Preparation Of Hierarchical Cobalt-based Composites And Its Supercapacitor Performance

Supercapacitors have been paid close attention by many researchers because of their high power density,fast charging and fast discharging,but their low energy density limits their wide application.Therefore,the design of excellent electrode materials becomes the key to improving the performance of supercapacitors.Transition metals have been widely studied by researchers due to their more valence states and higher theoretical value of specific capacity.However,the conductivity of transition metals is not ideal,which limits their widespread application in energy storage devices.In order to solve the problem of poor conductivity of supercapacitors and improve the energy density of supercapacitors,transition metal phosphides are widely used in the materials of supercapacitors.In this thesis,starting with the selection and structural design of electrode materials,the flexible three-dimensional conductive carbon cloth as the base,transition metal as the supporting material,through phosphorization,respectively prepared CoP、CoO@Ni Mo-O（P）and CoP@Ni Mn-P.Finally,research their energy storage properties,from the structural design and energy storage characteristics of electrode materials,the following researches were conducted:（1）Preparation of CoP nanomaterials and its supercapacitor properties:Co（NO）₃·6H₂O was selected as the raw material to prepare Co（OH）₂nanowires with the reaction time of 2.5 h and 4 h on carbon cloth by one-step hydrothermal reaction,and then they were phosphated at low temperature.Finally,CoP（t=2.5h）and CoP（t=4h）were produced.The morphology characterization of Co（OH）₂（t=2.5h）、Co（OH）₂（t=4h）、CoP（t=2.5h）、CoP（t=4h）were analyzed by SEM、XRD、XPS、TEM.Secondly,electrochemical reaction tests were conducted on these four substances.After comparison,CoP（t=4h）has the best performance,with 358.8 F g^-1specific capacity at 1 A g^-1current density and 280 F g^-1specific capacity at 10 A g^-1current density.After 1000 charge-discharge cycles,the capacitance retention at 84.8%.Although the CoP electrode material has some capacitance characteristics,its specific capacity and cycle stability are not ideal.（2）Preparation of hierarchical CoO@NiMo-O（P）composites and its supercapacitor properties:Firstly,Co（OH）₂nanowires were prepared by one-step hydrothermal reaction,and then two transition nickel and molybdenum were selected for secondary hydrothermal reaction on Co（OH）₂nanowires,Co（OH）₂@Ni Mo-O（OH）which were prepared by Ni Mo-O（OH）nanowires coated with Co（OH）₂nanowire.Finally,the hierarchical composite electrode material CoO@Ni Mo-O（P）was prepared successfully by annealing and phosphating.The morphology and structure of CoO@Ni Mo-O（P）were tested by SEM、TEM and other characterization means,and the phase analysis of this electrode materials was carried out by XRD and XPS.Finally,through electrochemical testing analysis,the electrode material CoO@Ni Mo-O（P）had a specific capacity of 1304.6 F g^-1at 1 A g^-1current density and 854.6 F g^-1at 10 A g^-1current density.After 1000 charging and discharging cycles,the capacitance retention was 87%.Compared with CoP,CoO@Ni Mo-O（P）electrode materials not only improve the specific capacity,but also improve the cycle stability.However,the preparation technology and the safety of CoO@Ni Mo-O（P）electrode materials still need to be improved.（3）Preparation of hierarchical CoP@NiMn-P composites and its supercapacitor properties:Co（OH）₂nanowires were prepared on conductive carbon cloth by hydrothermal reaction,and then Ni Mn-OH nanowires were prepared on Co（OH）₂nanowires by electrodeposition method.Finally,CoP@Ni Mn-P hierarchical composites nanomaterials were synthesized by low-temperature phosphorization.The morphology,structure and valence state of electrode materials were tested and analyzed by means of SEM、XRD、XPS、TEM and other material characterization methods,and then the electrochemical performance of the electrode material CoP@Ni Mn-P was tested.The results showed that it has a specific capacity of 2162.2 F g^-1at the current density of 1 A g^-1and 1975.6 F g^-1at the high current density of10 A g^-1.After 5000 cycles of charging and discharging,the capacitance retention was 83.3%.The final CoP@Ni Mn-P was assembled as a CoP@Ni Mn-P//AC device with a specific capacity of 59.4 F g^-1at 1 A g^-1current density and an energy density of 21.1 Wh Kg^-1at a power density of 804.3 W Kg^-1.At the current density of 2 A g^-1,the capacitance retention was81.8%after 3500 charge-discharge cycles.Compared with CoO@Ni Mo-O（P）,CoP@Ni Mn-P electrode materials can simplify the preparation process and improve the performance of supercapacitors,CoP@NiMn-P electrode material has a good application prospect.