3D Hierarchical Co–Al Layered Double Hydroxides with Long-Term Stabilities and High Rate Performances in Supercapacitors
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Three-dimensional (3D) flower-like Co–Al layered double hydroxide (Co–Al-LDH) architectures composed of atomically thin nanosheets were successfully synthesized via a hydrothermal method in a mixed solvent of water and butyl alcohol. Owing to the unique hierarchical structure and modification by butyl alcohol, the electrochemical stability and the charge/mass transport of the Co–Al-LDHs was improved. When used in supercapacitors, the obtained Co–Al-LDHs deliver a high specific capacitance of 838 F g−1 at a current density of 1 A g−1 and excellent rate performance (753 F g−1 at 30 A g−1 and 677 F g−1 at 100 A g−1), as well as excellent cycling stability with 95% retention of the initial capacitance even after 20,000 cycles at a current density of 5 A g−1. This work provides a promising alternative strategy to enhance the electrochemical properties of supercapacitors.
KeywordsCo–Al layered double hydroxides (Co–Al-LDHs) Nanosheets 3D hierarchical architectures Butyl alcohol Supercapacitors
3D Flower-like Co–Al layered double hydroxides (Co–Al-LDHs) built up of atomically thin nanosheets were successfully synthesized via a hydrothermal method in a mixed solvent of water and butyl alcohol.
Owing to the unique hierarchical structure and modification by butyl alcohol, the electrochemical stability and the charge/mass transport of the Co–Al-LDHs was improved, therefore leading to high specific capacitance, excellent rate performance and good cycling stability in supercapacitors.
To meet the increasing demand for clean energy technologies, many energy storage and conversion devices, such as fuel cells, batteries, and supercapacitors, have been developed [1, 2, 3, 4, 5]. Compared with other chemical energy storage devices, supercapacitors have attracted extensive attention owing to their fast charge/discharge rate, high power density, and long cycle lifetime [6, 7, 8, 9, 10]. Up to now, carbon-based capacitors have been widely studied due to their cost-effectiveness and excellent rate and cyclic stability . However, the relatively low capacitance (<300 F g−1) cannot meet the demand for high energy density.
It has been reported that pseudocapacitive transition metal oxides/hydroxides possess high capacitances derived from their reversible faradic reactions [11, 12, 13, 14]. Layered double hydroxides (LDHs), which are made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules, are promising electrode materials for supercapacitors due to the synergistic effects of bi-metal cations, such as reciprocal activation [15, 16]. However, the migration of metal cations can be limited by other cations, which can suppress the aggregation and growth of the active materials [17, 18]. Co–Al-LDHs with divalent Co2+ ions and trivalent Al3+ ions are one of the most commonly studied LDHs because of their excellent electrochemical properties [19, 20, 21]. However, the specific capacitance, rate capability, and stability are usually poor because of the limited conductivity and the re-stacking of 2D nanosheets [22, 23]. Compositing with highly conductive substrates, such as Ni foil or carbon materials, is considered an effective method to improve the performance of Co–Al-LDHs. For example, the porous Co–Al-LDHs/GO (GO, graphene oxide) nanocomposite exhibits a specific capacitance of 1043 F g−1 at 1 A g−1 . H-OH intercalated Co–Al-LDHs on Ni foil shows a capacitance of 1031 F g−1 at 1 A g−1 and an ultrahigh rate capability with 66% capability retention at 100 A g−1 . However, the cycling stability of LDHs is usually less than 5000 cycles (Table S1), which is far from the practical demand of 100,000–200,000 cycles. Therefore, the stability of Co–Al-LDHs is the most prominent problem to overcome.
In general, active materials for electrodes with larger surface areas show higher capacitances and stabilities. Two-dimensional (2D) monolayer LDH nanosheets with extremely large surface areas can be prepared by a top-down method, in which LDH nanoplates are first prepared and then exfoliated in liquid medium by ultrasonic treatment . However, the nanosheets prefer to re-stack to reduce the surface free energy, which is detrimental to the capacitance and stability of the electrodes. It has been accepted that three-dimensional (3D) hierarchical structures composed of 2D nanosheets are more stable than 2D nanosheets [27, 28]. The unique structure is beneficial to charge and mass transport and the mitigation of volume change during the charge/discharge process . Furthermore, 3D hierarchical structures can supply more points to connect the conductive matrix in the electrodes, which can provide more electron paths and suppress the separation of active materials [30, 31, 32]. On the other hand, the stability of the layered compounds can be improved by modification with organic compounds because they can intercalate and/or adsorb into the layers to reduce the surface energy [33, 34, 35, 36] and further prevent the re-stacking of nanosheets . For example, Xiao et al. found that MoS2/PEO [poly(ethylene oxide)] nanocomposites had high reversible capacities with long-term reversibility because the incorporation of PEO can stabilize the disordered structure of MoS2 .
Herein, 3D hierarchical Co–Al-LDHs were fabricated in a rationally designed reaction system. Owing to the unique hierarchical structures composed of atomically thin nanosheets and the modification by butyl alcohol, the electrochemical stability and the charge/mass transport of the 3D Co–Al-LDH architectures were improved. When used in supercapacitors, high specific capacitance and good cycling stability were achieved.
3 Experimental Section
3.1 Synthesis of 3D Hierarchical Co–Al-LDHs
In a typical procedure, Co(NO3)2·6H2O (2.4 mmol, 0.698 g) and Al(NO3)3·9H2O (0.8 mmol, 0.3 g) were dissolved in 40 mL deionized water and 40 mL butyl alcohol and stirred for 30 min. Then, 0.384 g of urea and 15 mg of citric acid trisodium salt dehydrate were added and further stirred for another 30 min. Next, the mixtures were sealed in a 100-mL Teflon-lined steel autoclave and hydrothermally treated at 120 °C for 12 h. After being cooled to room temperature naturally, the samples were filtered and washed with deionized water and ethanol several times and then freeze-dried (5 × 10−2 mbar at T ≤ −46 °C) for 24 h to obtain the 3D Co–Al-LDHs. For comparison, 2D Co–Al-LDHs were prepared using deionized water as the solvent, and zero-dimensional (0D) Co–Al-LDHs were prepared using butyl alcohol as the solvent under similar reaction conditions.
3.2 Material Characterization
The crystal structure and phase were characterized on an X-ray powder diffractometer (XRD, Shimadzu-6000) and X-ray photoelectron spectrometer (XPS, VG Scientific ESCLAB 220iXL). The size and morphology of the as-synthesized products were determined by a transmission electron microscope (TEM, JEOL-1200) and field emission scanning electron microscope (FESEM, JEOL, JSM-7401F) with an accelerating voltage of 5 kV. Atomic force microscopy (AFM) measurements were collected on a Multimode atomic force microscope (Veeco Instruments, Inc.). Typically, a freshly diluted ethanol solution of the NiFe-LDH samples was ultrasonically treated and then deposited onto a clean mica wafer by drop-casting. The nitrogen adsorption–desorption measurement was conducted on a Micromeritics ASAP 2010 analyzer, and the specific surface areas of samples were determined by Brunauer–Emmett–Teller (BET) analysis. FT-IR spectra were recorded on a PerkinElmer Spectrum 100 Fourier transform infrared spectrometer using KBr pellets.
3.3 Electrochemical Measurements
The electrochemical experiments were performed using a standard three-electrode configuration with the as-synthesized sample electrode as the working electrode, platinum as the counter electrode, and Hg/HgO as the reference electrode. The electrolyte was a 2 mol L−1 aqueous KOH solution. The working electrodes were prepared as follows: 75 wt% active materials were mixed with 7.5 wt% acetylene black, 7.5 wt% KS-6, and 10 wt% polyvinylidene fluoride in NMP. The slurry was pressed on Ni foam (2 cm ×1 cm × 1 mm) and dried at 80 °C under vacuum for 6 h. Each working electrode contained approximately 1 mg of active material. CV and galvanostatic charge/discharge tests were performed on an electrochemical workstation (Zahner Zennium CIMPS-1, Germany) in the potential range of 0–0.55 V and 0–0.45 V, respectively. Electrochemical impedance spectroscopy (EIS) was carried out by applying a 5 mV amplitude over a frequency range of 0.01 Hz to 100 kHz at open circuit potential.
4 Results and Discussion
The high performance of the 3D Co–Al-LDH material can be ascribed to its unique 3D hierarchical structure. First, the atomically thin building units with thicknesses of approximately 1.6 nm (152 m2 g−1) can provide a large amount of electrochemically active sites to result in high capacitance. Additionally, the 3D hierarchical structures can prevent the re-stacking of nanosheets, and the surface modification of organic molecules can enhance the stability of 3D Co–Al-LDHs (Figs. S7, S8), leading to long-term cyclic stability. Furthermore, the pores in the 3D hierarchical structure are readily accessible for electrolyte, facilitating the transport of ions from the liquid to the active surface of the LDH. Finally, the 3D hierarchical structure can also supply more points to connect the conductive matrix in the electrode, which is beneficial to the conductivity and rate capability of the electrodes.
A facile synthetic route was developed to directly prepare 3D hierarchical Co–Al-LDHs composed of atomically thin nanosheets. The as-obtained hierarchical Co–Al-LDHs show a high specific capacitance of 801 F g−1 at 5 A g−1, excellent rate performance with a capacitance of 677 F g−1 at 100 A g−1, and good cycling stability with only 5% decline after 20,000 cycles. Such excellent performance is derived from its atomically thin building units modified by organic molecules and its unique 3D hierarchical structure. This work may provide a promising alternative strategy to prepare other LDHs with enhanced electrochemical properties for supercapacitors.
This work was supported by the National Basic Research Program of China (2014CB239702), Research project of environmental protection in Jiangsu province (2016060), and Science and Technology Commission of Shanghai Municipality (14DZ2250800).
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