Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials

Abstract

While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe-N-C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe-N-C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe-N-C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe-N-C materials. These new insights open the path to bottom-up synthesis approaches and studies on site-support interactions.

Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe‐N‐C Catalysts

“…These observations were confirmed by extended X‐ray absorption fine structure (EXAFS), with a lower intensity of the Fe‐Fe backscattering signal for FeNC‐dry‐1 vs. FeNC‐wet‐1 (Figure 1 d). As previously reported,10b a strong Fe‐N(O) interaction without Fe‐Fe interaction for FeNC‐dry‐0.5 confirms the sole presence of FeN x C y moieties. Quantification of the absolute content of each Fe component was derived from the Mössbauer spectra fittings (Figure 1 e) 6c.…”

“…In contrast, high PRR current on FeNC‐dry‐0.5 demonstrates that FeN x C y moieties are PRR‐active. This is a first major finding of the present study, enabled by the synthesis of Fe‐N‐C catalysts free of Fe particles 10b, 15…”

“…All four of the Fe‐N‐C catalysts showed high ORR activity (Figure 2 a), comparable to that of other high‐performing Fe‐N‐C catalysts 3c. The NC catalyst showed poor ORR kinetics, as expected in acidic medium 4, 10a,10b. Rotating ring disk electrode (RRDE) measurements at 800 μg cm −2 loading showed low H 2 O 2 production (<6 %) during ORR for the four Fe‐N‐C catalysts (Figure 2 b and S4).…”

“…In the continuation of our recent studies,10b, 13 the catalysts were synthesized by pyrolysis of Fe II acetate, 1,10‐phenanthroline (Phen), and a Zn II zeolitic imidazolate framework (ZIF‐8). Three catalysts were first prepared, named FeNC‐wet‐1, FeNC‐dry‐1, and FeNC‐dry‐0.5, the labeling referring to homogenization conditions and Fe content before pyrolysis (see Methods in the Supporting Information).…”

“…We also demonstrated that FeN x C y moieties and Fe@N‐C species are moderately active toward PRR, proving their possible roles in both direct (major path) and indirect 4 e − ORR pathways. Since the synthesis of Fe‐N‐C catalysts with only FeN x C y moieties,10b, 15 only Fe@N‐C particles,10a or their combination3b,3d, 17 is now controllable, the understanding of the nature of active sites for PRR provided herein offers new insights for the rational design of advanced Fe‐N‐C catalysts with high selectivity and expectedly improved durability in polymer electrolyte fuel cells. …”