Search

Efficient reactant distribution and water removal are critical during polymer electrolyte fuel cell (PEFC) operation. The bipolar plate and its corresponding flow field design are vital among the PEFC components for enhancing reactant transport and water removal. The issues arising in the PEFC during the high current operation, such as reactant starvation and water removal, can be alleviated by improving the flow channel geometry. In this study, we analyze the variation in overall PEFC performance and corresponding reactant transport phenomenon for two independent design cases. The converging gradient design without channel traps at 0.4 V operating voltage exhibited a current density increment of 6.85% against the conventional design. Moreover, at 0.4 V, including channel traps enhanced the current density, as we observed a current density increment of 7.1% for the converging design with channel traps against the conventional design without channel traps. Likewise, at 0.4 V, the diverging design with channel traps exhibited a current density increment of 5.85% against the diverging design with no channel traps. Further, enhanced reactant distribution is observed in the catalyst layer upon introducing channel traps in the flow field design. [Display omitted] • Flow field design with the inclusion of channel traps is explored for performance. • Flow channel trap inclusion leads to cell performance increment. • Enhanced reactant distribution under the channel trap region is observed. • Converging design (case-2) attains a maximum current density of 1.38 A/cm2. • Converging (case-2) gains 7.0% current density compared to conventional design. [ABSTRACT FROM AUTHOR]

A modified Metal‐Organic Framework UiO‐66‐NH2‐based photocathode in a zero‐gap gas phase photoelectrolyzer was applied for CO2 reduction. Four types of porous carbon fiber layers with different wettability were employed to tailor the local environment of the cathodic surface reactions, optimizing activity and selectivity towards formate, methanol, and ethanol. Results are explained by mass transport through the different type and arrangement of carbon fiber support layers in the photocathodes and the resulting local environment at the UiO‐66‐NH2 catalyst. The highest energy‐to‐fuel conversion efficiency of 1.06 % towards hydrocarbons was achieved with the most hydrophobic carbon fiber (H23C2). The results are a step further in understanding how the design and composition of the photoelectrodes in photoelectrochemical electrolyzers can impact the CO2 reduction efficiency and selectivity. [ABSTRACT FROM AUTHOR]

Local evolution of delignification and xylan removal inside wood chips was investigated throughout the initial stages of kraft cooking. Model chips of birch sapwood were pulped at 145, 155 and 165 °C, utilizing white liquors with hydroxide content ranging from 0.25 to 0.55 mol/kg. The composition of different sections in each cooked sample was then determined. Xylan was isolated from selected samples and analyzed using size exclusion chromatography and HSQC NMR. Most changes in concentration and structure of residual xylan occurred early in the process (<45 min). Furthermore, xylan samples isolated from the tissue of different cooked chips had similar average molecular weights, indicating that temperature and alkali content had little impact over the extent of reactions affecting residual xylan. In contrast, xylan dissolution was significantly dependent on pulping conditions, increasing with hydroxide concentration. The lignin profile inside the cooked chips also varied with alkali content and temperature, and it was shown to be more uniform when applying low cooking temperatures (145 °C). Finally, increased delignification and xylan removal were detected close to the transverse surfaces of chips (likely due to the fast mass transport in vessels/lumen), implying that anatomical features of wood can have a significant impact on pulping. [ABSTRACT FROM AUTHOR]

Wood is a heterogeneous material with significant variation among species. This inherent complexity poses a challenge to the continuous expansion of our understanding of the kraft process; yet previous pulping research has mainly been limited to a few species. This study investigates variations among some less studied species and their cell wall level delignification behaviour during kraft pulping. Ground wood of birch, beech, aspen, and alder were pulped at near-constant composition and temperature conditions. Minor, yet significant, differences in the rates of their delignification were observed: aspen had a pronounced fast rate during the initial stage, whereas alder delignified more slowly relative to its high initial lignin content. The dissolution of xylan was substantially faster for birch. In contrast, no substantial differences were detected between the species in the molecular weight and structure of the dissolved wood components, suggesting that the different delignification behaviours stem from variations in the residual phase. The molecular weight distribution of dissolved lignin was uniform during the initial stage of pulping, which is indicative of rapid and extensive fragmentation. Subsequently, the weight increased continuously for the remainder of the process, suggesting that the mass transfer within the cell wall influenced the overall delignification kinetics. [ABSTRACT FROM AUTHOR]

Hydrogen is a favored alternative to fossil fuels due to the advantages of cleanliness, zero emissions, and high calorific value. Large‐scale green hydrogen production can be achieved using proton exchange membrane water electrolyzers (PEMWEs) with utilization of renewable energy. The porous transport layer (PTL), positioned between the flow fields and catalyst layers (CLs) in PEMWEs, plays a critical role in facilitating water/gas transport, enabling electrical/thermal conduction, and mechanically supporting CLs and membranes. Superior corrosion resistance is essential as PTL operates in acidic media with oxygen saturation and high working potential. This paper covers the development of high‐performance titanium‐based PTLs for PEMWEs. The heat/electrical conduction and mass transport mechanisms of PTLs and how they affect the overall performances are reviewed. By carefully designing and controlling substrate microstructure, protective coating, and surface modification, the performance of PTL can be regulated and optimized. The two‐phase mass transport characteristics can be enhanced by fine‐tuning the microstructure and surface wettability of PTL. The addition of a microporous top‐layer can effectively improve PTL|CL contact and increase the availability of catalytic sites. The anticorrosion coatings, which are crucial for chemical stability and conductivity of the PTL, are compared and analyzed in terms of composition, fabrication, and performance. [ABSTRACT FROM AUTHOR]

Hydrogen transport in the anode of a proton‐exchange membrane fuel cell (PEMFC) has been studied with a modulation technique relating the hydrogen flow‐rate (Q˜H2 ${{\tilde{Q}}_{H2}}$) and the faradaic current (I˜ ${\tilde{I}}$), called Current‐modulated Hydrogen flow‐rate Spectroscopy (CH2S). A simple analytical expression for the transfer function, H(jω)=n F Q˜H2 ${{\tilde{Q}}_{H2}}$ /I˜ ${\tilde{I}}$ , is provided, showing a skewed semicircle in Nyquist representation (−H'' vs. H'), extending from H'=0 to H'=1, and with the maximum frequency at ωmax=2.33(DH2 /Li2), where DH2 is the effective hydrogen diffusivity and Li the thickness of the anode gas diffusion layer (GDL). The expression for CH2S is also calculated with an existing reversible chemical reaction in the GDL. Experimental results under different operation conditions show two transport processes limiting the anode reaction, one attributed to molecular diffusion through the partially saturated GDL, and the other to the microporous layer (MPL), or its interfaces with GDL or with the catalyst layer (CL). CH2S provides the hydrogen diffusivities (DH2,i) associated to each process under the different conditions. Current density decreases slightly the diffusivity of the GDL, while it becomes activated in the MPL; using two GDLs in the anode improves both GDL and MPL diffusivities; humidification decreases the diffusivity in both, GDL and MPL; finally, a superhydrophobic anodic CL prepared by electrospray improves hydrogen diffusivity in GDL and MPL. [ABSTRACT FROM AUTHOR]

The demand for innovative batteries with high specific energy densities has increased. Li‐metal batteries employing Li‐metal anodes, regarded as the ultimate anodes with a high theoretical capacity, have been extensively studied over the past few decades. However, the poor reversibility and safety concerns regarding Li‐metal anodes remain unresolved. The importance of the electrode/electrolyte interface, especially the solid electrolyte interphase (SEI), for achieving reversibility of Li metal anodes has been extensively studied. Herein, we focused on the impact of the Li ion transport properties in oligoether (glyme)‐based electrolytes on the deposition/dissolution efficiency of Li metal anodes. Analysis of the low‐frequency impedance spectra of Li‐plated Cu/Li cells revealed that the diffusion resistance of Li ions (Rdiffusion) may be a dominant contributor to the internal resistance of the cells employing glyme‐based electrolytes. A higher Rdiffusion in poor‐mass‐transport electrolytes with a lower Li ion transference number resulted in larger polarization during Li deposition/dissolution, leading to more pronounced unfavorable side reactions and lower Coulombic efficiency. Rdiffusion rather than interfacial resistance affected the reversibility of the Li metal anode. Enhancing the Li ion mass transport ability of electrolytes is important for achieving highly reversible charge‐discharge performance of Li metal anodes at high current densities. [ABSTRACT FROM AUTHOR]

The practical use of solid‐state lithium metal batteries is hindered by the rapid growth of lithium dendrites through the solid electrolyte, leading to a short lifespan, and poor performance at high rates. The mass transport in anode bulk is proposed as a critical reason related to the chemo‐mechanical failure of the interface and growth of lithium dendrites. In this study, a lithium‐composite anode with a smartly integrated LiC6 phase is developed using a thermodynamic reaction between molten Li and thermally expanded graphite. The gravity segregation effect creates a gradient dispersion of LiC6 in Li, resulting in improved bulk Li diffusion kinetics (70 times higher than that of a pure Li anode). This composite electrode exhibits low interfacial resistance (≈2.3 Ω cm2) and an ultra‐high critical current density of 2.4 mA cm−2 in all‐solid‐state symmetric cells at room temperature. During repeated Li stripping/plating cycles, the enhanced bulk Li diffusion kinetics maintain the close interfacial contact, leading to excellent long‐term cycling stability for over 4000 h with a high cumulative areal capacity. These findings provide valuable insights that promoted mass transport in lithium metal anode is of great benefit to high‐performance solid‐state lithium metal batteries. [ABSTRACT FROM AUTHOR]

The mechanics of capillary force in biological systems have critical roles in the formation of the intra- and inter-cellular structures, which may mediate the organization, morphogenesis, and homeostasis of biomolecular condensates. Current techniques may not allow direct and precise measurements of the capillary forces at the intra- and inter-cellular scales. By preserving liquid droplets at the liquid-liquid interface, we have discovered and studied ideal models, i.e., interfacial liquids and marbles, for understanding general capillary mechanics that existed in liquid-in-liquid systems, e.g., biomolecular condensates. The unexpectedly long coalescence time of the interfacial liquids revealed that the Stokes equation does not hold as the radius of the liquid bridge approaches zero, evidencing the existence of a third inertially limited viscous regime. Moreover, liquid transport from a liquid droplet to a liquid reservoir can be prohibited by coating the droplet surface with hydrophobic or amphiphilic particles, forming interfacial liquid marbles. Unique characteristics, including high stability, transparency, gas permeability, and self-assembly, are observed for the interfacial liquid marbles. Phase transition and separation induced by the formation of nanostructured materials can be directly observed within the interfacial liquid marbles without the need for surfactants and agitation, making them useful tools to research the interfacial mechanics. [ABSTRACT FROM AUTHOR]

Fluid dynamics models complement expensive experiments with limited measurement accuracy that investigate the mass transport in PEM water electrolysis. Here, a first-principle microscale model for oxygen transport is successfully validated that accounts for (1) uncertain transport processes in catalyst layers, (2) numerically challenging capillary-dominated two-phase flow and (3) bubble detachments in channels. We developed algorithms for the stochastic generation of geometries and for the coupling of flow and transport processes. The flow model is based on the volume of fluid method and reproduces experimentally measured pressure drops and bubble velocities within minichannels with a 30% and 20% accuracy, respectively, provided that the capillary number is above 2.1 × 10−7. At lower capillary numbers, excessive spurious currents occur. Correspondingly, two-phase flow simulations within the porous transport layers are stable at current densities above 0.5 A cm−2 and match operando gas saturation measurements within a 20% margin at relevant locations. The simulated bubble detachments occur at pore throats that agree with porosimetry and microfluidic experiments. The presented model allows explaining and optimizing mass transport processes in channels and porous transport layers. These were found to be negligibly sensitive to transport resistances within the catalyst layer, providing information on boundary conditions for future catalyst layer models. [ABSTRACT FROM AUTHOR]

Molecular electrocatalysts have demonstrated potential for the hydrogen evolution reaction (HER) due to their well-defined structures and high intrinsic activities. Achieving rapid production of hydrogen requires molecular electrocatalysts to operate at high current densities, which still presents a challenge. In this work, we demonstrate that molecularly dispersed electrocatalysts of cobalt phthalocyanine anchored on carbon nanotubes (CoPc MDEs) are superior candidates due to the efficient charge transport between the substrate and the active site. The intrinsic activity can be enhanced by introducing functional groups on phthalocyanine. To facilitate mass transport, di(ethylene glycol) substituted CoPc molecules are further anchored on a three-dimensional self-supported electrode (CoPc-DEG MDE@CC), enabling continuous operation for 25 h at −1000 mA/cm2 in 1.0 M KOH. Our study demonstrates the potential of molecular electrocatalysts for HER and emphasizes the importance of adjusting intrinsic activity, and charge and mass transport capacity for practical molecular electrocatalysts. [ABSTRACT FROM AUTHOR]

The ordered membrane electrode assembly (MEA) has gained much attention because of its potential in improving mass transfer. Here, a comprehensive study was conducted on the influence of the patterned microporous layer (MPL) on the proton exchange membrane fuel cell performances. When patterned MPL is employed, grooves are generated between the catalyst layer and the gas diffusion layer. It is found that the grooves do not increase the contact resistance, and it is beneficial for water retention. When the MEA works under low humidity scenarios, the MEA with patterned MPL illustrated higher performance, due to the reduced inner resistance caused by improved water retention, leading to increased ionic conductivity. However, when the humidity is higher than 80% or working under high current density, the generated water accumulated in the grooves and hindered the oxygen mass transport, leading to a reduced MEA performance. [ABSTRACT FROM AUTHOR]

Operation at high current density of polymer electrolyte membrane electrolysis cell (PEMEC) can reduce its electrode area and capital expense. The high current density operation is limited by the limiting current density (LCD), where electrolysis voltage abruptly increases. Effects of operating temperature and pressure on the LCD were evaluated in a lab-scale PEMEC within a range from 70 to 90 °C and from 0.1 to 0.3 MPa. Also, a theoretical model was developed to predict the LCD, and to clarify what determines LCD and how the operating conditions influence the LCD. The theoretical analysis suggests that when current density is close to the LCD, water mass transport at anode reaches a limitation, which indicates that water saturation reaches almost zero at anodic catalyst layer. The analysis also finds that lower operating temperature and higher operating pressure can reduce oxygen gas saturation and raise the water saturation in porous transport layer at anode, and finally enlarge the LCD. • Limiting current density (LCD) is decided by mass transport limitation. • Oxygen gas saturation near catalyst layer is close to 1 at LCD. • Bubble overvoltage abruptly rises at LCD. • Lower temperature and higher pressure enlarge LCD. [ABSTRACT FROM AUTHOR]

Anodic oxidation is a promising method for removing organic pollutants from water due to its high nonselectivity and effectiveness. Nevertheless, its widespread application is limited due to its low current efficiency, high energy consumption and low treatment rates. These problems may be overcome by the optimization of the process parameters, reactor design and electrode geometry, by coupling the experimental investigations with mathematical modeling. Here we review the modeling of anodic oxidation with focus on basics of this process, the competition phenomenon in real wastewater, flow cells and batch cells, historical aspects, general modeling equations, modeling with plate electrodes, modeling with porous 3-dimension electrodes and the density functional theory. Mathematical modeling can provide current, voltage and concentration distributions in the system. Mathematical modeling can also determine the effects on the performance of parameters such as diffusion layer thickness, flow velocity, applied current density, solution treatment time, initial concentration and diffusion coefficients of organic pollutants, electrode surface area, and oxidation reaction rate constant. Mathematical models allow to determine whether the limiting factor of the process is kinetics or diffusion, and to study the impact of competition of phenomena. The density functional theory provides information on probable reaction pathways and by-products. [ABSTRACT FROM AUTHOR]

IrRu bimetallic oxides are recognized as the promising acidic oxygen evolution reaction (OER) catalysts, but breaking the tradeoff between their activity and stability is an unresolved question. Meanwhile, addressing the issues of mass transport obstruction of IrRu bimetallic oxides under high current remains a challenge for the development of proton exchange membrane water electrolysis (PEM-WE). Herein, we prepared an IrRuOx nanomeshes (IrRuOx NMs) with high coordination number (CN) of Ir-O-Ru bonds in a mixed molten salt with high solubility of the Ir/Ru precursor. X-ray absorption spectroscopy analysis revealed that the IrRuOx NMs possess high coordination number of Ir-O-Ru bonds (CNIr-O-Ru = 5.6) with a distance of 3.18 Å. Moreover, the nanomesh structures of IrRuOx NMs provided hierarchical channels to accelerate the transport of oxygen and water, thus further improving the electrochemical activity. Consequently, the IrRuOx NMs as OER catalysts can simultaneously achieve high activity and stability with low overpotential of 196 mV to reach 10 mA·cm−2 and slightly increase by 70 mV over 650 h test. Differential electrochemical mass spectrometry tests suggest that the preferred OER mechanism for IrRuOx NMs is the adsorbent evolution mechanism, which is beneficial for the robust structural stability. [ABSTRACT FROM AUTHOR]

Proton exchange membrane (PEM) electrolysis is widely considered an integral part of the energy transition. Thinner membranes can reduce Ohmic losses and increase efficiency. However, hydrogen crossover limits the use of thinner membranes. For the first time, this review provides a comprehensive overview of both model and experimental works on hydrogen crossover and crossover mitigation. By combining the experimental data and state-of-the-art models, we discuss the recent progress in understanding crossover mechanisms. The dominant mechanism for hydrogen crossover is the diffusion driven by both the high hydrogen partial pressures at the cathode and supersaturation in the catalyst layer due to transport limitations. The reviewed models successfully reproduce the experimental data obtained by different groups. A variety of strategies for including recombination catalysts in the cell are currently investigated in academic and corporate research. Some of these show promising results with potential for near-term industrial application. • H 2 crossover conventionally limits the use of thin membranes in PEM electrolysis. • Supersaturation and electroosmotic water drag can significantly affect gas crossover. • Mechanistic crossover models describe various experimental results accurately. • Recombination catalysts mitigate safety issues associated with H 2 crossover. • Multiple approaches for including recombination catalysts in MEAs are promising. [ABSTRACT FROM AUTHOR]

Hybrid water electrolysis offers a groundbreaking approach to energy‐saving hydrogen production by utilizing thermodynamically favorable and value‐added reactions to replace sluggish anodic oxygen evolution in overall water splitting. However, numerous anodic processes generate intermediates or products incompatible with aqueous environments, thereby degrading anodic performance to limit the efficiency and durability of hybrid water electrolysis. Herein, this difficulty is addressed by applying anodic mass transport intensification in conjunction with catalyst engineering, realizing a long‐lasting hybrid seawater electrolysis coupling sulfion oxidation reaction (SOR) for efficient hydrogen production below 1.2 V at an industrial‐level current density of 500 mA cm−2. An exceptional longevity of 2000 h is achieved for the electrolysis by eliminating anode passivation by sulfur deposition during SOR, a general concern in the sulfur‐involved industrial sector. This technology cut the electricity expense of hydrogen production to 2.80 kWh m−3 H2, undercutting alkaline water electrolysis by 34.9–51.1%. Simultaneously, fast upcycling of sulfion pollutants to value‐added sulfur is achieved to add additional economic and environmental profits, enabling hydrogen production at a midpoint expense ($ 0.93) of the United States Department of Energy's 2026 target ($ 2.0) for the cost per gallon of gasoline‐equivalent. [ABSTRACT FROM AUTHOR]

Among natural hazards occurring offshore, submarine landslides pose a significant risk to offshore infrastructure installations attached to the seafloor. With the offshore being important for current and future energy production, there is a need to anticipate where future landslide events are likely to occur to support planning and development projects. Using the northern Gulf of Mexico (GoM) as a case study, this paper performs Landslide Susceptibility Mapping (LSM) using a gradient-boosted decision tree (GBDT) model to characterize the spatial patterns of submarine landslide probability over the United States Exclusive Economic Zone (EEZ) where water depths are greater than 120 m. With known spatial extents of historic submarine landslides and a Geographic Information System (GIS) database of known topographical, geomorphological, geological, and geochemical factors, the resulting model was capable of accurately forecasting potential locations of sediment instability. Results of a permutation modelling approach indicated that LSM accuracy is sensitive to the number of unique training locations with model accuracy becoming more stable as the number of training regions was increased. The influence that each input feature had on predicting landslide susceptibility was evaluated using the SHapely Additive exPlanations (SHAP) feature attribution method. Areas of high and very high susceptibility were associated with steep terrain including salt basins and escarpments. This case study serves as an initial assessment of the machine learning (ML) capabilities for producing accurate submarine landslide susceptibility maps given the current state of available natural hazard-related datasets and conveys both successes and limitations. [ABSTRACT FROM AUTHOR]

Magnetic nanofluids, also referred to as ferromagnetic particle levitation systems, are materials with highly responsive magnetic properties. Due to their magnetic responsiveness, excellent controllability, favorable thermal characteristics, and versatility, magnetic nanofluids have sparked considerable interest in both industrial manufacturing and scientific research. Magnetic nanofluids have been used and developed in diverse areas such as materials science, physics, chemistry and engineering due to their remarkable characteristics such as rapid magnetic reaction, elastic flow capacities, and tunable thermal and optical properties. This paper provides a full and in-depth introduction to the diverse uses of ferrofluids including material fabrication, fluid droplet manipulation, and biomedicine for the power and machinery sectors. As a result, magnetic nanofluids have shown promising applications and have provided innovative ideas for multidisciplinary research in biology, chemistry, physics and materials science. This paper also presents an overview of the device construction and the latest developments in magnetic-nanofluid-related equipment, as well as possible challenging issues and promising future scenarios. [ABSTRACT FROM AUTHOR]

Electrocatalysis plays a key role in the development of renewable energy technologies. Along with the design of electrocatalysts, the microenvironment around catalytic sites has received increasing attention because it affects the distribution and mass transport of reaction species and impacts the reaction kinetics. In this Concept article, we highlight some mechanistic insights into the effect of microenvironment on gas‐involving electrocatalytic reactions, including CO2 reduction, 2‐electron oxygen reduction, and hydrazine oxidation, demonstrating their sensitivity to the wetting properties of microenvironment. For reactions with a gaseous reactant, a moderately hydrophobic microenvironment can greatly enhance the mass transport of gaseous species to accelerate the reaction kinetics while improving the stability of gas‐diffusion electrodes. In contrast, for reactions with a liquid reactant and gaseous product, a hydrophilic microenvironment improves the exposure of catalytic sites to the reactant, while a hydrophobic microenvironment benefits the reaction on the other end by accelerating the diffusion and detachment of generated gas bubbles, which would otherwise block the catalytic sites from the reactant. These understandings and insights can provide important guidelines on the control and optimization of catalyst microenvironment for the development of efficient electrolyzers. [ABSTRACT FROM AUTHOR]

In this study, porous, and hollow nanorods of NiTiO 3 (NiTiO 3 NRs-p: 2–3 μm in length and 400 nm in diameter) are successfully prepared using an ethylene glycol-mediated approach at room temperature (25 °C), followed by calcination at 700 °C in air environment. Their rough surfaces and accumulated holes are advantageous for adsorption and subsequent photoreduction of carbon dioxide (CO 2) under UV–vis irradiation. The NiTiO 3 NRs-p exhibit superior performance in terms of photoconversion of CO 2 (purity 99.999 %, 100 mL/min) into gaseous CO (57.833 μmolg−1 h−1) and CH 4 (24.33 μmolg−1 h−1) with an overall CO 2 selectivity (S CO2) of 89 %. In contrast, the solid NiTiO 3 nanorods (NiTiO 3 NRs) exhibit moderate performance in CO 2 reduction, producing CO (30.33 μmolg−1 h−1) and CH 4 (11.50 μmolg−1 h−1) with a CO 2 selectivity of S CO2 = 60 %. NiTiO 3 NRs-p exhibits superior photocatalytic activity against CO 2 over other NiTiO 3 morphologies with similar physical and chemical properties (such as nanoparticles (NPs) and nanofibers (NFs). This enhancement in photocatalytic activity can be attributed to the porous texture and hollow rod-like structure of NiTiO 3 NRs-p, which provides a larger number of active sites to promote rapid mass transport. This unique one-dimensional (1D) structure also facilitates the separation of photogenerated electron-hole pairs, as demonstrated experimentally. This study opens a new room for a simple and facile routes for the photocatalytic conversion of CO 2 into solar fuels. [ABSTRACT FROM AUTHOR]

Vision relies on the continuous exchange of material between the photoreceptors, retinal pigment epithelium and choriocapillaris, a dense microvascular bed located underneath the outer retina. The anatomy and physiology of the choriocapillaris and their association with retinal homeostasis have proven difficult to characterize, mainly because of the unusual geometry of this vascular bed. By analysing tissue dissected from 81 human eyes, we show that the thickness of the choriocapillaris does not vary significantly over large portions of the macula or with age. Assessments of spatial variations in the anatomy of the choriocapillaris in three additional human eyes indicate that the location of arteriolar and venular vessels connected to the plane of the choriocapillaris is non‐random, and that venular insertions cluster around arteriolar ones. Mathematical models built upon these anatomical analyses reveal that the choriocapillaris contains regions where the transport of passive elements is dominated by diffusion, and that these diffusion‐limited regions represent areas of reduced exchange with the outer retina. The width of diffusion‐limited regions is determined by arterial flow rate and the relative arrangement of arteriolar and venular insertions. These analyses demonstrate that the apparent complexity of the choriocapillaris conceals a fine balance between several anatomical and functional parameters to effectively support homeostasis of the outer retina. Key points: The choriocapillaris is the capillary bed supporting the metabolism of photoreceptors and retinal pigment epithelium, two critical components of the visual system located in the outer part of the retina.The choriocapillaris has evolved a planar multipolar vascular geometry that differs markedly from the branched topology of most vasculatures in the human body.Here, we report that this planar multipolar vascular geometry is associated with spatially heterogenous molecular exchange between choriocapillaris and outer retina.Our data and analyses highlight a necessary balance between choriocapillaris anatomical and functional parameters to effectively support homeostasis of the outer retina. [ABSTRACT FROM AUTHOR]

Using Lagrangian coherent structures (LCSs), the mass transport process of large-scale coherent structures in a buoyant jet diffusion flame is studied numerically to gain a key understanding of combustion instability. First, the reacting flow fields of the jet diffusion flame are numerically simulated for two cases, a stable case without considering the buoyancy effect and an unstable case that includes the buoyancy effect corresponding to combustion instability. In particular, large-scale Kelvin-Helmholtz instability (KHI) vortex structures outside the flame surface are captured in the unstable case. Then, the ridges in finite-time Lyapunov exponent (FTLE) fields that sketch the distinct regions in the reacting flow field are extracted as LCSs. The mass transport process is studied by comparing the distribution of LCSs with reaction fields. The flame surface is found to coincide with the attracting LCSs, which separate the fuel and air on opposite sides of the flame surface. In contrast, the reaction products, including species and reaction heat, are confined in the boundaries consisting of repelling LCSs on the inner and outer sides of the flame surface. Finally, the evolution of LCSs is tracked to analyze the generation of KHI vortices and species transport in the jet diffusion flame. With the aid of LCSs, it is found that there exist some saddle-type flow features separating the distinct flow structures into several regions, and the jet fuel and airflow enter into the separated regions from different open boundaries. Importantly, the attracting LCSs in the separated regions then bulge outwards, forming KHI vortices as the attracting LCSs further stretch and fold. Furthermore, the vortices continue moving upwards while species flow from repelling LCSs to attracting LCSs, leading to mixing. These results show that attracting and repelling LCSs can act as the flame surface and the mass and energy transport boundary of the reaction products, respectively. In summary, the work presented can provide a new method in combustion controlling to estimate the location of the flame surface and the transport boundaries of species from the velocity field, by using LCSs. [ABSTRACT FROM AUTHOR]

In the production of catalyst‐coated membranes (CCMs) for proton‐exchange membrane fuel cells and electrolyzers, the ink formulation and its processing are key factors in determining the resulting catalyst layer. Catalyst inks often contain a multicomponent solvent mixture. Selective drying, which can occur with solvent mixtures, changes the composition in the solidifying film and thus influences the microstructure of the layer that forms. The selectivity depends on the material‐specific thermodynamics of the solvents and the process‐related drying parameters. Different 1‐propanol/water mixtures serve as the state of the art material system considered and commonly used for CCM inks. Typical solvent mixtures can be dried selectively or non‐selectively, depending on the initial ink composition and humidity of the drying air. In mixtures that contain more 1‐propanol than the azeotropic or arheotropic composition, the 1‐propanol content accumulates in the remaining liquid; if there is less, it decreases. Increasing the preloading of the drying air with water leads to a relative water enrichment and shifts the tipping point to higher initial alcohol fractions. This behavior can be transferred to the real CCM production. [ABSTRACT FROM AUTHOR]

Abstract Hydrogen is a favored alternative to fossil fuels due to the advantages of cleanliness, zero emissions, and high calorific value. Large‐scale green hydrogen production can be achieved using proton exchange membrane water electrolyzers (PEMWEs) with utilization of renewable energy. The porous transport layer (PTL), positioned between the flow fields and catalyst layers (CLs) in PEMWEs, plays a critical role in facilitating water/gas transport, enabling electrical/thermal conduction, and mechanically supporting CLs and membranes. Superior corrosion resistance is essential as PTL operates in acidic media with oxygen saturation and high working potential. This paper covers the development of high‐performance titanium‐based PTLs for PEMWEs. The heat/electrical conduction and mass transport mechanisms of PTLs and how they affect the overall performances are reviewed. By carefully designing and controlling substrate microstructure, protective coating, and surface modification, the performance of PTL can be regulated and optimized. The two‐phase mass transport characteristics can be enhanced by fine‐tuning the microstructure and surface wettability of PTL. The addition of a microporous top‐layer can effectively improve PTL|CL contact and increase the availability of catalytic sites. The anticorrosion coatings, which are crucial for chemical stability and conductivity of the PTL, are compared and analyzed in terms of composition, fabrication, and performance.

Semi-transparent platinum electrodes were designed and optimized to implement photoluminescence (PL) and electrochemiluminescence (ECL) in microchannels under flow conditions. The luminescence properties of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+) were used to map steady-state emitted light through semi-transparent microchannel electrodes. Several diffusive-convective regimes were thus imposed in order to experimentally highlight the operating conditions of the electrodes, in particular the active zones at their upstream edges contributing to the Faradaic current. PL and ECL profiles were established on the electrode surfaces as a function of flow rate. The PL profiles confirmed the control of the process by mass transport. The data were validated by numerical simulations within the limits of experimental accuracy. ECL emission in presence of the co-reactant tri-n-propylamine (TPA) was also limited by mass transport. However, in comparison the characteristics of the ECL profiles demonstrated the complexity of the underlying mechanism involving Ru(bpy)32+ regeneration and TPA consumption.

Abstract The demand for innovative batteries with high specific energy densities has increased. Li‐metal batteries employing Li‐metal anodes, regarded as the ultimate anodes with a high theoretical capacity, have been extensively studied over the past few decades. However, the poor reversibility and safety concerns regarding Li‐metal anodes remain unresolved. The importance of the electrode/electrolyte interface, especially the solid electrolyte interphase (SEI), for achieving reversibility of Li metal anodes has been extensively studied. Herein, we focused on the impact of the Li ion transport properties in oligoether (glyme)‐based electrolytes on the deposition/dissolution efficiency of Li metal anodes. Analysis of the low‐frequency impedance spectra of Li‐plated Cu/Li cells revealed that the diffusion resistance of Li ions (Rdiffusion) may be a dominant contributor to the internal resistance of the cells employing glyme‐based electrolytes. A higher Rdiffusion in poor‐mass‐transport electrolytes with a lower Li ion transference number resulted in larger polarization during Li deposition/dissolution, leading to more pronounced unfavorable side reactions and lower Coulombic efficiency. Rdiffusion rather than interfacial resistance affected the reversibility of the Li metal anode. Enhancing the Li ion mass transport ability of electrolytes is important for achieving highly reversible charge‐discharge performance of Li metal anodes at high current densities.

Abstract Hydrogen transport in the anode of a proton‐exchange membrane fuel cell (PEMFC) has been studied with a modulation technique relating the hydrogen flow‐rate ( Q˜H2 ) and the faradaic current ( I˜ ), called Current‐modulated Hydrogen flow‐rate Spectroscopy (CH2S). A simple analytical expression for the transfer function, H(jω)=n F Q˜H2 / I˜ , is provided, showing a skewed semicircle in Nyquist representation (−H'' vs. H'), extending from H’=0 to H’=1, and with the maximum frequency at ωmax=2.33(DH2 /Li2), where DH2 is the effective hydrogen diffusivity and Li the thickness of the anode gas diffusion layer (GDL). The expression for CH2S is also calculated with an existing reversible chemical reaction in the GDL. Experimental results under different operation conditions show two transport processes limiting the anode reaction, one attributed to molecular diffusion through the partially saturated GDL, and the other to the microporous layer (MPL), or its interfaces with GDL or with the catalyst layer (CL). CH2S provides the hydrogen diffusivities (DH2,i) associated to each process under the different conditions. Current density decreases slightly the diffusivity of the GDL, while it becomes activated in the MPL; using two GDLs in the anode improves both GDL and MPL diffusivities; humidification decreases the diffusivity in both, GDL and MPL; finally, a superhydrophobic anodic CL prepared by electrospray improves hydrogen diffusivity in GDL and MPL.

Performance of symmetric-architecture metal-supported solid oxide fuel cells was improved significantly by optimizing the catalyst infiltration process and metal support structure. Optimization of component structure and processing parameters was performed during tape-casting and fabrication of button cells. Mass transport of oxygen in the metal support was identified as a major limitation. To overcome this limitation, pore former loading and thickness of the metal support (130–250 μm) were optimized. The catalyst infiltration process was also improved by studying the impact of firing temperature (400 °C–900 °C) and infiltration cycle numbers (1–15). The maximum power density of the optimized cell was 0.9 W cm−2 at 700 °C using hydrogen as a fuel, a three-fold increase over the baseline cell performance. The degradation rate of optimized cells at 550 °C, 600 °C, and 700 °C was 2%, 4.5%, and 5.5% per 100 h, respectively. The phenomena of mass transport, catalyst coarsening, and chromium poisoning on the catalyst were analyzed by electrochemical impedance spectroscopy and scanning electron microscopy.

The phenomenon of carbon isotopic fractionation, induced by the transport of methane in tight sedimentary rocks through processes primarily involving diffusion and adsorption/desorption, is ubiquitous in nature and plays a significant role in numerous geological and geochemical systems. Consequently, understanding the mechanisms of transport-induced carbon isotopic fractionation both theoretically and experimentally is of considerable scientific importance. However, previous experimental studies have observed carbon isotope fractionation phenomena that are entirely distinct, and even exhibit opposing characteristics. At present, there is a lack of a convincing mechanistic explanation and valid numerical model for this discrepancy. Here, we performed gas transport experiments under different gas pressures (1–5 MPa) and confining pressures (10–20 MPa). The results show that methane carbon isotope fractionation during natural gas transport through shale is controlled by its pore structure and evolves regularly with increasing effective stress. Compared with the carbon isotopic composition of the source gas, the initial effluent methane is predominantly depleted in 13C, but occasionally exhibits 13C enrichment. The carbon isotopic composition of effluent methane converges to that of the source gas as mass transport reaches a steady state. The evolution patterns of the isotope fractionation curve, transitioning from the initial non-steady state to the final steady state, can be categorized into five distinct types. The combined effect of multi-level transport channels offers the most compelling mechanistic explanation for the observed evolution patterns and their interconversion. Numerical simulation studies demonstrate that existing models, including the Rayleigh model, the diffusion model, and the coupled diffusion-adsorption/desorption model, are unable to describe the observed complex isotope fractionation behavior. In contrast, the multi-scale multi-mechanism coupled model developed herein, incorporating diffusion and adsorption/desorption across multi-level transport channels, effectively reproduces all the observed fractionation patterns and supports the mechanistic rationale for the combined effect. Finally, the potential carbon isotopic fractionation resulting from natural gas transport in/through porous media and its geological implications are discussed in several hypothetical scenarios combining numerical simulations. These findings highlight the limitations of carbon isotopic parameters for determining the origin and maturity of natural gas, and underscore their potential in identifying greenhouse gas leaks and tracing sources.

The Keyhole Plasma Arc Welding (KPAW) process utilizes arc plasma highly constricted by a water-cooled cupper nozzle to produce great arc pressure for opening a keyhole in the weld pool, achieving full penetration to the thick plate. However, advanced control of welding is known to still be difficult due to the complexity of the process mechanism, in which thermal and dynamic interactions among the arc, keyhole, and weld pool are critically important. In KPAW, two large eddies are generally formed in the weld pool behind the keyhole by plasma shear force as the dominant driving force. These govern the heat transport process in the weld pool and have a strong influence on the weld pool formation process. The weld pool flow velocity is much faster than those of other welding processes such as Tungsten Inert Gas (TIG) welding and Gas Metal Arc (GMA) welding, enhancing the heat transport to lower the weld pool surface temperature. Since the strength and direction of this shear force strongly depend on the keyhole shape, it is possible to control the weld pool formation process by changing the keyhole shape by adjusting the torch design and operating parameters. If the lower eddy is relatively stronger, the heat transport to the bottom side increases and the penetration increases. However, burn-through is more likely to occur, and heat transport to the top side decreases, causing undercut. In order to realize further sophistication of KPAW, a deep theoretical understanding of the process mechanism is essential. In this article, the recent progress in studies regarding the interaction mechanism of arc, keyhole, and weld pool in KPAW is reviewed. [ABSTRACT FROM AUTHOR]

[Display omitted] Establishing alternative methods for freshwater production is imperative to effectively alleviate global water scarcity, particularly in land-locked arid regions. In this context, extracting water from the ubiquitous atmospheric moisture is an ingenious strategy for decentralized freshwater production. Sorption-based atmospheric water harvesting (SAWH) shows strong potential for supplying liquid water in a portable and sustainable way even in desert environments. Herein, the latest progress in SAWH technology in terms of materials, devices, and systems is reviewed. Recent advances in sorbent materials with improved water uptake capacity and accelerated sorption–desorption kinetics, including physical sorbents, polymeric hydrogels, composite sorbents, and ionic solutions, are discussed. The thermal designs of SAWH devices for improving energy utilization efficiency, heat transfer, and mass transport are evaluated, and the development of representative SAWH prototypes is clarified in a chronological order. Thereafter, state-of-the-art operation patterns of SAWH systems, incorporating intermittent, daytime continuous and 24-hour continuous patterns, are examined. Furthermore, current challenges and future research goals of this cutting-edge field are outlined. This review highlights the irreplaceable role of heat and mass transfer enhancement and facile structural improvement for constructing high-yield water harvesters. [ABSTRACT FROM AUTHOR]

Photothermal catalytic CO2 hydrogenation holds great promise for relieving the global environment and energy crises. The "nano‐greenhouse effect" has been recognized as a crucial strategy for improving the heat management capabilities of a photothermal catalyst by ameliorating the convective and radiative heat losses. Yet it remains unclear to what degree the respective heat transfer and mass transport efficiencies depend on the specific structures. Herein, the structure‐function relationship of the "nano‐greenhouse effect" was investigated and optimized in a prototypical Ni@SiO2 core‐shell catalyst towards photothermal CO2 catalysis. Experimental and theoretical results indicate that modulation of the thickness and porosity of the SiO2 nanoshell leads to variations in both heat preservation and mass transport properties. This work deepens the understandings on the contributing factor of the "nano‐greenhouse effect" towards enhanced photothermal conversion. It also provides insights on the design principles of an ideal photothermal catalyst in balancing heat management and mass transport processes. [ABSTRACT FROM AUTHOR]

We present a numerical approach to address mass transport problems in multiphase systems, where a diffusing species concentration may exhibit discontinuities across phase boundaries. The approach employs a fixed structured grid, non-conforming with the probably complex or even evolving phase interfaces, in the same spirit as in numerous works in the literature focused on the dynamics of multiphase flows containing solid particles, immiscible fluids, elastic embedded structures, etc. The distinctive feature of the proposition is that in the transport equation, solved over the entire domain, the discontinuities are captured by including a distribution of source-dipoles along the phase boundaries. Moreover, the magnitude of the discontinuities and the source-dipole field strength do not need to be predetermined but are found as parts of the solution by a compatibility condition on the composite concentration field. A numerical implementation based on the finite element method is presented and examples are discussed demonstrating the validity of the approach. In addition, several types of multiphase mass transport problems are discussed, and simple examples are also presented, where it could find application. [ABSTRACT FROM AUTHOR]

The endeavor to develop high‐performance electrochemical energy applications has underscored the growing importance of comprehending the intricate dynamics within an electrode's structure and their influence on overall performance. This review investigates the complexities of electrode‐ionomer interactions, which play a critical role in optimizing electrochemical reactions. Our examination encompasses both microscopic and meso/macro scale functions of ionomers at the electrode‐ionomer interface, providing a thorough analysis of how these interactions can either enhance or impede surface reactions. Furthermore, this review explores the broader‐scale implications of ionomer distribution within porous electrodes, taking into account factors like ionomer types, electrode ink formulation, and carbon support interactions. We also present and evaluate state‐of‐the‐art techniques for investigating ionomer distribution, including electrochemical methods, imaging, modeling, and analytical techniques. Finally, the performance implications of these phenomena are discussed in the context of energy conversion devices. Through this comprehensive exploration of intricate interactions, this review contributes to the ongoing advancements in the field of energy research, ultimately facilitating the design and development of more efficient and sustainable energy devices. [ABSTRACT FROM AUTHOR]

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]

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.

Metal alloy catalysts (e.g., Pt-Co) are widely used in fuel cells for improving the oxygen reduction reaction kinetics. Despite the promise, the leaching of the alloying element contaminates the ionomer/membrane, leading to poor durability. However, the underlying mechanisms by which cation contamination affects fuel cell performance remain poorly understood. Here, we provide a comprehensive understanding of cation contamination effects through the controlled doping of electrodes. We couple electrochemical testing results with membrane conductivity/water uptake measurements and impedance modeling to pinpoint where and how the losses in performance occur. We identify that (1) ∼44% of Co2+ exchange of the ionomer can be tolerated in the electrode, (2) loss in performance is predominantly induced by O2 and proton transport losses, and (3) Co2+ preferentially resides in the electrode under wet operating conditions. Our results provide a first-of-its-kind mechanistic explanation for cation effects and inform strategies for mitigating these undesired effects when using alloy catalysts.

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.