Preparation And ORR Performance Of Carbon Nanotube-doped Fe, Ni-N/C Catalysts

Electrochemical oxygen reduction has attracted a lot of attention due to its application in Metal-air cell and proton exchange membrane fuel cell.Precious metal material(PMG)has been a high activity electrocatalyst for ORR reaction.As a cathode catalyst with good catalytic activity,precious metal Pt based catalyst(PMG)has not been widely used due to itsscarcity and slow reaction kinetics.Fe-based catalysts have the characteristics with high content and low price,becoming a promising non-precious metal(PMG-free)electrocatalyst.However,Fe-N/C catalyzed intermediate H2O2 which will react with Fe2+,destroy the Carbon Matrix and seriously affect the stability of the catalyst.In order to improve the activity and stability of Fe-based catalysts,this paper Fe nanoparticles(Fe,FeCx,FeNx)were coated in the graphene layer by Ni salt functionalized ZIF-8,in this paper,the influence of Ni doping on the morphology,structure and ORR performance of Fe-N/C catalyst was studied by functionalizing ZIF-8 with Ni salt.The structure of the catalyst was characterized by powder X-ray diffraction(XRD),physical adsorption(BET),field emission scanning electron microscopy(FESEM),field emission transmission electron microscopy(TEM),X-ray Photoelectron Spectroscopy(XPS).The formation mechanism of the catalyst and its electrocatalytic activity in oxygen reduction reaction were studied.(1)Fe,Ni-N/C catalysts with CNTs structure were derived by using functionalized ZIF-8 with nickel acetate as self sacrificial template.SEM and STEM analysis show that the intermediate pyro lysate Ni-N/C has a large diameter and "bamboo like" carbon nanotube structure,which facilitates large-scale transportation of O2 and exposures the internal active sites of Ni-Nx.While Fe,Ni-N/C has a large diameter CNTs and maintains a good dodecahedral structure.Fe,Ni-N/C forms graphene coated Fe-NX active sites and FeNi alloy.XPS shows that there is a synergistic coupling between FeNi alloys,and Fe,Ni-N/C have lower Fe0 binding energy than traditional Fe-N/C catalysts,which changes the electronic structure of surface carbon and provides more CN defect sites with a positive charge.In ORR reaction,the initial potential(E0)of Fe,Ni-N/C in 0.1 mA cm-2 was 1.08 V,which was higher than Pt/C and better than Ni-N/C(E0=0.91 V)and Fe-N/C(E0=0.97 V).The stability of Fe,Ni-N/C is better than Pt/C and Fe-N/C in 36000s durability cycle.(2)The effects of Ni source on the structure and ORR Properties of Ni-N co-doped carbon(Ni-N/C)and FeNi-N co-doped carbon(Fe,Ni-N/C)were investigated.Firstly,Ni-N/C-a?Ni-N/C-b?Ni-N/C-c which Ni(CH3COO)2?NiCl2?Nickel acetylacetonate as nickel sources respectively were synthesized by self-template method.BET and STEM show that Ni-N/C is a kind of nano-sized sheet mesoporous material with large amount of CNTS,which is beneficial to mass transfer and gas diffusion.Ni-N/C-b with thin porous nanosheets and unique "honeycomb" CNTS structure has obtained high ORR performance with a starting potential of 0.94 V.Secondly,FeNi-N co-doped carbon(Fe,Ni-N/C-a,Fe,Ni-N/C_b,Fe,Ni-N/C-c)were prepared.XPS results show that the Fe0 binding energy of Fe,Ni-N/C-a is lower than Fe,Ni-N/C-b and Fe,Ni-N/C-c,FeNi has the greatest synergistic coupling effect.In ORR test,Fe,Ni-N/C-b starting potential(E0=1.0 V),stability(current density decreased to 74.9%of initial value after 33000 s)and methanol toxicity were slightly lower than Fe,Ni-N/C-a(E0=1.08 V and current density decreased to 84.5%of initial value after 33000 s).The results show that Ni(CH3COO)2 as a nickel source for further synthesis of Fe,Ni-N/C-a has the best performance in improving ORR performance.With the best graphene coating structure,Fe,Ni-N/C-a is superior to commercial Pt/C.