Assembling MnO₂ Nanobelts On Skeleton-protected Nitrogen-Doped Graphene Aerogel And Properties

MnO2 with low cost,high abundance and high theoretical specific capacitance,is widely used in areas such as energy storage devices and removal of organic pollutants in water.However,MnO2 suffers from low conductivity and easily tends to aggregate,which should be solved by controlling morphology or combining with highly conductive matrix.To overcome disadvantages of aggregation and poorly electronic conductivity of MnO2 nanostructures for improving performances,MnO2 nanostructures are often loaded on the matrix of carbon materials(like graphene and its derivatives)with big surface area and good conductivity.When introducing MnO2 into conductive carbon materials,MnO2 is often obtained by unsaturated sp2 hybridized carbon(Csp2)reducing KMnO4,where Csp2 as reducing agent or sacrificial agent.This leads to serious problems:(1)destroying C=C and conductivity of carbon materials;(2)ruining the structural integrity of carbon skeleton and stability of carbon materials.The structure of material is closely related to its properties,so it is necessary to bring in a new reducing agent replacing Csp2 to reduce KMnO4,protecting the carbon skeleton and improving stability and application performance.In this thesis,carbon skeletonprotected MnO2 nanoribbons/nitrogen-doped graphene aerogel(MNRs/NGA)is studyed both for supercapacitor and removing organic dye(methylene blue,MB),proving the importance of carbon skeleton protection,emphasizing the importance of"optimized chemical reaction procedure".In chapter 3,Mn2+ in MnSO4 replacing Csp2 in NGA with a big specific surface area,reacts with KMnO4.MNRs grow in-situ on the structurally protected and consequently conductively protected NGA,with about 500 nm long,50 nm wide and 11.1 nm thick.The exposure ratio of surface unit cells is calculated to be so high as 35.6%.MNRs/NGA with a big surface area,hierarchical pores(1.89-173.94 nm)and high exposure ratio of surface unit cells shows excellent supercapacitor performance.Benifiting from not only the electric double layer capacitance but also pseudocapacitance,MNRs/NGA shows excellent electrochemical performance with large specific capacitance(712.4 F g-1),rate capability(600.1 F g-1 at 20 A g-1)and good cycle stability(96.3%after 10000 cycles).Based on good electrical conductivity,big surface area and hierarchical pores of MNRs/NGA,the asymmetric supercapacitor(ASC)with MNRs/NGA as positive electrode and activated carbon(AC)as negative electrode shows higher specific capacitance(215.2 F g-1),specific energy(123.2 W h kg-1)specific power(1000 W kg-1)and cycle stability(97.1%after 10000 cycles)than reported carbon materials and MnO2-based materials.MNRs/NGA provides a new strategy for preparation and application of bi-functional materials.In chapter 4,the high efficient removal of MB performance of carbon skeletonprotected MNRs/NGA is studyed.95.7%MB can be removed within 30 min.After 10 cycles,the removal rate is still as high as 89.5%without significant changes in morphology and crystal phase of MNRs.The amount of MB by physical adsorption(1.7 mgMB g(MNRs/NGA)-1)and chemical oxidative degradation(23.9 mgMB g(MNRs/NGA)-1)are quantitatively measured and MB enriches on MNRs/NGA by physical adsorption firstly,which accelerates the degradation rate.The oxidative degradation plays a major role for complete removal of MB.The yields of ·OH and ·O2-free radicals are 76.08 and 2.92 mg·gMnO2-1 by quantitatively measured,respectively.The high yield of ·OH can promote the high efficient removal of MB by MNRs/NGA.High performance of supercapacitor and highly efficient removal of organic pollutants are just examples of practical applications,effectively proving the focus of this thesis-carbon skeleton protection,emphasizing the concept of "optimized chemical reaction procedure",which has been negleted in the area of the preparation and application of MnO2/carbon composites.Compared with MnO2 nanosheets/carbon skeleton-unprotected and oxidized(MNSs/ox-NGA)and MnO2 nanoneedles(MNNs)/ox-NGA,MNRs/NGA shows a smaller ID/IG(1.52)in Raman spectra,a larger ratio of C=C(50.1%)in C 1s XPS and a smaller series resistance(Rs)(2.18)and charge transfer resistance(Rct)(1.12)in electrochemical impedance spectroscopy(EIS),further confirming that carbon skeleton of MNRs/NGA is protected.Meanwhile,structural protected MNRs/NGA shows better supercapacitor and removal of MB performances than MNSs/ox-NGA and MNNs/ox-NGA,demonstrating that the importance of structural protection for improving the performances of MnO2/carbon composites.