Study On Preperation And Performance Optimization Of MnO₂Aqueous Supercapacitors

Manganese dioxide has become a research hotspot of supercapacitor electrodematerials because of its advantages of abundant resources, low price, no pollutionand high theoretical specific capacity etc. But MnO2still exists many problems asa supercapacitor electrode active material. This has affected the performance ofmanganese dioxide seriously. Low specific surface area, low electronic and ionicconductivity are the most obvious problems of the MnO2electrode material. In orderto optimize the electrochemical performance of super capacitor, we studied theManganese dioxide crystal, preparation of composite electorde, the conductive agentand the electrode compaction density. The structures and particle sizes of theas-synthesized MnO2were characterized by XRD, TG and SEM, et al. Theelectrochemical performances of MnO2eletrodes were evaluated by eletrochemical test.Four types of manganese oxides that differing in crystal structures are prepared,which contain amorphous MnO2, α-, β-and γ-type. At the scanning rate of5mA/s. Theβ-MnO2has a high specific capacitance (129.7F/g). The electrode has thesmallest resistance (0.8Ω). An asymmetric carbon/MnO2supercapacitor made byβ-MnO2has a capacitance value of5.2F, with minimum resistance(3.8Ω), andmaximum specific energy (4.13W·h/kg), the capacitance decreases to90.4%of initialvalue after5000cycles, which shows a good stability.The MnO2/AC or MnO2/RuO2composite electrodes, whose electrochemicalperformance are better than MnO2electrodes. With20wt%AC, the specific capacitancehave a maximum value, reaching144.6F/g, which is44%higher than a single MnO2electrode. A supercapacitor contains20%AC has the largest capacitance value (5.6F),with maximum specific energy (4.35W·h/kg). The specific capacitance of MnO2/RuO2composite electrodes, will continue increases with the amount of RuO2, with the trendof increasing gradually slow down, the electric resistance would decreases to remainstable. It is believed that pseudocapacitive reactions occurring in the electrodes areenhanced by the existence of RuO2.The four different conductive carbon as conductive agent were studied. Theresults show that the main factor influencing the performance of MnO2materials isthe dispersed uniformly of conductive agent in the electrode material. In four types,acetylene black is better than others as the conductive agent, with which the MnO2composite electrodes get the largest capacitance value, reaching133.9F/g. Theequivalent series resistance comes to a minimum value(1.8Ω), the electrode charge anddischarge efficiency reached96.7%by the addition of acetylene black. The capacitance of a electrode with acetylene black remains69.7%while the charging current growingfrom5mA to100mA, which is the best high-rate discharge performance.The compaction density caused by different pressure on20%MnO2/AC compositeelectrode. The results show that when the compaction density reaches0.8g/cm3electrode has the largest value(98F/g). The maximum value is4.21W·h/kg when thecompaction density reaches0.6g/cm3. A electrode with a higher compaction density willhas a better capacity retention rate and stable resistance after5000cycles, which meanselectrodes with higher compaction density are more stable.