Preparation And Performance Research Of Carbon-based Oxygen Reduction Catalyst Derived From Metal Organic Framework

Oxygen Reduction Reaction（ORR）is important in new energy conversion technologies such as proton exchange membrane fuel cell（PEMFC）and metal air battery（MAB）.Compared with the anode reaction,the ORR reaction is slower kinetically and requires more Pt catalyst,which limits the overall performance and commercial application of PEMFC.Therefore,it is urgent to develop a cheap and efficient non-noble metal ORR electrocatalyst.Carbon-based catalysts are the most studied non-noble metal catalysts at present.Metal organic Frameworks（MOFs）are considered as the most ideal precursors.MOF-derived carbon materials have been proved to have great potential in ORR electrocatalysis and have been extensively studied as catalysts.In this paper,different MOFs were used as precursors to prepare MOF-derived carbon-based ORR catalysts,and their ORR properties in acid and alkaline solutions were studied.The main contents of the study are as follows:（1）A porous N-doped carbon-based catalyst was prepared by KCl-assisted pyrolysis of ZIF-8.In the process of pyrolysis preparation,KCl as etching agent and activator,permeates into ZIF-8 derived carbon materials through capillaries,which is beneficial to the formation of microporous and mesoporous carbon nanosheets.The nitrogen configuration and graphitization degree of carbon materials can be effectively controlled by adjusting the amount of KCl and pyrolysis temperature.The optimized NCNS-10-900 catalyst has a specific surface area up to～2495 m²·g^-1 and a rich porous structure,which is conducive to fully exposing the active site,improving the mass transfer process and enhancing the electron transfer rate.These synergies promote the half-wave potential of NCNS-10-900(E_1/2=0.88 V)to be higher than 20%Pt/C(E_1/2=0.86 V)in alkaline solution.The electron transfer number is～4.0,which is 4 electron paths,showing excellent ORR catalytic activity.Accelerated cycle durability test shows the half-wave potential drop of NCNS-10-900(ΔE_1/2=17 m V)smaller than that of Pt/C(ΔE_1/2=34 m V),have excellent long-term stability.（2）MnCl₂ coordination with HMT to obtain Mn HMT coordination polymer（Mn HMT）,then the atomically dispersed Mn-N-C catalyst was prepared by pyrolysis.The FT-IR and elemental analysis confirmed that HMT is a double-toothed bridging ligand,in which two N atoms were coordinated with Mn²⁺to form a serrated chain structure of the[MnCl₂（HMT）_x（H₂O）_y]_n unit.With different molar ratios of MnCl₂/HMT,Mn HMT presents two different morphologies:massive and nanocrystalline.The characterization confirms that the prepared Mn atoms dispersed carbon nanosheets（Mn NCS）from direct pyrolysis of Mn HMT shows the morphology of highly crumpled paper ultra-thin carbon nanosheets with atomically dispersed Mn-N_x sites,with high specific surface area(～2006 m²·g^-1)and hierarchical porous structure.The half-wave potential(E_1/2=0.89 V)of the optimized Mn NCS-4-800 electrocatalyst under alkaline condition is better than that of 20%Pt/C(E_1/2=0.87 V).In addition,it also showed good ORR activity(E_1/2=0.76 V)in acidic medium,which was close to 20%Pt/C(E_1/2=0.80 V).The reaction path was almost direct 4-electron path,which could effectively reduce O₂ to H₂O.Accelerate circulation test shows the E_1/2 of Mn NCS-4-800 decreased(alkaline:ΔE_1/2=11 m V,acidic:ΔE_1/2=20 m V)to less than 20%Pt/C(alkaline:ΔE_1/2=20 m V,acidic:ΔE_1/2=35 m V).At the same time,Fenton test also showed that the reactive oxygen level of Mn NCS-4-800 was much lower than that of Fe-N-C,which confirmed the excellent long-term stability of Mn NCS-4-800.The rechargeable Zn-air battery composed of Mn NCS-4-800 shows an OCP of 1.50 V and a peak power density of130 m W·cm^-2.Ddensity functional theory（DFT）calculations show that O₂ can be activated and protonated at the Mn-N₄-C₁₂ site,which can optimize its binding ability with the reaction intermediates（*O,*OH and*OOH）,thus accelerating the whole ORR process,indicating that the efficient 4e ORR mechanism occurs on Mn-N₄-C₁₂ site.This work is of great significance for the synthesis of Mn-based MOF and Mn-N-C ORR catalysts and their applications in energy conversion systems.We believe that the construction of Metal-HMT（MHMT）through different coordination methods provides lots of possibilities for the topological structure,which is a fast and efficient method to prepare two-dimensional carbon materials derived from MHMT,especially M-N-C catalysts.