The Research Of Pyrolyzed Fe-g-C₃N₄@C As An Efficient Non-precious Metal Catalyst For Fuel Cell Oxygen Reduction Reaction

The cathode oxygen reduction reaction (ORR) of proton exchange membrane fuelcells(PEMFCs) suffers from sluggish kinetics, which accounts for approximately80%of the Pt loading in fuel cell electrodes. However, the prohibitive cost, limited supplyand weak durability of Pt have significantly limited the commercialization ofPEMFCs. Consequently, as alternative materials, the development of non-preciousmetal catalyst(NPMCs) with effecient activity and durability for ORR is highlysought for fuel cell applications.Herein, we prepared a novel NPMC with high ORR activity and stability in acidicmedia, by pyrolyzing a precursor mixture containing carbon support, monomers ofgraphitic carbon nitride (g-C3N4) and Fe ions. The carbon black (Ketjenblack EC600JD, denoted as KB) and a composite of carbon black/reduces graphene oxide(KB/rGO) were used as carbon supports, accordingly, the resulting catalysts weredenoted as p-Fe-g-C3N4@KB and p-Fe-g-C3N4@KB/rGO, respectively. In this paper,the effects of heat-treating temperature、Fe content and the ratio of composite carbonsupport on ORR performance were investigated. The ORR activity and four-electronselectivity of Fe-N/C catalysts were characterized using a rotating disk electrode(RDE) and a rotating ring disk electrode (RRDE) respectively. The durability ofcatalysts was determined through an accelerated durability test (ADT). In order toexplore the surface morphology, crystal phase structure、surface element compositionas well as the possible ORR active sites of these catalysts, a variety ofcharacterizations, including EDS、XRD、IR、TG、XPS、SEM、TEM、STEM、N2adsorption-desorption isotherms were carried out.The ORR performance results reveal that: the p-Fe-g-C3N4@KB catalyst pyrolyzedat750°C with10%Fe content displays the optimal ORR activity and selectivity, witha mass activity of7.2mA mg-1at0.75V, furthermore, p-Fe-g-C3N4@KB exhibitssuperior durability in comparison to that of commercial20wt%Pt/C in acid media;while when using KB/rGO as composite carbon support, the ORR performance of theresulting catalyst was significantly improved, the mass activity reached up to18.5 mA mg-1at0.75V, which is2.5times the value of p-Fe-g-C3N4@KB. Furthermore,p-Fe-g-C3N4@KB/rGO is much more stable than p-Fe-g-C3N4@KB in acid media.The physical characterization results showed that:(1)The specific surface area,type of N doping, particle size and distribution of the metal particles, and thenanostructure of resulting catalysts are key factors affecting ORR performance;(2)Both N-doped metal particles and N-doped carbon supports are ORR active sites;(3)When using KB/rGO as composite carbon support, some novel nanostructures insitu grow on/between the graphene sheets interspaced by carbon nanosphereaggregates. The in-situ formed nanostructure includes metal particals encapsulated incarbon nanotubes (CNTs) and core-shell structures of a few layers of graphene shellstructure wrapped metal particles. These in-situ formed nanostructures, on one handcan prevent the rGO sheets from stacking, on the other hand can well protect metalparticles against acid corrosion, thus improving the ORR activity and long termstability;(4)The co-doping of O and S elements has a synergistic effect to improvethe ORR activity of the catalysts.