Study On Preparation Of Nickel/Cobalt Oxide-based Compounds For Supercapacitors By Hydrothermal Electrochemical Deposition

As a new type of energy storage devices, supercapacitors have attracted much attention from word-wide researchers because of their high power density, long cycle life and fast charging and discharging capacity. At present, supercapacitor electrode materials are mainly divided into three types. The first type is carbon materials. They store energy by electric double layer with have large specific surface area, which limits their low energy storage capacity for applications in supercapacitors. The second type is conductive polymers, which exhibit a high voltage window, but their cyclic stability is poorer than the others because of the easiness to fall off of active substance during charging and discharging. The third type is transition metal compounds, such as cobalt, nickel, manganese and so on, which have rich sources,low cost, and low environmental impact. Furthermore, these compounds have various valence states, and thus are considered to be ideal supercapacitor electrode materials.There are many soft methods for the preparation of transition metal compounds, such as solution method, hydrothermal deposition, electrochemical method, sol-gel,template method, and so on. However, these methods suffered from a relatively weak adhesion between the film and its substrate, which lead to low load mass of the compounds, and thus low area specific capacitance. This paper summarized the progress in electrode materials and preparation technologies for supercapacitors.According to these, a technoloy called as hydrothermal electrochemical deposition was put forward to prepare nickel and cobalt oxide-based membrane electrodes with large capacity for supercapacitors. The main research contents are as follows:In different reaction systems, we prepared the nickel and cobalt based membrane electrodes for supercapacitors by hydrothermal electrochemcial deposition, and studied the influence of deposition voltage on the electrochemical properties of the membrane. The electrodes were characterized by different methods, such as SEM,TEM, XRD, XPS, BET, and so on, surface morphologies, structural features and pore-size distribution were analyzed in detail. And, with a three-electrode electrochemical workstation, its electrochemical performance was analyzed.In the system of nickel nitrate, ammonium fluoride, and urea solution, a three-dimensional nanoporous ?-Ni(OH)2 membrane electrode was successfully prepared. The specific capacitance declined gradually in the range from 1.0 V to 1.3V. At a deposition voltage of 1.0 V, the prepared membrane electrode materials showed the excellent electrochemical performance. it could deliver high capacitance of 9.39 F/cm2 at 5 m A/cm2. After 1000 cycles, the area capacity maintained 54% at 8m A/cm2.In the system of cobalt nitrate, ammonium fluoride, and urea solution, a needle Co(CO3)0.5(OH)·0.11 H2 O membranel was successfully prepared. The thin film electrode material presented an ultra high cycle life.When the current density was 5, 8,16, 20, and 30 m A/cm2 respectively, the areaspecific capacitance reached 4.1, 3.5, 2.8,2.6 and 2.3 F/cm2 respectively, that is 577, 493, 394, 366 and 324 F/g. After 15000 cycles, the capacitance remained in 4.2 F/cm2 at 8 m A/cm2.Subsequently, the current density was changed to 30 m A/cm2, the specific capacitance decreased from 2.8F/cm2 to1.4 F/cm2 after cycling 39000 times, its ratio still kept as high as 50%.The effect of deposition voltages on the capacitor performance were further studied. It was discovered that at a constant-voltage deposition of 1.2 V, the first discharge capacity was 6.67 F/cm2 at the current density of 30 m A/cm2. After 10873 cycles, the capacitance retention rate was still 55.4%.In the system of nickel chloride, cobalt nitrate, and urea solution, a membrane with nickel doped containing Co(OH)2, Co O-Co2O3, Co O compounds on a nickel foam was prepared. The effects of deposition voltages on the electrochemical performance of the prepared membrane were studied. The results showed that at the deposition voltage of 1.2 V, the membrane electrode had the best electrochemical performance. When the current density was 5, 8, 16, 20 and 30 m A/cm2 respectively,the specific capacities were 26.7, 20, 20, 19.6 and 17.6 F/cm2. After 3000 cycles, thespecific capacitance dropped to 17.5 F/cm2 at the current density of 30 m A/cm2, and the capacitance retention rate was 99%.