Study On The Stability Of Iron Nitrides And Their Carbon Coating Mechanism

Iron nitride is the most promising non-noble metal electrode material in the electrochemical storage.It displays excellent mechanical properties,wear resistance,corrosion resistance and magnetic properties.However,iron nitride exhibits thermal instability,which is easily oxidized in air.The relative stability of iron nitride phases was investigated by using density functional theory（DFT）.The calculated results show that Fe₂N and Fe₃N having the higher formation energies are more stable than other Fe-N phases.Fe₈N and Fe₂₄N₁₀ phases are metastable,which would be further decomposed into other more stable phases,i.e.Fe₈N→Fe₄N+Fe₄ and Fe₂₄N₁₀→6Fe₂N+4Fe₃N.Among these Fe-N phases,the ionic bond in Fe₈N phase is the strongest.Fe₄N nanoparticles were prepared using high-pressure gas-solid reaction bed system.The phases,morphology,and composition of Fe₄N nanoparticles were detected by X-ray diffraction（XRD）,scanning electron microscope（SEM）,energy disperse spectroscopy（EDS）,respectively.Fe₄N nanoparticles with uniform size and high purity were obtained.The oxidation process of Fe₄N nanoparticles were investigated by thermogravimertry-differential scanning calorimetry（TG-DSC）.It can be seen that the adsorption and dissociation processes of O₂ on Fe₄N occurred before releasing of nitrogen oxides.In the temperature range of 80-280°C,the peak strength of Fe₄N became weaker with increasing temperature.When the temperature was higher than 280°C,the product phase began to change obviously.When the temperature was higher than 430°C,the main product phase was Fe₂O₃.The adsorption mechanism of O₂ on iron nitride was investigated by DFT.The correctness of TG-DSC thermal analysis results was verified.For O₂Fe₄N cluster with ground state,one O atom tended to locate at the Fe-Fe bridge site,and the other O atom tended to locate at the center of the Fe-Fe-Fe plane far from the N atom.It indicates that O₂ molecule decomposes on the surface of Fe₄N molecule,and this is a two-step process.All O₂Fe₄N clusters still exhibit high kinetic activity.The adsorption of O₂ on Fe₄N by is a physical adsorption process.The carbon coating can prevent the oxidation of iron nitride.The carbon-coated iron nitride particles were prepared using high-pressure gas-solid reaction bed system.The phases,morphology,and composition were determined by XRD,SEM and EDS,respectively.Combined with the EDS results,in the sample at 300°C,part of the iron nitride has begun to be oxidized and formed intermediate species,but the reaction is not complete and nitrogen element has not completely disappeared.Compared to the fact that the iron nitride uncoated carbon has been converted into iron oxide at 280°C,it shows that the carbon layer in carbon coated iron nitride has a certain suppression on the thermal decomposition of iron nitride.The adsorption mechanism of Fe N on HCNO molecules was investigated by first-principles.It can be found that the Fe atoms of Fe N molecules tended to insert into the N and O atoms of HCNO molecules,showing the higher adsorption energy and chemical stability.The interaction mechanism between Fe_xN（x=1-4）molecules and carbon C_y（y=50、60、70）cages was investigated by DFT.It can be found that the N atoms prefer to occupy the centers of C_y cages.In Fe_xN@C_y（x=1-4,y=50,60,70）core@shell clusters,the Fe atoms were inclined to approach the centers of the hexagonal rings.The Fe N@C₅₀,Fe₂N@C₆₀ and Fe₂N@C₇₀ clusters display more stable than other clusters.It is of certain theoretical significance to understand the oxidation mechanism of iron nitride and analyze the heterogeneity of surface carbon layer and the difference of stability of different carbon-coated iron nitride particles.