Preparation And Capacitor Properties Of Cellulose-based Activated Carbon Fibers

As a green and efficient energy storage device,the performance of supercapacitor is mainly affected by electrode materials.Currently,activated carbon（AC）is still the most widely used electrode material for double electric layer capacitor due to its relatively low price.Compared with AC,activated carbon fiber（ACF）has the characteristics of developed microporous structure and exposed on the surface of the fiber,fast adsorption and desorption speed,especially the ultrafine activated carbon fiber（UACF）with larger specific surface area and shallower pores.In addition,ACF can also be processed into paper,cloth,felt and other forms,which can be directly used as self-supporting electrodes.However,commercial ACF mainly uses petrochemical products as raw materials,the non-renewability of these raw materials has exacerbated the consumption of petroleum resources.Meanwhile,the UACF prepared by electrospinning has problems such as low production efficiency,high cost,and complicated operation that have limited its wide application.In view of the above problems,in this paper,cellulose-based ACF was prepared by using cotton fiber as raw material,through fibrillation and wet forming processes,combined with carbonization and activation method.The effects of different fibrillation degree on the structure and electrochemical properties of ACF were investigated in detail.Through the screening of different activation methods,the synergistic activation method was optimized to control the pore structure of ACF,and the relationship between ACF pore structure and capacitance performance was revealed by theoretical models.On this basis,a high-performance cellulose-based activated carbon fiber paper（ACFP）was designed and prepared,which can be directly used as a self-supporting electrode without adding any binder.This work provided a theoretical basis and new ideas for the preparation of ACF from natural fibers and the fabrication of cost-effective paper-based electrode materials.The main research results of this paper were as follows:（1）Using cotton pulp as raw material,it was subjected to fibrilization treatment by PFI milling method,and then prepared by wet papermaking and CO₂ activation to prepare cellulose-based ACF.The effects of different degrees of fibrillation treatment on the structure and electrochemical performance of ACF were explored in detail.The results indicated that with the increase of fibrillation degree of cotton fiber,the specific surface area and fibrillation degree of cotton fiber increased gradually,the air permeability of cotton paper decreased gradually.The specific surface area,pore volume and specific capacitance of prepared ACF all showed a trend of increasing first and then decreasing slowly.Among them,40°SR ACF had the largest specific surface area（1267 m²/g）and specific capacitance（0.5 A/g,129.9 F/g）.Through the fitting analysis of ACF pore structure and specific capacitance,the results showed that the pore structure parameters of ACF and specific capacitance were linearly positively correlated,he correlation coefficient with specific surface area and micropore volume was R²≥0.98.In addition,compared with coconut shell AC prepared under the same activation conditions,ACF has more developed microporous structure,higher specific surface area and specific capacitance,and can be used as the preferred raw material for preparing supercapacitor electrodes.（2）Using 40°SR cotton fibers as raw materials,the effects of CO₂ activation,H₃PO₄activation,and H₃PO₄-CO₂ synergistic activation on the microscopic morphology,surface characteristic,pore structure,and electrochemical performance of ACF were investigated.The results showed that the ACF prepared by the three methods were all amorphous carbon structures,but there were obvious differences in ACF pore structure and double layer capacitance performance.The ACF prepared by CO₂ activation was mainly microporous（microporosity up to 85%）with narrow pore size distribution and the average pore size was in1.6 nm.The ACF had relatively high conductivity（159 S/m）and specific capacitance（103～129.9 F/g,0.5 A/g～5 A/g）.The ACF prepared by H₃PO₄ activation can obtain high specific surface area（1277 m²/g）and rich micro-mesoporous structure（microporosity up to51%）at low temperature.However,its conductivity was low（0.13 S/m）,resulting in high impedance and low capacitance retention（71%）.The specific surface area prepared by the synergistic activation method was the highest,the pore size distribution was wide,the average pore size was 2.3 nm,the mesoporous can reach 61%,and it showed a high specific capacitance（135～111.5 F/g,0.5 A/g～5 A/g）and capacitance retention（82.6%）.（3）Based on the research on the synergistic activation of H₃PO₄-CO₂,the pore structure of ACF was further regulated by controlling the technological parameter during the activation process,and the theoretical model was used to clarify the effect of ACF pore structure and capacitance performance relation.The results showed that the optimal process parameters of the synergistic activation method were impregnation ratio of 1:1.5,activation temperature of850°C,and activation time of 60 min.The specific surface area of ACF prepared under these conditions was 2098 m²/g and the total pore volume was 1.23 cm³/g,mass specific capacitance was 114～149.0 F/g,its performance was better than YP50F（105～141 F/g）,indicating that ACF had a certain commercial value.In addition,the specific capacitance was fitted by the simplified micro/mesoporous model,and the correlation coefficient R² was greater than 0.966.It can be seen from the model calculation that the contribution of micropores to specific capacitance was64.7%higher than that of mesopores.With the increase of the current density（0.1 A/g～5.0 A/g）,the area specific capacitance and volume specific capacitance of the micropores decreased faster than that of the mesopores.It showed that when charging and discharging at high current,and the capacitance retention rate of mesopore was bigger under high current density.Using this model to predict the specific capacitance of ACF with known pore structure had certain reference value.（4）Based on the above research,using the fibrillated cotton fiber（40°SR）and CFs as raw materials,the cellulose-based ACFP was designed and prepared by wet forming process and H₃PO4-CO₂ synergistic activation method.The effect of CFs content on the structure and electrochemical performance of self-supporting paper-based electrodes was explored.The results showed that when the CFs content was 10%～15%,the prepared ACFP had high specific surface area（808～1106 m²/g）,high electrical conductivity（1640～1786 S/m）,good tensile properties（4.6～6.4 MPa）and flexible processability.ACFP can be used directly as a self-supporting electrode without any binder.The maximum specific capacitance based on the entire ACFP electrode was 48.8 F/cm³（or 165 F/g）,and ACFP had excellent cycle stability.The ACFP-15 can be assembled into an all-solid-state paper-based supercapacitor that revealed excellent electrochemical performances.The energy density can reach 6.01 Wh·kg^-1.In addition,ACFP can be used as substrate material,and Mn O₂ can be uniformly loaded on the surface of ACFP.The specific capacitance was increased by 1.8 times compared with unloaded ACFP-15.