Preparation And Applications Of Ginkgo Leaf-derived Porous Carbon Materials

Porous carbon materials have been widely used in fields of electrocatalysis,wastewater purification,and electrochemical energy storage due to their appealing properties of high specific surface areas,good porosities,high electric conductivities,and good chemical stabilities.However,current synthesis methods for porous carbon materials commonly have disadvantages,including high cost,complex process,and poor controllability.Therefore,it is of importance to find low-cost,environmentally friendly,highly efficient and controllable synthetic methods for the porous carbon materials to realize their scalable preparation and applications.Natural biomass materials are rich in resource with low cost.Rich carbon and other non-carbon elements enable biomass materials to be ideal precursors for heteroatom-doped porous carbon materials.Therefore,many researchers in recent years have focused on preparation of porous carbon materials with controllable structures and compositions using biomass materials as precursors via environmentally friendly and low-cost methods,further realizing their wide applications in electrocatalysis,adsorption,and electrochemical energy storage.In this paper,ginkgo leaves are used as precursors to prepare porous carbon materials(referred to as GLCs)with well-developed pore structures,high specific surface areas,and rich heteroatoms(including O,N,and S)by pre-carbonization and KOH activation methods.Carbonization and pore-forming mechanisms of GLCs are presented by studying structural composition change of ginkgo leaves under different preparation conditions.Based on different chemical compositions and porosities,different GLCs are used as adsorbents for methylene blue adsorption in wastewater solution and as electrode materials for supercapacitor and Li-ion capacitors,respectively.In addition,Co,N-doped GLCs are also used as electrocatalysts for ORR.The conclusion is as follows:(1)GLCs are prepared by one-step method and precarbonization-activation method through adjusting carbonization process,activation agent ratio and activation temperature.It is shown that aromatization of ginkgo leaves occurred at 400?.Due to low specific surface area and poor porosity of GLC obtained by one-step method,KOH is further used to optimize porosity of GLC.The number of oxidization-induced defects increased with the temperature in the precarbonization,thus enhancing the following KOH activation process.Specific surface area and pore structure of GLC can be optimized by impregnation instead of physical mixing,increasing activation temperature,and adding KOH amount.As a result,a GLC is fabricated with a high specific surface area of 2523 m2g-1,a pore volume of 1.62 cm3 g-1(with a macro-/mesopore volume ratio value of 75.1%),and rich C/N/O/P/S atoms(with content of 85.71%,1.49%,11.8%,0.27%,0.78%,respectively).The corresponding condition is determined with a pre-carbonized temperature of 400?,a heating rate of 5? min-1,a carbon and activation agent ratio value of 1:1,and a subsequent activation temperature of 900?.(2)The Co,N-doped GLC electrocatalysts with good electrocatalytic activity is fabricated by pyrolyzing cobalt acetate(as Co resource),melamine(as N resource)and GLC(as support material)at 600 ? followed by acid-wishing and secondary heat-treatment at 800?.The optimized synthetic condition and dopant content are detailly investigated as well.The results indicate that the acid-wishing process is beneficial for uniform dispersion of metal particles on GLC and removal of impurity.With the increasing of melamine amount,the GLC electrocatalyst exhibits high initial potential and half-wave potential,showing better performance than commercial Pt/C electrocatalyst.(3)The obtained GLCs are also used as adsorbents for methylene blue in wastewater and effects of specific surface area and porous structure of GLC,concentration of initial adsorbate,and pH value of solution on adsorption performance are studied in detail.It is shown that high surface area and numerous micropores of GLCs provide rich adsorption sites for methylene blue adsorption,improving the adsorptive property.The GLC with the highest surface area displays the best adsorptive property with a adsorption capacity of 1206 mg g-1 at pH=2.Investigations of adsorption kinetics and thermodynamics indicates indicate that the methylene blue adsorption by the GLC is a spontaneous endothermic process and more fitted to the pseudo-second-order model and the equilibrium data are favorably described by Langmuir isotherm,revealing the existence of chemical adsorption.(4)GLC was further used as electrode materials for supercapacitors,and the influence of specific surface area,porosity and heteroatom content on capacitance and rate capability were investigated.Results indicate that rich heteroatoms not only provide additional pseudo capacitance for GLC electrode but also improve the wettability of GLC electrode to KOH electrolyte,giving high efficient surface area for ionic storage.Among the heteroatoms,N atoms can improve electric conductivity of GLC while O atoms will decrease the electric conductivity of GLC,thus affecting rate performance of GLC electrode.As compared with micropore-dominant and mesopore-dominant porosities,hierarchical porosity contributes both high specific surface area and open structures which endow the GLC with high specific capacitance and good rate performance.It is shown that the GLC prepared at 600? exhibited the best capacitive performance among all the samples.The GLC electrode shows a high capacitance of 130 F g-1 at the current density of 5 A g-1.(5)To meet electrode requirement for high-performance Li-ion capacitors,two GLCs(GLC-400-L-3-600 and GLC-400-L-1-900)are prepared by dipping-precarbonization-activation method and further used as anode and cathode materials for Li-ion capacitors.Beneficial from rich heteroatoms and high specific surface area,the GLC-400-L-3-600 anode exhibits a reversible capacity of 776 mAh g-1 at a current density of 0.1 Ag-1,and a reversible capacity of 299 mAhg-1 is still retained after 500 cycles at 2 A g-1.The GLC-400-L-1-900 cathode delivers a specific capacity of 98 mAh g-1 at a current density of 0.1 A g-1,and a capacity retention of 80%can be reached after 5000 cycles at 2 A g-1.The good cathode performance is ascribed to the high surface area,rich heteroatoms and hierarchically porous structure.The assembled Li-ion capacitor based on these two GLC electrodes shows a high energy density of 118 Wh kg-1 and a power density of 31.6 kW kg-1 as well as superior long cycling performance(with a capacity retention of 79%even after 6000 cycles).