Reduced Graphene Oxide Aerogel inside Melamine Sponge as an Electrocatalyst for the Oxygen Reduction Reaction

A graphene oxide aerogel (GOA) was formed inside a melamine sponge (MS) framework. After reduction with hydrazine at 60 °C, the electrical conductive nitrogen-enriched rGOA-MS composite material with a specific density of 20.1 mg/cm3 was used to fabricate an electrode, which proved to be a promising electrocatalyst for the oxygen reduction reaction. The rGOA-MS composite material was characterized by elemental analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. It was found that nitrogen in the material is presented by different types with the maximum concentration of pyrrole-like nitrogen. By using Raman scattering it was established that the rGOA component of the material is graphene-like carbon with an average size of the sp2-domains of 5.7 nm. This explains a quite high conductivity of the composite obtained.


Experimental (GO synthesis)
For the GO synthesis, the modified Hummers method was used [1]. A vessel with a Teflon mixer and a thermometer was filled with 20 g of graphite powder and 650 ml of concentrated Н2SO4. During the mixing of the load, 10 ml of concentrated HNO3 were added, and the mixture was heated in a water heater at 45 °С. Next, it was cooled down to 10-15 °С, and 72 g of KMnO4 were added gradually over 5 h while maintaining the temperature below 20 °С. The resulting mixture was heated to 40 °С, then mixed to a pastelike state and left for 24 hours. It was then cooled down to 10-15 °С, and 120 ml of water were slowly added at a temperature below 10-15°С. The mixture was kept at 45 °С for 1 h, then one litre of water was added when cooling down the mixture. The suspension obtained was then poured into a 3-litre glass vessel and slowly (in order to avoid foam formation) 70 ml of a concentrated Н2О2 (28 mass%) were added. The suspension colour turned to bright yellow. The warm mixture was centrifuged, the deposit was dispersed in a 3% solution of HCl (2 litres) and centrifuged again. This operation was repeated 4 or 5 times. After the operation was completed, the GO sample was diluted with 25 litres of distilled water and washed until the acidity of the washing water was below 4 pH and no SO4 2-and Clions remained. In order to prepare the water suspension of GO nanosheets, 300 mg of GO were mixed with 400 ml of water, followed by the suspension processing unit being placed in an ultrasonic bath for 2-4 h, and then finally centrifuging the suspension for 15 minutes at 3000 g.

Elemental Content of graphene oxide
The composition of graphene oxide depends on the method of their production. The graphene oxide selling by the Graphene Supermarket Company contains 79% of carbon and 20% of oxygen [1]. The remaining one percent left is likely contributed by hydrogen. In the pioneer work, Brodie [2] determined the graphite oxide composition C:O:H as 61.04:37.1:1.85 or, in molecular representation, as C2.19O1.00H0.80. Our fresh samples of graphite oxide obtained using the Hummers method contained by mass 50.10% of carbon, 44.81% of oxygen and 2.69% of hydrogen. The sum of these quantities is somewhat smaller than S1 100% because the sample contains some technological impurities, which may reside in closed pores of graphite oxide and therefore cannot be washed out with distilled water. [1] https://graphene-supermarket.com/Dispersion-in-Water-Single-Layer-Graphene-Oxide-175-ml.html.

X-Ray Photoelectron Spectra of Graphene Oxide
X-ray photoelectron spectroscopy [1] (XPS) is widely used for the characterization of carbon-based materials and graphene oxide in particular [2]. It is natural to consider that for a single-layer graphene oxide nanosheets composition determined by XPS, should coincide with the bulk composition of the film consisting of monolayer nanosheets. A survey spectrum of a GO sample presented by a film obtained by precipitation of GO suspension is presented in Figure S1. The oxygen concentration is estimated in 18 -25 at. %, whereas concentrations of sulfur, nitrogen, and chlorine (technological impurities) are 0.6 -1.3 аt. %, > 0.3 аt. % and > 0.2 at. %, respectively. Figure S1. The survey XPS spectrum of a GO film. The Cl 2p peak is located at ~200 eV. Figure S2. The C 1s XPS spectra of GO films and their decomposition.

S2
High-resolution XPS C 1s core level spectra of GO films shown in Figure S2 contain three peaks (Table S1): peak 1 at 284.5 eV which is due to carbon atoms in the graphene network, peak 2 at 286.5 eV which is due to carbon atoms singly boned with oxygen atoms, and peak 3 at 288.5 eV which is due to carbon atoms of carbonyl groups. This peak assignment is in agreement with assignments made elsewhere in the literature [3][4][5][6][7].

Raman Spectra of Graphene Oxide
Raman spectra are often invoked for characterization of various carbon compounds. The Raman spectrum of a diamond contains [1][2][3][4] a narrow peak at 1332 cm -1 , whereas the most intense peak in Raman spectra of "good" graphite, e.g., highly-oriented pyrolytic graphite (HOPG), usually denoted as the peak G is located [5] at 1580 cm -1 . If graphite has a large number of defects, then the peak D named as a disorder peak appears [6][7][8][9][10][11] at ~1350 cm -1 . Raman spectrum of graphene oxide under study is shown in Figure 3 and the peak energies are presented in Table 2. The peak designations therein correspond to the designations commonly used in the literature. The ratio intensities ID/IG of peaks G and D, can be related [4] to graphene crystallite sizes in the basal plane La according to the following equation: La = (2.4 × 10 -10 ) λ 4 L (ID/IG) -1 (1) where λL is the laser wavelength in nm. When computed according to Eq. 1 sizes La of the sample, whose ID/IG ratio is given in Table 2, is 20 nm. Figure S3. A Raman spectrum of GO. The excitation wavelength was 532 nm. This work a λ is the laser wavelength.

GO sheet and film morphology
The dimensions of GO nanosheets depend significantly on the initial graphite, the duration and temperature of subsequent washing, drying and storage operations. Figure 4 shows SEM images of individual large sheets of exhaust gas on a smooth surface. Figure S4. Microphotographs of GO nanosheets.
Thick GO films separated from substrates are sometimes named as graphene oxide papers (GOP). A microphotograph of both GOP sides is presented in Figure S5. It follows from Figure S6, which displays the GOP cross-section, that the paper structure is layered. The GO nanosheets were found to possess inhomogeneous packing because of the folds.