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

Supercapacitors are become widely concerned owing to their high-power density,ultra-long cycle capability,and low maintenance cost.However,the energy density of supercapacitors is unsatisfactory.Improving the energy density of supercapacitors without sacrificing power density is a current research focus.According to the energy density formula E=1/2 CV²,there are two main methods to improve the energy density:（1）increase the specific capacitance（C）of the electrode material;（2）increase the working voltage（V）of the supercapacitor.Therefore,researchers are devoted to developing new green,low-cost,and easily modified high-performance electrode materials,selecting of suitable electrolytes,and designing the overall device.Biomass-based carbon aerogels are carbon materials with a three-dimensional network structure,which are prepared from the rich natural polymer cellulose or hydrogel-like plants,via sol-gel,freeze-drying,and carbonization.Owing to their low density,high porosity,and easy to modify,biomass-based carbon aerogel has great potential in the preparation of supercapacitor electrode materials.In this paper,the hierarchical porous carbon aerogels were prepared via CO₂activation method,which using biomass cellulose as raw materials and ice crystal as a template.The carbon aerogels were optimized by heteroatomic nitrogen doping and metal oxide doping,and their pore structure,surface chemical composition and graphitization degree were analyzed,and the relationship between their main physical and chemical properties and electrolyte system on the electrochemical performance of supercapacitor was studied.The all-cellulose-based asymmetric thick electrode supercapacitor devices with high bulk energy density were constructed.Besides,the nitrogen-doped hierarchical porous carbon aerogels were obtained by a one-step carbonization activation method using its own nitrogen-containing hydrogel-like plant（banana）as the raw material.The main research contents of the paper are as follows:（1）Hydrogels were prepared by the sol-gel method using microcrystalline cellulose as raw materials,and cellulose carbon aerogels with the three-dimensional hierarchical porous structure were constructed by freeze-drying and one-step carbonization activation method.The results indicated that the specific surface area and total pore volume of carbon aerogels increased with the increase of CO₂ activation temperature or CO₂ activation time.When the activation temperature of CO₂ was 1000°C and the activation time was 2 h,the specific surface area and total pore volume of sample CA-1000-2 reached the maximum value of 1118 m²·g^-1 and 0.53cm³·g^-1,and the micropore volume also reached 0.41 cm³·g^-1.The microstructure exhibited an interconnected three-dimensional porous structure with many worm-like micropores on the thin walls.In the three-electrode test system in 6 M KOH electrolyte,the sample CA-1000-2exhibited a specific capacitance of 223.9 F·g^-1 at a current density of 0.5 A·g^-1,and when assembled into a 2032 model cell for testing,its capacity retention was 99.97%after 5000 cycles,and the energy density was 3.23 Wh·kg^-1 at a power density of 50 W·kg^-1.（2）Based on the controlled pore structure technique,hierarchical porous nitrogen-doped carbon aerogels with permeable honeycomb structures were synthesized in situ by adding polypyrrole as the nitrogen source during the sol-gel process and using ice crystals as the template.The ice crystals were sublimated after freeze-drying,which leaving a large number of15-20μm honeycomb macropore structures.After carbonization and CO₂ activation,the specific surface area and total pore volume of the carbon aerogel sample N-CA-2 were 1196 m²·g^-1 and0.58 cm³·g^-1,respectively,and the pore size distribution was mainly concentrated in 1.2 nm micropores and 8.4 nm mesopores.The microcrystalline structures of the samples are all dominated by amorphous carbon,with a small amount of graphitized carbon present.The atomic percentage of N of sample N-CA-2 was 4.98 at.%,and the surface nitrogen elements were mainly present in the bonded form of graphitic N,pyridine N,and pyrrole N.In the three-electrode test system in 6 M KOH electrolyte,the capacitance and charge transfer resistance of N-CA-2 were144 F·g^-1 and 1.4Ω,respectively.When assembled into a symmetrical 2032 model cell for the two-system test,using 6.0 M KOH as the electrolyte,the specific capacitance of the device reached 129.8 F·g^-1 at a current density of 0.1 A·g^-1,the energy density of the device was 4.5Wh·kg^-1 at a power density of 50 W·kg^-1.When using the ionic liquid EMIMBF₄ as an electrolyte,the energy density of the supercapacitor was 21.9 Wh·kg^-1 at a power density of 145 W·kg^-1,and the capacitance retention rate was 82.6%after 3000 cycles at a current density of 2 A·g^-1.（3）Taking advantage of the three-dimensional network porous structure and wettability of the precursor aerogels,the waste dye Rhodamine B was used as a nitrogen source for adsorption treatment.0.1 g of aerogels could completely adsorb 5 m L of a high concentration of 5 g·L^-1 of RB dye within 30 s,and the adsorption amount could be as high as 250 mg·g^-1.The nitrogen-doped aerogels were dried and carbonization as precursors,and the heteroatomic nitrogen was successfully doped into the carbon aerogels skeleton with atomic percentages of nitrogen of 2.15at.%,and mainly in the bonded forms of graphitic N,pyridine N and pyrrole N.In the three-electrode test system in 6 M KOH electrolyte,the equivalent series resistance of the nitrogen-doped carbon aerogel sample was only 0.5Ω,and its specific capacitance value was 185 F·g^-1 at a current density of 1 A·g^-1.（4）Self-contained nitrogen hydrogel-like plant（Banana）was directly used as the raw material,and the nitrogen-doped carbon aerogels with hierarchical porous structure were prepared by one-step carbonization and CO₂ activation method.After CO₂ activation,the specific surface area and total pore volume of the carbon aerogel（BCA-2）could reach 1414.9 m²·g^-1 and0.746 cm³·g^-1,and the pore size distribution was concentrated in the range of 1.9-4.0 nm,with a worm-like porous structure on the thin wall.Besides,after carbonization and CO₂ activation,the N element was well retained with a content of about 2.55 at.%,which was presented in the combined form of graphite N,pyridine N,and pyrrole N.In the three-electrode test system in 6M KOH electrolyte,the specific capacitance value of BCA-2 was 178.9 F·g^-1 at a current density of 1 A·g^-1.When assembled into a symmetrical 2032 model cell for the two-system test,the capacitance retention of was 98%after 10000 cycles at a current density of 5 A·g^-1,and the energy density of the device reached 3.23 Wh·kg^-1 when the power density was 50 W·kg^-1.（5）The composite Ru O₂-doped cellulose carbon aerogels（CA-R-3）were obtained by redox method by mosaicking Ru O₂ nanoparticles on the skeleton of hierarchical porous cellulose carbon aerogel(specific surface area of 726 m²·g^-1 and total pore volume of 0.75 cm³·g^-1).The results showed that the microscopic pore structure of CA-R-3 was clogged,and its specific surface area and total pore volume decreased to 96.71 m²·g^-1 and 0.19 cm³·g^-1.In the three-electrode test,the specific capacitance value of sample CA-R-3 was 244.1 F·g^-1 at a current density of 0.5 A·g^-1 in 3 M KOH electrolyte,and its capacitance retention was 88.6%after 500cycles at a current density of 5 A·g^-1.（6）Asymmetric supercapacitor devices based on all-cellulose-based ultra-thick electrodes were constructed by making full use of the high porosity of cellulose aerogels and the permeable hierarchical porous properties of carbon aerogels:cellulose aerogels as the separator,sheet-like1.2 mm-thick carbon aerogel（CA）and 1 mm-thick Ru O₂-doped carbon aerogel（CA-R）were directly used as the anode and cathode of the supercapacitors,without conductive agent and adhesive added to the electrodes.The sample CA exhibited many cracked macropores（100-300nm）with specific surface area and pore volume of 1130 m²·g^-1 and 0.83 cm³·g^-1,respectively,and the microcrystalline structure is amorphous.When the 1 mm thick CA was directly used as an electrode in a three-system test under 1 M Na₂SO₄aqueous electrolyte,the area specific capacitance was 1039 m F·cm^-2 at a current density of 1 m A·cm^-2,and the capacitance retention was 95%at a current density of 5 m A·cm^-2 after 3000 cycles.The specific surface area and pore volume of CA-R were 829.01 m²·g^-1 and 0.77 cm³·g^-1,respectively,and the microcrystalline structure was also amorphous.When the 1 mm-thick CA-R was directly used as the electrode and tested in three systems under the same electrolyte,its area specific capacitance reached 4284m F·cm^-2 at a current density of 2 m A·cm^-2,and its capacitance retention rate only decreased by0.3%after 3000 cycles at a current density of 10 m A·cm^-2.When assembled into a supercapacitor device,the area specific capacitance and volume specific capacitance can reach 1.36 F·cm^-2 and6.18 F·cm^-3 in a 1 M Na₂SO₄ electrolyte test(current density of 5 m A·cm^-2).The volumetric energy density is up to 3.4 Wh·L^-1 at a power density of 23 W·L^-1.Besides,the capacitance retention is up to 98%after 2500 cycles at a current density of 6 m A·cm^-2.