Rationally designed nanosheet-based CoMoO₄–NiMoO₄ nanotubes for high-performance electrochemical electrodes

Abstract

Ultrathin nanosheet-based CoMoO₄–NiMoO₄ nanotubes were designed and synthesized by a hydrothermal treatment. The nanotubes were composed of highly ordered ultrathin nanosheets. When used as electrodes in electrochemical capacitors, the nanosheet-based CoMoO₄–NiMoO₄ nanotubes demonstrated a high specific capacitance of 751 F g⁻¹ at a current density of 1 A g⁻¹ and good cycling ability with 94% initial specific capacitance retention after 2000 cycles. The electrode showed a high energy density of 30.86 W h kg⁻¹ at a specific power of 0.27 kW kg⁻¹ and still maintained the energy density of 16 W h kg⁻¹ at the power density of 4.85 kW kg⁻¹. The prominent electrochemical performances are attributed to the rational combination of two electroactive materials and the unique nanostructure of the nanotubes constructed by nanosheets that increases the rate of electrolyte diffusion and electron transport. The nanosheet-based CoMoO₄–NiMoO₄ nanotubes with remarkable electrochemical properties could be considered a prospective electrode material for the application of electrochemical capacitors.

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1. Introduction

Concerns over the rapid depletion of fossil fuels and increasingly worsening environmental pollution have evoked unprecedented endeavors in developing clean, efficient and renewable energy sources to maintain a healthy, prompt and sustainable economic development. Indeed, the development has led to a rapid increase of green energy production such as solar and wind resources. However, the solar and wind resources are unstable and intermittent, indicating that energy storage systems to store the electrical energy generated from these sources^1–6 is becoming one of the great challenges in the twenty-first century to maintain a healthy economic development. Among various energy storage systems, the most important ones are batteries and electrochemical capacitors (ECs). Especially, ECs are widely recognized as a more efficient power device compared with batteries and fuel cells,^7–10 because they are superior in many areas such as maintenance-free, possessing a longer cycle life, requiring a very simple charging circuit, green environmental protection, and generally much safer.¹¹ According to different ways of charge storage, ECs can be divided into two types, electric double layer capacitors storing electrical energy via reversible ion absorption at electrode/electrolyte interface and pseudocapacitors (PCs) based on fast and reversible surface redox reactions.^12,13 PCs can provide higher energy densities than the electrical double layer capacitors with carbon-based active materials as electrodes. Therefore, most current researches have been focused on PCs. Early transition metal oxides such as MnO₂,¹⁴ Co₃O₄ ¹⁵ and NiO,¹⁶ are a group of very promising PCs electrode materials that have been widely explored and developed. However, ECs based on such transition metal oxides still deliver an unsatisfactory energy density (lower than that of batteries) for practical application,¹⁷ due to the low electrical conductivity and improper nanostructure. Therefore, it is still a great challenge to design and fabricate novel electrode materials with excellent electrochemical properties.

Recently, MMoO₄ (M = Co, Ni), as the potential electrode material, has attracted much research interest due to its natural abundance and excellent electrochemical performance resulting from good electrical conductivity and high electrochemical activity.¹⁸ For examples, Daoping Cai, et al.¹⁹ synthesized NiMoO₄ nanorod as electrode materials with capacitance of 974.4 F g⁻¹ at a current density of 1 A g⁻¹. Di Guo, et al.²⁰ synthesized NiMoO₄ nanowires supported on carbon cloth with ultrahigh capacitance of 1587 F g⁻¹ at a current density of 5 mA cm⁻². Zhuoxun Yin, et al.²¹ synthesized NiMoO₄ nanotubes that shows the specific capacitances of 864 F g⁻¹ at 1 A g⁻¹. However, the rate capability and stability of NiMoO₄ are still low, which impede its practical application in energy storage devices. On the other hand, CoMoO₄ has excellent rate capability due to its large cell parameters (a = 10.21 A, b = 9.268 A, c = 7.022 A),²² though its specific capacitance remains to be further increased.

To meet the requirement of high stability and high specific capacitance, one promising way is to develop novel hybrid pseudo-capacitive systems via combining the metal oxides with binary metal oxide/hydroxides.^23–26 Indeed, mixed metal molybdates provide an improvement in electrochemical performance as result of the synergistic effect of individual metal molybdates. For examples, the heterostructured MnMoO₄–CoMoO₄ nanowires have been reported to exhibit a specific capacitance of 181.7 F g⁻¹ and high cycling stability.²⁷ The NiMoO₄@CoMoO₄ hierarchical nanospheres on nickel foam have been reported to display a high specific capacitance of 1601.6 F g⁻¹ as well as better cycling stability and rate capability.²⁸ The improvement in electrochemical performance for CoMoO₄–NiMoO₄·xH₂O bundles have been demonstrated.²⁹ The CoMoO₄–NiMoO₄·xH₂O with grain-like nanocomposite has been investigated, which shows enhancement in specific capacitance.³⁰

In our study, we have designed and fabricated ultrathin nanosheet-based CoMoO₄–NiMoO₄ nanotubes (NTs) by a hydrothermal route. This CoMoO₄–NiMoO₄ NTs electrode has many apparent advantages: (1) the thin nanosheets give the rise to a high surface area, providing more active reaction sites and improving the utilization of active materials not only on the active material surface but also throughout the bulk. (2) The tubular structure and the large open space between the nanosheets can greatly enhance the kinetics of ion and electron transport inside the electrodes. (3) What's more, it is reasonably attributed to synergistic effects between CoMoO₄ and NiMoO₄. Benefiting from these advantages, the as-prepared nanosheet-based CoMoO₄–NiMoO₄ NTs applied in ECs reveals excellent electrochemical behavior such as the high specific capacitances and the good rate capability.

2. Experimental section

All of chemical reagents were analytically pure and used without further purification.

2.1. Synthesis of nanosheet-based CoMoO₄–NiMoO₄ NTs

Nanosheet-based CoMoO₄–NiMoO₄ NTs were synthesized by a two-step hydrothermal method coupled with a post annealing treatment. In brief, 1 g of ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) was dissolved in a HNO₃ solution (20 mL concentrated HNO₃ added in 100 mL H₂O). After fully dissolved, this reaction solution was transferred into a Teflon-lined stainless steel autoclave (50 mL capacity) and heated at 180 °C in an electric oven for 8 h.³¹ After cooling, the light gray product was harvested by centrifugation and washed thoroughly with ultrapure water before drying at 60 °C overnight. The MoO₃ nanowires were obtained. 0.0738 g of MoO₃ nanowires were dispersed in 40 mL of water–ethanol (volume ratio 1 : 1) solvent containing 0.43 g nickel acetate and 0.22 g cobalt acetate under stirring for 30 min. The above mixture was then transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 mL for hydrothermal treatment at 90 °C for 5 h. As the autoclave cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with distilled water and absolute ethanol, and dried in air. The products above were annealed in an air atmosphere at 450 °C for 4 h, and nanosheet-based CoMoO₄–NiMoO₄ NTs were then obtained.

2.2. Materials characterization

The general morphologies of the samples were characterized by scanning electron microscopy (SEM, Hitachi SU 70) and transmission electron microscope (TEM) equipped with by an energy dispersive X-ray (EDX) spectrometer. Their phase structures were determined by X-ray diffraction (XRD, Rigaku D/max-2600 PC) using the Cu Kα radiation (λ = 1.5406 Å). The specific surface area was measured with a Micromeritics ASAP 2010 instrument and analyzed by the Brunauer–Emmett–Teller (BET) method.

2.3. Electrochemical performance measurements

The electrochemical properties were measured on a Lab-logic electrochemical workstation (VMP3, France) using a three-electrode configuration in 3 M KOH. The working electrodes were prepared with the mixture containing 75 wt% of the active material, 15 wt% of acetylene black, and 10 wt% of polytetrafluoroethylene. The mixture was coated on the Ni foam and dried overnight at 60 °C. The loading weight of active CoMoO₄, NiMoO_4, and CoMoO₄–NiMoO₄ composite nanotubes are about 2 mg cm⁻². And a Pt foil was used as the counter electrode and a Hg/HgO electrode as the reference electrode in order to precisely control electrochemical potentials. Cycling voltammetry (CV), galvanostatic charge/discharge (GCD), electrochemical impedance spectroscopy (EIS), and galvanostatic cycling were performed.

The specific capacitance (C_s), the energy density (E), and the power density (P) was calculated using the following equation

where I is the discharge current, Δt is the discharge time, m is the loading mass of active material, and ΔV is the potential window during the discharge process.

3. Results and discussion

3.1. Characterization of CoMoO₄–NiMoO₄ NTs structures

Fig. 1 shows typical SEM images of the as-synthesized MoO₃ nanowires (NWs) and CoMoO₄–NiMoO₄ NTs. Fig. 1a demonstrates a typical low-magnification SEM image of the MoO₃ NWs. Fig. 1b is the enlarged SEM image of a local area in the Fig. 1a. It can be seen from the magnified SEM images, the NWs appear to have a relative uniform and cylindrical in shape, having an average diameter of ∼250 nm. Noteworthily, the surface of the NWs is very smooth. When such smooth MoO₃ NWs reacts with nickel acetate and cobalt acetate, CoMoO₄–NiMoO₄ NTs formed. The formation of CoMoO₄–NiMoO₄ NTs can also be explained by the mechanism of the Kirkendall effect.³² Fig. 1c demonstrates a representative low-magnification SEM image of the CoMoO₄–NiMoO₄ NTs with a diameter of ∼400 nm. Fig. 1d shows a magnified SEM image of the local area in the Fig. 1c. The nanotubes are combined with ultrathin nanosheets, which will provide more surface area for easy diffusion of the electrolyte into the inner interface of the electrode materials. Such a unique hierarchical nanostructure has interconnected nanosheets and hollow nanotubes, which could ensure high accessibility of electrolytes into the whole electrode and improve the utilization rate of electrode materials. Additional SEM images of CoMoO₄ NTs and NiMoO₄ NTs can be found in ESI (Fig. S1†).

Fig. 1 (a) Low and (b) high-magnification SEM images of MoO₃ nanowires. (c) Low and (d) high-magnification SEM images of CoMoO₄–NiMoO₄ NTs.

The XRD patterns of MoO₃ NWs and the CoMoO₄–NiMoO₄ NTs are shown in Fig. 2. The pattern of MoO₃ NWs is in good agreement with the standard patterns for MoO₃ (PDF, card no. 65-2421 in Fig. 2a). In the Fig. 2b, the diffraction peaks of the composites are well matched with the standard diffraction patterns of NiMoO₄ (PDF, card no.9-175) and CoMoO₄ (PDF, card no.21-868). There are no impurity peaks, such as those for NiO, Co₃O₄, or MoO₃. The pattern of CoMoO₄–NiMoO₄ NTs contains the diffraction peaks of both CoMoO₄ and NiMoO₄, indicating the formation of the CoMoO₄–NiMoO₄ composite.

Fig. 2 (a) XRD pattern of the MoO₃ NWs. (b) XRD pattern of the CoMoO₄–NiMoO₄ NTs.

Fig. 3a shows a TEM image of a single CoMoO₄–NiMoO₄ NT with a diameter of ∼400 nm and the hollow-tube of ∼150 nm, which is consistent with the SEM observations. The surface is combined with ultrathin nanosheets. The Fig. 3b is an EDX spectrum taken from this nanotube, which identifies the presence of Ni, Co, Mo and O (C and Cu peaks are from the sample grid), confirming that the nanosheet is mainly made of CoMoO₄ and NiMoO₄. This is consistent with the XRD. To clarify the composition distribution of the product and structure properties, the spatial distribution of the compositional elements within the structures is obtained by using HRTEM–EDX line scans along the nanotube's axial direction. The HRTEM–EDX line scans spectra of sample reveals that the Co, Ni, Mo and O distribute the whole region of the products, and their quantities are much higher in the external region than in the inner one, also demonstrating that the products possess the tubular character.

Fig. 3 (a) The TEM image of a single CoMoO₄–NiMoO₄ nanotube. (b) The EDS pattern of CoMoO₄–NiMoO₄ nanotube. (c) HRTEM-EDX line scans spectra of sample.

Fig. 4 shows nitrogen adsorption–desorption isotherms of CoMoO₄, NiMoO₄ and CoMoO₄–NiMoO₄ NTs and corresponding pore size distribution curves. The specific surface area of the samples are calculated by the multipoint Brunauer–Emmett–Teller (BET) method. 96.8 m² g⁻¹ for CoMoO₄–NiMoO₄ NTs is higher than that of NiMoO₄ NTs (74.37 m² g⁻¹) and CoMoO₄ NTs (71.45 m² g⁻¹). The corresponding pore size distribution curve is obtained by the BJH method, and indicated the size distribution of the relatively narrow pore centered at about 6–10 nm (Fig. 4b). The higher surface area of CoMoO₄–NiMoO₄ NTs allow not only more surface for electrolyte to permeate but also more active sites, which is favorable for improving the surface adsorption of high concentration alkali ions, decreasing the electrolyte starvation near the electrode surface, reducing the internal resistance of the electrode, and thus making increased utilization of active materials and pseudocapacitance.³³ The relation between specific surface area of NiMoO₄ NTs, CoMoO₄ NTs, CoMoO₄–NiMoO₄ NTs and the performance of supercapacitors was shown in Table 1.

Fig. 4 N₂ absorption–desorption isotherm (a) and pore-size distribution curves (b) of the sample CoMoO₄–NiMoO₄.

Table 1 The specific surface area, specific capacitance, rate capability and cycling performance of NiMoO₄ NTs, CoMoO₄ NTs, and CoMoO₄–NiMoO₄ NTs

	NiMoO4 NTs	CoMoO4 NTs	CoMoO4–NiMoO4 NTs
Specific surface area	74.37 m^2 g^−1	71.45 m^2 g^−1	96.8 m^2 g^−1
Specific capacitance at 1 A g^−1	815 F g^−1	353 F g^−1	751 F g^−1
Rate capability	29%	64%	66%
Cycling performance for 2000 cycles	46%	84%	94%

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3.2. Electrochemical characterization of CoMoO₄–NiMoO₄ NTs

In order to explore the electrochemical properties of CoMoO₄–NiMoO₄ NTs composite electrode, both CV and GCD measurements were carried out with 3 M KOH aqueous solution in a three-electrode system. Fig. 5a shows the CV curves of CoMoO₄–NiMoO₄ NTs composite electrode within the potential range of 0–0.8 V at various sweeping rates ranging from 5–100 mV s⁻¹. Obviously, a pair of well-defined redox peaks is visible in each CV curve, demonstrating that the capacitance characteristics are mainly ascribed to the faradic capacitive behavior. The anodic peak is related to the oxidation process, and the catholic peak is connected with the reduction process. The shape of the CV curves is not significantly influenced by the increase of the scan rates. This indicates the favorable electron and ionic conduction and the excellent high-rate performance of the nanostructures. To estimate the rate capability, charge–discharge measurements were performed at various current densities, as shown in Fig. 5b. All of the curves have the obvious voltage plateaus in the charge and discharge process. The potential plateaus observed in the discharge curves correspond to the reductive process and match well with the reduction peaks observed in the CV curves, indicating good pseudocapacitive behaviors.³⁴

Fig. 5 (a) CV curves of the CoMoO₄–NiMoO₄ NTs composite electrodes at different scan rates ranging from 5 to 100 mV s⁻¹ in 3 M KOH aqueous solution. (b) GCD curves of the CoMoO₄–NiMoO₄ NTs composite electrodes collected at different current densities from 1 A g⁻¹ to 20 A g⁻¹ (c) CV curves of the CoMoO₄, NiMoO₄ and CoMoO₄–NiMoO₄ NTs composite electrodes at a scan rate of 50 mV s⁻¹, respectively. (d) Special capacitances as a function of current density. (e) Cycle performance of the CoMoO₄, NiMoO₄ and CoMoO₄–NiMoO₄ NTs composite electrodes during 2000 cycles at a current density of 2 A g⁻¹. (f) Nyquist plots of the CoMoO₄, NiMoO₄ and the CoMoO₄–NiMoO₄ NTs composite electrodes, respectively.

In order to determine the contribution of CoMoO₄ and NiMoO₄ to the electrochemical properties of the CoMoO₄–NiMoO₄ NTs composite electrode, CV curves of CoMoO₄, NiMoO₄, and CoMoO₄–NiMoO₄ were performed at the scan rate of 50 mV s⁻¹, as shown in Fig. 5c. The CV curves of the three electrodes exhibit a pair of redox peaks, indicating that the capacitance characteristics are primarily governed by faradaic reactions. In addition, from the area integrated within the current–potential curves, it could be deduced that both the CoMoO₄ and NiMoO₄ electrodes contribute to the total capacitance of CoMoO₄–NiMoO₄ composite electrodes. More importantly, the enclosed area of the CoMoO₄–NiMoO₄ electrode is obviously larger than that of CoMoO₄ and NiMoO₄. We consider that this increased area is mainly contributed by enlarged surface area and synergistic effects between CoMoO₄ and NiMoO₄.

To understand the rate capability of CoMoO₄, NiMoO₄, and CoMoO₄–NiMoO₄ electrodes, charge–discharge measurements were performed at various current densities, as shown Fig. S2 and S3 (see in ESI†). The specific capacitances of CoMoO₄, NiMoO₄, and CoMoO₄–NiMoO₄ electrodes at controlled current densities are shown in Fig. 5d. The pure CoMoO₄ exhibits good rate capability (64% capacitance remained at a high current density of 20 A g⁻¹), but a low specific capacitance of 353 F g⁻¹ at a current density of 1 A g⁻¹. The NiMoO₄ shows a high specific capacitance of 815 F g⁻¹ at a current density of 1 A g⁻¹, but only 29% of this value remained at a high current density of 20 A g⁻¹. And the CoMoO₄–NiMoO₄ composite shows a high specific capacitance of 751 F g⁻¹ at a current density of 1 A g⁻¹, and 66% of this value remained at a high current density of 20 A g⁻¹, which combines the advantages of the good rate capability of CoMoO₄ and the high specific capacitance of NiMoO₄. Though the specific capacitance of the composite was slightly lower than that of NiMoO₄ at the current density of 1 A g⁻¹, but was higher than those of NiMoO₄ at higher current densities. The rate capability of composite electrodes (from 1 to 20 A g⁻¹) is better than many other pseudocapacitor electrodes such as α-NiMoO₄ (49% capacitance retention from 1 A g⁻¹ to 12 A g⁻¹)³⁰ and NiMoO₄ NTs electrode (70% capacitance retention from 1 A g⁻¹ to 5 A g⁻¹).³⁵ It is concluded that the high specific capacitances of the CoMoO₄–NiMoO₄ NTs composite electrode were mainly contributed by the NiMoO₄, while the good rate capability was contributed by the CoMoO₄ component. The combination of the high specific capacitance and good rate capability makes the composite a promising electrode material for ECs.

The durability of the electrode materials is a critical aspect for practical applications. The cycling performance was conducted in the range of 0–0.55 V in 3 M KOH aqueous solution. Fig. 5e characterizes cycling performance of 2000 cycles. CoMoO₄–NiMoO₄ NTs electrode reveals a good cycling ability with 94% of the initial specific capacitance remained after 2000 cycles. In contrast, the capacitance of CoMoO₄ and NiMoO₄ electrodes only retained 84% and 46% of their respective initial values. The cycling stability of CoMoO₄–NiMoO₄ NTs electrode is higher than some electrode material such as Co₃O₄@NiMoO₄ (after 3000 cycles, the capacitance retentions is about 77%)¹⁹ and NiMoO₄ NTs electrode (after 1000 cycles, the capacitance retentions is about 71%).²¹ Such good rate capability of the CoMoO₄–NiMoO₄ NTs composite nanostructures can be mainly ascribed to enhanced electrical conductivity and synergistic effect between CoMoO₄ and NiMoO_4.

Nyquist plots were shown in Fig. 5f. All the impedance curves include an arc in the middle and high frequency region and a sloped line in the low-frequency region, but shows differences in details. Firstly, the internal resistance (R_b) can be obtained from the high frequency intercept on the real axis. Obviously, R_b of CoMoO₄–NiMoO₄ electrode 0.7 Ω is smaller than those of CoMoO₄ electrode 0.8 Ω and NiMoO₄ electrode 1.1 Ω. Secondly, the CoMoO₄–NiMoO₄ electrode also shows a small pseudo charge transfer resistance (R_ct), which corresponds to the small semicircle in the impedance plots. Both lower R_b and R_ct indicate the improved electron conductivity, which can be explained by the impurity bands, based on the band theory, introduced after mixed semiconductive oxides together, thus improving electron conductivity of the composite. What's more, the slope of the straight line in the low frequency region of the CoMoO₄–NiMoO₄ electrode is the highest compared with that of NiMoO₄ and CoMoO₄ electrodes, suggesting a more superior electrochemical capacitive behavior in the CoMoO₄–NiMoO₄ electrodes. The above electrochemical analysis clearly demonstrates the remarkable supercapacitive performance of CoMoO₄–NiMoO₄ NTs composite electrode.

Energy density and power density are two key factors to evaluate the power applications of electrochemical capacitors. For further evaluation of the properties of the CoMoO₄–NiMoO₄ electrode, the Ragone plots have been tested as shown in Fig. 6. A good ECs is expected to provide high energy density or high specific capacitance at high current densities. The CoMoO₄–NiMoO₄ electrode delivers an energy density of 30.86 W h kg⁻¹ at a power density of 0.27 kW kg⁻¹, and energy density of 16 W h kg⁻¹ at a power density of 4.85 kW kg⁻¹. These energy density and power density values are significantly greater than CoMoO₄ electrode (energy density is 8.17 W h kg⁻¹, when power density is 5.15 kW kg⁻¹) and NiMoO₄ electrode (energy density is 5.12 W h kg⁻¹, when power density is 3.85 kW kg⁻¹). It reveals that the application of CoMoO₄–NiMoO₄ as the electrode can improve the specific capacitances of the ECs.

Fig. 6 The Ragone plots for the CoMoO₄, NiMoO₄ and CoMoO₄–NiMoO₄ NTs composite electrodes.

We consider that there are three contributions to the excellent supercapacitive performance of the CoMoO₄–NiMoO₄ composed electrode material. First and foremost, the thin nanosheets give rise to a high surface area as high as 96 m² g⁻¹, which provide more active reaction sites and improve the utilization of active materials not only on the active material surface but also throughout the bulk. The tubular structure and the proper open space between the nanosheets can greatly enhance the kinetics of ion and electron transport inside the electrodes. Secondly, the CoMoO₄–NiMoO₄ NTs composite nanostructures have the preferable electrical conductivity. Moreover, it is reasonably attributed to synergistic effects between CoMoO₄ and NiMoO₄.

4. Conclusions

In summary, we adopted a facile hydrothermal method to successfully synthesize CoMoO₄–NiMoO₄ NTs for PCs. The CoMoO₄–NiMoO₄ NTs composite electrode exhibits excellent electrochemical performance with a high capacitance of 751 F g⁻¹ at 1 A g⁻¹ and desirable rate performance, as well as outstanding cycling life (remained 94% of its initial specific capacitance after 2000 cycles). Meanwhile, the electrode delivers a high energy density of 16 W h kg⁻¹ at a power density of 4.85 kW kg⁻¹. The electrochemical performances are significantly superior to those of pristine CoMoO₄ and NiMoO₄, due to the enlarged specific surface area, better electronic conductivity and synergistic effects. It reveals that the CoMoO₄–NiMoO₄ NTs composite electrode is promising pseudocapacitive material for high performance ECs. We also confirms the feasibility of the rational design of advanced integrated electrode materials for high-performance ECs.

Acknowledgements

This work was partially supported by the Graduate Students' Research Innovation Project of Harbin Normal University (HSDSSCX2014-05).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27401j

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