| Supercapacitors（SCs）have the advantages of short charging time,large power density,excellent cycle performance and high safety performance.They have a broad application prospect in military industry,energy vehicle,electronics and other fields.Developing high-performance materials and asymmetric device is the key factor to broaden the scale application of SCs.In recent years,people have conducted a lot of research on electrode materials.Metal oxides and hydroxides with pseudo-capacitive properties show obvious advantages over the double-layer carbon materials.However,the actual specific capacitance of metal oxides/hydroxides is far lower than their theoretical value,due to their low conductivity and specific surface area not conducive to the transmission of electron/ion.The contradiction between small-sized nanomaterials and unstable structure makes it difficult to guarantee the multiplier performance and cyclic stability,which seriously affects the performance of devices.In order to solve these problems,a novel three-dimensionally porous electrode material constructed by MOFs-derived layered double hydroxide（LDH）nanosheets containing rich oxygen vacancies by a novel strategy of electrochemically assisted preparation of LDH material derived from metal-organic frameworks（MOFs）.First,Co-MOFs grow on the three-dimensional conductive skeleton of a nickel foam（NF）via liquid phase deposition,and then bimetallic Co Ni-MOFs is obtained by using ion exchange.Finally,the 3D porous electrode material constructed by Co Ni-LDH with oxygen vacancies is obtained via electrochemical transformation.Meanwhile,the formation mechanism of oxygen vacancies is discussed,and the reasons for the performance enhancement is analyzed.An asymmetric all-solid-state supercapacitor device is assembled by using Co Ni-LDH as the positive electrode together with the alkene hydrogel as the negative electrode,and its electrochemical performance is also studied systematically.The main contents are summarized as follows:（1）Co-MOFs nanostructures grow on the ligaments of a 3D nickel foam（NF）by using a liquid phase deposition method at room temperature.The reaction parameters（the concentrations of solution,reaction time,and so on）are explored to analyze their effects on the resulting morphology and specific nanostructures of Co-MOFs as well as their electrochemical performance.Research results show that Co-MOFs with a lamellar cross-weaving skeleton grow on the ligaments of NF.When the concentrations of 2-methylimidazole and cobalt nitrate hexahydrate are 0.8 M and 0.1M,respectivley.The reaction time is 5 h,the resulting Co-MOFs nanostructures have a strong interface bonding force with the NF.The results of electrochemical tests show that specific capacitance of the optimized Co-MOF electrode material is 600.9m F cm-2at a current density of 0.5 m A cm-2.When the current density increases by 20times,the capacitance value decreases to 386.2 m F cm-2.At a high current density of20 m A cm-2,the coulomb efficiency reaches up to 94.2%after 4,000 charge-discharge cycles,but specific capacitance retention rate is only 56.2%of the initial capacitance value.Based on the results,we discussed the involved reasons for the low rate capability and poor cycling stability of the NF-supported Co-MOFs.（2）Based on the optimized Co Ni-MOFs,a novel preparation strategy of combining ion exchange and electrochemical assistance is proposed.Namely Ni2+ions are introduced into the Co-MOFs material by ion exchange,and partial Co2+are replaced from Co-MOFs to convert into bimetal Co Ni-MOFs,and then the Co Ni-MOF is successfully converted into three-dimensional porous electrode material constructed by oxygen vacancies-contained Co Ni-LDH nanosheets by means of micro-current CV electrochemical synthesis.The Co Ni-LDH nanosheets growing on a lamellar cross-weaving skeleton.The generation mechanism of oxygen vacancies and its influence on the electrochemical performance are investigated.The results show that the formed nanosheets increase the contact area between electrode material and the electrolyte,and shorten the moving paths of electrons and ions.Meanwhile,the appearance of oxygen vacancies increases the active site of the electrode material,effectively improves the conductivity of the electrode material and reduces the internal resistance of the electrode material.At a current density of 1 m A cm-2,the specific capacitance is 3688.6 m F cm-2,which is about 7 times of that of Co-MOF electrode material.After charge-discharge for 10,000 cycles under 20 m A cm-2,the capacitance retention is 87.1%of the initial value,and the Coulomb efficiency reaches up to 92.1%.Additionally,an asymmetric all-solid supercapacitor device is assembled by using Co Ni-LDH as the positive electrode and graphene hydrogel as the negative electrode.The assembled device has a work voltage window of 1.5 V,and the energy density reaches 114.3 m Wh cm-3(the power density 2.7 W cm-3).This work demonstrates the effective preparation technique,and provides a new route for the design of high-performance LDHs electrode materials for all-solid-state SCs. |
