Design Of Supercapacitor Based On LDHs And Research On Energy Storage Characteristics

Due to the intermittent and time-dependent energy generated by clean energy,it is increasingly important to develop energy storage systems that are compatible with clean energy.Over the past few decades,supercapacitors（SCs）as energy conversion and storage systems with excellent stability and efficiency have attracted wide attention from researchers in industry and academia.However,the main problem facing the development and application of SCs is low energy density.The key factors affecting the energy density of SCs include electrode material and voltage window.The energy density of the device can be further improved by developing high performance electrode materials and extending the voltage window of the assembled device.Therefore,efforts have been made to develop electrode materials with higher specific capacitance and wider voltage windows.In order to improve the specific capacitance of the electrode material,further improving the conductivity and the active site is the key point.Among many high performance electrode materials,layered double hydroxides（LDHs）are one of the most widely used in the field of electrochemical energy storage due to their two-dimensional layered structure and high theoretical capacity.In order to enhance the conductivity of LDHs,the cationic proportion in the host layer of LDHs was adjusted by using the physical and chemical properties of LDHs itself;In order to further enhance the active site,the crystal structure was improved by phase transformation while regulating the electrical conductivity of LDHs itself;On this basis,in order to improve the conductivity and the active site at the same time,the quantum dots of high conductivity was introduced into the LDHs framework to improve the redox reaction;In order to improve the voltage window of the device,a hybrid SCs with double layer capacitor material as the negative electrode is assembled by using water electrolyte based on the charge conservation principle.Therefore,reasonable design and the development of high-performance LDHs-based SCs with modified electrode materials lay a solid theoretical and practical foundation for further conducting and promoting the industrial application of SCs.The main research results are as follows:（1）Dopping LDHs based supercapacitors and research on energy storage characteristics（a）In order to improve the conductivity of LDHs and further enhance the specific capacitance of electrode materials,different Ni_xCo₂Al-LDHs@CC were synthesized by adjusting the content of Ni²⁺using the physical and chemical properties of ternary LDHs.The effects of Ni²⁺on the morphology,surface area and conductivity of the electrode were investigated by corresponding characterization and electrochemical tests.Due to the increase of Ni²⁺content,the morphology of LDHs changes from nanosheets to a mixture of nanosheets and nanowires with high specific surface area.When the current density is 1.0 m A cm^-2,the specific capacitance of Ni₁Co₂Al-LDHs@CC is 1772.5 m C cm^-2.After 12000 cycles,the capacitance retention rate is still 93.3%.The capacitance and conductivity of electrode are significantly better than that of the bimetallic Co Al-LDHs@CC.Hybrid supercapacitors（HSCs）were assembled in KOH aqueous electrolyte with Ni₁Co₂Al-LDHs@CC as anode and activated carbon as cathode.When the power density of the device is 0.75m W cm^-2,the energy density is 0.102 m Wh cm^-2.The capacitance of the device still maintains 90.98%of the original capacity after 12000cycles.（b）As the above experiments have proved that bivalent cationic metals can regulate the morphology and conductivity of ternary LDHs,in order to more comprehensively clarify the influence of metal ions on the storage mechanism of LDHs,Ni₁Co₃Ga_x-LDHs@CC with different trivalent cationic concentrations was synthesized by one-step hydrothermal method.The structure,morphology,crystallinity and conductivity of the electrode materials were optimized by adjusting the molar concentration of Ga³⁺.The results show that with the increase of Ga³⁺content,the interlayer space becomes more open and the dynamics of ion diffusion accelerates.When the current density is 1.0 A g^-1,the electrochemical capacitance of Ni₁Co₃Ga₁-LDHs@CC is 2012.5 F g^-1.To further expand the voltage window,HSCs is constructed using mesoporous carbon（MPC）as the cathode and Ni₁Co₃Ga₁-LDHs@CC as the anode,yielding an energy density of 84.22 Wh kg^-1 at the power density of 800.1 W kg^-1.The fabricated Ni₁Co₃Ga₁-LDHs@CC//MPC devices can light multiple LEDs in series for more than one minute.（2）Phosphating LDHs to design multicomponent hybrid based supercapacitors and research on energy storage characteristics（a）In order to enhance the active site of LDHs,LDHs were selectively phosphating by phase transition technique.First,Ni Ga-LDH/NF was prepared by hydrothermal method.The Ni Ga-LDH/NF with the highest electrochemical activity was obtained by adjusting the molar concentration ratio of Ni-Ga metal ions.Then,Ni₂P/Ni Ga₂O₄/Ni Ga-LDH/NF was prepared by Na H₂PO₂-induced partial phosphating.Electrochemical tests show that the specific capacitance of Ni_0.5Ga_0.5-LDH/NF is 295m Ah g^-1(2124 F g^-1)at 1.0 A g^-1.After phosphating,the specific capacitance of Ni₂P/Ni Ga₂O₄-1.2/Ni Ga-LDH/NF can reach 454.7 m Ah g^-1(3274 F g^-1 at 1.0 A g^-1).The results show that the capacitance of the electrode material increases obviously after phosphating.In order to comply with the better charge balance of the device,the metal-organic frame was calcined at high temperature to prepare Co₃O₄-C with excellent capacitance as the cathode.The assembled Ni₂P/Ni Ga₂O₄-1.2/Ni Ga-LDH/NF//Co₃O₄-C HSCs obtained a remarkable energy density of 72 Wh kg^-1at a power density of 800 W kg^-1.（b）In order to further expose the active sites at the reaction interface of LDHs,Ni Zn Co-P nanoarray with 2D-3D hierarchical structure were prepared by a template fixation method.The prepared 2D-3D Ni Zn Co-P/CC electrode has the porous three-dimensional framework and two-dimensional layered characteristics of subsequently derived Ni Zn Co-LDH/CC composite template.The results show that the optimal electrode of SCs device can reach 2816 F g^-1 at 1.0 A g^-1,which is 2.15 times that of three-dimensional electrode and 1.27 times that of two-dimensional electrode.In order to expand the voltage window of the device,the negative electrode of Zn-doped carbon nanosheets（Zn-CNS）was prepared by calcing Zn-MOF.The specific capacitance of Zn-CNS is 398 F g^-1 when the current density is 1.0 A g^-1.The assembled 2D-3D Ni Zn Co-P//Zn@CNS devices have excellent energy density and stability.The device can light multiple series LEDs for more than 4 minutes.（3）Quantum dot coupling LDHs based supercapacitors and research on energy storage characteristics（a）In order to improve both the electrical conductivity and the active site of LDHs,Ni-MOF nanosheets were prepared on nickel foam（NF）by room temperature coprecipitate method,and Ni-MOF/NF was soaked in N-GQD solution.Then Ni Ga-LDH/N-GQD/NF composite electrode containing N-GOD quantum dots was prepared by hydrothermal method.The prepared Ni Ga-LDH/N-GQD/NF has an excellent specific capacitance of 2160 F g^-1 when the current density is 1.0 A g^-1.This is mainly because MOF provides conductive networks and fast electron transport for composite electrodes.N-GQD enhances the hydrophilicity and the reaction site of the composite electrode,accelerating the diffusion of OH^-on the electrode surface due to the large amount of nitrogen-doped functional groups.In order to enlarge the voltage window,the Co-MOF derived carbon NSs as anode is prepared with a specific capacitance of 298 F g^-1 at a current density of 1.0 A g^-1.When the power density of HSCs devices is 1432.7 W kg^-1,the energy density is 78.8 Wh kg^-1,and the capacitance retention rate reaches 81.2%after 8000 cycles.（b）In order to further increase the number of active sites between LDHs layers,a composite electrode containing Cd S quantum dots between ultra-thin Ni Fe-LDH layers was prepared by simple electrochemical cycling and low temperature vulcanization.By adjusting reaction temperature,reaction time and number of cycles,the layer spacing of LDHs is increased from 0.70 nm to 0.81 nm(Cd S_inter.-Ni Fe),and the performance of LDHs is greatly improved.By electrochemical test,the capacitance of Cd S_inter.-Ni Fe electrode is 3245 F g^-1when the current density is 1.0 A g^-1,which is 1.6 times that of Cd S_surf.-Ni Fe electrode and 2.5 times that of the Ni Fe-LDH electrode.In order to expand the voltage window of the device,the porous carbon（Zn Co-PC）cathode was prepared by calcining Zn Co-MOF at high temperature with a specific capacitance of 320 F g^-1 at a current density of 1.0 A g^-1.In addition,the HSCs are assembled with Cd S_inter.-Ni Fe as anode and Zn Co-PC as cathode to obtain a remarkable energy density of 121.56 Wh kg^-1 and a capacitance retention rate of 91.5%during 10000 cycles.The device can light multiple series LEDs for more than ten minutes.