Investigation On Energy Storage Mechanism And Performance Enhancement Of Iron Oxide Anodes In Aqueous Electrolyte

As a safe,environmentally friendly and low-cost energy storage device,aqueous based energy storage devices have high commercial value for large-scale applications.However,due to the limitation of decomposition potential window of water,the discharge voltage of aqueous battery is usually low,which is around 1.2 V.The working potential of most positive electrode materials is below the oxygen evolution potential of water and can be used directly in aqueous-electrolyte systems.In order to increase the working voltage of the battery,the negative electrode material with a low working potential plays a large role.Fe₂O₃has high hydrogen evolution overpotential and low charge storage potential which is expected to achieve the characteristic of high capacity and low working potential as a negative electrode for aqueous battery.However,there is a serious capacity fading phenomenon for Fe₂O₃anodes working in aqueous electrolyte.Aiming at this problem,we have carried out research on the energy storage mechanism of iron oxide in different aqueous electrolytes,and then improved of its performance.The main work is as follows:（1）The finger-ringα-Fe₂O₃is synthesized by one-step hydrothermal method,and the three-electrode test is carried out in neutral and alkaline electrolyte to test its energy storage characteristics.It is found that it has a relatively high initial capacity(180 m Ah g^-1)in alkine electrolyte,but the capacity decays rapidly during the cycle;In the neutral electrolyte,the initial capacity is only 110 m Ah g^-1,and the capacity continues to drop sharply to 0 after 100cycles of circulation,which is accompanied by precipitation.With careful analysis,it is proposed that part of Fe₂O₃is reduced to be resoluble Fe²⁺followed by the formation of Fe（OH）₂around the electrode in low potential range owning to the insertion of（H₃O）⁺,and subsequently converted into Fe OOH as precipitation,which would cause irreversible mass loss and hence fast capacity decay.It is found that Fe₂O₃is irreversibly reduced to be Fe₃O₄during the initial discharging process in alkine electrolyte.Structural collapse of Fe₃O₄and agglomerating into larger particles during the following cycles should take main responsibility for the poor durability.Therefore,how to effectively suppress irreversible mass loss in neutral electrolyte,and agglomeration in alkaline electrolyte is the key to the development of Fe₂O₃anodes.（2）Aiming at the agglomeration of active materials in alkaline electrolyte,using a stirring method to compound Fe₂O₃with GO to explore the composite-electrode electrochemical performance in alkaline electrolyte.It is found that GO is reduced to r GO during the cycling.The presence of GO can significantly improve the specific capacity and cycle stability of the composite electrode.The specific capacity of FG10（GO with 10%mass ratio content）can reach 223 m Ah g^-1at a current density of 1 A g^-1,but there is still a significant attenuation during cycling.With the increase of GO content,the specific capacity of electrode has decreased slightly,but its cycle stability has gradually improved.The capacity retention rate of FG200 can reach 95%after 400 cycles.The electrochemical performance of the sample compounded with GO is significantly better than that of pure Fe₂O₃,indicating that the mixing of GO can effectively inhibit the agglomeration of Fe₃O₄,thereby improving the stability of the electrode.（3）The micron spherical Ni（OH）₂was prepared by hydrothermal method as the positive electrode.Using the optimized FG100 as the negative electrode,the Ni-Fe battery was assembled in an alkaline aqueous electrolyte.The Ni-Fe battery has a working voltage range of 1.7 V and a discharge platform at 1.1 V.The capacity of the full battery can reach 66 m Ah g-1（calculate the mass of the positive and negative active materials）at a current density of 1A g^-1with a good cycle stability.