Fabrication And Electrochemical Studies Of High-Energy-Density Supercapacitors With Porous Carbon-based Electrode Materials

Supercapacitors(SCs)show the advantages of fast charge/discharge rate,high power density and great cycling stability,being widely applied in electrochemical energy storage field.Porous carbon materials are the most common electrodes in SCs,with advanced pore structure and high specific surface area for ion adsorpotion to achieve electrochemical double layer energy storage.However,the corresponding SCs exhibit the low energy density,which limits further development and appliment of SCs.It is a challenging and crucial problem that how to design new carbon-based materials and fabricate new-type supercapacitors to achieve high power density and high energy density simultaneously.Our work focuses on multiple dimentions to provide solutions:for materials design,porous carbon-based materials are structured and composited to facilitate ion diffusion and improve capacitance;for energy storage mechanism,composite machanisms are introduced by combining double layer capacitance with pseudocapacitance or redox capacity;from the aspect of electrolytes,high-valance cations and redox active electrolytes are adopted to construct new-type high-energy-density SCs with porous carbon-based electrode materials.The main results in this work are as follows:(1)Fabrications of mesoporous carbon microspheres(MCMs)in flow capacitorsFlow capacitors are fabricated by MCMs as electrode materials based on electrochemical double-layer energy storage mechanism,to achieve high energy density at high power density benefiting from mesoporous structure.MCMs are obtained by a simple but high-efficiency spray-drying method,with temperature of 120?,spray pressure of 0.4 MPa and feeding rate of 750 mL/h.MCMs display smooth surface,great spherical shape and high dispersibility.The corresponding MCM suspension electrodes exhibit excellent flowability and good stability.Pore structure of MCMs can be easily controlled by adjusting the size of silica sol,the mesopore size of MCM in this work is controlled between 7 and 30 nm,the larger mesopore size,the faster ion diffusion.Taking MCM-3 as an example,MCM-3 possesses the average pore size of 29.9 nm,the specific surface area of 1150 m2/g and the pore volume of 4.1 cm3/g,with fast ion diffusion in materials without sacrificing ion adsorption ability.The electrochemical performance of MCMs in flow capacitors shows the capacitance retention of 75%when scan rate increases 50 times,and energy density of 1 Wh/kg at 1.8 kW/kg,nearly once higher than commercial microporous carbon spheres.It indicates the superiority of mesoporous electrodes for SCs with high-energy-density at high-rate.(2)Fabrication of MCM/V2O5 electrode materials in aqueous Al-ion capacitorsAqueous Al-ion capacitors are fabricated by MCM/V2O5 as electrode materials combining electrochemical double-layer capacitance with pseudocapaitance,to achieve efficiently Al3+insertion and energy density improvement.With the advantages of high surface area and typical mesoporous structure,MCMs are used as excellent matrix for V2O5 growth,providing abundant growth sites,adequate impregnation space and effective aggregation inhibitation.MCM/V2O5 is synthesized by a simple wet-impregnation method,with superfine nanocrystal V2O5 evenly dispersed in MCMs,with fast ion diffusion rate intriguing redox reactions.The electrochemical results show Al3+can achieve flexibly insertion/extraction from V2O5.MCM/V2O5 in 1M Al2(SO4)3 electrolyte displays the energy density is improved to 18 Wh/kg,four times higher than that of MCM.Compared to electrolyte systems of Mg2+and Na+,Al3+with higher external charge leads to more charge transfer during ion insertion/extraction,and thus enhanced energy density.It provides a feasible plan to achieve high energy by fabricating carbon-based composites in high-valence cation contained aqueous electrolyte systems.(3)MCM cathode and NiFe-LDH/rGO anode for asymmetric organic Li-ion capacitorsAsymmetric organic Li-ion capacitors are fabricated by MCM as cathode and NiFe-LDH/rGO as anode.It combines electrochemical double-layer mechanism in capacitors with redox reactions mechanism in batteries,which achieves energy density improvement of an order of magnitude and simultaneously maintains high power density.Anode material of NiFe-LDH/rGO is synthesized by one-step hydrothermal method.The role of rGO is deeply understood,reflecting in not only conductivity improvement but also modifying NiFe-LDH crystal growth behavior by providing heterogeneous nucleation sites.The role of growth modification is unique that cannot be observed in physically mixed NiFe-LDH+rGO composite.NiFe-LDH/rGO shows highly dispersed NiFe-LDH nanosheets on rGO without evident aggregation.In Li-ion batteries,NiFe-LDH/rGO exhibits high capacity of 1151 mAh/g at 0.1 A/g.In asymmetric organic Li-ion capacitors with mass ratio of cathode:anode of 6:1,NiFe-LDH/rGO//MCM shows the energy density of 133 Wh/kg and the power density of 4016 W/kg,with long cycling stability more than 5000 cycles.(4)Activated carbon felt electrode for asymmetric redox additive supercapacitorsAsymmetric redox additive supercapacitors are fabricated by ACF-900 as electrodes and introducing two different redox additives into electrolyte,which extends the working voltage window and thus achieves high energy density.ACF-900 possesses great wettability and abundant ion adsorption sites,directly used as electrodes without complex slurry preparation and coating process.In 2M KOH without additives,ACF-900 exhibits typical capacitive behavior with the energy density of 3.6 Wh/kg.Adding K3[Fe(CN)6]and 2,6-DHAQ into 2M KOH can improve the energy density to 11.8 and 8.9 Wh/kg,perform redox behavior in positive and negative voltage,respectively.It inspires us to fabricate the asymmetric supercapactor adding K3[Fe(CN)6]in positive electrolyte and 2,6-DHAQ in negative electrolyte.The aqueous asymmetric supercapacitor shows the extended voltage window of 2 V and the enhanced energy density of 39.1 Wh/kg,ten times as that in 2M KOH and more than three times higher than that of systems with one additive added.It provides a solution to the problem of narrow voltage window and limited energy density of aqueous supercapacitors.