Effect of Iron Precursors on the Structure and Oxygen Reduction Activity of Iron–Nitrogen–Carbon Catalysts

Relevant Properties of Carbon Support Materials in Successful Fe-N-C Synthesis for the Oxygen Reduction Reaction: Study of Carbon Blacks and Biomass-Based Carbons

Fe-N-C materials are promising non-precious metal catalysts for the oxygen reduction reaction in fuel cells and batteries. However, during the synthesis of these materials less active Fe-containing nanoparticles are formed in many cases which lead to a decrease in electrochemical activity and stability. In this study, we reveal the significant properties of the carbon support required for the successful incorporation of Fe-N-related active sites. The impact of two carbon blacks and two activated biomass-based carbons on the Fe-N-C synthesis is investigated and crucial support properties are identified. Carbon supports having low portions of amorphous carbon, moderate surface areas (>800 m2/g) and mesopores result in the successful incorporation of Fe and N on an atomic level and improved oxygen reduction reaction (ORR) activity. A low surface area and especially amorphous parts of the carbon promote the formation of metallic iron species covered by a graphitic layer. In contrast, highly microporous systems with amorphous carbon provoke the formation of less active iron carbides and carbon nanotubes. Overall, a phosphoric acid activated biomass is revealed as novel and sustainable carbon support for the formation of Fe-Nx sites. Overall, this study provides valuable and significant information for the future development of novel and sustainable carbon supports for Fe-N-C catalysts.

From Enzymes to Functional Materials-Towards Activation of Small Molecules

The design of non-noble metal-containing heterogeneous catalysts for the activation of small molecules is of utmost importance for our society. While nature possesses very sophisticated machineries to perform such conversions, rationally designed catalytic materials are rare. Herein, we aim to raise the awareness of the overall common design and working principles of catalysts incorporating aspects of biology, chemistry, and material sciences.

Ordered mesoporous carbons codoped with nitrogen and iron as effective catalysts for oxygen reduction reaction

Doping with foreign atoms is an effective approach to significantly enhance the catalytic performance of carbon materials for oxygen reduction reaction (ORR). In this paper, a colloidal silica template method was employed to synthesize nitrogen and iron codoped ordered mesoporous carbon for ORR electrocatalysis. The carbon materials were thoroughly characterized by transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy measurements. The porosity was quantified by nitrogen adsorption/desorption measurements that showed the formation of ordered mesoporous structures with a BET specific surface area up to 953.8 m² g^-1 and the mesopores mostly centered at ca. 25 nm, close to the size of the colloidal silica. The resulting mesoporous carbon exhibited apparent ORR activity in alkaline media, which was highly comparable to that of commercial Pt/C (20 wt%), with the onset potential at +0.99 V vs. RHE. This was ascribed largely to nitrogen dopants, with additional contributions from the trace amounts of iron dopants, and the reactions appeared to be facilitated by the formation of a mesoporous structure. Moreover, the mesoporous carbon showed better stability, resistance against fuel crossover, and selective activity than Pt/C. This work demonstrates a new paradigm for the preparation of heteroatom-doped carbon materials that are promising alternatives to Pt-based catalysts for fuel cells.