Co₃O₄-based Electrode Materials And Their Supercapacitive Performances

A new kind of energy and energy-saving devices have attracted more and more attention because of the rapid depletion of fossil fuel and further deterioration of ecological environment. Supercapacitors as a new type of energy storage devices have being used in communications, computer market, military equipment and many other fields, owing to high power density, long lifetime, fast charge/diacharge process and wide operating temperature. However, it has a lower energy density than secondary battery. Thus, the key to the application and development of supercapacitors is to enhance its energy density without the sacrifice of power density. Relatively speaking,metal oxides electrode materials have some advantages, especially in terms of energy density, which hopfully achieve a high energy density close to that of secondary battery. Among various metal oxides, Co₃O₄, with the feature of low-cost, abundant resources, high reaction activity and stable structure, is regared as potential pseudocapacitive electrode materials. Meanwhile, current collector is also a part of supercapacitor electrode and directly affacts electrical conductivity of active species,which could futher change the electrochemical performances of electrode materials.So it is of great significance to build integrative electrode with large specific surface area and seamless link between electrode materials and current collector. And in situ growth method can make Co₃O₄ and other metal oxides form seamless link on substrate, which could effectively reduce the contact resistance.In the text, we prepared Co₃O₄ with different structures by changing experimental conditions, which makes them have different charge storage capabilities. We have paied high attention to the relationship between micro-structure and electrochemical performance and discussed that in detail. In this paper, we grew Co₃O₄ on nickel foam,carbon fiber cloth and graphene hydrogel acted as substrate by using hydrothermal method and subsequent thermal treatment to obtain electrode materials with enhaced performances. At last, we used the resultant electrode materials to assemble supercapacitors and studied their electrochemical performances.The main contents can be summarized as:1. With the skeletal nickel foam as the substrate, we used hydrothermal method to grow Co₃O₄ with three-dimensional cage-like structure composed of Co₃O₄ nanowires?Co₃O₄/NF?. A series of experimental techniques including X-ray diffraction?XRD?,fourier infrared spectrum?FT-IR?, field emission scanning electron microscopy?FESEM?, transmission electron microscopy?TEM? were employed to investitage the structure, morphology and composition of the samples. Meanwhile, three-dimensional cage-like Co₃O₄/NF composite can exhibited a specific capacitance of 642 F g^-1 at 1 A g^-1 in the three-electrode configuration with 3 M KOH solution as electrolyte. In order to display the practical energy storage property, we assembled asymmetric supercapacitor by using Co₃O₄/NF and graphene hydrogels?GH? as positive and negative electrodes, respectively, and tested its electrochemical performances. The result showed that the supercapacitor played a high energy density for 31 Wh kg^-1 at the power density for 854 W kg^-1.2. We used hydrothermal method and subsequent heat treatment to prepare nanoneedle Co₃O₄/CC composite, in which carbon cloth was regared as current collector. The result showed the Co₃O₄ grew evenly on the carbon cloth. Meanwhile,special treatment for the carbon cloth is important to enhance the electrochemical performances for supercapacitors. The electrochemical results showed that the specific capacitance of electrode can reach 333 F g^-1 at the scan rate of 1 A g^-1 under the potential window of 0-0.6 V. Additionally, we assembled asymmetric supercapacitor by employing GH and tested its electrochemical performances. The result revealed that the supercapacitor has a energy density of 10 Wh kg^-1 at the power density of 837 W kg^-1.3. We employed hydrothermal processes to prepare Co?OH?₂/r GH composite in which cobalt nitrate and GO were used as starting materials. Then we further changed the precursor into Co₃O₄/rGH via subsequent heat treatment.X-ray diffraction?XRD?.The field emission scanning electron microscopy?FESEM? and elemental analysis?EDS? were used to characterize the structure and morphology. The results showed that the Co₃O₄ nanowires staggered with reduced graphene oxide hydrogel to form a hierarchical structure, which made the electrode material process excellent performances. The result showed a specific capacitance of 294 F g^-1 at a current density of 1 A g^-1.