Design,microstructure And Electrochemical Properties Of Carbon-based Nanomaterials

The increasing use of fossil fuels has inevitably caused serious enegy crisis and environmental pollution issue.Therefore,it is very necerassry to develop new enery storage and conversion materials with excellent electrochemical properties.Among various energy materials,carbon-based nanomaterials have many advantages,such as wide source,diverse structure,low cost,high conductivity,adjustable structure,environmental friendliness,good selectivity and chemical stability and thus have been considered as one of the most promising candidates of sulfur host materials for lithium-sulfur batteries and electrocatalytic materials for nitrogen reduction reaction.As the promising host materials of sulfur for lithium-sulfur battery,carbon-based nanomaterials can effectively anchor sulfur and polysulfides,provide transport channels for electrons/ions,alleviate volume changes during charging/discharging and inhibit shuttle effect.On the other hand,at present,ammonia is mainly synthesisized industrially by Haber-Bosch process,which leads to huge energy cosumption and serious environmental pollution.The unique pore structure,high specific surface area and good conductivity of carbon-based nanomaterials are conducive to the adsorption and activation of nitrogen,effectively reducing the reaction barrier and promoting the reduction of nitrogen to ammonia.In this thesis,a series of heteroatom-doped carbon-based nanomaterials were systhesized and the structure,micromorphology and electrochemical properties of the materials used as cathode host materials for lithium-sulfur batteries and electrocatalysts for nitrogen reduction reaction were investigated in detail.The main contents and results are as follows:1.An in-situ N（1.67 at%）and O（12.1 at%）dual-doped porous carbon derived from rapeseed meal has been synthesized by KOH activation and subsequent carbonization at high temperatrue.The obtained material possesses abundant micropores and mesopores with a relatively high specific surface area of 2434.9 m²g^-1 and a large pore volume of 1.49 cm³ g^-1,contributing to physical and chemical dual-adsorption for polysulfides.The carbon material composited with sulfur delivers a high reversible capacity of 1259 mAh g^-1 at 0.1 C and exhibits excellent cycling stability(512 mAh g^-1 after 200 cycles at the areal mass loading of S of 2.4 mg cm^-2).2.Inspired by traditional Chinese steamed buns,a unique porous microcellular carbon composed of cross-linked nanopores has been synthesized by an eco-friendly biological fermentation using banana peel as a carbon precursor.The hierarchical carbon framework obtained under the aerobic respiration and anaerobic breathing of biological yeast during fermenting and simultaneously the inherent doping of N（3.28 at%）produce a promising carbon host material to stabilize the structure of electrodes and restrict the dissolution of polysulfides during charging and discharging.The amount of biological yeast has an important influence on the microstructure of the biomass carbons and the electrochemical properties of carbon/sulfur electrodes.The optimal amount of biological yeast is 3.0 wt%,where the carbon/sulfur composite electrode possesses the sulfur loading of 74.34 wt% and achieves a large initial reversible capacity of 1174 mAh g^-1 at 0.1 C and a high capacity hold of 58.35%after100 cycles.3.Sulfur-doped carbon nanosphere（S-CNS）was prepared via a hydrothermal reaction followed by Ar annealing.Heteroatom S can tailor the electronic structure of carbon atoms,which offers an effective method to promote the electrocatalytic performance.The S-CNS can acts as an efficient and stable nitrogen reduction reaction（NRR）catalyst for ambient N₂ to NH₃ conversion with excellent selectivity.In 0.1 M Na₂SO₄,the S-CNS achieves a large NH₃ yield of 19.07μg h^-1 mg_cat.^-1 and a high Faradic efficiency of 7.47% at–0.7 V versus reversible hydrogen electrode,much higher than those of undoped CNS(3.70μg h^-1 mg_cat.^-1,1.45%).Notably,this catalyst also demonstrates high electrochemical and structure stability.4.Sulfur-doped graphene（S-G）was prepared by high-temperature annealing of graphene oxide and benzyl disulfide under Ar atmosphere.The S-G is proposed as an efficient and stable electrocatalyst to drive the nitrogen reduction reaction（NRR）at ambient conditions.In 0.1 M HCl,such S-G attains a remarkably large NH₃ yield of27.3μg h^–1 mg_cat.^–1 and a high Faradic efficiency of 11.5% at–0.6 and–0.5 V vs.reversible hydrogen electrode,respectively,much higher than those of undoped G(6.25μg h^–1 mg _cat.^–1;0.52%).Density functional theory calculations reveal that carbon atoms close to substituted sulfur atoms are the underlying catalytic active sites for NRR on S-G,and related NRR mechanism is also explored.