Carbon-Based Catalysts for Oxygen Reduction Prepared By Ball Milling

Abstract

Carbon-based materials, for example graphite, graphene, graphene oxide (GO), have been widely used as substrates in various fields including electrocatalysis and electroanalysis. More specifically, nanocarbons have been considered as electrode materials including but not limited to (bio)fuel cells, supercapacitors and (bio)sensors. The main advantages of the electrodes based on carbon nanomaterials are their low cost compared to metals, mechanical stability and a wide potential window in electrochemistry.1 The oxygen reduction reaction (ORR), which takes place for example in fuel cells, metal-air batteries and (bio)sensors, greatly depends on the electrode material. Therefore, a major focus is on the development of efficient cathode catalysts for the ORR. It is well-known that the reduction of oxygen is rather inhibited on pristine carbon materials.2,3 However, a range of carbon-based materials like heteroatom-doped carbon or transition metal-nitrogen-carbon have shown great promise as ORR electrocatalysts in alkaline media.1,2 Up to date, in most cases, complicated synthesis procedures have been employed to produce the ORR catalysts. Therefore, on the one hand, it is very important to develop a catalyst with high ORR activity. But on the other hand, it is crucial to use a simple but efficient approach for the preparation of good ORR catalyst. In this regard, ball milling has been usually used to reduce the size of the carbon material, but ball milling has also been employed in order to modify the surface of the nanocarbon.4,5 These advantages make ball milling an attractive method to produce catalysts for the ORR. In this study, a very simple method based on ball milling is employed in order to synthesize ORR catalysts. In more specific, transition metal and nitrogen co-doped or solely nitrogen-doped nanocarbon catalysts were synthesized by ball milling using different carbon materials in the presence of metal-nitrogen or nitrogen precursor followed by pyrolysis at 800 °C. For the physico-chemical characterization of the doped carbon-based catalysts, scanning electron microscopy, energy dispersive X-ray spectroscopic measurements and X-ray photoelectron spectroscopy were used. The SEM images revealed different morphology for the catalysts. In addition, the elemental analysis of the materials showed that depending on the catalyst preparation procedure, metal and nitrogen or only nitrogen was detected in the catalyst material. The electrochemical measurements in O2-saturated 0.1 M KOH conducted by the rotating disk electrode method exhibited good electrocatalytic activity of all of these catalysts toward the ORR compared to the unmodified carbon materials. This shows that the simple method based on ball milling proposed herein is suitable for the synthesizing ORR catalysts.