| Ni(OH)2 nanopowders were prepared by hydrothermal method at different reaction temperatures and times.From the X-ray diffraction(XRD),the main phase of the obtained sample was α-Ni(OH)2,and in addition,the second phase β-Ni(OH)2 was also present.The increase of reaction temperature has no obvious effect on the crystallinity of the sample,but with the increase of reaction time,the crystallinity of the sample increases.The Ni(OH)2 samples prepared at different temperatures have similar surface morphology and all show a fluffy spherical structure formed by self-assembly of nanowires.As the reaction time increases,the grains transition from spherical to lamellar.Cyclic voltammetry(CV)method,galvanostatic charge and discharge(GC)method and electrochemical impedance spectroscopy(EIS)were used to test the electrochemical performance of the samples.The results showed that the Ni(OH)2(N 120-2)sample obtained at 120℃ for 24 h had the highest specific capacitance and reached 563 F·g-1 at the current density of 0.5 A·g-1.When the current density increases to 3 A·g"1,the specific capacitance retention rate is 28%.Under this condition,a Ni(OH)2(N120-2/NF(foam nickel))nanowire structure sample was directly grown on the foam nickel,and the rate performance of this sample was improved by 10%compared with the N120-2 sample.Ni1-xCox(OH)2/NF(x=0,0.11,0.25,and 0.5)samples were prepared by hydrothermal method using foamed nickel as a substrate.According to XRD,the main phase of the Ni(OH)2/NF sample was a-Ni(OH)2,and the secondary phase was β-NiOOH.With the increase of Co doping,the main phase of the sample is a-Ni(OH)2,and the secondary phase gradually transitions from β-NiOOH to β-Ni(OH)2.The results of field emission scanning electron microscopy(FESEM)and transmission electron microscopy(TEM)showed that the Ni(OH)2/NF sample was a nanosheet with a wrinkled surface,and the specific surface area was 398 m2·g-1.With the increase of Co doping,the nanosheet gradually curled to form nanorods,and the specific surface area also gradually decreased.According to X-ray photoelectron spectroscopy(XPS)analysis,Co is incorporated into the Ni(OH)2 lattice in a+2 valence form.The electrochemical properties of Ni1-xCox(OH)2/NF samples were characterized by CV,GC and EIS.The results show that the specific capacitance of Ni1-xCox(OH)2/NF sample increases first and then decreases with the increase of doping amount,which is higher than that of undoped Ni(OH)2/NF sample.When the Co doping amount is 25%,the highest specific capacitance is 2111 F·g-1(at a current density of 0.5 A·g-1),and its rate performance is also significantly improved.When the Co doping amount is 25%,the highest specific capacitance is 2111 F·g-1(at a current density of 0.5 A·g-1),and its rate performance is also significantly improved.The mechanism by which Co doping significantly improves the capacitance performance of Ni(OH)2 is discussed.Ni1-xCox(OH)2(x=0,0.25)and Ni0.75Co0.25(OH)2/RGO(reduced graphene oxide)composites were prepared by microwave-assisted hydrothermal method.Ni1-xCox(OH)2(x=0,0.25)samples consist of two phases:β-Ni(OH)2 and α-Ni(OH)2.After compounding with reduced graphene oxide(RGO),its crystal structure did not change.Ni(OH)2 is in the form of flakes with a specific surface area of 51 m2·g-1.The Ni0.75Co0.25(OH)2 samples formed nanorods,and the Ni0.75Co0.25(OH)2/RGO composites with RGO formed a petal-like structure on the RGO surface with a specific surface area of 67 m2·g-1.At the same time,the wettability of the Ni0.75Co0.25(OH)2/RGO sample was also significantly improved.The electrochemical performance characterization results show that the specific capacitance and rate performance of Ni0.75Co0.25(OH)2/RGO sample are significantly improved.At the current density of 0.5 A·g-1,the specific capacitance reaches the maximum,which is 911.23 F·g-1.When the current density increases to 5 A·g-1,the specific capacitance retention rate reaches 66%.The mechanism of Co-doping and RGO compounding significantly improves the capacitance performance of Ni(OH)2 electrode materials. |
PDF Full Text Request