Study On The Preparation Of Corncob-Based Electrode Material For Supercapacitor By Microwave Method

Supercapacitors（SCs）,due to the excellent power performance,long cycle life,simple operation mode and easy integration into electronic products,have been widely used in consumer electronics,storage backup systems and industrial power and energy management.With the implementation of“carbon peak,carbon neutral”strategy,the development of supercapacitors will be urgently needed and very important for the clean energy storage.However,the electrode material of supercapacitor-activated carbon is still mainly imported in China,which will seriously hinder the development and application of supercapacitor industry in China.In this paper,corn cob,a renewable agricultural waste,is used as the precursor for activated carbon production.High efficiency,energy saving and green microwave heating method is used to prepare corn cob based electrode materials for supercapacitor.The production technology,formation mechanism,structure and electrochemical performance of as-prepared electrode materials are studied,which the theoretical basis of preparing activated carbon with low cost and high efficiency for supercapacitor application is developled.The main research contents and conclusions are presented as follows:（1）Corncob is used as carbon source,microwave and electric heating method are used to prepare electrode materials for supercapacitor under the same technological conditions respectively.Compared with the electrode materials prepared by electrothermal heating method（CEAC）,microwave activation process has a fast heating speed and uniform temperature distribution,which will not lead to coking and overheating result in the pore structure blocked by carbon deposition.The corn cob based electrode material prepared by microwave activation（MWAC）has higher specific surface area（2508 m²/g）,richer porosity,higher graphitization degree and richer hetero-atom doping.In 6 mol/L KOH electrolyte and at the current density of 0.5 A/g,the specific capacities of MWAC and CEAC are 266 F/g and 192 F/g,respectively.After 10000cycles charge-discharge,the capacity retention rates are 96.8%and 87.3%.When the power density is 250 W/kg,the energy densities of MWAC and CEAC are 9.23 Wh/kg and 6.66 Wh/kg,respectively.In the organic electrolyte of 1 mol/LTEA.BF₄/AN,at the current density of 0.5 A/g,the specific capacities of MWAC and CEAC are 116 F/g and 103 F/g,respectively.After 10000cycles charge-discharge,the capacity retention rates are 85.1%and 80.3%,severally.When the power density is 500 W/kg,the energy densities are 16.2 Wh/kg and 14.3 Wh/kg,respectively.In EMIMBF₄ ionic liquid electrolyte,the specific capacities of MWAC and CEAC supercapacitors are 106.7 F/g and 71.7 F/g at the current density of 0.5 A/g,respectively.When the power density is 180 W/kg,the energy density is 56.1Wh/kg and 46.8Wh/kg.Thus,microwave heating is beneficial to the preparation of corn cob based electrode material,which is obviously better than electrical heating treatment.（2）In view of the poor performance of electrode materials prepared by electrothermal activation due to uneven heating,the corncob-based electrode materials are innovatively prepared by microwave KOH activation method.The effects of microwave activation power,radiation time and alkali carbon ratio on specific surface area and pore structure of electrode materials are studied;Structure-activity relationship between structure and electrochemical properties of materials is further studied.It is found that with the specific surface area firstly increases and following decreases gradually with the increase of microwave power,due to the rapid increase of mesoporous ratio.With increasing the microwave radiation timeand the alkali-carbon ratio,the larger the specific surface area is reached.The specific surface area of the electrode material was high（2152m²/g）with the microwave power of 1000 W,radiation time of 20 min and alkali-carbon ratio of2.0:1.In the three-electrode system,the specific capacity of the electrode material prepared under these conditions is up to 414 F/g at the current density of 1.0 A/g.The specific capacity of the as-prepard button supercapacitor is up to 264 F/g at the current density of 1.0 A/g,and the capacity retention rate is 99.07%after 10000 cycles charge-discharging.Microwave KOH activation can prepare corncob based activated carbon electrode material with high specific surface area,well-developed pore structure and high specific capacity.（3）In order to address the issue of serious damage to equipment and environment during the preparation of electrode materials by microwave KOH activation,corncob-based electrode materials are innovatively prepared by microwave CO₂ activation method.The effects of microwave activation power,radiation time and CO₂ flow rate on specific surface area and pore structure of electrode materials are studied.Structure-activity relationship between structure and electrochemical properties of materials.The results show that the mesoporous ratio and specific surface area increase with the increasing of the microwave power;The specific surface area first increases and then decreases with the increasing of mesoporous ratio.The specific surface area and mesoporous ratio increased first and then decrease with the increase of CO₂flow rate.When the microwave activation power is 1100 W,radiation time is 30 min and CO₂ flow rate is 150m L/min,the specific surface area of the electrode material is 1322 m²/g and mesoporous ratio reaches 15.5%.In the three-electrode system,the specific capacity of the electrode material prepared under the optimal conditions is up to 205 F/g at the current density of 1.0 A/g in 6 mol/L KOH electrolyte.At the current density of 1.0 A/g,the specific capacity of the button supercapacitor reaches 122 F/g,and the capacity retention rate is 101.3%after 10000 cycles charge-discharge.Microwave CO₂ activation not only avoids the adverse effects of alkali on equipment and environment,but also can prepare corncob based activated carbon electrode materials with high mesoporous ratio and excellent cycling performance.（4）Faced with the problem of low specific surface area and specific capacity of electrode materials prepared by microwave CO₂ activation,corncob-based activated carbon electrode materials are innovatively prepared by microwave KOH-CO₂ activation.It is found that compared with KOH activation,KOH-CO₂ activation method greatly reduces the amount of alkali and alleviates the issues of equipment corrosion and environmental pollution caused by KOH to a large extent.Compared with CO₂ activation method,the issue of lower specific surface area and specific capacity of electrode materials is addressed.Furthermore,the structures,electrochemical properties and kinetics of the electrode materials prepared by one-step and two-step microwave KOH-CO₂ methods are compared.The results show that compared with the electrode material（MWAC-2）prepared by the two-step method,the one-step microwave KOH-CO₂ activation method has a faster heating rate,uniform heating,volatile matter and tar can quickly spill out,and more defects are formed.The higher activation energy and pre-exponential factor during carbonization indicated that more active points were formed.The electrode material（MWAC-1）prepared by one-step microwave KOH-CO₂ activation method has high specific surface area（1948m²/g）,abundant porosity,high graphitization degree and abundant heteroatomic doping.In 6mol/L KOH electrolyte and at 0.5 A/g current density,the specific capacities of MWAC-1 and MWAC-2 supercapacitors are 281 F/g and 198 F/g,respectively,and the capacity retention rates are 99.8%and 100%after 10000 cycles charge-discharge.In the organic electrolyte of 1mol/LTEA.BF₄/AN,the specific capacities of MWAC-1 and MWAC-2 supercapacitors are 139F/g and 103 F/gat 0.5 A/g current density,respectively.After 10000 cycles charge-discharge,the capacity retention rate is 99.6%.When the power density is 250 W/kg,the energy densities were19.3 Wh/kg and 14.4 Wh/kg,respectively.In EMIMBF₄ ionic liquid electrolyte,the specific capacities of MWAC-1 and MWAC-2 supercapacitors are 109 F/g and 97 F/g at the current density of 0.5 A/g.When the power density is 900 W/kg,the energy densities are 48.2 Wh/kg and 43.5Wh/kg.One-step microwave KOH-CO₂ activation method is not only simple to operate,but also can prepare corncob based activated carbon electrode materials with large specific surface area,high mesoporous ratio,high specific capacity and excellent cycling performance.