Oxygen Free-Radical Reduction of Brain Capillary Rubidium Uptake

Free radicals are proposed to play a role in the injury following cerebral ischemia in which cerebral edema is a prominent feature. To determine whether free radicals might alter the movement of ions and water across the blood-brain barrier, we examined their effect on brain capillary transport. Rat brain capillaries were isolated, incubated with a system that generates free radicals, and various capillary transport systems were studied. Rubidium uptake was reduced 74% whereas rubidium efflux, glucose transport, and capillary water space were unchanged. The results following the addition of radical scavengers indicated that hydrogen peroxide or a related free radical was the toxic species. These data suggest that free radicals can impair capillary endothelial cell mechanisms that help maintain homeostasis of electrolytes and water in brain.     

Egg‐Derived Mesoporous Carbon Microspheres as Bifunctional Oxygen Evolution and Oxygen Reduction Electrocatalysts

A bifunctional evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts are developed, based on codoped mesoporous carbon microspheres from ecofriendly biomass of eggs without the introduction of extrinsic dopants, via a facile and high‐throughput spray‐drying process. The obtained egg‐derived mesoporous carbon microspheres (egg‐CMS) present large specific surface area and high pore volume, as well as abundant dopant types including nitrogen, phosphorous, and iron that are originated from the innate protein and small organic molecule contents. When fabricated as OER or ORR catalysts, these egg‐CMS exhibit low onset potentials, high current densities, small Tafel slopes, and excellent stabilities. As a proof‐of‐concept, a rechargeable Zn‐air battery is demonstrated using the high‐active egg‐CMS as a bifunctional OER and ORR catalyst, suggesting the capability of utilizing full biomass materials for efficient energy storage and utilization.     

Active MnOx Electrocatalysts Prepared by Atomic Layer Deposition for Oxygen Evolution and Oxygen Reduction Reactions

The ability to deposit conformal catalytic thin films enables opportunities to achieve complex nanostructured designs for catalysis. Atomic layer deposition (ALD) is capable of creating conformal thin films over complex substrates. Here, ALD‐MnO_x on glassy carbon is investigated as a catalyst for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), two reactions that are of growing interest due to their many applications in alternative energy technologies. The films are characterized by X‐ray photoelectron spectroscopy, X‐ray diffraction, scanning electron microscopy, ellipsometry, and cyclic voltammetry. The as‐deposited films consist of Mn(II)O, which is shown to be a poor catalyst for the ORR, but highly active for the OER. By controllably annealing the samples, Mn₂O₃ catalysts with good activity for both the ORR and OER are synthesized. Hypotheses are presented to explain the large difference in the activity between the MnO and Mn₂O₃ catalysts for the ORR, but similar activity for the OER, including the effects of surface oxidation under experimental conditions. These catalysts synthesized though ALD compare favorably to the best MnO_x catalysts in the literature, demonstrating a viable way to produce highly active, conformal thin films from earth‐abundant materials for the ORR and the OER.     

Hydrogen-Dependent Oxygen Reduction by Homoacetogenic Bacteria Isolated from Termite Guts

Although homoacetogenic bacteria are generally considered to be obligate anaerobes, they colonize the intestinal tracts of termites and other environments that are not entirely anoxic in space or time. In this study, we investigated how homoacetogenic bacteria isolated from the hindguts of various termites respond to the presence of molecular oxygen. All strains investigated formed growth bands in oxygen gradient agar tubes under a headspace of H₂-CO₂. The position of the bands coincided with the oxic-anoxic interface and depended on the O₂ partial pressure in the headspace; the position of the bands relative to the meniscus remained stable for more than 1 month. Experiments with dense cell suspensions, performed with Clark-type O₂ and H₂ electrodes, revealed a large capacity for H₂-dependent oxygen reduction in Sporomusa termitida and Sporomusa sp. strain TmAO3 (149 and 826 nmol min⁻¹ mg of protein⁻¹, respectively). Both strains also reduced O₂ with endogenous reductants, albeit at lower rates. Only in Acetonema longum did the basal rates exceed the H₂-dependent rates considerably (181 versus 28 nmol min⁻¹ mg of protein)⁻¹). Addition of organic substrates did not stimulate O₂ consumption in any of the strains. Nevertheless, reductive acetogenesis by cell suspensions of strain TmAO3 was inhibited even at the lowest O₂ fluxes, and growth in nonreduced medium occurred only after the bacteria had rendered the medium anoxic. Similar results were obtained with Acetobacterium woodii, suggesting that the results are not unique to the strains isolated from termites. We concluded that because of their tolerance to temporary exposure to O₂ at low partial pressures (up to 1.5 kPa in the case of strain TmAO3) and because of their large capacity for O₂ reduction, homoacetogens can reestablish conditions favorable for growth by actively removing oxygen from their environment.     

Hierarchical Polyelemental Nanoparticles as Bifunctional Catalysts for Oxygen Evolution and Reduction Reactions

Efficient electrocatalysts are critical in various clean energy conversion and storage systems. Polyelemental nanomaterials are attractive as multifunctional catalysts due to their wide compositions and synergistic properties. However, controlled synthesis of polyelemental nanomaterials is difficult due to their complex composition. Herein, a one‐step synthetic strategy is presented to fabricate a hierarchical polyelemental nanomaterial, which contains ultrasmall precious metal nanoparticles (IrPt, ≈5 nm) anchored on spinel‐structure transition metal oxide nanoparticles. The polyelemental nanoparticles serve as excellent bifunctional catalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The mass catalytic activity of the polyelemental nanoparticles is 7‐times higher than that of Pt in ORR and 28‐times that of Ir in OER at the same overpotentials, demonstrating the high activity of the bifunctional electrocatalyst. This outstanding performance is attributed to the controlled multiple elemental composition, mixed chemical states, and large electroactive surface area. The hierarchical nanostructure and polyelemental design of these nanoparticles offer a general and powerful alternative material for catalysis, solar cells, and more.     

Efficiently Enhancing Oxygen Reduction Electrocatalytic Activity of MnO2 Using Facile Hydrogenation

Improving the electrocatalytic oxygen reduction reaction (ORR) activity of transition metal oxides is important for the development of non‐noble metal catalysts that are used in metal‐air batteries and fuel cells. Here, a novel facile strategy of hydrogenation to significantly enhance the ORR performance of MnO₂. The hydrogenated MnO₂ (H‐MnO₂), which is prepared through a simple heat treatment in hydrogen gas, shows characteristics of modified lattice/surface structures and increased electrical conductivity. In 0.1 M KOH aqueous solution, the prepared H‐MnO₂ exhibits high activity toward the oxygen electrocatalysis with more positive onset potential (≈60 mV), ≈14% larger of limiting current, lower yield of peroxide species, and better durability than the pristine oxide. Further conductivity testing and density functional theory (DFT) studies reveal the faster kinetics of ORR after hydrogenation is due to the formation of hydrogen bonds and altered microstructure and improved electronic properties. These results highlight the importance of hydrogenation as a facile yet effective strategy to improve the catalytic activity of transition metal oxides for ORR‐based applications.     

Molecular Design of Single‐Atom Catalysts for Oxygen Reduction Reaction

Fuel cells are highly attractive for direct chemical‐to‐electrical energy conversion and represent the ultimate mobile power supply solution. However, presently, fuel cells are limited by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR), which requires the use of Pt as a catalyst, thus significantly increasing the overall cost of the cells. Recently, nonprecious metal single‐atom catalysts (SACs) with high ORR activity under both acidic and alkaline conditions have been recognized as promising cost‐effective alternatives to replace Pt in fuel cells. Considerable efforts have been devoted to further improving the ORR activity of SACs, including tailoring the coordination structure of the metal centers, enriching the concentration of the metal centers, and engineering the electronic structure and porosity of the substrate. Herein, a brief introduction to fuel cells and fundamentals of the ORR parameters of SACs and the origin of their high activity is provided, followed by a detailed review of the recently developed strategies used to optimize the ORR activity of SACs in both rotating disk electrode and membrane electrode assembly tests. Remarks and perspectives on the remaining challenges and future directions of SACs for the development of commercial fuel cells are also presented.     

Extending Cassava Root Shelf Life via Reduction of Reactive Oxygen Species Production

One of the major constraints facing the large-scale production of cassava (Manihot esculenta) roots is the rapid postharvest physiological deterioration (PPD) that occurs within 72 h following harvest. One of the earliest recognized biochemical events during the initiation of PPD is a rapid burst of reactive oxygen species (ROS) accumulation. We have investigated the source of this oxidative burst to identify possible strategies to limit its extent and to extend cassava root shelf life. We provide evidence for a causal link between cyanogenesis and the onset of the oxidative burst that triggers PPD. By measuring ROS accumulation in transgenic low-cyanogen plants with and without cyanide complementation, we show that PPD is cyanide dependent, presumably resulting from a cyanide-dependent inhibition of respiration. To reduce cyanide-dependent ROS production in cassava root mitochondria, we generated transgenic plants expressing a codon-optimized Arabidopsis (Arabidopsis thaliana) mitochondrial alternative oxidase gene (AOX1A). Unlike cytochrome c oxidase, AOX is cyanide insensitive. Transgenic plants overexpressing AOX exhibited over a 10-fold reduction in ROS accumulation compared with wild-type plants. The reduction in ROS accumulation was associated with a delayed onset of PPD by 14 to 21 d after harvest of greenhouse-grown plants. The delay in PPD in transgenic plants was also observed under field conditions, but with a root biomass yield loss in the highest AOX-expressing lines. These data reveal a mechanism for PPD in cassava based on cyanide-induced oxidative stress as well as PPD control strategies involving inhibition of ROS production or its sequestration.     

Highly Efficient Oxygen Reduction Reaction Activity of Graphitic Tube Encapsulating Nitrided CoxFey Alloy

Nonprecious metals are promising catalysts to avoid the sluggish oxygen reduction reaction (ORR) in next‐generation regenerative fuel cells or metal–air batteries. Therefore, development of nonprecious metal catalysts for ORR is highly desirable. Herein, precise tuning of the atomic ratio of Fe and Co encapsulated in melamine‐derived nitrogen‐rich graphitic tube (NGT) is reported. The Co_1.08Fe_3.34 hybrid with metal? nitrogen bonds ( 1 : Co_1.08Fe_3.34@NGT) shows remarkable ORR catalytic activities (80 mV higher in onset potential and 50 mV higher in half‐wave potential than those of state‐of‐the‐art commercial Pt/C catalysts), high current density, and stability. In acidic solution, 1 also shows compatible performance to commercial Pt/C in terms of ORR activity, current density, stability, and methanol tolerance. The high ORR activity is ascribed to the co‐existence of Fe? N, Co? N, and sufficient metallic FeCo alloys which favor faster electron movement and better adsorption of oxygen molecules on the catalyst surface. In the alkaline anion exchange membrane fuel cell setup, this cell delivers the power density of 117 mW cm^?2, demonstrating its potential use for energy conversion and storage applications.     

Efficient Oxygen Reduction Catalysts of Porous Carbon Nanostructures Decorated with Transition Metal Species

Developing substitutes of noble metal catalysts toward oxygen reduction reaction (ORR) at the cathode is of vital importance for promoting low‐temperature polymer electrolyte membrane fuel cells. Transition metal species have been one of the hot areas of interest due to their low cost, high activity, and long‐term stability. The design of porous carbon nanostructures decorated with transition metal species plays a vital role in enhancing ORR catalytic performance. Here, the recent breakthroughs in porous carbon nanostructures decorated with transition metal species (including nanoparticles and atomically dispersed supported metal) are discussed. The porous nanostructures can provide large surface area as well as abundant pore channels, leading to sufficient exposure of active sites and efficient mass transfer. These nanostructures can be synthesized by several approaches, including the templated method, the self‐templated method, the impregnation process, and so on. Furthermore, the ORR performance and the exploration of active sites are also discussed for further enhancement of the ORR catalysts. Finally, the challenges and prospects are discussed, which would push forward the development of ORR catalysts in the near future.     

Involvement of Molecular Oxygen in the Enzyme-Catalyzed NADH Oxidation and Ferric Leghemoglobin Reduction

Ferric leghemoglobin reductase (FLbR) from soybean (Glycine max [L.] Merr) nodules catalyzed oxidation of NADH, reduction of ferric leghemoglobin (Lb⁺³), and reduction of dichloroindophenol (diaphorase activity). None of these reactions was detectable when O₂ was removed from the reaction system, but all were restored upon readdition of O₂. In the absence of exogenous electron carriers and in the presence of O₂ and excess NADH, FLbR catalyzed NADH oxidation with the generation of H₂O₂ functioning as an NADH oxidase. The possible involvement of peroxide-like intermediates in the FLbR-catalyzed reactions was analyzed by measuring the effects of peroxidase and catalase on FLbR activities; both enzymes at low concentrations (about 2 μg/mL) stimulated the FLbR-catalyzed NADH oxidation and Lb⁺³ reduction. The formation of H₂O₂ during the FLbR-catalyzed NADH oxidation was confirmed using a sensitive assay based on the fluorescence emitted by dichlorofluorescin upon reaction with H₂O₂. The stoichiometry ratios between the FLbR-catalyzed NADH oxidation and Lb⁺³ reduction were not constant but changed with time and with concentrations of NADH and O₂ in the reaction solution, indicating that the reactions were not directly coupled and electrons from NADH oxidation were transferred to Lb⁺³ by reaction intermediates. A study of the affinity of FLbR for O₂ showed that the enzyme required at least micromolar levels of dissolved O₂ for optimal activities. A mechanism for the FLbR-catalyzed reactions is proposed by analogy with related oxidoreductase systems.