Morphology, Structure Control And Catalytic Performance Of Fe-NC Carbon-based Catalysts

The fuel cell can directly convert chemical energy into electrical energy with high efficiency,and is not limited by the efficiency of the heat engine Carnot.Currently,the cathode oxygen reduction reaction(ORR)of fuel cells is slower than the anode oxidation(HOR)reaction kinetics,and more precious metal platinum(Pt)is needed to promote the cathode oxygen reduction(ORR)reaction.However,the scarcity of platinum resources makes platinum-based catalysts expensive,which limits the large-scale commercial application of fuel cells.Therefore,it is of great significance to develop non-noble metal catalysts(NPMCs)used in fuel cells to replace platinum-based catalysts.Among the many non-precious metal catalysts NPMCs,carbon-based catalysts composed of cheap and readily available earth-rich transition metals(M=Fe,Co,Cu,etc.)and heteroatoms(N,P,B,etc.)and carbon elements are widely used attention.In particular,the Fe-N-C catalyst oxygenation(ORR)obtained by doping Fe and N has the best activity,and its activity and stability are close to the commercial carbon-supported platinum catalyst(Pt/C).Although the prospect of Fe-N-C catalysts has always been optimistic,however,one of the main problems of Fe-N-C catalyst is that when there is a large amount of Fe,it dissolves in the electrolyte to form Fe²⁺,and the free radicals generated by Fe²⁺and H₂O₂through the Fenton reaction cause damage to the organic film in the fuel cell,resulting in the battery performance is degraded or even fails.In recent years,in order to reduce the side effects caused by the Fenton reaction,three-dimensional porous nitrogen-doped carbon materials without Fe or very little Fe content have attracted much attention as fuel cell cathode oxygen reduction(ORR)catalysts.To solve the above problems,in this paper,chemical oxidative polymerization method is used to adjust the amount of different types of initiators to induce the polymerization of aniline monomers and obtain iron salt-polyaniline conjugated precursors with different morphologies and structures.Then,the iron salt-polyaniline conjugated precursor was pyrolyzed under a nitrogen atmosphere,and the structure of the conjugated precursor was successfully inherited into the final carbon nanostructure of the Fe-N-C catalyst.We used FTIR,Raman,TEM,SEM,XRD,XPS and other instruments to characterize the iron salt-polyaniline precursor and Fe-N-C catalyst,combined with the electrochemical test results of Fe-N-C catalyst to get the following conclusions:(1)By adjusting the molar ratio of initiator to aniline,iron salt-polyaniline conjugated precursors with different morphologies and structures,such as polyaniline nanoparticles,elongated polyaniline fibers and stubby polyaniline fibers,were obtained;(2)The average diameter(La)of the graphite sheet,and the thickness(Lc)of graphite crystallites,and the ratio of disordered carbon,and specific surface area of that finally obtained the Fe-N-C catalyst were controlled indirectly by adjusting the molar ratio of ammonium persulfate to aniline,where La was between 5.2 and 7.2 nm increases first and then decreases,Lc first increases and then decreases in the range of 1.2?2.2nm,and the number of stacked layers obtained by Lc/d₀₀₂varies between 3 and 6 layers;(3)The type and content of nitrogen(pyridine nitrogen and graphite nitrogen)in the Fe-N-C catalyst can be adjusted,by adjusting the molar ratio of hydrogen peroxide(H₂O₂)to aniline,the contents of pyridine nitrogen and graphite nitrogen increase with the increase of the amount of hydrogen peroxide;(4)The best-performing Fe-N-C catalyst prepared with ammonium persulfate as the initiator has a half-wave potential(E_1/2)higher than the Pt/C(JM-20%)catalyst 50m V in an alkaline environment;The best-performing Fe-NC catalyst prepared by hydrogen oxide has a half-wave potential(E_1/2)in an alkaline environment lower than 73m V of the Pt/C(JM-20%)catalyst,both of which have good durability and methanol tolerance.