MOF-based Heteroatom-doped Porous Carbon Composites And Their Electrocatalytic Properties

Because the crisis of fossil energy depletion and serious environmental pollution caused by the use of fossil fuel,the search for alternative clean and sustainable energy has become a research hotspot.As a clean and efficient power generation device,fuel cell has broad application prospect.Oxygen reduction reaction（ORR）is the cathode reaction of fuel cell,and its sluggish kinetics is one of the main problems faced by the cathode of fuel cell.The disadvantages of cathode Pt/C catalyst,such as scarcity of resources,high cost and poor stability,have restricted the large-scale commercialization of fuel cells.In addition,when methanol is used as fuel,methanol solution will permeate from anode to cathode through proton exchange membrane,reducing the open circuit voltage and current efficiency of the fuel cell.Therefore,it is necessary to develop non-platinum catalysts with high performance,high stability and methanol resistance.Metal-organic framework（MOF）materials have many advantages such as diverse structure,easy functionalization and adjustable aperture.As a kind of MOF,zeolite imidazolium frameworks（ZIFs）have become a hot topic due to their high specific surface area,regular pore size,uniform distribution of metal atoms and rich nitrogen sources.In this paper,Co-ZIF and Zn-ZIF-derived transition metal/heteroatom doped carbon catalysts are taken as the research object,and their catalytic performance for ORR is improved through structural regulation and non-metallic element doping.The main research contents are summarized as follows:（1）The flower-and leaf-like core-shell structured FL-Zn-ZIF@ZIF-67 is successfully prepared by growth of ZIF-67 on the surface of flower-and leaf-like Zn-ZIF（FL-Zn-ZIF）.By calcining at high temperature in inert atmosphere and acid etching,FL-Zn-ZIF@ZIF-67 is transformed into cobalt-and nitrogen-doped carbon nanotube-grafted flower-and leaf-like hierarchical porous carbon materials（Co,N/FLC@CNT）.The CNTs formed by autocatalysis are distributed on the surfaces and edges of leaves and flowers,which is beneficial to improve the electrical conductivity of the materials.Co,N/FLC@CNT possesses porous structure with external main mesopores and internal main micropores,which can promote the diffusion of O₂ in the ORR process and shorten the transfer path of ions and electrons.Co,N/FLC@CNT catalyst shows good ORR catalytic performance in alkaline solution in terms of high onset potential（0.95 V）,half-wave potential（0.82 V）,and current density at E_1/2(2.9 m A cm^-2).Its limiting diffusion current density is comparable to that of commercial Pt/C.The Co,N/FLC@CNT material catalyzes ORR through 4e^-path.In chronoamperometry（CA）test,Co,N/FLC@CNT displays excellent stability with 99.2%relative current density after 20,000 s.In addition,Co,N/FLC@CNT catalyst has better methanol tolerance and durability than commercial Pt/C catalyst.（2）Zn Co-ZIFs are grown on the surface of CNT,and Zn Co-ZIFs are linked up by CNTs.S/Zn Co-ZIFs/CNT is prepared by in-situ sulfur doping.A series of Zn Co/NSC/CNT catalysts are formed after high temperature heat treatment of S/Zn Co-ZIFs/CNT with different thiourea contents.Zn Co particles with an average particle size of about 12 nm are coated by NSC and distributed on the surface of CNT.The addition of CNT improves the conductivity of the material,while S doping can increase the graphite-N content in the catalyst.The graphite-N content in Zn Co/NSC/CNT-5 is the highest.The electrochemical active surface area（ECSA）of Zn Co/NSC/CNT-5 is also the largest,implying the most active sites.In alkaline electrolyte,the ORR performance of Zn Co/NSC/CNT-5 is comparable to that of commercial Pt/C catalyst.The onset potential,half-wave potential,and the limit diffusion current density of the LSV curve of ORR on Zn Co/NSC/CNT-5 is 0.985 V,0.825 V,and 5.13 m A cm^-2,respectively.The Tafel slope for ORR on Zn Co/NSC/CNT-5 is 85.6 m V dec^-1.The transfer electron number（n）during ORR is 3.8,which is close to that of commercial Pt/C（3.95）.Zn Co/NSC/CNT-5 catalyst has better methanol tolerance and long-term stability than commercial Pt/C catalyst.The LSV curves of ORR on Zn Co/NSC/CNT-5 before and after accelerated durability tests（ADT）tests are almost the same.