Control of manganese dioxide crystallographic structure in the redox reaction between graphene and permanganate ions and their electrochemical performance

Abstract

MnO₂ with α + γ-, δ-, and α-phases were synthesized by using graphene as sacrificial template in a proposed KMnO₄–graphene–H₂SO₄ reaction system. The as-prepared products were characterized with X-ray diffraction technique, Raman spectroscopy, and transmission electron microscopy. The structural analysis reveals that the cation concentration, i.e. H⁺ and K⁺, has a profound effect on both the crystallographic structures and morphologies of the final products. The relatively higher K⁺ concentration but lower H⁺ concentration facilitates the formation of δ-phased MnO₂ with a petal-like structure, and the lower concentration of both K⁺ and H⁺ cations is more conducive to the formation of a mixed phase of (α + γ) MnO₂. A further increase in the concentration of H⁺, forming the α-phased MnO₂ nanorods is preferred. The electrochemical properties for supercapacitors indicate that the electrochemical performances of MnO₂ strongly depend on their crystallographic structures, and they present a Faradaic reactivity sequence of δ- > α- > α + γ-MnO₂.

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1. Introduction

The limited availability of fossil fuels coupled with the growing demand for sustainable and renewable development has led to the significant attention in developing regenerative energy conversion/storage devices.^1–7 As one of the most effective and practical technologies for energy storage components, supercapacitors (SC) have attracted much attention due to their high power density, excellent reversibility and long cycle-life. SC can be classified into electrical double-layer capacitance (EDLC) and Faradic pseudocapacitance depended on the charge storage mechanism.^3,6,8 Compared with the former one, Faradic pseudocapacitance can provide more specific capacitance than the EDLC of carbon-based electrode materials,⁹ due to a fast and reversible Faradic redox reaction.³ Hence, great efforts have been focused on searching for pseudocapacitance materials with improved electrochemical performance.⁸

Among the various Faradic pseudocapacitance electrode materials, MnO₂ possesses many significant advantages, such as the high theoretical pseudo-capacitance (∼1400 F g⁻¹),⁹ wide voltage windows, low-cost and environmental compatibilities.^{1,5,6,8,10,11} However, MnO₂ obtained by traditional method always possesses a low specific surface area,¹² leading to a poor performance in their applications in SC.¹³ Therefore, searching for special synthesis method of MnO₂ materials with a significantly increased specific surface area is urgently needed.

Graphene, the thinnest two-dimensional carbon structure, has shown exceptional physicochemical properties that has been making a tremendous impact in the areas of physics, chemistry and materials science.^14–19 Its unique structure suggests it may have great potential for providing new approaches and critical improvements in the material synthesis. For example, the sacrificial template method by means of in situ replacement with the framework of graphene to yield the corresponding materials can dramatically increase the specific surface area and performance of the prepared materials. Typically, several approaches have been developed for the fabrication of MnO₂ pseudocapacitance materials by using graphene as the sacrificial template. Chen et al.²⁰ reported a method by in situ replacing the framework of graphene with MnO₂ in which the layered structure of graphene was transmitted to the as-prepared metal oxides. Such lamella-like structure bestowed the as-prepared MnO₂ sample an excellent electrochemical stability as the supercapacitor electrode. Zhao et al.¹³ synthesized the MnO₂ nanosheets with graphene oxide as a sacrificial template. The morphology transmission from graphene oxide to δ-type MnO₂ nanosheets results in an especially high surface area (157 m² g⁻¹) and a high performance in Faraday pseudocapacitances.

Although the single-phased MnO₂ nanomaterials with controllable morphology synthesized by graphene template method have been reported by several groups, as mentioned above,^10,13,20 it is still hard to control their crystal structure by using such method. In fact, besides the morphologies, the electrochemical properties of MnO₂ are also largely depended on its crystallographic structure.^21,22 In current work, three types of MnO₂ (α + γ-, δ-, and α-phases) with high specific surface area were synthesized using graphene as a sacrificial template in a KMnO₄–graphene–H₂SO₄ reaction system. The control of crystal phase and morphology was realized by simply varying the cation type (K⁺, H⁺) and cation concentration in the reaction system. To our knowledge, it is for the first time to obtain the different crystallographic structures of MnO₂ by using graphene as a sacrificial template. The further studies on the crystallographic structure-dependent electrochemical reactions and energy storage behaviors of the obtained MnO₂ electrode materials show that the δ-MnO₂ electrode presents the highest specific capacitance than that of the α-MnO₂ and α + γ-MnO₂.

2. Experimental section

2.1. Material synthesis

Graphene was purchased from the Institute of Shanxi Coal Chemistry, Chinese Academy of Sciences. All chemical reagents in this work were analytical grade and used as received. Three kinds of MnO₂ with different crystalline structures were prepared by using graphene as the sacrificial template.

Synthesis of α + γ-phased MnO₂. The MnO₂ samples were prepared as Chen's method after a minor modification.²³ In a typical procedure, graphene (10 mg) was ground with KMnO₄ (100 mg) by using an agate mortar and pestle to get a powder mixture. 10 mL deionized (DI) water was then added and stirred at room temperature (16 °C) for 20 min. Then, 50 μL concentrated H₂SO₄ (95–98 wt%) was added to the system and stirred at room temperature for additional 1 h. The solution was then heated in an oil bath at 85 °C for 1 h. After that, the reaction mixture was removed from the heat source, and cooled down to room temperature. The precipitates were collected by centrifugation for several times and then the product was dried at 65 °C for 12 h.

Synthesis of δ-phased MnO₂ nanosheets. The δ-phased MnO₂ nanosheets were prepared under the same conditions for the α + γ-phased MnO₂ nanorod, except that the amount of KMnO₄ was increased to 150 mg.

Synthesis of α-phased MnO₂ nanorods. The α-phased MnO₂ nanorods were prepared under the same conditions with the α + γ-phased MnO₂ sample, except that the volume of H₂SO₄ was 100 μL. In addition, to explore the formation mechanism of MnO₂ with different crystalline structures, the influence of H₂SO₄ and KMnO₄ concentration were also investigated.

2.2. Material characterization

The crystal structure of the samples was characterized by powder X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer) with a Cu Kα X-ray source (λ = 1.5405 Å) in the range between 15 and 80°. The morphology and microstructure were examined by scanning electron microscopy (SEM, JEOL JSM-7000F) and transmission electron microscopy (TEM, JEOL JEM-3010). Nitrogen adsorption/desorption was carried out at 77.3 K by means of an ASAP 2020M Xtended Pressure Sorption Analyzer. The Raman spectrum was recorded on the HORIBA JOBIN YVON Raman spectrometer using laser excitation at 514 nm from an argon ion laser source. Thermogravimetric analysis (TGA) curves were measured by using a Q1000DSC + LNCS + FACS Q600SDT thermoanalyzer systems from room temperature to 800 °C in an air atmosphere. The electronic structure of the prepared samples was analyzed by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA, Kratos Analytical Ltd.) analysis.

2.3. Electrochemical measurement

The electrodes of the electrochemical capacitors were fabricated by mixing the prepared powder with 10 wt% acetylene black and 5 wt% polytetrafluoro-ethylene (PTFE) binder of the total electrode mass. A small amount of alcohol was added to the mixture, forming the homogeneous paste. The mixture was pressed onto the nickel foam current collectors (3.0 × 1.0 cm²) to make the electrodes. Every electrode contains the active material approximately 5.0 mg cm⁻². Electrochemical characterization was carried out in a conventional three-electrode cell in 1 M Na₂SO₄ electrolytes. Platinum foil and Ag/AgCl electrode were used as the counter and reference electrode, respectively. Cyclic voltammetry (CV) and galvanostatic charge/discharge measurements were conducted using a CHI 660e electrochemical workstation (Shanghai Chenhua Co. Ltd., China). Long-term galvanostatic charge/discharge cycling tests were performed on a multi-channel battery analyzer (Land, CT2001A).

3. Results and discussion

The graphene features a transparent layered structure and a high surface area of 625.2 m² g⁻¹ as proven by SEM, TEM and nitrogen sorption measurements (Fig. 4a for BET measurement and Fig. S1 in ESI,† SF, for SEM and TEM images), respectively. Due to the thin-layered structure, almost all the carbon atoms in the graphene are reactive, thus it can be oxidized by MnO₄⁻ ions in an acid solution through the following chemical reaction eqn (1):²³

4KMnO₄ + 3C + 2H₂SO₄ → 4MnO₂ + 3CO₂ + 2K₂SO₄ + 2H₂O (1)

The phase structure of the prepared samples was characterized by the XRD analysis and shown in Fig. 1. The pristine graphene only shows a weak and broad peak in the range of 20–28° (Fig. 1a). The XRD pattern in Fig. 1b can be assigned to the tetragonal phase of α-type MnO₂ (JCPDS 44-0141).²⁴ The α-MnO₂ consists of interlinking double chains of edge-sharing basic units octahedral [MnO₆], which are linked at corners to form (2 × 2) and (1 × 1) tunnels (Fig. 1), and the size of (2 × 2) and (1 × 1) tunnels is 4.6 × 4.6 and 2.3 × 2.3 Å, respectively.¹ The broad peaks at 2θ = 37.1, and 65.5° in Fig. 1c can be indexed to the poorly crystalline phased δ-type MnO₂ (JCPDS 18-0802).²² The δ-phased MnO₂ is a 2D layered structure with an interlayer separation of ∼7 Å between the MnO₆ octahedra, in which a large amount of stabilizing cations, such as K⁺ was considered to fill those space.²² Unlike the above patterns, the diffraction pattern in Fig. 1d, consists of the peaks resulting from both the α- and γ-phased MnO₂ (JCPDS 14-0644),^22,25,26 suggesting that the sample possesses a mixed phase. In all cases, the broad peak of graphene located in the range of 20–28° disappeared, indicating that the framework of the graphene was replaced by MnO₂.

Fig. 1 XRD patterns of the (a) graphene, (b) α-, (c) δ- and (d) α + γ-MnO₂ samples, and the corresponding schematic illustration of MnO₂ crystal structures.

The morphology and microstructure of the synthesized MnO₂ samples were characterized by SEM and TEM, as shown in Fig. 2. The SEM and TEM observations clearly indicate that both the nanorod- and plate-like structures present in the mixed phase of α + γ-MnO₂ sample (Fig. 2a and d). The high-resolution transmission electron microscopy (HRTEM) image (Fig. 2g) based on a single nanorod reveals the lattice fringe spacings of 0.312 nm, corresponding to the (310) crystal planes of the tetragonal phase of α-type MnO₂.²³ The selected area electron diffraction (SAED) pattern (Fig. 2h) indicates the poly-crystalline structure of the sample. The SEM and TEM images (Fig. 2b and e) show that the as-prepared δ-phased MnO₂ sample exhibits a porous and petal-like structure. The formation of the pores can be attributed to the petal-like MnO₂ sheets randomly stacking together. Fig. 2i shows the HRTEM image of the δ-phased MnO₂ sample. The lattice spacing of 0.245 nm between adjacent lattice planes can be assigned to the (006) facets of δ-phased MnO₂, which is in agreement with 0.244 nm as reported in the literature for δ-type MnO₂.²² The corresponding SAED pattern, as shown in Fig. 2j, indicates a poly-crystalline nature of the δ-type MnO₂ sample. The morphology of the as-prepared α-phased MnO₂ is shown in Fig. 2c and f. It can be seen that the sample mainly consisted of rod-like MnO₂ nanoparticles (Fig. 2c and f). Fig. 2k shows the HRTEM image of a single nanorod. The lattice space is measured to be about 0.312 nm, corresponding to the (310) plane of α-MnO₂. The SAED pattern (Fig. 2l) indicates a poly-crystalline structure of the α-MnO₂ sample. Besides the Si signals coming from the silicon substrate, Mn, O, and K were detected from the α + γ-, δ- and α-MnO₂ samples in the energy dispersive spectra (EDS), which confirm that most of the graphene reduced KMnO₄ to MnO₂ in all the samples (Fig. 2m–o).

Fig. 2 SEM images of the (a) α + γ-, (b) δ- and (c) α-MnO₂ samples. TEM images of the (d) α + γ-, (e) δ- and (f) α-MnO₂ samples. HRTEM images of the (g) α + γ-, (i) δ- and (k) α-MnO₂ samples. SAED images of the (h) α + γ-, (j) δ- and (l) α-MnO₂ samples. A representative EDS spectrum of the (m) α + γ-, (n) δ- and (o) α-MnO₂ samples.

The structural change upon redox reaction between graphene and permanganate ions was further characterized by the Raman spectroscopy. Fig. 3a shows the Raman spectra of the graphene, α + γ-, δ- and α-phased MnO₂ samples. The complete disappearance of the D and G bands of the graphene and the appearance of the peaks located at 630 cm⁻¹, which are commonly attributed to the symmetric stretching vibration Mn–O of MnO₆ groups,¹⁴ confirm the formation of MnO₂ and the most of graphene being consumed in the final samples. These results also suggest the replacement of graphene to MnO₂, instead of forming the graphene–MnO₂ composites.

Fig. 3 (a) Raman spectra of the graphene and as-prepared samples. (b) TGA of the graphene, α + γ-, δ- and α-MnO₂ samples in an air atmosphere. (c) TGA of the α + γ-, δ- and α-MnO₂ samples. XPS spectra of the (d) survey scan, (e) Mn 3s region, and (f) Mn 2p region for the α + γ-MnO₂ sample. XPS spectra of the (g) survey scan, (h) Mn 3s region, and (i) Mn 2p region for the δ-MnO₂ sample. XPS spectra of the (j) survey scan, (k) Mn 3s region, and (l) Mn 2p region for the α-MnO₂ sample.

Fig. 3b and c show the TGA of the graphene, α + γ-, δ- and α-MnO₂ samples in an air atmosphere. The weight loss of 5–14% (5.3% for α + γ- and α-MnO₂, 14% for δ-MnO₂) below 200 °C is attributed to the evaporation of the absorbed water in all the samples.¹⁴ A significant weight loss can be observed in the range of 400–500 °C for graphene. This feature is associated with the thermal decomposition of the carbon skeleton. Distinctly different from the curves for graphene, only little weight loss in the range of 400–500 °C in the curves of α + γ-, δ- and α-MnO₂ samples is observed (Fig. 3b and c). These results imply that most of the carbon has been removed during the direct redox reaction between MnO₄⁻ and graphene. The weight loss around 545–650 °C for the α + γ-, δ- and α-MnO₂ samples is due to the transformation of MnO₂ into Mn₂O₃ by the evolution of oxygen.¹³ These TGA results indicates that most of the graphene has transferred into MnO₂.

The electronic structure of the prepared α + γ-, δ- and α-MnO₂ samples were investigated by XPS analysis. Fig. 3d–l show the representative XPS spectra of the prepared MnO₂ samples. The manganese oxidation state was verified from the multiplet splitting of the Mn 3s peak.²⁷ This splitting arises from the parallel spin coupling of the 3s electron with the 3d electron during the photoelectron ejection. The energy separation between the two peaks is related to the mean manganese oxidation state.²⁸ The splitting width is 4.5 eV for α + γ- and α-MnO₂ samples, and 4.8 eV for δ-MnO₂, which are in accordance with a previous report of MnO₂ (4.78 eV).²⁸ The peaks of Mn 2p_1/2 and 2p_3/2 which are centered at 653.8 and 642.0 eV for α + γ- and α-MnO₂ samples, and centered at 653.9 and 642.1 eV for δ-MnO₂ samples, respectively, with a spin-energy separation of 11.8 eV,²⁰ suggesting the formation of MnO₂. Moreover, the peak of O 1s (Fig. S2† in SF) includes a spiking (centered at 529.7 eV for α + γ-MnO₂, 529.6 eV for α- and δ-MnO₂) and a relatively weak peak (centered at 531.9 eV for α + γ-MnO₂, 531.2 eV for δ-MnO₂, and 531.4 eV for α-MnO₂). The two peaks are related to two kinds of oxygen: one is correspond to the [MnO₆] octahedra in the lattice of MnO₂, and the other is the H₂O which is exist between the interlayer of MnO₂ crystal structure.¹³ The peaks of the C1s in (d), (g) and (j) result from the carbon conductive tape being used as the substrate.

A typical type-IV isotherm characteristic can be observed for all the MnO₂ samples.²⁹ The nitrogen adsorption/desorption isotherms show a substantial hysteresis loop in the P/P₀ range above 0.45 for graphene, δ- and α-MnO₂, and 0.8 for α + γ-MnO₂, respectively (Fig. 4). This feature indicates the presence of a lot of relatively large mesopores in the framework of the obtained MnO₂ samples.^24,29 The pore size distributions for all the samples determined by BJH and HK methods are shown in the inset of Fig. 4. It shows that the α + γ-phased MnO₂ sample consists of micropores (1–2 nm) and a large distribution of mesopores with pore radii between 2 and 50 nm. The δ-phased sample mainly consists of micropores centered at 1 nm and mesopores centered at 3–4 nm. The α-phased sample mainly consists of micropores (1–2 nm) and mesopores (centered at 3–4 nm). The BET surface area of the α + γ-, δ- and α-phased MnO₂ samples were calculated to be 93.7, 140.3 and 84.0 m² g⁻¹, respectively. Remarkably, these values are much higher than the MnO₂ produced by traditional method,¹² indicating the graphene is an ideal template for the synthesis of MnO₂ nanostructures with a high surface area.

Fig. 4 The nitrogen adsorption–desorption isotherms of (a) graphene, (b) α + γ-, (c) δ- and (d) α-MnO₂ samples. The insert shows BJH and H–K pore-size distributions.

To further investigate the role of graphene in the reaction process, the same procedure and recipe were employed as the α + γ-MnO₂ sample but without graphene. As shown in Fig. S1c-A,† the solution colour still kept in dark purple after the reaction and no MnO₂ was obtained, indicating that the reduction of MnO₄⁻ only in the presence of water was impossible. After the replacement reactions, the solution colour for the α + γ-, δ- and α-MnO₂ reaction systems changed from dark purple to black/brown (Fig. S1c† in SF), implying the growth of MnO₂ at the cost of the carbon atoms of graphene.

The SEM images and XRD patterns indicate that the amount of H₂SO₄ and the presence of potassium ions have a profound effect on the morphologies and crystallographic structures of the as-prepared MnO₂ nanomaterials. When the mass ratio of graphene to KMnO₄ was 1 : 10 without adding any H₂SO₄, the obtained MnO₂ sample seems to inherit the structure of graphene, which displays a folded and wrinkled sheet-like character with relatively small amounts of flower-like particles decorating on their surface (Fig. 5a and S3† in SF). The XRD pattern of the sample exhibits the characteristic diffraction peaks corresponding to the pure δ-phased MnO₂ (Fig. 5d, sample 2^#). When 100 μL or 200 μL H₂SO₄ was added into the above mixture (10 mg graphene and 100 mg KMnO₄), the rod-like MnO₂ nanoparticles with a length around 150 nm and a diameter about 20–30 nm can be observed (Fig. 2c and 5b). When the volume of H₂SO₄ was increased from 500 μL to 5 mL, the rod-like MnO₂ structure gradually diminished (Fig. S4† in SF), which should be due to the MnO₂ being corroded by the excessive acid, while the crystalline structure of such samples still display a tetragonal phase of α-type MnO₂ (Fig. 5d, samples 3^#–5^#). When the amount of KMnO₄ was increased to 300 mg and the graphene was still maintained at 10 mg with adding 50 μL H₂SO₄, the obtained sample displays the petal-like morphology, as shown in Fig. 5c. The XRD pattern of MnO₂ sample exhibits broad peaks at 2θ = 37.0 and 65.5° with a pure δ-MnO₂ phase (Fig. 5d, sample 1^#), which is consistent with the above obtained δ-MnO₂ sample (10 mg graphene and 150 mg KMnO₄ was used with adding 50 μL H₂SO₄), indicating that the content of potassium ions may be the key factor to form the δ-phased MnO₂.

Fig. 5 (a) SEM image of the sample prepared when the mass ratio of graphene: KMnO₄ = 1: 10 at 85 °C without adding any H₂SO₄, (b) SEM image of the sample prepared when 10 mg graphene and 100 mg KMnO₄ was used at 85 °C with 200 μL H₂SO₄, (c) SEM image of the sample prepared when 10 mg graphene and 300 mg KMnO₄ was used at 85 °C with 50 μL H₂SO₄, (d) XRD patterns of the as-prepared samples (1^# was the sample prepared when 10 mg graphene and 300 mg KMnO₄ was used at 85 °C with 50 μL H₂SO₄. 2^# was the sample prepared when 10 mg graphene and 100 mg KMnO₄ was used at 85 °C without adding any H₂SO₄. 3^# was the sample prepared when 10 mg graphene and 100 mg KMnO₄ was used at 85 °C with 200 μL H₂SO₄, 4^# was the sample prepared when 10 mg graphene and 100 mg KMnO₄ was used at 85 °C with 500 μL H₂SO₄, 5^# was the sample prepared when 10 mg graphene and 100 mg KMnO₄ was used at 85 °C with 5 mL H₂SO₄). (e) Schematic illustration of the formation process for varies of MnO₂ materials with different crystal structures prepared from graphene.

Based on the above results, the possible formation mechanisms of MnO₂ samples with different morphologies and phase structures are illustrated in Fig. 5e. The reaction process includes two stages, the graphene-driven in situ redox reaction and the ion-driven crystallization.¹³ Graphene possesses a lamellar structure with a 2D layer of sp²-bonded carbon. In the reaction system, carbon atoms of graphene act as a sacrificial reductant when exposed to the KMnO₄ solution.¹⁴ The reaction is featured by electron transfer from the carbon to the MnO₄⁻ ions,³⁰ and the C–C bonds could be broken and the carbon atoms were fully oxidized to generate CO₂. Simultaneously, elimination the carbon atoms of graphene may lead to the MnO₄⁻ being reduced and in situ forming the [MnO₆] octahedra.¹³ The in situ transformation from the carbon atoms of graphene to [MnO₆] octahedra of MnO₂ may retain some structure property of graphene (such as high surface area) to the final product. In order to minimize the total energy, the neighboring [MnO₆] octahedra may edge-share for energy stabilization.¹³

However, the question present here is how the [MnO₆] octahedra interlink each other in different way, thus to form various crystal structures (tunnel or layer), when the concentration of the ions (K⁺, H⁺) are changed. It has been reported that the K⁺ cation can favor the formation of different crystallographic structures of MnO₂.^31,32 The KMnO₄ in our reaction system can provide K⁺, which thus contribute to obtain various crystal structures of MnO₂. We also introduced H₂SO₄ in our reaction system because (i) MnO₄⁻ are more reactive in the acidic ambient than the H₂SO₄-free system, and hence the graphene is more vulnerable to be oxidated; (ii) the crystallographic structure of the MnO₂ can also be controlled by H⁺. It is expected that by using two kinds of crystal oriented cations would crystallize different kinds of MnO₂. We found that the relatively high K⁺ concentration and low H⁺ concentration facilitates the formation of δ-type MnO₂. Since the spacing in the δ-type MnO₂ is larger than that in the α-type MnO₂, more cations might be required to stabilize their layered structure than the tunneled α-type MnO₂. The experimental results also show that the structural transformation from the layered phase to the tunneled tetragonal phase is strongly dependent on the acidity of the reaction system. With the volume of the H₂SO₄ was further increased, i.e. 100 μL in the reaction solution, the tunnel-structured α-type MnO₂ is formed. It is worth pointing out that the MnO₄⁻ ions show higher oxidation ability when increase the amount of H₂SO₄, which is necessary to form a more stable phase of α-MnO₂. In addition, the higher acidity may bring about the contraction of the unit cell, which causes the formation of a more compactness structure that enables it to accommodate the harsh ambient (higher acidity). While the relatively lower concentration of both K⁺ and H⁺ ions facilitates the formation of a mixed phase of α + γ-MnO₂. It has been proved that the α-phased MnO₂ with the 2 × 2 and 1 × 1 tunnels are more stable than the γ-phased MnO₂, which has a less symmetrical structure of the 1 × 2 tunnel.³³ Therefore, a weaker oxidation ability of MnO₄⁻ ions resulting from a lower acidity surrounding may induce the formation of the less stable γ-phase MnO₂. In addition, as Sun et al. reported that the lack of K⁺ and H⁺ can induce the formation of γ-MnO₂.³⁴ In our experiment, there still exist the low concentration of K⁺ and H⁺ cations, which thus favors the formation of a mixed phase of α + γ-MnO₂.

MnO₂, as one of important functional materials, has been widely researched and applied in the field of energy storage, especially in SC. The total charge storage capacity of MnO₂ electrode is dependent on two concurrent parts: (i) the non-Faradic capacitance (i.e., EDLC) accumulated charges at the surface of MnO₂ electrodes via the material surface adsorption or desorption of cations in the electrolyte; (ii) the Faradic capacitance (i.e., pseudocapacitance) that is generated by the intercalation/deintercalation of cations into the MnO₂ lattice during the electrochemical reduction and oxidation process.³⁵ The charge storage mechanism of MnO₂ materials can be explained by the following reactions in the aqueous electrolytes:³⁶

(1) Non-Faradic:

(MnO₂)_surface + C⁺ + e⁻ ⇄ (MnO₂C⁺)_surface, (C = H⁺, Na⁺) (2)

(2) Faradic:

MnO₂ + C⁺ + e⁻ ⇄ MnOOC, (C = H⁺, Na⁺) (3)

In general, the main part of the capacitance for MnO₂ materials comes from the pseudocapacitive redox process, which strongly depends on their crystallographic structures. The electrochemical properties of the as-prepared α + γ-, δ- and α-MnO₂ samples toward supercapacitor electrode materials were tested under the same conditions. As shown in the cyclic voltammetry (CV) curves in Fig. 6a, symmetric and rectangular CV curves without obvious redox peaks are observed for all the three electrodes, suggesting the fast reversible Faradic reaction and ideal capacitive behavior. The area under the CV curve for δ-MnO₂ electrode is larger than that of α + γ- and α-MnO₂ electrode, suggesting that δ-MnO₂ electrode has the highest specific capacity values among the three samples, and this may due to the large interlayer spacing of the δ-type MnO₂.

Fig. 6 (a) CV curves of the α + γ-, δ- and α-MnO₂ electrodes at the scan rate of 10 mV s⁻¹. (b) Galvanostatic charge–discharge of the α + γ-, δ- and α-MnO₂ electrodes at the current density of 1.0 A g⁻¹. (c) Variation of specific capacitance against current density for the α + γ-, δ- and α-MnO₂ electrodes. (d) Cycling performance of the α + γ-, δ- and α-MnO₂ electrodes.

The capacitive performance was further investigated by the galvanostatic charge/discharge experiments (Fig. 6b). According to the galvanostatic discharge curves, the corresponding specific capacitances of the samples can be calculated using the following expression: C_sc = (IΔt)/(mΔV), where I is the discharge current, Δt is the total discharge time, m is the mass of active material, ΔV is the potential drop during discharging process and C_sc is the specific capacitance.^4,11,37 From the slope of the galvanostatic charge/discharge curves, the total specific capacitance of α + γ-, δ- and α-MnO₂ electrodes are calculated to be 60, 160 and 75 F g⁻¹, respectively. These specific capacitance values are higher than the previously reported values of 32.8 F g⁻¹ for α-MnO₂, 24 F g⁻¹ for γ-MnO₂ prepared by the microwave–hydrothermal method.³² In order to accurately reflect the energy storage behaviors and crystallization-dependent electrochemical reaction of the prepared MnO₂ materials, the pure double layer capacitance (based on the specific surface area of the prepared MnO₂ materials) was calculated by using the BET surface area and the average value of 21 μF cm⁻² and give the pure double layer specific capacitance of 19.8, 29.5 and 17.6 F g⁻¹ for α + γ-MnO₂, δ-MnO₂ and α-MnO₂,³⁸ which are far lower than the measured value from the galvanostatic charge/discharge measurement, indicating that the main part of the capacitance for MnO₂ materials comes from the pseudocapacitive redox process. Because the pseudocapacitance properties are related to the intercalation/deintercalation of cations into the MnO₂ electrode lattice, thus only the crystallographic structures, which possess a large tunnel size to accommodate these ions, can make a contribution to the capacitance.^1,22 Therefore, the electrochemical property of MnO₂ greatly depended on their crystallographic structures with different tunnels or interlayers. Based on the testing data, it is found that the specific capacitance of the as-crystallized MnO₂ supercapacitors have Faradaic reactivity sequence of δ- > α- > α + γ-MnO₂. This means that the larger interlayer size of δ-MnO₂ (an interlayer separation of ∼7 Å) is more feasible for the intercalation of cations (Na⁺ ions in this case), while the narrow tunnels of α- and γ-phased MnO₂ (2 × 2, 4.6 × 4.6 Å and 1 × 2, 2.3 × 4.6 Å, respectively) did not facilitate the intercalation of Na⁺ ions into the material as well as the δ-MnO₂.

Fig. 6c compares the variation of specific capacitance against current density for the α + γ-, δ- and α-MnO₂ electrodes. It can be seen from the Fig. 6c, the δ-MnO₂ electrode shows the highest specific capacity values among the three samples at different scan rate. To evaluate the long-life stability of α + γ-, δ- and α-MnO₂ material, cycle charge–discharge testing is employed to examine the service life of the α + γ-, δ- and α-MnO₂ electrodes. The cutoff voltage of all the electrodes is controlled from 0 to 0.9 V at a current density of 1 A g⁻¹ in 1 M Na₂SO₄ electrolyte. The variations of specific capacitance of the obtained MnO₂ electrodes during cycling are illustrated in Fig. 6d. The capacitance of all the three MnO₂ electrodes increases slightly during the cycling process, most likely due to the electrolyte penetrate into the bulk of electrode materials during the initial cycling process. As for the δ-MnO₂, the specific capacitance after 500 cycles is 147 F g⁻¹, which is 20% higher than the initial value and 6% lower than the maximum value. As for the α + γ- and α-MnO₂, the specific capacitance after 500 cycles are 46.8 F g⁻¹ and 65.5 F g⁻¹, which are 14% and 3% higher than the initial values, indicating that the three MnO₂ working electrodes can withstand 500 cycles with a slight increase in the specific capacitance, exhibiting a high electrochemical stability.

4. Conclusions

MnO₂ materials with different crystallographic structures (i.e. α + γ-, δ-, and α-phase) were prepared by using the graphene as a sacrificial template. It was found that higher K⁺ cation concentration favors the crystallization of δ-MnO₂, while the α-MnO₂ was produced with higher H⁺ cation concentration. Controllable synthesis of optimized crystallographic structures electrode materials are important for the applications of supercapacitors. Electrochemical measurements show that the performance of the as-crystallized MnO₂ supercapacitors is greatly depended on their crystallographic structures. The obtained MnO₂ materials have a Faradaic reactivity sequence of δ- > α- > α + γ-MnO₂. More importantly, the large BET surface areas and controllable crystallographic structures of MnO₂ materials are believed to largely improve its special applications, including catalysis, sensors, adsorbent, etc. We believe that our research may enlarge the potential application of graphene, and provide a possible methodology to product affordable nanomaterials for high-performance supercapacitors.

Acknowledgements

We thank Dr Chuansheng Ma and Dr Guang Yang from the International Center for Dielectric Research, Xi'an Jiaotong University, for their support with SEM, TEM and HRTEM. This work is supported by National Natural Science Foundation of China (no. 51271135), the program for New Century Excellent Talents in university (no. NCET-12-0455), the Fundamental Research Funds for the Central Universities, and the project of Innovative Team of Shanxi Province (no. 2013KCT-05).

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Footnote

† Electronic supplementary information (ESI) available: SEM and TEM images of the pristine graphene. Photographs demonstrating the reaction between graphene and KMnO₄ solution. XPS spectra of the O 1s region for the prepared samples. TEM images of the sample prepared when the amount ratio of graphene: KMnO₄ = 10 mg: 100 mg at 85 °C without adding any H₂SO₄. Raman spectra of the graphene and as-prepared samples. SEM image of the sample prepared when 10 mg graphene and 100 mg KMnO₄ was used at 85 °C with 500 μL H₂SO₄ and 5 mL H₂SO₄. See DOI: 10.1039/c5ra01455g

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