Nitrogen-doped Carbon Nanotubes Research Articles

Uniform Co9S8 nanosheets on carbon nanotube arrays grown on Ni mesh as free-standing electrodes for asymmetric supercapacitors

A free-standing 3D core/shell composite material CNTs@Co9S8 on nickel mesh has been prepared by a simple two-step method. Carbon nanotubes (CNTs) grown vertically on the surface of nickel mesh by chemical vapor deposition (CVD) method act as conductive skeleton. Then Co9S8 nanosheets uniformly aligned on the surface of the CNTs by solvothermal method are interconnected to form a honeycomb structure. Owing to the close contact of the CNTs@Co9S8 hybrid with the nickel mesh and the hierarchical 3D porous structure, the prepared Ni@CNTs@Co9S8 electrodes exhibit low resistance and excellent charge storage capability with a specific capacitance of 1305.4 F g−1 at 1 A g−1. The composite delivers an excellent rate capability with 84.3 % capacitance retention at 10 A g−1. An asymmetric supercapacitor composed of Ni@CNTs@Co9S8 and nitrogen-doped carbon nanotubes (NCNTs) shows an energy density of 23.8 Wh kg−1 and a high power density of 8007.4 W kg−1. The specific capacitance of the ASC device remained 87.31 % after 10,000 charge-discharge cycles and two tandem ASCs can power two red LED diodes for at least 7 min, suggesting the potential application of the obtained free-standing electrode for supercapacitors.

Enhancing electron interaction between Pt and support for superior electrochemical performance through atomic layer deposition of tungsten oxide

The stabilization of platinum (Pt) catalysts through strong metal-support interactions is crucial for their successful implementation in fuel cell applications. Tungsten oxide (WO3) has demonstrated excellent CO tolerance and has been recognized as a promising substrate for anchoring and stabilizing Pt nanoparticles (NPs). However, the limited specific surface area of conventional tungsten oxide restricts its effectiveness in dispersing noble metal NPs. In this study, we present a pioneering approach by employing atomic layer deposition (ALD) to create a WO3 interlayer between Pt NPs and a carbon substrate. Using nitrogen-doped carbon nanotubes (NCNT) as the foundation, WO3 nanoparticles (2–5 nm) were selectively synthesized, followed by the subsequent deposition of Pt NPs using a bottom-up approach. The Pt-WO3-NCNT catalyst, with a WO3 bridge layer effectively inserted between the active site and carbon support, has displayed a notable augmentation in electrocatalytic activity and notable stability when compared to commercial Pt catalysts in oxygen reduction reaction (ORR). The detailed microstructure and the enhanced electrochemical reaction mechanism of Pt-WO3-NCNT catalyst has been investigated by X-ray adsorption spectrum and density functional theory (DFT) calculations. This work presents an innovative approach for enhancing the stability of Pt catalysts through the utilization of the ALD technique.

N-Doped Graphene Nanofibers with Porous Channel Comprising Fe x S y Nanocrystals and Intertwined N-Doped CNTs as Efficient Interlayers for Li-S Batteries

Hierarchically porous and conductive nanofibers (NFs) comprising nitrogen-doped reduced graphene oxide (N-rGO) and iron sulfide (Fe x S y ) nanocrystals along with highly intertwined nitrogen-doped carbon nanotubes (N-CNTs) abbreviated as “P-rGO@Fe x S y /N-CNT NFs” were synthesized via electrospinning technique as multifunctional interlayers via coating on the commercial separator for stable lithium-sulfur (Li-S) batteries. The porous N-rGO framework acts as a backbone to enhance the structural integrity of the nanostructure. The N-CNTs and N-rGO guarantee various conductive channels for quick electron transfer thus allowing fast redox reactions. The thermal breakdown of polystyrene (PS) resulted in the formation of continuous longitudinal channels. Correspondingly, the Li-S cell incorporating the P-rGO@Fe x S y /N-CNT NF-modified separator and S electrode (2.41 mg cm-2) exhibited boosted electrochemical properties such as justifiable rate capability and steady cycling performance (after 800 charge-discharge, cell exhibits a capacity of 464 mA h g-1 with 44% retention at 0.1 C). The novel synthesis strategy discussed herein will provide significant intuitions to the advancement of innovative nanostructures for different rechargeable purposes.

Metal-encapsulated nitrogen-doped carbon nanotube arrays electrode for enhancing sulfion oxidation reaction and hydrogen evolution reaction by regulating of intermediate adsorption

For treatment of sulfion-containing wastewater, coupling the electrochemical sulfion oxidation reaction (SOR) with hydrogen evolution reaction (HER) can be an ideal way for sulfur and H2 resources recovery. Herein, we synthesize a metal-modified carbon nanotube arrays electrode (Co@NCNTs/CC) for SOR and HER. This electrode has excellent performance for SOR and HER attributed to the unique array structure. It can achieve 99.36 mA/cm2 at 0.6 V for SOR, and 10 mA/cm2 at 0.067 V for HER. Density functional theory calculations verify that metal modification is able to regulate the electronic structure of carbon nanotube, which is able to optimize the adsorption of intermediates. Employed Co@NCNTs/CC as bifunctional electrodes to establish a hybrid electrolytic cell can reduce about 67% of energy consumption compared with the traditional water splitting electrolytic cell. Finally, the hybrid electrolytic cell is used to treat actual sulfion-containing wastewater, achieving the sulfur yield of 30 mg h−1 cm−2 and the hydrogen production of 0.64 mL/min.

Synthesis, magnetoresistance, and thermoelectrical properties of environmentally stable n-type nitrogen-doped multiwalled carbon nanotubes

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) are known as a perspective material for a variety of applications in nanoelectronic devices, sensors, catalysts for carbon dioxide reduction, and flexible thermoelectrics. However, up to date most of the reports on the properties of N-MWCNTs are focused on a narrow niche of research, for example, a study of low-temperature magnetoresistance or room-temperature thermoelectrical properties. In this work, N-MWCNTs were synthesized using benzene:pyridine precursor in different ratios, and both magnetoresistance and thermoelectrical properties of the synthesized N-MWCNTs were systematically investigated in the temperature range 2-300 K and compared with the properties of undoped MWCNTs. Unexpected switching of the magnetoresistance of the N-MWCNTs at low temperatures from negative to positive values was observed, and the processes underlying this effect are discussed. The study of the thermoelectrical properties revealed n-type conductance in the N-MWCNTs, which was attributed to the impact of nitrogen defects incorporated in the MWCNT structure. Performed for the first-time investigations of the thermal stability of the Seebeck coefficient of N-MWCNTs in air revealed that the Seebeck coefficient retains its negative values and even increases after annealing of the N-MWCNTs in air at 500 °C. These findings illustrate the high potential of the presented in this work N-MWCNTs for applications in different devices in a wide range of temperatures.

Nitrogen‐Doped Carbon Nanotubes with Large Interplanar Distances toward Fast Potassium Storage

AbstractOne of the main challenges potassium‐ion batteries (PIBs) face is the lack of structurally stable anodes with high reactivity and fast kinetics for reversible potassium insertion/extraction. Herein, nitrogen‐doped carbon nanotubes (N‐CNTs) are synthesized using a straightforward method assisted by a Co‐based catalyst. The as‐synthesized N‐CNTs possess interlayer distances up to 0.38 nm and nitrogen‐doping content of 2.2 at.%. Compared to the undoped CNTs (U‐CNTs), N‐CNTs exhibit a promising initial specific capacity of 568 mAh g−1 at 0.1 A g−1, as well as excellent long‐term cycling performance of 104, 82, and 76 mAh g−1 at a high current of 0.5, 1, and 2 A g−1 for 500 cycles. The cyclic voltammetry (CV) measurements reveal the potassium storage mechanism of N‐CNTs, which combines the major capacitive mechanisms and the secondary diffusion, and successfully avoids inconspicuous voltage plateau. The kinetic analysis of the galvanostatic intermittent titration technique and ex situ X‐ray photoelectron spectroscopy spectra show fast reaction kinetics and low side effects and degradation for the N‐CNTs anodes during the potassiation/de‐potassiation process. This study provides a straightforward method to synthesize heteroatom‐doped carbonaceous anode materials and achieves superior electrochemical properties with cost‐effectiveness and material sustainability.

CoNiFe alloy nanoparticles encapsulated into nitrogen-doped carbon nanotubes toward superior electrocatalytic overall water splitting in alkaline freshwater/seawater under large-current density.

Developing bifunctional catalysts for overall water splitting with high activity and durability at high current density remains a challenge. In an attempt to overcome this bottleneck, in this work, unique CoNiFe-layered double hydroxide nanoflowers are in situ grown on nickel-iron (NiFe) foam through a corrosive approach and following a chemical vapor deposition process to generate nitrogen-doped carbon nanotubes at the presence of melamine (CoNiFe@NCNTs). The coupling effects between various metal species act a key role in accelerating the reaction kinetics. Moreover, the in situ formed NCNTs also favor promoting electrocatalytic activity and stability. For oxygen evolution reaction it requires low overpotentials of 330 and 341 mV in 1M KOH and 1M KOH + seawater to drive 500 mA cm-2. Moreover, water electrolysis can be operated with CoNiFe@NCNTs as both anode and cathode with small voltages of 1.95 and 1.93V to achieve 500 mA cm-2 in 1M KOH and 1M KOH + seawater, respectively.

Integrating V-doped CoP on Ti3C2Tx MXene-incorporated hollow carbon nanofibers as a freestanding positrode and MOF-derived carbon nanotube negatrode for flexible supercapacitors

Novel architectural electrode materials that are freestanding and exhibit improved electrochemical performances in flexible asymmetric supercapacitors are urgently needed; however, designing these materials is challenging. Herein, a Ti3C2TX MXene-aligned hollow carbon fiber (MX/HCF) is engineered and vanadium-doped cobalt phosphide nanorod arrays are grown on it (V-CoP@MX/HCF) and applied for supercapacitor positrode. V doping modulates the surface structure and electronic environment of CoP nanorods grown over conductive and flexible MX/HCFs. As expected, the optimized V-CoP@MX/HCF exhibits a better electrochemical performance (1896.8Fg−1) than that of CoP@MX/HCF and CoP@HCF. For the negative electrode, zeolitic imidazolate framework-67 (ZIF-67) grown on electrospun polyacrylonitrile (PAN) fibers is converted to cobalt nanoparticle-encapsulated nitrogen-doped carbon nanotubes at carbon nanofibers (Co-CNT@CNF) by a simple heat treatment without the burden of external catalysts and reducing gases. The as-designed freestanding Co-CNT@CNF delivers a specific capacitance of 405.5Fg−1 with superior cycling stability. A flexible asymmetric supercapacitor (V-CoP@MX/HCF//Co-CNT@CNF) is designed that unveil remarkable electrochemical properties, such as a high energy density of 72.4 Wh kg−1 at 800.12 W kg−1 and good capacitance retention (91.4 %) after 10,000 charge/discharge cycles. Furthermore, the device can maintain a consistent performance regardless of the bending degree. More importantly, this work provides new opportunities to rationally design novel freestanding cathodes and anodes for high-performance flexible asymmetric supercapacitors.

Nitrogen-Doped Single-Walled Carbon Nanotubes by Floating-Catalyst CVD Process

Due to the properties of single-walled carbon nanotube (SWCNT), thin film SWCNTs have been used for many versatile applications. Direct synthesis of the nanotube films with desired electronic structures without prerequisite of post treatment process then becomes one important step. Here, we present the synthesis of nitrogen-doped SWCNTs by floating-catalyst chemical vapor deposition process. With low nitrogen level incorporated into the sp2 carbon network, substitutional and pyridinic nitrogens become predominance. The electronic structure of N-doped SWCNT film could be changed into n-type doping. The localized structures of carbon and nitrogen bonding environments were also revealed by x-ray absorption spectroscopy. The formation of p-n junction was also obtained from the I-V characteristic of N-doped SWCNT heterojunction diode revealing n-type behavior of the sample.

Adding Fe/dicyandiamide to Co-MOF to greatly improve its ORR/OER bifunctional electrocatalytic activity

Exploring efficient non-precious metal oxygen electrocatalysts for oxygen reduction/evolution reactions (ORR/OER) is of great importance in electrochemical energy conversion devices like metal-air battery. Among them, carbon-based composites derived from metal-organic framework (MOF) exhibit excellent catalytic performance in ORR/OER. In this paper, Co-MOF-67 was firstly prepared, and then mixed with FeCl3·6 H2O and dicyandiamide, followed by a one-step pyrolysis to obtain bamboo-like nitrogen-doped carbon nanotubes. For the samples, Fe and Co nanoparticles were encapsulated in the front end of the carbon nanotubes and the presence of new (n)-diamond carbon was unexpectedly found. The obtained catalyst (FeCo@CNTs-60) has an onset potential of 1.04 V for ORR in alkaline solution, which is close to the commercial Pt/C (40 wt% Pt) of 1.05 V. The onset potential of 1.02 V in quasi-neutral solution exceeds that of Pt/C of 0.99 V. In addition, FeCo@CNTs-60 exhibited a low potential difference of 0.81 V between OER and ORR potentials. The alkaline zinc-air batteries, fabricated with the sample as the electrocatalyst of air electrode, exhibit higher peak power density and stability than Pt/C, while the rechargeable batteries present superior cycling discharge/charge stability at 5 mA·cm−2 and 10 mA·cm−2. In particular, the battery with FeCo@CNTs-60 was conducted for continuous over 1300 h of ultra long-term cycle charging / discharging test at a current density of 5 mA·cm−2, and it reveals excellent stability with high voltage efficiency and also presents superior cycle charging / discharging stability after replacing the Zn plates and electrolyte.

One-Pot Synthesis of Ultra-Small Pt Nanoparticles-Loaded Nitrogen-Doped Mesoporous Carbon Nanotube for Efficient Catalytic Reaction.

In this study, Pt nanoparticles-loaded nitrogen-doped mesoporous carbon nanotube (Pt/NMCT) was successfully synthesized through a polydopamine-mediated "one-pot" co-deposition strategy. The Pt source was introduced during the co-deposition of polydopamine and silica on the surface of SiO2 nanowire (SiO2 NW), and Pt atoms were fixed in the skeleton by the chelation of polydopamine. Thus, in the subsequent calcination process in nitrogen atmosphere, the growth and agglomeration of Pt nanoparticles were effectively restricted, achieving the in situ loading of uniformly dispersed, ultra-small (~2 nm) Pt nanoparticles. The method is mild, convenient, and does not require additional surfactants, reducing agents, or stabilizers. At the same time, the use of the dual silica templates (SiO2 NW and the co-deposited silica nanoclusters) brought about a hierarchical pore structure with a high specific surface area (620 m2 g-1) and a large pore volume (1.46 cm3 g-1). The loading process of Pt was studied by analyzing the electron microscope and X-ray photoelectron spectroscopy of the intermediate products. The catalytic performance of Pt/NMCT was investigated in the reduction of 4-nitrophenol. The Pt/NMCT with a hierarchical pore structure had an apparent reaction rate constant of 0.184 min-1, significantly higher than that of the sample, without the removal of the silica templates to generate the hierarchical porosity (0.017 min-1). This work provides an outstanding contribution to the design of supported noble metal catalysts and also highlights the importance of the hierarchical pore structure for catalytic activity.

Precision tuning of highly efficient Pt-based ternary alloys on nitrogen-doped multi-wall carbon nanotubes for methanol oxidation reaction

The electrochemical methanol oxidation is a crucial reaction in the conversion of renewable energy. To enable the widespread adoption of direct methanol fuel cells (DMFCs), it is essential to create and engineer catalysts that are both highly effective and robust for conducting the methanol oxidation reaction (MOR). In this work, trimetallic PtCoRu electrocatalysts on nitrogen-doped carbon and multi-wall carbon nanotubes (PtCoRu@NC/MWCNTs) were prepared through a two-pot synthetic strategy. The acceleration of CO oxidation to CO2 and the blocking of CO reduction on adjacent Pt active sites were attributed to the crucial role played by cobalt atoms in the as-prepared electrocatalysts. The precise control of Co atoms loading was achieved through precursor stoichiometry. Various physicochemical techniques were employed to analyze the morphology, element composition, and electronic state of the catalyst. Electrochemical investigations and theoretical calculations confirmed that the Pt1Co3Ru1@NC/MWCNTs exhibit excellent electrocatalytic performance and durability for the process of MOR. The enhanced MOR activity can be attributed to the synergistic effect between the multiple elements resulting from precisely controlled Co loading content on surface of the electrocatalyst, which facilitates efficient charge transfer. This interaction between the multiple components also modifies the electronic structures of active sites, thereby promoting the conversion of intermediates and accelerating the MOR process. Thus, achieving precise control over Co loading in PtCoRu@NC/MWCNTs would enable the development of high-performance catalysts for DMFCs.

Effects of peroxide types on the removal performance and mechanism of sulfonamide antibiotics using graphene-based catalytic membranes

This study reports in situ catalytic oxidation processes using graphene-based catalytic membranes to activate peroxydisulfate (PDS), peroxymonosulfate (PMS), and hydrogen peroxide (H2O2). Sulfonamide antibiotics in water, including sulfamethoxazole (SMX), sulfadiazine (SDZ), sulfamerazine (SMR), and sulfamethazine (SM2), were used as target pollutants. The composites of reduced graphene oxide and nitrogen-doped carbon nanotubes were loaded onto nylon microfiltration membranes at 8 g·m–2 mass dosage. The PDS-based systems exhibited the highest efficiency in removing SDZ (≥ 98%) and SMX (> 93%) during continuous 10 h filtration. In contrast, PMS was more suitable for oxidizing SMR (≥ 99%) and SM2 (≥ 90%) in a single pass. Based on functional group characterization and density functional theory calculations, structural defects and pyridinic/graphitic nitrogen species in carbon mats were identified as the main active centers for peroxide activation. Moreover, PDS, PMS and H2O2 exhibited distinct adsorption behaviors on defective and nitrogen-doped graphitic carbon, influencing the cleavage of peroxide bonds to generate reactive oxygen species. The reaction mechanism was investigated through electron paramagnetic resonance and chemical quenching tests. Surface-active species and singlet oxygen dominated in the PDS- and PMS-based systems, representing non-radical pathways, that selectively oxidize sulfonamides in real water matrices more effectively than the hydroxyl radicals involved in H2O2-based systems. PDS activating exhibited significant performance in organic fouling polishing, lowering the transmembrane pressure during continuous filtration. These findings offer valuable insights into implementing in situ catalytic oxidation processes in actual water purification.

NiS2 nanoparticles anchored on Co-carbon nanotubes for supercapacitor and overall water splitting

Exploiting low-cost and efficient electrode materials with catalytic activity toward water splitting and supercapacitor has a crucial role with sustainable energy development. Herein, a composite structure is created in which nickel sulfide nanoparticles are loaded on the surface of nitrogen-doped carbon nanotubes (Co-CNTs) encapsulated with cobalt nanoparticles as multifunctional catalysts. The growth in situ of NiS2 nanoparticles results in the Co-CNTs@NiS2 composite with ideal interface effect which remarkable improves the performance in supercapacitor, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity. As a supercapacitor electrode, Co-CNTs@NiS2 activated carbon asymmetric supercapacitor performs a high specific capacitance of 370 F g−1 at 1 A g−1, revealing high retention of 68.8% after 3000 cycles. As an electrocatalyst, it is just needed to afford the overpotential of 235 and 309 mV in HER and OER to reach a current density of 10 mA cm−2 without iR correction. The Co-CNTs@NiS2 composite with optimized exposed surface just requires an overall cell voltage of 1.73 V to achieve 10 mA cm−2 in electrolyzer. The results supply great potential in the application of renewable energy.

Flash Nitrogen-Doped Carbon Nanotubes for Energy Storage and Conversion.

In recent years, nitrogen-doped carbons show great application potentials in the fields of electrochemical energy storage and conversion. Here, the ultrafast and green preparation of nitrogen-doped carbon nanotubes (N-CNTs) via an efficient flash Joule heating method is reported. The precursor of 1D core-shell structure of CNT@polyaniline is first synthesized using an in situ polymerization method and then rapidly conversed into N-CNTs at ≈1300 K within 1 s. Electrochemical tests reveal the desirable capacitive property and oxygen catalytic activity of the optimized N-CNT material. It delivers an improved area capacitance of 101.7 mF cm-2 at 5mV s-1 in 1m KOH electrolyte, and the assembled symmetrical supercapacitor shows an energy density of 1.03 µWh cm-2 and excellent cycle stability over 10000 cycles. In addition, the flash N-CNTs exhibit impressive catalytic performance toward oxygen reduction reaction with a half-wave potential of 0.8V in alkaline medium, comparable to the sample prepared by the conventional long-time pyrolysis method. The Zn-air battery presents superior charge-discharge ability and long-term durability relative to commercial Pt/C catalyst. These remarkable electrochemical performances validate the superiorities of the Joule heating method in preparing the heteroatom-doped carbon materials for wide applications.

Outstanding glucose sensing properties of N-doped carbon nanotubes boosted by Co(II) and Cu(II) ions in alkaline electrolytes

Electrochemical glucose detection co-catalyzed by bimetallic ions is carried out by introducing Co(II) and Cu(II) into the alkaline solution. The technique avoids the complicated electrode design and catalyst poisoning. By using nitrogen-doped carbon nanotubes (NCNT) loaded on carbon cloth (CC) as the working electrode, the bimetal co-catalytic system achieves a sensitivity of 6,909 mA M−1 cm−2, a detection limit of 0.4 μM (S/N = 3), as well as good selectivity and stability. By performing systematic electrochemical and structural analysis, the mechanism of Co(II) and Cu(II) co-catalysis for glucose detection in the solution is elucidated. Co(II) supplies the dynamic reaction sites and source of active intermediate (CoOOH), while Cu(II) promotes the formation of CoOOH. The results provide mechanistic insights into bimetal co-catalysis of glucose detection and other electrochemical reactions.

Synthesis of highly thermal conductive carbon nanotubes on cellulose nanofiber film-loaded polyethylene glycol phase change material with flame retardancy by layered double hydroxide

The nitrogen-doped carbon nanotubes (CNTs) were obtained by using methyl orange as a soft template and high temperature treatment, on which the layered double hydroxide (LDH) was grown in situ by hydrothermal method, and finally mixed with the phase change materials (PCM) by vacuum filtration to obtain a series of novel composite films with great flame retardancy and thermal conductivity. The use of cellulose nanofiber (CNF), polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) as PCM increased the film-forming properties, and the CNTs could improve the thermal conductivity of composites, while reducing the agglomeration of LDH nanolayers and improving the flame retardancy of the composite films. The results showed that the LDH grown on the nanotubes had a typical hexagonal lamellar structure, and its uniform distribution was the fundamental reason for the flame retardancy of the films which was verified by defined combustion experiments, the shortest self-extinguishing time and the least unburned area were achieved at a mass ratio of 7.14 % of LDH material. The melt and crystallization enthalpies of the thermally conductive flame-retardant films were 74.65 J/g−1 and 74.82 J/g−1, and the thermal conductivity of the material was increased to 0.04129 W/(m·K) after the use of 2.5 % thermally conductive filler. It can reduce the temperature of LED by up to 5.9 °C during the LED opening process and up to 5.2 °C during the LED closing process.

Electrical properties of hierarchical nitrogen-doped multi-walled carbon nanotubes obtained using cobalt nanoparticles

In this work, for the first time, both initial and secondary branches of hierarchical nitrogen-doped multi-walled carbon nanotubes (h-N-MWCNTs) were obtained using chemical vapor deposition due to the decomposition of ace-tonitrile over Co-based nanoparticles. The results of a study of the electrical conductivity of h-N-MWCNTs for a wide temperature range of 300−4.2 K are presented. It was shown that fluctuation-assisted tunneling between delocalized states is the dominant conduction mechanism at temperatures above 50 K. An analysis of the temperature and electric field dependences of the resistance indicates that the transfer of electric charges below 50 K occurs due to hoppings between localized states located in the vicinity of the Fermi level. It was shown that the Coulomb interaction affects the charge carrier transport. The width of the Coulomb gap estimated from the temperature dependence of the resistance is 0.5 meV. The electrical conductivity of h-N-MWCNTs in the temperature range of 50−4.2 K occurs by the Efros–Shklovskii variable range hopping conduction mechanism. It was found that the charge localization length is ≈150 nm. Electrical property analysis indicates that the dielectric constant of the h-N-MWCNTs is ≈ 100.

Direct synthesis of nitrogen-doped narrow-diameter carbon nanotubes through floating-catalyst chemical vapor deposition with high hydrogen flow rate

The precise control of substitutional nitrogen (N) doping into a carbon nanotube (CNT) lattice is key to tuning their unique one-dimensional electronic properties. Here we report a direct synthesis of high-quality N-doped single-wall CNTs (N-SWCNTs) with ∼1 nm in diameter using a floating-catalyst chemical vapor deposition under a high flow rate of hydrogen as a carrier gas. The high hydrogen flow rate enhances the total N content in the CNT lattice. The N-SWCNTs exhibit an n-type doping behavior induced by enriched graphitic-N as confirmed by Raman analysis. Our finding will be beneficial to tailoring the doping state of N-SWCNTs.

Enhanced Four-Electron Selective Oxygen Reduction Reaction at Carbon Nanotubes-Supported Sulfonic Acid-Functionalized Copper Phthalocyanine.

In the present work, oxygen reduction reaction (ORR) is explored in an acidic medium with two different catalytic supports (multi-walled carbon nanotubes (MWCNTs) and nitrogen-doped multi-walled carbon nanotubes (NMWCNTs)) and two different catalysts (copper phthalocyanine (CuPc) and sulfonic acid functionalized CuPc (CuPc-SO3-)). The composite, NMWCNTs-CuPc-SO3- exhibits high ORR activity (assessed based on the onset potential (0.57 V vs. reversible hydrogen electrode) and Tafel slope) in comparison to the other composites. Rotating ring disc electrode (RRDE) studies demonstrate a highly selective four-electron ORR (less than 2.5% H2O2 formation) at the NMWCNTs-CuPc-SO3-. The synergistic effect of the catalyst support (NMWCNTs) and sulfonic acid functionalization of the catalyst (in CuPc-SO3-) increase the efficiency and selectivity of the ORR at the NMWCNTs-CuPc-SO3-. The catalyst activity of NMWCNTs-CuPc-SO3- has been compared with many reported materials and found to be better than several catalysts. NMWCNTs-CuPc-SO3- shows high tolerance for methanol and very small deviation in the onset potential (10 mV) between the linear sweep voltammetry responses recorded before and after 3000 cyclic voltammetry cycles, demonstrating exceptional durability. The high durability is attributed to the stabilization of CuPc-SO3- by the additional coordination with nitrogen (Cu-Nx) present on the surface of NMWCNTs.