Design,Synthesis And Catalytic Mechanism Study Of Iron-Containing Oxygen Reduction Catalysts

Fuel cells have the advantages of high theoretical energy density,good safety,environmentally friendly,and etc.Therefore,it has broad development prospects in the context of increasing energy demand.However,the slow oxygen reduction reaction（ORR）kinetic process on cathode is the main factor limiting the large-scale application of various fuel cells.Therefore,design and synthesis of high activity ORR catalysts and in-depth study of the catalytic mechanism are of great importance to promote the development of fuel cell industry.In this paper,the morphological structure optimization,electronic structure modulation and active site design of iron-containing ORR catalysts were achieved through precursor design and preparation process modulation.The prepared catalysts displayed excellent electrochemical performances.And the catalytic mechanisms and the corresponding active sites were revealed by combing with advanced spectroscopic techniques and theoretical calculations.The main research contents and conclusions of this paper are listed as follows:（1）Aerogel precursors were prepared by freeze-drying after the reaction of sodium alginate with iron ions.The Fe-N₄ single-atom catalysts with abundant microporous structures were synthesized by pyrolyzing the as-obtained precursors.The homogeneous dispersed Fe-N₄ sites and the abundant microporous structure enable the Fe NC-900-8 catalyst to exhibit excellent ORR activity and stability.The electrochemical results show that the half-wave potential of Fe NC-900-8 can achieve 0.88 V,which is better than the commercial Pt/C of 0.84 V.（2）The Fe-containing multicomponent ORR catalysts were prepared by pyrolysis porous carbon that loaded with iron polyphthalocyanine.The ORR activity of the catalyst was further enhanced by the introduction of Fe₃O₄ and Fe₃N on the basis of Fe-N₄ sites.The synergetic effect between active components and multilevel pore structure is the main factor for multicomponent catalysts to demonstrate excellent ORR activity in electrochemical tests.The optimized multicomponent Fe₃O₄/Fe₃N/Fe-N-C@PC-2.5 catalyst displayed a half-wave potential of 0.90 V,showing excellent ORR activity.（3）Firstly,Cu-N₄ doping into the pyrrolic-N type Fe-N₄ was predicted to lower the ORR energy barrier by theoretical calculations.Subsequently,a series of model catalysts were prepared to verify the conclusions of the theoretical calculations.It was demonstrated by experimental results that the doped Cu-N₄ sites can effectively regulate the valence state of Fe atoms by increasing the electron cloud density around Fe atoms,thus further enhance the ORR activity of the Fe-N₄ sites.The half-wave potential of the obtained Fe Cu-N-C@PC sample increased to 0.91 V compared with 0.88V of Fe-N-C@PC without doping.（4）The Fe-doped porous bifunctional catalysts were synthesized by pyrolyzing trimetallic ZIF precursors.The introduction of Fe-N_x sites can optimize the morphological structure while further improve the ORR/OER bifunctional activity of the catalyst.The obtained Co@Co Fe_0.01-N-C catalyst exhibited excellent performance and stability in primary/secondary Zn-air batteries.（5）Iron polyphthalocyanine with Fe-N₄ sites were dispersed on r GO surface byπ-πinteractions using high-energy ball milling.The charge shift caused byπ-πstacking between the iron polyphthalocyanine and r GO realizes the electronic structure regulation of Fe-N₄ sites.The half-wave potential of this pyrolysis-free catalyst reached to 0.90 V,demonstrating an excellent ORR activity.