Synthesis Of Biomass-based Hetroatom(N/O/S)Doped Carbons Materials And Its Capacitance Performance

The fifth generation mobile communication technology and artificial intelligence technology have promoted extensive researches on the new energy storage and energy conversion devices.Supercapacitors as green and efficient energy storage device have attracted much attention in scientific researches and commercial applications due to some advantages of fast charging and discharging,high power density and good cycling.It is well known that electrode materials play the key role and affect the electrochemical performance of supercapacitors.Among the electrode materials,biomass-based carbon materials have become the research focuses owing to outstandings including good chemical stability,excellent conductivity and environmental friendliness.In this paper,kelp,corn and peanut were used as carbon sources,O/S/N decorated hierarchical porous,O/N co-doped carbon flakes as well as porous carbon were designed and developed.The effects of specific surface area,pore structure and chemical composition on electrochemical performance were studied to improve the rate performance and cycling of supercapacitors.The main research contents are summarized as follows:（1）To develop simple methods of preparing biomass-based porous carbon materials is of great scientific significance.Peanuts rich in nitrogen were carbonized at 700℃to obtain N-doped carbon materials（N-C）.Two methods are used to prepare porous carbon:the first method was to carbonize N-C at 700℃,and the obtained porous carbon was named N-PC;In the second method,N-C was first soaked in H₂O₂solution for 5 h,and then carbonized at 700℃,finally,the obtained porous carbon was denoted as N-PC-5.The chemical composition,graphitization degree,specific surface area and pore size distribution of the obtained porous carbons through the above two methods were investigated,and their electrochemical properties were also studied.The results show that N-PC-5 has good electrochemical performance.In a three-electrode system,N-PC-5 has the capacitance of 86 F g^-1 at 0.5 A g^-1,which is better than that of N-PC(45 F g^-1).In the two-electrode system,N-PC-5 has single-electrode capacitance of 52 F g^-1 at 0.2 A g^-1,and its energy density reaches 1.79 Wh kg^-1 at a power density of 49.99 W kg^-1.At 1 A g^-1,its capacitance retention rate is nearly 96%after 10000 cycles.（2）According to previous researhes,nitrogen atoms in carbon materials can improve the conductivity of carbon materials and increase electrochemical performance.O-doped carbon flakes was first prepared from corn by pyrolysis.g-C₃N₄（G）was heated at 600℃and 700℃,respectively,and the obtained sample was designated as GT（T=600,700℃）,which is used as nitrogen source.Then,carbon flakes and GT with a mass ratio of 1:1were mixed together and pyrolyzed at T,and the O/N co-doped carbon flakes（N-C-GT）was prepared.The results show that N-C-G700 carbon flakes with O（10.40 at.%）and N（7.66 at.%）have good capacitance performance and excellent cycling.In the three-electrode 6 mol L^-1 KOH system,the optimal sample（N-C-G700）has the specific capacitance of 120.5 F g^-1 at 0.2 A g^-1 while it still keep 50.5%even at 10 A g^-1.In the two-electrode system,N-C-G700 shows the single-electrode capacitance of 100 F g^-1at0.2 A g^-1.N-C-G700 can deliver energy density of 3.47 Wh kg^-1 at a power density of52.05 W kg^-1.Especially,its capacitance retention rate is nearly 96%after 10,000 cycles,suggesting N-C-G700 good cycling stability.（3）Kelp is composed of rich inorganic salts,O/S/N doped hierarchical porous carbons（HPCT,T=600,700 or 800℃）were fabricated using kelp as the precursor by two-step carbonization.The effects of carbonization temperature and carbonization steps on the pore structure,specific surface area and chemical composition of porous carbon were studied,and the formation mechanism was proposed.The research results show that the optimal sample obtained by two-step carbonization at 800℃（denoted as O/S/N-HPC800）shows good rate performance and excellent cycling.In the three-electrode system,O/S/N-HPC800 has a specific capacitance of 276 F g^-1 at 1 A g^-1,as the current density is 100 A g^-1,it still keeps 72.4%.When assembled in a two-electrode system,O/S/N-HPC800 offers a maximum capacitance of 166 F g^-1 at 0.5 A g^-1,and exhibits a maximum energy density of 5.76 Wh kg^-1 at a power density of 124.92 W kg^-1.Specifically,it can still keep a 97%retention of the initial specific capacitance at 3 A g^-1after 10000 charge-discharge cycles and its coulombic efficiency is nearly 100%.