A mesoporous Mn-Co oxide for supercapacitors was derived from a mixed oxalate Mn 0.8Co 0.2C 2O 4·nH 2O, 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 m 2·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.
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 [
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 [
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 [
In this work, a mesoporous Mn-Co oxide was derived from a mixed Mn0.8Co0.2C2O4×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.
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.
A Philips X’pert Pro X-ray diffractometer with CuKa 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.
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.