Control over the morphology and phase of MnO_x formed in the modified Hummers' method and impact on the electrocapacitive properties of MnO_x–graphite oxide composite electrodes

Abstract

We report an one-pot method to prepare manganese oxide decorated graphite oxide as composite electrode materials for supercapacitors. Dispersed manganese oxide particles were precipitated on the graphite oxide surfaces from KMnO₄ added during the synthesis of graphite oxide by using a modified Hummers' method. The morphology and crystalline phase of the manganese oxide can be controlled by adjusting the mass ratio of KMnO₄/graphite. The sample prepared with a KMnO₄/graphite mass ratio of 2 contained nanoflowers of MnO₂ phase and exhibited a specific capacitance of 120 F g⁻¹ in 6 M KOH at a current density of 0.2 A g⁻¹. This composite retained 85% of its original capacitance after 2000 cycles, indicating an excellent stability against cycling.

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1. Introduction

Transition-metal oxides such as RuO₂,¹ MnO₂,² NiO,³ and Fe₃O₄ ⁴ are promising electrode materials for advanced pseudocapacitors. Manganese oxides (MnO_x), particularly MnO₂, are among the most attractive pseudoelectrode materials due to their low cost and environmental friendliness.⁵ Studies have shown that the electrocapacitive properties of MnO_x depend on crystalline phase^6–8 and morphology.^9,10 As a result, a great deal of research efforts has been devoted to controlling the crystalline phase and morphology of MnO_x.^11–14

Among different phases, MnO₂ is the most desirable one for pseudocapacitor applications.¹⁵ Both the crystalline phase and morphology of MnO₂ are reported to affect the electrochemical performance. For example, Brousse et al.⁸ reported that an amorphous MnO₂ electrode had a higher capacitance than two-dimensional (2D) birnessite and one-dimensional (1D) γ- and β-MnO₂. On the other hand, the MnO₂ morphology will affect the number of active sites available for redox reactions, as well as transport kinetics of electrolyte ions.¹⁶ Nanostructured MnO₂ (nanoneedles,¹⁷ nanorods,¹⁸and nanowires¹⁹) has been reported to display improved electrocapacitive properties compared to the bulk counterparts.

A drawback of MnO₂ is its poor electrical conductivity, which leads to a low power density of MnO₂-based electrochemical capacitors. To overcome this problem, MnO₂ has been dispersed on various carbon supports (e.g., graphene, carbon nanotubes, and activated carbon fibres).^20–24 In the work of Chen et al.,¹⁷ needle-like α-MnO₂ nanocrystal was deposited on graphene oxide sheets by using the hydrothermal method. The composite electrode exhibited a capacitance of around 200 F g⁻¹. Zhang et al. prepared birnessite-type MnO₂/activated carbon nanocomposite electrodes, which delivered a specific capacitance of 50.6 F g⁻¹ with an energy density of 28.1 W h kg⁻¹.²⁵ In one of our previous studies,²² we compared the effect of carbon supports on the performance of MnO₂ and found that graphene oxide (GO) was the best support among the carbon materials studied. In this work, graphite oxide instead of GO was used to support MnO_x directly formed from the modified Hummers' system.

The modified Hummers' method is popularly used to prepare GO.²⁶ Graphite is chemically oxidised by KMnO₄ to form graphite oxide, followed by an exfoliation to obtain GO. Reduction of GO yields reduced graphene oxide. In this work, the Mn species presented in the modified Hummers' synthesis system were directly precipitated to form MnO_x dispersed on graphite oxide as schematically illustrated in Fig. 1a. With different mass ratios of KMnO₄/graphite in the synthesis system, different MnO_x phases (MnO, MnO₂, Mn₃O₄) and morphologies (needle, flower, fractal, and sphere) were obtained (Fig. 1b). Electrochemical characterization results showed that a MnO₂–graphite oxide sample prepared with a KMnO₄/graphite mass ratio of 2 exhibited the best electrochemical performance. This study also highlights the relationship between morphology/phase and electrocapacitive property of MnO_x–graphite oxide composite materials.

Fig. 1 Schematic illustration of MnO_x–graphite oxide synthesis approach based on modified Hummers' method (a), and tailored nanostructures of MnO_x achieved with the various KMnO₄/graphite mass ratios (b).

2. Experimental

2.1 Chemicals

Graphite flakes, polytetrafluoroethylene (PTFE, 60 wt% in water), and potassium permanganate (KMnO₄, 99 wt%) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 32 wt%), sulfuric acid (H₂SO₄, 98 wt%) and ammonia solution (NH₃·H₂O, 25 wt%) were purchased from EMSURE. Hydrogen peroxide (H₂O₂, 30 wt%), sodium nitrate (NaNO₃, 95 wt%) and potassium hydroxide (KOH) were purchased from Chem-supply. Carbon black (Vulcan XC 72R) was provided by Cabot, Co. USA. Nickel foam was purchased from Shenzhen Biyuan Electronic, Co. Ltd, China. All chemicals were used as received without further purification.

2.2 Preparation of MnO_x–graphite oxide samples

In a typical experiment, 5 g of graphite flakes and 2.5 g of NaNO₃ were added to 120 mL of 98 wt% H₂SO₄ under stirring at 6 °C. After 30 min, a given amount of KMnO₄ was added slowly to maintain the system temperature below 30 °C (5 g, 10 g, 15 g and 25 g were used to achieve mass ratios of KMnO₄/graphite = 1, 2, 3, and 5, respectively), and the mixture was stirred for 16 h at 6 °C. Then 150 mL of deionized (DI) water was added, and the mixture was stirred for another 24 h at room temperature. After the addition of 50 mL of 30 wt% H₂O₂, the solution was further stirred for 24 h. To precipitate manganese species, the system pH was adjusted to 10 by using a 25 wt% ammonia solution. The solids were collected by centrifugation and washed with deionized water, then dried in air at 70 °C for 24 h. Samples obtained are denoted as G-Mn-X, where X stands for the mass ratio of KMnO₄/graphite used in the modified Hummers' method.

2.3 Characterizations

The samples were characterized by using transmission electron microscopy (TEM, JEOL 1010, FEI Tecnai F30), scanning electron microscopy (SEM, JEOL F7001) equipped with energy dispersive X-ray spectrometer (EDS), and X-ray photoelectron spectroscopy (XPS, Kartos Axis Ultra photoelectron spectrometer) using monochromatic Al Kα excitation source (1486.6 eV, 225 W, 15 kV and 15 mA). The XPS spectra were calibrated to a carbon C 1s excitation at a binding energy of 284.5 eV and quantitative analysis was performed with CasaXPS after Shirley background subtraction. Thermo-gravimetric analysis (TGA, Shimadzu Simultaneous DT-1 analyser) was performed under a constant air flow of 20 mL min⁻¹ from room temperature to 900 °C at a heating rate of 10 °C min⁻¹. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-Ray diffractometer with Ni-filtered Cu Kα radiation (λ = 1.54056 Å) at a scan rate of 3 °C min⁻¹ under a voltage of 40 kV and a current of 30 mA.

2.4 Electrochemical characterization

A symmetric electrochemical cell was used to evaluate the electrocapacitive properties of the samples. The electrodes were fabricated by mixing 80 wt% of active material (G-Mn-X), 15 wt% carbon black as a conductive additive and 5 wt% polytetrafluorene ethylene (PTFE) as a binder. The mixture was pressed onto a KOH-cleaned Ni foam. The loading of the active material was about 3 mg cm⁻². The electrolyte was 6 M KOH aqueous solution. Electrodes were soaked for at least 24 h prior to electrochemical measurements. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements were conducted at room temperature and atmospheric pressure using an Autolab PGSTAT 3020N. The CV scans were performed at a potential window of 0–0.4 V and scan rates in the range of 10–50 mV s⁻¹. The GCD cycles were performed at current densities from 0.2 to 2 A g⁻¹ in the potential window of 0–0.4 V. Electrochemical impedance spectroscopy (EIS) was measured at open circuit potential with an amplitude of 5 mV in the frequency range of 10⁻³ to 10⁵ Hz. 1 M Na₂SO₄ aqueous solution was also used as an electrolyte for comparison purpose.

The specific capacitance of a single electrode was calculated from the equation,

where I is the current (in A), Δt is the discharge time (in s), ΔV is the potential change during discharge (in V), and m is the total mass loading on both electrodes (in g).²⁷ To allow comparison of our results with other capacitance values reported, the capacitances for the two-electrode cells were converted to the equivalent value in a three-electrode configuration.

3. Results and discussion

The XPS spectra (Fig. 2) for the MnO_x–graphite oxide samples show both Mn 2p_3/2 and Mn 2p_1/2 peaks. These peaks can indicate the difference between surface valence states of these materials.²⁸ The specific binding energies and their corresponding valence states are summarized in Table 1. Mn(VII) species from KMnO₄ were reduced to Mn(II) ions; and these Mn(II) ions preferentially bond with oxygen-containing functional groups including carboxyl, hydroxyl and epoxyl groups on the graphite oxide surface.^14,29 More oxygen-containing groups were generated on the graphite oxide surface at the higher ratios of KMnO₄/graphite to attract manganese species for deposition. Therefore, the obtained manganese oxide varies in phase.³⁰ The peak at 641.2 eV corresponding to Mn 2p_3/2 and the peak at 653.0 eV corresponding to Mn 2p_1/2 in G-Mn-1 can be assigned to the Mn species in MnO.^31,32 As evidenced by the 2p_3/2 and 2p_1/2 peak position shift, the crystalline phase of manganese oxide changed to MnO₂ (Fig. 2b)^19,33 and Mn₃O₄ (Fig. 2c and d).^34–36

Fig. 2 XPS spectra in Mn 2p region of G-Mn-1 (a), G-Mn-2 (b), G-Mn-3 (c), and G-Mn-5 (d).

Table 1 Binding energies of MnO_x–graphite oxide samples

Sample	Binding energy (eV)			Mn state	Ref
	Mn 2p3/2	Mn 2p1/2	Species
G-Mn-1	641.2	653.0	MnO	2+	31 and 32
G-Mn-2	642.4	653.8	MnO2	4+	19 and 33
G-Mn-3	641.5	653.1	Mn3O4	2+, 3+	34 and 35
G-Mn-5	641.7	653.5	Mn3O4	2+, 3+	35 and 36

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The MnO_x phases can be confirmed by the XRD patterns shown in Fig. S1.† No obvious MnO peaks are observed from the XRD pattern of G-Mn-1, which may be attributed to the low MnO_x loading and low degree of crystallinity in this sample. For G-Mn-2, the characteristic XRD peaks correspond to MnO₂, which has a diffraction peak at about 19 degrees two-theta (Fig. S1†). In contrast, G-Mn-3 and G-Mn-5 exhibited characteristic peaks of Mn₃O₄ (Fig. S1†). Continuous phase change occurred in these composites from MnO to MnO₂, and finally to the stable state of Mn₃O₄. In the previous studies that utilized Mn as a resource in a modified Hummers' method,^37,38 only Mn₃O₄ was obtained. However, in our system, by simply adjusting the KMnO₄/graphite mass ratio, the resultant MnO_x crystalline phase can be easily tuned.

The loading of manganese oxide in the nanocomposites was determined by using the TGA technique. The TGA curves (Fig. 3a) show the residual masses of MnO_x (which was converted to Mn₃O₄) in the four G-Mn-X samples were 13.3%, 21.8%, 29.2%, and 46.3%, respectively. Each TGA curve features an initial weight loss due to desorption of adsorbed water and gases, followed by three weight loss events. The weight loss event between 150 and 300 °C is mainly attributed to the evolution of CO, CO₂, and water from the thermal decomposition of oxygen-containing groups on the graphite oxide,³⁹ which does not show much difference among these four samples. The weight loss event in the temperature range of 300–550 °C is due to loss of oxygen from high valence MnO_x to lower valence states,⁴⁰ as well as decomposition of surface functionalities on the graphite oxide. The weight loss above 600 °C is attributed to the combustion of carbon from the samples.⁴¹ From Fig. 3b, it can be seen that there is an exothermic peak in the temperature range from 300 °C to 550 °C. This peak shifts to lower temperatures with increased KMnO₄/graphite mass ratio. This temperature at which the exothermic peak is located is corresponding to the weight loss events in Fig. 3a. The difference between the four samples mainly lies in the oxidation degree of the graphite oxide.

Fig. 3 TGA (a) and DSC (b) analysis of G-Mn-1, G-Mn-2, G-Mn-3 and G-Mn-5.

Fig. 4 shows the TEM images of the samples prepared with different KMnO₄/graphite mass ratios. It can be seen that the morphology varies from one sample to the other. At X = 1, needle-shaped particles of low population deposited on the graphite oxide surface can be seen (Fig. 4a). The particles are typically of 20 nm in width and 100 nm in length (Fig. 4e). The contrast of the TEM images in Fig. 4a and e is poor, suggesting a low degree of crystallinity of the manganese oxide. By increasing the mass ratio to 2, MnO₂ nanoparticles with a flowerlike morphology can be observed (Fig. 4b). The Mn₃O₄ nanoparticles in sample G-Mn-3 have an irregular morphology (Fig. 4c and g). At X = 5, the graphite oxide surface is covered with spherical Mn₃O₄ nanoparticles with particle sizes of around 20 nm (Fig. 4h). These observations demonstrate that the morphology of MnO_x nanoparticles deposited on graphite oxide can be controlled by tuning the mass ratio of KMnO₄/graphite.

Fig. 4 TEM images at different magnifications of G-Mn-1 (a and e), G-Mn-2 (b and f), G-Mn-3 (c and g), G-Mn-5 (d and h). The magnification of images (a–d) should be the same, and (e–h) as well.

In the modified Hummers' method, KMnO₄ is used to oxidize graphite. A high mass ratio of KMnO₄/graphite will lead to a high degree of oxidation of the graphite oxide surface. It has been demonstrated that the degree of oxidation of a carbon substrate affects the growth and morphology of particles deposited on the carbon substrate.^30,42 On the other hand, the defect sites on the carbon substrate provides nucleation sites.⁴³ The experimental results in this work indeed confirmed that a very high ratio of KMnO₄/graphite led to rapid growth and aggregation of MnO_x particles. A low mass ratio of KMnO₄/graphite can produce relatively few oxygen-containing functional groups and poor dispersion of MnO_x.⁴² Therefore, an appropriate oxidation can realize good dispersion and uniform size of MnO_x particles. This study suggests an optimum KMnO₄/graphite mass ratio of 2 to produce well-dispersed MnO₂ on graphite oxide.

The SEM images shown in Fig. 5 provide further evidence of the variation in morphology of MnO_x nanoparticle on graphite oxide. The energy dispersive X-ray spectroscopy (EDS) results provided a quantitative estimation of the amount of MnO_x deposited on graphite oxide: 10.4 wt% MnO on G-Mn-1, 18.0 wt% MnO₂ on G-Mn-2, 23.1 wt% Mn₃O₄ on G-Mn-3, and 37.0 wt% Mn₃O₄ on G-Mn-5. These EDS data are consistent with the TGA results.

Fig. 5 SEM images and EDS spectra of G-Mn-1 (a and b), G-Mn-2 (c and d), G-Mn-3 (e and f), and G-Mn-5 (g and h).

Fig. 6a highlights that G-Mn-2 achieved the highest specific capacitance, about 120 F g⁻¹ at 0.2 A g⁻¹. Hereinafter, we will focus our discussion of electrochemical results on this sample. The capacitive property of G-Mn-2 was evaluated by using cyclic voltammetry (CV) (Fig. 6b), galvanostatic charge–discharge (GCD) method (Fig. 6c), and electrochemical impedance spectroscopy (EIS) (Fig. 6d and e). Table 2 provides a summary of specific capacitance and ESR of the four G-Mn-X samples. The details of electrochemical measurements and results discussion of three other composites are provided in the ESI.† Another common electrolyte reported for MnO_x supercapacitors is Na₂SO₄. To allow comparison of results in this neutral electrolyte, we measured the electrocapacitve properties of G-Mn-2 in 1 M Na₂SO₄, showing specific capacitance of 35 F g⁻¹ at 0.2 A g⁻¹ (Fig. S5†), which is lower value than that measured in 6 M KOH (120 F g⁻¹ at 0.2 A g⁻¹). This trend of electrochemical performances in Na₂SO₄ and KOH is similar to other reports (e.g. Ma et al.⁴⁴) and 6 M KOH has been chosen in many studies as an electrolyte for graphene–MnO_x composites.^45,46

Fig. 6 Specific capacitance of G-Mn-1, G-Mn-2, G-Mn-3 and G-Mn-5 at current density of 0.2 A g⁻¹ (a), CV curves of G-Mn-2 at various scan rates (b), galvanostatic charge–discharge curves of G-Mn-2 at different current densities (c), electrical impedance spectra (d), Nyquist plot in the high frequency region (e), and cycle performance at a current density of 1 A g⁻¹ for 2000 cycles (f).

Table 2 Capacitance and ESR data of samples

Sample	Specific capacitance at 0.2 A g^−1 (F g^−1)	Equivalent series resistance, ESR (ohm)
G-Mn-1	108	0.5
G-Mn-2	120	1.9
G-Mn-3	100	2.5
G-Mn-5	98	2.8

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The measured G-Mn-2 capacitance is much higher than pure graphite oxide,⁴⁷ and the excellent performance of G-Mn-2 is due to pseudocapacitance contributions from redox reaction of MnO₂:⁴⁸

MnO₂ + C⁺ + e⁻ ↔ MnOOC

where C⁺ = K⁺ for the KOH electrolyte in this study. The CV curves at different scan rates of electrode G-Mn-2 exhibited a nearly rectangular shape, which is consistent with previous studies.^45,49,50 These CV curves do not feature strong redox peaks as seen in other pseudocapacitive systems. This may be due to the low content of manganese oxide in the composites (18–37 wt%) and the dominating contribution of electric double layer capacitance. Another possible reason may come from the poor characterization of the redox transitions at the electrode/electrolyte surface using the CV technique as has been documented.¹⁴

The charge–discharge behaviour of the composite electrodes is shown in Fig. 6c. The shape of the GCD curves is symmetrical, in good accordance with the CV results, indicating a fairly reversible charge–discharge process. No linear drop was observed in the discharge curves until the current density increased to 2 A g⁻¹, after which the potential drop may result from the internal resistance of the electrode associated with the electrical connection resistance, bulk solution resistance, and resistance to ion migration in electrode materials.⁵¹

To further study the electrochemical performance of these composites, electrochemical impedance spectroscopy measurements were conducted in the frequency range of 10⁻³ to 10⁵ Hz. The small semicircle existing in the high frequency range implies a low charge transfer resistance (Fig. 6d and e). The nearly vertical line in the low frequency again validates the excellent capacitive properties.

The G-Mn-2 electrode was electrochemically stable against charge–discharge cycling at a current density of 1 A g⁻¹ (Fig. 6f). After 2000 consecutive cycles, 85% of the initial capacitance of the electrode was retained, indicating this graphite oxide supported manganese dioxide composite is a promising electrode material for supercapacitor applications.

4. Conclusions

This research demonstrates that the manganese species presented in the synthesis system of graphene oxide from graphite using the modified Hummers' method can be converted to manganese oxides of different crystalline phases and morphologies depending upon the mass ratio of KMnO₄/graphite employed. Manganese oxides dispersed on graphite oxide displayed interesting electrocapacitive properties. A sample containing MnO₂ with a flower morphology on graphite oxide prepared using a KMnO₄/graphite ratio of 2 exhibited the best capacitive behaviour in a 6 M KOH electrolyte, and is stable against charge–discharge cycling.

Acknowledgements

Australian Research Council (ARC) is acknowledged for supporting the work (Project DP130101870). The authors thank the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, The University of Queensland. Xiaoming Sun acknowledges China Scholarship Council (CSC) and The University of Queensland for providing the scholarships.

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns and additional electrochemical characterizations. See DOI: 10.1039/c6ra05577j

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