Preparation And Performance Of M/N/C Electrocatalysts For Oxygen Reduction Reaction

Proton exchange membrane fuel cell(PEMFC)and metal-air battery have attracted extensive research due to their low or zero emission and high efficiency.Oxygen reduction reaction(ORR)is a key reaction in these advanced energy technologies.However,the ORR involves multi-electron transfer process,whose kinetics is slow and requires a large number of electrocatalysts to lower the reaction energy barrier and speed up the reaction rate.The practical application of Platinum based materials(PGM)catalysts is still hindered by their high prices,scarce resources,and low stabilities.At present,low-cost and earth-abundant non-platinum catalysts are facing the challenges of low activity and low stability.Therefore,it is very important to develop an efficient and stable non-platinum catalyst.For the controllable design and performance control of cathodic non-noble metal catalysts for PEMFC,the main contents of this paper are as follows:(1)The preparation and study of high bifunctional performance Fe/N-C oxygen catalyst(ⅰ)We designed an atomically dispersed Fe/N-codoped double-shelled hollow carbon nanosphere catalyst(mark as AD-Fe/N-C)by self-template and polypyrrole encapsulation method.The porous Fe3O4 hollow nanospheres served as the templates and Fe source,and pyrrole was N source.The polypyrrole wrapped Fe could effectively prevent the iron metal agglomeration to ensure high dispersion,meanwhile,the AD-Fe/N-C catalyst synthesized by pyrolyzing the Fe-containing polypyrrole formed specific open double-shelled hollow architecture.(ⅱ)The electrochemical experiments reveal that AD-Fe/N-C-900 catalyst exhibits a high bifunctional catalytic activity of 0.655 V in potential between ORR(E1/2=0.927 V vs.RHE)and OER(Ej=10=1.582 V vs.RHE)difference,along with good stability(only 3 mV loss after 10,000 cycles)for the ORR.Based on our results and analysis,the enhanced electrocatalytic activity of AD-Fe/N-C-900 catalyst might be attributed to its high content of mesopore,high density of Fe-Nx active sites,and open double-shelled hollow structure.(2)The preparation and study of high acticity and high stability FeCo/N-C catalyst(ⅰ)We designed a CoFe@C nanoparticles supported on FeCo/N-C catalyst through pyrolyzing the precursor which was made by in situ doping Fe-Co bimetal during the growth of Zn-MOF.RDE test results show that the Eonset of Fe3Co15/N-C catalyst is 0.981 V(vs.RHE),the E1/2 is 0.851 V(vs.RHE),and jm@0.8V is close to 60 A g-1,showing excellent stability in the AST.(ⅱ)Through XPS,TEM,SEM,XRD and so on,we found that:1)the doping of Co could form the new active sites,which further improve the ORR activity of Fe3/N-C catalyst;2)Fe-Co-codoping could form rich pore structure,increase the exposure ofactive sites,improve proton transport,and thus improve the ORR activity of the catalysts;3)DFT calculation demonstrated that the Fe4/Fe2Co2 could stabilize the high-spin FeN4 structure and make O2 tend to adsorb on FeN4 in horizontal rows,thus enhancing O2 adsorption,activating the O-O bond(elongating the O-O bond),greatly reducing the oxygen dissociation energy barrier,and making the O-O bond break more easily.(ⅲ)Through the ABTS probe and XPS results before and after AST,we found that the doping of Co could weaken the Fenton reaction of the Fe-based catalysts,reduce the content of the reactive oxygen species,partly inhibit the carbon corrosion,slow the loss of metal center,and thus improve the stability of the catalysts.