Oxygen Reduction on Graphene-Based Materials of Different Origin

Abstract

Graphene, the two-dimensional carbon nanomaterial with interesting properties, is still under an intense focus of research. The traditional method to obtain a high-quality graphene is by mechanical exfoliation. However, it has several drawbacks. For example, it is very time consuming method in addition to the low productivity. To overcome those problems, various alternative methods for graphene synthesis have been developed. Currently, one of the most extensively used method to produce graphene is to first, exfoliate graphene oxide (GO) from graphite oxide by solution-based processes followed by reduction of GO by chemical, thermal or UV-assisted approaches in order to obtain a reduced GO (rGO), which should have similar properties to pristine graphene. Also, an electrochemical exfoliation of graphite for the production of graphene has attracted considerable interest due to its high productivity as well as simplicity.1 The use of graphene-based nanomaterials as electrocatalysts for oxygen reduction reaction (ORR) occurring at the cathode of fuel cells and metal-air batteries, has received a great attention.2 Numerous studies have been carried out to prepare graphene-based materials doped with heteroatoms or non-precious metals with improved electrocatalytic activity toward the ORR. However, comparative studies on the ORR on pristine graphene of different origin are still insufficient. Based on the literature, the electrochemical behavior of various graphene-based materials (e.g. CVD-grown graphene, rGO, etc.) toward the ORR depends on the origin of the graphene.3,4 The main purpose of this work is to study the effect of graphene-based materials of different origin on the ORR activity in alkaline medium. Herein, two commercially available graphene samples and pre-synthesized rGO were selected. rGO samples were prepared from GO which in turn was synthesized using two different approaches: modified Hummer´s method and electrochemical exfoliation. Since GO is the starting material of rGO, glassy carbon (GC) electrodes were coated with GO under exactly the same conditions as were prepared with commercially available graphene samples and rGO for comparison purpose. The materials were studied through various characterization techniques such as transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. In the literature, there are only a few reports in which OH‒ ionomer has been used instead of Nafion ionomer as a binder during the catalyst ink preparation. Therefore, it was of special interest to study the effect of OH‒ ionomer on the ORR process as well. For comparison purposes, the catalyst ink made from N,N-dimethylformamide (DMF) was also used. To evaluate the catalytic activity of these graphene-based materials for ORR in O2-saturated 0.1 M KOH solution, several techniques including linear sweep voltammetry and the rotating disk electrode method were applied. Since the rotating ring-disk electrode (RRDE) method gives additional information about intermediate peroxide species (OOH– in the alkaline solution) formed during the ORR process, hence, in some experiments, RRDE method was employed as well. Firstly, the results revealed that the ORR activity depended on the origin of the catalyst material. In more specific, the electrocatalytic behavior of commercially available graphene-based electrodes toward the ORR was higher than that of the rGO-modified GC electrode. In addition, the graphene-based electrodes prepared from the suspension in DMF showed better ORR activity than those made using the suspensions in 2-propanol containing OH‒ ionomer, referring to the inhibiting effect of the ionomer. However, the results also showed that the amount of OH‒ionomer in the catalyst layer should be optimized. Otherwise, the electrocatalytic activity of the catalyst material toward the ORR was significantly reduced. It was also clear that the electrochemical reduction of oxygen on all the studied materials followed a two-step ORR process with the formation of peroxide as an intermediate.