Controllable Synthesis And Electrochemical Performance Of Derivatives From Flake-like ZIFs

Among various energy storage technologies,supercapacitors(SCs)are considered as promising energy storage technology due to its excellent power density and long cycling lifespan.For supercapacitors,the influence of electrode materials on their electrochemical performance is very important.At present,it is generally regarded that the microstructure of electrode materials plays an important role in the performance of SCs.Therefore,the design of electrode materials with reasonable structure and controllable microstructure is the key to improve the performance of SCs.In this paper,flake-like Zeolitic Imidazolate Frameworks(ZIF-67)precursors were synthesized on different substrates,and then nanosheet arrays with different morphologies and phases were prepared by different methods.The phase and structure transformation of ZIF-67during phosphating,carbonization,oxidation,sulfuration the were studied,and the performance of pseudocapacitor was improved through the design and control of microstructure.The specific contents are as follows:(1)The transition metal phosphide is an excellent candidate for supercapacitor due to its superior electrical conductivity and high theoretical capacity.In addition,compared with traditional 3D nano-materials,2D nanosheets possess greater specific surface area and shorter electron transport distance.In this study,a reasonable approach is proposed to synthesis of ZIF-67 nanosheet on nickel foam through further phosphorization by chemical vapor deposition(CVD)to obtain the flake-like Co P combined with Ni₂P(NCP/NF),of which nickel foam serves as the current collector as well as the resource of Ni to form Ni₂P.Benefiting from nanosheet array of Co P,the NCP/NF can improve the capacity of Ni₂P from 0.57 C cm^-2 to 1.43 C cm^-2 at 1 m A cm^-2.Furthermore,the NPC/NF/reduced graphene oxide(RGO)asymmetric supercapacitor(ASC)shows an energy density of 26.9?Wh cm^-2 at a power density of0.896 m W cm^-2,and excellent cycling performance with a capacity retention of 93.75%after 5000 cycles at 10 m A cm^-2.(2)In order to further explore the transformation characteristics of flake-like ZIF-67,and find out more different types of derivatives of flake-like ZIF-67 in the application of supercapacitors,in this work,a two-step thermal conversion different from traditional approaches is reported to synthesize tightly aligned Co₃O₄/carbon materials.The architecture of cross-linked nanosheets and uniformly distributed Co₃O₄nanoparticles(NPs)is obtained by in-situ reduction of cobalt cations in flake-like zeolitic imidazolate frameworks(ZIFs)within inert gas at 500?and subsequent mild oxidation of metallic cobalt NPs to Co₃O₄ NPs(Co₃O₄@C-500).As a result,this strategy effectively prevents structural collapse and defect formation,which are usually caused by direct oxidation of ZIF-67 nanosheets.In supercapacitor tests,Co₃O₄@C-500 shows an appreciable specific capacitance(703.3 F g^-1 at 1 A g^-1)and excellent cycling stability with the capacitance retention of 100%after 10000 cycles at 10 A g^-1.Moreover,the Co₃O₄@C-500//reduced graphene oxide(RGO)asymmetric supercapacitor(ASC)exhibits an energy density of 43.99 Wh kg^-1 at a power density of 824.8 W kg^-1,and excellent cycling performance with a capacity retention of 88%after 10000 cycles.Such excellent performance might benefit from the rational composition control and unique structure of Co₃O₄ NPs embedded in the carbon skeleton,which results in short electrolyte diffusion channels as well as electron transport distance and effectively reduce negative effect of volume change of electrode materials during the charge/discharge process.Co@C-500,the carbonized product of flake-like ZIF-67,is used as precursor,through one-step hydrothermal treatment,the results show that Co@C-500 Conversion to two-dimensional(2D)Mo S2 coated Co3S4 nanoparticle sheet arrays(Co Mo S@C).Co@C-500 as a precursor,the flake structure composed of nanoparticles was perfectly maintained during hydrothermal treatment.Compared with the Co₃S₄ nanosheets without Mo S₂(Co₃S₄@C),Co Mo S@C at the current density of 1 A g^-1,the specific capacity of the electrode is 2.9 times than that of the former.Co Mo S@C The asymmetric supercapacitor with RGO exhibits excellent electrochemical energy,with a high energy density of 78.9 whkg^-1 at a power density of 800.1 W kg^-1.The capacity is only 0.4%lost after 10000 cycles.Its excellent electrochemical performance is attributed to existence of 2D Mo S₂ greatly increases the electron transfer efficiency.The multilayer 2D structure promotes the electrolyte to enter the material quickly,which makes the Co₃S₄ nanoparticles in the inner layer react more quickly under the same charging current;The bimetallic sulfide phase can provide more reaction sites for redox reaction;The carbon skeleton inhibits the volume expansion of Co Mo S@C during the charging and discharging process.