Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors

We report a simple yet versatile method to simultaneously achieve the exfoliation and reduction of graphite oxide. By treating graphite oxide powders in a commercial microwave oven, reduced graphite oxide materials could be readily obtained within 1 min. Extensive characterizations showed that the as-prepared materials consisted of crumpled, few-layer thick and electronically conductive graphitic sheets. Using the microwave exfoliated graphite oxide as electrode material in an ultracapacitor cell, specific capacitance values as high as 191 F/g have been demonstrated with KOH electrolyte. 2010 Elsevier Ltd. All rights reserved. First prepared almost 150 years ago [1], graphite oxide (GO) has re-emerged as an intensive research interest in recent years due to its role as a precursor for the affordable and large-scale production of graphene-based materials [2]. GOderived materials include chemically functionalized [3] or reduced [4] graphene oxide (exfoliated from GO) sheets, assembled paper-like forms [5,6], and graphene-based composites [7]. Due to the attached oxygen functional groups, GO is electrically insulating and various reduction methods have been developed to restore its electrical conductivity. Chemical reduction using agents such as hydrazine or dimethylhydrazine [3,4,8,9], hydroquinone [10] and NaBH4 [11,12] have been used to reduce GO or graphene oxide exfoliated from GO. Direct thermal treatment at elevated temperatures provides yet another method to reduce GO while eliminating the use of potentially hazardous reducing agents. ‘Thermal shock’ of GO powders at temperatures up to 1050 C has been used [13,14] to obtain exfoliated and conductive graphitic materials. A flash-assisted reduction of films composed of graphene er Ltd. All rights reserved R.S. Ruoff). icrowave assisted exfoliati oxide platelets and their polymer composites has been reported [15]. As a convenient and rapid heating source, microwave irradiation has been used to prepare exfoliated graphite (EG) from a wide range of graphite intercalation compounds (GICs) [16– 19]. EG has also been prepared by directly heating natural graphite with nitric acid and potassium permanganate in a microwave oven [20]. Recently, microwave assisted chemical reduction of graphene oxide has been realized by heating its suspensions in aqueous or organic media [21]. Herein, we report preparation of exfoliated and reduced GO by treating the GO precursor in a microwave oven for less than 1 min. This facile and efficient process has provided a straightforward method to generate what we will call MEGO (microwave exfoliated graphite oxide) that could be used, among other applications, as a high-performance electrode material in energy storage devices such as ultracapacitors. GO powders (Fig. 1a) made from the modified Hummers method [22] were treated in a microwave oven (GE, Model