Supercapacitive carbon nanotube-cobalt molybdate nanocomposites prepared via solvent-free microwave synthesis

Abstract

Cobalt molybdate (CoMoO₄) nanoplatelets with a crystalline-amorphous core-shell structure anchored via multi-walled carbon nanotubes were prepared by a solvent-free microwave synthesis method. The entire procedure took only 15 min. The nanocomposite shows a promising capacitance of 170 F g⁻¹ with a potential window of 0.8 V, degrading by only 6.8% after 1000 cycles.

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Transition metal molybdates play an important role in the fields of catalysis, including desulphurization,^1–3 CO₂ hydrogenation,⁴ selective oxidation,^5,6 and selective dehydrogenation.⁷ Recently researchers have shown that they can also be used as electrode active materials for high performance Li-ion batteries and supercapacitors.^8–12 Electrochemical supercapacitors have many applications, such as power back-up, electric vehicles, pacemakers, load cranes and forklifts. A variety of nanostructured systems have been recently explored.^13–20 Many studies concerning the utilization of metal oxides and nitrides, including RuO₂,^13,14 MnO₂,¹⁵ Co₃O₄,^16,17 NiO,¹⁸ MoO₃¹⁹ MoO₂,²⁰ and VN,²¹as the electrochemical active materials have been reported. However, the high cost of RuO₂, the cycling instability of MnO₂, MoO₃ and MoO₂, and the narrow potential windows of Co₃O₄ and NiO prohibit their use in many applications. A recent exciting study has demonstrated that hierarchical MnMoO₄/CoMoO₄ heterostructured nanowires possess promising specific capacitance of about 200 F g⁻¹ at a current density of 0.5 A g⁻¹.⁸ However, the size of individually synthesized MnMoO₄ or CoMoO₄ is larger. In particular, the CoMoO₄ delivered only a modest capacitance of 85 F g⁻¹ at the same current density.

Finer structures with higher surface to volume ratios are desirable for improving either the catalytic or electrochemical activities. Several methods for the preparation of nanoscale transition metal molybdates with different shapes have been employed. These include solid-state reactions in a disperse medium,³ liquid phase synthesis,²² hydrothermal,²³ and sol–gel methods.²⁴ These methods are carried out in solution using either organic materials or water, therefore both are somewhat time consuming and possess a negative environmental impact if scaled up to commercial quantities.

Microwave synthesis has emerged as an important technique for preparing materials, offering higher reaction rates, better selectivities and shorter reaction time compared to conventional thermal methods.^25–29 Though several kinds of nanoscale molybdates have been successfully synthesized using this technique,^30,31 the reported methods involve utilizing a solvent.

In this study, the synthesis of cobalt molybdate was achieved via ambient microwave treatment without the use of any solvent. The overall synthesis process can be completed within 15 min (see ESI†). The acid functionalized-multiwalled carbon nanotubes (AF-MWCNTs) were utilized as both microwave susceptors and heterogeneous nucleation sites for CoMoO₄ nanocrystal formation. They also act as ancillary electrical conductive materials to lower the ohmic resistance of the electrodes.

Raman spectroscopy was used to investigate the obtained products. The bands around 331, 353, 806, 869 and 926 cm⁻¹ are assigned to CoMoO₄.^8,32 Among them, the bands around 353, 806, 869, and 926 cm⁻¹ can be assigned to Co–Mo–O stretching vibrations in CoMoO₄ species.³² (Fig. 1 A, red line). The bands in the positions of around 1345, 1576 and 2662 cm⁻¹ are the D band, G band and 2D band of AF-MWCNTs, respectively.^29,33 The ratio of D and G band intensity can be used to estimate the degree of disorder of the graphite materials. The I_D/I_G ratio of AF-MWCNTs is ∼0.11 (Fig. 1 A, black line), while the I_D/I_G ratio of the AF-MWCNTs in the CoMoO₄/MWCNTs is ∼0.19, indicating that some additional disorder was introduced by synthesis procedure. The crystal structure of the product was further investigated via XRD analysis. The relatively broad peak at 2θ = 26.50° is the combination of CoMoO₄ (00-021-0868, C2/m (12), a = 10.21, b = 9.27, c = 7.022) and AF-MWCNT peaks, corresponding to the (002) reflection of α-CoMoO₄ and the (001) reflection of AF-MWCNTs.^34,35 The peak around 2θ = 42.60° is assigned to be a combination of the (002) plane of AF-MWCNTs,^34,35 and the (132) of α-CoMoO₄. The other peaks can be ascribed to α-CoMoO₄ (Fig. 1 B).

Fig. 1 (A) Raman spectra and (B) XRD pattern of CoMoO₄/MWCNTs (CoMoO₄, black colored numbers; AF-MWCNTs, red colored numbers).

The chemical environment and composition of the products were determined by XPS analysis and TG-DSC. XPS showed the composite is composed of 75.2% CoMoO₄ and 24.8% AF-MWCNTs by weight†.

Scanning electron microscope (SEM) images highlight the obtained product. The CoMoO₄ phase is in the shape of pseudo hexagonal nanoplatelets with that are approximately 40 nm in thickness (Fig. 2A and 2B). Theses nanoplatelets appear to be oriented relatively randomly. The low magnification bright field (BF) TEM image shows the CoMoO₄ assembly was damaged and CNTs were exposed after a strong ultrasonic vibration in ethanol (Fig. 2C), confirming the nanocomposite structure of CoMoO₄/MWCNTs. Fig. 2D shows a TEM micrograph of a typical CoMoO₄ crystallite. It can be observed that the CoMoO₄ assembly has a “core-shell” structure. The bulk is well-crystallized with minimal defects. Continuous lattice fringes are clearly observed in a high-resolution micrograph (Fig. 2E). The interplanar distance of 0.44 nm (obtained from FFT) agrees well with the d-spacing between the {111} planes of α-CoMoO₄. The shell of the crystallite is amorphous, being roughly 12 nm in thickness (Fig. 2F).

Fig. 2 (A, B) SEM and (C) BF TEM micrograph of CoMoO₄/MWCNTs. (D) TEM micrograph of an individual CoMoO₄ crystallite. HRTEM images of (E) the bulk and (F) the edge of CoMoO₄ crystallite.

The precursors utilized for growing CoMoO₄/MWCNTs, (NH₄)₆Mo₇O₂₄·4H₂O and CoCl₂·6H₂O, are polar molecules that absorb microwave energy. In addition, AF-MWCNTs are well known to possess excellent microwave absorbing ability. At the moderate temperature achieved during microwave synthesis (measured to be 260 °C), (NH₄)₆Mo₇O₂₄·4H₂O and CoCl₂·6H₂O will react to form CoMoO₄, NH₃, HCl and H₂O. The reaction sequence can be described as the following:

(NH₄)₆Mo₇O₂₄·4H₂O(s) + 7CoCl₂·6H₂O(s) → 7CoMoO₄(s) + 6NH₃(g) + 14HCl(g) + 42H₂O(g) (1)

At 260 °C the reactants are in a molten state. Therefore their concentration is much higher than that achievable via conventional solvent-based synthesis. Conversion during microwave synthesis, which happens quite rapidly, results in a high degree of supersaturation. There can be hydrogen bonding between the O atoms of CoMoO₄ and the H atoms of AF-MWCNTs. In addition, possible coordination bonding is between the Co atoms of CoMoO₄ and the O atoms of AF-MWCNTs. These allow the CoMoO₄ molecules to easily bind to the acidified surface of the AF-MWCNTs, facilitating rapid nucleation. The high nucleation density allows for the CoMoO₄ assemblies to form nanoscale structures. The reference CoMoO₄ synthesized under identical conditions but without any carbon additive displays an irregular but very low surface area morphology (Fig. S4†). The CoMoO₄ synthesized using conventional MWCNTs is not nanoscale either (Fig. S5†), providing further evidence for the strong influence of the AF-MWCNT surface functionality on nucleation density.

The BET specific surface area of CoMoO₄ assemblies is 41.2 m² g⁻¹ (inferred from the BET surface area of CoMoO₄/MWCNTs, as shown in ESI†), which is much larger than that of CoMoO₄ nanorods synthesized via a hydrothermal approach.²³ The CoMoO₄ assemblies have pseudo hexagonal nanoplatelets that are arranged randomly. The entire assembly possesses a “honeycomb-like” structure that includes significant nanoscale porosity between the contacting platelets. Such a structure provides adequate contact area of the active material with an electrolyte. Such an assembly of the electrochemically active phase, combined with a carbon nanotube “skeleton” for an enhanced electrical conductivity through the bulk of the electrode, should make this system favorable for faradaic pseudocapacitive applications.

The electrochemical supercapacitive performance of the CoMoO₄/MWCNTs was evaluated in a three-electrode cell using 1M KOH as the electrolyte. The cyclic voltammogram (CV) recorded at 2 mV s⁻¹ shows an anodic peak at around −0.05 V (vs. Hg/HgO) and a cathodic peak at about −0.1 V (Fig. 3A). The specific current of the redox peaks increases with the sweep rates. The specific current is an order of magnitude higher than that achieved with the same mass loading of bulk CoMoO₄ (Fig. S6†). At higher sweep rates, the anodic and the cathodic peaks shift to higher and lower potentials, respectively. CoMoO₄/MWCNTs exhibit a high capacitive current and a rectangular “box-like” shape throughout the potential window of 0.8 V. It is well-known that in systems like RuO₂, the “box-like” shape of a CV curve, which resembles a CV obtained for a carbon-based electrical double layer (EDL) system, is attributed to multiple redox peaks. Surface areas in excess of 1000 m² g⁻¹ are required to achieve specific capacitances of 100 F g⁻¹ based on EDL. Therefore it is logical to attribute the favorable symmetric shape of the CV curve of CoMoO₄/MWCNTs to multiple Co and Mo redox couples. This is a significant advantage of using CoMoO₄/MWCNTs for high-energy density supercapacitors: All the CVs are quite symmetric, indicating that the redox reactions are highly reversible. The charging-discharging curves of CoMoO₄/MWCNTs at different currents are shown in Fig. 3B. The results yield a capacitance of 170 F g⁻¹ at 0.1 A g⁻¹, 115 F g⁻¹ at 0.5 A g⁻¹, and 96 F g⁻¹ at 1 A g⁻¹ (Fig. 3C). To our knowledge, 170 F g⁻¹ is the highest capacitance ever reported for CoMoO₄. After 1000 cycles at a current density of 2 A g⁻¹, the specific capacitance shows only a 6.8% fade (Fig. 3D). This decay is much less compared with that reported for typical metal oxides.^8,20

Fig. 3 (A) CV curves of CoMoO₄/MWCNTs at various scan rates. (B) Galvanostatic charge/discharge curves; (C) Specific capacitance at the different charge/discharge specific currents. (D) Capacitance retention versus cycle number.

In summary, we demonstrate an efficient and environmental-friendly microwave solid synthesis method to prepare CoMoO₄/MWCNTs in as fast as 15 min. The obtained pseudo hexagonal CoMoO₄ nanoplateles have crystallized cores and amorphous shells. A specific capacitance of 170 F g⁻¹ in a wide potential window of 0.8 V is achieved. Considering the high density of CoMoO₄ (5.9 g cm⁻³), its volume capacitance is much higher than the commercial carbon-based supercapacitors. The existence of MWCNTs further enhances the conductivity of the composite and therefore improves its performance at high current loading. Furthermore, only 6.8% capacitance fading is observed after 1000 cycles at 2 A g⁻¹, which makes it a promising candidate for next generation supercapacitors.

Acknowledgements

This work was funded by NSERC Discovery and NINT NRC.

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Footnote

† Electronic Supplementary Information (ESI) available: TEM image of AF-MWCNTs, synthesis of CoMoO₄/MWCNTs and reference CoMoO₄ samples. Characterization, electrochemistry, XPS analysis, thermal analysis, BET surface area, Raman shifts and SEM images of the reference CoMoO₄ samples. CV curves of the reference CoMoO₄ and CoMoO₄/MWCNTs. See DOI: 10.1039/c2ra01300b/

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