Influences of graphene oxide support on the electrochemical performances of graphene oxide-MnO2 nanocomposites
© Yang et al; licensee Springer. 2011
Received: 14 June 2011
Accepted: 27 September 2011
Published: 27 September 2011
MnO2 supported on graphene oxide (GO) made from different graphite materials has been synthesized and further investigated as electrode materials for supercapacitors. The structure and morphology of MnO2-GO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Nitrogen adsorption-desorption. As demonstrated, the GO fabricated from commercial expanded graphite (denoted as GO(1)) possesses more functional groups and larger interplane gap compared to the GO from commercial graphite powder (denoted as GO(2)). The surface area and functionalities of GO have significant effects on the morphology and electrochemical activity of MnO2, which lead to the fact that the loading amount of MnO2 on GO(1) is much higher than that on GO(2). Elemental analysis performed via inductively coupled plasma optical emission spectroscopy confirmed higher amounts of MnO2 loading on GO(1). As the electrode of supercapacitor, MnO2-GO(1) nanocomposites show larger capacitance (307.7 F g-1) and better electrochemical activity than MnO2-GO(2) possibly due to the high loading, good uniformity, and homogeneous distribution of MnO2 on GO(1) support.
As one of the green supercapacitor electrode materials, MnO2 shows potential to replace RuO2 due to its high specific capacitance, environmental compatibility, low cost, and abundance in nature. In general, the fabrication of MnO2 can be readily realized on large scale using traditional chemical co-precipitation methods [1, 2]. However, MnO2 powders produced by these methods suffer some disadvantages, like low specific surface area, and thus low specific capacitance in most cases. To improve the electrochemical performance, the strategy of direct deposition of MnO2 on large-surface-area materials, such as carbon blacks, carbon nanotubes, activated or mesoporous carbons [3–9], is quite promising. Recently, graphene oxide (GO), a shining-star material, has been widely investigated as a suitable support for MnO2 loading [10, 11]. Thanks to the large accessible surface area provided by GO, more ions can transport onto the material surface, achieving high electric-double-layer capacitance in aqueous electrolytes. Furthermore, nanostructured MnO2 modified on GO support can effectually prevent the aggregation of GO nanosheets caused by van der Waals interactions. As a result, the available electrochemical active surface area for energy storage can be greatly enhanced.
Structurally, a single-layer of graphite oxide, also called GO, consists of a honeycomb lattice of carbon atoms with oxygen-containing functional groups which are proposed to present in the form of carboxyl, hydroxyl, and epoxy groups [12, 13]. These functional groups can enlarge the gap between adjacent GO sheets. For instance, the (002) diffraction peak of pristine graphite is located at approximately 26°, and the interplane distance is 0.34 nm. After oxidation of graphite, the diffraction peak shifts to a lower angle, indicative of a larger interplane gap. The functional groups and the larger interplane gap enable GO sheets to be easily decorated or intercalated by polymers, quantum dots, and metal/metal oxide nanoparticles (NPs), etc. [10, 11, 14–20], which are favorable and much desired for various applications. Nevertheless, until now, few reports have focused on the effects of functional groups and interplane gap of GO on the loading amount of quantum dots or metal/metal oxide nanoparticles, and so on.
In this work, we study the influences of GO supports on the electrochemical behavior of MnO2-GO nanocomposites. Two types of GO nanosheets, denoted as GO(1) and GO(2), made from commercial expanded graphite (CEG) and commercial graphite powder (CGP), respectively, are employed as MnO2 supports for comparative study. GO(1) nanosheets are proved to have more functional groups and larger interplane gap compared to GO(2) nanosheets, which might be capable of enhancing the loading amount of MnO2. As a result, MnO2-GO(1) nanocomposite exhibits higher energy and powder densities in neutral aqueous electrolytes.
Commercial expanded graphite, commercial graphite powder, 98% H2SO4, 30% H2O2, potassium permanganate (KMnO4) and NaNO3 were used as received. Distilled water was used in all the processes of aqueous solution preparation and washing.
Scanning electron microscopy images were obtained on a field-emission scanning electron microscope (FE-SEM JEOL JSM-6700F; JEOL, Tokyo, Japan). Transmission electron microscopy (TEM) analyses were carried out using an electron microscope (JEM 2010F; JEOL, Tokyo, Japan) operating at 120 kV. The Raman spectra were recorded using a WITEC-CRM200 Raman system (WITEC, Germany). The excitation source is 532-nm laser (2.33 eV). X-ray photoelectron spectroscopy (XPS) measurement, was carried out on a thermo scientific ESCALAB 250 (Thermo Fisher Scientific, UK). The nanocomposites X-ray diffraction (XRD) studies were charactered by a Bruker D8 ADVANCE XRD (Bruker AXS, Germany). Nitrogen adsorption-desorption experiments were investigated at 77 K on an automatic volumetric sorption analyzer (Quantachrome, NOVA1200; Micromeritics, USA). The surface area was calculated using the Brunauer-Emmett-Teller equation. Pore size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method using the adsorption branches. Quantitative elemental determinations were performed by firstly dissolving the solid samples with a CEM Mars microwave digester (Matthews, NC, USA), followed by analysis with a Thermo Scientific iCAP 6000 series inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo Scientific, England).
Synthesis of GO
Two kinds of graphite were used for synthesizing GO by a modified Hummers method [21–23]. In brief, 2 g of CEG (or CGP) and 1.5 g of NaNO3 were added into 150 mL of 98% H2SO4 solution in a flask which was immersed in an ice bath. Afterwards, 9 g of KMnO4 was slowly added in the solution. Meanwhile, the temperature of the mixed solution was maintained below 20°C for 2 h to avoid overheating and explosion. The mixture was stirred for 5 days. Then, 10 mL of 30% H2O2 was added into the solution in order to completely react with the remaining KMnO4, leading to a bright yellow solution. Finally, the resulting mixture was washed by 3% H2SO4 and H2O until the pH value of the solution was approximately 5-6. GO powder was obtained after freeze drying the suspension, labeled as GO(1)/GO(2).
Synthesis of MnO2-GO nanocomposites
The MnO2-GO nanocomposites were prepared by an in situ reduction method . The detailed procedure was as follows: 200 mg of GO(1) (or GO(2)) was blended with 150 mL of 0.02 M KMnO4 solution in a three-necked round-bottomed flask. The as-obtained mixture was refluxed at 120°C for 12 h with sustained magnetic stirring. The nanocomposites, labeled as MnO2-GO(1) (or MnO2-GO(2)), was then centrifuged, washed, and finally dried in air at 55°C overnight.
The working electrode of the electrochemical capacitors was fabricated by mixing the nanocomposites (15 mg) with 15 wt.% acetylene black and 5 wt.% polytetrafluorene-ethylene binder of the total electrode mass. A small amount of ethanol was added to the mixture for more homogeneous paste. The mixture was then pressed onto nickel foam current collector (1.0 × 1.0 cm) (washed by acetone and 0.1 M HCl carefully before use) to make electrodes. Electrochemical characterizations were carried out in a conventional three-electrode cell with 1 M Na2SO4 as the electrolyte. A platinum foil and saturated Ag/AgCl electrode were used as the counter and reference electrode, respectively. All electrochemical measurements were conducted using CHI 660 electrochemical workstation.
Results and discussion
The inset HRTEM images in Figure 3 show the lattice fringes of the MnO2-GO(1) and MnO2-GO(2) nanocomposites. Three distinct sets of lattice spacing of ca. 0.237, 0.29, and 0.48 nm are shown, corresponding to the (211), (001), and (200) planes of α-MnO2, respectively. The inset image in Figure 3a and 3 the upper inset image in Figure 3b present the orientation of the three epitaxial growths of MnO2 nanoflakes on GO(1) and GO(2). Both epitaxial growths for the formation of the MnO2 nanorods on GO(2) were revealed by the lower inset image in Figure 3b. The presence of clear lattice fringes in the HRTEM images confirms the crystalline nature of the α-MnO2 nanorodes and nanoflakes. The following Raman and XPS characterization also prove the polymorph of the MnO2 is α-MnO2.
The element analysis was further studied by inductively coupled plasma (ICP) to prove the different amount of Mn in the nanocomposites. ICP-OES analysis of the concentrations of Mn in the nanocomposits confirmed that MnO2-GO(1) (230.3 mg g-1) has higher Mn content than MnO2-GO(2) (153.6 mg g-1), which will affect the morphology and electrochemical performances of the nanocomposites.
The spectra in Figure 6c and 6d illustrate the existence of MnO2 by the peaks assigned to Mn 2p3/2 (642.7 eV) and Mn 2p1/2 (653.9 eV), respectively. They have a spin-energy separation of 11.2 eV, further confirming the presence of α-MnO2 in the nanocomposite [42, 43].
Besides the oxygen (O 1s, 532.4 eV) signals from graphene sheets in Figure 6e and 6f, the O 1s peak observed at 530.0 eV is assigned to the oxygen bonded with manganese . On the basis of the quantitative analysis of XPS data, the corresponding atomic ratios of Mn to C for MnO2-GO(1) and MnO2-GO(2) in the nanocomposite are estimated to be 1:1.61 and 1:1.81 by integrating the area of each element peak areas, with their relative sensitive factor taken into account as well. It is worth noting that most carbon atoms in graphene sheets have not been substituted by Mn. However, MnO2-GO(1) still has more replacement Mn position in the nanocomposite than MnO2-GO(2). All of the data further confirm the existence of α-MnO2 and the loading of MnO2 is higher in MnO2-GO(1) than that in MnO2-GO(2).
Based on the investigation of the chemical structure, morphology, and electrochemical behavior of MnO2-GO(1) and MnO2-GO(2), we conclude that the initial properties of GO have notable influences on the morphology and electrochemical activity of the GO-MnO2 nanocomposites. The GO synthesized from the CEG has more functional groups and lager interplane distance. Therefore, MnO2 nanoparticles can distribute homogeneously on GO(1) with high quantity. Because of the high surface area of MnO2-GO(1) and high loading efficiency of MnO2, the specific capacitance of MnO2-GO(1) is almost twice of MnO2-GO(2). The surface chemistry and structural properties of GO is of significant importance as nanoparticles carrier for various applications, such as catalyst, energy storage devices, etc.
This work is supported by the Singapore National Research Foundation under NRF RF Award no. NRF RF2010-07 and MOE Tier 2 MOE2009-T2-1-037. HPY gratefully thanks Professor Richard D. Webster for his fruitful discussions.
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