Study On Single Atom Manganese/Zinc-nitrogen Co-doped Carbon-based Non-noble Metal Oxygen Reduction Reaction Catalyst

In recent years,oxygen reduction reaction（ORR）is one of the most important reactions of green energy power generation systems such as fuel cell and metal-air cell etc.Developing non-noble metal catalysts for ORR to replace Pt based precious catalysts is highly desirable to promote development of green energy technology.Numerous researchers have made great efforts on studying highly efficient and low-cost cathodic ORR catalysts to replace the expensive commercial Pt/C catalysts.Transition metal-nitrogen co-doped carbon catalyst（M-N-C）have set off as one of the most promising non-noble metal electrocatalysts for ORR.Based on the strong complexing ability of dimine pyridine polymer precursor to manganese and zinc,a variety of monatomic dispersed Mn-N-C Zn-N-C and Mn/Zn-N-C catalysts were prepared.The result shows that their oxygen-reducing activity in alkaline and acid solutions and their application performance in zinc-air batteries are superior to that of commercial Pt/C.At the same time,MN_xis confirmed as the main active sites for ORR and their specific mechanical mechanisms was explored.The main contents in this work is as follow:（1）2,6-diacetylpyridine and 2,6-diaminopyridine were selected as reactants to synthesize a dimine pyridine based polymer precursors.The manganese diimine pyridine organic polymer was obtained by the complexation reaction of the polymer to manganese ions.The high density and highly dispersed manganese single atom coordination nitrogen doped carbon-based catalyst was prepared by pyrolysis.The active site of Mn N_xgroup in Mn-N-C catalyst plays a major role in improving catalytic activity for ORR.In addition,the Mn-N-C catalyst has a high specific surface area and hierarchical pore structure,which is conducive to the transport of exposed active sites and O₂.These properties make Mn-N-C900 exhibits superior ORR activity and stability than that of Pt/C electrocatalyst.（2）Dual-pyridine ligands was prepared by using 2,6-diacetyl-pyridine and2,6-diaminopyridine（DMP）as reactants.The catalyst was regulated by adding different manganese salt precursors(Mn Cl₂,Mn NO_3,Mn（CH₃COO）₂).Mn-N-C catalysts with carbon matrix with different graphitization degrees were prepared.XRD,Raman and XPS results show that the graphitization degree of MN-N-C Mn（CH₃COO）₂catalyst is relatively the highest.The electrochemical stability test also shows that the catalyst has the best stability.However,it is worth noting that the ORR activity of the catalyst is relatively poor（half-wave potential 0.8V）,which is 80m V lower than that of the MN-N-C Mn Cl₂catalyst.This indicates that the improvement of graphitization degree is beneficial to the stability of the catalyst,but may affect the activity of the catalyst to some extent.Therefore,to obtain a more efficient and stable cathode catalyst,it is necessary to increase the number of active sites or enhance the intrinsic activity of active sites while improving the graphitization degree of catalyst carrier.It also provides a new idea for the research of high efficiency and low cost cathode non-noble metal catalyst.（3）Dual-pyridine ligands was prepared by using 2,6-diacetyl-pyridine and2,6-diaminopyridine（DMP）as reactants.The manganese dual-pyridine organic polymer was synthesized by complexing manganese and zinc ions.The final catalyst（Mn/Zn-N-C）was obtained by pyrolysis.By means of XRD,TEM,XPS,XAS and other test methods,it is proved that the metal in the catalyst is monatomic dispersed in the form of Mn N_x,Zn N_xmoieties.Monatomic metal active sites were directly observed in HAADF-STEM mode.It was further proved that Mn N_x,Zn N_xwas the main active site through toxicological experiments.In addition,through the preparation of MN-N-C,Zn-N-C monometallic catalyst and the comparative analysis of bimetallic catalyst,we found that the synergistic effect of Mn and Zn atoms increased the number of active sites in the catalyst.So the bimetal catalyst has higher ORR activity.The electrochemical test showed that Mn/Zn-N-C catalyst had excellent ORR activity（half wave point was higher than Pt/C 40 m V）.The zinc-air battery test further proved that the catalyst had high ORR activity with a maximum power density of 153 m W cm^-2,which also verified the practicability of the catalyst.