Design And Performance Of Porous Transition Metal Nitrogen-Carbon Catalysts ZnFe And ZnFeCo For Oxygen Reduction Reaction Performance Study

With the aggravation of energy shortage and environmental pollution,we have been urged to develop novel clean energy conversion and energy storage devices.Zinc-air batteries have been called the“green energy for the 21st century”due to their high energy density,environmental friendliness,high reliability,and high cost-effectiveness.However,the oxygen reduction reaction（ORR）at the air cathode of the zinc-air battery suffers from a slow kinetic process that requires overcoming a high energy barrier,limiting its further development.Precious metals are highly effective for ORR but limited by the scarcity of reserves.Therefore,it is of significance to develop and design an inexpensive,efficient,and practical non-precious metal ORR catalyst to replace the conventional precious catalysts.In this paper,two types of Zn/Fe-NC and Zn Fe₂₅Co₂₀-NC catalysts with hierarchical porous structures were synthesized based on MOF materials and template method.The efficient ORR single-atom catalysts were prepared in terms of the catalyst synthesis method,carrier structure and active sites.The studies presented in this paper are as follows.1)Dual-atom dispersed Zn/Fe sites anchored in nitrogen-doped porous carbon catalysts（Zn/Fe-NC）were synthesized using ZIF-8 as the carbon skeleton.ZIF-8 played a key role in stabilizing Fe atoms,resulting in a high loading of accessible atomically dispersed Zn and Fe anchored in porous carbon.Benefiting from the appropriate porous structure and the synergistic effect between the Zn and Fe sites,the obtained Zn/Fe-NC catalyst presented excellent ORR efficiency in 0.1 M KOH solution with a half-wave potential(E_1/2)of 0.875V,and excellent long-term stability with a negligible drop in E_1/2 after 10,000 accelerated cycles（8 m V）,as well as significant resistance to methanol.Density flooding theory（DFT）calculations further confirm that the adjacent Zn sites increase the charge density of the Fe sites facilitating the key species OH^*desorption process,thus improving the overall ORR performance.The Zn-air battery assembled with Zn/Fe-NC as air electrode exhibits remarkable power density(186.2 m W cm^-2)and outstanding specific capacity(814.6 m Ah g^-1).2)A three-metal Zn Fe₂₅Co₂₀-NC catalyst with high specific surface area was synthesized by a two-step pyrolysis method.Firstly,Si O₂ was used as a template and Zn Fe bimetallic MOF as a precursor,followed by Zn volatilization under high temperature carbonization to form a nitrogen-doped carbon skeleton(Zn Fe₂₅-NC)loaded with Zn Fe bimetallic.Then,urea was introduced as an additional nitrogen source and Co salt for secondary thermal activation to obtain Zn Fe₂₅Co₂₀-NC catalyst with a honeycomb-like structure.The large number of pore structures not only supports more active sites but also facilitates the mass transport of oxygen,oxygen-containing species and water.The nitrogen on the carbon skeleton can chemically coordinate with metals to form active sites,and the Zn Fe and Co sites dominate the oxygen reduction and oxygen evolution catalytic activities,respectively.The electrochemical test results showed that Zn Fe₂₅Co₂₀-NC displayed excellent performance as a bifunctional oxygen catalyst.The Zn-air battery assembled with Zn Fe₂₅Co₂₀-NC as air electrode exhibits remarkable power density(146.6 m W cm^-2)and outstanding specific capacity(780.2 m Ah g^-1)compared to commercial noble metals（Pt/C+Ru O₂）.