Role of polymeric surfactant in the synthesis of cobalt molybdate nanospheres for hybrid capacitor applications

Abstract

The role of Pluronic F127, a triblock copolymer, i.e. poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) PEO–PPO–PEO, adsorbed on cobalt molybdate (CoMoO₄), and its influence on the physico-chemical properties have been investigated. As a surfactant, Pluronic F127 is able to alter the surface properties of CoMoO₄ during synthesis. Through a facile synthesis at 300 °C, F127 adsorbs at the interface and self-assembles into a micellar aggregate, resulting in the formation of CoMoO₄ nanospheres. A cluster of nano-particles with an average size of 250 nm is obtained when F127 is added to CoMoO₄, while rod-shaped particles (1 μm) are obtained in the absence of F127. The surfactant-assisted CoMoO₄ is associated with enhanced pore accessibility and electronic conductivity, having a dual role in potential applications. The objective of this study is to test the as-synthesized CoMoO₄ with regard to application in energy storage by tuning the surface properties. The hybrid capacitor (F127 added to CoMoO₄ vs. activated carbon) showed an excellent electrochemical performance with a specific capacitance of 79 F g⁻¹ and an energy density of 38 W h kg⁻¹ in 2 M NaOH electrolyte, which was much higher than that for pure CoMoO₄ (23 F g⁻¹). The long-term cycling stability of the modified CoMoO₄ was tested and it was found to retain almost 80% of its initial capacity after 2000 cycles. The results obtained suggest that F127-modified CoMoO₄ would be a suitable candidate for fabricating a cost-effective energy storage device.

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1. Introduction

Water-soluble, Pluronic (F127), poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), PEO–PPO–PEO, tri-block copolymers are known to adsorb onto molybdenum species, such as cobalt molybdate (CoMoO₄). Above the critical micellization concentration (CMC), the added F127 tri-block copolymer forms micelles and self-assembles into a variety of aggregates. The role of the polymeric surfactant on CoMoO₄ during the synthesis and its effect on the physico-chemical and electrochemical properties have been studied for the first time. Such studies on the interactions between the polymer and the CoMoO₄ morphology provide fundamental insights into the surface properties and its use in energy storage applications.

Electrochemical supercapacitors exhibiting rapid charging, excellent cycling stability, and a miniature size are attractive candidates for next-generation power devices.¹ However, electrochemical capacitors possess low energy densities compared to rechargeable battery systems.² Up to now, much work has been devoted to improving the energy density of capacitors to meet the demand for next-generation supercapacitor applications.^3,4 Commercial supercapacitors are comprised of two identical activated carbon (AC) electrodes. These supercapacitors are of the electric double-layer capacitor (EDLC) type. Being fully dependent on the double layer, these supercapacitors suffer from poor energy density (5–6 W h kg⁻¹).⁵ To overcome this, efforts have been made to investigate and study the electrochemical properties of supercapacitors based on dissimilar electrodes, such as an activated carbon (AC) negative electrode and a metal oxide positive electrode.^6–9

There are a range of candidates that can be employed as electrodes for batteries and supercapacitors. Transition metal oxides (RuO₂, MnO₂)^10–12 and phosphate materials (LiNiPO₄,¹³ LiCoPO₄, and¹⁴ LiMnPO₄ (ref. 15)) were widely studied and, following this, there has recently been a renewed interest in molybdates. Transition metal oxides and phosphates played an important role in tuning the redox potential, which resulted in improved performance characteristics of energy storage devices.^10–15 Various transition metal oxides have been proposed for application in supercapacitors. Among them, ruthenium oxide exhibits high capacitance, good electrochemical properties and excellent reversibility. However, its application is limited due to high cost and low porosity.¹⁰ Subsequently, manganese dioxide (MnO₂) was considered to be another promising candidate for supercapacitors due to its high energy density and low cost. But, MnO₂ possesses a poor cycling stability and poor electrical conductivity.¹² Thereon, research has been focused on overcoming the disadvantages of MnO₂ by introducing different additives to the active MnO₂ material.¹⁵ On the other hand, phosphate materials are quite poor in terms of conductivity.

Alternatively, it is well known that molybdate materials have excellent catalytic activity and good electrochemical properties, and they are environmentally friendly.² Due to these specifications, much effort has been devoted to investigating the possibility of using metal molybdates in energy storage devices.^{6–9,16–20} Among various molybdate materials, MnMoO₄/CoMoO₄ heterostructured nanowires showed a specific capacitance of 187 F g⁻¹ at a current density of 1 A g⁻¹, with excellent reversibility after 1000 cycles. These nanowires were reported to have a larger surface area and defects that promoted good electron transport and efficient electrochemical reactions.¹ A CoMoO₄ nanoplate exhibiting a capacitance of 170 F g⁻¹ over 1000 cycles has also been reported.¹⁶ In another study, CoMoO₄ nanoplates synthesised via a hydrothermal route showed a specific capacitance of 1.26 F cm⁻². The reported performance was correlated to the open network structure of CoMoO₄ coated on Ni foam.²¹ Following this, Xu et al.¹⁷ reported an increase in the electrical conductivity of a CoMoO₄ composite material through in situ addition of graphene, which resulted in a specific capacitance of 322 F g⁻¹. Apart from the single-cell characteristics, an asymmetric hybrid device constructed from AC and CoMoO₄ in 2 M LiOH has also been reported with a specific capacitance of 105 F g⁻¹.¹⁸ However, the fabricated hybrid device exhibited a meagre energy density of 14 W h kg⁻¹. More recently, NiMoO₄@CoMoO₄ nanospheres were synthesised through a hydrothermal technique.²² Zhang's group showed that the nanospheres exhibited a specific capacitance of 77 F g⁻¹ at a current density of 14 A g⁻¹ when tested for a symmetric capacitor device.²² The enhanced electrochemical properties of the material, compared to pure CoMoO₄, were attributed to the fast diffusion of ions resulting from the effect of Ni and Co molybdates deposited onto a conductive substrate.²² Overall, almost all the reported work on CoMoO₄ material has been focused on conventional synthetic methods, such as microwave, combustion, hydrothermal, and co-precipitation, and has tested for single electrode characteristics employing cyclic voltammetric techniques. In addition, the working potential window of most of the reported materials is limited, which affects their application in energy storage. No work has been done so far on the influence of polymeric surfactants and their micellar formation, resulting in nanocomposites, which are a prerequisite for capacitor applications. The one-step facile approach employed in our work is cost-effective and scalable for commercialization.

Herein, we successfully prepared an asymmetric hybrid device comprising AC∥α-CoMoO₄. To the best of our knowledge, this is the first time an asymmetric AC∥α-CoMoO₄ full cell has been built in an aqueous system and tested under galvanostatic conditions, in which a unique polymeric surfactant (Pluronic F127) is employed. The tri-block copolymer surfactant, F127, belongs to a group of commonly used water-soluble surface-active compounds. The objective of having surfactant present during the chemical synthesis is to (a) achieve a nanostructured morphology through the aid of a structure-directing agent, such as F127, (b) enhance the adsorption properties of F127 on CoMoO₄, which controls the shape of the micelles and their interaction with the molybdate species and (c) have a uniform distribution of conductive carbon in the CoMoO₄ matrix to achieve a better conductivity. This enhances the effective electrolyte diffusion and charge transport, resulting in improved specific capacitance and rate capability. Aqueous systems are a preferable electrolyte choice for a number of reasons. They are far less expensive than organic solvents and have fewer disposal and safety issues. The ionic conductivity of NaOH (aq.) is two orders of magnitude greater than that of organic electrolytes, allowing higher discharge rates and lower voltage drops due to electrolyte impedance. Hence, a solution of 2 M NaOH (aq.) was used as an electrolyte in the current asymmetric capacitor study.

2. Experimental

2.1 Materials

Cobalt molybdate (α-CoMoO₄) was synthesised using analytically pure Co(NO₃)₂·6H₂O (6.648 g), (NH₄)₆Mo₇O₂₄·4H₂O (4.033 g) and 1.7 g Pluronic F127, known as F127, supplied by Sigma Aldrich. Pluronic F127 poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer has M_W = 12 600 with 70% PEO content. The formula for this copolymer is EO₁₀₆PO₇₀EO₁₀₆. For comparison purposes, CoMoO₄ was also synthesised in the absence of surfactant. Details of the synthesis procedure are given in our previous work.¹⁸ Fig. 1 shows a schematic diagram of the synthesis of CoMoO₄ in the presence of F127 surfactant. Activated carbon (AC) was commercially bought from Calgon Carbon.

Fig. 1 Schematic of the synthesis of a CoMoO₄ sample.

2.2 Characterization

Modified CoMoO₄ was characterised by physical characterisation methods and compared to the pure sample. Details of the physical and electrochemical characterisation of the activated carbon are given in Fig. S1–S3 (ESI†). XRD was used to identify the crystal structure of the prepared materials using a Siemens D 500 X-ray diffractometer 5635 with a Cu-Kα source at a scan speed of 1° min⁻¹. The voltage and current were 35 kV and 28 mA, respectively. In addition, attenuated total reflectance Fourier transform infrared (ATR-FTIR) studies were performed using a Bruker IFS 125/HR spectrometer at Australian Synchrotron to study the nature of bonding and chemical structure of the modified CoMoO₄. Far IR and mid IR pellets were prepared by mixing samples with paraffin and KBr, respectively. A high magnification Zeiss Neon 40ESB Field Emission Scanning Electron Microscope (FE-SEM) was also used to acquire topographical and elemental information on the samples prepared. The morphology and lattice imaging of the CoMoO₄ powder were obtained by transmission electron microscopy (TEM), using a JEOL 200F TEM operated at 200 kV. TEM specimens were prepared by grinding a small amount of powder in methanol and dispersing on a holey carbon film. Brunauer–Emmett–Teller (BET) surface area measurements and porosity analysis were also carried out using a Micromeritics Tristar II surface area and porosity analyser. For porosity measurements, all samples were degassed at 100 °C overnight before analysis. For the three-electrode tests, a platinum wire of 10 cm length and 1 mm diameter and mercury–mercuric oxide (Hg/HgO) served as the counter and reference electrodes, respectively. The capacitor device was constructed with cobalt molybdate as the positive electrode and activated carbon as the negative electrode.

2.3 Device fabrication and measurements

For electrochemical measurement, the positive and negative electrodes were prepared by mixing either CoMoO₄ or AC (75 wt%), carbon black (15 wt%) and PVDF (10 wt%) with 0.4 mL of NMP to make a slurry. The slurry was coated on a small graphite sheet (area of coating, 1 cm²). The remainder of the graphite strip was thoroughly masked using an insulation film to obtain a coated surface area of 1 cm² exposed to the 2 M NaOH electrolyte. Cyclic voltammetry of the samples was carried out using an EG&G Princeton Applied Research Versa Stat III model. With respect to single electrode characteristics, for cyclic voltammetry (CV) tests, the working electrode was cycled between 0 and 0.65 V at scan rates of 1, 2, 5 and 10 mV s⁻¹. Galvanostatic charge–discharge cycles were performed over a potential range identical to that of the CV test but using an 8-channel BioLogic VSP-300 battery analyser from MTI Corp., USA at a current density of 0.1 A g⁻¹. For AC, the working electrode was cycled between 0 and −1.0 V at a scan rate of 2 mV s⁻¹. The mass balance was calculated using eqn (1)

m₊/m₋ = (C₋ × ΔE₋)/(C₊ × ΔE₊) (1)

where C₋ and C₊ are the specific capacitances for the AC and CoMoO₄, respectively; ΔE₋ and ΔE₊ are the potential differences from the open circuit voltage (OCV) to the AC and CoMoO₄ charge–discharge potential stability limits, respectively.

The specific capacitances of the AC and CoMoO₄ after subtracting the contributions of carbon black (acetylene black) and the binder were calculated to be 135 F g⁻¹ and 170 F g⁻¹, respectively. Based on the single-electrode characteristics, from eqn (1), the optimal mass ratio between AC and CoMoO₄ was determined to be 1.6 for the fabricated hybrid capacitor. Therefore, the masses of the AC and CoMoO₄ material were 16.0 and 10.0 mg, respectively. An aqueous solution of 2 M NaOH was employed as an electrolyte for all electrochemical measurements. A hybrid capacitor was fabricated and the charge–discharge studies were carried out at various current densities ranging from 0.1 to 1.0 A g⁻¹. The cut-off charge and discharge voltages were 1.6 and 0.2 V, respectively. The specific capacitance and energy density of the device were calculated at the end of each charge–discharge test. Electrochemical impedance spectroscopy (EIS) was carried out with an amplitude of 5 mV over a frequency range from 10 mHz to 700 kHz at open-circuit potential.

3. Results and discussion

3.1 Physico-chemical characterization of CoMoO₄

The synthesis of cobalt molybdate (CoMoO₄) was performed using a cost-effective one-step, polymeric surfactant-assisted combustion method. The synthesized CoMoO₄ was analysed for crystal structure and phase determination using X-ray diffraction (XRD). The X-ray diffraction (XRD) patterns of both the pure and modified (F127 added) CoMoO₄ samples are in good agreement with the reported values¹⁸ and Powder Diffraction File (PDF) card number 21-0868, indicating the formation of a single-phase crystalline structured CoMoO₄. A typical diffraction pattern for modified CoMoO₄ is shown in Fig. 2. The pattern shows one major peak at 26.5° (220) along with several minor peaks labelled in the figure. The absence of any secondary phases confirms that the modified CoMoO₄ is of high purity. A crystallite size of 9.5 nm was calculated using the Debye–Scherrer formula, using the major diffraction peak (220), which had a full-width at half-maximum (FWHM) of 0.9°.

Fig. 2 X-ray diffraction (XRD) pattern of the modified CoMoO₄ sample.

Fig. 3 shows the far and mid IR spectra of the modified CoMoO₄. To the best of our knowledge, the far IR region has not been reported earlier for CoMoO₄ material in the literature. The absorption band observed (Fig. 3A) at 433 cm⁻¹ and several other shoulder-like regions correspond to the Co–Mo–O bands. However, in this study, no attempts are made to characterise the bands observed in Fig. 3A by assigning frequencies of M–O, M–O–M, or M–M (where M indicates Co and Mo and O indicate oxygen) as no database is available. In the mid IR region (Fig. 3B), peaks observed at 710, 760, 855, 950, 1400, 1600, 2220, 2354 and 3350 cm⁻¹ are in good agreement with the reported values for CoMoO₄.^5,6,19,20 The peaks in the region 700–950 cm⁻¹ correspond to Mo–O bonds, while the peak at 1400 cm⁻¹ is related to MoO₄. To further explore the adsorption characteristics of the modified CoMoO₄, the spectra obtained for the mid infra-red region were compared with that of pure CoMoO₄. The differences in the mid-infra-red region are shown in Fig. S4 (ESI†). The IR spectrum of pure CoMoO₄ showed strong peaks at 1300 and 1500 cm⁻¹ and a band at 3400 cm⁻¹, corresponding to the characteristic absorption peaks for CoMoO₄. The band at 3400 cm⁻¹ could be assigned to O–H bonds. For the modified sample, an intense peak at 1620 cm⁻¹ and a high-intensity band at 3250 cm⁻¹ have been observed. The observed peak shifts in the two spectra could be attributed to the adsorption of non-ionic surfactant from the precursor solution onto the molybdate species, involving hydrogen bonding.²³ The peak at 1620 cm⁻¹ can be assigned to Mo⁶⁺–OH bending vibrations. A band at 3250 cm⁻¹ can be attributed to hydrogen-bonded water molecules adsorbed on the surface of CoMoO₄.²⁴ Overall, the IR results suggest that the adsorption occurred mainly via hydrogen bonding between the oxygen atoms of the hydrophilic chain in F127. The XRD pattern and IR results suggest that the obtained product is CoMoO₄ and bonding variations are confirmed for the surfactant-modified sample. However, information about the formation of nanostructures, their chemical composition, and adsorption properties cannot be gained.

Fig. 3 Far (A) and mid (B) infrared transmittance spectra (IR) of the modified CoMoO₄ material using a synchrotron source.

3.2 Morphological studies of CoMoO₄

(a) FESEM studies. Microscopic analyses provide vital information on the size, homogeneity and intermolecular aggregation of modified CoMoO₄. Field Emission SEM (FE-SEM) images of the pure and modified samples are compared in Fig. 4. Elemental analysis corresponding to CoMoO₄ samples, depicted in Fig. S5 (ESI†), confirms the presence of mainly Co and Mo. As can be seen from the FE-SEM images, there was a significant change in morphology when polymeric surfactant was added to CoMoO₄ during the synthesis. The pure CoMoO₄ shows a fused rod-like morphology (Fig. 4A and B), while the modified CoMoO₄ (Fig. 4C and D) shows clusters of nanospheres with an average size of 250 nm (with an individual crystallite size of ∼10 nm). The observed trend is characteristic of F127, which has a tendency to self-assemble into a variety of micellar aggregations, forming nanostructures due to the increased size of the PEO blocks.²⁵ A similar type of spherical mesopore was observed for silica composites synthesised using Pluronic F127.²⁶ The modified sample shows nanostructured particles that are highly porous in nature. The presence of micelles and CoMoO₄–surfactant interactions favoured by the synthetic temperature, allows the formation of nanospheres (see Fig. 1). The irregular shape seen in Fig. 4C and D indicates that the micelles become unstable at 300 °C and are unable to form regular spheres. The elemental composition of the modified sample (Fig. S5 (ESI†)) showed a carbon peak confirming the distribution of residual carbon decomposed from the F127 surfactant, producing CoMoO₄/C nanocomposites. The formation of mesopores and carbon that facilitates fast ionic diffusion with reduced internal assistance²⁷ results in a dual role with respect to improving the specific capacitance of the modified CoMoO₄ material. As a result, it can be concluded that the morphology and pore accessibility of the sample can be altered by the addition of the F127 surfactant. The synthetic temperature and concentration of surfactant, which are the main parameters affecting the modification of the poly-ethylene-oxide chains in the product and its degradation, will be discussed in detail in our subsequent publication.

Fig. 4 Field emission SEM micrographs of (A and B) pure and (C and D) modified (F127 added) CoMoO₄ at lower and higher magnifications.

(b) TEM imaging. To further confirm the morphology and its lattice imaging, transmission electron microscopy associated with energy-dispersive X-ray spectroscopy (EDS) has been performed and the corresponding images are shown in Fig. 5 and S6 (ESI†). The observed differences in the images ascertain the change in morphology of CoMoO₄ samples in the presence and absence of F127 surfactant, which agrees well with the SEM images observed in Fig. 4. Rod-like particles with sizes of 0.5 μm are observed for pure CoMoO₄, as shown in Fig. 5A and B, under different magnifications. Pure CoMoO₄ showed a blend of porous and non-porous crystals with a particle size ranging from 100–600 nm. In the case of modified CoMoO₄ (Fig. 5C), a cluster of nanospheres was observed with sizes of around 200 nm, which are uniformly distributed and porous in nature. Interestingly, Fig. 5D shows hexagonal particles of the obtained CoMoO₄ product. The corresponding lattice fringes in the image in Fig. 5D (inset) show a spacing of 0.3 nm, which can be indexed to the (220) plane of CoMoO₄, and this is in good agreement with the XRD results. The TEM/EDS approach has been employed to quantitatively analyse the chemical composition of the CoMoO₄ sample. The TEM images and composition profile are shown in Fig. S6 (ESI†). The spectra performed at different locations in the samples invariably showed the presence of Co, Mo and O. However, for the modified CoMoO₄ sample, the proportion of Mo has been decreased, suggesting that the F127 surfactant adsorbed on the molybdate moiety. Morphological studies confirm that the pure and modified CoMoO₄ are quite distinct in terms of surface chemistry. It is expected that the modified material will have improved adsorption properties at the interfaces, hence the surface area of the material is vital and is discussed in the next section.

Fig. 5 TEM imaging of (A and B) pure and (C and D) modified (F127 added) CoMoO₄ at lower and higher magnifications. Hexagonal particles are shown in (D). The inset in (D) indicates a fringe-like pattern relating to the CoMoO₄ particles.

(c) BET surface area of CoMoO₄. As the energy storage capability originating from the capacitive behaviour of the synthesised electrode depends on the specific area and pore volume, the pore structure and the presence of mesopores were investigated by N₂ adsorption–desorption.

Nitrogen adsorption–desorption isotherms and BJH pore size distributions (inset) for pure and modified CoMoO₄ samples are presented in Fig. 6. The typical N₂ adsorption–desorption graph for CoMoO₄ samples shows an increase in the rate of N₂ adsorption as the relative pressure increases. Both samples (Fig. 6a and b) show a typical type-IV adsorption–desorption isotherm with H₂-type hysteresis loops characteristic of mesoporous materials.²⁸ However, the area of the loop for the modified sample (Fig. 6b) is found to be higher than that for the pure CoMoO₄. This suggests that the percentage of mesoporous particles is higher for the modified sample and, as a result, the ion diffusion pathway is shorter for an intercalation reaction to occur reversibly. This indicates that the modified CoMoO₄ will have enhanced pseudocapacitance for energy storage applications. As concluded in Table 1, the pure and modified CoMoO₄ have surface areas of 11 and 20 m² g⁻¹, respectively. The surface area of the pure sample was quite low, with no hysteresis in the adsorption isotherm. The relatively high surface area obtained for the modified CoMoO₄ provides more faradaic active sites and facilitates the contact of the active sites with the electrolyte. The modified sample possesses a higher surface area compared to that of the pure sample, which could be due to the fact that the presence of polymeric surfactants during the synthesis controlled the particle size. Such a unique nanostructure is highly desirable for faradaic and non-faradaic reactions occurring during charge–discharge processes, with regard to adsorption properties, fast ion diffusion and electron transportation.²⁹ The physico-chemical results strongly suggest that F127-modified CoMoO₄ is a suitable cathode material for supercapacitor applications.

Fig. 6 Nitrogen adsorption–desorption isotherms of (a) pure and (b) modified CoMoO₄ samples. Insets are pore-size distribution curves.

Table 1 Physical properties of pure and modified CoMoO₄ samples

Sample	BET surface area, m^2 g^−1	Average particle size, nm	Average pore diameter, nm
Pure	11.4	525.5	29.2
Modified	20.8	288	30.6

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3.3 Electrochemical studies of CoMoO₄

(a) Cyclic voltammetry studies. To evaluate the energy storage (capacitive) properties of the pure and modified CoMoO₄, potentiostatic cyclic voltammetry (CV), galvanostatic charge–discharge (CD), and electrochemical impedance spectroscopy (EIS) studies were carried out in aqueous 2 M NaOH electrolyte.

To acquire the redox peaks of pure and modified CoMoO₄ samples, cyclic voltammetry (CV) studies were initially carried out at a scan rate of 1 mV s⁻¹ and the results are shown in Fig. 7A and B. As shown in Fig. 7A, oxidation (A1) and corresponding reduction peaks (C1 and C2) are quite weak and ill-defined, indicating that the pure cobalt molybdate material is less electrochemically active. However, for the modified sample (Fig. 7B), the cyclic voltammogram shows a pair of strong redox peaks in each CV curve, indicating typical pseudocapacitive characteristics that are mainly diffusion-controlled and governed by faradic reactions. One well-defined oxidation (anodic) peak (A1 = 0.28 V) during the positive scan and another, while reversing the potential, in the negative scan, corresponding to two reduction (cathodic) peaks (C1 = 0.11 and C2 = 0.53), are observed. A small shoulder (A2 = 0.45) is also seen during the oxidation reaction. The product formed during oxidation of the modified CoMoO₄ undergoes two separate reduction processes (C1 and C2) upon reversing the potential. In the presence of F127 surfactant, the peak currents are prominent. Compared to the pure sample, the areas under the peaks are much larger for the modified material, illustrating that the material is electrochemically reversible and suitable for energy storage. The F127 non-ionic polymeric surfactant influences the electrolyte surface tension and improves ionic transport, which results in well-defined redox peaks and an additional shoulder, A2. The electrochemical performance indicates better faradaic behaviour for the CoMoO₄ nanospheres within the voltage window of 0.6 V. A pair of redox peaks (C1 and A1) seen in Fig. 7B is related to the formation of a different cobalt oxide, indicating that the pseudocapacitance behaviour is reversible for multiple cycles. The pseudocapacitance is correlated to the faradic process involving the ability of OH⁻ to be intercalated into the oxidised form of CoMoO₄ (Co₃O₄) for improved charge storage. The electrochemical reactions of CoMoO₄ during oxidation (A1) and reduction (C1) can be summarised in eqn (2) and (3).⁸

3[Co(OH)₃]⁻ ↔ Co₃O₄ + 4H₂O + OH + 2e⁻ (2)

Co₃O₄ + H₂O + OH⁻ ↔ 3CoOOH + e⁻ (3)

Fig. 7 Cyclic voltammetry curves of (A) pure and (B) modified CoMoO₄. (C) CV curves of modified CoMoO₄ at variable sweep rates (indicated in the figure). (D) Variation of specific capacitance with current for modified CoMoO₄, inset in (D) shows charge–discharge cycles (for 20, 40, 60, 80 and 100 cycles), illustrating the reversibility of the cell tested in the three-cell configuration.

However, peak C2, raised from the adsorption of ions on the surface, corresponding to the non-faradaic process, is found to be quasi-reversible with a shoulder of A2 during the subsequent oxidation process. The CV profile suggests that the addition of F127 to CoMoO₄ leads to a change in surface functional groups, in which carbon composites are produced by the decomposition of the polymeric surfactant at an elevated synthesis temperature of 300 °C. Fig. 7C shows the cyclic voltammetry curves of the modified CoMoO₄ at different scan rates. At higher scan rates, the oxidation and reduction peaks become more prominent, suggesting fast kinetics and ease of ionic transport. The linear dependence of the current on the scan rate implies that the capacitor is suitable for high-power applications. The specific capacitance taken from the galvanostatic charge–discharge time within the voltage window of 0.6 V was calculated for each scan rate and the results are plotted in Fig. 7D and summarised in Table 2. The inset in Fig. 7D shows that the continuous charge–discharge cycling between 0 and 0.6 V is reversible. The curves have a typical symmetrical shape with no distortion, implying capacitive behaviour with 100% coulombic efficiency. The specific capacitance (SC) of the cobalt molybdate was calculated using eqn (4).

SC (F g⁻¹) = IΔt/mΔV (4)

where I (A) is the applied current used for charge–discharge, Δt (sec) is the time elapsed for the discharge cycle, m (g) is the mass of the active material and ΔV (V) is the voltage interval of the discharge.

Table 2 Calculated specific capacitance for modified CoMoO₄ obtained by the potentiodynamic method at different sweep rates

Scan rate/mV s^−1	1	2	5	10	25
Specific capacitance/F g^−1	170	111	78	71	64

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From the three-cell configuration, a specific capacitance of 170 F g⁻¹ was observed for the modified CoMoO₄. However, the specific capacitance decreases as the current increases. This could be due to a faster sweep rate occurring on the surface than in the bulk material at higher discharge currents, resulting in low transportation of ions.

(b) Charge–discharge studies of the hybrid capacitor (AC vs. CoMoO₄). To further understand the capacitive behaviour of modified CoMoO₄ and to examine its suitability for energy storage devices, a charge–discharge study was employed on the hybrid device. The term hybrid represents the combination of a capacitive electrode (negative) and a faradaic electrode (positive), as shown in the schematic diagram in Fig. 8A. Details of the electrochemical characteristics for the positive electrode (CoMoO₄) are shown in Fig. 7. The performance characteristics for the negative electrode, activated carbon (AC), are shown in Fig. S3 (ESI†) and the results are in accordance with those reported in the literature.²⁹ Based on the three-electrode configuration, the safe working voltage windows for the CoMoO₄ and AC electrodes are found to be 0.6 and 1.0 V, respectively. Hence, for the fabricated hybrid device (Fig. 8B), CoMoO₄ coupled with AC, the total cell working voltage should be 1.6 V. The energy density (E) was calculated using eqn (5).

E (W h kg⁻¹) = specific capacity (mA h g⁻¹) × mid charge potential (V) (5)

Fig. 8 (A) Schematic of the hybrid device fabricated in this study and (B) galvanostatic charge–discharge profiles of modified CoMoO₄ tested at different currents.

Based on the initial results, galvanostatic studies of the device were performed only for modified CoMoO₄. Fig. 8B shows the charge–discharge characteristics of modified AC∥CoMoO₄ at various current rates imposed on the device. The observed specific capacitance and energy densities at different currents for the device, AC∥CoMoO₄, are given in Table 3. The modified CoMoO₄ electrode delivers specific capacitances of 79, 76, 73 and 69 F g⁻¹ at currents of 0.1, 0.2, 0.5 and 1.0 A g⁻¹, respectively, and maintains almost 90% of its specific capacitance when the charge–discharge rate increases from 0.1 to 1.0 A g⁻¹. The observed decrease in capacitance is linear with increasing current rate, which is typical for electrochemical capacitors. This capacity retention could be due to ion percolation (electrolyte penetration) on the large surface area of the mesoporous electrode and it is found to be versatile for multiple cycles.

Table 3 Calculated specific capacitance and energy densities for modified AC∥CoMoO₄

Specific current/A g^−1	Specific capacitance/F g^−1	Energy density/W h kg^−1
0.1	79	38
0.2	76	33
0.5	73	31
1.0	69	29

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Table 4 summarises the performance characteristics of reported CoMoO₄ materials and compares those results with our current work. The modified CoMoO₄ shows comparable electrochemical behaviour with those in the studies reported in Table 4. Moreover, the currently reported material is cost-effective and facilely synthesised compared to microwave-assisted synthesised samples and reduced graphite CoMoO₄ materials. This allows the modified CoMoO₄ presented in this study to be a possible candidate for future studies related to next-generation supercapacitors.

Table 4 Comparison of the electrochemical performances of different CoMoO₄ materials

Electrode material	Specific capacitance/F g^−1	Electrolyte	Configuration (two/three-electrode)	Reference
MnMoO4/CoMoO4	187.1	2 M NaOH	3	1
CoMoO4·0.75H2O	380	1 M Na2SO4	3	5
CoMoO4·chitosan	135	2 M NaOH	2	8
CoMoO4	169	3 M KOH	3	9
CoMoO4/MWCNTs	170	2 M NaOH	3	16
RGO/CoMoO4	322.5	6 M NaOH	3	17
CoMoO4	105	2 M Li(OH)	2	18
NiMoO4–CoMoO4·XH2O	80	2 M NaOH	2	30
CoMoO4	133	2 M KOH	3	31
CoMoO4	170	2 M NaOH	3	This study
CoMoO4	79	2 M NaOH	2	This study

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(c) Electrochemical impedance spectroscopy studies. Electrochemical impedance spectroscopy (EIS) was carried out in order to further ascertain the electrode/electrolyte interfacial resistance of the pure and modified CoMoO₄ electrodes. Nyquist plots for the pure and modified samples can be seen in Fig. 9A. At high frequencies, the intercept of the semicircle with the X-axis represents the equivalent series resistance (ESR). The ESR includes the ionic resistance of the electrolyte, the resistance of the active material and the resistance between the electrode and electrolyte. The ESR for the modified electrode is almost 2 Ω cm⁻² whereas that for the pure sample is 29 Ω cm⁻². The lower value indicates a faster charge–discharge ability for the modified electrode than for the pure sample.¹⁶ The diffusive resistance (Warburg impedance) of the modified electrode was lower than that of the pure sample. The indicator is the straight line in the lower frequency region, which is due to diffusion of electrolyte ions. The closer the line is to 90° (an indicator of an ideal supercapacitor) the better is the capacitance behaviour of the sample.

Fig. 9 (A) Electrochemical Impedance Spectra (EIS) of pure and modified CoMoO₄ materials. (B) Variation of specific capacitance vs. number of cycles for modified CoMoO₄ material. Inset shows the charge–discharge cycles (for 20, 40, 60, 80 and 100 cycles), illustrating the reversibility of the device tested in a two-cell configuration.

(d) Cycling stability of the hybrid capacitor (CoMoO₄ vs. AC). To further examine the capacity retention of modified CoMoO₄ vs. the AC device, a long-term cyclability test over 2000 cycles was carried out and result is presented in Fig. 9B. A galvanostatic technique has been used to evaluate the cycling stability of the hybrid device. The modified CoMoO₄ material showed excellent cycling stability, after an initial decrease in capacitance, in the given potential window of 1.6 V, while retaining almost 80% of its initial capacitance after 2000 cycles. The available discharge capacitance after 2000 cycles was about 60 F g⁻¹. The observed loss in capacitance is due to the reduced electron conductive carbon coating on the surface, which increased the ion diffusion resistance upon cycling. The continuous charge–discharge cycling curves for the first 100 cycles are shown in the inset, illustrating that the shape of the curves has been retained and that the electrochemical process involved both faradaic (electron transfer) and non-faradaic (adsorption) reactions. These results confirm the data obtained with BET measurements, suggesting that the porous structure of the modified sample leads to a reduction of the mass transfer resistance, improving the penetration of the electrolyte and ion diffusion in the electrode material. Overall, the physico-chemical and electrochemical results suggests that the porous nature of the F127-modified CoMoO₄ enhanced the performance through high ion diffusion and conductivity.

4. Conclusions

In summary, we have demonstrated the fabrication and performance of the polymeric surfactant-modified CoMoO₄ nanospheres. The F127 surfactant changes the rod-like structure of CoMoO₄ to nanospheres with a size of 250 nm. The surfactant–cobalt molybdate interaction favoured by the chosen concentration aids the growth of nanospheres with a carbon coating for good electronic conductivity. The mesoporous cobalt molybdate demonstrated excellent pseudocapacitance and cycling stability (almost 80% capacity retention over 2000 cycles) when utilised as an electrode in a hybrid device vs. an activated carbon electrode. The charge–discharge test revealed that the surfactant-assisted CoMoO₄ device had a specific capacitance of 79 F g⁻¹ and an energy density of 38 W h kg⁻¹ at a current of 1 mA. The results confirm that the unique morphology and mesoporosity made the CoMoO₄ material suitable for next-generation supercapacitors.

Acknowledgements

This research was funded from ARC's Discovery Projects funding scheme DP1092543. The views expressed herein are those of the authors and are not necessarily those of the Australian Research Council. The authors would like to acknowledge Australian Synchrotron (P9499) for providing beam time to enable work on IR, Curtin University for SEM facilities and AINSE Research Grant (ALNGRA15051) to carry out the microscopy work at ANSTO.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02628a

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