0.05M; 2-0.1M; 3-0.2M; 4-0.3M; 5-0.4M.

C. 3.3 Structure characterization of the manganese oxide film electrodes The X-ray diffraction pattern of the manganese oxide

film (prepared at 0.55V voltage, pH value equals to 12, the concentration of KMnO4 solution equals to 0.4M, electrolyze 1.5h) is shown in Fig. 5. It is an amorphous structure. Fig. 6 is the scanning electron micrograph of the same manganese oxide film. From the images we observed the film is flat rough, it mainly contains Mn and O elements. The S and Na elements may come from the Na2SO4 solution. Fig. 7 shows the FTIR spectra of MnO2 film, the regions from 800 – 500 cm−1 correspond to the Mn–O stretching and bending vibrations. The absorption of around 1700 and 1400 cm−1 in the spectrum is the H–O bending vibrations. The regions from 2800 – 3700 cm−1 correspond to the H–O stretching vibration [17].

IV. CONCLUTION In summary, MnO2 film can been obtained on stainless

steel surface by cathodic potentiostatic electrodeposition method at 0.55V voltage in KMnO4 aqueous solution. The capacitance property of MnO2 film was quite well in 0.1M Na2SO4. The film electrolyzed 1.5h under 0.55V voltage in 0.4M KMnO4 solution with pH value of 12 had the largest specific capacitance of 232.94 F.g-1. The X-ray diffraction pattern shows that the film is an amorphous structure.

0 20 40 60 80 1004000

6000

8000

10000

12000

14000

16000

18000

inte

nsity

(a.u

.)

2θ

Figure 5. XRD patterns of the manganese oxide films

1 2 3 4 5 6 7 8 9 10

Mn

(b)

S

C

KAlNa

OMn

Mn

Inte

nsity

Energy/keV

Figure 6. Scanning electron micrographs of the manganese oxide film. (a) Surface image, (b) EDX image.

4000 3500 3000 2500 2000 1500 1000 50060

65

70

75

80

85

90

95

100

trans

mitt

ance

(% )

Wavenumbers(cm-1)

Figure 7. FTIR spectra of MnO2 film electrode

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