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

Janssen, André; Martinaiou, Ioanna; Wagner, Stephan; Weidler, Natascha; Shahraei, Ali; Kramm, Ulrike I.

doi:10.1007/s10751-017-1481-z

Cited by 17 publications

(9 citation statements)

References 30 publications

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“…In recent studies we varied the amount of sulfur within this synthesis [22]. The trends in kinetic current density as a function of S/Me ratio were similar to trends observed previously for the combination of iron oxalate with CoTMPP [23], with a maximum in ORR activity at S/Fe = 0.8.…”

Section: Introductionsupporting

confidence: 70%

“…However, in contrast to these earlier finding, our new catalysts still contained considerable fractions of iron carbide in the optimum of ORR activity. Only for the highest S/Me ratios iron carbide formation was inhibited but iron sulfide species were formed [22]. This showed us, that while sulfuraddition strongly affects the achievable ORR activity, sulfur itself helps not to prevent iron carbide formation in our selected system.…”

Section: Introductionmentioning

confidence: 73%

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

Schardt¹,

Weidler²,

Wallace³

et al. 2018

Preprint

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Abstract: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 agent (SFA), 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 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, whereat 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 2 nd 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.

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Section: Introductionsupporting

confidence: 70%

Section: Introductionmentioning

confidence: 73%

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

Schardt¹,

Weidler²,

Wallace³

et al. 2018

Preprint

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“…The selection includes FeNC catalysts prepared from various preparation approaches: starting from porphyrins, [49,[134][135][136][137][138][139][140][141][142] metal organic frameworks [38,41,143] and polyaniline in combination with an iron source, [38,[144][145][146][147] formation of active sites by an ammonia treatment, [38,137,148] and others. [149,150] Note that the labels S i correspond to characteristic doublets known as, for example, D1 or D2 in the original references; however, because some distinct signals carried the same or similar labels in different references, we have chosen an independent nomenclature here. Signals labeled with roman numerals are obtained with various treatments, for example, poisoning by sodium sulfite (IV), [142] presence of excess sulfur during pyrolysis (II and III), or unknown origin [38,143] of their specific character.…”

Section: Implications For Fenc Systemsmentioning

confidence: 99%

Calibration of computational Mössbauer spectroscopy to unravel active sites in FeNC catalysts for the oxygen reduction reaction

Gallenkamp

Kramm

Proppe

et al. 2020

Int J of Quantum Chemistry

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Single-atom catalysts with iron ions in the active site, known as FeNC catalysts, show high activity for the oxygen reduction reaction and hence hold promise for access to low-cost fuel cells. Because of the amorphous, multiphase structure of the FeNC catalysts, the iron environment and its electronic structure are poorly understood. While it is widely accepted that the catalytically active site contains an iron ion ligated by several nitrogen donors embedded in a graphene-like plane, the exact structural details, such as the presence or nature of axial ligands, are unknown. Computational chemistry in combination with Mössbauer spectroscopy can help unravel the geometric and electronic structures of the active sites. As a first step toward this goal, we present a calibration of computational Mössbauer spectroscopy for FeN 4-like environments. The uncertainty of both the isomer shift and the quadrupole splitting prediction is determined, from which trust regions for the Mössbauer parameter predictions of computational FeNC models are derived. We find that TPSSh, B3LYP, and PBE0 perform equally well; the trust regions with B3LYP are 0.13 mm s −1 for the isomer shift and 0.36 mm s −1 for the quadrupole splitting. The calibration data is made publicly available in an interactive notebook that provides predicted Mössbauer parameters with individual uncertainty estimates from computed contact densities and quadrupole splitting values. We show that a differentiation of common FeNC Mössbauer signals by a separate analysis of isomer shift and quadrupole splitting will most likely be insufficient, whereas their simultaneous evaluation will allow the assignment to adequate computational FeNC models.

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“…In the following, the influence of sulfur on the structure and thus on activity and durability was studied, by adapting a novel synthesis route. While the maximum in activity was similar in comparison to previous studies, our S-added most active catalyst still contained iron carbides, in contrast to previous reports [213]. Furthermore, the investigation of the addition of an Ionic Liquid to a S-free and a S-added catalyst under a Load Cycle durability protocol for application in alkaline media showed that this modification improved the activity and the stability of the S-free catalyst, however it did not further improve the performance of the S-added catalyst [215].…”

Section: Discussionsupporting

confidence: 86%

Degradation studies of Me-N-C catalysts for the oxygen reduction reaction in fuel cells

Μαρτιναίου¹

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The increasing demand forrenewable energy along with the requirement of decreasing CO2 emissions is a major challenge for the scientific community. Fuel cells are among the most promising electrochemical devices because of their low operating temperature and high power density. The main advantage of a fuel cell is that electrical power can be produced continuously as long as the fuel supply is provided. Another important advantage is high efficiency. The efficiency of fuel cells is superior to that of combustion engines, particularly at low loads, which makes low-temperature fuel cells (0―100 °C) attractive for automotive propulsion. State of the art catalyst for the anode as well as the cathode is typically based on platinum-supported on carbon. However, the platinum catalyst alone would account for 38-56% of the stack cost [1]. Thus the higher efficiency, in comparison to combustion engines, comes with a higher price that makes the commercialization not competitive now. As a large quantity of the precious metal is required to catalyse the oxygen reduction reaction (ORR), current research is focused on this reaction and especially on the development of alternative non-precious metal catalysts (NPMC). In order for these catalysts to be a commercially viable solution for replacing platinum-based catalysts, they should meet two criteria, improving both activity and stability of these catalysts. Despite, several milestones that have been achieved regarding the activity of these catalysts [2– 5], stability is still relatively poor in comparison to platinum-based systems. This dissertation focuses on the investigation of the stability of non-precious metal catalysts for oxygen reduction reaction mainly in acidic media for application in Proton Exchange Membrane Fuel Cells (PEMFCs) and Direct Methanol Fuel Cells (DMFCs). Α part of this study also deals with performance determination of NPMC in alkaline media, regarding their application in Alkaline Fuel Cells (AFCs). The electrochemical tests were performed with a Rotating Disk Electrode technique. Stability refers to the ability of a system to maintain performance at constant current (or voltage) conditions, while durability refers to the ability to maintain performance following a voltage cycling. First, a systematic study on the impact of the metal centre on durability was conducted. Thirteen MeN-C catalysts were examined with a Start/ Stop (SSC) durability protocol in the potential range of 1.0 V – 1.5 V. Raman spectroscopy was performed before and after the durability tests and a correlation between electrochemical evaluation and Raman spectroscopy in this potential region was found. The carbon oxidation is related to the disintegration of active MeN4 sites that might be initiated by both: the oxidation of the surrounding graphene sheets and by a displacement of the metal out of the N4 plane and this was evidenced by a decrease in the D3 band. Furthermore, a novel synthesis protocol was developed in our group and a Fe-N-C catalyst was optimized with the addition of sulfur(S) in the precursor. With respect to activity the best-off S-added catalyst and the S-free one were then examined for durability under a Load Cycle (LC) protocol (0.6– 1.0 V) in alkaline media. A modification of both catalysts with ionic liquid (IL) was introduced by the group of Professor B. J.M. Etzold within a cooperation framework. The durability of the modified S-free catalyst was found superior to the durability of the non-modified catalyst. In the case of the Sadded catalyst, the IL modification did not further improve its durability. Finally, a third synthesis approach was developed, leading to an active Fe-N-C catalyst also with sulfur in the precursor. The stability of this catalyst was investigated in a DMFC within a research stay abroad project in collaboration with Professor S. Specchia from Politecnico di Torino and subsequently examined by post mortem Mössbauer spectroscopy. This catalyst was further evaluated with a Load Cycle durability protocol and post mortem Raman spectroscopy in our laboratories.

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

Cited by 17 publications

References 30 publications

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

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

Calibration of computational Mössbauer spectroscopy to unravel active sites in FeNC catalysts for the oxygen reduction reaction

Degradation studies of Me-N-C catalysts for the oxygen reduction reaction in fuel cells

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

Cited by 17 publications

References 30 publications

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

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

Calibration of computational Mössbauer spectroscopy to unravel active sites in FeNC catalysts for the oxygen reduction reaction

Degradation studies of Me-N-C catalysts for the oxygen reduction reaction in fuel cells

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

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