Effective Gas Diffusion Research Articles

Elucidating the impact of gas transport on high-performing air electrodes: a 1D physically based model unveiling the correlation between microstructure and impedance response

Abstract A 1D physically-based model on high performing air electrodes for Solid Oxide Cells is used to unravel the physical mechanisms lying behind the resistive peaks observed in experimental impedance data, posing particular attention to the low frequency contribution. In particular, the latter is commonly observed when analyzing the impedance response of high performing air electrode materials but its physical interpretation is still questioned. The model construction is grounded on the microstructural characteristics of conventional screen-printed electrodes. These properties were extracted by combining the statistical analysis of experimental 2D images taken with a scanning electron microscope and a validated microstructural model able to generate the synthetic 3D reconstructions of homogeneous electrode architectures. The implemented electrochemical model was tailored on the specific characteristics of a reference high performing SmBa0.8Ca0.2Co2O5+δ electrode material. Specifically, the model was used to reproduce its stationary and dynamic behavior between 650-750 °C, with an inlet oxygen partial pressure between 0.1-1 atm. The performed simulations unraveled the physical mechanisms lying behind the resistive contributions emerging from the impedance data. In particular, the effect of gas transport was analyzed in detail to understand the impact of the electrode microstructure on its electrochemical behavior. A sensitivity analysis on the effective gas diffusion coefficient highlighted that, in the investigated operating conditions, the electrochemical performance of classic screen-printed air electrodes is not limited by the gas diffusion. On the contrary, the low frequency contribution evidenced in the Nyquist plots was addressed to the impact of gas conversion. The developed electrochemical model successfully completed the correlation between microstructural and electrochemical properties and the results included in this work can be extended to different electrode materials tested in similar operating conditions.

Steam Electrolysis Using Solid Acid Electrolytes at Intermediate Temperatures

Solid acid electrolysis cells (SAECs), operating at intermediate temperatures (150-300 °C), has potentials to perform electrochemical conversions using renewable energy resources with a small cell voltage. One of the examples is a steam electrolysis to produce hydrogen and oxygen. The composite of CsH2PO4 solid acid and SiP2O7 matrix is a popular proton-conducting electrolyte in SAECs. The formation of CsH5(PO4)2 at the interface of CsH2PO4 and SiP2O7 provides high conductivity in a wide temperature range from 150 °C to 250 °C.[1] However, CsH5(PO4)2 becomes a liquid phase above 150 °C with a strong acidity, which leads to a low mechanical strength of the electrolyte and a fast cell degradation. In this study, SiO2 precursor was explored to obtain a SiP2O7 matrix, which reduced the generation of CsH5(PO4)2 in the composite and improved the stability of steam electrolysis. Additionally, the cell voltage was greatly reduced by modifying Aquivion proton conducting ionomer on a IrO2/Ti anode, which is likely due to the increasing OER (oxygen evolution reaction) active sites on the IrO2 surface.Commonly used SiP2O7 was prepared from a fumed silica which has a particle size of 3 nm and a surface area of 395 m2/g.[1] Firstly, the chronopotentiometry test was conducted using the traditional CsH2PO4/SiP2O7 electrolyte (molar ratio: 1/2) at 250 °C, the cell voltage rapidly increased after operating for 3 hours (Figure 1. black curve). X-ray diffraction (XRD) patterns showed TiP2O7 generated on the IrO2/Ti anode after the test, suggesting that the corrosion of Ti anode substrate by the strong acid of CsH5(PO4)2 might be the reason for cell degradation. The liquid CsH5(PO4)2 in the electrolyte also caused a severe deformation of the electrolyte pellet, highlighting the necessity to control the amount of CsH5(PO4)2 in the electrolyte. The SiO2 precursor was replaced to the one with particle size of 200 nm and a surface area of 10 m2/g while adjusting the heating time during SiP2O7 synthesis. The resultant SiP2O7 matrix contained a high proportion of hexagonal crystal structure (SiP2O7-hex). Scanning electron microscope (SEM) and differential thermal analysis (DTA) results exhibit that the SiP2O7-hex has a larger particle size and a lower hygroscopicity compared to the traditional SiP2O7. Chronopotentiometry tests was conducted for CsH2PO4/SiP2O7-hex electrolyte with small doping (16%) of CsH5(PO4)2 on the electrolyte surface to ensure a good contact between the electrolyte and the electrode. The steam electrolysis was stably operated for more than 20 hours (red curve in Figure 1), and the electrolyte pellet maintained its original shape after the test. These results suggest that SiP2O7-hex has a low reactivity with CsH2PO4 under humidified conditions, which limits the generation of liquid phase CsH5(PO4)2, and prevents the electrolyte deformation and the cell degradation.However, as seen from Figure 1, the cell voltage increased in the initial stage and finally stabilized at 2 V under only 10 mA cm-2. It is reported that both CsH5(PO4)2 and CsH2PO4 solid acid migrate towards the anode side during OER.[2,3] Energy dispersive X-ray spectroscopy (EDS) confirmed that the Cs salt almost fully covered the IrO2 after the test, which might lead to the high cell voltage. Initially, there were abundant triple phase interfaces (gas-electrolyte-IrO2 interface) where H2O and O2 can be effectively transported. However, as the solid acid migrated on the IrO2 surface with time, the effective gas diffusion pathway was blocked, and the cell voltage gradually increased. By modifying the IrO2/Ti with Aquivion, which is a popular ionomer used in proton exchange membrane fuel cells (PEMFCs) to provide proton pathways and improve water management on the electrode, the cell voltage at 10 mA cm-2 was decreased to 1.6 V. The achievable current density also increased from 30 mA cm-2 to 50 mA cm-2. It is hypothesized that the modification of Aquivion on IrO2 helps to maintain the active sites at the electrolyte-IrO2 interface, thus improving the efficiency of steam electrolysis.Overall, this study highlights the significance to control the amount of conductive liquid phase in the solid acid electrolyte and to maintain the accessibility to the electrochemical active sites. Reference: [1] T. Matsui et al., J. Electrochem. Soc, 2006, 153, A339.[2] M. Wagner et al., Sustainable Energy Fuels, 2020, 4, 5284.[3] N. Fujiwara et al., ChemSusChem, 2021, 14, 417. Figure 1

Simulation method of heat-mass transfer between hot wind and wet coating in the manufacturing process of pole piece

In order to improve the complex numerical calculation process for wet coating of pole piece, a three-dimensional numerical model of heat-mass transfer characteristics is established between hot wind and wet coating based on multi-field coupling theory of porous media and meshless parallel method, and its reliability is verified through literature data. Meanwhile, the difference calculation method and evaluation process are proposed for heat-mass transfer characteristics to solve this numerical model. The results show that the maximum errors are less than 14.7 % for this model. In addition, a scale modeling method is proposed to improve computational efficiency based on similarity theory. Meanwhile, the influences of different scale factors on temperature and humidity parameters are analyzed for wet coating. The results show that this method is accurate and reliable because the determination coefficient R2 of all parameters is more than 0.99. Meanwhile, compared with the calculation time of original model, the calculation efficiency increases by 44.6 % for the model with scale factor selected as 800. Finally, the variation and distribution of internal parameters are analyzed for wet coating during pore emptying process based on above simulation method. The results show that the variation trends of liquid film thickness, pore emptying rate, water saturation, gas saturation and effective gas diffusion coefficient are corresponding with the accelerated, uniform and decelerated drying processes of porous media. Meanwhile, the location of four-corner edges for wet coating is the weak link in heat-mass transfer process. Therefore, the heat-mass transfer characteristics for wet coating can be improved by increasing its initial thickness or adjust reasonable drying position in engineering.

Numerical investigation of effect of mechanical compression on the transport properties of fuel cell microporous layer using a pore-scale model

The microporous layer (MPL) plays an important role in water and thermal management of proton exchange membrane fuel cells (PEMFCs). An in-depth investigation of the mechanical compression effect on transport properties in the MPL can help optimize cell performance. In this work, the microstructure of the MPL is numerically reconstructed and the finite element method is applied to simulate mechanical behavior. Besides, the distribution of stress-strain, porosity, and pore size in the MPL under ten different levels of mechanical compression strains are studied. Lastly, the pore-scale model is employed to investigate the effective transport properties of the MPL as a function of compression strain. The analysis reveals that as the MPL strain increases from 0% to 40%, there is a 29% decrease in porosity, a 50% reduction in average pore diameter, a 60% decrease in effective gas diffusivity, a 100% increase in tortuosity, and an 80% increase in electrical and thermal conductivity. With the escalation of mechanical compression, both the magnitude and uniformity of stress-strain-displacement concurrently rise. Mechanical compression strains below 20% exhibit a lesser impact on transport properties. Beyond this threshold, exceeding the 20% compression strain point, mechanical stress assumes a critical role in influencing MPL transport properties.

Molecular dynamics simulation of strained controlled gas diffusion in polyimide membranes

The study of the diffusion and separation performance of small molecules within polyimide membranes under uniaxial stress is crucial for their applications under high-temperature and high-pressure conditions. In this study, we systematically investigated the diffusion behavior and diffusion selectivity of N2, CF4, C2F6, SF6, C2H4, C2H6, CH4, and SO2 gases, as well as their gas mixtures, within polyimide membranes under different tensile temperatures (300 K, 400 K, 500 K, 600 K) and tensile rates (1011 s−1, 1010 s−1, 109 s−1, 108 s−1) using molecular dynamics simulations. Our findings indicate that the diffusion coefficients of gas molecules within the polymer membrane increase with increasing strain or temperature. Excessive or overly rapid tensile rates are not favorable for the most efficient diffusion of molecules within the membrane. The strain has a relatively small impact on the diffusion coefficients of gas molecules that strongly adsorb to the polymer membrane. In contrast, the differences in the kinetic diameters of gas molecules affect diffusion selectivity under varying strain. These results suggest that a combination of molecular kinetic diameter, gas-polymer interactions, internal free volume, and pore distribution influences the effective gas diffusion within polyimide membranes. By controlling strain appropriately, the diffusion separation performance of this membrane material can be effectively improved.

PORE-SCALE STUDIES OF THE EFFECTIVE GAS DIFFUSIVITY IN MICROSCALE POROUS MEDIA BY THE DIRECT SIMULATION MONTE CARLO (DSMC) METHOD

The diffusion of gases in microscale porous media plays a pivotal role in multiple engineering applications. Accurate prediction of gas diffusivity in these media is crucial for optimizing such processes. In this research, we utilized the direct Monte Carlo simulation (DSMC) to analyze gas diffusion in microscale porous media, reconstructed using the quartet structure generation set (QSGS) method. We examined the influence of gas pressure, porosity, tortuosity, and porous microstructure on the effective gas diffusivity in microscale porous media. The findings indicate that the dimensionless effective gas diffusivity is inversely related to gas pressure, given a consistent microstructure. The tortuosity, which depends on the microstructure, significantly influences the gas diffusivity. Specifically, as the tortuosity increases, the effective gas diffusivity decreases at the same porosity. In addition, the anisotropy has a substantial effect on the gas diffusivity in a certain direction; however, it has almost no influence on the effective gas diffusivity. Finally, concluding from extensive numerical data, we introduce a predictive model for effective gas diffusivity in microscale porous media. This model considers the effects of Knudsen (Kn) and tortuosity and is able to predict the gas diffusivity in isotropic and anisotropic porous accurately with porosity ranging from 65% to 95% and Kn ranging from 0.1 to 10.

(Invited) Developing Electrodes for Solid Oxide Fuel Cells What I Learned from Anil Virkar about Designing High Performance Electrodes

The performance of state-of-the-art solid oxide fuel cells (SOFC) and electrolyzers (SOEC) are largely limited by the overpotentials at the electrodes, both anode and cathode. The electrode overpotentials can be categorized by i) concentration of gas species, ii) charge transfer, and, iii) ionic transport near the interface, with the magnitude and contribution of each dependent on the cell geometry and operating conditions. One of the strategies of reducing overpotentials and improving electrode performance is the use of functionally grade electrodes wherein individual layers are optimized for porosity, conductivity, and three phase boundaries. Outer current collecting layers are designed with substantial porous microstructures and from highly electronically conductive materials resulting in low sheet resistance. Whereas the electrode layer near the electrolyte interface can be comprised of a two phase electrolyte-electrocatalyst composite in order to extend the effective three phase boundary layer beyond the electrolyte surface.The effects of various microstructural and material properties on the various electrode overpotentials were investigated. The influence of porosity and pore structure on the effective N2-O2 binary gas diffusion in the air electrode and resulting concentration polarization was studied. Likewise, highly porous anode supports were developed and characterized with the intent to decrease gas transport concentration overpotentials. Given electrodes that are adequately thin or have sufficient porosity in order to facilitate gas diffusion and reduce concentration polarizing, a substantial portion of the electrode overpotential can be attributed to the charge transfer and transport of the oxide ion from the electrode into the electrolyte, especially at lower temperatures. The critical transition region between the electrode and the electrolyte may span only a few microns but is essentially the equivalent of the ‘last mile problem’ for a fuel cell.Composite electrode layers nearest to the electrolyte were studied with regard to microstructure and materials selection. The activation overpotential was measured and related to the electrode properties including, three phase boundary density, the ionic transport within this layer, and the charge transfer of the electrocatalyst. In particular, the influence of the ionic conductivity of the electrolyte phase in the composite electrode on the electrode polarization was investigated. In addition, the microstructure of this electrolyte phase within the composite phase was studied and the sensitivity of necks between sintered electrolyte particles on ionic transport was estimated. A unique fabrication method was developed that produced an electrolyte skeleton with improved particle-to-particle necking and yet extremely porous, that could subsequently be infiltrated with an electrocatalyst material. Applying and combining the various strategies outlined above, highly efficient electrodes were developed and demonstrated for solid oxide fuel cells.

(Invited) Developing Electrodes for Solid Oxide Fuel Cells What I Learned from Anil Virkar about Designing High Performance Electrodes

The effects of various microstructural and material properties on the electrode overpotentials of solid oxide fuel cells (SOFC) were investigated. A review of the influence and sensitivity of porosity and pore structure on the effective gas diffusivity and resulting concentration polarization of electrodes is presented. The doping of Ni in the anode support as a means to improve redox cycling and stability was demonstrated. Composite electrode layers nearest to the electrolyte were studied with regard to microstructure and materials selection. In particular, the influence of the ionic conductivity of the electrolyte phase in the composite electrode on the electrode polarization was investigated. A fabrication method was developed that produced an electrolyte skeleton with improved particle-to-particle necking and yet extremely porous, that could subsequently be infiltrated with an electrocatalyst material. Applying and combining the various strategies outlined, highly efficient electrodes were developed and demonstrated for solid oxide fuel cells.

Measurement of Effective Hydrogen-Methane Gas Diffusion Coefficients in Reservoir Rocks

Summary If hydrogen is stored in depleted gas fields, the remaining hydrocarbon gas can be used as cushion gas. The composition of the backproduced gas depends on the magnitude of mixing between the hydrocarbon gas and the hydrogen injected. One important parameter that contributes to this process of mixing is molecular diffusion. Although diffusion models are incorporated in the latest commercial reservoir simulators, effective diffusion coefficients for specific rock types, pressures, temperatures, and gas compositions are not available in the literature. Thus, laboratory measurements were performed to improve storage performance predictions for an underground hydrogen storage (UHS) project in Austria. An experimental setup was developed that enables measurements of effective multicomponent gas diffusion coefficients. Gas concentrations are detected using infrared light spectroscopy, which eliminates the necessity of gas sampling. To test the accuracy of the apparatus, binary diffusion coefficients were determined using different gases and at multiple pressures and temperatures. Effective diffusion coefficients were then determined for different rock types. Experiments were performed multiple times for quality control and to test reproducibility. The measured binary diffusion coefficients without porous media show a very good agreement with the published literature data and available correlations based on the kinetic gas theory (Chapman-Enskog, Fuller-Schettler-Giddings). Measurements of effective diffusion coefficients were performed for three different rock types that represent various facies in a UHS project in Austria. A correlation between static rock properties and effective diffusion coefficients was established and used as input to improve the numerical model of the UHS. This input is crucial for the simulation of backproduced gas composition and properties which are essential parameters for storage economics. In addition, the results show the impact of pressure on effective diffusion coefficients, which impacts UHS performance.

Determination of Concentration-Dependent Effective Diffusivity of Each Gas Component of a Binary Mixture in Porous Media Saturated with Heavy Oil under Reservoir Conditions

Summary One frequently used enhanced heavy oil recovery technique is gas injection, during which heavy oil viscosity is reduced due to diffusion of gaseous components and heavy oil swelling in porous media. Effective diffusivities of gas components are generally assumed to be constants, while no attempts have been made to determine both the concentration-dependent effective diffusivity in porous media saturated with heavy oil and the preferential contribution of each component in a binary/ternary gas mixture. In this study, a pragmatic and robust technique has been proposed to determine the concentration-dependent effective diffusivity of each gas component by reproducing the experimental measurements during pressure decay tests for CO2-C3H8-heavy oil systems in porous media. Experimentally, CO2 and C3H8 are utilized to diffuse into sandpacks fully saturated with heavy oil. Under a constant temperature within a thermostatic chamber, the pressures of the aforementioned gas(es)-heavy oil systems are consistently tracked and saved while gas samples are taken at the start and end of the diffusion tests for gas chromatography analyses. Theoretically, a mass transfer model is formulated to determine effective gas diffusivity in heavy oil as a concentration-dependent function by incorporating Fick’s second law and the modified Peng-Robinson equation of state (PR EOS). The concentration-dependent effective diffusivity for each gas component is ascertained when the measured pressure profiles and gas compositions are matched well to their correspondingly calculated values with minimum deviations. Compared to either a constant assumption or a linear concentration-dependent relation with respect to diffusivity, an exponential concentration-dependent relation leads to more accurately reproducing the measured pressure profiles. Compared with pure CO2, its effective diffusivity in a binary (i.e., CO2 and C3H8) gas system is found to be larger, indicating that C3H8 accelerates the CO2 mass transfer into heavy oil under the same circumstances. Furthermore, this study confirms that a larger tortuosity of a porous medium leads to a longer diffusion path with less contact between gas and liquid phases and that a lower concentration of a gaseous component yields a lower effective diffusivity.

Cauliflower-like copper zinc tin sulfur for ppb-level NO2 sensing at room temperature

Structure nanocrystallization dramatically enhances the specific surface area and is one of the most popular methods for preparing high-performance gas sensing materials. However, most of the preparation methods are suffering high-cost and long time-consuming. This work synthesized Cu2ZnSnS4 (CZTS), a cost-effective and stable direct-bandgap semiconductor with cauliflower-like morphology by facile solution drop coating and low-temperature annealing. The morphology and gas sensing performance of CZTS films could be modulated by controlling the annealing time. The gas sensors showed a significant response toward ppb-level NO2 (1.287 toward 50 ppb) with a short response, recovery time, and high selectivity at room temperature (25 °C). It is worth mentioning that the limit of detection is 50 ppb, which is the first report on the ppb-level detection ability of CZTS-based NO2 gas sensors. The enhanced sensitivity of the cauliflower-like CZTS nanocrystal-based gas sensor can be ascribed to the small grain size, large specific surface area for effective gas diffusion, and excellent conductivity of the spherical packed structure.

X-ray micro-CT based computation of effective diffusivity of metabolic gases in tomato fruit

The effective diffusivity is a key engineering property of fruit that affects transport of metabolic gases and, hence, ripening. It strongly depends on the microstructure of the fruit. In bulky fruit such as tomato, porosity and pore structure differ between tissues and change during maturation and ripening. Knowledge of the relationship between gas diffusivity and microstructure of tomato will aid management and control of postharvest operations. By using mathematical models of mass transport that incorporate the 3-D microscopic tissue geometry obtained from micro-computed tomography (μ-CT), the relationship between microstructural features and gas diffusivity may be computed in a direct way. In this study, a previously validated pore-scale network model (PNM) was used to simulate the effective gas diffusivity of O2, CO2 and ethylene for five types of tomato fruit tissues (i.e., outer mesocarp, inner mesocarp, septa, placenta and columella) at different maturation and ripeness stages. The effective gas diffusivity of the placenta and columella was large during the entire ripening process as these tissues contained more interconnected open intercellular spaces. The effective diffusivity of the inner mesocarp increased with increasing porosity during ripening while the microstructural changes of the outer mesocarp lagged compared to those of the inner mesocarp region, resulting in a delayed increase of the effective gas diffusivity with ripening. Regression models between the effective diffusivity of O2, CO2 and ethylene as a function of porosity, open porosity or tortuosity were established. This study provides a quantitative basis for further gas exchange modeling in intact tomato fruit to predict their ripening process.

Continuous-flow electrosynthesis of ammonia by nitrogen reduction and hydrogen oxidation.

Ammonia is a critical component in fertilizers, pharmaceuticals, and fine chemicals and is an ideal, carbon-free fuel. Recently, lithium-mediated nitrogen reduction has proven to be a promising route for electrochemical ammonia synthesis at ambient conditions. In this work, we report a continuous-flow electrolyzer equipped with 25-square centimeter-effective area gas diffusion electrodes wherein nitrogen reduction is coupled with hydrogen oxidation. We show that the classical catalyst platinum is not stable for hydrogen oxidation in the organic electrolyte, but a platinum-gold alloy lowers the anode potential and avoids the decremental decomposition of the organic electrolyte. At optimal operating conditions, we achieve, at 1 bar, a faradaic efficiency for ammonia production of up to 61 ± 1% and an energy efficiency of 13 ± 1% at a current density of -6 milliamperes per square centimeter.

Mass transfer performance inside Ca-based thermochemical energy storage materials under different operating conditions

CaCO3/CaO is a promising thermochemical energy storage material to achieve the continuous and stable operation of renewable energy, due to its unique merits such as high energy storage density and long storage time. However, it makes stringent demands on mass transfer to obtain a high energy discharging performance. This study investigated the synergetic effect of complex pore structures and different operating conditions on the micro-flow diffusion mass transfer performance inside CaO materials. We found that the effective gas diffusion coefficient increased with increasing porosity, and decreased with an increase in fractal dimension. In addition, under different operating conditions, the difference in the effective gas diffusion coefficient inside CaO materials with high porosity and low fractal dimensions was much larger than that inside CaO materials with low porosity and high fractal dimensions. This indicated that the synergetic effect should be considered for mass transfer performance inside CaO materials with high porosity and low fractal dimensions. To better understand this synergetic effect, a prediction model was proposed based on machine learning, where its average error was around 12%, and the root means square error was around 0.04138, which was better than that of the traditional Maxwell model. This proposed model may provide theoretical guidance for the design of Ca-based materials with high performance, and it could also be used in reactor design or system thermodynamic investigations.

Rational Design of Covalent Organic Frameworks as Gas Diffusion Layers for Multi-atmosphere Lithium-Air Batteries.

Non-aqueous Li-air batteries, despite their high energy density and low cost, have not been deployed practically due to their instability in ambient air, where moisture causes parasitic reactions and shortens their life drastically. Here, we demonstrate the rational design of nanoporous covalent organic frameworks (COFs) as effective gas diffusion layers (GDLs) to address this constraint. The COF GDLs, with a tailor-made pore size of ~1.4 nm and superhydrophobicity can limit the intrusion of organic electrolytes and moisture into the gas diffusion channels, enabling high capacity, fast kinetics, and excellent stability of the Li-air batteries. Moreover, we achieve multi-atmosphere Li-air batteries, which can stably cycle under open ambient air (relative humidity up to 95%) and even in various atmospheres with looping oxygen, humid air, and carbon dioxide. The design principles of our COF GDLs can be universally applied in energy storage and electrochemical systems using organic electrolytes.

Rational Design of Covalent Organic Frameworks as Gas Diffusion Layers for Multi‐atmosphere Lithium‐Air Batteries

AbstractNon‐aqueous Li‐air batteries, despite their high energy density and low cost, have not been deployed practically due to their instability in ambient air, where moisture causes parasitic reactions and shortens their life drastically. Here, we demonstrate the rational design of nanoporous covalent organic frameworks (COFs) as effective gas diffusion layers (GDLs) to address this constraint. The COF GDLs, with a tailor‐made pore size of ≈1.4 nm and superhydrophobicity, can limit the intrusion of organic electrolytes and moisture into the gas diffusion channels, enabling high capacity, fast kinetics, and excellent stability of the Li‐air batteries. Moreover, we achieve multi‐atmosphere Li‐air batteries, which can stably cycle under open ambient air (relative humidity up to 95 %) and even in various atmospheres with looping oxygen, humid air, and carbon dioxide. The design principles of our COF GDLs can be universally applied in energy storage and electrochemical systems using organic electrolytes.

In-situ estimation of water transfer parameters in a proton exchange membrane fuel cell

A reduced model to describe steady state transport of water through a PEM fuel cell is proposed. It includes three rigorously estimated parameters: the effective gas diffusion coefficient of water vapor in hydrogen, of water vapor in air and of water in sorbed phase in the membrane. First, it is proved that the parameters are well decorrelated. Then, uncertainty on estimates is calculated by simulating an experiment with random noise. Finally, the parameters are estimated as a function of the average humidity. It is observed that the gas phase diffusion coefficients do not depend on humidity. The effective diffusion coefficient in sorbed phase decreases slightly with increasing average relative humidity. It is shown that the effect of electrodes on water transport is negligible by comparing the water fluxes through an MEA and a membrane alone. A temperature gradient is imposed between the two plates and allows to obtain the condensation of water on one side of the membrane. A discontinuity of the water flow through the MEA is observed and results are interpreted by an increase of the effective diffusion coefficient in sorbed phase and/or by an increase of the water content of the membrane in contact with liquid water.

Gas diffusion in an automotive catalyst in an unsteady state

The influence of gas diffusion is often ignored because the washcoat layer is thin. Therefore, in addition to developing new catalytic materials, modifying the washcoat pores is important for improving effective gas diffusivity. The gas diffusion phenomena in an unsteady state in an experimentally simulated washcoat layer were evaluated by the pulse injection method. Wicke–Kallenbach diffusion cell was used. Effective diffusion coefficients obtained in an unsteady state were compared to those obtained in a steady state. The effective diffusion coefficients were similar to the values obtained under a steady state, except for low-molecular-weight gasses, due to the difference in mean molar speed between the evaluating gas and atmospheric gas.

Ultrahigh sensitive NO sensor based on WO3 film with ppb-level sensitivity

Nitric oxide (NO) is one of the hazardous gases which directly affect the vital organs of human beings. An accurate identification is essential for air quality assurance, environmental monitoring, automotive technology, and other industrial processes. Herein, we report nanostructured tungsten oxide (WO3) thin-film grown on SiO2 substrate under controlled oxidation conditions for NO sensing. The nanostructured-WO3 film's monoclinic phase has an average crystallite size of 12.89 nm, a lattice strain of 20.56, and an optical band gap of 2.89 eV. The fabricated device demonstrates high selectivity, stability, and faster response/recovery time with a sensor response of 1.26 for 100 ppb NO concentration, which increases to an ultra-high response (>25) for 1 ppm at 250 °C with a detection limit of ∼4 ppb. Ultrahigh response of the sensors is due to high specific surface area and effective gas diffusion ability, which lead to potential applications in low concentration NO detection.

Modelling the effect of morphology on thermal aging of low-density closed-cell PU foams

Low-density closed-cell polyurethane (PU) foams are widely used in thermal insulation in diverse applications, therefore predicting their thermal aging has a distinct practical importance. Several approaches have been developed allowing forecasting the long-term changes of gas composition in foams, which manifest as the thermal aging. The accelerated aging test methods of foams need to be applied to each foam morphology of interest, while diffusion modelling-based methods, employing characterisation of the gas transport properties of PU polymer or monitoring the evolution of gas composition in foam cells, are relatively complicated. In this study, the experimentally determined evolution of PU foam conductivity is used for estimating the effective diffusivities of gases, based on which permeability of the PU polymer is subsequently evaluated by a structural model assuming Kelvin cell geometry of foam cells. The proposed approach enables prediction of aging dynamics of foams developed from the same PU polymer but possessing a markedly different morphology.