Exploring Spin Distribution and Electronic Properties in FeN4-Graphene Catalysts with Edge Terminations.

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.

Chemical Kinetic Method for Active-Site Quantification in Fe-N-C Catalysts and Correlation with Molecular Probe and Spectroscopic Site-Counting Methods.

Mononuclear Fe ions ligated by nitrogen (FeNx) dispersed on nitrogen-doped carbon (Fe-N-C) serve as active centers for electrocatalytic O2 reduction and thermocatalytic aerobic oxidations. Despite their promise as replacements for precious metals in a variety of practical applications, such as fuel cells, the discovery of new Fe-N-C catalysts has relied primarily on empirical approaches. In this context, the development of quantitative structure-reactivity relationships and benchmarking of catalysts prepared by different synthetic routes and by different laboratories would be facilitated by the broader adoption of methods to quantify atomically dispersed FeNx active centers. In this study, we develop a kinetic probe reaction method that uses the aerobic oxidation of a model hydroquinone substrate to quantify the density of FeNx centers in Fe-N-C catalysts. The kinetic method is compared with low-temperature Mössbauer spectroscopy, CO pulse chemisorption, and electrochemical reductive stripping of NO derived from NO2- on a suite of Fe-N-C catalysts prepared by diverse routes and featuring either the exclusive presence of Fe as FeNx sites or the coexistence of aggregated Fe species in addition to FeNx. The FeNx site densities derived from the kinetic method correlate well with those obtained from CO pulse chemisorption and Mössbauer spectroscopy. The broad survey of Fe-N-C materials also reveals the presence of outliers and challenges associated with each site quantification approach. The kinetic method developed here does not require pretreatments that may alter active-site distributions or specialized equipment beyond reaction vessels and standard analytical instrumentation.

Electrochemical carbonyl reduction on single-site M-N-C catalysts.

Electrochemical conversion of organic compounds holds promise for advancing sustainable synthesis and catalysis. This study explored electrochemical carbonyl hydrogenation on single-site M-N-C (Metal Nitrogen-doped Carbon) catalysts using formaldehyde, acetaldehyde, and acetone as model reactants. We strive to correlate and understand the selectivity dependence on the nature of the metal centers. Density Functional Theory calculations revealed similar binding energetics for carbonyl groups through oxygen-down or carbon-down adsorption due to oxygen and carbon scaling. Fe-N-C exhibited specific oxyphilicity and could selectively reduce aldehydes to hydrocarbons. By contrast, the carbophilic Co-N-C selectively converted acetaldehyde and acetone to ethanol and 2-propanol, respectively. We claim that the oxyphilicity of the active sites and consequent adsorption geometry (oxygen-down vs. carbon-down) are crucial in controlling product selectivity. These findings offer mechanistic insights into electrochemical carbonyl hydrogenation and can guide the development of efficient and sustainable electrocatalytic valorization of biomass-derived compounds.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches.

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

Unraveling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO2 Reduction.

Single-atom catalysts with a well-defined metal center open unique opportunities for exploring the catalytically active site and reaction mechanism of chemical reactions. However, understanding of the electronic and structural dynamics of single-atom catalytic centers under reaction conditions is still limited due to the challenge of combining operando techniques that are sensitive to such sites and model single-atom systems. Herein, supported by state-of-the-art operando techniques, we provide an in-depth study of the dynamic structural and electronic evolution during the electrochemical CO2 reduction reaction (CO2RR) of a model catalyst comprising iron only as a high-spin (HS) Fe(III)N4 center in its resting state. Operando 57Fe Mössbauer and X-ray absorption spectroscopies clearly evidence the change from a HS Fe(III)N4 to a HS Fe(II)N4 center with decreasing potential, CO2- or Ar-saturation of the electrolyte, leading to different adsorbates and stability of the HS Fe(II)N4 center. With operando Raman spectroscopy and cyclic voltammetry, we identify that the phthalocyanine (Pc) ligand coordinating the iron cation center undergoes a redox process from Fe(II)Pc to Fe(II)Pc-. Altogether, the HS Fe(II)Pc- species is identified as the catalytic intermediate for CO2RR. Furthermore, theoretical calculations reveal that the electroreduction of the Pc ligand modifies the d-band center of the in situ generated HS Fe(II)Pc- species, resulting in an optimal binding strength to CO2 and thus boosting the catalytic performance of CO2RR. This work provides both experimental and theoretical evidence toward the electronic structural and dynamics of reactive sites in single-Fe-atom materials and shall guide the design of novel efficient catalysts for CO2RR.

Structural and Reactivity Effects of Secondary Metal Doping into Iron-Nitrogen-Carbon Catalysts for Oxygen Electroreduction.

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-Nx sites 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-Nx and Sn-Nx or Co-Nx sites, while no evidence was found for binuclear Fe-M-Nx sites. 57Fe Mössbauer spectroscopy revealed that the bimetallic catalysts feature a higher D1/D2 ratio of the spectral signatures assigned to two distinct Fe-Nx sites, 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.

Operando Spectroscopic Analysis of Axial Oxygen-Coordinated Single-Sn-Atom Sites for Electrochemical CO2 Reduction.

Sn-based materials have been demonstrated as promising catalysts for the selective electrochemical CO2 reduction reaction (CO2RR). However, the detailed structures of catalytic intermediates and the key surface species remain to be identified. In this work, a series of single-Sn-atom catalysts with well-defined structures is developed as model systems to explore their electrochemical reactivity toward CO2RR. The selectivity and activity of CO2 reduction to formic acid on Sn-single-atom sites are shown to be correlated with Sn(IV)-N4 moieties axially coordinated with oxygen (O-Sn-N4), reaching an optimal HCOOH Faradaic efficiency of 89.4% with a partial current density (jHCOOH) of 74.8 mA·cm-2 at -1.0 V vs reversible hydrogen electrode (RHE). Employing a combination of operando X-ray absorption spectroscopy, attenuated total reflectance surface-enhanced infrared absorption spectroscopy, Raman spectroscopy, and 119Sn Mössbauer spectroscopy, surface-bound bidentate tin carbonate species are captured during CO2RR. Moreover, the electronic and coordination structures of the single-Sn-atom species under reaction conditions are determined. Density functional theory (DFT) calculations further support the preferred formation of Sn-O-CO2 species over the O-Sn-N4 sites, which effectively modulates the adsorption configuration of the reactive intermediates and lowers the energy barrier for the hydrogenation of *OCHO species, as compared to the preferred formation of *COOH species over the Sn-N4 sites, thereby greatly facilitating CO2-to-HCOOH conversion.

FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta-Coordinated Sites.

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O2 ) reduction at the cathode of proton exchange membrane fuel cells are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O2  reduction, their controlled synthesis and stability for practical applications remain challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilization remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, this issue is addressed by coordinating Fe in a highly porous nitrogen-doped carbon support (≈3295 m2  g-1 ), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine and a Mg2+ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54 × 1019  sites gFeNC -1  and a record 52% FeNx electrochemical utilization based on in situ nitrite stripping are achieved. The Fe single atoms are characterized pre- and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which are further studied by density functional theory calculations.

Doped Graphene To Mimic the Bacterial NADH Oxidase for One-Step NAD+ Supplementation in Mammals.

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.

Oxygen Reduction Reaction in Alkaline Media Causes Iron Leaching from Fe-N-C Electrocatalysts.

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 FeNxCy and 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.

What is Next in Anion-Exchange Membrane Water Electrolyzers? Bottlenecks, Benefits, and Future.

As highlighted by the recent roadmaps from the European Union and the United States, water electrolysis is the most valuable high-intensity technology for producing green hydrogen. Currently, two commercial low-temperature water electrolyzer technologies exist: alkaline water electrolyzer (A-WE) and proton-exchange membrane water electrolyzer (PEM-WE). However, both have major drawbacks. A-WE shows low productivity and efficiency, while PEM-WE uses a significant amount of critical raw materials. Lately, the use of anion-exchange membrane water electrolyzers (AEM-WE) has been proposed to overcome the limitations of the current commercial systems. AEM-WE could become the cornerstone to achieve an intense, safe, and resilient green hydrogen production to fulfill the hydrogen targets to achieve the 2050 decarbonization goals. Here, the status of AEM-WE development is discussed, with a focus on the most critical aspects for research and highlighting the potential routes for overcoming the remaining issues. The Review closes with the future perspective on the AEM-WE research indicating the targets to be achieved.

Quantification of Active Site Density and Turnover Frequency: From Single-Atom Metal to Nanoparticle Electrocatalysts.

Single-atom catalysts (SACs) featuring atomically dispersed metal cations covalently embedded in a carbon matrix show significant potential to achieve high catalytic performance in various electrocatalytic reactions. Although considerable advances have been achieved in their syntheses and electrochemical applications, further development and fundamental understanding are limited by a lack of strategies that can allow the quantitative analyses of their intrinsic catalytic characteristics, that is, active site density (SD) and turnover frequency (TOF). Here we show an in situ SD quantification method using a cyanide anion as a probe molecule. The decrease in cyanide concentration triggered by irreversible adsorption on metal-based active sites of a model Fe-N-C catalyst is precisely measured by spectrophotometry, and it is correlated to the relative decrease in electrocatalytic activity in the model reaction of oxygen reduction reaction. The linear correlation verifies the surface-sensitive and metal-specific adsorption of cyanide on Fe-N x sites, based on which the values of SD and TOF can be determined. Notably, this analytical strategy shows versatile applicability to a series of transition/noble metal SACs and Pt nanoparticles in a broad pH range (1-13). The SD and TOF quantification can afford an improved understanding of the structure-activity relationship for a broad range of electrocatalysts, in particular, the SACs, for which no general electrochemical method to determine the intrinsic catalytic characteristics is available.

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites.

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has largely been impeded by the low oxygen reduction reaction activity of M-N-C due to low active site density and site utilization. Herein, we overcome these limits by implementing chemical vapour deposition to synthesize Fe-N-C by flowing iron chloride vapour over a Zn-N-C substrate at 750 °C, leading to high-temperature trans-metalation of Zn-N4 sites into Fe-N4 sites. Characterization by multiple techniques shows that all Fe-N4 sites formed via this approach are gas-phase and electrochemically accessible. As a result, the Fe-N-C catalyst has an active site density of 1.92 × 1020 sites per gram with 100% site utilization. This catalyst delivers an unprecedented oxygen reduction reaction activity of 33 mA cm-2 at 0.90 V (iR-corrected; i, current; R, resistance) in a H2-O2 proton exchange membrane fuel cell at 1.0 bar and 80 °C.

Selective electrochemical reduction of nitric oxide to hydroxylamine by atomically dispersed iron catalyst.

Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product selectivity remains a great challenge. Here we show iron-nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. Using in operando spectroscopic techniques, the catalytic site is identified as isolated ferrous moieties, at which the rate for hydroxylamine production increases in a super-Nernstian way upon pH decrease. Computational multiscale modelling attributes the origin of unconventional pH dependence to the redox active (non-innocent) property of NO. This makes the rate-limiting NO adsorbate state more sensitive to surface charge which varies with the pH-dependent overpotential. Guided by these fundamental insights, we achieve a Faradaic efficiency of 71% and an unprecedented production rate of 215 µmol cm-2 h-1 at a short-circuit mode in a flow-type fuel cell without significant catalytic deactivation over 50 h operation.

Potential-Induced Spin Changes in Fe/N/C Electrocatalysts Assessed by In Situ X-ray Emission Spectroscopy.

The commercial success of the electrochemical energy conversion technologies required for the decarbonization of the energy sector requires the replacement of the noble metal-based electrocatalysts currently used in (co-)electrolyzers and fuel cells with inexpensive, platinum-group metal-free analogs. Among these, Fe/N/C-type catalysts display promising performances for the reduction of O2 or CO2 , but their insufficient activity and stability jeopardize their implementation in such devices. To circumvent these issues, a better understanding of the local geometric and electronic structure of their catalytic active sites under reaction conditions is needed. Herein we shed light on the electronic structure of the molecular sites in two Fe/N/C catalysts by probing their average spin state with X-ray emission spectroscopy (XES). Chiefly, our in situ XES measurements reveal for the first time the existence of reversible, potential-induced spin state changes in these materials.

Narrow resection margins are not associated with mortality or recurrence in patients with Merkel cell carcinoma: A retrospective study.

BACKGROUND: Wide local excision constitutes the standard of care for Merkel cell carcinoma, but the optimal margin width remains controversial. OBJECTIVES: To assess whether narrow margins (0.5-1 cm) were associated with outcome. METHODS: Patients were recruited from a retrospective French multicentric cohort and included if they had had excision of primary tumor with minimum lateral margins of 0.5 cm. Factors associated with mortality and recurrence were assessed by multivariate regression. RESULTS: Among the 214 patients included, 58 (27.1%) had undergone excision with narrow margins (0.5-1 cm) versus 156 (72.9%) with wide margins (>1 cm). During a median follow-up of 50.7 months, cancer-specific survival did not differ between groups (5-year specific survival rate 76.8% [95% confidence interval 61.7%-91.9%] and 76.2% [95% confidence interval 68.8%-83.6%], respectively). Overall survival, any recurrence-free survival, and local recurrence-free survival did not significantly differ between groups. Cancer-specific mortality was associated with age, male sex, American Joint Committee on Cancer stage III, and presence of positive margins. LIMITATIONS: Retrospective design, heterogenous baseline characteristics between groups. CONCLUSION: Excision with narrow margins was not associated with outcome in this cohort, in which most patients had clear margins and postoperative radiation therapy. Residual tumor, mostly found on deep surgical margins, was independently associated with prognosis.

Engineering Fe-N Doped Graphene to Mimic Biological Functions of NADPH Oxidase in Cells.

NADPH oxidase (NOX) as a transmembrane enzyme complex controls the generation of superoxide that plays important roles in immune signaling pathway. NOX inactivation may elicit immunodeficiency and cause chronic granulomatous disease (CGD). Biocompatible synthetic materials with NOX-like activities would therefore be interesting as curative and/or preventive approaches in case of NOX deficiency. Herein, we synthesized a Fe-N doped graphene (FeNGR) nanomaterial that could mimic the activity of NOX by efficiently catalyzing the conversion of NADPH into NADP+ and triggering the generation of oxygen radicals. The resulting FeNGR nanozyme had similar cellular distribution to NOX and is able to mimic the enzyme function in NOX-deficient cells by catalyzing the generation of superoxide and retrieving the immune activity, evidenced by TNF-α, IL-1ß, and IL-6 production in response to Alum exposure. Overall, our study discovered a synthetic material (FeNGR) to mimic NOX and demonstrated its biological function in immune activation of NOX-deficient cells.