Preparation Of Co-based MOF Metal Phosphides And Research Of The Electrochemical Characteristics In Composite Materials

Compared with traditional lithium-ion batteries and fuel cells,supercapacitors have received much attention as a new type of energy storage device that meets the requirements of safety,stability,high power density and fast charging and discharging.Among the components of supercapacitor,electrode material is the most critical factor to determine the performance of supercapacitor.In addition,rational design of the nanostructure and chemical composition of materials from a microscopic perspective can be used to achieve functionalization and practicality of materials.Metal organic frameworks（MOFs）are high-performance framework materials composed of metal ions and their organic ligands,and their large specific surface area,high porosity,and variable structure provide new opportunities for the development of high-performance electrode materials,which can be designed as porous structural materials.When used as basic template electrode materials,their spatial structure can effectively mitigate the volume expansion caused by ion embedding and de-embedding.In this paper,high performance transition metal phosphide electrode materials are designed from three aspects:changing morphological structure,elemental doping,and composite materials,using MOFs materials as sacrificial templates:（1）Co-MOF dodecahedra were synthesized by the solvothermal method,and CoP/C polyhedral nanoparticles were prepared by high-temperature phosphorylation using them as precursor materials,which were physically characterized by XRD,XPS,SEM and TEM tests.The results of electrochemical experiments showed that the CoP/C exhibited a specific capacity of 116.6 m Ah/g at a current density of 1 A/g and a cycle retention rate of 63.7%after 5000 consecutive charges and discharges at a high current density of 10 A/g.The hybrid supercapacitor assembled with CoP as the positive electrode and AC as the negative electrode exhibited an energy density of 32 Wh/kg at800 W/kg and a cycle retention rate of 73.8%.（2）The Co-MOF precursor was firstly prepared by a simple solvothermal method,and then by adding a nickel source to the precursor,when the H+generated by the hydrolysis of Ni²⁺entered the Co-MOF skeleton,the coordination bond between Co²⁺and 2-MIM was broken,and then part of the free state Co²⁺was oxidized by the dissolved O² and NO^3-in the solution to the free state Co³⁺,Co²⁺,Co³⁺and Ni Co-LDH was generated by co-precipitation of Ni²⁺.The different ionic diffusion rates of the two metal elements would lead to the formation of hollow structures,followed by the synthesis of Ni CoP precursors by high temperature phosphorylation,which were physically and electrochemically characterized.Electrochemical results showed that Ni CoP was obtained with a high specific capacity of 134.1 m Ah/g at a current density of 1A/g,and the capacity retention was 60.6%when the current density was increased to 10 A/g.The assembled HSC device has an energy density of 35.9 Wh/kg at a power density of 699.9 W/kg,as well as a stability of 71.1%after 8000 cycles at a high current density of 10 A/g.（3）NiCoP hollow nanoparticles were rationally prepared by a simple method within three-dimensional（3D）reduced graphene oxide（r GO）.In this unique layered structure,r GO is used as a highly conductive interconnecting network matrix,thus allowing for fast ion and electron transport at hybrid electrodes.Thanks to the synergistic effect between Ni CoP and r GO,the hybrid material shows an excellent energy storage capacity in a three-electrode system with a high cycling stability（77.4%）with a high capacitance retention of 62.7%.The hybrid supercapacitor was prepared using Ni CoP/r GO-20 composite as the positive electrode and AC as the negative electrode.The hybrid supercapacitor has a high energy density of 63.6 Wh/kg and excellent cycling performance with 83.7%capacitance retention in 10000 cycles.This provides a good way to design advanced hybrid materials of transition metal phosphides for high-energy density supercapacitors.