A Greener Synthesis of Fe-N-C Catalysts for the Oxygen Reduction Reaction

Abstract

The world is faced with an ever-increasing energy demand that, under the aspect of global warming, must be covered by renewable sources such as wind- and solar energy. However, the main challenge linked to these sources is to provide a continuous energy supply, demanding highly efficient energy storage and conversion devices.One promising conversion technology are fuel cells. [1] These usually rely on expensive and rare platinum group metal (PGM) catalysts for the sluggish oxygen reduction reaction (ORR). In order to serve the human energy demand, cheap and scalable fuel cell systems are required. The class of M-N-C (M = Fe, Mn, Co) type catalysts shows particularly good catalytic behaviour comparable to PGM based catalysts. However, promising catalysts are usually synthesised from toxic materials with solvent and energy intensive methods. [2]Here, we present a synthetic approach towards Fe-N-C type catalysts, that uses only non-hazardous chemicals, produces no wastewater and is more energy efficient than classical approaches. We used iron hydroxide as iron source for its lower environmental impact as compared to ferric chloride. Cyanoguanidine (non-toxic) was chosen over cyanamide (toxic) as a pore forming agent and nitrogen source. Tryptophan was used over PANI and the like for its availability from natural sources with no need for toxic precursors. Instead of classical wet chemical approaches we used a solvent free mechanochemical synthesis. And by using vulcan carbon as electron conducting scaffold, we were able to carbonize at 750 °C, increasing the overall energy efficiency. The catalytic activity towards the ORR of the obtained catalysts was determined by rotating disc electrode measurements in acidic and basic media. With a 100 mV lower onset potential, their performance in an alkaline environment is good but not quite comparable to Pt based catalysts.Future studies will focus on the correlation of nitrogen content as well as porosity of the catalysts with their electrocatalytical behaviour.[1] I. Staffell, D. Scamman, A. Velazquez Abad, P. Balcombe, P. E. Dodds, P. Ekins, N. Shah, K. R. Ward, Energy Environ. Sci. 2019, 12, 463–491.[2] H. T. Chung, D. A. Cullen, D. Higgins, B. T. Sneed, E. F. Holby, K. L. More, P. Zelenay, Science 2017, 357, 479-484. Figure 1