Co3O4 nanoparticles anchored on nitrogen-doped reduced graphene oxide as a multifunctional catalyst for H2O2 reduction, oxygen reduction and evolution reaction

Designing Nanocomposite-Based Electrochemical Biosensors for Diabetes Mellitus Detection: A Review

This review will unveil the development of a new generation of electrochemical sensors utilizing a transition-metal-oxide-based nanocomposite with varying morphology. There has been considerable discussion on the role of transition metal oxide-based nanocomposite, including iron, nickel, copper, cobalt, zinc, platinum, manganese, conducting polymers, and their composites, in electrochemical and biosensing applications. Utilizing these materials to detect glucose and hydrogen peroxide selectively and sensitively with the correct chemical functionalization is possible. These transition metals and their oxide nanoparticles offer a potential method for electrode modification in sensors. Nanotechnology has made it feasible to develop nanostructured materials for glucose and H2O2 biosensor applications. Highly sensitive and selective biosensors with a low detection limit can detect biomolecules at nanomolar to picomolar (10-9 to 10-12 molar) concentrations to assess physiological and metabolic parameters. By mixing carbon-based materials (graphene oxide) with inorganic nanoparticles, nanocomposite biosensor devices with increased sensitivity can be made using semiconducting nanoparticles, quantum dots, organic polymers, and biomolecules.

Use of Co3O4 nanoparticles with different surface morphologies for removal of toxic substances and investigation of antimicrobial activities via in vivo studies

Co3O4 nanoparticles (NPs) were formed using hydrothermal synthesis method and various surfactants to study the effect of changing surface morphology on catalytic and antibacterial activities. FT-IR, TEM, SEM, BET, XRD, and XPS analyses were performed to characterize the NPs. It was observed that as the morphology of Co3O4 changes, it creates differences in the reduction efficiency of organic dyes and p-nitrophenol (p-NP), which are toxic to living organisms and widely used in industry. The reaction rate constants (Kapp) for Co3O4-urea, Co3O4-ed, and Co3O4-NaOH in the reduction of p-NP were found to be 1.86 × 10-2 s-1, 1.83 × 10-2 s-1, and 2.4 × 10-3 s-1, respectively. In the presence of Co3O4-urea catalyst from the prepared nanoparticles, 99.29% conversion to p-aminophenol (p-AP) was observed, while in the presence of the same catalyst, 98.06% of methylene blue (MB) was removed within 1 h. The antibacterial activity of Co3O4 particles was compared with five standard antibiotics for both gram-positive and gram-negative bacteria. The results obtained indicate that the antimicrobial activity of the synthesized Co3O4 particles has a remarkable inhibitory effect on the growth of various pathogenic microorganisms. The current work could be an innovative and beneficial search for both biomedical and wastewater treatment applications.

CeO2/CuO/NiO hybrid nanostructures loaded on N-doped reduced graphene oxide nanosheets as an efficient electrocatalyst for water oxidation and non-enzymatic glucose detection

In this work, the three-component heterostructure of CeO₂/CuO/NiO was synthesized by a co-precipitation procedure and heating at a temperature of 750 °C. Then, CeO₂/CuO/NiO nanoparticles were successfully supported on N-doped reduced graphene oxide (N-rGO) by a hydrothermal method. The obtained nanomaterials were used as effective electrocatalysts for the oxygen evolution reaction and glucose sensing in an alkaline medium. The results indicated that when CeO₂/CuO/NiO is anchored on N-rGO nanosheets, active catalytic sites increase. On the other hand, N-doped rGO enhances electrical conductivity and electron transfer for water or glucose oxidation. CeO₂/CuO/NiO@N-rGO has a large electrochemically active surface area and more active catalytic positions, and thus exhibits high activity for the OER with a low overpotential of 290 mV, a suitable Tafel slope of 110 mV dec^-1, and superior stability and durability for at least 10 hours. CeO₂/CuO/NiO@N-rGO can also detect glucose with a high sensitivity of 912.7 μA mM^-1 cm^-2, a low detection limit of 0.053 μM, a wide linear range between 0.001 and 24 mM, and a short response time of about 2.9 s. Moreover, the high selectivity and stability of this electrode for glucose sensing show its potential for clinical applications.

Electrocatalytic Oxygen Reduction Reaction of Graphene Oxide and Metal-Free Graphene in an Alkaline Medium

Graphene is a well-known two-dimensional material with a large surface area and is used for numerous applications in a variety of fields. Metal-free carbon materials such as graphene-based materials are widely used as an electrocatalyst for oxygen reduction reactions (ORRs). Recently, more attention has been paid to developing metal-free graphenes doped with heteroatoms such as N, S, and P as efficient electrocatalysts for ORR. In contrast, we found our prepared graphene from graphene oxide (GO) by the pyrolysis method under a nitrogen atmosphere at 900 °C has shown better ORR activity in aqueous 0.1 M potassium hydroxide solution electrolyte as compared with the electrocatalytic activity of pristine GO. At first, we prepared various graphene by pyrolysis of 50 mg and 100 mg of GO in one to three alumina boats and pyrolyzed the samples under a N₂ atmosphere at 900 °C. The prepared samples are named G50-1B to 3B and G100-1B and G100-2B. The prepared GO and graphenes were also analyzed under various characterization techniques to confirm their morphology and structural integrity. The obtained results suggest that the ORR electrocatalytic activity of graphene may differ based on the pyrolysis conditions. We found that G100-1B (E_onset, E_1/2, J_L, and n values of 0.843, 0.774, 4.558, and 3.76) and G100-2B (E_onset, E_1/2, and J_L values of 0.837, 0.737, 4.544, and 3.41) displayed better electrocatalytic ORR activity, as did Pt/C electrode (E_onset, E_1/2, and J_L values of 0.965, 0.864, 5.222, and 3.71, respectively). These results display the wide use of the prepared graphene for ORR and also can be used for fuel cell and metal-air battery applications.

Highly Efficient Electrochemical Sensing of Acetaminophen by Cobalt Oxide-Embedded Nitrogen-Doped Hollow Carbon Spheres

With respect to sensor application investigations, hollow mesoporous carbon sphere-based materials of the spinel type of cobalt oxide (Co₃O₄) and heteroatom-doped materials are gaining popularity. In this contribution, dopamine hydrochloride (DA) and cobalt phthalocyanine (CoPc) precursors were employed to construct a highly homogeneous Co₃O₄-embedded N-doped hollow carbon sphere (Co₃O₄@NHCS) by a straightforward one-step polymerization procedure. The resulting Co₃O₄@NHCS materials may effectively tune the surface area, defect sites, and doping amount of N and Co elements by altering the loading amount of CoPc. The relatively high surface area, greater spherical wall thickness, enriched defect sites, and better extent of N and Co sites are all visible in the best 200 mg loaded Co₃O₄@NHCS-2 material. This leads to significant improvement in pyridine and graphitic N site concentrations, which offers exceptional electrochemical performance. Electrochemical analysis was used to study the electrocatalytic activity of Co₃O₄@NHCSs towards the sensing of pharmacologically active significant compounds (acetaminophen). Excellent sensor properties include the linear range (0.001-0.2 and 1.0-8.0 mM), sensitivity, limit of detection (0.07 and 0.11 μM), and selectivity in the modified Co₃O₄@NHCSs/GCE. The authentic sample (acetaminophen tablet) produces a satisfactory result when used practically.

Co-based Catalysts for Selective H2 O2 Electroproduction via 2-electron Oxygen Reduction Reaction

Electrochemical production of hydrogen peroxide (H₂ O₂ ) via two-electron oxygen reduction reaction (ORR) process is emerging as a promising alternative method to the conventional anthraquinone process. To realize high-efficiency H₂ O₂ electrosynthesis, robust and low cost electrocatalysts have been intensively pursued, among which Co-based catalysts attract particular research interests due to the earth-abundance and high selectivity. Here, we provide a comprehensive review on the advancement of Co-based electrocatalyst for H₂ O₂ electroproduction. The fundamental chemistry of 2-electron ORR is discussed firstly for guiding the rational design of electrocatalysts. Subsequently, the development of Co-based electrocatalysts involving nanoparticles, compounds and single atom catalysts is summarized with the focus on active site identification, structure regulation and mechanism understanding. Moreover, the current challenges and future directions of the Co-based electrocatalysts are briefly summarized in this review.

Electrophoretic Deposition of Co3O4 Particles/Reduced Graphene Oxide Composites for Efficient Non-Enzymatic H2O2 Sensing

In this work, the facile fabrication of Co₃O₄ particles/reduced graphene oxide (Co₃O₄/rGO) composites on Indium tin oxide (ITO) slide was achieved by an electrophoretic deposition and annealing process. The deposition time and ratio of the precursors were optimized. Structural characterization and chemical composition investigation indicated successful loading of Co₃O₄ particles on graphene sheets. When applied as a non-enzymatic H₂O₂ sensor, Co₃O₄/rGO showed significant electrocatalytic activity, with a wide linear range (0.1-19.5 mM) and high sensitivity (0.2247 mA mM^-1 cm^-2). The good anti-interference ability, reproducibility, and long-term stability of the constructed sensor were also presented. The application of Co₃O₄/rGO in real sample analysis was evaluated in human urine sample with satisfactory results, indicating the feasibility of the sensor in physiological and medical applications.

Square-Facet Nanobar MOF-Derived Co3O4@Co/N-doped CNT Core-Shell-based Nanocomposites as Cathode Materials for High-Performance Supercapacitor Studies

The binary as well as ternary nanocomposites of the square-facet nanobar Co-MOF-derived Co₃O₄@Co/N-CNTs (N-CNTs: nitrogen-doped carbon nanotubes) with Ag NPs and rGO have been synthesized via an easy wet chemical route, and their supercapacitor behavior was then studied. At a controlled pH of the precursor solution, square-facet nanobars of Co-MOF were first synthesized by the solvothermal method and then pyrolyzed under a controlled nitrogen atmosphere to get a core-shell system of Co₃O₄@Co/N-CNTs. In the second step, different compositions of Co₃O₄@Co/N-CNT core-shell structures were formed by an ex-situ method with Ag NPs and rGO moieties. Among several bare, binary, and ternary compositions tested in 6 M aqueous KOH electrolyte, a ternary nanocomposite having a 7.0:1.5:1.5 stoichiometric ratio of Co₃O₄@Co/N-CNT, Ag NPs, and rGO, respectively, reported the highest specific capacitance (3393.8 F g^-1 at 5 mV s^-1). The optimized nanocomposite showed the energy density, power density, and Coulombic efficiency of 74.1 W h.kg^-1, 443.7 W.kg^-1, and 101.3%, respectively, with excellent electrochemical stability. After testing an asymmetrical supercapacitor with a Co₃O₄@Co/N-CNT/Ag NPs/rGO/nickel foam cathode and an activated carbon/nickel foam anode, it showed 4.9 W h.kg^-1 of energy density and 5000.0 W.kg^-1 of power density.

Perovskite-Based Nanocomposite Electrocatalysts: An Alternative to Platinum ORR Catalyst in Microbial Fuel Cell Cathodes

Microbial fuel cells (MFCs) are biochemical systems having the benefit of producing green energy through the microbial degradation of organic contaminants in wastewater. The efficiency of MFCs largely depends on the cathode oxygen reduction reaction (ORR). A preferable ORR catalyst must have good oxygen reduction kinetics, high conductivity and durability, together with cost-effectiveness. Platinum-based electrodes are considered a state-of-the-art ORR catalyst. However, the scarcity and higher cost of Pt are the main challenges for the commercialization of MFCs; therefore, in search of alternative, cost-effective catalysts, those such as doped carbons and transition-metal-based electrocatalysts have been researched for more than a decade. Recently, perovskite-oxide-based nanocomposites have emerged as a potential ORR catalyst due to their versatile elemental composition, molecular mechanism and the scope of nanoengineering for further developments. In this article, we discuss various studies conducted and opportunities associated with perovskite-based catalysts for ORR in MFCs. Special focus is given to a basic understanding of the ORR reaction mechanism through oxygen vacancy, modification of its microstructure by introducing alkaline earth metals, electron transfer pathways and the synergistic effect of perovskite and carbon. At the end, we also propose various challenges and prospects to further improve the ORR activity of perovskite-based catalysts.

Carbonaceous Oxygen Evolution Reaction Catalysts: From Defect and Doping-Induced Activity over Hybrid Compounds to Ordered Framework Structures

Oxygen evolution reaction (OER) is expected to be of great importance for the future energy conversion and storage in form of hydrogen by water electrolysis. Besides the traditional noble-metal or transition metal oxide-based catalysts, carbonaceous electrocatalysts are of great interest due to their huge structural and compositional variety and unrestricted abundance. This review provides a summary of recent advances in the field of carbon-based OER catalysts ranging from "pure" or unintentionally doped carbon allotropes over heteroatom-doped carbonaceous materials and carbon/transition metal compounds to metal oxide composites where the role of carbon is mainly assigned to be a conductive support. Furthermore, the review discusses the recent developments in the field of ordered carbon framework structures (metal organic framework and covalent organic framework structures) that potentially allow a rational design of heteroatom-doped 3D porous structures with defined composition and spatial arrangement of doping atoms to deepen the understanding on the OER mechanism on carbonaceous structures in the future. Besides introducing the structural and compositional origin of electrochemical activity, the review discusses the mechanism of the catalytic activity of carbonaceous materials, their stability under OER conditions, and potential synergistic effects in combination with metal (or metal oxide) co-catalysts.

A highly durable CoOx/N-doped graphitized-nano-diamond electrocatalyst for oxygen reduction reaction

Oxygen reduction reaction (ORR) occupies a pivotal position in fuel cell applications, and it is a challenge to obtain highly durable ORR catalysts. Herein, porous cobalt oxide microsphere growing at the surface of on nitrogen-doped graphitized-nano-diamond (CoO_x/N-GND) was prepared using hydrothermal and subsequent heat treatment process. Porous cobalt oxide of high specific surface area could expose more surface Co²⁺that act as active sites than bulk one does. The doping of nitrogen also promotes the catalytic activity. Besides, nano-diamond (ND) ofsp³hybrid structure was used as an electronic conduction carriers of ultrahigh stability to improve the durability of catalytic composite. Prepared CoO_x/N-GND shows a satisfactory half-wave potential of 0.82 V (versus RHE), which is close to that of Pt/C (0.85 V), an excellent methanol tolerance and a lower activity loss after 5000 cycles. These merits inspire the application of CoO_x/N-GND as the cathode of Zn-air battery and the battery performance was evaluated in this work. In general, this work highlights an innovate approach to design and prepare highly durable catalyst.

Carbon-based 0D/1D/2D assembly with desired structures and defect states as non-metal bifunctional electrocatalyst for zinc-air battery

For the design of electrocatalysts, the combination between components and the regulation of material structures tend to be neglected, giving rise to the constraint of catalytic performance and durability. Herein, we developed a graphene oxide quantum dots (GOQDs) with enhanced oxygen content by a one-step cutting method. Then, one-dimensional (1D) carbon nanotubes and two-dimensional (2D) reduced graphene oxide are crosslinked and self-assembled, thus attracting unsaturated-bond-riches GOODs (0D) to uniformly attach to the skeleton, simultaneously achieving nitrogen and sulfur co-doping. To the best of our knowledge, there is no report to prepare bifunctional electrocatalyst with GOQDs. Electrochemical tests show that even without metal-doping, the novel non-metal bifunctional electrocatalyst (N,S-GOQD-RGO/CNT) exhibits a higher half-wave potential (0.84 V) and enhanced limiting current density (5.88 mA cm^-2) than commercial Pt/C catalyst. The density functional theory is implemented to reveal the coordination of nitrogen and sulfur co-doping on GOQDs, which results in the improvement of overall catalytic active sites. Furthermore, the rechargeable zinc-air battery based on N,S-GOQD-RGO/CNT exhibits a maximum power density of 134.3 mW cm^-2, open circuit potential of 1.414 V, which is better than Pt/C+Ru/C mixed material. The obtained N,S-GOQD-RGO/CNT will provide a perspective application in fuel cells.

Design Strategies of Non-Noble Metal-Based Electrocatalysts for Two-Electron Oxygen Reduction to Hydrogen Peroxide

Hydrogen peroxide (H₂ O₂ ) is a highly value-added and environmentally friendly chemical with various applications. The production of H₂ O₂ by electrocatalytic 2e^- oxygen reduction reaction (ORR) has drawn considerable research attention, with a view to replacing the currently established anthraquinone process. Electrocatalysts with low cost, high activity, high selectivity, and superior stability are in high demand to realize precise control over electrochemical H₂ O₂ synthesis by 2e^- ORR and the feasible commercialization of this system. This Review introduces a comprehensive overview of non-noble metal-based catalysts for electrochemical oxygen reduction to afford H₂ O₂ , providing an insight into catalyst design and corresponding reaction mechanisms. It starts with an in-depth discussion on the origins of 2e^- /4e^- selectivity towards ORR for catalysts. Recent advances in design strategies for non-noble metal-based catalysts, including carbon nanomaterials and transition metal-based materials, for electrochemical oxygen reduction to H₂ O₂ are then discussed, with an emphasis on the effects of electronic structure, nanostructure, and surface properties on catalytic performance. Finally, future challenges and opportunities are proposed for the further development of H₂ O₂ electrogeneration through 2e^- ORR, from the standpoints of mechanistic studies and practical application.

Solid-State Ball-Milling of Co3O4 Nano/Microspheres and Carbon Black Endorsed LaMnO3 Perovskite Catalyst for Bifunctional Oxygen Electrocatalysis

Developing a highly stable and non-precious, low-cost, bifunctional electrocatalyst is essential for energy storage and energy conversion devices due to the increasing demand from the consumers. Therefore, the fabrication of a bifunctional electrocatalyst is an emerging focus for the promotion and dissemination of energy storage/conversion devices. Spinel and perovskite transition metal oxides have been widely explored as efficient bifunctional electrocatalysts to replace the noble metals in fuel cell and metal-air batteries. In this work, we developed a bifunctional catalyst for oxygen reduction and oxygen evolution reaction (ORR/OER) study using the mechanochemical route coupling of cobalt oxide nano/microspheres and carbon black particles incorporated lanthanum manganite perovskite (LaMnO3@C-Co3O4) composite. It was synthesized through a simple and less-time consuming solid-state ball-milling method. The synthesized LaMnO3@C-Co3O4 composite was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction spectroscopy, and micro-Raman spectroscopy techniques. The electrocatalysis results showed excellent electrochemical activity towards ORR/OER kinetics using LaMnO3@C-Co3O4 catalyst, as compared with Pt/C, bare LaMnO3@C, and LaMnO3@C-RuO2 catalysts. The observed results suggested that the newly developed LaMnO3@C-Co3O4 electrocatalyst can be used as a potential candidate for air-cathodes in fuel cell and metal-air batteries.

Zeolite-Y encapsulated cobalt(ii) Schiff-base complexes employed for photocatalytic dye-degradation and upcycling CO2

Planar cobalt(ii) Schiff-base complexes show modified structural and functional properties after encapsulation inside zeolite-Y.

Sacrificial ZnO nanorods drive N and O dual-doped carbon towards trifunctional electrocatalysts for Zn–air batteries and self-powered water splitting devices

Integrated energy systems (IES) have attracted increasing attention in recent years.

Facile Synthesis of Co3O4@CoO@Co Gradient Core@Shell Nanoparticles and Their Applications for Oxygen Evolution and Reduction in Alkaline Electrolytes

We demonstrate a facile fabrication scheme for Co₃O₄@CoO@Co (gradient core@shell) nanoparticles on graphene and explore their electrocatalytic potentials for an oxygen evolution reaction (OER) and an oxygen reduction reaction (ORR) in alkaline electrolytes. The synthetic approach begins with the preparation of Co₃O₄ nanoparticles via a hydrothermal process, which is followed by a controlled hydrogen reduction treatment to render nanoparticles with radial constituents of Co₃O₄/CoO/Co (inside/outside). X-ray diffraction patterns confirm the formation of crystalline Co₃O₄ nanoparticles, and their gradual transformation to cubic CoO and fcc Co on the surface. Images from transmission electron microscope reveal a core@shell microstructure. These Co₃O₄@CoO@Co nanoparticles show impressive activities and durability for OER. For ORR electrocatalysis, the Co₃O₄@CoO@Co nanoparticles are subjected to a galvanic displacement reaction in which the surface Co atoms undergo oxidative dissolution for the reduction of Pt ions from the electrolyte to form Co₃O₄@Pt nanoparticles. With commercial Pt/C as a benchmark, we determine the ORR activities in sequence of Pt/C > Co₃O₄@Pt > Co₃O₄. Measurements from a rotation disk electrode at various rotation speeds indicate a 4-electron transfer path for Co₃O₄@Pt. In addition, the specific activity of Co₃O₄@Pt is more than two times greater than that of Pt/C.

Recent Developments for Aluminum–Air Batteries

Abstract

Environmental concerns such as climate change due to rapid population growth are becoming increasingly serious and require amelioration. One solution is to create large capacity batteries that can be applied in electricity-based applications to lessen dependence on petroleum. Here, aluminum–air batteries are considered to be promising for next-generation energy storage applications due to a high theoretical energy density of 8.1 kWh kg−1 that is significantly larger than that of the current lithium-ion batteries. Based on this, this review will present the fundamentals and challenges involved in the fabrication of aluminum–air batteries in terms of individual components, including aluminum anodes, electrolytes and air cathodes. In addition, this review will discuss the possibility of creating rechargeable aluminum–air batteries.

Graphic Abstract

The development of an artificial neural network – genetic algorithm model (ANN-GA) for the adsorption and photocatalysis of methylene blue on a novel sulfur–nitrogen co-doped Fe2O3 nanostructure surface

In this research an S-N doped Fe₂O₃ nanostructure is synthesized and its adsorption ability and photocatalytic activity were evaluated. The optimum experimental conditions were obtained and an ANN-GA model was proposed for predicting experimental values.

Cobalt oxide nanoparticle-decorated reduced graphene oxide (Co3O4–rGO): active and sustainable nanoelectrodes for water oxidation reaction

Herein, cobalt oxide (Co₃O₄)-decorated reduced graphene oxide (rGO)-based nanoelectrodes were fabricated by the chemical reduction method for electrocatalytic water oxidation reactions.

Hierarchical Co3O4 nanorods anchored on nitrogen doped reduced graphene oxide: a highly efficient bifunctional electrocatalyst for rechargeable Zn–air batteries

The electrocatalytic activity of the N-rGO/Co₃O₄ nanocomposites was tuned towards highly efficient bifunctional air-cathodes for Zn–Air batteries.

Two-Dimensional Materials on the Rocks: Positive and Negative Role of Dopants and Impurities in Electrochemistry

Two-dimensional (2D) materials, such as graphene and transition-metal chalcogenides, were shown in many works as very potent catalysts for industrially important electrochemical reactions, such as oxygen reduction, hydrogen and oxygen evolution, and carbon dioxide reduction. We critically discuss here the development in the field, showing that not only dopants but also impurities can have dramatic effects on catalysis. Note here that the difference between dopant and impurity is merely semantic-dopant is an impurity deliberately added to the material. We contest the general belief that all doping has a positive effect on electrocatalysis. We show that in many cases, dopants actually inhibit the electrochemistry of 2D materials. This review provides a balanced view of the field of 2D materials electrocatalysis.

Atomic layer deposition of Co3O4 nanocrystals on N-doped electrospun carbon nanofibers for oxygen reduction and oxygen evolution reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are considered as the two crucial reactions in key renewable-energy technologies including fuel cells and water splitting. Despite promising research progress in the preparation of various non-noble metal based electrocatalysts, it is still highly challenging but desirable to develop novel fabrication strategies to synthesize highly active and cost-effective ORR/OER bifunctional electrocatalysts in a precisely controlled manner. Herein, we report atomic layer deposition (ALD) of highly monodisperse Co3O4 nanocrystals of different sizes on N-doped electrospun carbon nanofibers (nCNFs) as high performance bifunctional catalysts (Co@nCNFs) for the ORR and OER. Co@nCNFs (with an average Co3O4 particle size of ∼3 nm) show high ORR performance exhibiting an onset potential of 0.87 V with a low Tafel slope of 119 mV dec-1 approaching that of commercial Pt/C. Similarly, the Co@nCNF electrocatalyst showed remarkable catalytic activity in the OER. The turnover frequency (TOF) value determined at an overpotential of 550 mV for the Co@nCNFs is ∼0.14 s-1 which is ca. 3 and ca. 15-fold higher than those of bulk Co (∼0.05 s-1) and the standard state-of-the-art IrO x (0.0089 s-1) catalyst, respectively. This work will open new possibilities for fabrication of inexpensive non-noble metal materials in highly controlled manner for applications as bifunctional ORR/OER electrocatalysis.

Oxygen Electrocatalysis at MnIII-O x-C Hybrid Heterojunction: An Electronic Synergy or Cooperative Catalysis?

The interface at the metal oxide-carbon hybrid heterojunction is the source to the well-known "synergistic effect" in catalysis. Understanding the structure-function properties is key for designing more advanced catalyst-support systems. Using a model Mn^III-O _x single-layer catalyst on carbon, we herein report a full elucidation to the catalytic synergism at the hybrid heterojunction in the oxygen reduction reaction (ORR). The successful fabrication of the single-layer catalyst from bottom-up is fully characterized by the X-ray absorption fine structure and high-resolution transmission electron microscopy. For oxygen electrocatalysis over this model hybrid heterostructure, our results, from both theory and experiment, show that the synergistic ORR truly undergoes a cooperated two-step electrocatalysis with catalytic promotion (Δ E_onset = 60 mV) near the heterojunction and over the single-layer catalyst through an interfacial electronic interplay, rather than an abstruse transition towards a one-step dissociative pathway. Finally, we report a superior peroxide-reducing activity of 432.5 mA cm^-2 mg_(M)^-1 over the Mn^III-O _x single-layer.

Co/rGO synthesized via the alcohol-thermal method as a heterogeneous catalyst for the highly efficient oxidation of ethylbenzene with oxygen

Co₃O₄ nanoparticles uniformly dispersed on reduced graphene oxide (Co/rGO) were synthesized by the alcohol-thermal method as a highly efficient catalyst with initiator NHPI for the selective oxidation of ethylbenzene to acetophenone using O₂ as a green oxidant.

Mn3O4 Quantum Dots Supported on Nitrogen-Doped Partially Exfoliated Multiwall Carbon Nanotubes as Oxygen Reduction Electrocatalysts for High-Performance Zn-Air Batteries

Highly efficient and low-cost nonprecious metal electrocatalysts that favor a four-electron pathway for the oxygen reduction reaction (ORR) are essential for high-performance metal-air batteries. Herein, we show an ultrasonication-assisted synthesis method to prepare Mn₃O₄ quantum dots (QDs, ca. 2 nm) anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes (Mn₃O₄ QDs/N-p-MCNTs) as a high-performance ORR catalyst. The Mn₃O₄ QDs/N-p-MCNTs facilitated the four-electron pathway for the ORR and exhibited sufficient catalytic activity with an onset potential of 0.850 V (vs reversible hydrogen electrode), which is only 38 mV less positive than that of Pt/C (0.888 V). In addition, the Mn₃O₄ QDs/N-p-MCNTs demonstrated superior stability than Pt/C in alkaline solutions. Furthermore, a Zn-air battery using the Mn₃O₄ QDs/N-p-MCNTs cathode catalyst successfully generated a specific capacity of 745 mA h g^-1 at 10 mA cm^-2 without the loss of voltage after continuous discharging for 105 h. The superior ORR activity of Mn₃O₄ QDs/N-p-MCNTs can be ascribed to the homogeneous Mn₃O₄ QDs loaded onto the N-doped carbon skeleton and the synergistic effects of Mn₃O₄ QDs, nitrogen, and carbon nanotubes. The interface binding energy of -3.35 eV calculated by the first-principles density functional theory method illustrated the high stability of the QD-anchored catalyst. The most stable adsorption structure of O₂, at the interface between Mn₃O₄ QDs and the graphene layer, had the binding energy of -1.17 eV, greatly enhancing the ORR activity. In addition to the high ORR activity and stability, the cost of production of Mn₃O₄ QDs/N-p-MCNTs is low, which will broadly facilitate the real application of metal-air batteries.

Electrocatalytic N-Doped Graphitic Nanofiber - Metal/Metal Oxide Nanoparticle Composites

Carbon-based nanocomposites have shown promising results in replacing commercial Pt/C as high-performance, low cost, nonprecious metal-based oxygen reduction reaction (ORR) catalysts. Developing unique nanostructures of active components (e.g., metal oxides) and carbon materials is essential for their application in next generation electrode materials for fuel cells and metal-air batteries. Herein, a general approach for the production of 1D porous nitrogen-doped graphitic carbon fibers embedded with active ORR components, (M/MO_x , i.e., metal or metal oxide nanoparticles) using a facile two-step electrospinning and annealing process is reported. Metal nanoparticles/nanoclusters nucleate within the polymer nanofibers and subsequently catalyze graphitization of the surrounding polymer matrix and following oxidation, create an interconnected graphite-metal oxide framework with large pore channels, considerable active sites, and high specific surface area. The metal/metal oxide@N-doped graphitic carbon fibers, especially Co₃ O₄ , exhibit comparable ORR catalytic activity but superior stability and methanol tolerance versus Pt in alkaline solutions, which can be ascribed to the synergistic chemical coupling effects between Co₃ O₄ and robust 1D porous structures composed of interconnected N-doped graphitic nanocarbon rings. This finding provides a novel insight into the design of functional electrocatalysts using electrospun carbon nanomaterials for their application in energy storage and conversion fields.

Facile synthesis and catalytic performance of Co3O4 nanosheets in situ formed on reduced graphene oxide modified Ni foam

A three-dimensional (3D) catalyst electrode of Co₃O₄ nanosheets in situ formed on reduced graphene oxide modified Ni foam (Co₃O₄/rGO@Ni foam) for H₂O₂ electroreduction is prepared by a two-step hydrothermal method. In the first step, graphene oxide sheets are reduced and formed on the skeleton of Ni foam and Co₃O₄ nanosheets are synthesized intermixed with the rGO sheets through the second step. The Co₃O₄ nanosheets are made up of plentiful nanoparticles and there are many nanoholes among these nanoparticles which are beneficial for the sufficient contact between H₂O₂ and the catalyst. The morphology and phase composition of the Co₃O₄/rGO@Ni foam electrode are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrocatalytic activity of the as-prepared electrode is investigated by cyclic voltammetry (CV) and chronoamperometry (CA). From the results, it can be seen that in 2 mol L^-1 NaOH and 0.5 mol L^-1 H₂O₂, the reduction current density of H₂O₂ on the Co₃O₄/rGO@Ni foam electrode is 450 mA cm^-2 at -0.8 V which is much higher than that on Co₃O₄ directly supported on Ni foam. This obvious increase of the current density can be attributed to the increase of the surface area of the electrode after the addition of rGO. Also, the interpenetration of rGO and Co₃O₄ nanosheets improves the electron and ion transport ability of the electrode which leads to a good electrocatalytic activity and stability of the Co₃O₄/rGO@Ni foam electrode.

The CoOOH-TMB oxidative system for use in colorimetric and test strip based determination of ascorbic acid

The authors report that cobalt oxyhydroxide (CoOOH) nanoflakes possess intrinsic oxidizing ability to directly oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to form a blue colored product (oxTMB) even in the absence of H₂O₂ and oxygen. In the presence of ascorbic acid (AA), less of the blue product is formed because AA reduces oxTMB. These findings constitute a new scheme for colorimetric detection of AA. Absorbance, best measured at 652 nm, linearly drops in the 10 nM to 1 μM AA concentration range, and the limit of detection is 5 nM (at an S/N ratio of 3). The reaction is complete within <5 s and highly selective. A strip test has been designed for fast and on-spot visual detection of AA. The method was applied to the direct estimation of AA in the microdialysate of brain, and also in soft drink samples. The strip test is considered to be a promising tool for the rapid screening of AA in brain and commercial samples. Graphic abstract Schematic of the CoOOH-TMB colorimetric system that exhibits a high selectivity for ascorbic acid (AA). A strip test has been designed for fast and on-spot visual detection of AA.

Architecting a Mesoporous N-Doped Graphitic Carbon Framework Encapsulating CoTe2 as an Efficient Oxygen Evolution Electrocatalyst

To improve the efficiency of cobalt-based catalysts for water electrolysis, tremendous efforts have been dedicated to tuning the composition, morphology, size, and structure of the materials. We report here a facile preparation of orthorhombic CoTe₂ nanocrystals embedded in an N-doped graphitic carbon matrix to form a 3D architecture with a size of ∼500 nm and abundant mesopores of ∼4 nm for the oxygen evolution reaction (OER). The hybrid electrocatalyst delivers a small overpotential of 300 mV at 10 mA cm^-2, which is much lower than that for pristine CoTe₂ powder. After cycling for 2000 cycles or driving continual OER for 20 h, only a slight loss is observed. The mesoporous 3D architecture and the strong interaction between N-doped graphitic carbon and CoTe₂ are responsible for the enhancement of the electrocatalytic performance.