Interface synthesis of mesoporous MnO2 and its electrochemical capacitive behaviors

doi:10.1016/j.jcis.2008.02.055

Journal of Colloid and Interface Science

Volume 322, Issue 2, 15 June 2008, Pages 545-550

https://doi.org/10.1016/j.jcis.2008.02.055 Get rights and content

Abstract

Mesoporous MnO₂ has been synthesized by means of a novel, facile, and template-free method by virtue of a soft interface between CCl₄ and H₂O without any surfactants or organometallic precursors or ligands. X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy analysis, scanning electron microscopy, and an ASAP2010 autoadsorption analyzer were applied to investigate the composition and microstructure of the as-synthesized MnO₂. The structure characterizations indicated a good mesoporous structure for as-prepared MnO₂ with an adsorption average pore diameter of 9.7 nm, mesoporous volume of 0.58 cm³ g⁻¹, and Brunauer–Emmett–Teller specific surface area of 239 m² g⁻¹. Electrochemical properties of the mesoporous MnO₂ were elucidated by cyclic voltammograms, galvanostatic charge–discharge, and electrochemical impedance spectroscopy in 1 M Na₂SO₄ electrolyte. Electrochemical data analysis demonstrated that as-synthesized MnO₂ had good capacitive behavior due to its unique mesoporous structure. A specific capacitance of ca. 220 F g⁻¹ could still be delivered for the mesoporous MnO₂ even at a scan rate of 100 mV s⁻¹.

Graphical abstract

Mesoporous MnO₂ with good electrochemical performance has been synthesized by means of a facile and template-free method by virtue of soft interface between CCl₄ and H₂O.

Introduction

Electrochemical capacitors (ECs) have been known for many years, but only recently are emerging as a kind of attractive energy-storage/conversion device for future use in hybrid electric vehicles in combination with rechargeable batteries or fuel cells [1]. Generally, ECs can be broadly classified into two categories: electrical double layer capacitors (EDLCs), which build up electrical charge at the electrode/electrolyte interface as described by the Gouy–Chapman–Stern–Grahame (GCSG) model, and pseudocapacitors, which utilize electrochemical redox reactions at the interfaces at certain potentials. Both rely on the physicochemical changes occurring at the electrode/electrolyte interface [1].

Tetravalent manganese oxide (MnO₂) is a most attractive candidate for EC electrode materials due to its environmentally benign nature, low cost, and favorable pseudocapacitive characteristics [2], [3], [4], [5], [6], [7]. Beyond these advantageous properties, MnO₂ is also very promising in a neutral electrolyte system [8], [9], [10]. Although this compound exhibits a pseudocapacitive behavior with specific capacitance (SC) as high as 698 F g⁻¹ for MnO₂ thin films [11], 100–200 F g⁻¹ is still delivered for powder-based MnO₂ electrodes [12], [13], [14], which are far from the theoretical value of ca. 1400 F g⁻¹ [15]. In addition, compared with RuO₂ with high SC values ranging from 720 to 760 F g⁻¹ [16], MnO₂ powder also exhibits a lower electrochemical SC. Therefore, it is significant and necessary to improve the capacitive performance of MnO₂ powder. It is well known that the electrochemical properties of nanostructured MnO₂ strongly depend on its dimensionality, powder morphology, crystalline structure, and bulk density [4]. So far, two mechanisms have been proposed for the charge-storage mechanism in MnO₂-based electrodes: (a) the redox process, as shown by Eq. (1), is mainly governed by the insertion and deinsertion of electrolyte cations C⁺ (Na⁺ and/or H⁺) from the electrolyte into the MnO₂ matrix [5]; (b) the mechanism is based on the surface adsorption of C⁺ on MnO₂ shown by Eq. (2) [10]:MnO₂ + C⁺ + e⁻ ⇌ MnOOC,(MnO₂)_surface + C⁺ + e⁻ ⇌ (MnO⁻₂ C⁺)_surface.

Regardless of which mechanism proposed here is more reasonable and acceptable, one thing is consistent: the SC of MnO₂ results from its surface reaction; that is, only a limited fraction of the MnO₂ is electrochemically active. The charge storage might involve only the surface atoms of the MnO₂ crystallites or a very thin layer. Therefore, the pseudocapacitance is critically dependent on the effective surface area associated with the pore-size distribution (PSD) and pore volume.

It has been well established that electroactive materials possessing a high effective specific surface area with a suitable PSD can enhance the charge-storage capacity for ECs [17], [18], because such electroactive materials with unique mesoporous structures are expected to lead to electrolyte ion diffusion more easily into porous electrode materials such that the electroactive surface reactions (including both as shown by Eqs. (1), (2)) occur as much as possible.

Thus, the potential and logical strategy is how to control the surface morphology and structure of the electroactive material. Currently, extension of the soft or/and hard templating procedure [2], [19], [20] to the formation of mesoporous MnO₂ with larger specific surface area and proper PSD has attracted great interest. However, the use of templates is undesirable as it increases the production cost and complexity. Therefore, an easily controlled method without the need for templating to prepare mesopore-structure MnO₂ is more significant and feasible, and advocated due to its simplicity and low cost. Recently, an organic-aqueous soft interface has been established as an alternative useful approach to conventional homogeneous synthesis [21]. Moreover, it has been employed to obtain some particles, such as, Au, Ag, Cu, CuS, CuSe, CuO, Cu(OH)₂, TiO₂, and MnO₂ in the literature [21], [22], [23], [24], [25], [26], [27], [28], [29]. However, commonly, in these procedures, surfactants are needed as direction agents, or expensive organometallic compounds are used as the precursors, or at least two steps are necessary.

Herein, a novel, facile, and template-free strategy, for the first time, was proposed to prepare MnO₂ with a mesoporous architecture by means of a unique CCl₄/H₂O soft interface without the addition of any surfactant or organometallic precursors or ligands. All the electrochemical tests demonstrated that the as-prepared mesoporous MnO₂ possessed good electrochemical capacitance performance.

Section snippets

Materials

All the chemicals used in this study are of analytical grade. MnCl₂, KMnO₄, and CCl₄ were obtained from Nanjing Chemical Company (Nanjing, China). All aqueous solutions were freshly prepared using high-purity water (18 MΩ cm resistance) from an Ampeon 1810-B system (Jiangsu, China).

Preparation of the mesoporous MnO₂

A schematic representation of the typical reaction setup is shown in Fig. 1. Thirty milliliters of 1 M MnCl₂ solution was added into 80 mL CCl₄ solution, and 5 min later, the obvious CCl₄/H₂O interface was obtained.

X-ray diffraction pattern and FTIR spectroscopy for the as-prepared MnO₂

To identify the nature of the MnO₂ sample, powder XRD and FTIR measurements were recorded. Fig. 2a illustrates the wide-angle X-ray diffraction pattern of the synthesized mesoporous MnO₂. Peaks at $2 θ = 26.2$ , 37.2, and 65.1° are obviously observed. Among the peaks, the peaks at $2 θ = 26.2$ and 37.2° are assigned to α-MnO₂ and 65.1° to γ-MnO₂ [3], [4], [5], [6], [19], respectively. It indicates that the crystalline state of the sample is a mixture of α- and γ-MnO₂. Lack of clear peaks, low intensity,

Summary

Mesoporous MnO₂ with adsorption average pore size of 9.7 nm, mesoporous volume of 0.58 cm³ g⁻¹, and BET specific surface area of 239 m² g⁻¹ have been successfully synthesized by means of a soft interface between H₂O and CCl₄. The as-prepared MnO₂ was proven to be a mixture of α- and γ-MnO₂ by X-ray diffraction techniques. Electrochemical properties of the mesoporous MnO₂ were studied by CV, galvanostatic charge/discharge, and EIS measurements in a three-electrode cell with 1 M NaSO₄ aqueous

Acknowledgments

This work was supported by the National Basic Research Program of China (973 Program) (No. 2007CB209703), the National Natural Science Foundation of China (No. 20403014, No. 20633040), the Natural Science Foundation of Jiangsu Province (BK2006196), and Graduate Innovation Plan of Jiangsu Province (CX07B-089Z).

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Interface synthesis of mesoporous MnO2 and its electrochemical capacitive behaviors

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Preparation of the mesoporous MnO2

X-ray diffraction pattern and FTIR spectroscopy for the as-prepared MnO2

Summary

Acknowledgments

J. Power Sources

J. Power Sources

J. Mater. Process. Technol.

J. Solid State Chem.

J. Power Sources

Mater. Chem. Phys.

Micropor. Mesopor. Mater.

Chem. Phys. Lett.

Electrochim. Acta

Electrochem. Commun.

Colloids Surf. A

Electrochim. Acta

Electrochemical Supercapacitors, Scientific Fundamental and Technological Applications

Chem. Mater.

J. Electrochem. Soc.

J. Phys. Chem. B

J. Electrochem. Soc.

Interface synthesis of mesoporous MnO₂ and its electrochemical capacitive behaviors

Preparation of the mesoporous MnO₂

X-ray diffraction pattern and FTIR spectroscopy for the as-prepared MnO₂