Preparation And Supercapacitor Properties Of Zr-MOFs And Carbon Quantum Dot Composites

The energy and environmental crisis has seriously affected the current economic and social development.The development of new energy sources and energy storage devices has become an important way to solve the energy crisis.Supercapacitors are a kind of electrochemical energy storage devices,which have received widespread attention due to their high power density and long cycle life.Electrode materials play a key role in their performance due to its core role for the development of supercapacitors.As a new class of porous materials,Metal-Organic Frameworks（MOFs）constructed from metal ions and organic ligands usually have high specific surface areas and variable pore sizes,which could be used in supercapacitors.The main reason limiting their application in supercapacitors is the poor electrical conductivity.In order to enhance the overall energy storage performance of MOFs as an electrode material,this paper focused on the introduction of carbon quantum dots（CQD）and doping of transition metals to modulate the composition of MOFs/CQD with high specific capacitance and good stability.1.In order to investigate the supercapacitance performance of MOFs/CQD composites,CQD@UiO-66 and CQD@UiO-66-NH₂electrode materials were prepared by the hydrothermal method using UiO-66 and UiO-66-NH₂as templates,and their supercapacitance performance had been further characterised.The structural characterisation showed that CQD were successfully introduced into MOF.The constant current charge/discharge results show that the specific capacitance of CQD-5@UiO-66 is 1394.1 F g^-1at a current density of 1 A g^-1,which is 1302.9 F g^-1higher than that of UiO-66(91.2 F g^-1),while the cycling stability of the material reaches 91.23%after 2000 charge/discharge tests.The material was assembled with activated carbon to form an asymmetric supercapacitor with an energy density of 19.79 Wh kg^-1at a power density of 750 W kg^-1.Connecting three devices in series can make two red LEDs glow continuously for for two minutes.With the addition of CQDs,CQD-5@UiO-66 is able to accelerate electron transfer for fast and reversible chemical reactions.2.In order to further improve the supercapacitance performance of the composites,Zr Ni-UiO-66,CQD@Zr Ni-UiO-66 and CQD@Ni-UiO-66-NH₂composites with different metal ratios were synthesized by a bimetallic doping method on the basis of CQD@UiO-66and CQD@UiO-66-NH₂.The related structural characterization and supercapacitor performance of these composites had been tested.The electrochemical test results showed that CQD-5@Zr₁Ni₁-UiO-66 improved the specific capacitance by 177%compared to CQD-5@UiO-66.The specific capacitance of CQD-5@Zr₁Ni₁-UiO-66 is 2468.75 F g^-1at a current density of 1 A g^-1,along with cycling stability of 94.28%after 2000 charge/discharge tests.The material was assembled with activated carbon to form an asymmetric supercapacitor with an energy density of 34.79 Wh kg^-1at a power density of 750 W kg^-1,Connecting three devices in series can make two red LEDs glow continuously for six minutes.The introduced Ni²⁺could enrich redox active site,which has a synergistic effect with CQD to improve the conductivity and facilitate charge transfer.3.In order to investigate the effect of heteroatoms on the performance of supercapacitors,DUT-69 and Zr-CAU-28 were prepared by substituting terephthalic acid with thiophene dicarboxylic acid and furan dicarboxylic acid,respectively.According to the previoussysnthesisconditions,CQD-5@DUT-69,CQD-5@Ni-DUT-69,CQD-5@Zr-CAU-28 and CQD-5@Ni-Zr-CAU-28,were synthesized,and their electrochemical performance had been characterized.The results show that CQD-5@Ni-DUT-69 has the best specific capacitance of 400 F g^-1@1 A g^-1and maintains a specific capacity of 80.45%after 2000 cycles,along with an improvement of 102.5 F g^-1over DUT-69(297.5 F g^-1@1 A g^-1).