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A group of researchers from China has discovered a scalable, safe and green method to synthesize graphene oxide using water electrolytic oxidation of graphite. The details of their method were published in Nature Communications earlier this month.
Graphene oxide has unique, desirable properties
Graphene oxide is an oxidized form of graphene. The exact structure of graphene oxide depends on the synthesis method used, but it typically consists of graphene sheets laced with covalently bonded oxygen-containing functional groups including hydroxyl, epoxy, and carboxyl groups.
Graphene oxide is often used as a precursor for graphene. The oxygen-containing functional groups of graphene oxide make it hydrophilic so it can be dispersed in water, allowing easy assembly of graphene macrostructures. Recently, the unique properties of graphene oxide have led to a variety of applications beyond acting as a graphene precursor. Graphene oxide can be functionalized, providing a tunable chemical platform. As a result, graphene oxide has found applications in supercapacitors, batteries, fuel cells, biomedicine, and polymer composites.
Traditional graphene oxide production presents risks to safety and the environment
Graphene oxide is typically produced from graphite using strong oxidizing agents. The current routes to graphene oxide have significant disadvantages. The Brodie and Staudenmaier methods, which use KClO3 and nitric acid to oxidize graphite, both produce dangerous gases and present an explosion risk. The more commonly used Hummers’ method uses concentrated H2SO4 and KMnO4 for oxidation. However, reactive intermediates produced by the Hummers’ method can also cause explosions. All three methods produce copious amounts of chemical waste and can produce graphene oxide that is contaminated with metal ions. For this reason, researchers have been searching for new, greener, and more efficient methods to produce graphene oxide.
Electrochemistry provides a green, safe alternative for graphene oxide synthesis
Recently, the potential of electrochemical methods to provide environmental-friendly, efficient routes to graphene oxide has been highlighted. However, until now, graphene oxides produced electrochemically have suffered from low degrees of oxidation. The research group from China have now reported an electrochemical method to produce graphene oxide with a high degree of oxidation. The method is efficient, rapid, environmentally friendly, and does not present an explosion risk.
The reported method used a two-step electrochemical process. First, flexible graphite paper is subjected to electrochemical intercalation in concentrated H2SO4 to form stage-I graphite intercalation compound paper (GICP). The GICP then undergoes water electrolytic oxidation in dilute H2SO4. The graphite oxide produced is then exfoliated in water to produce graphene oxide.
The electrolytic oxidation of the GICP uses water as the oxidation agent, while the H2SO4 acts as a control agent. No other oxidants are used so there was no risk of explosion and the graphene oxide produced is not contaminated with metal ions. The graphene oxide produced can be washed with much less water than is used for the chemical oxidation methods, so less contaminated wastewater is produced. Furthermore, the electrochemical oxidation rate is over 100 times faster than the commonly used chemical oxidation methods, and the team can control the degree of oxidation of the graphene oxide by altering the concentration of the H2SO4 during electrolysis.
The electrochemically synthesized graphene oxide has similar properties to graphene oxide produced using the Hummers’ method, with a similar structure, degree of oxidation, and oxygen-containing functional groups. The team demonstrated the use of their graphene oxide in transparent conductive films, flexible and strong papers, and ultra-light elastic aerogels
In conclusion, researchers from the Chinese Academy of Sciences have reported an important new synthesis method for graphene oxide that is safer, greener, and more efficient than traditional chemical oxidation methods. Although their process needs to be scaled up to industrial levels, their work has important implications for applications of both graphene and graphene oxide.
References and further reading:
Songfeng Pei, Qinwei Wei, Kun Huang, Hui-Ming Cheng, Wencai Ren, ‘Green synthesis of graphene oxide by seconds timescale water electrolytic oxidation’ Nature Communications 9:145, 2018.
https://www.azonano.com/article.aspx?ArticleID=4044 Accessed January 10th, 2017.