Preparation And Supercapacitor Properties Of L-Tyrosine-Based Deep Eutectic Solvent-Derived Carbon Materials

The effect of pure carbon as a supercapacitor electrode material has been proved unsatisfactory because the electric double layer energy storage mechanism results in low specific capacitance and energy density which greatly limits the application of carbon electrode materials in supercapacitors.Several functional modification strategies are considered to be effective methods to regulate the pore structure,specific surface area,surface chemical state and composition,and conductivity,and ultimately improve the electrochemical performance of carbon materials,such as optimizing structure,heteroatom doping and compositing.As a new type of green solvent discovered at the beginning of this century,deep eutectic solvent（DES）has many advantages,such as negligible vapor pressure,rich and cheap raw material,biodegradability,non-toxicity and recycle.In recent years,the use of DES for the preparation of hierarchical pore carbon materials has become a new research direction.Based on this,in this thesis,a series of DES-derived doped porous carbon electrode materials were prepared with L-Tyrosine as the hydrogen bond donor of DES,carbon source and hetero atom source,ZnCl₂as the hydrogen bond acceptor of DES and catalyst activator.The effects of raw material ratio and carbonization temperature on the morphology,structural and composition,and final capacitance performance of the material were discussed in detail.The main research contents are as follows:1.In order to test the effect of ZnCl₂ as a hydrogen bond acceptor,we first selected the phenolic polymerization system to prepare DES-derived porous carbon materials.Firstly,we used phloroglyphenol as a hydrogen bond donor and ZnCl₂ to form DES.Formaldehyde was added to the DES and the phenolic resin precursor was then obtained by condensation polymerization reaction with phosphoric acid as the catalyst and heteroatom source.Finally,the P-doped porous carbon materials were prepared through two steps of high-temperature carbonization activation.The effects of carbonization temperature,KOH activation ratio and activation temperature on the morphology,composition and structure,and final capacitance of the materials were investigated in detail.The experimental results showed that when the carbonization temperature was 800℃,the mass ratio of KOH to carbon was 2:1,and the activation temperature was 800℃,the sample PPC-800-2 exhibited the largest specific capacitance(406.2 F g^-1 at 5 mV s^-1),better rate performance(65.8%at 500 mV s^-1),large area specific capacitance(11.35 F cm^-2)and excellent cycle stability（100,000 cycles of 100%capacitance retention）.The excellent capacitance performance could be attributed to large specific surface area,hierarchical porous structure and P atoms doping of PPC-800-2.2.N-doped carbon materials were prepared by high-temperature carbonization of the DES composed of L-Tyrosine as hydrogen bond donor and ZnCl₂ as hydrogen bond acceptor.The effects of the ratio of L-Tyrosine and ZnCl₂ and the carbonation temperature on the morphology,composition and structure,and capacitance of the material were investigated in detail.The experimental results show that when the ratio of L-Tyrosine to ZnCl₂ was 1:2.5 and the carbonization temperature was 800℃,the sample NCs-800-2.5 displayed high specific capacitance(226.9 F g^-1 at 0.5 A g^-1),better rate performance(57.4%at 100 A g^-1)and cycle stability（10,000 cycles of capacitor retention of 80%）.This could be attributed to larger specific surface area(1073 m² g^-1),a suitable pore size distribution and contribution derived from doped N（1.84%）and O（7.4%）of NCs-800-2.5.3.Although the L-Tyrosine-ZnCl₂ type DES-derived carbon material has a large specific surface area,N and O atoms doping content are low.In addition,the specific capacitance and cycle stability of the material are not satisfactory.Therefore,in order to further improve the capacitance performance of the material,based on the second part of the research work,urea with high nitrogen content was introduced as a hydrogen bond donor of DES,and as a carbon and nitrogen source.L-Tyrosine-Urea-ZnCl₂ type DES were formed at first and then doped carbon material with high nitrogen doping amount were obtained through high-temperature carbonization of DES.The effects of the mass ratio of L-Tyrosine,urea and ZnCl₂ and the carbonization temperature on the morphology,composition and structure,and capacitance of nitrogen-doped carbon materials were investigated in detail.The results showed that when the mass ratio of L-Tyrosine,urea and ZnCl₂ was 1:2:5 and the carbonization temperature was 900℃,the sample NCs-900-2-5 exhibited superior specific capacitance(306.5 F g^-1 at 0.5 A g^-1),better rate performance(65.6%at 50 A g^-1)and the excellent cycle stability（50,000 cycles of capacitance without attenuation）.This could be attributed to large specific surface area(1621m² g^-1),suitable hierarchical porous structure and high N（6.95%）,O（10.57%）atom doping amount of NCs-900-2-5.4.It is a common method for preparing polysaccharide-based porous carbon materials by firstly preparing aerogels and then high-temparature treatment.However,gel precursors often require freeze-drying or supercritical drying techniques and the drying technology is required to be high.In this part,we added L-Tyrosine-ZnCl₂ type DES to the common polysaccharide-agar which was easy to gel.The precursor was prepared via conventional drying of hydrogel.Finally,Agar-based nitrogen-doped porous carbon was obtained by one-step high-temperature carbonization activation of the precursor.The effects of the amount of activator and carbonization temperature on the morphology,composition and structure and the final capacitance performance of the material were investigated in detail.The experimental results showed that when the activation ratio of KOH to agar gel was 1:3 and a carbonization temperature of 800℃,the sample Agar-NC_800-3 exhibited remarkable specific capacitance(320.7 F g^-1 at 0.5 A g^-1),good rate performance(69%at 20 A g^-1),excellent cycle stability（10,000 cycles capacitance retention of 100%）and high energy density(23.8 Wh kg^-1).This could be attributed to the large specific surface area,porous structure and N,O doping of Agar-NC800-3.