Theoretical And Experimental Study On The Preparation Of Hollow Prismatic Iron Oxide

With the rapid development of materials science today,designing and controlling the synthesis of materials with special morphology is the driving force for the continuous progress and development of current material science field.Ferric oxides with hollow structure not only have the characteristics of common iron oxides,but also have the advantages of low density,large specific surface area,high load capacity and rich porosity,which make them more popular in the field of energy storage.There are still many difficulties in the preparation of hollow iron oxides,such as excessive dependence on templates and poor structural stability.In response to the above problems,this paper uses density functional theory（DFT）to study the formation mechanism and morphology control mechanism of hollow prismatic ferrous oxalate.The hollow prismatic ferrous oxalate with regular shape was synthesized by self-template etching at room temperature.The thermodynamic analysis of the thermal decomposition system of ferrous oxalate was carried out and the advantage zone diagram of its thermal decomposition was drawn.Based on the advantage zone diagram,the experimental scheme was designed.Using the self-made ferrous oxalate as the precursor to synthesize hollow Fe₃O₄ and Fe₂O₃ with the precursor morphological in one step by thermal decomposition,and make a preliminary exploration on the electrochemical properties of hollow products.The main conclusions are as follows:（1）By calculating and analyzing various complex groups in the reaction system of ferrous oxalate at different pH values,it was found that the pH value of reaction solution changes from slightly alkaline to acidic is an important condition for the formation of hollow ferrous oxalate.Compared with H₂C₂O₄,Na⁺in Na₂C₂O₄ has a"occupying"effect,which affects the morphology and dispersibility of ferrous oxalate.Ethylene glycol（EG）added to the system can"wrap"ferrous oxalate and effect the size and dispersion of ferrous oxalate.（2）Using sodium oxalate and ferrous sulfate as raw materials,and different proportions of EG and H₂O as reaction solutions,hollow prismatic ferrous oxalate with regular morphology were successfully prepared by self-template etching method,which verified the rationality of the theoretical calculation results.At the same time,the experiment also verified that the introduction of Na⁺into the system can not only make the crystal morphology of ferrous oxalate prismatic,but also improve the dispersibility of ferrous oxalate powder.EG acts as a surfactant,controlling the size and dispersibility of ferrous oxalate particles.（3）The final product of thermal decomposition of ferrous oxalate under aerobic conditions is ferric oxide,and there are three stages in the reaction.Pure Fe₃O₄ can be obtained by controlling the reaction temperature to 350℃,setting P_CO=0.01Pa,P_CO2=10³Pa in（CO+CO₂）mixed atmosphere.The basic morphology of all thermal decomposition products is similar to precursors,which conforms to the"morphological inheritance"effect.（4）At high current density of 10 A·g^-1,the specific capacitance of hollow prisms Fe₂O₃ is up to 281 A·g^-1,which is 1.5 times the specific capacity of commercial ferric oxide.After 50 cycles of charge and discharge stability test,the capacity retention rate of hollow prisms Fe₂O₃ is far more than 40%,which is about 1 times of solid iron oxide.Compared with solid and commercial ferric oxide,hollow prisms Fe₂O₃ showed better cyclic stability and specific capacitance.