Evaluation Of Sr、Co Doped LaFeO₃ As Supercapacitor Electrode Materials

Supercapacitor is a new type of efficient energy storage devices, between traditional capacitor and fuel cell/ batteries. Supercapacitors have high power density, good reversibility, high cycle efficiency, long cycling life and good capability of security. Therefore Supercapacitor is considered as a promising type of environmental friendly energy storage devices. Electrode materials have been a core problem in the fields of electrochemical capacitors, supercapacitive electrode materials are mainly divided into carbon element, conductive polymers, metallic oxide and perovskite type oxides and so on. Perovskite type oxide is a new promising type of electrode materials in recent years, because of unique structure of ABO3 with good electronic conductivity and ionic conductivity. The main object of this thesis is the synthesis of LaFeO₃-based perovskite oxide electrode materials and the tests of the physical characteristics and electrochemical properties of the home-made electrode materials.LaFeO₃ powders were used as electrode active material by ultrasonic dispersion dipping method, and the carbon paper was used as current collector. The study found that the reasonable loading of electrode active material is about 0.6⁰.7 mg cm-2. Ball-milling the electrode powders for 15 hours prior to the dipping process can improve the electrochemical performance of electrode materials. Because the specific surface area of LaFeO₃ material is smaller, and only Fe element can occur the oxidation-reduction reactions in LaFeO₃ material, so the specific capacitance of material is not high. The specific capacitance of LaFeO₃ electrode is about 102 F g-1 at 2 m V s-1, and almost invariant at the constant current of 2 m A for 1000 cycles. LaFeO₃ electrode shows a good cycling stability which is essentially required for supercapacitors. However, the undoped LaFeO₃ shows a low specific capacitance due to a reduced specific surface area resulting from the high synthesis temperature. Furthermore, Fe is the only variable valence element in the LaFeO₃, which will limit the improvement of the specific capacitance of the electrode via the oxidation-reduction reactions.In order to improve the specific capacitance, LaFeO₃ was co-doped with Sr and Co in the following study. Sol-gel method and Glycine-nitrate combustion synthesis method were utilized to synthesize La_0.6Sr_0.4Co_0.2Fe_0.8O₃ powders. The room-temperature conductivity of LSCF6428 powders is around 60 S cm-1 which shows good electronic conductivity. The results show that La_0.6Sr_0.4Co_0.2Fe_0.8O₃ electrode synthesized by glycine-nitrate combustion synthesis method has higher specific surface area, more uniform porosity, higher specific capacitance and ionic conductivity.The glycine-nitrate synthesized La_0.6Sr_0.4Co_0.2Fe_0.8O₃ powders were used as electrode active material, and the electrochemical properties were tested in 1 M Na2SO4 electrolyte solution and 6 M KOH electrolyte solution respectively. The electrochemical window of LSCF6428 electrode was-0.1⁰.9 V and-1.2⁰.4 V respectively in Na₂SO₄ electrolyte solution and KOH electrolyte solution. The specific capacitance of LSCF6428 electrodes are about 168 and 268 F g-1 in Na2SO4 and KOH at 2 m V s-1 scanning rate, respectively. Compared with the LaFeO₃ electrode, the LSCF6428 electrode shows a wider electrochemical window and a higher specific capacitance.LSCF electrodes have the Faradaic pseudo-capacitance because of the valence variance of Co and Fe. The results show that the room-temperature conductivity of LSCF electrodes are around 60 200 S cm-1, the conductivity increases with the increasing of Co. Changing the proportion of Co and Fe, LSCF6428 and LSCF6482 powders were used as electrode active material, analyzing the electrochemical properties in 1 M Na2SO4 electrolyte solution and 6 M KOH electrolyte solution respectively. The electrochemical window of LSCF6482 electrode became-1.2⁰.3 V in 6 M KOH electrolyte solution because of the increasing amount of Co. The specific capacitance of LSCF6428 electrodes are about 168 and 268 F g-1 in 1 M Na2SO4 and 6 M KOH at 2 m V s-1 scanning rate, respectively. The specific capacitance of LSCF6482 electrodes are about 200 and 308 F g-1 in 1 M Na2SO4 and 6 M KOH at 2 m V s-1 scanning rate, respectively. The increasing amount of Co has enhanced the Faradaic pseudo-capacitance and redox reaction. The specific capacitance, electronic and ionic conductivity have been enhanced simultaneously via co-doping of Sr and Co in LaFeO₃. The results explain that the ability of storage charge of Co is stronger than Fe, and the contribution of Co is higher than Fe in the Faradaic pseudo-capacitance and redox reaction. The specific capacitance of LSCF6428 and LSCF6482 electrons were almost invariant at the constant current of 2 m A for 1000 cycles. It is reasonable to expect that Sr and Co co-doping LaFeO₃ will be a potential electrode material for the supercapacitor application.