Synthesis of porous Cu_7.2S₄ sub-microspheres by an ion exchange method for high-performance supercapacitors

Abstract

The porous Cu_7.2S₄ sub-microspheres were synthesized by ion exchange reaction using Cu₂O as a precursor. The morphology and structure of the obtained samples were characterized by XRD, SEM, and TEM analysis in detail and the formation mechanism was studied. The pseudo-capacitive properties were evaluated by CV and galvanostatic charge–discharge tests in 6 M KOH solution. The as-prepared porous Cu_7.2S₄ sub-microspheres electrode has a specific capacitance of 491.5 F g⁻¹ at 1 A g⁻¹ and exhibits good electrochemical performance as an electrode material for supercapacitors.

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1. Introduction

The fossil fuel crisis and environmental pollution have attracted significant interest towards developing green and sustainable energy storage devices. Supercapacitors (SCs), with high power densities (>10 kW kg⁻¹) and long cycle lives (>10⁶ cycles) represent a unique sort of energy storage device which can bridge the gap between batteries and conventional capacitors.^1,2 According to the energy-storage mechanisms, supercapacitors are divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors.^3–6 EDLCs assembled using carbon-based materials exhibit lower specific capacitance and inferior energy density. In contrast, pseudocapacitors can obviously enhance the specific capacitance and energy density by using interfacial reversible faradaic reactions to store energy.⁷

Electrode materials are the most important factor in determining the properties of pseudocapacitors. Many active materials such as metal oxides, hydroxides and conducting polymers have been widely applied in supercapacitors based on their pseudocapacitive properties.^8–10 More recently, transition metal sulfides, such as CuS, NiS, CoS₂ and MoS₂, have been considered as one of the most promising pseudocapacitor electrode materials because of their high electrochemical activity and low electronegativity.^11–14 Among them, copper sulfides exhibit many different stoichiometric composition in a wide range from Cu₂S at the copper-rich side to CuS₂ at the copper-deficient side, including chalcocite (Cu₂S), djurleite (Cu_1.96S), digenite (Cu_1.8S), anilite (Cu_1.75S), geerite (Cu_1.6S), spionkopite (Cu_1.39S), covellite (CuS) and villamaninite (CuS₂).¹⁵ Up to now, various morphologies of copper sulfides including stick-like hierarchical structure, nanoflake, nanowire, nanoplatelet and nanosheet have been successfully fabricated by different synthetic methods.^16–21 However, it remains a challenge to develop simple methods to synthesize copper sulfides with designed chemical composition, novel structures, and controlled morphologies, for use as the electrode materials of supercapacitors.

Here, we demonstrate that porous Cu_7.2S₄ sub-microspheres can be prepared via a simple hydrothermal ion exchange route by using Cu₂O sub-microspheres as precursor. The capacitive properties of Cu_7.2S₄ sub-microspheres were investigated in detail. The measurement results show the sample exhibits outstanding electrochemical properties, offering great potential as a high-performance supercapacitor electrode material.

2. Experimental

2.1 Chemical and synthesis

All the reagents were analytical grade and used without any further purification. The Cu₂O sub-microspheres were prepared by a reported method with slight modification.²² In a typical process, 250 mg of Cu(CH₃COO)₂·H₂O and 1.4 g of β-cyclodextrin (β-CD) were dissolved in 120 mL distilled water under magnetic stirring. Then 100 mg of L-ascorbic acid was added to the above solution with constant stirring. Afterward, the mixture was rapidly exposed to ultrasonic irradiation for 5 min (JY98-IIIDN ultrasonic cell crusher instrument, 20 kHz, 100 W cm⁻²). The yellow precipitates were filtered, washed with distilled water and absolute alcohol several times and then dried in 60 °C oven for 12 h.

The porous Cu_7.2S₄ sub-microspheres were prepared through a hydrothermal method. In a typical process, 240 mg of Cu₂O and 4 mmol thiourea were added in 40 mL distilled water under stirring. The homogeneous suspension was transferred to and sealed in a Teflon-line stainless steel autoclave of 50 mL capacity. The autoclave was heated to 150 °C and maintained at this temperature for 10 h. After cooling, the black products were filtered, washed with distilled water and absolute alcohol several times and then dried at 60 °C for 12 h.

2.2 Characterization of materials

The crystal structures and morphologies of the products were characterized by X-ray diffraction (XRD, Rigaku/Max-2550 with Cu Kα radiation), scanning electron microscopy (SEM, JEOL JSM-6330F) and transmission electron microscopy (TEM) (JEOL 2010, 200 kV). Fourier transform infrared spectroscopy (FTIR) analysis was carried out by using a Nicolet MAGNA-IR 750 FTIR spectrometer at ambient conditions. The nitrogen adsorption–desorption isotherm was determined using the Brunauer–Emmett–Teller (BET) equation by a surface area analyser SSA-4200 (Builder).

2.3 Electrochemical measurements

The electrochemical measurements were all conducted on a CHI 660E electrochemical workstation (Shanghai, Chenhua). The electrochemical properties were investigated in a three-electrode system at ambient temperature, with 6 M KOH as the electrolyte. The working electrodes were fabricated by mixing the active material (porous Cu_7.2S₄ sub-microspheres), acetylene black and polytetrafluoroethylene (PTFE) in a weight ratio of 80 : 10 : 10. The formed paste was pressed at 7 MPa to a piece of nickel foam (1.0 cm × 1.0 cm), and dried in 80 °C for 12 h. A platinum plate and Hg/HgO were used as the counter electrode and the reference electrode, respectively.

3. Results and discussion

3.1 Structure and morphology of the Cu₂O templates and Cu_7.2S₄ samples

The XRD patterns of the Cu₂O precursor and Cu_7.2S₄ samples are shown in Fig. 1. All diffraction peaks shown in Fig. 1 can be readily indexed to the cubic phase of Cu₂O (Fig. 1A) and Cu_7.2S₄ (digenite, Fig. 1B), which are in good agreement with the standard diffraction data (JCODS card file no. 78-2076 for Cu₂O and 24-0061 for Cu_7.2S₄). Besides, no peak from Cu₂O crystallized phases can be detected in Fig. 1B, indicating that all the Cu₂O precursors have been transferred to the pure Cu_7.2S₄. The pattern inset in Fig. 1B gives the crystal structure of Cu_7.2S₄. The digenite Cu_7.2S₄ is a p-type semiconductor due to the presence of Cu vacancies in the lattice and the band gap value is about 1.5 eV.¹⁵

Fig. 1 XRD patterns of Cu₂O (A) precursors and Cu_7.2S₄ (B).

The morphology and microstructure of the Cu₂O precursors and Cu_7.2S₄ samples were characterized by SEM and TEM. The panoramic SEM image in Fig. 2A indicates that the Cu₂O precursors are mainly composed of sub-microspheres with uniform diameter (about 800 nm). The high-magnification SEM image (Fig. 2B) and TEM image (Fig. 2C) show that the sub-microspheres are porous and composed of many irregular nanoparticles. Using the above Cu₂O sub-microspheres as the precursors, the Cu_7.2S₄ samples have been successfully synthesized via a hydrothermal sulfur process. Fig. 2D and E reveal the representative SEM images of the obtained Cu_7.2S₄ samples. It can be seen that the samples are more porous and very similar to the Cu₂O templates in morphology and size. The TEM image (Fig. 2F) further demonstrates pores were distributed on both the surface and the interior of Cu_7.2S₄ sub-microspheres.

Fig. 2 SEM and TEM images of Cu₂O precursor (A–C) and porous Cu_7.2S₄ sub-microspheres (D–F).

3.2 Formation mechanism of the Cu_7.2S₄ samples

β-Cyclodextrin (β-CD) served as a capping agent in the formation of porous Cu₂O sub-microspheres. β-CD is a torus-shaped cyclic oligosaccharide consisting of seven 1,4-linked α-D-glucose units.²³ Fig. 3 is the chemical structure of β-CD. The hydroxyls on β-CD molecules would strongly coordinate with metal ions such as Cu²⁺ ions to form the [Cu(β-CD)_n]²⁺complex.²⁴ Then, the [Cu(β-CD)_n]²⁺ was reduced to Cu₂O by L-ascorbic acid under the condition of high-intensity ultrasound irradiation and the strong interaction between Cu²⁺ ions and the hydroxyl to allow β-CD to attach to Cu₂O crystals to form [Cu₂O(β-CD)_n] complex.^24,25 Fig. 4 gives the FTIR spectra of the products prepared at 150 °C for 10 h (Fig. 4A) and the pure β-CD (Fig. 4B), respectively. By comparing the FTIR spectra, it can be found that β-CD still existed in the final products after hydrothermal treatment and not decomposed. This result also confirmed that there was a strong interaction between Cu²⁺ ions and the hydroxyl to allow β-CD to attach to final Cu_7.2S₄ sub-microspheres. Thiourea is a common and well sulfur source for the formation of metal sulfide. Under hydrothermal treatment, the thiourea added in the solution can be decomposed and released sulfur species to react with [Cu₂O(β-CD)_n] complex to form Cu_7.2S₄. The possible reaction process can be described as follows:

Cu²⁺ + nβ-CD → [Cu(β-CD)_n]²⁺ (1)

[Cu(β-CD)_n]²⁺ + L-ascorbic acid → [Cu₂O(β-CD)_n] (2)

NH₂CSNH₂ + 2H₂O → 2NH₃ + H₂S + CO₂ (3)

[Cu₂O(β-CD)_n] + H₂S → Cu_7.2S₄(β-CD)_n + H₂O (4)

Fig. 3 The chemical structure of β-cyclodextrin.

Fig. 4 FTIR of (A) the products prepared at 150 °C for 10 h and (B) pure β-CD.

The morphology and phase transformation could be described by Scheme 1. The formation of Cu_7.2S₄ form Cu₂O is an ion exchange process. The diffusion effect during this process can be employed to create interior voids and prepare materials with porous structures (Fig. 2F).²⁶

Scheme 1 Growth mechanism for the Cu_7.2S₄ sub-microspheres.

The porous and pore structures can increase the electrode/electrolyte contact area and provide more electroactive sites, while the abundant porosity can offer numerous channels for fast transport of ions and electrons and alleviate the volume change during the charge–discharge process.²⁷ Therefore, the obtained porous Cu_7.2S₄ sub-microsphere may be a potential material for supercapacitors. The N₂ adsorption and desorption isotherms for the porous Cu_7.2S₄ samples are provided in Fig. 5, with the specific surface area 19.7 m² g⁻¹. Moreover, the corresponding pore size distribution (the inset of Fig. 5) shows a peak pore diameter of 90 nm with pores up to 110 nm in size.

Fig. 5 N₂ adsorption–desorption isotherm curves obtained by BET analysis, and the curves in the insets depict the corresponding pore size distribution based on the BJH method during desorption.

3.3 Electrochemical performance of the Cu_7.2S₄ sub-microspheres

The electrochemical properties of the supercapacitor electrodes based on Cu_7.2S₄ samples are investigated by means of CV, galvanostatic charge–discharge (GCD) and EIS techniques in a three-electrode system with 6 M KOH aqueous solution as electrolyte. CV is considered to be an ideal technique to investigate the capacitive behavior of electrode materials. Fig. 6A shows CV curves of Cu_7.2S₄ electrode at various scan rates ranging from 2 to 50 mV s⁻¹. The shape of the CV curves clearly exhibit one pair of well-defined redox peaks within 0–0.6 V (vs. Hg/HgO) and almost symmetric, indicating good reversibility of oxidation and reduction processes. This typical pseudocapacitive behavior caused by electrochemical reactions is obviously distinct from the electric double layer capacitance characterized by nearly rectangular CV curves. The redox peaks corresponding to the reversible redox reaction could be expressed as:²⁸

Cu_7.2S₄ + OH⁻ ↔ Cu_7.2S₄OH + e⁻ (5)

Fig. 6 (A) CV curves of porous Cu_7.2S₄ sub-microspheres at different scan rate. (B) GCD curves of porous Cu_7.2S₄ sub-microspheres at different current density. (C) Cyclic behavior for Cu_7.2S₄ at 3 A g⁻¹ (D) EIS spectra of prepared products before and after cycle tests.

The specific capacitances are calculated from the GCD curves at various current densities according to the following equation:

where C (F g⁻¹) is the specific capacitance, I (A) represents the discharge current, and m (g), Δt (s), and ΔV (V) are mass of active materials, total discharge time and potential drop during discharge, respectively. The specific capacitance at various current densities is shown in Fig. 6B. The porous Cu_7.2S₄ sub-microspheres show a specific capacitance of 491.7 F g⁻¹ at 1 A g⁻¹ and 324.6 F g⁻¹ at 10 A g⁻¹. The highest specific capacitance of 491.7 F g⁻¹ obtained at the current density of 1 A g⁻¹ was higher than some previous works reported other copper sulfides of CuS nanowire arrays (305 F g⁻¹ at 0.6 mA cm⁻¹),¹⁹ CuS nanoplatelets (72.85 F g⁻¹ at 5 mV s⁻¹),²⁰ CuS nanoneedles/CNT (122 F g⁻¹ at 1.2 A g⁻¹),²⁹ CuS/polypyrrole composites (427 F g⁻¹ at 1 A g⁻¹)³⁰ and CuS hollow spheres (111.2 F g⁻¹ at 1 A g⁻¹).³¹ To get more information of the electrochemical stability of the porous Cu_7.2S₄ sub-microspheres electrode, consecutive GCD cycles were undertaken at a current density of 3 A g⁻¹ within a voltage range from 0 to 0.52 V (vs. Hg/HgO) in 6 M KOH electrolyte. The capacitance retention for 1000 cycles is shown in Fig. 6C. The specific capacitance was increased in the first 60 cycles, which may be due to the active materials have not been fully used at the initial stage and additional cycles were needed to fully activate Cu_7.2S₄ material.^31,32 Then it slowly decreased in the next 340 cycles and kept basically unchanged in the last 600 cycles, retaining 82% of the initial value at the 1000 cycles.

In order to investigate the detailed electrochemical characteristics of the Cu_7.2S₄ electrode before and after 1000 cycles, EIS measurements were carried out. The resultant Nyquist plots are shown in Fig. 6D. All electrodes were measured in the frequency range of 0.01 to 10⁵ Hz at open circuit potential with an AC perturbation of 5 mV. The Nyquist plots consist of a semicircle in the high frequency range and a relatively straight line in the low frequency region. The equivalent circuit proposed to fit the spectra is shown in the insets of Fig. 6D, in which R_s, R_ct, C_dl, W, and C_F represent bulk solution resistance, faradaic interfacial charge-transfer resistance, double layer capacitance, the interfacial diffusive resistance (Warburg impedance), and faradaic pseudocapacitance, respectively.³³ The R_s values can be obtained by high-frequency intercept on the real axis (Z′) and the semicircle diameter at high-frequency range is relevant to the charge-transfer resistances (R_ct) which charge transfer reaction at the interface of electrolyte (OH⁻)/Cu_7.2S₄ electrode.³⁴ It can be seen that the R_s values were similar before and after cycles. However, the R_ct values change from 3.2 Ω to 44.3 Ω before and after 1000 cycles, which may be caused by the collapse and agglomeration of active materials during the cycles. The linear part in the impedance plots at low frequencies correspond to the Warburg impedance (W), which is described as dependence of ion diffusion from the electrolyte to the electrode interface.³⁵ Both the straight lines in the EIS spectra inclined at an angle over 70° to the Z′-axis, indicated that porous Cu_7.2S₄ electrode is more advantageous to ion diffusion and the electrochemical capacitive behavior is not controlled by a diffusion process.³⁶

4. Conclusions

In summary, the porous Cu_7.2S₄ sub-microspheres have been successful prepared by a solution-based ion exchange reaction. The XRD, SEM, and TEM characterization indicate that the as-prepared Cu_7.2S₄ samples preserve the structure and morphology of the precursor Cu₂O. Further electrochemical measurements indicated that the porous Cu_7.2S₄ sub-microspheres electrode has a specific capacitance of 491.5 F g⁻¹ at 1 A g⁻¹ and good cycling stability, suggesting its potential for high-performance supercapacitor application.

Acknowledgements

The authors are grateful to the National High Technology Research and Development Program of China (No. 2009AA03Z319) and the Fundamental Research Funds for the Central Universities of China (No. DUT12LK04).

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