Elucidating the Structural Composition of an Fe–N–C Catalyst by Nuclear‐ and Electron‐Resonance Techniques

Wagner, Stephan; Auerbach, Hendrik; Tait, Claudia E.; Martinaiou, Ioanna; Kumar, Shyam C. N.; Kübel, Christian; Sergeev, I. A.; Wille, Hans‐Christian; Behrends, Jan; Wolny, Juliusz A.; Schünemann, Volker; Kramm, Ulrike I.

doi:10.1002/ange.201903753

Cited by 29 publications

(27 citation statements)

References 48 publications

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“…Meanwhile, g ≈ 4.3 originates from Fe 3+ with a rhombic ligand field. [ 21 ] As stated in Figure 2d, iron content in Fe 2+ @NCS‐A is higher than that in Fe 3+ @NCS‐A. Thus the missing Fe signal in EPR suggests a higher Fe 2+ content in the Fe 2+ @NCS‐A than Fe 3+ @NCS‐A.…”

Section: Resultsmentioning

confidence: 84%

Iron, Nitrogen Co‐Doped Carbon Spheres as Low Cost, Scalable Electrocatalysts for the Oxygen Reduction Reaction

Feng

Cai

Magliocca

et al. 2021

Adv Funct Materials

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Atomically dispersed transition metal-nitrogen-carbon catalysts are emerging as low-cost electrocatalysts for the oxygen reduction reaction in fuel cells. However, a cost-effective and scalable synthesis strategy for these catalysts is still required, as well as a greater understanding of their mechanisms. Herein, iron, nitrogen co-doped carbon spheres (Fe@NCS) have been prepared via hydrothermal carbonization and high-temperature post carbonization. It is determined that FeN 4 is the main form of iron existing in the obtained Fe@ NCS. Two different precursors containing Fe 2+ and Fe 3+ are compared. Both chemical and structural differences have been observed in catalysts starting from Fe 2+ and Fe 3+ precursors. Fe 2+ @NCS-A (starting with Fe 2+ precursor) shows better catalytic activity for the oxygen reduction reaction. This catalyst is studied in an anion exchange membrane fuel cell. The high open-circuit voltage demonstrates the potential approach for developing high-performance, low-cost fuel cell catalysts.

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Section: Resultsmentioning

confidence: 84%

Iron, Nitrogen Co‐Doped Carbon Spheres as Low Cost, Scalable Electrocatalysts for the Oxygen Reduction Reaction

Feng

Cai

Magliocca

et al. 2021

Adv Funct Materials

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“…The doublets do not allow the assignment of pyrrolic or pyridinic Fe−N 4 sites. 16,41,42 However, the absence of side phases such as metallic iron, iron carbide, or iron nitride confirms that no graphitization has occurred, despite the high temperature of 1000 °C and the high total Fe content. Obviously, the stability of pyrrolic Fe−N bonds (at least in the tetrapyrrolic Fe−N 4 sites originating from Zn 2+ imprinting) is high enough to overcome temperatures of 1000 °C, at least for some time, without undergoing carbothermal reduction, although the quantity and quality of the nitrogen doping is still affected by the heating (Table S3, Figure S3).…”

Section: ■ Results and Discussionmentioning

confidence: 98%

Resolving the Dilemma of Fe–N–C Catalysts by the Selective Synthesis of Tetrapyrrolic Active Sites via an Imprinting Strategy

Menga

Low

et al. 2021

J. Am. Chem. Soc.

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Combining the abundance and inexpensiveness of their constituent elements with their atomic dispersion, atomically dispersed Fe–N–C catalysts represent the most promising alternative to precious-metal-based materials in proton exchange membrane (PEM) fuel cells. Due to the high temperatures involved in their synthesis and the sensitivity of Fe ions toward carbothermal reduction, current synthetic methods are intrinsically limited in type and amount of the desired, catalytically active Fe–N4 sites, and high active site densities have been out of reach (dilemma of Fe–N–C catalysts). We herein identify a paradigm change in the synthesis of Fe–N–C catalysts arising from the developments of other M–N–C single-atom catalysts. Supported by DFT calculations we propose fundamental principles for the synthesis of M–N–C materials. We further exploit the proposed principles in a novel synthetic strategy to surpass the dilemma of Fe–N–C catalysts. The selective formation of tetrapyrrolic Zn–N4 sites in a tailor-made Zn–N–C material is utilized as an active-site imprint for the preparation of a corresponding Fe–N–C catalyst. By successive low- and high-temperature ion exchange reactions, we obtain a phase-pure Fe–N–C catalyst, with a high loading of atomically dispersed Fe (>3 wt %). Moreover, the catalyst is entirely composed of tetrapyrrolic Fe–N4 sites. The density of tetrapyrrolic Fe–N4 sites is more than six times as high as for previously reported tetrapyrrolic single-site Fe–N–C fuel cell catalysts.

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“…Nowadays, most researches regarding atomically dispersed M-NC materials are being focused on the characterizations of active sites by using cutting-edge characterization techniques [22][23][24][25], and developments of new types of metal centers to fit more applicable fields [26][27][28][29], leaving large space for finely tuning the electronic properties of M-N x moieties via local heteroatom doping. Currently, there have been some literatures reporting that the catalytic performance of atomically dispersed M-NC materials is highly related with rational design and optimization of the atomic structures of M-N x moieties through adjusting their electronic properties, such as by breaking symmetry with x ≠ 4 [30][31][32], doping metal center or N periphery with heteroatoms [33][34][35][36].…”

Section: Introductionmentioning

confidence: 99%

Atomically Dispersed Fe-N4 Modified with Precisely Located S for Highly Efficient Oxygen Reduction

et al. 2020

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Immobilizing metal atoms by multiple nitrogen atoms has triggered exceptional catalytic activity toward many critical electrochemical reactions due to their merits of highly unsaturated coordination and strong metal-substrate interaction. Herein, atomically dispersed Fe-NC material with precise sulfur modification to Fe periphery (termed as Fe-NSC) was synthesized, X-ray absorption near edge structure analysis confirmed the central Fe atom being stabilized in a specific configuration of Fe(N3)(N–C–S). By enabling precisely localized S doping, the electronic structure of Fe-N4 moiety could be mediated, leading to the beneficial adjustment of absorption/desorption properties of reactant/intermediate on Fe center. Density functional theory simulation suggested that more negative charge density would be localized over Fe-N4 moiety after S doping, allowing weakened binding capability to *OH intermediates and faster charge transfer from Fe center to O species. Electrochemical measurements revealed that the Fe-NSC sample exhibited significantly enhanced oxygen reduction reaction performance compared to the S-free Fe-NC material (termed as Fe-NC), showing an excellent onset potential of 1.09 V and half-wave potential of 0.92 V in 0.1 M KOH. Our work may enlighten relevant studies regarding to accessing improvement on the catalytic performance of atomically dispersed M-NC materials by managing precisely tuned local environments of M-Nx moiety.

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Elucidating the Structural Composition of an Fe–N–C Catalyst by Nuclear‐ and Electron‐Resonance Techniques

Cited by 29 publications

References 48 publications

Iron, Nitrogen Co‐Doped Carbon Spheres as Low Cost, Scalable Electrocatalysts for the Oxygen Reduction Reaction

Iron, Nitrogen Co‐Doped Carbon Spheres as Low Cost, Scalable Electrocatalysts for the Oxygen Reduction Reaction

Resolving the Dilemma of Fe–N–C Catalysts by the Selective Synthesis of Tetrapyrrolic Active Sites via an Imprinting Strategy

Atomically Dispersed Fe-N4 Modified with Precisely Located S for Highly Efficient Oxygen Reduction

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