Oxygen-Deficient Defects Induced High Performance NiCo₂O₄ Nanowires And Exploring Its Mechanism For Next Generation Supercapacitors

With the continuous depletion of traditional fossil fuels,renewable energy development and utilization has become the mainstream.As a kind of energy storage devices,supercapacitors have attracted much attention of researchers because of their high power density,safe and reliable cycle life and environmental friendliness.In recent years,as a power supply for spare power supply,electronic devices and even electric vehicles,supercapacitors have been applied in the fields of social life and industrial production.However,poor conductivity and relatively low energy density have been restricting the development of supercapacitors.In this paper,we study the effect and mechanism of oxygen vacancies in the NiCo₂O₄ electrode material.The main contents of this paper is concluded as follows:?1?In situ growth of NiCo₂O₄ nanoelectrode material on the surface of nickel foam was prepared by hydrothermal method using cobalt nitrate as raw material,urea and ammonium fluoride as additives.The XRD,TEM,EDX,SEM and other characterization techniques were used to analyze the morphology and structure of NiCo₂O₄ electrode materials under different treatment conditions.It was found that the growth of pristine NiCo₂O₄ electrode material was mainly grew on the nickel foam framework,and the morphology was mainly based on the nanowires grown on the nanosheets.The electrochemical properties of the pristine and hydrogenated samples were tested.It was found that the specific capacity reached the maximum value at 3 h hydrogenation.The conductivity of the samples was found to be boosted after hydrogenation,which indicated that the hydrogen reduction could enhance the electrochemical performance of NiCo₂O₄.?2?The NiCo₂O₄ nanowires were grown as the contrast material?denoted as NCO?,and the NiCo₂O₄ precursors were annealed under different atmosphere to prepare NiCo₂O₄ with different crystallinity.Processed NiCo₂O₄ nanowires were characterized by XRD,TEM,SEM and so on.The effect of different annealing atmosphere on the morphology and electrochemical properties of NiCo₂O₄ nanowires was studied.With the comparative analysis of the electrochemical performance test,it is found that the samples after nitridation in mixed atmosphere?N2:Air=9:1?have best comprehensive properties.In addition,the different nitrided time samples were characterized by high resolution transmission electron microscopy.With the increasing proportion of N₂ in annealing atmosphere,the crystallinity of NiCo₂O₄nanowires is getting worse,showing that the NiCo₂O₄ precursor can't get enough oxygen in the reaction during annealing,resulting in a product with oxygen vacancies defect.We further observed the post-mortem morphology of NCO and NCO-N?annealing precursor in pure nitrogen?nanowires,found that the electrochemical reaction of NCO-N samples have been changed from the common surface/interface reaction into the bulk reaction,indicating crystal defects of electrode materials can promote the electrochemical reaction with electrolyte.?3?On the basis of previous studies on the electrical properties of transition metal oxides with oxygen vacancies defects in the crystal structure,the mechanism of oxygen vacancies improving the performance of NiCo₂O₄ nanowires is revealed.The hydrogenated and nitrided samples were analyzed by XPS curve,and oxygen vacancies of NiCo₂O₄ nanowires were found in the surface of hydrogenated and nitride samples.Based on density functional theory analysis,the existence of oxygen vacancies in transition metal oxides could change its band structure.The introduction of oxygen vacancies can generate impurity band between the semiconductor electrode materials the band,decrease the band gap and reduce energy required by the electron transition to the conduction band.The electronic conductivity would be improved caused by the oxygen vacancy existed in the electrode materials.In addition,through the analysis of XPS data of nitrided NiCo₂O₄,found that the existence of a large number of oxygen deficiency in the nanowire surface makes the O atom with low coordination number state O^- ion?bond energy of 531.8 eV?,the binding force of this ion and metal cations is very weak,effectively reducing the redox reaction barrier.More electrode materials can participate in the electrochemical reaction,enhance the utilization of active material.