Abstract
In this work, we study the preparation of nanostructured manganese oxide and the charge storage mechanism of supercapacitors. The results of this study are separated into three parts. The first part discusses different charge storage mechanisms between different potential regions and the effects of additive in the electrolyte on manganese oxide. The second part suggests a simple way to synthesize mesoporous MnOx and the textural characteristics and electrochemical properties are investigated. In the third part, the effects of additive on the activation and cycle-life test of manganese oxide are examined. Besides, we also assembled MnOx/graphene asymmetric supercapacitor in the electrolyte with or w/o additive. In the first part, the electrochemical behavior and the corresponding mass variations of amorphous manganese oxide (denoted as a-MnOx) are examined simultaneously in neutral electrolyte containing 10 mM Na2SO4 without or with NaHCO3 or Na2HPO4 by cyclic voltammetry with a quartz crystal microbalance (QCM). From this EQCM study, a-MnOx is unstable between 1.0 and 1.2 V in 10 mM Na2SO4 because of significant dissolution of a-MnOx due to oxygen evolution. From the MCR (mass-to-charge ratio) value, the major ion involves is H+ when the potential window is between -0.2V and 0.2V. But when it is between 0.2V and 0.8V, H3O+ are considered to be the main ions intercalate/deintercalate within the manganese oxide. The dissolution phenomenon and oxygen evolution are successfully suppressed by the formation of insoluble manganese carbonate or the adsorption of phosphate by adding NaHCO3 or Na2HPO4 in the Na2SO4 electrolyte, enlarging the potential window for the charge/dichrage of a-MnOx. In the second part, the Mn3O4 is synthesized through calcination of the mixture of methanol (or GO/methanol) and manganese acetate. The instrument X-ray diffractometer(XRD), scanning electron microscope(SEM), transmission electron microscope(TEM), Thermogravity analysis (TGA), surface area and pore size analyzer (BET), and cyclic voltammetry (CV) are employed to characterized the sample. It is discovered that manganese oxide synthesized with graphene oxide suspended in methanol possesses the properties of narrower pore size distribution, larger surface area and pore volume. Therefore, the higher capacitance is attributed to the larger pore volume which facilitates ion transportation. In the third part, the effects of bicarbonate on the activation and cycle life of manganese oxide are studied. It is found that higher capacitance value can be gained when activating manganese oxide in the electrolyte with bicarbonate. From the results of ICP-MS, the concentration of manganese in the electrolyte with additive is much fewer than that in bare sodium sulfate. Besides, the assembly of asymmetric supercapacitor MnOx/graphene reaches 2.5V which is the largest maximum cell voltage in aqueous electrolyte to the best of our knowledge. In addition, the energy density reaches 18.66Wh/kg and power density of 20kW/kg at 2.4V cell voltage in 0.1M Na2SO4 with adding 3mM NaHCO3. Furthermore, it exhibits an excellent charge-discharge behavior after 1000 cycles at the cell voltage of 2.4V at current density of 10A/g.