Controlled Nitrogen/Oxygen Heteroatom Configuration And Electrochemical Mechanisms Of Carbon Materials

Integration of heteroatoms（e.g.nitrogen,oxygen,sulphur,phosphorus,boron,etc.）into electrically conductive carbon materials to regulate their charge distribution and to tune surface wettability has been widely regarded as one of the most effective means to promote electrochemical performance of carbon materials.However,heteroatom-doped carbon materials are fabricated always involving inevitable high-temperature thermal treatment,bringing about an uncontrolled conversion of different heteroatom,which seriously limits accurate control of configuration,precise identification of electrochemical active sites,fundamental understanding of electrochemical mechanisms,and significant improvement of electrochemical performance of heteroatom-doped carbon materials.Therefore,it becomes a key scientific issue to achieve precise regulation of heteroatom doping configurations in carbon materials for electrochemical applications.A strategy of low-temperature carbonization and subsequent alkaline-activation of phenolic resins was proposed to obtain single pyrrole nitrogen-doped carbon materials and hydroquinone/catechol-enriched carbon materials,achieving controllable doping of nitrogen and oxygen configurations in carbon materials.Furthermore,this work also studied the pseudocapacitive energy storage mechanism of pyrrole nitrogen configuration and hydroquinone/catechol groups in carbon material and the aerobically autoxidized self-chargeable mechanism derived from synergistic effect of pyrrolic nitrogen and hydroquinone/catechol configuration in carbon cathode material.The specific content is as follows:（1）The single pyrrolic-nitrogen-doped carbon material（SPNCMs）was synthesized based on a facile engineering process that combines the low-temperature carbonization and alkaline-activation of 3-halogenated phenol-3-aminophenol-formaldehyde obtain by one step hydrothermal method.In these processes,the low-temperature heat treatment effectively avoided uncontrolled conversion of nitrogen configuration,and the low-temperature dehalogenation effect facilitates the formation of conjugated planar between aromatic rings by removing hydrogen halide at low temperature to ensure enough graphitization of carbon materials.Based on the pseudocapacitance positively correlated to pyrrolic nitrogen content of carbon materials,it can be preliminarily inferred that the pyrrolic nitrogen configuration is the pseudocapacitance active site of nitrogen doped carbon electrode materials.The ex-situ XPS results of SPNCMs after charging and subsequent discharging further demonstrate that the pseudocapacitance mechanism of pyrrole nitrogen in carbon materials is the gradual reversible conversion between pyrrolic nitrogen and oxidized pyrrolic nitrogen during the charging and discharging process.Improtantly,a single pyrrolic-nitrogen-doped carbon material was synthesized for the first time in this work and the pseudocapacitance mechanism of pyrrolic nitrogen in acidic electrolytes in carbon materials was also confirmed.（2）The hierarchically porous N-doped carbon material with dominantly pyrrolic nitrogen was synthesized based on a facile engineering process that combines the low-temperature carbonization and alkaline-activation of hydrothermally synthesized3-fluorophenol-3-aminophenol-formaldehyde resin using a self-sacrificial SBA-15template.In addition,the combined effect of the self-sacrificial SBA-15 template and KOH-activation resulted in a high specific surface area(582 m² g^-1)and hierarchical microporous/mesoporous architecture,thereby maximising the exposure of electrochemically active sites.The electrochemical characteristics of hierarchically porous N-doped carbon materials were similar to those of SPNCMs,further confirming the pseudocapacitance mechanism of pyrrole nitrogen,and the introduction of mesoporous structure effectively improves the rate performance of electrode materials.（3）The oxygen-enriched carbon material was synthesized based on a facile method that combines the low-temperature carbonization and alkaline-activation of hydrothermally synthesized phenol-formaldehyde resin,successfully achieving controllable doping of hydroquinone/catechol groups.Based on aerobic autoxidation of hydroquinone/catechol groups,the aerobically autoxidized self-charge concept based on hydroquinone/catechol-enriched carbon cathode material was proposed.Meanwhile,through air oxidation self-charging performance and ex-situ XPS results of oxygen-enriched carbon material,the air oxidation self-charging mechanism was clarified as follows:the hydroquinone/catechol groups were firstly oxidized by environmental O₂ to para-quinone/ortho-quinone involving proton removal with an increased potential in the pristine oxygen-enriched carbon material in the air oxidation self-charging process,and the quinone groups electrochemically reduced to corresponding quinone groups involving proton introduction with a decreased potential in the subsequent galvanostatic discharge process.（4）The nitrogen and oxygen co-doped carbon material（N-5,O co-doped carbon）was synthesized based on a facile engineering process that combines the low-temperature carbonization and alkaline-activation of hydrothermally synthesized3-fluorophenol-3-aminophenol-formaldehyde resin obtain by one step hydrothermal,successfully achieving controllable doping of pyrrolic nitrogen and catechol groups.Furthermore,an aerobically autoxidized self-chargeable concept derived from synergistic effect of pyrrolic nitrogen and catechol configuration in carbon cathode material was proposed for the first time.Meanwhile,through oxidation self-charging performance and ex-situ XPS results of N-5,O co-doped carbon material,the air oxidation self-charging mechanism was clarified as follows:the pyrrolic nitrogen and catechol groups were firstly oxidized by environmental O₂ to oxidized pyrrolic nitrogen and ortho-quinone involving proton removal with an increased potential in the pristine oxygen-enriched carbon material in the air-breathing chemical self-charge process,and the oxidized pyrrolic nitrogen and quinone groups electrochemically reduced to pyrrolic nitrogen and catechol groups involving proton introduction with a decreased potential in the subsequent galvanostatic discharge process.Based on the synergistic effect of pyrrolic nitrogen and hydroxyl species with strong electron-donating effect to conjugated carbon-based backbone,N-5,O co-doped carbon exhibited the highest HOMO energy level（-5.12 e V）than those of O-enriched carbon and N-doped carbon,and were most susceptible to O₂ oxidation.