Molecular Dynamics Simulation of Oxygen Diffusion in (PuxTh1−x)O2 Crystals

Polymer electrolyte fuel cells (PEFCs) are expected as a next-generation power sources due to their low emissions and high efficiencies. However, it is required to improve the power density at lower Pt loading. It is known that the losses of the power density are caused by activation losses, ohmic losses and mass transport losses, and that the mass transport losses are dominant at higher current density. Moreover, it is suggested that one of the causes of the mass transport losses is the shortage of oxygen in the cathode side of PEFC. In the cathodic catalyst layer, there are Pt catalysts on supported carbon microparticles, and those particles are covered with ionomer films which are composed of polymer electrolytes and water molecules. The ionomer has two properties for the water-generating reactions in the cathodic catalyst layer: the proton conductivity and the oxygen permeability. In particular, the dependence of the oxygen permeability on water content has not been clear. In this study, we analyzed water content dependence of oxygen permeation properties in ionomer on Pt surface using molecular dynamics simulations. Regarding the calculation system, Nafion was adopted to the polymer electrolyte in the ionomer, and water content, λ, was defined as the ratio of the number of sulfo groups in Nafion to the number of water molecules. We set λ = 3, 7, 11 and constructed a system of an equilibrium state of ionomer on Pt surface at each water content. Then oxygen molecules were inserted above the ionomer and permeate the ionomer along the thickness direction at steady state. Firstly, the density distributions were obtained to evaluate the structural properties of the ionomer. As a result, the region of the ionomer is divided into three region: ionomer/gas interface, bulk region, ionomer/Pt interface. Next, the number of permeated oxygen molecules through the ionomer was counted, and the oxygen permeability of the ionomer was estimated at different water content. As a result, the oxygen permeability decreases with increasing water content. Gas permeability is estimated using the permeation coefficient which is obtained by the product of the diffusion coefficient and solubility coefficient of gas molecules, indicating that the diffusivity and solubility are governing factors of the permeability. Therefore, the oxygen diffusivity and solubility was evaluated to discuss the oxygen permeation mechanism in the ionomer. The oxygen diffusion coefficient was obtained by the density distribution of the oxygen molecules along the thickness direction using the Fick’s law. As a result, the oxygen diffusivity in each region of the ionomer decreases at lower water content while slightly increases at higher water content, with the increase in water content. On the other hand, the oxygen solubility coefficient was calculated using the test-particle-insertion method, and the solubility in each region of the ionomer decreases with increasing water content. Finally, the oxygen permeability was evaluated using the diffusivity and solubility in each region. Consequently, the oxygen permeability decreases with increasing water content in each region, and the permeability in the ionomer/Pt interface is the smallest, which indicates that the oxygen dissolution in the ionomer/Pt interface is dominant in the oxygen permeation through the ionomer.

The diffusion of oxygen in the cornea

Diffusion of oxygen into pitch

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To interpret the diffusion behavior of oxygen in epoxy asphalt with different cross‐linking degrees, epoxy asphalt‐oxygen diffusion layer models were constructed. The effect of temperature on oxygen diffusion and the number of oxygen molecules penetrating the epoxy asphalt was quantitatively analyzed. Fick's second law was used to determine the oxygen diffusion coefficient. The Mean square displacement (MSD) was extracted to analyze the effect of cross‐linking degree on the molecular motion of the base asphalt. The results show that as the cross‐linking degree increases, the number of oxygen in the epoxy asphalt decreases, accompanied by a reduction in the oxygen diffusion coefficient. Temperature significantly affects the rate of oxygen diffusion, with higher temperatures leading to greater diffusion depths over the same simulation time. The oxygen diffusion coefficient of epoxy asphalt ranges from 4.60 × 10−11 to 2.20 × 10−10 m2/s at temperatures of 298–373 K. The epoxy system markedly restricts the movement of base asphalt molecules, with saturate being most affected by cross‐linking degree and asphaltene the least. A high cross‐linking network impedes the asphalt‐oxygen contact oxidation, enhancing epoxy asphalt's anti‐aging capacity. These findings provide support for optimizing epoxy asphalt formulations and guiding the development of anti‐oxidant epoxy asphalt.

Molecular dynamics simulations have been widely adopted to study oxygen ion diffusion mechanisms in materials for application in solid oxide fuel cells. Indeed, understanding the fundamental aspects of oxygen diffusion is important to develop new materials for this application. In this work, Nd1-xAExBaInO4-x/2 (AE = Ca, Sr, Ba) compounds have been studied by MD simulations focusing on oxygen diffusion mechanisms. Two general clustering methods were used, namely a convex hull classification method and a DBSCAN machine learning algorithm, to identify oxygen ion diffusion pathways. Here, relevant details are provided for an efficient use of these two approaches during MD analysis of ion conductors. The calculations show that Ca is the most favorable dopant for substituting Nd in NdBaInO4, while Ba is the least desired. Indeed, the substitution of Nd by Ca hardly changes the pristine lattice parameters of NdBaInO4 and leads to the highest oxygen diffusion coefficient compared to other dopants. The oxygen vacancies induced by doping mainly locate on two specific oxygen sites over four oxygen sites available. Concerning the diffusion process, jumps involving these two sites play the main role and are associated with smaller migration enthalpies. For the main diffusion path, ions migrate along the b (2 routes) and c (4 routes) directions. Some other oxygen sites can be considered as barriers for the diffusion process inducing a strong anisotropy in the diffusion process. Additionally, the residence time analysis of oxygen ions confirms that ions at different sites have different motion abilities. As a whole, the approach presented here can be extrapolated to other ion conductors for gaining detailed information about the diffusion process.

This study employs ab initio calculations to systematically evaluate the stability, equilibrium volumes, activation energy barriers, and oxygen diffusion properties of La 2-x A x NiO 4+δ (A = Ca, Sr, Ba; x = 0, 0.125, 0.25). The findings indicate that Ca and Sr doping leads to a reduction in the equilibrium volume, while Ba induces a slight increase. Additionally, doping with any of these three elements increases the formation energy, thereby reducing the overall stability of the material. The activation energy barrier is primarily dictated by ionic size, with smaller ions such as Ca lowering the barrier, while larger ions, including Sr and Ba, increase it. The dominant oxygen diffusion pathway varies with dopant type, with Path #1 (Intersticialcy mechanism along the ab plane) being the preferred mechanism in Sr- and Ba-doped compounds. Ca doping improves both the oxygen diffusion coefficient and ionic conductivity, whereas Sr and Ba result in reduced values for these properties. These results provide valuable insights for the optimization of oxygen transport in La 2-x A x NiO 4+δ . • The stability and activation energy barrier of La 2-x A x NiO 4+δ with different A-site dopants were predicted. • The partition function was used to estimate the average energy barrier across diffusion paths. • Ca doping enhances oxygen diffusivity in La 2-x A x NiO 4+δ . • La 2-x Ca x NiO 4+δ shows the highest attempt frequency among the studied compounds.

(La 0.8Sr 0.2)(Mn 1− yFe y)O 3± δ oxides for ITSOFC cathode materials? : Electrical and ionic transport properties

Porosity effects on oxygen ions diffusion in the yttria-stabilized zirconia (YSZ) by molecular dynamics simulation

Previous oxygen microelectrode studies have shown that the oxygen diffusion coefficient (DO₂) increases during extracellular PO₂ decreases, while intracellular PO₂ remained unchanged and thus cell function (spike activity of neurons). Oxygen dependency of complex multicellular organisms requires a stable and adequate oxygen supply to the cells, while toxic concentrations have to be avoided. Oxygen brought to the tissue by convection diffuses through the intercellular and cell membranes, which are potential barriers to diffusion. In gerbil brain cortex, PO₂ and DO₂ were measured by membrane-covered and by bare gold microelectrodes, as were also spike potentials. Moderate respiratory hypoxia was followed by a primary sharp drop of tissue PO₂ that recovered to higher values concomitant with an increase of DO₂. A drop in intracellular PO₂ recovered immediately. Studies on the abdominal ganglion of aplysia californica showed similar results.Heterogeneity is a feature of both normal oxygen supply to tissue and supply due to a wide range of disturbances in oxygen supply. Oxygen diffusion through membranes is variable thereby ensuring adequate intracellular PO₂. Cell-derived glucosamine oxidase seems to regulate the polymerization/depolymerisation ratio of membrane mucopolysaccharides and thus oxygen diffusion.Variability of oxygen diffusion is a decisive parameter for regulating the supply/demand ratio of oxygen supply to the cell; this occurs in highly developed animals as well as in species of a less sophisticated nature. Autoregulation of oxygen diffusion is as important as the distribution/perfusion ratio of the capillary meshwork and as the oxygen extraction ratio in relation to oxygen consumption of the cell. Oxygen diffusion resistance is the cellular protection against luxury oxygen supply (which can result in toxic oxidative species leading to mutagenesis).

Diffusion properties of oxygen in the α2-Ti3Al alloy

The chemical stability of a ferritic alloy (Fe-Cr alloy) was examined as interconnect under CH 4 -H 2 O gases to simulate the real fuel gas conditions. Oxide scale layers were formed on the alloy surface by reaction with CH 4 -H 2 O gases at 1073 K. The surface morphology varied with the formation of needle-like or plate-like Cr-Mn-(Fe) oxide spinels and Cr 2 O 3 . Secondary ion mass spectrometry (SIMS) was used to measure the distribution of minor and major elements around the oxide scale/alloy interface. High concentration of Mn on the oxide scale surface suggested fast diffusion of Mn in the oxide scale to form spinels. Annealing in CH 4 -H 2 O made the oxide scale thicker with time. Growth of the oxide scale layer thickness followed the parabolic growth relationship with time. The parabolic growth rate constants were in the order of 10 -6 μm 2 s -1 for all experimental conditions. Isotope oxygen ( 18 O 2 ) diffusion coefficients of oxide scale were estimated by the 18 O diffusion profiles at 1073 K, which was in the order of 10 -4 μm 2 s -1 . Therefore, the oxygen diffusion was not the rate determining step for oxide scale formation. The electrical conductivity of oxidized alloy showed no dependence on p(CH4) in the reaction gases.

Yttria-stabilized zirconia (YSZ) is of interest to the aerospace community, notably for its application as a thermal barrier coating for turbine engine components. In such an application, diffusion of both oxygen ions and cations is of concern. Oxygen diffusion can lead to deterioration of a coated part, and often necessitates an environmental barrier coating. Cation diffusion in YSZ is much slower than oxygen diffusion. However, such diffusion is a mechanism by which creep takes place, potentially affecting the mechanical integrity and phase stability of the coating. In other applications, the high oxygen diffusivity of YSZ is useful, and makes the material of interest for use as a solid-state electrolyte in fuel cells. The kinetic Monte Carlo (kMC) method offers a number of advantages compared with the more widely known molecular dynamics simulation method. In particular, kMC is much more efficient for the study of processes, such as diffusion, that involve infrequent events. We describe the results of kinetic Monte Carlo computer simulations of oxygen and cation diffusion in YSZ. Using diffusive energy barriers from ab initio calculations and from the literature, we present results on the temperature dependence of oxygen and cation diffusivity, and on the dependence of the diffusivities on yttria concentration and oxygen sublattice vacancy concentration. We also present results of the effect on diffusivity of oxygen vacancies in the vicinity of the barrier cations that determine the oxygen diffusion energy barriers.

The mass transfer characteristics of gas diffusion layer (GDL) are closely related to cell performance in polymer electrolyte fuel cell (PEFC). It is necessary to characterize the liquid water distribution, the microscopic conformation, and the oxygen diffusion coefficient in a GDL containing moisture. The configurations of a novel hybrid type GDL, in which two porous media with different wettabilities are arranged alternately (1),(2) and two porous media with different pore size distributions were alternately arranged (3), have been proposed with the aim of improving the oxygen diffusivity by controlling the moisture distribution in GDL porous media. However, the basic principles of hybrid type GDLs were examined in the former reports as a pilot study by using thicker media than those used in practice. It is thus necessary to elucidate the performance of the hybrid type GDLs with thinner sizes for the practical use. The present study examined a hybrid type GDL to control the liquid water behavior in the porous media. A carbon paper GDL used in practice for PEFCs with a wettability distribution due to adjacent hydrophilic and hydrophobic region was used as a test material. The measurement of oxygen diffusivity and visualization of liquid water in the GDL with wettability distribution were performed simultaneously using a galvanic battery type oxygen absorber and X-ray radiography, respectively.

Effects of oxygen diffusion and reaction kinetics in photodynamic therapy are considered in the context of a multicell tumour spheroid model. Steady-state measurements of oxygen made with a Clark-style microelectrode (4 mu m diameter tip) enable the authors to determine the rate of metabolic oxygen consumption and the oxygen diffusion coefficient in 500 mu m diameter EMT6/Ro spheroids. These values are 5.77 mu mol 1-1 s-1 and 1460 mu m2 s-1, respectively. Time-dependent electrode measurements of oxygen concentration during laser irradiation of individual Photofrin-sensitized spheroids are fitted to numerical solutions of a pair of diffusion-with-reaction equations. The analysis yields the rate of photodynamic oxygen consumption and a parameter that governs the oxygen sensitivity of photodynamic therapy. These experimentally derived quantities are used to calculate the temporal and spatial distributions of oxygen and the rate of oxygen consumption in a spheroid during irradiation at several fluence rates. The spatial distribution of photodynamic oxygen consumption is strongly fluence rate dependent. Using the experimental and theoretical results developed in this report, previously published survival data are analysed. The analysis indicates that the threshold dose of reacting singlet oxygen in the EMT6/Ro spheroid is 323+or-38 mu mol 1-1 (mean+or-SEM).