Electrocatalytic oxygen reduction activity of boron-doped carbon nanoparticles synthesized via solution plasma process

Synergistic N/F Dual-Doped MoC/C Catalyst Synthesized via Liquid Phase Plasma for Sustainable Ammonia Production

NH3 is a versatile solution for the storage and distribution of sustainable energy, offering high energy density and promising applications as a renewable hydrogen carrier. However, electrochemical NH3 synthesis under ambient conditions remains challenging, such as low selectivity and efficiency, owing to the inertness of N≡N and competing reactions. In this study, a catalyst (MoC/NFC) comprising molybdenum carbide evenly dispersed on carbon doped with N and F heteroatoms was successfully synthesized using liquid-phase plasma. The MoC/NFC catalyst exhibited a maximum NH3 yield of 115 μg h-1 mg-1cat. with a faradaic efficiency of 1.15% at -0.7 V vs reversible hydrogen electrode in 0.1 M KOH electrolyte. Pyridinic- and pyrrolic-N atoms adjacent to the carbon pores served as active sites for N2 adsorption and enabled N2 triple bond cleavage. In addition, F doping contributed to N2 activation owing to the high electronegativity of 3.98, resulting in the attraction of more electrons. These findings demonstrate a significant advancement in the development of efficient catalysts for electrochemical ammonia synthesis, potentially paving the way for scalable and sustainable NH3 production methods that can support the growing demand for renewable energy storage solutions.

Structural Regulation and Oxygen Reduction Reaction Activity of [Co(bpy)2BO2(OH)] and [Cu(bpy)(OH)]2- Complex Templated Borates

Two templated borates, [Co(bpy)₂BO₂(OH)]·[B₅O₆(OH)₄]·H₃BO₃·H₃O·H₂O (1) and [Cu(bpy)(OH)]₂·[B₅O₆(OH)₄]₂·H₂O (2), have been synthesized successfully and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and Fourier transform infrared. The [Co(bpy)₂BO₂(OH)] complex in 1 shows a very rare coordination mode between Co²⁺ and BO₂(OH)^2-. The structures of 1 and 2 can be adjusted by changing the reagent. The oxygen reduction reaction activity of these Co- and Cu-based catalysts was studied. The E_1/2 values of Co-C-750 and Cu-C-750 are 0.864 and 0.837 V, respectively.

Exploration of the oxygen reduction reaction activity of four transition metal borates: synthesis, structure and characterization

Four new transition metal borates, [Co(1-MI)₂]·B₅O₇(OH)₃ (1, 1-MI = 1-methylimidazole), Co₂(PZ)₈(SO₄)₂·2H₃BO₃ (2, PZ = pyrazole), K₇{(BO₃)Co[B₁₂O₁₈(OH)₆]}·H₂O (3) and K₂{[Ni(PY)₂]₂·[(C₄H₂O₆)₂B]}₂ (4, PY = pyridine), have been successfully synthesized by a mild method. Their structures were determined by X-ray single crystal diffraction. These compounds were further characterized by FTIR, PXRD and UV-vis-NIR. It is worth noting that the catalytic properties of catalysts based on these four borates were explored, and that organically templated borate 1- and 2-based catalysts showed better ORR catalytic performance compared to 3- and 4-based catalysts. This may be caused by the supramolecular skeletons of 1 and 2 providing sufficient space and porosity.

Simultaneous synthesis of graphite-like and amorphous carbon materials via solution plasma and their evaluation as additive materials for cathode in Li-O2 battery

Cathode materials are essential for enhancing electrocatalytic activity in energy-conversion devices. Carbon is one of the most suitable cathodic materials for Li–O₂ batteries owing to its chemical and thermal stability. Carbon materials synthesized from tributyl borate (TBB) using a nonthermal solution plasma method were characterized using x‐ray diffraction, Raman, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy, and x-ray photoelectron spectroscopy and were evaluated as additive materials for cathodes in a Li–O₂ battery. Two separate carbon materials were formed at the same time, a carbon dispersed in solution and a carbon precipitate at the bottom of the reactor, which had amorphous and graphite-like structures, respectively. The amorphous carbon contained boron and tungsten carbide, and the graphite-like carbon had more defects and electronic conductivity. The crystallinity and density of defects in the graphite-like carbon could be tuned by changing the SP operating frequency. The Li–O₂ battery with the amorphous carbon containing boron and tungsten carbide was found to have a high capacity, while the one with the graphite-like carbon showed an affinity for the formation of Li₂O₂, which is the desired discharge product, and exhibited high cycling performance.

Hybrid Molybdenum Carbide/Heteroatom-Doped Carbon Electrocatalyst for Advanced Oxygen Evolution Reaction in Hydrogen Production

Hydrogen energy is one of the key technologies that can help to prevent global warming. A water electrolysis process can be used to produce hydrogen, in which hydrogen is produced at one electrode of the electrochemical cell, and oxygen is produced at the other electrode. On the other hand, the oxygen evolution reaction (OER) requires multiple reaction steps and precious-metal-based catalysts (e.g., Ru/C, Ir/C, RuO2, and IrO2) as electrocatalysts to improve the reaction rate. Their high cost and limited supply, however, limit their applications to the mass production of hydrogen. In this study, boron, nitrogen-doped carbon incorporated with molybdenum carbide (MoC-BN/C) was synthesized to replace the precious-metal-based catalysts in the OER. B, N-doped carbon with nanosized molybdenum nanoparticles was fabricated by plasma engineering. The synthesized catalysts were heat-treated at 600, 700, and 800 °C in nitrogen for one hour to enhance the conductivity. The best MoC-BN/C electrocatalysts (heated at 800 °C) exhibited superior OER catalytic activity: 1.498 V (vs. RHE) and 1.550 V at a current density of 10 and 100 mA/cm2, respectively. The hybrid electrocatalysts even outperformed the noble electrocatalyst (5 wt.% Ru/C) with higher stability. Therefore, the hybrid electrocatalyst can replace expensive precious-metal-based catalysts for the upcoming hydrogen economy.

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.

Synergistic effect on BCN nanomaterials for the oxygen reduction reaction – a kinetic and mechanistic analysis to explore the active sites

The rGO doped with boron and nitrogen reduce the oxygen via the dissociative four-electron pathway whereas the two-electron oxygen reduction reaction is more predominant on the rGO doped with either of the two individual heteroatoms.

A Review on the Promising Plasma-Assisted Preparation of Electrocatalysts

Electrocatalysts are becoming increasingly important for both energy conversion and environmental catalysis. Plasma technology can realize surface etching and heteroatom doping, and generate highly dispersed components and redox species to increase the exposure of the active edge sites so as to improve the surface utilization and catalytic activity. This review summarizes the recent plasma-assisted preparation methods of noble metal catalysts, non-noble metal catalysts, non-metal catalysts, and other electrochemical catalysts, with emphasis on the characteristics of plasma-assisted methods. The influence of the morphology, structure, defect, dopant, and other factors on the catalytic performance of electrocatalysts is discussed.

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.

Copper nanoparticles anchored onto boron-doped graphene nanosheets for use as a high performance asymmetric solid-state supercapacitor

There is a high demand for high energy and power density in the field of energy storage devices. To rectify these limitations, a novel asymmetric solid-state supercapacitor (ASSC) was designed and fabricated using a copper anchored boron doped graphene nanosheet (CuBG) as a negative electrode and reduced graphene nanoplatelets as a positive electrode with H2SO4/PVA as the quasi-solid electrolyte. The CuBG was prepared using a two step hydrothermal process followed by pyrolysis at different temperatures using chemical vapour deposition (CVD), using copper sulphate (CuSO4) and boron-trioxide (B2O3) as precursors, for doping in graphene oxide. Owing to the remarkable structure and morphology of Cu nanoparticles on nanosheets of boron intercalated with graphene oxide, the nanosheets exhibit a high specific capacitance of 483 Fg-1 at 1 Ag-1 with a capacitance retention of 96% after 5000 cycles, respectively, in a two-electrode system. In addition, the designed and fabricated solid state ASSC device of rGO//CuBG exhibited a high energy and power density of 132.5 W h kg-1 and 1000 W kg-1, respectively, in a wide potential window of 2.0 V, with an excellent stability, retaining 91% of its initial specific capacitance after 5000 cycles. The electrochemical capacitance of CuBG was also evaluated in a three and two electrode system using a KOH and KOH/PVA solid electrolyte respectively. A specific capacitance of 87.5 Fg-1 was achieved at 1 Ag-1 using the fabricated asymmetric device with a 31.1 W h kg-1 energy density at a corresponding power density of 800 W kg-1 and an 85% capacitance was retained after 5000 cycles. The kinetics of the interfacial charge transport phenomena were analysed using a Nyquist plot of the electrochemical impedance analysis.

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.

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.

Fabrication of bacterial cellulose-ZnO composite via solution plasma process for antibacterial applications

Zinc oxide (ZnO) was successfully synthesized by applying a solution plasma, a plasma discharge in a liquid phase, without the addition of a reducing agent and simultaneously deposited into a bacterial cellulose pellicle that functioned as a template. By the reasons of its nano-sized structure as well as favorable porous configuration, the BC pellicle has been proved to be a splendid upholding template for the coordination of ZnO. In addition, the ZnO-deposited BC composites demonstrated strong antibacterial activity without a photocatalytic reaction against both Staphylococcus aureus and Escherichia coli. Hence, the ZnO-deposited BC composites can be used as an antibacterial material in wound dressing and water disinfection applications.

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.

Effects of halogen doping on nanocarbon catalysts synthesized by a solution plasma process for the oxygen reduction reaction

Halogen (F, CI, and Br)-containing carbon materials were successfully synthesized by solution plasma process. The effects of halogen doping on chemical structure and electrocatalytic activity were investigated.

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.