Fabrication And Electrochemical Properties Of Different Morphologies Co-based MOFs As Electrode Materials

On account of the scarcity of conventional energy sources and the environmental problems,people urgently need to find novel energy storage technologies.Supercapacitor occupies an essential position in the field of electrochemical energy storage due to the advantages of excellent power density and environmental friendliness.As a part of the supercapacitor,electrode materials play a vital role in the performance of supercapacitor.Metal-organic frameworks（MOFs）materials possess great potential applications for supercapacitor owing to superior specific surface area,flexible and designable structures,and multi-level pore structure.Since the active site of metal cobalt has redox properties in the electrochemical conversion reaction,it can chelate with a variety of molecular ligands and with high coordination number.Thereby the MOFs materials formed by the coordination reaction can maintain pleasurable chemical and thermal stability.Therefore,in this work,a series of Co-based MOFs with different morphologies were fabricated and their electrochemical properties were studied and analyzed in detail by varying the types and proportions of organic ligands and metal salt hydrates providing transition metal centers Ni²⁺and Co²⁺,reaction temperatures,and reaction times using a one-step solvothermal method.The details are as follows.（1）The Co-MOFs were synthesized by a facile one-step solvothermal method using terephthalic acid（PTA）as the organic ligand and Co（NO₃）₂·6H₂O providing the metal central Co²⁺.And by adjusting the mass ratio of PTA to Co（NO₃）₂·6H₂O that provides metal center Co²⁺,the reaction temperature and time to study the influence on the electrochemical performance of Co-MOFs electrode.The electrochemical test results show that the strip-like Co-MOFs possesses outstanding electrochemical properties at the mass ratio of PTA to Co（NO₃）₂·6H₂O of 1:2,and reaction condition at 150℃for 6 h.The specific capacitance of the striped-like Co-MOFs electrode is 300 F/g at a current density of 0.5 A/g,and the specific capacitance is maintained at 266 F/g at 5 A/g,maintaining its initial capacitance89%.The Co-MOFs//AC asymmetric supercapacitor assembled with strip-like Co-MOFs displays an energy density of 19.5 Wh/kg at a power density of 389 W/kg.（2）The Ni/Co-MOFs were fabricated by a one-step solvothermal method using PTA as the ligand and Ni（NO₃）₂·6H₂O and Co（NO₃）₂·6H₂O to provide the metal centers Ni²⁺and Co²⁺.By varying the mass ratio of Ni（NO₃）₂·6H₂O to Co（NO₃）₂·6H₂O,the mass of PTA,the volume of ethylene glycol,the reaction temperature and the reaction time,the electrochemical properties of Ni/Co-MOFs materials was investigated.The electrochemical test results depict that the obtained juniperus sabina-like Ni/Co-MOFs possesses rosy electrochemical properties,which was fabricated at the mass ratio of Ni（NO₃）₂·6H₂O to Co（NO₃）₂·6H₂O of 2:1,the mass of PTA of 0.15 g,the volume of EG of 25 mL,the reaction temperature of 180°C,and the reaction time of 8 h.The juniperus sabina-like Ni/Co-MOFs exhibits a specific capacitance of 982.5 F/g at 0.5 A/g,and the retention rate of specific capacitance is 87%when the current density was increased 10 times.Meanwhile,the Ni/Co-MOFs//AC asymmetric supercapacitor assembled with the juniperus sabina-like Ni/Co-MOFs as the positive electrode shows a specific capacitance of 132 F/g at 0.5 A/g,and corresponding power density of 400 W/kg,the energy density of 46.9 Wh/kg.（3）1,3,5-benzenetricarboxylic acid（H₃BTC）was used as the organic ligand,and Ni Cl₂·6H₂O and Co（NO₃）₂·6H₂O provided the metal centers Ni²⁺and Co²⁺to synthesized Ni/Co-MOFs.And the effect on the electrochemical properties of Ni/Co-MOFs was investigated by adjusting the reaction temperature.The electrochemical test results demonstrate that the specific capacitance of the novel spherical-like Ni/Co-MOFs is 779 F/g at 0.5 A/g when the reaction temperature was 120°C.The Ni/Co-MOFs maintains 79%of the initial capacitance value when the current density was increased 5 A/g.In addition,the asymmetric supercapacitor assembled with the novel spherical-like Ni/Co-MOFs as the positive electrode displays a specific capacitance of 107 F/g at 0.5 A/g,a power density of400 W/kg,and an energy density of 38 Wh/kg.