Controlled Preparation Of Fe-Co Based Electrodes For Supercapacitor Applications

As a reliable electrochemical energy storage system,supercapacitors have the characteristics of higher power density,longer service life and green environmental protection than batteries,and have gradually become the focus of research on energy storage materials and devices.Among them,binary cobalt-based metal oxide is an important pseudocapacitor material and is the first choice for supercapacitor electrode materials.The iron element has a wide range of sources,low cost,and divalent iron ions are more active than the corresponding divalent nickel,manganese,and zinc ions.It has higher electrochemical activity and conductivity and can be used as electrode material for supercapacitors.However,the single iron-cobalt oxide has low ion diffusion rate,limited capacitance,poor rate and cycle stability,and cannot meet the requirements of high-performance electrode materials.The electrode materials in this paper optimize the configuration of iron-cobalt-based supercapacitors from the aspects of improvement of synthesis methods,electrolyte control and heterostructure to prepare high-performance supercapacitors.The details are as follows:（1）Construction of FeCo₂O_4-xS_x/carbon fiber membrane composite material.Combining electrospinning technology and hydrothermal vulcanization process,a three-dimensional network framework FeCo₂O_4-xS_x/carbon fiber membrane composite material was prepared.The material has a large surface area and a high porosity,which can effectively increase the electron transport path,reduce the charge transport resistance,reduce the agglomeration of nanoparticles and increase the number of active sites,thereby achieving a synergistic enhancement effect between the electrochemically active substance and the carbon fiber membrane base material.Electrochemical studies have shown that at a current density of 1 A g^-1,the specific capacitance of FeCo₂O_4-xS_x/carbon fiber membrane can reach 1039 F g^-1,which is 306F g^-1 higher than the FeCo₂O₄ carbon fiber membrane electrode under the same conditions The rate performance is 11%higher.It is further assembled into an asymmetric supercapacitor by using it as the positive electrode and activated carbon as the negative electrode.The asymmetric capacitor has good cycle stability.After5000 cycles at a current density of 10 A g^-1,it can still maintain a capacitance retention rate of 86%.（2）A redox additive is introduced into the aqueous electrolyte.The reversible redox couples in K₃[Fe（CN）₆]can accelerate the reaction kinetics of FeCo₂O₄ nanoneedle electrode materials and increase the pseudocapacitance contribution of electrode materials.Studies have shown that when the current density is 2 A g^-1 with 3 M KOH+0.05 M K₃[Fe（CN）₆]as the electrolyte,the specific capacitance of FeCo₂O₄ can reach 1557 F g^-1,and the capacitance after 5000 cycles of charge and discharge The retention rate was 80.7%.At the same time,FeCo₂O₄//AC hybrid supercapacitors have high specific capacitance(188 F g^-1@1 A g^-1)and high energy density(750.1 W kg^-1@58.8 Wh kg^-1).This indicates that the successful addition of electrolyte additives has improved the pseudocapacitance contribution,rate performance and cycle life of the material.（3）Heterostructure of iron cobaltate@molybdenum sulfide.Induced by solvothermal method with ethylene glycol solvent,the surface of FeCo₂O₄ nanoneedle array is coated with non-crystalline MoS_x to synthesize the iron cobaltate nanoneedle@molybdenum sulfide nanoparticle heterostructure electrode.Phase characterization found that the heterostructure has a large number of defects and abundant unsaturated sulfur active sites,which can prevent the agglomeration of electrode materials,shorten the migration distance of ions,and relieve internal stress during charging and discharging.Electrochemical analysis shows that the FC@MS-EG electrode has a specific capacitance as high as 1956 F g^-1 at 1 A g^-1,and has excellent cycle stability（capacitance retention rate reaches 91.1%after 6000cycles）.DFT calculations show that the introduction of non-crystalline MoS_x increases the conductivity of the composite,which is beneficial to increase the electrochemical active sites.Further assembled into an FC@MS-EG//AC asymmetric supercapacitor,it was found that the specific capacitance reached 281.9 F g^-1 at 1 A g^-1,and the power density reached 750 W kg^-1 at an energy density of 88.09 Wh kg^-1.The assembled flexible all-solid supercapacitor can be easily bent and twisted to different angles without causing capacitance attenuation.This heterogeneous structure provides new ideas for the rational design and utilization of electrodes.