Synthesis And Electrochemical Performance Of Ni/Co Based Transition Metal Compounds As Electrode Materials For Supercapacitors

Due to their unique characteristics in terms of faster charge and discharge processes, long cycling life, high power density and great potency as power sources for applications in electric/hybrid electric vehicles and portable electronics, supercapacitors are widely considered to be one of the most perspect ive choices available for the rapid development of energy storage devices. There are two kinds of supercapacitors based on different ways of charge storage: electrical double-layer capacitors(EDLCs) and pseudocapacitors. Although EDLCs based on high surface area carbonaceous materials are widely used as commercial supercapacitors(SCs), pseudocapacitors with much higher specific capacitance, utilize fast and reversible surface or near surface reactions for charge storage, are competitive for high performance supercapacitors. Among various transition metal oxide electrode materials of pseudocapacitors, nickel or cobalt based oxides have attract our attention due to their higher theoretical capacitance, low cost, friendly to environment and good corrosion resistance in alkaline solutions. However, their poor conductivity and lower actual specifie capacitance still can‘t meet the requirements of practical applications. In this paper, starting with Co3O4 or Ni O, and then developing into N i/Co based hybrid microstructure aiming at the disadvantages of N i/Co based oxide. As-synthesized microstructures can improve electric conductivity and specific capacitance of pseudocapacitors to some extent when used as electrode materials. The main contents of our research are as follows:1. Hierarchical N i O ultrafine nanowires grown on mesoporous N i O nanosheets with Ni foam as a substrate synthesized by a facile and effective hydrothermal method showed exceptionally high specific capacitance with 1493 F g-1 at a current density of 3 A g-1. The specific capacitance can preserve ~58% retention of the initial capacitance from 3 A g-1 to 50 A g-1. The specific capacitance has ~87% retention after 2000 cycles, demonstrating superior electrochemical cycling stability. The remarkable electrochemical performance can make the unique hierarchical N i O nanosheets/nanowires to be a perspective electrode material for electrochemical energy storage applications.2. A facile method to synthesize hierarchical architectures of Co3O4 nanowires@Co3O4 ultrafine nanowires grown on Ni foam was developed. The unique architectures consisting of numerous ultrafine Co3O4 nanowires(shell) well grown on the surface of an Co3O4 nanowire(core) delivered remarkable electrochemical performance with ultrahigh specific capacitance with 1640 F g-1 at a current density of 2 m A cm-2. The specific capacitance has ~ 66% retention of the initial capacitance when the current densities increased from 2 m A cm-2 to 50 m A cm-2, demonstrating superior rate capability. In addition, the specific capacitance has no degradation even after 10000 cycles. Such fascinating capacitive behaviors could be related to ultrafine microstructures which are interconnected with each other, leading a unique core/shell hierarchical feature with rational interspaces among adjacent nanostructures. Thus, the as- formed Co3O4 nanowires@Co3O4 ultrafine nanowires are highly accessible to the electrolyte and engage in the reversible faradic reaction adequately, leading the improvement of electrochemical performance.3. We demonstrate the synthesis of highly ordered mesoporous Ni Co2O4 by nanocasting method and examine its electrochemical performance by means of cyclic voltammetry and galvanostatic charge-discharge method. The highly ordered mesoporous N i Co2O4 prepared using mesoporous silica KIT-6 as a template presents exceptionally high specific capacitance with 1699 F g-1 at a current density of 1 A g-1. Importantly, specific capacitance of the electrode has 104.1% retention after 10000 cycles. In addition, other 3D mesoporous nanostructures, mesoporous Co3O4 and mesoporous N i O, synthesized by similar nanocasting method also show outstanding pseudocapacitive performance. Thus, the effective design of highly ordered mesoporous electrodes demonstrated in this work offers a promising strategy for supercapacitors with superior electrochemical properties.4. We present a facile two-step hydrothermal method for the fabrication of hierarchical heterostructures of Ni O@MMo O4(M = Co, Ni) nanosheet arrays on Ni foam. In this hierarchical structure, numerous MMo O4 nanoflakes grow on a Ni O nanosheet and the integration of MMo O4 can improve the whole electrode‘s conductivity, leading an ideal pathway for electron and ion transport. The hierarchical N i O@MMo O4(M = Co, Ni) heterostructure electrode demonstrated remarkable electrochemical performance with high specific capacitance and predominant cycling stability, making it one of the perspective electrode materials for high performance supercapacitors.