Preparation Of In-situ S Doped Atomic Dispersion Iron-nitrogen-carbon New Strategy And Mechanism Of Promoting Oxygen Reduction Reaction

Metal-air batteries and fuel cells are devices that directly convert chemical energy into electric energy,which are ideal power sources due to their inherent characteristics of high energy density,high energy efficiency,and CO₂-free emission.Oxygen reduction reaction（ORR）plays a predominant role in determining the overall energy conversion efficiency of these devices.Currently,Pt is the most effective catalyst for ORR.However,the high cost originating from the scarcity of Pt resources seriously hinders the large-scale commercial application of these technologies.As non-noble metal catalysts for ORR,metal-nitrogen-carbon（M-N-C）have received extensive attention as a potential alternative to platinum.However,the overall performance of the M-N-C catalyst still cannot meet practical requirements.Recently,heteroatom doping has been considered as one of the effective ways to further tune electronic structures of active sites in transition metal-nitrogen-carbon to improve catalytic activity for the oxygen reduction reaction.In particular,sulfur（S）-doped Fe-N-C demonstrated great potential to enhance ORR activity.Nevertheless,S-doping is considered to be more challenging than incorporating N due to its larger atom size and lower electronegativity.Therefore,in this work,we develop a precise synthesis of S-doped M-N-C via a molecular-level design and reveal their interaction mechanism for electrocatalysis.The main achievements are as follows:（1）We developed an in-situ S doping strategy of tuning the electronic structure of the center Fe atom in Fe-N-C to promote ORR,in which newly designed Fe（II）ion coordinated copolymers with N and S co-containing bis（imino）-pyridine based ligands were synthesized using a Schiff reaction of 2,6-diacetylpyridine with two diacetyl groups as the core structural unit and multi-amonia aromatic compounds as N source and 2,5-dithiobiuret as S source.It is advantageous for the formation of uniform coordination of the transition metal ion with N and incorporation of S dopant to use these kinds of copolymers capable of complexing transition metal ions（Fe,Co,Mn）.In addition,Na Cl is used as a sacrificial template for the copolymerization in the method,which is conducive to form open-structured Fe-N-C nanosheets,facilitating the fast mass transportation in ORR.The S-doped Fe-N-C（Fe-N-C-S）nanoflakes synchronized with DAP,TIT and DTB exhibited significant ORR activity at 850°C,and the half-potentials is 0.904V and 0.80V vs RHE in alkaline and acidic media,respectively.（2）A series of S-doped Fe-N-C catalysts with different ratios of N and S were prepared to study the electronic regulation effect of S-doping on Fe-N-C active site by using different molar ratios of carbamethidine and 2,5-dithiourea.The results show that the introduction of 2,5-dithiourea could promote the formation of Fe-pyrrole-N.When the S:N ratio reaches 3:2,the pyrrole-N relative content reaches the maximum,and the catalytic activity is also higher than other specific catalysts.This work provides a new idea and research direction for heteroatom-doped carbon material M-N-C catalyst.