Synthesis Of Polyatomic Co-self-doped Biochar By Rigid Support Strategy And Its High Energy Density Supercapacitor Performance

Electrochemical energy storage has become a research hotspot in various renewable energy storage technologies due to its high efficiency and flexibility.Among them,supercapacitors have the advantages of high power density,fast charging and discharging speed,long cycle life,and environmental protection,and are considered to be one of the clean and efficient energy storage devices with the most development potential.However,the lower energy density limits its further development.The use of cheap biomass with the advantages of renewable,natural pores,and uniform distribution of heteroatoms to prepare electrode materials is expected to make up for this shortcoming.In this paper,a series of heteroatom self-doped porous carbon materials were prepared by a simple filling supported channel without alkali activation process using different biomass as carbon source.The microscopic morphology,pore structure and heteroatom doping amount of the obtained porous carbon materials were adjusted by changing the carbonization temperature,etc.,and supercapacitors with excellent electrochemical performance were assembled based on this.The main research contents are as follows:（1）Electrochemically active heteroatom self-doped porous carbons were prepared using Undaria pinnatifida as carbon precursor and low-valent recyclable Na Cl as pore filler,which effectively suppressed the shrinkage of Undaria pinnatifida-derived carbon frameworks.The whole preparation process is simple,green and does not involve strong acid,strong base and other strong oxidants.The typical sample SUPC-800 has high specific surface area(713 m²g^-1)and pore volume(0.51 cm³ g^-1),and retains heteroatoms（N:4.91 at.%,O:8.38 at.%,P:0.22 at.%）.Benefiting from the rich pore structure of the material,it shows high mass specific capacitance(355 F g^-1@0.5 A g^-1)and good rate capability.The symmetric supercapacitor assembled in 1.0 M H₂SO₄ electrolyte can provide a competitive high energy density(16.9Wh kg^-1@165 W kg^-1),and the specific capacitance does not have any degradation after10,000 GCD tests,showing excellent cycle life.（2）Four kinds of heteroatom N,O,P,S self-co-doped porous activated carbon materials（HPAC）were prepared by carbonizing Undaria pinnatifida dipped in magnesium acetate solution.Compared with Na Cl,which is chemically inert,the in-situ generation of Mg O with high melting point from magnesium acetate can act as a rigid support to effectively prevent the collapse of pores,increase mesopores and keep the pores open.Meanwhile,the typical sample HPAC-700 has higher doping contents of N（7.06 at.%）,O（8.45 at.%）,P（0.2 at.%）,and S（0.62 at.%）.It has a high specific surface area of 1116 m² g^-1,a large pore volume of1.63 cm³g^-1 and a pore size distribution that matches the H₂SO₄ electrolyte.At a current density of 0.5 A g^-1,its mass specific capacitance can reach 580 F g^-1 and 306 F g^-1 at 100 A g^-1,and exhibit excellent rate characteristics.In the two-electrode system,the monolithic mass specific capacitance is still as high as 489 F g^-1 at a current density of 0.5 A g^-1;and at a power density of 164 W kg^-1,it exhibits a high energy density of 28.7 Wh kg^-1.In addition,the button-type symmetric supercapacitor assembled from it showed no energy loss after 8000charge-discharge cycles at a current density of 5 A g^-1,and successfully lit LEDs.（3）Two kinds of heteroatom self-doped porous biochars（STGC and MTGC）were prepared by using the heteroatom-rich Tricholomagambosum as the carbon precursor,referring to the above-mentioned filling methods,to further verify the supporting effect of Na Cl and Mg O templates on pores and the practicability and universality of the preparation process.They all have similar pore size distribution and pore structure,and the heteroatom contents in typical biochar STGC are N（5.12 at.%）,O（7.76 at.%）,P（0.97 at.%）and S（0.60at.%）,respectively.While the biochar MTGC also has a high amount of heteroatom doping（N:4.71 at.%,O:13.66 at.%,P:1.41 at.%,S:0.82 at.%）.In the three-electrode and two-electrode systems with 1.0 M H₂SO₄ as the electrolyte,the mass specific capacitance of biochar STGC is 434 F g^-1 and 342 F g^-1,respectively,and the mass specific capacitance of biochar MTGC is higher,446 F g^-1 and 385 F g^-1,respectively;they also exhibit good rate performance and excellent cycling stability.Furthermore,biochar STGC and MTGC exhibited high energy densities of 20.1 Wh kg^-1 and 22.6 Wh kg^-1 at power densities of 164 W kg^-1 and 162 W kg^-1,respectively.