As a new type of energy storage device, supercapacitor has attracted more
and more attention due to its irreplaceable advantages and has become a global
research hotspot. Cobalt oxide is a potential supercapacitor electrode material
with a theoretical specific capacitance of more than 3000F/g.
LinChuan et al. used an alkoxide hydrolysis method to prepare ultrafine
Co2O3 electrode active materials, with a single electrode specific capacitance
reaching 291F/g. In addition, the composite of CoOx and other metal oxides has
also been widely studied. The author used citric acid as the template and cobalt
nitrate as the raw material to prepare the precursor using the hydrothermal
method. It was thermally decomposed at 200°C to obtain Co3O4, and studied its
neutralization of -0.1 to 0.5V (vs. SCE, the same below) in a 6mol/LKOH
solution. ) potential range, electrochemical capacitance performance such as
cyclic voltammetry and constant current discharge.
1 experiment
1.1 Preparation of Co3O4
Weigh cobalt nitrate [Co(NO3)3·6H2O], sodium hydroxide, and citric acid
(C6H8O7·H2O) in a certain proportion and dissolve them in 30 mL of deionized
water. Stir magnetically at room temperature until the solution becomes clear.
Then the mixture was transferred into a polytetrafluoroethylene-lined stainless
steel high-pressure reactor with a filling degree of 80%, sealed, and kept at a
constant temperature of 220°C for 24 hours. After the reaction is complete, cool
to room temperature to obtain a purple-red precipitate, filter it with suction,
rinse repeatedly with deionized water and absolute ethanol, and dry under vacuum
at 60°C for 24 hours. The obtained powder was heat treated in a muffle furnace
at 200°C for 3 hours to obtain Co3O4 black powder.
1.2 Physical characterization of materials
XRD measurement was performed using a Japanese MaxM18ce X-ray
diffractometer. Experimental conditions: CuKα radiation (λ=0.151418nm), tube
voltage 40kV, tube current 100mA, scanning range 2θ of 10 to 80°, scanning speed
10(°)/min; The morphology and particle size of the samples were observed with a
German Leo1430VP scanning electron microscope.
1.3 Electrochemical testing of materials
Co3O4 powder is mixed with acetylene black and binder
polytetrafluoroethylene at a mass ratio of 70:25:5 to form a paste. Apply it
evenly on the nickel mesh. After drying at room temperature, press it into an
electrode sheet with an area of 1cm2. A saturated calomel electrode is used as
The reference electrode uses a platinum electrode as the auxiliary electrode. In
the 6mol/LKOH solution and in the potential range of -0.1~0.5V, the cyclic
voltammetry test and discharge test of the electrode were studied (conducted on
the CHI660 electrochemical workstation). The constant current charge and
discharge test is completed by the Arbin battery tester, charging and
discharging from 0 to 0.4V at a constant current of 10mA/g.
2Results and discussion
2.1 Physical characterization of materials
2.1.1 XRD analysis of materials
Figure 1 shows the XRD spectrum of Co3O4 obtained by heat treatment at
200°C. As can be seen from Figure 1, the diffraction peaks in the XRD pattern
appear at eight positions: 18.8, 31.1, 36.8, 44.7, 55.3, 59.2, 65.2, and 78.0°.
The spectrum has a low-angle peak at 18.8°, followed by another peak at 31.1°, a
stronger peak at 36.8°, and weaker diffraction peaks at 55.3 and 78.0°.
According to the peak intensity, it can be judged that the obtained product is
amorphous, and the amorphous material is suitable as a supercapacitor electrode
material.
2.1.2 Morphology analysis of materials
Figure 2 is the SEM photo of Co3O4. It can be seen from Figure 2a that a
few of the generated products have a spherical structure, and most of them are
flakes that are compacted together to form a bundle-like structure. It can be
seen from Figure 2b that the bundle-like structure is composed of several
layers, each layer is about 2 μm long, 0.5 μm wide, and 0.1 μm thick. The energy
storage mechanism of metal oxide electrode materials is mainly based on the
reversible Faradaic reaction involving electron transfer that occurs between the
electrode material and the electrolyte. This reaction causes the electrode to
generate a high quasi-capacitance, thereby achieving the purpose of storing
energy. The larger the specific surface area of the electrode material and the
larger the contact area with the electrolyte, the faster the reversible Faradaic
reaction of electron transfer. According to the scanning electron microscope
photos, this structure can provide a large specific surface area, which is
conducive to the entry and migration of electrolyte ions, and at the same time
allows the material to be fully and effectively utilized. It can be speculated
that it has a high specific capacitance.
2.2 Electrochemical performance characterization
2.2.1 Cyclic voltammetry test
Figure 3 is the cyclic voltammogram of Co3O4 electrode at different
scanning speeds. As can be seen from Figure 3, the electrode material has a pair
of obvious oxidation peaks near 0.3V and 0.4V, and a pair of obvious reduction
peaks near 0.1V and 0.2V. Among them, the P1 peak is the process of oxidation of
Co2+ to Co3+, and the P2 peak It is the reverse process; P3 peak is the process
of oxidation of Co3+ to Co4+, and P4 peak is the reverse process. The redox
current is large, and it can be judged that the Co3O4 electrode material
obtained by heat treatment at 200°C shows better capacitance characteristics
within the working range. In addition, the response of potential to current
mainly depends on the change of potential and not on the current. That is, the
current of the cyclic voltammetry curve increases with the increase of the scan
rate. This correspondence can also indicate that the material has fast charge
and discharge characteristics. .
2.2.2 Constant current discharge test
Figure 4 shows the charge and discharge curves of Co3O4 electrode at
current densities of 5, 10, and 20 mA/g respectively. It can be seen from Figure
4 that the charging curve and discharge curve have ideal symmetry. In addition,
as the current doubles during the charge and discharge process, the charge and
discharge time corresponding to the same current is almost doubled, which
illustrates that the reaction of the electrode material in the electrolyte is
approximately reversible. This result is mutually confirmed with the results of
cyclic voltammetry.
3Conclusion
Using citric acid as a template, the precursor was prepared using a
hydrothermal method. After heat treatment at 200°C, the Co3O4 electrode material
obtained had good cycle performance. When the charge and discharge current
density is 5mA/g, the single electrode specific capacitance reaches 442F/g.
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