Making Graphene Oxide Green
The growing demand on graphene oxide raises substantial questions regarding environmental concerns. A group led by Ren Wencai at the Institute of Metal Research of the Chinese Academy of Sciences developed a green and fast method which allows to produce graphene oxide on large scales.
Graphene oxide (GO) became an independent key product in the graphene market during the last years with an estimated account for approximately 40% of the total industry revenue share by 2024. Fueled by the fast development in car, aerospace and electronics industries, which discovered GO as valuable electrode material or make use of the unique properties of GO in light weight composite materials, the market is estimated to exceed 200 million USD by 2024. This growing trend poses an urgent demand on the development of large scale and ecological friendly production methods for GO.
Large scale production of GO relies on the exfoliation of graphite oxide, which was prepared first in 1859 by Brodie by reaction of graphite with strong oxidants. Since then, the oxidation methods for graphite experienced only slight modifications. Thus, not surprisingly, the three main methods for large scale graphite oxide production nowadays still rely on oxidizing graphite with concentrated H2SO4 in presence of strong oxidizing acids, such as fuming nitric acid (Staudenmaier method), concentrated nitric acid (Hofmann method) or in presence of NaNO3 and KMnO4 as strong oxidizing agents (Hummers method).
This poses not only challenges in minimizing environmental pollution but also raises health and safety issues. The Staudenmaier method produces hazardous gases such as NOX and carcinogenic ClO-, while a mixture of KMnO4 and H2SO4 in Hummers method can cause severe explosions at elevated temperatures. Additionally, removing the excess acid and KMnO4 needs large amounts of water and therefore produces a considerable amount of waste, contaminated with metal ions and strong oxidizing chemicals. Besides the environmental concerns, these three methods also suffer from long reaction times, which scale unfavorably with production cost.
A team led by Ren Wencai at the Institute of Metal Research of the Chinese Academy of Sciences (IMR, CAS) addressed these shortcomings by developing a green production method which is based on electrochemical oxidation of graphite. In their study, which was published in Nature Communications  lately, they first produce a stage-I-graphite intercalation compound (GICP) by electrochemical intercalation of sulphuric acid molecules in flexible graphite paper (FGP). This intercalation compound is then used as an anode material and oxidized in an electrochemical reaction in diluted H2SO4. Oxidation occurs within few seconds, thus drastically reduces the production time of several hundred hours in traditional methods, and can be visually traced by a color change from blue to yellow. Moreover, during electrochemical oxidation, the formed graphite oxide exfoliates and the GO flakes can be easily collected by filtration before being washed with water. Strikingly, compared to the traditional methods the amount of water, which is used during the purification step, could be reduced by 85% and does not contain metal ion contamination, making the new synthesis method a first step to green GO production.
But how does the electrochemically produced GO (EGO) compare to traditional GO in terms of quality?
The researchers compared the properties of GO sheets obtained by Hummers method (HGO) and by their green method and found that electronic, optical and structural properties are fairly identical. The C/O ratio is around 1.7, which is lower than the typical value of 2.0 observed in HGO. However, the type of oxygen-containing functional groups is essentially the same. Importantly, the degree of oxidation can be tuned between 1.5 and 2 by simply changing the concentration of the sulphuric acid in the oxidation step.
Besides the green aspect, up-scaling and fast production is similar important for an industrial process implementation. Therefore the Ren group developed a continuous process, where a sacrificial GICP anode is continuously supplied to the electrolyte with a specific rate. In this way they could achieve a productivity of 12 g h-1 in a 250 ml lab scale experiment, with the prospective to up-scale the process.
This study addresses important environmental questions in GO production, which will become an increasing challenge, as the graphene market grows. Likewise Ren and co-workers demonstrate an elegant, fast and scalable production method, meeting the demands for industrial production.
 Pei, S. et al. Green synthesis of graphene oxide by seconds timescale water electrolytic oxidation. Nat. Commun. (2018) 9:145, DOI: 10.1038/s41467-017-02479-z