Preparation Of Cobalt-based Composites And The Research On Their Capacitive Performances

Cobalt-based materials are suitable for supercapacitors because of their high theoretical specific capacitance,but their rate performance and power density are limited by many factors in application.Moreover,because the energy storage mode of cobalt-based materials is based on the redox reaction of cobalt ions,the materials' structure is prone to change,which affects the cycling stability of the electrode.Therefore,a series of new cobalt-based materials have been designed and constructed in this paper and the contents are as follows:(1)Kelp was used as the carbon precursor to prepare a biomass carbon with meshy microstructure,and Co3O4 nanoparticles were further modified on the meshy biochar(SGBC)by using solvothermal and pyrolysis methods in turn.The prepared Co3O4@SGBC composite combined the special structure of SGBC with uniformly distributed Co3O4 nanoparticles,which provided large specific surface and good conductivity.Different methods including scanning electron microscopy,transmission electron microscopy,X-ray powder diffraction,Raman spectroscopy,X-ray photoelectron spectroscopy and nitrogen adsorption-desorption were used to characterize the structure and component of composite.Co3P4@SGBC was employed to construct an electrode for supercapacitor,which had better electrochemical performance than that of pristine Co3O4 nanoparticles and SGBC electrodes.The highest specific capacitance(1212.4 F·g-1 at 0.5 A·g-1)with excellent cycle stability could be obtained and the capacity retention was 99.5%at 2 A·g-1 over 10000 cycles,which could be attributed to the synergistic effects of both pseudocapacitance and electric double layer capacitance under optimized carbon-cobalt ratio.Furthermore,an asymmetric supercapacitor system was constructed by utilizing Co3O4@SGBC as positive electrode with SGBC as negative electrode.This two-electrode system presented a good performance with energy density of 59.82 Wh.kg-1 at a power density of 700 W·kg-1,highlighting the potential application in efficient energy storage device.(2)A carbon-supported cobalt oxide composite(Co3O4@EPC)was synthesized using biochar derived from core shell of Lucuma nervosa and cobalt(?)phthalocyanine through solvothermal and thermal decomposition methods.Characterizations demonstrated that CO3O4@EPC was a stable combination of 50-80nm Co3O4 spherical grains with EPC consisting mainly of amorphous carbon.N2 adsorption-desorption isotherm was used to determine the specific surface area of Co3O4@EPC at 87.88 m2·g-1.EIS,CV and constant current charge-discharge(GCD)methods were used to analyze the capacitive performance of CO3O4@EPC.The maximum specific capacitance of Co3O4@EPC was about 781.56 F·g-1 at a current density of 0.5 A·g-1,The two-electrode system(Co3O4@EPC//EPC)constructed directly using Co3O4@EPC and EPC as the positive and negative electrodes has an energy density of 32.8 Wh·kg-1 at power density of 1333 W·kg-1,and the capacitance retention rate of Co3O4@EPC//EPC was 96.7%over 20000 cycles at the current density of 10 A·g-1.(3)A series of sodium alginate-derived carbon-coated Co3V2O8 composites for supercapacitors were synthesized.The characterization results show that the Co3V2O8 component of the composite material was orthorhombic,and the larger the proportion of Co3V2O8,the smaller the specific surface area and pore volume of the composites.Electrochemical testing confirmed that the different ratios of carbon and Co3V2O8 in the composite led to differences in the performance from each other.Among them,CoVC2 had the best capacitance performance.The capacitance of the CoVC2 electrode was 1198.45 F·g-1 at a current density of 0.5 A·g-1.The energy density of CoVC2//CoVCO device can reach up to 60.95 Wh·kg-1 at power density of 700 W·kg-1 and even with power density up to 5600 W·kg-1 as the energy density still maintained 30.39 Wh.kg-1.and the capacitance still retained 80.15%of initial capacitance over 10000 cycles.(4)A precursor containing phosphorus and cobalt with spiny structure was firstly prepared by hydrothermal method.After calcination at 500? and 700?,two products named as 500P-Co and 700P-Co were obtained.The precursor and two products had been characterized by various instruments and showed that they were Co(PO3)2 and Co2P2O7 with different degrees of crystallinity.The capacitance electrodes constructed with 500P-Co and 700P-Co had been tested and showed that the specific capacitance was 731.52 and 230.45 F·g-1(at 1 A·g-1),respectively.The larger specific capacitance of 500P-Co was ascribed to the carbon residue derived from the organic constituent in 500P-Co,which contributed part of the capacitance.At current density of 6 A·g-1,the specific capacitance of 500P-Co and 700P-Co electrodes maintained 109.5%and 98.1%of the initial capacitance over 4000 cycles,indicating that both electrodes had good stable performance.The 500P-Co//AC asymmetric supercapacitor device,which was assembled by employing 500P-Co as positive electrode and porous activated carbon as negative electrode,can provide an energy density of 171.6 Wh.kg-1 at power density of 3259.8 W·kg-1,which was better than that of 700P-Co//AC(64.1 Wh·kg-1 at 2110.9 W·kg-1).(5)A precursor,which was a tetragonal bipyramidal complex synthesized by N-hydroxyl succinimide and Co2+,had been successfully thermal decomposited in nitrogen atmosphere and air atmosphere environment to generate series Co3O4@N-Cx(x=6,7,8,9)materials.The results showed that Co3O4@N-Cx were cubic crystal of Co3O4 doped with N and C heteroatoms,and the morphology were flower-like that composed by extremely thin lamella.Among them,Co3O4@N-C8 had the best electrochemical performance,its specific capacitance was 1123.45 F·g-1 at 1 A·g-1.The Co3O4@N-C8//EPC device constructed with Co3O4@N-C8 positive electrode and EPC(A carbon derived from core shell of Lucuma nervosa,Refer to Chapter 2)negative electrode had excellent performance.Its energy density can reach 32.8 Wh·kg-1 at power density of 600 W·kg-1.The capacitance at current density of 10 A·g-1 remained 95.76%of the initial value over 5000 cycles.