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Organisationseinheit der BAM
理论模拟与实验协同的材料表征与机理研究
(2025)
In this academic salon among Chinese-speaking researchers within Germany and EU, I presented my work on utilizing computational chemistry to support experimental characterization and mechanism studies. The discussion focused on metal- and nitrogen-doped carbon (M-N-Cs) with well-defined coordination geometry as an excellent reference material for synergistic theory-experimental research. Some key topics included the analysis of structural preferences of imprinting ions, spectroscopic characterization of specific active sites and mechanism studies pertaining oxygen reduction reaction and hydrogen peroxide chemistry at these active sites.
Development and fundamental understanding of precious-group-metal-free electrocatalysts is hampered by limitations in the quantification of the intrinsic activity of different catalytic sites and understanding the different reaction mechanisms. Comparing isomorphic nitrogen-doped carbons, Zn-N-Cs and Fe-N-Cs with the common tetrapyrrolic motif, a catalyst-independent outer-sphere rate-determining step in the alkaline oxygen reduction reaction is observed. Density functional theory (DFT) simulations on tetrapyrrolic model structures indicate the highest occupied molecular orbital (HOMO) level as a good descriptor for the catalytic activity. Contour plots suggest that the electron transfer occurs directly from the tetrapyrrolic coordination site, rather than from the metal center. Metal-free tetrapyrrolic N4 sites are discovered to be highly active oxygen reduction reaction (ORR) active sites in alkaline that reach turnover frequencies (TOF) of 0.33 and 1.84 s−1 at 0.80 and 0.75 VRHE in the order of magnitude of tetrapyrrolic Fe–N4 sites in the acidic ORR. While Zn-coordination lowers the HOMO level and therefore the catalytic activity, Fe-coordination lifts the HOMO level resulting in TOF values of 0.4 and 4 s−1 for tetrapyrrolic Fe–N4 sites at 0.90 and 0.85 VRHE, respectively. At higher mass activities, the peroxide reduction becomes rate-limiting, where highest peroxide production rates are observed for the nitrogen-doped carbon.
In der vorliegenden Arbeit wird eine Kern-Schale-Strategie vorgestellt, die das grundlegende Problem gängiger nichtgraphitischer Hartkohlenstoff-Anoden adressiert: Hohe reversible Kapazitäten gehen bislang typischerweise mit erheblichen irreversiblen Verlusten in den ersten Zyklen einher. Analog zu Graphit, das sowohl Lithiumspeicherung als auch die Abtrennung von Elektrolytlösungsmitteln in einer homogenen Struktur vereint, zeigen wir, dass sich diese beiden Funktionen auch in nichtgraphitischen Kohlenstoffen gezielt in einer heterogenen Architektur kombinieren lassen. Hochporöse Aktivkohlen werden durch kinetisch kontrollierte Gasphasenabscheidung mit einer dünnen Schicht nichtgraphitischen Kohlenstoffs überzogen, sodass eine funktionale Kern-Schale-Struktur entsteht. Gasadsorptionsmessungen an Kern-, Schalen-, Kern-Schale- und mechanisch beschädigte Kern-Schale-Partikeln, bestätigen, dass die Porosität des Kerns erhalten bleibt und die Schale semipermeabel ist. Die Sorption von Diethylcarbonat wird als geeignetere Methode im Vergleich zu N2- oder CO2-Sorptionsmessungen eingeführt, um die irreversiblen Verluste des ersten Zyklus mit der tatsächlichen Flüssig-Fest-Grenzfläche von Kohlenstoffanoden zu verknüpfen. Die funktionalen Kern-Schale-Partikel zeigen eine stark reduzierte Aufnahme von Diethylcarbonat, was hohe reversible Kapazitäten bei deutlich geringeren Erstzyklusverlusten ermöglicht. Bei einer reversiblen Kapazität von 400 ± 24 mAh g−1 und einer initialen Coulombeffizienz von 82 ± 2% zeigt sich, dass die dreistufige Natriumspeicherung in der gezielt entwickelten Kern-Schale-Architektur den größeren Ionenradius von Natrium gegenüber Lithium (372 mAh g−1 in Graphit) kompensieren kann. Die entwickelten Kern-Schale-Anoden erreichen damit ein Leistungsniveau, das für eine kommerzielle Anwendung vielversprechend ist.
A core-shell strategy is introduced to overcome the dilemma of common non-graphitic hard carbon anodes, linking high reversible storage capacity to practically unacceptable irreversible losses in the first cycle(s). Just as Graphite homogeneously combines effective lithium storage with an electrolyte solvent-sieving function, we show that both of these functions could be strategically integrated into non-graphitic carbons in a heterogeneous structure. Highly porous activated carbons are sealed by kinetically tuned gas-phase deposition of non-graphitic carbon to form a functional core-shell structure. Gas sorption porosimetry on core, shell, core–shell, and cracked core-shell particles confirms preserved core porosity and a semi-permeable shell. Diethyl carbonate sorption analysis is introduced as a more suitable probe than N2 or CO2 sorption, linking first-cycle losses to the liquid–solid interface of carbon anodes. The functional core-shell particles with much reduced diethyl carbonate uptake allow for high storage capacity and reduced first cycle losses. Delivering 400 ± 24 mAh g−1 with 82 ± 2% first-cycle reversibility, it is shown that three-stage Na storage in designed core-shell anodes can compensate for the larger size of sodium compared to lithium stored in graphite anodes (372 mAh g−1). The designed core-shell anodes show state-of-the-art performance with commercial promise.
Elucidating the Intrinsic CO2RR performance at MN4 sites using Morphologically Comparable M-N-Cs
(2025)
Metal- and nitrogen-doped carbons (M-N-Cs) are a promising class of low-cost electrocatalysts derived from earth-abundant elements for various electrochemical applications including ORR and CO2RR [1,2]. Traditionally, M-N-Cs containing active metals (M = Fe, Co, and Ni) are synthesized through direct pyrolysis of inorganic and organic precursors. However, this process often leads to the undesired formation of inorganic side phases via carbothermal reduction, hindering both the efficient incorporation of active metal and the intrinsic activity comparison among active sites.
To overcome these challenges, we developed an active-site imprinting strategy where active metals are introduced post-pyrolysis through ion-exchange [3-5]. In this work, we applied a Mg imprinting strategy to synthesize Co-N-Cs and Ni-N-Cs with comparable morphology and metal dopant concentrations, enabling a more direct comparison of their intrinsic activities. Notably, Ni-N-Cs produced via this method consistently demonstrated superior activity and selectivity compared to isomorphic Co-N-Cs, achieving CO Faraday efficiencies of up to 95% at relatively low overpotentials. Furthermore, the Ni-N-C catalyst exhibited excellent stability at -0.65 VRHE, retaining 92.5% of its current density and 97.6% of its CO selectivity after 100 hours of continuous operation.
A distinct advantage of the imprinting strategy lies in the suppression of inorganic side phases, enabling an accurate local structure characterization of the porphyrin-like tetrapyrrolic MN4 coordination structure using Extended X-ray Absorption Fine Structure (EXAFS). The characterization of the tetrapyrrolic NiN4 site is especially puzzling, as density functional theory (DFT) simulations often predict these sites to be inert to both CO2RR and HER. Drawing on recent studies which highlight the crucial role of cations for CO2RR on noble metal catalysts like Cu, Ag and Au [6-7], we performed mechanistic investigations using DFT to extend the relevance of cations to the CO2RR/HER performance at the MN4 sites, and further illustrate the advantage of the pyrrolic N atoms in anchoring them near the active metals.
Metal-and nitrogen-doped carbons (MN-Cs) represent a promising class of low cost electrocatalysts derived from nature-abundant elements for various electrochemical processes including CO 2 RR [1, 2]. Traditionally, MN-Cs containing active metals (M= Fe, Co, Ni) are synthesized by direct pyrolysis of inorganic and organic precursors, a process that often results in the undesired formation of inorganic side phases through carbothermal reduction, impeding the effective integration of active metals like Fe, Co and Ni. Furthermore, comparing the intrinsic activities of different MN-Cs can be complicated due to variations in catalyst morphology and active site concentration that arise during the pyrolysis.
To address these challenges, we developed an active-site imprinting strategy in which active metals are introduced post-pyrolysis via ion-exchange [3-5]. In this work, we employed the Mg imprinting strategy to produce Co-N-Cs and Ni-N-Cs with comparable morphology and metal dopant concentration. Our approach allows for a more direct comparison of the intrinsic activities that arise from the metal dopant. The Ni-N-Cs derived this way are consistently higher in activity and selectivity than the corresponding Co-N-Cs, exhibiting a CO Faraday efficiency of up to 95% at potentials between-0.5 to-0.8 V RHE. The Ni-NC catalyst maintains high stability at-0.65 V RHE, with 92.5% retention of current density and 97.6% retention of CO selectivity after 100 hours of continuous operation.
A water-free ionothermal synthesis of porous magnesium-imprinted nitrogen-doped carbon (Mg–NC) materials is introduced to prepare a platform material to investigate electrocatalytic structure-performance relations. Atomically dispersed Co- and Fe-NCs isomorphic to the pristine Mg-NCs are prepared by ion-exchange reactions. The current Mg-templating strategy enables relatively high pyrolysis product yields of up to 50 wt% and resultant Fe-NC and Co-NC catalysts contain high and comparable active metal loading of up to 2.52 wt% Fe and 2.29 wt% Co, respectively. A combination of X-ray spectroscopies with DFT studies reveals a tetrapyrrolic structure of the coordination sites, originating from a pyrolytic magnesium template ion reaction within the ionothermal synthesis. Two sets of highly active isomorphic tetrapyrrolic Fe-NCs and Co-NCs are utilized to understand the differences in intrinsic electrocatalytic performance of Co-NCs and Fe-NCs towards the alkaline oxygen reduction reaction (ORR). Despite their superior valence electronic properties to facilitate the initial outer-sphere electron transfer to O2, Co-NCs show significantly lower performance than Fe-NC with comparable loading. Although the generally discussed weaker binding of peroxide intermediates to CoN4 sites compared to FeN4 sites is evident, experimental and theoretical investigation reveal that it is the underlying peroxide oxidation activity that suppresses the oxygen reduction activity of M-NCs. The high peroxide oxidation activity of Co-NCs explains their reduced alkaline ORR relative to Fe-NCs, shedding light on the understated significance of controlling peroxide chemistry for the optimizing cathodic performance.
Metal and nitrogen-doped carbons (M-N-Cs) with M-N4 moieties as the generally accepted active sites, have gradually reached promising levels of activity towards crucial electrochemical reactions like the oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR). Even the metal-free counterparts, the nitrogen-doped carbons (NDCs), are common and well-studied electrocatalysts for ORR in alkaline media. In
these disordered porous materials, the characterization and quantification of (M)N4 sites can be challenging and time-consuming, with many spectroscopic methods drastically overestimating their site accessibility for catalytic applications. In this study, we explore an alternative approach towards active site characterization and quantification in M-N-Cs based on multi-Langmuir analysis of CO2-sorption isotherms at low pressures, with support fromelectronic DFT calculations.
