Cobalt oxide nanocubes interleaved reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction in alkaline medium

A new approach of magnetic field application in miniaturized pipette-tip extraction for trace analysis of four synthetic hormones in breast milk samples

Herein, a novel homemade electrical device was designed, including two pieces of external neodymium magnets, providing a reciprocating magnetic field to introduce a magnetic-assisted dispersive pipette-tip micro solid-phase extraction. To evaluate the performance efficiency of the proposed method, a novel magnetic calcined GO/SiO₂@Co-Fe nanocube sorbent was synthesized, filled into the pipette-tip, exposed to the reciprocating magnetic field, and applied for the preconcentration of some hormone therapy drugs in human biological matrices. The effective adsorption and desorption parameters were optimized using a rotatable central composite design and one-variable-at-a-time approaches. Under the optimized conditions, the target analytes' detection limits were found to be below 0.02 ng mL^-1. Moreover, the calibration curves were linear in the range of 0.03-500.00 ng mL^-1 (R² > 0.9966), with RSDs% less than 7.8 %. Eventually, the established method was applied to extract the analytes from breast milk samples, followed by LC-ESI-MS/MS analysis.

Microwave assisted synthesis of Mn3O4 nanograins intercalated into reduced graphene oxide layers as cathode material for alternative clean power generation energy device

Mn₃O₄ nanograins incorporated into reduced graphene oxide as a nanocomposite electrocatalyst have been synthesized via one-step, facile, and single-pot microwave-assisted hydrothermal technique. The nanocomposites were employed as cathode material of fuel cells for oxygen reduction reaction (ORR). The synthesized product was thoroughly studied by using important characterization, such as XRD for the structure analysis and FESEM and TEM analyses to assess the morphological structures of the material. Raman spectra were employed to study the GO, rGO bands and formation of Mn₃O₄@rGO nanocomposite. FTIR and UV–Vis spectroscopic analysis were used to verify the effective synthesis of the desired electrocatalyst. The Mn₃O₄@rGO-10% nanocomposite with 10 wt% of graphene oxide was used to alter the shiny surface of the working electrode and applied for ORR in O₂ purged 0.5 M KOH electrolyte solution. The Mn₃O₄@rGO-10% nanocomposite electrocatalyst exhibited outstanding performance with an improved current of − 0.738 mA/cm² and shifted overpotential values of − 0.345 V when compared to other controlled electrodes, including the conventionally used Pt/C catalyst generally used for ORR activity. The tolerance of Mn₃O₄@rGO-10% nanocomposite was tested by injecting a higher concentration of methanol, i.e., 0.5 M, and found unsusceptible by methanol crossover. The stability test of the synthesized electrocatalyst after 3000 s was also considered, and it demonstrated excellent current retention of 98% compared to commercially available Pt/C electrocatalyst. The synthesized nanocomposite material could be regarded as an effective and Pt-free electrocatalyst for practical ORR that meets the requirement of low cost, facile fabrication, and adequate stability.

Lewis Acid Site Assisted Bifunctional Activity of Tin Doped Gallium Oxide and Its Application in Rechargeable Zn-Air Batteries

The enhanced safety, superior energy, and power density of rechargeable metal-air batteries make them ideal energy storage systems for application in energy grids and electric vehicles. However, the absence of a cost-effective and stable bifunctional catalyst that can replace expensive platinum (Pt)-based catalyst to promote oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air cathode hinders their broader adaptation. Here, it is demonstrated that Tin (Sn) doped β-gallium oxide (β-Ga₂ O₃ ) in the bulk form can efficiently catalyze ORR and OER and, hence, be applied as the cathode in Zn-air batteries. The Sn-doped β-Ga₂ O₃ sample with 15% Sn (Sn_{x =0.15} -Ga₂ O₃ ) displayed exceptional catalytic activity for a bulk, non-noble metal-based catalyst. When used as a cathode, the excellent electrocatalytic bifunctional activity of Sn_{x =0.15} -Ga₂ O₃ leads to a prototype Zn-air battery with a high-power density of 138 mW cm^-2 and improved cycling stability compared to devices with benchmark Pt-based cathode. The combined experimental and theoretical exploration revealed that the Lewis acid sites in β-Ga₂ O₃ aid in regulating the electron density distribution on the Sn-doped sites, optimize the adsorption energies of reaction intermediates, and facilitate the formation of critical reaction intermediate (O*), leading to enhanced electrocatalytic activity.

Spray-Pyrolytic Tunable Structures of Mn Oxides-Based Composites for Electrocatalytic Activity Improvement in Oxygen Reduction

Hybrid nanomaterials based on manganese, cobalt, and lanthanum oxides of different morphology and phase compositions were prepared using a facile single-step ultrasonic spray pyrolysis (USP) process and tested as electrocatalysts for oxygen reduction reaction (ORR). The structural and morphological characterizations were completed by XRD and SEM-EDS. Electrochemical performance was characterized by cyclic voltammetry and linear sweep voltammetry in a rotating disk electrode assembly. All synthesized materials were found electrocatalytically active for ORR in alkaline media. Two different manganese oxide states were incorporated into a Co3O4 matrix, δ-MnO2 at 500 and 600 °C and manganese (II,III) oxide-Mn3O4 at 800 °C. The difference in crystalline structure revealed flower-like nanosheets for birnessite-MnO2 and well-defined spherical nanoparticles for material based on Mn3O4. Electrochemical responses indicate that the ORR mechanism follows a preceding step of MnO2 reduction to MnOOH. The calculated number of electrons exchanged for the hybrid materials demonstrate a four-electron oxygen reduction pathway and high electrocatalytic activity towards ORR. The comparison of molar catalytic activities points out the importance of the composition and that the synergy of Co and Mn is superior to Co3O4/La2O3 and pristine Mn oxide. The results reveal that synthesized hybrid materials are promising electrocatalysts for ORR.

Flame Spray Pyrolysis Co3O4/CoO as Highly-Efficient Nanocatalyst for Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is the rate-limiting reaction in the cathode side of fuel cells. In the quest for alternatives to Pt-electrodes as cathodes in ORR, appropriate transition metal oxide-based electrocatalysts are needed. In the present work, we have synthesized Co3O4 and CoO/Co3O4 nanostructures using flame spray pyrolysis (FSP), as electrocatalysts for ORR in acidic and alkaline media. A detailed study of the effect of (Co-oxide)/Pt ratio on ORR efficiency shows that the present FSP-made Co-oxides are able to perform ORR at very low-Pt loading, 0.4% of total metal content. In acid medium, an electrode with (5.2% Pt + 4.8% Co3O4), achieved the highest ORR performance (Jmax = 8.31 mA/cm2, E1/2 = 0.66 V). In alkaline medium, superior performance and stability have been achieved by an electrode with (0.4%Pt + 9.6% (CoO/Co3O4)) with ORR activity (Jmax = 3.5 mA/cm2, E1/2 = 0.08 V). Using XRD, XPS, Raman and TEM data, we discuss the structural and electronic aspects of the FSP-made Co-oxide catalysts in relation to the ORR performance. Cyclic voltammetry data indicate that the ORR process involves active sites associated with Co3+ cations at the cobalt oxide surface. Technology-wise, the present work demonstrates that the developed FSP-protocols, constitutes a novel scalable process for production of co-oxides appropriate for oxygen reduction reaction electrodes.

Enhancing oxygen reduction reaction in air-cathode microbial fuel cells treating wastewater with cobalt and nitrogen co-doped ordered mesoporous carbon as cathode catalysts

The sluggish oxygen reduction reaction (ORR) on the cathode severely limits the energy conversion efficiency of microbial fuel cells (MFCs). In this study, cobalt and nitrogen co-doped ordered mesoporous carbon (Co_x-N-OMC) was prepared by heat-treating a mixture of cobalt nitrate, melamine and ordered mesoporous carbon (OMC). The addition of cobalt nitrate remarkably improved the ORR reactivity, compared to the nitrogen-doped OMC catalyst. By optimizing the dosage of cobalt nitrate (x = 0.6, 0.8 and 1.0 g), the Co_0.8-N-OMC catalyst displayed excellent ORR catalytic performances in neutral media with the onset potential of 0.79 V (vs. RHE), half-wave potential of 0.59 V and limiting current density of 5.43 mA/cm², which was comparable to the commercial Pt/C catalyst (0.86 V, 0.60 V and 4.76 mA/cm²). The high activity of Co_0.8-N-OMC catalyst was attributed to the high active surface area, higher total nitrogen amount, and higher relative distribution of graphitic nitrogen and pyrrolic nitrogen species. Furthermore, single chamber microbial fuel cell (SCMFC) with Co_0.8-N-OMC cathode exhibited the highest power density of 389 ± 24 mW/m², chemical oxygen demand (COD) removal of 81.1 ± 2.2% and coulombic efficiency (CE) of 17.2 ± 2.5%. On the other hand, in the Co_1.0-N-OMC catalyst, increasing the cobalt dosage from 0.8 to 1.0 g resulted in more oxidized-N species, and the reduced power generation in SCMFC (360 ± 8 mW/m²). The power generated by these catalysts and results of electrochemical evaluation were strongly correlated with the total nitrogen contents on the catalyst surface. This study demonstrated the feasibility of optimizing the dosage of metal to enhance wastewater treatment capacity.

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.

Recent Progresses in Oxygen Reduction Reaction Electrocatalysts for Electrochemical Energy Applications

Abstract

Electrochemical energy storage systems such as fuel cells and metal–air batteries can be used as clean power sources for electric vehicles. In these systems, one necessary reaction at the cathode is the catalysis of oxygen reduction reaction (ORR), which is the rate-determining factor affecting overall system performance. Therefore, to increase the rate of ORR for enhanced system performances, efficient electrocatalysts are essential. And although ORR electrocatalysts have been intensively explored and developed, significant breakthroughs have yet been achieved in terms of catalytic activity, stability, cost and associated electrochemical system performance. Based on this, this review will comprehensively present the recent progresses of ORR electrocatalysts, including precious metal catalysts, non-precious metal catalysts, single-atom catalysts and metal-free catalysts. In addition, major technical challenges are analyzed and possible future research directions to overcome these challenges are proposed to facilitate further research and development toward practical application.

Graphic Abstract

Reduced graphene/nanostructured cobalt oxide nanocomposite for enhanced electrochemical performance of supercapacitor applications

We demonstrate the preparation of nanostructures cobalt oxide/reduced graphene oxide (Co₃O₄/rGO) nanocomposites by a simple one-step cost-effective hydrothermal technique for possible electrode materials in supercapacitor application. The X-ray diffraction patterns were employed to confirm the nanocomposite crystal system of Co₃O₄/rGO by demonstrating the existence of normal cubic spinel structure of Co₃O₄ in the matrix of Co₃O₄/rGO nanocomposite. FTIR and FT-Raman studies manifested the structural behaviour and quality of prepared Co₃O₄/rGO nanocomposite. The optical properties of the nanocomposite Co₃O₄/rGO have been investigated by UV absorption spectra. The SEM/TEM images showed that the Co₃O₄ nanoparticles in the Co₃O₄/rGO nanocomposites were covered over the surface of the rGO sheets. The electrical properties were analyzed in terms of real and imaginary permittivity, dielectric loss and AC conductivity. The electrocatalytic activities of synthesized Co₃O₄/rGO nanocomposites were determined by cyclic voltammetry and charge-discharge cycle to evaluate the supercapacitive performance. The specific capacitance of 754 Fg^-1 was recorded for Co₃O₄/rGO nanocomposite based electrode in three electrode cell system. The electrode material exhibited an acceptable capability and excellent long-term cyclic stability by maintaining 96% after 1000 continuous cycles. These results showed that the prepared sample could be an ideal candidate for high-energy application as electrode materials. The synthesized Co₃O₄/rGO nanocomposite is a versatile material and can be used in various application such as fuel cells, electrochemical sensors, gas sensors, solar cells, and photocatalysis.

Graphene oxide decorated with zinc oxide nanoflower, silver and titanium dioxide nanoparticles: fabrication, characterization, DNA interaction, and antibacterial activity

The fabrication, characterization, and antibacterial activity of novel nanocomposites based on graphene oxide (GO) nanosheets decorated with silver, titanium dioxide nanoparticles, and zinc oxide nanoflowers were examined. The fabricated nanocomposites were characterized by various techniques including X-ray diffraction, ultraviolet-visible light absorption and fluorescence spectroscopy, Brunauer–Emmett–Teller theory analysis, Fourier transform infrared, and scanning electron microscopy. The antibacterial activity of the GO–metal oxide nanocomposites against two Gram-positive and two Gram-negative bacteria was examined by using the standard counting plate methodology. The results showed that the fabricated nanocomposites on the surface of GO could inhibit the growth of microbial adhered cells, and consequently prevent the process of biofilm formation in food packaging and medical devices. To confirm the antibacterial activity of the examined GO-nanocomposites, we examined their interactions with bovine serum albumin (BSA) and circulating tumor DNA (ctDNA) by steady-state fluorescence spectroscopy. Upon addition of different amounts of fabricated GO-nanocomposites, the fluorescence intensities of the singlet states of BSA and ctDNA were considerably quenched. The higher quenching was observed in the case of GO–Ag–TiO₂@ZnO nanocomposite compared with other control composites.

The fabrication, characterization, and antibacterial activity of novel nanocomposites based on graphene oxide (GO) nanosheets decorated with silver, titanium dioxide nanoparticles, and zinc oxide nanoflowers were examined.

Co/CoOx Nanoparticles Embedded on Carbon for Efficient Catalysis of Oxygen Evolution and Oxygen Reduction Reactions

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are important electrochemical reactions to realize clean energy technologies. Herein, we prepared a hybrid electrocatalyst consisting of Co/CoO_x nanoparticles embedded in amorphous carbon through the simple pyrolysis of cobalt-based zeolitic terephthalate frameworks. The pyrolysis temperature significantly influenced the structure morphology and catalytic behavior. Good contact between Co/CoO_x and amorphous carbon resulted in a high catalytic efficiency. The hybrid obtained under pyrolysis temperature of 600 °C exhibited the highest performance for OER, offering a stable current density of 10 mA cm^-2 at 277 mV in basic media. Besides good OER behavior, it also showed good ORR performance [onset potential: ∼0.87 V vs. the reversible hydrogen electrode (RHE), diffusion-limiting current density: ∼4.9 mA cm^-2 ]. This work describes a novel and efficient catalyst, and greatly expands the scope of low-cost Co-based electrocatalysts for various electrochemical reactions without the need for N-containing ligands.

Supramolecular gel assisted synthesis of Co2P nanosheets as an efficient and stable catalyst for oxygen reduction reaction

Co₂P/C, with nanosheet morphology, is prepared through a facile supramolecular-gel assisted strategy which presents excellent electrocatalytic performance for ORR.

Graphene oxide–metal oxide nanocomposites: fabrication, characterization and removal of cationic rhodamine B dye

The fabrication and characterization of graphene oxide (GO) nanosheets and their reaction with Fe₃O₄ and ZrO₂ metal oxides to form two nanocomposites, namely graphene oxide–iron oxide (GO–Fe₃O₄) and graphene oxide–iron oxide–zirconium oxide (GO–Fe₃O₄@ZrO₂), have been examined. The fabricated nanocomposites were examined using different techniques, e.g.transmission electron microscopy, X-ray diffraction, zeta potential measurement and Fourier transform infrared spectroscopy. Compared to GO, the newly fabricated GO–Fe₃O₄ and GO–Fe₃O₄@ZrO₂ nanocomposites have the advantage of smaller band gaps, which result in increased adsorption capacity and photocatalytic effects. The results also showed the great effect of the examined GO–metal oxide nanocomposites on the decomposition of cationic rhodamine B dye, as indicated by steady-state absorption and fluorescence, time correlated single photon counting and nanosecond laser photolysis techniques. The antibacterial activity of the fabricated GO and GO–metal oxides has been studied against Gram-positive and Gram-negative bacteria.

The fabrication and characterization of graphene oxide–iron oxide and graphene oxide–iron oxide–zirconium oxide nanocomposites have been reported. The decomposition of cationic rhodamine B dye by both nanocomposites has been examined.

B,N,S tri-doped reduced graphite oxide–cobalt oxide composite: a bifunctional electrocatalyst for enhanced oxygen reduction and oxygen evolution reactions

Cobalt oxide with B,N,S tri-doped reduced graphite oxide exhibits synergistic effects to enhance ORR activity.