The Application And Research Of Lithium Iron Phosphate And Its Composites In Oxygen Eiectrode

Oxygen electrode contains two key reactions involing oxygen evolution reaction(OER)and oxygen reduction reaction(ORR),which plays a vital role during the process of the preparation of hydrogen energy and the application of fuel cells.To improve the OER/ORR electrocatalytic activity and durability of the catalysts,we focused on LiFePO4 as OER catalyst and support of ORR catalyst to carry out our researches concerning the design of its electronic structure and micro-morphology.In this thesis,we fabricated a series of LixFePO4/C(0<x?1)and Pt/LiFePO4 composites with diffetent ratios of Pt,which were used to catalyze OER and ORR,respectively.The main contents are as follows:(1)Fe is active element for OER and its valence has different effects on the catalytic performance.In view of the above-mentioned facts,we choosed the inexpensive commercial LiFePO4/C cathode materials of lithium ion battery to tune the amount of Li+ by electrochemical method,thus the electron structure of Fe element was changed.We made a research of the electrocatalytic activity of the above mentioned catalysts in 0.1 mol/L PBS solution at pH 7.No apparent changes of structure and morphology of LiFePO4 were observed before and after in-situ delithium.Therefore,we regarded the valence of transition metal Fe as the only variable.The overpotential of LiFePO4/C,Li0.85FePO4/C,Li0.66FePO4/C,Li0.40FePO4/C and Li0.12FePO4/C catalysts was gradually reduced at the current density of 10 mA/cm2 and the aboved catalysts exhibited increasing turnover frequencies.We drew a conclusion that the catalytic activity was significantly improved by continuously extracting lithium ions out of LiFePO4/C,namely Fe3+ was more favored than Fe2+ to catalyze OER in neutral medium.The enhancement was attributed to the unique electronic structure of Fe transforing from Fe2+ to Fe3+ after delithiation.(2)High cost and low stability are the main problems of commercial Pt-based ORR catalyst.We synthesized bowknot LiFeP04 support by solvothermal method and obtained Pt/LiFePO4 composites with different ratios of Pt.The ORR tests of the above Pt/LiFePO4 catalysts were implemented in 0.1 mol/L KOH solution.The onset potential and durability of Pt/LiFePO4(7.8 wt.%)for ORR approached to the performance of commercial Pt/C(20 wt.%).But the mass percent of Pt decreased to a great extent,which was attributed to the higher specific surface area of the mesoporous bowknot LiFePO4 catalyst support.The unique morphology catalyst support not only benefitted the uniform dispersion of Pt nanoparticles,but also benefitted gas diffusion and kept in touch with electrolyte closely.Additionally,from the XPS data of the shifted binding energy of Fe 2p3/2 and Pt 4f7/2 peak,we found that electrons transferred from Fe of support to the Pt catalyst,changing the electron structure of Pt.The enhanced catalytic performances were attributed to the synergistic effect of electron transport between catalyst and support.In this thesis,we centered around LiFePO4 to research the effect of Fe electron structure on OER catalytic activity and act as an ORR catalyst support.It provided guidance for the future design of Fe-based water splitting catalyst and novel non-carbon ORR catalyst support.