| As one of the most promising electrochemical energy storage systems,aprotic lithium-oxygen?Li-O2?battery with an ultrahigh theoretical energy density has captured widespread attention.The cathodes of Li-O2 batteries undergo oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?with slow kinetics during charge and discharge.Therefore,it is urgent to design a porous cathode catalyst with high catalytic activity and stability.The metal nitrogen-doped carbon?M-N-C?materials obtained by pyrolyzing metal-organic frameworks?MOFs?precursors in an inert atmosphere not only retained the high surface area and porous structure of the pristine MOFs to a great extent,but also significantly improved the conductivity and stability.Furthermore,the M-N-C catalysts possessed improved electrocatalytic activity and remarkable stability under harsh conditions.Therefore,two typical zeolite imidazole frameworks?ZIFs?,ZIF-9 and ZIF-67,were selected as the main research objects in this work.The catalytic activity of M-N-C catalysts was improved by incorporating heterogeneous metals and constructing hierarchical porous structure.The main contents of this thesis are as follows:?1?First,a new MOF structure was discovered during synthesis Co-ZIF-9.Most of Co-ZIF-9 reported in previous literatures were not pure.However,no definite structure has been presented until now.Herein,pure Co-ZIF-9 was successfully prepared under microwave method,pure Co-ZIF-9 and new pure Co-MOF?Co?bim?2,“bim”represents benzimidazolate?were prepared under solvothermal procedures.The factors affecting the growth of crystal structures during the synthesis process were investigated.The X-ray single crystal diffractometer,PXRD,FTIR were used to characterize the structure of the new metal-organic framework?Co-MOF?.This work solved the problem of the non-pure phase of Co-ZIF-9reported in the previous literatures.?2?Then,Fe ions were successfully incorporated into the ZIF-9 structure by mixing FeSO4·7H2O under nitrogen atmosphere to obtain pure phase FeCo-ZIFs-X?X represents Fe/Co molar ratio in bimetallic zeolite imidazole frameworks FeCo-ZIFs?crystals.The FeCo-N-C-X catalysts were obtained by pyrolysis of FeCo-ZIFs precursors under N2atmosphere.Cyclic voltammetry?CV?and linear scan voltammetric?LSV?measurements confirmed that the FeCo-N-C-0.2 catalyst carbonized under 800°C with a halfwave potential at 0.826 V?versus RHE?exhibited the best ORR performance.This phenomenon might be attributed to the high content of nitrogen in the catalyst and the high level of graphitization,as well as the metal nanoparticles were less agglomerated when FeCo-ZIFs pyrolyzed at 800°C.The incorporation of Fe effectively improved the ORR catalytic activity compared to the Co-N-C catalyst obtained by pyrolysis of Co-ZIF-9,but there was still a certain gap compared with the commercial Pt/C catalyst.It might be due to the large size of the catalyst particles,and the active sites such as metal nitrogen were not sufficiently exposed.?3?Finally,micro-meso-macroporous FeCo-N-C-X?denoted as“M-FeCo-N-C-X”,M represents micro-meso-macroporous structure?catalysts derived from hierarchical M-FeCo-ZIF-67 was prepared.The micropores in M-FeCo-N-C-X have strong capability in O2 capture as well as dictate the nucleation and early-stage deposition of Li2O2,the mesopores provided channels for the electrolyte wetting,and the macroporous structure promoted more available active sites when used as cathode for Li-O2 batteries.More importantly,M-FeCo-N-C-0.2 based cathode showed a high initial capacity(18,750 mAh g-1@0.1 A g-1),good rate capability(7,900 mAh g-1@0.5 A g-1),and cycle stability up to 192 cycles.Interestingly,the FeCo-N-C-0.2 without macropores suffered relatively poorer stability with only 75 cycles,although its discharge capacity was still as high as 17,200 mAh g-1(@0.1 A g-1).The excellent performance attributed to the synergistic contribution of homogeneous Fe,Co nanoparticles and N co-doping carbon frameworks with special micro-meso-macroporous structure. |
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