For catalysing dioxygen reduction, iron–nitrogen–carbon (Fe–N–C) materials are today the best candidates to replace platinum in proton-exchange membrane fuel cell (PEMFC) cathodes. Despite tremendous ...progress in their activity and site-structure understanding, improved durability is critically needed but challenged by insufficient understanding of their degradation mechanisms during operation. Here, we show that FeN x C y moieties in a representative Fe–N–C catalyst are structurally stable but electrochemically unstable when exposed in an acidic medium to H 2 O 2 , the main oxygen reduction reaction (ORR) byproduct. We reveal that exposure to H 2 O 2 leaves iron-based catalytic sites untouched but decreases their turnover frequency (TOF) via oxidation of the carbon surface, leading to weakened O 2 -binding on iron-based sites. Their TOF is recovered upon electrochemical reduction of the carbon surface, demonstrating the proposed deactivation mechanism. Our results reveal for the first time a hitherto unsuspected key deactivation mechanism during the ORR in an acidic medium. This study identifies the N-doped carbon surface as the Achilles' heel during ORR catalysis in PEMFCs. Observed in acidic but not in alkaline electrolytes, these insights suggest that durable Fe–N–C catalysts are within reach for PEMFCs if rational strategies minimizing the amount of H 2 O 2 or reactive oxygen species (ROS) produced during the ORR are developed.
Catalysis is a key technology for the synthesis of renewable fuels through electrochemical reduction of CO2. However, successful CO2 reduction still suffers from the lack of affordable catalyst ...design and understanding the factors governing catalysis. Herein, we demonstrate that the CO2 conversion selectivity on Sn (or SnOx/Sn) electrodes is correlated to the native oxygen content at the subsurface. Electrochemical analyses show that the reduced Sn electrode with abundant oxygen species effectively stabilizes a CO2.− intermediate rather than the clean Sn surface, and consequently results in enhanced formate production in the CO2 reduction. Based on this design strategy, a hierarchical Sn dendrite electrode with high oxygen content, consisting of a multi‐branched conifer‐like structure with an enlarged surface area, was synthesized. The electrode exhibits a superior formate production rate (228.6 μmol h−1 cm−2) at −1.36 VRHE without any considerable catalytic degradation over 18 h of operation.
Not exactly what it says on the tin: Rational design principles for tin electrodes to be used in selective CO2 reduction to formate are suggested using hierarchical tin dendrite electrodes (multi‐branched conifer‐like structure) that show remarkable activity and stability. The initial oxygen content of the tin electrode is set as “selectivity descriptor” and the architecture is manipulated to maximize the number of active sites.
Flexible thermoelectrics that enable conformal contact with heat sources of arbitrary shape are indispensable for self‐powered wearable electronics. Scalable integration of flexible thermoelectric ...(TE) materials into functional devices has improved over the past few years, however, the practical applications of flexible TE materials are still hindered by low performance. Herein, highly aligned carbon‐nanotube yarns (CNTYs) are proposed, combined with selective doping via picoliter scale inkjet printing. Coagulation assisted by van der Waals forces ensures a highly aligned structure of the CNTY, thus achieving the ultrahigh power factors of 4091 and 4739 µW m−1 K−2 for the p‐ and n‐type, respectively. The proposed TE materials can be effortlessly up‐scaled into highly integrated modules via inkjet printing. A highly integrated, flexible CNTY‐based TE generator (TEG) with 600 PN pairs generates unparalleled milliwatt‐scale power at ΔT = 25 K, which is a few orders of magnitude higher than those of previously reported flexible material‐based TEGs. This TEG successfully powers a red light‐emitting diode using body heat alone, requiring no external power sources. For the seamless operation of practical applications requiring high power, this work explores the key design parameters for flexible TEGs with high performance and manufacturability and presents new platforms for self‐powered wearable electronics.
Flexible thermoelectrics (TE) that enable conformal contact with heat sources of arbitrary shape are indispensable for self‐powered wearable electronics. The authors propose highly aligned carbon‐nanotube yarns combined with selective doping via picoliter scale inkjet printing, offering effortless scale‐up into highly integrated modules. The TE module with 600 PN pairs generates unparalleled milliwatt‐scale power at ΔT = 25 K.
N-doped carbon materials are considered as next-generation oxygen reduction reaction (ORR) catalysts for fuel cells due to their prolonged stability and low cost. However, the underlying mechanism of ...these catalysts has been only insufficiently identified, preventing the rational design of high-performing catalysts. Here, we show that the first electron is transferred into O2 molecules at the outer Helmholtz plane (ET-OHP) over a long range. This is in sharp contrast to the conventional belief that O2 adsorption must precede the ET step and thus that the active site must possess as good an O2 binding character as that which occurs on metallic catalysts. Based on the ET-OHP mechanism, the location of the electrode potential dominantly characterizes the ORR activity. Accordingly, we demonstrate that the electrode potential can be elevated by reducing the graphene size and/or including metal impurities, thereby enhancing the ORR activity, which can be transferred into single-cell operations with superior stability.
Graphene has been highlighted recently as a promising material for energy conversion due to its unique properties deriving from a two-dimensional layered structure of sp super(2)-hybridized carbon. ...Herein, N-doped graphene (NGr) is developed for its application in oxygen reduction reactions (ORRs) in acidic media, and additional doping of B or P into the NGr is attempted to enhance the ORR performance. The NGr exhibits an onset potential of 0.84 V and a mass activity of 0.45 mA mg super(-1) at 0.75 V. However, the B, N- (BNGr) and P, N-doped graphene (PNGr) show onset potentials of 0.86 and 0.87 V, and mass activities of 0.53 and 0.80 mA mg super(-1), respectively, which are correspondingly 1.2 and 1.8 times higher than those of the NGr. Moreover, an additional doping of B or P effectively reduces the production of H sub(2)O sub(2) in the ORRs, and shows much higher stability than that of Pt/C in acidic media. It is proposed that the improvement in the ORR activity results from the enhanced asymmetry of the spin density or electron transfer on the basal plane of the graphene, and the decrease in the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the graphene through additional doping of B or P.
Phosphorus and/or sulfur are additionally doped into N-doped carbon (NDC) using phosphoric acid and cysteine. The resulting catalysts demonstrate excellent oxygen reduction activities coupled with ...high stabilities in acidic media. Specially, additional S-doping in NDC reveals nearly 2.5 times higher activity than that of NDC at 0.75 V (vs. RHE).
An understanding of the deactivation mechanism of Pt electrocatalysis towards the ammonia oxidation reaction (AOR) to dinitrogen is key to the successful introduction of the nitrogen energy cycle, ...which can be used as a source of hydrogen fuels. Herein, we study AOR electrocatalysis on NOx-adsorbed polycrystalline Pt electrodes, i.e. NO, NO2− and NO3−, and find that all the NOx species can significantly reduce the catalytic activity of Pt. Combined stationary/transient voltammograms reveal that the poisonous NOx species are produced by NH3 oxidation on the bare Pt surface, but the adsorbed NOx species can be transformed to N2 by the Langmuir–Hinshelwood mechanism with *NO as a key intermediate.
Display omitted
•NO, NO2− and NO3− reduce the catalytic activity of Pt in NH3 oxidation reaction.•The NOx species are produced by NH3 oxidation on Pt at high potentials.•NOx species accumulate on Pt surface at high potentials.•The NOx species can be transformed to N2 via the Langmuir–Hinshelwood mechanism.
Neurotrophin 4 (NT-4) belongs to the family of neurotrophic factors, and it interacts with the tyrosine kinase B (trkB) receptor. NT-4 has neuroprotective effects following cerebral ischemia. Its ...role might be similar to brain-derived neurotrophic factor (BDNF), because both interact with trkB. Exercise also improves neural function by increasing neurotrophic factors. However, expression profiles of NT-4 in the brain during exercise are unknown. Here, we assessed the expressions of NT-4 and its receptor, trkB, following cerebral ischemia and hypothesized that exercise changes the expressions of NT-4 and trkB. Results showed that in a permanent middle cerebral artery occlusion rat model, ischemia decreased NT-4 and trkB expression. Immunohistochemistry showed their immunoreactivities around the region of the ischemic area. Treadmill exercise changed the expression of NT-4, which increased in the contralateral hemisphere in rats with ischemic injury. TrkB also showed similar patterns to its neurotophins. The change in NT-4 suggested that exercise might have primed NT4 production so that further injury causes slightly greater increases in NT4 compared with non-exercise controls.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Photo(electro)catalysis methods have drawn significant attention for efficient, energy‐saving, and environmental‐friendly organic contaminant degradation in wastewater. However, conventional ...oxide‐based powder photocatalysts are limited to UV‐light absorption and are unfavorable in the subsequent postseparation process. In this paper, a large‐area crystalline‐semiconductor nitride membrane with a distinct nanoporous surface is fabricated, which can be scaled up to a full wafer and easily retrieved after photodegradation. The unique nanoporous surface enhances broadband light absorption, provides abundant reactive sites, and promotes the dye‐molecule reaction with adsorbed hydroxyl radicals on the surface. The superior electric contact between the nickel bottom layer and nitride membrane facilitates swift charge carrier transportation. In laboratory tests, the nanostructure membrane can degrade 93% of the dye in 6 h under illumination with a small applied bias (0.5 V vs Ag/AgCl). Furthermore, a 2 inch diameter wafer‐scale membrane is deployed in a rooftop test under natural sunlight. The membrane operates stably for seven cycles (over 50 h) with an outstanding dye degradation efficiency (>92%) and satisfied average total organic carbon removal rate (≈50%) in each cycle. This demonstration thus opens the pathway toward the production of nanostructured semiconductor layers for large‐scale and practical wastewater treatment using natural sunlight.
A nitride semiconductor nanostructured membrane is fabricated for a solar‐driven dye degradation utility. The reusable wafer‐scale membrane operates stably at outdoor with an outstanding dye degradation efficiency of >92%, providing satisfied and repeatable total organic carbon removal rate of ≈50%. The pathway for practical wastewater treatment using natural sunlight is thus demonstrated.
Display omitted
•Fe-N-C catalyst was developed from chemically-exfoliated carbon nanofibers.•A porous network for efficient mass transports was constructed in the electrode.•The catalyst showed a ...considerable oxygen reduction reaction activity.
Chemically-exfoliated graphene materials have been widely investigated for various applications. The graphene materials are typically synthesized from graphite granules, but their success as support materials of Fe-N-C electrocatalysts for the oxygen reduction reaction is highly ambiguous due to their large sheet-like morphology impeding the efficient transport of reactants/products. In this study, we synthesize a graphene-based Fe-N-C catalyst from cabon nanofibers as parent materials, which consist of stacked-cup carbon surrounded by a few graphitic walls. This Fe-N-C catalyst shows a much improved catalytic activity compared with that synthesized from graphite granules. The physical and electrochemical characterizations reveal that the carbon nanofibers are simultaneously transferred into a mixture of small graphene ribbons and flakes after chemical exfoliation and surface modification, forming a porous network structure in a fabricated electrode with a modified electronic structure. The electrocatalytic pathway, methanol-tolerance, and active site of the graphene-based Fe-N-C catalyst are also investigated. The results show that the carbon nanofibers are promising parent carbon materials for preparation of graphene-based Fe-N-C electrocatalysts.
PDF
