Effect of Carbon Matrix and Fe-species on the Activity and Stability of Fe-N-C Catalysts for PEMFC

Kurzfassung

Fe-N-C materials are promising non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). However, they suffer from lower stability compared to Pt/C catalysts. Especially the presence of unstable metallic particles in Fe-N-Cs can lead to low ORR activities and further promote the formation of reactive oxygen species through Fenton-like reactions, leading to strong catalyst degradation. In this study, four different carbon materials including oxidized Vulcan® (ox-V) and Blacks Pearls® (ox-BP), KOH- and H3PO4-activated rye straw (aRS) are investigated as carbon matrix for Fe-N site formation during synthesis. It will be shown that Fe-N-Cs based on ox-V and aPRSKOH contain large amounts of metallic Fe-particles and Fe3C species (Fig. 1 a), whereas ox-BP and aRSH3PO4 based Fe-N-Cs show uniform distribution of N and Fe. By physical analysis it will be revealed that a moderate surface area (> 800 m²/g) and pore volume (> 0.8 cm³/g), mesoporosity and low amounts of amorphous carbon are beneficial for Fe-N site incorporation.[1] These findings are further complemented by rotating ring-disc electrode measurements in acidic electrolyte (Fig. 1 b). The Fe-N-C catalysts which contain Fe particles or Fe3C display low mass activities (MA) of 0.4 0.9 A/g (at 0.75 V vs. RHE), whereas the atomically distributed Fe-N-C catalysts reveal much higher MAs (4.2 5.2 A/g).[1] Furthermore, the polarization curves after an accelerated stress test (AST) indicate differences in stability in terms of changes in MA. Especially, the Fe-N-aRSH3PO4 catalyst shows little degradation, which can be attributed to incorporated P and more stable Fe-N sites. This study will help to understand the impact of carbon matrix towards the incorporation of active and stable Fe-N sites. In future work the most promising catalysts will be implemented in high temperature-PEMFC.