Thin Ce Research Articles

(Invited) Nanomembrane Coating for Advanced Electrocatalysis

Advanced electrocatalysis that possesses higher selectivity and durability is more and more important for the electrification of chemical processes, which is expected in the renewable energy society in the future. Electrochemical reactions are often determined by the competition between the thermodynamic redox potential and the applied potential. Because the redox potential is characteristic for determining the product, it is often difficult to adjust the selectivity just by surface chemistry. Against this background, surface modification of electrodes is a well-studied field. Surface modification improves membrane functionality and selectivity, resulting in electrocatalysts with higher productivity and selectivity. Surface protective layers are also associated with material durability, as they prevent toxic and harmful substances from approaching the surface and prevent dissolution of the electrode material into the electrolyte. We are investigating the use of "nanomembrane" coatings on electrodes to achieve better selectivity and durability.Electrochemical reduction deposition of ultrathin layers of Cr and Mo on Pt cathodes has been shown to yield O2-tolerant hydrogen-evolving electrodes in the appropriate pH range. Electrochemical measurements indicate that these surface layers act as a membrane film. Many bulky ions and O2 molecules cannot pass through these surface layers, but H2 molecules can, consistent with electrochemical and spectroscopic measurements. First, the electrochemical properties of Pt modified with other elements such as Cr and Mo were investigated and monitored for catalytic activity under different atmospheres. Next, in-operando X-ray absorption spectroscopy (XAS) was used to investigate the working state of the material: electrochemical measurements in H2 and O2 confirmed that the oxygen reduction reaction (ORR) was severely suppressed and the hydrogen evolution reaction (HER) was maintained. XAS results also showed that the oxidation state of Pt can be freely changed by a potential sweep with or without a coating layer. We extended this work to nanofilm coating on oxygen-evolving anodes for water splitting, and succeeded in anodic deposition of a thin Ce layer on Ni-Fe electrodes; the Ce layer not only prevents unwanted ions from reaching the electrode surfaces, but also prevents Fe ions from leaching from the electrodes, thus improving the durability of the electrodes. In this presentation, key findings of these nanomembrane coatings and future prospects for selective and durable electrodes will be discussed.

Black flies (Diptera: Simuliidae) of central and northeastern Washington, United States of America, with cytogenetic emphasis on the Simulium arcticum complex

AbstractWe made 28 collections of black flies (Diptera: Simuliidae) at 24 locations in central and northeastern Washington state, United States of America, and identified 10 species in three genera, including Simulium arcticum Malloch, which we studied cytogenetically. We analysed 745 larvae of S. arcticum cytogenetically from nine of the 11 sites where it occurred; five sites had small sample sizes. For the collections with large sample sizes, the distribution of S. arcticum may have a geographic pattern. Larvae in western tributaries of the Columbia River have the sex-linked IIL-2 inversion and heterozygotes for the IS-1 autosomal polymorphism in abundance but lack the IIL-21 sex-linked inversion, whereas larvae in eastern tributaries of the Columbia River possess the IIL-21 inversion but lack IIL-2 and the IS-1 inversions. A cytotype new to science, S. arcticum IIL-81, occurs in some larvae at the Methow River in the eastern Cascades region. All females, regardless of location, possess enhanced (Ce Ce) centromere bands in their IIL-chromosomes, whereas all males possess the enhanced, thin (Ce Ct) centromere band dimorphism. The Methow River had nine types of chromosomally identified males in 2019 and eight types in 2020.

FemtoMAX - an X-ray beamline for structural dynamics at the short-pulse facility of MAX IV.

The FemtoMAX beamline facilitates studies of the structural dynamics of materials. Such studies are of fundamental importance for key scientific problems related to programming materials using light, enabling new storage media and new manufacturing techniques, obtaining sustainable energy by mimicking photosynthesis, and gleaning insights into chemical and biological functional dynamics. The FemtoMAX beamline utilizes the MAX IV linear accelerator as an electron source. The photon bursts have a pulse length of 100 fs, which is on the timescale of molecular vibrations, and have wavelengths matching interatomic distances (Å). The uniqueness of the beamline has called for special beamline components. This paper presents the beamline design including ultrasensitive X-ray beam-position monitors based on thin Ce:YAG screens, efficient harmonic separators and novel timing tools.

Long Term (4 Years) Performance of Co/Ce Coated 441 for SOFC Interconnect Applications

Ferritic stainless steel interconnectors are widely used in solid oxide fuel cells (SOFC) due to a combination of low cost, compatible thermal expansion properties and ease of manufacturing. However, their viability is hindered by some key technical hurdles. Most stainless steels suggested for SOFC applications rely on the formation of a fairly protective Cr2O3 scale. However, in air side environments Cr species evaporation leads to material failure and insufficient lifetimes. Coatings offer a way to mitigate Cr evaporation and in some cases decrease oxide scale growth. Numerous research papers have investigated different coating compositions and deposition techniques, however, the most commonly suggested material is (Mn,Co)3O4 (MCO). Instead of depositing MCO metallic Co can be applied. During operation Co is converted into Co3O4 and subsequently enriched in Mn from the underlying steel substrate. It has been shown that this type of coating effectively impedes Cr volatilization, even when only very thin layers (600 nm) are applied. Moreover the major advantage of metallic Co coatings are cost savings, since Co can be applied in a large scale roll-to-roll process before stamping the interconnect into its final shape. Furthermore, our previous work has shown that an additional layer of 10 nm Ce in combination with the Co coating results in the aforementioned mitigation of volatilization of Cr species, and also effectively reduces oxide scale growth. Oxide scale growth is detrimental for two reasons. First, a depletion of oxide scale forming element (Cr) can result in the formation of poorly protective iron oxide. Second, even a protective Cr2O3 scale grows with time. Since Cr2O3 is only a moderate electronic conductor this results in an increase of Area Specific Resistance (ASR) as the oxide scale grows in thickness. This eventually creates prohibitively high resistances. Both effects can be mitigated by addition of a thin Ce layer. In the present work Ce/Co (10/600 nm) coated AISI 441 has been exposed for 4 years (35 000 h) at 800 °C. The mass gain of the samples over the course of exposure has been recorded at selected intervals. After 35 000 h a mass gain of 3.76 mg/cm2 has been measured. Over the entire exposure parabolic mass gain kinetics are observed indicating the absence of a poorly protective iron oxide. After 35 000 h the samples were moved to a different setup to measure Cr(VI) evaporation. It was found that despite the fact that the coating was only 600 nm in thickness, it effectively inhibits volatilization of Cr even after 4 years of exposure. Furthermore, samples were removed at selected intervals to measure ASR.

Ejecta Transport, Breakup and Conversion

We report experimental results from an initial study of reactive and nonreactive metal fragments—ejecta—transporting in vacuum, and in reactive and nonreactive gases. We postulate that reactive metal fragments ejected into a reactive gas, such as H $$_2$$ , will break up into smaller fragments in situations where they are otherwise hydrodynamically stable in a nonreactive gas such as He. To evaluate the hypothesis we machined periodic perturbations onto thin Ce and Zn coupons and then explosively shocked them to eject hot, micron-scale fragments from the perturbations. The ejecta masses were diagnosed with piezoelectric pressure transducers, and their transport in H $$_2$$ and He was imaged with visible and infrared (IR) cameras. Because $${\text {Ce}} + {\text {H}}_2 \mapsto {\text {CeH}}_2 + \Delta H$$ , where $$\Delta H$$ is the enthalpy of formation, an observed increase of the relative IR (radiance) temperature $$T_{{R}}$$ between the Ce–H $$_2$$ and Ce–He gas systems can be used to estimate the amount of Ce that converts to CeH $$_2$$ . The experiments sought to determine whether dynamic chemical effects should be included in ejecta-transport models.

Temperature-dependent Faraday rotation and magnetization reorientation in cerium-substituted yttrium iron garnet thin films

We report on the temperature dependence of the magnetic and magneto-optical properties in cerium-substituted yttrium iron garnet (Ce:YIG) thin films. Measurements of the Faraday rotation as a function of temperature show that the magnetic easy axis of thin Ce:YIG films reorients from in-plane to out-of-plane on cooling below −100 °C. We argue that the temperature-dependence of the magnetostriction and magnetocrystalline anisotropy of Ce:YIG is the dominant factor contributing to the change in easy axis direction, and we describe the changes in the magneto-optical spectra with temperature.

Pre-Coated Ce/Co-Coated Steel: Mitigating Cr Vaporization, Increasing Corrosion Resistance at Competitive Cost

Cr vaporization and oxide scale growth are probably the two most detrimental degradation mechanisms associated to Cr2O3-forming alloys as interconnect material in SOFC. Different type of perovskite or spinel coatings are commonly used to mitigate Cr vaporization. Furthermore a number of studies have suggested that reactive element coatings (Ce, La, Hf) can reduce the oxidation rate of Cr2O3-forming alloys. However, the use of coatings increases cost due to addition processing and handling costs. The present study investigates the effectiveness of thin Ce and Co Physical Vapor Deposited (PVD) coatings with respect to Cr vaporization, corrosion resistance and Area Specific Resistance (ASR). The coatings were applied in a continuous roll-to-roll process which allows for high volumes. The use of pre-coated interconnects is thus more economic than a batch process. However, a consequence of the pre-coated concept is that the coating is subject to deformation when the pre-coated steel is pressed into the interconnect shape. Therefore this study also investigates the effect of deformation on the coating. Coated steel strips were bi- and uniaxially deformed. Furthermore a commercially used interconnect shape is investigated and the effects on Cr evaporation and corrosion are studied: Cr vaporization was measured in an air-3% H2O environment at 850 °C for 336 h. The results revealed that when the pre-coated steel was deformed, large cracks were formed. However, upon exposure those cracks did heal forming a continuous surface oxide rich in Co and Mn. As an effect of the rapid healing, no increase in Cr vaporization was measured for the pre-coated material.

Pre-Coated Ce/Co-Coated Steel: Mitigating Cr Vaporization, Increasing Corrosion Resistance at Competitive Cost

Cr vaporization and oxide scale growth are probably the two most detrimental degradation mechanisms associated to Cr2O3-forming alloys as interconnect material in SOFC. Both issues can be improved by the application of coatings. However, the use of coatings increase material cost. To reduce coating costs it is suggested that large amounts of steel can be coated before shaping the pre-coated material into an interconnect. The present study investigates the effectiveness of thin Ce and Co coatings on the cost effective steel AISI 441 with respect to Cr vaporization and corrosion resistance. Furthermore, the effect of mechanical deformation of the coated steel is examined. The results revealed that when the pre-coated steel is deformed, large cracks are formed. However, upon exposure those cracks can heal forming a continuous surface oxide rich in Co and Mn. As an effect of the rapid healing, no increase in Cr vaporization was measured.

Development of a Dual-Sided Readout DOI-PET Module Using Large-Area Monolithic MPPC-Arrays

We are proposing a novel design for a module with depth of interaction (DOI) capability for gamma rays by measuring the pulse-height ratio of double-sided Multi-Pixel Photon Counters (MPPCs) coupled at both ends of a scintillation crystal block. Thanks to newly developed monolithic MPPC arrays consisting of 4 × 4 channels with a three-side buttable package, the module is very thin and compact, thereby enabling less dead space between each module when arranged into a fully designed gantry. To demonstrate our concept of a DOI measuring technique, we first made a 1-D crystal array consisting of five Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) cubic crystals measuring 3×3×3 mm3 in size, separated by a layer of air approximately 10 μm-thick. When the light signals output from both ends are read with the 3×3 mm2 MPPCs, the position of each crystal is clearly distinguished. The same measurements were also made using Ce-doped (Lu,Y)2(SiO4)O (Ce:LYSO), achieving a similarly good separation. We then fabricated thin Ce:GAGG 2-D crystal arrays consisting of two types: [A] 4 × 4 matrix of 3×3×3 mm3 pixels, and [B] 10 t 10 matrix of 0.8×0.8×5 mm3 pixels, with each pixel divided by a BaSO4 reflector 0.2 mm-thick. Then four arrays are laid on top of each other facing the DOI direction through a layer of air 10 μm-thick. We demonstrated that the 3-D position of each Ce:GAGG pixel is clearly distinguished in both the 2-D and DOI directions for type A and B when illuminated by 662 keV gamma rays. Average energy resolutions of 9.8 ± 0.8% and 11.8 ± 1.3% were obtained for types A and B, respectively. These results suggest that our proposed method is simple and offers promise in achieving both excellent spatial and energy resolutions for future medical imaging, particularly in positron emission tomography (PET).

Operation of ceria-electrolyte solid oxide fuel cells on iso-octane–air fuel mixtures

Reduced-temperature solid oxide fuel cells (SOFCs) – with thin Ce 0.85Sm 0.15O 1.925 (SDC) electrolytes, thick Ni–SDC anode supports, and composite cathodes containing La 0.6Sr 0.4Co 0.2Fe 0.8O 3 (LSCF) and SDC – were fabricated and tested with iso-octane/air fuel mixtures. An additional supported catalyst layer, placed between the fuel stream and the anode, was needed to obtain a stable output power density (e.g. 0.6 W cm −2 at 590 °C) without anode coking. The Ru-CeO 2 catalyst produced CO 2 and H 2 at temperatures <350 °C, while H 2 and CO became predominant above 500 °C. Power densities were substantially less than for the same cells with H 2 fuel (e.g. 1.0 W cm −2 at 600 °C), due to the dilute (≈20%) hydrogen in the fuel mixture produced by iso-octane partial oxidation. Electrochemical impedance analysis showed a main arc that represented ≈60% of the total resistance, and that increased substantially upon switching from hydrogen to iso-octane/air.

Solid oxide fuel cells operated by internal partial oxidation reforming of iso-octane

Two types of solid oxide fuel cells (SOFCs), with thin Ce 0.85Sm 0.15O 1.925 (SDC) or 8 mol% Y 2O 3-stabilized ZrO 2 (YSZ) electrolytes, were fabricated and tested with iso-octane/air fuel mixtures. An additional Ru–CeO 2 catalyst layer, placed between the fuel stream and the anode, was needed to obtain a stable output power density without anode coking. Thermodynamic analysis and catalysis experiments showed that H 2 and CO were primary reaction products at ≈750 °C, but that these decreased and H 2O and CO 2 increased as the operating temperature dropped below ≈600 °C. Power densities for YSZ cells were 0.7 W cm −2 at 0.7 V and 790 °C, and for SDC cells were 0.6 W cm −2 at 0.6 V and 590 °C. Limiting current behavior was observed due to the relatively low (≈20%) H 2 content in the reformed fuel.

Resistance of (La, Sr)CoO3 / YSZ Interface with a Ceria Interlayer

Electrochemical impedance of dense (La,Sr)CoO, (LSCO) electrode on single crystalline YSZ electrolyte was investigated in order to clarify the effect of ceria interlayer. Pulsed laser deposition technique was used to fabricate a dense LSCO electrode (∼1.5 pm) and a thin (∼100 nm) Ce 0.9 Gd 0.1 O 1.95 (CGO) interlayer on a (1 0 0) surface of YSZ. Unlike the direct contact of LSCO on YSZ, LSCO/CGO/YSZ interface showed no significant degradation during the operation. The main part of the resistance was attributed to oxygen exchange reaction at the electrode surface as the case of LSCO electrode on CGO electrolyte. However, a possible minor contribution of the interface resistance was revealed by equivalent circuit analysis. The interface impedance was a single depressed semicircle, in most cases, in a complex impedance plane. It did not depend strongly on oxygen potential and temperature. It ranged from 0.3 to 1 Ωcm -2 , which may be significant in a practical porous electrode with smaller contact area.

Self-healing mechanism of a protective film prepared on a Ce(NO 3) 3-pretreated zinc electrode by modification with Zn(NO 3) 2 and Na 3PO 4

Self-healing mechanism of a protective film against corrosion of zinc at scratches in an aerated 0.5 M NaCl solution was investigated by polarization measurements, X-ray photoelectron spectroscopy (XPS) and electron-probe microanalysis (EPMA). The film was prepared on a zinc electrode by treatment in a Ce(NO 3) 3 solution and addition of aqueous solutions containing 9.98 or 19.9 μg/cm 2 of Zn(NO 3) 2 · 6H 2O and 55.2 μg/cm 2 of Na 3PO 4 · 12H 2O. After the coated electrode was scratched with a knife-edge crosswise and immersed in the NaCl solution for many hours, polarization measurements, observation of pit formation at the scratches, XPS and EPMA were carried out. This film was remarkably protective and self-healing against zinc corrosion on the scratched electrode. The cathodic and anodic processes of zinc corrosion were markedly suppressed by coverage of the surface except for scratches with a thin Ce 2O 3 layer containing a small amount of Ce 4+ and the surface of scratches with a layer composed of Zn 3(PO 4) 2 · 4H 2O, Zn(OH) 2 and ZnO mostly.

Intermediate-temperature SOFCs with thin Ce 0.8Y 0.2O 1.9 films prepared by screen-printing

Thin electrolyte films of yttria-doped ceria (YDC, Ce 0.8Y 0.2O 1.9) were fabricated on green substrates of NiO–YDC by a screen-printing technique. Dense YDC thin layers were subsequently formed on the porous NiO–YDC substrates after the green bi-layers were co-sintered at 1350 °C. The YDC films were about 15 μm as characterized by scanning electron microscope. With Sm 0.5Sr 0.5CoO 2.75 as cathodes, single cells were tested from 450 to 650 °C with humidified (3% H 2O) hydrogen as fuel and air as oxidant. Open circuit voltage of 0.97 V was obtained at 450 °C, indicating negligible gas permeation through the YDC thin films. Maximum power density of about 360 mW/cm 2 at 650 °C was achieved with current density of about 800 mA/cm 2. Electrochemical characterizations of the single cells indicate that it is critical to reduce the electrode polarization by developing new electrode materials as well as fabrication processes for solid oxide fuel cells that are operated at intermediate temperature.

Ce-Pt(111) surface alloys:: Electronic structure by photoemission

Rare-earth intermetallic systems are of considerable interest with respect to their fascinating properties due to localized 4f electrons or mixed valency. Therefore, we deposited thin Ce films on Pt(111) at room temperature and annealed it to 900 K thus forming a well-ordered Ce–Pt surface alloy. Using angle-resolved photoemission Fermi surfaces have been mapped for the Pt(111)–(2×2)Ce alloy and the clean Pt(111). We observe both strong deviations and striking similarities.

Microstructure — ionic conductivity relationships in ceria-gadolinia electrolytes

Impedance spectroscopy has been used to separate grain interior and grain boundary conductivities in a series of measurements on thin Ce 0.8 Gd 0.2 O 2 − δ ceramic electrolytes with a range of different grain size distributions. The “brick layer” microstructural model has been used to provide an estimate of a true grain boundary conductivity and to relate the electrical properties of the ceramic to microstructural parameters. High resolution electron microscopy images show that insulating intergranular phases do not necessarily have to be present in order to cause a large grain boundary resistance. A clear relationship between grain boundary resistance and the number of grain boundaries was observed for samples of mean grain size of 3 μm or larger. For samples of sub-micron average grain sizes, measured grain boundary conductivities were inconsistent with model predictions, being significantly higher than predicted.

Formation of the CeSix/Si(111) interface

The deposition of thin Ce films on Si(111) and subsequent or simultaneous annealing leads to the formation of various structurally, electronically, and chemically different CeSix phases, as revealed by scanning-tunneling microscopy, low-energy electron diffraction, and photoelectron spectroscopy. Several phases can coexist on the same sample, and with increasing annealing temperatures larger homogeneous domains are observed. This behavior is assigned to a mismatch of the lattices of the Si-rich CeSix bulk compounds with that of the Si(111) substrate surface, resulting at temperatures above 700 °C in the formation of a thin film of hexagonal CeSi1.67, which does not exist as a bulk compound.

Low temperature oxidation and reduction of ceria particles on Rh(111)

X-ray photoelectron spectroscopy (XPS) was used to study the oxidation and reduction of Ce vapor deposited onto a Rh(111) single crystal in a system which couples a moderate pressure reactor to an ultrahigh vacuum (UHV) analysis chamber. This article compares the behavior of thick (≥50 Å) ceria layers to that of thin (4–8 Å) ceria islands. Using variable takeoff angle XPS and difference spectra we find that the interior of both the thick Ce layers and the thin Ce islands oxidize completely to CeO2 with 1 Torr of oxygen at 475 K; however, the surfaces of both of the cerias are a mixture of Ce+3 and Ce+4. As for reduction, under our conditions (≤525 K) both the thick layers and the thin islands undergo a surface reduction that generates Ce+3. This Ce+3 surface is quite stable and no reduction of the bulk of the material occurs. Whereas the thick ceria layers require high pressures (8 Torr) of CO in order to reduce the surface, the thin islands reduce under UHV conditions—CO adsorbed at 300 K forms CO2 at 475 K during desorption. Further reduction of the thin ceria islands does not occur if they are treated with 8 Torr of CO at 525 K. We conclude that intimate contact with Rh can enhance ceria reduction via reaction with CO which initially adsorbs on Rh. Further, these two cerias are in fact very similar in that the amount of oxygen that they store is proportional to their surface area.

An X-ray photoemission study of Ce-Rh, Ce-Pd and Ce-Ag interfaces

X-ray photoelectron spectroscopy (XPS) has been employed to study electronic states in Ce-Rh, C-Pd, and Ce-Ag interfaces. Thin Ce films were deposited onto evaporated substrates of Rh, Pd and Ag, and conversely, thin films of Rh, Pd and Ag were deposited onto evaporated Ce films. Tetravalent Ce emission was observed in the Rh and Pd systems. Substrate core level attenuation curves show a deviation from exponential decay as material is deposited. Substrate core level shifts as well as the appearance of a tetravalent Ce signal are indicative of interface alloy formation; however, clustering of the evaporated species may also take place.

Cerium silicide formation in thin Ce–Si multilayer films

Alternating layers of Si(200 Å thick) and Ce(200 Å thick) up to 26 layers altogether were deposited by electron evaporation under ultrahigh vacuum conditions on Si(100) substrate held at 150 °C. Isothermal, rapid thermal annealing has been used to react these Ce–Si multilayer films. A variety of analytical techniques has been used to study these multilayer films after annealing, and among these are Auger electron spectroscopy, Rutherford backscattering, X-ray diffraction, and high resolution transmission electron microscopy. Intermixing of these thin Ce–Si multilayer films has occurred at temperatures as low as 150 °C for 2 h, when annealed. Increasing the annealing temperature from 150 to 400 °C for 1 h, CeSi2 forms gradually and the completion of reaction occurs at ∼300–400 °C. During the formation of CeSi2 from 150–400 °C, there is some evidence for small grains in the selected area diffraction patterns, indicating that CeSi2 crystallites were present in some regions. However, we have no conclusive evidence for the formation of epitaxial CeSi2 layers, only polycrystals were formed when reacted in the solid phase even after rapid thermal anneal at 900 °C for 10 s. The formation mechanism has also been discussed in combining the results of the La–Si system.