Oxygen Stoichiometry Research Articles

Nanoscopic oxygen control of functional oxide nanoparticles by electro-chemical route at ambient temperature

La0.67Ca0.33MnOδ nanoparticles of approximate size ∼ 4 nm have been prepared by the chemical solution deposition method to investigate effect of oxygen stoichiometry in the nanoparticles without changing their sizes. Electrochemical oxidation method has been used to change the oxygen stoichiometry δ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\delta$$\\end{document} at room temperature, which unlike conventional methods to change oxygen stoichiometry by heating in controlled ambience, does not lead to any significant change in size. This has allowed us to investigate the effects of stoichiometry variations in the nanoparticles with no change in size. The unit cell volume, lattice constants and orthorhombic strains of the as prepared sample (with δ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\delta$$\\end{document} = 2.74) are changed by incorporation of oxygen by electrochemical oxidation which in turns affects the magnetic properties. In addition, oxidation leads to change in oxygen stoichiometry of the magnetically “dead” surface layer on the nanoparticles which also affects their magnetization and coercive field.

Experimental and Modeling High-Pressure Study of Ammonia-Methane Oxidation in a Flow Reactor.

The present work deals with an experimental and modeling analysis of the oxidation of ammonia-methane mixtures at high pressure (up to 40 bar) in the 550-1250 K temperature range using a quartz tubular reactor and argon as a diluent. The impact of temperature, pressure, oxygen stoichiometry, and CH4/NH3 ratio has been analyzed on the concentrations of NH3, NO2, N2O, NO, N2, HCN, CH4, CO, and CO2 obtained as main products of the ammonia-methane mixture oxidation. The main results obtained indicate that increasing either the pressure, CH4/NH3 ratio, or stoichiometry results in a shift of NH3 and CH4 conversion to lower temperatures. The effect of pressure is particularly significant in the low range of pressures studied. The main products of ammonia oxidation are N2, NO, and N2O while NO2 concentrations are below the detection limit for all of the conditions considered. The N2O formation is favored by increasing the CH4/NH3 ratio and stoichiometry. The experimental results are simulated and interpreted in terms of an updated detailed chemical kinetic mechanism, which, in general, is able to describe well the conversion of both NH3 and CH4 under almost all of the studied conditions. Nevertheless, some discrepancies are found between the experimental results and model calculations.

Control of Liquid Water Transport in Cathode of PEFC Using Wettability-Patterned Electrodes

During the operation of polymer electrolyte fuel cells (PEFCs), the product water that accumulates in the cathode electrode blocks the oxygen transport to the reaction sites and causes severe performance degradation. This phenomenon, known as “flooding,” is one of the critical issues to be solved for achieving high efficiency and high power density. To alleviate water flooding, it’s necessary to design an electrode/channel structure that could actively remove liquid water from the porous electrode. Zhang et al. [1] reported that the wettability patterning of a gas diffusion layer (GDL) enables it to provide the liquid water pathway in the cathode electrode. Furthermore, Nishida et al. [2] demonstrated that the hydrophilization of cathode channel walls promotes water removal from the GDL to the flow channel. This study fabricated a wettability-patterned GDL impregnated with a hydrophilic silica-based solvent and newly proposed an electrode/channel structure combining the patterned electrode and hydrophilized flow channel. The effect of its structure on the liquid water distribution within the cathode GDL of an operating PEFC was also investigated using X-ray imaging.The authors fabricated the experimental fuel cell with an effective electrode area of 1.8 x 1.6 cm2. The membrane electrode assembly (MEA) was sandwiched between two carbon separators with a single-serpentine flow field (number of channels: 7, channel width: 1.0 mm, depth: 1.0 mm, total length: 88.5 mm). Fig. 1 shows the regions measured by X-ray imaging in the cross-sectional view of the cathode. Since the cathode GDL is irradiated with an X-ray beam in the in-plane direction, the through-plane distributions of water can be observed in the GDL. The measurement regions were set under the 4th channel and under the land between the 3rd and 4th channel. The GDL in the cathode was impregnated with the silica-based solvent in the range of 0.3 mm width x 10 mm length x 0.1 mm depth. The hydrophilized region was positioned along the right sidewall of the 4th channel. The channel walls of the cathode were also coated with the same hydrophilic solvent. Fig. 2 represents the distribution of water saturation in the (a) uncustomized and (b) wettability-patterned GDL. The current density was increased stepwise from 0.14 to 1.02 A/cm2 for 8 min at room temperature and non-humidified condition. All images were captured at t=6 min after starting the operation. Dry hydrogen (stoichiometry: 1.5@2.0A/cm2) and oxygen (stoichiometry: 3.0@2.0A/cm2) were supplied to the anode and cathode, respectively, in a counter-flow arrangement. As shown in Fig. 2(b), it was noted that the water saturation around the hydrophilized region was effectively reduced due to the in-plane water suction. The patterned GDL partially impregnated with the highly hydrophilic solvent enables to induce the product water from the hydrophobic to hydrophilic region, resulting in the reduction of flooding. To further accelerate the through-plane water transport from the hydrophilized region in the GDL to the channel sidewall, it’s essential to introduce a difference in their wettability and optimize their combinations.

Phase Transition Engineering of Vanadium Dioxide Induced by Oxygen Vacancies

A reconfigurable optical functional system is a pioneering and comprehensive approach for portable future flexible electronics, irrespective of traditional silicon-based technology. VO2 shows tremendous temperature-dependent structural and electrical phase transitions as a complex correlated metal oxide, presenting it as a potential candidate for bolometers, thermal camouflage, and optoelectronics applications. However, the structural stability and high transition temperature restrict room-temperature applications. The present study reports the oxygen vacancies-assisted phase transition engineering in the VO2/muscovite heterostructure. The ion bombardment alters the oxygen stoichiometry of VO2/muscovite thin film at room temperature. A significant decrease in lattice constant has been observed during the transition of the insulating M1 (monoclinic) phase into the metallic R (tetragonal) phase, which was confirmed through high-resolution XRD. The change in oxygen vacancy concentration determined using X-ray photoelectron spectroscopy (XPS) actively suppressed and stabilized the epitaxial thin film of VO2 near room temperature. We investigated whether applying increasing power-mediated ion bombardment for a few minutes affects the oxygen vacancy formation on the VO2/muscovite film’s surface. Consequently, a change in KPFM and 4 orders of magnitude change in resistance of VO2 thin film have been observed. This report reveals the fundamental understanding of controlling the MIT in epitaxial VO2 thin film via facile and clean phase transition engineering techniques for advanced flexible electronic devices.

Influence of the synthesis method on the microstructural properties of Ta modified AgNbO3 ferroelectric thin films

Lead-free Ag(Nb0.80Ta0.20)O3 (ANT) thin films were synthesized via a chemical solution deposition process by the spin-coating technique. The effect of two different heat-treatment methods on the structural and microstructural properties was systematically investigated. X-ray diffraction and Raman spectroscopy results revealed that the orthorhombic perovskite structure was successfully obtained for the film crystallized by the conventional heat-treatment cycle. Secondary phases, however, associated with (Nb,Ta)2O5, Ag8(Nb,Ta)26O69, Ag2(Nb,Ta)8O21, Ag2(Nb,Ta)4O11 phases and metallic Ag ion were detected for the rapidly annealed film. The surface microstructure is noticeably influenced by the crystallization method, revealing a change in the morphology of the grains for the studied samples. The expected cationic and oxygen stoichiometries, as well as the homogeneity of microstructure in-depth, were confirmed using the Rutherford backscattering. The nanoscale ferroelectric response has been investigated from piezoelectric force microscopy analysis, detecting a clear domain structure for the studied ANT films.

Event-triggered dynamic prescribed performance control for oxygen stoichiometry and stack temperature in PEM fuel cell systems with actuators faults

Oxygen stoichiometry (OS) and stack temperature (ST) significantly affect the output performance and durability of proton exchange membrane fuel cells. This paper investigates the prescribed performance tracking control strategy of OS and ST under external disturbances, modeling errors, actuators faults and limited communication resources. The total disturbances are compensated by a variable-bandwidth extended state observer to simplify the control design and enhance the system robustness. To tolerate excessive disturbances and relax the strict requirement on the initial conditions, a new event-triggered adaptive sliding mode controller that integrates modified dynamic performance constraints and nonsingular error transformation is developed to ensure the user-designed transient and steady-state performance constraints, while saving the communication costs and avoiding the chattering and jump in control signals. The numerical validation results under combined working scenarios indicate that the proposed controller always ensures the preset error constraints and possesses superior performance in tracking the optimal OS value 2 and the optimal ST value 353.15K than other three representative controllers, with the integral absolute errors of 0.732 for OS tracking and 12.94 for ST tracking; besides, it achieves satisfactory robustness to uncertain model parameters and relatively large initial tracking errors.

Doped HfOx Nanoclusters: Polar and Resistive Switching in the Smallest Functional Units

In the study presented in this article, the impact of proton doping on the structural and electronic properties of hafnium oxide nanoclusters is investigated, with a focus on their potential for use in resistive and polar switching devices. In the results, it is shown that the incorporation of protons can stabilize the cage‐like crystalline structures of clusters, leading to reversible changes in electronic properties by varying oxygen stoichiometry. However, the full coverage of hafnia atoms by hydrogen removes in‐gap states, highlighting the importance of controlled moisture content in redox‐based memristive devices and neuromorphic units. In addition, in this study, the polar properties of these clusters are explored, illustrating possible polar switching in metastable pure , low‐barrier antiferroelectric‐like switching in carbon‐stabilized , and low‐barrier polar switching in . In these findings, the potential of clusters is revealed as active components for next‐generation high‐capacity nonvolatile electronic memory and beyond von Neumann computing in sub‐nanometer scale.

Physical Origin of Threshold Switching in Amorphous Chromium‐Doped V2O3

Devices made of amorphous thin films of the prototypical Mott‐insulator chromium‐doped V2O3 show a threshold and negative differential resistance effect after an electroforming step. Here, it is demonstrated that this effect is caused by the formation of a crystalline filament, in which a thermal runaway effect can occur. A compact model is developed that can describe the switching behavior as well as its inherent variability. The influence of the doping concentration and oxygen stoichiometry on the switching behavior is characterized and by fitting the model to the experimental data, the underlying physical changes are extracted.

Tantalum Oxide Thin Films Sputter-Deposited by Oxygen Gas Pulsing

Tantalum oxide thin films are deposited by DC reactive magnetron sputtering from a tantalum metallic target and argon + oxygen. The oxygen gas is pulsed during the deposition with a constant pulsing period T = 10 s, whereas the introduction time of the reactive gas, namely the tON injection time, is systematically changed from 0 to 100% of T. Therefore, composition of as-deposited TaOx films is continuously changed from pure metallic tantalum to the over-stoichiometric Ta2O5 material. Films adopt the body-centered cubic structure (metallic Ta) for the lowest tON injection time values (oxygen stoichiometry x < 1.0) and become amorphous for the longest ones. It is shown that the tON injection time is a key parameter to deposit either homogeneous tantalum oxides, or periodic Ta/TaOx multilayers with alternations close to 3 nm. Optical transmittance in the visible region of the film/glass substrate system and electrical conductivity vs. temperature both exhibit a gradual evolution from metallic (σ300K = 8.17 × 105 S m−1 with an opaque behavior) to semiconducting (σ300K = 1.97 × 103 S m−1 with a semi-transparent transmittance) and finally to dielectric properties (σ300K < 10−5 S m−1 for interferential films) as a function of the oxygen concentration in the films.

Electronic Bulk and Surface Properties of Titanium Dioxide Studied by DFT-1/2.

Transparent conductive oxides, such as TiO2, are important functional materials for optoelectronic and photovoltaic devices. We investigate the electronic bulk properties of the TiO2 phases rutile and anatase with the DFT-1/2 method and obtain a quantitatively good description of their electronic band structures. We then applied this method to the (001) surfaces of rutile and anatase and calculated their ionization potentials (IPs) and work functions (WF). To relate these calculated surface properties with values from experiments, we evaluated the effect of varying the oxygen stoichiometry at the surface on both IP and WF.

Grain size effect on the critical behavior of the ferromagnetic transition in electronically phase separated La0.5Ca0.5MnO3

Mixed valence manganese oxides (manganites) are known to exhibit intrinsic inhomogeneous magnetic states, which are influenced by a great variety of factors, including oxygen stoichiometry, nanostructuration and ceramic grain size. In this context, the prototypical half-doped compound La0.5Ca0.5MnO3was extensively studied. Here we reloaded the phase separated scenario, focusing on the thermodynamical critical behavior of this system at the paramagnetic - ferromagnetic transition and its correlation with the evolution of its ordering capabilities controlled through the ceramic grain size.Our findings establish a clear connection between the correlation length and the grain size, shedding light on the role of this parameter in the control of inhomogeneous magnetic ground states in manganites. Therefore it is possible to tailor fundamental physical properties as the scaling laws of the system just tuning the ceramic grain size.

Structure‐property Relationship of Double Perovskite Oxide towards Trifunctional Electrocatalytic Activity: Strategy for Designing and Development

AbstractIn the present scenario, the paramount significant roles of various heterogeneous catalysts stimulate the modern technologies to underpin the benchmark requirements for the generation of sustainable energy by reducing toxic fossil fuel emissions. Such critical role necessitates further development of cost‐effective highly efficient and earth‐abundant multifunctional or trifunctional electrocatalysts to promote the advancement of electrochemical overall water splitting performances, yet it is extremely desirable. In this review context, we present the development of double perovskite (DP) oxides as robust trifunctional catalysts for electrochemical oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reactions (HER) by rational design of multiple cationic redox sites with stoichiometric oxygen amount. Particularly, we highlight the importance of the structural modifications via doping, surface structure and oxygen stoichiometry as key parameters to tune the electrocatalytic activities and understand the insight into activity and mechanism of this oxide family. This perspective also describes controlled synthesis protocols including the surface structure of double perovskite oxides are key techniques for realizing a correlation between structure‐activity relationships of these materials. Finally, it is concluded by outlining the several aspects of optimization strategies and computational opportunities can expand the future scope of double perovskite oxides as robust trifunctional electrocatalysts.

Cation-heterogeneity in internally gelated U1-zCezO2-x, 0.15 ≤ z ≤ 0.3 microspheres

Internal gelation of aqueous mixtures of metal (M = Ln, An) nitrate with Uranyl Nitrate is generally assumed to yield cation homogeneity and a fluorite type single phase U1-zMzO2±x solid solution. As-sintered (U,Ce)O2 internally gelated microspheres, manufactured with target z values up to 0.3 using Ce(NO3)3, were observed to exhibit systematic peak broadening and splitting at higher 2θ angles in their X-Ray diffraction (XRD) patterns, correlating with increasing z≥0.15. This was interpreted as an unexpected departure from a single phase material. Thermogravimetry was used to make an initial assessment whether these peak anomalies were caused by an oxygen hypostoichiometry. Results indicated global oxygen stoichiometry for all compositions. The subsequent detailed characterization study via Electron Probe Micro Analysis of cross-sections of the as-sintered microspheres revealed the systematic presence of spherical Ce concentration gradients, as well as µm-sized highly Ce-enriched features. EDS and TEM studies on focused ion beam lamellae extracted from the cross-sections of as-sintered microspheres revealed a hexagonal Ce4.67(SiO4)3O minor phase manifesting as single grain precipitates and clusters uncovering the presence and critical role of Silicon as an unexpected contaminant and Ce-scavenger from surrounding (U,Ce)O2 grains. Characterization at intermediate heat treatment steps revealed that the systematic U/Ce heterogeneity features are already present post-gelation and are independent of the superimposed trace Ce-Si-O phase. This work constitutes the first systematic cation distribution study on cross-sections of (U,Ce)O2±x microspheres, executed on a series of compositions, using a combination of elemental mapping techniques.

Oxygen diffusion and surface exchange coefficients measurements under high pressure: Comparative behavior of oxygen deficient versus over‐stoichiometric air electrode materials

AbstractMixed ionic electronic conductors (MIECs) oxides are used as electrode materials for solid oxide cell (SOC) application, as they combine high electronic conductivity as well as high oxygen diffusivity and oxygen surface exchange coefficients. The ionic transport properties can be directly determined thanks to the isotopic exchange depth profiling (IEDP) method. To date, the reported measurements have been performed at ambient pressure and below. However, for a higher efficiency of hydrogen production at the system level, it is envisaged to operate the cell between 10 and 60 bar. To characterize the MIEC oxides properties in such conditions, an innovative setup able to operate up to a total pressure of 50 bar and 900°C has been developed. The main goal of this study was to compare the behavior of two types of reference materials: the oxygen deficient La‐Sr‐Fe‐Co perovskites, and the overstoichiometric lanthanide nickelates Ln2NiO4+δ (Ln = La, Pr, Nd). Diffusion and surface exchange coefficients obtained under 6.3 bar of oxygen are measured and their evolution discussed in light of the change in oxygen stoichiometries. This analysis allows better understanding of the dependency of the surface exchange coefficient with the oxygen partial pressure.

Influence of oxygen stoichiometry on [formula omitted] spin state transitions and magnetic properties in [formula omitted] cobaltites

The layered EuBaCo2O5+δ (δ= 0.47) (EBCO) cobaltite shows various magnetic phases at high temperature and magnetic glassy state at low temperature. Further, the various magnetic phases of the EBCO are coupled with the spin state transition of Co3+ octahedra. In the present study, we report the structural and magnetization properties of the EBCO by changing the oxygen stoichiometry. The various oxygen compositions were obtained after post annealing the as-prepared EBCO cobaltite under argon and oxygen atmospheres and the final stoichiometry was detected from the iodometric titrations. The δ= 0.13 and 0.67 samples show the tetragonal symmetry (space group, P4/mmm), whereas the δ= 0.41, 0.47 and 0.58 samples show the orthorhombic symmetry (space group, Pmmm). The magnetization measurements on the δ= 0.13 sample shows the paramagnetic to antiferromagnetic ordering at TN= 288 K and the electrical resistivity analysis shows the Co2+ and Co3+ charge ordering at TCO= 180 K. Further, the δ= 0.13 sample show the absence of the ferromagnetic phase and Co3+ spin state transitions. The δ= 0.58 and 0.67 samples show the coexistence of ferromagnetic and antiferromagnetic phases, however, the features of Co3+ intermediate spin (IS) to low spin (LS) state transition is suppressed, which is observed in δ= 0.47 sample at 151 K. Our results indicate a tuning of the crystal structure along with the magnetic properties and also the Co3+ spin state transitions by oxygen stoichiometry variation in EuBaCo2O5+δ cobaltites.

Validation of and Simulations using a High-Temperature Electrolysis Model

Efforts for increasing the share of renewable power generation are dramatically boosted since electrical power from fossil fuels has been used as weapon [1]. Therefore, efficient conversion systems are of great importance to conserve most of the delivered power by nature. High-temperature electrolysis is such an efficient type of energy conversion system compared to similar technologies [2-4]. To bridge the gap to an industrial scale application, the electrochemical and chemical processes have to be understood in detail which is enlightened by some publications from our group [5-8]. In this contribution, a multiphysics model for three types of electrolysis processes (steam, CO2 and co-electrolysis) is presented and evaluated. Relevant parameters like the exchange current density or the anodic / cathodic transfer coefficient including parameter variations for the inlet gas stream, flow rate of the gas, temperature and applied current are investigated. For the microscale properties in the functional layer, Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) is used with the “Slice and View” technique [9,10]. Extracting information like the Triple Phase Boundary Length (TPBL), the fraction of electronic and ionic phase in mixed ionic-electronic conducting (MIEC) materials or the porosity and tortuosity of the (La,Sr)CoO3-δ (LSC) air electrode and the Ni / 8 mol% yttria stabilized zirconia (Ni-8YSZ) fuel electrode help in enhancing the modeling quality of high-temperature SOC model. A conversion from the TPBL (s. Figure 1) to an active area to volume ratio for the computation is needed.Calculated iV-characteristics and experimental curves are compared and serve as a validation for (I,V) data points at higher currents. Depending on the degree of agreement, the (missing) main contributions to loss mechanisms can be identified and initiate a feedback loop for material optimization. Sensitivity analyses are carried out for parameters that are difficult to vary in experiments (e.g. porosity). Porosity and tortuosity parametrize the porous structure for a homogeneously distributed property assumption across the material layers. The ratio between electronic and ionic phase fine-tunes the positioning of iV-characteristics within the potential regime of Ohmic and polarization losses. Calculated electrochemical impedance spectra (EIS) enhance the insight into the theoretical system and may be compared to experimental spectroscopy data via Equivalent Circuit Models (ECM). Calculated data from the model are acquired as in the used experimental setup. As an example, calculated EIS data from steam electrolysis show good agreement with a R-(RQ)4 ECM which as well describes most of the corresponding experimental data in our group. Effects of degradation are envisioned to be incorporated into the model as well.[1] Hosp, G.; Höltschi, R.; Keusch, N.; Schürpf, T.: https://www.nzz.ch/wirtschaft/rohstoffe-als-waffen-die-neusten-entwicklungen-ld.1681268 (28th October 2022, 2.13 pm)[2] Foit, S.R.; Dittrich, L.; Duyster, T.; Vinke, I.; Eichel, R.-A.; de Haart, L.G.J. (2020): Direct Solid Oxide Electrolysis of Carbon Dioxide: Analysis of Performance and Processes. In: Processes 8 (11), 1390[3] Foit, S.R.; Vinke, I.C.; de Haart, L.G.J.; Eichel, R.-A. (2017): Power-to-Syngas: An Enabling Technology for the Transition of the Energy System? In: Angew. Chem. Int. Edit. 56 (20), 5402–5411[4] Tanaka, Y.; Hoerlein, M.P.; Schiller, G. (2016): Numerical simulation of steam electrolysis with a solid oxide cell for proper evaluation of cell performances. In: Int. J. Hydrogen Energ. 41, 752–763[5] Wolf, S.E.; Dittrich, L.; Nohl, M.; Duyster, T.; Vinke, I.C.; Eichel, R.-A.; de Haart, L.G.J. (2022): Boundary Investigation of High-Temperature Co-Electrolysis Towards Direct CO2 Electrolysis. In: J. Electrochem. Soc. 169, 034531[6] Unachukwu, I.D.; Vibhu, V.; Vinke, I.C.; Eichel, R.-A.; de Haart, L.G.J. (2022): Sr Substituted La2-xSrxNi0.8Co0.2O4+ d (0 ≤ x ≤ 0.8): Impact on Oxygen Stoichiometry and Electrochemical Properties.In: Energies 15, 2136[7] Mebrahtu, C.+; Nohl, M.+; Dittrich, L.; Foit, S.R.; de Haart, L.G.J.; Eichel, R.-A.; Palkovits, R. (2021): Integrated Co-Electrolysis and Syngas Methanation for the Direct Production of Synthetic Natural Gas from CO2 and H2O. In: ChemSusChem 14, 2295 – 2302[8] Dittrich, L.; Nohl, M.; Jaekel, E.E.; Foit, S.R.; de Haart, L.G.J. (Bert), Eichel, R.-A. (2019): High-Temperature Co-Electrolysis: A Versatile Method to Sustainably Produce Tailored Syngas Compositions. In: J. Electrochem. Soc. 166 (13), F971 – F975[9] Wilson, J.R.; Kobsiriphat, W.; Mendoza, R.; Chen, H.; Hiller, J.M.; Miller, D.J.; Thornton, K.; Voorhees, P.W.; Adler, S.B.; Barnett, S.A. (2006): Three-dimensional reconstruction of a solid-oxide fuel-cell anode. In: Nat. Mater. 5, 541–544[10] Gostovic, D.; Smith, J.R.; Kundinger, D.P.; Jones, K.S.; Wachsman, E.D. (2007): Three-Dimensional Reconstruction of Porous LSCF Cathodes. In: Electrochem. Solid St. 10 (12), B214–B217Figure 1: Histogram of the TPBL from a 400 image sample set of a FIB-SEM cut of a Ni-8YSZ electrode. Figure 1

Contrasting Intermediate-Temperature La0.7Sr0.3VO3.86-⸹ (LSV) Porous Catalyst Elevated Sulfur Tolerance Using Gaseous and Liquid Fuel Explored through Experimental and Modeling Characterization Over Time

The U.S. Army is investigating Solid Oxide Fuel Cells (SOFCs), which can produce clean, secure, and sustainable energy to replace existing power sources. Today SOFCs are known to provide high density, high efficiency power with long term stability at low cost and environmental impact, utilizing a variety of naturally sourced fuels such as hydrogen, hydrocarbons, and alcohols. This fuel flexibility makes SOFCs attractive for use with multiple applications and helps bridge the gap between current hydrogen-fueled SOFC technologies and future hydrocarbon-fueled based use. Despite this potential, many technical hurdles remain, such as thermal stress cracking and coarsening, delamination, and catalyst contamination. Catalyst contamination is of particular concern for the U.S. Army, which operates on JP-8 kerosene-based fuel via AR-70 single fuel policy, as sulfur impurities commonly found in naturally sourced hydrocarbon-based fuels, can poison the catalyst, permanently degrading SOFC system performance, even at low concentrations. As the U.S. Army begins to investigate SOFCs to enable additional capabilities like silent watch, advanced radios, and exportable power in their fleet, catalyst poisoning from sulfur presents a concerning technology gap that must be addressed, especially since it is also possible that sulfur contaminated fuel, scavenged in theater, may be used, and could contain elevated levels (>300ppm) of sulfur.While lower temperature SOFC operation is also actively being pursued to help alleviate physical material degradation issues, with the introduction of hydrocarbon-based fuels, catalyst development to reduce sulfur poisoning degradation also needs further exploration. This study continues to experimentally and theoretically investigate the response of sulfur tolerant anode catalyst La0.7Sr0.3VO3.86-⸹ (LSV) at currently targeted SOFC intermediate operating temperatures (400-600°C). We have previously investigated low and moderate hydrogen sulfide (H2S) concentrations (30ppm, 300ppm) in balance hydrogen and methane gas environments for up to 100 hours. In this work, LSV showed significant sulfur tolerance in hydrogen and methane balance gases when compared to the industry standard Ni-YSZ anode, where sulfur adsorption rates were 278-287x lower in some cases. The lowest rates occurred between 600-700°C, which is attributed to the cubic structure the material presents in this temperature range. Higher adsorption rates were observed in the monoclinic/tetragonal structure the material presents at temperatures between 400-500°C. Average adsorption rates were overall higher in methane. When compared against Ni-YSZ, which is known to accumulate sulfur from hydrogen sulfide via two-step dissociative adsorption, LSV was observed to accumulate sulfur through weak chemisorption (H2S*, 0.43 eV max) of molecular hydrogen sulfide on oxygen deficient surfaces and one-step dissociative adsorption (H2S*, 0.34 eV, max, HS*+H*, 8.34 eV, max) on oxygen sufficient surfaces. The weak chemisorption behavior on oxygen deficient surfaces is attributed to the reduced oxygen stoichiometry of the material, in which only vanadium V2+/3+ couples exist. Overall, molecular adsorption/dissociation was observed to occur near strontium defects on LaO/SrO terminations. The propensity of H2S adsorption on LSV surfaces was observed to be significantly less for oxygen deficient LSV, and significantly higher for oxygen sufficient LSV, when compared to H2S adsorption calculations on Ni (100) surfaces, which follow a two-step dissociative adsorption reaction (H2S* → HS*+H* → H*+S*), resulting in a strongly adsorbed S* species (5.96 eV, max).Under the Army’s single fuel policy, it is more likely SOFCs would be fueled using hydrogen and/or methane, which are both gases, however SOFCs in the future could also be directly fueled using liquid alcohol fuels, or as scavenged from theater. Typical small alcohol molecules (methanol or ethanol) do not contain high concentrations of sulfur; however, alcohols can be contaminated with sulfur-containing hydrocarbons. This work will investigate sulfur adsorption using methanol contaminated with 300ppm thiophene using the same operating temperature and heating duration conditions with hydrogen and methane. These results will be contrasted against the 300ppm H2S hydrogen and methane results previously reported. Figure 1

Enhancement of Electrocatalytic and Pseudocapacitive Properties as a Function of Structural Order in A2Fe2O5 (A = Sr, Ba).

Significant enhancements of electrocatalytic activities for both half-reactions of water-electrolysis, i.e., oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), as well as pseudocapacitive charge-storage properties are demonstrated upon changing the structural order in a perovskite-type system. The structural change is prompted by the increase in the ionic radius of the A-site ion in A2Fe2O5. The structure of Sr2Fe2O5 consists of alternating layers of FeO6 octahedra and FeO4 tetrahedra, whereas Ba2Fe2O5 comprises seven different coordination geometries for Fe. We note that the catalytically active metal, i.e., iron, and the oxygen stoichiometry are the same for both materials. Nevertheless, the change in the structural order results in significantly greater electrocatalytic activity of Ba2Fe2O5, manifested in smaller overpotentials, smaller charge-transfer resistance, greater electrocatalytic current, and faster reaction kinetics. In addition, this material shows significantly enhanced pseudocapacitive properties, with greater specific capacitance and energy density compared to Sr2Fe2O5. These findings indicate the important role of structural order in directing the electrochemical properties.

Investigation of the measurement uncertainty with regard to oxygen stoichiometry on health status of proton exchange membrane fuel cell via machine learning method

The electrochemical impedance spectrometry (EIS) measurement coupled with equivalent-circuit (EC) model is widely used to monitor the health status of proton exchange membrane fuel cell (PEMFC). However, limited studies are available to investigate the impact of measurement uncertainty on the evaluation of EC parameters. To fill this gap, this study aims at establishing a machine learning-based method to improve identification of EC parameters based on EIS measurement for PEMFC health monitoring. This computational method was employed to estimate the uncertainty of EC parameters with regard to oxygen stoichiometry that could be quantified by means of charge transfer resistance and limited current density. Gaussian process classification and combined sampling methods together with theoretical calculation were adopted to generate the synthetic data with an R2 of 0.991 to determine the measurement uncertainty of EC parameters. The effectiveness of this computational approach was validated against high-level noisy experimental data. The result shows that this approach can effectively reduce the variances of EC parameters from over 400% to less than 30% and satisfactorily quantify the measurement uncertainty within 35 s. The introduction of synthetic data allows Gaussian process regression to predict the most reliable EIS spectra in the absence of measured oxygen stoichiometry with the computational time optimized. This study expands our capacity in improving the PEMFC health management via machine learning-based computational methods.

Discovery of charge order in a cuprate Mott insulator

Copper oxide superconductors universally exhibit multiple forms of electronically ordered phases that break the native translational symmetry of the CuO2 planes. In underdoped cuprates with correlated metallic ground states, charge/spin stripes and incommensurate charge density waves (CDWs) have been experimentally observed over the years, while early theoretical studies also predicted the emergence of a Coulomb-frustrated 'charge crystal' phase in the very lightly doped, insulating limit of CuO2 planes. Here, we search for signatures of CDW order in very lightly hole-doped cuprates from the 123 family RBa2Cu3O7 - δ (RBCO; R: Y or rare earth), by using resonant X-ray scattering, electron transport, and muon spin rotation measurements to resolve the electronic and magnetic ground states fully. Specifically, Pr is used to substitute Y at the R-site to systematically suppress the superconductivity and access the extremely low hole-doping regime of the cuprate phase diagram without changing the oxygen stoichiometry. X-ray scattering data taken on Pr-doped YBCO thin films reveal an in-plane CDW order that follows the same linear evolution of wave vector versus hole concentration as oxygen-underdoped YBCO but extends all the way to the insulating and magnetically ordered Mott limit. Combined with the recent observation of charge crystal phase on an insulating surface of Bi2Sr2CaCu2O8 + z, our results in RBCO suggest that this electronic symmetry breaking is universally present in very lightly doped CuO2 planes. These findings bridge the gap between the Mott insulating state and the underdoped metallic state and underscore the prominent role that Coulomb-frustrated electronic phase separation plays among all cuprates.