Preparation And Supercapacitive Capabilities Of Heteroatom-Doped Porous Carbon Materials

Porous carbon materials,by virtue of high specific surface area,high electrical conductivity,high physico-chemical stability and rich sources,have become the first choice for supercapcitor electrodes.However,conventional porous carbon materials usually suffer from low specific capacitance,heteroatom-doping(e.g.N,O,B,S,etc.),pore engineering and morphology control of scuh materials can significantly enhance their supercapacitive performances.In this thesis,several heteroatom-doped porous carbon materials(HDCMs)have been successfully synthesized by using low-cost and heteroatom-enriched precursors that ranged from biomasses to small organic molecule and covalent organic frameworks.The supercapacitive performances of these HDCMs acting as advanced supercapacitor electrodes have also been investigated based on their compositions and structures as well as morphology control.The main research works include:(1)Preparation of O-doped porous carbons with outstanding supercapacitance performance.O-doped porous carbons have been synthesized from the lotus seedpods(LS)precursor by hydrothermal treatment of the precursor in KOH solution followed by simultaneous pyrolysis and activation.The porous carbon prepared under optimal conditions(LSC-800)exhibited an ultrahigh O content(31.06 wt%),a relatively high specific surface area(1813 m² g^-1),a high total pore volume(1.05 cm³g^-1)and an appropriate mesopore-size(2?5 nm)that facitiates for rapid transport of electrolyte ions.The LSC-800 electrode exhibited ultrahigh capacitive performance(431 F g^-1 at0.2 A g^-1),good rate capability(75.5%capacity retention at 20 A g^-1)as well as high stability(about 95.4%of capacitance retention after 10,000 cycles at 2 A g^-1)in 6 MKOH electrolyte.(2)Oxygen and nitrogen co-doped porous carbon nanosheets for high volumetric performance supercapacitors.O/N-co-doped porous carbon nanosheets have been prepared by the direct pyrolysis of perilla frutescens(PF)leaves.Under the optimum pyrolysis temperature(700oC),the PF-derived carbon nanosheets(PFC-700)having O,N contents of 18.76 at%and 1.70 at%,respectively,exhibited a hierarchical pore structure with a specific surface area(655 m² g^-1)and a relatively low pore volume(0.44 cm³ g^-1).Owing to these merits,PFC-700 displayed both high gravimetric capacitance(270 F g^-1 at 0.5 A g^-1)and high volumetric capacitance(287 F cm^-3 at0.5 A g^-1).In addition,the PFC-700-based symmetric supercapacitor offered a high volumetric energy density(14.8 Wh L^-1 at 490 W L^-1)together with a high stability(about 96.1%of capacitance retention after 10,000 cycles at 2.0 A g^-1).(3)MnO₂ nanostructures deposited on O/N-co-doped graphene-like porous carbon nanosheets for high-energy density supercapacitors.Firstly,the salvia splendens-derived,O/N-codoped graphene-like carbon nanosheets(GPCN-SS)were fabricated by the combination of salt sealing and pyrolysis.The MnO₂/GPCN-SS composite was then fabricated by the deposition of MnO₂ nanostructures on the GPCN-SS surface under hydrothermal conditions.The as-fabricated MnO₂/GPCN-SS composite possessed a relatively high specific surface area(483 m² g^-1)and an appropriate mesopore size(2?5 nm).Thanks to the synergistic effect of GPCN-SS and MnO₂ nanostructures,the MnO₂/GPCN-SS composite electrode exhibited high specific capacitance of 438 F g^-1 at 0.5 A g^-1(almost twice as high as the pristine GPCN-SS)and high rate capability(67.8%capacity retention at 50 A g^-1)in Na₂SO₄electrolyte.More importantly,an asymmetric supercapacitor assembled with MnO₂/GPCN-SS composite cathode and GPCN-SS anode in neutral electrolyte displayed excellent rate capability(77.8%capacity retention from 0.5 to 50 A g^-1)and high energy density(50.2 Wh kg^-1 at 516 W kg^-1).(4)B/N-co-doped graphene-like porous carbon nanosheets for high-rate flexible solid-state supercapacitors.B/N-codoped graphene-like carbon nanosheets have been successfully fabricated via simply pyrolyzing a mixture of tri(beta-chloroethyl)amine hydrochloride(CAH)and ammonium borate.In this synthesis,CAH serves as carbon precursor and N source,the ammonium borate serves as both 2D template and B/N sources.The as-fabricated B/N-codoped,graphene-like porous carbon nanosheets(BN-GPCN-850)possessed a graphene-like sheet structure,a relatively high specific surface area(833 cm³ g^-1),a hierarchically porous structure,together with a high N/B contents(9.61 at%N and 3.92 at%B).The BN-GPCN-850 flexible film served as binder-free electrode displayed a high specific capacitance of 361 F g^-1 at 0.5 A g^-1,an high-rate capability(83.1%of capacitance retention at 1?100 A g^-1)in 6.0 MKOH electrolyte.The BN-GPCN-850 film-based flexible solid-state supercapacitors exhibited a high specific capacitance(281 F g^-1 at 1 A g^-1),an high-rate capability(78.3%of capacitance retention at 1?50 A g^-1)in PVA-KOH electrolyte and a high energy density(26.4 Wh kg^-1)together with a long cycle-life(91.3%of capacitance retention after 10,000 cycles at 5 A g^-1)in PVA-Na₂SO₄ electrolyte.(5)O/N co-doped,layered porous carbon with mesoporosity up to 99%for ultrahigh-rate capability supercapacitors.O/N-codoped mesoporous carbons have been successfully fabricated by one-step Na₂CO₃-activation of covalent triazine-based frameworks that derived from 2,4,6-tris(4-cyanophenoxy)-1,3,6-triazine monomer(CTFO)at 800?1000°C.The as-obtained O/N-codoped carbon(CTFC-900)with up to 99%mesoporosity possessed a layered structure together with a high specific surface area(2425 m² g^-1)and a high N/O contents(6.98 at%N and 10.32 at%O).Owing to these merits,the supercapacitor based on CTFC-900 electrode exhibited a high specific capacitance of 287 F g^-1 at 1.0 A g^-1 and an ultrahigh rate capability of66.1%at 1?1000 A g^-1 in alkaline electrolyte.Moreover,the CTFC-900-based supercapacitor in neutral electrolyte achieved both high energy density(32.2 Wh kg^-1)and high power density(187 kW kg^-1).