Preparation And Electrochemical Properties Of Biomass-based Electrode Materials For Supercapacitors

Supercapacitors,as a new type of energy storage device,have attracted widespread attention,but the relatively low energy density is an urgent problem for their commercial applications.The design of advanced high-energy-density supercapacitors requires that the electrode material possess a hierarchically nanoporous structure with high specific surface area to promote the adsorption,storage and diffusion of electrolyte ions.In recent years,due to the advantages of low cost,wide sources,sustainable regeneration and environmental friendliness,the conversion of various biomass and its derivatives as precursors to porous carbon materials has been extensively studied.In this article,using sodium carboxymethyl cellulose（CMC）as the precursor,a novel ice template approach was designed to prepare the CMC-derived hierarchically porous carbon（CHPC）electrode material,and the composite preparation of CHPC/MXene and CHPC/MnO was carried out.The details are as follows:Firstly,a novel freezing-extraction-thermally drying approach was developed to prepare biomass porous carbon.By the procedures of dissolving CMC into a hydrogel,quickly freezing it in liquid nitrogen and immersing it in absolute ethanol at room temperature,the micro-ice crystal templates were formed,dissolved and extracted out of the material system.After a short period of thermally drying to remove residual ethanol,the CMC solid porous framework was obtained.Finally,CHPC material was prepared by one-step carbonization.This scalable method can be easily combined with the activation method.By dissolving CMC and a small amount of Cu（CH₃COO）₂,KOH or other activators into a hydrogel,the carbon with better structure and performance can be obtained through the same steps.Among all the samples,CHPC-3 has the most ideal hierarchical pore structure and the best electrochemical performance.The specific surface area is as high as 1594 m² g^-1,and the specific capacitance reaches 263.8 F g^-1at 1 A g^-1,even at a high current density of 40 A g^-1,it still maintains a specific capacity of 80.1%.After10,000 charge-discharge cycles,the specific capacity remains 99.3%of the initial value.The assembled CHPC-3//CHPC-3 ASS device achieves a specific capacity of 170.7 F g^-1 at 1 A g^-1,and an energy density of 5.9 Wh kg^-1 at a power density of 249.9 W kg^-1.Secondly,CHPC was functionalized with H₂SO₄/HNO₃ solution,and then mixed with MXene 2D material ultrasonically,and the CHPC/MXene composite flexible free-standing film was easily obtained after vacuum filtration and freeze-drying process,which realizes the application of CHPC in flexible electrodes.The results show that CHPC/MXene-2 has good flexibility,and no fracture occurs after bending,winding,and folding.Moreover,it has a satisfying conductivity of 256.4 S cm^-1.The double electrolyte transport channel of porous carbon and MXene interlayer gap promotes the adsorption,diffusion and migration of ions.Based on this feature,its area specific capacitance reaches 744.1 m F cm^-2 at 3 m A cm^-2,and after 5000 cycles of GCD testing,the capacitance retention is 93.3%,showing the excellent.cycle life.Finally,the CHPC/MnO composite was prepared in situ by the ice template method.By dissolving CMC in Mn（CH₃COO）₂ solution to form a hydrogel and freezing it in liquid nitrogen,followed by freeze-drying,to get the CMC/Mn（CH₃COO）₂ solid porous framework,after which being carbonized,the desired CHPC/MnO composite was obtained.It is found that the CHPC/MnO-800 sample has the most ideal carbon/manganese composite structure.The carbonaceous has a hierarchical pore structure with a specific surface area of up to 849.1 m² g^-1.MnO nanocubic crystal phase is uniformly distributed in the carbonaceous framework,and the electrolyte can be quickly transported to the active site of MnO through the porous carbon channel,which improves the utilization of the active material.At a current density of 1 A g^-1,its specific capacitance reached 343.2 F g^-1.After 5000 cycles of GCD testing,the capacitance retained 87.1%.