Diffusion Coefficients Of Species Research Articles

Correlation Between Microscopic Current Fluctuations Observed at Ultra-Microelectrodes and Macroscopic Bulk Electrolysis Performance in Redox-Active Microemulsions

Abstract Microemulsions (µEs) have been proposed as redox flow battery (RFB) electrolytes that maximize ionic conductivity and charge capacity by synergizing two immiscible phases. However, charge transfer during electrolysis in µEs is poorly understood. Here, we show that ultramicroelectrode electrolysis of ferrocene-loaded µEs - 20%, 60%, and 90% water - reveals stochastic current fluctuations. These are differentiated in the scanning electrochemical microscopy (SECM) geometry, where power spectral density analysis showed distinct changes in the frequency contributions. SECM in the substrate generation-tip collection mode showed that fluctuations arise under mass-transfer control. Significant differences in the diffusion coefficient of ferrocene species were deducted from SECM approach curves, suggesting phase transfer behavior. Using bulk electrolysis, we calculated the charge accessibility and cycling behavior in the µEs. A decrease in the stochastic behavior of the µEs seems to correlate to a higher accessibility and cycling performance, with the 90% water µE displaying the best reversibility and the 60% the lowest. Altogether, these results suggest that Marangoni-type convection driven by concentration gradients and/or µE restructuring during charge transfer play a role in the electrochemical performance of µEs. This presents opportunities for screening and diagnosing the performance of these emerging RFB electrolytes.

Analysis of Entropy Generation via Non-Similar Numerical Approach for Magnetohydrodynamics Casson Fluid Flow with Joule Heating.

This analysis emphasizes the significance of radiation and chemical reaction effects on the boundary layer flow (BLF) of Casson liquid over a linearly elongating surface, as well as the properties of momentum, entropy production, species, and thermal dispersion. The mass diffusion coefficient and temperature-dependent models of thermal conductivity and species are used to provide thermal transportation. Nonlinear partial differential equations (NPDEs) that go against the conservation laws of mass, momentum, heat, and species transportation are the form arising problems take on. A set of coupled dimensionless partial differential equations (PDEs) are obtained from a set of convective differential equations by applying the proper non-similar transformations. Local non-similarity approaches provide an analytical approximation of the dimensionless non-similar system up to two degrees of truncations. The built-in Matlab (Version: 7.10.0.499 (R2010a)) solver bvp4c is used to perform numerical simulations of the local non-similar (LNS) truncations.

Solving ZIB Challenges: The Dynamic Role of Water in Deep Eutectic Solvents Electrolyte

Zinc-Ion batteries (ZIBs) are considered one of the most promising technologies in the realm of post-lithium-ion batteries. Rechargeable chemistries with zinc anodes hold significant technological potential due to their high theoretical energy density, lower manufacturing costs, abundant raw materials, and inherent safety. ZIBs typically feature neutral or weakly acidic aqueous electrolytes and exhibit typical drawbacks of Zn anodes: hydrogen evolution, passivation, and shape changes.In aqueous electrolytes, two types of water molecules can be described: free water molecules and solvated water molecules that participate in Zn2+ solvation structure [Zn(H2O)6]2+. The free water easily reacts with metallic Zn at the electrode/electrolyte interface, leading to the aforementioned irreversibilities. Alternative electrolytes such as Deep Eutectic Solvents (DESs) can be used to modulate Zn solvation shell, preserving the green and safe characteristics of aqueous-based ones. DESs show relevant characteristics for battery applications, such as low vapor pressure, good thermal and chemical stability, non-flammability, easy moldability, wide electrochemical window, and structured interface. However, a main drawback for electrochemical applications is the high viscosity, low diffusivity, and sluggish kinetics.Water molecules can be added as a cosolvent to eutectic mixtures to increase the diffusion coefficient of electroactive species and reduce the electrolyte viscosity without destroying the eutectic nature by up to 40%. The hydration percentage of DES can modify the thickness of the electrolyte-electrode interface and the Zn2+ solvation structure. Thus, the tuning capability, on one hand, of the solution chemistry of Zn2+ containing electroactive species, and on the other hand, of the electrode–electrolyte interfacial structure and chemistry by the combined use of DES and water provides a flexible tool to control Zn plating and stripping processes.In this study, we investigate the electrochemistry of Zn in ethaline DES with different percentages of water (0%, 5%, 10%, 25%, 50%, 70%, respectively), transitioning from a water-in-salt structure to a salt-in-water one. Fundamental electrokinetic and electrocrystallization studies based on cyclic voltammetry and chronoamperometry at different cathodic substrates were complemented with galvanostatic cycling tests of Zn|Zn symmetric CR2032 coin cells, SEM imaging of electrodes, and in situ Surface Enhanced Raman spectroscopy (SERS).Moreover, the Zn solvation structure was investigated using spontaneous Raman spectroscopy . The employed methodology provided valuable insights into the Zn plating reaction. Water molecules are present in a "confined" state rather than a free one, directly interacting with choline cations and being trapped in the eutectic framework if the molar fraction is kept below 40%.When ZnSO4 is dissolved in anhydrous ethaline, the [ZnCl4]2− species is formed and stabilized by choline. The peculiarities of Zn2+ coordination were found to result in unusual voltammetric behavior, characterized by a cathodic peak in the cathodic branch of the anodic-going scan.Although tetrachlorozincate is the dominant species, it does not seem to be the complex from which Zn is deposited; indeed, [ZnCl4]2− has a very negative reduction potential. Mechanistic studies of Zn electrodeposition onto different cathodic materials have concluded that the metal is formed by the reduction of an intermediate Zn-containing species. The two organic components of the eutectic solvents can be dehydrogenated at a negative potential, forming RO-, which can replace one or more chloride ligands in [ZnCl4]2−.Hydrated DES exhibits faster kinetics, which, in turn, leads to a lower nucleation overpotential. This phenomenon has been explained with two different mechanisms occurring at the same time. Firstly, by increasing the hydration percentage in the eutectic framework, the coordination of Zn2+ changes, forming [ZnCl3(H2O)]-. The water molecule in the Zn2+ sheath has a lower energy barrier than chlorine in the step-by-step desolvation of Zn2+. Consequently, the [ZnCl3(H2O)]- complex exhibits a lower dissociation energy than [ZnCl4]2- present in anhydrous ethaline, resulting in a faster desolvation process that can reduce nucleation overpotentials.Secondly, water molecules hydrogen-bonded to DES components can be reduced at cathodic potential, forming H· radicals that can then participate in the decomposition of the organic component. This apparent side reaction can aid in the formation of RO- ligands that participate in the Zn-species undergoing reduction, thereby increasing the amount of reactants.This multi-technique approach, also applicable to the study of new electrolytes, leads to the optimization of DES hydration, reducing the presence of free water and addressing the main drawbacks associated with eutectic electrolytes. The nuanced understanding of the interplay between solvent composition, Zn solvation, and electrodeposition mechanisms paves the way for the development of more efficient and sustainable ZIB technologies. Figure 1

Study of salt free-diffusion by 1D transport numerical simulations and shadowgraph experiments

Applying the Nernst-Planck formalism of 1D diffusive flux equations for ionic species of salts dissolved in water, Pitzer formalism for activity coefficients and PHREEQC software for species diffusion coefficients, with ionic strength-dependence coefficients optimized, we obtained expressions for apparent diffusion coefficients of sodium chloride, calcium chloride, and sodium sulphate, prevalent in deep aquifers. PHREEQC 1D transport simulations of free-diffusion experiments yielded temporal evolution of the concentration and gradient vertical profiles. The dynamics of concentrations non-equilibrium fluctuations during the free-diffusion experiments have been recorded by shadowgraphy. Thanks to the calculated gradient profiles the thickness of the sample has been integrated in the analysis equations of the structure functions. From the earliest stages of the diffusion process we measured the diffusion coefficients for the three salts, in very good agreement with reference measurements, validating the technique and method of analysis for studying free-diffusion in reservoirs targeted for CO2 and energy vectors storage.

Numerical spectral approach for studying activation energy behavior in viscoelastic fluid flow through non-Darcian medium

The work examines the effects of MHD incompressible viscoelastic fluid under nonlinear thermal radiation while flowing in a non-Darcy porous medium. The utilization of the Darcy-Brinkmann-Forchheimer model has thus been implemented in the formulation of the momentum equation. The study posits the existence of a chemical phenomenon which involves elements A and B as two chemical species. The diffusion coefficient of chemical species exhibits inequality when considered for viscoelastic fluid. Numerical solutions are generated using SRM, spectral relaxation technique for governing flow problems. The current algorithm is formulated and implemented using the computational software MATLAB. The results exhibit a considerable degree of agreement with formerly established numerical outcomes within the framework of a specific scenario. The observations from this study provide evidence that chemical species B operates as a catalyst in a subsurface environment that is oriented horizontally. As a result, the connection between the homogeneous-heterogeneous scheme can be observed in the context of isothermal cubic autocatalytic occurrences and processes of the first order, subsequently. The utilization of the heterogeneous reaction parameter offers notable advantages in reducing the concentration of the bulk-fluid while simultaneously enhancing the concentration of the catalyst situated over the surface. The current findings contribute to the exploration of the computational study of diffusivities in MHD viscoelastic fluid flow. Specifically, this research inquires the effects of nonlinear thermal radiation and homogeneous-heterogeneous reactions.

Thermal distribution evolution model of SEI in lithium metal anodes

Lithium metal batteries are considered an advantageous choice for high energy density batteries due to their extremely high theoretical specific capacity. However, the interface instability has always been the biggest challenge for the commercial development of lithium metal batteries. The evolution mechanism of the thermal field is a key factor affecting the cyclic life during the evolution of lithium metal interfaces. Herein, the interface evolution mechanism for the impact of thermal distribution on Li dendrites is revealed and quantified by the thermal distribution evolution model of the solid electrolyte interphase (SEI). Three influencing factors are outlined as follows: 1) the ratio of the SEI to electrolyte diffusion capability. 2) The SEI diffusion coefficients for both anionic and cationic species. 3) The anisotropy in the SEI. The results indicate that under the appropriate ratio, the temperature gradient at the tip of lithium dendrites and the SEI-electrolyte interface are lower, which is favorable for a more even distribution of heat within the SEI. Additionally, the diffusion of anions can also influence the temperature gradient during the evolution process of lithium dendrites. Furthermore, introducing anisotropy to the SEI can induce lateral growth of lithium dendrites and reduce the temperature gradient at the tip of the lithium dendrites and the SEI-electrolyte interface. This work provides valuable insights for the interface design of lithium metal batteries.

An experimentally derived calibration model for hydrogen-fueled solid oxide fuel cells at intermediate temperature

The characteristic polarization curve (e.g. I–V curve) of a solid oxide fuel cell (SOFC) is a valuable outcome of experimental tests because it conveys crucial information about the SOFC's current-potential operational range and optimum performance point. Numerical analysis of the experimentally achieved I–V curve can yield information that is not readily available from it, such as parameters of operation, kinetics of electrochemical reactions, and mechanisms of mass and current transfer. Such details can be used to choose optimal operating conditions and improve the cell manufacturing process. Using experimental polarization curves of a commercial SOFC fed with hydrogen and air, this paper develops a calibration algorithm and extends the analysis to evaluate further operational parameters of the cell. The calibrated model was developed for a commercial SOFC operating at 700 °C and 800 °C. As target parameters, the calibrated model defined the anode and cathode exchange current densities as well as the effective diffusion coefficients of gas species within both electrodes and their respective pore radii. A comprehensive SOFC analysis model requires a detailed understanding of the morphology of the porous material inside the cell, and these parameters provided that insight. Using experimental and analytically optimized polarization curves, target parameters are defined with an accuracy of more than 98%. Based on the developed model, it was also possible to predict the maximum current density the cell could reach at temperatures of 800 °C and 700 °C, which was about 2 Acm−2 and 1.7 Acm−2, respectively. Compared with SOFC parameters reported in literature, the obtained values were fully consistent. In spite of this, the algorithm outcomes are considered “equivalent” parameters since they are solutions to a multivariable problem, and only their combination with other known parameters can result in fitting a benchmark curve. Additionally, their uniqueness criterion is not yet investigated as a means of describing actual cell parameters.

Aza[5]helicene‐Derived Semiconducting Polymers for Improved Performance in Perovskite Solar Cells: Exploring Energetic and Morphological Influences

AbstractThe strategic design of solution‐processable semiconducting polymers possessing both matched energy levels and elevated glass transition temperatures is of urgent importance in the progression of thermally robust n‐i‐p perovskite solar cells with efficiencies exceeding 25 %. In this work, we employed direct arylation polymerization to achieve the high‐yield synthesis of three aza[5]helicene‐derived copolymers with distinct HOMO energy levels and exceptional glass transition temperatures. Upon integration of these semiconducting polymers into formamidinium lead triiodide‐based perovskite solar cells, marked disparities in photovoltaic parameters manifest, primarily stemming from variations in the electrical conductivity and film morphology of the hole transport layers. The p‐A5HP‐E‐POZOD‐E copolymer, featuring a main chain comprising alternating repeats of aza[5]helicene, ethylenedioxythiophene, phenoxazine, and ethylenedioxythiophene, attains an initial average efficiency of 25.5 %, markedly surpassing reference materials such as spiro‐OMeTAD (23.0 %), PTAA (17.0 %), and P3HT (11.6 %). Notably, p‐A5HP‐E‐POZOD‐E exhibits a high cohesive energy density, resulting in enhanced Young's modulus and diminished external species diffusion coefficients, thereby conferring perovskite solar cells with exceptional 85 °C tolerance and operational stability.

Aza[5]helicene-Derived Semiconducting Polymers for Improved Performance in Perovskite Solar Cells: Exploring Energetic and Morphological Influences.

The strategic design of solution-processable semiconducting polymers possessing both matched energy levels and elevated glass transition temperatures is of urgent importance in the progression of thermally robust n-i-p perovskite solar cells with efficiencies exceeding 25 %. In this work, we employed direct arylation polymerization to achieve the high-yield synthesis of three aza[5]helicene-derived copolymers with distinct HOMO energy levels and exceptional glass transition temperatures. Upon integration of these semiconducting polymers into formamidinium lead triiodide-based perovskite solar cells, marked disparities in photovoltaic parameters manifest, primarily stemming from variations in the electrical conductivity and film morphology of the hole transport layers. The p-A5HP-E-POZOD-E copolymer, featuring a main chain comprising alternating repeats of aza[5]helicene, ethylenedioxythiophene, phenoxazine, and ethylenedioxythiophene, attains an initial average efficiency of 25.5 %, markedly surpassing reference materials such as spiro-OMeTAD (23.0 %), PTAA (17.0 %), and P3HT (11.6 %). Notably, p-A5HP-E-POZOD-E exhibits a high cohesive energy density, resulting in enhanced Young's modulus and diminished external species diffusion coefficients, thereby conferring perovskite solar cells with exceptional 85 °C tolerance and operational stability.

In-situ concentration measurement of soluble-soluble redox couple in molten chlorides utilizing intense natural convection effect

Herein we reported the in-situ electrochemical concentration measurement of soluble-soluble redox species (Eu3+/Eu2+ and Sm3+/Sm2+) in molten LiCl-KCl, based on the notable natural convection in molten salts. The combined simulation and experiments confirmed the presence of strong natural convection, which resulted in steady-state current in cyclic voltammetry tests at low scan rates. Interestingly, this natural convection effects offered a novel and simple way to calculate the diffusion coefficient and concentration ratio of soluble redox species.

Continuum Modeling of Gas and Ions Transport in a High-Surface-Area Carbon (HSC) Nanopore

PEM fuel cell (PEMFC) is one of the promising energy conversion devices for emission-free transportation applications. It is beneficial in faster charging and better scalability compared to battery-powered vehicles, making PEMFC a viable option for heavy-duty vehicles which require higher efficiency and longer lifetimes. To economically compete with the combustion engine and improve commercialization, the PEMFC cathode desires optimization in enhancing the utilization of the expensive Pt catalyst. Integration of high-surface-area carbon (HSC) has shown to increase accessible Pt surface by diminishing ionomer poisoning as Pt particles can be loaded inside the nanoscale micropores (<5 nm) that ionomer cannot penetrate due to size restriction. However, separating Pt catalysts from ionomers requires a water pathway to deliver the proton to the interior catalyst surface, leading to RH-dependent electrochemical surface area (ECSA). In this work, we develop a steady-state, isothermal continuum model to study the gas and ions transport within HSC nanopores. The pore is represented as a straight cylindrical channel filled with water with an ionomer domain at the pore mouth serving as a proton reservoir. The ionomer phase contains H+, OH-, and background stationary SO3 - while the water phase only includes H+ and OH-. The Poisson-Nernst-Plank equations describe the flux of ions, and O2 transport is described by Fick’s law. Non-ideal behaviors including nanoconfinement effects and dielectric saturation are considered in the pore water phase, reflected by corrections in ion chemical potentials, diffusion coefficients of transport species, and dielectric constant. The electrical double layer (EDL) at the water-electrode interface is depicted as discrete finite domains applying the Gouy-Chapman-Stern (GCS) theory, with the oxygen reduction reaction (ORR) occurring at a finite reaction plane at the outer Helmholtz plane (OHP) through both acidic and alkaline reaction pathways. Donnan potential drop is solved at the ionomer-pore water interface and governs potential distribution in the pore water phase, further determining ionic concentrations at the catalyst surface and thus the kinetic region of polarization curves. The strong-adsorbed water adlayers due to nanoconfinement effects create a transport barrier for O2 approaching the catalyst surface, leading to higher mass transport resistance. A comparison between a flooded pore scenario (only a water phase exists in the pore) and a wetted pore scenario (the pore wall is covered by a water film while a gas phase also exists in the pore) shows that mass transport is hindered in the flooding pore as O2 dissolves in the pore water through a limited pore-ionomer interfacial area, while pore gas phase in the wetted pore serves as an extra O2 resource. This suggests that the management of HSC water uptake needs to avoid pore flooding while ensuring enough wettability to allow proton transport. The model is expected to couple with HSC pore size distribution measured from experiments and upscale to a cell-level model in future work.

Understanding Characteristic Electrochemical Impedance Spectra of Redox Flow Batteries with Multiphysics Modelling

Electrochemical impedance spectroscopy (EIS) is a non-destructive technique for analyzing electrochemical systems (such as redox flow batteries – RFBs). Battery researchers widely adopt equivalent circuit models (ECMs) to analyze EIS spectra of RFBs and attempt to use the circuit to deduce prevalent transport and kinetic properties [1]. Straightforward adoption of these ECMs suffers from some setbacks which need rectifying. There is a major issue of poor or inconsistent physical interpretation of the different circuit elements. ECMs of batteries are commonly used in real-time applications due to their simplicity and importance for determining properties like state-of-charge. However, prediction performance in low state-of-charge areas has to be improved [2], which reinforces the need to relate recorded spectra to the physical properties of cell components.Using an experimentally validated full Multiphysics model of single-cell vanadium RFB analyzed in the frequency domain, we approach understanding the EIS spectra characteristics based on the influence of cell features and operating parameters. To achieve our analyses, we investigate the effects of different cell properties and operating parameters on the Multiphysics model produced EIS spectra and the resulting ECM circuit element parameters that fit the spectra. Five different values for each cell property or operating parameter are considered for the following components: Electrode: Electrical conductivity, porosity vis a vie specific surface area and hydraulic permeability, reaction rate constants for the positive and negative sides.Membrane: Ionic conductivity, fixed charge concentration, porosity, and hydraulic permeability.Electrolyte: State-of-charge, Composition vis a vie bulk diffusion coefficients of species, density, and viscosity.

Diffusion of Hydroxylated Si Vacancies in Olivine, and Its Relevance to Si Diffusion

AbstractThe diffusion coefficient associated with the 3,612 cm−1 infrared absorbance band in H‐bearing olivine, attributed to a defect involving a vacant Si site balanced by four H+ (i.e., ), was determined in H in‐diffusion experiments at 1200°C–1400°C and 1.5–3.0 GPa in piston‐cylinder and multi‐anvil apparatuses. The retrieved values are in good agreement with experimentally determined diffusion coefficients ascribed to the same process from previous experiments, despite those being conducted at atmospheric pressure and representing out‐diffusion rather than in‐diffusion. Overall, the diffusivity of the Si vacancy is several orders of magnitude lower than that of the M‐site vacancy (∼4 orders of magnitude at 1000°C), with a higher activation energy (∼450–500 vs. 200–250 kJ mol−1), when both are charge‐compensated solely by H+. From these data, the diffusion coefficient of Si in H‐bearing olivine can be estimated using the relationship between vacancy concentration, vacancy diffusivity, and diffusivity of an ion moving by a vacancy mechanism. However, these diffusivities are inconsistent with published results for Si diffusivity, with a considerable discrepancy of around two orders of magnitude. This suggests that simple relationships between diffusion coefficients of vacancies and species moving by vacancy diffusion mechanisms may be inappropriate, highlighting again the complexity of H incorporation and diffusion mechanisms in olivine.

Cross-frontal mode: An alternative methodology for Taylor dispersion analysis of monomodal sample

Taylor dispersion analysis (TDA) is a technique dedicated to the determination of the molecular diffusion coefficient (D) of species, using band broadening of an analyte in a laminar flow. Two modes are commonly used to perform TDA: pulse and frontal modes. In each case, a fitting of the signal is required. We propose here a third mode denoted as cross-frontal mode, combining two crossed sample fronts without modification of a classical CE device for the rapid and accurate determination of D of caffeine, reduced glutathione (GSH), insulin from bovine pancreas, bovine serum albumin (BSA) and citrate-capped gold nanoparticles (AuNP). Theoretical aspects and methodology are described, showing a good correlation between the so-called cross-frontal mode and usual frontal mode. Limitations of the techniques are also assessed, and are similar to regular modes while no fitting is required. This new methodology allows improving the sensitivity toward low concentrated sample compared to pulse mode, and an alternative mathematical treatment compared to regular TDA modes.

Phosphate aggregation, diffusion, and adsorption on kaolinite in saline solutions by molecular dynamics simulation

Large amounts of phosphates are required for the survival of cells. However, undesirable algae overgrowth requires phosphorus levels to be kept under control. For large bodies of water, cost-effective and ecosystem-friendly ways to export phosphate are needed. The use of clays appears to be the most promising method in seawater. However, for a broader application, the mechanisms of phosphate adsorption on kaolinite, the conditions for effective flocculation, and the impact of salt must first be addressed. In this study, molecular dynamics simulations are used to determine the adsorption and anchoring mechanisms of two species of phosphates that predominate at neutral pH, the dihydrogen phosphate ion H2PO4− and the hydrogen phosphate ion HPO42−, in a 1:1 ratio, on each of the three kaolinite principal planes, the basal surfaces, octahedral gibbsite (001) and tetrahedral siloxane (001¯), and the edge sites (010), in aqueous NaCl at a concentration of 0.6 M to mimic seawater. Phosphate aggregation is enhanced by Na bridges, although at very high concentrations, the number of aggregates and the size of the largest cluster decrease. Calculated diffusion coefficients for both phosphate species drop sharply in hypersaline solutions, although the decay rate is higher for the HPO42− species. Diffusion results are in close agreement with experimental data. Phosphate adsorption on kaolinite occurs in clusters. The hydrophilic surface (001) adsorbs increasing amounts of each of the phosphate species as the salt concentration increases, more HPO42− than H2PO4−. The hydrophobic (001¯) surface remains non-reactive to any species. The hydrophilic (010) edge surface increases adsorption of H2PO4− and drastically decreases that of HPO42− as salt concentration increases. Bidentate adsorption of HPO42− on the much rougher edge surface, even if saturated with adsorbed Na ions, is sterically impeded. The dominant adsorption mechanism is through cationic bridges on the gibbsite surface and at the edges. The adsorption capacity of kaolinite, including all its surfaces, increases with salt concentration, reaching its highest value in synthetic seawater. The values are of the order, although lower than the experimental ones obtained with less-accessible clays with atomic substitutions. The use of clays to control phosphate content in coastal waters is expected to contribute to public health and sustainability of marine industrial activities.

Role of surface catalyzed reaction in the flow of temperature-dependent viscosity fluid over a rotating disk

In many engineering applications, it is important to consider the temperature dependence of a material's properties, such as its viscosity and thermal conductivity. Aiming the same, here, in the existing exploration, viscosity and thermal conductivity are taken as variables instead of constant. This study discusses the flow of a viscous fluid over a rotating disk with a surface catalyst. The homogeneous-heterogeneous reactions are supplemented by surface catalysis that boosts the chemical reaction rate in the minimum possible time. The novelty of the problem is enhanced by the incorporation of the variable diffusion coefficients of both chemical species. The Von Karman transformation is used and the system is scrutinized by the bvp4c function of the MATLAB software. The results are portrayed in the form of illustrations and tables. The outcomes divulged that the mass diffusion rate and wall concentration for the porosity and surface catalyzed parameter show a decreasing behavior.

The Effects of Differential Diffusion on Turbulent Non-Premixed Flames LO2/CH4 under Transcritical Conditions Using Large-Eddy Simulation

In this paper, a large-eddy simulation (LES) of turbulent non-premixed LO2/CH4 combustion under transcritical conditions is performed based on the Mascotte test rig from the Office National d’Etudes et de Recherches Ae´rospatiales (ONERA), and the aim is to understand the effects of differential diffusion on the flame behaviors. In the LES, oxygen was injected into the environment above the critical pressure while the temperature was below the critical temperature. The flamelet/progress variable (FPV) approach was used as the combustion model. Two LES cases with different species diffusion coefficient schemes—i.e., non-unity and unity Lewis numbers—for generating the flamelet tables were carried out to explore the effects of differential diffusion on the flame and flow structures. The results of the LES case with non-unity Lewis numbers were in good agreement with the experimental data. It was shown that differential diffusion had evident impacts on the flame structure and flow dynamics. In particular, when unity Lewis numbers were used to evaluate the species diffusion coefficient, the flame length was underestimated and the flame expansion was more significant. Compared to laminar counterflow flames, turbulence in jet flames allows chemical reactions to take place in a wider range of mixture fractions. The density distributions of the two LES cases in the mixture fraction space were very similar, indicating that differential diffusion had no significant effects on the phase transition under transcritical conditions.

Using Molecular Dynamics Simulations to Understand the Effect of Fatty Acids Chain Length on Structural and Dynamic Properties of Deep Eutectic Solvents Based on Choline Chloride and Fatty Acids

AbstractThe binary mixtures of choline chloride and a series of fatty acids such as Caprylic acid (C8), Capric acid (C10), Lauric acid (C12), Myristic acid (14), and Palmitic acid (C16) have been prepared. The structural and dynamical properties of the eutectic mixture with the constant molar ratio of Ch+Cl−: FAs=1 : 1 was evaluated at 353 K. To analyze the intermolecular interactions in the binary mixtures, the 100 ns MD simulation was implemented using the NPT ensemble. The structure and properties of the binary mixtures were investigated to understand their molecular interactions. The results confirmed that the interaction between chloride ions and fatty acids has played an important role in interrupting the main interaction between the acids in the pure state. However, Interactions between chloride anion and the hydroxyl group of cations are more weakened in the presence of FAs with a shorter chain length. It is well known that there is a similar relationship between the intermolecular interactions and dynamics properties in distinctive structures of DES. Therefore, the other theme of this study is exploring the possible effect of intermolecular interactions on the self‐diffusion coefficients of species and determining the fatty acids that have stronger interactions with choline chloride salt to form the eutectic solvents. The diffusion coefficient of species was calculated by Einstein's equations. The results of dynamics properties showed the strong Hydrogen bonding network of the binary mixture of Caprylic acid and Choline chloride strongly affects molecular migrations.

Voltammetric and electrodeposition study for the recovery of antimony from effluents generated in the copper electrorefining process

Antimony is a metalloid with limited availability as a primary resource, but it is commonly found as an impurity in effluents generated in the copper metallurgy. Thus, the development of clean and selective processes to recover antimony from these wastewaters would improve the sustainability of the copper production. In this work, an emulated effluent of the copper electrorefining industry that contains antimony and hydrochloric acid was characterized by means of voltammetric and electrodeposition tests using two different cell configurations: a static cell, and a dynamic cell with a rotating disk electrode (RDE). Voltammograms were obtained at varying hydrochloric acid and antimony concentrations, inversion potentials, scan rates and RDE rotation rates. Two main conclusions were drawn: (a) the deposition of antimony is a mass transfer-controlled process; and (b) an increase in hydrochloric acid concentration improves the deposition of antimony. The diffusion coefficient of antimony species was obtained applying the Randles-Ševčík and the Levich equations; both of them providing very similar values (5.29 ± 0.20 · 10−6 cm2 s−1). The effective electrodeposition of antimony from highly concentrated hydrochloric acid solutions was demonstrated. The surface examination of the electrodes revealed that compact and adherent deposits of antimony could be obtained under operating conditions that minimize the hydrogen evolution reaction in both potentiostatic and galvanostatic modes. Intensified convective regimes by using the RDE improve the supply of dissolved antimony towards the electrode surface, thus leading to a notorious increase in current density and, consequently, in the rate of antimony deposition.

Electrochemical Measurement of Interfacial Distribution and Diffusion Coefficients of Electroactive Species for Ion-Exchange Membranes: Application to Br2/Br- Redox Couple.

A novel method has been proposed for rapid determination of principal transmembrane transport parameters for solute electroactive co-ions/molecules, in relation to the crossover problem in power sources. It is based on direct measurements of current for the electrode, separated from solution by an ion-exchange membrane, under voltammetric and chronoamperometric regimes. An electroactive reagent is initially distributed within the membrane/solution space under equilibrium. Then, potential change induces its transformation into the product at the electrode under the diffusion-limited regime. For the chronoamperometric experiment, the electrode potential steps backward after the current stabilization, thus inducing an opposite redox transformation. Novel analytical solutions for nonstationary concentrations and current have been derived for such two-stage regime. The comparison of theoretical predictions with experimental data for the Br2/Br- redox couple (where only Br- is initially present) has provided the diffusion coefficients of the Br- and Br2 species inside the membrane, D(Br-) = (2.98 ± 0.27) 10-6 cm2/s and D(Br2) = (1.10 ± 0.07) 10-6 cm2/s, and the distribution coefficient of the Br- species at the membrane/solution boundary, K(Br-) = 0.190 ± 0.005, for various HBr additions (0.125-0.75 M) to aqueous 2 M H2SO4 solution. This possibility to determine transport characteristics of two electroactive species, the initial solute component and its redox product, within a single experiment, represents a unique feature of this study.