The Synthesis And Structure Control Of Porous Carbons For High-Performance Supercapacitors

The porous carbons are the most popular electrode materials for supercapacitors due to the high surface area,appropriate porous structure,controllable surface chemistry and electrochemical stability.To increase the energy and power density of porous carbons,the popular methods are the introduction of mesopores and nitrogen.This thesis mainly focused on the design of the electrode materials and the composition of the electrolytes,to develop low-cost,green and efficient hierarchically porous carbon electrode material.The effect of the nitrogen sources on the physical/chemical properties and electrochemical performances of nitrogen-doped porous carbons as well as the composition of electrolyte solute and solvent on the electrochemical performance of supercapacitor were detailedly studied.The main results of this thesis are summarized as follows:?1?The effect of nitrogen precursors on the electrochemical performance of nitrogen doped porous carbon.Nitrogen doped porous carbons have been synthesized via a hydrolysis-crosslinking-carbonization process,using four nitrogen precursors to investigate the effect of nitrogen precursors on the electrochemical performance.The nitrogen exists as pyridinic N,pyrrolic N and graphitic N in carbon material.The pyridinic N which decided by the amount of C=N in precursor could contribute most to the electrochemical performance because the pyridinic N could serve as an electron donor in the lattice to improve the carbon wettability,surface polarity,adsorptive ability and conductivity.The nitrogen doped carbon?CM?prepared with melamine as nitrogen precursor owns favourable electrochemical performance with specific cell capcitances of 44.4,41.5 and 37.3 F g^-1 in 3 M H2S04,6 M KOH and 1M Et₄NBF₄/PC respectively.?2?Hierarchical porous carbons with high surface area were prepared by direct carbonization of the polymers which were synthesized via a typical sol-gel method,using terephthalaldehyde and resorcinol as carbon precursors,and metal chloride as reaction assistant.Such hierarchical porosity plays an important role for rapid ion diffusion,resulting in excellent rate capability and low diffusion resistance.Moreover,it should be noted that ZnCl₂ could also act as in-situ activation agent during the carbonization process to generate large surface area of 1106 m²·g^-1 and pore volume of 1.2 cm3·g^-1 In 3 M H₂SO₄,high specific capacitance of 174.6 F·g^-1 by using the ZnCl₂-assited hierarchical porous carbon as electrode material is obtained and it could retain ca.84%when the current density increases from 0.1 A·g^-1 to 20 A·g^-1.The facile production and excellent electrochemical performance suggested an effictive method to prepare hierarchical porous carbon for energy storage.?3?Template-free synthesis of nitrogen-doped hierarchical porous carbons for supercapacitor electrodes.Nitrogen-doped hierarchical porous carbons?NHPCs?with controllable nitrogen content were prepared via a template-free method by direct carbonization of melamine-resorcinol-terephthaldehyde networks.The specific surface area increases with the nitrogen content increasing with the largest specific surface area of 1150 m²·g^-1.Nitrogen doped in the porous carbon hosts has also been proved to be an effective approach to improve the electrochemical performance by increasing wettability,adding faraday pseudocapacitive reaction and enhancing electric conductivity.NHPC3 with suitable nitrogen content of 10.6 wt.%delivers high specific capacitance of 214.0 F·g^-1 and keeps 134.5 F·g^-1 when the current density increases from 0.1 to 20 A·g^-1 in 3 M H₂SO₄.Furthermore,the capacitance could retain 96.5%after 5000 cycles.The appropriate nitrogen doping and well-developed hierarchical porosity could increase capacitance and accelerate the ion diffusion for high energy and power density.?4?The effect of composition on electrolyte properties and electrochemical performance of super capacitors.When the solute of electrolyte is SBPBF₄,the viscosity is lower as well as the conductivity is higher than that of Et₄NBF₄/PC.The molecular structures of the two kinds of salt were simulated with materials studio software.The consequences indicated that the ionic radius of SBP+was smaller than Et4N+,leading easier adsorption and desorption of the ions on the surface of activated carbon.At-20 °C,the specific capacitance using SBPBF₄ as electrolyte could reach 24.7 F·g^-1 and the capaicance could retain 84%after 5000 cycles.The relaxation time??0?is 71.5 s,indicating the ions could diffuse fast in electrode.The solvent of electrolyte should own low viscosity to accelerate ion diffusion and high dielectric constant to promote ionization,as well as,low melting point and high boiling point to broaden the operating temperature range.The addition of dimethyl carbonate can obviously reduce the viscosity and improve the conductivity of the electrolyte.When the ratio of PC and DMC is 7:3,the supercapacitor shows the best performance.At-20?,when the current is 2 A·g^-1,the specific capacitance could increase to 26.7 F·g^-1,the relaxation time decease to 32.6 s and the capaicance could retain 89%after 5000 cycles.