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While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe–N–C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe–N–C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH 3 -pyrolysed Fe–N–C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN 4 porphyrinic architectures with different O 2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe–N–C materials. These new insights open the path to bottom-up synthesis approaches and studies on site–support interactions. Although Fe–N–C materials are promising catalysts for oxygen electroreduction in polymer fuel cells, the structure of their active sites is unclear. Quantitative analysis of Fe–N–C now reveals the existence of porphyrin-like FeN 4 C 12 moieties.

Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co–N–C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoN 4 C 12 , CoN 3 C 10,porp and CoN 2 C 5 . The O 2 electro-reduction and operando X-ray absorption response are measured in acidic medium on Co–N–C and compared to those of a Fe–N–C catalyst prepared similarly. We show that cobalt moieties are unmodified from 0.0 to 1.0 V versus a reversible hydrogen electrode, while Fe-based moieties experience structural and electronic-state changes. On the basis of density functional theory analysis and established relationships between redox potential and O 2 -adsorption strength, we conclude that cobalt-based moieties bind O 2 too weakly for efficient O 2 reduction. Nitrogen-doped carbon materials with atomically dispersed iron or cobalt are promising for catalytic use. Here, the authors show that cobalt moieties have a higher redox potential, bind oxygen more weakly and are less active toward oxygen reduction than their iron counterpart, despite similar coordination.

Understanding the spin distribution in FeN4-doped graphene nanoribbons with zigzag and armchair terminations is crucial for tuning the electronic properties of graphene-supported non-platinum catalysts. Since the spin-polarized carbon and iron electronic states may act together to change the electronic properties of the doped graphene, we provide in this work a systematic evaluation using a periodic density-functional theory-based method of the variation of spin-moment distribution and electronic properties with the position and orientation of the FeN4 defects, and the edge terminations of the graphene nanoribbons. Antiferromagnetic and ferromagnetic spin ordering of the zigzag edges were considered. We reveal that the electronic structures in both zigzag and armchair geometries are very sensitive to the location of FeN4 defects, changing from semi-conducting (in-plane defect location) to half-metallic (at-edge defect location). The introduction of FeN4 defects at edge positions cancels the known dependence of the magnetic and electronic proper-ties of undoped graphene nanoribbons on their edge geometries. The implications of the reported results for catalysis are also discussed in view of the presented electronic and magnetic properties.

Herein, we study the London dispersion forces between organic structure directing agents (OSDAs)—here tetraalkyl-ammonium or -phosphonium molecules—and silica zeolite frameworks (FWs). We demonstrate that the interaction energy for these dispersion forces is correlated to the number of H atoms in OSDAs, irrespective of the structures of OSDAs or FWs, and of variations in charges and thermal motions. All calculations considered—DFT-D3 and BOMD undertaken by us, and molecular mechanics from an accessible database—led to the same trend. The mean energy of these dispersion forces is ca. −2 kcal.mol−1 per H for efficient H-O contacts.

While Fe–N–C materials are a promising alternative to platinum for catalysing the oxygen reduction reaction in acidic polymer fuel cells, limited understanding of their operando degradation restricts rational approaches towards improved durability. Here we show that Fe–N–C catalysts initially comprising two distinct FeN x sites (S1 and S2) degrade via the transformation of S1 into iron oxides while the structure and number of S2 were unmodified. Structure–activity correlations drawn from end-of-test 57 Fe Mössbauer spectroscopy reveal that both sites initially contribute to the oxygen reduction reaction activity but only S2 substantially contributes after 50 h of operation. From in situ 57 Fe Mössbauer spectroscopy in inert gas coupled to calculations of the Mössbauer signature of FeN x moieties in different electronic states, we identify S1 to be a high-spin FeN 4 C 12 moiety and S2 a low- or intermediate-spin FeN 4 C 10 moiety. These insights lay the groundwork for rational approaches towards Fe–N–C cathodes with improved durability in acidic fuel cells. Fe–N–C materials are a promising alternative to platinum for catalysing the oxygen reduction reaction in acidic polymer fuel cells. Now, a 57 Fe Mössbauer study reveals that while these catalysts initially comprise two distinct FeN x sites, a high-spin FeN 4 C 12 and a low- or intermediate-spin FeN 4 C 10 , only the latter is durable in operating conditions.

While Fe–N–C materials have shown promising initial oxygen reduction reaction (ORR) activity, they lack durability in acidic medium. Key degradation mechanisms include FeN 4 site demetallation and deactivation by reactive oxygen species. Here we show for mainstream Fe–N–Cs that adding 1 wt.% Pt nanoparticles via a soft polyol method results in well-defined and stable Pt/Fe–N–C hybrids. The Pt addition strongly reduces the H 2 O 2 production and Fe leaching rate during ORR, while post mortem Mössbauer spectroscopy reveals that the highly active but unstable Fe(III)N 4 site is partially stabilized. The similar H 2 O 2 electroreduction activity of Pt/Fe–N–C and Fe–N–C and other analyses point toward a long-distance electronic effect of Pt nanoparticles in stabilizing FeN 4 sites. Computational chemistry reveals that spin polarization of distant Pt atoms mitigates the structural changes of FeN 4 sites upon adsorption of oxygenated species atop Fe, especially in high-spin state. The durability of Fe-N-C single-atom-catalysts for oxygen reduction in acidic medium remains elusive. Here, the authors report how a low amount of platinum stabilizes the FeN4 sites via a long-range electronic effect.

deMon2k is a readily available program specialized in Density Functional Theory (DFT) simulations within the framework of Auxiliary DFT. This article is intended as a tutorial-review of the capabilities of the program for molecular simulations involving ground and excited electronic states. The program implements an additive QM/MM (quantum mechanics/molecular mechanics) module relying either on non-polarizable or polarizable force fields. QM/MM methodologies available in deMon2k include ground-state geometry optimizations, ground-state Born–Oppenheimer molecular dynamics simulations, Ehrenfest non-adiabatic molecular dynamics simulations, and attosecond electron dynamics. In addition several electric and magnetic properties can be computed with QM/MM. We review the framework implemented in the program, including the most recently implemented options (link atoms, implicit continuum for remote environments, metadynamics, etc.), together with six applicative examples. The applications involve (i) a reactivity study of a cyclic organic molecule in water; (ii) the establishment of free-energy profiles for nucleophilic-substitution reactions by the umbrella sampling method; (iii) the construction of two-dimensional free energy maps by metadynamics simulations; (iv) the simulation of UV-visible absorption spectra of a solvated chromophore molecule; (v) the simulation of a free energy profile for an electron transfer reaction within Marcus theory; and (vi) the simulation of fragmentation of a peptide after collision with a high-energy proton.

Machine learning approaches can drastically decrease the computational time for the predictions of spectroscopic properties in materials, while preserving the quality of the computational approaches. We studied the performance of kernel-ridge regression (KRR) and gradient boosting regressor (GBR) models trained on the isotropic shielding values, computed with density-functional theory (DFT), in a series of different known zeolites containing out-of-frame metal cations or fluorine anion and organic structure-directing cations. The smooth overlap of atomic position descriptors were computed from the DFT-optimised Cartesian coordinates of each atoms in the zeolite crystal cells. The use of these descriptors as inputs in both machine learning regression methods led to the prediction of the DFT isotropic shielding values with mean errors within 0.6 ppm. The results showed that the GBR model scales better than the KRR model.

Understanding the spin distribution in FeN[sub.4]-doped graphene nanoribbons with zigzag and armchair terminations is crucial for tuning the electronic properties of graphene-supported non-platinum catalysts. Since the spin-polarized carbon and iron electronic states may act together to change the electronic properties of the doped graphene, we provide in this work a systematic evaluation using a periodic density-functional theory-based method of the variation of spin-moment distribution and electronic properties with the position and orientation of the FeN[sub.4] defects, and the edge terminations of the graphene nanoribbons. Antiferromagnetic and ferromagnetic spin ordering of the zigzag edges were considered. We reveal that the electronic structures in both zigzag and armchair geometries are very sensitive to the location of FeN[sub.4] defects, changing from semi-conducting (in-plane defect location) to half-metallic (at-edge defect location). The introduction of FeN[sub.4] defects at edge positions cancels the known dependence of the magnetic and electronic proper-ties of undoped graphene nanoribbons on their edge geometries. The implications of the reported results for catalysis are also discussed in view of the presented electronic and magnetic properties.

This contribution reports the discovery and analysis of a p -block Sn-based catalyst for the electroreduction of molecular oxygen in acidic conditions at fuel cell cathodes; the catalyst is free of platinum-group metals and contains single-metal-atom actives sites coordinated by nitrogen. The prepared SnNC catalysts meet and exceed state-of-the-art FeNC catalysts in terms of intrinsic catalytic turn-over frequency and hydrogen–air fuel cell power density. The SnNC-NH 3 catalysts displayed a 40–50% higher current density than FeNC-NH 3 at cell voltages below 0.7 V. Additional benefits include a highly favourable selectivity for the four-electron reduction pathway and a Fenton-inactive character of Sn. A range of analytical techniques combined with density functional theory calculations indicate that stannic Sn( iv )N x single-metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytically active moieties. The superior proton-exchange membrane fuel cell performance of SnNC cathode catalysts under realistic, hydrogen–air fuel cell conditions, particularly after NH 3 activation treatment, makes them a promising alternative to today’s state-of-the-art Fe-based catalysts. For oxygen reduction and hydrogen oxidation reactions, proton-exchange membrane fuel cells typically rely on precious-metal-based catalysts. A p -block single-metal-site tin/nitrogen-doped carbon is shown to exhibit promising electrocatalytic and fuel cell performance.