Synthesis And Applications Of Hierarchically Porous Carbons

The exceptional physicochemical properties,including good electrical conductivity,high gas/liquid permeability,and excellent chemical/thermal stability,make porous carbons a versatile materials platform for solving global energy and environmental issues.For many important applications,such as energy storage and conversion,gas storage,heterogeneous catalysis,and water treatment,constructing hierarchically porosity in carbon architectures is generally regarded as an effective strategy to improve its performance.This is because hierarchically porous carbons(HPCs)integrate merits of different-sized pore systems:small-sized pore structures(micropores and small mesopores)endow HPCs with high specific surface areas(SSAs)for uniformly dispersing functional sites and providing a large accessible surface area;meanwhile large pore structures(large mesopores and macropores)can provide favorable diffusion paths for efficient mass transport and adequate storage space for guest species.Although the performance of carbons can be generally improved by constructing the hierarchically porous structure,the pore structure characteristics(specific surface area,pore volume,and pore size distribution)and composition of HPCs also require elaborate designs to achieve maximum performance for different applications.Therefore,the development of controllable methods for HPCs synthesis is critical for the rational engineering of HPCs with desired properties.Besides,to promote the rapid development of HPCs,the preparation method should be simple and universal.However,currently established methods for HPCs synthesis,such as the activation method,template method,ionothermal method,and metal-organic framework carbonization method,are difficult to meet the above requirements at the same time.On the other hand,the application research of HPCs is still in the development stage.How to use the structural advantages of HPCs to further expand its application space is also facing challenges.This dissertation starts from the above two points and focuses on the synthesis of HPCs and its applications.The main research results can be summarized as follows.A simple,general,and controllable method for HPC synthesis by carbonization of non-porous coordination polymers was developed.The non-porous coordination polymers can be conveniently prepared by coordinating metal cations with ligands in alkaline aqueous solution at room temperature;meanwhile,by selecting zinc ions(Zn2+)as the metal source,HPCs can be obtained by one-step carbonization of non-porous coordination polymer.A variety of ligands including carboxylates,N-heterocyclic compounds,phenolic compounds,sulfonates,and their hybrids can be thermally converted into the coordination polymers for HPCs synthesis.Depending on the structure and composition of ligands,the specific surface area and pore volume of carbons can reach up to 2647 m2 g-1 and 2.39 cm3 g-1,respectively;meanwhile in-situ nitrogen doping and sulfur doping can also be realized.The versatility of this method not only allows us to control the pore structure and composition of HPCs by selecting different ligand molecules,but also enables us to achieve controllable porosity synthesis by using the mixed ligand strategy where linear dicarboxylic acids are deliberately added during the coordination polymer synthesis.Based on the above results,we demonstrated that the hierarchically porous nature of the ligands derived carbons is independent of the porosity of coordination polymers but originates from a generic thermal decomposition behavior of coordination polymers.Combined with a series of ex-situ physical characterizations and on-line thermal analyses,we figured out that the formation of porous structures in final carbons is associated with the in-situ templating effect and thermal activation effect of thermally evolved zinc oxide.A universal method for preparing highly efficient Fe-Nx-C oxygen reduction reaction(ORR)catalyst using HPCs as the support was established.Two HPCs with high specific surface areas(?2000 m2 g-1)were prepared by ionothermal methods.The synthetic steps of the catalysts include the wet impregnation of Fe-phen complex into HPCs,and pyrolysis treatment of the composite at high temperature.The Fe-containing species in two HPCs derived catalysts both exclusively exist in the form of atomically dispersed Fe-Nx sites with high ORR activity;meanwhile,the two catalysts can also inherit the unique textural characteristics of their original support,including high specific surface area and mesoporous/macroporous structure.In contrast,additional ORR-inactive Fe-containing nanoparticles were presented in the catalyst prepared using commercial conductive carbon black(KJ600)as the carbon support,while its specific surface area and mesopore volume are also relatively small.Both half-cell and proton exchange membrane fuel cell tests demonstrated that the two HPCs derived catalysts have higher ORR activity than the KJ600-derived catalyst.This can be attributed to the fact that the HPCs with the high specific surface area can effectively control the evolution of the precursor molecular complex to the Fe-Nx site during the pyrolysis and avoid the formation of Fe-containing nanoparticles,thus resulting in a high conversion ratio of active sites;on the other hand,the hierarchically porous structure helps to expose numerous active sites and simultaneously allows rapid mass transport,which thereby together improve the utilization of active sites.