Preparation And Electrochemical Properties Of Transition Metal Oxides Supported On Nickel Foam

Supercapacitors,also known as electrochemical capacitors,are recognized as important energy storage and conversion device among conventional physical capacitors,secondary batteries and fuel cells.In future,supercapacitors has a promising future as potential power source for electric vehicles.The commercial electrical double layer capacitors at present use carbon-based materials as electrode materials.However,the low specific capacitance and energy density(<10 Wh·kg^-1)limits its application in electric vehicles areas.Recently,in order to achieve higher specific capacitance and energy density,lots of researches have been dedicated to the investigation of transition metal oxides pseudocapacitive materials.In recent years,many researches focus on the preparation of the nanostructured transition metal oxides materials directly grown on the conductive substrates.These conducting additive-free and binderless materials directly used as electrodes for supercapacitors exhibit excellent electrochemical capacitances.In this dissertation,CuO?Co₃O₄ and Ni?OH?₂ materials supported on nickel foam have been prepared by different methods,and directly used as electrodes for supercapacitors.We used Ag to modify and improve the poor electrical conductivity of CuO material.Finally,an asymmetric supercapacitor has been fabricated successfully using Ni?OH?₂/Ni-foam as positive electrode and activated carbon as negative electrode in 6 mol·L^-1 KOH aqueous electrolyte.The microstructure and morphology of materials are examined by X-ray diffraction spectroscopy?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and so on.Electrochemical properties are investigated by cyclic voltammetry,galvanostatic charge/discharge test and electrochemical impedance spectroscopy.Firstly,monoclinic CuO sheet arrays freely standing on porous nickel foam are prepared via a ammonia complexation-deposition method.Results show that the array of CuO sheets forms a uniform film of around 5?m in thickness,which completely covers the nickel foam skeleton.The CuO sheets are composed of nanoparticles with diameters of around 24 nm.Based on studies about electrochemial behaviors of CuO in different concentration of KOH electrolytes,we proposed that the pseudocapacitance of CuO/Ni-foam electrode for supercapatitors maybe involved in the faradaic reactions between Cu?I?and Cu?II?species.Electrochemical studies show that the highest electrochemical activity and specific capacitance of CuO/Ni-foam electrode are obtained in 6 mol·L^-1 KOH electrolyte.The electrode exhibits a specific capacitance of 569 F·g^-1 at a charge/discharge current density of 5 mA·cm^-2.When the current density increases to 30 mA·cm^-2,the specific capacitance maintains at 221 F·g^-1.The specific capacitance retention is 82%after 500 charge/discharge cycles at 10 mA·cm^-2.In order to improve the electrical conductivity of CuO,CuO/Ni-foam is modified by Ag via the silver mirror reaction.The specific capacitance of the modified Ag-CuO/Ni-foam electrode is 689 F·g^-1 at a charge/discharge current density of 1 A·g^-1,and increased by 271 F·g^-1 compared to CuO/Ni-foam electrode.The specific capacitance was found to be 426 F·g^-1 after the 2000 cycles.When the current density increases to 10 A·g^-1,the specific capacitance maintains at 299 F·g^-1 and the maximum energy density of 23.9 Wh·kg^-1 is obtained.Studies show that the Ag improves the electron transport path between the current collectors and the active materials,leading to improved utilization of the CuO materials.Furthermore,the electrochemical activity,reversibility,specific cpacitance,high-rate capacitance and energy density have been greatly improved.Moreover,in order to obtain higher specific capacitance and energy density,Co₃O₄ nanowires with spinel structure have been synthesized on nickel foam by a hydrothermal method.The Co₃O₄ nanowires completely covers the nickel foam skeleton and the lengths up to 1?2 ?m.The nano wires are composed of nanoparticles with diameters of around 10 nm.Electrochemical studies show that Co₃0₄/Ni-foam electrode has excellent electrochemical activity.The specific capacitance of Co₃O₄/Ni-foam electrode is 1019.58 F·g^-1 at 3.38 A·g^-1.When the current density increases to 33.80 A·g^-1,the specific capacitance also maintains at 466.06 F·g^-1,and the charge transfer resistance is only 0.31 ?.The capacitance retention is as high as 95%after 1000 charge/discharge cycles at 3.38 A·g^-1 and with excellent cycling stability.In addition,the maximum energy density and power density are 28.6 Wh·kg^-1 and 7.6 kW·kg^-1,respectively.Finally,?-Ni?OH?₂ sheets are directly grown on nickel foam by a ammonia complexation-deposition method.The Ni?OH?₂/Ni-foam electrode has high electrochemical activity and excellent reversibility.The Ni?OH?₂/Ni-foam electrode exhibits a high specific electric quantity of 790.3 C·g^-1,and high specific capacitance of 2258 F g"1 at a charge/discharge current density of 5 mA·cm^-2.When the current density increases to 30 mA·cm^-2,the specific capacitance also maintains at 1520 F·g^-1 and the capacitance retention is 67%.An asymmetric supercapacitor has been fabricated successfully using Ni?OH?₂/Ni-foam as positive electrode and activated carbon as negative electrode in 6 mol·L^-1 KOH electrolyte.The Ni?OH?₂//AC supercapacitors could be cycled reversibly in the high-voltage region of 0-1.6 V and the charge transfer resistance is 24.1 ?.The maximum specific capacitance of 105.8 F·g^-1 and energy density of 37.6 Wh·kg^-1 are obatined at a charge/discharge current density of 2 mA·cm^-2.This supercapacitor also exhibits an excellent cycling stability along with 92%specific capacitance retained after 1000 cycles at 5 mA·cm^-2.