Study On MOF-derived Iron Oxide Modified Carbon-based Oxygen Reduction Catalyst

The metal-air battery is a device that converts chemical energy into electrical energy with the characteristics of clean and high efficiency,high energy density,and no harmful emissions.It can solve increasingly prominent energy and environmental problems.However,the slow kinetics of cathodic oxygen reduction reaction（ORR）and the cost of expensive catalysts and the lack of stability of precious metal catalysts restrict its large-scale application.It is of great practical significance to develop a cathodic oxygen catalyst that is cost-effective and easy to be scaled-up.Compared with precious metal catalyst,transition metal oxide catalysts with abundant reserve,low price and excellent intrinsic catalytic activity show high potential for application in metal-zir battery.While it has been demonstrated in the previous literature that it is difficult to obtain highly dispersed metal oxide catalyst due to Ostwald ripening and particle agglomeration.u Its conductivity is also needed to be improved.This paper aims to synthesize nano-scale oxides that were loaded on highly conductive carbon materials,and uses metal organic framework（MOF）as a precursor to synthesize oxides-carbon based composites and single atomic catalysts with different contents and compositions.A systematically investigation of relationship among the chemical composition,microstructure and electrochemical performance.The newly developed catalysts are also applied to the real zinc-air battery.First,a aqueous solution instead of toxic organic solvent is used to prepared Fe³⁺modified ZIF-8 material,which was pyrolyzed at high temperature to synthesizes Fe₃O₄-Fe-N/C-CNT（Fe NC）and used as high-efficiency cathode ORR catalyst.We deeply studied the effect of the Fe₃O₄ loading on the material morphology and electrochemical performance of the resultant catalyst.The Fe NC gives higher half-wave potential,limiting current density,reaction kinetic,operation stability under constant voltage,methanol anti poisoning resistance,and the electrochemical performance of Zn-air battery over commercial Pt/C materials and the composition device.The presence of carbon materialmen able the sufficient electronic conductivity and charge transfer between active site and carbon are facilitated by CNT,and the active expose and mass transfer are improved by the mesoporous structure of the material,plus the high content of metal-N,pyridine and graphitized nitrogen in the composite are the sources of excellent ORR performance.In addition,we also found that although the direct ORR activity of surface Fe₃O₄ limitted,its presence significantly improves the wettability of the surface hydrophobic carbon-based catalyst,and significantly promotes the chemical adsorption and further activation of O₂ on the catalyst surface,which significantly improve the ORR activity.Secondly,a single active site Zn-N-C single atom catalyst is obtained from metal organic framework（ZIF8）as precursor through a two-step of high temperature treatment and one intermediate steps of room temperature ammonia treatment.Compared with the material treated with strong acid or alkali,the material treated with weak alkali has better ORR catalytic activity.The half-wave potential of this material can reach 0.861V in 0.1M KOH solution,and the Tafel slope is 84.36m V dec^-1,and follow a 4 electronic reduction process according to K-L equation calculation.More importantly,Zn-N-C cathodic catalyst shows a distinguish ORR durability because of constant element valence of Zn.In the 12h test,there is only a 2.8%attenuation（vs.36%of Pt/C）.The etching of the exposed Zn O and Zn groups increases the pore volume of the ZIF-derived carbon material.The room temperature etching with NH₃ and the subsequent high temperature treatment also significantly increase the active site of the catalyst.This work provides a new preparation technology with extremely simple preparation process to obtain single site ORR catalyst with excellent catalytic activity.Finally,we combining the work of the above two chapters,the hydrophilic hydrophilic iron oxide was loaded onto the surface of hydrophobic stable single-atom Zn-NC material and applied as ORR catalyst.Even exceptional ORR activity as follows E_onset is 1.15 V,E_1/2 reaches0.90 V,J_lim=5.44 m A/cm²,and tafel slope of 122.65 m V/dec is obtained in alkaline solution with the optimized loading and calcination temperature.The activity is one of the best electrochemical performance in the literature for non-previous metal electrocatalysts.Which basically reaches the best performance value in the literature.