Surface Functionalization Of Porous Carbon Materials And Their Electrochemical Properties

As a new type of energy storage device,supercapacitors have been widely studied due to their excellent power density,rapid charging-discharging rate and stable cycling performance.However,supercapacitors exhibit lower energy density compared to the conventional batteries,which greatly limits their further applications.Therefore,the reasonable design and controllable preparation of electrode materials with excellent electrochemical performance are the main direction of effort to greatly improve the energy density of supercapacitors.Carbon-based nanomaterials are widely used in the field of energy storage due to their abundant reserves,easy processability,operable porous structure and good electrical conductivity.Nevertheless,as the electrode materials of supercapacitors,the energy storage mechanism of nano-carbon materials is mainly based on the electric double layer theory,which always exhibits inferior capacitance.Currently,there are two ways to solve this problem,improving the electrical conductivity or optimizing the pore structure of carbon materials.It can not only improve the effective specific surface area and surface wettability,but also provide additional pseudocapacitance by faradic reaction with the method of heteroatom doping,chemical oxidation and surface grafting of carbon surface.From this point of view,carbon-based electrode materials were rationally designed and synthesized with three-dimensional network architecture through template method in this thesis.After that,the surface properties and electrochemical performances of the synthesized materials were further improved by heteroatom functionalization or activation.The structural design of carbon materials focused on restricting the active small molecules,which lays a foundation for the assembly of super capacitors with high energy density.The main research contents are as follows:(1)N-doped aligned carbon nanosheet network(N-ACN)with high specific surface area was prepared using MgO as the template and potassium citrate as both the carbon source and the activator.N-ACN is composed of interconnecting carbon nanosheets with numerous pores on the carbon surface,which exhibits continuous three-dimensional conductive network for the rapid transfer of electrons.Meanwhile,multilevel pore channels which are fabricated on the surface of nanosheets are beneficial for the fast ion diffusion.N-doping not only improves the wettability of materials,but also provides additional pseudocapacitance.As a result,the N-ACN10 exhibits a high specific capacitance of 331 F g-1 at 2 mV s-1,and a superior rate capability of 203 F g-1 at 5 V s-1.Furthermore,NACN//NACN was fabricated in neutral electrolyte,which delivers an energy density of 20.6 Wh kg-1 at a power density of 200 W kg-1 and remains 98%of capacitance retention after 10,000 cycles.(2)Pillared-porous carbon nanosheets architecture(PPCN-4)with high specific surface area was synthesized by the carbonization with magnesium oxide as template and mesophase asphalt as carbon source.Abundant oxygen functional groups was further introduced in carbon surface by chemical oxidation method,and a simple vacuum adsorption of electrochemical additive K3[Fe(CN)6]was used to fill small molecule into pillared-porous carbon with 20 nm thickness.Pillared-porous carbon nanosheets architecture modified by K3[Fe(CN)6](F-OPPCN)was obtained by stably anchoring carbonyl and carboxyl on the surface of carbon materials.The interlayer hole between the supporting carbon nanosheets can not only provide rapid transport channels for electrolyte ions diffusion,but also provide storage space for electrochemical additive.Thus,the K3[Fe(CN)6]can provide additional pseudocapacitance by rapid redox reaction.As a result,the specific capacitance of F-OPPCN is three times higher than that of PPCN.In addition,both the oxygen functional groups on the surface and the narrow interlayer spacing can effect together to form a chemical/physical dual constraint on electrochemical additive,which inhibits the loss of electrochemical active.The capacitance retention rate of as-assembled asymmetric supercapacitors based on F-OPPCN can still maintain 91%after 10,000 cycles.(3)Bubble-like porous carbon(BC-0.5)with high oxygen content was synthesized by SiO2 as the template and manganese-oleate complex as carbon source.After the removal of SiO2,numerous mesopores were formed inside the material,which can act as reservoirs to store electrochemical additive.By filling the electrochemical additive small molecule phenylenediamine into the bubble reservoir of porous carbon,a covalent bond can be formed between oxygen functional groups on the surface of BC and PPD(PPD-BC).P-phenylenediamine acts as a stable pseudocapacitance unit and provides additional specific capacitance on the surface of carbon material.The specific capacitance of PPD-BC is nearly three times higher than that of BC-0.5.In addition,the narrow micropores of bubble-like porous carbon can inhibit the loss of p-phenylenediamine during the charging/discharging process,and thus ensuring the cycling stability of the electrode material,which can maintain 92%capacitance retention after 5000 cycles.The asymmetric supercapacitor was assembled in the alkaline electrolyte with PPD-BC as the negative electrode and Ni(OH)2 as the positive electrode,which delivers a high energy density of 94 Wh kg-1 at the power density of 423 W kg-1.