Capacity Improvement Strategies And Electrochemical Performances Of NiFe₂O₄ Supercapacitor Electrode Materials

In recent years,in order to achieve the carbon peak and neutrality targets and promote the transformation of energy consumption structure to low-carbon and sustainable development,it is imperative to develop high-efficiency energy storage devices.Supercapacitors have become one of the key research directions because of their fast charge-discharge rate,excellent cycle stability and high power density.Supercapacitors with high power density can maximize energy utilization efficiency when used with batteries with high energy density,and have a wide range of application prospects in the fields of electric vehicles,portable electronic devices and smart grids.However,the development of supercapacitors is still limited by their low energy density,which is also a key problem that researchers need to solve urgently.As a part closely related to the energy density of supercapacitors,electrode materials have attracted much attention.More and more materials with excellent properties have been developed and proved to have good application prospects.NiFe2O4 materials are one of them.NiFe2O4 materials have excellent pseudocapacitance performance.In addition,the presence of Ni and Fe bimetallic elements provides more reaction sites for electrochemical energy storage.However,due to the inherent low conductivity of transition metal oxides,NiFe2O4 has poor performance at high rates and on electrochemical capacity.Therefore,the electrochemical performances of NiFe2O4-based electrode materials were enhanced by elemental doping,oxygen vacancy modification and combination with carbon materials.(1)In order to enhance the electrochemical capacity and increase the reactive sites,NiFe2O4 materials were improved by partial sulfur doping method.Partial sulfur doping NiFe2O4 materials were successfully prepared by hydrothermal method.The effect of Na2S was studied.Morphology of partial sulfur doping NiFe2O4 materials were nanosheet.The phase was relatively pure and with no impurities.The shift of the XRD diffraction peak proved the successful doping of sulfur.The XPS results showed that the samples showed obvious characteristic peaks of S element after partial sulfur doping.The results of the three-electrode electrochemical performance showed that the specific capacitance of partial sulfur doping NiFe2O4 materials(284.0 F/g at 1 A/g current density)was better than that of untreated NiFe2O4 materials(182.2 F/g at 1 A/g current density).And the rate performance was better(capacity retention rate 82.2%vs 68.3%at 10 A/g current density),which proved that partial sulfur doping treatment could effectively improve the conductivity and electrochemical capacity of NiFe2O4 materials.Partially sulfur doping N iFe2O4//activated carbon asymmetric supercapacitors showed excellent practical application.The energy density could reach 21.14 Wh/kg at a power density of 375 W/kg.Its electrochemical capacity was increased by 50%after 20,000 cycles of charge-discharge process.(2)In order to enhance the conductivity and increase the OH-adsorption site of the sample in the process of electrochemical energy storage,the NiFe2O4 materials were improved by oxygen vacancies engineering method.The modified NiFe2O4-6 materials were prepared by a further heat treatment with activated carbon bed.And the effects of different heat treatment temperatures and activated carbon addition amounts on the samples were studied.The morphology of the untreated NiFe2O4 and the sample treated at 400℃ were both nanosheet.While the nanosheets of the sample treated at 500℃ were significantly broken.And the sample treated at 600℃ showed recrystallization.The amount of activated carbon had no effect on the morphology of the sample.Fe2O3 impurities existed in both untreated NiFe2O4 samples and samples treated at 400℃.While the samples treated at 500 and 600℃ showed pure phase.The oxygen vacancy content in the samples after heat treatment with activated carbon was significantly improved.The conductivity of the sample after heat treatment with activated carbon was significantly improved.The electrochemical results showed that the specific capacitance of the modified NiFe2O4-δ material(808.0 F/g at 1 A/g)was much larger than that of the untreated NiFe2O4 sample(219.6 F/g at 1 A/g).Meanwhile,the modified NiFe2O4-δmaterial had better rate performance(capacity retention rate 66.6%vs 52.6%at 10 A/g current density),which proved that the oxygen vacancies engineering process could effectively improve the conductivity and electrochemical capacity of NiFe2O4 material.The oxygen vacancies engineering NiFe2O4-δ//activated carbon asymmetric supercapacitor showed excellent performance.The energy density could reach 17.7 Wh/kg at a power density of 375 W/kg,and the electrochemical capacity could be maintained at 98.7%after 5000 cycles.(3)In order to improve the poor conductivity of NiFe2O4 materials and decrease the electrochemical capacity loss due to volume expansion during cyclic charge and discharge,NiFe2O4 materials were improved by combining with slit modified hollow carbon fibers.NiFe2O4/slit modified hollow carbon fiber composites were prepared by coaxial electrospinning and Al2O3 hard template sacrifice method.The effects of Al2O3 additions on the samples were studied.The effect of Al2O3 additions on the morphology of the samples was mainly reflected in the width of the slits on the hollow carbon fibers.The slits on the surface of the sample with 0.1 g Al2O3 were narrow,which was not conducive to electrolyte infiltration.While the slits on the surface of the sample with 0.3 g Al2O3 were too wide,and the NiFe2O4 material was easy to fall off.while the sample with 0.2 g Al2O3 showed the most suitable morphology.No impurities are present in the composite.The graphitization degree of composite samples prepared by different Al2O3 additions was similar.The electrochemical performance results showed that the specific capacitance of NiFe2O4/slit modified hollow carbon fiber composites was 225.4 F/g at a current density of 1 A/g,while the NiFe2O4 particles were only 134.8 F/g.In addition,NiFe2O4/slit modified hollow carbon fiber composites had better cycle stability(capacity retention rate 100%vs 80%after 5000 times of charge and discharge).This proved that combining with slit modified hollow carbon fibers could effectively improve the cycling stability of the sample.Energy density of NiFe2O4/slit modified hollow carbon fiber composite//activated carbon asymmetric supercapacitor could reach 13.44 Wh/kg at a power density of 750 W/kg.(4)In order to improve the capacity,NiFe2O4 materials was improved by combining with carbon nanodots.NiFe2O4/carbon nanodot composites were prepared by hydrothermal method.The effects of different carbon nanodot dispersions and heat treatment temperatures on the samples were studied.Ni2O3 impurities existed in the samples prepared under heat treatment at 300℃,while the samples prepared under heat treatment at 400℃ were relatively pure.The prepared NiFe2O4/carbon nanodot composites all exhibited nanosheet morphology,and the amount of nanodot dispersion added had no effect on the morphology of the composites.It can be seen in the TEM image that carbon nanodots exist.Different composites prepared at 400℃showed similar graphitization degrees,and all had characteristic peaks belonging to NiFe2O4.A large number of hydrophilic functional groups such as carboxyl groups were adsorbed on the surface of the composite,which was conducive to the infiltration of electrolyte.The results of the three-electrode electrochemical performance showed that the sample with a carbon nanodot dispersion added amount of 20 mL had the best electrochemical performance with a specific capacitance of 706.4 F/g at a scanning rate of 1 mV/s and a specific capacitance of 548.1 F/g at a current density of 1 A/g.NiFe2O4/carbon nanodot composites//activated carbon asymmetric supercapacitors exhibited excellent performance.The energy density could reach 20.8 Wh/kg at a power density of 375 W/kg.