J. Cent. South Univ. Technol. (2007)05-0638-05

DOI: 10.1007/s11771-007-0122-0                                                                                     

Effect of Ni-doping on electrochemical capacitance of MnO₂ electrode materials

ZHANG Zhi-an(张治安), LAI Yan-qing(赖延清), LI Jie(李  劼), LIU Ye-xiang(刘业翔)

(School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China)

Abstract: Mn/Ni composite oxides as active electrode materials for supercapacitors were prepared by solid-state reaction through the reduction of KMnO₄ with manganese acetate and nickel acetate at low temperature. The products were characterized by X-ray diffractometry(XRD) and transmission electron microscopy(TEM). The electrochemical characterizations were performed by cyclic voltammetry (CV) and constant current charge-discharge in a three-electrode system. The effects of different potential windows, scan rates, and cycle numbers on the capacitance behavior of Mn_0.8Ni_0.2O_x composite oxide were also investigated. The results show that the composite oxides are of nano-size and amorphous structure. With increasing the molar ratio of Ni, the specific capacitance goes through a maximum at molar fraction of Ni of 20%. The specific capacitance of Mn_0.8Ni_0.2O_x composite oxide is 194.5 F/g at constant current discharge of 5 mA.

Key words: MnO₂; supercapacitor; capacitance; Ni doping; composite oxide                  

1 Introduction

Supercapacitor is a promising energy storage device for meeting the high-power electric market^[1-4]. Complementary to battery, supercapacitor can provide superior power density and cyclability, thus it is regarded as an intermediate device between traditional capacitor and battery. Supercapacitors are generally categorized into two types based on the charge-discharge mechanism. The electric double-layer capacitance (EDLC) arises from electrostatics separation of charges at the interface between electrode and electrolyte. On the other hand, pseudocapacitance, typically 10 times greater than EDLC, results from either rapid adsorption/desorption superficial reaction or multi-electron-transfer faradic reactions with fast charge/discharge properties. Many transition metal oxides have been shown to be excellent electrode materials for supercapacitors whose charge storage mechanism is primarily based on pseudocapacitance. RuO₂, for example, delivers a high capacity up to 700 F/g and an excellent cyclability. But RuO₂-based supercapacitors are too expensive to be commercially attractive^[5-6]. Non-noble oxides such as NiO, Co₃O₄, MnO₂ are very promising candidates for electrode materials in supercapacitors^[7-13]. These oxides show specific capacitance varying between 100 and  200 F/g. Recent research is focused on increasing the specific capacitance of the oxides by introducing other oxides technology^[14-15].

ZHANG et al^[16] reported that MnO₂ prepared by solid-state reaction at low temperature shows ideal capacitive behavior in Na₂SO₄ solution in the potential range from –0.2 to 0.8 V(vs SCE). However, the relatively low specific capacitance (150 F/g) needs to be improved for supercapacitor application. In this study, Ni-doped MnO₂ composite oxides as electrode materials were prepared by solid-state reaction through the reduction of KMnO₄ with manganese acetate and nickel acetate at low temperature. The physical properties of the oxides, such as crystallinity and particle size were characterized by X-ray diffractometry(XRD) and transmission electron microscopy(TEM). The capacitive characteristics of the composite oxides as the active electrode materials for supercapacitors were also investigated by cyclic voltammetry and chronopotentiometry.

2 Experimental

2.1 Preparation of electrode materials

The electrode materials were synthesized by reduction of KMnO₄ with manganese acetate and nickel acetate. First, a controlled amount of Mn(Ac)₂?4H₂O and Ni(Ac)₂·3H₂O were mixed and ground for 5 min in an agate mortar before mixing with the powder of KMnO₄; then, the mixture was ground at room temperature for  0.5 h; next, it was heated in a water bath at 65?C for 6 h to make the reaction proceed completely; finally, the as-prepared powder was washed and filtered several times with deionized water and annealed in a vacuum oven at 200 ℃ overnight. The composite oxides(in molar ratio, A: n(Mn)/n(Ni)=10?0, B: n(Mn)/n(Ni)=9?1, C: n(Mn)/n(Ni)=8?2, D: n(Mn)/n(Ni)=7?3, E:n(Mn)/ n(Ni)=6?4) can be obtained.

2.2 Preparation of electrode

Electrodes were prepared by mixing 75%(mass fraction) of the composite oxide powder as active material with 20% acetylene black and 5% polytetrafluoroethylene (PTFE) binder. The former two constituents were firstly mixed together to obtain a homogeneous black powder. The binder was then added with a few drops of ethanol. The mixture was ground completely to form a rubber-like paste, and pressed with a hand oil press under 10 MPa onto a Ni-foam current collector as a working electrode, followed by drying at 80 ℃ under vacuum for 12 h.

2.3 Material characteristics

The X-ray diffraction patterns of the products were recorded using a Philips X’pert Pro with Cu K_α radiation (λ=0.154 056 nm) operated at 40 kV and 40 mA. The morphology and particle size of the products were characterized by transmission electron microscope (TEM, JEM-100CX JEOL, Japan).

2.4 Electrochemical measurement

A Ps-14 potentiostat/galvanostat was used for all electrochemical measurements. A beaker type electrochemical cell equipped with the working electrode, an SCE electrode reference and 20 mm×20 mm Pt film counter electrode were used. A Luggin capillary, whose tip was set at a distance of 1-2 mm from the surface of the working electrode, was used to minimize errors due to IR drop in the electrolyte. Aqueous solutions of      1 mol/L Na₂SO₄, employed as the electrolyte, were degassed with purified argon before measurement. The solution temperature was maintained at 25 ℃. The cyclic voltammogram (CV) and the galvanostatic charge- discharge curves of the electrode were recorded after 20 cycles. All the specific capacitances are reported per gram of active material unless otherwise specified. The specific capacitance in F/g evaluated by chrono- potentiometry can be calculated

         (1)

where  Q is the charge on the electrode, V is the potential, I is the discharge current, t is the discharge time, and m is the mass of the active material in an electrode.

3 Results and discussion

3.1 Structure characteristics of material

The XRD patterns of the undoped MnO₂ and the Ni- doped MnO₂ composite oxides are shown in Fig.1. For the pure MnO₂, the obvious peaks are obtained, indicating excellent crystalline. However, for the Ni- doped MnO₂ composite oxides, the peaks are broadened and lowered. When the molar ratio of Mn to Ni (n(Mn)/n(Ni)) is 9?1, the width of the peaks starts to increase, indicating the presence of amorphous phase. When n(Mn)/n(Ni) is 8:2, the main peak appears at 2θ= 37? and other peaks are broadened and lowered, indicating that the structure changes to amorphous. When n(Mn)/n(Ni) is 7?3 or 6?4, no obvious peaks are observed. Previous research showed that amorphous nanostructured oxides exhibiting pseudocapacitance were considered to be promising materials for high-energy-density application^[12-13].

Fig.1 XRD patterns of pure MnO₂ and Mn/Ni composite oxides

n(Mn)/n(Ni): 1—10?0; 2—9?1; 3—8?2; 4—7?3; 5—6?4

The morphologies of the undoped MnO₂ and the Ni- doped MnO₂ composite oxides were examined by TEM. Typical images of the pure MnO₂ and of the Mn/Ni composite oxides are shown in Fig.2. For the pure MnO₂, the particle size is not larger than 40 nm. The typical morphology is irregularly spherical. When n(Mn)/n(Ni) is 8?2, the particle size of the Mn/Ni composite oxide is reduced to 15 nm and the composite oxide disperses well. The morphology is spherical or needle. When n(Mn)/n(Ni) is 6?4, some agglomerations are found.

Fig.2 TEM images of pure MnO₂ and Mn/Ni composite oxides

(a) MnO₂; (b) Mn_0.8Ni_0.2O_x; (c) Mn_0.6Ni_0.4O_x

3.2 Effect of Ni-doping on specific capacitance of composite oxides

To investigate the performance of the composite oxide electrode, all of these electrodes were charged/discharged at different discharge currents of 5, 10 and 20 mA using a potential window between -0.2 and 0.8 V(vs SCE). Fig.3 shows the dependence of the specific capacitance on molar ratio of Ni. By 5 mA constant current charges-discharge, the pure MnO₂ has the specific capacitance of 150 F/g. With increasing the molar ratio of Ni, the specific capacitance goes through a maximum at approximately 20% molar fraction of Ni. The specific capacitance reaches 194.5 F/g for the Mn_0.8Ni_0.2O_x composite oxide. With further increase of Ni content of the composite oxide, the specific capacitance starts to decrease gradually.

Fig.3 Specific capacitances of different Mn/Ni composite oxides at different constant discharges currents

I/mA: 1—5; 2—10; 3—20

From the above results, it can be inferred that the specific capacitance depends on the material microstructure of the composite oxide. For the pure MnO₂, the particle size is relatively large, and the surface area is relatively low. When n(Mn)/n(Ni) is 8?2, the oxide exhibits an amorphous structure, the particle size decreases, the surface area increases, and the active site area and the utilization of the oxide are enlarged, which results in an increase of the capacitance of the composite oxide when compared with the capacitance of the pure MnO₂. And, when Ni content increases, agglomerations will lead to a decrease of the surface area and the oxide utilization.

Moreover, for all the composite oxides, the specific capacitance of the nanocomposite oxide decreases with the increase of discharge current, which can be explained by considering utilization of the active electrode material. When the charge-discharge current is low, the time of charge-discharge process is long, and the reversible faradic reaction is carried out thoroughly, which results in high utilization of the active electrode material. When charge-discharge current is increased, the time of charge-discharge process is shortened, and the utilization of the active electrode material is lowered. Thus, the specific capacitance of the nanocomopiste decreases with increasing charge-discharge current.

3.3 Discharge curves of composite oxides

Fig.4 shows the discharge curves of the pure MnO₂  and the Ni-doped composite oxide at the discharge currents of 5, 10 and 20 mA. It can be found that the discharge curve is approximately linear at 5 mA. When the discharge current is 10 mA, the inflexion appears at the potential of +0.22 V (vs SCE), corresponding to a short discharge platform. When the discharge current is 20 mA, the inflexion also appears at the potential of +0.18 V (vs SCE), corresponding to a short discharge platform.

Fig.4 Discharge curves of MnO₂(a) and Mn_0.8Ni_0.2O_x(b) at different constant discharge currents

I/mA: 1—5; 2—10; 3—20

It can be found that the discharge curves for Mn_0.8Ni_0.2O_x oxide are approximately linear at 5, 10 and 20 mA. From Fig.4, it can be seen that the discharge time of Mn_0.8Ni_0.2O_x oxide is longer than that of the pure MnO₂. Hence, the composite is a promising electrode material in the application of supercapacitors.

3.4 Effect of Ni-doped on CV curves

Fig.5 shows CV curves of the pure MnO₂ and the Ni-doped composite oxide at the different scan rates in  1 mol/L Na₂SO₄.

Fig.5 CV curves of MnO₂(a) and Mn_0.8Ni_0.2O_x(b) at 5, 10 and 20 mV/s

Scan rate/(mV?s^-1): 1—5; 2—10; 3—20

For the pure MnO₂, the shape of CV curves is rectangle at the scan rate of 5 mV/s, exhibiting excellent capacitance behavior of the oxide. With the increase of the scan rate, the curves of CV show distorted. The responding current fluctuates with the potential. When the scan rates are 10 and 20 mV/s, respectively, the reduction current appears and fluctuates at the potential of 0.2 V (vs SCE), which corresponds to the previous results, whereas for the Mn_0.8Ni_0.2O_x, all the CV curves show no obvious redox peaks. It can be seen that the currents change fast when the direction of scan is changed, exhibiting no obvious electrochemical polarization.

3.5 CV characteristics of Mn_0.8Ni_0.2O_x oxide at different potential windows

The electrochemical reversibility of Mn/Ni composite oxide is usually examined by varying the upper potential limit of CV. Typical CV curves of Mn_0.8Ni_0.2O_x composite oxide electrode between -0.2 and 1.0 V (vs SCE), measured at 5 mV/s in 1 mol/L Na₂SO₄, are shown in Fig.6. When the potential window is between -0.2 and 1.0 V(vs SCE), the response currents vary with the potential. When the potential window range is between -0.2 and 0.9 V(vs SCE), all the curves show rectangle-like shape and no obvious redox

Fig.6 Effect of potential windows on CV curves for Mn_0.8Ni_0.2O_x at scan rate of 5 mV/s

Potential window/V: 1— -0.2-0.4; 1— -0.2-0.5; 3— -0.2-0.6; 4— -0.2-0.7; 5— -0.2-0.8; 6— -0.2-0.9;   7— -0.2-1.0

peaks. Note that all voltammetric currents approximately follow the same trace on the positive sweeps of all CV curves in this figure. The currents on all CV curves reaching the plateau values are very fast when the direction of potential sweep is just changed. This shows typical capacitive-like characteristics. These results indicate good electrochemical reversibility of the composite oxide in 1 mol/L Na₂SO₄, resulting in high-power characteristics of the composite oxide.

3.6 Cycle life

The cycle life stability of the active material in the electrolyte was tested using cyclic voltammetry at the scan rate of 5 mV/s. The CVs of the nanocomposite Mn_0.8Ni_0.2O_x oxide measured at the 30th,100th and 200th cycle are shown in Fig.7. When the cycle number is 30, the corresponding current varies with the potential; when the cycle number is 100, the corresponding current is stable, no significant change of CV is observed after 200 cycles for the Mn_0.8Ni_0.2O_x oxide electrode, indicating good electrochemical stability, high electrochemical reversibility and long cycle life. Thus a very promising electrode material for supercapacitors has been prepared using the solid-state reaction described.

Fig.7 Effect of cycle number on CV curves for Mn_0.8Ni_0.2O_x at scan rate of 5 mV/s

Cycle number: 1—30; 2—100; 3—200

4 Conclusions

1) Mn/Ni composite oxides as active electrode materials for supercapacitor were synthesized by using a method based on solid-state reaction of KMnO₄ with Mn(Ac)₂·4H₂O and Ni(Ac)₂·3H₂O at low temperature.

2) The particles of the composites are nano size, and the size decreases with increasing Ni content. With increasing the molar ratio of Ni, the specific capacitance goes through a maximum at molar fraction of Ni of 20%. The specific capacitance of Mn_0.8Ni_0.2O_x composite oxide is 194.5 F/g at constant discharge current of 5 mA. Therefore, the Mn/Ni composite oxide is a promising electrode material for supercapacitor.

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Foundation item: Project(20060390889) supported by China Postdoctoral Science Foundation; Project (2006FJ4236) supported by Hunan Postdoctoral Scientific Program

Received date: 2007-03-10; Accepted date: 2007-05-15

Corresponding author: ZHANG Zhi-an, PhD; Tel: +86-731-8876454; E-mail: zhianzhang@sina.com

 (Edited by CHEN Wei-ping)