Preparation Of Niobium-based Compound Composite Electrode Material And Research On Its Capacitance Performance

Supercapacitors are widely used in the fields of national defense,aerospace,transportation and electronic information due to their advantages such as high power density,fast charge-discharge rate and long cycle life.The rapid development of flexible wearable electronic devices has promoted the requirement of the flexible energy storage devices that tend to be portable,lightweight and flexible.The preparation and design of electrode materials are the key factors to determine the energy storage performance of flexible supercapacitors.Transition metal oxide has a higher theoretical pseudocapacitance,and Nb2O5,as an electrode material,has a unique pseudocapacitance energy storage mechanism,but its poor conductivity,low multiplier performance and poor stability during the cycle limit its wide application.To address those problems,in chapter ?,by using electrochemical deposition technique combination with thermal annealing method,we fabricate Nb2O5 nanorods on the surface of carbon cloth to achieve T-Nb2O5 composite electrode(CC-TN).The lowest conductivity of the composite electrode was 10 S cm-1,which was significantly improved compared with that of Nb2O5(3 × 10-6 S cm-1).The CC-TN electrode delivers a maximum specific capacitance of 1540 mF cm-2 in 3.0 M KOH electrolyte,and retains 86%of its initial capacitance when the current density increased to 50 mA cm-2.The design of niobium-carbon cloth composite electrode increases the conductivity of the electrode,improves the bonding force between the transition metal oxide and the substrate,and effectively solves the problems of low rate performance and poor cycling stability.In the process of electrochemical testing,it is found that the mechanism of energy storage is different from traditional pseudocapacitance mechanism.Further study indicates that the electrochemical charge storage process consists of two steps,electrochemically induced phase transition and intercalation pseudocapacitance behavior.Firstly,T-Nb2O5 could transform into[Nb6O19]8-in the strong KOH alkali electrolte.Then,under the interaction of electrochemical induction,the K+ were embedded in the ionic layer during the charging process to form a stable intermediate product of K3Nb7O19.While in the discharging process,K+ were de-intercalated and the product is K3Nb8O21.Therefore,the charge storage mechanism of the electrode relies on the K+intercalation reaction between K3Nb7O19 and K3Nb8O21.According to the above-mentioned reaction mechanism of CC-TN,in Chapater IV,we prepared the high-performance Nb-containing ternary compound electrodes by the electrochemical conversion of Nb2O5 electrode in different alkaline mediums.We found the atomic ratios,phases and electrochemical properties of the potassium niobate strongly depend on the alkaline conditions as well as the potassium precursor.The CC-KNbO3 composite electrodes by electrochemical conversion of Nb2O5 in KOH electrolyte deliver area capacitance of 638 mF cm-2.The electrode prepared in the mixed electrolyte of KNO3 and ammonia gives a higher area capacitance of 788 mF cm-2 with a slightly enhanced rate capability.In the mixed electrolyte of KCl and K2S2O8,the electrochemical conversion of Nb2O5 produced cubic KNb3O8.Its area capacitance further increases to 1844 mF cm-2 at the current density of 2 mA cm-2.When the current density increased to 50 mA cm-2,the capacitance retention rate is 53%.The high specific capacitance of KNb3O8 electrode is clearly due to its sheet-lihe morphology which could improve the utilization efficiency of electrode,and the presence of abundant spaces that allow easy penetration of the electrolyte into the interior electrode.