Interface synthesis of mesoporous MnO2 and its electrochemical capacitive behaviors
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 , 37.2, and 65.1° are obviously observed. Among the peaks, the peaks at 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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