Mass Transport Behavior Research Articles

Accurately predicting transport properties of porous fibrous materials by machine learning methods

AbstractMachine learning algorithms trained on data gathered from stochastically generated gas diffusion layers (GDLs) were used to predict key transport properties that govern effective mass transport behaviour in polymer electrolyte membrane fuel cells. Specifically, we present the largest database in the present literature of stochastically generated fibrous GDL substrates (containing over 2000 unique materials) and the associated structural and transport properties determined via pore network modelling. Seven established machine learning algorithms were trained to predict the effective single‐phase permeability (ksp) and diffusivity (Dsp), and the relative permeability (kr) and diffusivity (Dr) of the generated materials using well‐defined material properties as input features. Gradient boosting regression (GBR), artificial neural network, and support vector regression were the best performing predictors of the single‐phase properties, all of which exhibited statistically insignificant differences in error. GBR provided the best prediction accuracy of relative transport properties.

Mismatching integration-enabled strains and defects engineering in LDH microstructure for high-rate and long-life charge storage

Layered double hydroxides (LDH) have been extensively investigated for charge storage, however, their development is hampered by the sluggish reaction dynamics. Herein, triggered by mismatching integration of Mn sites, we configured wrinkled Mn/NiCo-LDH with strains and defects, where promoted mass & charge transport behaviors were realized. The well-tailored Mn/NiCo-LDH displays a capacity up to 518 C g−1 (1 A g−1), a remarkable rate performance (78%@100 A g−1) and a long cycle life (without capacity decay after 10,000 cycles). We clarified that the moderate electron transfer between the released Mn species and Co2+ serves as the pre-step, while the compressive strain induces structural deformation with promoted reaction dynamics. Theoretical and operando investigations further demonstrate that the Mn sites boost ion adsorption/transport and electron transfer, and the Mn-induced effect remains active after multiple charge/discharge processes. This contribution provides some insights for controllable structure design and modulation toward high-efficient energy storage.

Stratified Flow of Micropolar Nanofluid over Riga Plate: Numerical Analysis

In this article, we present a numerical analysis of the energy and mass transport behavior of microrotational flow via Riga plate, considering suction or injection and mixed convection. The thermal stratified parameters of nanofluid are captured using an interpretation of the well-known Keller box model, which helps us to determine the characteristic properties of the physical parameters. The formulated boundary layer equations (nonlinear partial differential equations) are transformed into coupled ODEs with nonlinearities for the stratified controlled regimes. The impact of embedded flow and all physical quantities of practical interest, such as velocity, temperature, and concentration profile, are inspected and presented through tables and graphs. We found that the heat transfer on the surface decreases for the temperature stratification factor as mass transfer increases. Additionally, the fluid velocity increases as the modified Hartmann number increases.

Visualization of Ion Fluxes in Nanopipettes: Detection and Analysis of Electro-osmosis of the Second Kind.

Nanopipettes are finding increasing use as nano "test tubes", with reactions triggered through application of an electrochemical potential between electrodes in the nanopipette and a bathing solution (bath). Key to this application is an understanding of how the applied potential induces mixing of the reagents from the nanopipette and the bath. Here, we demonstrate a laser scanning confocal microscope (LSCM) approach to tracking the ingress of dye into a nanopipette (20-50 nm diameter end opening). We examine the case of dianionic fluorescein under alkaline conditions (pH 11) and large applied tip potentials (±10 V), with respect to the bath, and surprisingly find that dye ingress from the bath into the nanopipette is not observed under either sign of potential. Finite element method (FEM) simulations indicate this is due to the dominance of electro-osmosis in mass transport, with electro-osmotic flow in the conventional direction at +10 V and electro-osmosis of the second kind acting in the same direction at -10 V, caused by the formation of significant space charge in the center of the orifice. The results highlight the significant deviation in mass transport behavior that emerges at the nanoscale and the utility of the combined LSCM and FEM approach in deepening understanding, which in turn should promote new applications of nanopipettes.

Heat and mass transport performance of MHD elastico‐viscous fluid flow over a vertically oriented magnetized surface with magnetic and thermo diffusions

AbstractThe key objective in this study is to examine the heat and mass transport behavior of magnetohydrodynamic elastic‐viscous fluid flow over a vertically oriented magnetized surface placed in a uniform permeable regime with magnetic and thermo diffusions. The fluid is partially ionized and permeated to flow in the presence of a strong magnetic field domain. Hence the Hall current effect is considered in this investigation. The significance of rotation and induced magnetic field on the flowing nature are also scrutinized in this study. The mathematical model of the problem is converted to a similar model by introducing suitable nondimensional variables. To obtain the closed‐form solutions of the flow leading equations, the regular perturbation analysis is utilized. For the exhibition of results, figures and tables are generated with the assistance of scientific computation software MATHEMATICA. Computed results are validated with the existing result in the limiting case. Such an investigation is important in evaluating the flow characteristics of low magnetic diffusive viscoelastic fluid. A noteworthy result seen is that magnetic diffusion significantly controls the fluid flow by altering the magnetic drag force. Mass diffusion factor brings an increase in the fluid velocity. Furthermore, we observed that the surface current density along the principal flow direction is significantly reduced by magnetic diffusion and mass diffusion factor.

(Invited) A Pore Scale Approach to Designing Polymer Electrolyte Membrane Water Electrolyzers

Polymer electrolyte membrane (PEM) water electrolyzers can be used to produce hydrogen from the decomposition of water, thereby enabling the use of a variety of renewable and emissions-free energy sources, such as solar, wind, tidal, and nuclear energy to produce clean hydrogen. While electrolyzers are currently commercially available, the technology has not yet been optimized. In order to reach cost targets for widespread commercial adoption, we must realize materials that enable more effective multiphase flow phenomena than what currently exists. Mass transport losses in PEM water electrolyzers pose challenges, yet designing these materials requires the a priori knowledge of how the heterogeneous properties of the porous materials and their interfacial contacts influence electrochemical performance. I will discuss the critical design factors and how they influence mass transport behaviour in PEM electrolyzers. In particular, I will discuss the existence of a new performance indicator for gas evolving electrochemical energy conversion devices – the critical current density. The critical current density pinpoints a performance inflection point whereby both the gas saturations and mass transport overpotentials suddenly dominate cell performance and exacerbate failure. I will discuss the mass transport behavior of a PEM water electrolyzer using in operando neutron imaging at operating current densities as high as 9 A·cm-2.

Aqueous Ammonia Wetting & Evaporation on Gas-Diffusion Layers

Recent interest in electrochemical technologies for low temperature ammonia production motivates the need to understand key mass transport behavior at the electrode/flow-field interface. This work measures wetting properties as well as evaporation rates of aqueous ammonia solutions from the surface of porous carbon based gas-diffusion layers (GDL). These materials are commonly used in membrane-electrode assemblies (MEA) which are at the heart of electrolytic ammonia synthesis and direct ammonia fuel cell (DAFC) systems. Aqueous ammonia solutions of 0%, 10%, and 20% NH3 concentration by weight were micropipetted onto the surfaces of Toray 120 GDL. Droplet dimensions and mass were recorded with respect to time using a digital scale and automated goniometer. An environmental chamber mounted to the goniometer was used to conduct the higher temperature tests at 60 ⁰C and 100 ⁰C. The impact of concentration, and temperature on key wetting behavior and evaporation rates was probed. Droplet evaporation rates, initially dominated by NH3, fall as the ammonia concentration decreases. Higher NH3 concentrations lead to higher initial evaporation rates. Elevated temperatures increased the evaporation rates in all cases. Slow nucleate boiling was observed in the droplets for the 100 ⁰C cases. Overall, wetting throughout the evaporation process is dominated by pinning effects as the droplet contact widths remained constant with falling height and contact angles. The impact on concentration was also investigated on breakthrough pressure and adhesion force which have been previously demonstrated to be useful metrics for gauging material performance by the polymer-electrolyte-membrane fuel cell (PEFC) community. Higher NH3 solutions reduced the breakthrough pressure and also lead to higher adhesion force to the GDL surface. The effect of GDL hydrophobic coatings (PTFE wt. %) on these metrics was found to be insignificant over 10%.

Computational analysis of vincristine loaded silk fibroin hydrogel for sustained drug delivery applications: Multiphysics modeling and experiments

In this paper, silk fibroin hydrogel is used as a drug carrier for vincristine. To optimize drug delivery, a multi-physics model is proposed that couples the deformation and diffusion fields. We applied inverse analysis and general continuum mechanics to define material parameters and mechanical properties. To examine the mass transport and chemical behavior, an affinity-based diffusion and degradation of a drug-loaded polymer matrix is employed. Some experiments are carried out to examine the capability of the presented model. After preparing the vincristine loaded silk hydrogel syringes, they were injected into PBS and enzyme solutions to monitor the drug release rate for 40 days. Obtained results from the computational simulation and laboratory tests showed that the silk fibroin hydrogel was deswelled after about 40 days in enzyme solution. Degradation led to faster and higher doses of vincristine drug release in comparison to the case of PBS solution. Results revealed that more than 80% of the drug was released in the first 5 days in the enzyme solution, but in PBS solution only 10% of the drug was released during 40 days. The model predictions of deswelling behavior and drug release rate were in good agreement with those of experimental results. Therefore, it can be employed as a reliable tool for further predictions.

Modeling and evaluating performance of full-scale reverse osmosis system in industrial water treatment plant

Practical modeling for assessing the efficiency of a full-scale reverse osmosis (RO) system may be a challenging task. This is because the operating conditions of RO systems can change significantly in actual practice owing to high seasonal variations and different progress of membrane fouling during long-term filtration. Accordingly, it is difficult to reliably model the RO performance if such conditions are excluded. In this study, we model a full-scale installation of a RO membrane system, considering actual operations of the industrial water treatment plant. A numerical model is built to describe spatiotemporal behavior of (water, salt, and foulant) mass transport inside a full-dimension pressure vessel. By performing a global sensitivity analysis, we evaluate the relative importance of key influential factors on model accuracy and specific energy consumption (SEC). The model and its parameters are optimized based on the sensitivity result and validated using best-fitted time-series measurement data of 3875 h. The results demonstrate the practical behaviors of fouling development and separation performance of the primary RO process. A regression tree analysis of SEC for 27 different operational scenarios in simulations may benefit decision making for energy efficient RO. Results reveal the high dependence of SEC on cleaning frequency in the feed temperature range.

Multiscale modeling of interface-mediated mechanical, thermal, and mass transport in heterogeneous materials: Perspectives and applications

In this perspective, we: (a) characterize the multiscale nature of the mechanical (plasticity, fracture, twinning, and phase transformations) and thermal behavior, as well as the mass transport behavior in a variety of materials with microstructure complexities; (b) examine the applicability of several representative experimental/computational techniques/approaches in identifying the mechanisms underlying the interface-dictated mechanical, thermal, and mass transport; (c) highlight the need for the development of multiscale methods that can address atomistic and continuum descriptions of materials within one framework, together with our preliminary attempts in this regard. This perspective, together with the relevant papers collected in this focus issue, will inspire researchers to further develop advanced theories, algorithms, and software implementation for bottom-up predictive simulation of the deformation, thermal, and diffusion behavior of advanced materials. Such methods can support the design and development of materials with desired combinations of properties, for example high strength/ductility/toughness, low/high thermal/ionic conductivity, corrosion-/irradiation-resistance.

Determination of Vapor and Momentum Roughness Lengths Above an Undulating Soil Surface Based on PIV‐Measured Velocity Profiles

AbstractAccurately predicting bare‐soil evaporation requires the proper characterization of the near‐surface atmospheric conditions. These conditions, dependent on factors such as surface microtopography and wind velocity, vary greatly and therefore require high‐resolution datasets to be fully incorporated into evaporation models. These factors are oftentimes parameterized in models through the aerodynamic resistance (ra), in which the vapor roughness length (z0v) and the momentum roughness length (z0m) are two crucial parameters that describe the transport near the soil‐atmosphere interface. Typically, when evaluating bare‐soil evaporation, these two characteristic lengths are assumed equal, although differences are likely to occur especially in turbulent flows over undulating surfaces. Thus, this study aims to investigate the relationship between z0v and z0m above undulating surfaces to ultimately improve accuracy in estimating evaporation rate. To achieve this goal, four uniquely designed wind tunnel—soil tank experiments were conducted considering different wind speeds and undulation spacings. Particle image velocimetry (PIV) was used to measure the velocity field above the undulating surface in high resolution. Using the high‐fidelity data set, the logarithmic ratio of z0v to z0m is determined and used to estimate ra. Results confirm that these lengths differ significantly, with the logarithmic ratio roughly ranging from −15 to −5 under the conditions tested. PIV‐measured results demonstrate this ratio is closely tied to the mass and momentum transport behaviors influenced by surface undulations. Using the data‐integrated formulation of ra, predictions of evaporation rate were prepared for both the laboratory and lysimeter experiments, demonstrating the efficacy of the proposed approach in this study.

Unsteady Separated Stagnation Point Flow of Nanofluid past a Moving Flat Surface in the Presence of Buongiorno's Model

This paper explores energy and mass transport behavior of unstable separated stagnation point flow of nanofluid over a moving flat surface along with Buongiorno’s model. Characteristic of Brownian diffusion and thermophoresis are considered. Additionally, characteristics of chemical reaction is taken into account. A parametric investigation is performed to investigate the outcome of abundant parameters such as temperature, velocity and concentration. An appropriate equation is converting into a set of ODEs through employing appropriate transformation. The governing equations has been solved numerically by using the classical fourth-order Runge-Kutta integration technique combined with the conventional shooting procedure after adapting it into an initial value problem. Our findings depict that the temperature field θ(ζ) improves for augmenting values of theromophoresis parameter (Nt) with dual solutions of attached flow without inflection and flow with inflection. Also, the difference of Brownian motion parameter (Nb) with two different solutions of attached flow exists with energy profile. It can be found that an energy profile θ(ζ) elevates due to augmenting values of (Nb). It has been perceived that thermal boundary layer thickness elevates due to large amount of Brownian motion parameter (Nb).

Temperature enhances the ohmic and mass transport behaviour in membrane electrode assembly carbon dioxide electrolyzers

The operating temperature is a critical parameter for improving the performance of a carbon dioxide electrolyzer. Specifically, the power density reduced by up to 35% (at 215 mA/cm2) when increasing the operating temperature from 25 °C to 60 °C, and increasing the cell temperature led to significantly lower ohmic resistances and mass transport limitations. A 5–fold reduction in ohmic resistance (from 3.35 Ω·cm2 to 0.64 Ω·cm2) was achieved by increasing the cell temperature from 25 °C to 60 °C at 215 mA/cm2. These reductions in ohmic overvoltages were attributed to higher cation exchange membrane water content at higher operating temperatures observed via synchrotron X-ray radiography. The higher water content at higher temperatures was attributed to the relaxation of the polymer backbone of the membrane. The dominating mechanisms for mass transport limitations at high current densities (≥305 mA/cm2) were temperature dependent. Specifically, at 40 °C, liquid water in the electrode inhibited reactant carbon dioxide transport to the reaction sites; whereas, at 60 °C, the product gas saturation in the electrode inhibited mass transport. However, a higher temperature led to consistently lower mass transport losses at each current density (e.g., a reduction from 0.75 V to 0.37 V at 395 mA/cm2 when increasing the temperature from 40 °C to 60 °C) since less water accumulated in the cathode gas diffusion layer.

Investigation of transient mass transport induced deformation of PEGDA hydrogel in photocurable solution

Hydrogels have attractive characteristics such as notable biocompatibility, outstanding hydrophilia and efficient mass transport (MT), which makes hydrogel suitable for tissue engineering scaffold materials. Stereolithography is a prevalent method to prepare hydrogel scaffold with complex structure. However, it is difficult to ensure dimension accuracy in printing complex hydrogel scaffold since solvents can migrate in gel and render MT behavior, which consequently induce deformation and dimensional inaccuracy of printed hydrogel. Little investigation of MT of gel in mixture like solution is conducted. Aiming to depict multiple solvents MT in gel, a model is established by mainly modifying the representation of free energy of mixing to the multi-component one. This model is feasible to describe concurrent multiple solvents migration and mechanical deformation for hydrogel in liquid mixtures, which fulfills understanding of multi-component MT. In this work, the transient MT behaviors of poly(ethylene glycol) diacrylate gel in its photocurable solution are investigated by the model. The numerical model is validated by experimental results and then applied to simulate free and confined MT process of the system. It is found that the deformation trend is not monotonous and total gel deformation can be regulated by adjusting printing time. The correspondence between numerical and experimental results proved that this model can promisingly illustrate multi-component MT induced deformation and provide instructions on improving dimensional accuracy for printing hydrogels.

Rapid screening of nanopore candidates in nanoporous single-layer graphene for selective separations using molecular visualization and interatomic potentials.

Nanoporous single-layer graphene is promising as an ideal membrane because of its extreme thinness, chemical resistance, and mechanical strength, provided that selective nanopores are successfully incorporated. However, screening and understanding the transport characteristics of the large number of possible pores in graphene are limited by the high computational requirements of molecular dynamics (MD) simulations and the difficulty in experimentally characterizing pores of known structures. MD simulations cannot readily simulate the large number of pores that are encountered in actual membranes to predict transport, and given the huge variety of possible pores, it is hard to narrow down which pores to simulate. Here, we report alternative routes to rapidly screen molecules and nanopores with negligible computational requirement to shortlist selective nanopore candidates. Through the 3D representation and visualization of the pores' and molecules' atoms with their van der Waals radii using open-source software, we could identify suitable C-passivated nanopores for both gas- and liquid-phase separation while accounting for the pore and molecule shapes. The method was validated by simulations reported in the literature and was applied to study the mass transport behavior across a given distribution of nanopores. We also designed a second method that accounts for Lennard-Jones and electrostatic interactions between atoms to screen selective non-C-passivated nanopores for gas separations. Overall, these visualization methods can reduce the computational requirements for pore screening and speed up selective pore identification for subsequent detailed MD simulations and guide the experimental design and interpretation of transport measurements in nanoporous atomically thin membranes.

Correction to “Ceramic Fiber-Based Structures as Catalyst Supports: A Study on Mass and Heat Transport Behavior Applied to CO2 Methanation

Correction to “Ceramic Fiber-Based Structures as Catalyst Supports: A Study on Mass and Heat Transport Behavior Applied to CO₂ Methanation

A Computational Study of the Mass Transport Behavior of Hydrogen in In-Service Carbon Steel Pipeline Fillet Welds During Deposition and Desorption

Numerical calculations were used to study the diffusion and thermotransport of hydrogen during deposition and diffusion of hydrogen during desorption of in-service carbon steel pipeline fillet welds prone to hydrogen cracking. During deposition, only a small amount of hydrogen migrates into the HAZ and little migrates into the pipe. Thermotransport decreases the hydrogen concentration in the HAZ and regions of the weld closer to the HAZ, and increases the concentration toward the outer edge of the weld. This effect increases the rate of desorption of hydrogen from the weld, which increases with weld size/pipe wall thickness. The activation energy for desorption varies with the fraction of hydrogen removed and appears to be a function of the activation energy for diffusion and mass fraction of hydrogen in the weld, HAZ, and pipe. The differences in the desorption rate due to thermotransport appear to be due to differences in the activation energy for desorption and hydrogen concentration gradients. Maintaining a 121 °C preheat/interpass temperature during welding significantly reduces post-weld heating times necessary to reach hydrogen levels considered to be safe from hydrogen cracking. Calculated results and experimental results from another study show good agreement for desorption at 132 °C for up to 30 minutes.

Different dominating mass transport mechanisms for drying and sorption of toluene-PMMA films – Visualized with Raman spectroscopy

The phase equilibrium as well as the diffusion kinetics of toluene-poly (methyl methacrylate) have been investigated over a concentration range from 0 to 75 v% toluene with independent measurements in order to describe drying and sorption processes under different conditions. The phase equilibrium of a micrometer thick layer, measured using a magnetic suspension balance can be confirmed with quartz crystal microbalance measurements of layers in the nanometer range. The diffusion kinetics has been determined to be concentration- and temperature-dependent using experimental data from drying experiments gained via inverse micro Raman-spectroscopy. A different mass transport behavior can be observed for sorption experiments. The determined mass transport behavior of both, drying and sorption measurements can be verified by experiments with a magnetic suspension balance. In total, the determined data permits the modeling of an integral drying process under different drying conditions as well as the prediction of concentration profiles in the film.

Mass Transport Behaviors in Graphene and Polyaniline Heterostructure–Based Microsupercapacitors

The development of miniaturized energy storage components with high areal performance for emerging electronics depends on scalable fabrication techniques for thick electrodes and an in‐depth understanding of the intrinsic properties of materials. Based on the coprecipitation behavior of electrically exfoliated graphene and reduced graphene oxide–templated polyaniline (PANi) nanoflake, this work develops a simple, green, low‐cost, and scalable drop‐casted technique to easily fabricate uniform thick electrodes (up to 80 μm) on various substrates. Through using a direct laser writing process, planar microsupercapacitors can be readily attained. As‐fabricated flexible all‐solid‐state microsupercapacitors exhibit an ultrahigh areal capacitance of 172 mF cm−2 at 0.1 A cm−2 and excellent cycling stability of 91% capacitance retention over 2000 cycles at a high current density of 1.0 A cm−2. Furthermore, based on the electrochemical quartz crystal microbalance research result, the pseudocapacitance contribution is mostly provided by the adsorption/desorption of SO42− anions during the protonation process of PANi. This work offers a simple strategy toward superior‐performance micro‐sized energy devices and a new perspective to understand the origin of the capacitance of composites and heterostructures.

A molecular-level strategy to boost the mass transport of perovskite electrocatalyst for enhanced oxygen evolution

Perovskite oxides are of particular interest for the oxygen evolution reaction (OER) due to their high intrinsic activity. However, low surface area and nonpores in bulk phase generally limit the mass transport and thereby result in unsatisfactory mass activity. Herein, we propose a “molecular-level strategy” with the simultaneous modulation of the ordered pores on the oxygen-deficient sites along with sulfur (S) substitution on oxygen sites at the molecular level to boost the mass transport behavior of perovskite electrocatalyst for enhanced mass activity. As a proof of concept, the elaborately designed brownmillerite oxide Sr2Co1.6Fe0.4O4.8S0.2 (S-BM-SCF) shows approximately fourfold mass activity enhancement in 1 M KOH compared with the pristine SrCo0.8Fe0.2O3-δ (SCF) perovskite. Comprehensive experimental results, in combination with theoretical calculations, demonstrate that the intrinsic molecular-level pores in the brownmillerite structure can facilitate reactive hydroxyl ion (OH−) uptake into the oxygen-vacant sites and that S doping further promotes OH− adsorption by electronic structure modulation, thus accelerating mass transport rate. Meanwhile, the S-BM-SCF can significantly weaken the resistance of O2 desorption on the catalyst surface, facilitating the O2 evolution. This work deepens the understanding of how mass transport impacts the kinetics of the OER process and opens up a new avenue to design high-performance catalysts on the molecular level.