Zinc-based Templating Strategy Toward The Synthesis And Application Of Porous Carbon

Porous carbon is composed of randomly stacked defect graphene layers containing a large number of interconnected nanovoids,which leads to its impressive specific surface area(SSA,>2000 m2 g-1)and pore volume(>1 cm3 g-1).Due to good electrical conductivity,high stability,and easily tunable physical and chemical properties,porous carbon materials have been widely used in many fields,including environmental remediation,biomedicine,and gas separation.In the past two decades,porous carbon has been indispensable for novel electrochemical energy storage and conversion technologies,such as lithium-ion batteries,supercapacitors,and fuel cells.Porous carbon consists of carbonaceous and porous components,so the carbon precursors and pore-formation methods are two key factors for preparing porous carbon.At present,the precursor scope for carbon synthesis is mainly centralized in various carbon-rich organic matters,including natural and synthetic polymers.However,it is difficult to control the textural and compositional characteristics of the resulting carbons using natural polymers.For synthetic polymers,the complicated and slow polymerization process brings great inconvenience to the production of carbon materials.In contrast,"bottom-up-like" methods starting from small organic molecules could provide a promising pathway toward the facile and controllable preparation of carbons;although small molecules are inexpensive and diverse in structure,their high saturated vapour pressure and poor thermal stability generally make them difficult to be employed as carbon precursors.In this paper,the preparation and performance study of porous carbon is conducted from two aspects of small organic molecule precursors and poremaking methods.The specific contents are as follows:1.A facile and efficient synthetic strategy is developed for transforming small organic molecules into porous carbons.In this method,Zn salts are added to an alkaline aqueous solution of organic small molecules at room temperature to obtain metal complexing compounds,which are then pyrolyzed to porous carbon materials.This method is applicable to various small phenolic molecules with at least one monodentate chelating site because their metal complexing compounds possess sufficient thermal stability to generate carbons materials at high temperatures.Additionally,during the high-temperature carbonization process,metal ions(Zn2+)in the coordination compounds can evolve into nanosized particle templates,promoting the formation of porous structures.The carbons prepared from the bottom-up templates possesse hierarchical porous structure,high specific surface areas(SSAs,2753 m2 g-1)and large pore volumes(3.36 cm3 g-1).3.ZnO plays an important role in the formation of the above porous structures.Therefore,we further explore ZnO and develope a top-down template strategy for the synthesis of porous carbon.This method allows a simple and low-cost preparation of porous carbon by direct carbonization of a mixture of commercially available ZnO powders and organic precursors.During the carbonization,the ZnO powders can evolve into nanoparticles that serve as in situ templates for the formation of porous structures.We understand that the unconventional thermal evolution behavior of ZnO involves two steps:1)ZnO decompose into atomically dispersed Zn species at high temperatures;2)atomically dispersed Zn species are aggregated into nanoparticles.The top-down templating method is widely applicable to different organic molecule precursors and metal oxides to produce carbon materials,so the porosity and heteroatom doping of the prepared carbon can be regulated by varying the organic precursors and/or the metal oxides.The prepared porous carbons exhibit high SSAs(up to 2183 m2 g-1)and pore volumes(up to 2.21 cm3 g-1).In particular,this method can also be extended to prepare nitrogen/sulfur-doped porous carbon and fuel cell catalysts with abundant atomically dispersed metal-nitrogen sites.3.Based on the migrating templates in ZnO top-down templating strategy,we further expand more diverse precursors,breaking the limitation of liquid precursors in the traditional hard-template method,and ZnO can also be obtained by pyrolysis of zinc salts.Therefore,a practical,simple and low-cost zinc salts-assisted method is developed for preparing porous carbon.Diverse biomass or wastes are converted into high-value hierarchical porous carbons(HPCs)by zinc salts-assisted high-temperature carbonization.During the pyrolysis of the organic precursors/zinc salts mixture,Zn species in the zinc salts thermally converte into ZnO nanoparticles that serve as in-situ templates,creating nanopores in the carbon matrix.The prepared HPCs possess high SSAs(up to 2432 m2 g-1),large pore volumes(up to 4.30 cm3 g-1),and broad pore size distributions.This method is applicable to various zinc salts and organic precursors(including biomass,agricultural/industrial waste,etc.);thus,the porous structures and heteroatom doping of the HPCs can be controlled accordingly.Additionally,zinc salts can be recovered and recycled,which is conducive to the low-cost and large-scale production of HPCs.4.We explored the adsorption and electrocatalytic performance of the porous carbon materials described above.The carbon material derived from the complexing compounds of small phenolic molecules possesses a hierarchical porous structure,and exhibites superior adsorption performance for pollutants than conventional KOHactivated carbon,showing a faster adsorption rate(2.1×10-2 mg g-1 min-1)and a higher adsorption capacity(1251 mg g-1).The functional carbon materials synthesized by the top-down templating method containe abundant metal-nitrogen active sites and exhibite outstanding H2-air proton exchange membrane fuel cell performance,with a peak power density of 545 mW cm-2.