Shape-controlled synthesis of MnCO₃ nanostructures and their applications in supercapacitors

Abstract

MnCO₃ nanospheres and nanocubes have been successfully synthesized by a facile precipitation method utilizing ethylene glycol. When these MnCO₃ samples were employed as electrodes in the pseudocapacitor with an aqueous NaClO₄ electrolyte, they exhibited excellent supercapacitor characteristics. The MnCO₃ nanospheres showed a specific capacitance of 129 F g⁻¹ at 0.15 A g⁻¹, while the nanocube electrode had a specific capacitance of 91 F g⁻¹ at the same current density. The as-prepared MnCO₃ electrode materials had a high retention of 87% and 92% after 1000 cycles at 0.3 A g⁻¹, indicating a high-rate electrochemical cycling life. This study has highlighted the promising prospects of MnCO₃ as a novel electrode material used for supercapacitors.

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1. Introduction

Supercapacitors (SCs), refer to a family of electrochemical capacitors which bridge the gap between conventional capacitors and rechargeable batteries.^1–3 They have many advantages compared with traditional capacitors, such as high specific power, high available capacitance values per unit volume, long cycle life which is adaptable to a wide range of temperatures, high electrical efficiency and environmental protection.^4–6 As a result, SCs could be potentially applied in various fields such as portable electronic devices and hybrid electric vehicles.^7–9 SCs can be classified into two categories, electric double-layer capacitors (EDLCs) and pseudocapacitors.¹⁰ The EDLCs accumulate charges at the electrode/electrolyte interface,^11,12 while fast and reversible faradic processes would take place in the pseudocapacitors.¹³ Because of highly reversible faradic reaction, the pseudocapacitors have higher specific capacitance and energy density than EDLCs with many potential applications.^14–16 Thus, novel electrode materials are often pursued in the study of high-performance pseudocapacitors.

Ruthenium oxide (RuO₂) was considered to be one of the most advantageous candidates as the electrode material for SCs due to its high capacitance, reversible charge–discharge feature as well as superior electrical conductivity.^17–19 However, the high cost of ruthenium, the low porosity and toxic nature of the RuO₂ limit its popularity.^20–22 Recently, much effort has been focused on transitional metal compounds containing nickel, cobalt and manganese with different morphologies. Mai et al. have synthesized MnMoO₄/CoMoO₄ heterostructured nanowires with good electrochemical properties.²³ Qu and co-workers designed and synthesized a novel three-dimensional frame architecture of cobalt oxide nanostructure, which exhibits high comprehensive electrochemical performance.²⁴ Hercule et al. created interconnected nanorods-nanoflakes Li₂Co₂(MoO₄)₃ structure serves as electroactive material for SCs and it delivers a high rate capability and long cycle stability.²⁵ These transitional metal compounds with high specific surface area have superior electrochemical performance, low cost and non-toxic characters had been widely used as electrode materials for SCs.^23–31 Although some transitional metal oxides, hydroxides and even sulphides are being used as electrode materials for supercapacitors, only a few products with metal carbonates are reported.^32,33 This work explores the feasibility of using manganese carbonate as a new kind of promising raw materials for supercapacitors, since it is environmental friendly, and can be easily fabricated with excellent capacitance performance.

Here we presented a facile co-precipitation method assisted with ethylene glycol for synthesizing shape-controlled MnCO₃ nanostructures.^34,35 Electrochemical results indicated that the specific capacitance of MnCO₃ nanospheres (NSs) and nanocubes (NCs) are 129 F g⁻¹ and 91 F g⁻¹ at the current density of 0.15 A g⁻¹, respectively. They also exhibited excellent charge–discharge cycling stability. Only 13% and 8% of the values of the original specific capacitance were lost after 1000 cycles at current density of 0.3 A g⁻¹ for the MnCO₃-NSs and NCs, respectively.

2. Experimental

Manganese sulfate monohydrate (MnSO₄·H₂O; from Tianjin Hengxing Chemical Reagent Co. Ltd), and sodium bicarbonate (NaHCO₃; from Tianjin Hengxing Chemical Reagent Co. Ltd) were received and directly used. 0.015 M of MnSO₄ solution in a mixture of water and ethylene glycol (EG) were under vigorous agitation before 0.15 M of NaHCO₃ solution was added. The MnCO₃-NSs and MnCO₃-NCs were then obtained by adjusting the concentrations of water and EG with different volume ratios of 30% and 50% v/v, respectively. The mixed solution of MnSO₄ and NaHCO₃ was stirred for 4 hours at room temperature to get the turbid liquid. The obtained precipitates were filtered and washed with deionized water and ethanol before drying at 60 °C for 12 hours.

Structure, composition and morphology of the as-synthesized samples were examined by X-ray diffraction (XRD) using a Rigaku (RU300) diffractometer with Cu Kα radiation (λ = 0.1540598 nm); scanning electron microscopy (SEM) using a Quanta F400 FE-SEM at an accelerating voltage of 20 kV; electron transmission microscopy using a Tecnai 20 FEG TEM equipped with an energy-dispersive X-ray (EDX) spectrometer at 120 kV. The Brunauer–Emmmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analyses were also conducted by using the Gemini VII 2390.

The electrochemical characterizations of the as-prepared electrode materials included cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), cycling stability and electrochemical impedance spectroscopy (EIS). All of these measurements were carried out at room temperature by using a CHI760 electrochemical working station with a three-electrode system. During the test, an 0.1 M NaClO₄ aqueous electrolyte solution was used. A platinum foil served as counter electrode, while a saturated calomel electrode (SCE) served as the reference electrode. The working electrode material was prepared by mixing the as-synthesized MnCO₃, carbon black and PVDF (as binder) in a weight ratio of 70 : 20 : 10. The prepared electrode material was pasted directly onto a nickel foam current collector (1.0 cm × 1.0 cm). The formed collector was pressed under 10 MPa and dried at 60 °C under vacuum for 12 hours.

3. Results and discussion

3.1 Characterization

Fig. 1 shows the XRD patterns of as-synthesized samples (MnCO₃-NSs and MnCO₃-NCs). All the diffraction peaks are indexed to the phase of MnCO₃ (JCPDS card no. 44-1472) and no irrelevant peaks are detected in the XRD pattern which confirms MnCO₃ nanostructures are pure.

Fig. 1 XRD patterns of the as-synthesized samples (MnCO₃-NSs and MnCO₃-NCs prepared with 30% and 50% of EG, respectively).

The SEM images of MnCO₃-NSs and MnCO₃-NCs structures are shown in Fig. 2(a–d). We can find that the well monodispersed spheres in Fig. 2(a and b) and cubes in Fig. 2(c and d) under different magnifications. The diameter of as-prepared MnCO₃-NSs is ranging from 600 to 800 nm. While the MnCO₃-NCs are smaller, ranging from 500 to 600 nm in size. From the high magnification images Fig. 2(b and d), we observed that the MnCO₃-NSs and NCs structures were formed by the piling up of layers nanoflakes, which indicating that the nanostructures apparently formed by depositing of the smaller aggregates onto larger ones.³⁶ Our synthetic process also provides a possible way to tune the particle morphology. It is reported that anions of the reactants obviously play an important role in determining the final morphology of the carbonates products.^37,38 For instance, the existing SO₄²⁻ ions in solution tend to facilitate the formation of spherical structure.³⁹ On the other hand, the ethylene glycol organic additive has the suitable co-ordination capability to interact with the metal ions and changes their activity in the solution.⁴⁰ As a result, the morphology of MnCO₃ nanostructures can be controlled by adding different anions and ethylene glycol organic additive.

Fig. 2 (a and c) Low- and (b and d) high-magnification SEM images of MnCO₃-NSs and MnCO₃-NCs samples.

In order to illustrate the morphology and architecture of MnCO₃ in detail, typical TEM images of MnCO₃-NSs and MnCO₃-NCs are obtained and shown in Fig. 3(a–d). In detail, Fig. 3(a and b) and (c and d) show the sphere and cube which consists of numerous interconnected nanoflakes, which forming the continuous monodisperse structures. Moreover, the HR-TEM images of the NSs (insert of Fig. 3(b)) and for the NCs (insert of Fig. 3(d)) reveal their crystalline character of the as-prepared MnCO₃ phase. The well defined fringes can be seen on both NSs and NCs samples; and the interplanar spacing is about 0.29 nm, which corresponds to the (104) plane of the MnCO₃.

Fig. 3 TEM images of MnCO₃-NSs (a and b) and MnCO₃-NCs (c and d), respectively. The insert of (b and d) are the high resolution TEM images of the two samples.

The nitrogen adsorption/desorption isotherms and the BJH pore size distribution curves of MnCO₃-NSs and MnCO₃-NCs are shown in Fig. 4. According to the IUPAC classification scheme, it is evident that the obtained isotherms belong to type IV. The specific surface area of MnCO₃-NSs is 28 m² g⁻¹, which is higher than that of the MnCO₃-NCs with 21 m² g⁻¹. The nitrogen sorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distributions of these samples are ranged from 2 to 200 nm.

Fig. 4 Nitrogen adsorption/desorption isotherms of MnCO₃-NSs and MnCO₃-NCs. Insert are the corresponding pore size distribution curves of the MnCO₃-NSs and MnCO₃-NCs.

3.2 Electrochemical measurements

A three-electrode system was used to characterized the electrochemical properties of the as-synthesized MnCO₃-NSs and MnCO₃-NCs in their application as electrode materials in the 0.1 M NaClO₄ aqueous electrolyte. Fig. 5(a and b) shown the cyclic voltammetry (CV) results of the as-prepared MnCO₃-NSs and NCs electrodes in the voltage potential between 0 and 0.9 V under various sweep rates from 2 to 20 mV s⁻¹. It was obvious that the CV curves showed rectangular-like shapes under different scan rates, indicating good charge propagation within the electrodes and exhibiting characteristics for supercapacitors with excellent capacitance behavior. Typically, CV curves in Fig. 5(a and b) for both MnCO₃-NSs and MnCO₃-NCs tended to deviate from ideal rectangular shape at high scan rate of 20 mV s⁻¹ due to the sluggish ion incorporation into the electrode material. The CV measurements were also performed in a 6 M KOH aqueous electrolyte. The results are shown in Fig. 5(c) and (d), where the curves exhibit pairs of pseudo redox peaks under different scan rates from 2 to 20 mV s⁻¹. The results are consistent with some previous result reported.³²

Fig. 5 (a and b) CV curves of MnCO₃-NSs and MnCO₃-NCs electrodes in 0.1 M NaClO₄ aqueous electrolyte under different scan rates at voltage potentials between 0 and 0.9 V, (c and d) CV curves of MnCO₃-NSs and MnCO₃-NCs electrodes in 6 M KOH aqueous electrolyte under different scan rates, (e and f) charge/discharge curves of NSs and NCs in 0.1 M NaClO₄ aqueous electrolyte, (g) rate performance of the two types of electrodes, (h) cycling performance of electrodes at the current density of 0.3 A g⁻¹.

To further evaluate the electrochemical properties of the MnCO₃-NSs and MnCO₃-NCs, we had measured the galvanostatic charge/discharge behaviors for both kinds of electrodes in 0.1 M NaClO₄ aqueous electrolyte, which are shown in Fig. 5(e) and (f). It was noted that the discharge curves for both the MnCO₃-NSs and NCs samples could be divided into three regions. In the first region, region I, due to the internal resistance of electrode material, there exhibited a sudden drop in current at the start of the discharge. In region II, a linear variation of the time dependence of the potential indicated the double-layer capacitance behavior, which was probably caused by the charge separation which was taking place between the electrode and electrolyte interface. In region III, a slope variation of the time dependence of the potential indicated a typical pseudo-capacitance behavior, which was a result of the electrochemical adsorption/desorption or redox reaction taking place at the interface between electrode and electrolyte.⁴¹ The specific capacitance value of the MnCO₃ nanostructure could be calculated by:

where C_m is the specific capacitance, I is the constant charge/discharge current, Δt is the discharge time, m is the total mass of electrode material and ΔV is the potential difference of the electrode. In our samples, the specific capacitance contribution from the nickel foam current collector was ignored. It was justifiable because its capacitance was determined to be 0.1 F g⁻¹ under the current density of 0.1 A g⁻¹ in a 0.1 M NaClO₄ aqueous electrolyte, too small to be effective. Fig. 5(g) shows the specific capacitance values of MnCO₃-NSs and MnCO₃-NCs obtained under different charge/discharge current density from 0.15 A g⁻¹ to 2 A g⁻¹. At the low current density 0.15 A g⁻¹, the specific capacitance of the as-synthesized MnCO₃-NSs electrode exhibited the specific capacitance approximating to 129 F g⁻¹, which was higher than that previously reported.³² As shown in Fig. 5(g), the specific capacitance of MnCO₃-NSs electrode was larger than that of the MnCO₃-NCs electrode. The reason for such difference was owing to the diverse specific surface areas of these two kinds of electrodes, which could be evaluated and confirmed by the BET results in Fig. 4. The BET surface area of MnCO₃-NSs was found to be 28 m² g⁻¹ which was larger than those in MnCO₃-NCs having the value of 21 m² g⁻¹.

In order to estimate the stability of the as-synthesized MnCO₃ electrodes, we had also measured the cycling performance of the electrode materials in 0.1 M NaClO₄ aqueous electrolyte at a current density of 0.3 A g⁻¹ and the results were revealed in Fig. 5(h). After 1000 cycles, the capacitance of the MnCO₃-NSs remained at 87% and that of MnCO₃-NCs electrode remained at 92% of the original value. This demonstrated an excellent long-term stability of the as-synthesized materials as the electrodes for supercapacitors.

The electrochemical performances of these two kinds of electrodes were further confirmed by the electrochemical impedance spectroscopy (EIS) measurements performed in a 0.1 M NaClO₄ aqueous electrolyte over the frequency range 0.01–100 000 Hz. The results are shown on Fig. 6. The EIS data were analyzed using the Nyquist plots. For both spectroscopies, a semicircle in the high-frequency region and a straight line at low frequency region could be observed. An equivalent circuit consisting of a bulk solution resistance R_s, a charge-transfer R_ct, a pseudocapacitive element C_p from the redox process, and a constant phase element (CPE) to account for the double-layer capacitance figure was inserted in Fig. 6.²⁴ The initial non-zero intercept in high frequency region at the beginning of the semicircle was identical in both the curves which was resulted from the bulk solution resistance R_s, while the semicircle represented the charge transport resistance R_ct. After calculated by the Zview software, the solution resistance R_s of these two kinds of electrodes (MnCO₃-NSs and NCs) was determined to be 16.86 Ω and 21.6 Ω, while the charge-transfer resistance R_ct of the MnCO₃-NSs was 131.8 Ω, which was smaller than 216.6 Ω of the MnCO₃-NCs. Evidently, this also indicated that the lower resistance of MnCO₃-NSs resulted in a better electrochemical performance. It was also mentioned that, owing to the relatively large charge-transfer resistances of both MnCO₃-NSs and MnCO₃-NCs compared with other kinds of electrode materials, the specific capacitances of MnCO₃-NSs and MnCO₃-NCs would not be as good under high current density situation.^42,43

Fig. 6 The electrochemical impedance spectra (EIS) of the electrodes at room temperature; the insert shows the equivalent circuit for the electrochemical impedance spectra.

4. Conclusions

In summary, we have successfully designed and synthesized the monodispersed MnCO₃ nanosphere and nanocube samples by a facile co-precipitation method. The structure and morphology of the as-synthesized material were characterized by XRD, SEM, TEM and BET measurements. The electrochemical characterizations of MnCO₃ nanostructures, including cyclic voltammetry, galvanostatic charge/discharge, electrochemical impedance spectroscopy and cycling stability were measured by a three-electrodes system. The MnCO₃-NSs and NCs nanostructures exhibited well supercapacitor characteristics, relatively high specific capacitance and excellent long-term stability. The as-obtained MnCO₃ nanomaterials are expected to be one of the promising electrode materials for supercapacitors.

Acknowledgements

This work was supported by the HKSAR Government RGC-GRF Grant (CUHK14303914) and by the Direct Grant (Project Code: 4053076) from the Faculty of Science, The Chinese University of Hong Kong.

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