Alginate derived Co3O4/Co nanoparticles decorated in N-doped porous carbon as an efficient bifunctional catalyst for oxygen evolution and reduction reactions

Electrochemical Crosslinking of Alginate-Towards Doped Carbons for Oxygen Reduction

Electrochemical crosslinking of alginate strands by in situ iron oxidation was explored using a potentiostatic regime. Carbon-based materials co-doped with iron, nitrogen, and/or sulfur were prepared via electrolyte composition variation with a nitrogen-rich compound (rivanol) or through post-treatments with sodium sulfide. Nanometer-sized iron particles were confirmed by transmission and field emission scanning electron microscopy in all samples as a consequence of the homogeneous dispersion of iron in the alginate scaffold and its concomitant growth-limiting effect of alginate chains. Raman spectra confirmed a rise in structural disorder with rivanol/Na2S treatment, which points to more defect sites and edges known to be active sites for oxygen reduction. Fourier transform infrared (FTIR) spectra confirmed the presence of different iron, nitrogen, and sulfur species, with a marked difference between Na2S treated/untreated samples. The most positive onset potential (-0.26 V vs. saturated calomel electrode, SCE) was evidenced for the sample co-doped with N, S, and Fe, surpassing the activity of those with single and/or double doping. The mechanism of oxygen reduction in 0.1 M KOH was dominated by the 2e- reduction pathway at low overpotentials and shifted towards complete 4e- reduction at the most negative explored values. The presented results put forward electrochemically formed alginate gels functionalized by homogeneously dispersed multivalent cations as an excellent starting point in nanomaterial design and engineering.

Comparison of the Fluidized State Stability from Radioactive Particle Tracking Results

Currently, various industrial processes are carried out in fluidized bed reactors. Knowing its internal dynamics is fundamental for the intensification of these processes. This work assesses the motion of fluidized calcium alginate spheres under the influence of an upward fluid flow within a 1.2 m high and 0.1 m inner diameter acrylic column. The liquid–solid fluidized bed was compared with a gas–liquid–solid fluidized bed operation mode in terms of mixing behavior. The radioactive particle tracking technique is a proper methodology to study the internal dynamics of these kinds of equipment. Data gathered were analyzed with Shannon entropy as a dynamic mixing measure. Mixing times were found to be between 1 and 2.5 seconds for both fluidization modes. The liquid–solid fluidized bed presents a rather smooth mixing time profile along the column. On the other hand, the gas–liquid–solid fluidized bed showed high sensitivity of entropy production with height, reaching a sharp tendency break at the second quartile of the column. The Glansdorff–Prigogine stability measure can accurately capture flow regime transitions of the gas–liquid–solid fluidized bed, allowing it to be used to construct reliable operative windows for fluidization equipment.

Elucidating the Role of Oxide-Oxide/Carbon Interfaces of CuOx-CeO2/C in Boosting Electrocatalytic Performance

Herein, we report the synthesis and bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of a CuO_x-CeO₂/C electrocatalyst (EC) with rich oxide-oxide and oxide-carbon interfaces. It not only demonstrates a smaller Tafel slope (65 mV dec^-1) and higher limiting current density (-5.03 mA cm^-2) but also exhibits an onset potential (-0.10 V vs Ag/AgCl) comparable to that of benchmark Pt/C. Besides undergoing the favorable direct four-electron ORR pathway, it unveils a loss of 23% of its initial current after 6 h of a stability test and a negative shift of 4 mV in the half-wave potential after the accelerated durability test compared to the corresponding current loss of 28% and negative shift of 20 mV for Pt/C. It also reveals remarkable OER activity in an alkaline medium with a low onset potential (0.20 V) and a smaller Tafel slope (177 mV dec^-1). The bifunctional ORR/OER activity of CuO_x-CeO₂/C EC can be ascribed to the synergistic effects, its unique structure with enriched oxygen vacancies owing to the presence of Ce⁴⁺/Ce³⁺, robust oxide-oxide and oxide-carbon heterointerfaces, and homogeneous dispersion of oxides over the carbon bed, which facilitates faster electronic conduction.

Pt-Co3O4 Superstructures by One-Pot Reduction/Precipitation in Bicontinuous Microemulsion for Electrocatalytic Oxygen Evolution Reaction

Bicontinuous microemulsions (BCME) were used to synthesize hierarchical superstructures (HSs) of Pt-Co3O4 by reduction/precipitation. BCMEs possess water and oil nanochannels, and therefore, both hydrophilic and lipophilic precursors can be used. Thus, PtAq-CoAq, PtAq-CoOi, PtOi-CoAq and PtOi-CoOi were prepared (where Aq and Oi stand for the precursor present in aqueous or oily phase, respectively). The characterization of the Pt-Co3O4-HS confirmed the formation of metallic Pt and Co3O4 whose composition and morphology are controlled by the initial pH and precursor combination, determining the presence of the reducing/precipitant species in the reaction media. The electrocatalytic activity of the Pt-Co3O4-HSs for oxygen evolution reaction (OER) was investigated using linear sweep voltammetry in 0.1 M KOH and compared with Pt-HS. The lowest onset overpotentials for Pt-Co3O4-Hs were achieved with PtOi-CoOi (1.46 V vs. RHE), while the lowest overpotential at a current density of 10 mA cm−2 (η10) was obtained for the PtAq-CoAq (381 mV). Tafel slopes were 102, 89, 157 and 92 mV dec−1, for PtAq-CoAq, PtAq-CoOi, PtOi-CoAq and PtOi-CoOi, respectively. The Pt-Co3O4-HSs showed a better performance than Pt-HS. Our work shows that the properties and performance of metal–metal oxide HSs obtained in BCMEs depend on the phases in which the precursors are present.

Advanced non-noble materials in bifunctional catalysts for ORR and OER toward aqueous metal-air batteries

The catalyst in the oxygen electrode is the core component of the aqueous metal-air battery, which plays a vital role in the determination of the open circuit potential, energy density, and cycle life of the battery. For rechargeable aqueous metal-air batteries, the catalyst should have both good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance. Compared with precious metal catalysts, non-precious metal materials have more advantages in terms of abundant resource reserves and low prices. Over the past few years, great efforts have been made in the development of non-precious metal bifunctional catalysts. This review selectively evaluates the advantages, disadvantages and development status of recent advanced materials including pure carbon materials, carbon-based metal materials and carbon-free materials as bifunctional oxygen catalysts. Preliminary improvement strategies are formulated to make up for the deficiency of each material. The development prospects and challenges facing bifunctional catalysts in the future are also discussed.

Coordinating Adsorption and Catalytic Activity of Polysulfide on Hierarchical Integrated Electrodes for High-Performance Flexible Li-S Batteries

Lithium-sulfur (Li-S) batteries have been known to be a promising substitute because of their much higher theoretical energy density than that of traditional Li-ion batteries. However, the low utilization of sulfur caused by the poor conductivity of sulfur and shuttle of lithium polysulfides (LiPSs) severely restrict the commercial application of Li-S batteries, especially in flexible wearable devices. Herein, a hierarchical nitrogen-doped carbon nanotube (NCNT)@Co-Co₃O₄ nanowire array (NWA)-integrated electrode was developed based on the rational design of density functional theory calculations, which shows simultaneous confinement adsorption and catalysis conversion of LiPSs. In situ Raman spectra further proved that the NCNT@Co-Co₃O₄ NWAs exhibit sufficient adsorption capacity and high catalytic conversion of LiPSs. As a result, the NCNT@Co-Co₃O₄@S electrode exhibited the desirable specific capacity and excellent cyclic stability at both low and high sulfur loadings. Moreover, pouch cells with the NCNT@Co-Co₃O₄@S cathode show higher capacity under flat or bending states and longer cycle stability than that of the reported results. This work provides a new approach for the development of high-performance Li-S batteries toward future wearable electronics.

A Co3O4/MnCO3 heterojunction on three-dimensional nickel foam for an enhanced oxygen evolution reaction

A Co₃O₄/MnCO₃ heterojunction on NF with a unique architecture exhibits prominent OER activity and stability that is superior to most Co₃O₄-based catalysts.

Catalytically Active Carbon From Cattail Fibers for Electrochemical Reduction Reaction

Catalytically active carbons derived from plant biomass are conducive to the construction of renewable energy source system and utilization of sustainable resources. In this article, natural cattail fibers are used to fabricate porous nitrogen-doped carbon via direct chemical activation and heteroatom modification treatments. The graphene-like sheets from biomass pyrolysis are assembled into three-dimensional carbon frameworks. The chemical activation of KHCO₃ generated unique porous structure and N-containing molecules pyrolysis modification provided nitrogen doping atoms. High surface area up to 2,345 m²·g⁻¹ with simultaneous hierarchical pores (from micro to meso and macro) with abundant edge defects are achieved for these carbon materials. These materials have a very large external surface area up to 1,773 m²·g⁻¹. The above strategy exhibits a significant synergistic effect on the improvement of catalytic properties toward hydrogen evolution reaction and oxygen reduction reaction. The small over potentials and Tafel slopes of these catalytically active carbons demonstrate excellent potential applications in renewable energy conversion and storage systems. This research established a new link among environmental improvement, biomass conversion and renewable energy utilization.

Amperometric sensing of catechol by using a nanocomposite prepared from Ag/Ag2O nanoparticles and N,S-doped carbon quantum dots

This work describes the synthesis of a nanocomposite consisting of Ag₂O, silver nanoparticles and N,S-doped carbon quantum dots (Ag₂O/Ag@NS-CQD). The NS-CQD were prepared by hydrothermal treatment of p-aminobenzenesulfonic acid. They act as both the reducing and stabilizing agent for synthesis of Ag₂O/Ag@NS-CQD. The composite was characterized by UV-vis spectroscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The glassy carbon electrode (GCE) was modified by coating it with Ag₂O/Ag@NS-CQD. It exhibits excellent amperometric response to catechol, typically at a low working potential of around 0.25 V. Under the best experimental conditions, the sensor has a wide linear response (0.2 to 180 μM) and a low detection limit (13 nM; at S/N = 3). The method was applied to analysis of spiked water samples and gave satisfactory results. Graphical abstract Schematic representation of the preparation of the Ag/Ag2O@N,S-doped carbon quantum dots composite using p-aminobenzenesulfonic acid and silver nitrate as the starting materials. The corresponding modified glassy carbon electrode exhibits the excellent amperometric sensing performance toward catechol at pH 7.0 with low detection limit and good selectivity.