Synthesis of micro/nano-structured Mn₃O₄ for supercapacitor electrode with excellent rate performance

Abstract

Micro/nano-structured Mn₃O₄ with an open three-dimensional flower-like morphology were fabricated by a facile solvothermal approach using hexadecyltrimethylammonium bromide as a surfactant and CH₃CH₂OH as a solvent. The Mn₃O₄ microspheres were self-assembled by one-dimensional nanowires; in addition, a dandelion-structure formation mechanism of the Mn₃O₄ microspheres is discussed. The Mn₃O₄ microspheres used as a supercapacitor electrode in 1 mol L⁻¹ Na₂SO₄ electrolyte have a specific capacitance value of 286 F g⁻¹ at a low current density of 0.5 A g⁻¹, and can still retain 80% (230 F g⁻¹) and 73% specific capacitance (210 F g⁻¹) when the current densities are increased ten-fold (5 A g⁻¹) and twenty-fold (10 A g⁻¹), respectively. In addition, the capacitance retention is 90% after 1000 cycles at a current density of 5 A g⁻¹. In comparison with Mn₃O₄ synthesized in N,N-dimethylformamide solvent at 0.5 A g⁻¹ and 5 A g⁻¹, the specific capacitance obtained increased by 18.7% and 17.4%, respectively.

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1 Introduction

Due to their inexpensive cost, higher capacitance and longer cycle life, transition metal oxides such as Co(OH)₂,¹ Ni(OH)₂,² NiO,³ MnO₂,⁴ Mn₃O₄ ⁵ have attracted significant attention. Among these transition metal oxides, Mn₃O₄ is the most promising electrode material for commercial supercapacitors because of its environmental friendliness.^6,7 However, poor electronic conductivity limits its application in high-power electrochemical capacitors.^8–11 Though high surface areas and readily accessible mesopores can be achieved by embedding or depositing Mn₃O₄ nanoparticles on to a highly conductive porous matrix, such as graphene,^12–14 carbon¹⁵ or other materials,¹⁶ there are some shortcomings too, such as the synthesis complexity, the relatively low capacitance, and low stability. Wu et al.¹³ synthesized Mn₃O₄/graphene nano-composites by a simple solvothermal process, and they exhibited good rate capability, with a capacity of 116 F g⁻¹ at 5 A g⁻¹, as compared with a capacity of 22.5 F g⁻¹ at 5 A g⁻¹ for Mn₃O₄ without graphene, i.e. nearly a fivefold improvement. Although the high surface area can provide more electrochemically active sites for charge storage/delivery, and although the large tunable porosity is beneficial for ion transportation and electrolyte permeation,^17,18 the downsizing strategy and the porosity of the materials reduce the volumetric energy density,^19,20 which is an undesirable feature for the use of Mn₃O₄ as an electrode material in commercial applications.

In our previous work,²¹ porous nanostructured Mn₃O₄ particles with a size of about 10 nm were synthesized by a hydrothermal method by adding the surfactant hexadecyltrimethylammonium bromide (CTAB) in to N,N-dimethylformamide (DMF) solvent, and they exhibited a high specific capacitance of 232.5 F g⁻¹ at 0.5 A g⁻¹ and a good rate capability of 190 F g⁻¹ at 5 A g⁻¹. In consideration of the toxicity and the viscidity of DMF, DMF was replaced by CH₃CH₂OH in the solvothermal method. In the present investigation, the Mn₃O₄ electrode materials were synthesized by a solvothermal method using CTAB as a surfactant and CH₃CH₂OH as a solvent. The purpose of the present work is to attain a Mn₃O₄ electrode material with a high rate discharge-ability, and also to gain more insights into the formation mechanism of the micro/nano-structures.

2 Experimental

Mn₃O₄ microspheres were prepared by a solvothermal synthesis; the experimental procedures were as follows: (1) CTAB (0.364 g) was dissolved in 40 mL CH₃CH₂OH and stirred for 15 min; (2) Mn(CH₃COO)₂·4H₂O (2.45 g) was dissolved in 10 mL H₂O; (3) Mn(CH₃COO)₂ solution was added to the CTAB solution and stirred for 30 min; (4) carbamide (0.6 g) was then added and the solution was stirred for 30 min, followed by pouring into a 100 mL PTFE-lined stainless container, and subsequently heated at 140 °C for 4 h in a vacuum drying oven.

The phase constituents were determined on a Rigaku D/max 2500pc X-ray diffractometer (XRD). X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 250Xi. The morphologies of the synthesized materials were characterized by an S-4800 field emission scanning electron microscopic (FE-SEM). The microstructures of the samples mounted on a Cu grid were observed by a JEM-2100 at 200 kV.

All electrochemical measurements were performed in a three-electrode system. The Mn₃O₄ activated material was used as the working electrode, and was prepared by mixing the activated material with acetylene black and PVDF at a mass ratio of 7 : 2 : 1; the mass loading of each electrode was about 2 mg cm⁻². Pt electrode was used as the counter electrode, and a Hg/Hg₂Cl₂ electrode was used as the reference electrode in 1 mol L⁻¹ Na₂SO₄ electrolyte solution. The electrochemical performance was tested on a BTS-5V10 mA system in the range of 0–1 V at 0.5 A g⁻¹ and 5 A g⁻¹ current densities. The electrochemical impedance spectroscopy (EIS) was conducted on a CHI 660E electrochemical workstation with an oscillation voltage of 5 mV and the applied frequency ranged from 0.01 Hz to 100 kHz.

3 Results and discussion

3.1 Structure characterization

Fig. 1 is the XRD pattern of the prepared sample. It can be seen from Fig. 1 that there is only a single Mn₃O₄ phase with a space group I4₁/amd. Hence, it can be determined that the Mn₃O₄ microspheres used as the supercapacitor electrode present a body-centered tetragonal manganese oxide Mn₃O₄, with the lattice parameters a = 5.750 nm, b = 5.750 nm and c = 9.420 nm. According to Scherrer's equation, the calculated particles sizes of Mn₃O₄ from the half-width of the (211), (103), (112), and (101) diffraction peaks are 7.83 nm, 7.05 nm, 7.86 nm and 7.51 nm, respectively. So, it is thought that prepared Mn₃O₄ particles are nanoparticles with an average size of about 7.56 nm.

Fig. 1 XRD pattern of Mn₃O₄ electrode material.

Fig. 2a and b are the XPS spectra of the as-prepared Mn₃O₄ sample in a region of 0–1200 eV and in the Mn 2p region, respectively. As seen from Fig. 2a, no impurities are present except for the contaminant carbon. It can be seen from Fig. 2b that the binding energy values of Mn 2p_2/3 and Mn 2p_1/2 are 641.43 eV and 653.53 eV, respectively. The Mn 2p_2/3 (641.43 eV) value corresponds to Mn₃O₄.^13,22 In addition, it can be observed that the spin–orbit splitting width (ΔE) of the two peaks (i.e. Mn 2p_2/3 and Mn 2p_1/2) is 12.10 eV, which is in accordance with the spectrum of Mn₃O₄.^23,24

Fig. 2 XPS spectrum: (a) survey scan, (b) Mn 2p region of Mn₃O₄ sample.

3.2 Morphology characterization

Fig. 3a–d are the SEM micrographs of the Mn₃O₄ microspheres at different magnifications. It can be seen from Fig. 3a that most of the Mn₃O₄ consist of spherical particles with a flower-like morphology, and have a diameter of 3–5 μm. At higher magnifications, these flower-like microspheres with porous structures are composed of many petals with an average thickness of less than 50 nm, see Fig. 3b–d. Fig. 3e and f, show the interior structure of the prepared Mn₃O₄ microspheres, and it can be concluded that the micro/nano-structured Mn₃O₄ microspheres are self-assembled by nanoparticles.

Fig. 3 SEM micrographs of the Mn₃O₄ electrode material: (a) overall morphology of the products, (b–d) SEM micrographs of the microsphere at high magnifications, (e and f) the constituent detail in the microsphere.

Fig. 4 shows the TEM images of a single prepared Mn₃O₄ particle and its surface morphology. It can be seen from Fig. 4a that the Mn₃O₄ particle is a microsphere with a diameter of about 3–5 μm. As seen from Fig. 4b and in the inset in Fig. 4b, the surfaces of the Mn₃O₄ microsphere seem to be fabricated by one-dimensional nanowires, and the diameter of the nanowires is less than 10 nm. It can be deduced that one-dimensional nanowires first assemble into two-dimensional petals in the internal part of the materials, and then assemble into three-dimensional micro/nano-structures. When the micro/nano-structured Mn₃O₄ is used as a supercapacitor electrode, it can be expected to exhibit some advantages, such as: (1) nanoparticles are favorable for electron transport at the interface of the electrode and electrolyte; (2) the porous structures can maintain high electrolyte penetration/diffusion rates; and (3) the large volumetric energy density with the micro/nano-structure is useful for commercial applications.

Fig. 4 TEM images of the flower-like Mn₃O₄ microspheres: (a) overview of a single Mn₃O₄ microsphere, (b) high-magnification TEM image at the edge of a Mn₃O₄ microsphere with the inset revealing the one-dimensional nanowires and the porous detail of the microsphere.

3.3 Formation mechanism

Based on the surface morphology (Fig. 4) and the interior structure (Fig. 3e and f), the dandelion-structure formation mechanism of the Mn₃O₄ microsphere is discussed, as illustrated in Fig. 5. First, a CTAB solution is obtained utilizing CH₃CH₂OH as the solvent (CTAB, as a typical surfactant, is composed of a hydrophilic group and hydrophobic group); second, due to the hydrophilic nature of Mn(CH₃COO)₂, the hydrophilic group of CTAB is aggregated on Mn(CH₃COO)₂ and an O/W micelle is produced with the addition of Mn(CH₃COO)₂ solution. As the hydrophobic groups stay on the outside of the micelle, they attract each other, which results in the growth of micelles. The aggregation of these micelles subsides progressively with the addition of carbamide, which is used as a precipitant. Finally, Mn₃O₄ with a porous structure forms with the loss of the hydrophobic groups around 140 °C.

Fig. 5 Schematic illustration of the growth mechanism of Mn₃O₄ microspheres.

3.4 Electrochemical properties

Fig. 6a shows the cyclic voltammetry (CV) curves of the Mn₃O₄ microspheres at a scan rate of 5–100 mV s⁻¹ in the potential range of 0.0 V to 1.0 V. All the tests were conducted in 1 mol L⁻¹ Na₂SO₄ solution. It is evident that all the curves present a symmetrical shape, including at extremely high rates (100 mV s⁻¹). Compared with data from Mn₃O₄ synthesized in DMF solvent,²¹ the curve at the scan rate of 50 mV s⁻¹ has deviates from a symmetrical shape, suggesting that the Mn₃O₄ microspheres fabricated using CH₃CH₂OH as the solvent in the present work have much better capacitive behavior.

Fig. 6 Cyclic voltammograms (a) and charge–discharge curves (b) of Mn₃O₄ electrode.

Fig. 6b shows the galvanostatic charge–discharge curves of the Mn₃O₄ microspheres at the current density ranging from 0.5 A g⁻¹ to 10 A g⁻¹. It can be seen that the prepared Mn₃O₄ microspheres have a specific capacitance value of 286 F g⁻¹ at a current density of 0.5 A g⁻¹, and can still retain 80% specific capacitance (230 F g⁻¹) and 73% specific capacitance (210 F g⁻¹) when the current densities are increased tenfold (5 A g⁻¹) and twentyfold (10 A g⁻¹), respectively. In comparison with data from Mn₃O₄ synthesized in DMF solvent,²¹ the specific capacitance obtained in the present work increased by 18.7% and 17.4% at 0.5 A g⁻¹ and 5 A g⁻¹, respectively. In comparison with the values reported by Zhang et al.,⁵ the specific capacitance (286 F g⁻¹) increased by about 40% over that of the Mn₃O₄ electrode (205 F g⁻¹) at a low current density of 0.5 A g⁻¹, while the specific capacitance (230 F g⁻¹) increased by about 72% over that of the Mn₃O₄ electrode (134 F g⁻¹) at a high current density of 5 A g⁻¹.

The high rate performance of capacitance is one of the most important electrochemical parameters for the application of supercapacitors, especially for the electrode materials used in electric vehicles. Fig. 7a shows the relationship between the specific capacitance and the current density. It can be seen that the Mn₃O₄ electrode has an excellent high-rate performance, which is probably ascribed to the one-dimensional nanowire structure of the prepared Mn₃O₄ microspheres. The nanoparticles and the porous structure can provide a larger surface area and more Mn₃O₄/electrolyte contact areas.

Fig. 7 Discharge capacity of Mn₃O₄ electrode at different current densities (a) and cycling stability profile of Mn₃O₄ electrode at a current density of 5 A g⁻¹ (b).

The cycle stability test was carried out by the galvanostatic charge–discharge technique at a current density of 5 A g⁻¹. The specific capacitance as a function of the cycle number is presented in Fig. 7b. It can be seen from Fig. 7b that the capacitance retention is 90% after 1000 cycles. Although the capacitance retention decreases with cycle number, the coulombic efficiency on the charge–discharge cycle numbers almost remains constant at 97%. It is noticed that the Mn₃O₄ electrode prepared in CH₃CH₂OH solvent has a specific capacitance of 230 F g⁻¹ on the 12^th charge–discharge cycle, while the specific capacitance of the Mn₃O₄ synthesized in DMF solvent increases continuously for the first 600 cycles. It is clear that the Mn₃O₄ electrode material obtained in the present work has a better activated property than the Mn₃O₄ prepared in DMF. This suggests that the dandelion-structure self-assembled by one-dimensional nanowires can provide more active sites for redox reactions.

Fig. 8 presents the electrochemical impedance spectra (EIS) of the as-prepared Mn₃O₄ electrode materials. The EIS data are fitted by Zview software according to an equivalent circuit (see the inset in Fig. 8), and for details of the equivalent circuit, refer to the literature.²¹ The semicircle near the high-frequency region (see the inset in Fig. 8) corresponds to the charge transfer resistance R_ct at the interface of the electrode and electrolyte. The fitting result shows that the charge-transfer resistance R_ct is 2.43 Ω for the Mn₃O₄ electrode. The charge-transfer resistance (2.43 Ω) is decreased by about 4.1 times than that reported by Wu et al.,¹³ while the charge-transfer resistance (2.43 Ω) is increased by about 2.7 times than that of the Mn₃O₄ electrode (0.91 Ω) synthesized in DMF solvent, which has a porous structure with a large surface area.²¹ Thus, more work is needed to improve the porous structure of the Mn₃O₄ electrode materials in the future.

Fig. 8 Electrochemical impedance spectrum of Mn₃O₄ electrode.

4 Conclusions

Mn₃O₄ microspheres were prepared by a solvothermal synthesis. Micro/nano-structured Mn₃O₄ used as a supercapacitor electrode exhibits a relatively high specific capacitance of 230 F g⁻¹ and 210 F g⁻¹ at a current density of 5 A g⁻¹ and 10 A g⁻¹, which is about 80% and 73% of the specific capacitance of 286 F g⁻¹ at a current density of 0.5 A g⁻¹, respectively. The micro/nano-structured Mn₃O₄ used as a supercapacitor electrode exhibits a large interface of the electrode and electrolyte, high electrolyte penetration/diffusion rates and a large volumetric energy density.

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