Functionalization Design And Electrochemical Performance Study Of Wheat Straw-derived Porous Carbon

Biomass-derived porous carbon materials are widely used in electrochemical energy storage due to their high specific surface area,hierarchical pore structure and high graphitization degree.However,due to the charge storage mechanism of the electric double layer,its lower electrochemical capacity is limited in the application of high-performance electrode materials.Therefore,functional design of biomass porous carbon to improve its specific capacity has become the key to break through the bottleneck of application,such as regulating specific surface area and pore structure,heteroatom doping or surface modification,and compounding with pseudocapacitive materials.Among them,the coupling strategy with pseudocapacitive materials is one of the effective methods to improve the capacitance performance of biomass carbon-based materials.However,due to the weak interaction between biomass porous carbon and pseudocapacitive materials,the composite architecture lacks stability,showing unsatisfactory cycle stability and rate performance.In this work,the surface modification of wheat straw derived porous carbon materials was carried out on the basis of regulating microstructure.Combined with different redox active substances,the effect of enhanced interaction between carbon materials and redox active substances on the electrochemical properties of composites was investigated.The main research contents and conclusions are as follows:（1）Biomass porous carbon（WS-2）with high specific surface area and hierarchical pore structure was prepared by controlling the alkali carbon ratio（1:1,2:1,3:1）with wheat straw as precursor and KOH as activator.Secondly,aminated porous carbon（FWS）was synthesized by hydrothermal reaction using ethylenediamine as amination reagent.The results showed that the surface amination modification did not destroy the pore structure and specific surface area of biomass porous carbon,and the surface wettability of FWS was significantly improved.WS and FWS show good electric double layer capacitance performance in both acidic and alkaline electrolytes.The performance of FWS is slightly better than that of WS.In 2 M KOH electrolyte,FWS has a specific capacitance of 287.1 F g^-1 at 1 A g^-1,and the rate performance is 75.9%at10 A g^-1.The specific capacitance in 1 M H₂SO₄ electrolyte is 253.5 F g^-1,and the rate performance is 78.1%at 10 A g^-1.（2）Aminated porous carbon@Ni²⁺composite electrode(FWS@Ni²⁺)was obtained by potentiostatic electrosorption of Ni²⁺with pseudocapacitive properties using FWS as working electrode.The results showed that the adsorption capacity of FWS to Ni²⁺is significantly higher than that of WS due to the coordination complexation of Ni²⁺with abundant nitrogen-containing functional groups.The electrochemical properties of FWS@Ni²⁺composite electrode materials are better than those of WS@Ni²⁺.The optimized FWS@Ni²⁺-300 composite electrode has a specific capacitance of 512.8 F g^-1 at 1 A g^-1,a capacitance retention rate of 53.7%at 10 A g^-1,and a capacitance retention rate of 76.5%after 5000 charge-discharge cycles at 5 A g^-1.（3）The abundant amino functional groups on the surface of FWS can anchor the redox active organic compound 2,6-diaminoanthraquinone（DAQ）through strong hydrogen bonding.The DAQ@FWS composite exhibits excellent electrochemical performance in 1 M H₂SO₄electrolyte.When the mass ratio of DAQ to FWS is 0.5:1,the specific capacitance of DAQ@FWS is 424.9 F g^-1 at 1 A g^-1.When the current density increases to 30 A g^-1,it still has a specific capacitance of 300.4 F g^-1.At a current density of 10 A g^-1,the capacitance retention rate is 93.1%after 10,000 cycles of charge and discharge.Through first-principles calculations,DAQ@FWS(E_ads=-1.66 e V)has lower adsorption energy and smaller band gap than DAQ@WS system(E_ads=-0.31 e V),indicating that DAQ@FWS has better ion/electron transport kinetics and electrochemical stability.The symmetric supercapacitor assembled with DAQ@FWS as electrode has high energy density(36.4 Wh kg^-1)and power density(7001 W kg^-1),and the capacitance retention rate is 88.3%after 10,000 cycles.（4）The surface of biomass porous carbon was modified by anionic surfactant sodium dodecyl sulfate（SDS）.The negatively charged SDS modified porous carbon（SWS）was combined with cationic dye azure A（AA）by strong electrostatic adsorption to obtain composite electrode material（AA@SWS）.The results showed that the enhanced electrostatic adsorption and intra-pore confinement between AA and SWS significantly improved the electrochemical stability of electrode material.The AA@SWS composite exhibits good electrochemical performance in both alkaline and acidic electrolytes.In 2 M KOH,the specific capacitance of AA@SWS at 1 A g^-1 is 557.9 F g^-1,which is 1.9 times that of SWS(289.3 F g^-1).When the current density increases to 30 A g^-1,the capacitance retention rate is 70.4%,and the capacity retention rate is 92.4%after 12,000 cycles at 10 A g^-1.Density functional theory calculations proved the enhanced interaction between SWS and AA.The symmetric supercapacitor assembled with AA@SWS as electrode exhibits good energy density(32.1 Wh kg^-1),power density(6500 W kg^-1)and long-term cycle stability（88.9%capacitance retention after 10,000cycles）.