Study Of Structure Regulation Of Porous Carbon In Molten Salt And The Capacitive Performances

As a class of green electrochemical devices,supercapacitors are currently attracting intensive attention due to their excellent power density and long cycle life,however,the low specific capacitance and energy density still hinder their applications.Electrode materials are usually considered to be crucial for supercapacitors and carbon materials are most widely used as electrode materials.As for carbon electrodes,two-dimensional thin carbon sheets can reduce diffusive resistance to mass transport on the large electrode/electrolyte interfaces.Meanwhile,hierarchical porous structures including macro-,meso-and micro-pores respectively can provide ion-buffering reservoirs,ion-transporting channels and ion-storage sites.To obtain hierarchical porou carbon materials,chemical activation methods activated with KOH,ZnCl₂,etc are usually adopted.However,these techniques usually suffer from toxic or strongly corrosive chemical reagent to obtain the final products.Additionally,up to now the synthesis of biomass-derived porous carbons is performed predominantly under inert gas atmosphere?like N₂ and Ar?,which requires large amounts of resource consumption as well as harsh technical and equipment requirements,hindering the commercial production.With respect to these concerns,a facile and versatile salt-sealing technique is presented to prepare functional porous carbon sheets under air atmosphere from biomass.Low cost and non-toxicity potassium chloride was adopted as dual function agents to protect the generated carbon structure from being destroyed by air at high temperatures and to achieve high specific surface area by building hierarchical porous structures during one-step carbonization.The electrochemical performances of the samples are investigated in detail.The detailed contents of the study are illustrated as follows.?1?Highly porous carbon sheets were prepared from fresh clover stems under air atmosphere via a facile potassium chloride salt-sealing technique,which not only avoids using the high cost inert gas protection but also spontaneously introduce multi-level porosity into the carbon structure taking advantage of the trace of oxygen in the molten salt system.The as-obtained porous carbon sheets possess high specific surface area of 2244 m² g^-1 and interconnected hierarchical pore structures from micro-to macro-scale,which provide abundant storage active sites and fast ion diffusion channels.In addition,the spontaneously formed N doping sites not only improve the electron conductivity of the electrode but also enhance the specific capacitance by introducing pseudocapacitance.When employed as supercapacitor electrodes,a high specific capacitance of 436 F g^-1 at 1 A g^-1 and an excellent rate capacity with capacitance remaining 290 F g^-1 at 50 A g^-1 are demonstrated.Furthermore,the assembled symmetric supercapacitor delivers a high specific capacitance of 420 F g^-1 at 0.5 A g^-1,excellent rate capacity and cycling stability which retains99.4%of the initial capacitance at 5 A g^-11 after 30000 cycles.?2?A green and scalable route is developed to prepare hierarchical porous carbon sheets?HPCS?from biomass?cornstalk?directly under air atmosphere instead of inert gas protection.In addition,the method adopts non-toxic NaCl and KCl mixed salt as reaction media to regulate the activation process,which avoids the corrosive and poisonous KOH and ZnCl₂ reagents.Meanwhile,the pore structures of HPCS can be well-manipulated by the salt templates and the etching effects of the oxygen atom as well as the high energy molten salt media.The as-obtained HPCS consist of ultra-thin sheet structure with abundant hierarchical pores which facilitates the electrolyte diffusion and ion transfer.In addition,the high specific surface area of1588 m² g^-1 m² g^-1 provides sufficient active sites.Therefore,the as-prepared HPCS exhibit a high specific capacitance of 407 F g^-1 at 1 A g^-11 in a three-electrode system.Moreover,the assembled symmetrical supercapacitor in a two-electrode system also exhibits an outstanding specific capacitance of 413 F g^-1 at 0.5A g^-1,excellent rate capacity and a long-term cycling stability with 92.6%of initial capacitance retention after 20 000 cycles at 5 A g^-1.?3?A facile potassium chloride salt-locking technique combined with hydrothermal treatment on precursors was explored to prepare nitrogen-doped hierarchical porous carbon sheets in air from biomass.Benefiting from the effective synthesis strategy,the as-obtained carbon possesses a unique nitrogen-doped thin carbon sheet structure with abundant hierarchical pores and large specific surface areas of 1459 m² g^-1.The doped nitrogen in carbon framework has a positive effect on the electrochemical properties of the electrode material,the thin carbon sheet structure benefits for fast ion transfer,the abundant meso-pores provide convenient channels for rapid charge transportation,large specific surface area and lots of micro-pores guarantee sufficient ion-storage sites.Therefore,applied for supercapacitors,the carbon electrode material exhibits an outstanding specific capacitance of 451 F g^-1 at 0.5 A g^-11 in a three-electrode system.Moreover,the assembled symmetric supercapacitor based on two same carbon electrodes also displays high specific capacitance of 309 F g^-1 at 0.5 A g^-1,excellent rate capacity and remarkable cycling stability with 99.3%of the initial capacitance retention after 10,000 cycles at 5 A^-1.