Facile Synthesis Of Novel Heteroatom-doped Porous Carbon And Its Composites For High-performance Solid-state Supercapacitors

Carbon-based materials have attracted many attentions as electrode materials for supercapacitors.To date,significant progress has been made in the synthesis of different carbon-based electrodes.However,carbon-based supercapacitors always have a low energy density due to the limited ion accessible area and relatively high equivalent series resistance.Moreover,some templates or structure-directed agents are often employed in their preparation.It not only boosts their production cost but also increases the complexity of the need for the template removal step,which restricts the further large-scale application of most carbon-based electrode materials.In addition,chitosan and gelatin,cost-effective and environmentally friendly natural biopolymers with abundant N-containing groups,are considered as a sustainable precursor for the self-doped carbon framework.This research work mainly focuses on how to prepare heteroatom-doped hierarchical porous carbon and its composites through simple and effective synthesis methods,and improving the electrochemical performances of carbon-based electrodes by various methods,including constructing hierarchical porous structures,doping heteroatoms,increasing the degree of graphitization and combining with metal oxides.In this paper,some novel synthesis strategies are proposed to simplify the synthetic procedures and reduce the production cost,which are expected to achieve large-scale production.And their formation mechanisms and energy-storage mechanism are also investigated.Additionally,the according solid-state supercapacitor devices are assembled and achieve high power densities and high energy densities.The main contents of the dissertation are as follows:1.Conventionally,porous carbons are prepared by a two-step process,including carbonization and activation of carbon precursors,which give rise to high energy consumption and tedious procedures.Inspired by the“bread leavening”,an activator-assisted pyrolysis strategy was developed to prepare porous carbon materials in this work.Chitosan was selected as a nitrogen-containing carbon precursor.And the self-N-doped hierarchical porous carbon foams were prepared by the one-step carbonization of activator-embedded chitosan aerogels.Moreover,the activation effects of different types and ratios of activators on the chitosan-derived carbon materials were discussed,including the effects on micro-morphologies,specific surface areas,pore structures and electrochemical performances.The optimized HPCF has a large specific surface areas(1916.6 m² g^-1)and exhibits a high specific capacitance of 304 F g^-1 at 1 A g^-1,as well as good rate performance(235 F g^-1 at 20A g^-1)and cycling stability(96%of capacitance retention after 10000 cycles).The as-assembled solid-state EDLC device can power a red LED efficiently..2.As is known,the high carbonization temperature can improve the graphitization degree which is related to conductivity.And the good conductivity is the basis of the electrode materials.However,with the carbonization temperature of increased,heteroatoms doped in the carbon materials,such as nitrogen and sulfur,will be greatly lost.Therefore,on the basis of the first work,N,S co-doped grapheme-enhanced hierarchical porous carbon foam(N,S-GHPCF)was prepared via a facile and low-cost one-step method.This novel strategy not only enables the synchronous carbonization and activation via a one-step high-temperature annealing process,but also simultaneously dopes nitrogen and sulfur as well as graphene into the hierarchical porous carbon to improve its electrochemical performance.Specifically,the thiourea is not only the dopant of nitrogen and sulfur but also the further activator to achieve an ultrahigh specific surface area.Moreover,the introduction of graphene without the agglomeration and restacking effectively enhance the electrical conductivity of the carbonaceous matrix,and also play a crucial role in the formation of hierarchical porosity,especially the mesopores,via the synergistic effect with the activating agents.The resultant uniform doped nitrogen and sulfur can promote the wettability of carbons and cause defects in carbon lattice to increase the active sites,as well as contribute to additional pseudocapacitance.As expected,the N,S-GHPCF exhibited a high specific capacitance(405 F g^-1 at 1 A g^-1),excellent rate capability(295 F g^-1 at 100 A g^-1)and outstanding cycle performance(98.8%of capacitance retention after 10000 cycles).In addition,the assembled symmetric devices deliver a high energy density of 18.4 W h kg^-1 at a power density of 300 W kg^-1 and have long cycle life,implying the potential application of N,S-GHPCF in high-performance energy storage devices.3.Graphene oxide is easy to agglomeration and stacking due to the strong intermolecular force,and its preparation process is complicated and high-cost.Moreover,the pseudocapacitance contributed by the doped heteroatoms in the alkaline electrolyte needs to be further improved.Therefore,a new N,B co-doped hierarchical porous graphitic carbon(N,B-HPGC)was prepared by carbonization of N-containing gelatin in the presence of K₃Fe(C₂O₄)₃ and H₃BO₃.The hydrosoluble K₃Fe(C₂O₄)₃is selected as both the activating agent(K₂C₂O₄)and graphitizing catalyst(Fe)to realize simultaneous activation and graphitization.And H₃BO₃ is employed as both boron source for B doping and the template for improving the hierarchical porosity.N as the electron-donor and B as the hole-donor can change the electronic structure and density of states of C.N,B co-doping not only produce additional surface defects on carbon skeleton,but also increase the surface adsorption energy of the carbonaceous material,which can promote the Faradaic reaction in alkaline electrolyte.The as-prepared N,B-HPGC with interconnective macro-.meso-,and micropores possess an high surface area(1742.4 m² g^-1),effective heteroatom(N and B)modification and enhanced electrical conductivity,which are beneficial for the fast ion/electron transport and charge storage.As expected,the N,B-HPGC not only exhibits the electric double layer capacitance characteristics,but also shows the obvious pseudocapacitance features.And the N,B-HPGC demonstrates excellent electrochemical performances in both the three-electrode system and the solid-state device.4.The composites based on graphitic carbon and transitional metal oxides are regarded as one of the most promising electrochemical materials owing to the synergistic combination of the advantages of both superior electrical conductivity and high pseudocapacitance.In this work,a simple one-pot template-free strategy for the preparation of three-dimensional hierarchical porous nitrogen-doped carbon framework in situ armored NiO nanograins(NCF/NiO)by an ammonia-induced method assisted by the pyrolysis of a decomposable salt is reported.Specifically,the pyrolysis of ammonium nitrate released abundant gas to form macropores,mesopores,and even micropores in the as-prepared 3D nanostructure by the ammonia induction,which enhanced the hierarchical porosity of the composites.And,the ammonia-induced method can not only efficiently yield plentiful anisotropic pores but also effectively promote the uniform formation of metal hydroxide nano-particles.When evaluated as the electrode in supercapacitors,the as-prepared NCF/NiO-2 hybrid exhibits a high specific capacitance(1074 F g^-1 at 1 A g^-1)and outstanding cycling performance(almost no decay after 5000 cycles at 5 A g^-1).Moreover,the as-assembled solid-state asymmetric supercapacitor can achieve a high cell potential of1.6 V and deliver a maximum energy density of 40.18 W h kg^-1at a power density of800 W kg^-1.The economical,facile,and green preparation method offers a new avenue for fabricating metal oxides/carbon composites with low cost and efficiency.