Study On Preparation Of Porous Carbon From Rice Husk-Derived Heavy Bio-Oil And Its Supercapacitor Properties

Rice husk,as a renewable biomass with huge output in China,has uniform size and is suitable for poly-generation and utilization through large-scale pyrolysis.Rice husk-derived heavy bio-oil is an industrial by-product produced in the process of rice husk pyrolysis.It has poor physical and chemical properties which hinder its effective utilization by upgrading.In addition,heavy bio-oil contains polycyclic aromatic hydrocarbons with certain toxicity,which may cause environmental pollution.Therefore,From the perspective of environmental protection and resource utilization,it is urgent to explore ways of high-value utilization of rice husk-derived heavy bio-oil.In this paper,rice husk-derived heavy bio-oil was used as raw material to prepare high-performance porous carbons for supercapacitor electrode materials by using its characteristics of high carbon content,low ash content and excellent thermos-plasticity.Focusing on the problems existing in the current research of porous carbon electrode materials,such as low specific surface areas,few hierarchical pores and insufficient surface heteroatomic functional groups,different preparation processes were developed to control the pore structure and surface properties of the porous carbons.The transformation from rice husk-derived heavy bio-oil to high specific surface carbon materials,three-dimensional interconnected hierarchical porous carbon materials and nitrogen-doped hierarchical porous carbon materials was realized to improve their electrochemical properties.The main conclusions obtained in this paper are as follows:Firstly,heavy bio-oil derived carbon materials with high specific surface areas were prepared by Na OH activation method.The effects of the Na OH ratio and activation temperature on the specific surface area and pore structure of the carbons were studied.The microstructure,phase structure and surface element composition of the carbons were investigated.The effects of the different specific surface areas on the supercapacitor properties of the carbon materials were investigated in the three-electrode and two-electrode systems.The results showed that when the mass ratio of Na OH to the carbon precursor was3:1 and the activation temperature was 800°C,the prepared carbon material possessed the ultra-high specific surface area of 2826 m²/g,the total pore volume of 1.780 cm³/g,and the pore size distribution was concentrated at about 2 nm.The carbon material was a kind of amorphous carbon with low graphitization degree.The surface of the carbon material contained oxygen functional groups and almost no nitrogen.With the increase of the specific surface area,the electrochemical properties of the carbon materials were obviously improved.In the three-electrode system,the specific capacitance of the carbon material could reach up to287 F/g（0.5 A/g）.In the two-electrode system,the assembled supercapacitor achieved the energy density of 12.95 W h/kg at a power density of 300 W/kg.These results showed that the transformation from rice husk-derived heavy bio-oil to the carbon material with high specific surface area and its application in supercapacitor electrodes were feasible.Then,a novel two-step biological template assisted molten salt activation method was devised to synthesize heavy bio-oil derived carbon materials with hierarchical pore structures.The regulating effect of the egg shell as biological template on the formation of the hierarchical pore structures was studied.By adjusting the mass ratio of KCl and KHCO₃,and the activation temperature,the synergistic activation mechanism of the two molten salts was revealed.The hierarchical pore structures of the prepared carbon materials were characterized and the influences of the pore structure on the supercapacitor performances were further explored.The results showed that the carbon precursor obtained by biological template method had uniform mesoporous structure.On this basis,the further activation could regulate the pore size distribution of the carbon materials and form hierarchical pore structure.In the process of synergistic activation,KCl was the molten phase and built-in template,KHCO₃was the main activator.When their mass ratio was regulated to be 1:3,the prepared carbon material had the best matching hierarchical pore structure.The distribution peaks of micropores and meso-macropores were located at 0.4 nm and 10.6 nm respectively.The hierarchical pores were wide and interconnected.The total pore volume of the carbon material was as high as 1.900 cm³/g,the microporosity was 50.58%,and the specific surface area was2200 m²/g.In the three-electrode system,the specific capacitance of the hierarchical porous carbon could reach 327 F/g（0.5 A/g）.Compared with the carbon material activated by Na OH,the electrochemical properties of the hierarchical porous carbon were significantly improved.In the two-electrode system,the assembled supercapacitor achieved the energy density of14.25 W h/kg at a power density of 300 W/kg.Finally,based on the new devised preparation process,by introducing urea as the nitrogen source,heavy bio-oil derived nitrogen-doped hierarchical porous carbons were prepared.The effects of the nitrogen-doping position,mass ratio and temperature on the amount of the surface nitrogen atoms and the pore structure of the carbon materials were studied.The types and contents of the surface nitrogen-containing functional groups of the carbon materials were investigated.The effects of the doped nitrogen structures on the supercapacitor properties of the carbon materials were studied.The results showed that the surface properties and structural properties of the carbon materials could be controlled synchronously by carrying out the nitrogen-doping process and activation process simultaneously.When the mixing mass ratio of urea to the mesoporous carbon precursor was2:1,the amount of nitrogen on the surface of the carbon material was as high as 6.17 at%（atomic percentage content）.In addition,the carbon material had developed hierarchical pore structure,high specific surface area of 2388 m²/g and a total pore volume of 2.125 cm³/g.The N-X structure formed on the surface of the carbon material could introduce pseudo capacitance to improve its energy storage capacity.In the three-electrode system,the specific capacitance of the nitrogen-doped hierarchical porous carbon could reach 405 F/g（0.5 A/g）,which is 2.1 times that of the commercial activated carbon.In the two-electrode system,the assembled supercapacitor achieved the energy density of 17.90 W h/kg at a power density of300 W/kg.