Facile hydrothermal synthesis of porous MgCo₂O₄ nanoflakes as an electrode material for high-performance asymmetric supercapacitors

Abstract

In this work, porous MgCo₂O₄ nanoflakes (MgCo₂O₄ NFs) and MgCo₂O₄ nanocubes (MgCo₂O₄ NCs) have been successfully synthesized through a simple hydrothermal method combined with a post calcination process of the precursor in air. The morphology of the MgCo₂O₄ samples can be easily tuned by changing the hydrothermal temperature and reaction time, respectively. The porous MgCo₂O₄ NFs with an average pore size of 12.5 nm had a BET specific surface area up to 64.9 m² g⁻¹, which was larger than that of MgCo₂O₄ NCs (19.8 m² g⁻¹). The MgCo₂O₄ NFs delivered a specific capacitance of 734.1 F g⁻¹ at 1 A g⁻¹ and exhibited a considerable rate performance with 74.0% capacitance retention at 12 A g⁻¹. About 94.2% of its original capacitance could be retained after 5000 charge–discharge cycles at a constant current density of 5 A g⁻¹. An asymmetric supercapacitor (ASC) was assembled by using MgCo₂O₄ NFs as the positive electrode and AC as the negative electrode, and the ASC had a wide operation voltage of 1.7 V and a high energy density of 33.0 W h kg⁻¹ at a power density of 859.6 W kg⁻¹. Such outstanding electrochemical performances make the MgCo₂O₄ NFs a promising candidate for supercapacitor applications. In addition, the simple and scalable synthesis method can be extended to the preparation of other metal oxide-based electrode materials.

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1. Introduction

The energy crisis and environmental pollution have become more serious in recent years, and so making use of clean and sustainable energies such as solar power, wind energy and bio-fuel is an important issue. Various energy storage and conversion systems with high performance and environmental friendliness have been developed so far. For example, lithium-ion batteries, fuel batteries, supercapacitors, and so on have received increasing attention.^1–3 Among them, supercapacitors have attracted tremendous interest due to their merits of low cost, long cycling stability, fast charge rate, and high power density. With the development of new technology for materials synthesis and device assembly, the performances of supercapacitors have been improved rapidly and they have been applied in a wide range of fields such as hybrid electric vehicles, memory backup systems, emergency lighting systems, and portable military devices.^4,5 However, the relatively low energy density of supercapacitors compared with that of lithium-ion batteries is a serious problem, which may hinder their further applications. It is widely known that the electrode material is a key component in supercapacitors and determines the electrochemical performance to a certain extent.^6,7 The supercapacitors with excellent performance usually possess the characteristics of high specific capacitance and rate capability, long-life cycling stability, and high energy density. It should be noted that these features are closely associated with the specific surface area, electronic/ionic conductivity, and mechanical stability of electrode materials.^8,9 Large specific surface area can provide more active sites for redox reactions, and superior electronic/ionic conductivity ensures that redox reactions can proceed quickly at high current density or scan rate. In addition, good mechanical stability is helpful for the electrode material to retain its structural integrity during the long-term cycling process.

According to the energy storage mechanism, supercapacitors can be divided into two categories including electric double-layer capacitors (EDLCs) and pseudo-capacitors (PCs). EDLCs store charges by ion accumulation at the interface between the electrode material and electrolyte, which is a pure physical process. Carbonaceous materials with large specific surface area, such as activated carbon, graphene, and carbon nanotubes (CNTs), are ideal candidates of electrode materials for EDLCs.^10,11 PCs store energy through reversible faradaic reactions using metal oxides or conducting polymers as common electrode materials. Such energy storage behavior is observed in many chemical reactions, so it is a chemical process. In general, PCs can deliver higher specific capacitance and energy density than EDLCs.^12–15 Among the various electrode materials for PCs, transition metal oxides (TMOs) such as NiO, MnO₂ and Co₃O₄ are considered as the most promising candidates owing to their high theoretical specific capacitance.¹⁶ Co₃O₄ can provide relatively huge capacitance owing to its rich redox reactions. However, cobalt is rare in nature and expensive, and such situation may hinder its wide applications in the field of energy storage. A possible and effective method to change this is to partially replace cobalt cations in the Co₃O₄ crystal structure with other cheap metals such as Mg, Mn, Cu, Fe, Ni, Zn, etc.¹⁷ The formed cobalt-based binary transition metal oxides (BTMOs) may have better electrical conductivity and superior electrochemical performances than pure Co₃O₄ owing to the synergistic effect between different metallic ions.¹⁸ For instance, NiCo₂O₄ nanowires (NWs) and Co₃O₄ NWs were synthesized by the microemulsion technique combined with a post thermal treatment, and under the same current density of 0.5 A g⁻¹, the specific capacitances could reach 1481 and 990 F g⁻¹, respectively.¹⁹ Another example is ZnCo₂O₄ nanoflakes, which could deliver a specific capacitance up to 1220 F g⁻¹ at 2 A g⁻¹, which was about 500 F g⁻¹ higher than that of Co₃O₄ under the same conditions.²⁰

MgCo₂O₄ with a spinel structure stands out among a variety of BTMOs because of its high theoretical specific capacitance (∼3122 F g⁻¹) and other good electrochemical properties. In the MgCo₂O₄ crystal structure, the Mg cation occupies the tetrahedral site and the Co cation occupies the octahedral site. When used as an electrode material for supercapacitors, elemental Mg does not participate in the redox reactions, and all the reactions come from cobalt with various valence states. However, the magnesium has better conductivity than cobalt, and the specific capacitance of Co₃O₄ is expected to improve if one of the Co cations is replaced by Mg. In other words, MgCo₂O₄ probably exhibits better electrochemical performance than Co₃O₄ in practical applications. Several studies focused on the synthesis of MgCo₂O₄ with novel morphologies such as submicron prisms,²¹ NWs,²² and nanoneedles²³ have been reported. Typically, the MgCo₂O₄ particles synthesized by the molten salt method exhibited a specific capacitance of only 321 F g⁻¹ at a current density of 0.5 A g⁻¹.²⁴ Krishnan et al. reported that MgCo₂O₄ cuboidal microcrystals had a capacity of ∼345 C g⁻¹ at 1 A g⁻¹.²⁵ A double-urchin-like MgCo₂O₄ microstructure was hydrothermally prepared, and it delivered a specific capacitance of 508 F g⁻¹ at 2 A g⁻¹.²⁶ Powdered MgCo₂O₄ electrode materials usually have lower specific capacitance than the theoretical value due to the use of binder reagents in the electrode fabrication process, and the structural integrity of the electrode materials often cannot be totally preserved under pressure.²⁷ Improving the electrochemical performance of MgCo₂O₄ has become an urgent issue, and has greatly accelerated the research progress on binder-free synthesis or formation of MgCo₂O₄-based composites. For example, nickel foam supported MgCo₂O₄ nanocone arrays were directly obtained through a one-step hydrothermal method, and such electrode delivered a capacitance of 750 F g⁻¹ at 1 A g⁻¹.²⁸ The specific capacitance of the MgCo₂O₄@ppy urchin-like core–shell structure could reach 1079.6 F g⁻¹ at 1 A g⁻¹, which was mainly contributed by the synergistic effect between MgCo₂O₄ and ppy.²⁹ Gao et al. prepared a MgCo₂O₄@NiMoO₄ composite with a double-urchin-like hierarchical shape on nickel foam through two-step hydrothermal reactions, and such composite delivered a specific capacitance of 1775 F g⁻¹ at 1 A g⁻¹ as well as an energy density of 37.5 W h kg⁻¹ at a power density of 480 W kg⁻¹.³⁰ Although research on MgCo₂O₄ as an electrode material and its applications in supercapacitors has achieved considerable progress, the specific capacitance is still far below the theoretical value. As for the formation of MgCo₂O₄-based composites, the preparation process is generally complicated and not suitable for mass production. Therefore, it is still a big challenge to improve the electrochemical performances of MgCo₂O₄ under the premise of easy synthetic operation, large-scale production and low cost.

Herein, we propose a facile hydrothermal method accompanied by a post annealing process to synthesize porous MgCo₂O₄ nanoflakes (MgCo₂O₄ NFs). To the best of our knowledge, powdered MgCo₂O₄ electrode materials with similar shapes have rarely been reported so far. Such porous MgCo₂O₄ NFs delivered a specific capacitance of 734.1 F g⁻¹ at 1 A g⁻¹ and still retained 543.4 F g⁻¹ even at a high current density of 12 A g⁻¹. In addition, about 94.2% of the original capacitance was retained after 5000 continuous charge–discharge cycles at 5 A g⁻¹, suggesting the excellent cycling stability of the MgCo₂O₄ NF-modified electrode. For comparison, porous MgCo₂O₄ nanocubes (MgCo₂O₄ NCs) were also hydrothermally prepared at 150 °C while the other synthetic conditions remained the same. The MgCo₂O₄ NC-based electrode exhibited a specific capacitance of 541.0 F g⁻¹ at 1 A g⁻¹ and 440.9 F g⁻¹ at 12 A g⁻¹, and the two values were lower than those of MgCo₂O₄ NFs, respectively. Furthermore, an asymmetric supercapacitor (ASC) was fabricated by using activated carbon (AC) as the negative electrode and MgCo₂O₄ as the positive electrode, respectively. The MgCo₂O₄ NFs//AC ASC exhibited higher specific capacitance and energy density than the MgCo₂O₄ NCs//AC ASC. Such results illustrate that the MgCo₂O₄ NFs have excellent electrochemical performance and can serve as a promising electrode material for energy storage in the future.

2. Experimental section

2.1. Materials and methods

All reagents used in this work were of analytical grade and were used directly without extra purification. In a typical synthesis, 0.214 g of (CH₃COO)₂Mg·4H₂O, 0.498 g of (CH₃COO)₂Co·4H₂O and 0.6 g urea were dissolved in 40 mL of distilled water with magnetic stirring. Then, the obtained solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave, sealed, and maintained at 120 °C for 10 h in an electric oven. After cooling down to room temperature naturally, the precipitate was collected by centrifugation, rinsed several times with distilled water and absolute ethanol, respectively, and then dried at 60 °C. Finally, the powdered precursor was calcined at 400 °C for 3 h with a ramp rate of 5 °C min⁻¹.

2.2. Characterization

The XRD pattern was collected on a powder X-ray diffractometer (XRD, Bruker D8 Advance) using a Cu-Kα (λ = 0.1548 nm) X-ray source and the 2θ value ranged from 10 to 70°. The morphology of the sample was observed under a field-emission scanning electron microscope (FESEM, JEOL JSM7100F). The transmission electron microscopy (TEM) image, high-resolution TEM (HRTEM) image and selected-area electron diffraction (SAED) pattern were obtained using a transmission electron microscope (JEOL JEM2100F) with an accelerating voltage of 200 kV. In order to perform the TEM characterization, the sample was dispersed in absolute ethanol and then dropped onto a carbon-coated copper mesh. The surface chemical composition and elemental valence states were investigated by X-ray photoelectron spectroscopy (XPS), which was performed on an ESCALAB 250 spectrometer using an Al Kα (1486.6 eV) X-ray source. N₂ adsorption–desorption isotherms were obtained on a Quantachrome Autosorb 1-C adsorption analyzer at 77 K to evaluate the porous structure including the specific surface area, average pore size and pore distribution, which were analyzed by using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.

2.3. Electrochemical tests

The working electrode was fabricated as follows. The active material, conductive acetylene black and polyvinylidene fluoride (PVDF) binder with a mass ratio of 80 : 15 : 5 were mixed and dispersed in N-methyl-2-pyrrolidone (NMP) solvent, then the resulting slurry was coated on nickel foam with a loading area of about 1 × 1 cm². After drying at 95 °C in a vacuum oven, the nickel foam with loaded MgCo₂O₄ (2.0 mg for NFs and 2.1 mg for NCs) was pressed under a pressure of 10 MPa. All the electrochemical tests including cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS), and cycling were carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., China). The measurements were performed in a typical three-electrode system using 2 M KOH aqueous solution as the electrolyte, and Pt wire and a saturated calomel electrode (SCE) as the counter and reference electrodes, respectively. The potential window for CV measurements was fixed at 0–0.6 V and the scan rate increased from 5 to 50 mV s⁻¹. The GCD tests were carried out within a potential window of 0–0.43 V while the current density varied in the range of 1–12 A g⁻¹. Under an open circuit potential with an AC amplitude of 5 mV, the EIS test was performed in the frequency range of 10⁵ to 10⁻² Hz. In addition, 2 M KOH aqueous solution was also used as an electrolyte for the two-electrode tests. The ASC was assembled by using activated carbon (AC) as the negative electrode and MgCo₂O₄ as the positive electrode, respectively.

According to the GCD curves, the specific capacitance, energy density and power density of the MgCo₂O₄ electrode material can be calculated based on the following equations:

C = IΔt/mΔV (1)

E = CΔV²/7.2 (2)

P = 3600E/Δt (3)

where C (F g⁻¹) is the specific capacitance, Δt (s) indicates the discharge time, I (A) represents the discharge current, m (g) is the mass of the active material, ΔV (V) is the potential window, and E (W h kg⁻¹) and P (W kg⁻¹) represent the energy density and power density, respectively. As for the tests in the two-electrode system, m (g) indicates the total mass of active materials within both the negative and positive electrodes.

3. Results and discussion

The composition of the precipitate after hydrothermal reaction was investigated by XRD, and the typical XRD pattern is presented in the ESI (Fig. S1).† The precursor contained Mg₂CO₃(OH)₂·0.5H₂O (JCPDS no. 37-0454) and Co(CO₃)_0.5(OH)·0.11H₂O (JCPDS no. 48-0083). After thermal treatment of the precursor at 400 °C for 3 h, the crystal structure and phase purity of the final MgCo₂O₄ were also investigated by the XRD technique. As shown in Fig. 1, eight well-defined diffraction peaks were located at 18.8°, 31.5°, 36.9°, 38.3°, 44.9°, 55.4°, 59.2°, and 65.2°, which matched well with previously reported data.³¹ Such result demonstrated the cubic spinel structure of MgCo₂O₄ (JCPDS no. 81-0667, a = b = c = 8.10 Å). All the diffraction peaks were sharp, suggesting good crystallization of the sample. In addition, there were no other peaks ascribed to impurities, revealing high purity of these MgCo₂O₄ NFs.

Fig. 1 The XRD pattern of the porous MgCo₂O₄ nanoflakes obtained in the typical synthesis.

The SEM technique was used to investigate the morphology and size of the obtained product. Fig. 2 presents several SEM images of MgCo₂O₄ NFs with different magnifications. From the panoramic image shown in Fig. 2a, it was observed that the sample was composed of a large number of nanoflakes. All these nanoflakes had a rectangle-like shape, and each flake was about 3.1 μm in length and 1.9 μm in width (Fig. 2b and c). The magnified SEM image in Fig. 2d further indicated the presence of many pores in the flake. In order to investigate the elemental composition and distribution, the SEM image and its corresponding elemental mapping images were obtained and are shown in Fig. 2e–h. The similar shape of the mapping images suggested that elemental Mg, Co, and O were uniformly distributed throughout the flakes.

Fig. 2 (a–d) SEM images of MgCo₂O₄ NFs obtained in the typical synthesis with a post annealing process in air, and (e–h) SEM image of MgCo₂O₄ NFs and the corresponding elemental mapping images of Mg, Co, and O elements, respectively.

The detailed microstructure of MgCo₂O₄ NFs was further examined by TEM, and Fig. 3a shows a typical TEM image that was taken from the corner position of an arbitrarily selected flake. It directly indicated that the MgCo₂O₄ NF was actually composed of numerous NPs along with many pores. The porous structure was formed during the calcination process due to the decomposition of the precursor. The selected-area electron diffraction (SAED) pattern of the porous MgCo₂O₄ NF is shown in Fig. 3b; several ED rings can be clearly observed, suggesting the polycrystalline characteristic of the MgCo₂O₄ flake. The ED rings can be indexed to the (111), (220), (311), (400), and (422) crystal planes of MgCo₂O₄, respectively, and match well with the XRD data. Fig. 3c shows a high-resolution TEM (HRTEM) image of the MgCo₂O₄ NF, from which the lattice can be clearly seen with regular orientations. Fig. 3d and e present the magnified HRTEM images of the circled positions in Fig. 3c, and the lattice spacings were calculated to be 0.24 and 0.20 nm, corresponding to the (311) and (400) lattice planes of cubic MgCo₂O₄, respectively.

Fig. 3 (a) TEM image, (b) SAED pattern, (c) HRTEM image, and (d and e) magnified HRTEM images of the MgCo₂O₄ NFs.

The chemical composition and elemental valence states of MgCo₂O₄ NFs were investigated by XPS measurements. The peak of C 1s located at 284.6 eV was used as a reference to calibrate all the XPS spectra in this work. The full survey spectrum is illustrated in Fig. 4a, and the peaks indicating the presence of Mg, Co, O, and C elements can be clearly observed. The high resolution spectrum of Mg 1s is shown in Fig. 4b; it can be seen that the peak was located at about 1303 eV, suggesting the existence of Mg²⁺ in the MgCo₂O₄ NFs.²⁹Fig. 4c displays the Co 2p XPS spectrum, and the two main peaks were centered at binding energies of 779.9 and 794.9 eV, corresponding to the Co 2p_3/2 and Co 2p_1/2, respectively.³² The energy separation between the two main peaks was 15.0 eV, suggesting the presence of Co³⁺ in the MgCo₂O₄ NFs.³³ After Gaussian fitting, the two main peaks could be separated into two spin–orbit doublets, suggesting the coexistence of Co²⁺ and Co³⁺ in the sample. The peaks at binding energies of 779.4 and 794.4 eV corresponded to Co³⁺, and the other two peaks at binding energies of 780.8 and 796.0 eV were ascribed to Co²⁺.^34,35 Furthermore, two weak peaks at 786.7 and 803.2 eV could be identified as satellite peaks (marked as “Sat.”), and were consistent with previously reported data.³⁶Fig. 4d presents the high resolution spectrum of O 1s, and two fitting peaks were located at binding energies of 529.3 eV (O1) and 531.2 eV (O2), respectively. The peak at 529.3 eV was ascribed to metal–oxygen (Mg–O and Co–O) bonds and another one at 531.2 eV was attributed to the –OH group.^37,38

Fig. 4 (a) XPS survey spectrum and high-resolution spectra for (b) Mg 1s, (c) Co 2p, and (d) O 1s of the MgCo₂O₄ NFs.

Time-dependent experiments were conducted to investigate the shape evolution process of these MgCo₂O₄ NFs. In the first 2 h of hydrothermal reaction, the product was mainly composed of an urchin-like microstructure, which was constructed from many nanorods (Fig. 5a). As the reaction proceeded for 6 h, some of these nanorod-based urchin-like microstructures were transformed into thin nanoflakes (Fig. 5b). After 10 h, the sample was dominated by all the flakes, and the urchins completely disappeared. If the duration of hydrothermal reaction was continuously prolonged to 15 h, some flakes tended to be stacked and formed a loosely layered structure in order that the total surface free energy could be decreased (Fig. 5c). After that, the shape of MgCo₂O₄ had no obvious change even when the reaction time was prolonged to 24 h, as revealed by the SEM image in Fig. 5d. Based on the above SEM observations, the formation process of MgCo₂O₄ NFs was believed to follow a typical Ostwald ripening mechanism. Nanoparticles (NPs) with a small size precipitated out at the initial stage, and these NPs could serve as seeds for the growth of the urchin-like structure or flakes in the following steps. The MgCo₂O₄ microstructures with a larger size were formed at the expense of the small particles. The detailed growth mechanism needs more investigation, and some related work is currently underway.

Fig. 5 SEM images of the MgCo₂O₄ samples prepared with hydrothermal reaction durations of (a) 2, (b) 6, (c) 15, and (d) 24 h, respectively. All the samples were annealed at 400 °C for 3 h.

It was also found that temperature played an important role in the formation of MgCo₂O₄ NFs. Some samples were prepared at 105, 135, and 150 °C for 10 h while other procedures and parameters were the same. At low temperatures of 105 and 120 °C, MgCo₂O₄ flakes were produced as the dominant structure in the final sample (Fig. 6a and 2a). As the temperature increased to 135 °C, the total number of flakes reduced and few layered cubes coexisted (Fig. 6b). The reaction accelerated greatly at higher temperature, and more energy could be provided for the chemical reactions. When the temperature was high enough, the microstructures with less anisotropic shape would be formed finally due to the fast crystal growth. To obtain anisotropic microstructures, the temperature should be low and the nucleation step should be separated from the subsequent crystal growth step. So if the temperature was further increased to 150 °C, a huge quantity of less anisotropic quasi-cubes with an average edge length of about 7 μm were formed (Fig. 6c). The magnified SEM images in Fig. 6d and e indicated that the nanocubes (NCs) were also assembled with many NPs, and so the NCs exhibited a porous structure. The XRD pattern of the MgCo₂O₄ NCs is presented in Fig. 6f. All the diffraction peaks can be well indexed to the cubic spinel structure of MgCo₂O₄ (JCPDS no. 81-0667), suggesting high purity of the obtained MgCo₂O₄ NCs.

Fig. 6 SEM image of the MgCo₂O₄ samples prepared at (a) 105, (b) 135, and (c–e) 150 °C, respectively, and (f) the XRD pattern of the MgCo₂O₄ cubes. All the samples were annealed at 400 °C for 3 h.

A large specific surface area of an electrode material is one of the most important factors influencing its electrochemical performances. N₂ adsorption–desorption measurement was performed at 77 K to determine the specific surface area of MgCo₂O₄ NFs and NCs. The adsorption–desorption isotherms of MgCo₂O₄ NFs are illustrated in Fig. 7a, and the isotherms could be classified as typical type-IV with a type H3 hysteresis, suggesting the mesoporous structure of MgCo₂O₄ NFs.³⁹ The specific surface area of MgCo₂O₄ NFs was calculated to be 64.9 m² g⁻¹, which was larger than that of a double-urchin-like MgCo₂O₄ hierarchical structure (26.45 m² g⁻¹)²⁶ and flower-like MgCo₂O₄ microstructure (55 m² g⁻¹).⁴⁰ A porous structure with a large specific surface area is beneficial for the ion diffusion, and may enhance the electrochemical performances of an electrode material. Fig. 7b reveals the pore size distribution of MgCo₂O₄ NFs using the BJH model. The pore size was mainly distributed at 2.8 nm and the average pore size was calculated to be 12.5 nm, further proving that the MgCo₂O₄ NFs possessed mesoporous features. For comparison, the specific surface area and pore size of MgCo₂O₄ NCs were also investigated, and the results are presented in Fig. 7c and d. The MgCo₂O₄ NCs exhibited a mesoporous characteristic with an average pore size of 9.8 nm, but the BET specific surface area was only 19.8 m² g⁻¹, far lower than that of MgCo₂O₄ NFs. It is widely known that the mesoporous structure is beneficial for the transportation of both electrons and ions, and full contact between the electrode material and electrolyte can promote the faradaic reactions. Furthermore, more actives sites can be provided by an electrode material with a larger specific surface area, so when used for supercapacitors, the MgCo₂O₄ NFs are expected to exhibit better electrochemical performance than MgCo₂O₄ NCs.

Fig. 7 Nitrogen adsorption–desorption isotherms and the corresponding BJH pore-size distribution plots of (a and b) the MgCo₂O₄ NFs and (c and d) the MgCo₂O₄ NCs, respectively.

Based on the above discussion, the MgCo₂O₄ NFs may serve as a promising electrode material for supercapacitors, so the electrochemical performance is evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS), and cycling measurements. All the tests were conducted in a typical three-electrode system at room temperature and 2 M KOH aqueous solution was used as the electrolyte. As shown in Fig. 8a, several CV curves of the MgCo₂O₄ NF-modified electrode were obtained over a potential window of 0–0.6 V as the scan rates varied from 5 to 50 mV s⁻¹. Unlike the rectangular CV curves for electric double-layer capacitors, a couple of redox peaks can be observed in each CV curve, demonstrating the pseudo-capacitive behavior of MgCo₂O₄ NFs. The occurrence of redox peaks can be ascribed to the following reversible faradaic reactions:^29,41

MgCo₂O₄ + 2H₂O ↔ 2CoOOH + Mg²⁺ + 2OH⁻ (4)

CoOOH + H₂O + e⁻ ↔ Co(OH)₂ + OH⁻ (5)

Fig. 8 Electrochemical tests of the MgCo₂O₄-NFs-modified electrode in 2 M KOH solution: (a) CV curves obtained at scan rates from 5 to 50 mV s⁻¹, (b) GCD curves measured at different current densities from 1 to 12 A g⁻¹, (c) specific capacitance at different current densities, and (d) cycling performance and coulombic efficiency at 5 A g⁻¹ for 5000 cycles with the inset showing the last 10 cycles.

It was noticed that the anodic and cathodic peaks moved towards more positive and negative directions when the scan rate increased, which was attributed to the polarization and ohmic internal resistance of the electrode material.⁴² The position of oxidation peaks moved from 0.34 to 0.47 V and the reduction peaks shifted from 0.19 to 0.12 V as the scan rate increased from 5 to 50 mV s⁻¹. The active sites at the inner surface of the electrode material cannot fully participate in the redox reactions because of the limited ion diffusion at high scan rate. Basically, all the CV curves obtained at different scan rates retained a symmetrical shape, indicating the excellent reversibility of the redox reactions taking place in the MgCo₂O₄ NF-based electrode.

The GCD tests were performed over a potential window of 0–0.43 V with the current density increasing from 1 to 12 A g⁻¹. As shown in Fig. 8b, the platforms on all the discharge curves can be clearly observed, suggesting the pseudo-capacitive characteristic of MgCo₂O₄ NFs. It is worth mentioning that the charge and discharge curves in all GCD curves at different current densities are symmetrical, indicating the high stability and coulombic efficiency of MgCo₂O₄ NFs. According to eqn (1), the specific capacitances were calculated to be 734.1, 670.7, 633.3, 590.1, 568.5, and 543.4 F g⁻¹ at current densities of 1, 3, 5, 8, 10, and 12 A g⁻¹, respectively. The rate capability of MgCo₂O₄ NFs was about 74.0% as the current density increased from 1 to 12 A g⁻¹, suggesting a considerable rate performance. The specific capacitance decreased gradually with the increase of current density, which was directly observed from the curve of specific capacitance vs. current density in Fig. 8c. It demonstrated that the kinetics of redox reactions was mainly dominated by ion diffusion in the electrolyte. The use of PVDF binder and conductive carbon lead to a worse voltage drop at high current density, resulting in a decrease in specific capacitance. The capacitance value (734.1 F g⁻¹ at 1 A g⁻¹) of MgCo₂O₄ NFs was higher than that of MgCo₂O₄ powder (321 F g⁻¹ at 0.5 A g⁻¹),²⁴ double-urchin like MgCo₂O₄ microstructure (508 F g⁻¹ at 2 A g⁻¹),²⁶ and MgCo₂O₄ twinned-hemispheres (626.5 F g⁻¹ at 1 A g⁻¹).⁴³ It is no doubt that the electrode materials obtained using a binder-free method or BTMO-based composites usually exhibit better electrochemical performances. For instance, MgCo₂O₄ nanoneedle arrays directly grown on Ni foam delivered a specific capacitance of 804 F g⁻¹ at a current density of 1 A g⁻¹,⁴⁴ and MgCo₂O₄@MnO₂ core–shell arrays on graphene-coated Ni foam delivered a specific capacitance of 887.3 F g⁻¹ at 1 A g⁻¹.⁴⁵ Considering the mass production and simple synthetic process, the MgCo₂O₄ NFs have considerable advantages as a promising electrode material for practical applications in supercapacitors.

Cycling stability is another important issue for an electrode material. Especially, a material with long-term cycling durability probably has a greater likelihood of being considered and used in supercapacitor devices. Hence, the cycling performance of MgCo₂O₄ NFs was evaluated through a continuous 5000 cycle GCD test at a constant current density of 5 A g⁻¹, and the result is shown in Fig. 8d. About 95.6% of the initial capacitance was retained after 1000 cycles, and at the end of the 5000^th cycle, about 94.2% of the initial value was retained, suggesting the excellent cycling performance of MgCo₂O₄ NFs. The electrochemical reversibility and cycling stability of electrode materials is also reflected by coulombic efficiency to some extent, which can be calculated from the following equation:

where η represents the coulombic efficiency, and t_d and t_c are the discharge and charge times, respectively. The value of η was close to 100% during the whole 5000 cycle process, which suggested the good reversibility of faradaic reactions (Fig. 8d). In addition, the last 10 cycles are presented in the inset of Fig. 8d, from which a highly symmetric and linear shape with relatively small resistance drops can be observed, and it further confirmed the good electrochemical reversibility of the MgCo₂O₄ NF-modified electrode. After cycling, the MgCo₂O₄ NFs were examined by SEM and XRD (Fig. S2†), and the overall structure and shape of these NFs were well preserved (Fig. S2a†). It seemed that the surface of NFs became very rough, and it probably resulted from the continuous electron transfer and ion diffusion in the cycling process. The XRD data illustrated that the MgCo₂O₄ phase had almost no change (Fig. S2b†).

For comparison, the electrochemical performance of MgCo₂O₄ NCs was also investigated under the same test conditions. The CV test was performed over a potential window of 0–0.6 V. As the scan rate increased from 5 to 50 mV s⁻¹, all the CV curves exhibited similar shapes to those of the MgCo₂O₄ NF electrode (Fig. 9a). It also suggested that the MgCo₂O₄ NCs possessed pseudo-capacitance. Over a potential window of 0–0.43 V, the GCD curves were obtained at different current densities of 1–12 A g⁻¹ (Fig. 9b). It can be clearly observed that the shape of these curves had no significant difference from that of MgCo₂O₄ NFs. The specific capacitances were calculated to be 541.0, 509.4, 488.1, 464.4, 452.5, and 440.9 F g⁻¹ at current densities of 1, 3, 5, 8, 10, and 12 A g⁻¹, respectively, which were indeed lower than those of MgCo₂O₄ NFs (Fig. 9c). The rate capability of MgCo₂O₄ NCs was about 81.5% as the current density increased from 1 to 12 A g⁻¹. The GCD curves of MgCo₂O₄ NFs and NCs at the same current density of 1 A g⁻¹ are shown in Fig. 9d, and it clearly demonstrated that the MgCo₂O₄ NFs delivered higher specific capacitance at 1 A g⁻¹ due to the longer discharge time. The MgCo₂O₄ NFs possess larger specific surface area than NCs, and may provide more active sites for faradaic reactions. In addition, the ion diffusion and electron transfer can proceed more easily in the thin NFs than in the NCs, and so the inner impedance may be smaller for NFs. This probably is the basic reason that MgCo₂O₄ NFs deliver higher specific capacitance than the NCs.

Fig. 9 (a) CV curves of MgCo₂O₄ NCs obtained at scan rates from 5 to 50 mV s⁻¹, (b) GCD curves of MgCo₂O₄ NCs measured at different current densities from 1 to 12 A g⁻¹, (c) the plot of specific capacitance vs. current density for MgCo₂O₄ NFs and MgCo₂O₄ NCs, respectively, and (d) GCD curves at 1 A g⁻¹ for MgCo₂O₄ NFs and MgCo₂O₄ NCs, respectively. All the tests were performed in 2 M KOH aqueous electrolyte.

The electrochemical impedance spectroscopy (EIS) test was conducted over the frequency range from 10⁵ Hz to 10⁻² Hz under open-circuit potential to explore the electrical conductivity of the MgCo₂O₄ NF electrode. The Nyquist plot is presented in Fig. 10, which consisted of a semicircle in the high frequency region and a straight line in the low frequency region. The value of internal resistance (R_s) can be directly obtained from the intercept on the real axis (Z′) at high frequency, which is the sum of ionic resistance of the electrolyte, intrinsic resistance of the material, and contact resistance between the electrode material and electrolyte.⁴⁶R_ct indicates the charge transfer resistance during faradaic reactions, which can be represented by the diameter of the semicircle at high frequency. The slope of the straight line in the low frequency region is attributed to the Warburg impedance (Z_w), which is mainly produced from the diffusion resistance of OH⁻ ions.⁴⁷ In this work, the values of R_s and R_ct were calculated to be 0.75 and 0.4 Ω, respectively. Such results suggest that the MgCo₂O₄ NF-based electrode possesses excellent electrical conductivity, and can serve as a promising electrode material for next-generation advanced supercapacitors.

Fig. 10 Nyquist plot of the MgCo₂O₄-NF-modified electrode and the inset is the enlarged plot in the high frequency region as well as the corresponding equivalent circuit.

In order to evaluate the practical applications of such MgCo₂O₄ NFs in supercapacitors, we fabricated an asymmetrical supercapacitor (ASC) device using MgCo₂O₄ NFs as a positive electrode and activated carbon (AC) as a negative electrode (MgCo₂O₄ NFs//AC ASC). Fig. S3† presents the electrochemical performances of the AC electrode, which was tested in 2 M KOH aqueous solution at room temperature. The specific capacitances of the AC electrode were calculated from GCD curves, and the values were 174.9, 168.2, 162.7, 160.4, 158.4, and 156.9 F g⁻¹ at current densities of 1, 2, 4, 6, 8, and 10 A g⁻¹, respectively. It demonstrates that AC is an excellent negative electrode material for supercapacitors. It is worth noting that the positive and negative electrodes should satisfy the charge balance (q⁺ = q⁻), and then the ASC may achieve optimal performances. Several CV curves of the MgCo₂O₄ NFs//AC ASC obtained at a scan rate of 10 mV s⁻¹ with different voltage windows are shown in Fig. 11a, and it is obvious that the operation voltage of such ASC could be enlarged to 0–1.7 V. It is well known that a larger voltage window may lead to higher energy density. In order to further confirm that this voltage window is appropriate, GCD tests were performed with different voltage windows at a current density of 1 A g⁻¹ (Fig. 11b), and the voltage window of 0–1.7 V was found to be the most suitable for the ASC. The specific capacitances calculated from GCD curves with different voltage windows are shown in Fig. 11c. As the voltage window was enlarged from 0–1.0 to 0–1.7 V, the specific capacitance increased from 43.4 to 82.2 F g⁻¹ and the energy density accordingly improved from 6.0 to 33.0 W h kg⁻¹. Hence, the appropriate voltage window for the electrochemical tests of the MgCo₂O₄ NFs//AC ASC is 0–1.7 V.

Fig. 11 (a) CV curves of the MgCo₂O₄ NFs//AC ASC measured over different voltage windows at a scan rate of 10 mV s⁻¹, (b) GCD curves of the MgCo₂O₄ NFs//AC ASC measured over different voltage windows at a current density of 1 A g⁻¹, and (c) specific capacitance as a function of voltage windows.

Fig. 12a shows the CV curves of the MgCo₂O₄ NFs//AC ASC that were obtained with scan rate increasing from 5 to 50 mV s⁻¹ in the voltage window of 0–1.7 V. The distorted and rectangular shape of all the curves illustrated that both the pseudo-capacitance from MgCo₂O₄ NFs and the electric double-layer capacitance from AC contributed to the total capacitance of the ASC. The GCD curves obtained at different current densities (1–10 A g⁻¹) are presented in Fig. 12b, and the specific capacitances were calculated to be 82.2, 74.4, 68.5, 65.0, 63.3, and 61.5 F g⁻¹ at 1, 2, 4, 6, 8, and 10 A g⁻¹, respectively (Fig. 12c). The capacitance retention was about 74.8% as the current density increased from 1 to 10 A g⁻¹, suggesting a good rate capability of the MgCo₂O₄ NFs//AC ASC. The MgCo₂O₄ NCs//AC ASC was also fabricated through the same procedure using MgCo₂O₄ NCs and AC as the positive and negative electrodes, respectively, and its electrochemical performance was evaluated under the same conditions. As shown in Fig. 12d and e, the CV and GCD curves suggested that the pseudo-capacitance of MgCo₂O₄ NCs and the electrode double-layer capacitance of the AC electrode also contributed to the capacitance of the MgCo₂O₄ NCs//AC ASC. The specific capacitances were calculated from the GCD curves and are shown in Fig. 12f. The values were 70.2, 68.5, 64.1, 63.2, 62.0, and 61.3 F g⁻¹ at 1, 2, 4, 6, 8, and 10 A g⁻¹, respectively, which were lower than those of the MgCo₂O₄ NFs//AC ASC. From the above analysis, it can be concluded that the MgCo₂O₄ NFs possess better electrochemical properties and may have great potential for supercapacitors.

Fig. 12 Electrochemical tests for the MgCo₂O₄ NFs//AC ASC in 2 M KOH solution: (a) CV curves at different scan rates in the voltage window of 0–1.7 V, (b) GCD curves at different current densities, and (c) the specific capacitance at various current densities. Electrochemical tests for the MgCo₂O₄ NCs//AC ASC: (d) CV curves at different scan rates in the voltage window of 0–1.7 V, (e) GCD curves at different current densities, and (f) the specific capacitance at various current densities.

The long-term cycling stability is also important for the ASC device. The cycling performances of the MgCo₂O₄ NFs//AC ASC and MgCo₂O₄ NCs//AC ASC were evaluated at a current density of 5 A g⁻¹, and the results are shown in Fig. 13. After a continuous 4000 cycle GCD process, the capacitance of the MgCo₂O₄ NFs//AC ASC maintained approximately 110.4% of its initial value, and the coulombic efficiency remained at almost 100% during the whole cycling process (Fig. 13a). The last 5 GCD curves are displayed in Fig. 13b, and the shape of the curves had no significant change and remained symmetrical. It suggested the excellent cycling performance of the MgCo₂O₄ NFs//AC ASC. As shown in Fig. 13c, the capacitance retention of the MgCo₂O₄ NCs//AC ASC was about 104.8% after 4000 cycles, and the coulombic efficiency also remained close to 100%. The last 5 GCD curves shown in Fig. 13d also had a symmetrical shape.

Fig. 13 (a) Consecutive 4000 cycle GCD curves at 5 A g⁻¹ and the related coulombic efficiency, and (b) the last 5 GCD curves of the MgCo₂O₄ NFs//AC ASC, (c) continuous 4000 cycle GCD curves at 5 A g⁻¹ and the related coulombic efficiency, and (d) the last 5 GCD curves of the MgCo₂O₄ NCs//AC ASC.

Energy and power densities are vital factors in determining the practical applications of supercapacitors; in this case, the energy and power densities of the MgCo₂O₄ NFs//AC ASC as well as the MgCo₂O₄ NCs//AC ASC were calculated based on GCD curves, and the Ragone plot is illustrated in Fig. 14. The MgCo₂O₄ NFs//AC ASC exhibited an energy density of 33.0 W h kg⁻¹ at a power density of 859.6 W kg⁻¹, and retained 24.7 W h kg⁻¹ when the power density increased to 9527.6 W kg⁻¹. In comparison, the MgCo₂O₄ NCs//AC ASC exhibited an energy density of 28.2 W h kg⁻¹ at a power density of 860.4 W kg⁻¹. The energy density of the MgCo₂O₄ NFs//AC ASC is higher than those of several previously reported ASCs based on MgCo₂O₄ or other TMOs, such as NiCo₂O₄ nanosheets/CuCo₂O₄ nanocones/NF//AC (15 W h kg⁻¹ at 814 W kg⁻¹),⁴⁸ NiCo₂S₄ NPs//AC (28.3 W h kg⁻¹ at 245 W kg⁻¹),³⁹ MgCo₂O₄ nanosheets/NF//AC (12.99 W h kg⁻¹ at 448.7 W kg⁻¹),⁴² and Co₃O₄/NF//carbon aerogel microspheres (17.9 W h kg⁻¹ at 750 W kg⁻¹).⁴⁹ However, these values are still lower than those of some composites or other cobalt-based oxides directly grown on conductive substrates, such as urchin-like MgCo₂O₄@ppy/NF//AC (33.4 W h kg⁻¹ at 320 W kg⁻¹),²⁹ double-urchin-like hierarchical MgCo₂O₄@NiMo₂O₄ core–shell nanomaterial/NF//AC (37.5 W h kg⁻¹ at 480 W kg⁻¹),³⁰ ZnCo₂O₄ NW cluster arrays/NF//AC (41 W h kg⁻¹ at 384 W kg⁻¹),⁵⁰ and MgCo₂O₄ nanoneedles/NF//rGO (81 W h kg⁻¹ at 1350 W kg⁻¹).⁴⁴ From the viewpoint of large-scale synthesis of powdered electrode materials and low cost, the MgCo₂O₄ NFs//AC ASC fabricated in this work achieves a wide operation voltage of 0–1.7 V and delivers an energy density as high as 33.0 W h kg⁻¹, and all the results indicate that the MgCo₂O₄ NFs can be considered as a promising electrode material for supercapacitors.

Fig. 14 Ragone plots of the MgCo₂O₄ NFs//AC ASC, MgCo₂O₄ NCs//AC ASC and other ASCs reported previously.

4. Conclusions

In summary, MgCo₂O₄ nanoflakes with a rectangular shape were successfully synthesized via a hydrothermal method followed by an annealing treatment of the precursor. These MgCo₂O₄ NFs have a mesoporous structure with an average pore diameter of 12.5 nm and a specific surface area up to 64.9 m² g⁻¹. The electrode modified with MgCo₂O₄ NFs delivered a specific capacitance of 734.1 F g⁻¹ at 1 A g⁻¹ and 543.4 F g⁻¹ at 12 A g⁻¹. After 5000 cycles at 5 A g⁻¹, the specific capacitance remained at about 94.2% of its initial value, suggesting a superior cycling stability of such electrode. In addition, the MgCo₂O₄ NFs//AC ASC delivered an energy density of 33.0 W h kg⁻¹ at a power density of 859.6 W kg⁻¹. Such ASC also exhibited excellent cycling performance, and almost 110.4% of the original capacitance was retained after 4000 cycles at a current density of 5 A g⁻¹. All the results illustrate that the MgCo₂O₄ NFs have superior electrochemical performance and great application potential as an electrode material for supercapacitors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the research project supported by the Shanxi Scholarship Council of China, financial support from the International Cooperation of Science and Technology Projects in Shanxi Province (201703D421040 and 201803D421092), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Shanxi Province.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0na00353k

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