Effect of Synthesis Routes on the Performances of MnO2 for Electrochemical Capacitors

The effect of synthesis routes on the performances of MnO2 used as an active material in electrochemical capacitors was studied by two different routes using 0.15 mol.L-1 manganese acetate and 0.1 mol.L-1 potassium permanganate as starting materials. Method 1 was addition of manganese acetate solution to potassium permanganate solution and in method 2, the solutions were swapped. The XRD diffractions and the SEM images of samples by two routes showed that the obtained MnO2 were both amorphous like and their particle sizes are ca. 200 similar to 300 nm. By BET method, MnO2 particles using method 1 have higher surface area value (329 m(2).g(-1)), uniform size distribution (in the range of 6 similar to 12 nm) and lower pore volume (0.45 cm(3).g(-1)), while method 2 produced particles with wide continuous pore size distribution (from micropores to macropores, mean pore size 11.4 nm), lower surface area (298 m(2)/g) and higher pore volume (0.66 cm(3).g(-1)). Impedance spectroscopy studies revealed that the charge transfer resistance of MnO2 by method 2 has higher value than that of MnO2 by method 1. The results of cyclic voltammograms showed that the specific capacity of MnO2 by method 2 at low scan rate (203 F.g(-1) at 2 mV.s(-1)) was larger than that of MnO2 by method 1 (189 F.g(-1) at 2 mV.s(-1)). With the increase of the scan rate, the loss of the specific capacitance of MnO2 by method 1 is smaller than that of MnO2 by method 2. The charge-discharge measurements showed that MnO2 by method 1 exihibits better rate capability, for the specific capacity of MnO2 by method 1 at 2 A.g(-1) is 96.3% of its capacity at 0.1 A.g(-1), while the specific capacity of MnO2 by method 2 at 2 A.g(-1) is 92.5% of its value at 0.1 A.g(-1). All the results indicated that the differences of the performances of MnO2 from the different synthesis routes arised from the different microstructure characteristics of MnO2.