Research Of Iron Based Ion-type Pseudocapacitors And Their Devices

As an advanced and efficient energy storage device,supercapacitors have merits of high power density,short charging/discharging time,and long cycling life.These advantages make them highly promising for use in electric vehicles,hybrid electric vehicles,portable electronics,and other high power equipments.However,the low energy density of supercapacitor is one key issue which restricts its development and application in the society.In this thesis,we carry our fundamental researches,focusing on the energy storage characteristics of iron based electrode materials and devoting to enhance the energy density of supercapacitors.We have designed the electrode material in systematic and ionic levels,in order to realize the theoretical capacitance of the electroactive materials.Commercial FeCl3·6H2O,Fe(NO3)3·9H2O were adopted as electroactive materials directly to fabricate supercapacitor electrodes,in which Fe3+/Fe2+ cations were essentially electroactive materials and the energy storage mechanism was Faradaic pseudocapacitance.Meanwhile,we have studied the electrochemical reactions and energy storage performance of the electoactive Fe3+/Fe2+ cations in alkaline and aqueous electrolytes.The main contents are summarized as bellow:1.As an electroactive material,FeCl3·6H2O can be adopted to fabricate supercapacitor electrodes directly with high performance.Fe3+ in FeCl3·6H2O can be effectively transformed into Fe2O3·H2O colloids by an in-situ crystallization process in KOH electrolyte assisted by an external electric field.The charge storage mechanism was redox reaction between Fe3+ and Fe2+ and the oxidation and reduction potentials were 0.39 V and 0.23 V,respectively.In the positive potential region from a to 0.45 V,the specifIc capacitance of the Fe3+ electrode material was 977 F/g and the energy density was 23.6 Wh/kg at the power density of 3.4 kW/kg.The obtained energy density was far beyond the commercial supercapacitors(?5Wh/kg).After 1000 cycling test at 30A/g,the specific capacitance can maintain 60%.Meanwhile,the obtained specific capacitance was the highest among the reported values in literatures.2.Commercial Fe(NO3)3·9H2O was used as electroactive materials directly to fabricate the supercapacitor electrode,which is electroactive both in the positive(0?0.45 V)and negative potential region(-1.2?0 V).At the current density of 1 A/g,this Fe3+ cathode exhibited 393 F/g specific capacitance,and the energy density was 10.4 Wh/kg at the power density of 0.22 Wh/kg.At 1 A/g current density,this Fe3+ anode exhibited 120 F/g specific capacitance,and the energy density was 23.6 Wh/kg at the power density of 0.6 Wh/kg.After 1000 cycling test,the specific capacitance maintain 54%and 38%in the positive and negative potential regions,respectively.The charge storage mechanism is between Fe3+ and Fe2+ in the both regions.The assembled Fe(NO3)3·9H2O symmetric supercapacitor device had excellent electrochemical performance in the alkaline and aqueous electrolyte.The working potential region was as large as 1.6 V,and the maximum specific capacitance of the device can reach 100 F/g at the current density of 1 A/g.The according energy density was around 40 Wh/kg at the power density of 0.8 kW/kg.After charging for 1 minute,a tandem device with two such supercapacitor cells can power on a 3.0 V LED for more than 30 minutes.3.To further explore the electrochemical characteristics of the electroactive materials FeCl3·6H2O and Fe(NO3)3·9H2O,we fabricated supercapacitor electrodes by combining the electroactive materials,conducting material,and the binder with different mass ratios,and in KOH electrolyte with 1-6 M concentration.The experimental results show that the redox potentials of the electrode materials decrease with the increasing of the KOH electrolyte concentration,and the values can match well with the thermodynamic calculation results.The electrochemical behavior of the Fe3+ can be affected by the ligands(OH-,NO3-,H2O,Cl-)and in our designed electrode the solid state Fe3+ cations,such as in FeOOH and Fe4NO3(OH)11,,were active in the negative potential region.The free Fe3+ cations were active in the positive potential region.Besides,the pseudocapacitive performance of the Fe(NO3)3·9H2O can be enhanced by the increase ratio of the conducting reagent.The according specific capacitance was 220 F/g at the current density of 10 A/g,and the energy density was 58.4 Wh/kg at the power density of 8.4 kW/kg.4.The maximum working potential of supercapacitor is mainly decided by the adopted electrolytes,and irreversible damage would occur when the applied voltage across the supercapacitor cell exceeds the decomposition potential of the electrolyte.The nominal voltage of commercial carbon based supercapacitor with organic electrolyte is around 3.0 V.The 3.0 V nominal voltage and 5 Wh/kg energy density of supercapacitor are often too low to meet the application requirements.Thus,supercapacitors need to be connected in series and in parallel to construct a supercapacitor matrix.Due to technical restriction and environmental effect,the tolerance in the characteristics of supercapacitor components leads to imbalanced voltage during the charge/discharge cycle.This may influence adversely the supercapacitors performance and lifetime.To avoid this phenomenon,a concise switch capacitance balancing circuit model was designed.The experimental results demonstrate that it can equalize the voltage of supercapacitor cells.