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Nanoscale heterostructures of covalent intermetallics should give birth to a wide range of interface-driven physical and chemical properties. Such a level of design however remains unattainable for most of these compounds, due to the difficulty to reach a crystalline order of covalent bonds at the moderate temperatures required for colloidal chemistry. Herein, we design heterostructured cobalt silicide nanoparticles to trigger magnetic and catalytic properties in silicon-based materials. Our strategy consists in controlling the diffusion of cobalt atoms into silicon nanoparticles, by reacting these particles in molten salts. By adjusting the temperature, we tune the conversion of the initial silicon particles toward homogeneous CoSi nanoparticles and core–shell nanoparticles made of a CoSi shell and a silicon-rich core. The increased interface-to-volume ratio of the CoSi component in the core–shell particles yields distinct properties compared to the bulk and homogeneous nanoparticles. First, the core–shell particles exhibit increased ferromagnetism, despite the bulk diamagnetic properties of cobalt monosilicide. Second, the core–shell nanoparticles act as efficient precatalysts for alkaline water oxidation, where the nanostructure is converted in situ into a layered cobalt silicon oxide/(oxy)hydroxide with high and stable oxygen evolution reaction (OER) electrocatalytic activity. This work demonstrates a route to design heterostructured nanocrystals of covalent intermetallic compounds and shows that these new structures exhibit very rich, yet poorly explored, interface-based physical properties and reactivity.

The protective coating of the electrode materials is a known source of improvement of the cycling performances in battery devices. In the case of the LiMn2O4cathodes, the coating with a thin alumina layer has been proven to show performance efficiency. However, the precise mechanism of its effect on the performance improvement of the electrodes is still not clear. In this work we investigate alumina-coating-induced effects on the structural dynamics of the active materials in correlation to the modified solid electrolyte interface dynamics. The local structures of coated and uncoated samples at different galvanostatic points are studied by both soft X-ray absorption measurements at the Mn L-edges and O K-edge (in total electron yield mode) and hard X-ray absorption at the Mn K-edge (in transmission mode). The different probing depths of the employed techniques allowed us to study the structural dynamics both at the surface and within the bulk of the active material. We demonstrate that the coating successfully hinders the Mn3+disproportionation and, hence, the degradation of the active material. Side products (layered Li2MnO3and MnO) and changes in the local crystal symmetry with formation of Li2Mn2O4are observed in uncoated electrodes. The role of alumina coating on the stability of the passivation layer and its consequent effect on the structural stability of the bulk active materials is discussed.

Nicotinamide adenine dinucleotide (NAD) is a critical regulator of metabolic networks, and declining levels of its oxidized form, NAD+, are closely associated with numerous diseases. While supplementing cells with precursors needed for NAD+synthesis has shown poor efficacy in combatting NAD+decline, an alternative strategy is the development of synthetic materials that catalyze the oxidation of NADH into NAD+, thereby taking over the natural role of the NADH oxidase (NOX) present in bacteria. Herein, we discovered that metal-nitrogen-doped graphene (MNGR) materials can catalyze the oxidation of NADH into NAD+. Among MNGR materials with different transition metals, Fe-, Co-, and Cu-NGR displayed strong catalytic activity combined with >80% conversion of NADH into NAD+, similar specificity to NOX for abstracting hydrogen from the pyridine ring of nicotinamide, and higher selectivity than 51 other nanomaterials. The NOX-like activity of FeNGR functioned well in diverse cell lines. As a proof of concept of the in vivo application, we showed that FeNGR could specifically target the liver and remedy the metabolic flux anomaly in obesity mice with NAD+-deficient cells. Overall, our study provides a distinct insight for exploration of drug candidates by design of synthetic materials to mimic the functions of unique enzymes (e.g., NOX) in bacteria.

The present work investigates the cationic distribution of a complex CuS compound belonging to a promising class of materials with significant prospects for thermoelectric generator and photovoltaic applications. We propose the use of powder resonant X-ray scattering in a combinatorial experimental approach with X-ray powder diffraction and single crystal , as a valid, general, and practical method to unravel the complex cation ordering encountered in the synthetic germanite Cu22Fe8Ge4S32. The generation and testing of all the possible structural models is rapid and rigorous as it is based on two modules written in python language: a module that takes care of model creation and ordering (permutation algorithm associated with some filtering and ordering algorithms) and a python parser for the refinement software (FullProf) input and output. Each possible cationic model is tested through combined Rietveld refinement of resonant X-ray data at selected edges and analyzed considering its global χ2(Bragg contribution) agreement factor. This approach can easily integrate information coming from other techniques, as in our case EXAFS spectroscopy and single crystal X-ray diffraction. In spite of the complexity of the germanite, we were able to define the main characteristics of the cationic ordering (space group P4̅3n): (i) the “interstitial” site 2ais fully occupied by Fe, (ii) Ge is located only on the 6dsite (or the symmetry equivalent one 6c), and (iii) the remaining Fe atoms are located preferentially on the 12fsite and possibly on the 6d(or 6c) site along with Ge. Moreover, we single out a probable enrichment of Ge at the expense of Fe. The approach developed in this case study can be used as a guideline for the crystal structure resolution of analogous compounds.

The electrochemical activity of modern Fe–N–C electrocatalysts in alkaline media is on par with that of platinum. For successful application in fuel cells (FCs), however, also high durability and longevity must be demonstrated. Currently, a limited understanding of degradation pathways, especially under operando conditions, hinders the design and synthesis of simultaneously active and stable Fe–N–C electrocatalysts. In this work, using a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry setup, Fe dissolution is studied under conditions close to those in FCs, that is, with a porous catalyst layer (CL) and at current densities up to −125 mA·cm–2. Varying the rate of the oxygen reduction reaction (ORR), we show a remarkable linear correlation between the Faradaic charge passed through the electrode and the amount of Fe dissolved from the electrode. This finding is rationalized assuming that oxygen reduction and Fe dissolution reactions are interlinked, likely through a common intermediate formed during the Fe redox transitions in Fe species involved in the ORR, such as FeNxCyand Fe3C@N–C. Moreover, such a linear correlation allows the application of a simple metric─S-number─to report the material’s stability. Hence, in the current work, a powerful tool for a more applied stability screening of different electrocatalysts is introduced, which allows on the one hand fast performance investigations under more realistic conditions, and on the other hand a more advanced mechanistic understanding of Fe–N–C degradation in CLs.

The active forms of Au and Pt in CeO2-based catalysts for the water–gas shift (WGS) reaction are an issue that remains unclear, although it has been widely studied. On one hand, ionic species might be responsible for weakening the Ce–O bonds, thus increasing the oxygen mobility and WGS activity. On the other hand, the close contact of Au or Pt atoms with CeO2oxygen vacancies at the metal–CeO2interface might provide the active sites for an efficient reaction. In this work, using in situX-ray absorption spectroscopy, we demonstrate that both Au and Pt remain unoxidized during the reaction. Remarkable differences involving the dynamics established by both species under WGS atmospheres were recognized. For the prereduced Pt catalyst, the increase of the conversion coincided with a restructuration of the Pt atoms into cuboctahedrical metallic particles without significant variations on the overall particle size. Contrary to the relatively static behavior of Pt0, Au0nanoparticles exhibited a sequence of particle splitting and agglomeration while maintaining a zero oxidation state despite not being located in a metallic environment during the process. High WGS activity was obtained when Au atoms were surrounded by oxygen. The fact that Au preserves its unoxidized state indicates that the chemical interaction between Au and oxygen must be necessarily electrostatic and that such an electrostatic interaction is fundamental for a top performance in the WGS process.

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-NH3catalysts displayed a 40–50% higher current density than FeNC-NH3at 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)Nxsingle-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 NH3activation treatment, makes them a promising alternative to today’s state-of-the-art Fe-based catalysts.

Pyrolysis is indispensable for synthesizing highly active Fe–N–C catalysts for the oxygen reduction reaction (ORR) in acid, but how Fe, N, and C precursors transform to ORR-active sites during pyrolysis remains unclear. This knowledge gap obscures the connections between the input precursors and the output products, clouding the pathway toward Fe–N–C catalyst improvement. Herein, we unravel the evolution pathway of precursors to ORR-active catalyst comprised exclusively of single-atom Fe1(II)–N4sites via in-temperature X-ray absorption spectroscopy. The Fe precursor transforms to Fe oxides below 300 °C and then to tetrahedral Fe1(II)–O4via a crystal-to-melt-like transformation below 600 °C. The Fe1(II)–O4releases a single Fe atom that diffuses into the N-doped carbon defect forming Fe1(II)–N4above 600 °C. This vapor-phase single Fe atom transport mechanism is verified by synthesizing Fe1(II)–N4sites via “noncontact pyrolysis” wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.

Porous noble metal nanostructures providing high surface areas and large pore volumes are attractive for numerous applications, especially catalysis. However, the synthesis of such architectures with nanoscale features is challenging because of e.g., high surface energy. Previous reports are based on multi-step synthetic approaches with low yield and are thus not compatible with an industrial scale. Herein, we demonstrate that hierarchical nanostructured noble metals and alloys, with thermal stability up to 800 °C, can be fabricated by a high throughput, green and straightforward aerosol-assisted synthesis using bi-functional reducing templates. We unravel the reduction mechanism at the solid/solid interface at the single-particle level using state-of-the-art techniques such as in situtransmission electron microscopy, and in situsynchrotron-based X-ray absorption spectroscopy. We propose a mechanism based on a radical process thermally triggered by the depolymerization of organic components. From a fundamental point of view, this study sheds light on the reduction processes of noble metals in the presence of organic compounds with important fundamental implications for catalysis and electrocatalysis.

Nitrogen-doped carbon materials featuring atomically dispersed metal cations (M–N–C) are an emerging family of materials with potential applications for electrocatalysis. The electrocatalytic activity of M–N–C materials toward four-electron oxygen reduction reaction (ORR) to H2O is a mainstream line of research for replacing platinum-group-metal-based catalysts at the cathode of fuel cells. However, fundamental and practical aspects of their electrocatalytic activity toward two-electron ORR to H2O2, a future green “dream” process for chemical industry, remain poorly understood. Here we combined computational and experimental efforts to uncover the trends in electrochemical H2O2production over a series of M–N–C materials (M = Mn, Fe, Co, Ni, and Cu) exclusively comprising atomically dispersed M–Nxsites from molecular first-principles to bench-scale electrolyzers operating at industrial current density. We investigated the effect of the nature of a 3d metal within a series of M–N–C catalysts on the electrocatalytic activity/selectivity for ORR (H2O2and H2O products) and H2O2reduction reaction (H2O2RR). Co–N–C catalyst was uncovered with outstanding H2O2productivity considering its high ORR activity, highest H2O2selectivity, and lowest H2O2RR activity. The activity–selectivity trend over M–N–C materials was further analyzed by density functional theory, providing molecular-scale understandings of experimental volcano trends for four- and two-electron ORR. The predicted binding energy of HO* intermediate over Co–N–C catalyst is located near the top of the volcano accounting for favorable two-electron ORR. The industrial H2O2productivity over Co–N–C catalyst was demonstrated in a microflow cell, exhibiting an unprecedented production rate of more than 4 mol peroxide gcatalyst–1h–1at a current density of 50 mA cm–2.

We show here how the nature of various divalent cations M2+(Ca2+, Zn2+, or Fe2+) influences the structure and mechanical properties of ionotropic polygalacturonate (polyGal) hydrogels designed by the diffusion of cations along one direction (external gelation). All hydrogels exhibit strong gradients of polyGal and cation concentrations, which are similar for all studied cations with a constant ratio R = [M2+]/[Gal] equal to 0.25, showing that every M2+cation interacts with four galacturonate (Gal) units all along the gels. The regions of the hydrogels formed in the early stages of the gelation process are also similar for all cations and are homogeneous, with the same characteristic mesh size (75 ± 5 Å, as measured by small angle neutron scattering (SANS)) and the same storage modulus G′ (∼5 × 104Pa). Conversely, in the regions of the gels formed in later stages of the process there exist differences in mechanical properties, turbidity, and local structure from one cation to another. Zn(II)-polyGal and Fe(II)-polyGal hydrogels display mesoscopic heterogeneities, more marked in case of Fe than for Zn, that are not present in Ca(II)-polyGal hydrogels. This comes from the mode and the strength of association between the cation and the Gal unit (bidentate for Ca2+and monodentate “egg-box” for Zn2+and Fe2+). Cross-links formed by Zn2+and Fe2+have a higher stability (lower ability to untie and reform) that induces the formation of local heterogeneities in the early stages of the gelation process whose size progressively increases during the gel growth, a mechanism that does not occur for cross-links made by Ca2+that are less stable and enable possible reorganizations between polyGal chains.

The effect of ions on the structure of liquid water is still not completely understood, despite extensive experimental and theoretical studies. A combined XANES and molecular dynamics investigation on diluted Zn2+and Hg2+aqueous solutions reveals that the influence of a single ion on the bonding pattern of water molecules is strongly dependent on the nature of the ion. While the structure of water is not altered by the presence of the Zn2+ion, the Hg2+cation has a strong impact on the hydrogen-bond network of water that extends beyond the first coordination shell.

Two Fe-N-C catalysts comprising only atomically-dispersed FeNx moieties were prepared, differing only in the fact that the second catalyst (Fe0.5-NH3) was obtained by subjecting the first one (Fe0.5-Ar) to a short pyrolysis in ammonia. While the initial ORR activity in acid medium in rotating disk electrode is similar for both catalysts, the activity in alkaline medium is significantly higher for Fe0.5-NH3. Time-resolved Fe dissolution reveals a circa 10 times enhanced Fe leaching rate in acidic electrolyte for Fe0.5-NH3 relative to Fe0.5-Ar. Furthermore, for the former, the leaching rate is strongly enhanced when the electrochemical potential is in the range 0.75-0.3 V vs. RHE. This may explain the reduced stability of ammonia-pyrolyzed Fe-N-C catalysts in operating PEMFCs. In alkaline medium in contrast, Fe0.5-NH3 is more active and more stable, with minimized Fe leaching during electrochemical operation in load-cycling mode. Operando X-ray absorption spectroscopy measurements in alkaline electrolyte reveals similar trends of the XANES and EXAFS spectra as a function of the electrochemical potential for both catalysts, but the magnitude of change is much less for Fe0.5-NH3, as evidenced by a Du analysis. This is interpreted as a lower average oxidation state of FeNx moieties in Fe0.5-NH3 at open circuit potential.

Since a few years, non-precious metal catalysts with iron or cobalt as active centers show sufficient activity to be viable candidates as electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFC). They can sustain substantial current densities when operated at low potentials. However, their stabilization at high cathode potentials, necessary for high energy efficiency, remains a daunting task. Here an Fe-N-C catalyst is stabilized over the whole potential range through functionalization with minute amounts of platinum. With the addition of 1 to 2 wt% Pt, the present Pt/Fe-N-C hybrid catalysts show a similar current density at 0.8 V than Fe-N-C but are much more stable during operation in PEMFC. Various characterization techniques, including CO stripping, demonstrate that platinum in these hybrid catalysts is ORR-inactive, not only initially but also after the PEMFC potentiostatic test. It is proposed that the present platinum species protects the Fe-based active sites from the ORR by-product H2O2, or reactive oxygen species produced from its reaction with surface Fe. This proof-of-concept paves the way for a new class of hybrid catalysts, where the activity and stability of Me-N-C catalysts can be independently addressed.

While improved activity was recently reported for bimetallic iron-metal-nitrogen-carbon (FeMNC) catalysts for the oxygen reduction reaction (ORR) in acid medium, the nature of active sites and interactions between the two metals are poorly understood. Here, FeSnNC and FeCoNC catalysts were structurally and catalytically compared to their parent FeNC and SnNC catalysts. While CO cryo-chemisorption revealed a twice lower site density of M-Nxsites for FeSnNC and FeCoNC relative to FeNC and SnNC, the mass activity of both bimetallic catalysts is 50–100% higher than that of FeNC due to a larger turnover frequency in the bimetallic catalysts. Electron microscopy and X-ray absorption spectroscopy identified the coexistence of Fe-Nxand Sn-Nxor Co-Nxsites, while no evidence was found for binuclear Fe-M-Nxsites. 57Fe Mössbauer spectroscopy revealed that the bimetallic catalysts feature a higher D1/D2 ratio of the spectral signatures assigned to two distinct Fe-Nxsites, relative to the FeNC parent catalyst. Thus, the addition of the secondary metal favored the formation of D1 sites, associated with the higher turnover frequency.

Fe-N-C and Co-N-C materials are promising catalysts for reducing oxygen in fuel cells. The degradation of such catalysts induced by H2O2 was investigated by contacting them ex situ with various amounts of H2O2. The degradation increased with increasing amounts of H2O2. The effect was most severe for Cr-N-C followed by Fe-N-C and last by Co-N-C. Treatment with H2O2 leads to diminished oxygen reduction activity at high potential and/or reduced transport properties at high current density in fuel cell. From spectroscopic characterization, it was found that 66 and 80% of the CoNxCy and FeNxCy moieties present in pristine catalysts survived the extensive H2O2 treatment, respectively. In parallel, the activity for oxygen reduction was divided by ca 6-10 for Fe-N-C and by ca 3 for Co-N-C. The results suggest that the main degradation mechanism in fuel cell for such catalysts is due to a chemical reaction with H2O2 that is generated during operation. The super-proportional decrease of the oxygen reduction activity with loss of FeNxCy and CoNxCy moieties suggests either that only a small fraction of such moieties are initially located on the top surface, or that their turnover frequency for oxygen reduction was drastically reduced due to surface oxidation by H2O2.

The structural properties of the hydrated lanthanoid(III) ions in aqueous solution and in the isostructural trifluoromethanesulfonate salts have been investigated by a quantitative analysis of the X‐ray absorption near‐edge structure (XANES) spectra at the K‐ and L3‐edges. The XANES analysis has provided a clear description of the variation of lanthanoid(III) hydration properties across the series. It was found that all of the lanthanoid(III) hydration complexes retain a tricapped trigonal prism (TTP) geometry, and along the series two of the capping water molecules become less and less strongly bound, before finally, on average, one of them leaves the hydration cluster. This gives rise to an eight‐coordinated distorted bicapped trigonal prism with two different LnO capping distances for the smallest lanthanoid(III) ions. This systematic study has shown that for lanthanoid compounds more accurate structural information is obtained from the analysis of the L3‐edge than from K‐edge XANES data. Moreover, whereas the second hydration shells provide a detectable contribution to the L3‐edge XANES spectra of the lighter lanthanoid ions, the K‐edge spectra are insensitive to the more distant coordination spheres.

Iron–nitrogen–carbon (Fe–N–C) materials emerged as one of the best non-platinum group material (non-PGM) alternatives to Pt/C catalysts for the electrochemical reduction of O2in fuel cells. Co-doping with a secondary metal center is a possible choice to further enhance the activity toward oxygen reduction reaction (ORR). Here, classical Fe–N–C materials were co-doped with Sn as a secondary metal center. Sn–N–C according to the literature shows excellent activity, in particular in the fuel cell setup; here, the same catalyst shows a non-negligible activity in 0.5 M H2SO4electrolyte but not as high as expected, meaning the different and uncertain nature of active sites. On the other hand, in mixed Fe, Sn–N–C catalysts, the presence of Sn improves the catalytic activity that is linked to a higher Fe–N4site density, whereas the possible synergistic interaction of Fe–N4and Sn–Nxfound no confirmation. The presence of Fe–N4and Sn–Nxwas thoroughly determined by extended X-ray absorption fine structure and NO stripping technique; furthermore, besides the typical voltammetric technique, the catalytic activity of Fe–N–C catalyst was determined and also compared with that of the gas diffusion electrode (GDE), which allows a fast and reliable screening for possible implementation in a full cell. This paper therefore explores the effect of Sn on the formation, activity, and selectivity of Fe–N–C catalysts in both acid and alkaline media by tuning the Sn/Fe ratio in the synthetic procedure, with the ratio 1/2 showing the best activity, even higher than that of the iron-only containing sample (jk= 2.11 vs 1.83 A g–1). Pt-free materials are also tested for ORR in GDE setup in both performance and durability tests.

Iron–sulfur (Fe–S) clusters are prosthetic groups of proteins biosynthesized on scaffold proteins by highly conserved multi-protein machineries. Biosynthesis of Fe–S clusters into the ISCU scaffold protein is initiated by ferrous iron insertion, followed by sulfur acquisition, via a still elusive mechanism. Notably, whether iron initially binds to the ISCU cysteine-rich assembly site or to a cysteine-less auxiliary site via N/O ligands remains unclear. We show here by SEC, circular dichroism (CD), and Mössbauer spectroscopies that iron binds to the assembly site of the monomeric form of prokaryotic and eukaryotic ISCU proteins via either one or two cysteines, referred to the 1-Cys and 2-Cys forms, respectively. The latter predominated at pH 8.0 and correlated with the Fe–S cluster assembly activity, whereas the former increased at a more acidic pH, together with free iron, suggesting that it constitutes an intermediate of the iron insertion process. Iron not binding to the assembly site was non-specifically bound to the aggregated ISCU, ruling out the existence of a structurally defined auxiliary site in ISCU. Characterization of the 2-Cys form by site-directed mutagenesis, CD, NMR, X-ray absorption, Mössbauer, and electron paramagnetic resonance spectroscopies showed that the iron center is coordinated by four strictly conserved amino acids of the assembly site, Cys35, Asp37, Cys61, and His103, in a tetrahedral geometry. The sulfur receptor Cys104 was at a very close distance and apparently bound to the iron center when His103 was missing, which may enable iron-dependent sulfur acquisition. Altogether, these data provide the structural basis to elucidate the Fe–S cluster assembly process and establish that the initiation of Fe–S cluster biosynthesis by insertion of a ferrous iron in the assembly site of ISCU is a conserved mechanism.

Two Fe-N-C catalysts comprising only atomically-dispersed FeNxmoieties were prepared, differing only in the fact that the second catalyst (Fe0.5-NH3) was obtained by subjecting the first one (Fe0.5-Ar) to a short pyrolysis in ammonia. While the initial ORR activity in acid medium in rotating disk electrode is similar for both catalysts, the activity in alkaline medium is significantly higher for Fe0.5-NH3. Time-resolved Fe dissolution reveals a circa 10 times enhanced Fe leaching rate in acidic electrolyte for Fe0.5-NH3relative to Fe0.5-Ar. Furthermore, for the former, the leaching rate is strongly enhanced when the electrochemical potential is in the range 0.75–0.3 V vs. RHE. This may explain the reduced stability of ammonia-pyrolyzed Fe-N-C catalysts in operating PEMFCs. In alkaline medium in contrast, Fe0.5-NH3is more active and more stable, with minimized Fe leaching during electrochemical operation in load-cycling mode. Operando X-ray absorption spectroscopy measurements in alkaline electrolyte reveals similar trends of the XANES and EXAFS spectra as a function of the electrochemical potential for both catalysts, but the magnitude of change is much less for Fe0.5-NH3, as evidenced by a Δμ analysis. This is interpreted as a lower average oxidation state of FeNxmoieties in Fe0.5-NH3at open circuit potential.

The combination of EXAFS spectroscopy and molecular dynamics simulations leads to the proposal of a new improved set of ionic radii for lanthanoid(III) cations in aqueous solution.