Studies On Preparation And Properties Of Electrode Materials Based On Metal-Organic Frameworks For Supercapacitor

Supercapacitor,also known as electrochemical capacitor or ultracapacitor,is a device which can store energy by polarizing electrolyte in the electrode-solution interface.It has been widely applied in telecommunications equipment,portable electronic equipment,standby power systems,hybrid cars and other fields due to the advantage of high power density,fast charge and discharge process,long cycle life and low pollution etc.Supercapacitors are new energy storage device,combining the advantages of the high specific power of dielectric capacitors and the high specific energy of rechargeable batteries.Recently,the progress of researches on supercapacitors has focused on using carbon material,metal oxides and conducing polymer as electrode materials.Metal-organic Fragments(MOFs),which have good thermal stability,discrete ordered structure,large internal surface area and abundant pores,can be directly or indirectly used as electrode materials for supercapacitors.In this paper,a series of MOFs complex and MOFs derivatives were prepared according to three basic types of MOFs materials,and then they were all applied to the electrode of supercapacitor.(1)A hybrid material of carbon nanotubes(CNTs)and Mn-based metal organic frameworks(Mn-MOFs)have been synthesized and used as Mn-based supercapacitor electrode materials.The incorporation of CNTs into Mn-MOFs leads to an inherent improvement in conductivity and an intrinsic increase in specific capacitance(from 43.2 F g-1 for pure Mn-MOFs to 203.1 F g-1 for CNTs@Mn-MOFs).Furthermore,the symmetrical supercapacitor based on the CNTs@Mn-MOFs exhibites excellent power density and outstanding stability even after 3000 cycles with 88%retention of the initial capacitance.This research exploits a new direction for developing Mn-based supercapacitor materials and provides an effective method to improve capacitive performance of MOFs materials.(2)An effective strategy for homogeneous and polyporous MoO3@CuO composite has been reported by heating a POMs@MOFs template(POMs=polyoxometalates),in which the Mo-POMs are incorporated into Cu-MOFs as secondary building units.The excellent doping of MoO3 to CuO leads to an obvious improvement in specific discharge capacity(from 15.4 mAh g-1 for CuO to 86.3 mAh g-1 for MoO3@CuO).The layered structure of MoO3 plays a key role in providing facilitated ion transport and electron diffusion pathways for the composite materials.This electrode demonstrates excellent electrochemical performance with a specific discharge capacity of 86.3 mAh g-1 at 1 A g-1 in 1 M LiOH.When this porous MoO3@CuO electrode is assembled into a symmetric all-solid-state device with PVA-LiOH gel polymer,the as fabricated device demonstrates good performance with an energy density of 7.9 Wh kg-1,power density of 8726 W kg-1,and excellent cycle life.(3)A composite of MoO2@Cu@C has been prepared using a thermolysis template of POMs@MOFs crystal(POMs = polyoxometalates),in which the Keggin-type Mo-based POMs with negative charges exist as secondary building units of Cu-based MOFs.In addition,the material is confirmed to exhibit an excellent capacity of 28.33 mAh g-1 at a current density of 1 A g-1,which is much higher than MoO2-based supercapacitors reported before.Moreover,we have fabricated a symmetrical flexible all-solid-state device with the MoO2@Cu@C electrode using PVA-KOH gel polymer as electrolyte and separator,and it shows outstanding stable performance retaining 91%of its highest charge capacity maintained after 5000 cycles.(4)A ZIF-8/CNTs template has been prepared using metal-organic frameworks(zeolitic imidazolate framework,ZIF-8)and carbon nanotubes(CNTs).ZnO quantum dots(QDs)/carbon/CNTs has been selectively synthesized from a single ZIF-8/CNTs template.The composite material has been used for supercapacitor electrode,and the performance of the electrode has been tested by three-electrode system.The results show that the electrochemical properties of the composite materials have obvious advantages compared with other ZnO-based supercapacitors.AC impedance test shows that the quantum dot shape and the doping of carbon materials can effectively improve the conductivity of the composites.In addition,the conductive network structure of the composites also brings a great improvement in its electrochemical performance.(5)A composite of porous N-doped carbon/CNTs with unique structure has been prepared by pyrolysis of ZIF-8/CNTs template.The composite carbon material has large specific surface area and special conductive network structure,which comes from the advantages of ZIF-8 and CNTs.The N-C/CNTs composite has been used as supercapacitor electrode,and its electrochemical performance has been tested under three-electrode system.The results show that the N-C/CNTs composite takes better performance than those of the carbon materials derived from the pure ZIF-8 template.The key factor is that the composite material has a special multichannel structure based on CNTs,and a three-dimensional conductive network structure formed by the cross winding of one dimensional CNTs.In addition,the two unique structures can lead to an effective CNTs channel,which will promote the charge transport and storage in the electrode materials.This unique structure ensures fast and efficient faradaic reaction in the electrode,resulting in excellent electrochemical performance,especially in higher capacitance value than those of the reported carbon materials derived from pure ZIF-8.