Facile synthesis of Co₃O₄ with different morphology and their application in supercapacitors

Abstract

In this article, direct growth of Co₃O₄ with different morphologies on nickel foam is successfully achieved via a simple hydrothermal method by changing the volume ratio between ethanol and water. The morphology and structure of the as-prepared samples are examined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The electrochemical performance of the Co₃O₄ electrodes is investigated as pseudocapacitor material by cyclic voltammetry and galvanostatic charge/discharge test in 3 mol L⁻¹ KOH solution. Results show that the solvent composition plays an important role not only in the morphology but also in the capacitance. Co₃O₄ with a honeycomb structure obtained from the volume ratio of C₂H₅OH/H₂O = 1 exhibits the highest capacitive performance, 2509.4 F g⁻¹ at 1 A g⁻¹ and 1754 F g⁻¹ at 10 A g⁻¹, which is much larger than that prepared in the pure water and pure ethanol solvent. The electrode also has a satisfactory cycling performance with capacity retention of 74% after 1000 cycles at 10 A g⁻¹. The enhanced electrochemical performance is ascribed to the honeycomb nanostructure allowing facile electrolyte flow which speeds up electrochemical reaction kinetics. These findings may open up the opportunity for optimizing the hydrothermal synthesis conditions to control the morphology and performance of the products.

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Introduction

Electrochemical capacitors, also known as supercapacitors or ultracapacitors, have drawn increasing attention as the most promising candidates for next-generation energy storage devices due to their high power densities, fast charge–discharge processes, long cycling life, and safe operation.^1,2 According to the different energy storage mechanisms, electrochemical supercapacitors are divided into two types. One is electrical double-layer capacitors (EDLCs) based on carbonaceous materials which store/deliver energy by non-faradaic surface interactions, leading to higher power density and long cycling life but low specific capacitance.^3,4 The other one is pseudocapacitors based on conducting polymers and transition metal oxides/hydroxide, where charge is stored/delivered via a fast and reversible surface faradic redox reaction.^5,6 Despite the specific capacitance of pseudocapacitors is several times larger than EDLCs, pseudocapacitive materials still present a compromise between the power performance and reversibility because of the fast faradic reaction leading to a high demand for the electron transport and fast ion transport.⁷ Therefore, a lot of research works have been done in the past few years to improve the performance of the electrochemical capacitor electrode.

Among these pseudocapacitive materials, Co₃O₄ is considered to be one of the most attractive materials owing to its lower toxicity, high theoretical specific capacitance (3560 F g⁻¹), good capability retention and efficient redox charge transfer.^8–13 Thus, various morphology of Co₃O₄ has been reported, such as nanoparticles,¹⁴ nanosheets,¹⁵ nanowires,¹⁶ nanotubes¹⁷ and nanoflowers¹⁸ etc. It has been found that Co₃O₄ based pseudocapacitors mainly depends on properties such as surface area, morphology, and specific orientation of different facets. Therefore, the control over the morphology of nanomaterials with well-defined shape and uniform size remains an important goal of modern synthetic chemistry, especially nanomaterials direct growth on conductive substrate. In comparison to the conventional Co₃O₄/carbon/PTFE electrode,^18–21 one-dimensional (1D) Co₃O₄ materials grow on current-collecting substrates could enhance the power density (or rate capability) and cycling stability since this structure not only have the ability to increase the contact area between the electrolyte and active material but also provide accessible channels that facilitate mass and charge transport.^22–24 In order to solve this issue, great efforts have been devoted to creating nanostructured materials with large surface area to enhance the kinetics of ion and electron transport and also shorten the diffusion path of ions and electrons.²⁵

There are several methods used to prepare cobalt oxides with the controlled morphology, however, most reports focus on the use of surfactant or template, which greatly increased the complicated process to remove these foreign substance. As a facile and effective synthesis method, hydrothermal is widely used for the preparation of Co₃O₄ nanomaterials and various structures have been obtained by changing the temperature, reaction time or reagents.^8–11 However, a very few reports have systematically investigated the effects of solvents composition on the morphology and electrochemical performance of Co₃O₄. Therefore, in this work, Co₃O₄ with different morphology has been successfully synthesized through adding different volume ratio ethanol. The obtained Co₃O₄ electrode exhibited a unique 3D hierarchical structure, which delivered large specific capacitances and good electrochemical stability at high rates. This work demonstrate that solvents seem to play a crucial role in shaping the morphology of Co₃O₄ crystals, which could be used to effectively control their structural organization and further improve the capacitive performance.

Experimental

Synthetic methods

The formation process of Ni foam supported Co₃O₄ electrodes were based on a hydrothermal methods followed by an annealing process. The experimental detail was as follows: the Ni foam was pretreated by degreasing with acetone, etching with 6.0 mol L⁻¹ HCl for 15 min, rinsing with water, prior to use. In a typical procedure, 4 mmol Co(NO)₃ and 10 mmol CO(NH₂)₂ were added to 24 mL C₂H₅OH/H₂O mixed solvent in a volume ratio of 1 : 0, 1 : 1 and 0 : 1 with vigorous stirring. After stirring for 15 min, the mixture was then transferred into a 30 mL Teflon-lined stainless steel autoclave. Afterwards, a piece of Ni foam was put into the autoclave and immersed in the solution. The autoclave was heated gradually to 90 °C and maintained at this temperature for 10 h to allow the growth of Co₃O₄. After that, the autoclave was cooled down to room temperature naturally, then, the Ni foam was removed from the growth solution and rinsed with distilled water several times before drying at 60 °C in an oven. Finally, the as-prepared samples were converted to Co₃O₄ via thermal decomposition at 300 °C for 3 h in air.

Materials characterization

The electrode morphology was examined by a scanning electron microscope (SEM, JEOL JSM-6480) and a transmission electron microscope (TEM, FEI Teccai G2S-Twin, Philips). The structure was analyzed using an X-ray diffractometer (Rigaku TTR III) with Cu Kα radiation. Chemical bonding information on metal-oxygen and precursor was investigated with Fourier transform infrared spectroscopy (FTIR, Equinos55, Bruker) using the potassium bromide pellet technique.

Electrochemical measurements

The electrochemical measurements were carried out in a three-electrode electrochemical cell containing 3 mol L⁻¹ KOH aqueous solution as the electrolyte, the prepared electrode as the working electrode, a saturated calomel electrode (SCE) as reference electrode and a Pt foil as counter-electrode. The cyclic voltammetry (CV) studies were performed on an electrochemical workstation (VMP3, BioLogic, France). Galvanostatic charge–discharge test was conducted using computer controlled cycling equipment (LAND, Wuhan China). All solutions were made with analytical grade chemical reagents and Milli-Q water (18 MΩ cm).

Results and discussion

By using different solvents composition, three samples with different morphologies were obtained. In order to distinguish with each other, the samples prepared using C₂H₅OH/H₂O volume ratios of 1 : 0, 1 : 1 and 0 : 1 were denoted as E-Co₃O₄, EW-Co₃O₄ and W-Co₃O₄, respectively. Their morphologies were detailed characterized by SEM and TEM. Fig. 1 shows the SEM images of as-prepared Co₃O₄ samples before and after calcined. It can be seen that all the three samples exhibit relatively uniform, well-dispersed morphology and have its own characteristic, completely different with each other. It can be clearly seen from Fig. S1† that Ni foam substrate was completely covered by Co₃O₄. Ni foam as the substrate for the direct growth of Co₃O₄ has many advantages, since that a 3D network structure with micro open cages and zigzag flow channels. As it shown in Fig. 1a and b, E-Co₃O₄ sample exhibits the sheet-like structure. The nanosheets interconnected with each other, creating loose porous nano-structures with abundant open space and electro-active surface sites, however, after calcined, the nanosheets have melted and bonded together, and cracks appeared on the surface. According to Fig. 1c and d, the morphology of EW-Co₃O₄ sample remained no obvious changes before and after calcination, which shows a honeycomb-like structure consists of dense nanowires and nanosheets attached to each other. For the sample prepared in pure deionized water (W-Co₃O₄), only nanowire structure was grown on the Ni foam (Fig. 1e and f).

Fig. 1 SEM images of E-Co₃O₄ before (a) and after (b) calcined, EW-Co₃O₄ before (c) and after (d) calcined, and W-Co₃O₄ before (e) and after (f) calcined. Insert is the high-magnified SEM images of the as-prepared samples after calcined.

The TEM images of the as-prepared Co₃O₄ samples before and after calcined are demonstrated in Fig. 2, which provide an insight of individual structures. The TEM images of E-Co₃O₄ (Fig. 2a) and W-Co₃O₄ (Fig. 2e) samples confirm its assembly as a sheet-like structure and only nanowire-like, respectively. While for the sample EW-Co₃O₄ (Fig. 2c), it can be clearly observed that the sample is composed of nanowire and nanosheet. However, all the three Co₃O₄ samples have an obvious change in the morphology that previously solid structure acquired after hydrothermal were converted to a porous structure composed of interconnected nanoparticles with about 20 nm in diameter after calcination. Those porous structures could enhance the accessible channels with the electro-active species, further facilitating mass and charge transport.

Fig. 2 TEM images of E-Co₃O₄ before (a) and after (b) calcined, EW-Co₃O₄ before (c) and after (d) calcined, and W-Co₃O₄ before (e) and after (f) calcined.

Fig. 3 shows the XRD patterns of as-prepared Co₃O₄ samples before (a) and after (b) annealing, respectively, and the standard date of spinel-phased Co₃O₄ crystal (JCPDS no. 43-1003) is also shown in Fig. 3b for comparison. To avoid the effect of Ni foam, all the samples were examined by the powders scratched from nickel foam. As shown, all the samples before the annealing are different and weak crystal. The peak of W-Co₃O₄ could be assigned to the Co(CO₃)_0.5(OH)·0.11H₂O (JCPDS no. 48-0083), which is agree with the literature reported and the major reactions occurred is as followed:^26,27

CO(NH₂)₂ + 4H₂O → H₂CO₃ + 2NH₃·H₂O (1)

NH₃·H₂O → NH₄⁺ + OH⁻ (2)

H₂CO₃ → 2H⁺ + CO₃²⁻ (3)

(x + y)Co²⁺ + yCO₃²⁻ + xOH⁻ + zH₂O → xCo(OH)₂·yCoCO₃·zH₂O (4)

However, when the ethanol was used as solvent, for the sample E-Co₃O₄, the peak appeared at 12.8° could be assigned to Co(OOCCH₃)₂·4H₂O (JCPDS no. 29-0465), indicating that ethanol began to be oxidized during the process of hydrothermal, further reacted with Co²⁺. The possible reaction is as followed:

CH₃CH₂OH + O₂ → CH₃COOH + H₂O (5)

2CH₃COOH + Co²⁺ → (CH₃COO)₂Co + 2H⁺ (6)

Thus, the precursor of EW-Co₃O₄ was a mixture of Co(OOCCH₃)₂·4H₂O and Co(CO₃)_0.5(OH)·0.11H₂O. After heat treatment, the peaks appeared at 2θ = 19.0°, 31.3°, 36.8°, 44.8°, 49.1°, 55.7°, 59.4°, 65.2° and 77.3° can be successfully indexed to (111), (220), (311), (222), (400), (331), (511), (440) and (533) plane reflections of the spinel Co₃O₄ phase (JCPDS no. 43-1003), implying that the crystal-line Co₃O₄ has been formed after annealing treatment (Fig. 3b).

Fig. 3 XRD patterns of as-prepared E-Co₃O₄, EW-Co₃O₄ and W-Co₃O₄ before (a) and after (b) annealing at 300 °C for 3 h in air, respectively.

Fig. 4a shows the FTIR spectra of the precursor of the three samples. As can been seen, the 1630 and 1384 cm⁻¹ bands appeared in both E-Co₃O₄ and EW-Co₃O₄ are the characteristic stretching vibration of the –COO– stretching vibration. However, as the CH₃CH₂OH ratio decreased, the relative strength of –C=O is decreased, revealing that the amount of –COOH is decreased in the precursor of EW-Co₃O₄. The peaks at 1630 and 1384 cm⁻¹ are due to the mode of the –COO– stretching vibration.²⁸ The EW-Co₃O₄ and W-Co₃O₄ precursor film contain stretching vibrations of νOCO₂(1499 cm⁻¹) and νCO₃²⁻(1384 cm⁻¹, 830 cm⁻¹ and 746 cm⁻¹)^29,30 as well as vibration bands of νCo–O (517 cm⁻¹) and νCo–OH (960 cm⁻¹).³¹ After annealing treatment, the bands of the precursor film disappeared and two very strong peaks centered at 661 and 565 cm⁻¹ characteristic of spinel Co₃O₄ are noticed, which are consistent with the XRD result. Combined the result of XRD, SEM, TEM and FTIR, it can be found that the solvent influences not only the morphology but also the composition of precursor.

Fig. 4 FT-IR spectra of as-prepared E-Co₃O₄, EW-Co₃O₄ and W-Co₃O₄ before (a) and after (b) annealing, respectively.

In order to demonstrate the excellent electrochemical properties of the as-prepared Co₃O₄ electrode, we scientifically evaluated its supercapacitance behavior through CV and galvanostatic charge–discharge in 3.0 mol L⁻¹ KOH aqueous electrolyte using the conventional three-electrode configuration with a Pt plate counter electrode and a SCE reference electrode. Fig. 5 shows the typical CVs of E-Co₃O₄ (a), EW-Co₃O₄ (b) and W-Co₃O₄ (c) in 3.0 mol L⁻¹ KOH solution with various sweep rates, respectively. The CV curves of the three Co₃O₄ with different morphology show two oxidation/reduction peaks in the potential range of −0.5 to 0.65 V. The peaks can be respectively attributed to the transition of the redox couple Co(II)/Co(III) and Co(III)/Co(IV),³² revealing their pseudo-capacitive characteristics. With the increase of the scan rate, the redox current density increased and the anodic and cathodic peaks shifted toward higher and lower potentials, respectively, indicating the quasi-reversible feature of the redox couples. Fig. 5d shows the comparison CVs of the as-prepared Co₃O₄ samples with the nickel foam after the solvothermal process at a scan rate of 10 mV s⁻¹ in 3.0 mol L⁻¹ KOH solution. It can been seen from the insert of Fig. 5d that Ni foam only showed a pair of redox peaks which is corresponding to the interconversion between Ni(II) and Ni(III).³³ However, compared with the as-prepared Co₃O₄ samples, the specific capacitance of nickel foam can be ignored which the peak currents reached less than 2 mA cm⁻². Clearly seen that the CV area of EW-Co₃O₄ sample is the largest among the three samples, declares the most ideal capacitive behavior.

Fig. 5 Cyclic voltammograms of E-Co₃O₄ (a), EW-Co₃O₄ (b) and W-Co₃O₄ (c) in 3.0 mol L⁻¹ KOH solution at different scan rates, respectively. (d) The cyclic voltammograms comparision of E-Co₃O₄, EW-Co₃O₄ W-Co₃O₄ and Ni foam after the solvothermal process in 3.0 mol L⁻¹ KOH at a 10 mV s⁻¹ scan rate.

Fig. 6 shows the discharge curves of E-Co₃O₄ (a), EW-Co₃O₄ (b) and W-Co₃O₄ (c) in 3.0 mol L⁻¹ KOH solution at different current densities. The specific capacitances were calculated according to the following equation:

C_m = (i × Δt)/(ΔV × m) (7)

where C_m (F g⁻¹) is the specific capacitance, i (A) is the discharge current, Δt (s) is the discharge time, ΔV (V) is the voltage window for discharge, and m (g) is the total mass of active material in the electrode. The specific capacitance per gram and per area versus current density for the as-prepared Co₃O₄ samples is shown in Fig. 6d. The mass loadings of E-Co₃O₄, EW-Co₃O₄ and W-Co₃O₄ (in mg of electrochemically active material per cm² of nickel foam) are 3.8, 2.1 and 3 mg cm⁻², respectively. As shown in the result, EW-Co₃O₄ sample displays an extremely performance for supercapacitors during the three samples and the specific capacitance values were measured to be 2509.4, 2004.2, 1754.9, 1221.6, 342.8 F g⁻¹ (5.26, 4.21, 3.69, 2.57 and 0.72 F cm⁻²) at the current density of 1, 5, 10, 20, and 50 A g⁻¹, respectively. It was also found that the capacitance decreased with an increase in discharge current density, which was caused by the electrode resistance and the relatively low utilization of the active material under higher discharge current densities.

Fig. 6 The discharge curves of E-Co₃O₄ (a), EW-Co₃O₄ (b) and W-Co₃O₄ (c) in 3.0 mol L⁻¹ KOH solution at different current densities. (d) Plots of specific capacitance per gram and per area versus current density for the as-prepared Co₃O₄ samples.

In order to better evaluation the electrochemical properties of the as-prepared Co₃O₄ materials, we prepared the electrode using the conventional Co₃O₄/carbon/PTFE technology. Typically, 85 wt% Co₃O₄ powder (scratched from nickel foam), 10 wt% acetylene black as a conducting agent and 5 wt% polytetrafluorethylene (PTFE) as a binder were homogeneously mixed and pressed into a nickel foam (1 cm × 1 cm) current collector under a pressure of 8 MPa. The mass loading of each electrode is about 6 mg cm⁻². Their supercapacitance behaviors were tested through CV and galvanostatic charge–discharge methods in 3.0 mol L⁻¹ KOH aqueous electrolyte using the conventional three-electrode. The results are shown in Fig. S2–S4.† Cleary, the electrochemical performance of the three samples is far below that of direct growth on the Ni foam, which may cause by these nanostructures, can improve the contact between electroactive materials and current collectors which facilitates better kinetics. However, among these three samples, EW-Co₃O₄ sample shows the highest performance, which is caused by EW-Co₃O₄ sample have a large specific surface area, further will benefit for enhancing the kinetics of ion and electron transport.

Cycling stability or cycling life is an important requirement for the as-prepared electrode materials in the practical supercapacitor applications. The cyclic performance of the EW-Co₃O₄ sample examined by galvanostatic charge–discharge tests at a current density of 10 A g⁻¹ for 1000 cycles is depicted in Fig. 7. The coulombic efficiency is also shown in Fig. 7, which was calculated based on the following equation:

η = t_d/t_c × 100 (8)

In this equation, t_c and t_d represent the time of charge and discharge, respectively. It is apparent that the specific capacitance decreased obviously in the first 200 with cycles. In order to explain the initial capacitance decay, EIS analysis is performed and the result is displayed in the insert of Fig. 7. All the impedance spectra were composed of one semicircle component at high-frequency followed by a linear component at the low-frequency. Obviously, the EW-Co₃O₄ electrode before cycles exhibits lower transfer resistance and closer straight line to 90°, demonstrating the lower diffusion resistance of ions and improved electrolyte ions diffusion.³⁴ After 1000 cycles, the specific capacitance remains 1225.7 F g⁻¹ with capacitance retention of 74%. The coulombic efficiency remains above 99% within 1000 cycles.

Fig. 7 The discharge specific capacitance and the coulombic efficiency of EW-Co₃O₄ sample for 1000 cycles at 10 A g⁻¹ in 3.0 mol L⁻¹ KOH solution. The insert is the EIS date of EW-Co₃O₄ sample before and after 1000 cycles.

As presented in Table 1, we have summarized the rate performance of recently reported Co₃O₄ based materials with our EW-Co₃O₄ electrode. Clearly, this is a rather high value compared to literature results (Table 1).^{15,18,19,21,31,35–39} The enhanced performance of the EW-Co₃O₄ electrode can be ascribed to the follow reasons: (1) the electro-active materials were directly grown onto nickel foam without any conducting additive and binder, therefore, the utilization of the active material is high. (2) The EW-Co₃O₄ sample provided a large specific surface area with many holes, benefitting from enhancing the kinetics of ion and electron transport and also shortening the diffusion path of ions and electrons.

Table 1 Comparison of the specific capacity at different current densities for recently reported Co₃O₄ materials

Current density A g^−1	1	2	5	10	20	Cycle	Method	Electrode	Reference
a Specific capacitance per gram.
Co3O4 nanowire array	—	323^a	—	290	272	Not given	Hydrothermal	Grown on Ni foam	31
Hollow Co3O4 nanowire	—	295	—	271	258	82% returned after 7500 cycles at 10 A g^−1	Hydrothermal	Grown on Ni foam	35
Co3O4/C flower-like nanostructures	290.9	255.3	—	—	—	Not given	Solvothermal	Powder + acetylene black + PTFE	18
Co3O4/rGO/CNTs Hybrid	—	364	330	—	—	Not given	Hydrothermal	Grown on rGO/CNTs hybrid paper	36
Porous Co3O4 nanoflake array	369	351	—	264	242	97.4% returned after 4000 cycles at 2 A g^−1	Hydrothermal	Grown on Ni foam	15
Co3O4-CNFs	552	524	—	—	—	99% returned after 2000 cycles at 4 A g^−1	Electrospinning	Powder + acetylene black + PVDF	37
Co3O4 hollow microspheres	—	394.4	386	360	319	92% returned after 500 cycles at 2 A g^−1	Annealing	Powder + acetylene black + PVDF	19
Brush-like Co3O4 nanowires	1525	—	—	—	—	94% returned after 5000 cycles at 1 A g^−1	Solvothermal	Grown on carbon fiber paper	38
Co3O4 nanofibers	340	—	315	296	—	94% returned after 1000 cycles at 1 A g^−1	Electrospinning	Powder + acetylene black + PVDF	21
Reduced Co3O4 NWs	—	978	770	484	—	90% returned after 2000 cycles at 2 A g^−1	Solution reduction	Grown on Ni foam	39
EW-Co3O4	2509	—	2004	1754	1221	74% returned after 1000 cycles at 10 A g^−1	Solvothermal	Grown on Ni foam	This work

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Conclusions

In summary, Co₃O₄ nanostructure with different morphologies has been easily obtained through a simple hydrothermal method with changing the solvents composition. The electrochemical performance of the as-prepared Co₃O₄ samples has been done without adding any binder or carbon additive. The specific capacitance of EW-Co₃O₄ electrode prepared in C₂H₅OH/H₂O volume ratios 1/1 is 2509.4 F g⁻¹ at 1 A g⁻¹ and 1754 F g⁻¹ at 10 A g⁻¹, which is much larger than that on the Co₃O₄ nanowire arrays prepared in the pure water and pure ethanol solvent, as well as a rather high value compared to literature results. The enhanced performance is possibly due to its unique structure that could greatly raise the utilization efficiency of active materials and guarantee a superior property for electrolyte diffusion. The as-prepared honeycomb-like Co₃O₄ electrode has a promising future for electrochemical supercapacitors application.

Acknowledgements

We gratefully acknowledge the financial support of this research by National Nature Science Foundation of China (21403044), the Heilongjiang Postdoctoral Fund (LBH-Z14054) and Fundamental Research Funds for the Central Universities (HEUCF20151004).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterizations. See DOI: 10.1039/c4ra17075j

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