Metal-free porous nitrogen-doped carbon nanotubes for enhanced oxygen reduction and evolution reactions

Carbon cloth surface engineering for simultaneous detection of ascorbic acid, dopamine, and uric acid in fetal bovine serum

Carbon cloth (CC) was electrochemically activated using three electrochemical methods in different electrolytes. The gases released during the activation process etched the surface of the CC, thereby increasing its surface area of the activated CC (FCC). Moreover, functional groups such as oxygen-containing groups and N-doping have been introduced. The quantity and type of functional groups introduced during the activation process were related to the anions in the solution. FCC electrodes were used to construct electrochemical sensors for the simultaneous determination of ascorbic acid (AA), dopamine (DA), and uric acid (UA). The PFCC electrode activated by advanced cyclic voltammetry in (NH4)2HPO4 showed linear ranges for AA, DA, and UA concentrations of 0.1 to 2.1 mM, 0.5 to 11 µM, and 0.5 to 11 µM, respectively. The detection limits are 72.93, 0.22, and 0.42 µM, respectively. The good flexibility the PFCC electrode made it suitable for the preparation of flexible sensors. The simultaneous determination of AA, DA, and UA in fetal bovine serum showed a reliable recovery ratio. This study provides a simple and green approach for activating carbon cloth and constructing flexible electrochemical sensors with the potential to simultaneously detect AA, DA, and UA.

A(CoFe)(S2)2/CoFe heterostructure constructed in S, N co-doped carbon nanotubes as an efficient oxygen electrocatalyst for zinc-air battery

Transition metal alloys can exhibit synergistic intermetallic effects to obtain high activities for oxygen reduction/evolution reactions (ORR/OER). However, due to the insufficient stability of active sites in alkaline electrolytes, conventional alloy catalysts still do not meet practical needs. Herein, by using polypyrrole tubes and cobalt-iron (CoFe) Prussian blue analogs as precursors, CoFe sulfides is in-situ formed on CoFe alloys to construct (CoFe)(S2)2/CoFe heterostructure in sulfur (S) and nitrogen (N) co-doped carbon nanotubes (CoFe@NCNTs-nS) via a low-temperature sulfidation strategy. The as-marked CoFe@NCNTs-12.5S exhibits a comparable ORR activity (half-wave potential of 0.901 V) to Pt/C (0.903 V) and a superior OER activity (overpotential of 272 mV at 10 mA cm-2) to RuO2 (299 mV). CoFe@NCNTs-12.5S also exhibits ultralow charge transfer resistances (ORR-6.36 Ω and OER-0.21 Ω) and an excellent potential difference of 0.617 V. The sulfidation-induced (CoFe)(S2)2/CoFe heterojunctions can accelerate interfacial charge transfer process. Tubular structure not only disperses the (CoFe)(S2)2/CoFe heterostructure, but also reduces the corrosion of active-sites to enhance catalysis stability. Zinc-air battery with CoFe@NCNTs-12.5S achieves a high specific capacity (718.1 mAh g-1), maintaining a voltage gap of 0.957 V after 400 h. This work reveals the potential of interface engineering for boosting ORR/OER activities of alloys via in-situ heterogenization.

Carbon Fiber Film with Multi-Hollow Channels to Expedite Oxygen Electrocatalytic Reaction Kinetics for Flexible Zn-Air Battery

The high oxygen electrocatalytic overpotential of flexible cathodes due to sluggish reaction kinetics result in low energy conversion efficiency of wearable zinc-air batteries (ZABs). Herein, lignin, as a 3D flexible carbon-rich macromolecule, is employed for partial replacement of polyacrylonitrile and constructing flexible freestanding air electrodes (FFAEs) with large amount of mesopores and multi-hollow channels via electrospinning combined with annealing strategy. The presence of lignin with disordered structure decreases the graphitization of carbon fibers, increases the structural defects, and optimizes the pore structure, facilitating the enhancement of electron-transfer kinetics. This unique structure effectively improves the accessibility of graphitic-N/pyridinic-N with oxygen reduction reaction (ORR) activity and pyridinic-N with oxygen evolution reaction (OER) activity for FFAEs, accelerating the mass transfer process of oxygen-active species. The resulting N-doped hollow carbon fiber films (NHCFs) exhibit superior bifunctional ORR/OER performance with a low potential difference of only 0.60 V. The rechargeable ZABs with NHCFs as metal-free cathodes possess a long-term cycling stability. Furthermore, the NHCFs can be used as FFAEs for flexible ZABs which have a high specific capacity and good cycling stability under different bending states. This work paves the way to design and produce highly active metal-free bifunctional FFAEs for electrochemical energy devices.

Fundamental understanding of nitrogen in biomass electrocatalysts for oxygen reduction and zinc-air batteries

Exploring high-efficiency catalysts for oxygen reduction reactions (ORRs) is essential for the development of large-scale applications of fuel cell and metal-air batteries technology. The as-prepared Fe-NC-800 via polymerization-pyrolysis strategy exhibited superior ORR activity with onset potential of 1.030 V vs. reversible hydrogen electrode (RHE) and half-wave potential of 0.908 V vs. RHE, which is higher than that of the Pt/C catalyst and most of other Fe-based catalysts. The different d-band center values can be attributed to the influence of different N-doped carbon, leading to the adjustment in the ORR activity. In addition, Fe-NC-800-based Zn-air battery showed better electrochemical performance with a high discharge specific capacity of 806 mA h g-1 and a high-power density of 220 mW cm-2 than that of the Pt/C-based battery. Therefore, the biomass Fe-NC-800 catalyst may become a promising substitute for Pt/C catalysts in energy storage and conversion devices.

Application and Research of Nursing Products Based on Single Wall Carbon Nanotubes in Personal Health Care

Single walled carbon nanotubes have special chemical and physical properties, at the same time, they have the advantage of macro maneuverability in maneuverability, so they have the potential of multi-functional applications. At present, composite materials, super capacitors and corresponding personal care products based on carbon nanotubes are widely used in all aspects of people's lives, and these products have obvious functional advantages. In this paper, based on the application of single-walled carbon nanotubes technology in personal care products, the separation and purification of ginsenoside pods, the raw material of high-end washing and care products, was studied. In order to enhance the conductivity and tensile strength of SWNTs, this paper innovatively introduces the purification method of current acid treatment to enhance the performance of SWNTs. Based on the improved single-walled carbon nanotubes technology, the extraction efficiency of the optimized single-walled carbon nanotubes was analyzed. In the end of this paper, the optimized extraction method of ginsenoside pods from SWNTs is compared with the traditional extraction method. The experimental results show that SWNTs has obvious time efficiency advantage.

Single Fe Atom on Hierarchically Porous S, N-Codoped Nanocarbon Derived from Porphyra Enable Boosted Oxygen Catalysis for Rechargeable Zn-Air Batteries

Iron-nitrogen-carbon materials (Fe-N-C) are known for their excellent oxygen reduction reaction (ORR) performance. Unfortunately, they generally show a laggard oxygen evolution reaction (OER) activity, which results in a lethargic charging performance in rechargeable Zn-air batteries. Here porous S-doped Fe-N-C nanosheets are innovatively synthesized utilizing a scalable FeCl₃ -encapsulated-porphyra precursor pyrolysis strategy. The obtained electrocatalyst exhibits ultrahigh ORR activity (E_1/2 = 0.84 V vs reversible hydrogen electrode) and impressive OER performance (E_{j = 10} = 1.64 V). The potential gap (ΔE = E_{j = 10} - E_1/2 ) is 0.80 V, outperforming that of most highly active bifunctional electrocatalysts reported to date. Furthermore, the key role of S involved in the atomically dispersed Fe-Nx species on the enhanced ORR and OER activities is expounded for the first time by ultrasound-assisted extraction of the exclusive S source (taurine) from porphyra. Moreover, the assembled rechargeable Zn-air battery comprising this bifunctional electrocatalyst exhibits higher power density (225.1 mW cm^-2 ) and lower charging-discharging overpotential (1.00 V, 100 mA cm^-2 compared to Pt/C + RuO₂ catalyst). The design strategy can expand the utilization of earth-abundant biomaterial-derived catalysts, and the mechanism investigations of S doping on the structure-activity relationship can inspire the progress of other functional electrocatalysts.

Efficient Metal-Free Electrocatalysts from N-Doped Carbon Nanomaterials: Mono-Doping and Co-Doping

N-doped carbon nanomaterials have rapidly grown as the most important metal-free catalysts in a wide range of chemical and electrochemical reactions. This current report summarizes the latest advances in N-doped carbon electrocatalysts prepared by N mono-doping and co-doping with other heteroatoms. The structure-performance relationship of these materials is subsequently rationalized and perspectives on developing more efficient and sustainable electrocatalysts from carbon nanomaterials are also suggested.

Three-Dimensional Heteroatom-Doped Nanocarbon for Metal-Free Oxygen Reduction Electrocatalysis: A Review

The oxygen reduction reaction (ORR) at the cathode is a fundamental process and functions a pivotal role in fuel cells and metal–air batteries. However, the electrochemical performance of these technologies has been still challenged by the high cost, scarcity, and insufficient durability of the traditional Pt-based ORR electrocatalysts. Heteroatom-doped nanocarbon electrocatalysts with competitive activity, enhanced durability, and acceptable cost, have recently attracted increasing interest and hold great promise as substitute for precious-metal catalysts (e.g., Pt and Pt-based materials). More importantly, three-dimensional (3D) porous architecture appears to be necessary for achieving high catalytic ORR activity by providing high specific surface areas with more exposed active sites and large pore volumes for efficient mass transport of reactants to the electrocatalysts. In this review, recent progress on the design, fabrication, and performance of 3D heteroatom-doped nanocarbon catalysts is summarized, aiming to elucidate the effects of heteroatom doping and 3D structure on the ORR performance of nanocarbon catalysts, thus promoting the design of highly active nanocarbon-based ORR electrocatalysts.

Bifunctional 3D n-doped porous carbon materials derived from paper towel for oxygen reduction reaction and supercapacitor

Designing and fabricating cheap and active bifunctional materials is crucial for the development of renewable energy technologies. In this article, three-dimensional nitrogen-doped porous carbon materials (NDPC-X, in which X represents the pyrolysis temperature) were fabricated by simultaneous carbonization and activation of polypyrrole-coated paper towel protected by a silica layer followed by acid etching. The material had a high specific surface area (1,123.40 m²/g). The as-obtained NDPC-900 displayed outstanding activity as a catalyst for the oxygen reduction reaction (ORR) as well as an electrode with a high specific capacitance in a supercapacitor in an alkaline medium. The NDPC-900 catalyst for the ORR exhibited a more positive reduction peak potential of -0.068 V (vs. Hg|HgCl₂) than that of Pt/C (-0.121 V), as well as better cycling stability and stronger methanol tolerance. Moreover, the NDPC-900 had a high specific capacitance of 379.50 F/g at a current density of 1 A/g, with a retention rate of 94.5% after 10,000 cycles in 6 mol/L KOH electrolyte when used as an electrode in a supercapacitor. All these results were attributed to the effect of a large surface area, which provided electrochemically active sites. This work introduces an effective way to use biomass-derived materials for the synthesis of promising bifunctional carbon material for electrochemical energy conversion and storage devices.

N-doped defective carbon with trace Co for efficient rechargeable liquid electrolyte-/all-solid-state Zn-air batteries

Simple synthesis of multifunctional electrocatalysts with plentiful active sites from earth-abundant materials is especially fascinating. Here, N-doped defective carbon with trace Co (1.5 wt%) was prepared via a scalable one pot solid pyrolysis process. The sample exhibits efficient bifunctional OER/ORR activity in alkaline, mainly ascribed to the unique micro-mesoporous structure (1-3 nm), high population of graphitic-N doping (up to 49.0%), abundant defects and the encapsulated Co nanoparticles with graphitized carbon. The according rechargeable liquid Zn-air batteries showed excellent performance (maximum power density of 154.0 mW cm^-2; energy density of 773 Wh kg^-1 at 5 mA cm^-2 and charging-discharging cycling stability over 100 cycles). As a proof-of-concept, the flexible, rechargeable all-solid-state Zn-air batteries were constructed, and displayed a maximum power density as high as 45.9 mW cm^-2, among the top level of those reported previously.

Polyaniline/β-MnO2 nanocomposites as cathode electrocatalyst for oxygen reduction reaction in microbial fuel cells

An efficient and inexpensive catalyst for oxygen reduction reaction (ORR), polyaniline (PANI) and β-MnO₂ nanocomposites (PANI/β-MnO₂), was developed for air-cathode microbial fuel cells (MFCs). The PANI/β-MnO₂, β-MnO₂, PANI and β-MnO₂ mixture modified graphite felt electrodes were fabricated as air-cathodes in double-chambered MFCs and their cell performances were compared. At a dosage of 6 mg cm^-2, the maximum power densities of MFCs with PANI/β-MnO₂, β-MnO₂, PANI and β-MnO₂ mixture cathodes reached 248, 183 and 204 mW m^-2, respectively, while the cathode resistances were 38.4, 45.5 and 42.3 Ω, respectively, according to impedance analysis. Weak interaction existed between the rod-like β-MnO₂ and surficial growth granular PANI, this together with the larger specific surface area and PANI electric conducting nature enhanced the electrochemical activity for ORR and improved the power generation. The PANI/β-MnO₂ nanocomposites are a promising cathode catalyst for practical application of MFCs.

High-performance oxygen reduction catalysts in both alkaline and acidic fuel cells based on pre-treating carbon material and iron precursor

We demonstrate a new and simple method for pre-treating the carbon material and iron precursor to prepare oxygen reduction reaction (ORR) catalysts, which can produce super-high performance and stability in alkaline solution, with high performance in acid solution. This strategy using cheap materials is simply controllable. Moreover, it has achieved smaller uniform nanoparticles to exhibit high stability, and the synergetic effect of Fe and N offered much higher performance in ORR than commercial Pt/C, with high maximum power density in alkaline and acid fuel cell test. So it can make this kind of catalysts be the most promising alternatives of Pt-based catalysts with best performance/price.

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).

Fine decoration of carbon nanotubes with metal organic frameworks for enhanced performance in supercapacitance and oxygen reduction reaction

Efficient electrode material is crucial for energy conversion from renewable sources such as solar electricity. We present a method for preparation of carbon nanotubes (CNTs) with zeolitic imidazolate frameworks (ZIFs, e.g., ZIF-8) via an in situ pyrolysis process. The resultant materials are completely new carbon composites with desirable hierarchical porosity and nitrogen-doped features. Electron microscopy images show that CNTs with small external diameters enable more uniform dispersion of ZIF-8-derived carbons, subsequently yielding a unique hierarchically porous structure. Such carbon shows superior activity in oxygen reduction reaction (ORR) and high performance of supercapacitance, making it a valuable metal-free electrode material and a competent alternative to the state-of-the-art Pt/C catalyst. The electrocatalytic performance of CNTs can be dramatically improved by the incorporation of ZIF-8-derived carbons, which is attributed to the combination of good conductivity, abundant accessible dopant species, as well as proper porosity. Our method offers a new avenue for constructing electrocatalysts by effective integration of ZIF-8-derived carbon and the CNTs skeleton.

Hierarchical Porous Carbon Doped with Iron/Nitrogen/Sulfur for Efficient Oxygen Reduction Reaction

Hierarchical porous Fe/N/S-doped carbon with a high content of graphitic nitrogen (FeNS/HPC) has been successfully synthesized by a facile dual-template method. FeNS/HPC shows not only macropores resulting from the dissolution of the SiO₂ template, but abundant mesopores were also obtained after removing the in situ generated Fe₂O₃ nanoparticles on the ultrathin (∼4 nm) carbon shell of the macropores. Moreover, micropores are produced during the thermal pyrolysis of the carbon precursors. With respect to the electrochemical performance in the oxygen reduction reaction (ORR), FeNS/HPC not only exceeds other prepared porous carbon materials completely but also shows higher onset potential (0.97 vs 0.93 V), half-wave potentials (0.87 vs 0.83 V), and diffusion current density (5.5 vs 5.3 mA cm^-2) than those of Pt/C. Furthermore, FeNS/HPC also exhibits outstanding stability and methanol tolerance, making it a competent candidate for ORR. The following aspects contribute to its excellent ORR performance. (1) High content of graphitic N (5.1%) and codoping of pyridinic N species, thiophene-S, FeN_x, and graphitic carbon-encapsulated iron nanoparticles, providing highly active sites. (2) The hierarchical porous mesh structure with micro-, meso-, and macroporosity, accelerating the mass transfer and facilitating full utilization of the active sites. (3) The high specific surface area (1148 m² g^-1) of the graphitic carbon shell, assuring a large interface and rapid electron conduction for ORR.

High-performance method of carbon nanotubes modification by microwave plasma for thin composite films preparation

In this work we present a simple and efficient method of nitrogen plasma modification of carbon nanotubes (CNTs). Nitrogen-doped CNTs and electroactive polymer were used for synthesis of nanocomposites.