Preparation And Electrochemical Performance Of Cobalt-based Metal-organic Framework Materials And Their Derivatives

Supercapacitors,a new type of energy storage devices between traditional capacitors and batteries,have attracted more and more attention due to their good charge-discharge efficiency,high power density,longevity and so on.Electrode material is the most important factor that determines electrochemical properties of supercapacitors through controlling the chemical structure and morphology of compound.Metal organic frameworks?MOFs?are widely applied in gas storage and separation,catalysis,drug delivery and electrochemistry,especially in supercapacitors,which can be attributed to their diversity,easy functionalization,controllable structure and developed channels.MOFs not only can be directly used as electrode materials for supercapacitors,but also as precursors of metal oxides and carbon electrode materials.In this paper,three different cobalt-based MOFs and their derivatives were prepared and applied in supercapacitors.The specific contents are as follows:1.A high N content cobalt-based metal organic frameworks?Co-MOFs?with nanorod structure were prepared by a simple solvothermal method for supercapacitors,which were utilized 4,4',4''-s-triazine-1,3,5-tri-p-aminobenzoic acid as organic ligand and cobalt acetate as metal coordination center.Doping N can enhance the wettability and conductivity of materials,while the regular nanorod structure can improve the stability of materials and facilitate the adsorption of electrolyte ions on the electrode surface to form electrical double layer.The synthesis temperature and feed ratio?molar ratio of Co to the ligand?were optimized to achieve the optimal Co-MOFs electrode materials.Under 140?and feed ratio of 1:2,the specific capacitance achieved 414.5F g^-1 at a current density of 0.5 A g^-1 and the capacity retention reached to 113%after18000 charge-discharge cycles.2.Based on the above 1 mentioned Co-MOFs,a S-doping Co based metal oxide was obtained through two steps of oxidation and sulfurization for supercapacitors.Introduction of S element can increase the electrochemical activity of electrode materials and bring pseudocapacitance effect,which is beneficial to electron diffusion and capacitance performance.The S-doping Co based metal oxide materials showed a capacitance of 562.1 F g^-1 at the 1 A g^-1 current density and retained the excellent stability of the precursor Co-MOFs,which reached to 111%retention after 20000charge-discharge cycles.3.A N-doped porous carbon composite containing Co@C core@shell structure with big surface area was prepared from ZIF-67 assisted polyvinylidene fluoride?PVDF?hollow fiber membranes through a simple calcination process for supercapacitors.The interaction between ZIF-67 and PVDF membranes solved problems that porous tunnels of PVDF membranes were easily collapsed and Co particles derived from ZIF-67 were heavily aggregated during carbonization procedure,which endowed the carbon composite with large surface and even mesopores.Appropriate surface area and pore size are beneficial for more active sites and electron diffusion.Furthermore,the unique Co@C core@shell structure can change the electron distribution between Co and C atom,which strengthens the electroactivity of carbon composite.In addition,doping N can increase the conductivity/wettability of electrode and generate pseudocapacitance.The best electrochemical properties of carbon composite were achieved by changing pyrolysis temperature,which delivered a high specific capacitance of 652 F g^-1 at 1 A g^-1 and a great stability of 97.1%retention after20000 cycles.