Effective Diffusion Coefficient Of Oxygen Research Articles

Molecular Dynamics Analysis of the Scattering Phenomena of Oxygen Molecules on an Ionomer Surface in Catalyst Layer of Fuel Cell

As energy demand increases and global warming progresses, expectations for fuel cells, which generate electricity through chemical reactions between hydrogen and oxygen, are rising. There are various types of fuel cells, which are classified according to the electrolyte. Polymer electrolyte fuel cells (PEFCs) are expected to be used in stationary power sources for home use and fuel cell vehicles because of their low operating temperature, short start-up time, and ease of miniaturization. The cost has been an issue in the widespread use of fuel cells. The amount of platinum used in the catalyst layer (CL) must be reduced to reduce costs. However, it increases the current density per unit area of platinum. There are three factors that contribute to voltage drop: ohmic polarization, activation polarization, and diffusion polarization. At high current densities, the voltage drop due to diffusion polarization is dominant. The main cause of diffusion polarization is the transport resistance of oxygen molecules in the CL, and improving the oxygen transport properties in the CL will lead to lower cost PEFCs. In this study, we analyzed the transport properties of oxygen molecules in the CLs of PEFCs using the Monte Carlo (MC) method. The effect of oxygen scattering on the overall oxygen transport properties was investigated by considering the behavior on the surface of the ionomer film (surface diffusion) into the MC method, which simulates the overall transport in the PEFC CL. The objective of this study is to reproduce accurate oxygen transport by comparing the results of MC and molecular dynamics (MD) methods. First, we used the MC method to analyze oxygen scattering phenomena. In this study, we used the 3D structure data of CL in the MC simulations. The three-dimensional structure of the sample was reconstructed from sequential slice images. Molecules were assumed to reflect specularly on the boundaries of the simulation domain. Oxygen molecules were randomly placed in the simulation system. The time step was set at 5 ps and the simulation time was set at 4 μs. In order to examine the effect of surface diffusion on the transport properties of oxygen molecules, the simulation was performed using various conditions of surface diffusion. We calculated the effective diffusion coefficient of oxygen in the CL as the transport property of oxygen molecules in the MC method. As the surface diffusion coefficient increases, the effective diffusion coefficient of oxygen increases. When the surface diffusion coefficient is small, the effective diffusion coefficient of oxygen increases due to a decrease in the time constant, which determines the probability distribution of the surface residence time. However, when the surface diffusion coefficient is large, a different trend emerges. As the time constant increases, the effective diffusion coefficient has a peak value and then decreases. Next, to verify the model of surface diffusion in the MC method, we analyzed the scattering phenomenon of oxygen molecules in a simulation system that simulates the pores in the CL using the MD method. We modeled the nano pores in the CL as a slit between the ionomer walls in the MD method. Nafion with the equivalent weight (EW) of 1146 was used as polymer model. The number of Nafion chains is set at 4, and the water content λ is set at 7. Three-dimensional periodic boundary conditions were applied to the simulation domain. After the system was equilibrated, 1 ns of NVT simulation was performed for production. The temperature was set at 297 K. The behavior of oxygen molecules impacting on the ionomer wall was analyzed. We analyzed the behavior of oxygen molecules colliding with the ionomer membrane using the MD method. The average residence time of surface diffusion on the ionomer thin film was of the order of 10-10s. When the calculation results were compared with the MC results, it was found that the average residence time obtained by the MD method was the value where the surface diffusion coefficient affects much on the effective diffusion coefficient. Comparing the results of the MD method with the surface diffusion model used in the MC method, there are some discrepancies between the results and the model. For example, the MC method used a model in which oxygen molecules reflect diffusively on the ionomer surface. However, in the MD method, it was found that oxygen molecules tend to reflect in the direction of travel (Fig. 1). Therefore, a more accurate model of the MC method needs to be developed in the future. Figure 1

Quantification Method of Mass Transfer Resistance in Cathode Catalyst Layer in PEFC

IntroductionTo spread PEFCs, it is important to optimize the cathode catalyst layer (CCL) because the oxygen reduction reaction (ORR) in CCL is sluggish. Our previous study proposed that the mass transfer resistance in the through-plane direction in CCL can be quantified by measuring ORR rate dependency on oxygen partial pressure, effective oxygen diffusion coefficient, ORR rate constant, or CCL thickness[1]. In this study, dependency of the polarization curve on the oxygen partial pressure was measured with commercial supported-platinum catalysts at fixed RH and temperature. The objective of this study is to establish the quantification methods of mass transfer resistances in CCL.TheoryIn our previous study[2, 3], the current density, i is expressed as i = 4F δ (C) k vcm p Oc F e [A/m2], (1)where δ (C) is the cathode catalyst layer (CCL) thickness, k vc is the reaction rate constant per unit volume of CCL [mol/(Pa·m3·s)] which does not include through-plane mass transport resistance. p O is the oxygen partial pressure [Pa]. The subscripts m and c denote PEM–CCL boundary and CCL–gas diffusion layer boundary, respectively. F e is the effectiveness factor, a function of the following 4 dimensionless moduli, M Om , M pm , P Om , and y Oc . F e is defined as the ratio of the observed reaction rate to the reaction rate which does not have the influence of through-plane mass transfer resistance. M Om denotes the ratio of O2 diffusion resistance to the reaction resistance. M pm denotes the ratio of proton transfer resistance to the reaction resistance. P Om is defined as the ratio of O2 diffusion resistance to the convection resistance. y Oc denotes the oxygen mole fraction. Definitions of the dimensionless four moduli are expressed as M Om =δ (C) (k vcm RT/D eO)0.5, (2) M pm = δ (C) (4F k vcm RT/(σ ep b c))0.5, (3) P Om = δ (C) N gm /(C g D eO), (4) y Oc = p Oc /P, (5)where R, T, σ ep, N gm and P denotes the gas constant [J/(mol K)], cell temperature [K], effective proton conductivity [S/m], total gas flux [mol/(m2 s)] and total pressure [Pa], respectively. When the cathode emf at the PEM–CCL boundary, E cm , T, and relative humidity (RH) are fixed, M Om is fixed. F e and M pm are proportional to i/p Oc and p Oc 0.5, respectively. The relationship between F e and M pm shown in Fig. 1 corresponds to that between i/p Oc and p Oc 0.5. Thus, F e and M pm can be determined just by fitting experimental data of i/p Oc plotted against p Oc 0.5 to the theoretical curves.Results and DiscussionPolarization curves shown in Fig. 2 were measured at 80 °C and 0.77 of inlet RH at varied p Oc . As expected from Eq. (1), current densities at a fixed E cm increases when p Oc increases. The current densities at the fixed E cm are plotted against p Oc as shown in Fig. 3. The current density is sublinear against p Oc . This is because M pm expressed in Eq. (3) increased when p Oc is higher. The ORR rate is lower than the value expected from the Tafel equation when mass transfer resistance increases.The dimensionless moduli were determined by fitting the theoretical relationship to the measured i/p Oc vs. p Oc 0.5 graph. From the determined dimensionless moduli, the ORR rate and mass transport parameters, k vc⊕, D eO and σ ep were also obtained and these values are shown in Table 1 with values in literature[4, 5].Effective proton conductivity, σ ep is relatively low compared with values in literature. This resulted from an experimental error in the current density at p Oc of 16 kPa. Lowering the current density at high p Oc leads to decrease in σ ep because M pm increases when p Oc is raised. By extensively measuring the polarization curves dependency on p Oc , the ORR rate parameters and mass transport parameters can be determined more accurately. Recalculated results using the obtained mass transport parameters are shown in Figs. 3 and 4. The recalculated results appropriately reproduced the ORR rate dependency on E cm and p Oc .ConclusionsThe through-plane mass transfer resistance in CCL was quantified by measuring the ORR rate dependency on the oxygen partial pressure. From the determined dimensionless moduli, mass transport parameters were obtained. Recalculated results appropriately reproduced the ORR rate dependency on cathode emf and oxygen partial pressure.AcknowledgementThis work was supported by the FC-Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high performance project (FY 2020–2023) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1

Efficient data generation for the determination of effective oxygen diffusion coefficients of cementitious materials

AbstractTo develop new cementitious materials with the goal of reducing the carbon footprint a rapid assessment of information about their resistance against damaging processes is of utmost importance to ensure long‐term durability. The Oxygen diffusion test is an efficient method to evaluate the ingress of gas in concretes and mortars without changing the porous structure of the specimen during testing. However, currently no standardized test method exists. We determined the effective Oxygen diffusion coefficient, De,O2 of mortar discs using a single diffusion cell method. Furthermore, we enhanced the procedure by extrapolating the oxygen saturation curves of the diffusion cells with a simple asymptotic function to identify a minimum required test duration where the extrapolated data still delivers reliable results and hence, shorten the overall required measurement time. In our case, we were able to significantly reduce the effort needed to obtain results that are within the range of the respective standard deviation of individual specimens. Our findings further strengthen the feasibility of time and cost efficient gas diffusion testing methods that could deliver complementary information for evaluating existing and designing new cementitious materials.

Quantification Method of Mass Transfer Resistance in Cathode Catalyst Layer in PEFC

The quantification method of mass transfer resistance in the cathode catalyst layer (CCL) should be established in order to appropriately evaluate the cell performance. Lots of PEFC research assumes that any resistances in PEFC are in series to reaction resistance. In this study, oxygen and proton transfer resistances in CCL were quantitatively separated from reaction resistance by applying the model where the through-plane mass- transport resistances were parallel to the reaction resistance. In order to evaluate oxygen and proton transfer resistance, a dimensionless cathode isothermal one-dimensional model was applied. The mass transfer resistance in CCL can be quantitatively evaluated by obtaining the value of five dimensionless moduli derived from mass balance. Polarization curves dependency on the oxygen partial pressure and relative humidity (RH) were measured at the temperature of 80 °C. Reaction rate parameters, effective oxygen diffusion coefficient, effective proton conductivity, and oxygen permeance through the ionomer layer were determined.

Effect of assembly pressure on the performance of proton exchange membrane fuel cell

The assembly force is a crucial factor in the process of proton exchange membrane fuel cell (PEMFC) stacking, and has significant effects on the fluid flow, mass transfer, and water and thermal management, which affect the fuel cell performance. In this study, from the most deformable component, the gas diffusion layer (GDL), combining with a finite-element analysis, and computational fluid dynamic method, the impact of the assembly force on a single-channel PEMFC is analyzed. A nonlinear stress–strain curve obtained from a microanalysis is creatively introduced into the two-dimensional compression model. The gas diffusion coefficient in the three-dimensional model is also obtained from the microscopic simulation. The simulated effective oxygen diffusion coefficient of the compressed GDL is approximately 0.86 times the Bruggemann estimated value. When the contact resistance is ignored, the output voltage at 2.5 MPa is decreased by approximately 15.4% at 1.7 A·cm−2 compared with that at 0.5 MPa. After the contact resistance is considered, the effects of the assembly pressure on the cell performance (V–I curve) are qualitatively different. The pressure drop of the 2.5 MPa case is 20% higher than that of the 1.4 MPa case at 1.7 A cm−2. O2 is hard to flow into the region under the rib where the porosity and permeability are lower. The results indicate that both liquid water and membrane water contents increase when the assembly force increases. The effect of the assembly force on the temperature is also analyzed.

Influence of liquid water accumulation on the impedance of a PEM fuel cell operating in dead end mode: Physical modeling and experimental validation

The dead-end mode operation of a PEM fuel cell reduces the reactants consumption and the system’s complexity, but also decreases its performance due to the accumulation of liquid water, which must be purged periodically. Despite extensive use of the EIS technique in studying PEM fuel cells’ internal processes, the inherently transient nature of dead-end mode operation has challenged the application and interpretation of this technique. In this paper, a novel analytical model is proposed to physically investigate the effect of liquid water accumulation on the impedance of a PEM fuel cell in dead-end mode operation. The model is well validated against experimental polarization curves and impedance diagrams in various operating conditions by employing a two-step fitting procedure. The findings of the model indicate the constant presence of a large amount of liquid water in the fuel cell due to the dead-end operation. Additionally, the buildup of liquid water reduces the active area of the catalyst layer and the effective oxygen diffusion coefficient in GDL, increasing both the charge and mass transfer resistances. Finally, with its low computational cost, the developed analytical model can provide a suitable framework for purposes such as fault diagnosis and degradation prognosis.

(Digital Presentation) Simulation of Agglomeration in Polymer Electrolyte Fuel Cell Catalyst Inks

Polymer electrolyte fuel cells (PEFCs) are expected to use as a power source for new generation automobiles, especially heavy-duty vehicles because of their low environmental affection. To be widely spread, it is essential to increase their power output and reduce the cost of catalyst platinum. Since oxygen reduction reaction (ORR) at the cathode is the dominant reaction, there is a need to improve the oxygen transport and proton conductivity. In recent years, porous carbon has been studied and it has been found its high activity and durability. However, the catalyst layer made of porous carbon gets a lower voltage than that made of non-porous carbon in the high current density area. This output reduction results from high oxygen transport resistance, but it is unclear how the porous structure influences the performance. So in this study, we assumed the following relation. Firstly, the difference in voltage at a high current density between carbons results in oxygen transport properties. Secondly, their properties depend on an effective oxygen diffusion coefficient, which is determined by the porous structure in the catalyst layer. And finally, the factor that attributes the catalyst layer structure is the size of the agglomerate in the catalyst ink because the catalyst layer is fabricated by the wet process. Therefore, we focused on the particle agglomeration process in catalyst ink and calculated the agglomerate behavior under various ink conditions using a numerical model. The differences in the agglomeration and its factors were also examined for carbon with different internal pores.A three-dimensional discrete element method (DEM) was used to calculate agglomeration. Initially, aggregates, the smallest unit of carbon black, were placed randomly in the cubic region. In this simulation, aggregates were regarded as spheres. The distance of movement was determined by solving the equation of motion. For each particle, the fluid drag, Brownian motion, and particle interactions were considered. The particle-particle interaction included the forces based on DLVO theory [1] and the ionomer effect. Several studies show that ionomer adsorbed on the carbon aggregates improves their stability. So it was assumed that the ionomer effect was the repulsive force and modeled using the same equation as for the EDL force. The dielectric constant dependence of this effect was used in the function previously reported by Magnus et al. [2]. The carbon particles assumed here were non-porous (Vulcan) and porous (Ketjen), and were coated with Pt particles. The initial average diameter of the aggregates was 300 nm. Hamaker coefficient was expressed as an average of the physical properties of platinum and carbon black, which was weighted by the percentage of platinum loaded on the surface of the carbon particles. The solvent was a mixture of 1-propanol (NPA) and water, and the dielectric constant of the ink was expressed as a weighted average of the composition ratio of water and NPA. The simulation was conducted under various water/alcohol ratios and ionomer/carbon ratios (I/C). From this model, it was confirmed that the carbon internal pores affect the size of the agglomerate in the catalyst ink.

Evaluation of Porous Media Gas Diffusion Models for PEMFC Applications

. Polymer electrolyte membrane fuel cells (PEMFCs) are considered as zero emission power sources for transportation and stationary power purposes. The membrane electrode assembly (MEA) is the core of PEMFC and is composed of a gas diffusion layer (GDL), catalyst layer (CL) and proton exchange membrane (PEM). GDL is a carbon-based, fibrous porous medium that simultaneously provides a path for heat, mass and electron transport, as well as providing a mechanically robust support for the CL. The gas diffusion in the GDL can be estimated by Fick’s law where the effective diffusion coefficient of gaseous species is used.There are many models in the literature based on correlations defining the effective diffusion coefficients through GDLs. Some of these models were originally derived to estimate the transport properties of a porous media composed of spherical particles, e.g. Bruggeman approximation and effective medium approximation. Inherently, such models tend to result to more inaccurate outcomes compared to the models which assume the GDL structure as cylindrical carbon fibers, i.e. the diffusion model based on percolation theory. The percolation theory model considers GDL as a medium composed of freely overlapping fibers oriented in different directions. However, there are several models available in the literature with less simplifying assumptions in GDL structure. The pore network model (PNM) reconstruct the porous media using topology and size information extracted from high resolution tomographic patterns. Also CFD based models can even investigate the actual GDL structure and reconstruct the 3D stochastic porous medium microstructure. These models combine pore-scale model with CFD approaches, e.g. lattice Boltzmann method (LBM) or direct numerical simulation (DNS). This study compares the available models for dry gas diffusion in GDL with experimental data acquired from symmetrical modified Loschmidt cell (SMLC). The SMLC is employed to measure the effective diffusion coefficient of oxygen passing through GDL samples, i.e. SGL SIGRACET 24BA, 24 BC, 25BA, 25BC and TGP-H-060. The SMLC result for effective diffusion coefficient in TGP-H-060 has less than 2% difference with the available data in the literature for this type of GDL. In order to evaluate the accuracy of effective medium models and percolation theory model, the experimental data for the above-mentioned GDLs is compared with the predictions of these models. The porosity of GDL samples are in the valid range of diffusion models. The diffusion models based on the effective medium approximation have the greater difference with SMLC data in compare with the percolation theory model. The model’s predictions are the worst for the GDLs with microporous layer (MPL), i.e. SIGRACET 24 BC and 25BC. Since the MPL imposes an extra resistance to gas diffusion which is not considered in any GDL diffusion models. The least error in model’s outcome is 30% which associates to the effective diffusion coefficient predicted by percolation theory model for SIGRACET 25BA.

Effect of long-term fertilization on bacterial community in a sandy loam soil and its relation to organic carbon accumulation

Fertilization can affect the transformation of soil organic carbon (SOC) and soil microbial community composition. However, thus far, how SOC accumulation in association with bacterial community is still unclear. We collected arable soils (aquic inceptisol) from a long-term fertilization experiment (20 years) including compost (CM), inorganic nitrogen + phosphorus + potassium (NPK), half compost N plus half inorganic fertilizer N (HCM), NP, NK, PK, and untreated (Control). We investigated the relationship between the SOC accumulation rate and bacterial community composition measured by high-throughput sequencing. The highest SOC accumulation rate was observed in the compost treatments. Furthermore, compost and balanced NPK treatments increased soil carbohydrate content significantly (P < 0.05), while no such enhancement was observed following NK and PK application. Compared with the Control, fertilization substantially reduced the effective diffusion coefficient of oxygen in soil. Meanwhile, fertilization lowered the relative abundance (RA) of Bacteroidetes but increased the RA of Proteobacteria. Compost application increased the RA of Firmicutes, while inorganic fertilizers reduced it. The RA of Proteobacteria and Firmicutes were significantly and positively correlated with the SOC and carbohydrate content and the SOC accumulation rate (P < 0.05). SOC accumulation was also accompanied with the reduction in the effective diffusion coefficient of oxygen in soil. Our results indicated that poor aeration may induce a shift in the microbial community composition and a transition from aerobic to anaerobic degradation of SOC, thereby favoring SOC accumulation.

Analysis of the Effect of Surface Diffusion on Effective Diffusivity of Oxygen in Catalyst Layer By Direct Simulation Monte Carlo

Because in the catalyst layers (CLs) of polymer electrolyte fuel cells (PEFCs) platinum (Pt) is used as catalyst, we need to reduce the amount of Pt for cost reduction. In order to maintain the same output even when the amount of Pt is reduced, it is necessary to increase the current density per unit area of Pt surface. At high current density, mass transport is a major factor in voltage drop. Therefore, improving the oxygen transport properties in CLs will lead to cost reduction of PEFCs. Although the transport properties of oxygen in the porous structure of CLs have been studied, the effect of surface diffusion, which is the motion of an oxygen molecule on the ionomer thin film, on transport properties has not yet been clarified. The purpose of the present study is to analyze how the motion of an oxygen molecule on the ionomer thin film affects the oxygen transport properties in the CLs of PEFCs.We used the direct simulation Monte Carlo (DSMC) method. In this study, a computational system was constructed based on an actual catalyst layer sample. The three dimensional structure of the sample was obtained as sequential slice images using a focused ion beam-scanning electron microscope (FIB-SEM). In order to examine the effect of surface diffusion on the transport properties of oxygen molecules, the simulation was performed using a various conditions of the surface diffusion. The behavior of surface diffusion was determined by a surface diffusion coefficient (D s) and a time constant (τ).Figure 1 shows the results of the effective diffusion coefficient of oxygen in the CL. The red line shows the effective diffusion coefficient of oxygen in the calculation without considering the surface diffusion, which is 2.75×10-6 m2/s. The plot marks shows the each surface diffusion coefficient. As the surface diffusion coefficient increases, the effective diffusion coefficient of oxygen increases. When the surface diffusion coefficient is small, the increase in the time constant results in the decrease in the effective diffusion coefficient of oxygen. When surface diffusion coefficient is large, the increase in the time constant leads to the larger effective diffusion coefficient of oxygen. However, when the surface diffusion coefficient is large, as the time constant increases, the effective diffusion coefficient of oxygen has a peak value and then decreases. Although the surface diffusion coefficient becomes larger, the effective diffusion coefficient does not continue to increase. This trend occurs presumably due to the tortuosity of the three dimensional structure. When an oxygen molecule moves along the bended three dimensional structure, the distance of the motion of the oxygen molecule become larger than the straight-line distance between the start and end points. This is the reason why the effective diffusion coefficient does not continue to increase.Fig. 1: Effective diffusion coefficient of oxygen in the catalyst layer as a function of the time constant of the surface diffusion. The difference in plot marks shows the difference in the surface diffusion coefficient. The red line shows the effective diffusion coefficient of oxygen in the calculation without considering the surface diffusion. Figure 1

Analysis of the Effect of Surface Diffusion on Effective Diffusivity of Oxygen in Catalyst Layer By Direct Simulation Monte Carlo

We analyzed the transport properties of oxygen molecules using the Monte Carlo method. We obtained the effective diffusion coefficient of oxygen molecules in a catalyst layer of polymer electrolyte fuel cells. In this calculation system, when oxygen molecules hit a wall, it diffuses on the ionomer surface for a period of time and then diffusely reflected. The simulation was performed using various conditions of the surface diffusion. The conditions are defined by the surface diffusion coefficient and the time constant that controls the residence time of oxygen molecules. When the surface diffusion coefficient is small, the increase in the time constant results in the decrease in the effective diffusion coefficient of oxygen. However, when the surface diffusion coefficient is large, as the time constant increases, the effective diffusion coefficient of oxygen has a peak value and then decreases. This phenomenon occurs due to the three-dimensional bending structure of the catalyst layer. When an oxygen molecule moves along the bending three-dimensional structure, the distance along the ionomer surface becomes larger than the straight line segment between the start and end points.

Determination Method of Dimensionless Moduli for Estimation of PEFC Performance

IntroductionTo predict PEFC performance without complicated simulation or a lot of experimental data is required for the optimum design of the cell, especially the cathode catalyst layer (CCL). In our previous study, the effectiveness factor of CCL was reported to be a function solely of 4 dimensionless moduli which were derived from the isothermal one-dimensional model of the cathode consisting of oxygen balance, oxygen transport, proton transport, reaction stoichiometry, and rate [1]. In this study, the dependency of the oxygen reduction reaction (ORR) rate on CCL thickness is analyzed and a method to determine dimensionless moduli is proposed.TheoryThe current density, i, is a product of the reaction rate without through-plane mass transfer resistances, k vcm p Oc , and the effectiveness factor, F e. k vc is the reaction rate constant per unit volume of CCL [mol/(Pa·m3·s)] and p O is the oxygen partial pressure [Pa], and the subscript m denotes PEM–CCL boundary and the subscript c denotes CCL–gas diffusion layer (GDL) boundary. i = 4 F δ k vcm p Oc F e [A/m2], (1)where δ is the CCL thickness, and F is the Faraday constant. F e is a function of 4 dimensionless moduli, M Om , M pm , P Om , and y Oc . M Om : Ratio of oxygen diffusion resistance to reaction resistance (Thiele modulus), M pm : Ratio of proton transport resistance to reaction resistance (our modulus), P Om : Ratio of oxygen diffusion resistance to convection resistance (Peclet number), and y Oc : Oxygen mole fraction. M Om and M pm are defined as follows: M Om = δ (k vcm RT / D eO)1/2 = c MO δ, (2) M pm = δ [4Fk vcm p Oc / (σ ep b c)]1/2 = c Mp δ, (3)where D eO is the effective oxygen diffusion coefficient [m2/s], and σ ep is the effective proton conductivity [S/m].Eqs. (1) and (2) indicate that the relationship between i and δ corresponds to the relationship between F e M Om and M Om when the other parameters are fixed. The former relationship is obtained from measurements and the latter is obtained from the theory.ExperimentalThe membrane electrode assembly (MEA) was consisted of the catalyst layers made of TEC10E30E (Tanaka Kikinzoku Kogyo K.K.) catalyst with Nafion® ionomer and membrane (DuPont NR-212). Ionomer/carbon weight ratio was 1.0, and the CCL thickness was 10, 28.3, and 46.8 mm (Pt loading was 0.15, 0.43, and 0.71 mg/cm2). H2 and O2 were humidified in bubblers at 75 ˚C. H2 flow rate was 600 cm3/min, and O2 flow rate was 300 cm3/min. The cell was operated at 80 °C and the pressure at the cell outlet was 1 atm.Results and DiscussionTo determine the dimensionless moduli from experimental data, dependency of ORR rate on p Oc , D eO, k vcm , and δ can be used for the analysis. Fig. 1 shows the effects of these variables on F e at fixed P Om and y Oc . By fitting the theoretical curve with the experimental data, dimensionless moduli, M Om and M pm can be determined. As shown in the figures, a wide range of data is desirable to determine dimensionless moduli. Since total pressure range is limited, it is not realistic to determine the dimensionless moduli from the total pressure dependency.The dependency of ORR rate on δ was measured as shown in Fig.2. Fig. 3 shows current density measured at E cm = 0.7 V using MEAs with different CCL thicknesses and dimensionless model analysis. F e M Om has a maximum point against M Om as expected [2]. As δ increases, the Pt amount increases proportionally but the transport resistance increases M Om and M pm and as a result, the effectiveness factor decreases. Consequently, the increase in the current density is limited. Tafel slope, b c = 0.0346 V was obtained from the experiment using Pt-sputtering catalyst for the analysis [3]. At E cm = 0.7 V, M Om and M pm were determined to be 0.38 in the case of δ = 10 mm. D eO = 7.27×10-8 m2/s and σ ep = 18.3 S/m are determined from Eqs. (2) and (3).Determination of M Om and M pm from the I-V measurements using MEAs with different δ's was demonstrated as an example. Measurements using a single MEA at different p Oc 's, measurements using MEAs with different δ's or using MEAs with different Pt/C catalysts can be used for the determination of the dimensionless moduli.When dimensionless moduli are determined, the optimum CCL structure such as CCL thickness and Pt/C ratio can be investigated. Direction for improving the performance can be suggested.

Fast manometric method for determining the effective oxygen diffusion coefficient through wine stopper

A fast manometric method has been designed for determining the oxygen transfer rate through cork stoppers. Thanks to this technique, it is possible to measure the effective diffusion coefficient of oxygen through a thin cork wafer in a few days, whereas months are required in the case of a full-length cork stopper. A calculation method has also been developed and validated to extrapolate the effective oxygen diffusion coefficient measured on thin wafers to a full-length stopper, based on the statistical analysis of the experimental data distribution. The question of the material heterogeneity and the representative thickness is addressed as well as the effective length of the stopper beyond which the gain in resistance to mass transfer becomes non-significant.

Optimizing Bifurcated Channels within an Anisotropic Scaffold for Engineering Vascularized Oriented Tissues.

Despite progress in engineering both vascularized tissues and oriented tissues, the fabrication of 3D vascularized oriented tissues remains a challenge due to an inability to successfully integrate vascular and anisotropic structures that can support mass transfer and guide cell alignment, respectively. More importantly, there is a lack of an effective approach to guiding the scaffold design bearing both structural features. Here, an approach is presented to optimize the bifurcated channels within an anisotropic scaffold based on oxygen transport simulation and biological experiments. The oxygen transport simulation is performed using the experimentally measured effective oxygen diffusion coefficient and hydraulic permeability of the anisotropic scaffolds, which are also seeded with muscle precursor cells and cultured in a custom-made perfusion bioreactor. Symmetric bifurcation model is used as fractal unit to design the channel network based on biomimetic principles. The bifurcation level of channel network is further optimized based on the oxygen transport simulation, which is then validated by DNA quantification assay and pimonidazole immunostaining. This study provides a practical guide to optimizing bifurcated channels in anisotropic scaffolds for oriented tissue engineering.

Transmembrane Protein Effects on Lipid Bilayer Oxygen Permeability

Oxygen (O2) diffusion across lipid bilayers is required for oxygen delivery to tissues. Diffusive transport is especially important in tissues where vessels are occluded or sparse. Atomistic molecular dynamics simulations have provided valuable insights into the mechanism and kinetics of oxygen transport at the membrane level. The role of lipids has been investigated in numerous studies, but proteins have generally not been included in the models. As a step toward a more authentic cell membrane model, we recently studied the effects of an ungated potassium channel protein on lipid bilayer oxygen permeability and related biophysical properties. That work provided initial support for the hypothesis that transmembrane proteins nonspecifically reduce the oxygen permeability of membranes, by reducing the fraction of permeable surface area and the effective diffusion coefficient for oxygen. Here, atomistic molecular dynamics simulations are used to test the hypothesis further, by considering several alpha-helical transmembrane proteins of varying size and biological function. Effects of the proteins are considered in the context of 1-palmitoyl,2-oleoylphosphatidylcholine (POPC) and POPC/cholesterol (1:1 mole ratio) bilayers. The current results support nonspecific reduction of permeability by transmembrane proteins that depends on both the in-plane area of the protein and the extent (circumference) of the protein-lipid interface. The protein effects are less dramatic in membranes incorporating cholesterol, perhaps due to packing defects at the protein-lipid interface in the relatively ordered POPC/cholesterol bilayers. The apparent solubility (partition coefficient) of oxygen within the membrane is diminished by cholesterol but enhanced by protein. These findings indicate that transmembrane proteins are a key factor in membrane permeability and that studies omitting them should be interpreted with caution.

Effective Oxygen Diffusion Coefficient of Till and Green Liquor Dregs (GLD) Mixes Used in Sealing Layer in Mine Waste Covers

Cover systems can efficiently limit acid mine drainage generation from sulfidic mine wastes by controlling oxygen diffusion. Their performance relies on their high degree of saturation, as oxygen diffusion is substantially reduced in water or saturated medium. However, natural soils available in the mine vicinities do not necessarily have the hydrogeological properties required for the construction of sealing layers. A common strategy is to improve the characteristics of local soils using bentonite amendment, but this usually induces high costs and environmental footprint. An alternative is to reuse (or valorise) waste materials, such as mine wastes or industrial wastes like green liquor dregs (GLD). Blends of till and GLD can have advantageous properties regarding water retention capacity and hydraulic conductivity. In this study, the effective oxygen diffusion coefficient De of till-GLD blends was evaluated using 81 diffusion tests. Various quantities and different types of GLD were tested. The diffusion coefficient was found to vary greatly depending on the degree of saturation. Even though the GLD contained naturally a substantial amount of water, a high water content of the till was still required to reach a low De. Measurements were also compared with modified Millington-Shearer predictive model which could generally predict the diffusion coefficient within an acceptable range. Results also indicated that the till-GLD mixes should not be exposed to evaporation as significant performance loss may rapidly occur upon drying. Main experimental results are presented in this paper together with recommendations in terms of cover design using till-GLD mixes.

Relation between oxygen gas diffusivity and porous characteristics under capillary condensation of water in cathode catalyst layers of polymer electrolyte membrane fuel cells

We present a coupled simulation method for evaluating oxygen diffusivity in cathode catalyst layers of polymer electrolyte membrane fuel cells under capillary condensation of water partially filling the pore network. The voxel data of a catalyst layer formed with platinum-supported carbon and Nafion ionomer are prepared from sequential cross-sectional images taken by a cryo-focused ion beam scanning electron microscope. Capillary condensation of water, which effectively alters the pore network, is simulated using the lattice density functional theory. Then, the effective porosity εeff, chord length distribution, and characteristic length of the partially wet catalyst layer are evaluated. The effective diffusion coefficients of oxygen in the Knudsen regime are evaluated using the mean-square displacement method. The ratio εeff/τ, with τ being the tortuosity factor, roughly exhibits intermediate behavior between the Bruggeman correlations of sphere- and cylinder-filling for εeff ≳ 0.2, while εeff/τ drastically deviates from the Bruggeman correlation for εeff ≲ 0.2. The effective oxygen diffusion coefficients at 350 K are predicted as a function of relative humidity and total pressure based on the kinetic theory of gases. The proposed method gives deeper insights into oxygen diffusivity under capillary condensation, contributing to more efficient material design.

A Particle Based Ionomer Attachment Model for a Fuel Cell Catalyst Layer

A particle model for ionomer attachment on carbon black in a Polymer Electrolyte Fuel Cell (PEFC) catalyst layer was developed based the random walk method. Two different methods of particle attachment were used that resemble different catalyst ink preparation conditions: the solution method and the colloidal method. In the solution method, the simulation of attachment is conducted on the aggregate structures and in the colloid method, the attachment is simulated on the agglomerate structures. The distribution of carbon black, ionomer and void space was used in a multiscale electrochemical simulator that calculated the mass/charge transfer and reaction in the catalyst layer. The results of effective oxygen diffusion coefficients are consistent with experimental result and show why the Bruggeman correlation often is a poor approximation for upscaling the effective diffusive and conductive components in PEFC porous media. The solution method allowed for a better proton conduction through the ionomer but resulted in a thicker ionomer film that increased the oxygen diffusive resistance. However, solution and colloidal method resulted in similar cell performances. Our model can aid in the design to develop fuel cell catalyst layers with improved performance.

Effects of Antioxidants Particle Size on Oxidation Resistance of MgO-C Refractory

In this study, Si and SiC powder with a critical particle size ranged from 0.1 to 0.4mm was added into MgO-C refractory as antioxidants. At 1200°C in air atmosphere, oxidation weight losses of cylindrical specimens with additives were measured and the effective diffusion coefficient of oxygen in refractories was calculated from the results. Thus, the effects of antioxidants particle size on the oxidation resistance were researched. The result shows that the particle size of antioxidant has a considerable influence on oxidation resistance of material. The oxidation resistance of MgO-C refractories increased at first as the critical particle size of Si powder increased from 0.1 to 0.2mm and then decreased as the critical particle size increased up to 0.4mm, while the oxidation resistance of MgO-C refractories decreased as the critical particle size of SiC additives increased from 0.1 to 0.4mm. The minimum effective diffusion coefficients of oxygen in MgO-C refractories added by Si and SiC were 10.90 and 14.09cm2/min, individually.

Effect of Fe addition on the microstructure and oxidation behavior of MgO–C refractory

With the aim to solve the problem of structure degradation of MgO–C refractory during high-temperature oxidation, iron powder (Fe) was introduced as both antioxidant and ceramic bond former, and its effect on the microstructure evolution, physical and mechanical properties was investigated. Oxidation kinetics analysis, based on the shrinking core model, was also performed to reveal the quantitative effect of Fe on the oxidation resistance. Compared to the traditional MgO–C refractory, both densification and bonding degree of Fe-containing refractory after high-temperature oxidation were greatly improved by the formation of Mg(Fe1·2Al0.8)O4 bridges and magnesio-wustite [(Mg,Fe)Oss]. This change of microstructure caused a dramatic decrease in the effective diffusion coefficient of oxygen (Deff, decreased from 18.3 to 5.0 cm2/min), which contributed the most to the improvement of the oxidation resistance. The maintenance of strength against oxidation time was another important finding for the newly designed refractory, providing a basis for developing new carbon-containing refractories with self-repairing function.