Capacitive Properties of Mesoporous Mn-Co Oxide Derived from a Mixed Oxalate

Yong Yan, Bingcai Wu, Cuihong Zheng, Daolai Fang

School of Materials Science and Engineering and Anhui Key Laboratory of Metal Materials and Processing, Anhui University of Technology, Ma’anshan, China.

DOI: 10.4236/msa.2012.36054                

Abstract

A mesoporous Mn-Co oxide for supercapacitors was derived from a mixed oxalate Mn_0.8Co_0.2C₂O₄·nH₂O, which was synthesized by a solid-state coordination reaction at room temperature. The synthesized mixed Mn-Co oxalate was decomposed in air at 250°C, resulting in a tetragonal spinel Mn-Co oxide with a primary particle size less than 100 nm. The obtained Mn-Co oxide presents a mesoporous texture with a specific surface area of 120 m²·g^﹣1. Electrochemical properties of the Mn-Co oxide electrode were investigated by cyclic voltammetry and galvanostatic charge/discharge in 6 mol·L^﹣1 KOH electrolyte. The Mn-Co oxide electrode delivered specific capacitances of 383 and 225 F·g^﹣1 at scan rates of 2 and 50 mV·s^﹣1, respectively. Subjected to 500 cycles at a current density of 1.34 A·g^﹣1, the symmetrical Mn-Co oxide capacitor showed specific capacitance of 179 F·g^﹣1, still retaining ~85% of its initial capacitance. The obtained Mn-Co oxide material showed good capacitive performance, which was promising for supercapacitor applications.

Keywords

Supercapacitors; Manganese Oxide; Solid-State Reaction; Cyclic Voltammetry; Capacitive Performance

1. Introduction

Electrochemical supercapacitors possess the unique energy-storage performance, such as greater power density and longer cycle life than secondary batteries, as well as higher energy density than conventional capacitors [1], showing great potential to be used in the areas of hybrid power sources, peak power sources, backup power storage, lightweight electronic fuses, starting power of fuel cells [2,3]. Generally the supercapacitors can be classified into two categories: electrical-double-layer capacitors (EDLCs), which build up electrical charge at the electrode/electrolyte interface [4,5], and pseudocapacitors, which are based on reversible faradic redox reactions at the interfaces at certain potentials [6-10].

Recently, with the purpose of achieving high energy density supercapacitors, great efforts have been devoted to search for excellent pseudocapacitive materials [11- 20]. Amorphous hydrated ruthenium oxide has been demonstrated to be an excellent pseudocapacitor material, which exhibits a high conductivity, good electrochemical stability, and a large specific capacitance (SC) of 720 - 760 F·g^-1 [21]. However, high cost and toxicity of the ruthenium oxide greatly limit its commercial applications. Thus, other less expensive and less toxic transition metal oxides, such as amorphous and crystallized MnO₂ [11- 14], nanostructured NiO [15,16], and mesoporous and nanowire-array Co₃O₄ [17,18], have been developed as electrode materials for supercapacitors. The electrode materials of the cheaper transition metal oxides exhibit remarkable capacitive nature, delivering a considerable SC. However, the resistivity and the equivalent series resistance (ESR) of these cheaper transition metal oxides are very large, which greatly limits their capacity and power density available.

On the other hand, it is reported that mixed oxide composites exhibit superior capacitive performance to single transition metal oxides. It has been demonstrated that the incorporation of Ni [19], Pb [20], Mo [22], and V [23] oxides into pristine manganese oxide can enhance specific capacitance and electrical conductivity of pristine manganese oxide. Nevertheless, the effect of Co oxide addition into pristine manganese oxide has not been extensively studied yet, and the results from different research groups vary significantly [24-26].

In this work, a mesoporous Mn-Co oxide was derived from a mixed Mn_0.8Co_0.2C₂O₄×nH₂O, which was synthesized by a solid-state coordination reaction. Its phase compositions and microstructures were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and N₂ adsorption-desorption measurements, and its electrochemical properties were analyzed by cyclic voltammetry and galvanostatic charge/discharge cycling. Considering the considerable content of cobalt added in the prepared Mn-Co oxides, and a remarkable pseudocapacitive behavior of cobalt oxide in alkaline aqueous electrolyte [17,18], KOH aqueous electrolyte was used to investigate the electrochemical properties of the Mn-Co oxide. The obtained Mn-Co oxide shows excellent capacitive performance in terms of specific capacitance (SC), power capability and cycling stability, which is promising for supercapacitor applications.

2. Experimental

2.1. Synthesis of the Mn-Co Oxide Powder

Analytical-grade manganese acetate Mn(CH₃COO)₂×4H₂O, cobalt acetate Co(CH₃COO)₂×4H₂O, and oxalic acid H₂- C₂O₄×2H₂O were used as starting materials. All the reagents were purchased from Sinopharm Group Chemical Reagent Company in China, and used as received. A proper amount of the starting materials was weighed accurately with a molar ratio of Mn(CH₃COO)₂×4H₂O:Co- (CH₃COO)₂×4H₂O:H₂C₂O₄×2H₂O of 0.8:0.2:1.1, then mixed and milled in a mortar to form a powder mixture. Subsequently, the obtained powder mixture was ball milled at room temperature for 5 h in a polyethylene container using zirconia balls as milling medium. The milled mixture was dried at 80˚C, resulting in a Mn-Co oxalate precursor. Finally, the Mn-Co oxalate precursor was calcined in air at 250˚C for 10 h to obtined Mn-Co oxide.

2.2. Structural Characterization

A Philips X’pert Pro X-ray diffractometer with CuK_a radiation (l = 1.5406 Å) was used to analyze the phase compositions of the dried milled mixture and the calcined Mn-Co oxide. Diffraction data were collected in the 2q range from 15˚ to 80˚, using the step-scan mode with a scanning speed of 0.02˚ step-size and 1 s per step. The morphology of the obtained Mn-Co oxide was observed by using a JSM 6490 scanning electron microscope (SEM). N₂ adsorption-desorption measurements were carried out at a Coulter SA 3100 system at 77.3 K. The surface area was calculated using the Brunauer-Emmett-Teller (BET) equation. Pore size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method. Samples were dried and degassed for 2 h at 150˚C to remove any surface organic or moisture contaminants.

2.3. Electrochemical Characterization

The electrodes for electrochemical measurements consisted of the prepared Mn-Co oxide, acetylene black (AB) and polytetrafluoroethylene (PTFE), whose weight ratio was 75:20:5. First, the Mn-Co oxide and the acetylene black were fully mixed and ground, then a proper amount of PTFE binder was added into the powder mixture to achieve a homogenous slurry, followed by painting the prepared slurry onto a nickel foam current collector with an area of 1 cm ´ 1 cm. Finally, the painted current collector was dried for 10 h at 80˚C, and pressed at a pressure of 10 MPa to form a reliable electrode. Each of the fabricated electrodes contained ~4 mg of the Mn-Co oxide electroactive material.

All the electrochemical measurements concerned were carried out in 6 mol·L^-1 KOH aqueous electrolyte. Cyclic voltammetric measurements were performed on a CHI6- 04C electrochemical workstation in a three-electrode cell set-up, using a Mn-Co oxide electrode as the working electrode, a platinum foil as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode. Cyclic voltammograms at various scan rates were recorded between -0.4 and 0.4 V vs. SCE.

For evaluation of galvanostatic charge/discharge performance of the obtained Mn-Co oxide electrodes, a symmetrical capacitor was assembled using a couple of the identical electrodes separated by a porous polypropylene separator. The galvanostatic charge/discharge performance at different current densities was determined by a battery test system of Neware BTS-3008W in a potential range from 0 to 0.8 V.

3. Results and Discussion

3.1. Phase Compositions and Microstructures

Figure 1(a) gives the XRD pattern of the Mn-Co oxalate synthesized by a solid-state coordination reaction. The XRD pattern of the synthesized Mn-Co oxalate matches that of α-form manganese oxalate (JCPDS, No. 25-0544), suggesting that the synthesized oxalate is a binary Mn-Co mixed oxalate [27]. Considering a molar ratio of Mn²⁺: Co²⁺ of 4:1 in the mixture of the starting materials used, and almost no loss of metal ions during ball-milling, the obtained Mn-Co oxalate can be expressed as a chemical formula of Mn_0.8Co_0.2C₂O₄×nH₂O. Figure 1(b) shows the

Conflicts of Interest

The authors declare no conflicts of interest.

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