Metal selenides, as a new class of metal chalcogenide, have recently attracted researcher's attention as superior electrode materials for energy storage. In this work, we have prepared the 3D nanosheet-assembled CoSe quasi-microspheres by a facile hydrothermal synthesis route and utilize them as supercapacitor electrode materials for the first time. The resulting 3D nanosheet-assembled CoSe quasi-microspheres manifest superior capacitive properties with a high specific capacitance of 440 F g−1 at 1 A g−1, excellent capacity retention rate of 85.9, 56.4 and 41.4% at 10, 50 and 100 A g−1, and good cycling stability with 102.3 and 96.3% of initial capacity retention rate at 2 and 10 A g−1 over 2000 and 5000 cycles, respectively. The good electrochemical performances can be ascribed to the unique 3D micro-/nanostructure with superior pore-size distribution, relatively large mesopores and exceptional electrical conductivity. The asymmetric supercapacitor is further assembled by employing 3D nanosheet-assembled CoSe quasi-microspheres as the cathode and activated carbon (AC) as the anode, respectively. The CoSe/AC asymmetric device displays a energy density of 17.6 Wh kg−1 at a power density of 684 W kg−1, and maintains 95.5% of its initial capacity at 3 A g−1 over 2000 cycles.
With fast economical development together with the increasing worsen environment and exhausted fossil fuels, it is extremely urgent to develop sustainable, efficient and clean energy storage technology.1–4 Supercapacitors, as a new energy storage device, have attracted growing attention because of their excellent supercapacitive properties with superior power density, quick charge-discharge rate and long cycling life when compared to batteries.5–7 Owing to the intriguing features, they show potential applications in many fields such as military project, aerospace, hybrid electric vehicles, new energy, consumer electronics, industrial power management.8–10
Based on the energy storage mechanism, supercapacitors are generally divided into two basic categories: electric double layer capacitor (EDLC) and pseudocapacitor.11 Pseudocapacitor can offer a higher specific capacity and energy density than that of EDLC, which renders it become the research focus in recent decade. These extensively researched pseudocapacitive materials mainly include metal oxides (RuO2, MnO2, NiO, Co3O4)12–18 and conducting polymers (PANI, PPy, PEDOT).19–21 Whereas, various new class of pseudocapacitive materials have recently been reported, including ferrum-based oxide (Fe2O3, Fe3O4),22,23 birnessite-MnO2 (KxMnO2·nH2O, NaMnO2),24,25 transition metal chalcogenide (metal sulfides, metal selenides).26 Among them, it is especially noteworthy that transition metal chalcogenide have been explored as high-performance pseudocapacitive materials because of the superior electrical conductivity and rich redox chemistry. Metal sulfides such as MoS2, NiS, CoS, MnS, ZnS have been investigated as potential electrode materials for use in supercapacitors,27–31 and they show superior capacitive properties, especially for the rate capability. Metal selenides, as another type of new metal chalcogenide, have been reported as superior supercapacitor electrode materials. Some metal selenides, including SnSe, GeSe, NiSe, Co0.85Se, CuSe, have been utilized as electrode material for application in supercapacitor.32–37 These pioneer researches have revealed that metal selenides appear highly promising candidate electrode materials for supercapacitor. The superior supercapacitive performances can be ascribed to the fact that metal selenides possess a narrower bandgap compared to their corresponding metal sulfides,38 resulting in the better electrical conductivity and the enhanced electrochemical capacitive properties. To the best of our knowledge, although CoSe has been used as electrocatalyst for oxygen evolution reaction,39 there is almost no reports about CoSe as supercapacitor electrode material. Therefore, it is very significant to investigate the electrochemical capacitive properties of CoSe.
In this study, we for the first time report the hydrothermal synthesis of 3D nanosheet-assembled CoSe quasi-microspheres and explore their use as supercapacitor electrode materials in three-electrode configuration. The electrochemical tests reveal that the as-resulted 3D nanosheet-assembled CoSe quasi-microspheres manifest superior electrochemical capacitive properties with a high specific capacitance, exceptional rate performance and good cycling stability. Furthermore, the asymmetric supercapacitor is fabricated based on the 3D nanosheet-assembled CoSe quasi-microspheres and AC as the cathode and anode, respectively, which achieves a energy density of 17.6 Wh kg−1 at a power density of 684 W kg−1, and superior cycling stability can be retained with only 4.5% loss of its initial capacity at 3 A g−1 over 2000 cycles.
Experimental
Materials preparation
In the synthesis of CoSe, 0.233 g of Co(NO3)2·6H2O, 0.089 g of SeO2 and 28 mL of distilled water were added to a Teflon-lined stainless steel autoclave, which was adequately stirred for 30 min. Subsequently, 16 ml of NH3·H2O (25∼28%) and 20 ml of N2H4·H2O (80 wt%) were added into the resulting solution in sequence under strong stirring. After continually stirring for another 30 min, the Teflon-lined stainless steel autoclave was heated to 180°C from room temperature and remained for 2 h. After naturally cooling to ambient temperature, the as-obtained product was washed with distilled water and absolute ethanol for several times via centrifugation process, and finally vacuum-dried at 60°C for 12 h.
Materials characterization
The phase and purity of the as-prepared CoSe sample were determined by the X-ray diffractometer (XRD, Rigaku D/max 2550 VB+). The chemical composition of the as-synthesized CoSe sample was obtained by X-ray photoelectron spectrometer (XPS, K-Alpha). The surface morphology and microstructure of the resulting CoSe sample were characterized by Field emission scanning electron microscopy (FESEM, MIRA3), Transmission electron microscopy and High resolution transmission electron microscopy (TEM and HRTEM, JEM-2100F) equipped with selected area electron diffraction (SAED). The BET specific surface area and pore structure of the as-resulted CoSe sample were acquired by Nitrogen adsorption/desorption measurement (BET, BELSORP-MINIII).
Electrochemical measurements
The supercapacitive properties of the CoSe electrode were tested using a Modulab electrochemical workstation in a conventional three-electrode configuration where the CoSe electrode, Pt foil, Hg/HgO and 2 M KOH utilized as the working electrode, counter electrode, reference electrode and electrolyte solution, respectively. The working electrode was made by mixing 70 wt% of the active material (CoSe), 20 wt% of conductive material (carbon black, super P) and 10 wt% of binder material (polyvinylidene difluoride, PVDF) with a few drops of solvent (N-methy-l-2-pyrrolidine, NMP). The resulting mixture was then coated onto a current collector (Ni foam) and dried at 100°C in a vacuum oven for 10 h. The working electrode was successfully fabricated by further pressing under a pressure of 10 MPa for 30 s. The mass loading of active materials is controlled at about 1.5 mg cm−2. Cyclic voltammetry (CV) was carried out at scan rates of 5∼200 mV s−1 in a potential window between 0 and 0.55 V. Galvanostatic charge-discharge (GCD) was performed at current densities of 1∼100 A g−1 in the potential range of 0∼0.5V. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz∼0.01 Hz with an ac voltage amplitude of 10 mV. The cycling stability was estimated by the repeated charge/discharge test in the potential ranging from 0 to 0.5 V at 2 and 10 A g−1 over 2000 and 5000 cycles, respectively.
The supercapacitive performances of the CoSe electrode were further evaluated by assembling an asymmetric supercapacitor with CoSe as the cathode and AC as the anode, respectively. The preparation process of AC was referring to our previously reported work.4,8 The mass ratio of the cathode (CoSe) and anode (AC) is calculated according to the equation: m+/m- = (C- × △V-)/(C+ × △V+), and the optimal mass ratio was calculated to be m(CoSe)/m(AC) = 0.84 in the asymmetric supercapacitor. The CV, GCD and cycling stability of the asymmetric device were conducted by utilizing Modulab electrochemical workstation.
Results and Discussion
X-ray diffraction (XRD) was utilized to determine the phase and purity of the as-prepared CoSe sample as demonstrated in Fig. 1. The X-ray diffraction peaks located at 33.2°, 44.8°, 50.5°, 60.1°, 61.8° and 69.7° can be corresponded to the (101), (102), (110), (103), (201) and (202) planes, respectively, which accords well with the freboldite phase CoSe (JCPDS No. 89-2004). In addition, no peaks related to any other impurities can be seen when compared to the standard XRD diffraction pattern, indicating that the pure CoSe product has been obtained in our work. X-ray photoelectron spectroscopy (XPS) was used to investigate the elemental composition of the as-resulted CoSe sample, as displayed in Fig. 2. Fig. 2a is the survey spectrum of the sample, indicating the presence of Co and Se elements as well as the unavoidable C and O elements. Fig. 2b reveals the presence of two main peaks at 797.1 and 781.1 eV corresponding to the Co 2p1/2 and 2p3/2 with two satellites at 802.9 and 786.0 eV, respectively.39 Fig. 2c shows the existence of two main peaks at 59.4 and 54.8 eV assigning to the Se 3d3/2 and 3d5/2 with one satellite at 61.3 eV, respectively.40 The XPS analysis further demonstrates the formation of CoSe, which is in good accordance with the results of XRD.
The morphology and microstructure of the as-resulted CoSe sample were investigated by FESEM, TEM, HRTEM and SAED, as presented in Fig. 3. The FESEM images (Figs. 3a and 3b) reveal a 3D quasi-microsphere structure with a size of 0.8–1 μm, which is composed of many packed nanosheets. As demonstrated in Figs. 3c and 3d, the TEM images further reveal the nanosheet-assembled 3D quasi-microsphere structure with a size of about 1 μm, which is in good agreement with the FESEM images. Moreover, the nanosheets interconnect with each other and assemble to form the 3D micro-/nanostructure. This type of unique 3D micro-/nanostructure can provide more contact area of electrolyte ions, contributing to the improvement of electrochemical properties. From the HRTEM image as shown in Fig. 3e, the adjacent lattice fringe can be clearly observable with spacing 0.27 nm, which accords well with the (101) plane of freboldite CoSe. The selected area electron diffraction (SAED) pattern (Fig. 3f) can be corresponded to the (101), (102), (110) and (103) planes, suggesting the polycrystalline characteristic of CoSe, which agrees well with the results of XRD analysis.
The BET specific surface area and pore-size distribution of the resulting 3D nanosheet-assembled CoSe quasi-microsphere were determined by nitrogen adsorption/desorption measurement, as presented in Fig. 4. According to the BET analysis, the specific surface area of the as-obtained 3D nanosheet-assembled CoSe quasi-microsphere is calculated to be 18.3 m2 g−1. Moreover, the nitrogen adsorption/desorption isotherm shows a type-IV curve with a H3 hysteresis loop, indicating the presence of mesoporous structure,41,42 which is further validated by the pore-size distribution curve displayed in the inset of Fig. 4. The pore size distribution is mainly focused on the relatively narrow range of 3–10 nm, and the average pore size is about 7.2 nm. The superior pore-size distribution and relatively large mesopores serve as "ion-buffering reservoirs" for electrolyte, which cannot only enhance the rate capability by facilitating the efficient and fast transport of ions and electrons, but also improve the cycling performance by buffering against the volume change during the charge/discharge processes.
The supercapacitive performance of the as-obtained 3D nanosheet-assembled CoSe quasi-microspheres was firstly studied by CV test at various scan rates of 5–200 mV s−1 as shown in Fig. 5a. As can be seen from the CV curves, a pair of redox peaks can be clearly observed for all the scan rates, suggesting the typical faradaic pseudocapacitive behavior occurred in the charge/discharge process, which is evidently different from the EDLC characteristic. The redox peaks can be attributed to the reversible faradaic redox reactions between Co2+/Co3+ and Co3+/Co4+,35 which can be expressed as the following equations:
Also, it can be observed that the peak current grows with the increasing scan rate from 5 to 200 mV s−1, and the shapes of various CV curves at different scan rates don't change significantly. Even at a high scan rate of 200 mV s−1, the shape of the CV curve doesn't show obvious distortion, which implies the low internal resistance and good rate capability of the CoSe electrode. The superior capacitive behavior can be ascribed to the unique 3D nanosheet-assembled CoSe quasi-microspheres structure with superior pore-size distribution, large mesopores and good electrical conductivity.
The GCD was utilized to further evaluate the capacitive property of the as-prepared 3D nanosheet-assembled CoSe quasi-microspheres at different current densities, as presented in Figs. 5b and 5c. The charge/discharge curves at all current densities show visible voltage plateaus, further revealing the pseudocapacitive characteristic, which accords well with the redox peaks observed from the CV curves. In addition, it can also be seen that the charge/discharge curves present the nearly symmetric profile, implying the good electrochemical reversibility. The specific capacitances of the as-obtained 3D nanosheet-assembled CoSe quasi-microspheres at different current densities are plotted in Fig. 5d, which are calculated from the GCD curves by the equation: Cs = (I × △t) / (△V × m),43 where Cs is the specific capacitance, I represents the discharge current, △t is the discharge time, △V denotes the potential window, and m refers to the mass of the electroactive material. The specific capacitances are 440, 436, 428, 417, 378, 326, 248, 206 and 182 F g−1 at current densities of 1, 2, 3, 5, 10, 20, 50, 80 and 100 A g−1, respectively. It is noteworthy that the as-synthesized 3D nanosheet-assembled CoSe quasi-microspheres manifest excellent rate capability with 85.9, 56.4 and 41.4% of capacitance retention rate when the current density varies from 1 A g−1 to 10, 50 and 100 A g−1, respectively. These values are comparable or even better than that of other reported Co- and Se-based materials in the literatures, as listed in the Table I.32,33,36,44–48 The superior capacitive properties can be ascribed to the superior pore-size distribution and large mesopores, which is helpful to the fast transport of ions and electrons during the charge/discharge process, resulting in the improvement of electrochemical kinetics. Besides, the reported exceptional electrical conductivity of CoSe can decrease the resistance and facilitate the quick transport of electrons, leading to the enhancement of rate performance.
Table I.Comparison of the electrochemical performances of the as-prepared 3D nanosheet-assembled CoSe quasi-microspheres in our work with previously reported Co- and Se-based materials in the literatures.
Sample | Specific capacitance | Rate capacity retention | Cycling capacity retention | Reference |
---|---|---|---|---|
CoCO3 nanoparticle | 440 F g−1 (1 A g−1) | 59.1% (10 A g−1) | 95.3% (4000 cycles at 5 A g−1) | 44 |
Porous Co3O4 microflowers | 240.2 F g−1 (0.625 A g−1) | 84.1% (6.25 A g−1) | 96.3% (2000 cycles at 3.75 A g−1) | 45 |
CoOOH nanoplates | 124 F g−1 (1 A g−1) | 80.6% (10 A g−1) | 79.4% (10000 cycles at 10 A g−1) | 46 |
CoS2 nanodendrite | 323.05 F g−1 (0.5 A g−1) | 65.3% (8 A g−1) | 80.22% (3000 cycles at 4 A g−1) | 47 |
CoS1.097 nanotube | 764 F g−1 (1 A g−1) | 31% (10 A g−1) | 79% (500 cycles at 2 A g−1) | 48 |
GeSe2 Nanostructures | 300 F g−1 (1 A g−1) | 60% (10 A g−1) | 99.3% (2000 cycles at 1 A g−1) | 33 |
SnSe nanosheets | 228 F g−1 (0.5 A g−1) | 51.3% (10 A g−1) | 99.2% (1000 cycles at 1 A g−1) | 32 |
Co0.85Se nanotubes | 216 F g−1 (1 A g−1) | 60.6% (8 A g−1) | 90.3% (2000 cycles at 1 A g−1) | 36 |
CoSe quasi-microspheres | 440 F g−1 (1 A g−1) | 85.9% (10 A g−1) | 96.3% (5000 cycles at 10 A g−1) | This work |
56.4% (50 A g−1) | ||||
41.4% (100 A g−1) |
The cycling performance of the as-prepared 3D nanosheet-assembled CoSe quasi-microspheres was tested at different current densities of 2 and 10 A g−1 by the repeated galvanostatic charge/discharge process in the potential range of 0∼0.5 V, as illustrated in Fig. 5e. It can be clearly observed that the specific capacitances at different current densities increase in the initial cycles, and then reach a maximum value as the continuous cycling, and finally gradually decrease until stable. This can be ascribed to the gradual penetration of the electrolyte ions into the inner electrode material during the cycling process, resulting in the sufficient activation after several hundred cycles. Whereas the activation process of the electrode is faster as the current density increases and therefore a shorter activation time can be observed at a higher current density of 10 A g−1 when compared to 2 A g−1. This phenomena is similar to that of many previously reported literatures.36,49–52 The resulting 3D nanosheet-assembled CoSe quasi-microspheres manifest excellent cycling stability with 102.3 and 96.3% of initial capacity retention rate at 2 and 10 A g−1 over 2000 and 5000 consecutive charge and discharge cycles, respectively. The cycling stability for the as-prepared 3D nanosheet-assembled CoSe quasi-microspheres is comparable or even better than that of other reported Co- and Se-based materials in the literatures, as shown in the Table I,32,33,36,44–48 which further suggests the promising candidate electrode materials for supercapacitor application.
To further elucidate the superior supercapacitive properties of the as-synthesized 3D nanosheet-assembled CoSe quasi-microspheres, the EIS test was performed in the frequency range of 100 kHz to 0.01 Hz and the Nyquist plot was presented in Fig. 5f. The Nyquist plot is composed of the high frequency region and low frequency region. As shown in Fig. 5f and the corresponding inset, a small real axis intercept and negligible semicircle in the high frequency region can be observed, implying the low internal resistance (Rs) and charge transfer resistance (Rct). In the low frequency region, the sloping of the straight line can reflect the capacitive behavior of the electrode material, and it is well-known that the sloping close to 90°C indicates the ideal capacitive behavior.53 Thus, the larger the sloping of the straight line is, the better the capacitive behavior is. As observed from the curve of the low frequency region, the sloping of the straight line is higher than 45°, suggesting the good capacitive behavior. Additionally, from the comparison of the EIS before and after cycling at a current density of 10 A g−1, we can see that the resistance and the sloping of the straight line of the electrode don't show the obvious change, which indicates that the as-obtained CoSe sample can remain good cycling stability. These results above confirm that why the as-resulted 3D nanosheet-assembled CoSe quasi-microspheres can obtain the excellent capacitive performances.
To further evaluate the as-prepared 3D nanosheet-assembled CoSe quasi-microspheres for practical application, the CoSe/AC asymmetric supercapacitor was fabricated by utilizing the 3D nanosheet-assembled CoSe quasi-microspheres as the anode and activated carbon as the cathode, respectively. Fig. 6a shows the CV curves of the asymmetric supercapacitor at scan rates of 5∼50 mV s−1 in a potential window of 0–1.5 V. The obvious redox peaks can be clearly observed from the CV curves, indicating the typical faradaic pseudocapacitive features, which is further confirmed by the nonlinear discharge curves and the voltage plateaus from the galvanostatic discharge curves as shown in Fig. 6b. Fig. 6c displays the Ragone plot of the energy and power density of the asymmetric supercapacitor, and it manifests a energy density of 17.6 Wh kg−1 at a power density of 684 W kg−1, and still retains at 6.5 Wh kg−1 at a high power density of 6840 W kg−1. As demonstrated in Fig. 6d, the cycling performance of the asymmetric supercapacitor was investigated at a current density of 3 A g−1 by the repeated GCD test in the potential range of 0∼1.5V for 2000 cycles. These results above indicate that the as-prepared 3D nanosheet-assembled CoSe quasi-microspheres might be highly promising candidate electrode materials for application in supercapacitors and other energy storage devices.
Conclusions
In summary, 3D nanosheet-assembled CoSe quasi-microspheres have been successfully prepared via a facile hydrothermal method, which is investigated for the use as supercapacitor electrode materials in a three-electrode configuration for the first time. Benefiting from the unique 3D micro-/nanostructure features with superior pore-size distribution, relatively large mesopores and exceptional electrical conductivity, the as-fabricated 3D nanosheet-assembled CoSe quasi-microspheres show a high specific capacitance (440 F g−1 at 1 A g−1), superior rate capability (85.9, 56.4 and 41.4% of capacity retention rate at 10, 50 and 100 A g−1 compared with 1 A g−1), and excellent cycling stability (102.3 and 96.3% of initial capacity retention rate at 2 and 10 A g−1 over 2000 and 5000 cycles). Moreover, the asymmetric supercapacitor is further assembled by using 3D nanosheet-assembled CoSe quasi-microspheres as the cathode and AC as the anode, respectively, which delivers a energy density of 17.6 Wh kg−1 at a power density of 684 W kg−1, and retains superior cycling stability with 95.5% of its initial capacity retention rate at 3 A g−1 over 2000 cycles. These results above demonstrate that the as-prepared 3D nanosheet-assembled CoSe quasi-microspheres can be highly promising candidate electrode materials for application in supercapacitors and even other energy storage systems.
Acknowledgments
The work is financially supported by National Natural Science Foundation of China (21601057), Project funded by China Postdoctoral Science Foundation (2017T100608, 2016M602425), and Scientific Research Fund of Hunan Provincial Education Department (16C0462).