Preparation Of Seaweed Biomass-Derived Porous Carbons And Their Electrochemical Energy Storage Performance

With the fast development and globalization of green electrochemical energy storage devices such as lithium-ion batteries,lithium-ion capacitors,and supercapacitors,people have to encounter the challenges of the cost,sustainability,and environmental friendliness of energy storage devices.Biomass-derived carbon materials,due to having many attractive advantages such as abundant reserves,good chemical stability,low-cost,and excellent electrical conductivity,have been extensively researched as the electrode active material of high-performance electrochemical energy storage devices.In this study,biomass-derived carbon materials with special morphologies were prepared by using two different preparation methods from seaweed,and they were used as novel electrode active materials in anode and cathode of lithium-ion batteries,negative and positive electrodes of lithium-ion hybrid capacitors(LIHCs),and electrode materials for aqueous and ionic liquid-based supercapacitors.The details are as follows:(1)Seaweed was used as raw material.It was py roly zed and activated with KOH subsequently.The two-steps synthesized seaweed-derived porous carbons retained the intrinsic structure of their precursor and contained some nitrogen,oxygen,and sulfur functional groups originating from seaweed.The sequential pyrolysis and activation of seaweed produced hierarchical porous hard pseudo-graphitic carbons,which maintained the original sheet-like morphology of seaweed and added obvious cavities to the material.The relationship between synthesis conditions and porosity is also studied here.The specific surface area of the carbonized sample at 600? and further activated with the KOH/C ratio of 3 was 2129 m2 g-1,the maximum pore volume was 1.024 cm3 g-1.As(electric double layer capacitor)EDLC electrode materials,the two-steps synthesized seaweed-derived porous carbons represented excellent charge storage behavior in aqueous media of 6 M KOH.Among them,the sample prepared with the KOH/C ratio of 3 had an outstanding rate performance and cycle performance at the same time.Its specific capacitance under the current density of 0.5A g-1 in an EDLC was 187.2 F g-1,while the sample with the KOH/C ratio of 4 showed 506.5 F g-1 specific capacitance in a three-electrode system in the same conditions.This material also exhibited 240.2 F g-1 specific capacitance in an ionic liquid-based at 2 A g-1.(2)Seaweed was mixed with KOH in an aqueous media and further heat treated in a nitrogen atmosphere to prepare one-step derived porous carbons.The one-step synthesized seaweed-derived porous carbons exhibited a 3D sponge-like structure,absolutely different from the original precursor's structure.Moreover,the synthesized materials contained some nitrogen,oxygen,and sulfur functional groups originating from seaweed.The amorphous porous carbons made of seaweed had a 3D skeleton as well as interconnected cavities in the materials' bulk.Along with the enhancement in KOH/seaweed ratio,the BET specific surface area decreased.The specific surface area of the sample heat-treated at 600? with the KOH/seaweed ratio of 0.5 was 1109 m2 g-1 and the maximum pore volume was 0.637 cm3 g-1.The one-step synthesized seaweed-derived porous carbons displayed excellent charge storage behavior as EDLC electrodes in the aqueous media of 6 M KOH.Among them,SWG-0.5 had a distinguishable rate performance and cycle performance simultaneously.In an EDLC,Its specific capacitance under the current density of 0.5 A g-1 was 122.4 F g-1,while it showed 358.5 F g-1 specific capacitance in a three-electrode system at 0.5 A g-1 and the same electrolyte.Moreover,the two-steps synthesized seaweed-derived porous carbons represented excellent charge storage behavior as EDLC electrodes with an ionic liquid electrolyte.Among them,the sample with the KOH/seaweed ratio of 0.5 had a remarkable specific capacitance of 197.7 F g-1 at a current density equal to 2 A g-1.(3)The porous carbons made of seaweed were assembled and tested as lithium ion battery anode and cathode as well as lithium-ion hybrid capacitor electrodes.The sample prepared via the two-steps synthesis method with the KOH/C ratio of 4 as the anode of lithium ion battery exhibited a prominent specific capacity of 1150 mAh g-1 at 0.1 A g-1 current density and the sample prepared via the same synthesis method with the KOH/C ratio of 3 tested as cathode lithium ion battery demonstrated 240 mAh g-1 specific capacity at the same current density.After 4000 and 5000 charge/discharge cycles at a current density of 1 A g-1,the anode kept 96.1%and 77.3%of its primitive specific capacity and after 1000 and 2000 charge/discharge cycles at a current density 5 A g-1,it maintained 78.6%and 66.7%of its initial specific capacity,respectively.The cathode underwent 4000 cycles at 5 A g-1 and kept 92.7%after 1000 cycles and 70.0%of its primary specific capacity after 4000 charge/discharge cycles.The resultant asymmetric LIHC made of the above-mentioned cathode and anode demonstrated excellent energy-power characteristics.At 81.4 and 990.5 W kg-1 power density,it displayed high energy densities of 123.2 and 76.1 Wh kg-1,respectively.More significantly,this asymmetric LIHC exhibited the-state-of-the-art cycling performance.After cycling 5000 times at 10 A g-1,the capacity of this device was remarkably retained at nearly 100%,which is superior as compared with the current progressive LIHCs.