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Abstract
A nitrogen-containing covalent organic framework obtained from the polymerization of 1,3-dicyanobenzene has been used as a starting material for the synthesis of Fe/N/C catalysts for the oxygen reduction reaction (ORR). In this work we report the effect of the thermal treatments on the nature and catalytic properties of the catalysts obtained after the thermal treatments. After the first thermal treatment, the catalysts obtained contain metallic iron and iron carbide particles, along with a minority fraction of inorganic FeNx sites. After acid leaching and a second thermal treatment, FeNx sites remain in the catalysts, along with a minor fraction of graphite-wrapped Fe3C particles. Both catalysts display high activity for the ORR, with the catalyst subjected to acid leaching and a second thermal treatment, 2HT-1,3DCB, displaying higher ORR activity and a lower production of H2O2. This observation suggests that iron particles, such as Fe3C, display ORR activity but mainly toward the two-electron pathway. On the contrary, FeNx ensembles promote the ORR via the four-electron pathway, that is, via H2O formation.
Highlights
Fuel cells generate electrical work by combining two redox reactions, namely, the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), generating a direct electrical potential difference with H2O as the only byproduct
To the best of our knowledge, such specific studies have not been performed in alkaline media, probably because it has been reported that Fe-free N−C moieties and isolated iron in metallic, carbide, or nitride species display activity for the ORR in an alkaline system.[38−40] Covalent triazine frameworks (CTFs) could be ideal candidates to become the starting core of an inexpensive nonprecious metal catalysts (NPMCs) synthesis when looking for a particular iron−nitrogen configuration
The relative content of carbon in 2HT-1,3DCB is significantly higher than that in 1HT-1,3DCB. This is because 1HT-1,3DCB contains a significant fraction of iron- and zinc-containing phases that are removed during the acid leaching and second pyrolysis treatment
Summary
Fuel cells generate electrical work by combining two redox reactions, namely, the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), generating a direct electrical potential difference (work) with H2O as the only byproduct. Iron−nitrogen ensembles can be coordinated in various forms,[34] such as Fe−N4, Fe−N2+2, N−Fe−N2+2, Fe− N4+1, Fe−N3, Fe−N2, and Fe−Nx−C4−x Among all of these configurations, it has been proposed that the low-spin ferrous FeN4 and the high-spin N−Fe−N2+2 (with a terminal protonated nitrogen) are the most active configurations in acidic media.[35,36] there is still controversy due to recent studies that declare that high-spin ferrous species Fe(III)N4C12 at the catalyst surface could be the main responsible species.[37] To the best of our knowledge, such specific studies have not been performed in alkaline media, probably because it has been reported that Fe-free N−C moieties and isolated iron in metallic, carbide, or nitride species display activity for the ORR in an alkaline system.[38−40] CTFs could be ideal candidates to become the starting core of an inexpensive NPMCs synthesis when looking for a particular iron−nitrogen configuration. We have designed two catalysts with high performance in alkaline media, and we have studied the effect of the different heat treatments on a nitrogen/ammonia atmosphere
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