Preparation Of Transition Metal MOFs And Its Composites And Study On The Properties Of Supercapacitors

In order to solve the problem of poor cycling stability and low specific capacitance of NiCo-MOFs,three kinds of NiCo-MOFs based composites（NiCo-MOFs/OCN,NiCo P/C and NiCo Mn-MOFs-S）have been prepared by doping,substituting or phosphating methods.Their electrochemical performances are studied.The detailed contents are as follows:（1）NiCo-MOFs grow in situ on g-C₃N₄（OCN）treated with carboxyl to obtained NiCo-MOFs/OCN.The XRD,SEM,XPS and electrochemical characterization are used to study the morphology and electrochemical properties of NiCo-MOFs/OCN.The results show that the NiCo-MOFs successfully grow on the surface of OCN with a layered structure.Electrochemical analysis indicates that the specific capacitance of NiCo-MOFs/OCN-5 is 832 F·g^-1at 1 A·g^-1,which is 20%higher than that of NiCo-MOFs.Under the current density of 2 A·g^-1,the specific capacitance of asymmetric supercapacitor NiCo-MOFs/OCN-5//AC is 97 F·g^-1;after 6800 charge and discharge cycles,its capacitance retention rate is as high as 92.1%.And its energy density is 44.3 Wh·kg^-1at a power density of 850 W·kg^-1.The mechanism study indicates that the carboxylated g-C₃N₄provides a curled plane and abundant O and N doped active sites,which plays a supporting role in the charge and discharge process of NiCo-MOFs/OCN and provides more pseudocapacitors.（2）Ni-Co phosphide（NiCo P/C）is obtained by phosphating NiCo-MOFs.The SEM analysis demonstrates that the molar ratio of Ni and Co has great influence on the morphology of NiCo P/C.When the ratio of Ni:Co is 2:1,the microtopography of NiCo P/C-21 becomes nanoparticles（25-65 nm）from amorphous.Electrochemical studies displays that the specific capacitance of NiCo P/C-21 reaches 1504 F·g^-1at 1 A·g^-1,which is 2.6 times that of non-phosphating（NiCo/C-21）,1.6 times that of Ni P/C and 3.9 times that of Co P/C.Under the power density of 723.5 W·kg^-1,the energy density of NiCo P/C-21//AC device is 50.9 Wh·kg^-1,and the capacitance retention rate is 71.6%after 5000cycles.The mechanism study illustrates that nanoparticles are beneficial to the penetration of electrolyte,and P with small electronegativity can accelerate the electron transfer rate and improve the redox reaction rate.And binary metals can provide more active sites.The synergistic effect of above factors significantly improves the specific capacitance of materials.（3）With the help of sodium dodecyl sulfate（SDS）,Mn ions are introduced into NiCo-MOFs to improve its cycle stability.The influence on the microstructure,sizes and electrochemical properties of NiCo-MOFs with different doping methods and contents of Mn are studied.The XRD and XPS analysis display that Mn partially replaces Co successfully.SEM analysis shows that the Ni₂Co_1-XMn _X-MOFs-S nanorods become regular and miniaturized after Mn partially replaces Co.Electrochemical studies illustrate that partial substitution of Mn is superior to direct doping.For example,the specific capacitance of Ni₂Co_0.75Mn_0.25-MOFs-S is 1428 F·g^-1at 1 A·g^-1,which is 8%higher than that of direct doping.After 3000 cycles,the capacitor retention rate is 83.5%.The mechanism analysis reveals that the Mn（replace Co）changes from low oxidation state(Mn³⁺)to high oxidation state(Mn⁴⁺)in electrochemical reaction,which attracts interlayer anions,leads to larger buffer space and reduce the irreversible transformation in crystal structure,thus improving the cycle stability of NiCo-MOFs.