The Design And Synthesis Of Heteroatom-doped Carbon-based Electrode Materials And The Study On Their Electrochemical Performance

With the sharp reserve decrease of fossil fuels as well as the continuous global demand of energy,increasing attention has been paid to the development of renewable energy carriers and the construction of effective energy conversion/storage devices.Carbon based nanomaterials are widely used in the energy conversion and storage field due to their low cost,abundant reserves and stable chemical properties.However,the carbon based electrochemical devices usually show low capacitance or poor electrocatalytic activity because of the inert instinct and limited physical properties of the carbon materials.Thus,the exploration of the novel carbon based materials with unique interface properties and electronic structure is of great significance to the studies on energy conversion and storage processes.Herein,aiming at remedying the shortage of the conventional carbon materials,a novel strategy based on in-situ impregnation/carbonization of polymer precursor is proposed for the synthesis of heteroatom doped carbon-based electrode materials.And the effect of heteroatom species or contents on carbon matrix,such as the surface and electronic properties,has been further studied.In addition,the prepared heteroatom doped carbon materials have been utilized as electrode materials for electrochemical energy storage/conversion systems,such as supercapacitors,hydrogen evolution reaction(HER),and lithium-sulfur batteries.These studies are foundational for a better understanding on correlation between heteroatom doping and electrochemical performance of the carbon based electrode materials.The main contents of this thesis can be summarized as three parts:(1)Nitrogen and oxygen co-doped hierarchical porous carbon with high faradaic activity for supercapacitors.The low specific capacitance of traditional porous carbon limits their further utilization as electrode materials for supercapacitors.Herein,by a facile method of in-situ impregnation-carbonization process,the design on nitrogen and oxygen co-doped porous carbon based supercapacitor materials with high faradaic activity derived from quinone-amine polymer(PAQ)has been proposed.According to the XPS and element analysis results,the total heteroatom content of the prepared material achieves 12.26 at.%and these doped heteroatom containing functional groups contribute pseudocapacitance to the carbon skeleton.Serving as a symmetrical supercapacitor electrode material in aqueous electrolyte,the nitrogen and oxygen co-doped porous carbon material yields excellent performance to deliver a high specific capacitance of 321 F g-1 at 1 A g-1,a superior energy density of 15.91 Wh kg-1 at the power density of 0.4 kW kg-1,and excellent long-term stability of remaining 98%capacitance after 15000 cycles at 5 A g-1.(2)Nitrogen and oxygen co-doped carbon encapsulating nickel nanoparticle as an effective and durable electrocatalyst for hydrogen evolution reaction.To enhance the poor electrocatalytic activities of the carbon electrocatalysts,a nitrogen and oxygen co-doped carbon encapsulating nickel nanoparticle composite with unique coated architecture and electronic structure has been designed and prepared for the electrocatalysis of HER.Based on the Mott-Schottky heterojunctions structure,the charge transfer between heteroatom doped carbon and Ni nanoparticle that increases the electron density of the carbon shell is significant to activate the inert carbon material,achieve a suitable adsorption to the intermediate state H*,and endow the proposed electrocatalyst with an intrinsic enhancement in the HER.Furthermore,the coated N,O co-doped carbon layer increases the specific surface area of the catalyst,provides more active sites for the electrocatalytic reaction,enhances the protection of the metallic nickel,and ensures the stability of the electrocatalyst.Serving as a HER electrocatalyst,the N,O co-doped carbon coated nickel shows excellent performance to deliver the overpotentials of 145 mV vs.RHE and 157 mV vs.RHE at a current density of 10 mA cm-2 in acid and alkaline electrolyte,respectively.In addition,the catalytic activity could remain essentially unchanged in both acidic and basic media at a current density of 20 mA cm-2 for 14 h,which exhibits much better stabilities in comparison with the commercial Pt/C catalyst and suggests its great potential for practical application.(3)Sulfur@nitrogen-doped porous carbon as cathode electrode for lithium-sulfur batteries.Lithium-sulfur batteries suffer from limited utilization of active material,low Coulomb efficiency and long-term stability due to the poor conductivity of sulfur and dissolution of polysulfide intermediates.Here,a nitrogen-doped porous carbon material has been prepared using the in-situ generated nickel nanoparticles as template under a relatively low carbonization temperature via the impregnation/carbonization strategy.The sulfur@nitrogen-doped porous carbon composite used for the cathode electrode of Li-S battery has been obtained through the high temperature melting method with the sulfur loading amount of 66.7 wt.%.With the connected hierarchical porous carbon network,the sulfur is confined within the architecture of the carbon based support,relieving the volume expansion of the electrode material during charging and discharging.Moreover,the doped nitrogen species could effectively enhance the chemical adsorption of carbon support to sulfur or polysulfide,which decreases the loss of active material and ensures the efficient electrochemical processes of the batteries.Serving as cathode electrode for the Li-S batteries,the prepared sulfur@nitrogen-doped porous carbon composite exhibits excellent rate performance and cycle stability to deliver an initial capacity of 1344 mAh g-1 at 0.2 C that remains 1100 mAh g-1 after 300 cycles tests and an initial capacity reached 836 mAh g-1 at 4.0 C that remains 473 mAh g-1 after 1000 cycles tests with a wonderful retention rate of 99.94%for each cycle.