Electrochemical Method for Graphene Preparation from Multi-Walled Carbon Nanotubes
Authors: Danilov Michail Olegovich , Rusetskii Igor A , Dovbeshko Galina I , Khyzhun Oleg Yu , Strelchuk V. , Kolbasov Genadii Ya 
Published: February 2020

There are the numerous methods of graphene synthesis from multi-walled carbon nanotubes (MWCNTs): redox reaction, modification via incorporation of nitrogen and alkaline earth atoms; plasma sputtering; unzipping by microwave radiation, ultrasonic and electrical treatment, treatment with metallic catalytic particles, treatment with laser irradiation and scanning tunnelling microscope tip hydrogenation in high-temperature conditions, as well as unrolling by electrochemical methods [1]. Among these technologically advanced methods, the electrochemical process seems to be most promising since it can be easily controlled by selection of three parameters, namely current, voltage and time.
As it is known from the literature, it is possible to carry out the electrochemical reduction of graphene oxide (GO) to electrochemical reduced graphene oxide (ERGO) in different media by a two-step electrochemical approach. Some investigators the processes of oxidation and reduction of graphene carried out in dilute sulfuric acid first at the anodic and then at cathodic polarization without changing the electrolyte. In our opinion, for more complete unzipping of nanotubes during their oxidation, more concentrated sulfuric acid should be used. The reduction process is preferably carried out in concentrated alkaline medium to neutralize the acid intercalated during oxidation. Additional cathode treatment in an alkaline medium will lead to an increase in the specific surface area and a more complete reducing to graphene. Therefore, the aim of our study was to develop a simple electrochemical method to produce ERGO as the catalyst carrier for oxygen electrodes for fuel cells using anodic oxidation of MWCNTs in an acidic medium and subsequent reduction in an alkaline medium.
MWCNT diameter was 10-30 nm, and the specific surface area was 230 m2g-1. The number of walls ranged from 8 to 15. The anodic oxidation in sulphuric acid and the electrochemical reduction in alkaline electrolyte carried out in a galvanostatic.
The materials synthesized via the electrochemical oxidation of MWCNTs with subsequent reduction were characterized using an electron microscopy (JEM-100 CXII), and X-ray diffraction (XRD) (DRON-4, CuKα radiation), Raman spectra and XPS spectra. Molecular species were analysed with FTIR spectroscopy.
The obtained materials were investigated as cathode material for the oxygen electrodes of power sources in alkaline electrolyte. We have studied electrochemical characteristics of the oxygen electrodes produced by oxidation of the carbon nanotubes over different time. The MWCNT anodic oxidation was carried out under +3 V over 1.5, 4, and 5 hours. We found that in over conditions the more suitable time for the obtained electrochemical parameters (current density) of the oxygen electrodes is 4-hour oxidation duration. We have also studied the MWCNTs oxidation at various potentials for 4 hours. The potentials were chosen in the range +1.8–+4.5 V. Anodic oxidation during 4 hours at +3.0 V creates an extremely active catalytic material, which best suits for oxygen electrodes compared to those made of electrochemically oxidized MWCNTs at other potentials. The materials obtained by anodic oxidation of the carbon nanotubes were subsequently reduced in the alkaline solution at -1.4 V potential over 4 hours.
The Raman spectra of all three measured samples show the presence of the Raman marker bands characteristic of the graphite structure, namely, D, G and 2D modes. Based on the analysis of the shape and position of these bands one can assume that after the anodic oxidation, the MWCNT crystallinity becomes worse while it improves after subsequent electrochemical reduction. This observation is also confirmed by corresponding XRD data. The frequencies for the D mode, G mode, and 2D mode suggest well-crystallized graphene structure. Closely positioned tags of C atoms in the XRD rings suggest that the synthesized sample consists of flat wavy sheets. As follows from the analysis of the Raman and IR spectra, XPS and XRD data, in the result of the MWCNT electrochemically treatment we obtained graphene nanoplatelets (Fig1), which contains carbon oxide groups on its surface. Electrodes based on electrochemically reduced graphene nanoplatelets produced from oxidized MWCNTs have the electrocatalytic characteristics close to those of the electrodes from MWCNTs containing deposited platinum. Obtained graphene nanoplatelets were stable over six months subject to testing in the fuel cell mock-up galvanostatic mode.
The electrochemical synthesis of graphene nanoplatelets via anodic oxidation of MWCNTs and it reduction has been confirmed by electron microscopy, XRD, X-ray fluorescence, IR and Raman spectroscopy. Electrochemical studies have shown that the obtained graphene are promising materials as a catalyst carrier for fuel cells oxygen electrodes.

M.O. Danilov, et al., Graphene Science Handbook. Fabrication Methods, (Eds. M. Aliofkhazraei, et al.), CRC Press/Taylor & Francis, 2016, pp. 205.

Figure 1