Preparation And Capacitive Performance Of Sodium Lignosulfonate Based Porous Carbons

Supercapacitor is a kind of energy storage system between secondary battery and physical capacitor,and it has gained widespread attention and application due to fast charge/discharge rate,high power density,wide operation temperature range,excellent cycling stability and outstanding safety.While compared with secondary batteries,energy density of supercapacitors is relative lower and needs to be further improved.Electrochemical performance of supercapacitors is determined by electrode materails,and porous carbons are the most widely investigated and used supercapacitor electrode materials,which have the advantages of high specific surface area,high electron conductivity,good chemical stability,wide operation temperature range,abundant precursors,low price,easy processing and nontoxicity.In this work,a sustainable biomass-deritive sodium lignosulfonate was used as carbon source,and a new,facile and low-cost strategy was proposed for developing and tunning pore structure of porous carbon in multiscale.Carbon materials with high specific surface area,micro-meso-macro porous structure and high packing density were prepared through in-situ templating and in-situ activation of sodium-containing salt,which was decomposed from sodium lignosulfonate,and high-performance supercapacitors in aqueous electrolyte with high volumetric specific capacitance and high volumetric energy density were constructed based on the as-obtained materials.The effect of preparation procedure on pore structure,micromorphology,microstructure and surface chemistry of the porous carbons were investigated;the relationship between structural property and electrochemical performance of the porous carbons was explored;and the pore structure evolution mechanism of sodium lignosulfonate based porous carbons was proposed.Firstly,high Na-containing sodium lignosulfonate was used as carbon precursor to prepare hierarchical porous carbons via one-step carbonization without any additional templating/activation agent.The highest specific surface area,total pore volume and mesopore ratio of the as-obtained porous carbon is 1526 m² g^-1,1.06 cm³ g^-1 and 29.2%,respectively.The carbon material has highly developed micropores with diameters of 1-2 nm and meso/macro pores with diameters of 10-100 nm.In a 7 mol L^-1 KOH aqueous electrolyte,the assembled symmetric supercapacitor shows a gravimetric specific capacitance of 247 F g^-1 at a current density of 0.05 A g^-1 and a capacitance retention of42.1%at 20 A g^-1.It also exhibits a maximum gravemitric energy density of 8.6 Wh kg^-1and power density of 5.7 kW kg^-1,as well as remarkable cycling stability after 20000charge/discharge cycles at current densities of 2 A g^-1 and 10 A g^-1.Secondly,in order to construct porous carbons with regular morphology and further improve its rate capability,pre-oxidation was introduced to treat high Na-containing sodium lignosulfonate,forming three-dimentional cross-linking structure in the precursor and maintaining its original sphere morphology in the carbonization process,and thus sodium lignosulfonate based porous carbon spheres were obtained.The highest specific surface area,total pore volume and mesopore ratio of the as-obtained material is 1939 m²g^-1,1.42 cm³ g^-1 and 26.1%,respectively.It has a micro?1-2 nm?/meso-macro?10-100nm?hierarchical porous structure and a particle size of 5-115?m.In a 7 mol L^-1 KOH aqueous electrolyte,the assembled symmetric supercapacitor shows a gravimetric specific capacitance of 225 F g^-1 at a current density of 0.1 A g^-1 and a capacitance retention of 65.3%at 20 A g^-1.It also exhibits a maximum gravemitric energy density of7.8 Wh kg^-1 and power density of 6.2 kW kg^-1,as well as remarkable cycling stability after 20000 charge/discharge cycles at current densities of 2 A g^-1 and 10 A g^-1.This porous carbon sphere presents superior volumetric specific capacitance of 68.3 F cm^-3and volumetric energy density of 2.4 Wh L^-1 than commercial activated carbons.Thirdly,in order to further improve volumetric specific capacitance and volumetric energy density of the carbon materials,spray drying was used to minimize particle size,a small amount of NaOH was used to tune mesopore size,as well as nitric acid oxidation was used to modify surface chemistry,and thus porous carbon hollow microspheres with hierarchical porous structure and abundant surface oxygen/nitrogen were obtained.The as-obtained carbon material has uniform spherical morphology,ultrathick shell?hundreds of nanometers?and small particle size?0.1-5?m?.The highest specific surface area and total pore volume of this material is 1342 m² g^-1 and 1.05 cm³ g^-1.Surface oxygen/nitrogen content increases significantly through nitric acid oxidation,and after oxidation the as-obtained material presents an oxygen content of 5.83-13.12 at.%and a nitrogen content of 0.59-0.97 at.%.In a 7 mol L^-1 KOH aqueous electrolyte,the assembled symmetric supercapacitor shows a gravimetric specific capacitance of 215 F g^-1 at a current density of 0.1 A g^-1 and a capacitance retention of 58.6%at 20 A g^-1.It also exhibits a maximum gravemitric energy density of 7.5 Wh kg^-1 and power density of 14.3 kW kg^-1,as well as an excellent capacitance retention of 93.4%after 10000charge/discharge cycles at a current density of 2 A g^-1.Volumetric specific capacitance and volumetric energy density of this carbon material has been improved to 84.8 F cm^-3and 2.9 Wh L^-1.Finally,the effect of physicochemical properties of different lignins,such as elemental and chemical compositions,on the pore structure of the obtained carbon materials was investigated.It was found that metal content in precursor plays a crucial role in developing pore structure,and specific surface area and total pore volume exhibits a proportional relationship with total metal content.Furthermore,through a scientific analysis of pyrolysis process,gas products,solid products of sodium lignosulfonate,as well as pore structure evolution in different carbonization stage,pore structure evolution with two mechanisms and three stages of sodium lignosulfonate based porous carbons has been proposed.The two mechanisms are in-situ templating?forming meso/macro pores of 10-100 nm?and in-situ activation?forming micropores and small mesopores?.The three stages include meso/macro pore forming stage?carbonization temperature?600??,micropore developing stage?600?<carbonization temperature?700??and small mesopore developing stage?800?<carbonization temperature?900??.