Preparation Of Iron Oxide Anode Material By Alcohol Solvothermal Method And Its Electrochemical Behavior

So far,great progress has been made in the invention of supercapacitors.Supercapacitors are one of the advanced forms of capacitors with higher energy density and power density,bridging capacitors and batteries.The energy storage through the formation of an electrical double layer is pivotal for supercapacitor technology.Accordingly,to further improve the energy density,surface faradaic（pseudocapacitive）processes are employed.In recent years,a tremendous effort has been imposed to enhance the energy density of supercapacitor and might be attained through asymmetric cell construction that offer wider potential window.To date,an important progress has been achieved in the manufacture of positive electrodes for asymmetric device.However,the electrochemical performance of negative electrode materials is fewer explored,especially transition metal oxide nanoparticles（TMONPs）.Nevertheless,pristine TMONPs limit the electrochemical performances of devices due to their intrinsic drawbacks,which may be overcome by employing different strategies through unique material engineering,i.e.,forming nanostructure,conducting composites,surface modulation,and morphology modification.Solvothermal reaction stands out among the common synthesis methods to obtain TMONPs because it can produce smaller particle size and novel morphology shape.The solvent in solvothermal reaction system is an important factor affecting the microstructure of TMONPs.Nevertheless,the related research on the relation between solvent and the structure and capacitive performance of TMONPs are rare till now.Taking these factors into consideration,four kinds of iron oxide NPs with different structures have been successfully prepared for the first in this paper by one-step solvothermal method,using methanol,ethyl alcohol,normal propyl alcohol and normal butanol as solvent respectively.Moreover,we demonstrate a simple Tween 20-assisted solvothermal route to synthesize 2D hierarchical iron oxide NPs successfully by methanol solvothermal method.In the end,the asymmetric supercapacitors were encapsulated using the above iron oxide NPs.The main results are outlined as following:1.A simple microstructure-controlled synthesis for iron oxide NPs has been developed.The influence of solvent on the sample morphology,particle size,elementary composition and electrochemical performance of iron oxide NPs were investigated.The chemical composition and microstructure of as-prepared iron oxide NPs were characterized by a variety of means,such as XRD,Raman,SEM and XPS.Among the as-prepared samples,Fe₃O₄/Fe₂O₃urchin-like composite,obtained from the methyl alcohol solvent,are composed of hierarchical needle-like nanosheets.Electrochemical measurements reveal that Fe₃O₄/Fe₂O₃urchin-like composite electrode is of the highest specific capacitance(@1 A g^-1,353.28 F g^-1)and prominent cycling stability(@5 A g^-1,92%capacitance retention after 5000 charge-discharge cycles)in 3 M KOH electrolyte.2.By employing tween20 as a surfactant,rationally designed hierarchical flower-like Fe₃O₄ has been successfully prepared for the first time via facile solvothermal reaction.The newly prepared Fe₃O₄ NPs featuring a novel 3D hierarchical flower-like structure and rough surface displays remarkable electrochemical performances,delivering high specific capacitance of 511.05 F g^-1(@1 A g^-1),outstanding cycling stability of 98.1%retention after5000 cycles,and brilliant rate capability of 59.6%even at 20 A g^-1.3.Furthermore,an asymmetric supercapacitor（Ni（OH）₂//Fe₃O₄）,using Ni（OH）₂ as positive electrode and Fe₃O₄ as negative electrode,delivers a high energy density 32.9 Wh kg^-1(@0.8 k W kg^-1)under potential window 1.6 V.Additionally,Ni（OH）₂//Fe₃O₄ exhibits good cycling performance(@5 A g^-1,81.1%capacitance retention after 3000 cycles).The performances suggest its potential application in supercapacitors.Formation mechanism,controlled synthesis and charge storage mechanism of as-prepared iron oxide NPs as electrode materials for asymmetry supercapacitor are systematically analyzed.This investigation provides a novel approach to prepare high-performance negative electrode for supercapaitor.Developing iron oxide NPs with highly capacitive capability and wonderful cycling stability will open up a new path for the development of advanced electrode materials for energy store systems.