Electrochemical Study Of Supercapacitors Based On Iron Ion Containing Redox Active Electrolyte

Supercapacitors?SCs?have attracted extensive attention in the energy storage field due to their virtue of facile preparation process,low pollution,low cost,fast charge/discharge,high efficiency and stability.Recently,in order to fabricate high-performance SCs with high power density and high energy density,the main research efforts have been switched from the development of various novel electrode materials to the exploration of new electrolytes and/or assembly configuration of the devices.The purpose of this work is to strengthen the electrochemical interaction between the electrode material and redox active electrolyte containing Fe²⁺/Fe³⁺and improve the electrochemical performance of SCs by designing and preparing various iron-containing electrode materials.Two kinds of such cathode materials,including Fe²⁺doped conductive polyaniline(Fe²⁺/PANI)and carbon nanofibers/iron oxide?CNFs/Fe₂O₃?electrode,as well as the tungsten oxide?WO₃?anode were prepared.The composition,chemical structure,morphology and micro-structure of these materials were characterized and their electrochemical behaviors were studied in Fe²⁺/Fe³⁺+H₂SO₄redox active electrolyte.Asymmetric SCs?ASCs?of WO₃//CNFs/Fe₂O₃were assembled,and the characteristics of ASCs,including specific capacitance performance,rate retention,power density,energy density and cycling stability,were studied and compared with the other ASCs.Firstly,the electro-deposition of PANI and PANI doped with Fe²⁺on the titanium mesh was carried out.The results indicated that the capacitive performance of Fe²⁺/PANI in H₂SO₄electrolyte were the best,in the case that the concentration of Fe²⁺in the electroplating solution was 0.4 M.The electrochemical measurements of symmetric SC indicated that the specific capacitance of the 0.4 M Fe²⁺/PANI electrode material in H₂SO₄electrolyte was 452 F/g at a current density of 5 A/g and the capacitance retention rate was 99.59%after 1000charging-discharging cycles at 20 A/g,which was better than that of pure PANI?87.5%?.Therefore,0.4 M Fe²⁺/PANI was used to assemble symmetric SC with the redox active electrolyte.Results indicated that the specific capacitance of the electrode material was the highest at the optimized Fe²⁺/Fe³⁺concentration?0.5 M?in the electrolyte,and the corresponding specific capacitance of the electrode material was up to 8468 F/g at a current density of 8 A/g,much higher than that of symmetric SC with H₂SO₄electrolyte.These demonstrated the effectiveness of the strategy to improve the capacitive performance of SCs through the combination of Fe²⁺doped electrode materials with Fe²⁺/Fe³⁺+H₂SO₄redox active electrolyte.According to the aforementioned results that the electrochemical performance of SCs and the electrochemical interaction between the electrode and the redox active electrolyte can be promoted by the combination of iron-containing electrode materials with the Fe²⁺/Fe³⁺+H₂SO₄redox active electrolyte,electrode materials suitable for redox active electrolyte system were designed and prepared by the direct flame growth of CNFs/Fe₂O₃on the surface of 304 stainless steel mesh?SS?.The effect of different combustion treatment times on the electrochemical performance of the as-produced electrodes was studied.The electrochemical testing results on the symmetric SCs assembled with CNFs/Fe₂O₃@SS electrode and Fe²⁺/Fe³⁺+H₂SO₄redox active electrolyte indicated that the electrode material fabricated at the flame treatment time of20 min exhibited the best capacitive performance,and a large specific area capacitance up to2412.6 m F/cm²at a current density of 3 m A/cm²and a high energy density of 14.57 m Wh/cm³at a power density of 65.22 m W/cm³could be achieved.The superiority of the preparation method of CNFs/Fe₂O₃@SS electrode and its application in the acidic redox active electrolyte were thus proved.Lastly,urchin-like WO₃nanomaterials on the surface of Ti mesh were prepared by hydrothermal reaction in an autoclave,and its electrochemical behaviors in the iron ion containing redox active electrolyte was also investigated.Moreover,WO₃/Ti electrode was used as anode and CNFs/Fe₂O₃@SS as cathode,and they were combined with Fe²⁺/Fe³⁺+H₂SO₄redox active electrolyte to assembly ASCs.Results indicated that the energy density of the assembled ASCs could be as high as 9.08 m Wh/cm³at a power density of 70.33 m W/cm³,and its capacitance still remained to be 85.1%after 1000 charging-discharging cycles.