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FeTMPPCl impregnated on a carbon black was heat treated to different temperatures. The obtained catalysts were characterized before and after acid leaching by structural and chemical analyses. On the basis of the structural characterization it was concluded that those FeN4 centres in which iron is mesomerically bonded to four nitrogen atoms, are ca...
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... pronounced if the porphyrin is pyrolyzed without the addition of carbon. 12 The comparison of both graphs Fig. 2a and b indicates that up to 600°C no decomposition of FeN 4 -centers occurs. It will be shown by Mössbauer measurements that a transformation of the initial FeN 4 -center is observed, which already occurs at a temperature of 400°C see Fig. ...
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... is initiated during a heat-treatment at T 600°C by a breaking of iron nitrogen bondings and not of nitrogen carbon bondings. From element analyses only a trend of the ratios of active and inactive species can be derived. In order to get a better insight into the nature of iron based structures, 57 Fe Mössbauer spectroscopy was performed. In Fig. 3 the spectra of the precursor FeTMPPCl + KB600 and the samples pyrolyzed at 400, 600, 800, and 1000°C with and without leaching are displayed. It should be stated that all measurements were made at room temperature as described in the experimental part and that the given temperatures are only re- lated to that of the pyrolysis step In ...
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... other peaks appear in the spectra caused by the pyrolysis of the material. The assignment, as given in the experimental part, is displayed in the spectrum of HT1000+ in Fig. 6. Evaluating the Mössbauer spectra it was found that besides the FeN 4 -center of the precursor, two further FeN 4 -modifications are present in the annealed catalysts see Fig. 3 and Table I. In our working model it was expected that the heat-treatment might cause a change of the mesomeric characteristic of the initial FeN 4 -center due to a transformation of the porphyrin molecules toward the car- bon matrix. If the four nitrogen atoms are not equally bonded to the iron center the different bonding distances ...
Citations
... Further optimization is required, that can be gained from a better understanding of the underlying and dominating degradation mechanisms and structure-function correlations. In principle, higher pyrolysis temperatures are known to enable better stability, 7,8 but at the same time, the number of ORR active sites decreases, 9 while the amount of side phases increases. Such side phases can be leached out during FC operation [10][11][12][13] and either contribute directly to the degradation or indirectly by poisoning of the membrane 14,15 or Fenton's reaction. ...
... While the effect of temperature on the stability of the catalyst is expected, 7,8 often lower pyrolysis temperatures were found beneficial in terms of activity. 9,49 The discrepancy might have its origin in the comparison of FC data (here) with RDE data in the related references. ...
FeNC catalysts are promising substitutes of platinum‐type catalysts for the oxygen reduction reaction (ORR). While previous research disclosed that high pyrolysis temperatures are required to achieve good stability, it was identified that a trade‐off needs to be made regarding the active site density. The central question is, if a good stability can also be reached at milder pyrolysis conditions but longer duration retaining more active sites, while enabling the defect‐rich carbon to heal during a long residence time? To address this, a variation of pyrolysis temperatures and durations is used in FeNC fabrication. Carbon morphology and iron species are characterized by Raman spectroscopy and Mössbauer spectroscopy, respectively. Fuel cell (FC) activity and stability data are acquired. The results are compared to ORR activity and selectivity data from rotating ring disc electrode experiments and resulting durability in accelerated stress tests mimicking the load cycle and start‐up and shut‐down cycle conditions. It is discussed how pyrolysis temperature and duration affect FC activity and stability. But, more important, the results connect the pyrolysis conditions to the required accelerated stress test protocol combination to enable a prediction of the catalyst stability in fuel cells. The best protocol of accelerated stress tests to mimic the degradation of FeNC in fuel cells depends on the preparation conditions. At mild preparation conditions both, carbon oxidation and active site disintegration overlay, but active site destruction contribute to a major extent. At high pyrolysis conditions carbon oxidation dominates, and an overall much better stability is obtained.
... Many studies have reported that the D1-related FeN 4 site is the active site for ORR in FeNC catalysts. 32,34,46,54,56 For PANI− Fe and ZIF−Fe, the FeN 4 species are found to be the majority species. Both species contribute to the ORR activity, and while D1 can lead to a higher TOF, D2 is the main contributor for the durability, 57 with only the difference in content for both samples probably being the partial reason for the higher TOF in ZIF−Fe due to its higher number of sites associated with D1. ...
The experimental development of catalytically ever-more active platinum group metal (PGM)-free materials for the oxygen reduction reaction (ORR) at fuel cell cathodes has been until recently a rather empirical iteration of synthesis and testing. Here, we present how kinetic reactivity maps based on kinetic descriptors of PGM-free single-metal-site ORR electrocatalysts can help to better understand the origin of catalytic reactivity and help to derive rational synthetic guidelines toward improved catalysts. Key in our analysis are the catalytic surface site density (SD) and the catalytic turnover frequency (TOF) in their role as controlling kinetic parameters for the ORR reactivity of PGM-free nitrogen-coordinated single-metal M-site carbon (MNC) catalysts. SD-TOF plots establish two-dimensional reactivity maps. We also consider the ratio between SD and the total number of single-metal sites in the bulk, referred to as the site utilization factor, which we propose as another guiding parameter for optimizing the synthesis of MNC catalysts. Exemplified by two sets of FeNC, CoNC, and SnNC catalysts prepared using two distinctly different N- and C-precursor material classes (Zn-based zeolitic imidazolate frameworks and covalent polyaniline), we comparatively diagnose the intrinsic kinetic ORR parameters as well as structural, morphological, and chemical properties. From there, we derive and discuss possible synthetic guidelines for further improvements. Our approach can be extended to other families of catalysts and may involve kinetic performance data of idealized liquid-electrolyte cells as well as gas diffusion layer-type flow cells.
... In the early study, the ORR activity is considered to be positively correlated to electron density of Fe center. For instance, Kramm et al. [195] suggested a relationship between the electron density of the FeN4 center and the catalytic activity. They found that electron transfer occurred from coordinated N atom to Fe metal in the FeN4 site induced by the pyrolysis of the FeTMPPCl 48 under higher temperatures, while an increase in electron density of the iron center enables an improvement in the turnover frequency during ORR. ...
... solution with scan rate of 10 mV s -1 at a rotation speed of 1600 rpm. 195 To unveil ORR activities trend on the samples with various amount of Fe contents, the correlation plot between the density of FeNx sites based on XPS analysis and ORR activity is established. As shown in Figure 13A, the ORR activity is proportional to amount of FeNx moieties. ...
Fe-N-C electrocatalyst is becoming a challenging research topic with great expectations for the next-generation energy devices due to its high intrinsic ORR activity comparable to platinum group metals (PGM) electrocatalyst. However, a high effective preparation strategy with improved catalytic performance and high robustness, employing noncritical components is of great importance for the development of Fe-N-C electrocatalyst, and the identification nature of active site for pyrolytic Fe-N-C material is still a topic of heated debate due to potential presence of multiple active sites.In this work we developed catalysts with N-decorated single atom Fe catalysts in porous carbon by high temperature pyrolysis using cost-effective, non-toxic raw components. Systematic variation of composition of N, and C precursors and various structuring agents, as well as conditions of different steps allowed us to determine the optimal parameters for a multi-step synthesis. Various characteristic technologies, including TGA/TPD-MS, Raman, BET, XRD, XPS, HAADF-STEM and XAS have been applied to study the composition and structure of synthesized Fe-N-C catalysts. The electrocatalytic performance of the catalysts were studied by (R)RDE measurements and accelerated durability tests. This study allowed us to demonstrate that Fe-N-C electrocatalysts with optimized structure and composition provide stable 4e- oxygen electroreduction performance in 0.1 M KOH electrolyte with activity outperforming benchmark 20% Pt/C catalysts. Most importantly, the nature of active sites and structure-performance correlation of as-obtained electrocatalysts are systematically investigated. The work aims to provide fundamental understanding of Fe-N-C catalyst from both material design and natura of active sites which is believed to favor the development of next generation electrocatalyst for fuel cell technology.
... H2O2 is the only stable intermediate or by-product of the ORR in acid medium and particularly stresses Metal-N-C catalysts [210]. [103,216]; however this effect is known to be non-durable [11,13,217]. Kumar et al. reported that Fe atoms are mobile and cluster on the doubly-pyrolysed Fe-N-C material, not on the Fe-N-C material submitted to a single pyrolysis after square-wave potential cycling between 0.6 and 1.0 V vs. RHE in Ar-saturated 0.1 M H2SO4 thermostated at 80 °C. ...
... Different Fe-NxCy moietiesFor Fe-N-C, the Fe-Nx moieties have been hypothesized to exist under various forms, e.g. (i) Fe-N4[103] (illustrated inFigure 1a), (ii) Fe-N2+2[104] and (iii) N-Fe-N2+2[104]. Furthermore, the Fe coordination is believed to change with its position in the carbon basal plane, leading to the presence of Fe-N2 (on the carbon edge), Fe-N3[105] or generically Fe-NxCy sites (e.g. ...
While supported metal nanoparticles cannot achieve full electrochemical utilization of metal atoms, catalysts featuring single‐metal atom sites may offer this possibility, along with advantages in selectivity. However, the passage from nanometric to atomic dimension is not without consequences. It first raises the question of efficient and robust synthesis methods, and underlines the need of cutting‐edge characterization techniques that can target single‐metal atoms. These analytical tools are also pivotal to gain insights into the structure of the active sites, and establish atomic structure–catalytic activity–selectivity–stability relationships. Herein, we illustrate these topics for electrocatalysis, with a particular focus on metal–nitrogen–carbon single‐metal atom catalysts, for which a fantastic leap forward has been achieved in the last 15 years, triggered by the growing interest in sustainable energy storage and conversion systems.
... It is important to keep in mind that the inclusion of the dopant is not only to stabilize SAs but also to modulate the electronic properties of active sites. Although most studies use essentially nitrogen, the addition of a second heteroatom has also been tested, such as sulfur [102,103] or phosphorus [104], by using monomers with double functionality such as 1-allyl-2-thiourea or mixtures as phytic acid/dopamine. By incorporating less electronegative atoms such as S and P compared with N, the coordination and thus the electronic density on the metal can be modulated, tuning the catalytic performance. ...
... So far, machine-learning studies on SAAs are focused on two aspects, i.e. SAA stability [102][103][104][105] and SAA catalytic performances on certain reactions [101,104,106]. As shown in Figure 4.5a, Chen and coworkers outlined a typical process of machine-learning-assisted first-principles investigation on SAAs [106]. ...
... The third step of machine learning is model fitting and generalization. Theoretical chemists tend to apply existing machine-learning models to SAAs such as the Gaussian process regression [101,102], the support vector machine [102,103], decision tree [103], random forest [106], and neural network [103]. In this step, researchers should carefully avoid underfitting and overfitting of machine-learning model. ...
Atomic catalysts supported on optimal supports exhibit an ideal strategy to maximize the utilization of active atoms for improving the catalytic efficiency and reducing the cost of catalysts. Among these atomic catalysts, double atom catalysts (DAC), and triple atom catalysts (TACs) are emerging materials, which represent the most basic active sites of the bridge and hollow sites and show high activity and high selectivity. In this chapter, the synthesis routes, characterization techniques, and important applications of DACs and TACs are summarized. Moreover, this chapter outlines the opportunities and challenges in developing DACs and TACs, which provide a comprehensive and distinct understanding of DACs and TACs and inspire further research in the field of catalysis.
... The suspension was placed into an ultrasonic bath for one hour and then left overnight for leaching. The aim of this treatment is the removal of possible inorganic side phases that might be formed by a partial decomposition of the porphyrin [29]. Afterwards, the product was filtrated, washed with distilled water and dried overnight. ...
In this work, the effect of porphyrin loading and template size is varied systematically to study its impact on the oxygen reduction reaction (ORR) activity and selectivity as followed by rotating ring disc electrode experiments in both acidic and alkaline electrolytes. The structural composition and morphology are investigated by ⁵⁷ Fe Mössbauer spectroscopy, transmission electron microscopy, Raman spectroscopy and Brunauer–Emmett–Teller analysis. It is shown that with decreasing template size, specifically the ORR performance towards fuel cell application gets improved, while at constant area loading of the iron precursor (here expressed in number of porphyrin layers), the iron signature does not change much. Moreover, it is well illustrated that too large area loadings result in the formation of undesired side phases that also cause a decrease in the performance, specifically in acidic electrolyte. Thus, if the impact of morphology is the focus of research it is important to consider the area loading rather than its weight loading. At constant weight loading, beside morphology the structural composition can also change and impact the catalytic performance.
This article is part of the theme issue ‘Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)’.
... 66 A demetallation without impairment of the nitrogen species occurs only for a negligible fraction of the metal centers, leading to an N-to-Mratio slightly larger than the initial value of four, as is in accordance with literature values. 67 Figure 2 shows the XP spectra of the N 1s region. The most pronounced feature was the presence of a double peak (398 and 400 eV) for the metal-free H 2 −N−C, whereas only one peak, situated at binding energies between those two peaks, was observed for all M−N−C. ...
... 81 This observation was further made for a pristine CoTMPP immobilized on an amorphous fumed silica. 67 In this case, however, we refrain from a definite assignment of this peak. Nevertheless, close to 100% of the total metal contents can be attributed to an M−N 4 coordination. ...
... For other metal phthalocyanines (Fe-, Ni-, Cu-, and Mn-Pc), however, Zhang et al. could not observe a significant CO formation, whereas we observed for our Ni−N−C and Fe−N−C catalysts the highest activities and FE(CO) of all investigated catalysts. On the one hand, this could indicate that the incorporation of the M−N 4 centers into the graphene network could improve their catalytic activity for the CO 2 RR compared to the molecular centers in isolated organometallic materials, as observations previously made for Fe-and Co−N−C catalysts for the ORR indicated.66,67,83,84 On the other hand is the nature of the local M environment during the reaction not yet investigated thoroughly enough and might therefore also slightly defer from M−N 4 .Fe−N−C shows the most positive onset potential for CO production among all investigated catalysts [−0.71 ...
In this work, the influences of various transition metal ions as active sites in high purity metal- and nitrogen-doped carbon catalysts (in short M−N−C), where M: Mn³⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, or Sn⁴⁺ in the catalyst powders, were systematically investigated for the electrochemical reduction of CO2 in the aqueous electrolyte. The almost exclusive presence of isolated M−N4 centers as catalytic sites was determined by X-ray photoelectron spectroscopy (XPS). The catalysts were electrochemically investigated in a gas diffusion electrode arrangement in bypass mode coupled in-line to a mass spectrometer. This allowed for the nearly simultaneous detection of products and current densities in linear sweep voltammetry experiments, from which potential-dependent specific production rates and faradaic efficiencies could be derived. Postmortem XPS analyses were performed after various stages of operation on the Cu−N−C catalyst, which was the only catalyst to produce hydrocarbons (CH4 and C2H4) in significant amounts. The data provided insights into the potential-induced electronic changes of the Cu−N−C catal st occurrin under operatin conditions. Our work further experimentall revealed the high affinity of M−N−C catalysts to convert CO2 to industrially relevant carbonaceous raw materials, while effectively suppressing the competing hydrogen evolution reaction. These results led to a better understanding of the role of the active sites, especially the central metal ion, in M−N−C and could contribute significantly to the improvement of selectivities and activities for the CO2RR in this catalyst class through tailor-made optimization strategies.
... By detecting the nuances of energy states of the 57 Fe nuclei, this technique even allows one to distinguish electronically different Fe species of similar coordination environment, such as FeN 4 moieties in phthalocyanines or porphyrins. 15,94,146,147 A representative Mössbauer spectrum from Kramm's works was fitted with the presence of one singlet, six doublets and three sextets. 94 The singlet and sextets were identified as superparamagnetic iron, iron metal, and nitride/carbide. ...
Fuel cell, a sustainable technology that assures a cleaner earth, once experienced disillusion due to the issue of economic viability associated with the massive usage of Pt-based catalysts. To address this bottleneck, research on cost-effective catalysts, especially on the cathode side, has been intensively carried out, where the latest achievements have made M–Nx/C material on its verge of application. Here, we present a comprehensive overview on the fundamental chemistry of oxygen reduction reaction (ORR) and the progress of the M–Nx/C catalysts. Efforts aimed at clarifying the nature of catalytic sites are summarized, highlighting the indispensable role of advanced characterization tools. A variety of successful attempts are exemplified to describe the synthetic methodologies, which are followed by the close tracking of newly emerging single-atom or dual-atom sites. Future directions for this research area are believed to be the rational and/or controllable synthesis of catalysts with adequate active sites, the exploitation of strategies to substantially alleviate catalyst degradation as well.
... H2O2 is the only stable intermediate or by-product of the ORR in acid medium and particularly stresses Metal-N-C catalysts [210]. [103,216]; however this effect is known to be non-durable [11,13,217]. Kumar et al. reported that Fe atoms are mobile and cluster on the doubly-pyrolysed Fe-N-C material, not on the Fe-N-C material submitted to a single pyrolysis after square-wave potential cycling between 0.6 and 1.0 V vs. RHE in Ar-saturated 0.1 M H2SO4 thermostated at 80 °C. ...
... Different Fe-NxCy moietiesFor Fe-N-C, the Fe-Nx moieties have been hypothesized to exist under various forms, e.g. (i) Fe-N4[103] (illustrated inFigure 1a), (ii) Fe-N2+2[104] and (iii) N-Fe-N2+2[104]. Furthermore, the Fe coordination is believed to change with its position in the carbon basal plane, leading to the presence of Fe-N2 (on the carbon edge), Fe-N3[105] or generically Fe-NxCy sites (e.g. ...
... According to the Sabatier principle, the fabrication of coordination environments in M-N x -C could change the delectron density of center metal and further influence the σ-type bonding between the orbitals of oxygen species (OOH * , OH * and O * ). This change of d-electron density in local structure of active sites could also be verified by Mossbauer spectroscopy [129]. ...
In this review, we surveyed the significance of local structure engineering on electrocatalysts and electrodes for the performance of oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Both on precious metal catalysts (PMC) and non-precious metal catalysts (NPMC), the main methods to modulate local structure of active sites have been summarized. By change of atomic coordination, modulation of bonding distortion and synergy effect from hierarchical structure, local structure engineering has influence on the intrinsic activity and stability of electrocatalysts. Moreover, we emphasized the intimate correlation between lyophobicity of electrocatalysts and membrane electrodes by local structure engineering. Our review aimed to inspire the exploration of advanced electrocatalysts and mechanism study for PEMFCs based on local structure engineering.[Figure not available: see fulltext.] © 2020, Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature.
