Electrochemical capacitors,also called supercapacitors,are considered potential energy storage devices due to their high power density,excellent reversibility,and long cycle life,and thus have attracted extensive research investigations in recent years ^{[1, 2, 3]}. A variety of electrode materials have been explored,such as carbon materials (e.g.,activated carbon black,carbon nanotubes, and carbon aerogel) ^{[4, 5]},transition metal oxides ^{[6, 7, 8]},and their salts (e.g.,RuO₂ ^[9],PbO₂ ^[10],MnO₂ ^{[11, 12]},NiO ^[13],Co₃O₄ ^[14], Na_xMnO₂ ^[15],and K_xMnO₂ ^[16]),and conducting polymers.

As a new type of thermoelectric material,NaCo₂O₄ has generated extensive interest because of its large thermoelectric power,low electrical resistivity,and thermal conductivity ^[17], and has been widely used in energy conversion and electronic devices. Some reports show that NaCo₂O₄ has a typical bronze oretype layered structure,consisting of Na⁺ and CoO₂ blocks alternately stacking along the c-axis to form layers ^{[18, 19]}. As this layered structure is advantageous for the material’s electron transmission and stability,NaCo₂O₄ could be applied in electrochemical capacitors to yield excellent electrochemical performance and good cyclic stability. However,to the best of our knowledge,NaCo₂O₄ has only been studied as a new thermoelectric material but not as a supercapacitor material.

In this study,we synthesize a sub-micron-scaled NaCo₂O₄ sample via a simple solid-state reaction,and then use this NaCo₂O₄ as the electrode material for supercapacitors. We also discuss the composition,morphology,and electrochemical capacitive behaviors of the synthesized NaCo₂O₄ sample. 2. Experimental 2.1. Synthesis of NaCo₂O₄ materials

The CoSO₄ 7H₂O,Na₂CO₃,NaOH,and graphite used were of analytical grade,while the acetylene black and PTFE were of battery grade.

100 mL of CoSO₄ solution (1.0 mol/L) was added to 60 mL of NaOH solution (4 mol/L) and the mixture was stirred for 30 min at 60℃ to yield a brown precipitate. Co₃O₄ particles were then obtained by filtration followed by washing and drying for 12 h at 100℃. NaCo₂O₄ was synthesized by ball-milling mixtures of Co₃O₄ and Na₂CO₃ (at a molar ratio of 2:3) for 2 h,followed by heating in an air atmosphere at 800℃ for 10 h. 2.2. Sample characterizations

The obtained particles were characterized on a Bruker D8 Advance powder X-ray diffractometer (XRD) with monochromatized Cu K_α radiation (λ= 0.154056 nm). The morphology of the NaCo₂O₄ was investigated with a scanning electron microscope (SEM) (LEO 1430VP,Germany). 2.3. Electrochemical measurements

NaCo₂O₄ (70 wt.%),graphite (15 wt.%),and acetylene black (10 wt.%) were mixed thoroughly with a certain amount of 5 wt.% PTFE aqueous solution to obtain a homogeneous slurry possessing adequate rheological properties. The slurry was scrape-coated onto a stainless steel mesh with a surface area of 1 cm² and dried at 80℃ for 12 h. Subsequently,the pasted electrodes were pressed at 10 MPa for 5 min to ensure electrical contact between the mesh and the active material. The total weight of the active material in the electrode was controlled at about 20 mg. Symmetric supercapacitors were assembled face to face using two identical electrodes with the same mass of active materials. The electrolyte used was a diaphragm paper with 6 mol/L NaOH.

Cyclic voltammetry (CV) testing was performed using a threeelectrode cell,in which a Ni-grid and a Hg/HgO electrode were used as the counter and reference electrodes,respectively. Cyclic voltammograms (CVs) were recorded between 0.15 V and 0.65 V at a scanning rate of 20 mV/s. Galvanostatic charge/discharge cycling was performed in a two-electrode system with identical electrodes having the same active electrode materials (i.e.,Type I symmetric supercapacitor). A constant current density of 50 mA/g was employed for charging/discharging the cell in a voltage range of 0.25-0.7 V. 3. Results and discussion 3.1. Crystal structure analysis

Fig. 1(a) shows the XRD pattern of the synthesized NaCo₂O₄. The characteristic peaks of the prepared sample are well in accordance with JCPDS-27-0682. No original oxides or other impurity phases were detected in these patterns,indicating singlephase NaCo₂O₄,which has a hexagonal layered structure in space group P6322. Peak (002) is found at 2θ= 16.2°and corresponds to the interlayer space (0.54 nm),as shown in Fig. 1(c). According to the Scherrer equation (D = Kλ/βcosθ),the average c-stacking height,calculated from (002) reflection,was 27.86 nm for layered NaCo₂O₄. Therefore,we could reasonably estimate that the average layer number of NaCo₂O₄ is about 52,indicating that crystal particles mainly grow along c-axis.

Fig. 1

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Fig. 1.(a) XRD pattern,(b) SEM image,and (c) schematic illustration of the microstructure of the obtained NaCo2O4.

NaCo₂O₄ consists of Na⁺ and CoO₂ blocks alternately stacking along the c-axis to form a layered structure; thus,the physical properties of NaCo₂O₄ are highly two-dimensional,as shown in Fig. 1(b). The unit cell of NaCo₂O₄ is built up with twisting sheets of edge-sharing CoO6 octahedrons,where Co is located in the centre of the octahedron and O at the six vertexes,and two octahedrons share one edge,forming traditional calcium titanate with a perovskite structure ^[20]. Na⁺ provides electrons for storage in the electrical layer and stabilizes the crystal structure. 3.2. Electrochemical performance

Fig. 2(a) shows the CV of the NaCo₂O₄ electrode within the first 100 cycles in 6 mol/L NaOH aqueous solution at a scan rate of 20 mV/s. It can be seen that the CVs are rectangular with only a pair of very weak redox peaks. As shown in Fig. 2(b) and (c),the weak redox peaks appear at 0.51 V and 0.37 V (vs. Hg/HgO). These redox peaks are similar to those of Na_xMnO₂,which can be definitivelyascribed to the intercalation/deintercalation of Na⁺ into and from the solid lattice ^{[11, 21, 22, 23]}. Moreover,in Fig. 2(a),the symmetrical CV curve of the NaCo₂O₄ electrode with respect to the zero-current axis means that the Faradic reactions of NaCo₂O₄ are reversible. In Fig. 2(a) we can also see that the CV curves of the NaCo₂O₄ electrode are very stable during the 100 cycles,indicating excellent electrochemical stability over the potential range of 0.15-0.65 V. This demonstrates that the layered structure of NaCo₂O₄ remains stable during the charge/discharge process.

Fig. 2

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Fig. 2.CV of NaCo2O4 electrode in 6 mol/L NaOH aqueous solution at a scan rate of 20 mV/s.

Fig. 3(a) illustrates the typical charge/discharge cycles for the NaCo₂O₄ electrodes at a current density of 50 mA/g. During both charge/discharge processes,the potential behavior shows good linear activity,and the charge/discharge curves present similar symmetrical triangular waves. These results demonstrate that the NaCo₂O₄ electrodes exhibit a good response for an electrochemical capacitor.

Fig. 3

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Fig. 3.(a) Charge/discharge curves and (b) cycling behavior of the NaCo2O4 electrodes at a current density of 50 mA/g.

The specific capacity of the NaCo₂O₄ electrode can be calculated from the charge/discharge curves,according to the following Eq. (1):

Fig. 3(b) shows the excellent cycling behaviors of the symmetric NaCo₂O₄/NaCo₂O₄ supercapacitor in 6 mol/L NaOH electrolyte at 50 mA/g current density between 0.25 V and 0.7 V. A decrease in specific capacitance of only 9% was observed in the initial 500 cycles at a current density of 50 mA/g,indicating that the structure of layered NaCo₂O₄ is very stable during the long-term intercalation/ deintercalation of Na⁺ ions. In addition,the coulombic efficiency of the capacitor is almost 100%,except during the initial several cycles. As the electrode material is not fully activated at the beginning,the specific capacity is lower for the first few cycles, after which the active substance and the electrolyte are in full contact and the electrode material is activated,resulting in an increased capacitance. The small grain of the NaCo₂O₄ material also contributes to its excellent cycling performance. 4. Conclusion

A new supercapacitor material,sub-micron-scaled NaCo₂O₄, was prepared via a solid-phase sintering method using Co₃O₄ and Na₂CO₃ as the starting materials. XRD and SEM show that the synthesized NaCo₂O₄ is single-phased with a hexagonal layered structure. The specific discharge capacity of the NaCo₂O₄ electrode was 337 F/g in a 6 mol/L NaOH solution between 0.15 V and 0.65 V at 50 mA/g current density. This demonstrates that the synthesized NaCo₂O₄ possesses a favorable electrochemical reversibility and excellent supercapacitor properties. The cycling performance of the synthesized NaCo₂O₄ is also admirable. It is concluded that NaCo₂O₄,a thermoelectric material,is a promising supercapacitor material. Acknowledgment

This work was financially supported by the Natural Science Foundation of China (No. 21071046) under approval. I