Preparation Of Iron-Based/Nitrogen-Doped Carbon Composites And Study On Supercapacitor Performance

Supercapacitors have become the key devices of the new generation of energy storage because of their advantages of high-power density,long cycle life,fast charging and discharging,environmental protection and no pollution.As the electrode materials of supercapacitor,metal oxide materials and carbon materials have not been applied in large scale due to their partial defects.In this paper,iron oxide/nitrogen doped carbon composite（Fe₂O₃/NC）and nitrogen doped carbon coated iron and manganese bimetallic oxide composite（Mn Fe₂O₄/NC）were synthesized by two different synthesis strategies.The electrochemical properties of Fe₂O₃/NC and Mn Fe₂O₄/NC were investigated by adjusting the carbonization temperature and time.These two composite materials combine the advantages of metal oxide and carbon materials,and introduce heteroatomic N element to improve the electrochemical performance of the composite materials.The hybrid supercapacitors Fe₂O₃/NC//NC and Mn Fe₂O₄/NC//AC were assembled from the composites prepared under the optimal conditions,and their electrochemical properties were tested.Polypyrrole（PPy）,which contains abundant C and N atoms,is selected as C source and N source because of its homogeneous distribution of N in the structure of carbon materials and the increase of reactive sites.At the same time,metal oxides were introduced into the preparation process to prepare the composite with excellent electrochemical performance.Based on this,polypyrrole nanotubes were synthesized by soft template method.Then a new type of iron oxide/nitrogen doped carbon composite（Fe₂O₃/NC）was synthesized by hydrothermal and high temperature carbonization.The results show that Fe₂O₃is uniformly loaded on NC in granular form,which provides a favorable channel for electron transfer.In addition,the porous structure of NC is conducive to rapid ion transport.When the carbonization temperature is 800℃and the carbonization time is 2 h,the Fe₂O₃/NC composites have high specific capacity(840.9 C·g^-1at 1 A·g^-1)and good specific capacity retention rate(85.1%at 1～10 A·g^-1).A hybrid supercapacitor with a working potential window of 0～1.5 V was assembled using Fe2O3/NC composite as positive electrode and NC as negative electrode respectively.Hybrid ultracapacitors have good rate capacity(70%specific capacity retention at 1～10 A·g^-1)and high energy density(46.5Wh·kg^-1at 765.8 W·kg^-1).Multicomponent transition metal oxides usually exhibit better electrochemical performance than single-component oxides due to their diverse chemical compositions and synergistic effects among components.Among all kinds of transition metal oxides,iron and manganese oxides show obvious advantages in electrochemical performance,cost and environmental friendliness.The FM-C precursor was prepared by co-precipitation method,and then a conductive polymer containing N element（polypyrrole）was introduced on the surface of the precursor by room temperature polymerization.Finally,after high temperature carbonization,nitrogen doped carbon coated Fe-Mn bimetallic oxide（Mn Fe₂O₄/NC）composite was obtained.The results show that the prepared Mn Fe₂O₄/NC composite exhibits good electrochemical properties when the carbonization temperature is700℃and the carbonization time is 2 h.Under 1 A·g^-1,the specific capacity of 938.6 C·g^-1is displayed.After 5000 times of charging and discharging at 10 A·g^-1,the specific capacity remains 80.5%.The prepared Mn Fe₂O₄/NC composite was used as the positive electrode and activated carbon（AC）as the negative electrode.Mn Fe₂O₄/NC//AC hybrid supercapacitor was assembled.The device maintains an energy density of 58.5 Wh·kg^-1at993.1 W·kg^-1power density.Mn Fe₂O₄/NC//AC hybrid supercapacitor still shows a high energy density of 31.3 Wh·kg^-1even at 7785.6 W·kg^-1high power density.The specific capacity retention rate of 5000 cycles is 86.7%.