Mass And Morphology-controllable Synthesis Of Ni-co Double Hydroxides And Their High Electrochemical Performance

Supercapacitor enjoys great popularity for its excellent electrochemical storage characteristics. In the capacitor component,the electrode material is its "heart" and plays a leading role. Common electrode materials consist of a carbon material,a conductive polymer and a transition metal oxide or hydroxide. By contrast, we found that the transition metal hydroxide or oxide electrode material depending on the capacity generated by Faraday reaction is much higher than carbon-based material relying on capacity generated by absorption and desorption, and it possesses excellent cycling and thermal stability and has broad application prospects. Currently, nickel hydroxide and cobalt hydroxide electrode material have been paid much extensive attention for their high theory specific capacitance.The nickel-cobalt double hydroxide electrode material produced by the recombination of them is the focus of researches in recent years. Thus, in this paper, we have designed chemical co-precipitation method to prepare nickel hydroxide electrode material, cobalt hydroxide electrode material, and by adjusting the molar ratio of cobalt and nickel to prepare nickel-cobalt double hydroxide composite materials and study their structures and electrochemical properties. The main researching contents are summarized as follows:Using Co(NO3)2·6H2O, Ni(NO3)2·6H2O as cobalt source,nickel source respectively and urea as precipitating agent, we have developed a facile chemical co-precipitation method to prepare Co(OH)2 and ?-Ni(OH)2 materials. Characterizing the both structure by XRD,SEM,TEM and found that there is a big difference in their morphology and structure. The surface of large flake Co(OH)2 is flat and has many pore structures with rather thick lamella and the stacking phenomenon does not appear. While the surface of silk-like a-Ni(OH)2 has wrinkles and doesn't take on pore structure with thin lamella and three-dimensional structure is formed by stacking. The differences in morphology are ascribed to different cobalt salts and nickel salts,thereby the performance of the materials are quite different. The electrochemical test results show that both operating voltage window is wider (0?0.8 V). When current density is 1 A/g,the electrode material of a-Ni(OH)2 is 11 times than that of Co(OH)2 with 1562.5 F/g capacitance. But when the current density is increased from 1 A/g to 8 A/g, the capacitance retention rate of a-Ni(OH)2 electrode material is only 58.6%,while Co(OH)2 electrode material is 67%. It indicates that when charging and discharging in an alkaline electrolyte, the stability of Co(OH)2 electrode material is superior to a-Ni(OH)2 electrode material.Under the condition of keeping the total molar of the cations, it successfully prepared a 3D nickel-cobalt double hydroxide composite materials with different morphologies by adjusting molar ratio of cobalt and nickel. With increasing nickel content, morphology of the composite material evolving from nano rod and nano linear to nano linear and at last to silk-like nanosheets,meanwhile,self-assemble into 3D network structure. By characterizing all nickel-cobalt double hydroxide composites from XRD, SEM, TEM, XPS, BET, we found that when Co2+ :Ni2+ =1:4, the product obtained is NiCo-LDH, which has a special layered structure and a large amounts of electrochemically active sites internally and is a good supercapacitor material. The electrochemical test results show that when Co2+ :Ni2+ =1:1, 1:2, 1:3, 1:4 under the condition of 1 A/g current density, the specific capacitances are 436.3 F/g, 1396.3 F/g,1512.5 F/g,1783.8 F/g respectively, which are higher than that of Co(OH)2 and a-Ni(OH)2 electrode material,and have a wide operating voltage window (0 ?0.8 V)compared with other papers, thus have a higher energy density. Compared with other electrode material nickel-cobalt double hydroxide, the speed of charge transfer and ion diffusion of NiCo-LDH electrode are faster. The specific capacitance of NiCo-LDH electrode material is up to 1783.8 F/g at a current density of 1 A/g, and after 1,000 times constant-current charge-discharge, the capacitance retention obtained is 84% at a current density of 10 A/g. It indicates that NiCo-LDH electrode material has a relatively stable cycle characteristics. But its minimum specific surface area is 20.3 mn/g, it shows that the specific surface area is not the sole basis for judging the specific capacitance; electrochemical properties of the electrode material are decided by a number of factors, such as structure,morphology,etc.NiCo-LDH material is used as positive electrode, activated carbon (AC) as negative electrode,water porous polypropylene film as diaphragm, 6 M KOH as electrolyte,assembled into simple NiCo-LDH//AC asymmetric super-capacitor, and the voltage window reaches 1.6 V. When current density is 1 A/g, a higher specific capacitance is 121.3 F/g, which shows good large-current discharging characteristics. NiCo-LDH//AC asymmetric super-capacitors have a higher energy density and power density. When the power density is 800 W/kg,the energy density can reach 43 Wh/kg, even at 7998 W/kg high-power density, it is also available for 18 Wh/kg energy density,and the diffusion impedance is lesser,so NiCo-LDH materials have greater potential for development in practical use.