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Influence of the synthesis conditions of reduced graphene oxide on the electrochemical characteristics of the oxygen electrode

M.O. Danilov*, I.A. Slobodyanyuk, I.A. Rusetskii, G.I. Dovbeshko, G.Ya. Kolbasov

Vernadskii Institute of General & Inorganic Chemistry of Nat. Acad. Sci., Kyiv, Ukraine

Abstract: Reduced graphene oxide (RGO) was obtained by chemical synthesis from multi walled carbon nanotubes. Using a suitable oxidant, we longitudinally “unzipped” nanotubes to form graphene oxide nanoribbons and then obtained RGO with a proper reductant. Standard redox potentials of carboxy groups were used for choosing oxidant and reductant. It has been shown that the required oxidant potential in acid medium should be more + 0.528 V and reductant potential in alkaline medium- less – 1.148 V. Current-potential curves for oxygen electrodes based on RGO, obtained by using the oxidants K₂Cr₂O₇, KMnO₄ and the reductants NaH₂PO₂, Na₂SO₃, were analyzed. The electrochemical characteristics of RGO in the oxygen reduction reaction were depended on the redox power of the reagents. We demonstrated that obtained RGO could be promising material for oxygen electrodes of fuel cells.

Keywords: reduced graphene oxide (RGO), synthesis, electrode materials, fuel cells, oxygen electrodes

Received October 04, 2014; Revised November 27, 2014; Accepted November 30, 2014

Citation: M.O. Danilov, I.A. Slobodyanyuk, I.A. Rusetskii, G.I. Dovbeshko, G.Ya. Kolbasov, Influence of the synthesis conditions of reduced graphene oxide on the electrochemical characteristics of the oxygen electrode. Nanoscience and Nanotechnology Research, 2 (1). 12-17. January 2014. doi:10.12691/nnr-2-1-3

1. Introduction

An application of air or oxygen electrode in device generating electrical energy is useful, it does not give rise to environmental problems and allows saving nonrenewable natural resources. Air and oxygen electrode is a three-phase electrode-electrolyte-gas system, where the generation of electric current is localized at the phase boundary. The current magnitude generated at such gas diffusion electrode depends on the triple contact zone of these three phases. In its turn, the electrode itself is composed of catalyst and carrier. The interaction between them determines the quantity of generated current, which depends on catalyst being used. It is known that nowadays the most effective catalyst for oxygen recovering is platinum, which is a very expensive material. A great number of works is oriented towards the investigation of other effective but less costly catalysts [1]. Another problem is catalytically active and stable carrier. In works [2,3,4] the benefits of carbon nanotubes used as the carrier are shown. At present time owing to the appearance of the new nanocarbon material graphene a series of works were dedicated to its studying as an electrode material for lithium-ionic accumulators [5] and also as a catalyst carrier for catalysts in fuel cells [6-10].

At the moment the following methods of synthesis graphene from carbon nanotubes are known: intercalation of alkaline earth elements [11,12] and nitrogen [13]; plasma etching [14,15,16]; microwave unzipping [17,18]; unzipping by catalytic metal nanoparticles [19,20]; ultrasonic unzipping [21,22]; opening-up by laser irradiation [23]; electrical unzipping [24]; hydrogenating at the high-temperature [25]; unzipping by influence of a scanning tunneling microscope [26]; electrochemical unrolling [27]; the redox chemical synthesis [28,29].

The chemical methods for the preparation of graphenes involve the stages of preparation graphene oxide (GO) and its subsequent reduction to give the so-called reduced graphene oxide (RGO). Synthesis, structure and chemical properties of GO and RGO were systematized and described in detail in reviews [28,30,31].

In view of this, the study of the dependence of electrochemical properties on the method for the preparation of RGO from multi wall carbon nanotubes (MWCNTs), which is used as a catalyst support for fuel cell oxygen electrodes, is of great interest.

2. Materials and Methods

As a precursor were selected MWCNTs with hard structure of graphene layers, which leads to a decrease in the binding energy between the carbon atoms in the graphene layer. Using a suitable oxidant, one cans longitudinally “unzipping” nanotubes to form GO nanoribbons and then obtain RGO by action with a reductant.

The outer diameter of MWCNTs was about 10-40 nm, the specific surface area was 230 m²g⁻¹, with a bulk density of 25-35 g dm⁻³. The number of walls was from 2-15. MWCNTs were purified of catalyst by means of hydrofluoric acid treatment.

The samples of RGO obtained by synthesis examined with the aid of a JEM-100 CXII electron microscope. The resulting synthesis RGO samples examined with an electron microscope JEM-100 CXII.The X-ray phase analysis was performed with the aid of a DRON-4 X-ray diffractometer with CuKα radiation. Raman spectra of initial MWCNTs and MWCNTs after treatment were registered by inVia Raman microscope (Renishaw) under the excitation of He-Ne laser with λex = 0.6328 Å. The position of referent Si sample at 520 cm⁻¹ was used as the reference for wavenumber calibration. WiRE3.4 software was used for Raman data acquisition and data analysis.

The synthesized materials were prepared by pressing two-layer oxygen electrodes. The hydrophobic layer contained 0.07 g/cm² acetylene black with 25% polytetrafluoroethylene, and the active layer contained 0.02 g/cm² RGO, with 5% polytetrafluoroethylene. The investigations were carried out on a fuel cell mockup, a zinc electrode being used as the anode. A mockup for the testing of gas-diffusion electrodes is described in [32]. The electrolyte was a solution of 6 M KOH. A silver-chloride electrode connected through a salt bridge was used as a reference electrode. The electrochemical characteristics were recorded under galvanostatic conditions. The oxygen source was a U-shaped electrolyzer with alkaline electrolyte. Oxygen was supplied to the gas electrodes under an excess pressure of 0.01 MPa. Before measurements, the oxygen electrode was blown through with oxygen for an hour.

Platinum is deposited by electrochemical method from an aqueous solution containing 3% H₂PtCl₆ and 0.2% lead acetate (II) at a voltage of 1 V for 2 minutes, the current direction is changed through 30 s.

3. Results and Discussion

Based on the standard redox potentials of carbon [33] (Table 1) the required oxidant potential in acid medium should be more + 0.528 V and oxidant potential in alkaline medium should be more – 0.603 V.

Table 1. Oxidation potentials of carbon in different media

The most famous strong oxidants (pH < 7 / pH > 7) are KMnO₄ (+1.69 V / +0.588 V); ozone (+2.075 V / +1.247 V); Сr₂O₇²⁻ (+1.36 V / —)); OsO₄ (+1.02 V / +0.17 V); FeO₄²⁻ (+2.07 V / +0.8 V), as well as several others [33].

However, if the process of breaking carbon bonds in nanotubes due to kinetic constraints, then the use of the thermodynamic redox scale for this process is not applicable.

Table 2. Oxidation potentials of carbon in different media

Accordingly, for the reduced of graphene oxide in alkaline medium should be used reducing agents, with potentials more less than -1.148 V (Table 2). In an acidic environment to reduced graphene oxide should be used reducing agents, with potentials more less than -0.320 V.

For the oxidation of MWCNTs in the acidic environment were used such oxidant as KMnO₄ (+1.69 V) and K₂Cr₂O₇ (+1.36 V). As reducing agents in an alkaline medium were used the solutions of sodium hypophosphite (E○ = -1.51 V) and sodium sulfite (E○ = -0.936 V) [33]. The method of synthesis RGO with using KMnO4 is described in [34].

One gram of MCNTs was dispersed for 1 h by stirring in 30 mL of concentrated sulfuric acid. Then K₂Cr₂O₇ was added under stirring for 72 h at a temperature of not over 21°C. Then the diluted mixture was filtered on a fine paper filter. The precipitate was washed with hydrochloric acid (10%) and distilled water. Reduction of the oxidized product was done by similar manner [34].

Figure 1 – Figure 2 shows micrographs of RGO, reduced by sodium sulfite (Figure 1) and sodium hypophosphite (Figure 2) from the GO obtained by oxidation of MWCNTs using potassium permanganate. Figure 3 shows the XRD analysis of products obtained by reduction of sodium hypophosphite (curve H-RGO) and sodium sulfite (curve S-RGO).

Figure 1. Micrograph of a sample of RGO obtained from MWCNTs oxidized by KMnO₄ and reduced with sodium sulfite

Figure 2. Micrograph of a sample of RGO obtained from MWCNTs oxidized by KMnO₄ and reduced with sodium hypophosphite

The XRD analysis showed the presence of two peaks, one of which corresponds to the reflection from the interplanar spacing between the graphene layers and is located at 2θ = 25.6°, and the second near 2θ = 21° corresponds to SiO₂ (substrate). Distance between the planes in graphene is found to be 3.43 Å, which is larger than the distance between planes in graphite (3.35 Å).

Figure 3. X-ray diagram of samples of graphene oxide reduced: (H-RGO) with sodium hypophosphite, (S-RGO) with sodium sulfite, (MWCNT) initial MWCNTs

Figure 4. Dependence of potential on current density for oxygen electrodes with the active layer in an amount of 0.02 g/cm² consisting of initial MWCNTs (1) and RGO which were reduced by the sodium hypophosphite from MWCNTs oxidized with potassium dichromate (2) and potassium permanganate (3)

Results obtained were analyzed using Bragg-Wolf equation given in [35]. The initial carbon nanotubes and samples of synthesized RGO have similar reflections; in the case of RGO reflection at 2θ = 25.6° becomes wider, the crystallinity of the RGO sample deteriorates, which indicates a decrease in the size of particles. Figure 4 shows plots of potential against current density for oxygen electrodes based on RGO reduced with sodium hypophosphite from the MWCNTs oxidized by using К₂Сr₂O₇ (RGO-Сr) (curve 2 in Figure 4) and MWCNTs oxidized by using potassium permanganate (RGO-Mn) (curve 3 in Figure 4).

As can be seen from the current-voltage curves (Figure 4), electrodes based on RGO-Mn have better characteristics than the electrodes based on RGO-Сr. Consequently, the electrochemical properties of oxygen electrodes correlated with the oxidizing power of oxidants used.

Figure 5. Raman spectra obtained at 633 nm and the intensity of 50% for initial MWCNTs (MWCNT) and RGO reduced by sodium hypophosphite from the nanotubes oxidized with potassium dichromate (MWCNT – Сr) and potassium permanganate (MWCNT – Mn)

The main feature of Raman spectra (Figure 5) of graphite structure is a so-called G band (1600 cm⁻¹) with E1g symmetry in the Г point of the Brillouin zone [36] that correlates with the ordering of graphite crystal lattice. The second feature of graphite-like materials is a D band that characterizes the disorder of graphene layer lattice [37], [38] and refers to breathing vibrations of rings of graphene layer in the K point of Brillouin zone. The second order mode of this vibration (2D band) is registered at 2600−2700 cm⁻¹, and it has an intensity which usually exceeds that of the second order vibrations and manifests a strong electron-phonon interaction. The characteristic feature of Raman spectra of MWCNT is the halfwidth equal to 50 cm⁻¹ for G-mode and above 60 cm⁻¹ for D-mode as well as D/G intensity ratio more than one, on the high- frequency wing of G mode one could register D′ mode [39] also. Raman spectrum of graphene monolayer contains G and 2D bands and does not contain D mode similar to graphite, intensity of 2D mode more than G mode. Raman spectrum of graphene nanoparticles contains G, D and 2D bands analogously to MWCNT. Position of the 2D band maximum could be used as a characteristic for determination the number of layers in the graphene sheets [38,39].

Initial MWCNT has D-mode at 1322 cm⁻¹, 2D-mode at 2647 cm⁻¹, G-mode consists of 2 modes: at 1570 cm⁻¹ and, so-called, D′-mode at 1598 cm⁻¹; the mode near 1130 cm⁻¹ and breathing modes at 226.5, 344,6 cm⁻¹. After treatment with KMnO₄ (К₂Сr₂O₇), the spectra changed and breathing modes located at 193 and 424 cm⁻¹ (235 and 349 cm⁻¹), correspondently, that should be the feature of non-homogeneity of the carbon nanotubes bungle or changes after treatment, however, the first supposition is better. After treatment with KMnO₄, the G and D modes became wider, and G-mode consists from one mode at 1587 cm⁻¹ and D-mode at 1328 cm⁻¹, the intensity of D-mode more than G-mode, number of defects became more and electron-phonon interaction became less, intensity of 2D-mode is small and anharmonicity became more. In initial MWCNT the value of anharmonicity is equaled to 3 cm⁻¹, in MWCNT-Mn - 13 cm⁻¹. Based on the spectra analysis, a size of particles became less after treatment with KMnO₄, than in initial samples. After treatment with К₂Сr₂O₇, changes the initial nanotubes are less than after treatment with KMnO₄. The spectra of the MWCNT-Cr looks similar to spectra of initial MWCNT with additional defects, however, a number of defects is less than in the case of MVCNT-Mn, the intensity of D-mode became more than those for G-mode, the intensity of 2D-mode is less than in the case of initial MWCNT and, the anharmonicity of 2D mode – 7 cm⁻¹, more than in the case of initial nanotubes (3 cm⁻¹) and less than in MWCNT after treatment with KMnO₄ (13 cm⁻¹). Estimation of size of particles from Raman spectra (ratio D/G at λ= 632.8 nm) according to [40] gives 38.5 nm for initial MWCNT. The size for RGO reduced with sodium hypophosphite from the MWCNTs oxidized by using К₂Сr₂O7 (Cr-MWCNT), is equaled to 29.6 nm. For RGO reduced with sodium hypophosphite from the MWCNTs oxidized by using potassium permanganate (RGO-Mn) we got a size equaled to 27.5 nm. These results were obtained with formulae taken from [40]:

where La is the crystallite sizes, El is the excitation energy used in the Raman experiment in eV, ID and IG intensity of the D and G modes, respectively.

After the oxidizing with potassium permanganate, the surface of nanotubes undergo to oxidation better than in the case of oxidizing with К₂Сr₂O7. In the case of exposure of MWCNT by HNO3, the surface of nanotubes became drastic disordered [41]. Our oxidants are stronger than HNO₃. So we suppose that after treatment by oxidizing with potassium permanganate, the MWCNT became more defective and disordered, Raman spectra drastically changed and looks like the amorphous carbon. The last fact lead to better longitudinal (sometimes transverse) breaks of carbon nanotubes, and as result, outer layers come off and their unzipping occur. We suppose that after treatment with potassium permanganate, initial MWCNT undergoes more changes in structure that those for К₂Сr₂O₇, the numbers of defects became more, sizes of particles became less and nanotubes seems to unzip better than in the case of oxidation of К₂Сr₂O₇, number of layers are equaled to those in carbon nanotubes or less.

Figure 6 shows plots of potential against current density for oxygen electrodes based on RGO obtained from the MWCNTs oxidized by potassium permanganate followed by reduction this product with sodium sulfite (curve 2) (RGO-SS) and sodium hypophosphite (curve 3) (RGO-SH).

Figure 6. Dependence of potential on current density for oxygen electrodes with the active layer in an amount of 0.02 g/cm² consisting of initial MWCNTs (1) and MWCNTs oxidized with potassium permanganate which were reduced to RGO with sodium sulfite (2) and sodium hypophosphite (3)

Figure 7. Dependence of potential on current density for oxygen electrodes with the active layer in an amount of 0.02 g/cm² consisting of initial MWCNTs (1), RGO obtained by the reduction of sodium hypophosphite MWCNTs oxidized with potassium permanganate (2), MWCNTs with deposited metallic platinum in an amount of 10 wt%

As can be seen (Figure 6), the electrodes based on RGO-SH at the same potential have large currents than the electrodes based on RGO-SS. Consequently, the electrochemical properties of the oxygen electrode correlated with a reduction power of the reducing agent. It should be noted that the standard redox potential of sodium sulfite in an alkaline medium среде (E○ = -0.936 V) more positive than required, in our view, the reduction potential of the GO in an alkaline medium E○ = -1.148 V. We can assume that sodium sulfite partially reduced some carboxy groups the reduction potential which is more than -0.936 V. In our opinion, this leads to a difference in the characteristics of the oxygen electrodes based on RGO-SS.

Figure 7 shows the comparative characteristics of the oxygen electrodes based on synthesized RGO and MWCNTs with deposited platinum.

As can be seen from the Figure 7, electrodes based on RGO-SH in their electrocatalytic characteristics approaching to electrodes based on carbon materials with deposited platinum. Oxygen electrodes based on reduced graphene oxide in galvanostatic mode were stable for six months at a current density 200 mA/cm².

4. Conclusions

We showed that a choice of oxidizing and reducing agents for the synthesis of reduced graphene oxide from multi-walled carbon nanotubes could be done on the basis of standard redox potentials. We found and concluded that the obtained reduced graphene oxide is a promising carrier of catalysts for the oxygen fuel cell electrode, which can replace the commercial electrode materials containing platinum.

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