Shape-Controllable Synthesis And Electrochemical Properties Of Nano-Structured Manganese Dioxides

Supercapacitors are new energy storage components between batteries and electrostatic capacitors, because of their higher power density, larger specific capacitor, broad operating temperature range and long cycle life, These merits could meet the high power need of electric vehicles when starting-up or accelerating. The fabrication of asymmetric supercapacitors with high energy density operated in aqueous electrolyte has become a hot topic in terms of cost reduction and environment-friendly. The study of supercapacitors mainly concentrated on the electrode materials. Amongst them, manganese dioxide (MnO2) aroused much attention thanks to its good electrochemical behavior, low cost, and environmental compatibility.MnO2 were widely used in the field of separation, catalyst, chemical power, due to their special physical and chemical properties.Many researches have demonstrated that the electronic and optical properties of the nano-structured MnO2 have highly relied on their morphology, crystallinity and particle size. Hence,it has gerat significance to study the controllable fabrieation conditions of MnO2 with different morphologies to meet different applications.This paper mainly consists of two sections, reviewe and experiments. The main researehing contents and results are as follows:Nanostructured a-MnO2 mesoporous materials with three-dimensional frameworks were prepared by mixing KMnO4 with Vitamin C (VC) under ambient conditions. The electrochemical performances indicated that the mesoporous MnO2 exhibit a high capacitance up to～200 F g-1. And for hybrid supercapacitor, it can obtained a 1.8 V practical cell voltage with capacitance of 23.1 F g-1. After 1200 cycles, the maximum energy density is 10.4 Wh kg-1 and power density is 14.7 kW kg"1.A series of high surface area mesoporous MnO2 with nano flower shape have been synthesized by the reduction of KMnO4with ethylene glycol under acidic conditions. The presence of the acid altered both the morphology and the structure of the MnO2. Based on the experiments, we proposed a "acid etch" mechanism. Using this material in a hybrid supercapacitor, a practical cell voltage of 1.8 V and a capacitance of 22.1 F g-1 after 1500 cycles is obtained.MnO2 nanorods were synthesized by co-precipitation in the presence of ammonium thiosulfate (APS) surfactant. The characterizations show that the sample synthesized without surfactant was 20-25 nm in diameter and ca.400 nm in length, whereas APS-assisted samples were shorter, ca.80 nm in length with diameter unchanged. The electrochemical tests data indicated that the sample synthesized with 0.125 g APS exhibited the best high-rate discharge performance.In addition, we studied the influence of alkaline electrolyte concentration on the performance of MnO2. We got the conclusion that 1M is the optimum alkaline electrolyte concentration for the electrochemical behavior.