The Effect Of Temperature On The Performance Of Supercapacitor Of Hydrothermal Electrochemical Deposition Of Cobalt-based Films

Supercapacitors,a new class of energy storage devices,have attracted much attention for their high power density,fast charging/discharging rate,long cycle life,and high reliability.Based on their charge storage mechanisms,supercapacitors can be categorized into two groups: electric double layer capacitors?EDLCs?and pseudocapacitors.Usually,in comparison with EDLCs,pseudocapacitors enable higher energy density.Among different transitional metal based materials,cobalt-based compounds for ECs exhibit a high theoretical specific capacitance?3460 F/g?,and thus arosed extensive studies,However low mass loading and low area capacitance limit their wide uses in our day life.Here,a hydrothermal electrochemical method was applied for the synthesis of Co?OH?₂ and Co?CO₃?_0.5?OH?·0.11H₂ O films at the different preparation temperature of 100 ??120 ??140 ? and 160 ? respectively.The morphology and microstructure of the as-synthesized samples were examined by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,Xray photoelectron spectrum?XPS?,transmission electron microscopy?TEM?and pore size analyze,and their electrochemical properties were characterized using Cyclic voltammetry?CV?and Galvanostatic charge/discharge?GCD?and Electrochemical impedance spectroscopy?EIS?methods by a three-electrode system.Based on the optimal Co?OH?₂ and Co?CO₃?_0.5?OH?·0.11H₂ O films,two asymmetric supercapacitors were fabricated through active carbon as a negative electrode.The results showed two compounds were formed at 100 °C,120 ?,140 ?,160 ?,respectively.At 100 °C,the as-formed compound is hexagonal-structuree Co?OH?₂,which displays a flower-like flaky morphology,and above 140 ?,it is tetragonal-structure Co?CO₃?_0.5?OH?·0.11H₂ O,presenting a needle-like morphology.Between the two temperature,the film consisted of two compounds.Among the three films,the mixed phase film showed the best capacitance than the others.It showed an special capacitance of 30.15 F/cm²?511 F/g?at 10 m A/cm²,and 20.94 F/cm²?354.9 F/g?at 60 mA/cm²,a retention ratio up to 86% even after 5000 cycles.After the mixed phase film was assembled to be an asymmetric supercapacitor with active carbon as a negative electrode,it could hold an capacitance of 3.071 F/cm²?357.1 F/g?at 5m A/cm² and 2.397 F/cm²?278.7 F/g?at 30 mA/cm²,good cycle stability?retention up to 77.4% after 5000 cycles?.A Co?CO₃?_0.5?OH?·0.11H₂O-AC asymmetric supercapacitor exhibited remarkable cycling stability?no capacitance fade after 3000 cycles?together with a promising capacitance of 3.406 F/cm²?143.4 F/g?at 5 mA/cm² and 1.348 F/cm²?56.8 F/g?at 30 mA/cm².