Carbon quantum dots/Ni–Al layered double hydroxide composite for high-performance supercapacitors

Abstract

Ultrathin nanoplate-like carbon quantum dots (CQDs)/Ni–Al layered double hydroxide (LDH) composite has been fabricated by a facile one-step solvothermal method. The obtained porous CQDs/NiAl-LDH composite displayed an enlarged specific surface and nanoscale size. As supercapacitor electrode material, the composite exhibited remarkable electrochemical performance with superior specific capacitance of 1794 F g⁻¹ (at 2 A g⁻¹), high rate capability of 967 F g⁻¹ (at 20 A g⁻¹) and excellent cycle performance (about 93% retention over 1500 cycles). The enhanced supercapacitor performance of CQDs/NiAl-LDH electrode should be attributed to the synergistic effect between NiAl-LDH nanosheets and CQDs.

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Introduction

Supercapacitors as one of the most promising electrochemical energy storage devices have attracted great attention in recent years due to their fast charge–discharge rate, high power density, long cycling life and low maintenance cost.^1–3 According to the charge-storage mechanism, supercapacitors can generally categorized into electrical double layer capacitors (EDLCs) and pseudocapacitors. Compared with EDLCs, pseudocapacitors, based on the transition metal oxides (such as RuO₂, MnO₂, NiO, Co₃O₄, Ni(OH)₂, Co(OH)₂ and their compound) and conducting polymers, can provide superior specific capacitance by utilizing fast and reversible electrochemical redox reactions,^4–10 but the relatively low mechanical stability and cycle life are major limitations for its application. Therefore, developing of alternative electrode that offer superior electrochemical performance and high stability is extremely desired.

Layered double hydroxides (LDHs) with positively charged hydrotalcite-like structure and transition metals have exhibited intriguing application potential in supercapacitors because of their abundant electrochemical active sites and flexible ion exchangeability.^11–14 However, the pristine LDH materials cannot display preferable supercapacitor performance due to their relatively poor electrical conductivity and low mass diffusion, which constrain high-rate electrolyte ion and electron transfer. Hence, a composite material with LDH and conductive agent would be a desirable solution for improving electrochemical performance.^15–17 Recently, many carbon materials such as carbon nanotubes (CNTs), activated carbon (AC), graphene have been reported to composite with LDHs as improved electrode materials of pseudocapacitors.^18–28 For example, Wei et al. fabricated CoMn-LDH nanowalls supporting on flexible carbon fibers (CFs), the resulting CoMn-LDH/CF electrode delivered a high specific capacitance (1079 F g⁻¹ at 2.1 A g⁻¹), and 82.5% capacitance retention even at 42.0 A g⁻¹.¹⁹ Wang et al. reported a new kind of ternary NiAl-LDH/CNT/GNS composite, exhibiting excellent specific capacitance (1562 F g⁻¹ at 5 mA cm⁻²), stable cycle life with only 3.5% deterioration after 1000 cycle test.²⁰

Carbon quantum dots (CQDs), known as a new class of nanocarbon materials with zero-dimensional graphite structures have drawn immense attention because of their unique nature: small particle sizes (about 5 nm), benign conductivity, rapid electron transfer, electron reservoir properties, easy large scale production and low cost.^29–32 In particular, CQDs have abundant functional groups on their surface that can solidly nucleate and anchor the pristine nanocrystals to achieve an intense electrostatic interaction. Accordingly, compositing of LDHs and CQDs could increase the load of the active material and strengthen the contact between the conductor and the active material, which will improve the electrochemical performance and stability.

However, little work is reported on CQDs nanohybrids for electrochemical energy storage, especially as supercapacitor electrode materials. In this paper, a novel CQDs/NiAl-LDH composite nanosheet as pseudocapacitor electrode materials was successfully synthesized through a facile and simple method. Compared with pristine NiAl-LDH, the as-obtained CQDs/NiAl-LDH composite represented a new assembly form, a mesopore size of 7.1 nm and higher specific surface area of 77.9 m² g⁻¹. In addition, the composite electrode exhibited high capacitance of 1794 F g⁻¹ (at the current density of 2 A g⁻¹), which has exceeded the specific capacitances of most previously reported NiAl-LDH based pseudocapacitance materials.^{18–20,23,26} Besides, the CQDs/NiAl-LDH electrode showed excellent rate capability (with the residual capacitance of 967 F g⁻¹ at the high current density of 20 A g⁻¹) and stable cycling capability (93% retention over 1500 cycles), which can be potentially used in energy storage/conversion devices.

Experimental

Preparation of CQDs

The CQDs were synthesized through a method of electrochemical ablation of graphite.³³ In a typical synthetic process, two identical graphite rods placed parallel (99.99%, Alfa Aesar Co. Ltd., 13 cm in length and 0.6 cm in diameter) were inserted into the electrolyte as both cathode and anode in a separation distance of 7.5 cm. The electrolyte was ultrapure water (18.4 MΩ cm⁻¹, 600 mL) without any other additives. Static potentials of 15–60 V were applied to the two electrodes using a direct current (DC) power supply for about ten days with stirring. Subsequently, a dark-yellow solution appeared gradually in the reactor. The solution was filtered with slow-speed quantitative filter paper, and the resultant solution was centrifuged at 22 000 rpm for 30 min to remove the precipitated graphite oxide and graphite particles. Finally, the obtained solution consisted of water-soluble CQDs.

Synthesis of CQDs/NiAl-LDH composite

In a typical synthesis, 20 mL of water-soluble CQDs (∼20 mg) aqueous solution obtained in above step were dispersed and immersed in ethanol by ultrasonic for 10 minutes. Then, 9 mL of 1.0 M Ni(NO₃)₂·6H₂O (9 mmol), 3 mL of 1.0 M Al(NO₃)₃·9H₂O (3 mmol) and 10 mL of 4.0 M CO(NH₂)₂ (40 mmol) aqueous solution were added to form a 80 mL solution. The solution was vigorously stirred for 1 h at room temperature. The above solution was then transferred to a sealed Teflon-lined stainless-steel autoclave, followed by heating in an oven under 140 °C for 14 h. The autoclave was then naturally cooled to room temperature. The obtained hybrid were subsequently rinsed several times with DI water and anhydrous ethanol, and dried at 60 °C for 8 h to obtain the final product. The CQDs content is 17% after calculation. In addition, by the same explored procedure, 10 mL and 30 mL CQDs aqueous solution were added into the system instead. The final product denoted as CQDs/NiAl-LDH-1 and CQDs/NiAl-LDH-2, respectively. As a comparison sample, the physical compound (labeled as NiAl-LDH + CQDs) was synthesized by mixed NiAl-LDH and CQDs in magnetic stirrer overnight.

Synthesis of pure NiAl-LDH microspheres

The flowerlike NiAl-LDH microspheres were synthesized by a facile solvothermal method as follows:³⁴ briefly, the above Al(NO₃)₃·9H₂O, Ni(NO₃)₂·6H₂O and CO(NH₂)₂ were dissolved in a 80 mL of ethanol solution and stirred for 1 h at room temperature. The above solution was then transferred to a sealed autoclave, and maintained at 140 °C for 14 h.

Characterization methods

The crystal structure of the resultant products were characterized by X-ray powder diffraction (XRD) by using an X'Pert-ProMPD (Holand) D/max-γA X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). Scanning electron microscopy (SEM) images was taken on a FEI-quanta 200 scanning electron microscope with acceleration voltage of 20 kV. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were collected on a FEI/Philips Tecnai 12 BioTWIN transmission electron microscope and a CM200 FEG transmission electron microscope, respectively. The Fourier Transform Infrared (FT-IR) spectra were obtained with a Varian Spectrum GX spectrometer. Raman spectra were collected on an HR 800 Raman spectroscope (J Y, France) equipped with a synapse CCD detector and a confocal Olympus microscope. The spectrograph uses 600 g mm⁻¹ gratings and a 633 nm He–Ne laser. X-ray Photoelectron Spectroscopy (XPS) data were obtained using a KRATOS Axis Ultra-DLD X-ray photoelectron spectrometer with a monochromatised Al Kα X-ray (hν = 1486.6 eV). The powder sample was attached to carbon tape and was set in the XPS chamber. Calibration of binding energy was carried out by setting binding energy of C 1s peak to 284.5 eV. BET specific surface areas and pore size distributions were determined on a Micromeritics ASAP 2050 porosimeter by plotting the adsorption isotherm of N₂ at liquid N₂ temperature (77 K).

Fabrication of nanomaterial electrodes

The nanomaterial electrodes were fabricated using the previously reported method:³⁵ the as-prepared samples (≤8 mg), acetylene black, and polytetrafluoroethylene (PTFE) were mixing with a mass ratio of 80 : 15 : 5, acetylene black and PTFE were respectively used as the electrical conductor and binder, and then was dispersed by a small amount of ethanol to produce a homogeneous paste. This paste was then pressed onto nickel foam (1 × 1 cm²) current-collectors to make electrodes. Before the performance measurements of the electrodes, as-obtained electrodes were dried at 50 °C for several hours.

Electrochemical measurement

The electrochemical properties of as-obtained products for pseudocapacitors were carried out at 25 °C in a conventional three-electrode system connected to CHI660E electrochemical workstation, with a LDH hybrid-based working electrode, a saturated calomel electrode (SCE) as reference electrode, and a platinum wire counter electrode. Freshly prepared 6 M KOH solution was used as the electrolyte. CV tests were performed between 0 and 0.6 V (vs. SCE) at 10 mV s⁻¹. Galvanostatic discharge curves were measured in the potential range of 0 to 0.42 V (vs. SCE) at different current densities. The electrochemical impedance spectroscopy measurements were performed over a frequency range from 0.05 to 10⁵ Hz with the ac perturbation of 5 mV.

Results and discussion

The morphologies and structures of the NiAl-LDH and CQDs/NiAl-LDH composites are investigated by scanning electron microscopy (SEM), as shown in Fig. 1. The pure NiAl-LDH exhibits flowerlike hierarchical porous microspheres with the particle size of several micrometers (see Fig. 1a). The surfaces of the microspheres are composed of ultrathin secondary LDH sheets with about 1–3 μm in length, which are assembled by lots of smaller primary LDH platelets.²⁰ The NiAl-LDH + CQDs compound shows similar size and shape with the pure LDH (Fig. S1†), which means that simple physical mixing of CQDs and NiAl-LDH will not influence the intrinsic structure of NiAl-LDH. While, the form of CQDs/NiAl-LDH sheets self-assembly has been changed (Fig. 1b and c). It could be clearly observed that the porous network structure consists of interlaced ultrathin nanoplates with a variable size of ca. 200–500 nm. Fig. 1d shows the typical transmission electron microscopy (TEM) image of the CQDs/NiAl-LDH composite. It is easy to see that the NiAl-LDH nanoplates and CQDs (∼5 nm) successfully formed a composite. The HRTEM image further revealed the composite was fabricated. The lattice fringes distance of 0.225 nm can be seen corresponding to the (009) plane characteristic of the NiAl-LDH nanocrystals. Another clearly defined lattice spacings of 0.321 nm corresponds to the (002) crystallographic planes of graphitic carbon. These results confirm the formation of composite structure in the CQDs/NiAl-LDH hybrid.

Fig. 1 SEM images of (a) NiAl-LDH and (b) and (c) CQDs/NiAl-LDH, (d) TEM images of the CQDs/NiAl-LDH composite. Arrows point to individual NiAl-LDH plates and smaller CQDs particles (trace of inset: HRTEM images of the LDH and CQDs in CQDs/NiAl-LDH composite).

The XRD patterns of LDH, CQDs and CQDs/NiAl-LDH are presented in Fig. 2a. All XRD patterns of the LDH-based compounds could be indexed to a typical rhombohedral phase Ni–Al hydrotalcite (JCPDS card 15-0087). The characteristic peak of CQDs at 26° is too weak to be observed in CQDs/NiAl-LDH composite, which resulted from the low amount of carbon and its relatively low diffraction intensity in the composites.³⁶ When observing individual CQDs/NiAl-LDH, it is easy to find a decrease intensity of (003) peak, indicative of decreased LDH crystallinity. The broadening diffraction peaks further indicate the particle size of NiAl-LDH become smaller and thinner with the addition of CQDs into the reaction system, which is consistent with the results of SEM images. Moreover, some main diffraction peaks [like (003) and (006) peaks] moved to lower angle, indicating a larger interlayer spacing than pure NiAl-LDH.¹² We can conclude that CQDs have a key role in forming the nanostructure of composite to obtain more efficient ion pathways.

Fig. 2 (a) XRD patterns and (b) FT-IR spectra of NiAl-LDH, CQDs and CQDs/NiAl-LDH.

The FT-IR spectrum of the as-prepared samples in the region from 4000 to 400 cm⁻¹ was presented in Fig. 2b. As to pure CQDs, several peaks located at about 3455 (hydroxyl OH), 1630 (carboxy C=O), 1386 (aromatic C=C), and 1108 cm⁻¹ (epoxide/ether C–O–C) corresponding to oxygen-containing groups and other new functional groups, indicating successful oxidation of graphite and the formation of hydrophilic groups. For NiAl-LDH, the broad peak centered at about 3450 cm⁻¹ corresponds to the O–H stretching vibration of water molecules exist in the interlayer and H-bonded OH groups, accompanied with the bending mode at 1633 cm⁻¹. The series of absorbance bands in the range of 1500–1300 cm⁻¹ and the slight peak at 1000 cm⁻¹ are due to the stretching mode of carbonate (CO₃²⁻). Other absorption bands below 800 cm⁻¹ are assigned to metal–oxygen (M–O) stretching in the brucite-like lattice. In the FT-IR spectra of CQDs/NiAl-LDH composite (red line), there is still some characteristic peaks of pure CQDs and NiAl-LDH. However, some dominant peaks shows a slight shift. For example, the C=O peak had ∼35 cm⁻¹ red shift which probably due to the strong electrostatic interactions between NiAl-LDH and CQDs or the formation of a chemical bound C–O–M (M represents Ni or Al). This crucial result indicating that not only physical absorption but more strong interactions may exist between CQDs and NiAl-LDH, which is much different with the FT-IR spectra of LDH + CQDs (Fig. S2†).

The characterization of the surface electronic states for the CQD/NiAl-LDH nanocomposite was also confirmed by XPS measurements (see Fig. 3). It is easy to see that except for the Ni, Al and O stemming from pristine NiAl-LDH, the C peaks corresponding to CQDs (CQDs content: ∼17%) can be distinctly detected in CQD/NiAl-LDH complex. For the C 1s XPS spectra shown in Fig. 3c, the dominant peak at 284.5 eV corresponding to the non oxygenated ring C (C–C), along with two peaks at 286.6 and 288.8 eV corresponding to the oxygen single-bonded carbon (C–O) and the carboxylic acid (O–C=O), which are in agreement with the FT-IR results as mentioned above. In addition, the peaks corresponding to Al, Ni 2p_3/2, and Ni 2p_1/2 (Fig. 3b and d) also confirm the presence of NiAl-LDH in the composite. Thus we can conclude that the CQD/NiAl-LDH nanocomposite has been successfully synthesized.

Fig. 3 (a and b) XPS survey spectrum, (c) C 1s and (d) Ni 2p of the CQDs/NiAl-LDH.

As we know, a large specific surface area and suitable pore size distribution are crucial in fast charge transfer and the formation of efficient ion pathways. Therefore, the products were further investigated by N₂ adsorption/desorption measurement for their surface area and pore structure (shown in Fig. 4). Owing to the addition of CQDs, the size of NiAl-LDH crystals becomes smaller and more exposed, a surface area of 77.9 m² g⁻¹ for the CQDs/NiAl-LDH composite is much higher than 24.1 m² g⁻¹ for pure LDH, which may result in more excellent electrochemical characteristics.²⁰ On the basis of the pore size distribution plot, two sizes of 2.4 and 7.1 nm can be observed, which indicates CQDs/NiAl-LDH composite is distinctive because of the existence of well-defined mesoporous. These may be attributed to its internal voids and interparticle spacing.³⁷ In view of such a distinct porous structure and high surface area, the composite may offer efficient transport pathways to their interior voids during the charge/discharge storage process, which is critical for the electrochemical performance.

Fig. 4 (a) N₂ adsorption/desorption isotherm and (b) pore size distribution of pristine NiAl-LDH and CQDs/NiAl-LDH.

In the subsequent experiments, the potential application of the CQDs/NiAl-LDH nanocomposite in pseudocapacitor was explored. The samples were used as electrode material and the electrochemical behaviors were characterized by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). Fig. 5a illustrates the CV curves of pristine NiAl-LDH and CQDs/NiAl-LDH electrodes within the potential window of 0 to 0.6 V in 6 M KOH aqueous solution at the scan rate of 10 mV s⁻¹. The CV patterns of both as-obtained electrodes all show typical pseudocapacitive nature, where a pair of well-defined redox peaks indicate the conversion between different oxidation states of Ni according to the following equation:³⁸

Ni(OH)₂ + OH⁻ = NiOOH + H₂O + e⁻

Fig. 5 (a) CV curves of pristine NiAl-LDH and CQDs/NiAl-LDH hybrid electrodes at a scan rate of 10 mV s⁻¹. (b) Discharge curves of NiAl-LDH and CQDs/NiAl-LDH at a current density of 2 A g⁻¹. The discharge curves of (c) pristine NiAl-LDH and (d) CQDs/NiAl-LDH at different current densities. (e) EIS plots of the pure NiAl-LDH and CQDs/NiAl-LDH composite electrode. (f) The specific capacitance retention with 1500 cycling numbers for CQDs/NiAl-LDH electrode tested at a current density of 10 A g⁻¹.

Furthermore, the CQDs/NiAl-LDH electrode shows higher redox peak currents and larger average areas of CV curves, indicating a superior electrochemical capacitance.

Fig. 5b shows the discharge curves of the as-prepared composites within the potential of 0–0.42 V at 2 A g⁻¹. It can be seen that the charge/discharge curves are nonlinear lines, also confirming the pseudocapacitance behavior of the samples due to quasireversible redox reactions at the electrode–electrolyte interface.¹⁵ The specific capacitance (C_s) was evaluated from the discharge curve as below:

C_s = IΔt/mΔV

where I is the discharge current (A), Δt is the discharge time (s), ΔV is the potential window of discharge (V) and m is the mass of the as-synthesized sample in the electrode.

The specific capacitance of CQDs/NiAl-LDH electrode was calculated to be 1794 F g⁻¹, which is four times larger than that of pristine NiAl-LDH (448 F g⁻¹) and NiAl-LDH + CQDs (shown in Fig. S3†). Such excellent specific capacitance should be attributed to the collaborative effects of the CQDs/NiAl-LDH nanostructures as follows: (i) the CQDs/NiAl-LDH hybrid possesses the lamellar structure with enlarged interlayer spacing (as Fig. 2 reveals). This characteristic nanostructure not only ensures the fast ion diffusion but also provides continuous electron pathways in the electrochemical reactions. (ii) Owning to the introduction of CQDs, pristine NiAl-LDH nanosheets become thinner during the self-assembly process, which may facilitate the access of the electrolyte and the exposure of active sites more easier. (iii) The presence of nano-sized CQDs improves the electrical conductivity and surface roughness of the hybrid electrode, which further boosts the ion transfer efficiency.

The discharge curves of the NiAl-LDH and CQDs/NiAl-LDH electrode vary with the current densities. The specific capacitance values of the CQDs/NiAl-LDH sample at current densities of 2, 4, 6, 8, 10, 20 A g⁻¹ were calculated to be 1794.6, 1619.1, 1316.9, 1291.7, 1138.4 and 967.3 F g⁻¹ respectively, which obviously exceeded the specific capacitances of LDH (449.4, 315.8, 311.5, 291.7, 252.9 and 208.3 F g⁻¹) (Fig. 5c and d). As the current density increases from 2 to 20 A g⁻¹, the specific capacitance of the pristine LDH retains 46.1%. In contrast, the CQDs/NiAl-LDH electrode, about 50% of the capacitance was maintained at even 40 A g⁻¹ demonstrating a better rate capability.

EIS analysis is one of the principal methods used to examine the fundamental properties of electrode materials for supercapacitors.³⁹ The EIS data of NiAl-LDH and CQDs/NiAl-LDH composites were analyzed by Nyquist plots as shown in Fig. 5e. Each impedance spectrum demonstrates similar form with a semicircle arc at high frequency region and a straight line at lower frequency. The intersection of the curve at the real part indicates the resistance of the electrochemical system (R_s), which includes the inherent resistance of the electroactive material, ionic resistance of electrolyte, and contact resistance at the interface between electrolyte and electrode. Obviously, contact resistance of the CQD/NiAl-LDH (0.84 Ω) is smaller than that of pristine NiAl-LDH sample (1.26 Ω). The semicircle diameter reflects the charge-transfer resistance (R_ct), corresponding to the charge transfer limiting process. Compared with the NiAl-LDH electrode, the 0.08 Ω of R_ct for the CQDs/NiAl-LDH hybrid electrode was lower. These results suggest the CQDs/NiAl-LDH composite may possess excellent capacitive performance because of the unique structure and superhigh conductive behaviour, which are convenient for the rapid and reversible charge transfer during electrochemical reactions.

Cycling performance is another important parameter in developing supercapacitor electrodes for practical applications. The cycling performance of the CQDs/NiAl-LDH electrode was tested by galvanostatic charge/discharge cycling at the current density of 10 A g⁻¹ (Fig. 5f). After 1500 cycles, the capacitance remains at 93% of its initial value, indicating little structural changes of the CQDs/NiAl-LDH thin nanosheets electrode during the repeated charge/discharge processes. Besides, there is a small increase of specific capacitance in the initial 200 cycles, which can be explained as the activation of the electrode material. In general, these results demonstrate good long-term electrochemical stability of the CQDs/NiAl-LDH hybrid electrode, which may be attributed to the high conductivity of CQDs and the network of the special porous structure in the hybrid.

Conclusions

In summary, we here have successfully fabricated a novel CQDs/NiAl-LDH nanocomposite with a relatively high BET surface area of 77.9 m² g⁻¹ and mesoporous structure via a simple hydrothermal method. The sample exhibited high specific capacitance, excellent rate capability, and extended cycling performance as electrode materials for supercapacitors. The enhanced supercapacitor performance may be ascribed to the synergistic effect between NiAl-LDH nanosheets and CQDs.⁴⁰ As the presence of nano-sized CQDs, the electrical conductivity, the surface roughness, and the active sites of composite effectively increased, further, the accessibility to electrolyte ions and ion transfer efficiency was improved. Overall, this work will not only broaden the application of CQDs, but provide a promising method for fabricating high-performance supercapacitors.

Acknowledgements

This work is supported by the National Basic Research Program of China (973 Program) (2012CB825803, 2013CB932702), the National Natural Science Foundation of China (21503020, 21373002, 21003081, 51132006).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02730j

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