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This Brief Communication presents a number of exact solutions describing the transient mixing of two gases of different molecular weights. Descriptions are given for both the concentration field and the associated induced motion in one-dimensional spherical, cylindrical, and planar configurations, including mixing layers, pockets, coflow jets, and concentrated mass sources. [ABSTRACT FROM AUTHOR]

The effective diffusivity of gaseous species in partially-saturated finite-size porous media is a valuable parameter for mathematical modeling of many processes, but it is difficult to measure experimentally. In this work, the effective diffusivity of carbon-fiber gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs) was determined by performing lattice Boltzmann (LB) simulations on X-ray tomographic reconstructions of invading water configurations. Calculations on dry GDLs were in close agreement with previous experimental data; the effective diffusivity was reduced by the addition of PTFE due to the loss of pore volume and the higher tortuosity of transport paths. The effect of water saturation was significantly larger. It was found that the resistance of water to gas transport was extremely dependent on the saturation distribution through the porous medium, particularly the peak saturation, and not just the average saturation as is typically considered in the literature. Through-plane diffusion was dramatically limited in materials with high-peak local saturations, even at low average saturation levels. No significant limitations were observed for diffusion in the material plane. The computed results demonstrate the strong sensitivity of finite-size porous media to local conditions, highlighting the difficulties of applying volume-averaged continuum-scale modeling techniques to micro-scale materials. [ABSTRACT FROM AUTHOR]

This study investigates the impact of electrolyte mixing inside the tanks of Vanadium Flow Battery (VFB) on capacity degradation. Heterogeneous mixing inside the tanks may lead to a severe capacity drop due to the existence of stagnant electrolyte regions and asymmetries in the State of Charge (SoC) of the positive and negative electrolytes reaching the cell. The study is based on a preliminary order-of-magnitude analysis involving the dimensionless Richardson and Reynolds numbers. A subsequent experimental campaign is carried out in which cell voltage and current response are screened for different tank designs and operating conditions to identify different fluid-dynamics events in the tanks. The results highlight the two main competing phenomena affecting the electrolyte fluid dynamics: buoyancy effects induced by density variations, and the inertia of the inlet submerged jet. The interplay between these two forces determines the flow field and therefore the SoC of the electrolyte feed to the cell, which affects the usable capacity and battery performance. Inertia dominated flows induce homogeneous electrolyte mixing, leading to a higher usable capacity, whereas buoyancy dominated flows result in a lower one. Furthermore, various inner structures were tested within the tanks to optimize mixing. These structures were created using additive manufacturing techniques. The implementation of helicoidal geometries within the tanks, promoting longer electrolyte paths and ample cross-sectional areas for convective mixing, markedly enhanced battery capacity compared to traditional empty tanks. These findings call for further investigation into optimizing the geometry of large-scale (VFB) storage tanks. • Heterogeneous mixing within the tanks may lead to a severe capacity drop in VFBs. • The flow dynamics results from the conflicting action of buoyancy and inertia. • Various 3D printed inner structures were tested within the tanks to optimize mixing. • An order of magnitude analysis enabled to proper understand the experimental results. • These findings call for further investigation to optimize large-scale tanks. [ABSTRACT FROM AUTHOR]

• Modelling of a RFB that uses two immiscible electrolytes. • Fluid-fluid interface is tracked using the Arbitrary Lagrangian-Eulerian method. • Maximum power densities between 1-1.75 mW/cm 2 in discharge and 1.5-4 mW/cm 2 in charge. • The limiting current density increases with the flow as j lim ∼ R e 0.3 . • The battery performance is limited by the availability of reactant near the electrodes. We present a mathematical model to study the steady-state performance of a membrane-less reversible redox flow battery formed by two immiscible electrolytes that spontaneously form a liquid-liquid system separated by a well defined interface. The model assumes a two-dimensional battery with two coflowing electrolytes and flat electrodes at the channel walls. In this configuration, the analysis of the far downstream solution indicates that the interface remains stable in all the parameter range covered by this study. To simplify the description of the problem, we use the dilute solution theory to decouple the calculation of the velocity and species concentration fields. Once the velocity field is known, we obtain the distribution of the mobile ionic species along with the current and the electric potential field of the flowing electrolyte solution. The numerical integration of the problem provides the variation of the battery current density I app with the State of Charge (SoC) for different applied cell voltages V cell. A detailed analysis of the concentration density plots indicates that the normal operation of the battery is interrupted when reactant depletion is achieved near the negative electrode both during charge and discharge. The effect of the electrolyte flow on the performance of the system is studied by varying the Reynolds, R e , and Péclet, P e , numbers. As expected, the flow velocity only affects the polarization curve in the concentration polarization region, when V cell is well below the equilibrium potential, resulting in limiting current densities that grow with R e as j lim ∼ R e 0.3 . In addition, both the single-pass conversion efficiency ψ and the product ψ j lim decrease with R e. Concerning the later, the decay rate with R e exhibits a power law with an exponent that almost doubles previous theoretical predictions obtained by imposing a prescribed velocity profile for the electrolyte in a membrane-less laminar flow battery with a liquid oxidant and gaseous fuel. The present work constitutes the first modelling attempt that simultaneously solves the fluid dynamical system formed by the two immiscible electrolytes and the electrochemical problem that determines the response of the membrane-less battery. The proposed model could be used as a valuable tool to optimize future flow battery designs based on immiscible electrolytes. [ABSTRACT FROM AUTHOR]

• 1D + 1D model for direct ethanol fuel cells based on detailed electro-oxidation kinetics. • The model is fitted to experimental polarization and product selectivity data. • The modeled cell performance includes I-V data and cell overall efficiency. • The along-the-channel evolution of acetaldehyde may exhibit non-monotonic behavior. • A parametric study for varying ethanol feed concentrations and flow rates is presented. This paper presents an isothermal, single-phase model for direct ethanol fuel cells. The ethanol electro-oxidation reaction is described using a detailed kinetic model that is able to predict anode polarization and product selectivity data. The anode kinetic model is coupled to a one-dimensional (1D) description for mass and charge transport across the membrane electrode assembly, which accounts for the mixed potential induced in the cathode catalyst layer by the crossover of ethanol and acetaldehyde. A simple 1D advection model is used to describe the spatial variation of the concentrations of the different species as well as the output and parasitic current densities along the flow channels. The proposed 1D + 1D model includes two adjustable parameters that are fitted by a genetic algorithm in order to reproduce previous experimental data. The calibrated model is then used to investigate the consumption of ethanol and the production, accumulation and consumption of acetaldehyde along the flow channels, which yields the product selectivity at different channel cross-sections. A parametric study is also presented for varying ethanol feed concentrations and flow rates. The results obtained under ethanol starvation conditions highlight the role of acetaldehyde as main free intermediate, which is first produced and later consumed once ethanol is fully depleted. The detailed kinetic description of the ethanol oxidation reaction enables the computation of the four efficiencies (i.e., theoretical, voltage, faradaic, end energy utilization) that characterize the operation of direct ethanol fuel cells, thus allowing to present overall fuel efficiency vs. cell current density curves for the first time. [ABSTRACT FROM AUTHOR]

Abstract Thin porous media are present in multiple electrochemical energy devices, where they provide key transport and structural functions. The prototypical example is gas diffusion layers (GDLs) in polymer-electrolyte fuel cells (PEFCs). While modeling has traditionally been used to explore PEFC operation, this is often accomplished using volume-averaged (VA) formulations, where the intrinsic inhomogeneities of the GDL are smoothed out and the lack of defining a representative elementary volume is an ever-present issue. In this work, the predictions of a single-phase VA PEFC model are compared to those of a pore-scale PEFC model using GDL tomograms as a part of the meshed domain to delineate important aspects that VA models cannot address. The results demonstrate that while VA models equipped with suitable effective properties can provide a good average estimate for overall performance, the lack of accounting for real structures limits their predictive power, especially for durability and degradation behavior where large deviations are found in the spatial distributions. Furthermore, interfacial effects between the GDL and the microporous layer are explored with the pore-scale model to understand the implications of the layered geometry. It is shown that the actual microstructure of the GDL/MPL transition region can significantly affect the fluxes across the sandwich, something that VA models cannot easily consider. Interfacial design is recognized as a key quality control parameter for large-scale MEA manufacturing and assembly. Graphical abstract Image 1 Highlights • Discrepancies of volume-averaged (VA) and pore-scale GDL models of a PEFC are examined. • VA models with suitable effective properties provide a good estimate for overall performance. • GDL inhomogeneities and interfacial phenomena influence predictions of VA models. • Impact of GDL microstructure on spatial distributions can affect durability modeling. • Interfacial design is a key control parameter for MEA manufacturing and assembly. [ABSTRACT FROM AUTHOR]

At the core of redox flow reactors, the design of the flow field geometry –which distributes the liquid electrolyte through the porous electrodes– and the porous electrode microstructure –which provides surfaces for electrochemical reactions– determines the performance of the system. To date, these two components have been engineered in isolation and their interdependence, although critical, is largely overlooked. Here, we systematically investigate the interaction between state-of-the-art electrode microstructures (a paper and a cloth) and prevailing flow field geometries (flow through, serpentine and four variations of interdigitated). We employ a suite of microscopic, fluid dynamics, and electrochemical diagnostics to elucidate structure-property-performance relationships. We find that interdigitated flow fields in combination with paper electrodes –which features a uniform microstructure with unimodal pore size distribution– and flow-through configurations combined with cloth electrodes –which have a hierarchical microstructure with bimodal pore size distribution– provide the most favorable trade-off between hydraulic and electrochemical performance. Our analysis evidences the importance of carrying out the co-design of flow fields and electrode microstructures in tandem. [Display omitted] • Coupled influence of electrodes and flow field geometries on reactor performance. • Woven electrodes perform best with flow through geometries. • Non-woven electrodes perform best with interdigitated designs. • Flow fields and electrodes must be designed in tandem. [ABSTRACT FROM AUTHOR]

Highlights • The applicability of the continuum hypothesis to transport in carbon-fiber papers is analyzed. • The LBM is combined with X-ray tomograms to simulate diffusion, convection and conduction. • A representative elementary volume cannot be defined due to the thin nature of these materials. • The anisotropic microstructure strongly reduces through-plane compared to in-plane transport. • The reduction is critical for solid-phase conductivity due to the bottleneck effect of the surface region. Abstract Understanding the transport processes that occur in carbon-fiber papers (CFPs) used in fuel cells, electrolyzers, and metal-air/redox flow batteries is necessary to help predict cell performance and durability, optimize materials and diagnose problems. The most common technique used to model these thin, heterogeneous, anisotropic porous media is the volume-averaged approximation based on the existence of a representative elementary volume (REV). However, the applicability of the continuum hypothesis to these materials has been questioned many times, and the error incurred in the predictions is yet to be quantified. In this work, the existence of a REV in CFPs is assessed in terms of dry effective transport properties: mass diffusivity, permeability and electrical/thermal conductivity. Multiple sub-samples with different widths and thicknesses are examined by combining the lattice Boltzmann method with X-ray tomography images of four uncompressed CFPs. The results show that a meaningful length scale can be defined in the material plane in the order of 1–2 mm, which is comparable to the rib/channel width used in the aforementioned devices. As for the through-plane direction, no distinctive length scale smaller than the thickness can be identified due to the lack of a well-defined separation between pore and volume-averaged scales in these inherently thin heterogeneous materials. The results also show that the highly porous surface region (amounting up to 20% of the thickness) significantly reduces the through-plane electrical/thermal conductivity. Overall, good agreement is found with previous experimental data of virtually uncompressed CFPs when approximately the full thickness is considered. [ABSTRACT FROM AUTHOR]

• Self-discharge reactions are a critical aspect of Micro Membraneless Vanadium Redox Flow Batteries. • If self-discharge reactions are very slow, the mathematical problem can be simplified to a diffusion-convection problem. • For sufficiently fast reactions the solution presents two outwards traveling reaction fronts. • Both limiting scenarios are controlled by diffusion and the differences between them in terms of flux of charged species lost are small. • Simple estimates are given that can be used to anticipate efficiency and capacity fading both in 2D and simple 3D geometries. Microfluidic-based membraneless redox flow batteries have been recently proposed and tested with the aim of removing one of the most expensive and problematic components of the system, the ion-exchange membrane. In this promising design, the electrolytes are allowed to flow parallel to each other along microchannels, where they remain separated thanks to the laminar flow conditions prevailing at sub-millimeter scales, which prevent the convective mixing of both streams. The lack of membrane enhances proton transfer and simplifies overall system design at the expense of larger crossover rates of vanadium ions. The aim of this work is to provide estimates for the crossover rates induced by the combined action of active species diffusion and homogeneous self-discharge reactions. As the rate of these reactions is still uncertain, two limiting cases are addressed: infinitely slow (frozen chemistry) and infinitely fast (chemical equilibrium) reactions. These two limits provide lower and upper bounds for the crossover rates in microfluidic vanadium redox flow batteries, which can be conveniently expressed in terms of analytical or semi-analytical expressions. In summary, the analysis presented herein provides design guidelines to evaluate the capacity fade resulting from the combined effect of vanadium cross-over and self-discharge reactions in these emerging systems. [ABSTRACT FROM AUTHOR]

It is known that both transcriptional and post-transcriptional mechanisms control messenger RNA (mRNA) levels. Compared to transcriptional regulations, our understanding of how post-transcriptional regulations adapt during fatty liver progression at the whole-transcriptome level is unclear. While traditional RNA-seq analysis uses only reads mapped to exons to determine gene expression, recent studies support the idea that intron-mapped reads can be reliably used to estimate gene transcription. In this study, we analyzed differential gene expression at both the exon and intron levels using two liver RNA-seq datasets from mice that were fed a high-fat diet for seven weeks (mild fatty liver) or thirty weeks (severe fatty liver). We found that the correlation between gene transcription and mature mRNA levels was much lower in mice with mild fatty liver as compared with mice with severe fatty liver. This result indicates broad post-transcriptional regulations for early fatty liver and such regulations are compromised for severe fatty liver. Specifically, gene ontology analysis revealed that genes involved in synapse organization and cell adhesion were transcriptionally upregulated, while their mature mRNAs were unaffected in mild fatty liver. Further characterization of post-transcriptionally suppressed genes in early fatty liver revealed that their mRNAs harbor a significantly longer 3' UTR, one of the major features that may subject RNA transcripts to nonsense-mediated RNA decay (NMD). We further show that the expression of representative genes that were post-transcriptionally suppressed were upregulated in mice with a hepatocyte-specific defect of NMD. Finally, we provide data supporting a time-dependent decrease in NMD activity in the liver of a diet-induced metabolic-dysfunction-associated fatty liver disease mouse model. In summary, our study supports the conclusion that NMD is essential in preventing unwanted/harmful gene expression at the early stage of fatty liver and such a mechanism is lost due to decreased NMD activity in mice with severe fatty liver.

Long non-coding RNA (lncRNA) genes represent a large class of transcripts that are widely expressed across species. As most human lncRNAs are non-conserved, we recently employed a unique humanized liver mouse model to study lncRNAs expressed in human livers. We identified a human hepatocyte-specific lncRNA, hLMR1 (human lncRNA metabolic regulator 1), which is induced by feeding and promotes hepatic cholesterol synthesis. Recent genome-wide association studies (GWAS) found that several single-nucleotide polymorphisms (SNPs) from the hLMR1 gene locus are associated with blood lipids and markers of liver damage. These results suggest that dietary and genetic factors may regulate hLMR1 to affect disease progression. In this study, we first screened for nutritional/hormonal factors and found that hLMR1 was robustly induced by insulin/glucose in cultured human hepatocytes, and this induction is dependent on the transcription factor SREBP1. We then tested if GWAS SNPs genetically linked to hLMR1 could regulate hLMR1 expression. We found that DNA sequences flanking rs9653945, a SNP from the last exon of the hLMR1 gene, functions as an enhancer that can be robustly activated by SREBP1c depending on the presence of rs9653945 major allele (G). We further performed CRISPR base editing in human HepG2 cells and found that rs9653945 major (G) to minor (A) allele modification resulted in blunted insulin/glucose-induced expression of hLMR1. Finally, we performed genotyping and gene expression analyses using a published human NAFLD RNA-seq dataset and found that individuals homozygous for rs9653945-G have a higher expression of hLMR1 and risk of NAFLD. Taken together, our data support a model that rs9653945-G predisposes individuals to insulin/glucose-induced hLMR1, contributing to the development of hyperlipidemia and NAFLD., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Electrochemical flow reactors are increasingly relevant platforms in emerging sustainable energy conversion and storage technologies. As a prominent example, redox flow batteries, a well-suited technology for large energy storage if the costs can be significantly reduced, leverage electrochemical reactors as power converting units. Within the reactor, the flow field geometry determines the electrolyte pumping power required, mass transport rates, and overall cell performance. However, current designs are inspired by fuel cell technologies but have not been engineered for redox flow battery applications, where liquid-phase electrochemistry is sustained. Here, we leverage stereolithography 3D printing to manufacture lung-inspired flow field geometries and compare their performance to conventional flow field designs. A versatile two-step process based on stereolithography 3D printing followed by a coating procedure to form a conductive structure is developed to manufacture lung-inspired flow field geometries. We employ a suite of fluid dynamics, electrochemical diagnostics, and finite element simulations to correlate the flow field geometry with performance in symmetric flow cells. We find that the lung-inspired structural pattern homogenizes the reactant distribution throughout the porous electrode and improves the electrolyte accessibility to the electrode reaction area. In addition, the results reveal that these novel flow field geometries can outperform conventional interdigitated flow field designs, as these patterns exhibit a more favorable balance of electrical and pumping power, achieving superior current densities at lower pressure loss. Although at its nascent stage, additive manufacturing offers a versatile design space for manufacturing engineered flow field geometries for advanced flow reactors in emerging electrochemical energy storage technologies., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The RNA Degradosome (RNAD) is a multi-enzyme complex, which performs important functions in post-transcriptional regulation in Escherichia coli with the assistance of regulatory sRNAs and the RNA chaperone Hfq. Although the interaction of the canonical RNAD components with RNase E has been extensively studied, the dynamic nature of the interactions in vivo remains largely unknown. In this work, we explored the rearrangements upon glucose stress using fluorescence energy transfer (hetero-FRET). Results revealed differences in the proximity of the canonical components with 1% (55.5 mM) glucose concentration, with the helicase RhlB and the glycolytic enzyme Enolase exhibiting the largest changes to the C-terminus of RNase E, followed by PNPase. We quantified ptsG mRNA decay and SgrS sRNA synthesis as they mediate bacterial adaptation to glucose stress conditions. We propose that once the mRNA degradation is completed, the RhlB, Enolase and PNPase decrease their proximity to the C-terminus of RNase E. Based on the results, we present a model where the canonical components of the RNAD coalesce when the bacteria is under glucose-6-phosphate stress and associate it with RNA decay. Our results demonstrate that FRET is a helpful tool to study conformational rearrangements in enzymatic complexes in bacteria in vivo., Competing Interests: Declaration of competing interest The authors declare no competing interests., (Copyright © 2021 Elsevier B.V. and Société Française de Biochimie et Biologie Moléculaire (SFBBM). All rights reserved.)

In the Staphylococcus aureus ATCC25923 strain, the flqB mutation in the 5'untranslated region (5'UTR) of the norA gene causes increased norA mRNA expression and high efflux activity (HEA). The involvement of the norA gene 5'UTR in HEA has not been explored in S. epidermidis; therefore, we examined the function of this region in S. epidermidis clinical isolates. The selection of isolates with HEA was performed based on ethidium bromide (EtBr) MIC values and efflux efficiency (EF) using the semi-automated fluorometric method. The function of the 5'UTR was studied by quantifying the levels of norA expression (RT-qPCR) and by identifying 5'UTR mutations by sequence analysis. Only 10 isolates from a total of 165 (6.1%) had HEA (EtBr MIC = 300 μg/ml and EF ranged from 48.4 to 97.2%). Eight of 10 isolates with HEA had the 5'UTR 95 Δ G mutation. Isolates carrying the 95 Δ G mutation had higher levels of norA expression compared with those that did not. To corroborate that the 95 Δ G mutation is involved in HEA, a strain adapted to EtBr was obtained in vitro. This strain also presented the 95 Δ G mutation and had a high level of norA expression and EF, indicating that the 95 Δ G mutation is important for the HEA phenotype. The 95 Δ G mutation produces a different structure in the Shine-Dalgarno region, which may promote better translation of norA mRNA. To our knowledge, this is the first report to demonstrate the participation of the 5'UTR 95 Δ G mutation of the norA gene in the HEA phenotype of S. epidermidis isolates. Here, we propose that the efflux of EtBr is caused by an increment in the transcription and/or translation of the norA gene., (Copyright © 2016 Elsevier Ltd. All rights reserved.)