Design, Preparation And Capacitance Properties Of 2D MOF Based Composite

Metal organic framework?MOF?,as a new material with large specific surface area,high porosity and low cost,has been widely used as electrode materials for supercapacitors in recent years.However,the low conductivity of MOF and the incomplete exposed electrochemical active sites that limit its capacitance performance,so it is very important to find an efficient way to further improve the electrochemical performance of MOF.So far,there are three methods to improve the electrical properties of MOF materials.The first way is to synthesize ultrathin 2D MOF nanoflakes.The nano thickness and high porosity can speed up the ion and electron transfer,and a large number of metal atoms exposed on the surface of2D MOF can be used as unsaturated active sites to generate redox reaction.The second way is to transform MOF into various metal compounds by using its own porosity as a template,so that its redox reaction can take place near the surface,increasing the number of charges involved in the electrochemical reaction.The third method is to add different conductive materials to improve the conductivity of the composite materials.According to the above three methods,three kinds of MOF composites with different structures are designed in this paper:1.NiCo-MOF?nickel cobalt bimetallic organic framework?nanosheets was in situ grown on one-dimensional polypyrrole tubes?PNT_S?,and NiCo-MOF@PNT_S composite was successfully prepared.The performance of NiCo-MOF@PNT_S was studied by controlling the amount of PNTs.The results showed that NiCo-MOF@PNT_S had the best electrochemical performance when the content of PNTs was 10 mg.And when the current density of NiCo-MOF@PNT_S is 0.5 A g^-1,the specific capacitance is as high as 1109 F g^-1.In addition,when the current density is increased by 40 times,i.e.20 A g^-1,the specific capacitance can still be kept at 888 F g^-1.This excellent performance can be attributed to the one-dimensional conducting structure of PNTs,which can be used as a fast channel of charge transfer and improve the electron transfer ability of materials.In addition,the pseudocapacitance of PNTS can reduce the capacitance loss caused by the conductive agent.2.Using two-dimensional Ti₃C₂T_x as template,NiCo-MOF nanocomposites with sandwich structure were successfully synthesized by in-situ growth of NiCo-MOF on the surface of the Ti₃C₂T_x via ultrasonication.The change of electrochemical properties of NiCo-MOF/Ti composites was analyzed by controlling the ratio of the two materials.The results show that the NiCo-MOF/Ti-5 has the best electrochemical performance when the mass of Ti₃C₂T_x is 5 mg.When the current density is 0.5 A g^-1,the specific capacitance of NiCo-MOF/Ti-5 is 835.8 F g^-1.In an asymmetric supercapacitor assembled by NiCo-MOF/Ti and activated carbon,when the power density is 562.5 W kg^-1,the energy density of 39.5 Wh kg^-1 can be obtained,and it also has good cycle stability?the capacitance retention rate after10000 cycles is 82.3%?.On the one hand,this excellent performance can be attributed to the high conductivity of Ti₃C₂T_x,on the other hand,the two-dimensional structure of Ti₃C₂T_x enables it to be evenly inserted into the 2D MOF lamellar gap,inhibit the aggregation of MOF,and expose more active sites.3,After heat treatment and flame treatment,melamine foam?MF?was used as raw material to transform it into nitrogen doped conductive hydrophilic carbon foam?NCF?and retain its original three-dimensional structure.Then,the Co-MOF@NCF composite with self-supporting structure was obtained by in-situ growth of Co-MOF on NCF.Finally,NiCo-LDH@NCF was obtained by etching Co-MOF with nickel ion via hydrothermal method.When the current density is 0.5 A g^-1,the specific capacitance of NiCo-LDH@NCF is 1512 F g^-1,and it has good rate performance.The asymmetric supercapacitor?ASC?assembled by NiCo-LDH@NCF and activated carbon has a high energy density of 41.5 Wh kg^-1 and a power density of 750 W kg^-1.In addition,after 10,000 cycles,ASC still showed good cycle stability,and the capacitance retention rate was 80.4%.The ultrahigh performance of the electrode material can be attributed to the fact that its conductive three-dimensional cross-linking structure stimulates more active sites,and the in-situ sacrificial template method is selected to make the active material more evenly distributed.Finally,the prepared electrode material can be used as electrode directly,reducing the loss of binder on capacitance,further improving its specific capacitance.