Study On Preparation And Capacitive Properties Of The Electrode Materials For Electrochemical Capacitors

As new energy storage devices, electrochemical capacitors have been applied in many fields because they possess high power density, high energy density, long cycle-life, et al. Two basic types of electrochemical capacitors can be realized using different charge-storage mechanism: electrochemical double-layer capacitors and faradaic pseudocapacitors. The former utilizes the capacitance arising from charge separation at an electrode/electrolyte interface, and the later utilizes the charge transfer pseudocapacitance arising from Faradaic reactions occurring at the electrode surface. The electrode materials are one of the key factors to determine capacitive properties of electrochemical capacitors. There are three kinds of the electrode materials: carbon, metal oxideand conductng polymers. This thesis was focused on the preparation of various metal oxides and metal oxides/carbon composites and their applications as the electrode materials in electrochemical capacitors. The main points of this thesis are summarized as follows:(1) Amorphous manganese oxide nanowires were potentiodynamically deposited onto graphite substrate at room temperature without any templates. The morphology and crystal structure of the prepared manganese oxide nanowires were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Moreover, the prepared manganese oxide nanowires/graphite (MnO_x-NWs/G) electrode was applied as electrode material for electrochemical capacitors and the corresponding capacitive properties were evaluated by cyclic voltammetrty (CV) and galvanostatic charge-discharge method. The results indicate that the MnO_x-NWs/G electrode has excellent capacitive properties: high specific capacitance (208 F g^-1 in 0.1M Na₂SO₄ aqueous solutions from 0 to 1 V at a current density of 1 mA cm^-2), high electrochemical reversibility and excellent long-term charge-discharge cycle stability.(2) The nickel-cobalt oxides/carbon nanotubes/graphite ((Co-Ni)O_x/CNTs/G) electrodes with different Ni/Co molar ratios were prepared by adding and thermally decomposing nickel and cobalt nitrates directly onto the surface of CNTs/G electrode to form nickel and cobalt oxides. CNTs used in this paper were grown directly on graphite substrate by chemical vapor deposition (CVD). The morphology and crystal structure of (Co-Ni)O_x/CNTs/G electrodes were investigated by SEM and XRD, respectively. The capacitive behavior of (Co-Ni)O_x/CNTs/G electrodes were investigated by CV and galvanostatic charge-discharge method in 1M KOH aqueous solutions. Additionally, the effect of Ni/Co molar ratios on capacitive behaviour of the (Co-Ni)O_x/CNTs/G electrode was also investigated. The results show that nickel-cobalt oxides are coated uniformly on the surface of CNTs and exist as NiO and Co3O4. When the Ni/Co molar ratio is 1:1, (Co-Ni)O_x/CNTs/G electrode shows the best capacitive properits: the highest specific capacitance (569 F g^-1 at 10mA cm^-2), excellent power characteristics and good charge-discharge cycle stability (only 3.6% losses of the specific capacitance are found after 2000 charge-discharge cycles at a discharge density of 10 mA cm^-2).(3) Manganese oxide/carbon nanotubes/graphite (MnO₂/CNTs/G) electrodes were synthesized by thermally decomposing manganese nitrates. CNTs used in this paper were grown directly on graphite disk by CVD. The morphology of MnO₂/CNTs/G electrode was characterized by SEM and transmission electron microscopy (TEM). The capacitive behavior of MnO₂/CNTs/G electrode was investigated by CV and galvanostatic charge-discharge method in 1M Na₂SO₄ aqueous solutions. Moreover, the effect of loading mass of MnO₂ on specific capacitance of the electrode was also investigated. The results show that MnO₂ are covered uniformly on the surface of CNTs and the layer thickness of MnO₂ is about 20 nm. When the loading mass of MnO₂ is 36.9μg cm^-2, the specific capacitance of MnO₂/CNTs/G electrode (based on MnO₂) at 1 mA cm^-2 equals 568 F g^-1. Additionally, good charge-discharge cycle stability (ca. 88% value of specific capacitance is remained after 2500 charge-discharge cycles at a discharge density of 10 mA cm^-2) and power characteristics of the MnO₂/CNTs/G electrode can be observed.(4) A porous Mn(OH)₂ thin film electrode with nanostructure was prepared successfully by electrochemically induced deposition method. The morphology and crystal structure of the prepared film were investigated by SEM and XRD, respectively. The effects of the composition of electrolyte and the deposition current on the morphology of the Mn(OH)₂ film were investigated and the possible deposition mechanism of the film was discussed. Moreover, the capacitive properties of the Mn(OH)₂ film electrode were evaluated by CV and galvanostatic charge-discharge method. The effects of the deposition condition (the deposition current density (iD), supplied mass (SM) and supplied rate (SR) of the supplied solution) on the capacitive properties of the Mn(OH)₂ film electrode were also examined. The results demonstrate that the morphology of the Mn(OH)₂ film depend on the amount and size of evolved H2 bubbles and can be effectively controlled by changing the composition of electrolyte and the deposition current. The Mn(OH)₂ film electrode prepared under the optimum deposition condition (iD = 23 mA cm^-2, SM = 1.25 mL and SR = 16.7μL min-1) shows excellent capacitive properties: high specific capacitance (493 F g^-1 in 0.1M Na₂SO₄ aqueous solution from 0 to 1 V at 1 mA cm^-2), high electrochemical reversibility and excellent long-term charge-discharge cycle stability (only 2.2% decreases of the specific capacitance are observed after 2000 cycles at a discharge density of 10 mA cm^-2).(5) Using carbon nanotubes grown directly on graphite substrate as supporting material, theγ-MnO₂/carbon nanotubes/graphite (γ-MnO₂/CNTs/G) electrode with high dispersibility ofγ-MnO₂ was prepared by electrochemically induced deposition method. The morphology and crystal structure of theγ-MnO₂/CNTs/G electrode were investigated by SEM and XRD, respectively. The capacitive properties of theγ-MnO₂/CNTs/G electrode were investigated by CV and the deposition process ofγ-MnO₂/CNTs/G electrode was also discussed. The results indicate thatγ-MnO₂ is only deposited uniformly on the surface of CNTs and form a rough film, which should attribute to the convection of solution caused by H2 bubble motion. Theγ-MnO₂/CNTs/G electrode has three-dimensional porous structure and shows excellent capacitive properties. A specific capacitance based onγ-MnO₂ as high as 579 F g^-1 is obtained at a scan rate of 10 mV s-1 in 0.1M Na₂SO₄ aqueous solution. Additionally, theγ-MnO₂/CNTs/G electrode shows good power characteristics and long-term cycle stability.(6) The well-aligned carbon nanotube arrays (ACNTs) were grown directly on the graphite substrate by plasma-enhanced hot filament chemical vapor deposition and used as supporting material. Theγ-MnO₂/ACNTs/graphite (γ-MnO₂/ACNTs/G) electrode with high dispersibilty ofγ-MnO₂ has been prepared by electrochemically induced deposition method. The morphology and crystal structure of theγ-MnO₂/ACNTs/G electrode were investigated by SEM, TEM and XRD, respectively. The capacitive properties ofγ-MnO₂/ACNTs/G electrode were characterized by CV and galvanostatic charge-discharge method. The results show thatγ-MnO₂ is coated uniformly on the surface of ACNTs and the layer thickness ofγ-MnO₂ is about 12 nm. Theγ-MnO₂/ACNTs/G electrode has three-dimensional porous structure and shows excellent capacitive properties. The specific capacitance of theγ-MnO₂/ACNT electrode based onγ-MnO₂ is as high as 784 F g^-1 in 0.1M Na₂SO₄ aqueous solution from 0 to 1 V when the charge-discharge current density is 1 mA cm^-2. Additionally, the electrode shows excellent power characteristics, high electrochemical reversibility and excellent long-term charge-discharge cycle stability (only 0.5% decreases of the specific capacitance are observed after 800 charge-discharge cycles at a discharge density of 1 mA cm^-2).(7) Using simple filter paper as supporting material, the MnO-C composite with high power density and energy density was prepared by the redox reaction between potassium permanganate and carbons then heat-treatment at high temperature. The morphology and crystal structure of the MnO-C composite were investigated by SEM and XRD. The capacitive properties of the MnO-C composite were evaluated by CV and galvanostatic charge-discharge method. The results indicate that manganese oxide in the prepared composite exists as MnO and the MnO-C composite has nano-shuttle or nanowire structure. The MnO-C composite/graphite (MnO-C/G) electrode has excellent capacitive properties. The specific capacitance of the MnO-C/G electrode based on MnO-C composite and MnO are as high as 248 F g^-1 and 636 F g^-1 at a scan rate of 50 mV s-1, respectively. Additionally, the MnO-C/G electrode has very high power characteristics (only 1.6% decreases in specific capacitance from 10 to 1000 mV s-1), excellent electrochemical reversibility and long-term charge-discharge cycle stability (only 6.4% decreases in specific capacitance for 6000 charge-discharge cycles at a discharge density of 10 mA cm^-2).