Basic Applied Research. Dioxide Manganese Electrode Materials Of Electrochemical Supercapacitors

Electrochemical supercapaciotrs have been recognized as unique energy storage devices which have both advantage of higher energy density and higher power density than rechargeable batteries and conventional dielectric capacitors.Besides,there are some other characteristics,such as higher capacity,fast charge-discharge and a longer cycle life,etc..Due to these properties,supercapacitors have potential applications in hybrid vehicles,portable devices,memory back-up systems and burst power generation,etc..So it has attracted attention widely all over the world.Study on supercapacitors is mostly focused on both high performance electrode materials and preparation of electrodes.At present,the electrode materials applied in industry is mainly carbon-based materials.Because of its double layer charges,the capacity is limited in advance.Many researchers pay more attention to the metal oxides in order to get higher capacity.In this paper, MnO₂ were researched as electrode materials in supercapacitors.The structure and ameliorated morphology of MnO₂ were characterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM).The impacts of specific capacitors were investigated in different concentration of electrolyte and in different proportion of active substances by cyclic voltammetry and constant current charge-discharge electrochemical methods.Firstly,MnO₂ powder were fabricated in both two ways,solid phase synthesis at a low temperature and liquid phase redox synthesis, both of which were suitable for electrode materials in supercapacitors.The majority of production powder were α-MnO₂ and Ramsdellite- MnO₂(R- MnO₂) affirmed by XRD,diameters of which were about 900nm～5μm and 200nm～ 900nm,respectively calculated by SEM.According to cyclic voltammetry and constant current charge-discharge curves in 6mol/L KOHelectrolyte,the two MnO₂ electrode materials had specific capacitance of 150.4F/g and 107.2F/g at the current density of -500mA/g,respectively;while the capacitance of 206.5 F/g and 112.9 F/g at the current density of-200mA/g,respectively.Investigations showed that the optimal test conditions were as follows:The mass of active substances was 25 ～ 50 mg/cm².The MnO₂ : graphite: PTFE ratio was 60 — 40:25 — 45:15(wt%).Concentration of KOH electrolyte is 6mol/L.Secondly,both calcine activation and acid activation impacted heavily on performance of MnO₂.The specific capacitors had risen to 159 F/g after MnO₂ was calcined at 300℃.But HNO₃ activation had a negative influence on MnO₂,which capacitance was lower,just 41 F/g.And 1mol/L H₂SO₄ activation made MnO₂ have a capacitance of 146 F/g.Finally,Some other element were doped into the MnO₂ powder by solid phase synthesis at a low temperature,including doped-Sn, doped Sn-Fe, doped Sn-Cr.The high specific capacitors were 173 F/g when the molar ratio of Mn to Sn was 100 to 20.If a little Fe element was added to,the capacitance would be higher to 197 F/g.And Cr element had a negative influence on MnO₂,but not so obviously.There was no use in liquid phase redox synthesis to dope Sn.The capacitances had no difference between MnO₂ doped-Sn and blank MnO₂.It's almost 89 F/g.The results from above experiments show that the MnO₂ have a higher specific capacitor of 150 ～200F/g compared with the carbon materials,a capacitance of 100F/g ca..The specific capacitors could be improved greatly to 173 F/g and 197 F/g,respectively,when the element Sn and Fe were doped into the MnO₂ powder.Therefore,it would be a important signification to do some studies on the MnO₂ electrode materials in supercapacitors and element doped into it.