One-Step Synthesis of Mesoporous Chlorine-Doped Carbonated Cobalt Hydroxide Nanowires for High-Performance Supercapacitors Electrode
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Self-stabilized and well-defined chlorine-doped carbonated cobalt hydroxide nanowires have been obtained as a binder-free electrode via a facile method. The Co material has a unique well-defined needle-like structure, composed of highly aligned monomer with the diameter of about 3–10 nm and numerous surface pores, which makes it have potential for high-performance electrochemical capacitors. The test results show the directly acquired Co-ClNWs(NiE) electrode in three-electrode system can reach the specific capacity of more than 2150 F/g under the current density of 1 A/g, accompanied by a good cycling stability of 94.3% capacitance retention after 500 cycles, and exhibits a high energy density of 41.8 W h/kg at the power density of 1280.7 W/kg when using it as the positive electrode of an asymmetric supercapacitor. After making a comparison of the current material with the conventional electrodes, we can find that a better electrochemical performance can be achieved with a more convenient one-step method. Therefore, we, in this work, may provide a new type of manufacturing concept for future electrode treatment.
KeywordsSupercapacitor Nickel foam One-step method High performance Porous nanomaterials
Chlorine-doped carbonated cobalt hydroxide nanowires
Co-ClNWs stuck on the nickel foam
Co-ClNWs grown on the nickel foam
Electrical double-layered capacitors
Galvanostatic charge and discharge measurements
- MBT and MAT
The electrochemical impedance spectra of Co-ClNWs(NiE) before and after cycling
As a kind of energy storage and conversion device, supercapacitor has attracted tremendous attention owing to its fast charging and discharging rate, high power density, long cycle life, and high reliability advantages [1, 2]. In recent years, it has supplemented the deficiency of the traditional energy storage and conversion equipments in many important applications and prospect fields such as military electronic equipment, electric vehicles, portable computers, etc. [3, 4, 5, 6, 7]. Generally, supercapacitors can be divided into two types according to their different electron storage mechanisms: traditional electrical double-layered capacitors (EDLCs) which store energy by accumulation of charges in the electrical double layer via electrostatic interactions, and pseudocapacitors which store energy via Faradaic redox reaction at electrode surface [8, 9, 10, 11]. Among the various pseudocapacitance materials, ruthenium oxide exhibits excellent electrochemical performance, but high cost, low porosity, and toxic nature severely limit its commercial application . Therefore, some cheaper and more environmentally friendly but highly capacitive metal oxides/hydroxides such as NiO, Co3O4, Fe3O4, Fe2O3, V2O5, Co(OH)2, and Ni(OH)2 have become the most promising candidates . Co(OH)2, displaying the obvious advantages of well-defined reversible redox reactions with high theoretical specific capacity, has been considered as a particularly attractive potential material . The study finds that high capacitance performance reflects in a special morphological structure with high specific surface area [6, 15, 16, 17, 18]. From previous reports, Mahmood and his co-workers synthesized chlorine-doped carbonated cobalt hydroxide (Co(CO3)0.35Cl0.20(OH)1.101.74H2O) nanowires with extraordinary capacitance and excellent energy density along with high rate capability and stability. Such high capacitance and energy densities are thought to be attributed to the unique structure of the Co(CO3)0.35Cl0.20(OH)1.10 nanowires, in which hydrophilic nature can significantly enhance the wettability of the electrode surface, and the existence of counter structure stabilizer anions (Cl− or/ and CO32−) effectively controls the polarization of the electrode . Inspired by the superiorities of such work, the prospect in terms of optimizing the structural and electrochemical properties by doping of some elements into Co(OH)2 is foreseen.
At the same time, in order to obtain high specific surface area and other special morphology, the researchers begin to innovate in structure [17, 20, 21, 22, 23]. When active material was attached to the other electrode material surface, it could form a parcel core-shell structure or layered three-dimensional structure, which could ensure the effect of the active material and electrolyte ion contact in improving the reaction efficiency. For example, Shude Liu and his co-workers proposed a supercapacitor electrode comprising a three-dimensional self-supported hierarchical MnCo-layered double hydroxides@Ni(OH)2 [MnCo-LDH@Ni(OH)2] core–shell hetero structure on conductive nickel foam . The resultant MnCo-LDH@Ni(OH)2 structure exhibited a high specific capacitance of 2320 F/g at a current density of 3 A/g, and a capacitance of 1308 F/g was maintained at a high current density of 30 A/g with a superior long cycle lifetime. However, due to the different characteristics of materials, the preparation method has been facing with the problems of complicated operation, harsh reaction conditions, and low success rate. Therefore, a more handy preparation measure to obtain uniform and orderly electrode materials with high electrochemical performances is highly desired .
In this paper, the mesoporous chlorine-doped carbonated cobalt hydroxide nanowires (Co-ClNWs) are directly grown on the nickel foam to prepare the nickel foam electrode (Co-ClNWs(NiE)) by a facile one-step hydrothermal method based on the performance advantages of Co(OH)2. The electrochemical performance test is performed with Co-ClNWs(NiE) directly as the working electrode, which provides a key measure to enhance both specific capacitance and energy density for its reasonable realization of the inner active sites of the bulk materials for storage energy. Meanwhile, the performance comparison is performed with the conventional electrode. It provides a feasible reference method for the increasement of capacitance and the application development of Co materials, and also offers new ideas for the structure and production of capacitor industrialization in the future.
Synthesis of Co-ClNWs on Ni Foam
Ni foam was obtained from Canrd Co., Ltd., China. Prior to use, it was treated with 0.5 M HCl under ultrasonic for 0.5 h, and then dried at 80 °C for 12 h after washing by a great amounts of deionized water and ethanol to remove surface ions. All other chemicals were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. in China without further treatment before use.
Firstly, 3.5 g CoCl2·6H2O and 0.9 g CO(NH2)2 were dissolved in 100 mL deionized water under magnetic stirring, lasting for 30 min until the solid was completely dispersed and dissolved. The obtained homogeneous solution was then transferred into stainless steel autoclave with several clean nickel foams fixed by stainless steel clips (the quality of the nickel foam was measured in advance), making sure the nickel foams are completely submerged, and placing at 120 °C with a thermal reaction of 20 h. After cooling to room temperature, the nickel foams were fetched out and washed with deionized water to remove impurities adhered on the surface. Finally, the samples were selected in the vacuum, drying in an oven for 10 h for use.
The structures and morphologies of the products were analyzed by field-emission scanning electron microscopy (SEM MIRA3 TESCA) and transmission electron microscope (TEM FEI Tecnai). X-ray diffraction (XRD) patterns were collected with a SIEMENSD500 diffractometer with Cu Kα radiation (λ = 0.15056 nm). X-ray photoelectron spectroscopy (XPS) was carried out on ESCALAB 250 with Al Kα radiation to examine the chemical compositions and chemical valence states of the samples. N2 adsorption-desorption isotherms were obtained by an ASAP 2020 instrument at 77 K. The BET and QSDFT methods were respectively used to determine the specific surface areas and the pore size distributions of the materials.
where C is the specific capacitance (F/g), I is the current (A), Δt is the discharge time (s), ΔV is the potential window (V), and m is the mass of the electroactive electrode (g).
where I (A) shows the charge/discharge current, m (g) represents the total active mass of the two electrodes, Δt (s) means the discharge time, ΔU (V) is the potential window, and Cc (F g−1), Ec (W h kg−1), and Pc (W kg−1) are the specific capacitance, energy density, and power density of the cell, respectively.
Results and Discussion
Characterization of the Co-ClNWs(NiE)
In order to further obtain the constitution of the as-synthesized Co-ClNWs(NiE), Raman spectrum of it is illustrated in the wavenumber range from 0 to 2000 cm−1 and shown in Fig. 4e. Four Raman bands for Co-ClNWs(NiE) observed at about 95, 813, 1045, and 1554 cm−1 can be assigned to the bending mode for Cl-Co-Cl, Co-O-H deformation mode, -OH deformation mode, and ν3 (CO3)2− antisymmetric stretching mode, respectively, suggesting that the main components are in agreement with the tests above [33, 34, 35]. The inset figure in Fig. 4f exhibits the N2 adsorption/desorption isotherm of Co-ClNWs(NiE), in which a type IV isotherm coupled with an obvious H3 character hysteresis loop can be observed, showing the existence of abundant meso- and macropores distribution onto the Co-ClNWs(NiE), in consistence with the result by TEM and the pore size distribution in Fig. 4f. This porous structure in terms of interconnected meso- and macropores is conductive to providing continuous channels for fast and unimpeded ion diffusion and thus ensuring a good accessibility of ion at the active sites. In addition, there are nearly no micropores existence in Co-ClNWs(NiE) because of nearly no N2 volume absorption under the pore sizes between 0 and 2 nm, which is responsible for the low specific surface area (about 5 m2/g) but for high crystallinity with rich active sites confirmed by XRD above.
Electrochemical Performance of the Co-ClNWs(E) Electrode
Electrochemical Performance of the Co-ClNWs(NiE) Electrode
Comparison of the performance of our work with the literatures based on three-electrode system
Current density (A g−1)
Specific capacitance (F g−1)
2 M KOH
3 M KOH
6 M KOH
6 M KOH
3 M KOH
6 M KOH
3 M KOH
1 M NaOH
6 M KOH
In order to further evaluate the charge storage capability of Co-ClNWs(NiE) in practice, an asymmetric supercapacitor (ASC) using Co-ClNWs(NiE) and AC respectively as positive electrode and negative electrode was fabricated. Figure 6g illustrates the CV curves of Co-ClNWs(NiE) and AC measured in a three-electrode system with the potential window of AC of − 1 to 0 V and Co-ClNWs(NiE) from 0 to 0.6 V. Therefore, it is expected that the as-fabricated ASC can be worked to 1.6 V. As shown in Fig. 6h, the CV curves of ASC under different scan rates show a pair of apparent peaks, demonstrating the typical faradaic characteristics . Additionally, a specific capacitance of 117.5 F/g can be obtained from the GCD curve at 1 A/g in Fig. 6i, in accordance with a high energy density of 41.8 W h/kg at the power density of 1280.7 W/kg, higher than many recently publicized works [45, 46]. When the current density is enlarged to 8 A/g, the ACS can still exhibited an energy density of 21.2 W h/kg under a high power density of 6397.3 W/kg. This result clearly suggests that the ACS with the Co-ClNWs(NiE) as positive electrode exhibits a high energy density without sacrificing the high power density though a bulk redox reaction is involved, reflecting a possible method to keep a high energy storage capability under fast charge and discharge processes.
In summary, a Co-ClNWs(NiE) electrode is fabricated via a facile one-step hydrothermal method. The active material Co-ClNWs is deposited on commercial nickel foam to form a free-standing supercapacitor electrode. After the optimization of the structure of the Co-ClNWs(E) electrode prepared by PTFE sticking method, the Co-ClNWs(NiE) electrode displays a high specific capacitance of 2150 F/g under the current density of 1 A/g, with a large energy density of 21.2 W h/kg under a high power density of 6397.3 W/kg even when the current density is up to 8 A/g. These results reveal that Co(CO3)0.35Cl0.20(OH)1.101.74H2O NWs are very promising candidates for the next generation of energy storage devices. On this basis, the structural advantages of nickel foam make the active materials fully reflect the capacitive properties. The electrode design concept described in this paper makes it possible to develop high-energy supercapacitors.
This work was funded by the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100162110068) and the National Natural Science Foundation of China (Grant No. 61275174).
YZ and SL prepared the materials and draft the manuscript. YZ, SL, HL, and WX designed the work. YZ, SL, and BZ carried out the structure analyses and electrochemical performance tests of the samples. JZ edited the whole manuscript. All authors had read and approved the final manuscript.
All authors are from School of Physics and Electronics, Central South University, Changsha 410083, China.
The authors declare that they have no competing interests.
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