Nitrogen-doped graphene nanoribbons as efficient metal-free electrocatalysts for oxygen reduction

Exploring the Potential of Heteroatom-Doped Graphene Nanoribbons as a Catalyst for Oxygen Reduction

In this study, we created a series of N, S, and P-doped and co-doped carbon catalysts using a single graphene nanoribbon (GNR) matrix and thoroughly evaluated the impact of doping on ORR activity and selectivity in acidic, neutral, and alkaline conditions. The results obtained showed no significant changes in the GNR structure after the doping process, though changes were observed in the surface chemistry in view of the heteroatom insertion and oxygen depletion. Of all the dopants investigated, nitrogen (mainly in the form of pyrrolic-N and graphitic-N) was the most easily inserted and detected in the carbon matrix. The electrochemical analyses conducted showed that doping impacted the performance of the catalyst in ORR through changes in the chemical composition of the catalyst, as well as in the double-layer capacitance and electrochemically accessible surface area. In terms of selectivity, GNR doped with phosphorus and sulfur favored the 2e- ORR pathway, while nitrogen favored the 4e- ORR pathway. These findings can provide useful insights into the design of more efficient and versatile catalytic materials for ORR in different electrolyte solutions, based on functionalized carbon.

Detection of nitrofurazone with metal-organic frameworks and reduced graphene oxide composites: insights from molecular dynamics simulations

Two-dimensional metal-organic framework (MOF) composites were produced by incorporating Fe-MOFs into reduced graphene oxide (rGO) nanosheets to form Fe-MOF/rGO composites by hydrothermal synthesis. SEM, TEM, XRD, XPS, and measurements of contact angles were used to characterize the composites. TEM studies revealed that the rod-like-shaped Fe-MOFs were extensively dispersed on the rGO sheets. Incorporating Fe-MOF into rGO significantly improves performance due to the large surface area, chemical stability, and high electrical conductivity. The response signals for the electrochemical sensing performance of Fe-MOF/rGO-modified electrodes to nitrofurazone (NFZ) were significantly enhanced. Differential pulse voltammetry was used to detect the NFZ, and the MOF/rGO sensor possesses a lower detection limit (0.77μM) with two dynamic ranges from 0.6-60 to 128-499.3 μM and high sensitivity (1.909 μA·mM^-1·cm^-2). Moreover, the anti-interference properties of the sensor were quite reproducible and stable. To understand the mechanism responsible for the enhanced sensing performance of the composite, grand canonical Monte Carlo calculations were performed for Fe-MOF/rGO composites with five unit cells of Fe-MOF and four layers of rGO. We attributed the improvement to the fact that the interface between the Fe-MOF and rGO absorbed increased NFZ molecules. The findings reported herein confirm that such Fe-MOF/rGO composites have significantly improved electrochemical performance and practical applicability of sensing nitrofurazone.

Recent advances in supramolecular self-assembly derived materials for high-performance supercapacitors

The key preponderance of supramolecular self-assembly strategy is its ability to precisely assemble various functional units at the molecular level through non-covalent bonds to form multifunctional materials. Supramolecular materials have the merits of diverse functional groups, flexible structure, and unique self-healing properties, which make them of great value in the field of energy storage. This paper reviews the latest research progress of the supramolecular self-assembly strategy for the advanced electrode materials and electrolytes for supercapacitors, including supramolecular self-assembly for the preparation of high-performance carbon materials, metal-based materials and conductive polymer materials, and its beneficial effects on the performance of supercapacitors. The preparation of high performance supramolecular polymer electrolytes and their application in flexible wearable devices and high energy density supercapacitors are also discussed in detail. In addition, at the end of this paper, the challenges of the supramolecular self-assembly strategy are summarized and the development of supramolecular-derived materials for supercapacitors is prospected.

Role of N-Doping and O-Groups in Unzipped N-Doped CNT Carbocatalyst for Peroxomonosulfate Activation: Quantitative Structure–Activity Relationship

We examined the relationship between the intrinsic structure of a carbocatalyst and catalytic activity of peroxomonosulfate (PMS) activation for acetaminophen degradation. A series of nitrogen-doped carbon nanotubes with different degrees of oxidation was synthesized by the unzipping method. The linear regression analysis proposes that pyridinic N and graphitic N played a key role in the catalytic oxidation, rather than pyrrolic N and oxidized N. Pyridinic N reinforce the electron population in the graphitic framework and initiate the non-radical pathway via the formation of surface-bound radicals. Furthermore, graphitic N forms activated complexes (carbocatalyst-PMS*), facilitating the electron-transfer oxidative pathway. The correlation also affirms that -C=O was dominantly involved as a main active site, rather than -C-OH and -COOH. This study can be viewed as the first attempt to demonstrate the relationship between the fraction of N-groups and activity, and the quantity of O-groups and activity by active species (quenching studies) was established to reveal the role of N-groups and O-groups in the radical and non-radical pathways.

Carbon-based 0D/1D/2D assembly with desired structures and defect states as non-metal bifunctional electrocatalyst for zinc-air battery

For the design of electrocatalysts, the combination between components and the regulation of material structures tend to be neglected, giving rise to the constraint of catalytic performance and durability. Herein, we developed a graphene oxide quantum dots (GOQDs) with enhanced oxygen content by a one-step cutting method. Then, one-dimensional (1D) carbon nanotubes and two-dimensional (2D) reduced graphene oxide are crosslinked and self-assembled, thus attracting unsaturated-bond-riches GOODs (0D) to uniformly attach to the skeleton, simultaneously achieving nitrogen and sulfur co-doping. To the best of our knowledge, there is no report to prepare bifunctional electrocatalyst with GOQDs. Electrochemical tests show that even without metal-doping, the novel non-metal bifunctional electrocatalyst (N,S-GOQD-RGO/CNT) exhibits a higher half-wave potential (0.84 V) and enhanced limiting current density (5.88 mA cm^-2) than commercial Pt/C catalyst. The density functional theory is implemented to reveal the coordination of nitrogen and sulfur co-doping on GOQDs, which results in the improvement of overall catalytic active sites. Furthermore, the rechargeable zinc-air battery based on N,S-GOQD-RGO/CNT exhibits a maximum power density of 134.3 mW cm^-2, open circuit potential of 1.414 V, which is better than Pt/C+Ru/C mixed material. The obtained N,S-GOQD-RGO/CNT will provide a perspective application in fuel cells.

A template-directed synthesis of metal-organic framework (MOF-74) ultrathin nanosheets for oxygen reduction electrocatalysis

Metal–organic frameworks (MOFs) have received wide attention for their promising applications in numerous fields due to their tailorable structure, metal centers and porosity. However, the low mass permeability, poor conductivity and blockage of active metal centers severely restrict the utilization of MOF systems in electrocatalysis. Two-dimensionalization can endow MOF materials extra unsaturated metal centers and enhanced electron-transfer ability, and could be an effective strategy to achieve high-performance MOF-based electrocatalysts. Herein, Ni-MOF-74 nanosheets are synthesized using layered double hydroxide (LDH) as a template to directly grow ultrathin structures. Benefiting from the two-dimensional structure, Ni-MOF-74 nanosheets with carbon substrate exhibit an enhanced ORR electrocatalytic property with positive half-wave potential (+0.83 V vs. RHE), a large current density (3.9 mA cm⁻²), four-electron selectivity and a promising long-term durability.

Layered double hydroxide (LDH) is employed as a layered template to synthesize ultrathin Ni-MOF-74 nanosheets. The two-dimensional structure brings numerous exposed unsaturated active sites and enhanced conductivity for Ni-MOF-74.

Fjord-Edge Graphene Nanoribbons with Site-Specific Nitrogen Substitution

The synthesis of graphene nanoribbons (GNRs) that contain site-specifically substituted backbone heteroatoms is one of the essential goals that must be achieved in order to control the electronic properties of these next generation organic materials. We have exploited our recently reported solid-state topochemical polymerization/cyclization-aromatization strategy to convert the simple 1,4-bis(3-pyridyl)butadiynes 3a,b into the fjord-edge nitrogen-doped graphene nanoribbon structures 1a,b (fjord-edge N₂[8]GNRs). Structural assignments are confirmed by CP/MAS ¹³C NMR, Raman, and XPS spectroscopy. The fjord-edge N₂[8]GNRs 1a,b are promising precursors for the novel backbone nitrogen-substituted N₂[8]_AGNRs 2a,b. Geometry and band calculations on N₂[8]_AGNR 2c indicate that this class of nanoribbons should have unusual bonding topology and metallicity.

Mn3O4 nanoparticle-decorated hollow mesoporous carbon spheres as an efficient catalyst for oxygen reduction reaction in Zn-air batteries

Hybrids of Mn₃O₄ nanoparticles and hollow carbon spheres prepared from templated pyrolysis of polydopamine were assembled via a straightforward sonication procedure. The resulting hybrids exhibit excellent catalytic activity toward the oxygen reduction reaction (ORR) in prototype Zn-air batteries. Impressively, these catalysts exhibit higher discharge potential and exceptional stability when compared to commercial Pt-Ru catalysts while simultaneously showing comparable onset potential and maximum current density.

Research Progress on Applications of Polyaniline (PANI) for Electrochemical Energy Storage and Conversion

Conducting polyaniline (PANI) with high conductivity, ease of synthesis, high flexibility, low cost, environmental friendliness and unique redox properties has been extensively applied in electrochemical energy storage and conversion technologies including supercapacitors, rechargeable batteries and fuel cells. Pure PANI exhibits inferior stability as supercapacitive electrode, and can not meet the ever-increasing demand for more stable molecular structure, higher power/energy density and more N-active sites. The combination of PANI and other active materials like carbon materials, metal compounds and other conducting polymers (CPs) can make up for these disadvantages as supercapacitive electrode. As for rechargeable batteries and fuel cells, recent research related to PANI mainly focus on PANI modified composite electrodes and supported composite electrocatalysts respectively. In various PANI based composite structures, PANI usually acts as a conductive layer and network, and the resultant PANI based composites with various unique structures have demonstrated superior electrochemical performance in supercapacitors, rechargeable batteries and fuel cells due to the synergistic effect. Additionally, PANI derived N-doped carbon materials also have been widely used as metal-free electrocatalysts for fuel cells, which is also involved in this review. In the end, we give a brief outline of future advances and research directions on PANI.

Amorphous Oxide Nanostructures for Advanced Electrocatalysis

Amorphous oxides have attracted special attention as advanced electrocatalysts owing to their unique local structural flexibility and attractive electrocatalytic properties. With abundant randomly oriented bonds and surface-exposed defects (e.g., oxygen vacancies) as active catalytic sites, the adsorption/desorption of reactants can be optimized, leading to superior catalytic activities. Amorphous oxide materials have found wide electrocatalytic applications ranging from hydrogen evolution and oxygen evolution to oxygen reduction, CO₂ electroreduction and nitrogen electroreduction. The amorphous oxide electrocatalysts even outperform their crystalline counterparts in terms of electrocatalytic activity and stability. Despite of the merits and achievements for amorphous oxide electrocatalysts, there are still issues and challenges existing for amorphous oxide electrocatalysts. There are rarely reviews specifically focusing on amorphous oxide electrocatalysts and therefore it is imperative to have a comprehensive overview of the research progress and to better understand the achievements and issues with amorphous oxide electrocatalysts. In this minireview, several general preparation methods for amorphous oxides are first introduced. Then, the achievements in amorphous oxides for several important electrocatalytic reactions are summarized. Finally, the challenges and perspectives for the development of amorphous oxide electrocatalysts are outlined.

Unraveling Photoexcited Charge Transfer Pathway and Process of CdS/Graphene Nanoribbon Composites toward Visible-Light Photocatalytic Hydrogen Evolution

Converting solar energy into chemical fuels is increasingly receiving a great deal of attention. In this work, CdS nanoparticles (NPs) are solvothermally anchored onto graphene nanoribbons (GNRs) that are longitudinally unzipped from multiwalled carbon nanotubes. The as-synthesized CdS/GNR nanocomposites with recyclability present GNR content-dependent activity in visible-light-driven hydrogen evolution from water splitting. In a range of 1-10 wt% GNRs, the CdS/GNR composites with 2 wt% GNRs achieves the greatest hydrogen evolution rate of 1.89 mmol h^-1 g^-1 . The corresponding apparent quantum efficiency is 19.3%, which is ≈3.7 times higher than that of pristine CdS NPs. To elucidate the underlying photocatalytic mechanism, a systematic characterization, including in situ irradiated X-ray photoelectron spectroscopy and Kelvin probe measurements, is performed. In particular, the interfacial charge transfer pathway and process from CdS NPs to GNRs is revealed. This work may open avenues to fabricate GNR-based nanocomposites for solar-to-chemical energy conversion and beyond.

Novel Porous Nitrogen Doped Graphene/Carbon Black Composites as Efficient Oxygen Reduction Reaction Electrocatalyst for Power Generation in Microbial Fuel Cell

To improve the power generation of a microbial fuel cell (MFC), a porous nitrogen-doped graphene/carbon black (NG/CB) composite as efficient oxygen reduction reaction (ORR) electrocatalyst was successfully synthesized by pyrolyzing graphene oxide (GO) encapsulated CB with cetyltrimethyl ammonium bromide as a bridge. This concept-to-proof synthesis can be considered as a template-like method. Based on this method, one composite named as NG/CB-10 was acquired using the optimized GO-to-CB mass ratio of 10:1. Electrochemical tests demonstrate that NG/CB-10 can catalyze ORR in neutral-pH medium through a four-electron pathway with positively shifted the onset potential, the enhanced current density and reduced charge transfer resistance. CB addition also prolongs the stability of NG/CB-10. The enhancement in electrochemical performance of NG/CB-10 was attributed to the enlarged surface area, abundant mesopores and high content of pyridinic nitrogen. The maximum power density of MFC equipping NG/CB-10 as cathode electrocatalyst reached 936 mW·m⁻², which was 26% higher than that of NG and equal to that of platinum/carbon. The cost of NG/CB-10 was reduced by 25% compared with that of NG. This work provides a novel method to synthesize promising ORR electrocatalyst for MFC in the future.

Enhanced lithium storage performance of graphene nanoribbons doped with high content of nitrogen atoms

Nitrogen doping can provide a large number of active sites for lithium-ion storage, thus can yield a higher capacity for lithium-ion batteries. However, most of the reported N-doped graphene-based materials have low nitrogen content (<10 wt%) as the introduction of nitrogen atoms prefer to be produced at edges and defects in the graphene lattices. Owing to the formation of edges and defects, the doped states or active sites can easily be located and nitrogen contents can be determined precisely. Here we present the preparation of N-doped graphene nanoribbons with high nitrogen contents (11.8 wt%) and a facile tunable configuration of doped states. The material can be used as an anode for lithium-ion batteries and shows a higher capacity (the electrode has a reversible capacity of 1100.34 mA h g^-1 at a charge/discharge rate of 100 mA g^-1, corresponds to a discharge time of about 9 h), better rate performance (the electrode has a reversible capacity of 471 mA h g^-1 at the current density of 2 A g^-1, corresponds to a discharge time of about 11.6 min) and improved cycling stability (87.37% of the initial capacity after 200 cycles). The experimental results and first-principle calculations suggest that the residual oxygen-containing functional groups of N-doped graphene nanoribbons promote the formation of pyrrolic nitrogen at edges and substantially increase the room for nitrogen doping. This work opens new strategies for designing and developing N-doped graphene anodes for high performance lithium-ion batteries.

Ultrasound promoted transition metal doped polyaniline nanofibers: Enhanced electrode material for electrochemical energy storage applications

Here in, we report a simple and facile method to synthesis morphology oriented transition metal (Nickel) doped polyaniline (Ni²⁺/PANI) by chemical oxidative polymerization with the assistance of ultrasonic irradiation. Physicochemical property of the materials examined through XRD and FT-IR. The morphological feature exposed that the sonochemical assisted Ni²⁺ doped PANI is differing from the conventional method and it reveals a notable electrochemical property as in the form of specific capacitance (370 F g^-1 at 0.5 A g^-1) with improved rate capability and sustained cycling performance due to its typical interconnected nano-fibrillar morphology than the other synthesized materials. These intriguing features realized from the properly arranged nanostructure with perfect doping and make as a promising candidate as an electrode material in supercapacitor applications.

Chemical Approaches to Carbon-Based Metal-Free Catalysts

Highly active and durable catalysts play a key role in clean energy technologies. However, the high cost, low reserves, and poor stability of noble-metal-based catalysts have hindered the large-scale development of renewable energy. Owing to their low cost, earth abundance, high activity, and excellent stability, carbon-based metal-free catalysts (CMFCs) are promising alternatives to precious-metal-based catalysts. Although many synthetic methods based on solution, surface/interface, solid state, and noncovalent chemistries have been developed for producing numerous CMFCs with diverse structures and functionalities, there is still a lack of effective approaches to precisely control the structures of active sites. Therefore, novel chemical approaches are needed for the development of highly active and durable CMFCs that are capable of replacing precious-metal catalysts for large-scale applications. Herein, a comprehensive and critical review on chemical approaches to CMFCs is given by summarizing important advancements, current challenges, and future perspectives in this emerging field. Through such a critical review, our understanding of CMFCs and the associated synthetic processes will be significantly increased.

Transition Metal–Nitrogen–Carbon (M–N–C) Catalysts for Oxygen Reduction Reaction. Insights on Synthesis and Performance in Polymer Electrolyte Fuel Cells

Platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) have attracted increasing interest as potential candidates to replace Pt, in the view of a future widespread commercialization of polymer electrolyte fuel cell (PEFC) devices, especially for automotive applications. Among different types of PGM-free catalysts, M–N–C materials appear to be the most promising ones in terms of activity. These catalysts can be produced using a wide variety of precursors containing C, N, and one (or more) active transition metal (mostly Fe or Co). The catalysts synthesis methods can be very different, even though they usually involve at least one pyrolysis step. In this review, five different synthesis methods are proposed, and described in detail. Several catalysts, produced approximately in the last decade, were analyzed in terms of performance in rotating disc electrode (RDE), and in H2/O2 or H2/air PEFC. The catalysts are subdivided in five different categories corresponding to the five synthesis methods described, and the RDE and PEFC performance is put in relation with the synthesis method.

Ternary N, S, and P-Doped Hollow Carbon Spheres Derived from Polyphosphazene as Pd Supports for Ethanol Oxidation Reaction

Ethanol oxidation reaction (EOR) is an important electrode reaction in ethanol fuel cells. However, there are many problems with commercial ethanol oxidation electrocatalysts today, such as poor durability, poor anti-CO poisoning ability, and low selectivity for C–C bond cleavage. Therefore, it is very meaningful to develop a high-performance EOR catalyst. Herein, we designed ternary N, S, and P-doped hollow carbon spheres (C–N,P,S) from polyphosphazene (PCCP) as Pd supports for EOR. Using SiO2 spheres as the templates, the PCCP was first coated on the surfaces of SiO2 spheres by in situ polymerization. Through high-temperature pyrolysis and hydrofluoric acid-etching, the hollow PCCP has a large surface area and porous structure. After loading Pd nanoparticles (NPs), the Pd/C–N, P, S catalysts with Pd NPs decorated on the surfaces of C–N, P, S can achieve a high mass peak current density of 1686 mA mgPd−1, which was 2.8 times greater than that of Pd/C. Meanwhile, the Pd/C–N, P, S catalyst also shows a better stability than that of Pd/C after a durability test of 3600s.

Controllable synthesis of carbon nanosheets derived from oxidative polymerisation of m-phenylenediamine

Synthesis of high-quality carbon nanosheets with superior physicochemical properties is of particular importance for environmental and catalytic applications. In this research, carbon nanosheets with tunable porosity were successfully synthesized using two-dimensional (2D) poly(m-phenylenediamine) (PmPD) as precursor. The flat polymer precursor was acquired by oxidative polymerisation of m-phenylenediamine coupled with iron ions coordination, which confined an anisotropic growth of polymer within the 2D directions. Moreover, the addition of H₂O after the polymerisation is able to indirectly regulate the porosity of the carbon nanosheets. The carbon nanosheets with controllable porosity realize comparable electrocatalytic activity for oxygen reduction reaction as compared with commercial Pt/C, indicative of great potential to serve as noble metals candidates in the application of zinc/air batteries.

Ultranarrow Graphene Nanoribbons toward Oxygen Reduction and Evolution Reactions

Identification of catalytic sites for oxygen reduction and evolution reactions (ORR/OER) is critical to rationally develop highly efficient bifunctional carbon-based metal-free electrocatalyst. Here, a unique defect-rich N-doped ultranarrow graphene nanoribbon with a high aspect ratio that exhibits excellent ORR/OER bifunctional activities and impressive long-term cycling stability in Zn-air batteries is successfully fabricated. Density functional theory calculations indicates that the topological defects (e.g., pentagons and heptagons) cooperated with pyridinic-N dopants on the edges are more favorable to electrocatalytic activity toward ORR and OER. This work provides a new design principle for carbon-based electrocatalytic nanomaterials.

Superior stability of a bifunctional oxygen electrode for primary, rechargeable and flexible Zn-air batteries

Central to commercializing metal-air batteries is the development of highly efficient and stable catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, a composite catalyst with a unique interpenetrating network (denoted as NiCo2O4@MnO2-CNTs-3) was synthesized and exhibited better bifunctional activity (ΔE = 0.87 V) and durability than both Pt/C and Ir/C catalysts. The improved performance arises from three factors: (i) MnO2 promotes the ORR while NiCo2O4 facilitates the OER; (ii) carbon nanotubes improve the electronic conductivity; and (iii) the highly porous structure enables the adsorption-desorption of O2 and enhances the structural stability. As a result, the primary and rechargeable Zn-air battery affords a high power density and specific capacity (722 mA h g-1), an outstanding discharge stability (255 mW cm-2 after 1000 cycles) and a high cycling stability (over 2280 cycles). Electron microscopy and electrochemical analysis revealed that the degradation of the rechargeable Zn-air battery performance resulted from the damage of the air electrode and the hydrogen evolution reaction on the zinc electrode. A flexible Zn-air battery employing a solid-state electrolyte showed an exciting stability (540 cycles) and high power density (85.9 mW cm-2), suggesting that the anion exchange membrane effectively prevents the migration of Zn2+ ions and the deposition of carbonates.

Heterostructures of MXenes and N-doped graphene as highly active bifunctional electrocatalysts

MXenes with versatile chemistry and superior electrical conductivity are prevalent candidate materials for energy storage and catalysts. Inspired by recent experiments of hybridizing MXenes with carbon materials, here we theoretically design a series of heterostructures of N-doped graphene supported by MXene monolayers as bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Our first-principles calculations show that the graphitic sheet on V2C and Mo2C MXenes are highly active with an ORR overpotential down to 0.36 V and reaction free energies for the HER approaching zero, both with low kinetic barriers. Such outstanding catalytic activities originate from the electronic coupling between the graphitic sheet and the MXene, and can be correlated with the pz band center of surface carbon atoms and the work function of the heterostructures. Our findings screen a novel form of highly active electrocatalysts by taking advantage of the fast charge transfer kinetics and strong interfacial coupling of MXenes, and illuminate a universal mechanism for modulating the catalytic properties of two-dimensional hybrid materials.

Stochastic DNA walker for electrochemical biosensing sensitized with gold nanocages@graphene nanoribbons

A target-driven stochastic DNA walking electrochemical biosensor sensitized with gold nanocages@graphene nanoribbons (Au NCs@GNRs) was explored for sensitive detection of target DNA. Benefited from the large surface area and excellent conductivity of Au NCs and GNRs, the proposed sensing platform not only improved the electron transfer kinetics involved in electrochemical reactions, but also enhanced the loading capability for stem-loop structural DNA segment (H). Upon the addition of target DNA, the hairpin structure of H was opened and H:target DNA duplex was formed based on toehold-mediated DNA strand displacement. In the presence of exonuclease III (Exo III), the H:target DNA duplex was digested. As a result, target DNA spontaneously dissociated from H:target DNA duplex and then hybridized with another H strand. Therefore, the continuous locomotion of target DNA unceasingly triggered new digestion process from near to far along the electrode surface, resulting in great signal amplification. The proposed strategy exhibited excellent detection performances for DNA analysis in complex matrix such as human serum, which illuminated the practical application field of the sensing platform.

Computational study of precision nitrogen doping on graphene nanoribbon edges

Nitrogen doping in graphene is important for applications spanning from electronics to metal-free electrocatalysts. Despite much experimental study, limited theoretic work has been done in understanding the mechanism of the doping process, especially from a precision perspective. Herein, we present a computational study on precision nitrogen doping on edges of graphene nanoribbons (GNRs) by combining molecular dynamics (MD) simulation at a time scale of 40 ns and density function theory (DFT) calculation. In the MD study both ammonia and acetonitrile were used as nitrogen sources. MD results revealed that the ammonia produces almost all amine-type dopants, while the acetonitrile produces a considerable amount of pyrrolic and pyridinic nitrogen dopants which are beneficial to electronics and electrocatalysts. Results also show that the concentration of pyrrolic and pyridinic dopants can be precisely controlled by the edge geometries of the GNRs. Furthermore, DFT calculation illustrated the reaction mechanism in these different types of the GNRs when using acetonitrile as the nitrogen source. The calculated energies in different reaction stages indicate the stability of dopants on various GNRs, agreeing well with the MD results. The disclosed mechanism of controllable nitrogen doping on the edges of the GNRs would provide guidance to experimental realization, paving new routes to widespread applications.

Magnetron Sputtering Deposition Cu@Onion-like N-C as High-Performance Electrocatalysts for Oxygen Reduction Reaction

The idea of a core-shell structure can promote the utilization of nonprecious metallic catalysts by enhancing their activity and stability for the oxygen reduction reaction (ORR). Developing a low-cost, high-efficiency, and high-reproducibility method for synthesizing core-shell-structured materials represents an urgent challenge. Here, we fabricate encapsulated Cu nanoparticles with nitrogen-doped onion-like graphite nanoshells (Cu@onion-like N-C) as an efficient ORR catalyst by magnetron sputtering, in which the graphite shells grow by an in situ self-assembly process activated by the surface-catalyzed behavior with Cu nanoparticles. The results show that the CuCN-650 °C catalyst achieves the optimized Cu@onion-like N-C structure with small-sized Cu nanoparticles and a few-layer nanoshells and exhibits excellent ORR electrocatalytic properties, including a half-wave potential and onset potential similar to those of commercial Pt/C, coupled with better stability and higher methanol tolerance than for commercial Pt/C in alkaline electrolytes. The internal Cu nanoparticles in the core-shell structure not only promote the formation of a high content of pyridinic N but also donate the electronic charges to outer N-doped C shells, and thus the synergistic effect between the encapsulated Cu nanoparticles and N-doped C shells is responsible for the excellent electrocatalytic activity for the ORR.

Effect of Dimensionality and Doping in Quasi-"One-Dimensional (1-D)" Nitrogen-Doped Graphene Nanoribbons on the Oxygen Reduction Reaction

Designing an efficient metal-free electrocatalyst for the oxygen reduction reaction (ORR) is a challenging research theme having enormous practical importance in several renewable energy technologies like fuel cell and metal-air batteries. Here we discuss a cost-effective and commercially viable strategy to develop high-performance nitrogen-doped graphene nanoribbon (N-GNR), which is a quasi-"one-dimensional" analogue of graphene. We have selected the N-GNR system to identify the doping-induced variation in the distribution of active catalytic sites experimentally in graphene-based electrocatalysts. N-GNR exhibits a comparable exchange current density (1.65 × 10^-9 vs 2.25 × 10^-9 A cm^-2), thermodynamic potential (0.80 vs 0.83 V), and smaller Tafel slope (55 vs 60 mV dec^-1) with respect to the benchmarking platinum/carbon (Pt/C), and also, more precisely, it goes through a four-electron pathway with low hydrogen peroxide yield. Although the exact mechanism is still not clear, the theme of the work is based on the identification of the possible active sites with the help of experimental evidence like X-ray photoelectron spectroscopy. These results support the assumption that an edge N (pyridinic N)-bonded adjacent C lowers the activation energy barriers of O₂ adsorption, predominantly to kinetically facilitate the ORR activity. We hope these results will be helpful in developing more efficient ORR catalysts.

A novel composite (FMC) to serve as a durable 3D-clam-shaped bifunctional cathode catalyst for both primary and rechargeable zinc-air batteries

Novel and highly durable air cathode electrocatalyst with three dimensional (3D)-clam-shaped structure, MnO₂ nanotubes-supported Fe₂O₃ (Fe₂O₃/MnO₂) composited by carbon nanotubes (CNTs) ((Fe₂O₃/MnO₂)_3/4-(CNTs)_1/4) is synthesized using a facile hydrothermal process and a following direct heat-treatment in the air. The morphology and composition of this catalyst are analyzed using scanning electronic microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). The morphology characteristics reveal that flower-like Fe₂O₃ particles are highly dispersed on both MnO₂ nanotubes and CNT surfaces, coupling all three components firmly. Electrochemical measurements indicate that the synergy of catalyst exhibit superior bi-functional catalytic activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as well as stability than Pt/C and IrO₂ catalysts. Using these catalysts for air-cathodes, both primary and rechargeable zinc-air batteries (ZABs) are assembled for performance validation. In a primary ZAB, this 3D-clamed catalyst shows a decent open circuit voltage (OCV, ∼1.48V) and a high discharge peak power density (349mWcm^-2), corresponding to a coulombic efficiency of 92%. In a rechargeable ZABs with this bifunctional catalyst, high OCV (>1.3V) and small charge-discharge voltage gap (<1.1V) are achieved along with high specific capacity (780mAhg^-1 at 30mAcm^-2) and robust cycle-life (1,390 cycles at cycle profile of 20mA/10min).

Distinctive morphology effects of porous-spherical/yolk-shell/hollow Pd-nitrogen-doped-carbon spheres catalyst for catalytic reduction of 4-nitrophenol

Pd-nitrogen-doped-carbon nanocatalysts (Pd-C/N) with different morphologies, such as porous spheres, yolk-shell and hollow structures, had been synthesized and compared. The yolk-shell Pd-nitrogen-doped-carbon nanocatalysts (YS-Pd-C/N) and hollow Pd-nitrogen-doped-carbon nanocatalysts (H-Pd-C/N) were prepared through different etch time using SiO₂ spheres as hard-templates. The as-prepared catalysts were characterized thoroughly by TEM, BET, XRD, FT-IR, and XPS. Importantly, the catalysts have moderate BET specific surface area in the range from 200 to 300m²g^-1 and pore volume between 0.2 and 0.3cm³g^-1. The reduction of 4-nitrophenol is chosen as a model reaction to research the morphology effects of these prepared Pd-C/N catalysts with the same chemical compositions. Interestingly, H-Pd-C/N exhibited the best catalytic performance, which could be attributed to its high nitrogen content, the uniform distribution of abundant active sites, as well as the synergistic effect of graphitic C/N shell and Pd species for the catalytic reaction. Especially, the unique hollow morphology and porous shell of H-Pd-C/N made it to be a nanoreactor, which was beneficial to improve the catalytic activities. In addition, H-Pd-C/N nanocatalysts exhibited favorable stability in the recycling reactions.

Enhancement of thermal conductivity in polyamide-6/graphene composites via a “bridge effect” of silicon carbide whiskers

It is urgent to manufacture a polymer composite that has high thermal conductivity (especially in the through-plane direction) and mechanical properties simultaneously to meet the heat dissipation requirement of electronic devices.

Synergy effect of specific electrons and surface plasmonic resonance enhanced visible-light photoelectrochemical sensing for sensitive analysis of the CaMV 35S promoter

A Ag/TiO₂/N-GNR ternary composite-based photoelectrochemical sensor for sensitive analysis of the CaMV 35S promoter.

Doping carbon nanotubes with N, S, and B for electrocatalytic oxygen reduction: a systematic investigation on single, double, and triple doped modes

Polydopamine-coated MWCNTs have been employed as reactive platforms for the anchoring of multiple heteroatom dopants.

Heteroatom-Doped Carbon Nanostructures Derived from Conjugated Polymers for Energy Applications

Heteroatom-doped carbon materials have been one of the most remarkable families of materials with promising applications in fuel cells, supercapacitors, and batteries. Among them, conjugated polymer (CP)-derived heteroatom-doped carbon materials exhibit remarkable electrochemical performances because the heteroatoms can be preserved at a relatively high content and keep stable under harsh working conditions. In this review, we summarized recent advances in the rational design and various applications of CP-derived heteroatom-doped carbon materials, including polyaniline (PANI), polypyrrole (PPy), and their ramification-derived carbons, as well as transition metal-carbon nanocomposites. The key point of considering CP-derived heteroatom-doped carbon materials as important candidates of electrode materials is that CPs contain only nonmetallic elements and some key heteroatoms in their backbones which provide great chances for the synthesis of metal-free heteroatom-doped carbon nanostructures. The presented examples in this review will provide new insights in designing and optimizing heteroatom-doped carbon materials for the development of anode and cathode materials for electrochemical device applications.