Reseach On The Design And Energy Storage Mechanism Of Cobalt-based Phosphide Hybrid Supercapacitors

In recent years,transition metal phosphides have been favored by researchers because of their excellent catalytic and energy storage properties.As a typical representative of transition metal phosphides,cobalt phosphide shows good conductivity,rich redox reaction and high theoretical specific capacity,which has attracted much attention as electrode materials.However,the poor rate ability and low cycle stability limit its practical application in alkaline electrolyte.In order to solve the above problems and improve the energy density and power density of supercapacitors,the following works are carried out in this paper:1. CoP/C materials with special structure are prepared by using MOF as the precursor. CoP was coated by carbon with good stability,which can effectively improve the cycle stability of electrode.Compared with the Co₃O₄/C electrode,hollow porous CoP/C nanocage has better conductivity and richer redox reactions.The good conductivity enhances the migration rate of electrons in electrode materials and electrolytes,which improves the electrochemical performance of CoP/C nanocage.At current density of 1 m A/cm²,the specific capacity of CoP/C is 343.1 m C/cm².And at a high current density of 10 m A/cm²,87.8%of the initial specific capacity is still retained.At 10 m A/cm²,the retention rate of specific capacity remains 76.8%after charging and discharging for 3000 times.The CoP/C electrode shows good rate ability and cycle stability.2. Modification of the electrode material by constructing interface structure is an effective method to improve the rate ability of electrode. Recombining metalloid CoP and n-type semiconductor Ce O₂ with high redox properties,the interface can promote electrons migration.A nove Ce O₂@CoP/NF positive material was in-situ grown on nickel foam?NF?,and the electrochemical performance was systematically investigated.The results show that the electrochemical performance of Ce O₂@CoP/NF electrode is significantly improved compared with CoP/NF electrode.At a current density of 1 A/g,the specific capacity of Ce O₂@CoP/NF electrode is 595.1 C/g.And at a large current density of 20 A/g,the specific capacity retains 75%.Theoretical calculations show that Schottky barrier is formed at the interface of Ce O₂@CoP,which promotes electrons migration and improves the electrochemical performance of electrode.The packed Ce O₂@CoP/NF//AC hybrid supercapacitor shows a high energy density of 55.4 Wh/kg at a power density of 955.9W/kg.The device performs good energy storage performance.The results show that the construction of heterojunction plays an important role in improving the rate ability of electrode.This work provides an experimental/theoretical method for understanding the electrons migration process in electrochemical reactions.3. Rare earth doping is also an effective strategy to improve the electrochemical performance of the electrode. The La element has large ion radius and rich electrons in the outer electronic laye,which can distort the crystal structure and increase the movable electrons in the doped crystal,promoting the rapid migration of ions in the CoP electrode.We prepared the binder-free La-CoP/NF electrode by hydrothermal-phosphatation method,and investigated the effect of La doping amount on the electrochemical performance of electrode.The results show that the diffusion rate of ions in the electrode material and electrolyte is fastest,when the La doping amount is 2.5%.La?2.5%?-CoP electrode material shows excellent electrochemical performance.The La?2.5%?-CoP/NF//AC hybrid supercapacitor also exhibits outstanding charge storage capacity.The packed La?2.5%?-CoP/NF//AC hybrid supercapacitor shows high energy densities of 45.9 and 17.8Wh/kg,when the power densities are 300 and 6000 W/kg,respectively.The results proved that doping rare earth La element can improve the electrons transport,which provides new ideas for optimizing electrode material.