Study On High-value Utilization Of Biomass Corncob In Water Purification And Energy Storage

China is a big agricultural country,and there is a large amount of agricultural and forestry waste,such as corncob with an annual output of about 30 million tons.Considering the cellulose content in corncob is as high as 32～36 wt.%,and the carbon elemental content is also rich,the corncobs can be used as a biomass renewable resource to transform and prepare eco-friendly functional materials,being applied in medical,environmental protection,energy storage and other fields.Focused on the high-value added conversion and utilization of corncob biomass,this paper explores the extraction of cellulose from corncob powder,and then the preparation of cellulose-based bioplastics,the construction of cellulose hydrogels for a novel solar-driven interfacial evaporation of water purification,and the in-situ high-temperature activation of residual corncob after extracting cellulose to produce nitrogen-containing hierarchically porous carbon materials for energy storage of supercapacitor,in order to realize the full utilization of corncob biomass.The main research contents and conclusions are as follows:（1）The valuable cellulose was extracted from corncob through"alkali/urea/water"solvent system at low-temperature approach.The influence of the particle sizes of raw corncob,extraction temperature,alkali type and alkali concentration and other process conditions were studied.It was found when at-12℃,the 4.6%Li OH/15%Urea/80.4%H₂O system could dissolve the cellulose with the larger molecular weight than Na OH-based system,thereby a higher yield of cellulose was obtained.The product was confirmed to be cellulose nanocrystals by XRD and FTIR analyses.Furthermore,the cellulose-based biofilm was constructed.Using dilute acid（5%H₂SO₄）as a coagulation bath and the cellulose hydrogel obtained from sufficient chemical and physical cross-linking as precursor,after dried in the air,a uniform,transparent and mechanical strong cellulose film was formed,which is naturally degradable and is expected to be a candidate for plastic products.（2）Cellulose hydrogels（CH）with three-dimensional cross-linking network were prepared from the extracted cellulose by chemical cross-linking method.When coupled with carbon nanotubes（CNTs）,an excellent light absorber in the full solar spectrum,the cellulose hydrogel composite（CNTs-CH）with photo-thermal conversion function was constructed.Taking advantage of the high water retention of CH,the efficient photo-thermal conversion ability and the excellent mechanical property of CNTs,CNTs-CH hydrogel exhibited superior performance in the field of solar-driven interfacial water evaporation purification.Under the optimal condition,when the addition of CNTs was only 0.2 wt.%,the average interfacial water evaporation rate about 1.52 kg m^-2 h^-1can be achieved for CNTs-CH under a standard sunlight,and the solar-steam conversion efficiency was as high as～92%.Moreover,CNTs-CH has been demonstrated possessing the anti-salt deposition and anti-fouling properties.Thus CNTs-CH is suitable for the purification of sea water,high-salt wastewater,and organic dye wastewater,etc.with a stable evaporation effect,and the purified water quality is much higher than the WHO and EPA standards for drinking water.（3）After extracting cellulose,corncob powder containing residual alkali and urea was further utilized to prepare nitrogen-doped porous carbon（NPC）,using alkali as chemical activator and urea as nitrogen source through an in-situ activation and carbonization process at high temperature.The biomass-derived NPC was used as electrode materials for supercapacitor.The effects of different alkaline solutions（Na OH and Li OH）for extracting cellulose and carbonization temperature（600-900℃）on the composition,pore structure and electrochemical properties of NPC were studied.The results showed that the appropriate activation and carbonization temperature was 700℃,and the specific capacitance of NPC-Na OH and NPC-Li OH obtained at 700℃was 192 and 214 F g^-1 respectively,at the current density of 0.5 A g^-1.Combined with the analyses of the pore structure and surface composition of two electrode materials,the different energy storage mechanism was revealed.For NPC-Na OH,a honeycomb hierarchical pore structure was conducive to the storage and transportation of ions.In the case of NPC-Li OH,the surface was smooth and mostly microporous and mesoporous,but its higher heteroatom N content made the larger pseudocapacitance contribution.