Preparation And Performances Of Nickel-based Metal-organic Framework Electrode Materials

As a new type of energy storage device,supercapacitor has attracted extensive attention owing to its fast charge/discharge rate,superior power density,long cycle life,safely and pollution-free characteristics.However,the low energy density of supercapacitor limits its large-scale industrial applications.At present,the main methods to improve the energy density of supercapacitor are developing of new electrode materials and the assembly of asymmetric supercapacitors.Metal-Organic Frameworks?MOFs?have become a potential new type of supercapacitor electrode material because of their controllable size,central metal ions with redox activity,high specific surface area and abundant pores.In this work,Ni-MOF and Ni-MOF composites with excellent electrochemical performance were prepared by employing the different synthetic strategies.We have studied the key factors in the electrochemical performance of Ni-MOF-based materials such as active additives,morphologies,sizes,phase composition,crytal structures,and so on.The further reasearh direction was put forwarded to promote the research and development of MOF-based materials in the field of electrochemistry.The Ni-MOF nanosheets were syntheisized by a simple solvent-thermal method,and its unique two-dimensional crystal structure was discussed in details.The charge stroage capacity of Ni-MOF was improved via using the redox addtives.Addition of K₄Fe?CN?₆ into electrolyte results in about 1.4 times increase in specific capacitance.The effect of K₄Fe?CN?₆ was investigated in details through the electrochemicl contrast experiments.We assembled Ni-MOF//CNTs-COOH supercapacitor with K₄Fe?CN?₆ electrolyte showing high energy density(55.8 Wh kg^-1),high power density(7000 W kg^-1)and long-term cycling stability,simultaneously.The Co/Ni-MOF and Zn/Ni-MOF electrode materials were constructed by adding cobalt nitrate or zinc nitrate.The XRD and FTIR tests showed that the introduced Co²⁺and Zn²⁺partly substituted for Ni²⁺in the MOFs,and the Ni-MOF two-dimensional crystal structure could be retained.The SEM measurements indicate that the Co/Ni-MOF and Zn/Ni-MOF have flower structure,which effectively alleviating the volume expansion caused by the electrochemical reaction.BET results indicated that the introduction of Co²⁺or Zn²⁺could improve the specific surface area and pore volume of electrode materials,and promote the effective contact between electrode materials and electrolyte ion.The charge storage capacity of Co/Ni-MOF and Zn/Ni-MOF electrode materials were better than that of Ni-MOF electrode material.Zn/Ni-MOF@PPy electrode material was prepared by a green and pollution-free“oxidant-free additive”synthetic strategy.The polymerization mechanism of pyrrole was investigated through contrast experiments.The results shown that the Zn/Ni-MOF could act as a Lewis acid catalyst to promote the oxidation of pyrrole in the air.XRD,FTIR,Raman and XPS characterization analysis proved that the oxidation polymerization of Py into PPy occurred not only on the surface of Zn/Ni-MOF but also between the layers of Zn/Ni-MOF crystal structure.The introduced PPy could be used as a good electron transfer channel to promote the electrochemical reaction.The effect of different composites amounts of PPy on the ionic diffusion resistance and conductivity of Zn/Ni-MOF@PPy electrode materials were systematically studied,and the content of PPy in the composite materials was optimized.The optimized Zn/Ni-MOF@PPy-0.15composite demonstrates excellent supercapacitor performance.A new type of flexible Ni-MOF-based homogeneous bipolar supercapacitor was developed,which could charge and store energy without the need of distinguishing the polarity of electrode,and effectively solve the irreversible damage caused to the asymmetric supercapacitor when the current is reversed.With this design,the controlled preparation of Ni-MOF materials grown on nickel foam was achieved by using Ni?OH?₂ self-sacrificing template method.The energy storage mechanism of Ni?OH?₂@Ni-MOF composite electrode material was revealed.A homogeneous bipolar supercapacitor with high energy density and high power density was constructed,which has important implications for the study of supercapacitors.