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To meet the demands for high power density, enhancing the water discharge capacity of proton exchange membrane fuel cells (PEMFCs) is imperative. The microporous layer (MPL), placed between a catalyst layer and gas diffusion layer, plays a crucial role in water management for PEMFCs. In this study, we fabricated electrospun MPLs of different pore sizes and characterized their morphology and pore structures. The mercury intrusion test showed that the mean pore sizes of electrospun MPLs are about 1.2–2.3 μm, much larger than commercial MPL's (about 70 nm). The electrospun MPLs were assembled in single PEMFCs to test their performance and Electrochemical Impedance Spectroscopy (EIS). The results demonstrate that the electrospun MPLs with a pore size exceeding 1.9 μm significantly enhance water discharge and oxygen transport at the cathode relative humidity (RH) of 50 % and 100 %. Moreover, the EIS testing and fitting outcomes indicate that when the PEMFC performance is dominated by mass transport under high current density and 100 % cathode RH, the electrospun MPLs with a pore size exceeding 1.9 μm exhibit a better water discharge capacity and lower mass transport resistance. • Electrospun MPLs have larger pore sizes (>1 μm) than commercial ones (∼100 nm). • Electrospun MPLs of a pore size exceeding 1.9 μm enhance water discharge. • Fuel cell performance was improved at cathode relative humidity >50 %. [ABSTRACT FROM AUTHOR]

The presence of vegetation within urban canyons leads to non-trivial patterns of the concentration of airborne pollutants, as a result of the complex structure of the velocity field. To investigate the relationship between concentration, velocity fields and vegetation density, we have performed wind-tunnel experiments in a reduced-scale street canyon, oriented perpendicular to the external wind flow, within which we placed a steady ground-level line source of a passive tracer. The aerodynamic behavior of vegetation was reproduced by inserting plastic miniatures of trees along the two long sides of the canyon, according to three different densities. The canyon ventilation was investigated by acquiring one-point simultaneous statistics of concentration and velocity over a dense grid of points within the canyon. The results show that the presence of trees hinders the upward mean vertical velocity at the rooftop, causes a reduction of the turbulent kinetic energy inside the canyon, and reduces the energy content of the large scales. The scalar concentration is conversely characterized by an enhanced level of turbulent fluctuations, whose magnitude is not dampened increasing the tree density. Within the canyon, high tree density inhibits turbulent mass fluxes, which are instead enhanced at roof level, where the mean component of the scalar flux is however hindered. A statistical analysis of concentration time series reveals that the lognormal distribution is suitable to model concentration fluctuations and extreme events, in dispersing plumes emitted by a linear source. [ABSTRACT FROM AUTHOR]

This paper investigates the multiple slip boundary conditions on mass transmission flow of non-Newtonian fluid due to unsteady stretching sheet. Mathematical models are constructed by utilizing boundary layer approach. The governing equations of PDE's are converted into ODE's by using similarity transformations. It is important to carefully consider the various constraints that may affect the flow field, temperature and mass transfer in the study. The Governing equations are solved numerically by applying fourth order RK shooting method. The purpose of study includes mass transmission analysis and it is very important to resolve the current industrial and scientific problems. In this research work the consequences of multiple slip on time independent MHD stream of an UCM fluid over an elongating surface with upper grade chemical reaction investigated. This study's significant findings are rate of mass transmission at the surface decreases with enhancing momentum. Concentration drops for increasing values of Sc. The concentration distribution depicts during (γ > 0) and increases in (γ < 0). It is also important to ensure that the assumptions and approximations made in the study are valid and reasonable in order to obtain accurate and reliable results. [ABSTRACT FROM AUTHOR]

Thread-based microfluidic fuel cells are promising micro power sources for portable and wearable electronic devices. In this study, a three-dimensional computational model is developed for a thread-based microfluidic fuel cell with carbon fiber electrodes. The interaction of fluid flow, species transport and electrochemical reactions is elucidated. The cell polarization curve obtained from modeling is validated by the experimental data. Moreover, effects of operational parameters and structural parameters on the cell performance and mass transport are further investigated. Results demonstrate that the presence of the intermediate flow channel can effectively prevent the reactant crossover and the parasitic current density is negligible. Appropriate increase in the reactant concentration is beneficial to yield uniform concentration distribution and enhance the mass transfer. Besides, the peak power density is only increased by 0.7% when the flow rate of anolyte and catholyte is increased by four times. Additionally, lowering the electrode spacing and the electrode porosity result in the superior cell performance. With the electrode spacing of 3 mm and the electrode porosity of 0.6, the peak power density of 34.6 mW cm−2 and the maximum current density of 112.5 mA cm−2 are achieved. This work provides theoretical guidance for future development and optimization for thread-based microfluidic fuel cells. • A three-dimensional computational model is developed for a thread-based microfluidic fuel cell. • The flow field and concentration distributions of reactants are calculated. • Effects of operational and structural parameters on the cell performance are discussed. • Cell performance is more sensitive to the structural parameter than the operational condition. [ABSTRACT FROM AUTHOR]

To increase the oxygen mass transport, water removal, and cell performance of proton exchange membrane fuel cells, tapered flow field configurations (FFCs) considering porous medium thickness (PMT) and thermal contact resistance (TCR) are proposed. In this work, the impact of the tapered FFC on the internal electrochemical process and overall cell performance is quantitatively explored using a three-dimensional multiphase fuel cell model. The tapered FFC is compared without and with account for both the PMT and the TCR between bipolar plates and gas diffusion layers. Compared to the conventional FFC, an increased ratio of the side length for the inlet to that for the outlet (L I/O) for the tapered FFC, addressing both the PMT and the TCR improves hydrogen, and oxygen delivery, water removal, and cell performance. The tapered FFC design with a L I/O of 1.2 is superior to all other tapered FFC designs in terms of uniformity of reactants, with an improvement in pressure drop by 68.74%, current density by 7.57%, and power density by 12.63%. These improvements result in an overall enhancement of cell performance. • A tapered FFC is proposed for fuel cells considering thermal contact resistance and porous medium thickness. • Thermal contact resistance reduced heat dissipation and water retention capability. • TCR and PMT between GDL and BP enhance PEM fuel cell performance. • The pressure drop improved for case 6 (L i/o 1.2) by approximately 68.74% from case 1 (L i/o 0.7). • The power density in case 6 (L i/o 1.2) is improved by 12.63% from case 1 (L i/o 0.7). [ABSTRACT FROM AUTHOR]

[Display omitted] The presence of ions in a solution is anticipated to induce distinct effects on macromolecules. Consequently, the tuning of adsorption and mass transfer of lignin molecules can be achieved by incorporating ions with chaotropic or kosmotropic characteristics. This study examines the adsorption and mass transfer behavior of lignin molecules across model cellulose membranes in presence of ions from the Hofmeister series. Experimental investigations encompassed the use of diffusion cells to quantify lignin's mass transfer through the membranes, and quartz crystal microbalance with dissipation (QCM-D) monitoring was used for adsorption studies. Notably, at high ion concentrations, the mass transport rate of lignin was observed to be lower in the presence of highly hydrated (kosmotropic) sulfate ions, conforming to the Hofmeister series. Intriguingly, this relationship was not apparent at lower ion concentrations. Furthermore, QCM-D experiments indicated that lignin displayed higher adsorption onto the cellulose surface when exposed to less hydrated (chaotropic) nitrate anions. This behavior can be rationalized by considering the system's increased entropy gain, facilitated by the release of adsorbed ions and water molecules from the cellulose surface upon lignin adsorption. This study highlights the complexity of ion-specific effects on mass transfer and adsorption processes and their dependency on ion concentrations. [ABSTRACT FROM AUTHOR]

Focused Ultrasound (FUS)-triggered nano-sized drug delivery, as a smart stimuli-responsive system for treating solid tumors, is computationally investigated to enhance localized delivery of drug and treatment efficacy. Integration of thermosensitive liposome (TSL), as a doxorubicin (DOX)-loaded nanocarrier, and FUS, provides a promising drug delivery system. A fully coupled partial differential system of equations, including the Helmholtz equation for FUS propagation, bio-heat transfer, interstitial fluid flow, drug transport in tissue and cellular spaces, and a pharmacodynamic model is first presented for this treatment approach. Equations are then solved by finite element methods to calculate intracellular drug concentration and treatment efficacy. The main objective of this study is to present a multi-physics and multi-scale model to simulate drug release, transport, and delivery to solid tumors, followed by an analysis of how FUS exposure time and drug release rate affect these processes. Our findings not only show the capability of model to replicate this therapeutic approach, but also confirm the benefits of this treatment with an improvement of drug aggregation in tumor and reduction of drug delivery in healthy tissue. For instance, the survival fraction of tumor cells after this treatment dropped to 62.4%, because of a large amount of delivered drugs to cancer cells. Next, a combination of three release rates (ultrafast, fast, and slow) and FUS exposure times (10, 30, and 60 min) was examined. Area under curve (AUC) results show that the combination of 30 min FUS exposure and rapid drug release leads to a practical and effective therapeutic response. [ABSTRACT FROM AUTHOR]

ABSTRACTMost microbial life on Earth is found in localized microenvironments that collectively exert a crucial role in maintaining ecosystem health and influencing global biogeochemical cycles. In many habitats such as biofilms in aquatic systems, bacterial flocs in activated sludge, periphyton mats, or particles sinking in the ocean, these microenvironments experience sporadic or continuous flow. Depending on their microscale structure, pores and channels through the microenvironments permit localized flow that shifts the relative importance of diffusive and advective mass transport. How this flow alters nutrient supply, facilitates waste removal, drives the emergence of different microbial niches, and impacts the overall function of the microenvironments remains unclear. Here, we quantify how pores through microenvironments that permit flow can elevate nutrient supply to the resident bacterial community using a microfluidic experimental system and gain further insights from coupled population-based and computational fluid dynamics simulations. We find that the microscale structure determines the relative contribution of advection vs diffusion, and even a modest flow through a pore in the range of 10 µm s−1 can increase the carrying capacity of a microenvironment by 10%. Recognizing the fundamental role that microbial hotspots play in the Earth system, developing frameworks that predict how their heterogeneous morphology and potential interstitial flows change microbial function and collectively alter global scale fluxes is critical.IMPORTANCEMicrobial life is a key driver of global biogeochemical cycles. Similar to the distribution of humans on Earth, they are often not homogeneously distributed in nature but occur in dense clusters that resemble microbial cities. Within and around these clusters, diffusion is often assumed as the sole mass-transfer process that dictates nutrient supply and waste removal. In many natural and engineered systems such as biofilms in aquatic environments, aggregates in bioremediation, or flocs in wastewater treatment plants, these clusters are exposed to flow that elevates mass transfer, a process that is often overlooked. In this study, we show that advective fluxes can increase the local growth of bacteria in a single microenvironment by up to 50% and shape their metabolism by disrupting localized anoxia or supplying nutrients at different rates. Collectively, advection-enhanced mass transport may thus regulate important biogeochemical transformations in both natural and engineered environments.

Abstract To achieve carbon neutrality, integrating intermittent renewable energy sources, such as solar and wind energy, necessitates the use of large-scale energy storage. Among various emerging energy storage technologies, redox flow batteries are particularly promising due to their good safety, scalability, and long cycle life. In order to meet the ever-growing market demand, it is essential to enhance the power density of battery stacks to lower the capital cost. One of the key components that impact the battery performance is the flow field, which is to distribute electrolytes onto electrodes. The design principle of flow fields is to maximize the distribution uniformity of electrolytes at a minimum pumping work. This review provides an overview of the progress and perspectives in flow field design and optimization, with an emphasis on the scale-up process. The methods used to evaluate the performance of flow fields, including both experimental and numerical techniques, are summarized, and the benefits of combining diverse methods are highlighted. The review then investigates the pattern design and structure optimization of serpentine- and interdigitated-based flow fields before discussing challenges and strategies for scaling up these flow fields. Finally, the remaining challenges and the prospects for designing highly efficient flow fields for battery stacks are outlined.

In last decades, there have been drastic environmental transformations and mutations happening all around the world. Due to the continuous mass transfer process, for example, CO2{{\rm{CO}}}_{2} mass transfer, which in this case, takes the form of greenhouse gas emissions, unusual and extreme kinds of phenomena have been occurring here and there, disturbing our ecosystems and causing damage and chaos on their paths. Reducing or stopping these gas emissions has become one of the major topics in our planet. We investigate the solvability of a mathematical model describing the mass transport process in nature and where additional perturbations parameters have been considered. Besides addressing the stability of the model, its convergence analysis is also given with the use of Crank–Nicholson numerical method, in order to assess its efficiency and perform some numerical simulations. The results obtained show that the model’s dynamic is characterized by many grouping (accumulation) zones, where mass (of CO2{{\rm{CO}}}_{2}, for instance) accumulates in an increasing way. This result is important in controlling how CO2{{\rm{CO}}}_{2} can be stored in this growingly perturbed environment that surrounds us.

Transport phenomena are key in controlling the performance of electrochemical energy-conversion technologies and can be highly complex, involving multiple length scales and materials/phases. Material designs optimized for one reactant species transport however may inhibit other transport processes. We explore such trade-offs in the context of polymer-electrolyte fuel-cell electrodes, where ionomer thin films provide the necessary proton conductivity but retard oxygen transport to the Pt reaction site and cause interfacial resistance due to sulfonate/Pt interactions. We examine the electrode overall gas-transport resistance and its components as a function of ionomer content and chemistry. Low-equivalent-weight ionomers allow better dissolved-gas and proton transport due to greater water uptake and low crystallinity but also cause significant interfacial resistance due to the high density of sulfonic acid groups. These effects of equivalent weight are also observed via in situ ionic conductivity and CO displacement measurements. Of critical importance, the results are supported by ex situ ellipsometry and X-ray scattering of model thin-film systems, thereby providing direct linkages and applicability of model studies to probe complex heterogeneous structures. Structural and resultant performance changes in the electrode are shown to occur above a threshold sulfonic-group loading, highlighting the significance of ink-based interactions. Our findings and methodologies are applicable to a variety of solid-state energy-conversion devices and material designs.

Alternate cobalt redox mediator based dye-sensitized solar cells (DSCs) are getting widespread attention taking advantage of their one-electron transfer mechanism compared to the conventional iodide/triiodide electrolyte. In the present study, we used indole fused heterocyclic organic sensitizers having indolo[3,2-b]indole as donor with three different π-spacers [(benzene (IID-1), thiophene (IID-2) and furan (IID-3)] along with cobalt bipyridine derivatives as redox mediators having different peripheral substituents {[Co(bpy)3]3+/2+, [Co(Me2bpy)3]3+/2+, and [Co(t-Bu2bpy)3]3+/2+}. A detailed investigation was carried out to understand the fundamental charge transfer processes and loss mechanism happening at the various interfaces as a function of structural variations in the present dye-electrolyte combinations. Among the investigated systems, higher performance was obtained for the association of furan substituted dye (IID-3) with [Co(t-Bu2bpy)3]3+/2+ electrolyte. The importance of choosing the right combination of sensitizer and electrolyte is critical to realize higher performance in dye-sensitized solar cells particularly while employing organic dyes and alternate metal complex redox electrolytes which was systematically investigated in the present manuscript.

Several heterogeneous catalysts have been proposed as recoverable catalysts for transesterification in place of homogeneous hydroxide or methoxide. Anion-exchange resin is a potential recoverable and reusable catalyst but its industrial use is still barred by its low catalytic activity. Transesterification of refined palm oil with methanol catalysed by anion-exchange resin was studied. A phenomenon limiting the reaction rates was discovered and is presented. Two types of anion-exchange resins were used as the catalysts: dense IRA402 and macroporous A26. With IRA402, an oil conversion of only 6.1 % was obtained from 4 h transesterification at 60 °C with a methanol-to-oil molar ratio of 9.5:1 and 4 cm3 resin per 42 cm3 reaction mixture. Severe mass transfer resistance in this three-phase system appeared to be the limiting factor. Addition of tetrahydrofuran (THF) as a co-solvent helped form a single liquid phase. The oil conversion was improved to 19.3 %. The reaction between oil and adsorbed methoxide at resin’s ion-exchange sites seemed to be the rate-determining step. When A26 was used, the rate of FAME production was boosted due to the porous nature of the resin. Nevertheless, the reaction rate fell sharply after a while into a slower second stage. The formation of a secondary liquid phae in resin pores triggered by glycerol generation was proposed as the cause of this drop in the reaction rate. A higher oil conversion of 63.4 % was obtained. Diluting the reaction mixture with more THF eliminated the slower second stage and sustained the firststage’s faster reaction rate. An oil conversion of 84.7 % was achieved from 3 h transesterification. A finite-volume model was developed to simulate the reaction and internal mass transport in A26 resin beads. The proposed secondary liquid phase formation and a branching pore structure in resin beads were required in the model to reproduce experimental data. The findings suggested improvement directions for anion-exchange resin as a catalyst for transesterification.

The precise prediction of moisture variations within concrete is of great significance because various deterioration phenomena spanning corrosion to carbonation are induced by mass transfer facilitated by the presence of moisture. Under actual environmental conditions, concrete is exposed to numerous wetting and drying cycles. Thus, to accurately predict moisture variations it is essential to consider the effect of these conditions. This study investigates the effect of alternate wetting and drying cycles on moisture penetration through concrete at the mesoscale using a 3D discrete network model based on a Rigid Body Spring Model (RBSM). For modeling moisture transport during wetting, the non-linear diffusion equation is used and the effects of capillary suction in moisture transport are also considered. On the other hand, moisture transport during drying is modeled as an evaporation-diffusion process. The simulation results obtained using this discrete network model are validated against the available experimental outcomes. During the wetting cycle, the moisture content in regions close to the exposed surface rapidly increases and approaches the maximum saturation. The effect of subsequent drying is seen only until a certain depth below the exposed surface, known as the influential depth. It is also observed that moisture continues to penetrate deeper into a specimen even during a drying cycle.

The effective management of oxygen transport resistance (OTR) within the cathode catalyst layer (CCL) is crucial for achieving a high catalyst performance at low platinum (Pt) loading. Over the past two decades, significant advancements have been made in the development of various high active platinum-based catalysts, aiming at enhancing oxygen mass transport and the oxygen reduction reaction (ORR). However, experimental investigations of transport processes in porous media are often computational costs and restrained by limitations in in-situ measurement capabilities, as well as spatial and temporal resolution. Fortunately, numerical simulation provides a valuable alternative for unveiling the intricate relationship between local transport properties and overall cell performance that remain unresolved or uncoupled through experimental approach. In this review, we elucidate the primary experimental and numerical efforts undertaken to improve OTR. We consolidate the available literature on OTR values and perform a quantitative comparison of the effectiveness of different strategies in mitigating OTR. Furthermore, we analyze the intrinsic limitations and challenges associated with current experimental and numerical methods. Finally, we outline future prospect for advancements in both experimental techniques and modelling methods.

AbstractFocused Ultrasound (FUS)-triggered nano-sized drug delivery, as a smart stimuli-responsive system for treating solid tumors, is computationally investigated to enhance localized delivery of drug and treatment efficacy. Integration of thermosensitive liposome (TSL), as a doxorubicin (DOX)-loaded nanocarrier, and FUS, provides a promising drug delivery system. A fully coupled partial differential system of equations, including the Helmholtz equation for FUS propagation, bio-heat transfer, interstitial fluid flow, drug transport in tissue and cellular spaces, and a pharmacodynamic model is first presented for this treatment approach. Equations are then solved by finite element methods to calculate intracellular drug concentration and treatment efficacy. The main objective of this study is to present a multi-physics and multi-scale model to simulate drug release, transport, and delivery to solid tumors, followed by an analysis of how FUS exposure time and drug release rate affect these processes. Our findings not only show the capability of model to replicate this therapeutic approach, but also confirm the benefits of this treatment with an improvement of drug aggregation in tumor and reduction of drug delivery in healthy tissue. For instance, the survival fraction of tumor cells after this treatment dropped to 62.4%, because of a large amount of delivered drugs to cancer cells. Next, a combination of three release rates (ultrafast, fast, and slow) and FUS exposure times (10, 30, and 60 min) was examined. Area under curve (AUC) results show that the combination of 30 min FUS exposure and rapid drug release leads to a practical and effective therapeutic response.

The growth of living systems is ubiquitous. Living organisms can continually update their sizes, shapes, and properties to meet various environmental challenges. Such a capability is also demonstrated by emerging self‐growing materials that can incorporate externally provided compounds to grow as living organisms. In this Minireview, we summarize these materials in terms of six aspects. First, we discuss their essential characteristics, then describe the strategies for enabling crosslinked organic materials to self‐grow from nutrient solutions containing polymerizable compounds. The developed examples are grouped into five categories based on their molecular mechanisms. We then explain the mechanism of mass transport within polymer networks during growth, which is critical for controlling the shape and morphology of the grown products. Afterwards, simulation models built to explain the interesting phenomena observed in self‐growing materials are discussed. The development of self‐growing materials is accompanied by various applications, including tuning bulk properties, creating textured surfaces, growth‐induced self‐healing, 4D printing, self‐growing implants, actuation, self‐growing structural coloration, and others. These examples are then summed up. Finally, we discuss the opportunities brought by self‐growing materials and their facing challenges. [ABSTRACT FROM AUTHOR]

Pulsed electrolysis endowed merits of high current density, low energy consumption, and simple operation; thus, a booming in their publication numbers has been witnessed in recent years. In this review, we aim to summarize the state-of-the-art pulsed current/potential strategy to enhance electrochemical reactions, such as oxygen reduction reactions (ORR), CO2 reduction (CO2RR), CO reduction (COR), etc. It begins with historic analysis of pulsed electrolysis via a bibliometric method, aiming at providing a progress over the course of around 40 years in a quantitative way. Then, the definition along with its influence of electronic effect, double layer effect and mass transport have been reviewed based on a mechanism point of view for the first time. To sum up the review, several current challenges and future prospects of pulsed electrolysis have provided for the future smart design of electrochemical process. [ABSTRACT FROM AUTHOR]

[Display omitted] Three-dimensional 1H UltraShort Echo Time magnetic resonance imaging (1H 3D UTE MRI) of the matrix tablet made of hydrophilic polymer hydrated in heavy water (D 2 O) will allow investigation of the hydration-induced spatiotemporal evolution of the material originally included in the matrix tablet during manufacturing (i.e., polymer chains and bound water). The oblong-shaped sodium alginate matrix tablets were used to verify the hypothesis. The matrix was measured before and during hydration in D 2 O for up to 2 h using the 1H 3D UTE MRI. Five echo times (first at 20 μs) were used, resulting in five three-dimensional images (one image for each echo time). In chosen cross-sections, two parametric images, i.e., amplitude and T 2 * relaxation time maps, were calculated using "pixel-by-pixel" mono-exponential fitting. The regions of the alginate matrix with T 2 * shorter than 600 μs were analyzed before (air-dry matrix) and during hydration (parametric, spatiotemporal analysis). During the study, only hydrogen nuclei (protons) pre-existing in the air-dry sample (polymer and bound water) were monitored because the hydration medium (D 2 O) was not visible. As a result, it was found that morphological changes in regions having T 2 * shorter than 300 μs were the effect of fast initial water ingress into the core of the matrix and subsequent polymer mobilization (early hydration providing additional 5% w/w hydration medium content relating to air-dry matrix). In particular, evolving layers in T 2 * maps were detected, and a fracture network was formed shortly after the matrix immersion in D 2 O. The current study presented a coherent picture of polymer mobilization accompanied by local polymer density decrease. We concluded, that the T 2 * mapping using 3D UTE MRI can effectively be applied as a polymer mobilization marker. [ABSTRACT FROM AUTHOR]

To achieve carbon neutrality, integrating intermittent renewable energy sources, such as solar and wind energy, necessitates the use of large-scale energy storage. Among various emerging energy storage technologies, redox flow batteries are particularly promising due to their good safety, scalability, and long cycle life. In order to meet the ever-growing market demand, it is essential to enhance the power density of battery stacks to lower the capital cost. One of the key components that impact the battery performance is the flow field, which is to distribute electrolytes onto electrodes. The design principle of flow fields is to maximize the distribution uniformity of electrolytes at a minimum pumping work. This review provides an overview of the progress and perspectives in flow field design and optimization, with an emphasis on the scale-up process. The methods used to evaluate the performance of flow fields, including both experimental and numerical techniques, are summarized, and the benefits of combining diverse methods are highlighted. The review then investigates the pattern design and structure optimization of serpentine- and interdigitated-based flow fields before discussing challenges and strategies for scaling up these flow fields. Finally, the remaining challenges and the prospects for designing highly efficient flow fields for battery stacks are outlined. [ABSTRACT FROM AUTHOR]

In this study, a staggered trapezoidal baffle structure was introduced in the flow channel of a proton-exchange membrane fuel cell, and its effect on the improvement of net power density was investigated. Under a constant inlet flow, the proposed structure adjusted the ratio of the channel width to the rib width. The staggered trapezoidal baffles on both sides achieved a guiding effect on the reaction gas. Furthermore, as the gas approached the central region of the flow channel, the convection effect intensified. For the flow channel with staggered trapezoidal baffles, the oxygen molar concentration at the center line of the interface between the gas diffusion layer and catalytic layer at the inlet reached 1.7 mol/m3, which was higher than the concentration of 1.5 mol/m3 in the basic flow channel. Additionally, the flow channel with staggered trapezoidal baffles exhibited a significantly faster decrease in the oxygen molar concentration in the latter half of the fuel cell. Furthermore, it demonstrated outstanding performance in effectively removing water with only 1/3 the volume fraction of water in the basic flow channel noted at the inlet of the optimized flow channel. In addition, genetic algorithm was used to optimize the height and depth of the trapezoidal baffles. Compared with the basic flow channel, the flow channel with staggered trapezoidal baffles had better mass transfer without a high pressure drop. The results showed that the staggered trapezoidal baffles can significantly increase the net power density. In particular, the optimal staggered trapezoidal baffles with the height and depth of 0.825 and 0.126 mm, respectively, achieved a net power density increase of 8.92%. [ABSTRACT FROM AUTHOR]

Proton exchange membrane fuel cells (PEMFCs) are considered a promising energy source in the field of transport and distributed power generation. Fundamental research into their key components is needed to improve PEMFC performance and accelerate commercialization. Binder addition and compression induced by assembly pressure can significantly change the microstructure of the gas diffusion layer and affect mass transport. A two-dimensional multicomponent lattice Boltzmann (LB) model considering the cathode electrochemical reaction was developed, and a GDL was reconstructed numerically and considering a binder structure. The effects of the binder and compression on mass transport and electrochemical performance within the GDL were investigated. The results showed that an increase in binder volume fraction led to more chain-like structures and closed pores that were unfavorable for mass transport. Compression increased the mass transfer resistance of the GDL in the region under the rib, leading to a decrease in oxygen concentration and local current density. [ABSTRACT FROM AUTHOR]

Porous electrodes govern the electrochemical performance and pumping requirements in redox flow batteries, yet conventional carbon‐fiber‐based porous electrodes have not been tailored to sustain the requirements of liquid‐phase electrochemistry. 3D printing is an effective approach to manufacturing deterministic architectures, enabling the tuning of electrochemical performance and pressure drop. In this work, model grid structures are manufactured with stereolithography 3D printing followed by carbonization and tested as flow battery electrode materials. Microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics are employed to investigate the resulting electrode properties, mass transport, and pressure drop of ordered lattice structures. The influence of the printing direction, pillar geometry, and flow field type on the cell performance is investigated and mass transfer vs. electrode structure correlations are elucidated. It is found that the printing direction impacts the electrode performance through a change in morphology, resulting in enhanced performance for diagonally printed electrodes. Furthermore, mass transfer rates within the electrode are improved by helical or triangular pillar shapes or by using interdigitated flow field designs. This study shows the potential of stereolithography 3D printing to manufacture customized electrode scaffolds, which could enable multiscale structures with superior electrochemical performance and low pumping losses. [ABSTRACT FROM AUTHOR]

In the framework of stable free modes, a criterion is derived for the localisation in latitude of the intensified sector of the mode, i.e. northwards, southwards, or equally distributed in the north and south. The criterion is based on the sign taken by the inverse of the total vorticity at the middle latitude of the reference basin, and resorts to the necessarily westward orientation of an unforced current according to one of Greenspan's theorems. Some solutions, also of non-linear modes, elucidate the result. [ABSTRACT FROM AUTHOR]

The charge and mass transport phenomena of the different types of commercial electrodes such as KFD 2.5, GFD 2.5, GFD 4.6 and GFA 6 in a vanadium redox flow battery with the single-cell are presented in this study, comparatively. In order to perform these comparisons, all properties of the four different types of electrodes are applied to the numerical model which validated experimental data. The results indicated that the biggest electrode potential difference between the positive electrode and negative electrode belongs to GFD 4.6 electrode with 1.20244 V, the smallest difference in the electrode potential belongs to KFD 2.5 electrode with 1.19832 V. Although KFD 2.5 and GFD 2.5 have the same electrode thickness, GFD 2.5 exhibits a better current density variation throughout both positive and negative electrode sides due to its higher electrical conductivity. While the V2+ and V5+ vanadium concentrations remain constant at 300 mol/m3 for KFD 2.5 and GFD 2.5 electrodes, these concentrations are approximately 310 mol/m3 for GFD 4.6 and GFA 6 after x/L = 0.5. [ABSTRACT FROM AUTHOR]

Micrometre-sized electroactive particles with high tapping density show significant potential for commercial application since they effectively alleviate low Coulombic efficiency and excessive solid electrolyte interphase (SEI) issues brought by nanostructures. Furthermore, optimizing the electrode architecture using novel design concepts can improve the energy density. Beyond the electrode material structure design strategy, binder plays a vital role in providing the mechanical stability and regulating the charge transport. This highlight presents the latest development to design high-capacity batteries by optimizing the binder structures in electrodes and emphasizes the significance of binder design for further commercial application.

Axisymmetric numerical simulations of the hydrodynamics around rising bubbles are performed in order to investigate the impact of surfactants on the bubble dynamics. Surfactants are assumed to be insoluble. The transport of the adsorbed surfactants is computed along the deforming surface at large surface Péclet number, and Marangoni stresses are taken into account. This simulation model leads to the stagnant-cap regime, with partially immobile interfaces. A parametric study is performed on cases at given Archimedes number, by varying the degree of contamination (Marangoni number) but maintaining a nearly constant Eötvös number. The presence of surfactants affects the rise velocity for oblate bubbles less than for spherical bubbles: the increase of the drag coefficient, due to interface contamination, is mitigated by a lower bubble deformation. When the cap angle $\theta _{cap}$ belongs to the southern hemisphere, the aspect ratio $\chi$ is found to decrease with contamination: the dynamic pressure responsible for the bubble distortion is lowered, related to the decline of kinetic energy. As soon as $\theta _{cap}$ lies in the northern hemisphere, the pressure stress causing distortion becomes independent on $\theta _{cap}: \chi$ no longer evolves with contamination, and already matches the prediction for fully immobile interfaces. Mass transfer of a passive scalar across the contaminated interface is also analysed. Surprisingly, the Sherwood number $Sh$ is found to follow the same law as for spherical shapes (Kentheswaran et al. , Intl J. Heat Mass Transfer , vol. 198, 2022, 123325), allowing us to predict the decrease in $Sh$ due to contamination. These results reveal the couplings between interface immobilisation, bubble deformation, rise velocity and interfacial mass transfer. [ABSTRACT FROM AUTHOR]

At high current densities, gas bubble escape is the critical factor affecting the mass transport and performance of the electrolyzer. For tight assembly water electrolysis technologies, the gas diffusion layer (GDL) between the catalyst layer (CL) and the flow field plate plays a critical role in gas bubble removal. Herein, we demonstrate that the electrolyzer's mass transport and performance can be significantly improved by simply manipulating the structure of the GDL. Combined with 3D printing technology, ordered nickel GDLs with straight‐through pores and adjustable grid sizes are systematically studied. Using an in situ high‐speed camera, the gas bubble releasing size and resident time have been observed and analyzed upon the change of the GDL architecture. The results show that a suitable grid size of the GDL can significantly accelerate mass transport by reducing the gas bubble size and the bubble resident time. An adhesive force measurement has further revealed the underlying mechanism. We then proposed and fabricated a novel hierarchical GDL, reaching a current density of 2 A/cm2 at a cell voltage of 1.95 V and 80 °C, one of the highest single‐cell performances in pure‐water‐fed anion exchange membrane water electrolysis (AEMWE). [ABSTRACT FROM AUTHOR]

This study develops a three-dimensional numerical model of a proton exchange membrane (PEM) fuel cell (PEMFC) to investigate the mass transport characteristics and performance of PEMFC with novel two-block structures in the cathode channel. The results reveal that the novel two-block structures improve the efficiency of mass transport and the performance of PEMFC. Effects of the arrangement of two-block structures and the operating conditions on the PEMFC performance are investigated. It is found that the block position closer to the outlet of the channel has greater improvement of the PEMFC performance. The PEMFC performance increases firstly with the block structure number and then decreases, and the best number of block structures is 6 for the present study. The block channel has more evident effect when the PEMFC operated under high current density and small stoichiometric ratio, and the block channel is effective in a wide relative humidity (RH) range. [Display omitted] • Effect of two-block channel on fuel cell mass transport and performance are studied. • Two-block channel increases cell performance with small pressure drop increment. • Fuel cell performance increases firstly with the block number and then decreases. • The optimal number of two-block structure is 6 for the best cell performance. • Applicability of two-block channel under different operating conditions are probed. [ABSTRACT FROM AUTHOR]

The delignification of birch chips during kraft pulping was investigated, targeting both the impregnation and cooking steps. Wood chips were impregnated using white liquor, white liquor + NaCl, water or NaCl aqueous solution. Then, the chips were cooked in batch autoclaves applying the same constant composition cooking conditions for all samples. Pulp and two fractions of black liquor (bulk liquor and centrifuged liquor representing the liquor inside the wood chips and fibers) were collected after different pulping times and analyzed for lignin and carbohydrate content. The dissolved wood components were precipitated from selected samples and characterized with respect to composition, molecular weight distribution and structural motifs. Cooking chemicals in the impregnation liquors led to faster delignification and xylan removal during cooking. Higher contents of lignin and xylan were measured in the lumen than in the bulk. The concentration profiles also showed accumulation of dissolved material in the lumen over time, suggesting significant mass transport limitation from lumen to bulk. Further analysis revealed higher fragmentation/degradation of dissolved material with increasing pulping time and in the bulk when compared to the lumen liquor, as demonstrated by the lower molecular weights and the changes in chemical shifts in the NMR spectra. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are a promising electrochemical platform for efficiently and reliably delivering electricity to the grid. Within the RFB, porous carbonaceous electrodes facilitate electrochemical reactions and distribute the flowing electrolyte. Tailoring electrode microstructure and surface area can improve RFB performance, lowering costs. Electrodes with spatially varying porosity may increase electrode utilization and provide surface area in reaction‐limited zones; however, the efficacy of such designs remains an open area of research. Herein, a non‐solvent‐induced phase‐separation (NIPS) technique that enables the reproducible synthesis of macrovoid‐free electrodes with well‐defined across‐thickness porosity gradients is described. The monotonically varying porosity profile is quantified and the physical properties and surface chemistries of porosity‐gradient electrodes are compared with macrovoid‐containing electrode, also synthesized by NIPS. Then, the electrochemical and fluid dynamic performance of the porosity‐gradient electrodes is evaluated, exploring the effect of changing the direction of the porosity gradient and benchmarking against the macrovoid‐containing electrode. Lastly, the performance is examined in a vanadium RFB, finding that the porosity‐gradient electrode outperforms the macrovoid electrode, is independent of gradient direction, and performs favorably compared to advanced electrodes in the contemporary literature. It is anticipated that the approach motivates further exploration of microstructurally tailored electrodes in electrochemical systems. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO2) gas diffusion is an important component of carbon cycling in soils. This process is particularly relevant in karst landscapes, which contain easily weathered rock, subsurface fractures, and cave networks. We instrumented three soil profiles--the shoulder, back slope, and toe slope of a sinkhole--above a karst cave in Virginia. Each profile had solidstate CO2 sensors and soil water content/temperature sensors at 20 and 60 cm depth that collected hourly measurements from 2017 to 2019. We calculated CO2 fluxes using Fick's first law along with measured soil and assumed atmospheric CO2 concentrations. With this approach, we identified occasional near-surface zero-flux planes, in which CO2 likely diffused both upward and downward. All profiles had upward CO2 fluxes during warm-season months, with maximum fluxes of 1.2 lmol CO2 m22 s21 in the shoulder and back slope versus 2.0 lmol CO2 m22 s21 in the toe slope. During cool-season months, upward CO2 fluxes were smaller (0-0.3 lmol CO2 m22 s21) and were often counteracted by downward fluxes in the toe slope, possibly driven by ventilation into the underlying cave. The toe slope had a cumulative annual efflux of 14.5 mol CO2 m22, which was .3 times greater than the other profiles. Fluxes were sensitive to soil porosity, with an order-of-magnitude difference when porosity was assumed to be 0.40 versus 0.56 cm3 cm23. The results of this study offer new insight into short-term and seasonal variations in diffusive CO2 gas transport in karst soils, and they may inform other investigations of non-uniform diffusion processes. [ABSTRACT FROM AUTHOR]

A good flow field design is important to the proton exchange membrane fuel cell (PEMFC) performance, especially under a high current density region, which is dominated by concentration polarization. Motivated by the variable cross-section channel idea, in this study, a novel flow field containing a converging-diverging (C-D) pattern is proposed. A three-dimensional multiphase model is established to study its performance. The numerical results show that it outperforms the conventional straight channel and only depth-variant channel. In the novel flow field the enhanced under land cross flow and higher effective mass transfer coefficient both improve the reactant transport. The effects of operating conditions, like stoichiometric ratio and operating pressure, on cell output performance are studied. It is found that a higher promotion rate can be obtained by increasing the stoichiometric ratio, but increasing the operating pressure has little effect. The droplet dynamic behavior in the C-D channel and straight channel are studied, and the results prove the better drainage capability of the novel flow field. • A mass transfer enhanced novel flow field is proposed to improve cell performance. • The power density of the novel flow field is increased by about 25.2% under 2.0 A/cm2. • A narrower C-D structure is more conducive to mass transfer and boosting performance. • The droplet dynamic behavior proves better drainage capacity of novel flow field. [ABSTRACT FROM AUTHOR]

Bone char has been successfully prepared from ostrich bone waste (OBC) via the physical activation under the presents of N2 gas and then was characterized by different techniques. The utility of the mass transport (MT) model to simulate the breakthrough curves of mercury(II) in a packed-bed adsorption column was evaluated and compared with the mathematical models (Bed Depth Service Time (BDST), Adams–Bohart (AB), Thomas, Dose–Response (DR), and Yoon–Nelson (YN)) under diverse operating factors such as influent concentration (35, 75, and 150 mg L−1), inlet flow rate (5, 10, 15, and 20 mL min−1), pH (2, 5, 7, and 9), and bed depth (10 and 20 cm). Based on the modeling results, it was concluded that the MT model had the best accuracy (R2 = 99.31%, MRE = 0.745% and NRMSE = 6.15%), followed by Th, BDST, YN, DR, and AB. The sensitivity analysis showed that the simulated breakthrough curves were more sensitive to maximum adsorption capacity (qT) than axial dispersion coefficient (DL) and apparent equilibrium constant (k). All models overestimated the breakthrough curves (MRE > 0). OBC was able to eliminate the Hg(II) from the real samples (greywater and petrochemical wastewater), which indicated that the presence of organic compounds in wastewater had no effect on the mercury(II) adsorption efficiency. The overall results showed that the dissolution–precipitation process and ion-exchange reaction were involved in the adsorption of the mercury(II) by OBC. [ABSTRACT FROM AUTHOR]

Currently, the durability of cement-based composites is primarily influenced by the presence and transport of corrosive fluids. In structural concrete, various factors such as mechanical loads and drying shrinkage can lead to the formation of cracks, which adversely affect its transport properties. While there is some consensus regarding the impact of individual cracks on transportation, our understanding of crack networks remains relatively limited. Furthermore, the maintenance of stress within the concrete can alter its porosity and influence the opening and closing of cracks, thereby affecting its transport properties. Due to limitations in experimental methods, characterizing the geometry of crack networks poses a challenge. However, numerical modeling offers an efficient and practical approach to investigate the impact of cracks, particularly microcracks, on the diffusivity of concrete. [ABSTRACT FROM AUTHOR]

A lot of applications for (semi-)crystalline triacylglycerol (TAG)-in-water dispersions exist in the life science and pharmaceutical industries. Unfortunately, during storage, these dispersions are often prone to changes in particle size due to unforeseen crystallization and recrystallization events. This results in the alterations of important product properties, such as viscosity and mouthfeel, or the premature release of encapsulated material. In this study, we investigated the effects and interplay of formulation, i.e., emulsifier combination, and processing parameters, i.e., cooling rate, on the colloidal stability of dispersed TAGs and aimed to improve their colloidal stability. We chose phospholipids (PLs) and β-lactoglobulin (β-lg) as the emulsifiers for our model systems, which are commonly applied in many food systems. When dispersions were characterized directly after cooling, we obtained smaller particles and narrower size distributions after fast cooling. Over the course of eleven weeks, the creaming behavior, particle size, melting behavior and polymorphism were characterized. The dispersions stabilized with solely β-lg exhibited a slight increase in particle size, whereas a decrease in size was found when PLs were added. Our results indicate that mass transport phenomena between TAG droplets and particles took place during storage. This migration of TAG molecules changed the composition and size distribution of the dispersed phase, especially at higher PL concentration (0.1 wt%). In our case, this could be prevented by using a lower concentration of PLs, i.e., 0.05 wt%. [ABSTRACT FROM AUTHOR]

Abstract Leveraging the interactions between ionomer and catalyst can increase the performance of proton exchange membrane fuel cells. The impacts of the equivalent weight (EW) of perfluorosulfonic acid–based ionomers on the platinum group metal‐free electrode structure and fuel cell performance have not been fully explored. Four membrane electrode assemblies (MEAs) were prepared by using a commercial Fe–N–C catalyst, two perfluorosulfonic acid ionomers with different EWs, that is, Aquivion 720 (A720) and Nafion 1100 (N1100), and two ionomer‐to‐catalyst (I/C) ratios. The four MEAs were characterized to understand the impact of the ionomer EW and content on the capacitance, proton conductivity, and mass transport on the cathode. The mass transport resistance was measured for the first time using a new oxygen reduction reaction limiting current method enabling to couple the effects of oxygen diffusion with liquid water generation. Low EW ionomer combined with a moderate I/C results in improved performance due to its enhanced proton conductivity. However, when used at high I/C, it can cause severe water flooding at high current density due to the enhanced liquid water uptake, especially at high relative humidity, resulting in lower catalyst utilization and higher mass transport resistance.

The current study focused on the charge and mass transport effect on the continuous electro-Fenton (EF) process treatment of synthetic Reactive orange 16 (RO16) dye using low-cost stainless-steel electrodes and sodium chloride (NaCl) supporting electrolytes, respectively. Lab-scale experiments were carried out in a 500 mL volume reactor cell at various initial RO16 dye concentrations (75-250 mg/L) and flow rates (0.05-0.4 L/h). The results showed that the decolorization rate increased quantitatively with an increment of the RO16 dye concentration and flow rate due to the mass transport limitation. Increasing the mass flow rate increased the mass transfer coefficient (km), improving the kinetics of the decay. It was found that regardless of inflow concentrations, the dye removal efficiency increased with the flow rate. Additionally, the degradation rate, elimination capacity, current efficiency (CE), and specific energy requirement were estimated for the process. A dimensionless current density relation was generated for the developed continuous stirred tank to describe the kinetics and mass transfer relationship towards the overall reaction rate contribution. It was found that the stainless-steel anode electrode proved to be preferable due to lower energy consumption (6.5 kWh m-3) and less iron sludge production. Additionally, the application of pyrite (FeS2) particulate electrode increased the process efficiency (~ 5%) for TOC removal and current mineralization while maintaining its sustainability for reuse.

Abstract The Earth's fluid budget is dominated by species in the system C–O–H–N–S together with halogens such as F and Cl. H2O is by far the most abundant. Such fluids are one of the two main mass transport agents (fluid and magma) in the Earth. Among those, in particular aqueous fluids are efficient solvents of geochemically important components at high temperature and pressure. The solution capacity of aqueous fluids can be enhanced further by dissolved halogens and sulfur. CO2 or nitrogen species has the opposite effect. Fluid-mediated transport in the Earth is by fluids passing through cracks at shallow depth and via percolation channels along grain boundaries at greater depth. Percolation velocity is linked to permeability, which, in turn is governed by rock porosity. Porosity is controlled by wetting angles, θ, at the interface between fluid and mineral surfaces. When θ

Mass transport of different species plays a crucial role in electrochemical conversion of CO2 due to the solubility limit of CO2 in aqueous electrolytes. In this study, we investigate the transport of CO2 and other ionic species through the electrolyte and the membrane, and its impact on the scale-up process of HCOO−/HCOOH formation. The mass transport of ions to the electrode and the membrane is modelled at constant current density. The mass transport limitations of CO2 on the formation of HCOO−/HCOOH is investigated at different pressures ranges from 5–40 bar. The maximum achievable partial current density of formate/formic acid is increased with increasing CO2 pressure. We use an ion exchange membrane model to understand the ion transport behaviour for both the monopolar and bipolar membranes. The cation exchange (CEM) and anion exchange membrane (AEM) model show that ion transport is limited by the electrolyte salt concentrations. For 0.1 M KHCO3, the AEM reaches the limiting current density more quickly than the CEM. For the BPM model, ion transport across the diffusion layer on either side of the BPM is also included to understand the concentration polarization across the BPM. The model revealed that the polarization losses across the bipolar membrane depend on the pH of the electrolyte used for the CO2 reduction reaction (CO2RR). The polarization loss on the anolyte side decreases with an increasing pH, while, on the cathode side, it increases with increasing catholyte pH. With this combined model for the electrode reactions and the membrane transport, we are able to account for the various factors influencing the polarization losses in the CO2 electrolyzer. To complete the analysis, we simulated the full cell polarization curve and fitted with the experimental data.