Study On Micro-nano Structure Regulation And Capacitive Properties Of Self-supporting CoFe₂O₄-based Anode Materials

Supercapacitor is a new type of energy storage device based on the electrochemical process of the electrode/electrolyte interface,which is between traditional physical capacitors and secondary batteries.It has broad applications prospects in high-power equipment,electric vehicles,micro-smart electronic equipment and other fields.At present,how to increase the energy density of a supercapacitor without sacrificing its power density and cycle life is a problem that needs to be solved urgently.In this paper,CoFe₂O₄-based compounds were taken as the research object.Firstly,in order to improve the specific capacity of electrode materials,the performance of electrode materials was optimized through porous structure and morphology control,and the porous nanocube structure CoFe₂O₄materials with high specific capacity were prepared.Secondly,based on the confinement effect of the self-assembled structure of Co₃O₄nanorods,a three-dimensional sea urchin-shaped micro-nano heterostructure Co₃O₄/CoFe₂O₄material was synthesized,which effectively improved the structural stability and cycle stability of the material.Finally,for the purpose of high working potential window,self-supporting grid-like Co₃O₄/CoFe₂O₄-NF nanosheet array electrodes and self-supporting dumbbell-shaped CoFe₂O₄/CoFe₂S₄-NF array electrodes were directly constructed on nickel foam,respectively.The aqueous and organic system hybrid supercapacitors with high working potential window was obtained,and the comprehensive evaluation of device performance was completed.In the alkaline buffer solution,Fe ions and Co ions were under high temperature liquid phase conditions,using cetyltrimethylammonium bromide as template and isopropanol as complexing agent to form nanocub CoFe₂O₄.Then,porous nanocube CoFe₂O₄with complete morphology and high specific surface area was obtained by calcination in air.As the electrode material for supercapacitors,its specific capacitance is 1313.06 F g^-1at 1 A g^-1in 2 mol L^-1KOH electrolyte.When the current density increases to 15 A g^-1,the specific capacitance remains 816.2 F g^-1,and the capacity remains 62.2%.In order to improve the specific capacitance and cycle stability of electrode materials,based on the confinement effect of self-assembly of Co₃O₄nanorods on the sea urchin-like structure,a three-dimensional sea urchin-like micro-nano structure Co₃O₄/CoFe₂O₄heterogeneous material was designed and synthesized.Firstly,with cobalt nitrate as the Co source,urea as the precipitator and morphology control agent,the sea urchin basic cobalt carbonate precursor with good structural stability was prepared in a controllable manner,and then the three-dimensional sea urchin Co₃O₄was obtained by high temperature annealing,and its formation mechanism was researched.Then,Fe³⁺was added and CoFe₂O₄particles were in-situ synthesized with Co₃O₄as the skeleton under high temperature reaction conditions.Finally,heterostructure Co₃O₄/CoFe₂O₄material with sea urchin-shaped was obtained and its electrochemical performance was studied.The open three-dimensional special morphology of the sea urchin and the porous structure on the nanorods increase the ion/electron transport path,shorten the ion/electron transmission distance,and thus improve the specific capacitance of the material.In addition,the confinement effect of sea urchin-shaped Co₃O₄can increase the structural stability of the material and improves its cyclic stability.When the current density is 1 A g^-1,the specific capacity of Co₃O₄/CoFe₂O₄composite electrode reaches 1152.8 F g^-1,and the capacity retention rate is still 84%after 5000 cycles.It can be seen from the above that ideal electrode materials must have both high effective specific surface area and excellent three-dimensional electron/ion transport channels.Therefore,we directly grew Co₃O₄nanorod arrays on the nickel foam substrate,and then compounded CoFe₂O₄nanosheets to obtain self-supporting grid-like Co₃O₄/CoFe₂O₄-NF nanosheet array electrodes.The electrochemical test results showed that the grid-like Co₃O₄/CoFe₂O₄nanosheets array had high specific capacity(1214.9 F g^-1at 2 A g^-1),and its capacity remained 95%after 5000 cycles.The all-solid hybrid capacitor assembled with it as the positive electrode and AC as the negative electrode shows a high working voltage window(1.5 V),and the specific capacitance is 168.6 F g^-1at 1.5 A g^-1.When the power density is 1125 W kg^-1,the energy density can reach 52.69 W h kg^-1.In order to improve the conductivity of CoFe₂O₄based materials,we prepared dumbbell-shaped CoFe₂O₄/CoFe₂S₄array electrode directly on nickel foam by a simple vulcanization method.It was found that this structural material could not only construct an excellent three-dimensional ion transport path,but also increase the electron transport capacity of the electrode.The CoFe₂O₄/CoFe₂S₄electrode achieves higher specific capacitance(1679.2 F g^-1at 1 A g^-1).The energy density of the assembled CoFe₂O₄/CoFe₂S₄//AC aqueous hybrid capacitor is 51.9 W h kg^-1at688.9 W kg^-1,and remains 26.0 W h kg^-1even at 7205 W kg^-1.In summary,this study systematically analyzed the three-dimensional ion channels constructed by porous structure with high specific surface area,and the mechanism of action of high intrinsic electrochemical active materials with fast electron transfer and multiple redox reaction sites on the charge storage of supercapacitor electrode materials,so as to guide the design and synthesis of self-supporting electrodes with high electrochemical performance,and lay a solid foundation for the application of supercapacitor technology.