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La reducción electroquímica de CO2 (CO2RR) destaca como una solución prometedora para la mitigación del cambio climático. Sin embargo, esta reacción presenta desafíos debido a la alta estabilidad de la molécula de CO2 y la difícil selectividad hacia diferentes productos, así como la competencia con la reacción de evolución de hidrógeno, haciendo necesario el desarrollo de catalizadores activos y selectivos. Actualmente, los catalizadores atómicamente dispersos se posicionan como una solución prometedora para modular la selectividad de esta reacción hacia CO. Entre ellos, los catalizadores M-N-C presentan sitios activos basados en átomos de un metal de transición coordinado con nitrógeno en una matriz carbonosa, lo que permite obtener catalizadores con alta porosidad, buena conductividad eléctrica y bajo coste. En cuanto a la identificación de los productos, si bien técnicas exsitu (cromatografía, espectrometría de masas), son comúnmente utilizadas, estas presentan ciertas limitaciones en términos de desfase temporal y detección de productos en concentraciones muy bajas. Para abordar este inconveniente, el uso de un electrodo rotatorio de disco anillo (RRDE) se propone como una alternativa rápida y factible para la identificación de los productos generados durante la CO2RR. En este método, los catalizadores se depositan en el disco del electrodo, donde tiene lugar la CO2RR, mientras que los productos generados son simultáneamente oxidados en el anillo. Este trabajo presenta el estudio de catalizadores Cu-N-C basados en una matriz carbonosa de xerogel de carbono dopado con nitrógeno. Se analizó la influencia del precursor de cobre y de la concentración de metal durante la síntesis de los catalizadores en su comportamiento electroquímico para la CO2RR mediante el uso de RRDE.

Materials and methods: Synthesis of C1N1.-- Synthesis of NC@Fe3C materials.-- Physicochemical characterization.-- Electrochemical characterization.-- AEMFC experiments.-- Structural Models.-- DFT computational details.-- Calculation methods for evaluating ORR activity.

Encapsulating Fe3C in carbon layers has emerged as an innovative strategy for protecting Fe3C while preserving its high oxygen reduction activity. However, fundamental questions persist regarding the active sites of encapsulated Fe3C due to the restricted accessibility of oxygen molecules to the metal sites. Herein, the intrinsic electron transfer mechanisms of Fe3C nanoparticles encapsulated in N-doped carbon materials are unveiled for oxygen reduction electrocatalysis. The precision-structured C1N1 material is used to synthesize N-doped carbons with encapsulated Fe3C, significantly enhancing catalytic activity (EONSET = 0.98 V) and achieving near-100% operational stability. In anion-exchange membrane fuel cells, an excellent peak power density of 830 mW cm−2 is reached at 60 °C. The experimental and computational results revealed that the presence of Fe3C cores dynamically triggers electron transfer to the outermost carbon layer. This phenomenon amplifies the oxygen reduction reaction performance at N sites, contributing significantly to the observed catalytic enhancement.

Encapsulating Fe3C in carbon layers has emerged as an innovative strategy for protecting Fe3C while preserving its high oxygen reduction activity. However, fundamental questions persist regarding the active sites of encapsulated Fe3C due to the restricted accessibility of oxygen molecules to the metal sites. Herein, the intrinsic electron transfer mechanisms of Fe3C nanoparticles encapsulated in N-doped carbon materials are unveiled for oxygen reduction electrocatalysis. The precision-structured C1N1 material is used to synthesize N-doped carbons with encapsulated Fe3C, significantly enhancing catalytic activity (EONSET = 0.98 V) and achieving near-100% operational stability. In anion-exchange membrane fuel cells, an excellent peak power density of 830 mW cm−2 is reached at 60 °C. The experimental and computational results revealed that the presence of Fe3C cores dynamically triggers electron transfer to the outermost carbon layer. This phenomenon amplifies the oxygen reduction reaction performance at N sites, contributing significantly to the observed catalytic enhancement. © 2024 Wiley-VCH GmbH.

The electrochemical CO2 reduction reaction (CO2RR) to controllable chemicals is considered as a promising pathway to store intermittent renewable energy. Herein, a set of catalysts based on copper-nitrogen-doped carbon xerogel (Cu-N-C) are successfully developed varying the copper amount and the nature of the copper precursor, for the efficient CO2RR. The electrocatalytic performance of Cu-N-C materials is assessed by a rotating ring-disc electrode (RRDE), technique still rarely explored for CO2RR. For comparison, products are also characterized by online gas chromatography in a H-cell. The as-synthesized Cu-N-C catalysts are found to be active and highly CO selective at low overpotentials (from −0.6 to −0.8 V vs. RHE) in 0.1 M KHCO3, while H2 from the competitive water reduction appears at larger overpotentials (−0.9 V vs. RHE). The optimum copper acetate-derived catalyst containing Cu-N4 moieties exhibits a CO2-to-CO turnover frequency of 997 h−1 at −0.9 V vs. RHE with a H2/CO ratio of 1.8. These results demonstrate that RRDE configuration can be used as a feasible approach for identifying electrolysis products from CO2RR.

Single atoms and nanoclusters of Fe, Ni, Co, Cu, and Mn are systematically designed and embedded in a well-defined C1N1-type material that has internal cavities of ≈0.6 nm based on four N atoms. These N atoms serve as perfect anchoring points for the nucleation of small nanoclusters of different metal natures through the creation of metal-nitrogen (TM-N4) bonds. After pyrolysis at 800 °C, TM@CNx-type structures are obtained, where TM is the transition metal and x < 1. Fe@CNx and Co@CNx are the most promising for oxygen reduction reaction and hydrogen evolution reaction, respectively, with a Pt-like performance, and Ni@CNx is the most active for oxygen evolution reaction (OER) with an EOER of 1.59 V versus RHE, far outperforming the commercial IrO2 (EOER = 1.72 V). This systematic and benchmarking study can serve as a basis for the future design of advanced multi-functional electrocatalysts by modulating and combining the metallic nature of nanoclusters and single atoms.

1. Stability of the G−N4−TM structures.-- 2. O2 Interaction with the Fe–N4 Center.-- 3. Calculation methods of evaluating ORR, OER and HER activity.-- 4. Density of states calculations.-- Supplementary Figures and Tables

Fig. S1. STEM (left) and TEM (right) images of Cu-N-C-Ac2 acquired using a Tecnay F30 microscope (300 kV). Fig. S2. XRD pattern of Cu-N-C-Ac2-3h obtained using the same experimental details than for Cu-N-CAc2 but with a lixiviation treatment of 3 hours. Fig. S3. N1s XPS spectra of the Cu-N-C. The material Cu-N-C-Ac1-BPT before the two-step post-treatment (acid leaching and 2nd carbonization) is also included for comparison purposes. Fig. S4. Effect of the two-step post-treatment (acid leaching and 2nd carbonization) on the nitrogen adsorption-desorption isotherms for Cu-N-C-Ac1. Fig. S5. Correlation between the Cu oxidation state and the energy position of the XANES spectrum, determined as the first maximum of the first derivative spectrum of Cu-N-C-Ac0.5 (red circle) and different copper reference compounds (black circles).Fig. S6. Variation with time of pH of the electrolyte (0.1 M KHCO3 aqueous solution) as a function of CO2 partial pressure. Balance with an inert gas for the equilibrium CO2 (aq) + H2O ↔ HCO3– + H+, pKa = 6.4, room temperature, 1 atm total pressure. Fig. S7. Oxidation of formic acid and methanol at the Pt ring of an RRDE, disc turned off, 1600 rpm. Fig. S8. Comparison of the CV in 0.1 M KHCO3 saturated with N2 (blue curve), methane oxidation voltammogram after saturation of the electrolyte with CH4 at the open circuit potential (red curve) and CH4 stripping voltammogram (Ead = 0.4 V vs. RHE, green curve) at the Pt ring of an RRDE, disc turned off, 1600 rpm. CV conditions from 0.05 to 1.6 V vs. RHE, 50 mV s-1. Room temperature. Fig. S9. CVs of the disc at 50 mV s-1 and 1600 rpm in CO2-saturated 0.1 M KHCO3 of Cu-N-C catalysts. Fig. S10. Chronoamperometric experiments at the disc for the Cu-N-C catalysts. Electrolyte: Fig. S11. CVs at the platinum ring for disc potentials reported in the legend. Table S1. Bulk chemical composition (wt. %) from EA and ICP of Cu-N-C electrocatalysts before and after the two-step post-treatment (acid leaching and 2nd

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 CeO2 oxygen vacancies at the metal-CeO2 interface might provide the active sites for an efficient reaction. In this work, using in situ X-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, Au0 nanoparticles 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

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 O2 in 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 H2SO4 electrolyte 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-N4 site density, whereas the possible synergistic interaction of Fe-N4 and Sn-Nx found no confirmation. The presence of Fe-N4 and Sn-Nx was 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.

We combined the hybrid density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) experiments in the study of the local atomic structure around Ir ions in ZnO thin films with different iridium content. This was then used in the first principles analysis of the thermoelectric properties of material. The emphasis has been put on the conditions for a positive Seebeck coefficient and p-type electrical conductivity as the functions of the Fermi level. We studied both computationally and experimentally several possible IrOx polyhedra (complexes) with a different number of surrounding oxygens and Ir oxidation states, including those with the formation of peroxide ions (O-2(2-)). In particular, octahedral coordination of iridium ions was identified by reverse Monte Carlo (RMC) simulations of the Ir L-3-edge EXAFS spectra of ZnO:Ir thin films as the predominant complex, which is supported by the calculated lowest interstitial oxygen incorporation energies. All the calculated IrOx (x = 4, 5, 6) complexes, regardless of Ir the oxidation state, demonstrate potential for p-type conduction if the Fermi level (mu(F)) falls in the range of 0-0.8 eV from the valence band maximum (VBM) and the Ir concentration is high enough (12.5% in the present DFT calculations). Even though the corresponding calculated Seebeck coefficient (S) around 80-89 mu V K-1 slightly exceeds the experimental values, we emphasise the presence of an important plateau in the dependence of S on mu(F) in this range for two complexes with the formation of peroxide ions (O-2(2-)). We predicted also that peroxide ions O-2(2-) are characterized by the calculated phonon frequencies of 810-942 cm(-1) in agreement with our previous Raman experimental results. In this light, we discuss the high sensitivity of calculated S(mu(F)) dependences to the atomic and electronic structure.

The electrochemical reduction of CO2 (eCO2RR) using renewable energy is an effective approach to pursue carbon neutrality. The eCO2RR to CO is indispensable in promoting C-C coupling through bifunctional catalysis and achieving cascade conversion from CO2 to C2+. This work investigates a series of M/N-C (M = Mn, Fe, Co, Ni, Cu, and Zn) catalysts, for which the metal precursor interacted with the nitrogen-doped carbon support (N-C) at room temperature, resulting in the metal being present as (sub)nanosized metal oxide clusters under ex situ conditions, except for Cu/N-C and Zn/N-C. A volcano trend in their activity toward CO as a function of the group of the transition metal is revealed, with Co/N-C exhibiting the highest activity at -0.5 V versus RHE, while Ni/N-C shows both appreciable activity and selectivity. Operando X-ray absorption spectroscopy shows that the majority of Cu atoms in Cu/N-C form Cu0 clusters during eCO2RR, while Mn/, Fe/, Co/, and Ni/N-C catalysts maintain the metal hydroxide structures, with a minor amount of M0 formed in Fe/, Co/, and Ni/N-C. The superior activity of Fe/, Co/, and Ni/N-C is ascribed to the phase contraction and the HCO3- insertion into the layered structure of metal hydroxides. Our work provides a facile synthetic approach toward highly active and selective electrocatalysts to convert CO2 into CO. Coupled with state-of-the-art NiFe-based anodes in a full-cell device, Ni/N-C exhibits >80% Faradaic efficiency toward CO at 100 mA cm-2.

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-NH3 catalysts displayed a 40–50% higher current density than FeNC-NH3 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)Nx 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 NH3 activation treatment, makes them a promising alternative to today’s state-of-the-art Fe-based catalysts.

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-NH3 catalysts displayed a 40–50% higher current density than FeNC-NH3 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)Nx 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 NH3 activation treatment, makes them a promising alternative to today’s state-of-the-art Fe-based catalysts.

In this article we show for the first time the formation of magnetic supported organic nanowires (ONWs) driven by self-assembly of a nonplanar Fe(III) phthalocyanine chloride (FePcCl) molecule. The ONWs grow by a crystallization mechanism on roughness-tailored substrates. The growth methodology consists of a vapor deposition under low vacuum and mild temperature conditions. The structure, microstructure, and chemical composition of the FePcCl NWs are thoroughly elucidated and compared with those of Fe(II) phthalocyanine NWs by a consistent and complementary combination of advanced electron microscopies and X-ray spectroscopies. In a further step, we vertically align the NWs by conformal deposition of a SiO2 shell. Such orientation is critical to analyze the magnetic properties of the FePcCl and FePc supported NWs. A ferromagnetic behavior below 30 K with an easy axis perpendicular to the phthalocyanine plane was observed in the two cases with the FePcCl nanowires presenting a wider hysteresis. These results open the path to the fabrication of nanostructured one-dimensional small-molecule spintronic devices.