The Investigation On Surface/Interface Of Fe-N-C Oxygen Reduction Electrocatalyst

Proton exchange membrane fuel cells(PEMFCs),as promising energy conversion devices,have received widespread attention thanks to the advantages of high energy efficiency,quick refueling and zero pollutant emission.However,the commercialization of PEMFCs is greatly impeded by the high cost of Pt catalysts.Due to the sluggish kinetics of cathodic oxygen reduction reaction(ORR),90%of Pt was required in cathode to reduce ORR overpotential.Therefore,exploring non-precious metal(NPM)catalysts to replace Pt catalyst is essential for large-scale applications of PEMFCs.Ternary Fe/N/C catalysts have been considered as promising NPM catalysts to substitute Pt catalysts for ORR.Remarkable progress has been achieved in the synthesis of active Fe/N/C-based catalysts in recent years.However,several challenges still remained for the commercialization of NPMCs:1)Mass activity or mass-transport property needs to be improved further;2)The mechanisms for initial rapid decay and the followed slow decay are still obscure and the efficient strategies to improve the durability of Fe-N-C are highly desirable.3)The main active site structure for pyrolyzed Fe-N-C catalysts is still under debate,meanwhile,we are still lack of the agreement on the understanding of the tolerance of Fe-N-C catalysts toward organics or gas impurities.In terms of practical meaning,this work intended to solve the following problems:Improve the activity and durability of Fe-N-C catalysts;make clear the mechanism of the initial rapid decay;make an agreement on the understanding of the tolerance of Fe-N-C catalysts toward organics.In terms of scientific issue,this work focused on the influence of surface/interface properties on kinetic activity and mass-transport properties of Fe-N-C catalysts.On the one hand,by adjusting the surface/interface properties of Fe-N-C catalysts,to improve mass activity of Fe-N-C catalysts and stabilize gas-liquid transport channels,and consequently boost the activity and durability of Fe-N-C catalysts.The details are as follows:1.We found that iron salts can greatly influence the ORR performance of pyrolyzed Fe/N/C catalyst,and Fe(SCN)3 can result in S-doping catalyst with high surface area,thereby leading to high ORR activity.The maximal power density of PEMFC using as-prepared S-doping Fe/N/C can reach as high as 1.03 W cm-2,demonstrating its promising application in PEMFCs2.We successfully built up a triple-phase interface in the micropores of a pyrolyzed Fe/N/C catalyst by controlling the distribution of hydrophobic DMS with just partial occupation inside of the micropores.This approach addressed effectively the problem of micropore water flooding of the Fe/N/C catalyst.As a result,the elaborately constructed Fe/N/C-based DMFC could yield a power density and short-term durability comparable to those of commercial Pt/C-based DMFCs3.We report a new strategy to significantly improve the stability of the Fe/N/C catalysts in PEMFCs by covalent grafting of a trifluoromethylphenyl(Ar-CF3)group.The hydrophobic character can effectively prevent water flooding of the Fe-N-C catalyst layer,and thus form robust mass-transport channels for gas liquid two-phase flow.Simultaneously,both electron withdrawing and hydrophobic properties considerably suppress the oxidative corrosion of the carbon matrix that hosts the catalytically active sites.Therefore,fluorinated Fe/N/C could perform stably over 100 h at 0.5 V with a current density of 0.56 A cm-2 in a H2-O2 PEMFC.Even when the cell voltage increased to 0.6 V,only 15%performance was lost after 100 h operation.On the other hand,by illustrating the influence of surface/interface on mass-transport properties of Fe-N-C catalysts,to provide a new mechanism for the initial rapid decay,meanwhile,make an agreement on the understanding of the tolerance of Fe-N-C catalysts toward organics.The details are as follows:4.We proposed a new mechanism for the rapid initial performance decay from the aspect of proton transport in micropores:Alkaline pyridinic N,located in micropores of Fe/N/C catalysts,is gradually protonated to form pyridinium cation,which not only can consume the proton of Nafion,but also form a space charge layer to electrostatically repel the proton,impeding its transport through micropore channels to active sites.This study shows new insight into the instability of Fe/N/C catalysts in PEMFCs5.We made an agreement on the understanding of the tolerance of Fe-N-C catalysts toward organics:a)In alkaline medium,the suppression effect of organic molecules on Fe/N/C catalysts originates from the blockage of transport channels by organic molecule adsorption,thus impeding the transport of ORR-related reactants and decreasing the ORR activity.The larger the molecular size or the lower polarity of organic molecules or the more micropores content of Fe-N-C catalysts,the more the ORR activity of target catalyst decreases.b)Unlike in alkaline medium,microporous-type Fe/N/C catalysts exhibited excellent resistance to organic molecules in acidic medium.c)Such drastic difference comes from the change of microporous surface polarity,caused by the protonation/deprotonation of pyridinic-N,thus altering the adsorption capacity of organic molecules in micropores.