Nx Sites Research Articles

Dynamic Control of Sacrificial Bond Transformation in the Fe-N-C Single-Atom Catalyst for Molecular Oxygen Reduction.

Atomically dispersed metal-nitrogen sites show great prospect for the oxygen reduction reaction (ORR), whereas the unsatisfactory adsorption-desorption behaviors of oxygenated intermediates on the metal centers impede improvement of the ORR performance. We propose a new conceptual strategy of introducing sacrificial bonds to remold the local coordination of Fe-Nx sites, via controlling the dynamic transformation of the Fe-S bonds in the Fe-N-C single-atom catalyst. Spectroscopic and theoretical results reveal that the selective cleavage of the sacrificial Fe-S bonds induces the incorporation of the electron-withdrawing oxidized sulfur on the Fe centers. The newly functionalized moieties endow the catalyst with superior ORR activity and remarkable stability, owing to the reduced electron localization around the Fe centers facilitating the desorption of ORR intermediates. These findings provide a unique perspective for precisely controlling the coordination structure of single-atom materials to optimize their activity.

Dynamic Control of Sacrificial Bond Transformation in the Fe−N−C Single‐Atom Catalyst for Molecular Oxygen Reduction

AbstractAtomically dispersed metal‐nitrogen sites show great prospect for the oxygen reduction reaction (ORR), whereas the unsatisfactory adsorption‐desorption behaviors of oxygenated intermediates on the metal centers impede improvement of the ORR performance. We propose a new conceptual strategy of introducing sacrificial bonds to remold the local coordination of Fe−Nx sites, via controlling the dynamic transformation of the Fe−S bonds in the Fe−N−C single‐atom catalyst. Spectroscopic and theoretical results reveal that the selective cleavage of the sacrificial Fe−S bonds induces the incorporation of the electron‐withdrawing oxidized sulfur on the Fe centers. The newly functionalized moieties endow the catalyst with superior ORR activity and remarkable stability, owing to the reduced electron localization around the Fe centers facilitating the desorption of ORR intermediates. These findings provide a unique perspective for precisely controlling the coordination structure of single‐atom materials to optimize their activity.

Polyacrylonitrile‐Derived Co or Fe Containing Nanofibre Catalysts Prepared Via Electrispinning for Alkaline Membrane Fuel Cell Cathode

An increasing worldwide energy demand drives the need for the development of more efficient and greener energy production and storage technologies (1). In recent years, carbon nanofibres (CNFs) have drawn a lot of attention and been widely studied to address these topical problems. For example, the CNF-based materials can be employed as an oxygen reduction reaction (ORR) catalyst at the fuel cell cathode. The CNFs are appealing because of their attractive properties e.g. high specific surface area, high length/diameter ratio, specific porosities and multiple functionalities. For the preparation of CNFs, a simple and versatile technique called electrospinning can be applied (2).In our previous papers regarding CNFs, we have investigated the pyrolyzed electrospun composite materials without added transition metals as ORR catalysts in 0.1 M KOH (3, 4). For further optimization of the catalyst material ORR activity, we have now included the Co or Fe precursors to the electrospinning solution. For the first time, the Fe- or Co-containing polyacrylonitrile (PAN)-based CNFs were studied as a cathode catalyst in the anion exchange membrane fuel cell (AEMFC) (5).The composite catalyst materials were prepared via electrospinning of Iron(II) acetate or Cobalt(II) acetate containing PAN solution with and without ionic liquid (IL). The electrospun nanofibres were pyrolysed at different temperatures and acid treated. The materials consisted of CNFs with diameter from ca. 200 to 700 nm. The composition of the catalysts included C, O, N and corresponding transition metal (Fe or Co). In 0.1 M KOH solution, the best performing ORR catalyst material was Fe‐containing CNF without IL prepared at 800 °C (Fe‐PAN‐A800). The best AEMFC performance was observed for Fe‐containing CNF with IL prepared at 1000 °C. The maximum power density value of this catalyst (Fe/IL‐PAN‐A1000) was 289 mW cm−2, which was 82 % of the value obtained with the commercial Pt/C cathode catalyst (Figure 1). The reasons behind the good performance at the AEMFC cathode were considered to be high specific surface area, mesoporous CNF cross‐linked structure and the presence of ORR‐active N species and Me−Nx sites (5).References A. Sarapuu, E. Kibena-Põldsepp, M. Borghei and K. Tammeveski, Journal of Materials Chemistry A, 6, 776 (2018). A. Barhoum, K. Pal, H. Rahier, H. Uludag, I. S. Kim and M. Bechelany, Applied Materials Today, 17, 1 (2019). M. Mooste, E. Kibena-Põldsepp, L. Matisen, M. Merisalu, M. Kook, V. Kisand, V. Vassiljeva, A. Krumme, V. Sammelselg and K. Tammeveski, Catalysis Letters, 148, 1815 (2018). M. Mooste, E. Kibena-Põldsepp, V. Vassiljeva, M. Merisalu, M. Kook, A. Treshchalov, V. Kisand, M. Uibu, A. Krumme, V. Sammelselg and K. Tammeveski, Journal of Materials Science, 54, 11618 (2019). M. Mooste, E. Kibena-Põldsepp, V. Vassiljeva, A. Kikas, M. Käärik, J. Kozlova, V. Kisand, M. Külaviir, S. Cavaliere, J. Leis, A. Krumme, V. Sammelselg, S. Holdcroft and K. Tammeveski, ChemCatChem, 12, 4568 (2020). Figure 1

Sulfur Doping versus Hierarchical Pore Structure: The Dominating Effect on the Fe–N–C Site Density, Activity, and Selectivity in Oxygen Reduction Reaction Electrocatalysis

Nitrogen doping has been always regarded as one of the major factors responsible for the increased catalytic activity of Fe–N–C catalysts in the oxygen reduction reaction, and recently, sulfur has emerged as a co-doping element capable of increasing the catalytic activity even more because of electronic effects, which modify the d-band center of the Fe–N–C catalysts or because of its capability to increase the Fe–Nx site density (SD). Herein, we investigate in detail the effect of sulfur doping of carbon support on the Fe–Nx site formation and on the textural properties (micro- and mesopore surface area and volume) in the resulting Fe–N–C catalysts. The Fe–N–C catalysts were prepared from mesoporous carbon with tunable sulfur doping (0–16 wt %), which was achieved by the modulation of the relative amount of sucrose/dibenzothiophene precursors. The carbon with the highest sulfur content was also activated through steam treatment at 800 °C for different durations, which allowed us to modulate the carbon pore volume and surface area (1296–1726 m2 g–1). The resulting catalysts were tested in O2-saturated 0.5 M H2SO4 electrolyte, and the site density (SD) was determined using the NO-stripping technique. Here, we demonstrate that sulfur doping has a porogenic effect increasing the microporosity of the carbon support, and it also facilitates the nitrogen fixation on the carbon support as well as the formation of Fe–Nx sites. It was found that the Fe–N–C catalytic activity [E1/2 ranges between 0.609 and 0.731 V vs reversible hydrogen electrode (RHE)] does not directly depend on sulfur content, but rather on the microporous surface and therefore any electronic effect appears not to be determinant as confirmed by X-ray photoemission spectroscopy (XPS). The graph reporting Fe–Nx SD versus sulfur content assumes a volcano-like shape, where the maximum value is obtained for a sulfur/iron ratio close to 18, i.e., a too high or too low sulfur doping has a detrimental effect on Fe–Nx formation. However, it was highlighted that the increase of Fe–Nx SD is a necessary but not sufficient condition for increasing the catalytic activity of the material, unless the textural properties are also optimized, i.e., there must be an optimized hierarchical porosity that facilitates the mass transport to the active sites.

Revealing the Effect of Nickel Nanoparticles for Li Plating and Stripping Processes on Ni−Nx Doped Hollow Carbon Sphere

AbstractIn this work, the effects of the coexistence of Ni−Nx sites and Ni metal nanoparticles of Ni−N−C (nickel‐nitrogen‐carbon) materials on the thermodynamic Li nucleation overpotential (η) and kinetic exchange current density (j0) are systematically studied. Ni‐Nx sites guarantee reduced nucleation resistance on the hollow carbon sphere (HCS) host, while the low amount residual of Ni nanoparticles plays an adverse role. The Li nucleation overpotential was down to 10.6 mV and the exchange current density increased from 1.139 to 2.325 mA cm−2 when the residual Ni nanoparticles in the prepared Ni−N−C composite were removed. As a result, pure Ni‐Nx sites displayed an average CE of 98.4 % over 300 cycles and a stable Li plating/stripping behavior for over 800 h.

Wide Potential CO2‐to‐CO Electroreduction Relies on Pyridinic‐N/Ni–Nx Sites and Its Zn–CO2 Battery Application

Although metal–N‐doped carbons are promising electrocatalysts for CO2‐to‐CO conversion due to their high conversion efficiency and current density, their commercial application is still challenging due to their narrow potential/current range. In general, a wide potential/current electrocatalyst is in high demand for industrial CO2 electrocatalysis. Controllable strategy to tune N configurations toward pyridinic‐enriched metal–Nx active sites is attractive. The synergetic effect of the ability of pyridinic N to capture Lewis acidic CO2 and the characteristics of Ni–N active sites to inhibit the hydrogen evolution reaction (HER) can achieve a wide potential window, which has high practical value. Herein, the well‐chosen precursors resulting in a pyridinic‐enriched Ni–Nx site can selectively catalyze CO2 toward CO generation in aqueous media with a high Faradaic efficiency (FE) of 100% with an overpotential of 680 mV. Continuous CO FE > 90% was recorded under a wide range of potentials without decay. Motivated by its practical characteristics in CO2 electrocatalysts, a Zn–CO2 battery is assembled achieving a CO2–CO conversion efficiency of more than 90% in a wide discharge current window. Therefore, the results highlight that pyridinic‐enriched Ni–Nx holds great promise as simultaneous CO‐selective electrocatalyst by suitably tuning M–N configurations as new perspective strategy.

Activation of peroxymonosulfate by single-atom Fe-g-C3N4 catalysts for high efficiency degradation of tetracycline via nonradical pathways: Role of high-valent iron-oxo species and Fe–Nx sites

A Fenton-like catalyst was prepared based on isolated Fe single-atom and Fe clusters anchored onto a g-C3N4 framework. It exhibited high activity and stability in the heterogeneous activation of peroxymonosulfate (PMS) for tetracycline (TC) degradation. Both experimental and density functional theory calculation results demonstrated that the unique N-coordinated single Fe atom (Fe–N4) served as active sites with optimal binding energy for PMS activation. The experimental analysis indicated that the single Fe atoms displayed superior catalytic activity to Fe clusters. Furthermore, both high-valent iron-oxo species and single oxygen-dominated nonradical processes caused TC degradation. Among them, single Fe atoms anchored onto Fe-g-C3N4 served as Fe–N4 reactive sites can directly activate PMS to produce high-valent iron-oxo species, which was the key active nonradical species causing TC degradation.

A surface regulation strategy to fabricate Cu-Nx sites of high homogeneity with countable activity towards oxygen reduction

Carbon catalysts with transition metal atoms anchored via nitrogen coordination have emerged as promising alternatives to Pt in catalyzing oxygen reduction reaction (ORR). However, the intangible evolution of structure in popular synthetic routes containing pyrolysis leads to discrete and irregular M−Nx sites, resulting in limited descriptors and thus ambiguous understandings of structure-activity relationships. We report here a facile method to synthesize Cu-Nx sites of high homogeneity anchored on carbon substrates through deposition and carbonization of glycoluril on metallic copper surface. The Cu-Nx sites take shape under the surface regulation of metallic copper and preferentially exposed after removal of excess Cu. The optimal catalyst presents efficient activity towards ORR with a half-wave potential of 0.84 V (vs. RHE). More importantly, linear relationships over the catalyst between current densities and electrochemical active surface areas (ECSAs) are derived, suggesting activities countable by ECSAs. The correlation clearly demonstrates the high homogeneity and catalytic accessibility of surface-regulated Cu-Nx sites. The well-elucidated structure–activity relationship identifies Cu-Nx sites as originates of catalytic ORR at an experimental level and sheds light on rational engineering of catalysts containing M−Nx sites.

Proton Capture Strategy for Enhancing Electrochemical CO2 Reduction on Atomically Dispersed Metal–Nitrogen Active Sites**

AbstractElectrocatalysts play a key role in accelerating the sluggish electrochemical CO2 reduction (ECR) involving multi‐electron and proton transfer. We now develop a proton capture strategy by accelerating the water dissociation reaction catalyzed by transition‐metal nanoparticles (NPs) adjacent to atomically dispersed and nitrogen‐coordinated single nickel (Ni−Nx) active sites to accelerate proton transfer to the latter for boosting the intermediate protonation step, and thus the whole ECR process. Aberration‐corrected scanning transmission electron microscopy, X‐ray absorption spectroscopy, and calculations reveal that the Ni NPs accelerate the adsorbed H (Had) generation and transfer to the adjacent Ni−Nx sites for boosting the intermediate protonation and the overall ECR processes. This proton capture strategy is universal to design and prepare for various high‐performance catalysts for diverse electrochemical reactions even beyond ECR.

Influence of doping nitrogen on the catalytic performance of carbon nanotubes encapsulating cobalt for selective oxidation of arylalkanes

Efficient selective oxidation of hydrocarbons to corresponding ketones has been a significant but challenging work in the past decades. Here, a nitrogen-doped bamboo-like carbon nanotubes encapsulating cobalt catalyst (NCNTs-X) using citric acid, melamine and CoCl2·6H2O as precursors was synthesized by a simple pyrolysis method. A series of characterization techniques were employed to investigate the physicochemical properties of samples. And the results show that the as-prepared catalysts achieve a high catalytic performance for selective ethylbenzene oxidation (78.3% ethylbenzene conversion) using molecular oxygen as oxidant, especially giving a high selectivity (93.8% selectivity to acetophenone). According to the investigation of the structure-activity relationship of the catalysts, it can be deduced that the Co–Nx sites have an obvious positive effect on catalytic performance thanks to the synergistic effect between metallic cobalt and pyridinic N.

Electronically Modified Atomic Sites Within a Multicomponent Co/Cu Composite for Efficient Oxygen Electroreduction

AbstractRational design of cost‐effective and active electrocatalysts is an essential step toward the large‐scale realization of hydrogen fuel cells and metal–air batteries. Its success requires a drastic improvement in the kinetics of the cathodic oxygen reduction reaction (ORR). Herein, a novel ORR catalyst formed by encapsulating thin Cu layer decorated Co nanoparticles inside graphitic carbon layers that are embedded with abundant CoNx and CuNx atomic sites (denoted as SA‐CoCu@Cu/CoNP), is reported. The multicomponent SA‐CoCu@Cu/CoNP composite exhibits a remarkable ORR catalytic activity, exceptional stability, and excellent methanol tolerance in alkaline media, outperforming the commercial platinum carbon under identical testing conditions and also being active in acidic media. The excellent ORR catalytic performance is ascribed to the modified electronic structure of the CoNx active sites due to an electron donating effect from the embedded nanoparticles and the nearby CuNx species, as revealed by X‐ray spectroscopic results and density functional theory computations. Moreover, the effective role of CuNx sites in suppressing the peroxide formation during ORR is also identified, endowing the resultant catalyst a prolonged stability and enhanced efficiency that are beneficial for practical applications.

Unraveling a Biomass-Derived Multiphase Catalyst for the Dehydrogenative Coupling of Silanes with Alcohols under Aerobic Conditions

Herein, a novel silver and chromium nanostructured N-doped carbonaceous material has been synthesized by a biomass-annealing approach using readily available chitosan as a raw material. The resulting catalyst AgCr@CN-800 has been applied for the dehydrogenative coupling reaction of various silanes with different alcohols to obtain the corresponding silyl ethers under aerobic and mild conditions. Besides excellent activity and selectivity, the as-prepared catalyst exhibits good stability and reusability. Characterization by X-ray diffraction, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and high-resolution transmission electron microscopy (TEM) in combination with careful examination of the structure with Cs-corrected high-angle annular dark-field scanning TEM revealed that the catalyst AgCr@CN-800 comprises Ag- and CrN-aggregated particles, as well as highly dispersed Ag–Nₓ and Cr–Nₓ sites embedded in N-doped graphitic structures. A comparative catalytic study using structure-related catalysts in combination with acid-leaching treatments has shown that the most active species are the Ag particles and that their activity is boosted by the presence of Cr-derived species. By in situ Raman spectroscopy experiments, it has been found that the dehydrogenative coupling of silanes with alcohols in the presence of catalyst AgCr@CN-800 takes place through an oxygen-assisted mechanism.

A dual-template strategy to engineer hierarchically porous Fe–N–C electrocatalysts for the high-performance cathodes of Zn–air batteries

MgO and ZnCl2 dual templates significantly optimize the accessibility of Fe–Nx sites and mass transfer.

Iron-doping on Cu–N–C composite with enhanced CO faraday efficiency for the electrochemical reduction of CO2

Fe–N–macrocycles have been viewing as the most promising catalyst for CO2ER. It is of great importance to explore the performance of composite CuFe–N–C in CO2ER. Fe-Cu–BTT precursor was prepared by introducing the ferrous ion to a microporous N-rich MOF. It exhibits a lower plateau temperature of 800 ℃ than the prototype. The pyrolysis product of FexCu–N–C increases the selectivity of CO2-to−CO due to the increase of the BET surface area, the total pore volume, and the Fe–Nx sites, as well as a lower density of Cu NPs in the carbon matrix. The Fe0.07Cu–N–C800 exhibits the highest FECO of 48.5 %.

Coordination Number Regulation of Molybdenum Single-Atom Nanozyme Peroxidase-like Specificity

Summary Nanozymes are promising alternatives to natural enzymes, but their use remains limited owing to poor specificity. Overcoming this and controlling the targeted enzyme-like performance of traditional nanozymes is extremely challenging due to the intrinsic structural complexity of these systems. We report theoretical design and experimental realization of a series of heterogeneous molybdenum single-atom nanozymes (named MoSA–Nx–C), wherein we find that the peroxidase-like specificity is well regulated by the coordination numbers of single Mo sites. The resulting MoSA–N3–C catalyst shows exclusive peroxidase-like behavior. It achieves this behavior via a homolytic pathway, whereas MoSA–N2–C and MoSA–N4–C catalysts have a different heterolytic pathway. The mechanism of this coordination-number-dependent enzymatic specificity is attributed to geometrical structure differences and orientation relationships of the frontier molecular orbitals toward these MoSA–Nx–C peroxidase mimics. This study demonstrates the rational design of peroxidase-specific nanozymes and precise regulation of their enzymatic properties.

Controlling active sites of Fe–N–C electrocatalysts for oxygen electrocatalysis

Abstract The electrochemical oxygen reduction reaction (ORR) is a critical reaction in many energy conversion and storage systems, including fuel cells and metal–air batteries. Nonprecious-metal-based catalysts have captured attention for realizing viable and sustainable devices. However, in Fe-based catalysts, the efficient utilization of active sites, Fe–Nx and Fe coated on a carbon layer (Fe@C), is challenging owing to difficulties in controlling these active sites during synthesis. In this study, FeNC electrocatalysts with varying Fe/C ratios show different Fe@C/Fe–Nx ratios and ORR activities. Increasing the carbon content increases the Fe–Nx site density while decreasing the size of the Fe nanoparticles and the thickness of the carbon coating layer, thus enhancing the ORR activity. As cathode materials for anion exchange membrane fuel cells and Zn–air batteries, the FeNC electrocatalysts exhibit excellent performance when compared to Pt catalysts and previously reported transition-metal-based catalysts. Based on the structural changes observed by in situ X-ray absorption fine structure analysis during electrochemical operation, these catalysts were found to contain electrocatalytically efficient Fe–N4 sites. This work provides efficient strategies for designing high-performance catalysts for the ORR.

An efficient pH-universal electrocatalyst for oxygen reduction: defect-rich graphitized carbon shell wrapped cobalt within hierarchical porous N-doped carbon aerogel

The design and facile synthesis of active, robust, cheap, and earth-abundant electrocatalysts to replace precious Pt catalysts is highly desirable for the cathodic oxygen reduction reaction (ORR) in fuel cells and rechargeable metal batteries. However, maximizing the catalytic activity and stability of the electrocatalysts in catalyzing ORR is significantly important but still remains challenge. To address this issue, we herein reported an effective strategy that fabricate the cobalt nanoparticles encapsulated with defect-rich graphitized carbon shell in the hierarchical porous N-doped carbon aerogel (Co/NCA), which can simultaneously enhance the electrochemical stability while boosting the electrocatalytic activity for ORR. The desirable combination of the Co nanoparticles encapsulated with defect-rich graphitic carbon shells coupling with Co−Nx sites, the large accessible surface-active sites, and the hierarchical porous structure give rise to the superior electrocatalytic activity and stability for ORR in wide pH range. As expect, the as-obtained catalyst exhibits high efficiency for ORR with an onset potential of 0.95 V (versus reversible hydrogen electrode, vs. RHE) and a half-wave potential of 0.83 V in 0.1 M KOH, comparable to the commercial Pt/C. Importantly, the catalyst also provides excellent ORR properties in acidic and neutral media with almost a 4e− pathway and superior durability and methanol tolerance, showing a potential as electrocatalysts for practical applications.

Fe, Cu-codoped metal-nitrogen-carbon catalysts with high selectivity and stability for the oxygen reduction reaction

Metal-nitrogen-carbon materials (M-N-C) are non-noble-metal-based alternatives to platinum-based catalysts and have attracted tremendous attention due to their low-cost, high abundance, and efficient catalytic performance towards the oxygen reduction reaction (ORR). Among them, Fe-based materials show remarkable ORR activity, but they are limited by low selectivity and low stability. To address these issues, herein, we have synthesized FeCu-based M-N-C catalysts, inspired by the bimetal center of cytochrome c oxidase (CcO). In acidic media, the selectivity was notably improved compared with Fe-based materials, with peroxide yields less than 1.2% (<1/3 of the hydrogen peroxide yields of Fe-N-C catalysts). In addition to Cu-N-C catalysts which can catalytically reduce hydrogen peroxide, the reduction current of hydrogen peroxide using FeCu-N-C-20 exceeded that of Fe-N-C by about 6% when the potential was greater than 0.4 V. Furthermore, FeCu-based M-N-C catalysts suffered from only a 15 mV attenuation in their half-wave potentials after 10,000 cycles of accelerated degradation tests (ADT), while there was a 30 mV negative shift for Fe-N-C. Therefore, we propose that the H2O2 released from Fe-Nx sites or N-doped carbon sites would be reduced by adjacent Cu- Nx sites, resulting in low H2O2 yields and high stability.

A rational synthesis of single-atom iron–nitrogen electrocatalysts for highly efficient oxygen reduction reaction

A feasible strategy was explored to achieve atomically dispersed Fe–Nx sites anchoring on porous carbon hybrid (Fe-SA/PC). The catalyst possessed excellent catalytic activity, high stability and methanol-tolerance toward ORR in alkaline solution.

Accelerated polysulfide conversion on hierarchical porous vanadium-nitrogen-carbon for advanced lithium-sulfur batteries.

With high theoretical specific density, low cost, and non-toxicity, Li-S batteries are regarded as a promising candidate for next-generation energy storage systems. However, the shuttling of soluble Li polysulfides (LiPSs) results in self-discharge and rapid capacity degradation. Herein, nitrogen-doped hierarchical porous carbon with embedded highly dispersed vanadium (v)-Nx sites (V-N-C) is developed as a high-performance Li-S battery cathode for the first time. The metal-organic polymer supramolecule structure formed by the electrostatic/hydrogen bond interaction of chitosan-VO3- strongly stabilizes V to generate a high density of V-Nx/C sites. During the discharge/charge process, the unique V-Nx/C active sites can serve as efficient catalysts to accelerate the redox kinetics of LiPSs, while the hierarchical porous carbon structure of V-N-C benefits the diffusion/transfer of Li+/e- and suppresses the shuttling of LiPSs. As a result, the S/V-N-C composite delivers a high specific capacity of 1111.2 mA h g-1 at 0.5C and maintains 573.6 mA h g-1 at 5C with a low capacity decay rate of 0.087% per cycle (over 500 cycles at 1C). The rate performance of the developed V-N-C cathode in Li-S batteries is superior to that of most of the reported M-N-C and carbon material/metal compound composite electrodes.