The Effect of Ball-Milling on the Oxygen Reduction Reaction Activity of Iron and Nitrogen Co-Doped Carbide-Derived Carbon Catalysts in Acid Media

Abstract

Storing renewable energy to complete the energy cycle is a crucial challenge in the movement from a fossil fuel-based economy to a circular one. In the hydrogen cycle, stored hydrogen is converted back to usable energy in a fuel cell. The biggest issue in proton exchange membrane fuel cells (PEMFC) is the replacement of noble metal catalysts with a non-precious metal catalysts (NPMC). Iron and nitrogen doped nanocarbon materials are among the most promising alternatives.In our previous works on iron and nitrogen doped carbide-derived carbons (CDC) we discovered that ball-milling of CDCs has a crucial effect on their porosity and the final activity of the catalyst.1,2 In this work,3 we study the effect of ball-milling conditions and compositions of catalyst precursors comprising a silicon carbide-derived carbon (SiCDC) on the properties of the final catalysts, most importantly their activity toward the oxygen reduction reaction (ORR). Ball-milling rates from 100 to 800 rpm were investigated with 400 rpm proving to be the optimum value. The effect of 1,10-phenanthroline-to-iron ratio in the precursor mixture was also studied, with a 12-1 molar ratio leading to the highest activity. Finally, ZnCl2 addition to the precursor mixture was explored as a pore former during pyrolysis. A ZnCl2-to-CDC mass ratio of 1 resulted in the highest ORR activity. 57Fe Mössbauer spectroscopy confirmed that most of the iron was atomically dispersed as Fe-Nx moieties (Figure 1, left). The most active catalyst showed a kinetic current density of 4.6 mA cm-2 at 0.8 V in rotating disk electrode tests and a current density of 18.6 mA cm-2 at 0.8 V in a single-cell proton exchange membrane fuel cell (Figure 1, right). References Ratso, N.R. Sahraie, M.T Sougrati, M. Käärik, M. Kook, R. Saar, P. Paiste, Q. Jia, J. Leis, S. Mukerjee, F. Jaouen, and K. Tammeveski, J. Mater. Chem. A, 6, 14663–14674 (2018).Ratso, M. Käärik, M. Kook, P. Paiste, J. Aruväli, S. Vlassov, V. Kisand, J. Leis, A. M. Kannan, and K. Tammeveski, Int. J. Hydrogen Energy, 44, 12636–12648 (2019).Ratso, M. T. Sougrati, M. Käärik, M. Merisalu, M. Rähn, V. Kisand, A. Kikas, P. Paiste, J. Leis, V. Sammelselg, F. Jaouen, and K. Tammeveski, ACS Appl. Energy Mater., 2, 7952–7962 (2019). Figure 1