Nitrogen-doped Graphene Layers Research Articles

Oxidative Cleavage of β-O-4 Linkage in Lignin via Co Nanoparticles Embedded in 3DNG as Catalyst.

The cleavage of β-O-4 linkage in lignin is one of the key steps for oxidative conversion of lignin to low-molecular-weight aromatics. Herein, Co nanoparticles embedded in three-dimensional network of nitrogen-doped graphene (Co/NG@3DNG-X) were prepared through an immersion-pyrolysis procedure, in which X denotes the pyrolysis temperature. The detailed characterization of Co/NG@3DNG-X shows that the Co nanoparticles are coated with a few layers of nitrogen-doped graphene (NG) sheets that are further embedded in 3DNG matrix. The catalytic activities of the Co/NG@3DNG-X for the oxidative cleavage of β-O-4 linkage in lignin model compounds with O2 as oxidant are explored. It is demonstrated that catalytic activities of as-prepared Co/NG@3DNG-X can be tuned by varying the pyrolysis condition, and the Co/NG@3DNG-900 shows the highest catalytic activity, which is attributed to the enriched Co-Nx species, the strong surface basicity, the high specific surface and the mesoporous motif of 3DNG network. More pronouncedly, the Co/NG@3DNG-900 can also effectively catalyze the oxidative cleavage of organosolv lignin, generating certain monomeric aromatics. Additionally, the intrinsic magnetic property of Co nanoparticles makes the Co/NG@3DNG-X be easily recovered from the reaction mixture, and the as-coated thin NG layer can protect Co nanoparticle from oxidation condition, which putting together afford the Co/NG@3DNG-X with good reusability and stability.

In-situ Synthesis of Nitrogen-doped Graphene Layer Encapsulated Palladium Nanoparticles for Highly Selective Hydrogenation of Vanillin

<i>In-situ</i> Synthesis of Nitrogen-doped Graphene Layer Encapsulated Palladium Nanoparticles for Highly Selective Hydrogenation of Vanillin

On-pot fabrication of binder-free composite of iron oxide grown onto porous N-doped graphene layers with outstanding charge storage performance for supercapacitors

An effective one-step electrochemical method was developed to synthesize three-dimensional N-doped porous graphene/magnetite nanoparticles hybrid onto Ni foam (Fe3O4@3D-NPG/NF electrode). In this method, 3D nitrogen-doped porous graphene layers are electrophoretically deposited onto Ni foam, accompanied by the simultaneous in situ electrochemical deposition (ECD) of magnetite particles onto 3D-NPG layers. For comparison, Fe3O4 particles and N-doped graphene were separately deposited onto Ni foam, and pristine Fe3O4/NF and 3D-NPG/NF electrodes were fabricated. The structure, composition and morphology of the fabricated electrode materials were systematically characterized by XRD, FT-IR, FE-SEM, Raman, TEM, BET, and TGA/DSC techniques. The formation mechanism of Fe3O4@3D-NPG hybrid through EPD/ECD was proposed and described in detail. The charge storage capabilities of the fabricated electrodes were analyzed as the supercapacitor electrode. The results GCD tests revealed that Fe3O4@3D-NPG electrode is able to deliver specific capacity value of 715 F g−1 at 2 A g−1 and cycle life of 94.3% after 5000 GCD cycles, where the pristine Fe3O4/NF electrode delivered only specific capacity of 219 F g−1 and 77.6% capacity retention. These findings implicated the positive synergistic effects between Fe3O4 and 3D-NPG in the hybrid electrode to exhibit higher supercapacitive performance. This simple strategy could find practical uses in the large-scale fabricating Fe3O4@3D-NPG electrode.

Pencil graphite electrode modified with nitrogen-doped graphene and molecular imprinted polyacrylamide/sol-gel as an ultrasensitive electrochemical sensor for the determination of fexofenadine in biological media

A sensitive and selective sensor was developed for the determination of fexofenadine (FEX) drug. The sensor was constructed based on the modification of a pencil graphite electrode (PGE) as a low cost, high available and versatile working electrode. The PGE was modified by nitrogen-doped graphene (NDG) and molecularly imprinted polymer (MIP) as receptor to increase the sensitivity and reducing the interference of other chemicals. NDG was electrochemically deposited on the electrode surface which is more homogeneous and facilitated than its conventional chemical synthesis. The molecular imprinted polymer was immobilized on the NDG layer by the sol-gel technique. In the optimum conditions, the imprinting factor was obtained equal to 4.8, indicating the optimal selectivity of the sensor for the determination of FEX. Differential pulse voltammetry (DPV) was used for the determination of FEX, which exhibited a linear calibration graph of Ip versus FEX concentration in the range of 5.0 × 10−10-7.8 × 10-6 M (mol L-1). The detection limit of the sensor was calculated equal to 1.5 × 10−10 M, which displayed a superior detection limit when compared with the other electrochemical sensors reported for the FEX determination. The developed sensor contained advantages of simple design, satisfactory reproducibility, appropriate determination recoveries and high selectivity. These features permitted the successful application of the sensor for the measurement of FEX in pharmaceutical and biological samples.

“Inside out” growth method for high-quality nitrogen-doped graphene

High-quality nitrogen-doped graphene on nickel is prepared by exploiting both the catalytic properties of nickel and the solubility of nitrogen atoms into its bulk. Following the standard chemical vapor deposition procedure, a previously nitrogen-doped nickel substrate is exposed to carbon-containing precursors so that nitrogen atoms, segregating to the surface, remain trapped in the growing graphene network. Morphological and chemical characterization by scanning tunneling microscopy and X-ray photoelectron spectroscopy demonstrates that the process yields a flat, wide, continuous nitrogen-doped graphene layer. Experimental results are combined with a thorough density functional theory investigation of possible structural models, to obtain a clear description at the atomic scale of the various configurations of the nitrogen atoms observed in the graphene mesh. This growth method is potentially scalable and suitable for the production of high-performance nano-devices with well-defined nitrogen centers, to be exploited as metal-free carbon-based catalysts in several applicative fields such as electrochemistry, energy storage, gas storage/sensing or wastewater treatment.

Nitrogen-doped ultrathin graphene encapsulated Cu nanoparticles decorated on SrTiO3 as an efficient water oxidation photocatalyst with activity comparable to BiVO4 under visible-light irradiation

Water oxidation is known as the bottleneck for both natural and artificial photosynthesis, and is still facing challenges in developing visible-light responsive photocatalyst with promoted efficiency. Herein, Cu nanoparticle encapsulated in nitrogen-doped ultrathin graphene (NC) layer decorated on SrTiO3 (STO) is designed as a high-efficient, low-cost photocatalyst for the photocatalytic water oxidation under visible-light irradiation. The O2 evolution rate over 1% Cu@NC/STO is approximately 2 times that of commercial WO3, and even comparable to BiVO4. Systematical studies and density functional theory simulations show that nitrogen-doped graphene layer not only protects Cu NPs from oxidation, but also modifies the electronic states of carbon layer in addition to enhancing charge carriers separation, which ultimately promotes photocatalytic water oxidation with reduced apparent activation energy. Furthermore, in universality test, Cu@NC/STO showed an excellent activity for thermocatalytic reduction of 4-nitrophenol. This work demonstrated the prospect of rational designing Cu-based photocatalysts with high stability and efficiency.

Multimetallic FeCoNiOx Nanoparticles Covered with Nitrogen-Doped Graphene Layers as Trifunctional Catalysts for Hydrogen Evolution and Oxygen Reduction and Evolution

This Article demonstrates the synthesis and catalysis of a functional material with one core and dual shells containing an alloyed FeCoNi core covered with a thin layer of spinel oxides and encapsu...

High efficient catalytic oxidation of 5-hydroxymethylfurfural into 2,5-furandicarboxylic acid under benign conditions with nitrogen-doped graphene encapsulated Cu nanoparticles

Abstract Selective oxidation of 5-hydroxymethylfurfual (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastics monomer is efficiently promoted by a simple system without noble-metal and base additives. In this work, graphene oxide (GO) was first synthesised by an electrochemical method with flexible graphite paper (FGP) as start carbon material, then, nitrogen-doped graphene (NG) layers encapsulated Cu nanoparticles (NPs) was prepared by one-step thermal treatment of GO supported Cu2+ in flowing NH3 atmosphere. Compared with NG supported Cu NPs prepared by the traditional impregnation method, enhanced catalytic activity was achieved over Cu/NG and an FDCA yield of 95.2% was achieved under mild reaction conditions with tert-butylhydroperoxide (t-BuOOH) as the oxidant. Control experiments with different catalysts and different addition procedure of t-BuOOH showed the yield of HMF and various intermediates during reaction. From the changing of intermediates concentrations and reaction rates, a reaction pathway through HMF-DFF-FFCA-FDCA was proposed. This work gives a more convenient, more green, more economical and effective method in encapsulated metal NPs preparation and high selectivity in HMF oxidation to FDCA under mild conditions.

Rational design of hierarchical FeSe2 encapsulated with bifunctional carbon cuboids as an advanced anode for sodium-ion batteries.

Earth-abundant transition-metal selenides (TMSs) have aroused great interest towards their application in sodium-ion batteries (SIBs). Herein, we present Fe-based Prussian blue analogs (PBA) modified by graphene oxide as precursors to synthesize FeSe2 nanoparticles within a nitrogen-doped carbon (NC) matrix and graphene layer (FeSe2/NC@G). The bifunctional carbon wrapped FeSe2/NC@G shows excellent sodium-storage performance with a large reversible capacity of 331 mA h g-1 at 5.0 A g-1 and a high cyclability of 323 mA h g-1 at the current density of 2.0 A g-1 after 1000 cycles (82% capacity retention). Furthermore, full SIBs are also fabricated and exhibit superior capacities and stabilities. The remarkable electrochemical properties result from the formation of an Fe-O-C chemical bond in the composite with enhanced electronic/ionic diffusion kinetics and structural integrity. This study paves the way for the successful synthesis of novel nanostructural TMSs which can be utilized in energy storage system application.

Theoretical models for hydrogen evolution reaction at combined Mo2C and N – doped graphene

Molybdenum carbide (Mo2C) based catalysts have been found to be one of the most promising catalysts for hydrogen evolution reaction (HER). As a first attempt to understand their prominent catalytic activity, different Mo2C based models are theoretically investigated, while their catalytic activities are compared with respect to existing experimental results. The first constructed model structures consist of small Mo2C clusters adsorbed on clean/nitrogen-doped graphene surfaces which have shown an improvement in the HER activity compared with pristine graphene and Mo2C bulk. Regarding the cluster structure as a simplified representation of an active side on the cathode in the electrochemical cell, the HER is investigated using the density functional theory (DFT). This small cluster study is designed to give a qualitative insight into the HER catalysis of Mo2C nanoparticles and also to show guidance for a new catalyst design. We consider as well model structures consisting on a Mo2C (001) surface covered by pure and nitrogen-doped (N-doped) graphene layers. This model surfaces show high HER activities in both acidic and alkaline electrolytes. The N-doped impurities in the graphene structures influence the HER activity. In this contribution the hydrogen adsorption at different active sites, the influence of the acidity on the HER activity and comparisons bewtween experimental and theoretical results are shown. Qualitatively, our theoretical results, based on a previously developed thermochemical approach, agree with recent experimental findings.

Hierarchical micro-flowers self-assembled from SnS monolayers and nitrogen-doped graphene lamellar nanosheets as advanced anode for lithium-ion battery

Although layered tin sulfide (SnS) nano-architectures are drawing much attention as anode materials in lithium ion batteries (LIBs) because of the high theoretical capacity, there are drawbacks such as the large volume expansion and low conductivity. To overcome these difficulties, attempts have been made to prepare tin sulfide/carbon (SnS/C) nano-hybrids by anchoring/coating nanostructured SnS on/in nano-sized carbon. However, the commonly reported SnS/C composites still suffer from initial Coulombic efficiency (ICE), high electrode swelling, and inferior long-term stability because of weak chemical affinity, and limited contacting interface between SnS and carbon specially at high SnS mass percentage. Herein, we report hierarchical micro-flowers self-assembled from SnS monolayers and nitrogen-doped graphene (NG) lamellar nanosheets (NG-SnS) which are suitable for LIBs anode. The NG-SnS with a high SnS mass percentage (93.9 wt%), obvious C–S bond, and increased contact provide to boost activity, utilization, conductivity, and reversibility of Li storage. Moreover, the elastic NG layers work as a mechanical supporter to decrease the outer volumetric expansion and restrain electrochemical agglomeration of SnS during (de)lithiation processes. The molecular engineered NG-SnS hybrid shows a high initial Coulombic efficiency of 88.2%, robust cycle stability with a high capacity of 790 mAh g−1 after 900 cycles at 0.5 C, considerable rate performance, and small electrode swelling of 12%.

Nitrogen-doped graphene layers for electrochemical oxygen reduction reaction boosted by lattice strain

Nitrogen-doped graphene is a promising non-precious metal electrocatalyst for the oxygen reduction reaction (ORR). We report that distorted nitrogen-doped graphene layers encapsulating Fe3Co nanoparticles supported on carbon (Fe3Co@NG-C) display excellent activity toward ORR in alkaline media. Compared with a commercial Pt/C electrocatalyst at a loading of 80 μgPt/cm2 on a rotating disk electrode, the Fe3Co@NG-C exhibits an onset potential positively shifted by 50 mV at 0.1 mA/cm2 and a nearly identical half-wave potential. The Fe3Co@NG-C has minimal activity degradation during accelerated durability testing and superior tolerance to methanol than commercial Pt/C. Density functional theory calculations combined with poisoning experiments reveal that the high activity of the Fe3Co@NG-C mainly arises from strain in nitrogen-doped graphene induced by encapsulation of Fe3Co nanoparticles with exposed (1 1 0) planes, which promotes stabilization of the key OOH* intermediate involved in ORR. Our study shows the rational design of improved carbon-based electrocatalysts for ORR can be achieved by using strain engineering.

(Invited) Improving Electrochemical Stability and Rate Capability of Layered Oxides in Li-Ion Batteries Via Controllable Formation of Two-Dimensional Heterointerface

The expanding variety of applications relying on portable autonomous power calls for innovative energy storage solutions with high performance. The performance of lithium-ion batteries (LIBs), the most widely used energy storage system today, is limited by the available cathode materials. Layered transition metal oxides show high redox activity in intercalation reactions and relatively high working potentials, making them especially attractive for use as cathodes in batteries. However, the low electronic conductivity of most oxides limits their performance. Expanding interlayer region combined with incorporation of the transition metal ions in high oxidation states, such as V5+, Nb5+, Mo6+, or W6+ in the structural layers, can be an efficient strategy to increase the specific capacity of the cathodes in intercalation batteries, a long sought for capability needed to realize batteries with higher energy densities compared to the performance characteristics achievable today. Utilizing such materials with high operation voltage/specific capacity and fast ion/electron transport as intercalation battery cathodes is attractive for creating next generation batteries with high energy and power density. An oxide material that meets both requirement of forming a layered structure with expanded interlayer spacing and containing a transition metal ion in the high oxidation state within the layers is bilayered (or δ-) vanadium oxide. Unusually for oxides, the two-dimensional bilayered V2O5 slabs are separated by a large interlayer spacing of 11.5 Å stabilized by structural water. It was shown that δ-V2O5·nH2O delivers high specific capacities in both lithium-ion and beyond lithium ion energy storage systems. However, bilayered vanadium oxide electrodes demonstrate poor capacity retention and moderate rate performance. In this presentation, I will demonstrate a new layered material with the regions in its structure built by the alternating layers of vanadium oxide and carbon. The vanadium oxide layers correspond to the bilayers present in δ-V2O5 structure. The carbon layers are created via thermal treatment of organic molecules containing nitrogen atoms, and therefore believed to resemble layers of the nitrogen-doped graphene (NDG). The presence of the NDG layers is supported by a combined scanning transmission electron microscopy (STEM) imaging, Raman spectroscopy, X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The interlayer distance between the vanadium oxide layer is determined to be 10.8 Å by both XRD and STEM, and the NDG layer forms in-between. δ-V2O5·nH2O electrodes, obtained via a sol-gel process, were used as a reference to evaluate electrochemical performance of layered δ-V2O5/NDG heterostructures in Li-ion batteries. δ-V2O5/NDG electrodes showed high energy storage performance, with capacities above 200 mAh g-1 and 36% improvement in capacity retention compared to δ-V2O5·nH2O electrodes in Li-ion cells. Moreover, this heterostructured phase displays significantly improved rate performance, with a 66% increase in capacity retention when current was increased from 20 mA g-1 to 300 mA g-1 compared to the δ-V2O5 reference in Li-ion batteries. The improved electrochemical performance is attributed to the enhanced electron transport through NDG layers and stabilized two-deimensional heterostructured interface between dissimilar layers. This work opens an opportunity to create next generation cathode materials with high specific capacity and advanced electrochemical stability in applications requiring stable Li-ion batteries that can tolerate high current rates.

Can Density Functional Theory Predict Mössbauer Spectra in Pyrolyzed Fe-N-C Catalysts?

57Fe Mössbauer spectroscopy, based on the recoil-free emission and resonant absorption of γ-rays, allows an easier separation of the signals arising from the different Fe environments. This is in the basis of the growing interest of using 57Mössbauer spectroscopy for identification of Fe-based species present in pyrolyzed Fe-N-C catalysts. Because of their high temperature synthesis, these catalysts often comprise different coordinations of atomically-dispersed Fe ions covalently bound to the nitrogen-doped carbon matrix (FeNxCy moieties), but also metallic iron and iron carbides with long-range order.1-3 In the 57Fe Mössbauer spectra (recorded in the absence of a strong external magnetic field) metallic iron and iron carbides result in sextets (except for the paramagnetic γ-Fe structure, leading to singlet spectral component) while the fingerprint of FeNxCy moieties is a doublet spectral component. Precise interpretation of the doublets (often labeled as D1 and D2) allowing to distinguish between iron coordination geometries, spin- and oxidation state remains however challenging and has so far lacked strong theoretical or experimental reference basis for their unambiguous assignments. D1 doublet has an isomer value ranging from 0.33 to 0.4 mm/s and quadrupole splitting (QS) ranging from 0.7 to 1.2 mm/s, which is usually attributed to Fe-N4 defects with Fe2+ in its low spin state. The assignment of the D2 doublet with isomer shifts ranging from 0.36 to 0.52 mm/s and with quadrupole splitting ranging from 2.4 to 2.7 mm/s is more ambiguous. It is attributed to Fe-N4 defects similar to iron phthalocyanine with Fe2+ in its intermediate spin state or to a Fe-N2+2 defect in which Fe binds to four pyridinic nitrogen atoms from two adjacent nitrogen-doped graphene layers. However, these assignments have been based on experimental records of carbon-supported FeN4 macrocycles, which differ greatly from the case in which the Fe-Nx moieties are integrated into the graphene matrix. It appears, therefore, obvious that theoretical Mössbauer spectra should be highly beneficial to rationally assign the D1 and D2 spectra of Fe-N-C materials. Theoretical studies (mostly obtained with density-functional-theory, DFT, based approaches) of 57Fe Mössbauer spectra, have evidenced that the choice of the exchange-correlation functional and atomic basis sets has a strong impact on the accuracy of QS values, in particular for intermediate and high spin iron complexes. It is therefore necessary to examine for every new class of Fe-containing compounds the variation of QS with the DFT functional and other computational details. Theoretical 57Fe Mössbauer spectra in pyrolyzed Fe-N-C catalysts have not yet been made available, which prompted us toward the present DFT study of the quadrupole splitting energies in Fe-N4 defects that are identified as the active sites in the pyrolyzed Fe-N-C materials. Extensive computations of QS in Fe-N4 models with Fe in various coordinations and with different oxidation/spin states have been carried out. The use of several DFT exchange-correlation functionals and basis sets in cluster and periodic approaches are compared and will be reported. The analysis of the DFT calculated values in comparison with experimental data are used to interpret quadrupole doublets ubiquitous in the Mössbauer spectra of pyrolyzed Fe-N-C catalysts. Acknowledgement: This work is supported by the LabEx CheMISyst ANR-10-LABX-05-01.

Big to Small: Ultrafine Mo2 C Particles Derived from Giant Polyoxomolybdate Clusters for Hydrogen Evolution Reaction.

Due to its electronic structure, similar to platinum, molybdenum carbides (Mo2 C) hold great promise as a cost-effective catalyst platform. However, the realization of high-performance Mo2 C catalysts is still limited because controlling their particle size and catalytic activity is challenging with current synthesis methods. Here, the synthesis of ultrafine β-Mo2 C nanoparticles with narrow size distribution (2.5 ± 0.7 nm) and high mass loading (up to 27.5 wt%) on graphene substrate using a giant Mo-based polyoxomolybdate cluster, Mo132 ((NH4 )42 [Mo132 O372 (CH3 COO)30 (H2 O)72 ]·10CH3 COONH4 ·300H2 O) is demonstrated. Moreover, a nitrogen-containing polymeric binder (polyethyleneimine) is used to create MoN bonds between Mo2 C nanoparticles and nitrogen-doped graphene layers, which significantly enhance the catalytic activity of Mo2 C for the hydrogen evolution reaction, as is revealed by X-ray photoelectron spectroscopy and density functional theory calculations. The optimal Mo2 C catalyst shows a large exchange current density of 1.19 mA cm-2 , a high turnover frequency of 0.70 s-1 as well as excellent durability. The demonstrated new strategy opens up the possibility of developing practical platinum substitutes based on Mo2 C for various catalytic applications.

The integration of Mo2C-embedded nitrogen-doped carbon with Co encapsulated in nitrogen-doped graphene layers derived from metal–organic-frameworks as a multi-functional electrocatalyst

High-performance multi-functional electrocatalysts are highly desirable for various energy storage and conversion devices. In this study, we report a one dimensional N-doped carbon nanostructure, containing Mo2C and Co nanoparticles encapsulated in nitrogen-doped graphene layers (Mo2C@NC/Co@NG) as a multi-functional electrocatalyst. The optimized Mo2C@NC/Co@NG exhibited high activities and stabilities towards oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The experimental results demonstrate that the high activities and stabilities of the multi-functional catalysts are ascribed to the synergistic effect between Mo2C, Co, and N dopants. Typically, the optimized Mo2C@NC/Co@NG catalyst delivered a HER current density of 10 mA cm-2 at a small overpotential of 142 mV. Furthermore, a rechargeable zinc-air battery with the optimized Mo2C@NC/Co@NG as OER and ORR bifunctional catalyst showed a high round-trip efficiency and robust cycling stability, superior to the battery assembled with the benchmarked mixture of Pt/C and IrO2 as the air electrode. This study provides a simple method to reasonably design and fabricate highly efficient and stable multi-functional catalysts for clean and sustainable energy storage and conversion devices.

Effect of nitrogen doping of graphene nanoflakes on their efficiency in supercapacitor applications

Pristine graphene nanoflakes (GNFs) and those doped with nitrogen up to a level of 10.9[Formula: see text]at.% were tested as an electrode material for electrochemical double-layer supercapacitor. Doping with nitrogen increased the specific capacitance of GNFs from 88 to 180[Formula: see text]F/g. The increase results from the enhanced interaction of nitrogen-doped graphene layers with electrolyte ions.

Ir/g-C3N4/Nitrogen-Doped Graphene Nanocomposites as Bifunctional Electrocatalysts for Overall Water Splitting in Acidic Electrolytes.

Nitrogen-doped graphene (NG) chemically coupled with graphitic carbon nitride (g-C3N4) may facilitate the kinetics of overall electrochemical water splitting. Herein, a facile strategy is adopted to synthesize monodispersed Ir nanoparticles on g-C3N4/NG layers. Benefiting from the synergistic effect between different components of the catalyst, the optimal Ir/g-C3N4/NG catalyst with a low content of Ir (5.9 wt %) exhibits highly active for electrochemical water splitting in acidic electrolyte. Specifically, as a hydrogen evolution reaction catalyst, the optimal Ir/g-C3N4/NG exhibits a Tafel slope of 22 mV·dec-1. The optimal catalyst requires an overpotential of 22 mV to reach the current density of 10 mA·cm-2, the value of which is superior to Ir/NG (32 mV) and 20 wt % Pt/C (28 mV) catalysts; as an oxygen evolution reaction catalyst, it also achieve the Tafel slope of only 72.8 mV·dec-1. At the overpotential of 300 mV, the mass activity of the optimal Ir/g-C3N4/NG catalyst is 2.8 times as large as that of 5.7 wt % Ir/NG catalyst. More significantly, as a bifunctional catalyst, the optimal Ir/g-C3N4/NG achieves a current density of 10 mA·cm-2 with a potential of only 1.56 V and displays good stability for overall water splitting. This work provides a new strategy to design highly efficient acidic catalysts for electrochemical overall water splitting.

Understanding Pyrolyzed Fe-N-C Catalysts By Bridging Density Functional Theory Calculations and Mössbauer Spectroscopy

Platinum group metal-free (PGM-free) catalysts for oxygen reduction are being pursued as a solution for scarcity and cost of catalysts utilizing platinum. Transition metal-nitrogen-carbon (M-N-C) materials are currently the most popular of PGM-free catalysts finding use in low temperature fuel cells. Iron-containing Fe-N-C are being synthesized lately in many variations ranging from atomically dispersed to metallic nano-particles containing types. Despite intense effort, much of the structure of these materials remains unknown and the interpretation of the activity and selectivity data rely on substantial hypothesis-based structural assumptions. 57Mössbauer spectroscopy is gaining significant importance in the experimental characterization of pyrolyzed Fe-N-C catalysts [1]. Isomer shifts and quadrupole splittings (QS), two main features of the 57Mössbauer spectra of Fe-bearing materials, vary systematically as a function of 57Fe oxidation, spin states, and coordination and can be used to accurately determine the local chemical environment of Fe ions in the Fe-N-C materials. All Fe-N-C catalysts have shown at least two clearlydistinct doublets in their Mössbauer spectra, often labeledas D1 and D2. The D1 doublet has an isomer shift value ranging from 0.33 to 0.4 mm/s and quadrupole splitting ranging from 0.7 to 1.2 mm/s, which is usually empirically attributedto Fe-N4defects with Fe2+in its low spin state. The assignment of the D2 doublet with isomer shifts ranging from 0.36 to 0.52 mm/s and with quadrupole splitting ranging from 2.4 to 2.7 mm/s is more ambiguous.It is attributedto Fe-N4 defects similar to iron phthalocyanine with Fe2+in its intermediate spin state or to a Fe-N2+2 defect in which Fe binds to four pyridinic nitrogen atoms from two adjacent nitrogen-doped graphene layers. However, the assignment of these doublets has been basedon experimental reports on carbon-supported FeN4macrocycles, which differs greatly from the case in which the Fe-Nxmoieties are integratedintothe graphene matrix. Therefore, there is a need for combining density functional theory (DFT) calculations of isomer shifts and quadrupole splittings on more realistic models, and compare them to those experimentally extracted with different fitting approaches from the Mössbauer spectraof real Fe-N-C materials. The DFT theory has been applied previously to calculate 57Fe isomeric shifts and quadrupolesplitting for different Fe-containing molecules, but has not yet been appliedtothe Fe-Nxdefects incorporated in the graphene matrix. In this work, both cluster and periodic approacheswere used to calculate thequadrupolesplitting in Fe-N4defects that are identified as the active sites in the pyrolyzed Fe-N-C materials. Because of the well-documented dependence of the DFT computed QS on the exchange-correlation functionals and basis sets [2],the methodology has been first tested for benchmark systems, shown in the Figure. Extensive computations of QS in Fe-N4models with Fe in various coordinationsand with different oxidation/spin states have also been carried out. Furthermore, the DFT calculated values were compared with experimental data to interpret quadrupole doublets ubiquitous in the Mössbauer spectra of pyrolyzed Fe-N-C catalysts. This paper will bring to discussion both computational approaches and correlations of 57Mössbauerspectroscopy “fingerprints” with observed and hypothesized structural motifs of Fe-containing active sites in PGM-free electrocatalysts for oxygen reduction reaction. Zitolo A., Goellner V., Armel V., Sougrati M.-T., Mineva T., Stievano L., Fonda E., Jaouen F., Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials, Nature Materials, 14 , 937 (2015). Pápai M. and Vankó G., On Predicting Mössbauer Parameters of Iron-Containing Molecules with Density-Functional Theory, J. Chemical Theory and Computation, 9, 5004 (2013). Figure 1

Hierarchical Porous Nanosheets Constructed by Graphene-Coated, Interconnected TiO2 Nanoparticles for Ultrafast Sodium Storage.

Sodium-ion batteries (SIBs) are considered promising next-generation energy storage devices. However, a lack of appropriate high-performance anode materials has prevented further improvements. Here, a hierarchical porous hybrid nanosheet composed of interconnected uniform TiO2 nanoparticles and nitrogen-doped graphene layer networks (TiO2 @NFG HPHNSs) that are synthesized using dual-functional C3 N4 nanosheets as both the self-sacrificing template and hybrid carbon source is reported. These HPHNSs deliver high reversible capacities of 146 mA h g-1 at 5 C for 8000 cycles, 129 mA h g-1 at 10 C for 20 000 cycles, and 116 mA h g-1 at 20 C for 10 000 cycles, as well as an ultrahigh rate capability up to 60 C with a capacity of 101 mA h g-1 . These results demonstrate the longest cyclabilities and best rate capability ever reported for TiO2 -based anode materials for SIBs. The unprecedented sodium storage performance of the TiO2 @NFG HPHNSs is due to their unique composition and hierarchical porous 2D structure.