Sonoelectrochemical nitrided graphene nanosheets with vacancies and their applications for catalysis and sensing of uric acid oxidation

A sonoelectrochemical method for preparing N-doped defective graphene nanosheets (N/O-dGNs) with point defects and 5-9 or 5-8-5 vacancies and oxygen-containing groups was successfully demonstrated. In this one-pot approach, the N-bonding configuration and N content of N/O-dGNs were finely tuned by the ultrasonic power (192, 320, and 640 W). The N content in atomic percentage (at%) for N/O-dGN (N/O-dGN320W) with point defects and 5-8-5 vacancy prepared at 320 W power was 5.6 at%, greater than 3.0 at% and 2.6 at% for N/O-dGN with point defects and 5-9 vacancies at 192 W and 640 W power (N/O-dGN192W and N/O-dGN640W), respectively. N-bonding sites on N/O-dGN320W were dominantly amine N (2.1 at%) and pyrrolic N (2.4 at%). Additionally, the electrocatalytic activity of N/O-dGN192W, N/O-dGN320W, and N/O-dGN640W was successfully demonstrated for the sequential uric acid (UA) oxidation reaction (UOR), in which N/O-dGN320W displayed a significant mass activity (2.51 A/g). As in the transient catalysis of UOR, N/O-dGN320W with amine N showed 400.8 μA mM-1 cm-2 in sensitivity within a wide linear analysis range (1.5 × 10-2-6 mM) for amperometrically sensing UA. The results of real sample experiments using serum samples further demonstrated the potential of N/O-dGN320W as a non-enzymatic UA sensor.

Plasma-Assisted Synthesis of Multicomponent Nanoparticles Containing Carbon, Tungsten Carbide and Silver as Multifunctional Filler for Polylactic Acid Composite Films

Multicomponent nanoparticles containing carbon, tungsten carbide and silver (carbon-WC-Ag nanoparticles) were simply synthesized via in-liquid electrical discharge plasma, the so-called solution plasma process, by using tungsten electrodes immersed in palm oil containing droplets of AgNO₃ solution as carbon and silver precursors, respectively. The atomic ratio of carbon:W:Ag in carbon-WC-Ag nanoparticles was 20:1:3. FE-SEM images revealed that the synthesized carbon-WC-Ag nanoparticles with particle sizes in the range of 20–400 nm had a spherical shape with a bumpy surface. TEM images of carbon-WC-Ag nanoparticles showed that tungsten carbide nanoparticles (WCNPs) and silver nanoparticles (AgNPs) with average particle sizes of 3.46 nm and 72.74 nm, respectively, were dispersed in amorphous carbon. The carbon-WC-Ag nanoparticles were used as multifunctional fillers for the preparation of polylactic acid (PLA) composite films, i.e., PLA/carbon-WC-Ag, by solution casting. Interestingly, the coexistence of WCNPs and AgNPs in carbon-WC-Ag nanoparticles provided a benefit for the co-nucleation ability of WCNPs and AgNPs, resulting in enhanced crystallization of PLA, as evidenced by the reduction in the cold crystallization temperature of PLA. At the low content of 1.23 wt% carbon-WC-Ag nanoparticles, the Young’s modulus and tensile strength of PLA/carbon-WC-Ag composite films were increased to 25.12% and 46.08%, respectively. Moreover, the PLA/carbon-WC-Ag composite films possessed antibacterial activities.

Application of a polyethylenimine-modified polyacrylonitrile-biomass waste composite fiber sorbent for the removal of a harmful cyanobacterial species from an aqueous solution

Cyanobacterial harmful algal blooms (Cyano-HABs) in water resources involving algal species such as Microcystis aeruginosa have become a serious environmental issue due to their severely negative effects. In the present study, an adsorption-based strategy was employed to control M. aeruginosa, with industrial waste-derived Escherichia coli biomass valorized to produce polyethylenimine-modified polyacrylonitrile-E. coli biomass composite fiber (PEI-PANBF). PEI-PANBF removed approximately 80% of M. aeruginosa cells from an aqueous solution without causing any cell damage. Interestingly, the thickness of PEI-PANBF had a strong influence on the efficiency of M. aeruginosa cell removal. In addition, PEI-PANBF simultaneously removed M. aeruginosa cells and their toxic secondary metabolite, microcystin-LR, from aqueous media. Thus, our proposed fiber represents a feasible utilization method of industrial waste biomass as a biosorbent for the control of Cyano-HABs.

In situ synthesis of copper nanoparticles encapsulated by nitrogen-doped graphene at room temperature via solution plasma

Metal–carbon core–shell nanostructures have gained research interest due to their better performances in not only stability but also other properties, such as catalytic, optical, and electrical properties. However, they are limited by complicated synthesis approaches. Therefore, the development of a simple method for the synthesis of metal–carbon core–shell nanostructures is of great significance. In this work, a novel Cu–core encapsulated by a N-doped few-layer graphene shell was successfully synthesized in a one-pot in-liquid plasma discharge, so-called solution plasma (SP), to our knowledge for the first time. The synthesis was conducted at room temperature and atmospheric pressure by using a pair of copper electrodes submerged in a DMF solution as the precursor. The core–shell structure of the obtained products was confirmed by HR-TEM, while further insight information was explained from the results of XRD, Raman, and XPS measurements. The obtained Cu-core encapsulated by the N-doped few-layer graphene shell demonstrated relatively high stability in acid media, compared to the commercial bare Cu particles. Moreover, the stability was found to depend on the thickness of the N-doped few-layer graphene shell which can be tuned by adjusting the SP operating conditions.

An excellent corrosion protection for copper nanoparticles by nitrogen-doped few-layer graphene via solution plasma process.

Cobalt Nanoparticles on Plasma-Controlled Nitrogen-Doped Carbon as High-Performance ORR Electrocatalyst for Primary Zn-Air Battery

Metal-air batteries and fuel cells have attracted much attention as powerful candidates for a renewable energy conversion system for the last few decades. However, the high cost and low durability of platinum-based catalysts used to enhance sluggish oxygen reduction reaction (ORR) at air electrodes prevents its wide application to industry. In this work, we applied a plasma process to synthesize cobalt nanoparticles catalysts on nitrogen-doped carbon support with controllable quaternary-N and amino-N structure. In the electrochemical test, the quaternary-N and amino-N-doped carbon (Q-A)/Co catalyst with dominant quaternary-N and amino-N showed the best onset potential (0.87 V vs. RHE) and highest limiting current density (-6.39 mA/cm2). Moreover, Q-A/Co was employed as the air catalyst of a primary zinc-air battery with comparable peak power density to a commercial 20 wt.% Pt/C catalyst with the same loading, as well as a stable galvanostatic discharge at -20 mA/cm2 for over 30,000 s. With this result, we proposed the synergetic effect of transitional metal nanoparticles with controllable nitrogen-bonding can improve the catalytic activity of the catalyst, which provides a new strategy to develop a Pt-free ORR electrocatalyst.

Self-Assembled Porous-Silica within N-Doped Carbon Nanofibers as Ultra-flexible Anodes for Soft Lithium Batteries

Silica is an attractive anode material for soft lithium batteries owing to its high specific capacity, but it suffers severe problems of large volume change and unstable solid-electrolyte interface. Moreover, it is a challenge to fabricate flexible silica anodes. Here, we report a low-cost and scalable strategy to create flexible anodes of N-doped carbon nanofiber-confined porous silica (p-SiO₂@N-CNF) by developing a sol-gel electrospinning process followed by carbonization. This approach causes the p-SiO₂ nanoparticles (NPs) to be self-assembled within the N-CNFs, which act like elastomer and electrolyte barrier to accommodate volume changes and to enhance the stability of SiO₂, whereas the NPs act as soft plasticizer providing strength to the CNF skeletons. Benefiting from the hierarchical structures, the anodes with high p-SiO₂ loadings (>1.6 mg/cm²) exhibit exceptional cycling performance (>1,000 cycles) in terms of bending, current rate, and capacity. Moreover, the batteries remain stable when discharging at 0.5 C and charging at 2 C.

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A scalable method is developed for the fabrication of flexible silica anodes

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The flexible mechanisms of carbon nanofiber and silica films are illustrated

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High-silica-loading anodes exhibit long cycle stability and high rate capability

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Soft silica anodes show appealing properties for soft batteries

Transition Metal (Fe, Co, Ni) Nanoparticles on Selective Amino-N-Doped Carbon as High-Performance Oxygen Reduction Reaction Electrocatalyst

Metal-air batteries are attracting increasing attention as a superior renewable energy conversion device due to their high performance and strong potential. However, the high cost and low stability of the current Pt catalyst is the main obstacle preventing wide industrial application. In this work, we applied a plasma process to fabricate aniline and a transition metals electrode (Fe, Co, Ni) as the carbon-nitrogen and the metal nanoparticle (NP) precursors, respectively, for selective metal/amino-N-doped carbon catalysts. All three as-synthesized catalysts exhibited dominant amino-N as the major C-N bonding state. In electrochemical testing, Co/amino-N-doped carbon showed positive E1/2 potential (0.83 V vs. Reversible Hydrogen Electrode (RHE)). In addition, the calculated electron transfer number (n) of Co/amino-N-doped carbon at 0.5 V vs. RHE was 3.81, which was only slightly less than that of commercial Pt/C (3.97). This superior performance of transition metal/amino-N-doped carbon promotes it as an economical oxygen reduction reaction (ORR) electrocatalyst to replace expensive Pt/C in metal-air batteries.

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.

Probing Active Sites on Metal-Free, Nitrogen-Doped Carbons for Oxygen Electroreduction: A Review

The Metal-free nitrogen-doped carbons represent an emerging low-cost nonprecious electrocatalyst for oxygen reduction reaction (ORR) that is a sluggish process at the cathode of polymer electrolyte membrane fuel cells (PEMFCs) and a verity of metal-air batteries. During the past few years, the ORR catalytic activity of nitrogen-doped carbons has been significantly increased, making them highly competitive alternatives to conventional precious metals based electrocatalysts for ORR. However, controversies remain in the unambiguous identification of the ORR active sites on nitrogen-doped carbons. This review summarizes the recent progress in probing the potential active sites on metal-free nitrogen-doped carbons for ORR, aiming to gain in-depth understanding of the ORR catalytic mechanism on nitrogen-doped carbons for further enhancing ORR activity.

Cytotoxicity against cancer cells of chitosan oligosaccharides prepared from chitosan powder degraded by electrical discharge plasma

Chitosan oligosaccharides, which obtain from degradation of chitosan, possess some interesting molecular weight-dependent biological properties, especially anticancer activity. Therefore, the conversion of chitosan to chitosan oligosaccharides with specific molecular weight has been continuously investigated in order to find effective strategies that can achieve both economic feasibility and environmental concerns. In this study, a novel process was developed to heterogeneously degrade chitosan powder by highly active species generated by electrical discharge plasma in a dilute salt solution (0.02 M) without the addition of other chemicals. The degradation rate obtained from the proposed process was comparable to that obtained from some other methods with the addition of acids and oxidizing agents. Separation of the water-soluble degraded products containing chitosan oligosaccharides from the reaction solution was simply done by filtration. The obtained chitosan oligosaccharides were further evaluated for an influence of their molecular weights on cytotoxicity against cancer cells and the selectivity toward cancer and normal cells.

Hydrothermal and plasma nitrided electrospun carbon nanofibers for amperometric sensing of hydrogen peroxide

Nitrogen-doped carbon nanofibers (CNFs) were prepared by an electrospinning method, this followed by a hydrothermal reaction or nitrogen plasma treatment to obtain electrode for non-enzymatic amperometric sensing of H₂O₂. The hydrothermally treated electrode performs better. Its electrochemical surface is 3.7 × 10^-3 mA cm^-2, which is larger than that of a nitrogen plasma treated electrode (8.9 × 10^-4) or a non-doped CNF (2.45 × 10^-4 mA cm^-2). The hydrothermally treated CNF with rough surface and a complex profile with doped N has a higher sensitivity (357 μA∙mM^-1∙cm^-2), a lower detection limit (0.62 μM), and a wider linear range (0.01-0.71 mM) than N-CNF_P at a working potential of -0.4 V (vs. Ag/AgCl). The electrode gave high recoveries when applied to the analysis of milk samples spiked with H₂O₂. Graphical abstract Nitrogen-doped carbon nanofibers prepared by an electrospinning method followed by a hydrothermal reaction (N-CNF_ht) or nitrogen plasma treatment (N-CNF_P) are directly used as non-enzymatic amperometric H₂O₂ sensors.

Plasma-Assisted Synthesis and Surface Modification of Electrode Materials for Renewable Energy

Renewable energy technology has been considered as a "MUST" option to lower the use of fossil fuels for industry and daily life. Designing critical and sophisticated materials is of great importance in order to realize high-performance energy technology. Typically, efficient synthesis and soft surface modification of nanomaterials are important for energy technology. Therefore, there are increasing demands on the rational design of efficient electrocatalysts or electrode materials, which are the key for scalable and practical electrochemical energy devices. Nevertheless, the development of versatile and cheap strategies is one of the main challenges to achieve the aforementioned goals. Accordingly, plasma technology has recently appeared as an extremely promising alternative for the synthesis and surface modification of nanomaterials for electrochemical devices. Here, the recent progress on the development of nonthermal plasma technology is highlighted for the synthesis and surface modification of advanced electrode materials for renewable energy technology including electrocatalysts for fuel cells, water splitting, metal-air batteries, and electrode materials for batteries and supercapacitors, etc.

A rational synthesis of hierarchically porous, N-doped carbon from Mg-based MOFs: understanding the link between nitrogen content and oxygen reduction electrocatalysis

Controlled mixtures of novel Mg-based metal-organic frameworks (MOFs) were prepared, with H(+) or K(+) as counterions. A linear relation was found between synthesis pH and K/H ratio in the resultant mixture, establishing the tunability of the synthesis. Upon pyrolysis, these precursor mixtures yield nitrogen-doped, hierarchically porous carbons, which have good activity towards the oxygen reduction reaction (ORR) at pH 13. The nitrogen content varies significantly along the homologous carbon series (>400%, 1.3 at% to 5.7 at%), to a much greater extent than microstructural parameters such as surface area and graphitization. This allows us to isolate the positive correlation between nitrogen content and electrocatalytic oxygen reduction ORR activity in this class of metal-free, N-doped, porous carbons.

The solution plasma process for heteroatom-carbon nanosheets: the role of precursors

The solution plasma process (SPP), known as non-equilibrium cold plasma at atmospheric pressure and room temperature, was used to investigate the synthesis of nitrogen-carbon nasnosheets (NCNs). To verify the effect of elementary composition and structure of N-methyl-2-pyrrolidone (NMP), various precursors were used in the SPP to synthesize NCNs via the bottom-up synthesis method for the first time. The NCNs were analyzed by transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Among the various precursors, SPP of 2-pyrrolidone was demonstrated to facilitate the formation of highly ordered NCNs. On the other hand, the SPP with cyclopentanone, cyclohexanone and pyrrole did not lead to the formation of carbon nanosheets. The results of this study would uncover new parameter fields for the growth of heteroatom-carbon nanosheets using this synthesis system. In addition, the study is expected to contribute toward research in improving the large-area growth and quality of two-dimensional nanostructures, such as heteroatom-carbon nanosheets or graphene, for various applications in other synthesis methods.

Fastest Formation Routes of Nanocarbons in Solution Plasma Processes

Although solution-plasma processing enables room-temperature synthesis of nanocarbons, the underlying mechanisms are not well understood. We investigated the routes of solution-plasma-induced nanocarbon formation from hexane, hexadecane, cyclohexane, and benzene. The synthesis rate from benzene was the highest. However, the nanocarbons from linear molecules were more crystalline than those from ring molecules. Linear molecules decomposed into shorter olefins, whereas ring molecules were reconstructed in the plasma. In the saturated ring molecules, C-H dissociation proceeded, followed by conversion into unsaturated ring molecules. However, unsaturated ring molecules were directly polymerized through cation radicals, such as benzene radical cation, and were converted into two- and three-ring molecules at the plasma-solution interface. The nanocarbons from linear molecules were synthesized in plasma from small molecules such as C2 under heat; the obtained products were the same as those obtained via pyrolysis synthesis. Conversely, the nanocarbons obtained from ring molecules were directly synthesized through an intermediate, such as benzene radical cation, at the interface between plasma and solution, resulting in the same products as those obtained via polymerization. These two different reaction fields provide a reasonable explanation for the fastest synthesis rate observed in the case of benzene.

Nitrogen-Doped Carbon Nanoparticle-Carbon Nanofiber Composite as an Efficient Metal-Free Cathode Catalyst for Oxygen Reduction Reaction

Metal-free nitrogen-doped carbon materials are currently considered at the forefront of potential alternative cathode catalysts for the oxygen reduction reaction (ORR) in fuel cell technology. Despite numerous efforts in this area over the past decade, rational design and development of a new catalyst system based on nitrogen-doped carbon materials via an innovative approach still present intriguing challenges in ORR catalysis research. Herein, a new kind of nitrogen-doped carbon nanoparticle-carbon nanofiber (NCNP-CNF) composite with highly efficient and stable ORR catalytic activity has been developed via a new approach assisted by a solution plasma process. The integration of NCNPs and CNFs by the solution plasma process can lead to a unique morphological feature and modify physicochemical properties. The NCNP-CNF composite exhibits a significantly enhanced ORR activity through a dominant four-electron pathway in an alkaline solution. The enhancement in ORR activity of NCNP-CNF composite can be attributed to the synergistic effects of good electron transport from highly graphitized CNFs as well as abundance of exposed catalytic sites and meso/macroporosity from NCNPs. More importantly, NCNP-CNF composite reveals excellent long-term durability and high tolerance to methanol crossover compared with those of a commercial 20 wt % supported on Vulcan XC-72. We expect that NCNP-CNF composite prepared by this synthetic approach can be a promising metal-free cathode catalyst candidate for ORR in fuel cells and metal-air batteries.

Heterocarbon nanosheets incorporating iron phthalocyanine for oxygen reduction reaction in both alkaline and acidic media

Heterocarbon nanosheets incorporating iron phthalocyanine as electrocatalysts have been successfully synthesized by a solution plasma process using high-repetition-frequency discharges at room temperature and atmospheric pressure.

Selective nitrogen bonding states in nitrogen-doped carbon via a solution plasma process for advanced oxygen reduction reaction

Selective nitrogen bonding within nitrogen-doped carbon nanoparticles was achieved by altering linear and heterocyclic precursor via solution plasma.

Fe–N-doped carbon-based composite as an efficient and durable electrocatalyst for the oxygen reduction reaction

An Fe–N-doped carbon nanoparticle–carbon nanofiber (Fe–N-CNP–CNF) composite was prepared by a solution plasma process followed by heat treatment. The Fe–N-CNP–CNF exhibits excellent catalytic activity, durability and methanol tolerance for the ORR.

Enhancement of conductivity in nano carbon balls by the addition of carbon tetrachloride via room temperature solution plasma process

(a) Reaction field in solution plasma zone and dehydrogenation by Cl radicals, (b) enhancement of conductivity of nanocarbon sphere by the addition of CCl₄ within benzene.

Hybrid polymer matrix composite containing polyaniline and Nafion as novel precursor of the enhanced catalyst for oxygen reduction reaction

A kind of hybrid polymer matrix composite serves as a novel nitrogen precursor for enhanced metal–nitrogen-doped carbon catalyst.

The evolution of hierarchical porosity in self-templated nitrogen-doped carbons and its effect on oxygen reduction electrocatalysis

Pyrolitic self-templating synthesis is an effective method for creating hierarchically porous N-doped carbons.

Synthesis of heteroatom-carbon nanosheets by solution plasma processing using N-methyl-2-pyrrolidone as precursor

Heteroatom-carbon nanosheets, composed of multi-layer graphene with turbostratic stacking, were successfully synthesized through a solution plasma processing (SPP) with N-methyl-2-pyrrolidone at room temperature and an atmospheric pressure.

Sucrose-derived activated carbons: electron transfer properties and application as oxygen reduction electrocatalysts

ORR electrocatalysts derived from sugar: activated carbons derived from sucrose showed electrocatalytic activity for the oxygen reduction reaction.