Investigating Oxygen Reduction Performance of Fe-N-C Catalysts for Proton Exchange Membrane Fuel Cells

Abstract

Non-precious Fe-N-C catalysts have reached high activity levels for acidic oxygen reduction reaction (ORR) in rotating ring disk electrode (RRDE) setup approaching to that of the platinum group metals (PGM) when compared at high catalyst loadings of 0.6-1.0 mgFe-N-C cm-2. The improvements in the ORR activity of Fe-N-Cs can be attributed to new synthetic routes developed in recent years that have led to increased Fe-N4 active site density and utilization and/or better intrinsic activity/turnover frequency (TOF) of those sites.1-3 Despite impressive activities in half-cells, Fe-N-Cs still require quite high loadings in the range of 4.0-5.0 mg cm-2 in proton exchange membrane fuel cells (PEMFCs), which makes the Fe-N-C cathodes 4-5x thicker than Pt-based cathodes (~100 µm for Fe-N-C layer vs ~10-20 µm for Pt layer). Thick Fe-N-C cathodes suffer from quick performance drop in medium to high current density region due to mass-transport related losses. Reducing the Fe-N-C loading levels in RRDE and in PEMFC without comprising the ORR activity would need further improvements in active site density, site utilization and TOF. In this talk, I will present our work on how morphological tailoring of carbon framework in Fe-N-Cs affects the utilization of active sites and water adsorption-desorption behaviour with the aim of improving the ORR performance and mass-transport properties. Active-site imprinted carbon frameworks are used to prepare Fe-N-Cs that consist of purely Fe-N4 sites without any other iron species.1,4,5 Besides activity, performance stability against extensive voltage cycling in RRDE and long-term durability in PEMFC single-cells is another important criterion that Fe-N-Cs need to fulfil for becoming a viable substitute to PGM catalysts. Very recently, encouraging results on the improvement of Fe-N-C durability in PEMFC tests have been reported using radical scavengers and by coating a thin carbon layer on the surface of Fe-N-C.6,7 However, significantly more efforts are required in that direction for reaching the durability targets of thousands of hours. Second part of the talk will discuss the performance stability of Fe-N-Cs.