Elucidating the structural composition of a Fe-N-C catalyst by nuclear and electron resonance techniques

Fe–N–C catalysts are very promising materials for fuel cells and metal–air batteries. This work gives fundamental insights into the structural composition of an Fe–N–C catalyst and highlights the importance of an in‐depth characterization. By nuclear‐ and electron‐resonance techniques, we are able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of α‐iron and, surprisingly, iron oxide. Our work makes it questionable to what extent FeN4 sites can be present in Fe–N–C catalysts prepared by pyrolysis at 900 °C and above. The simulation of the iron partial density of phonon states enables the identification of three FeN4 species in our catalyst, one of them comprising a sixfold coordination with end‐on bonded oxygen as one of the axial ligands

Influence of the Structure-Forming Agent on the Performance of Fe-N-C Catalysts

In this work, the influence of the structure-forming agent on the composition, morphology and oxygen reduction reaction (ORR) activity of Fe-N-C catalysts was investigated. As structure-forming agents (SFAs), dicyandiamide (DCDA) (nitrogen source) or oxalic acid (oxygen source) or mixtures thereof were used. For characterization, cyclic voltammetry and rotating disc electrode (RDE) experiments were performed in 0.1 M H₂SO₄. In addition to this, N₂ sorption measurements and Raman spectroscopy were performed for the structural, and elemental analysis for chemical characterization. The role of metal, nitrogen and carbon sources within the synthesis of Fe-N-C catalysts has been pointed out before. Here, we show that the optimum in terms of ORR activity is achieved if both N- and O-containing SFAs are used in almost similar fractions. All catalysts display a redox couple, where its position depends on the fractions of SFAs. The SFA has also a strong impact on the morphology: Catalysts that were prepared with a larger fraction of N-containing SFA revealed a higher order in graphitization, indicated by bands in the 2nd order range of the Raman spectra. Nevertheless, the optimum in terms of ORR activity is obtained for the catalyst with highest D/G band ratio. Therefore, the results indicate that the presence of an additional oxygen-containing SFA is beneficial within the preparation

On the effect of sulfite ions on the structural composition and ORR activity of Fe-N-C catalysts

Fe-N-C catalysts are the most promising group of non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). This study focusses on two different porphyrin-based Fe-N-C catalysts and a Fe-N-C catalyst prepared from alternative precursors under S-addition. Catalysts are subjected to a wet-chemical poisoning treatment by sulfite ions . A mechanism for the deactivation process of the active sites is proposed. ORR activity is evaluated for the original catalysts (OC) and for the poisoned catalysts in 0.1 M H2SO4. In addition, the structural composition of the catalysts is identified by Mobauer spectroscopy. Our results show that the sulfite ions bound irreversible to the catalysts and the catalysts lose significant fractions of their ORR activity while in Mobauer spectroscopy a new doublet appears. Based on the results, possible models for the binding of the ambident sulfite ion to the FeN4 centers are discussed

Influence of sulfur in the precursor mixture on the structural composition of Fe-N-C catalysts

Fe-N-C catalysts were prepared by a new synthesis protocol at 800 ∘C with subsequent acid leaching. The effect of sulfur was investigated by a systematic study in which the molar S/Fe ratio in the precursor was varied from 0.0 to 2.45. The obtained catalysts were evaluated for their ORR activity in 0.1 M H2 SO 4. In addition, the specific BET surface area was determined from N2 sorption measurements and structural characterization was made by Mößbauer spectroscopy. Catalysts contain FeN4 moieties and inorganic iron species. Structure activity correlation indicate a dominance of the ferrous low-spin FeN4 site for the ORR activity. This is in agreement with previous findings. In addition, the optimum in terms of ORR activity is in the same S/Me range as found for porphyrin-based catalysts. However, in contrast to previous conclusions of an avoidance of iron carbide formation by sulfur addition, a very high S/Fe ratio is required to obtain a catalyst free of iron carbide. Further work is required to identify the parameter that indeed enables inhibition of iron carbide formation

Influence of the Structure-Forming Agent on the Performance of Fe-N-C Catalysts

In this work, the influence of the structure-forming agent on the composition, morphology and oxygen reduction reaction (ORR) activity of Fe-N-C catalysts was investigated. As structure-forming agents (SFAs), dicyandiamide (DCDA) (nitrogen source) or oxalic acid (oxygen source) or mixtures thereof were used. For characterization, cyclic voltammetry and rotating disc electrode (RDE) experiments were performed in 0.1 M H2SO4. In addition to this, N2 sorption measurements and Raman spectroscopy were performed for the structural, and elemental analysis for chemical characterization. The role of metal, nitrogen and carbon sources within the synthesis of Fe-N-C catalysts has been pointed out before. Here, we show that the optimum in terms of ORR activity is achieved if both N- and O-containing SFAs are used in almost similar fractions. All catalysts display a redox couple, where its position depends on the fractions of SFAs. The SFA has also a strong impact on the morphology: Catalysts that were prepared with a larger fraction of N-containing SFA revealed a higher order in graphitization, indicated by bands in the 2nd order range of the Raman spectra. Nevertheless, the optimum in terms of ORR activity is obtained for the catalyst with highest D/G band ratio. Therefore, the results indicate that the presence of an additional oxygen-containing SFA is beneficial within the preparation

Elucidating the Origin of Hydrogen Evolution Reaction Activity in Mono- and Bimetallic Metal- and Nitrogen-Doped Carbon Catalysts (Me-N-C)

In this work, we present a comprehensive study on the role of metal species in MOF-based Me-N-C (mono- and bimetallic) catalysts for the hydrogen evolution reaction (HER). The catalysts are investigated with respect to HER activity and stability in alkaline electrolyte. On the basis of the structural analysis by X-ray diffraction, X-ray-induced photoelectron spectroscopy, and transmission electron microscopy, it is concluded that MeN4 sites seem to dominate the HER activity of these catalysts. There is a strong relation between the amount of MeN4 sites that are formed and the energy of formation related to these sites integrated at the edge of a graphene layer, as obtained from density functional theory (DFT) calculations. Our results show, for the first time, that the combination of two metals (Co and Mo) in a bimetallic (Co,Mo)-N-C catalyst allows hydrogen production with a significantly improved overpotential in comparison to its monometallic counterparts and other Me-N-C catalysts. By the combination of experimental results with DFT calculations, we show that the origin of the enhanced performance of our (Co,Mo)-N-C catalyst seems to be provided by an improved hydrogen binding energy on one MeN4 site because of the presence of a second MeN4 site in its close vicinity, as investigated in detail for our most active (Co,Mo)-N-C catalyst. The outstanding stability and good activity make especially the bimetallic Me-N-C catalysts interesting candidates for solar fuel applications

Activity and Degradation Study of an Fe-N-C catalyst for ORR in Direct Methanol Fuel Cell (DMFC)

In this work a comprehensive study of the activity and stability of a non-precious metal catalyst of type Fe- N- C in acidic media is reported. The catalyst was prepared from polyaniline, dicyandiamide and iron acetate as precursors. Temperature-dependent rotating-disk electrode experiments were performed to determine the activation energy of the catalyst. Besides, load cycle durability tests with and without the addition of methanol show that there is no additional deactivation caused by methanol addition. In a Direct Methanol Fuel Cell (DMFCs) our catalyst performed similarly good in comparison to other Fe-N-C catalysts. Raman and Mössbauer spectroscopy provide valuable information on the structural composition and chemical changes induced by durability and stability testing of the catalyst. While the maximum power density during DMFC operation decreases by 85 %, the qualitative distribution of iron sites might indicate the formation of iron and iron oxide clusters as decomposition product associated with the disintegration of FeN4 sites