Design Of Polyoxometalate-Modified MOF-Derived Porous Carbon-Based Materials And Study On Their Oxygen Reduction Performance

The rapid economy and technology growth have excessively consumed fossil energy and further caused environment/energy crisis.For dealing with these crises,the ecofriendly electrochemical energy storage and conversion devices,such as fuel cells and metal-air batteries,have attracted great attention.However,the slow kinetics of cathodic oxygen reduction reaction（ORR）,exorbitant overpotentials and poor stabilities have impeded fuel cells and metal-air batteries practical applications.Therefore,developing efficient,stable,and economical ORR catalysts are crucial for future fuel cells and metal-air batteries application.This thesis focuses on the preparation and regulation of highly efficient and stable oxygen reduction catalysts.We added the polyoxometalates（POMs）to the growth solution of metal-organic frameworks（MOFs）,and POMOF precursors were prepared by in-situ coating synthesis method.After calcination,non-precious metal-doped porous carbon-based materials were obtained.The experimental results show that catalysts we designed exhibit excellent performance comparable to the commercial Pt/C.The specific research contents are as follows:Mo/N co-doped polyhedral carbon framework（Co-NP/MNCF）loaded with Co nanoparticles was successfully prepared by pyrolysis of phosphomolybdic-modified metal-organic framework（MOF）precursor.The experimental results reveal that the catalyst has equivalent catalytic performance for the oxygen reduction reaction（ORR）to commercial Pt/C,with a half-wave potential of 0.836 V and a higher diffusion-limited current density.The catalyst only lost 3.26%current density after 20 hours of continuous voltage discharge,indicating good stability.The superior performance of the catalyst can be attributed to the modification of the pore size distribution of the MOF-derived polyhedral carbon skeleton and the introduction of heteroatoms via phosphomolybdic acid doping,which promotes the exposure of more active sites,optimizes the electronic structure,and accelerates electrolyte/electron/ion mass transfer.Furthermore,the catalyst has higher methanol tolerance than Pt/C,implying that it could find practical application in the field of fuel cells.The other parts of the works are Mo/N co-doped and W/N co-doped polyhedral carbon frameworks（Fe-NC/MNCF and Fe-NC/WNCF）,which were made by pyrolyzing Fe-doped ZIF-8 that had been similarly modified with phosphomolybdic and phosphotungstic.On the structure and characteristics of carbon-based catalysts produced from MOF,the impacts of various POM doping were investigated.With a half-wave potential of 0.89 V and a diffusion-limited current density of 6.4 m A cm^–2,Fe-NC/MNCF outperforms commercial Pt/C in terms of ORR catalytic performance.Additionally,the Fe-NC/MNCF-loaded electrode of the zinc-air battery（ZAB）demonstrates a high open circuit potential of 1.475 V and a power density of 111.6 m W cm^–2.The specific capacity reaches 794.8 m Ah g _Zn^-1,accounting for 96.9%of the theoretical capacity of ZAB.