Co-VN encapsulated in bamboo-like N-doped carbon nanotubes for ultrahigh-stability of oxygen reduction reaction

Co-modified polyoxovanadoborates derived Co/BN-CNT/VN based bifunctional electrocatalysts for rechargeable zinc-air batteries

Rational design of high-performance bifunctional electrocatalysts to accelerate the sluggish oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) in rechargeable Zn-air batteries remain an enduring challenge. The construction of multicomponent catalysts is a promising solution to achieve this goal. Herein, B and N co-doped interconnecting graphite carbon and carbon nanotube with the decoration of Co and vanadium nitride (VN) nanoparticles (Co/BN-CNT/VN) are synthesized using Co-modified polyoxovanadoborates as precursors. The optimized composite achieves superior bifunctional oxygen electrocatalytic activity and stability, which is comparable to noble metal catalysts and reported bifunctional electrocatalysts. Specifically, the half-potential of ORR reaches 0.85 V, and the overpotential of OER is low to 296 mV at a current density of 10 mA cm^-2. Strikingly, zinc-air batteries assembled based on Co/BN-CNT/VN demonstrate a small charge-discharge voltage gap of 0.873 V, a remarkable peak-power density of 156.3 mW cm^-2, and outstanding cycling durability (∼1000 cycles at 10 mA cm^-2). This work affords a new alternative strategy to create cost-effective and high-potency bifunctional oxygen electrocatalysts for advanced air batteries.

Metal-organic framework assisted vanadium oxide nanorods as efficient electrode materials for water oxidation

The water oxidation process, which comprises the oxygen evolution reaction (OER), is a critical catalytic mechanism for sustainable technologies like water electrolysis and fuel cells. Herein, we develop a unique metal-organic framework aided vanadium pentoxide nanorods (MOF-V₂O₅ NRs-500) that can be used as an OER electrocatalyst under alkaline solutions. The crystal structure, surface chemical state, and porosity of MOF-V₂O₅ NRs-500 can be altered by annealing in an oxygen atmosphere. The resultant MOF-V₂O₅ NRs-500 demonstrate high catalytic activity against OER in basic conditions, with a low overpotential of 300 mV at a derived current density of 50 mA cm^-2, and extraordinary durability of more than 50 h. Superior electrochemical performance might be attributed to the high exposure level of active sites emanating from porous MOF-V₂O₅ NRs-500. Furthermore, the porous MOF-V₂O₅ NRs-500 skeleton may provide homogenous mass transport channels as well as quick electron transfer.

Hollow nitrogen-doped carbon nanospheres as cathode catalysts to enhance oxygen reduction reaction in microbial fuel cells treating wastewater

Hollow nanospheres play a pivotal role in the electro-catalytic oxygen reduction reaction (ORR), which is a crucial step in microbial fuel cell (MFC) device. Herein, the hollow nitrogen-doped carbon nanospheres (HNCNS) were synthesized with the sacrifice of silica coated carbon nanospheres (CNS@SiO₂) as template. HNCNS remarkably enhanced the ORR activity compared to the solid carbon and solid silica spheres. By tuning calcination temperature (800-1100 °C), the surface chemistry properties of HNCNS were effectively regulated. The optimal HNCNS-1000 catalyst which was calcined at 1000 °C exhibited the highest ORR activity in neutral media with the onset potential of 0.255 V and half-wave potential of -0.006 V (vs. Ag/AgCl). Single chamber MFC (SCMFC) assembled with HNCNS-1000 cathode unveiled comparable activity to a conventional Pt/C reference. It showed the highest maximum power density of 1307 ± 26 mW/m², excellent output stability of 5.8% decline within 680 h, chemical oxygen demand (COD) removal of 94.0 ± 0.3% and coulombic efficiency (CE) of 7.9 ± 0.9%. These excellent results were attributed to a cooperative effect of the optimized surface properties (e.g., structural defects, relative content of pyrrolic nitrogen and specific surface area) and the formation of hollow nanosphere structure. Furthermore, the positive linear relationship of the structural defects and pyrrolic nitrogen species with the maximum power generation in SCMFC were clearly elucidated. This study demonstrated that the cost effective HNCNS-1000 was a promising alternative to commercial Pt/C catalyst for practical application in MFCs treating wastewater. Our result revealed the effectiveness of MFC fabricated with HNCNS-1000 cathode catalyst in terms of power generation and wastewater treatment.

Recent Advances in Waste Plastic Transformation into Valuable Platinum-Group Metal-Free Electrocatalysts for Oxygen Reduction Reaction

Plastic waste causes severe environmental hazards, owing to inadequate disposal and limited recycling. Under the framework of circular economy, there are urgent demands to valorize plastic waste more safely and sustainably. Therefore, much scientific interest has been witnessed recently in plastic waste-derived electrocatalysts for the oxygen reduction reaction (ORR), where the plastic waste acts as a cost-effective and easily available precursor for the carbon backbone. The ORR is not only a key efficiency indicator for fuel cells and metal-air batteries but also a major obstacle for their commercial realization. The applicability of the aforementioned electrochemical devices is limited, owing to sluggish ORR activity and expensive platinum-group metal electrocatalysts. However, waste-derived ORR electrocatalysts are emerging as a potential substitute that could be inexpensively fabricated upon the conversion of plastic waste into active materials containing earth-abundant transition metals. In this Minireview, very recent research developments regarding plastic waste-derived ORR electrocatalysts are critically summarized with a prime focus on the followed synthesis routes, physicochemical properties of the derived electrocatalysts, and their ultimate electrochemical performance. Finally, the prospects for the future development of plastic waste-derived electrocatalysts are discussed.

Cobalt encapsulated in bamboo-like N-doped carbon nanotubes for highly sensitive electroanalysis of Pb(II): enhancement based on adsorption and catalysis

Carbon nanotubes (CNTs) are recognized as desirable candidates to fabricate electrochemical sensing interfaces owing to their high surface area and excellent electron conductivity. However, the poor catalytic properties of CNTs significantly hinder their further application in electrochemical detection. Herein, for the first time we combined defective CNTs with catalytically active cobalt nanoparticles (Co NPs) to give cobalt encapsulated in a bamboo-like N-doped carbon nanotube nanocomposite (Co/N-CNTs). The novel constructed Co/N-CNTs are used as a modifier on a bare glass carbon electrode for the electrochemical detection of Pb(ii). As a result, the positive effect of adsorption and catalysis on Co/N-CNT shows a significant improvement in the electroanalytical performance towards Pb(ii) with a sensitivity of 69.74 μA μM-1 and a limit of detection of 0.039 μM. Moreover, the stability and practical applications of Co/N-CNTs towards Pb(ii) in real water samples obtained from a mining subsidence area were also considered. This method shows great promise, achieving an outstanding electroanalysis efficiency with noble-metal-free nanocomposite sensors based on the combination of carbon and transition metals.

Maximized Schottky Effect: The Ultrafine V2 O3 /Ni Heterojunctions Repeatedly Arranging on Monolayer Nanosheets for Efficient and Stable Water-to-Hydrogen Conversion

The Mott-Schottky heterojunction formed at the interface of ultrafine metallic Ni and semiconducting V₂ O₃ nanoparticles is constructed, and the heterojunctions are "knitted" into the tulle-like monolayer nanosheets on nickel foam (NF). The greatly reduced particle sizes of both Ni and V₂ O₃ on the Mott-Schottky heterojunction highly enhance the number of Schottky heterojunctions per unit area of the materials. Moreover, arranging the heterojunctions into the monolayer nanosheets makes the heterojunctions repeat and expose to the electrolyte sufficiently. The Schottky heterojunctions are like countless self-powered charge transfer workstations embedded in the tulle-like monolayer nanosheets, promoting maximum of the materials to participate into the electron transfer and become catalytic active sites. In addition, the tulle-like monolayer nanosheet structure can assist in pumping liquid phase electrolyte to the surface of catalysts, owing to the capillary force. The V₂ O₃ /Ni/NF Mott-Schottky catalyst exhibits excellent hydrogen evolution reaction (HER) performance with a low η₁₀ of 54 mV and needs -107 mV to get the current density of -100 mA cm^-2 . Furthermore, V₂ O₃ /Ni/NF Schottky electrocatalyst exhibits excellent urea oxidation reaction activity: 1.40, 1.51, and 1.61 V versus reversible hydrogen electrode (RHE) voltage are required to reach a current density of 100, 500, and 1000 mA cm^-2 , respectively.

Bimetallic Metal-Organic Framework-Derived Graphitic Carbon-Coated Small Co/VN Nanoparticles as Advanced Trifunctional Electrocatalysts

The rational design and construction of multifunctional electrocatalysts with high activity, low cost, and outstanding stability are highly desirable for the development of renewable energy but are still a big challenge. Bimetallic catalysts are a kind of promising candidates, like the hybrids of Co and VN nanoparticles (Co/VN). However, the inevitable aggregation during the preparation and electrochemical process lowers their reactivity and durability. Herein, small Co/VN nanoparticles (4-8 nm) embedded in porous graphitic carbon layers (Co/VN NPs@C) were obtained through the pyrolysis of metal-organic frameworks (MOFs). The synergistic effect of in situ generated Co and VN NPs together with fast electron transfer from graphitic carbon layers renders this catalyst to possess excellent trifunctional performance. More attractively, Co/VN NPs@C as both the anode and the cathode shows a low voltage of 1.58 V when the current density is up to 10 mA cm^-2, exceeding most electrocatalysts based on non-noble metals. The rechargeable Zn-air batteries constructed by Co/VN NPs@C deliver high round-trip efficiency together with a peak power density of 130 mW cm^-2, a specific capacity of 757 mAh g^-1, and desirable stability, outperforming the traditional Zn-air batteries based on the Pt/C and RuO₂ pair. This work opens a promising avenue toward constructing highly effective multifunctional electrocatalysts by designing small-sized nanoparticles with various active sites derived from MOFs.

Bimetallic carbon nanotube encapsulated Fe-Ni catalysts from fast pyrolysis of waste plastics and their oxygen reduction properties

Carbon-based bimetallic electrocatalysts were obtained by catalytic pyrolysis of waste plastics with Fe-Ni-based catalysts and were used as efficient oxygen reduction reaction (ORR) catalysts in this study. The prepared iron-nickel alloy nanoparticles encapsulated in oxidized carbon nanotubes (FeNi-OCNTs) are solid products with a unique structure. Moreover, the chemical composition and structural features of FeNi-OCNTs were determined. The iron-nickel alloy nanoparticles were wrapped in carbon layers, and the carbon nanotubes had an outer diameter of 20-50 nm and micron-scale lengths. FeNi-OCNT with a Fe/Ni ratio of 1:2 (FeNi-OCNT12) exhibited remarkable electrochemical performance as an ORR catalyst with a positive onset potential of 1.01 V (vs. RHE) and a half-wave potential of 0.87 V (vs. RHE), which were comparable to those of a commercial 20% Pt/C catalyst. Furthermore, FeNi-OCNT12 exhibited promising long-term stability and higher tolerance to methanol than the commercial 20% Pt/C catalyst in an alkaline medium. These properties were attributable to the protective OCNT coating of the iron-nickel alloy nanoparticles.

Preparation of Iron- and Nitrogen-Codoped Carbon Nanotubes from Waste Plastics Pyrolysis for the Oxygen Reduction Reaction

A novel method for the preparation of iron- and nitrogen-codoped carbon nanotubes (Fe-N-CNTs) is proposed, based on the catalytic pyrolysis of waste plastics. First, carbon nanotubes are produced from pyrolysis of plastic waste over Fe-Al₂ O₃ ; then, Fe-CNTs and melamine are heated together in an inert atmosphere. Different co-pyrolysis temperatures are tested to optimize the electrocatalyst production. A high doping temperature improves the degree of graphite formation and promotes the conversion of nitrogen into a more stable form. Compared with commercial Pt/C, the electrocatalyst obtained from pyrolysis at 850 °C shows remarkable properties, with an onset potential of 0.943 V versus RHE and a half-wave potential of 0.811 V versus RHE, and even better stability and anti-poisoning properties. In addition, zinc-air battery tests are performed, and the optimized catalyst exhibits a high maximum power density.

Review of Carbon and Graphene Quantum Dots for Sensing

Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluorescent, photoluminescent, chemiluminescent, and electrochemiluminescent properties are what gives them potential in sensing. In this Review, we summarize the basic knowledge on CQDs and GQDs before focusing on their application to sensing thus far followed by a discussion of future directions for research into CQDs- and GQD-based nanomaterials in sensing. With regard to the latter, the authors suggest that with the potential of these nanomaterials in sensing more research is needed on understanding their optical properties and why the synthetic methods influence their properties so much, into methods of surface functionalization that provide greater selectivity in sensing and into new sensing concepts that utilize the virtues of these nanomaterials to give us new or better sensors that could not be achieved in other ways.

Co,N-Codoped porous vanadium nitride nanoplates as superior bifunctional electrocatalysts for hydrogen evolution and oxygen reduction reactions

Developing efficient and low-cost bifunctional electrocatalysts as candidates for Pt-based materials to satisfy commercial applications in the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) is still very challenging. Herein, we show that Co,N-codoped porous vanadium nitride (VCoN) nanoplates are successfully synthesized via a simple one-step pyrolysis protocol without the use of NH3 gas. We also demonstrate that the crystallization, surface chemical state and porosity of vanadium nitride are well modulated by inventively using Co dopants as structural inducers. The resulting VCoN material exhibits an excellent catalytic activity towards the HER in alkaline media, with an extremely low onset potential of -0.03 V, an overpotential of 179 mV at 10 mA cm-2, and a remarkable durability for over 100 h. Moreover, it shows a superior ORR performance, which compares favorably with commercial 20% Pt/C, exhibiting an onset potential of ∼1.02 V, a half-wave potential of ∼0.91 V and a weak potential shift (-5 mV) after 2000 cycles at 1600 rpm in 0.1 M KOH. Such excellent electrocatalytic performance primarily contributes to the unique structural features of the heteroatom N (pyrrolic and graphitic N) and Co codoping in favor of improving the electrical conductivity and the high porosity contributing to exposing numerous catalytic active sites.

A coordination polymer-derived Co3O4/Co-N@NMC composite material as a Zn-air battery cathode electrocatalyst and microwave absorber

Zn-air batteries, promising energy storage equipment with high energy density, light weight and a compact structure, are a perfect power source for electric vehicles. For a Zn-air battery, the activity of the air cathode electrocatalyst plays an important role in its performance. Here, employing a coordination polymer as a precursor, a composite material built from Co3O4 and Co-N active centres with nitrogen-doped mesoporous carbon as a matrix has been synthesized successfully. This composite material possesses outstanding activity and stability in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes. It possesses a small half-wave potential (ORR1/2 = 0.786 V) and low overpotential (OER10 = 1.575 V) for the ORR and OER, respectively. With this composite material as an air cathode electrocatalyst, a rechargeable Zn-air battery was assembled successfully. During the discharge process, the maximum power density of this Zn-air battery is 122 mW cm-2 at 0.76 V. The specific capacity of this battery is 505 mA h g-1 at 25 mA cm-2. The voltage gap between the charge and discharge processes is only 0.744 V at 10 mA cm-2 and 1.308 V at 100 mA cm-2. This rechargeable battery also shows promising stability after long-term charge-discharge experiments. Furthermore, the composite material also exhibits outstanding microwave adsorption properties. Its maximum reflection loss (RL) arrives at -13.9 dB with a thickness of only 1.0 mm. Thus, we find that coordination polymers are an ideal precursor for Zn-air battery cathode electrocatalysts and microwave absorbers.