Bulk Oxygen Research Articles

La1-x(Sr, Na, K)xMnO3 perovskites for HCHO oxidation: The role of oxygen species on the catalytic mechanism

A-site substituted perovskites were prepared and evaluated for formaldehyde oxidation. Based on the T50, the catalytic activity ranking was: La0.8K0.2MnO3 > La0.8Na0.2MnO3 (LNMO) > La0.8Sr0.2MnO3 > LaMnO3. This improved activity is associated to (i) higher density of surface adsorption sites and (ii) higher bulk oxygen mobility. Stability tests, under humid and dry conditions, were also conducted. In dry conditions, a gradual deactivation of the substituted perovskites (from 50 % to 15 % HCHO conversion after 64 h, LNMO) was correlated to the loss of surface-active oxygen species and high-valence Mn4+. Besides, the presence of abundant adsorbed oxidation intermediates likely inhibited the reactivation of the catalyst surface. This inhibition was limited when humidity was added in the stream of gas, thus lowering the catalysts’ deactivation, demonstrating the importance of−OH surface groups on the catalyst surface in the oxidation mechanism. Consequently, this work highlights the design of perovskite-based catalysts with superior oxidation properties.

Oxidation and Storage Mechanisms for Nitrogen Oxides on Variously Terminated (001) Surfaces of SrFeO3-δ and Sr3Fe2O7-δ Perovskites.

The Ruddlesden-Popper (RP)-type layered perovskite is a candidate material for a new nitrogen oxide (NOx) storage catalyst. Here, we investigate the adsorption and oxidation of NOx on the (001) surfaces of RP-type oxide Sr3Fe2O7-δ for all of the terminations by comparing to those of simple perovskite SrFeO3-δ by the density functional theory (DFT) calculations. The possible (001) cleavages of Sr3Fe2O7 generate two FeO2- and three SrO-terminated surfaces, and the calculated surface energies indicated that the SrO-terminated surface generated by the cleavage at the rock salt layer is the most stable one. The oxygen of the FeO2-terminated surfaces could be removed with significantly low energy because the process involves the favorable reduction of the Fe4+ site. Consequently, the surface oxygen at the FeO2 site could easily oxidize adsorbed NO to NO2 by the Mars-van Krevelen mechanism. The resulting oxygen vacancy in the surface would be filled easily with lattice oxygen in bulk. The oxidation of NO with adsorbed molecular O2 was unfavorable by both the Langmuir-Hinshelwood and Eley-Rideal mechanisms because this process does not involve the reduction of the Fe4+ site. The oxygen of the SrO-terminated surfaces was tightly bound and acted as the adsorption site of NO and NO2. An electron transfer strengthened the NOx binding to the surface by forming nitrite (NO2-) or nitrate (NO3-) species. The DFT calculations revealed that the RP-type structure promoted NOx oxidation and storage properties by forming active oxygen due to the Jahn-Teller distortion and by exposing SrO-terminated surfaces due to the cleavage at the rock salt layer.

Comprehensive analysis of oxygen transport properties of a BaFe0.7Zr0.2Y0.1O3–δ-based mixed ionic-electronic conductor

Mixed ionic-electronic conductors (MIECs) represent a convenient means for separating oxygen from air, simultaneously combining high efficiency, low energy consumption and an environmentally safe electrochemical process. In the framework of this study, we propose two effective strategies for fabricating MIEC-based membranes: from a materials science viewpoint, a new perovskite-type BaFe0.7Zr0.2Y0.1O3–δ (BFZY) material was prepared as an alternative to Co-free oxides; from the technological viewpoint, the very simple tape-calendaring method was used to obtain a symmetrical-type membrane with a 150 μm-thick dense BFZY layer separating two porous and well-adhered layers of the same composition. According to experimental results, this membrane achieves maximal oxygen fluxes of 0.13- and 0.28-ml min−1 cm−2 at 750 °C and 850 °C, respectively, which are comparable with those obtained for related Fe- and even Co-containing MIEC membranes. A comprehensive analysis of transport properties conformed that bulk oxygen diffusion is a predominant process in the overall permeation process for the BFZY membrane. These findings provide a fresh approach for the rational design of MIEC materials and membrane cells for supporting the oxygen permeation process, including oxygen separation, oxygen production and electrochemical conversion.

The polymict carbonaceous breccia Aguas Zarcas: A potential analog to samples being returned by the OSIRIS‐REx and Hayabusa2 missions

AbstractOn April 23, 2019, a meteorite fall occurred in Aguas Zarcas, Costa Rica. According to the Meteoritical Bulletin, Aguas Zarcas is a brecciated CM2 chondrite dominated by two lithologies. Our X‐ray computed tomography (XCT) results show many different lithologies. In this paper, we describe the petrographic and mineralogical investigation of five different lithologies of the Aguas Zarcas meteorite. The bulk oxygen isotope compositions of some lithologies were also measured. The Aguas Zarcas meteorite is a breccia at all scales. From two small fragments, we have noted five main lithologies, including (1) Met‐1: a metal‐rich lithology; (2) Met‐2: a second metal‐rich lithology which is distinct from Met‐1; (3) a brecciated CM lithology with clasts of different petrologic subtypes; (4) a C1/2 lithology; and (5) a C1 lithology. The Met‐1 lithology is a new and unique carbonaceous chondrite which bears similarities to CR and CM chondrite groups, but is distinct from both based on oxygen isotope data. Met‐2 also represents a new type of carbonaceous chondrite, but it is more similar to the CM chondrite group, albeit with a very high abundance of metal. We have noted some similarities between the Met‐1 and Met‐2 lithologies and will explore possible genetic relationships. We have also identified a brecciated CM lithology with two primary components: a chondrule‐poor lithology and a chondrule‐rich lithology showing different petrologic subtypes. The other two lithologies, C1 and C1/2, are very altered and possibly related to the CM chondrite group. In this article, we describe all the lithologies in detail and attempt a classification of each in order to understand the origin and the history of formation of the Aguas Zarcas parent body.

Atmospheric pressure plasma jet fabricating of porous silver electrocatalyst as a promising approach to the creation of cathode layers of low temperature solid oxide fuel cells

Abstract Silver (Ag) electrocatalysts as cathode layers for low-temperature solid oxide fuel cells (LT-SOFCs) are prepared using an atmospheric pressure plasma jet (APPJ). The material characteristics of APPJ-coated silver are observed by an X-ray diffractometer and a field-emission scanning electron microscope. The porous cathode films, composed of Ag nanoparticles coated on YSZ electrolytes are observed. The porous Ni anodic layer is also prepared via an APPJ process on the other side of the YSZ. The values of area specific resistance and conductivities of Ag|YSZ are linearly increased with increasing operating temperatures. High conductivities of 139.6 and 143.2 S/cm are obtained at 550 and 600 °C, respectively. The exchange current and power density at 600 °C are 497.9 mA/cm2 and 108.4 mW/cm2, respectively. The AC impedance shows the two oxygen pathways, surface and bulk, of Ag electrocatalysts as reflected by the responses at low and high frequencies, respectively. Furthermore, LT-SOFCs which consist of cathode|electrolyte|anode are fabricated by APPJ technology in a fast-coating process.

Evaluating Microbial Interactions of Autotrophs and Heterotrophs in Partial Nitritation/Anammox (PN/A) Process by Experimental and Simulation Analyses

The microbial interactions between autotrophs and heterotrophs by the exchange of microbial products in a partial nitritation/anammox (PN/A) bioreactor were evaluated with both experimental and simulation analyses. Real-time quantitative PCR analysis showed that anammox bacteria (AMX) and ammonium oxidizing bacteria (AOB) made up 56.59% and 8.35% of total bacteria, respectively, while heterotrophs identified as Chloroflexi also constituted a large portion (32.76%) in the reactor, even without an external organic carbon supply. Furthermore, a mathematical model was developed to describe the growth of heterotrophs on soluble microbial products (SMP), which were released from the metabolism of autotrophs. After model calibration and validation, the simulation results were consistent with the experimental observations of the microbial composition and the nitrogenous transformations. According to the model analysis, the bulk oxygen concentration was determined to be the dominant factor governing the reactor performance and biomass fractions in the granule. Increasing granular size could decrease heterotrophic growth, but has little effect on the effluent concentration of SMP. Results of this study could establish a better understanding of eco-physiological interactions of autotrophs and heterotrophs in PN/A process.

Grey Fe-CeO2-σ for boosting photocatalytic ozonation of refractory pollutants: Roles of surface and bulk oxygen vacancies

Grey Fe doped CeO2-σ with tunable surface and bulk oxygen vacancies (OVs) were facilely prepared by a hydrothermal method. The light absorption, charge transfer efficiency, and catalytic activity and mechanism closely depended on the OVs. By tailoring the surface and bulk OVs, the catalytic activity of grey ceria was boosted due to the enhanced production of reactive oxygen species (i.e. OH, surface O). Density functional theory calculations show that the OVs, especially the bulk OVs, modulate the electronic structure for narrowing the bandgap and accelerating the charge transfer. Meanwhile, the surface OVs more promote the adsorption of ozone and the bulk OVs more induces the dissociation of ozone into reactive surface O. The calculation results agree well with the experimental observations. This study provides a simple method for engineering the surface and bulk OVs of ceria and gains insight into their roles in photocatalytic ozonation.

Tuning the concentration of surface/bulk oxygen vacancies in CeO2 nanorods to promote highly efficient photodegradation of organic dyes

To enhance the photodegradation ability of CeO2 for organic dyes, an effective strategy is to introduce oxygen vacancies (Vo). In general, the introduced Vo are simultaneously present both on the surface and in the bulk of CeO2. The surface oxygen vacancies (Vo-s) can decrease the band gap, thus enhancing light absorption to produce more photogenerated e− for photodegradation. However, the bulk oxygen vacancies (Vo-b) will inhibit photocatalytic activity by increasing the recombination of photogenerated e− and Vo-b. Therefore, regulating the concentrations of Vo-s to Vo-b is a breakthrough for achieving the best utilization of photogenerated e− during photodegradation. We used an easy hydrothermal method to achieve tunable concentrations of Vo-s to Vo-b in CeO2 nanorods. The optimized CeO2 presents a 70.2% removal of rhodamine B after 120 min of ultraviolet−visible light irradiation, and a superior photodegradation performance of multiple organics. This tuning strategy for Vo also provides guidance for developing other advanced metal-oxide semiconductor photocatalysts for the photodegradation of organic dyes.

Electrochemical Utilization of Iron IV in the Li1.3Fe0.4Nb0.3O2 Disordered Rocksalt Cathode

AbstractInterest in alkali‐rich oxide cathodes has grown in an effort to identify systems that provide high energy densities through reversible oxygen redox. However, some of the most promising compositions such as those based solely on earth abundant elements, e. g., iron and manganese, suffer from poor capacity retention and large hysteresis. Here, we use the disordered rocksalt cathode, Li1.3Fe0.4Nb0.3O2, as a model system to identify the underlying origin for the poor performance of Li‐rich iron‐based cathodes. Using elementally specific spectroscopic probes, we find the first charge is primarily accounted for by iron oxidation to 4+ below 4.25 V and O2 gas release beyond 4.25 V with no evidence of bulk oxygen redox. Although the Li1.3Fe0.4Nb0.3O2 is not a viable oxygen redox cathode, the iron 3+/4+ redox couple can be used reversibly during cycling.

Tunable oxygen defect density and location for enhancement of energy storage

Defect engineering is in the limelight for the fabrication of electrochemical energy storage devices. However, determining the influence of the defect density and location on the electrochemical behavior remains challenging. Herein, self-organized TiO2 nanotube arrays (TNTAs) are synthesized by anodization, and their oxygen defect location and density are tuned by a controllable post-annealing process. TNTAs annealed at 600 °C in N2 exhibit the highest capacity (289.2 mAh g−1 at 0.8C) for lithium-ion storage, while those annealed at 900 °C in N2 show a specific capacitance of 35.6 mF cm−2 and stability above 96% after 10,000 cycles for supercapacitor. Ex situ electron paramagnetic resonance spectra show that the surface-exposed oxygen defects increase, but the bulk embedded oxygen defects decrease with increasing annealing temperature. Density functional theory simulations reveal that a higher density of bulk oxygen defects corresponds to higher localized electrons states, which upshift the Fermi level and facilitate the lithium intercalation kinetic process. Meanwhile, differential charge density calculation indicates that the increase of surface oxygen defects in the anatase (1 0 1) plane leads to higher density excess electrons, which act as negative charge centers to enhance the surface potential for ion adsorption. This oxygen-deficient location and density tunable strategy introduce new opportunities for high-energy and high-power-density energy storage systems.

Smart oxygen vacancy engineering to enhance water oxidation efficiency by separating the different effects of bulk and surface vacancies

Oxygen vacancy engineering has been one of the most powerful strategies to enhance the efficiency of oxygen evolution reaction [OER]. Even though the positive effects of surface oxygen vacancy have been well proved, very controversial conclusions are also reported in perovskites. In this work, the roles of oxygen vacancies on the surface and in the bulk of La0.7Sr1.3Co2O6-δ (LSCO) are well separately investigated by X-ray absorption spectroscopy (surface sensitive) and magnetization measurement (bulk sensitive). The results suggest that massive surface oxygen vacancies have positive effects by enhancing the adsorption, while excessive oxygen vacancies in bulk is double-edged because of the interruptions of exchange interaction of cations with different valence states (Co–O–Co) and smooth transport of electrons. The sample prepared with massive surface oxygen vacancies while with less bulk oxygen vacancies yields 10 times higher specific activity at 1.6 V than commercial IrO2. This work uncovers the role of bulk oxygen vacancies in OER process and paves the way for smart oxygen vacancy engineering for higher OER efficiency.

Uniform Mesoporous Amorphous Cobalt-Inherent Silicon Oxide as a Highly Active Heterogeneous Catalyst in the Activation of Peroxymonosulfate for Rapid Oxidation of 2,4-Dichlorophenol: The Important Role of Inherent Cobalt in the Catalytic Mechanism.

Amorphous cobalt-inherent silicon oxide (Co-SiOx) was synthesized for the first time and employed as a highly active catalyst in the activation of peroxymonosulfate (PMS) for the rapid oxidation of 2,4-dichlorophenol (2,4-DCP). The characterization results revealed that the 0.15Co-SiOx possessed a high specific surface area of 607.95 m2/g with a uniform mesoporous structure (24.33 nm). The X-ray diffraction patterns indicate that the substituted cobalt atoms enlarge the unit cell parameter of the original SiO2, and the selected area electron diffraction pattern confirmed the amorphous nature of Co-SiOx. More bulk oxygen vacancies (Ov) existing in the Co-SiOx were identified to be one of the primary contributors to the significantly enhanced catalytic activation of PMS. The cobalt substitution both creates and stabilizes the surficial Ov and forms the adequately active Co(II)-Ov pairs which engine the electron transfer process during the catalytic activities. The active Co(II)-Ov pairs weaken the average electronegativity of Co/Si and Co/O sites, resulting in the prevalent changes in final state energy, which is the main driving cause of the binding energy shifts in the X-ray photoelectron spectroscopy (XPS) spectra of Si and O among all samples. The increase of the relative proportion of Co(III) in the spent Co-SiOx probably causes the binding energy shifts of the Co XPS spectrum compared to that of the Co-SiOx. The amorphous Co-SiOx outperforms stable and quick 2,4-DCP degradation, achieving a much higher kinetic rate of 0.7139 min-1 at pH = 7.02 than others via sulfate radical advanced oxidation processes (AOPs), photo-Fenton AOPs, H2O2 reagent AOPs, and other AOP approaches. The efficient degradation performance makes the amorphous Co-SiOx as a promising catalyst in removing 2,4-DCP or organic-rich pollutants.

Methane Bi-reforming for direct ethanol production over smart Cu/Mn- ferrite catalysts

The main goal of this research is direct synthesis of ethanol from methane bi-reforming reaction (H2O/CO2 methane reforming, SCMR) in a single step catalytic process. Smart nano-structured spinel ferrite catalysts with small ratio of Cu or Mn in composite with Fe cluster were prepared using ultrasonic assistance technique. Characterization data indicate the stability of the prepared ferrite structure during calcination step, so it can use for high temperature reaction. From XPS and Raman spectra, cupper ferrite shows deviation from ideal spinel structure, which increases the migration of bulk oxygen into surface to share in catalytic activity. The catalytic activity test was performed at moderate reaction condition (700 °C) using CO2 as soft oxidant to avoid methane combustion reaction and steam to release the alcoholic products. The two catalysts show unusual high catalytic activity and stability due to strong synergetic effect between metal and Fe. Cu–Fe gives high selectivity (∼50%) toward liquid product (ethanol), while Mn–Fe is more selective (∼70%) toward gas product (syngas). Furthermore, the chemical looping step is used to obtain ultra-stable SCMR reaction. The steam activation steps were performed for re-oxidation of reduced catalyst in addition to removal of carbon deposited.

Nickel-doped BaCo0.4Fe0.4Zr0.1Y0.1O3-δ as a new high-performance cathode for both oxygen-ion and proton conducting fuel cells

• BCFZY and Ni doped BCFZY cathodes are synthetized. • Ni doped BCFZY improves the oxygen ion and proton conductivity of the cathode. • BCFZYN has a good thermal stability and a reduced thermal expansion coefficient. • Ni doping enhances the ORR activity in both O-SOFCs and PCFCs. • BCFZYN exhibits a prominent stability in both symmetrical cell and single cell test. To develop a cathode with excellent oxygen reduction reaction (ORR) activity and durability at intermediate-to-low temperatures is significant to boost the advancement of solid oxide fuel cells (SOFCs), a fascinating energy conversion technology with low emissions and high efficiency. Perovskite oxides have been extensively developed as cathodes, and doping is an important strategy to alter the lattice diffusion and surface exchange properties of perovskites, to tailor catalytic performances for various redox reactions, including ORR for SOFCs. The reported BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3-δ (BCFZY) is a promising cathode for SOFCs. Herein, to further improve the performance of BCFZY at reduced temperatures, we systematically investigate the partial doping of B-sites with different metal elements, including Mn, Ni, Cu and Zn at a fixed content of 5%. Among them, it is found that Ba(Co 0.4 Fe 0.4 Zr 0.1 Y 0.1 ) 0.95 Ni 0.05 O 3-δ (BCFZYN) exhibits the lowest polarization impedance in both oxygen ion and proton conducting fuel cells. Based on conductivity relaxation experiments and oxygen and hydrogen permeation tests, it is found that nickel doping improves oxygen mobility, surface exchange kinetics, and bulk oxygen ion and proton conductivity. Thereby, a high ORR activity on oxygen ion and proton conducting electrolytes is achieved, reaching 0.038 and 0.607 Ω cm 2 at 550 °C, respectively. The cells with the BCFZYN electrode show outstanding operational stability (200 h of operation in a symmetrical cell and 1000 h of operation in a single cell). This suggests that the BCFZYN is a promising cathode of next-generation SOFCs.

Development of the Crank’s diffusion model for the case of material-gas feedback regime in gas flow reactors. Advanced methodology of oxygen partial pressure relaxation for the kinetics of oxygen exchange in nonstoichiometric oxides

• The Crank model may give incorrect k and D in case of vigorous reaction. • Extended samples require consideration of convective and diffusion gas transfer. • Models taking into account these factors are developed in this study. In order to determine the kinetic parameters of oxygen exchange between nonstoichiometric oxides and the gas phase, the rate constant of surface exchange k and the coefficient of bulk oxygen diffusion in the oxide D , one needs to model the kinetics of relaxation occurring under mixed diffusion-reaction control. It is shown that the widely used Crank model can misestimate the k and D values as it does not take into account such factors as (i) changes in oxygen partial pressure p O 2 in the reactor caused by vigorous reaction, (ii) the non-uniform distribution of p O 2 over the elongated oxide sample. Models taking into account these factors are discussed sequentially in this work: the ideal continuous stirred-tank reactor (CSTR) model where p O 2 varies over time as reaction proceeds; the tanks-in-series (TIS) model imitating the non-uniform distribution of p O 2 over the sample under approximation of convective gas transfer; and the model of tanks-in-series with diffusion-convective gas transfer (TSD). The TIS and TSD models (the latter one being used for low gas flow rates), which take into account all of the above factors, give the most reasonable results: the scatter of the values of the kinetic constants k and D determined at different rates of gas flow in the reactor does not exceed the reasonable experimental error. An expression is proposed for estimating k TIS , which corrects the k values obtained from the Crank model.

Atomic Origin for Hydrogenation Promoted Bulk Oxygen Vacancies Removal in Vanadium Dioxide.

Oxygen vacancies (VO), a common type of point defect in metal oxides materials, play important roles in the physical and chemical properties. To obtain stoichiometric oxide crystal, the pre-existing VO is always removed via careful post-annealing treatment at high temperature in an air or oxygen atmosphere. However, the annealing conditions are difficult to control, and the removal of VO in the bulk phase is restrained because of the high energy barrier of VO migration. Here, we selected VO2 crystal film as the model system and developed an alternative annealing treatment aided by controllable hydrogen doping, which can realize effective removal of VO defects in the VO2-δ crystal at a lower temperature. This finding is attributed to the hydrogenation accelerated oxygen vacancies recovery in the VO2-δ crystal. Theoretical calculations revealed that the H-doping-induced electrons are prone to accumulate around the oxygen defects in the VO2-δ film, which facilitates the diffusion of VO and thus makes it easier to be removed. The methodology is expected to be applied to other metal oxides for oxygen-related point defects control.

Ultrahigh purity conditions for nitride growth with low oxygen content by plasma-enhanced atomic layer deposition

Ultrahigh purity (UHP) reactor conditions provide a process environment for growth of nitride thin films with low oxygen content by plasma-enhanced atomic layer deposition (PEALD). In particular, UHP conditions correspond to partial pressures below 10−8 Torr for impurities within the PEALD process environment to limit incorporation before, during, and after film growth. In this article, we identify the various sources of background oxygen species and describe the measures taken to obtain UHP reactor conditions. For example, in situ ellipsometry results are presented that reveal the impact of oxygen incorporation on film resistivity during and after titanium nitride PEALD due to elevated levels of oxygen impurities in the argon process gas. A model is also developed that shows the significance of water permeation through elastomer vacuum seals. These examples demonstrate the importance of process gas purification and elimination of elastomer permeation toward achieving a UHP environment. X-ray photoelectron spectroscopy (XPS) depth profile data for titanium, aluminum, and silicon nitride by PEALD reveal bulk oxygen levels below 1 at. %, thereby demonstrating the effectiveness of UHP reactor conditions at reducing oxygen incorporation. Consistent with XPS, depth profile secondary ion mass spectroscopy results for titanium nitride PEALD confirm bulk oxygen content less than 1 at. %, further establishing the effectiveness of a UHP background for high purity nitride film growth.

Soot combustion over Ag catalysts supported on shape-controlled CeO2

Shape-controlled CeO2 nano-rods (CeO2-R), CeO2 nano-cubes (CeO2-C) and CeO2 nano-octahedrons (CeO2-O) were prepared as support of Ag/CeO2 for soot combustion. Ag/CeO2-C is more active than Ag/CeO2-R and Ag/CeO2-O, even with the presence of water, whose TOF value is 7.7, 11.4 times of Ag/CeO2-R and Ag/CeO2-O, respectively. The chemical state of Ag and the ability in oxygen activation is controlled by the support morphology through the interaction between Ag and CeO2. The weak interaction makes Ag in form of metallic state on Ag/CeO2-C which has better contact extent with soot than Ag oxides. Meanwhile, a high concentration of bulk oxygen vacancies on Ag/CeO2-C is also formed which efficiently provide active oxygen species by oxygen reverse spillover. It also explores a new pathway boosting soot combustion. As for CeO2-R and CeO2-O, although the introduced Ag improves the surface reducibility on CeO2, similar oxygen desorption behavior with the introduction of Ag reveals the unchanged way in oxygen activation and reaction pathway in soot combustion.

A reclassification of Northwest Africa 2900 from CV3 to CK3 chondrite

AbstractThe similarities between CV and CK chondrites are so substantial that some studies have argued for a common parent body origin. These similarities also mean that they are susceptible to misclassification as one another. It is, therefore, important to accurately classify CV and CK chondrites to properly compare the properties of the two groups and evaluate the single parent body hypothesis. In this study, we re‐evaluate the current classification of Northwest Africa (NWA) 2900 as a CV3 chondrite. Based on chondrule abundance (~13%), average chondrule diameter (1.11 ± 0.67 mm), iron content in chondrule olivine (Fa1 to Fa35 with a peak near ~Fa31) and matrix olivine (Fa33 to Fa36), magnetite abundance (~4 vol%), elemental abundances in olivine (Cr2O3, Al2O3, TiO2, MnO, NiO, and CaO) and magnetite (Cr2O3, Al2O3, TiO2, and NiO), and comparison with previously reported data for CV and CK chondrites, we propose that the classification of NWA 2900 be changed from CV3 chondrite to CK3 chondrite, with characteristics most similar to those of the CK3.8 subtype. We additionally suggest minor modifications to the compositional criteria used to distinguish between CV and CK chondrites and demonstrate that NWA 2900 extends the range of bulk oxygen isotope compositions of CK chondrites.

Experimental probing of effects of carbon support on bulk and local oxygen transport resistance in ultra-low Pt PEMFCs

Carbon support not only promises to dispersedly anchor Pt-based metal nanoparticles but also functions as the critical component of catalyst layers (CLs) in polymer electrolyte membrane fuel cells (PEMFCs). Thus, the geometrical and surface properties of carbon support are believed to impact the mass transport greatly, especially the local oxygen transport in ultra-low Pt PEMFCs. Herein, we explore influences of carbon support on oxygen transport resistance in cathode catalyst layers (CCLs) through combining the limiting current method with a dual-layer CCL design. Results demonstrate that the properties of carbon support have a more significant effect on the local oxygen transport resistance relative to the bulk one. Based on various advanced physicochemical characterizations, it is analyzed that the geometrical morphology of carbon support influences the bulk oxygen transport resistance via featuring the pore structure in the electrode, while the surface functional groups of carbon support influence the local oxygen transport resistance via determining the distribution of ultra-thin ionomer films on the catalyst surface. It is believed that the findings on the interaction between carbon support and cathode mass transport behavior give a critical enlightenment on the design of high performance ultra-low Pt membrane electrode assemblies (MEAs).