Research On Non-noble Metal MOF-based Derivative Catalysts For Fuel Cell Cathodes

Oxygen reduction reaction(ORR)is an important cathode reaction process in fuel cell.The kinetics of oxygen reduction is relatively slow.In order to reduce the energy barrier of the reaction,a high activity catalyst is needed to improve the reaction speed.The transition metal catalyst supported by nitrogen doped carbon carrier(M-N-C,M refers to transition metal such as Fe,Co,Mn,etc.)is one of the most promising ORR catalysts,but at present,it is necessary to increase the number of catalytic active sites of m-n-c catalyst and establish the structure-activity relationship of catalyst There are still challenges and lack of in-depth understanding.Based on the above problems,the bimetallic Mn Co-ISAs/CN catalyst was synthesized with MOFs with high specific surface area and high conductivity as carbon source and nitrogen source respectively.The Fe/SCN single atom catalyst was prepared by low temperature freezing pyrolysis method.Through a series of physical characterization and electrochemical performance tests,the following results were obtained:(1)The feasible strategy of anchoring Mn Co single atom catalyst(Mn Co-ISA_S/CN)on carbon and nitrogen materials was obtained by in situ derivation of metal organic framework(MOF).The synthesis method is based on the pyrolysis process of Zn/Co bimetallic MOF.Zn atoms decompose at high temperature lead to the formation of microporous carbon.Metal ions are trapped in the molecular cages to obtain the carbon nitrogen skeleton of Co modified MOF,and then the Mn atoms are adsorbed on the vacant sites through adsorption pyrolysis.Due to the doping of volatile Zn metal nodes,the space distance between adjacent metal ions is extended,and the agglomeration of metal ions is effectively prevented in the process of high-temperature pyrolysis.Finally,monodisperse Mn-N_x and Co-N_x coordination active sites are formed on the carbon nitrogen carrier.Through electrochemical test,Mn Co-ISA_S/CN showed excellent half wave potential(0.88 V)and strong catalytic stability in all reported non noble metal based electrocatalysts.Therefore,the conclusion is that the catalysts with Co-N_x and Mn-N_x sites have more effective ORR activity than those with only Co-N_x sites.This work will provide new opportunities for the design of ORR efficient catalysts.(2)Using a low-temperature freezing pyrolysis strategy,iron ions were directly bonded to the imidazolium ligands of zeolites imidazolium framework(ZIF)by using potassium thiocyanate as the S source,and the S-doped high-performance monoatomic Fe/SCN catalyst was derived,which greatly improved the ORR catalytic activity.The S atom was successfully doped into the carbon micropore by XPS.The half wave potential of S-doped Fe/SCN is 0.801 V(vs RHE)which is more positive than that of Fe/CN catalyst(0.776 V vs RHE)by 25 m V through electrochemical measurement,and it shows direct four electron reduction reaction and excellent catalytic activity of oxygen reduction.In acid condition,after 10000 cycles,Fe/CN catalyst decays 25 m V at half wave potential,while Fe/SCN catalyst decays only 12 m V at half wave potential,which shows that Fe/SCN has excellent stability.