A Comparative Study of Graphene-like Materials with and without Heteroatom Doping

Abstract

Graphene and graphene-like materials are still considered to be a “hot” subject of research in the area of electrocatalysis. Especially, since it is well-known that graphene has various fascinating properties like high specific surface area, very good mechanical strength and good electrical conductivity. Therefore, these properties make graphene suitable for the development of catalyst material for the energy conversion devices including but not limited to fuel cells and batteries.1 Methods for the mass production of graphene are highly necessary in order to meet the commercial demand in the fabrication of electrochemical energy conversion devices. For that, various approaches have been employed to synthesize graphene or graphene-like materials. For example, high-quality graphene can be obtained by the mechanical exfoliation of highly oriented pyrolytic graphite or chemical vapor deposition (CVD) method, however the first method is not suitable for the large scale production of graphene and the latter method is rather expensive. An alternative way to produce graphene-like materials is to use chemical approach or electrochemical exfoliation. For example, graphene oxide (GO) is one of the most commonly used precursors for fabricating graphene-like materials, for example reduced graphene oxide (rGO). And if optimized conditions are used, rGO with very similar properties to pristine graphene can be obtained. It is well-known that an ideal way, graphene is a two-dimensional single carbon layer with one atomic thickness in which the carbon atoms are packed in a hexagonal pattern. However, there are several materials which should be considered rather as graphene-like materials. For example, GO is chemically modified graphene prepared by oxidation and exfoliation and consisting of oxidative form of the basal plane surface, or rGO in which the oxygen content has been reduced using an appropriate method (e.g. chemical, thermal, photo-chemical, etc.). But very often, these materials are referred to as a graphene, even though these materials consist more than one graphene layer or oxygen groups are present on the surface (e.g. in case of GO).2 Different graphene synthesis procedures will lead to various materials with very different structures and which in turn exhibit different properties toward the oxygen reduction reaction (ORR), which is one of the most important processes taking place in fuel cells or metal-air batteries. It is widely known that pristine graphene is rather poor electrocatalyst for the ORR. For example, the ORR on CVD-grown graphene with mainly basal plane surface is rather inhibited.3 However, heteroatom-doped graphene has shown great promise toward the ORR.4 Therefore, the main purpose of this work was to study the ORR on graphene-like materials with and without heteroatom doping. First, in order to synthesize graphene-like materials, different approaches were used. For example, GO was prepared by the most commonly used method based on a modified Hummer´s method. In addition, GO was reduced using the chemical reduction method. Secondly, simple and more environmental friendly approach based on electrochemical exfoliation of graphite was also used in order to obtain GO. Thereafter, heteroatoms (for example nitrogen) were incorporated into the structure of graphene-like materials using also the electrochemical exfoliation method. Finally, the electrochemical behavior of these materials was investigated toward the ORR in alkaline medium by the rotating disk electrode method. The results revealed that depending on the synthesis procedure of GO, different electrocatalytic activity toward the ORR was obtained. Also, a comparison was made with commercially available graphene-based samples.5 The elemental composition of the graphene-like materials was studied by the X-ray photoelectron spectroscopy (XPS) and in case of heteroatom-doped graphene-like materials, nitrogen was detected by XPS.