Preparation Of Biomass-based Hierarchical Porous Carbon And Its Electrochemical Performance

With the increasing attention on renewable energy and environmental issues,the high-value utilization of biomass resources has aroused great interest and extensive research of many researchers.Biomass-based hierarchical porous carbon（BHPC）is a kind of carbon material with high specific surface area,good hierarchical porous structure,stable carbon skeleton,and low cost,so it is an ideal electrode material of electric double-layer capacitors（EDLCs）.In this paper,two BHPCs with good performance were prepared by simple carbonization and chemical activation method,using rice husk and straw cellulose as raw materials,and natural ordered pores as main ion channels.By studying the resource of different size pores,formation mechanism of hierarchical porous structure of rice husk hierarchical porous carbon（RHPC）and its electrochemical performance,the relationship between pore structure and electrochemical performance is established.High-performance and low-cost EDLC carbon electrode materials can be produced by adjusting and controlling the pore structures of BHPCs.In view of the problems of low tap density and volume energy density of BHPCs,the low-temperature carbon-fixation technology is used to significantly improve the carbonization yield of rice husk,the tap density of RHPC and its electrochemical performance.The effect of electrolyte on the electrochemical performance of rice straw cellulose hierarchical porous carbon（SCPC）was studied.The reasons for the different electrochemical performances of SCPC in different electrolytes are explained from electrolyte ion size,ionic conductivity,and the matching relationship between ions and pores.Based on the different capacitance characteristics in various electrolytes,the potential applications are analyzed.The main research results of this paper are as follows:1.RHPC was prepared by carbonization,desiliconization and activation with rice husk as raw material and its natural vascular bundle as main ion channels.RHPC shows good electrochemical performance in EDLCs.In order to clarify the source of different size pores（macropores,mesopores,and micropores）of RHPC and the formation mechanism of hierarchical porous structure,the four main components of rice husk（cellulose,lignin,hemicellulose,and silica）are effectively separated.By analyzing the pore structure changes of cellulose,lignin,and hemicellulose during carbonization,desiliconization,and activation processes,the source of different size pores of RHPC are determined.Micropores in RHPC mainly come from the contribution of cellulose,and the mesopores mainly come from the contribution of hemicellulose,lignin can provide some micropores and mesopores for RHPC at the same time.The macropores of RHPC are derived from the natural vascular bundles of rice husk.The larger size mesopores（3-50 nm）are mainly formed in the carbonization and desiliconization processes.Most micropores and smaller size mesopores（2-3 nm）of RHPC are mainly formed in the activation process.The macropores,mesopores and micropores are interconnected to form the hierarchical porous structure of RHPC.The macropores,mesopores and micropores are interconnected to form the hierarchical porous structure of RHPC.Via the study of the electrochemical performance of RHPC and the porous carbons from three rice husk components,the relationship between the structure and electrochemical performance of RHPC is established.Rice husk lignin porous carbon has the highest specific capacitance and excellent rate capability,which is due to its high specific surface area and mesoporosity.The outstanding cycling stability of rice husk cellulose porous carbon is attributed to its good electrical conductivity and stable carbon skeleton structure.The presence of hemicellulose contributes to the high specific capacitance and low internal resistance of the RHPC electrode to a certain extent.The synergistic effect of cellulose,lignin,and hemicellulose ensure RHPC has developed pores,excellent hierarchical porous structure,good electrical conductivity,and stable carbon skeleton,which makes RHPC has great application prospect in high-rate capability and long-life supercapacitors.This work provides a theoretical basis for the deep understanding of the formation mechanism of biomass-based hierarchical porous carbon,and provides a valuable reference for the controllable preparation and pore structure optimization of high-performance biomass-based hierarchical porous carbon.2.Based on the analysis of the pyrolysis characteristics of rice husk different components,LHAN was used as a modification reagent,the microstructure of RHPC was changed via impregnation,low-temperature carbon-fixation,and activation methods,and its electrochemical performance was also improved.This study provides an effective method for improving the volume energy density and yield of BHPCs,and provides an important theoretical basis and data support for large-scale production of high-performance BMPCs.（1）In KOH and Et₄NBF₄/PC electrolytes,the mass specific capacitance of RHPC-N10 electrodes after low-temperature carbon-fixation are 23.8%and 28.8%higher than those of RHPC electrodes before carbon-fixation,respectively.（2）The RHPC-N10electrode shows better rate capability(when the current density increased from 0.5 to20 A g^-1,the capacity retention rate is 97.9%)and better wettability in Et₄NBF₄/PC electrolyte.（3）The volume energy density(42.7 Wh L^-1)of RHPC-N10 capacitor is54.2%higher than that of RHPC capacitor,which effectively improves the application competitiveness of RHPC in EDLCs.（4）The total yield of RHPC is increased from7.4 wt%to 16.3 wt%after the low-temperature carbon-fixation treatment,which greatly improves the utilization of rice husk and saves production costs.This is of great significance for the large-scale production of RHPC.3.SCPC with good performance was prepared via a simple carbonization and activation methods using rice straw cellulose as raw material.SCPC has a high specific surface area,a good hierarchical porous structure,and a low resistivity,which make SCPC display excellent electrochemical performance for EDLCs.SCPCs have high specific capacitance of 312.6 F g^-1 in 6 M KOH at 0.5 A g^-1,excellent rate performance of 281.3 F g^-1 even at 15 A g^-1,and outstanding cycling stability of 92.9%capacitance retention after 20 000 cycles at 1 A g^-1.SCPC-based EDLC device can deliver an energy density of 8.7 Wh kg^-1 at a power density of 3.5 kW kg^-1 in 6 M KOH and an energy density of 28.6 Wh kg^-1 at a high power density of 14.1 kW kg^-1 in 1 M Et₄NBF₄/PC,which demonstrate the possibility of SCPC applied in EDLCs.4.In order to understand the relationship of the microstructure of SCPC and electrochemical performance and further tap the application potential of SCPC,the electrochemical performance of SCPC in different electrolytes was systematically studied.In H₂SO₄,the SCPC electrode exhibits outstanding cycling stability with 95.6%capacitance retention over 10 000 cycles.The SCPC electrode shows higher specific capacitance(309.0 F g^-1)and superior rate capability in KOH,and higher energy density of 17.9 Wh kg^-1 in Na₂SO₄,respectively.The SCPC electrode exhibits ideal capacitance characteristics,superior rate capability with capacitance retention of 95.8%,and high energy density of 36.0 Wh kg^-1 in Et₄NBF₄/PC.The significant difference of capacitive performance of SCPC electrode in various electrolytes is mainly attributed to the difference in the electrolyte ion size,the ionic conductivity,and the matching between the electrolyte ions and pore structure,respectively.The excellent hierarchical porous structure of SCPC is especially suitable for organic electrolytes with larger ion size.This work not only establishes the relationship between the structure of SCPC and its electrochemical performance in different electrolytes,but also provides more directions for the application of SCPC.