The development of platinum group metal (PGM)-free catalysts has become a pivotal focus in advancing oxygen reduction reaction (ORR) electrocatalysis for polymer electrolyte fuel cells (PEFCs). Among these, iron-based Me–N–C catalysts have emerged as promising candidates due to their high activity and low cost. However, their long-term stability remains a major challenge, largely attributed to the presence of inorganic iron phases such as metallic iron (α-Fe), iron carbide (Fe₃C), or other iron nitrides. These phases are often formed during high-temperature pyrolysis and may degrade under operational conditions, leading to performance loss through leaching and radical generation.
To understand the nature and distribution of iron species in these catalysts, Mössbauer spectroscopy (MS) is widely employed. This technique offers exceptional sensitivity to local electronic, magnetic, and structural environments of iron atoms. However, its effectiveness relies heavily on the use of ⁵⁷Fe-enriched precursors because natural iron contains only 2.2% ⁵⁷Fe, resulting in weak signals and long measurement times. While ⁵⁷Fe enrichment enables detailed characterization, it raises a critical question: does altering the isotopic composition of the precursor influence the final catalyst’s structure and activity?
This study systematically investigates the impact of varying degrees of ⁵⁷Fe-enrichment in iron acetate precursors on the composition and ORR performance of Fe–N–C catalysts synthesized from polyacrylonitrile (PAN) and Na₂CO₃. Four catalysts were prepared with identical initial iron content (0.5 wt%) but different ⁵⁷Fe enrichments (2%, 25%, 49%, and 95%). Electrochemical testing via rotating disk electrode (RDE) voltammetry revealed a consistent decrease in ORR activity with increasing ⁵⁷Fe content—up to a fourfold drop at 95% enrichment. Notably, this trend persisted even after normalizing activity by mass or by final iron content, ruling out differences in total iron loading or capacitance as causes.
Complementary characterization techniques provided insight into the underlying mechanisms.PP5 Antibody manufacturer Scanning transmission electron microscopy (STEM) clearly showed a significant increase in Fe-based inorganic particles in the fully enriched sample compared to the non-enriched one. These particles, typically encapsulated in a thin carbon shell, are likely inactive for ORR but detrimental to stability. Raman spectroscopy further supported this observation: the D/G band ratio decreased by approximately 25% with higher ⁵⁷Fe enrichment, indicating increased graphitization—a known catalytic effect of metallic iron species during pyrolysis.144875-48-9 medchemexpress
Mössbauer analysis confirmed the compositional shift quantitatively.PMID:35189699 The relative absorption area of Fe–Nx sites (assigned to doublets D1–D3) declined from ~60% to ~40%, while inorganic iron phases (singlet and sextets corresponding to superparamagnetic iron, α-Fe, and Fe₃C) increased proportionally. X-ray absorption spectroscopy (XAS), including XANES and EXAFS, corroborated these findings. The white line intensity decreased and pre-edge features intensified with higher ⁵⁷Fe content, aligning with an increasing contribution from Fe₃C-like species. Linear combination fitting (LCF) and multivariate curve resolution (MCR) analyses both indicated a rise in Fe₃C content from ~20% to ~60% and a corresponding decline in Fe–Nx contributions.
Interestingly, when the same ⁵⁷Fe-enrichment effect was tested on a porphyrin-based catalyst system (FeTMPPCl on carbon black), the trend reversed: ORR activity more than tripled with increasing ⁵⁷Fe content. This was linked to a redistribution of Mössbauer doublets, particularly an increase in the relative contribution of D1, which has been associated with highly active Fe–Nx sites. This contrast highlights that the influence of isotopic enrichment is not universal and depends on the synthesis pathway and precursor architecture.
These results demonstrate that the degree of ⁵⁷Fe-enrichment significantly affects both the chemical speciation and electrochemical performance of Fe–N–C catalysts. Therefore, conclusions drawn from studies using ⁵⁷Fe-enriched materials cannot be directly extrapolated to naturally abundant iron systems without verification. This underscores the necessity of comparing isotopically enriched and non-enriched samples to ensure accurate interpretation of catalytic behavior and material properties. For future research, it is imperative to account for potential isotope effects in catalyst design and evaluation, especially when aiming for real-world applications where natural isotopic abundance prevails.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com