Facile growth of Bi₂MoO₆ nanosheet arrays on Ni foam as an electrode for electrochemical applications

Abstract

A facile hydrothermal reaction has been developed for the large-scale growth of Bi₂MoO₆ nanosheets on conductive substrates with robust adhesion as high performance electrodes for electrochemical capacitors. It was found that the nickel foam facilitates the formation of a uniform nanosheet array with fast electron and ion transportation, a large electroactive surface area and an excellent structural stability. As a result, it shows a specific capacitance of 37.3 F g⁻¹ even at a current density of 2 A g⁻¹ and a high cycling stability with 89% retention of its initial specific capacitance after 1000 cycles in 1 M KCl solution. These results provide a way of fabricating Bi₂MoO₆ nanosheet arrays as efficient electrodes for electrochemical capacitors.

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1. Introduction

Supercapacitors with higher power densities and longer cycle lives have drawn considerable attention over the past few decades due to the increasing energy demands and environmental issues.^1–3 Up to now, numerous nanomaterials^4–8 with high specific surface areas have been investigated with improved charge accumulation and ion transportation for electrochemical utilization. For example, graphene,^9,10 possessing a two dimensional structure, enables ultrahigh charging–discharging of an electric capacitor. Besides, the fabrication of interesting nanoarchitectures is also considered as an effective method to boost the electrochemical activities.

Recently, the rational design of nanosheet,^11–13 nanowire^14–17 and nanorod arrays^18,19 directly on substrates has been confirmed as a great achievement in electrochemical (EC) research. It avoids the dead surface in a traditional slurry derived electrode and allows for more efficient charge exchange during the electrochemical process. In particular, the nanosheet array has attracted wide interest because of the large surface area and good electric transmission. Cheng and co-workers²⁰ prepared Ni–Mo oxide nanosheet arrays on Ni foam with a high EC performance. Xiao et al.²¹ obtained NiO nanosheet networks supported on Ni foams as high rate supercapacitor electrodes. Liu et al. synthesized mesoporous MnO₂ nanosheet arrays on Ni foam with improved capacitance.²²

Bismuth molybdate (Bi₂MoO₆) with an Aurivillius type structure²³ is a promising material that is widely used in photocatalytic applications due to its unique optical and electric properties. Moreover, it has been studied as a good electrode material for supercapacitors. As reported, Bi₂MoO₆ nanosheets²⁴ yield a capacitance of 25 F g⁻¹ at 5 mV s⁻¹ in 1 M NaCl electrolyte. However, its EC properties need to be improved for practical applications. To the best of our knowledge, there is no report on the construction of Bi₂MoO₆ nanosheet arrays for electrochemical applications.

Herein, we report a Bi₂MoO₆ nanosheet array structure directly grown on Ni foam, which was successfully prepared by the facile hydrothermal reaction illustrated in Scheme 1. Furthermore, this synthesis method is also suitable for growing Bi₂MoO₆ nanostructures on carbon cloth. The results show that the Ni foam based samples exhibit a superior EC activity with a high specific capacitance of 37.3 F g⁻¹ at a scan rate of 2 A g⁻¹ in 1 M KCl electrolyte. The nanosheet array structures could not only provide a large surface area for a high rate supercapacitor, but also exhibit an excellent electron transport performance. This work provides an effective way to fabricate a Bi₂MoO₆ nanosheet array on a substrate with research into its electrochemical properties.

Scheme 1 The schematic illustration of the Bi₂MoO₆ nanosheet array growth process on Ni foam and carbon cloth.

2. Experimental section

2.1 Synthesis of the Bi₂MoO₆ nanosheet array on Ni foam and carbon cloth

All the reagents were of analytical grade and were used without further purification. The nickel foam was acquired from Shanghai the d new materials co., Ltd with 320–380 g m⁻², 1.8 mm thickness and an aperture of 400 μm. The carbon cloth was obtained from physical and chemical (Hong Kong) co., Ltd (W0S1002) with 125 g m⁻² and 360 μm thickness. The Ni foam and carbon cloth (4 cm × 1 cm × 1 mm) were cleaned by sonication in acetone, and deionized water in sequence. In a typical synthesis, 0.84 g of Bi(NO₃)₃·5H₂O and 0.2 g of Na₂MoO₄·7H₂O were separately dissolved in 5 mL of ethylene glycol under magnetic stirring. Then, clear solutions formed and these two as-prepared solutions were mixed together in a second step. Finally, 25 mL of ethanol was added into the solution slowly with stirring for 10 min. The resulting clear solution (35 mL) was transferred into a 80 mL Teflon-lined stainless steel autoclave. The prepared Ni foam and carbon cloth were placed into the reaction solution, which was then maintained at 160 °C for 24 h. After cooling down to room temperature, the coated Ni foam and carbon cloth were washed with distilled water and dried at 60 °C. The substrate was cut to form a 2 cm × 1 cm × 1 mm shape for characterizing the electrochemical properties. The substrate was weighed before and after the reaction, and the masses of Bi₂MoO₆ were 9.8 mg and 10.2 mg grown on the Ni foam and carbon cloth substrates, respectively. For comparison, the electrochemical activities of pristine Ni foam (0.105 g) and carbon cloth (0.05 g) with 4 cm × 1 cm × 1 mm areas were measured in 1 M KCl aqueous electrolyte as well.

2.2 Characterization

SEM images were taken using a field-emission scanning electron microscope (JSM-6701F, JEOL) operated at an accelerating voltage of 5 kV. The X-ray diffraction (XRD) measurements were performed on a Rigaku RINT-2000 instrument using Cu Kα radiation (40 kV). The XRD patterns were recorded from 10° to 80° with a scanning rate of 0.067° s⁻¹. The oxidation states of the product were investigated using an ESCALAB 250Xi X-ray photoelectron spectrometer (XPS) with non-monochromatized Mg Kα X-rays as the excitation source.

2.3 Electrochemical measurements

The electrochemical properties were tested on a CHI660 (Chenhua, Shanghai, China) with a three electrode system. All of the measurements were carried out in 1 M KCl aqueous electrolyte solution at room temperature. The as-obtained Bi₂MoO₆ nanosheet coated Ni foam and carbon cloth were used as the working electrode separately. A saturated calomel electrode (SCE) and platinum sheet were used as the reference and counter electrodes, respectively. Cyclic voltammetry (CV) curves were measured in a potential range from −0.4 V to −0.1 V at different scan rates, varying from 10 to 50 mV s⁻¹. Galvanostatic charge–discharge (GCD) measurements were recorded at different current densities within the potential range from −0.4 V to −0.1 V. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range from 1 to 10⁵ Hz.

3. Results and discussion

The morphologies and structures of the as-obtained Bi₂MoO₆ nanosheet arrays were characterized by scanning electron microscopy (SEM), as shown in Fig. 1. It is clear that the samples with a high density were uniformly distributed on the skeleton of the Ni foam (Fig. 1A). These nanosheets interconnect with each other and form an ordered array structure, as shown in Fig. 1B. Moreover, Fig. 1C reveals that the thickness of a single nanosheet was approximately 20 nm. Additionally, Fig. 1D–F display typical SEM images of the Bi₂MoO₆ nanosheet array grown on carbon cloth. The general view shows the uniform distribution of samples on each carbon fiber. Compared with the nanosheets on Ni foam, overlapping nanosheets with a smaller size are deposited on the carbon cloth. Notably, the nanosheet array structure provides convenient diffusion channels for the penetration of electrolytes, which could dramatically facilitate energy storage.

Fig. 1 SEM images of the Bi₂MoO₆ nanosheet array grown on (A–C) Ni foam and (D–F) carbon cloth.

The X-ray diffraction (XRD) patterns of the Bi₂MoO₆ samples are shown in Fig. 2. Due to interference by the nickel foam, the Ni foam electrode displays weak diffraction peaks ascribed to γ-Bi₂MoO₆ (JCPDS #21-0102)²⁵ and strong diffraction peaks for nickel (JCPDS #65-2865).²⁴ For the carbon cloth electrode, the diffraction peaks of carbon and Bi₂MoO₆ are present in the pattern. Moreover, the element composition and oxidation state of the Bi₂MoO₆ grown on Ni foam were explored by X-ray photoelectron spectroscopy (XPS) (Fig. 2B–D). To reduce the interference caused by the nickel foam, the product was scratched from the nickel foam. As shown in Fig. 2B, the Bi 4f core level spectrum shows two peaks at 164.2 and 158.6 eV, corresponding to Bi 4f_5/2 and Bi 4f_7/2 of Bi³⁺. In the Mo 3d pattern (Fig. 2C), the binding energies at 231.9 eV and 235.3 eV can be assigned to Mo 3d_5/2 and Mo 3d_3/2 of Mo⁶⁺, respectively. The Bi–O bonds in Bi₂MoO₆ result in a peak at 530.9 eV in the O 1s pattern (Fig. 2D). This is consistent with the previous reports on γ-Bi₂MoO₆.²⁶

Fig. 2 (A) XRD pattern of the Bi₂MoO₆ nanosheet arrays and XPS spectra of the Bi₂MoO₆ nanosheet scratched from the Ni foam substrate (B) Bi 4f core level (C) Mo 3d core level and (D) O 1s core level.

To evaluate the electrochemical properties of the Bi₂MoO₆ nanosheet arrays, their cyclic voltammogram (CV) curves (Fig. 3A and B) and galvanostatic charge–discharge (GCD) patterns (Fig. 3C and D) were investigated in 1 M KCl aqueous electrolyte. It is clear that all the CV curves exhibit an approximately rectangular shape with redox peaks, indicating the strong pseudocapacitive capability of the samples.^27,28 Furthermore, the shapes of these curves do not significantly change with scan rate variation, revealing the good capacitive behavior and facilitated charge diffusion. More specifically, the Bi₂MoO₆ nanosheet array grown on Ni foam has a larger and more regular enclosed area than that of the array on carbon cloth, which is related to a more superior capacitance.²⁹ The specific capacitance values (C_s) of the samples were calculated from the CV curves according to the equation: C_s = ∫IdV/νmΔV, where I (A) is the response current, ν (V s⁻¹) is the scan rate, ΔV (V) is the potential window, and m (g) is the mass of the active electrode material. The maximum specific capacitance of the Ni foam electrode (33.8 F g⁻¹) is higher than that of the carbon cloth electrode (26.6 F g⁻¹) observed over this measurement range.^30,31 Compared with carbon cloth, Ni foam is more favorable for the formation of an ordered nanosheet array with a high rate capacitance.

Fig. 3 CV curves of the (A) Ni foam supported Bi₂MoO₆ nanosheet array and (B) carbon cloth supported Bi₂MoO₆ nanosheet array. Galvanostatic charge–discharge curves of the (C) Ni foam supported Bi₂MoO₆ nanosheet array and (D) carbon cloth supported Bi₂MoO₆ nanosheet array.

The GCD results of the samples obtained in a potential window between −0.4 and −0.1 V (vs. SCE) are shown in Fig. 3. The specific capacitance could be calculated based on the discharge data by the following equation: C = It/mΔV, where C (F g⁻¹) is the specific capacitance, I (A) is the discharge current, t (s) is the discharge time, ΔV (V) is the potential window during the discharging process, and m (g) is the mass of the active material.^32–34 Calculated from the equitation, the Ni foam supported Bi₂MoO₆ nanosheet array electrode exhibited a capacitance of 37.3 F g⁻¹ at 2 A g⁻¹, which is much higher than that of the carbon cloth supported electrode (25.1 F g⁻¹) under the same conditions. Therefore, the rational fabrication of a nanosheet array on a good conductive substrate exhibits obvious advantages for pseudocapacitive behavior. The nanosheet array structure directly grown on a substrate not only enhances the connection between the material and substrate, but also provides a larger surface area of the active sites.

In addition, the typical cyclic voltammogram (CV) curves (Fig. 4A and B) and galvanostatic charge–discharge (GCD) patterns (Fig. 4C and D) reveal the capacitance performance of the substrates. Calculated from the CV curves, the specific capacitance values (C_s) of the Ni foam and C cloth are 6.7 × 10⁻³ and 4.52 × 10⁻³ F g⁻¹, respectively, at 50 mV s⁻¹, which are much lower than those of the Bi₂MoO₆ array electrodes (33.8 F g⁻¹ for Bi₂MoO₆ supported on Ni foam and 26.6 F g⁻¹ for Bi₂MoO₆ supported on carbon fiber). Moreover, based on the discharge data, the specific capacitance values are 5 × 10⁻³ and 4 × 10⁻³ F g⁻¹ for the Ni foam and C cloth at 10⁻⁴ A g⁻¹, respectively. It is clear that the substrates make little contribution to the capacitance of the electrodes.

Fig. 4 CV curves of the (A) Ni foam and (B) carbon cloth, and galvanostatic charge–discharge curves of the (C) Ni foam and (D) carbon cloth.

The normalized CV curves of the two kinds of Bi₂MoO₆ nanosheet array grown on different substrates at 20 mV s⁻¹ give a review of the capacitance properties (Fig. 5A). Obviously, the Ni foam supported Bi₂MoO₆ exhibits a larger enclosed area than the other one, suggesting a larger areal capacitance. Furthermore, the capacitance values of the Ni foam based Bi₂MoO₆ are 38, 37.3, 34.7 and 33 F g⁻¹ at current densities of 1, 2, 3 and 5 A g⁻¹, respectively. However, the carbon cloth supported Bi₂MoO₆ possesses capacitance values of 27.7, 25.1, 23.4 and 21.7 F g⁻¹ at current densities of 1, 2, 3 and 5 A g⁻¹, respectively.

Fig. 5 (A) Normalized CV curves of the Bi₂MoO₆ nanosheet arrays at 20 mV s⁻¹, (B) capacitance of the samples versus current increase, (C) Nyquist plots of the Bi₂MoO₆ nanosheet arrays, and (D) cycling performance of the Bi₂MoO₆ nanosheet arrays at 2 A g⁻¹.

In order to reveal the origin of the electrochemical behaviors of the electrodes, the electrochemical impedance spectra (EIS) were characterized and the Nyquist plots are presented in Fig. 5C. As can be seen, the curve includes two parts: the curve in the low frequency area and the semicircle in the high frequency area. The more vertical curve and the smaller semicircle correspond to the lower Warburg impedance (W) and charge-transfer resistance between the electrode and electrolyte, respectively. W represents the diffusive resistance of the electrolyte into the interior of the electrode and ion diffusion into the electrode.^35–37 It is clear that the Ni foam supported Bi₂MoO₆ nanosheet array shows a better electron and ion transport performance than the carbon cloth supported Bi₂MoO₆ nanosheet array. Moreover, the cycling stability of the electrode is an important factor to investigate for the application of supercapacitors in energy storage. As shown in Fig. 5D, the Ni foam and carbon cloth supported Bi₂MoO₆ nanosheet arrays exhibit 89% and 87% retentions of the initial capacitance after 1000 cycles at 2 A g⁻¹, respectively, demonstrating the excellent cycling stability at high current densities. The different stabilities of these two electrodes may result from the different densities of the Bi₂MoO₆ nanosheets on the substrates. With a higher density of Bi₂MoO₆ nanosheets on the carbon cloth, the electrode is unfavorable for the penetration of electrolytes.³⁸ Above all, it is not hard to conclude that the Ni foam is more suitable as a substrate for the fabrication of the Bi₂MoO₆ nanosheet array.

4. Conclusion

In summary, Bi₂MoO₆ nanosheet arrays have been rationally designed on nickel foam and carbon cloth. The Ni foam supported electrode shows excellent electrochemical properties, which are superior to those of the carbon cloth based electrode. A high specific capacitance of 37.3 F g⁻¹ at 2 A g⁻¹ could be achieved with 11% capacitance loss after 1000 cycles. These enhanced electrochemical properties may result from the nanosheet array structure directly grown on the substrate, which provides a large surface area and excellent electric transport. The facile and rational strategy in this report affords an effective way to fabricate two-dimensional array nanostructures for the application of high performance supercapacitors.

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