Preparation And Supercapacitor Properties Of Heteroatom-doped Porous Carbon Materials Derived From L-Tyrosine Based Resin

As energy storage devices,Supercapacitors have received widespread attention in recent years due to the advantages of high-power density,fast charge-discharge speed and long cycle life,etc.The design and development of electrode materials is the research focus of supercapacitors.Among them,porous carbon materials are the most widely used electrode materials for supercapacitors.Organic polymer is a common polymer material in daily life.In recent years,it has become a common precursor for the preparation of porous carbon electrode materials for supercapacitors,mainly due to its advantages of high carbon conversion rate,cheap price and controllable pore structure.Some effective strategies have been proposed to improve the electrochemical performance of organic polymer based porous carbon,such as changing the raw materials,improving the preparation process,optimizing the activation condition,and doping the heteroatom,which are generally used to increase the specific surface area,adjust the pore size distribution,introduce surface functional groups,and improve conductivity.In this paper,the organic xerogel has been obtained from the polymerization reaction of tyrosine and pyrrole-2-carbaldehyde with phosphoric acid,and boric acid as the catalyst and doping source.After using ambient pressure drying,N/P and N/B doped porous carbon materials have been prepared through the carbonization and activation of organic xerogels.The structure and composition of the electrode material were characterized,the electrochemical performance was tested,and the preparation conditions were optimized.The main research content in this paper is as follows:（1）The organic xerogels were obtained from the polymerization reaction of tyrosine and pyrrole-2-carbaldehyde by using phosphoric acid as polymerization catalyst and phosphorus source.After using routine pressure drying,N/P co-doped hierarchical porous carbon materials were prepared through the carbonization and activation of organic xerogels.The influence of the ratio of KOH and carbonization temperature on the morphology,composition,structure and the final capacitance performance were investigated.The result showed that the sample NPC-800-2 that prepared by using the KOH/carbon xerogel mass ratio of 2 and the activation temperature of 800℃exhibited excellent electrochemical performance.The NPC-800-2 electrode demonstrated a high specific capacitance of 302 F g^-1 at 0.5 A g^-1and good rate capability with capacitance remaining of 214 F g^-1 at 20 A g^-1 in the KOH electrolyte.The maximum area capacitance was 14.38 F cm^-2,and the mass specific capacitance reached to 240 F g^-1 with 60 mg loaded.The NPC-800-2 electrode also displayed excellent cycling stability with no attenuation in the specific capacitance even after 20,000 cycles at 4 A g^-1.Moreover,the fabricated symmetric supercapacitor displayed a remarkable maximum energy density of 21.51 Wh kg^-1 at a power density of250 W kg^-1 in the Na₂SO₄ electrolyte,ultralong cycling life with no attenuation in the specific capacitance even after 100,000 cycles at 4 A g^-1 in the KOH electrolyte.The excellent electrochemical performances of the NPC-800-2 electrode could be mainly attributed to the characteristics of the large specific surface area(1877 m² g^-1),hierarchical structure with interconnected porous networks,and the N/P-doping（the doping amount of N and P is 1.21 at%and 1.56 at%,respectively）.（2）N/B co-doped porous carbon with rich micropore were prepared though one-step carbonization-activation of organic xerogels.The ratio of boric acid and the temperature of carbonization-activation were optimized.The result showed that the sample NBC-700-1 that prepared by using the boric acid/tyrosine mass ratio of 1,and the carbonization-activation temperature of 700℃had the characteristics of high SSA(1721 m² g^-1),rich micropore structure,and N content of 4.36 at%and B content of 0.24at%.These advantages made NBC-700-1 have excellent electrochemical performance.The NBC-700-1 electrode demonstrated a high specific capacitance of 377 F g^-1 at 0.5 A g^-1 and excellent rate capability with 73%capacitance retention at 20 A g^-1 in the KOH electrolyte.When 60 mg active material was loaded,the mass specific capacitance of the prepared electrode could reach to 292 F g^-1,and the area capacitance was up to17.53 F cm^-2.The fabricated symmetric supercapacitor delivered an excellent cycling life with no attenuation in the specific capacitance even after 100,000 cycles at 5 A g^-1.Moreover,a remarkable maximum energy density of 18.4 Wh kg^-1 at a power density of200.5 W kg^-1 was obtained in the Na₂SO₄ electrolyte.