Application Of Microwave Technology In The Preparation Of Electrochemical Energy Storage Materials

Exploring efficient and low-cost method of preparing electrochemical energy storage materials with excellent performance has always been a hot spot in the field of electric energy storage.Compared with the traditional material fabrication method,microwave irradiation is considered as a potential one in material synthesis due to the advantages of convenience and high energy efficiency.Porous carbons with high specific surface area,good conductivity,and excellent physical and chemical stability are ideal electrode materials for electric double layer supercapacitors,while transition metal phosphates owing to their rich valence states and active redox reactions,are great electrode materials of pseudo capacitors with excellent performance.This paper reports low-cost biomass-based porous carbon and transition metal phosphates fabricated by fast microwave irradiation,and studies the effects of microwave irradiation on the morphologies,specific surface area,pore size and electrochemical properties of porous carbons and transition metal phosphates.At the same time,systematically evaluating the electrochemical performance of assemble symmetric and asymmetric supercapacitors based on the electrode materials of biomass-based porous carbons and transition metal phosphates(1)An N and P co-doped mesoporous carbon is prepared in air by energy-efficient microwave irradiation using chitosan as precursor and nitrogen source,phosphoric acid as phosphorus source and microwave absorber.As-prepared mesoporous carbon exhibits ideal specific surface area(1203 m2 g-1)and good conductivity,and surface doping content of N and P elements reaches 3.82%and 0.99%,respectively.When used as an electrode material of supercapacitor,the obtained material shows good capacitance performance.Under current density of 0.5 A g-1,the specific capacitance reaches 248.4 F g-1,which is much higher than the control sample prepared by the traditional heating method(205.9 F g-1).The assembled symmetrical supercapacitor device based on N and P co-doped mesoporous carbon exhibits excellent rate and cycle performance.The capacitance retention rate reaches 88.4%even under high current density of 20 A g-1 and its capacitance only decreases by 1.5%after 10000 charge/discharge cycles.(2)An N and P co-doped microporous carbon is facilely fabricated by a strong polar nonaqueous solvent-assisted microwave method using chitosan as precursor and nitrogen source,polyphosphoric acid(PPA)as microwave absorber,activator and phosphorus source.Compared with traditional heating method,the reported material fabricated by microwave irradiation exhibits much higher specific surface area(2204 m2 g-1)and higher large micropore(1 nm?d?2 nm)proportion,larger charge storage capability and stronger conductivity also are procured through microwave irradiation.The specific capacitance reaches up to 317 F g-1 in 6M KOH solution under current density of 0.5 A g-1,which is about 1.33 times higher than that of control sample fabricated by traditional heating method.The symmetrical supercapacitor device based on the reported material exhibits excellent rate and cycle performance,whose capacitance retention arrives 88%even under high current density of 20 A g-1,and more than 95%of initial capacitance is retained after 10000 cycles.Meanwhile,when ionic liquid(EMIMBF4)is employed as electrolyte,the energy density of the supercapacitor device reaches up to 48.6 Wh kg-1 at a power density of 468.8 W kg-1.(3)NiHPO3·H2O is prepared in air by low-temperature microwave technology using anhydrous ethanol as solvent,nickel acetate and tricresol phosphate as raw materials.The as-prepared NiHPO3·H2O is constructed by micron-level irregular blocks and aggregation of nanoparticles with diameter of no more than 200 nm,the material exhibits specific surface area of 46.2 m2 g-1.At current density of 1 A g-1,the supercapacitor based on the obtained NiHPO3·H2O material presents excellent specific capacitance of 905.8 F g-1,and its capacitance retention rate arrives 73.4%after 1000 charge/discharge cycles.Using this material as a positive electrode,and N and P co-doped microporous carbon as negative electrode,the assembled hybrid supercapacitor device shows specific capacitance of 101.5 F g-1 at current density of 1 A g-1.At the power density of 955.3 W kg-1,the energy density of assembled asymmetric supercapacitor is 36.1 Wh kg-1,and its energy density still reaches 22.2 Wh kg-1 under the high power density of 8988.8 W kg-1.