Design And Construction Of Transition Metal(Cobalt,Nickle)/Carbon Electrodes For Supercapacitors

Supercapacitor with the excellent properties of high power density,fast charge-discharge rate and long recycling life has attracted significant research attention in energy fields.However,the heart of the matter that restrains their large-scale production applications is the relative low energy densities of currently reported supercapacitors.According to the energy density calculation formula of E=1/2 CV2,the energy density of a supercapacitor is collectively decided by the specific capacitance(C)of electrode materials and the operating voltage(V)of the devices.To obtain high C and V performances,we select transition metal(cobalt and nickel)as the electrode reaction active species and metal-organic frameworks(MOFs)as the porous carbon templates in this thesis,followed by high-temperature pyrolysis treatment in N2 atmosphere,to obtain high-performance electrode materials for supercapacitors.Further,we construct an aqueous asymmetric supercapacitor utilizing Co-and/or Ni-MOFs derived electrode materials,exhibiting high specific capacitance and extended working voltage window of the device.In summary,this thesis work consists of four chapters:the first chapter mainly summarizes the background of supercapacitors and MOFs materials,as well as the working principle of the supercapacitor,the structure characteristics of MOFs amd MOFs derived carbon-based materials and their applications as electrode materials for supercapacitors.Chapter two to four mainly introduces the research work during the postgraduate period,respectively.The specific research contents are summarized as follows:1.Taking advantage of the different coordination abilities of N-containing 4,4'-bipyridine and S-containing thiophene-2,5-dicarboxylate in different solvents,two kinds of Co-MOFs materials were synthesized in water and ethylene glycol,respectively.After pyrolysis treatment at 800? in N2 atmosphere for 2 h,these Co-MOFs materials can be readily converted into Co9S8/S,N co-doped porous graphitic carbon composites(Co9Ss@SNCB and Co9S8@SNCC,seperately).Compared with the bulk Co-MOFs precursor synthesized in water,the high temperature pyrolytic carbon-based material Co9Ss@SNCC drived from small-sized cubic Co-MOFs obtained in ethylene glycol,possesses smaller size and higher specific surface area.As supercapacitor electrode material,the synthesized Co9S8@SNCC exhibits a specific capacitance of 429 F g-1 at a current density of 1 A g-1.After 2000 times charge-discharge test at a current density of 5 A g-1,Co9S8/S,N co-doped porous graphitic carbon assembled supercapacitor indicates high applicable stability with a specific capacitance retention of 98%.2.On the basis of the above research,we further synthesize two dimensional Co/Ni bimetallic MOFs materials at room temperature through the precise regulation of the reaction precursor proportion and the adjustment of the preparation route.The formed CoNi alloy nanoparticles with?10 nm after pyrolysis treatment of Co/Ni MOFs precursor at 550 ? in N2 atmosphere are uniformly dispersed in S,N co-doped porous graphitic carbon layers.The resultant Co/Ni nanoparticles modified graphitic carbon composite was used as supercapacitor electrode material,exhibiting a capacitance of 1920 F g-1 at 1 A g-1,and the energy density can achieve 55.7 Wh kg-1 at a power density of 800 W kg-1.With the material structure characterization before and after eletrochemical measurements,the improvement of the storage capacity of Co/Ni nanoparticles modified graphitic carbon assembled supercapacitor can be due to the surface oxidation of CoNi alloy nanoparticles to form electrochemical active species,favourable for enhancing the binding ability toward OH-.3.A series of metal(Ni,Co,Zn,Mg,Cu etc.)-organic frameworks hydrogel materials were synthesized with large scale by a simple hydrothermal method utilizing metal sheets as metal ions source and acidic organic ligands as linkers.The as-synthesized MOFs hydrogel materials composed of one dimensional MOFs nanoribbons can be readily converted into lightweight aerogel materials after freeze drying.The experimental results demonstrate that under hydrothermal conditions,the formed metal ions dissolved from metal sheet can effectively coordinate with acidic organic ligands to self-assembly form one dimensional MOFs nanoribbons,further constructing three-dimensional MOFs hydrogels at room temperature.Taking the synthesized Ni-MOFs as an example,its aerogel derivatives as supercapacitor electrode material exhibit a specific capacitance of 240 F g-1 at 1 A g-1,indicating its great potential in energy devices.This thesis work demonstrates the feasibility of using low-cost and abundant metal sheets as metal ions source and acidic organic ligands as linkers to fabricate MOFs hydrogel and aerogel materials in large for environmental and energy applications.