| The development of electric vehicles has raised urgent demand for mobile electrochemical power sources with higher performance. Thus many researchers have focused on energy storage devices with high specific energy and large power density. Recently, lithium-air batteries have attracted intensive attention due to their extremely high theoretical energy density. However, lithium-air batteries are still far away from real application. Their poor cyclability and large overpotential during charge and discharge are the main hurdles. Various cathode catalysts, such as transition metal(Mn, Fe, Co, Ni) oxides, have been tried to alleviate these problems. Meanwhile, the supercapacitors have also received eye-catching attention because of their high specific energy and high power density. Pseudocapacitive metal oxides(such as Ru O2, Mn O2, Ni O, Co3O4, etc.) can produce higher specific capacitance than typical carbonaceous materials with electric double layer capacitance(EDLC). The rate capability and long term stability of pseudocapacitors have to be improved for the practical application. The difference in morphology and structure of electrode materials is likely to bring different electrochemical performances. Based on this point, we explored the morphological effect of Co3O4 on the performance of lithium-air batteries and supercapacitors.In this paper, the Co3O4 is selected as the electrode material because of its excellent properties in the above mentioned fields. Free-standing Co3O4 with two morphologies on Ni foam were prepared via a two-step strategy: hydrothermal process followed by a post-calcination process in air. Without the addition of surfactant SDS, the rod-shaped Co3O4 with the diameter ranging from 50 nm to 200 nm and 1um-3um in length was obtained. By using different concentrations of SDS, rose petals-shaped Co3O4 arrays with different crystallinity were obtained. The rose petals-shaped Co3O4 arrays are composed of well aligned uniform long-range(more than 2um in length) and thin(less than 10 nm in thickness) nanosheets. The cathode catalytic properties for Li-O2 batteries and the supercapacitive performances of all the prepared samples were tested:① Li-O2 batteries with Rod-shaped Co3O4 electrode yield relatively higher specific charge/discharge capacity due to its more reasonable hierarchical porous structure which is easy to store more solid product. Rose petals-shaped Co3O4 electrode exhibited relatively lower charge overpotential and better reversibility due to its high conductivity. Besides, the crystallinity had little effect on the overpotential, and only if the crystallinity is too poor, the specific capacity would decay.② As a supercapacitor electrode, both sample’s morphology and crystallinity have significant influence on the capacitive performance. All rose petals-shaped Co3O4 electrodes are much higher specific capacitance than Rod-shaped Co3O4. A rose petals-shaped electrode with proper crystallinity shows an outstanding specific capacitance of 2671 F/g(6357 m F/cm2) at 5m A/cm2, good power capability(high specific capaticance of 1370 F/g(3260 m F/cm2) at high current density of 100 m A/cm2 within discharging 17s), and excellent cycling stability(3000 cycles at 15 m A/cm2 or 50 m A/cm2 without capacitance decay).We studied Co3O4 powders with three different morphologies for Li-O2 batteries. The results show that the Co3O4 nanowire with relative open hierarchical pores exhibits the best capacitive performance. But its reversibility is worse than that of the honeycomb-liked Co3O4 which is composed of aggregate rough particles. The Co3O4 layer, composed of rectangular flakes, shows the worst performances even it have the largest specific surface area among them. In summary, the best morphology of the electrode materials for Li-O2 batteries cathode should have reasonable and relative open hierarchical porous structure and should contact sufficiently with the conductive materials or the current collectors without sacrifice of too much active sites. |
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