Egg Albumin-Assisted Hydrothermal Synthesis of Co3O4 Quasi-Cubes as Superior Electrode Material for Supercapacitors with Excellent Performances
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Novel Co3O4 quasi-cubes with layered structure were obtained via two-step synthetic procedures. The precursors were initially prepared via hydrothermal reaction in the presence of egg albumin, and then the precursors were directly annealed at 300 °C in air to be converted into pure Co3O4 powders. It was found that the size and morphology of final Co3O4 products were greatly influenced by the amount of egg albumin and hydrothermal durations, respectively. Such layered Co3O4 cubes possessed a mesoporous nature with a mean pore size of 5.58 nm and total specific surface area of 80.3 m2/g. A three-electrode system and 2 M of KOH aqueous electrolyte were employed to evaluate the electrochemical properties of these Co3O4 cubes. The results indicated that a specific capacitance of 754 F g−1 at 1 A g−1 was achieved. In addition, the Co3O4 cubes-modified electrode exhibited an excellent rate performance of 77% at 10 A g−1 and superior cycling durability with 86.7% capacitance retention during 4000 repeated charge-discharge process at 5 A g−1. Such high electrochemical performances suggest that these mesoporous Co3O4 quasi-cubes can serve as an important electrode material for the next-generation advanced supercapacitors in the future.
KeywordsCo3O4 Hydrothermal synthesis Electrochemical properties Supercapacitors
Electric double-layer capacitors
Electrochemical impedance spectroscopy
Field-emission electron microscope
Selected area electron diffraction
Saturated calomel electrode
Transmission electron microscope
Transition metal oxides
X-ray photoelectron spectroscopy
With the fast development of science and technology in modern society, relying solely on fossil fuels with limited storage is far from meeting the ever-increasing requirements of energy, so some new energy storage devices with environmental-benign types have been developed rapidly to solve this dilemma [1, 2, 3]. At present, batteries and supercapacitors are two types of the most promising energy storage systems because of their high performance and low cost. In particular, supercapacitors, also known as electrochemical capacitors, have attracted more attention in terms of their excellence in power density, long-term cycling life, charge-discharge rate, and other properties [4, 5, 6]. Attributed to such advantages, supercapacitors have been applied in emergency lighting, hybrid electric vehicles, military equipment, and short-term power sources [7, 8]. At the same time, the energy and power density of supercapacitors need to be continuously increased to accommodate the expansion of their application fields; as a result, tremendous efforts have been devoted to resolving this problem. Achieving noteworthy improvements in supercapacitors require a deep fundamental understanding of the charge storage mechanisms. It has been found that the shape, porosity, as well as mechanical properties of electrode materials have a crucial impact on the performances of supercapacitors [9, 10, 11]. For an ideal electrode material, the number of electrochemically active sites for charge transfer should be enhanced and ionic/electronic transport should be controlled at small diffusion length .
Supercapacitors have been differed on the categories on the basis of different energy storage mechanisms. One of them stores energy by charge accumulation at the interface of electrode and electrolyte, and it is known as electric double-layer capacitors (EDLCs). The other is pseudo-capacitors (PCs), which rely on fast Faradic reaction occurring near/on the surface of electrode materials to store energy [13, 14, 15, 16]. The carbonaceous materials, such as activated carbon, graphene, and carbon nanotubes (CNTs) that have large specific surface area and good conductivity, are ideal electrode materials for EDLCs. However, for the carbon-based materials, their inherently low specific capacitance is a sever defect that cannot be ignored, which leads to lower energy density than that of PCs . Conductive polymers as well as metal oxides are commonly used as electrode materials in PCs, due to their favorable pseudocapacitive characteristics of fast and reversible redox reactions. PCs can provide higher energy and power density, larger specific capacitance, and have attracted worldwide research interest . To date, metal oxides, especially transition metal oxides (TMOs), such as MnO2 [19, 20], NiO [21, 22], and Fe2O3 [23, 24], have attracted much attention as potential candidate for electrode materials, for they can provide rich redox charge transfer originated from their variety of oxidation states, which is beneficial to the Faraday reaction. Despite the virtues of low cost and high specific capacitance, the effects of these materials used as electrode in PCs are still not satisfactory, given the fact that they generally possess dramatic volume change, inferior rate-capability, and relatively high resistance; enormous efforts have been devoted to circumvent the hurdles . Among the series of TMOs, Co3O4 is considered as one of the most promising electrode materials. This kind of material possesses a theoretical specific capacitance as high as 3560 F g−1 . Besides, it is environmentally friendly, cheap, and rich in redox activity as well. Unfortunately, compared to its theoretical value, the specific capacitance of Co3O4 electrode achieves in practical applications is significantly low. Ascribed to the limitation transfer of electrons caused by the high internal resistance of Co3O4, only a part of active sites may be involved in the redox reaction, leading to low utilization of the active material and decrease in specific capacitance. Furthermore, the Co3O4 has a dramatic volume change trend during the process of rapid redox reactions, and the collapse of the electrode material leads to a reduction of the cycle life .
To address these problems, Co3O4 nanostructures with different morphologies, including nanorods, nanowires, nanoflakes, and nanoflowers, have been successfully prepared by controlling the synthesis process, aiming to increase the surface area and facilitate the redox reaction [28, 29, 30, 31]. The research results have shown that different morphologies have a significant effect on the performance of Co3O4 electrode, but merely changing the morphology is far from being able to improve its inherent poor conductivity and serious volume expansion defects. Researchers are devoted to combine Co3O4 with other highly conductive materials to obtain electrode materials with high charge transfer capabilities. In addition, the synergy between different materials can contribute to the redox reaction at the same time, to achieve the purpose of increasing the specific capacitance [32, 33, 34, 35]. From the point of practical applications and large production, it is significantly important to prepare powder electrode material through a simple synthetic process.
Solution method including hydrothermal/solvothermal route is one of the important synthetic strategies to prepare micro/nanomaterials on a large scale. In this method, surfactant is usually employed to control the rate of nucleation and crystal growth. So the final shape of nanostructures can be effectively tuned by the surfactant [36, 37, 38]. Several types of surfactant including cationic surfactant, anionic surfactant, nonionic surfactant, and so on can be used for the fabrication of nanomaterials. Among them, the biological molecules with functional groups have received increasing attention due to the environmental-benign of this kind of surfactant. The proteins can interact with inorganic nanoparticles and then to govern the nucleation of inorganic materials in aqueous solutions. Egg albumin, as an important protein, can be widely available from eggs. It has received much attention due to its gelling, foaming, and emulsifying characteristics. In addition, egg albumin is cost-effective and environmentally friendly, and the usage of such surfactant may not result in danger for both environment and the health of humans. Therefore, egg albumin can be employed for the preparation of nanomaterials with controlled morphology. For example, Geng et al. prepared single crystalline Fe3O4 nanotubes with high yields using egg albumin as a nanoreactor . ZnS nanosheets can be synthesized via egg albumin and microwave-assisted method . In addition, dumbbell-shaped BaCO3 superstructures and SnO2 biscuits can be obtained with the assistance of egg albumin by different research group [41, 42]. Overall, the reports on nanomaterials fabrication involving egg albumin have been rarely reported. In this work, porous Co3O4 cubes were synthesized with the assistance of egg albumin via a hydrothermal method and post calcination of the precursors. These Co3O4 porous cubes had average pore size of 5.58 nm, and the Brunauer-Emmett-Teller (BET) specific surface area was evaluated to be 80.3 m2/g. If such Co3O4 cubes were processed into a working electrode, a high capacitance of 754 F g−1 was obtained at 1 A g−1. Besides, if the current density was improved to 10 A g−1, the electrode showed a high rate capability up to 77%. A superior cycling performance with 86.7% capacitance retention (at 5 A g−1) was also achieved during 4000-cycle charge-discharge process. Such excellent electrochemical properties indicate that the porous Co3O4 cubes can serve as a promising electrode material for supercapacitors in the near future.
In this work, all reagents were in analytical pure grade and were used without any additional purification. Urea and cobalt (II) acetate tetrahydrate were purchased from Sinopharm Chemical Reagent Co., Ltd., and egg albumin was obtained from fresh eggs.
Preparation of Porous Co3O4 Cubes
To prepare the porous Co3O4 cubes, 3 mL of egg albumin, 2.4 g of urea, and 0.3 g of cobalt (II) acetate tetrahydrate were dissolved in 37 mL of de-ionized (DI) water with vigorous stirring. Then the mixture was loaded into an autoclave with 50 mL of capacity, and the autoclave was placed in an oven at 140 °C. Five hours later, the precipitates were harvested, rinsed, and dried at 60 °C overnight. The obtained precursor was annealed at 300 °C for 5 h in order that black powder was obtained. Control experiments were conducted with various hydrothermal time (1, 2, 15, and 24 h) and different amount of egg albumin, respectively, while keeping other parameters and procedures the same.
Fabrication of Working Electrode and Electrochemical Tests
The X-ray diffraction (XRD) pattern of the sample was collected on a powder X-ray diffractometer (Bruker D8 Advance), in which Cu-kα was used as X-ray source (λ = 0.1548 nm) and the range of 2θ was 25–100°. Field-emission electron microscope (FESEM) images were available from a JEOL JSM7100F scanning electron microscope, and transmission electron microscope (TEM) image was obtained on JEOL JEM2100F equipment with operation voltage of 200 kV. Before TEM measurement, the powder needs to be ultrasonically dispersed in ethanol for 10 min, then is dropped onto a carbon-coated copper grid. Raman examination was performed on RM 1000-Invia (Renishaw) spectrometer, and the wavelength for laser was chosen to be 514 nm. X-ray photoelectron spectroscopy (XPS) measurement was operated on ESCA 2000 spectrometer and Al Kα was employed as excitation source. According to nitrogen adsorption/desorption experiments conducted at 77 K, the Brunauer-Emmet-Teller (BET) surface area was obtained. In addition, pore size distribution (Barrett-Joyner-Halenda, BJH method) could be acquired from the related desorption isotherm.
Comparison for the specific capacitances of Co3O4-based electrode materials
BET specific surface area (m2/g)
Specific capacitance (F/g)
Co3O4 hollow boxes
278 F g−1 @ 0.5 A g−1
flower-like ZnO/Co3O4 nanobundle arrays
1983 F g−1 @ 2 A g−1
mesoporous Co3O4 nanoflake arrays on carbon cloth
450 F g−1 @ 1 A g−1
Hierarchical Co3O4 nanoflowers
198 F g−1 @ 1 A g−1
3D-nanonet hollow structured Co3O4
739 F g−1 @ 1 A g−1
352 F g−1 @ 1 A g−1
Flower-like Co3O4 microspheres
214 F g−1 @ 2 A g−1
Porous Co3O4 microspheres
342.1 F g−1 @ 0.5 A g−1
Hollow Co3O4 nanowire arrays
599 F g−1 @ 2 A g−1
Hollow fluffy Co3O4 cages
948.9 F g−1 @ 1 A g−1
Co3O4 hierarchical micro-and nanostructures
332.6 F g−1 @ 2 mA cm−2
3D Co3O4@MnO2 hierarchical nanoneedle arrays
1693.2 F g−1 @ 1 A g−1
Co3O4@highly ordered microporous carbon
1307 F g−1 @ 1 A g−1
Hierarchical Mo-decorated Co3O4 nanowire arrays
~ 2000 F g−1 @ 10 A g−1
754 F g−1 @ 1 A g−1
Porous Co3O4 quasi-cubes were prepared through an egg albumin-assisted hydrothermal method with a subsequent high-temperature treatment of precursor in air directly. The size and shape of final Co3O4 samples had a close relationship with the amount of egg albumin and hydrothermal reaction time, respectively. Such Co3O4 cubes possessed a mesoporous characteristic with surface area of 80.3 m2/g, average pore size of 5.58 nm, and main pore size distribution at 4.03 nm. Once these Co3O4 quasi-cubes were processed into a working electrode, it delivered a high specific capacitance of 754 F g−1 at 1 A g−1 and 581 F g−1 at the current density of 10 A g−1. After a continuous 4000 cycles at 5 A g−1, 86.7% capacitance retention could be obtained and it demonstrated a good cycling stability. The outstanding electrochemical properties of these Co3O4 cubes enable them to be promising electrode materials for advanced supercapacitors. In addition, the egg albumin-assisted synthesis route is expected to be extended to prepare other oxides-based electrode materials with novel morphology and superior electrochemical performances.
The work was supported by the financial supports from International Cooperation of Science and Technology Projects in Shanxi Province (201703D421040 and 201803D421092), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Shanxi Province.
JLS prepared the materials and the draft of the manuscript. YW, YFZ, and CJX carried out the structure analysis and electrochemical performance test of the samples. CJX and HYC designed the work. HYC approved the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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