Preparation Of Waste Biomass Derived Two-dimensional Carbon Materials And Carbon-based Composites And Their Supercapacitor Performance

At present,the biomass waste generated worldwide is increasing.Most of them are processed extensively,which will not only cause waste of resources,but also cause environmental pollution.Thus,how to realize the high-value utilization of biomass waste has become a popular research topic.As the most plentiful source of sustainable carbon,biomass is an ideal precursor for the preparation of carbon materials.However,carbon materials obtained by direct pyrolysis of biomass generally have a thick bulk structure with low specific surface area and porosity,which is not conducive to the rapid transport of ions and leads to poor electrochemical performance.Therefore,two-dimensional(2D)graphene-like carbon materials with high specific surface area and high conductivity were prepared by using biomass wastes from a wide range of sources and low prices and applied to the field of supercapacitors,which can not only realize the high-value utilization of biomass waste,but also obtain supercapacitor electrode materials with excellent performance.At present,many researchers have prepared 2D graphene-like carbon nanosheets(GCNs)using biomass carbon materials as precursors.From the perspective of dimensional transformation,the current research is mainly from the three-dimensional(3D)carbon material prepared by"thinning"treatment to obtain 2D GCNs(dimensionality reduction method).Inspired by dimensional shifts,we propose a new method for preparing 2D GCNs by one-dimensional(1D)"reassembly",which is named dimensionality increase method.Therefore,in this paper,biomass waste-the pappi of metaplexis japonica seed was selected as a carbon-containing precursor,and GCNs was prepared by dimensionality reduction method.1D nanocellulose fibers(NFCs)was separated from biomass waste-poplar sawdust,and GCNs was prepared by dimensionality increase method.The GCNs prepared by the two methods were tested for electrochemical properties.Pure carbon materials have a low capacity due to the limitation of electric double layer energy storage.In order to further increase the capacity of GCNs,Mn O₂ nanosheets that can provide pseudocapacitance were grown on the substrate of GCNs prepared by dimensionality increase method,GCNs/Mn O₂-X composites were prepared,and their electrochemical properties were explored.Finally,GCNs/Mn O₂-X//GCNs asymmetric supercapacitors(ASCs)were assembled.The main contents are as follows:(1)Dimensionality reduction method:biomass"thinning"treatment to prepare 2D GCNs.GCNs were prepared by one-step carbonization activation and hydrothermal treatment of nitric acid,using biomass waste(pappi of metaplexis japonica).The mechanism is that the carbon layers of the uncleaned samples after one-step carbonization and activation contain residual K₂CO₃,K₂CO₃ and HNO₃ will generate KNO₃ and CO₂ gas,and the CO₂ gas will be released between the carbon layers,thereby the carbon material was peeling off,which acts as a"thinning"of the carbon layer.The final prepared GCNs have few layers,and in 6 M KOH electrolyte can deliver a high specific capacity of 477 F g^-1 at 2 A g^-1,excellent rate capability(82.8%,when the current density reaches 200 A g^-1)and excellent cycling stability with a capacitance retention of 92%after 20,000 tests(20 A g^-1).(2)Dimensionality increase method:1D NFCs"reconstituted"to prepare 2D GCNs.1D NCFs were prepared from biomass waste(poplar sawdust).Carbon materials with different microscopic morphologies can be prepared by adjusting the concentration of the nanocellulose aqueous dispersion and the subsequent carbonization process.When the concentration of the nanocellulose aqueous dispersion is 0.4 mg m L^-1,due to hydrogen bonding during the freeze-drying process,a 2D sheet structure is formed after all the water was removed.And in the subsequent carbonization process,the original structure was maintained to obtain GCNs.The thickness of GCNs measured by atomic force microscopy was about 4 nm.Interlaced Carbon Fibers(ICFs,0.02 mg m L^-1),GCNs(0.4 mg m L^-1),and Dense Structure of Carbon Nanosheets(DCNs,20 mg m L^-1)were prepared.Among the three samples with different microscopic morphologies,GCNs have the best performance,with a specific capacity of 281 F g^-1(5 m V s^-1)in 1 M Na₂SO₄ electrolyte,even if the scan rate is 1000m V s^-1,its specific capacity can still be maintained to 193 F g^-1.(3)Preparation and electrochemical properties of GCNs/Mn O₂-X composites.The GCNs prepared by dimensionality increase method have the advantages of tunable morphology and easy operation.However,pure carbon materials are limited by the energy storage of the electric double layer and all have low specific capacity.Therefore,GCNs/Mn O₂-X composites were obtained by vertically growing Mn O₂nanosheets that can provide pseudocapacitance with GCNs prepared by dimensionality increase method as the substrate,which can improve the capacity.Then,the effect of reaction time X(X=2,4,6 h)on the electrochemical properties of GCNs/Mn O₂-X composites was explored,and the optimal reaction time was finally determined to be 4h.The GCNs/Mn O₂-4 exhibits a specific capacity of 299 F g^-1(5 m V s^-1),and the capacity remains?60%as the scan rate increases to 1000 m V s^-1.And the capacity can still maintain 92%of the initial value after 10,000 cycle tests(10 A g^-1).Finally,the ASCs consist of GCNs as the negative electrodes and GCNs/Mn O₂-X as the positive electrodes through the matching test.The GCNs/Mn O₂-4//GCNs ASC exhibits a high energy density of 14.0 Wh kg^-1 at a high power density(84.0 k W kg^-1),which is much higher than other reported ASCs.