Investigation Of The Pseudocapacitive Properties Of Nickel Cobalt Bimetallic Compounds

Electrochemical capacitors (also called supercapacitors) have attracted extensive attention due to their many advantages, such as high power density, short charging/discharging time and long lifespan, and hold great potentials as power sources for numerous applications, including high power facilities, electricvehicles and various electricdevices. Among the reported eletroactive materials, metallic compounds have been found to possess high specific capacitance and long lifespan, which can be used as the most promising electroactive materials for high-energy supercapacitors. In this paper, various Ni and Co based compounds, including Ni-Co based hydroxides, Ni-Co based oxides, Ni-Co based sulphides and Ni-Co based selenides, have been designed by coordinating with different anions and varying the Ni and Co ratios. Meanwhile, their supercapacitive performance has been explored. The main results is summarized as follows:Ultrathin Ni-Co based hydroxide nanoclusters with a phase have been prepared by a facile mixed solvothermal method. The Ni-Co based hydroxides with different Ni and Co ratios possess the same crystal structure, similar morphology and structure, but show much higher specific capacitance and rate capability, longer cycling lifespan compared to the α-Ni(OH)2when used as the electroactive materials for supercapacitors, indicating that the coordination of Ni and Co in Ni-Co hydroxides contributes to superior electrochemical activity and stable a phase. Furthermore, Ni-Co hydroxide arrays supported on Ni foam can be prepared by introducing Ni foam in the preparation process. Due to the specific arrays structure, enhanced supercapacitive performance and superior cycling stability is achieved.Ni-Co oxides with different Ni and Co ratios have been prepared via annealing the as-synthesized Ni-Co hydroxides samples at elevated temperature. The Ni-Co oxides can fully maintain the morphology and the structure of the precursor. The Ni-Co oxides show higher specific capacitance and rate capability, longer cycling stability compared to the CO3O4and NiO samples prepared in the same condition. Meanwhile, porous NiCo2O4flower-like nanostructure have been designed and prepared by a simple hydrothermal method. The as-prepared NiCo2O4sample possess at least two orders of magnitude higher conductivity than the Co3O4and NiO samples. Therefore, much higher specific capacitance and rate capability, longer cycling stability have been obtained when used as the electroactive materials for supercapacitors.Ni-Co sulhpides were first used as the electroactive materials for supcapacitors. Based on the anion-exchange reaction, urchin-like NiCo2S4nanostructures and NiCo2S4nanotube arrays on Ni foam have been synthesized. The NiCo2S4sample shows lower optical band gap energy and higher conductivity than the NiCo2O4sample with the same morphology and structure, which is benificial for the improvement of the the kinetics of the electrochemical process. The urchin-like NiCo2S4nanostructures show much higher specific capacitance and rate capability compared to the NiCo2O4with same morphology and structure as well as the Co9S8and NiS synthesized at the same condition. The NiCo2S4nanotube arrays on Ni foam achieve ultrahigh utilization efficiency at a high mass loading of6mg/cm2; therefore, ultrahigh specific capacitance have been achieved. Meanwhile, Ni-Co sulphides with different Ni and Co ratios have been prepared via a facile polyol process. The NiCo2, Ni1.5Co1.5, and Ni2Co sulphide samples exhibt high specific capacitance and rate capabilty compared to Co3S4and NiS, and show improved cyling performance than the NiS sample.CoSe, Co9Se8, NiSe, NiCo2Se4samples have been also prepared by a simple solvothermal method and outstanding supercapacitive performance has been obtaind when used as the electroacitve materials for supercapacitors. The as-synthesized CoS and Co9Se8samples exhibit ultrahigh cyling performance, no observable degradation is found after7,000and10,000cycles, repectively. The NiSe sample shows a specific capacitance as high as808.45F/g. The NiCo2Se4sample possesses a high specific capacitance, much higher rate capability, and excellent cycling performance. A specific capacitance as high as535.7F/g is obtained,67.0%of the capacitance can be maintained after50times increase in current density, and94.92%of the specific capacitance is obtained after7,000cycles cyling.