Investigations On The Stability And Fabrication Of Iron-nitrogen Coordination Oxygen Reduction Electrocatalyst

Fuel cells have received widespread attention for their high output power and environment friendly properties.The cathode reaction of fuel cells is oxygen reduction reaction（ORR）.Developing high-efficient and stable ORR electrocatalysts is one of the keys to realize the industrialization of fuel cells.In this paper,the bond strengths in Single atom catalyst（SAC）Fe SAC@G catalyst were studied by DFT calculation.Furthermore,an axial coordination structure was introduced,then a new kind of catalyst model Fe_xN@Fe was established and their ORR catalytic activity were investigated by DFT calculation.The failure processes of Fe SAC@G catalyst during discharging process in alkaline solutions were investigated and the failure mechanism was proposed.Based on the DFT calculations,Fe₄N@Fe catalysts were synthesized and related synthetic technology were explored.The morphology,structure,composition and performances of the catalysts were characterized by Raman,SEM,TEM,XPS,XRD and electrochemical measurements.DFT calculations show that the bond strength in Fe SAC@G catalyst increases in the following order:Fe-N<C-C_unit<C··N<C··C.Fe-N coordination bonds in Fe-N4active site are the weakest one,followed by C-C_unitcoordination bonds connecting the two adjacent structural units.To introduce multi-metal active sites and an axial coordination structure,Fe_xN@Fe catalyst models（x is an integer from 2 to 4）were established,in which the electrons tend to distribute toward inner part of Fe_xN@Fe.DFT calculations indicate that Fe₄N@Fe model possesses excellent structural stability,the best charge transfer number and proper oxygen adsorptive ability,which is suitable to act as an ORR catalyst.During potentiostatic discharge process,the ORR catalytic activity of Fe SAC@G catalysts decreases with the increase of KOH concentration,solution temperature and the negative shift of discharge potential.The failure of Fe SAC@G catalyst proceeds through the continue chemical bond breaking.A discharge failure mechanism about Fe SAC@G catalyst is proposed:as the discharge process proceeds,Fe-N coordination bonds in Fe SAC@G catalyst break firstly,resulting in the disappearance of Fe-N4 active sites and therefore the decrease of ORR catalytic activity.Whereafter,C-C_unitcoordination bonds,which connect the basic structural units in the structure of Fe SAC@G catalyst,fracture.Finally,C··N coordination bonds in the basic structural units rupture and organic fragments form,leading to the structural collapse of the catalyst and the formation of flower-like discharge by-products on the surface of the electrode.The designed Fe₄N@Fe catalyst was fabricated through a two-step synthetic method:Firstly,porous iron films were electrodeposited on the surface of copper films.Secondly,the surface layer of the porous iron films was nitride in ammonia atmosphere to form Fe₄N thin layer.The results indicate that electrodepositing current density and duration,the flow rate of ammonia,the nitriding temperature and duration all affect the formation of Fe₄N@Fe catalyst and therefore the ORR performance.The optimized synthetic technique was obtained:electrodepositing current density of 1.8 A cm^-2,electrodeposition time of 80 s,nitriding temperature of 300℃,nitriding time of210 min,ammonia flow rate of 30 sccm.The synthesized Fe₄N@Fe catalyst possesses high ORR performance stability,exhibiting excellent ORR performance approaching to commercial Pt/C catalysts.