Preparation Of Graded Porous Carbon And Its Electrochemical Energy Storage Performance

Porous carbon is widely used in electrical double-layer capacitors because of their low cost,tunable pore size distribution,good electrical conductivity,well-developed morphologies as well as better chemical stability.Micropores provide high specific surface area but have a low utilization rate and poor rate capability.Mesopores have a high surface utilization rate but low specific surface area.Macropores have limited specific surface area but it can serve as ion-buffering reservoirs which effectively reduce diffusion distance of ions.Design and fabrication of hierarchical porous structure can obtain excellent electrochemical performance.This paper mainly describes the preparation of hierarchical porous carbon,and the electrochemical properties are further investigated.The main progresses are summarized as follows:(1)Hierarchical porous carbon has been synthesized through the one-step carbonization-activation strategy with ferric citrate as template and carbon precursor,potassium hydroxide(KOH)as activation agent.When the mass ratio between KOH and residual carbon derived from ferric citrate is 2:1,the obtainedmicro-mesoporous carbon has high specific surface area(2055 m2/g),large pore volume(2.527 cm/g),and high mesopore volume proportion(95.1%).A high specific capacitance of 289 F/g is achieved at a current density of 0.2 A/g.Even at high current density of 20 A/g,the specific capacitance is still as high as 215 F/g.The capacitance retention is 74.4%,showing an excellent rate performance.(2)Using rapid heating and cooling process,hierarchical porous carbon has been prepared via the one-step template carbonization method with cobalt acetate as the template,citric acid as the carbon precursor.The as-prepared porous carbon hashigh specific surface area(753?890 m/g)and narrow pore size distribution that micropores and mesopores peak at 0.8 nm and 4 nm,respectively.The specific capacitance and rate performance are improving with the carbonization temperature increasing.The obtained porous carbon displays superior rate capability with specific capacitance of 102 F/g at 5 A/g,71%of its capacitance(144 F/g)at 0.2 A/g.(3)Nitrogen-doped hierarchical porous carbon is synthesized via one-pot carbonization of a mixture containing potassium citrate and ammonium chloride(NH4Cl).potassium citrate provides carbon source,while ammonium chloride acts as nitrogen dopant.The as-prepared porous carbon has high N content between 3.73?6.18%.Results shows the N-doped carbon with developed micropores with specific surface area(453?632 m/g),pore volume(0.327?0.457 cm/g).When the mass ratio of NH4Cl and potassium citrate is 1:2,the as-prepared N-doped carbon exhibits superior electrochemical properties in three-electrode tests.The N-doped carbon reaches 202 F/g at 0.2 A/g,and specific capacitance remains 121 F/g at 10 A/g with capacitance retention of 60%.The preparation of three hierarchical porous carbons in this paper is facile and operational easily.The as-obtained porous carbons with excellent electrochemical performance are promising electrode materials in the future.