Preparation Of Copper Oxide(Phosphide) Porous Films And The Size Effects On Their Energy Storage Properties

With the increasingly high demands for miniaturized portable and wearable electronics,new energy storage materials and devices are one of current reserch focuses.Supercapacitors as a new type of energy storage device have attracted great attention from international scholars and industrial filed becouse of their high power density,rapid charging-discharging and long cycle life.Large-scale and low-cost preparation of film electrodes with higher energy and power densities is the main trend of electrochemical energy storage.However,there is still a contradiction between the film thickness and capacitance:when the thickness is too large,most of electrode materials in a film cannot give contribution to energy storage,and even the film will be removed from a substrate.In this thesis,to address these issues,we designed two-dimensional nanosheets' arrays;by using copper slices and foams as raw materials and current collectors,porous film electrodes composed of intersecting CuO nanosheets and CuO-Cu3P hierarchical porous film electrodes were achieved successfully.Based on a systematical investigation of a quantitative relationship between the film thickness and specific capacitance,the electrochemical properties were optimized.The problem of constrained ion transfer in a thicker film electrode is thereforefore resolved,and the utilization ratio of active electrode materials is greatly improved.The main results of the thesis are summarized as follows:(1)Porous film electrodes composed of single-layer CuO nanosheets were in-situ grown on a thin copper foil through a facile hydrothermal oxidation synthesis where the Cu foil was used as the raw material and current collector.The unique nanostructure of resulting "standing" and uniformly distributed CuO nanosheets avoids the re-stacking of two-dimensional nanomaterials for the preparation of conventional electrodes.The thickness of a single standing CuO sheet is only 30 nm and there are many wavy holes on the edges of each sheet,which greatly improves the electrochemical area and active sites.At the same time,the standing CuO sheets are from an in situ growth during the surface oxidation reactions,which greatly improves electrical conductivity of the film and its bonding force with the substrate.By exploring the influence of reaction temperature,the reaction temperature was optimized to be 120 ?.In addition,effects of the mass-loading and the porous CuO films' thickness on electrochemical properties were studied.The results showed that the highest specific capacitance was 337.81 F g-1 at a current density of 0.25 mA cm-2 when CuO film was 0.24 mg cm-2.Comparing to other methods,our developed one-step hydrothermal oxidation technique is facile and easily scalable.(2)To significantly improve electrode performance,we applied copper foams with much lower mass density and higher specific surface as raw materials and current collector instead of copper foils.A porous covering composed of single-layer CuO nanosheets grew on the liagments of a Cu foam,and the energy-storage performance of the electrodes was optimized.The results show that the capacitance of decreases gradually with increasing the mass density of CuO,this is because the stacking CuO nanosheets become "dead volume" and decrease the utilization ratio of active materials,leading to the reduced specific capacitance.When CuO has a mass density of 7.3 mg cm-2,specific areal capacitance is optimized to be 869.57 mF cm-2,13 times higher than that of the CuO/Cu foil.This is because the 3D porous structure of the Cu foam greatly increases specific surface area,providing a large number of active sites and more channels for ions'diffusion.In particular,the enhancement of specific areal capacitance is beneficial for miniaturized and portable supercapacitor devices,which is of great significance to practical applications.(3)In order to further improve the electrochemical performance,we investigated chemical phosphatization of the copper foams supported CuO nanosheet arrays.By adjusting phosphating sodium hypophosphite mass(m),the phosphating degree of CuO can be controled,and the capacitance is thus optimized.When m<0.025 g,CuO nanosheets maintain their original appearance and nanostructure;As m increases,an adhesion of the adjacent standing CuO nanosheets occurs.When m>0.03 g,CuO nanosheets are completely phosphatized changed to irregular spherical morphology.At m?0.02 g,CuO was partially phosphatized to Cu3p,and the array structure is preserved.CuO-Cu3P composite nanosheets have been proved by the material characterizations.The electrochemical performance was optimized with a specific capacitance of 281.72 F g-1 at a current density of 1 mA cm-2,which was increased by 33.62%compared to the CuO/Cu foam electrode.Additionally,the capacitance remains 66.90%of its initial value when the current density is increased to 15 times,indicating a remarkable rate capability.The enhanced electrochemical performance should be attributed to the high conductivity of Cu3P and much more electrochemical activity sites.