Preparation Of High Performance Fe-N_x-C_y Based On Modified Carbon For Oxygen Reduction Reaction

As a key reaction in new energy conversion and storage devices(e.g.,fuel cells),oxygen reduction reaction(ORR)has the characteristics of slow reaction kinetics and high overpotential,which limits the commercial application of the devices.However,traditional platinum group metals(PGMs)are expensive and rare,so it is imperative to develop and design high-performance non-PGMs catalysts.Transition metal-nitrogen-carbon materials,especially iron-nitrogencarbon catalysts,have been studied extensively because of their excellent activity and low price.Fe-N macrocycle complexes,such as iron phthalocyanine(FePc),as the classic representatives of Fe-N-C electrocatalysts have been considered as the attractive alternative to Pt-based electrocatalysts for oxygen reduction reaction(ORR).FePc is a synthetically synthesized metal macrocyclic chelate with a highly conjugated system of 18 π electrons,and its central Fe-N4 structure shows catalytic activity for oxygen reduction.Due to the poor stability of molecular structure,FePc molecules rely on heat treatment to form more stable Fe-N-C/C coupling active sites.However,high temperature treatment will lead to the destruction and aggregation of active sites.Fe-N macrocyclic molecules have a contradiction in the reverse relationship between ORR catalytic activity and durability during heat treatment.Here in,we reported a feasible macromolecules structure stabilization strategy to controllable synthesize Fe-N active sites based on macromolecule iron precursor(polyphthalocyanine iron,FePPc)and nitrogen-rich organic polymer(polyaniline,PANI;polypyrrole,PPy;graphite phase carbon nitride,g-C3N4).Through in-situ polymerization of FePc on nitrogen-contained polymer modified carbon supports and subsequent hightemperature pyrolysis(700℃),we obtained a series of Fe-N-C electrocatalysts.Compared with FePc,the macromolecular polymer FePPc has higher stability due to the two-dimensional conjugated cross-linked structure.At the same time,the nitrogen source of the macromolecular structure we selected also has stronger thermal stability.In this work,the preparation of high-performance catalysts was achieved through reasonable selection of the precursor iron source and nitrogen source.First,carbon black BP2000 was used as a carrier,and polyphthalocyanine iron was loaded in situ to prepare C-FePPc material,and then pyrolyzed it at different temperatures to obtain a series of Fe-N-C structure catalysts.Studies have shown that the catalyst prepared based on FePPc has an intrinsic fourelectron reaction pathway,and has excellent oxygen reduction catalytic activity,but its catalytic activity decreases with the increase of pyrolysis temperature.The structure of polyphthalocyanine iron is unstable at high temperature(>700℃),and the active structure Fe-N4 is easily destroyed to reduce the intrinsic activity,but its activity still has a certain retention.In order to resolve the contradiction between the inverse relationship between the catalytic activity and durability of such catalysts,suppress the deactivation of the catalyst at high temperatures,and further reduce the release of the central metallic iron,carbon black is modified with high thermal stability nitrogen-containing macromolecular polymer.The modified carbon is used as a carrier to load polyphthalocyanine iron in situ.And the Fe-N-C type catalyst is prepared by pyrolysis.The strong coordination interaction between nitrogen and iron in the polymer anchors the iron,effectively inhibits the aggregation of iron and the destruction of the high coordination structure Fe-N4 during high temperature treatment,also provides more nitrogen to form Fe-N4 site.Compared with the non-added polymer electrocatalyst C-FePPc-700 and other electrocatalysts g-C3N4/C-FePPc-700 and PPy/C-FePPc-700,the PANI/CFePPc-700 catalyst shows the most ideal catalytic performance with a halfwave potential of 0.857 V in 0.1 M KOH,which is higher than Pt/C(0.837 V).Furthermore,the PANI/C-FePPc-700 performs well in catalytic stability and methanol resistance.