Preparation Of Co/Ni/Mn Based Transition Metal Pseudocapacitor Materials And Their Property Research For Supercapacitor

Supercapacitors(SCs)are widely used in the field of energy storage due to their high power density and good cycling life.However,the lower energy density of SCs has limited the further development of supercapacitors.Based on the pursuit of high energy density devices,the development of electrode materials with larger specific capacity and wider potential window becomes the most direct way to improve the specific energy of the device.This dissertation is mainly based on the preparation of electrode materials of transition metals Co/Ni/Mn for improving the energy density and electrochemical performance of the devices,which provides us a theoretical basis and technical support for the large-scale application of electrode materials.Due to the unique advantages of Co/Ni/Mn-based materials in the field of energy storage and the controllable structure of the metal-organic frameworks(MOFs),we prepared a two-dimensional sheet-stacked ternary Co-Ni-Mn MOF by a solvothermal method,and used it as the cathode material of supercapacitors;meanwhile,owing to the wide potential window of Mn in the aqueous electrolyte,we have also prepared sunflower-shaped Mn MOF and used it as the anode material.Electrochemical testing of the asymmetric capacitor Co-Ni-Mn MOF//Mn MOF revealed that the device had good electrochemical performance,which reached the specific capacitance of 94.9 F g^–1 at 1 A g^–1.While at a power density of 857.3 W kg^–1,it had a maximum energy density of 38.1 Wh Kg^–1.After the 5000 cycles at a current density of 5 A g^–1,the specific capacity retention of the device is only 70.2%,which suggested the cycle stability was needed to be improved.This may be due to the poor conductivity of traditional MOFs,or the coordination bonds of ternary MOFs are likely to break during the redox process,leading to structural collapse,which affects the cycling performance of the device.To further improve the cycle stability and pseudocapacitance of MOFs,we consider using reasonable chemical treatment methods to control the structures of MOFs.Compared with the ternary Co-Ni-Mn MOF in the previous work,the binary Co-Ni MOF exhibited a more regular lamellar structure and the interaction between the lamellars is closer.Hence,it was selected as the activated precursor,which was activated by Na BH₄and Na₂S.We found that more interacted small sheet networks formed in-situ in the surface of the boronized sample Co-Ni-B,which further enhanced the structural stability of the material and increases the specific surface area.Benificial from such self-supporting structure,Co-Ni-B-S,obtained from the further sulfurization,not only retained the flower-like structure of the material,but also made the partially reduced metal components oxidize into a stable low mixed valence,enhancing the electrochemical performance of the material.When assembled into a device with activated carbon,the energy density of Co-Ni-B-S//AC reached 50.0 Wh Kg^–1 at a power density of 857.7 W Kg^–1,and the device had a capacity retention rate of 87.7%after 5000 cycles at 12 A g^–1,showing improved cycle stability than the pure MOF device in the previous work.In addition to enhancing the pseudocapacitive properties of materials by chemical redox strategy,heat treatment is also a common method for activating MOFs.On this basis,combined with a simple quenching process,a core-shell electrode material was prepared.First,we also used the binary flower-shaped Co-Ni MOF with the self-supporting layer structure as the precursor.After heat treatment,the Co-Ni-500 sample with the retained flower-shaped structure was heated to 300 ^oC,and then was quenched in a Fe Cl₃/K₃[Fe(CN)₆]mixed solution to rapidly cover a layer of amorphous Prussian blue(PB)on the surface of Co-Ni-500.The double pseudocapacitor composite material Co-Ni-500/PB was obtained.Thanks to the Fe²⁺/Fe³⁺redox pseudo-capacitance of PB and the core-shell structure brought by quenching,the pseudo-capacitance characteristics of the Co-Ni-500/PB composite are significantly improved.At a current density of 1 A g^–1,the specific capacitance of Co-Ni-500/PB reached 879.3 F g^–1,and the capacity loss was only 14.1%after 5000 cycles at 10 A g^–1.Because quenching not only simplifies the preparation process and reduces the preparing time of the material,the obtained material also has structural advantages,so we consider extending such method.Flexible electrodes have always been the key development direction of supercapacitors,so the development of flexible electrode materials that can be quickly prepared can bring new breakthroughs to the development of flexible devices.We first prepared polyacrylonitrile membranes(PAN)by electrospinning and then carbonized to obtain PAN-C as a flexible substrate.Furthermore,potassium permanganate was selected as the quenching solution due to the wide potential window of manganese.Then a layer of Mn O₂ with high K⁺content and abundant defects was rapidly grown in-situ on the PAN-C surface by the quenching method,obtaining the Mn O₂@PAN-C electrode material.With this flexible electrode assembled into a symmetrical supercapacitor,the voltage of the device can be as high as 2.3 V.,And the energy density of this device can reach to 47.76 Wh Kg^–1 when the power density is 575 W kg^–1,exhibiting nice energy storage capability.Beneficial from the application of transition metal Co/Ni/Mn MOF derivatives and a new method for rapid preparation of electrode materials,the pseudo-capacitance characteristics of the material can be effectively enhanced,as well as the electrochemical performance of the material,providing new references for the industrial large-scale preparation of electrode materials of supercapacitors.