| Supercapacitors are considered as the promising energy storage device owing to their high power-density, high safety and good convenient operation in a wide temperature range, and show potential application in complementing lithium-ion batteries. The energy storage mechanism of supercapacitors commonly bases on either ion adsorption (electrochemical double layer capacitors, EDLCs) or fast surface redox reactions (pseudocapacitors). Usually, by comparasion of EDLCs, pseudocapacitors enable higher energy density. Manganese dioxide (MnO2) is among the most popular pseudocapacitive materials due to its low-cost, abundance, environmentally benign properties, high theoretical specific capacitance (～1370 F·g-1). For MnO2 nanostructures, many methods based on the size and shape-controlled synthesis has been reported. However, to improve specific capacitance and cycle stability of MnO2 nanostructure electrodes still remain challenges. In this work, we used glucose and potassium permanganate as raw material to prepare amorphous MnO2 nanoparticles via reflux-precipitation or hydrothermal reduction. The capacitive performances of MnO2 were optimized by tuning the reaction time and introducing foreign metal cations. The all-solid state supercapacitor was also fabricated by using the as-obtained MnO2 with high electrochemical performance as anode materials.Firstly, by using a simple reflux-precipitation way, amorphous Mn oxide nanoparticles sized in about 100 nm were prepared in aqueous solution. The structural characterizations and electrochemical measurements were excuted to compare the performance of the products obtained at different reaction time. The result showed that at 16 h reflux time, the product has the largest specific surface area and suitable pore size. Meanwhile, the best electrochemical performance was also achieved for 16 h product, in which the specific capaci-tane valus was 309 F·g-1 at a current density of 1 A·g-1 in Na2SO4 aqueous solution. The specific surface area and water content play important role in capacitane behavior of MnO2. The decrease in water content after sintering resulted in the decrease in electrochemical properties.Secondly, amorphous MnO2 with low K+ ion content was prepared using KMnO4 and glucose as raw materials via hydrothermal method. The large interlayer spacing of the product is in favor of the introduction of foreign ions. Hence, an intercalation of alkali metal cations into MnO2 bulk is carried out. With the increase of K+ content, the capacitance performance decreased. Na+-intercalated MnO2 was also synthesized via hydrothermal reduction, and shows the improved Na+diffusion coefficient and electrical conductivity. A specific capacitance based on Na+-intercalated MnO2 of 295 F·g-1 at a current density of 1 A·g-1 is obtained in Na2SO4 aqueous solution. Moreover, an all-solid-state symmetric supercapacitor based on Na+-intercalated MnO2 electrode is assembled. The supercapacitor device was connected to a red LED and successfully lighted it. Such results suggest that pre-intercalation of alkaline cations into Mn oxides is an effective method to enhance the electrochemical activity in alkaline electrolytes.Finally, all-solid-state symmetric and asymmetric supercapacitor devices were fabricated by using the Na+-intercalated MnO2 and/or commercial activated carbon as electrode materials. The influences of separator and electrolyte were discussed. By using Na2MoO4/PVA as electrolyte capacitor, an ideal supercapacitor was achieved. The areal capacitance of the as-assemblyed supercapacitor devices was 232 mF·cm-2 at a current density of 5 mA·cm-2,and the power density was 31.47 Wh·Kg-1. |
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