Investigation of Different Fe-N-Cs for the Oxygen Reduction Reaction by Operando X-Ray Absorption Spectroscopy

Abstract

Fe-N-C catalysts are one of the most promising alternatives to Pt for the oxygen reduction reaction in proton exchange membrane fuel cells (PEMFC). Although they already show high mass activities compared to Pt they lack from lower volumetric activity and stability. We have investigated the effect of different carbon supports in Fe N Cs on their activity and stability in high temperature PEMFC. As support for Fe-Nx sites Black Pearls® 2000 and phosphoric acid activated coconut shells were used and compared with a commercial Fe-N-C catalyst (PMF 011904, Pajarito Powder). Differences in stability during thin film measurements in 0.1 mol L-1 HClO4 were found showing much higher stability of coconut shell-based Fe-N-C (35 % mass activity loss) compared to black pearl-based Fe-N-C (67 %) after potential cycling (1.0-1.5 VRHE, 5000 cycles, N2). The higher stability was suspected to be attributed to the higher heteroatom doping (N, P) of the coconut shell-based Fe-N-C leading to stabilisation of active sites. [1] To get a deeper understanding of the impact of carbon support on the stability, operando X-ray absorption spectroscopy at the Fe K edge was used to characterise the atomic Fe active sites. Measurements were performed in diluted perchloric acid and oxygen saturation. The density and the nature of the Fe active sites during ORR depending on the electrode potential was analysed. The coconut shell-based catalyst exhibits a higher number of Fe-Nx sites compared to commercial PMF and black pearl based Fe-N-C which also contain metallic iron and iron carbide. Dynamic detection of the Fe K-edge intensity during potential cycling (1.2-0.1 VRHE) shows that the reversible iron redox potential of the coconut shell-based Fe-N-C is lower compared to the black pearl-based and commercial catalyst. This indicates lower intrinsic activity per active site, as similar mass activities of coconut shell-based and black-pearl-based catalyst were found. Moreover, the evaluation of the Fe K-edge shift and the EXAFS-region show that heteroatom doping (O, P, N) and undercoordination can be the reason for the lower redox potential. These findings underline the effect of the heteroatom doping on the catalyst’s activity and stability. Especially incorporation of P-species can be interesting for further optimisation of Pt-free M-N-C catalyst for PEMFC.

Literature: [1] J. Müller-Hülstede, D. Schonvogel, H. Schmies, P. Wagner, A. Dyck, M. Wark, ACS Appl. Energy Mater. 2021, 4, 7, 6912.