Mass Transport Coefficient Research Articles

Aluminium Recycling in Single- and Multiple-Capillary Laboratory Electrolysis Cells

This work is a contribution to the approach for Al purification and extraction from scrap using the thin-layer multiple-capillary molten salt electrochemical system. The single- and multiple-capillary cells were designed and used to study the kinetics of aluminium reduction in LiF–AlF3 and equimolar NaCl–KCl with 10 wt.% AlF3 addition at 720–850 °C. The cathodic process on the vertical liquid aluminium electrode in NaCl–KCl (+10 wt.% AlF3) in the 2.5 mm length capillary had mixed kinetics with signs of both diffusion and chemical reaction control. The apparent mass transport coefficient changed from 5.6∙10−3 cm.s−1 to 13.1∙10−3 cm.s−1 in the mentioned temperature range. The dependence between the mass transport coefficient and temperature follows an Arrhenius-type behaviour with an activation energy equal to 60.5 kJ.mol−1. In the multiple-capillary laboratory electrolysis cell, galvanostatic electrolysis in a 64LiF–36AlF3 melt showed that the electrochemical refinery can be performed at a current density of 1 A.cm−2 or higher with a total voltage drop of around 2.0 V and specific energy consumption of about 6–7 kWh.kg−1. The resistance fluctuated between 0.9 and 1.4 Ω during the electrolysis depending on the current density. Thin-layer aluminium recycling and refinery seems to be a promising approach capable of producing high-purity aluminium with low specific energy consumption.

A two-dimensional analytical unit cell model for redox flow battery evaluation and optimization

Cell performance optimization is important for improving the system efficiency of a redox flow battery. To gain better insights into key controlling factors of system efficiency, this work first proposed a theoretical model for a unit cell by extending a two-dimensional analytic model to a full cell. The model is then used for cell performance optimization after validating it with experimental and numerical modeling data. With the results, the activation, equilibrium, and pump energy losses are identified as the dominant battery energy losses. A guideline for reducing these sources is also proposed. Following the guideline, the mass transport coefficient is shown as a key control factor of equilibrium energy loss and Coulombic efficiency (CE). Approaches are then proposed to improve CE and system efficiency. The mass transport also controls pump rate optimization by reducing equilibrium energy loss. An optimal electrode porosity design can further improve a battery’s system efficiency based on an optimal porosity predicted by our model. The model also demonstrates distinct behaviors and overestimation in system efficiency when reduced to a zero-dimensional model. With the model, the guideline, and new insights, this work provides a reliable and efficient tool for the evaluation and optimization of redox flow batteries.

Electrochemical deposition of copper using a modified electrode with polyaniline film: Experimental analysis and ANN-based prediction

Wastewater from the electronics manufacturing industry contains relatively large amounts of dissolved copper in low concentrations (≤ 1000 mg L−1). Its discharge to the environment represents a danger to the bioenvironment. Among the different approaches proposed to separate toxic metal ions in low concentrations from solution, electroplating remains a practical method for removing metal ions. This work reports the potentiostatic and galvanostatic deposition of Cu(II) from dilute sulfate electrolyte containing 50–200 mg L−1 copper in a batch recycle reactor with a polyaniline-modified Pt-Ti mesh electrode. A copper plate electrode is also used for copper deposition and its performance is compared to that of the modified electrode (Pt-Ti-PANi). The experimental system is modeled by using artificial neural networks (ANN). The Pt-Ti-PANi electrode used for copper removal (99% in 2.5 h) is more efficient than the copper plate (96% in 4.5 h) in potentiostatic mode. The same is observed under the galvanostatic mode because it allows the optimization of copper deposition by applying controlled constant current steps under mass transport, thus minimizing energy consumption and process time. Six ANN configurations are tested to find the optimal architecture using process variables as inputs to the ANN models, such as initial concentration, potential, current, electrolysis time, and final concentration. The best ANN model (tansig-purelin) shows good agreement with the experimental data (ANN architecture 5:3:1 with adjusted determination coefficient 0.965 and mean square error of 1.083 × 10−10). The results show that the proposed ANN model successfully predicts the volumetric mass transport coefficient.

Study of engine-oil based CNT nanofluid flow on a rotating cylinder with viscous dissipation

An axisymmetric viscous carbon nanotube (CNT) nanofluid flow exterior to a rotating circular cylinder of radius a 0 along with viscous dissipation and magnetohydrodynamic (MHD) is considered here. The two-phased Buongiorno’s model is employed to observe the thermal energy transport in nanofluid with engine-oil as a background fluid and two distinct nanoparticles, i.e., single-walled (SWCNTs) and multi-walled (MWCNTs). The flow is driven because of the torsional motion and stretching of the cylinder along the z-direction, which constitutes the axial pressure gradient. It is seen that in the presence of a magnetic field, the axially dependent swirling motion of the cylinder creates a light transverse flow in the meridional plane, which is the principal cause for the wall jet phenomenon on the axial axis. The function computing the numerical solution is bvp5c, which is available in Matlab®. It is noted that for both SWCNTs/MWCNTs cases, thermal and mass transport coefficients are decreasing function of Prandtl and Schmidt numbers, respectively. However, they are augmented due to the presence of strong Lorentz force. The values for wall stress parameters (WSPs) are determined as a function of a magnetic parameter for 0.01 ≤ Re ≤ 102 (where Re is Reynolds number). It is also perceived that the axial WSP decreases due to an increase in values of Re, but an increase in numerical values of azimuthal WSP and Nusselt number is noticed.

Pre‐deposited dynamic membrane adsorber formed of microscale conventional iron oxide‐based adsorbents to remove arsenic from water: application study and mathematical modeling

AbstractBACKGROUNDThis study reports the development of a dynamic membrane (DM) adsorber by pre‐depositing powdered‐sizedfraction of iron oxide‐based adsorptive material on the surface of a microfiltration(MF) membrane. The aim is to use the developed DM adsorber for arsenate (As(V)) remediation from water by a combined mechanism of adsorptive and membrane filtration. The two applied iron oxide‐based adsorptive materials are micro‐sized granular ferric hydroxide and micro‐sized tetravalent manganese feroxyhyte, and are available at affordable price.RESULTSThe results show that As(V) removal efficiency strongly depends on the physicochemical properties of the depositing material such as specific surface area, isoelectric point and particle size of the pre‐depositing material. The experimentally determined As(V) removal rates were mathematically modeled using a homogeneous surface diffusion model, which incorporates the equilibrium parameters and mass transport coefficients of the adsorption process. The simulations showed that the mathematical model could describe the As(V) removal rates accurately over a broad range of operating conditions. The results further showed that the longer filtration times with very low normalized As(V) permeate concentration (C/Cf = 0.1, for example) can be prolonged by operating the DM adsorber at the lowermost membrane water flux of 31 L m−2 h−1 and a large amount of pre‐depositing material on the MF membrane surface (Ma = 14 mg cm−2).CONCLUSIONThe results presented in this study confirm that use of these inexpensive materials (side‐product of granular iron oxide‐based adsorbents) in treating As(V)‐polluted water would enhance the sustainability of the industrial production process of conventional granular adsorbents by utilizing the wastes created during the process of adsorbent production. © 2021 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

Impact of a gas diffusion layer's structural and textural properties on oxygen mass transport resistance in the cathode and performance of proton exchange membrane fuel cells

In this work, we used our previously developed method for determination of oxygen mass transport coefficients (k) to evaluate the effects of the gas diffusion layer (GDL) structure and texture on oxygen mass transfer in a cathode electrode and on proton exchange membrane fuel cell (PEMFC) performance. The method is based on measurements of limiting current distributions using oxygen mixtures with different diluents, which allows us to separate a contribution from gas phase molecular diffusion (km) and a combination of Knudsen diffusion and transport through ionomer/water films (kK+film). GDLs with varying microporous layer (MPL) loadings from 50 to 150% were used for the cathode electrode. The lack of an MPL in the GDL resulted in the highest values of km and kK+film but led to poor performance since the MPL plays a critical role in water management. Moreover, the application of a GDL without an MPL allowed the oxygen mass transport coefficient in the catalyst layer (kK+film, CL) to be directly determined. The contribution from the MPL to oxygen transport (kK, MPL) was separated by comparing the results for a cell with a GDL with and without an MPL. An increase in MPL loading caused a gradual decrease in O2 mass transport coefficients and improvement in PEMFC performance in the high power regime. The obtained structure-to-property correlations showed a trade-off in MPL content among mass transport properties, texture and high performance.

Studies of Chitosan at Different pH's in the Removal of Common Chlorinated Organics from Wastewater

This work explores the use of chitosan as an adsorbent of common chlorinated organics in waste water, namely 4-chlorophenol and tetrachloroethylene. We use both isothermal adsorption equilibrium and dynamic adsorption experiments to describe the phenomena of adsorption at pH 4, 5, 6, 7 and 9. Results show that 4-chlorophenol is adsorbed the greatest at pH 6 while tetrachloroethylene is adsorbed the greatest at pH 4. The Langmuir constant b values range from 0.00003 to 0.00012 and 0.00001 to 0.00007 for 4-chlorophenol and tetrachloroethylene, respectively, and overall it appeared that the binding energy of 4-chlorophenol to chitosan is higher than that of tetrachloroethylene. Dynamic adsorption experiments show that the adsorption of 4-chlorophenol on chitosan follows a second order process while tetrachloroethylene follows a first order process, where the external mass transport coefficient of 4-chlorophenol and tetrachloroethylene ranged from 0.081 - 0.755 and 0.009 - 0.285, respectively. Additionally, the internal pore diffusion rate of 4-chlorophenol inside the micropores of chitosan is shown to be approximately 2.5 times that of tetrachloroethylene.

Numerical Modelling of Membrane-Less Capillary-Driven Flow-Batteries

Rapid diagnostic electronic kits (RDKs) are increasingly being used all around the world for preliminary and/or emergency screening [1]. In addition to their low price, such kits can provide lab-quality results within minutes rather than days. Currently, point-of-care (POC) kits are powered by lithium button cells. Although these batteries are very reliable, the chemicals which are used to extract lithium from raw minerals can pollute local aquifers and thereby potentially poison nearby communities. There are also environmental concerns when kits powered by such batteries are disposed of. Consequently, there is an urgent need for new battery technologies which can mitigate these environmental concerns. In recent years, flow-batteries have emerged as a viable option for single-use applications. Among different alternatives, membrane-less capillary-driven flow-batteries are a promising solution, given that they can be fabricated exclusively from biodegradable materials. A paper-based, biodegradable flow-battery using quinone redox couples has already been designed and prototyped by Esquivel et al. [2]. Figure 1 schematically shows the architecture of this novel flow-battery called PowerPAD. The reactants are seen to enter the porous electrodes of this flow battery in the vertical direction and exit the electrodes in the same direction. The waste is then collected by an absorbent pad which is also responsible for maintaining the required flow rate through capillary forces. The power output of this flow battery is well within the reach of many POC kits. However, for such batteries to be considered as a viable replacement to conventional lithium batteries, their operational performance and power-density should be further enhanced. The power-density of a given redox flow battery can be enhanced through optimizing its electrode materials, reactant electrolytes, or cell architecture. For capillary-flow batteries, the material used for the absorbent pad or the cellulose paper can also be tuned for this purpose. The objective of the present work is to investigate the possibility of enhancing the power output of capillary-flow batteries through modifying their flow architecture. A numerical model is used for this purpose based on the microfluidic fuel cell mathematical model developed by Krishnamurthy et al. [3], featuring electrochemical kinetics in flow-through porous electrodes. For ease of comparing different flow architectures, the laminar flow is assumed to be quasi-steady and pressure-driven. This approach enables numerical determination of pressure drop in the cell for a wide range of flow rates, which can be correlated to the design of absorbent pad. Figure 1 shows the three flow cell architectures considered for the analysis in this work. They can be classified as “vertical-flow” design, “horizontal-flow” design, and “mixed-flow” design. The first architecture has already been used in [2] while the other two designs are new. It is speculated that the new designs can improve cell’s performance by giving the reactants more time to reach the surface of carbon fibers for the reactions to take place. To simulate the energy conversion performance of these designs, we rely on COMSOL Multiphysics software. The software first solves the fluid-flow equations to provide us with pressure drop and velocity field. The velocity field is then used in the mass-transport equation which is coupled with the electrochemical equations through Butler-Volmer equation. An iteration method is then used to compute the concentration and local current density fields. The results are finally expressed in terms of polarization curves. Due to the qualitative nature of the present work, we have decided to use vanadium species dissolved in sulfuric acid in our study. To be able to verify our numerical scheme with published data, the same geometrical and electrochemical parameters, and also the same carbon-fiber electrode (Toray TGP-H 120) as used in [3] have been used in this study. Our numerical results show that the original “vertical-flow” design used in [2] is not optimized. It is predicted that horizontal-flow design enhances the power-density of this flow battery by roughly 30% while the mixed-flow design is predicted to have the opposite effect. The good energy-converting performance of the horizontal design together with its higher fuel utilization efficiency can be attributed to its higher “mass transport coefficient”. This is not surprising realizing the fact that the Reynolds number of the horizontal design is larger than the vertical and mixed designs. However, it should be noted that, the cell’s pressure drop is roughly doubled for the horizontal design suggesting that it needs a better pad material to compensate for the extra pressure drop. All in all, it is concluded that cell flow architecture can be used as a powerful tool to enhance the power output of membrane-less capillary-driven flow-batteries. Figure 1

Development of an Oxygen Mass Transport Coefficient Measurement and Separation Method for Proton Exchange Membrane Fuel Cells

The cost and durability of proton exchange membrane fuel cells (PEMFCs) still limit their adoption as sustainable power systems [1]. Operation at high current densities, which decreases the inventory of costly materials [2], is still considered an option [3] in addition to concurrent reductions in Pt catalyst loadings. However, these approaches intensify mass transfer losses especially at the cathode because the oxidant stream is more dilute (21 % O2 in air) than the fuel (100 % H2) and the heavier O2 molecule moves more slowly. The improvement of characterization methods able to measure and separate oxygen mass transfer coefficients is desirable for more accurate identification and localization of resistances in membrane/electrode assemblies (MEAs).Average limiting currents iave are first measured with a dilute oxidant stream for different flow rates. Subsequently, data are fitted to a mathematical model [4] to obtain the overall mass transfer coefficient k: iave =ie (1–exp(–nFkpr /RTief)) (1)with ie the inlet reactant flow rate equivalent current density, n the number of electrons exchanged in the electrochemical reaction, F the Faraday constant, pr the dry inlet reactant stream pressure, R the ideal gas constant, T the temperature, and f the inert gas to reactant fraction in the dry inlet reactant stream. The oxidant diluent is also varied (He, N2, CO2) to separate the molecular diffusion mass transfer coefficient km by extrapolating to the origin the linear correlation between the overall mass transfer resistance and the diluent molecular mass M [4]: k –1=km (M)–1+k e+K (c)–1=km (M)–1+ke (c)–1+kK –1 (2)The ionomer permeability mass transfer coefficient contribution ke was separated from 1/k e+K by taking advantage of the constant Knudsen diffusion resistance (equation 2 above and equation 7 in [5]). For that last step, the linear correlation between the lumped ionomer permeability and Knudsen diffusion mass transfer resistance 1/k e+K and the oxygen concentration c (1 to 7 %) was extrapolated to the origin, which yielded the Knudsen diffusion mass transfer coefficient kK . This approach to isolate the Knudsen contribution is novel. Other techniques are used, including temperature variations [5].Experiments were conducted with a 50 cm2 active area cell and General Motors MEAs with a commercially relevant cathode catalyst loading of 0.05 to 0.15 mg Pt cm–2 (Pt catalyst supported on a high surface area carbon). All MEA tests were duplicated and completed for several temperatures (30 to 80 °C) and relative humidities (50 to 100 %). Four diagnostic methods were employed to supplement the analysis of mass transfer data. The use of O2, 21 % O2 in He, and air enabled the derivation of overpotentials from polarization curves [6]. The high frequency cell resistance was measured with a milliohmmeter. The catalyst area was extracted from the hydrogen adsorption region of cyclic voltammograms [7]. The overall cell water balance was also calculated to assess the presence of liquid water [8].The smallest mass transfer resistance was assigned to the ionomer permeability, which supported the need for repetitive measurements and statistics for an accurate quantification. The slope of the linear correlation between the ionomer permeability and Knudsen diffusion mass transfer resistance and the oxygen concentration was dependent on the water balance. A positive slope occurred under sub-saturated streams whereas a negative slope was correlated with the wettest operating conditions, which will require a method modification to extract the Knudsen diffusion resistance. A plot of the ionomer and molecular diffusion mass transfer resistance versus the total mass transfer overpotential showed two regimes that are differentiated by the absence/presence of liquid water (see figure). Catalyst layers, only several microns thick, have a larger mass transfer resistance and overpotential under flooding conditions.[1] Y. Wang, D. F. R. Diaz, K. S. Chen, Z. Wang, X. C. Adroher, Mater. Today, 32 (2020) 178.[2] A. Kongkanand, M. F. Mathias, J. Phys. Chem. Lett., 7 (2016) 1127.[3] N. Ramaswamy, W. Gu, J. M. Ziegelbauer, S. Kumaraguru, J. Electrochem. Soc., 167 (2020) article 064515.[4] T. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161 (2014) F1089.[5] N. Nonoyama, S. Okazaki, A. Z. Weber, Y. Ikogi, T. Yoshida, J. Electrochem. Soc., 158 (2011) B416.[6] J. St-Pierre, M. Angelo, K. Bethune, J. Huizingh, T. Reshetenko, M. Virji, Y. Zhai, Modern Fuel Cell Testing Laboratory, in Springer Handbook of Electrochemical Energy, Part D, Chapter 19, Edited by C. Breitkopf, K. Swider-Lyons, Springer, 2017, p. 611.[7] R. N. Carter, S. S. Kocha, F. T. Wagner, M. Fay, H. A. Gasteiger, ECS Trans., 11(1) (2007) 403.[8] J. St-Pierre, N. Jia, M. van der Geest, A. Atbi, H. R. Haas, United States patent 7,132,179, November 7, 2006. Figure 1

A Zero-Dimensional Electrochemical Model for Organic Flow Cells

We develop and experimentally validate a zero-dimensional model of the electrochemical performance of a flow cell. The model takes into account voltage losses due to Faradaic charge transfer, ohmic and mass transport resistance, as well as the effect of spatial variations in state-of-charge between the cell and electrolyte reservoir. We validate the model by comparing voltage-current data during cycling to equivalent electrochemical measurements from a symmetric cell with organic reactants. With the exception of the mass transfer coefficient, all relevant model parameters are determined independently prior to the experiment using electrochemical impedance spectroscopy and cyclic voltammetry measurements. We find excellent agreement between the model and experiment for constant-current and constant-voltage cycling, across a wide range of current densities (40 – 200 mA/cm2) and volumetric flow rates (~ 10 – 140 mL/min). The relationship between the fitted mass transport coefficient and flow rate conforms to expectations from the literature [1-3], and reasonably approximates the analogous relationship from single-reservoir cell measurements. This work supports the notion that zero-dimensional electrochemical models can yield simple but predictive frameworks for optimizing organic flow battery performance and understanding how reactant decomposition and crossover contribute to capacity fade.

Investigation of Mass Transport Properties of Fibrous Electrodes in Vanadium Redox Flow Batteries By Lattice Boltzmann Simulation

Redox flow batteries (RFBs) are gathering much attention as one of the promising candidates for a large-scale electrical energy storage device that is vital to utilize intermittent renewable energy in the grid [1]. In RFBs, fibrous electrodes have been widely applied presumably due to their large specific surface area and better electric conductance. It has been well recognized that the electrode structure profoundly affects cell performance that is attributed to the reaction and transport properties of the electrode [2,3].In this study, we developed a numerical simulation method to analyze pore-scale inhomogeneous behaviors of fluid and electrochemical reaction in fibrous electrodes using Lattice Boltzmann method (LBM). We focused our attention on porous structure affecting mass transport properties and overpotential in the electrodes for the vanadium redox flow battery.We carried out the numerical analysis in the negative electrode that simulates the RFB fibrous structure, as shown in Fig.1(a). As the calculation procedure first, the mass and the momentum conservation equation for the electrolyte solution were solved, and the chemical species conservation equation for the vanadium ion (V3+) in the electrolyte solution and the charge conservation equation for the electrode and the electrolyte solution were solved. All calculation was carried out by the Lattice Boltzmann code we developed. Numerical analysis of electrochemical reaction and transport field in the negative fibrous electrode under the charge at 50mA/cm2 was performed with different fiber diameter and porosity. As the pressure loss was fixed, the change in total overvoltage was evaluated as the change in energy loss.Figure 1 shows the calculation results of the electrochemical reaction and transport in the fiber electrode with a diameter of 3.75 μm and a porosity of 0.70. A path through which the electrolyte selectively flows is formed in the sparse area of the fibrous electrode and the inhomogeneous vanadium ion concentration and local reaction current density are identical in the electrode. This indicates that the electrolyte flow is locally biased due to the non-uniformity of the electrode. The reactive species concentration decreases due to the electrochemical reaction in the dense fiber region and causes local reaction current density.Based on a series of simulation with different diameter and porosity of the fibrous electrode, we obtained an equation for the electrolyte velocity and pressure drop in the electrode. We also evaluated averaged mass transport coefficients. We confirmed that the mass transfer characteristics are improved when the fiber diameter is large and the porosity is high. This is attributed to the larger the voids, the higher the permeability and the transport by advection was promoted. However, it is noteworthy that the larger diameter and the higher porosity leads to less electrode surface area. In our previous study [3], an optimized electrode architecture with a diameter of 2.4 μm and a porosity of 0.89 was presented by macro-scale simulations.In this fiber-scale simulations, we evaluated overpotential depending on the fiber diameter and the electrode porosity, and demonstrated the LBM simulation applied for optimization of fiber diameter and the electrode porosity in the RFB applications. Acknowledgements This research was supported by a Precursory Research for Embryonic Science and Technology (PRESTO) grant (grant number JPMJPR12C6) from the Japan Science and Technology Agency(JST).

Interplay between PEMFC Performance, Gas Diffusion Electrode Structure and Mass Transport Properties

Due to substantial research and development in the last two decades, energy conversion systems based on proton exchange membrane fuel cell (PEMFC) technology are emerging to the market. However, cost of the fuel cells is still high compared to the conventional energy generating systems like internal combustion engines. A decrease in Pt loading is a vital requirement for cost reduction and commercialization of the fuel cell technology. Operation of low-Pt PEMFCs showed that fuel cell performance is declining with a decrease in Pt loading and the voltage drop becomes very significant at high current density due to increasing mass transfer losses [1]. Thus, it is critical to understand this voltage loss phenomenon and mitigate it. Previously we developed a method for determining the oxygen mass transfer coefficient which is based on measurements of limiting current distributions using O2 mixtures with different diluents (from He to C3H8) [2]. In this work we studied effects of gas diffusion electrodes (GDEs) composition and structure such as cathode catalyst and microporous layer (MPL) loadings on oxygen mass transport and PEMFC performance.A segmented cell and data acquisition system were operated with commercially available 100 cm2 MEAs with cathode loadings of 0.1, 0.2 and 0.4 mgPt cm-2, while the anode Pt content was kept 0.1 mg cm-2. Cathode gas diffusion layer (GDL) was also varied to separate contributions from MPL and catalyst layer. 25BA (no MPL) and 25BC (with MPL) diffusion medias from Sigracet SGL were used for the cathode electrode. The experiments were performed at 80°C, fully humidified conditions and varied back pressures. Details of experimental procedures are presented in [2].A comparison of the IV curves showed that 25BA cannot provide good water and gas management at high current operation (Fig. 1 a). At the same time, an increase in Pt loading significantly improves the performance at low and high current regimes. The high performance of the sample MEA-0.4-25BC with Pt loading of 0.4 mgPt cm-2 and 25BC cathode GDL indicates on a possibility that part of catalyst layer acts as an MPL and actively participates in water removal from electrode structure to macro-porous layer of carbon paper.Using our previously described method we separated mass transport coefficients in gas phase (km, N2 ) and a combination of Knudsen diffusion and transport through ionomer/water films in overall GDE (kK+film ). Comparing results for MEA-25BA and MEA-25BC we extracted contribution from MPL (kK, MPL ) and cathode layer (CL) (kK+film, CL ). Further variation of back pressure allowed us to eliminate impact of Knudsen diffusion in CL and extract pressure independent contribution (kfilm,CL ) which represents a diffusion through the ionomer/water films in CL.The obtained values of pressure-independent oxygen mass transfer coefficients in CL (kfilm, CL ), Knudsen diffusion in MPL (kK, MPL ) and CL (kK, CL ) and molecular diffusion in N2 media (km, N2 ) are compared at Fig. 1 b. A reduction in Pt loading leads to a decrease in kfilm, CL as it was previously reported [3-6]. Interestingly, that MEA with the highest Pt content (MEA-0.4-25BC) is characterized by the greatest value of kK, MPL as well. All samples have nearly the same values of the O2 mass transfer coefficient in gas phase varying from 0.040 to 0.043 m s-1.Performances of the studied samples in terms of cell voltage at 1.5 A cm-2 and mass transport voltage losses (permeability and diffusion) are presented at Fig. 1 c. A direct comparison of the data shows that MEA-0.4-25BC has the best performance due to the greatest values of kfilm, CL and kK, MPL insuring a good gas transport within catalyst layer and MPL as well as water transport. Detailed analysis of the PEMFC performance, voltage losses and oxygen mass transfer coefficients at various operating conditions will be presented and discussed.ACKNOWLEDGEMENTSWe gratefully acknowledge funding from ONR (N00014-19-1-2159) and ARO (W911NF-15-1-0188). The authors are thankful G. Randolf and J. Huizingh for support of the operation as well as T. Carvalho for assisting with SEM.References A. Kongkanand, M. Mathias, J. Phys. Chem. Lett. 7 (2016) 1127.T. V. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161, F1089-F1100 (2014).Y. Ono, T. Mashio, S. Takaichi, A. Ohma, H. Kanesaka, K. Shinohara, ECS Trans. 28 (27) 69-78 (2010).N. Nonoyama, S. Okazaki, A.Z. Weber, Y. Ikogi, T. Yoshida, J. Electrochem. Soc. 158 (4) B416-B423 (2011).T. Greszler, D. Caulk P. Sinha, J. Electrochem. Soc. 159 (12) F831-F840 (2012).J.P. Owejan, J.E. Owejan, W. Gu, J. Electrochem. Soc. 160 (8) F824-F839 (2013). Figure 1

A revisit of the electro-diffusional theory for the wall shear stress measurement

This article intends to revisit the electro-diffusional theory for the wall shear stress measurement from mass transfer probes of rectangular shape by considering the existence of two components of the wall shear rate (i.e., axial and transversal). General analytical formulas for the effective transfer length and the dimensionless mass transport coefficient were derived as a function of two parameters: a dimensionless angle of the flow direction, relative to the leading edge of the probe, and the aspect ratio between the width and the length of the strip probe. The correctness of the analytical relations for arbitrary flow direction and the aspect ratio was confirmed by numerical solutions of the transport equation in the convective-diffusive regime. It has also been proved that the differences between the Lévêque solution and the general analytical formula exhibit a significant deviation for a specific range of parameters. In the case of the three-dimensional boundary layers, in addition to the magnitude of the wall shear stress, the direction of the fluid flow in the vicinity of the probe’s surface is of paramount importance. Accordingly, a measurement methodology is proposed using two strip probes with different aspect ratios. The resulting equations required to quantify the magnitude of the wall shear rate vector and the dimensionless angle are also derived.

Convective and Microwave Assisted Drying of Wet Porous Materials with Prolate Spheroidal Shape: A Finite-Volume Approach

Convective heating is a traditional method used for the drying of wet porous materials. Currently, microwave drying has been employed for this purpose, due to its excellent characteristics of uniform moisture removal and heating inside the material, higher drying rate, and low energy demand. This paper focuses on the study of the combined convective and microwave drying of porous solids with prolate spheroidal shape. An advanced mathematical modeling based on the diffusion theory (mass and energy conservation equations) written in prolate spheroidal coordinates was derived and the numerical solution using the finite-volume method is presented. Here, we evaluated the effect of the heat and mass transport coefficients and microwave power intensity on the moisture removal and heating of the solid. Results of the drying and heating kinetics and the moisture and temperature distribution inside the solid are presented and discussed. It was verified that the higher the convective heat and mass transfer coefficients and microwave power intensity, the faster the solid will dry and heat up.

Development of an Oxygen Mass Transport Coefficient Measurement and Separation Method for Proton Exchange Membrane Fuel Cells

In this work, we use a method to separate the total oxygen mass transport coefficient into molecular, Knudsen, and ionomer contributions. Therefore, limiting current density measurements are carried out as a function of the diluent gas (He, N2, CO2), temperature (30, 50, 80°C), relative humidity (50, 75, 100%), and oxygen concentration (1, 3, 5, 7%) using state of the art membrane electrode assemblies with three platinum loadings (0.05, 0.1, 0.15 mg/cm2). As expected, the molecular diffusion coefficient is independent of the platinum loading, but increases with temperature to a varying degree depending on the humidity level. On the other hand, the Knudsen diffusion coefficient increases with increasing electrochemical active surface area and temperature, and with decreasing relative humidity. The separation procedure includes a novel feature to isolate the ionomer mass transport resistance. Its interpretation as well as the method’s reliability are critically questioned using operating condition dependencies.

Unusual influence of binder composition and phosphoric acid leaching on oxygen mass transport in catalyst layers of high-temperature proton exchange membrane fuel cells

Oxygen mass transport in the cathode catalyst layer (CCL) of high-temperature proton exchange membrane fuel cells (HT-PEMFCs) plays an important role in promoting the fuel cell performance. The objective of this study is the development of suitable tools and analytical methods for quantitatively characterizing O2 mass transport coefficients in a binder used in HT-PEMFCs from a microscopic and local perspective. A microelectrode electrochemical system with high sensitivity and stability is built in-house. High-resolution cyclic voltammetry, potential-step chronoamperometry and sampled chronoamperometry at the nA scale are successfully performed in a shielding box to determine the O2 mass transport coefficients in a poly (ethersulfone)-poly (vinyl pyrrolidone) (PES-PVP) blend binder with different PVP contents. In addition, we investigate the effect of phosphoric acid (PA) leaching on O2 mass transport in the binder, which is generally used to explain the decline in membrane conductivity. Linear regression analysis reveals that the permeability of O2 in binder can be affected significantly by the PVP content and the PA doping level of the binder in CCL.

Enhanced separation of bioactive triterpenic acids with a triacontylsilyl silica gel adsorbent: From impulse and breakthrough experiments to the design of a simulated moving bed unit

A simulated moving bed (SMB) unit was designed to separate oleanolic and ursolic acids, two naturally occurring triterpenoids with structural isomerism, with remarkable nutraceutical and pharmacological properties. A triacontylsilyl silica gel adsorbent (stationary phase of an Acclaim C30 column) was considered and impulse tests with different solvents were performed to select a mobile phase, from which methanol/water 95/5 (%, v/v) emerged as the most suitable. Equilibrium and global mass transport coefficients were then determined through breakthrough experiments using pure compound solutions and the C30 column. Afterwards, these parameters were applied to the simulation of two model binary mixture separations, whose breakthrough curves were also experimentally measured.Finally, the SMB unit was designed and optimized. It was demonstrated that using the packing of an Acclaim C30 column and methanol/water 95/5 (%, v/v) as mobile phase it is possible to separate both acids with purities of 99.9 wt.%, a productivity of 1.705 kg/(m3adsorbent day), and a configuration of two columns per section (2–2–2–2). The simulated results obtained in this work with the C30 stationary phase represent a significant improvement over literature data.

Unsteady solute dispersion by electrokinetic flow in a polyelectrolyte layer-grafted rectangular microchannel with wall absorption

The dispersion of a neutral solute band by electrokinetic flow in polyelectrolyte layer (PEL)-grafted rectangular/slit microchannels is theoretically studied. The flow is assumed to be both steady and fully developed and a first-order irreversible reaction is considered at the wall to account for probable surface adsorption of solutes. Considering low electric potentials, analytical solutions are obtained for electric potential, fluid velocity and solute concentration. Special solutions are also obtained for the case without wall adsorption. To track the dispersion properties of the solute band, the generalized dispersion model is adopted by considering the exchange, the convection and the dispersion coefficients. The solutions developed are validated by comparing the results with the predictions of finite-element-based numerical simulations. Even though the solutions can take any form of initial solute concentration into account, the results are presented by considering a solute band of rectangular shape. The results reveal that, while the short-term transport coefficients are strongly affected by the initial concentration profile, the long-term values are not dependent upon the initial conditions. In addition, it is shown that the mass transport coefficients are strong functions of the channel aspect ratio; hence, approximating a rectangular geometry by the space between two parallel plates may lead to considerable errors in the estimation of mass transport characteristics. This is particularly important for the dispersion coefficient for which the long-term values for a slit microchannel are quite different from those for a rectangular channel of very high aspect ratio. It is also illustrated that the exchange and convection coefficients increase on increasing the Damkohler number, whereas the opposite is true for the dispersion coefficient. The convection and dispersion coefficients are generally increasing functions of the PEL fixed charge density and the PEL thickness and decreasing functions of the PEL friction coefficient. Last but not least, a thicker electric double layer is found to provide a larger degree of solute dispersion, which is the opposite of that observed in a microchannel with bare walls.

System-scale modeling and membrane structure parameter optimization for solar-powered sweeping gas membrane distillation desalination system

Sweeping Gas Membrane Distillation (SGMD) is a promising desalination method which achieves a desired balance between reducing conductive heat loss and improving mass transport coefficient by using flowing gas instead of condensing water and static air-gap in the permeate side. However, previous studies mostly focused on the effects of operation conditions on the permeate flux by using a module-scale model. The role of membrane properties in a pragmatic system-scale desalination platform is seldom examined. This study newly established a system-scale mathematical model for a solar-powered SGMD desalination system in real outdoor weather conditions and investigated the interaction between membrane resistance and driving force to moisture transfer. Three hollow fiber membrane humidifiers made of membranes with different membrane structure parameters (pore size, porosity and tortuosity of the membrane pore) were tested experimentally to validate the new model. The verified model can be employed to optimize membrane structure parameters in terms of mean pore diameter, porosity and tortuosity. The results show that the controller of membrane resistance gradually transforms from Knudsen diffusion to ordinary molecular diffusion at a normal operating temperature range of 50–80 °C when the pore size increases from 140 nm to 160 nm. It is no longer beneficial to improve system performance by increasing the pore diameters above 150 nm where ordinary molecular diffusion begins to dominate membrane resistance. The optimal porosity is about 0.75, and further increase in porosity does not yield significant improvements in terms of freshwater yield and system Coefficient of Performance (COP). The approximate linear variation of the system performance indices with the tortuosity indicates that smaller tortuosity is favorable for the system performance. The system-scale modeling can be successfully employed for predicting the practical SGMD system performance and optimizing the membrane structure parameters.

Flow Cell Characterisation: Flow Visualisation, Pressure Drop and Mass Transport at 2D Electrodes in a Rectangular Channel

Aiming to demonstrate the importance and facility of characterising the reaction environment in new commercial laboratory-scale flow cells, fluid flow, pressure drop and space averaged mass transport coefficient were studied in the C-Flow® Lab 5 × 5 cell. A flow-by configuration with smooth, planar electrodes in a rectangular channel was used. Electrolyte mean linear velocities of 2 to 10 cm s−1 past the electrode surface and channel Reynolds numbers of 53 to 265 were considered. The effect of a turbulence promoter next to the working electrode was evaluated. Flow distribution was explored by a qualitative flow visualization study, while the relevance of pressure drop was shown by measurements over the flow channel and the whole cell as a function of mean linear velocity. The electrochemical performance was quantified from the limiting current, permitting the determination of the mass transport coefficient at the electrodes over the same range of flow rates. Reactant conversion in the batch recirculation mode and normalised space velocity were predicted from the electrochemical plug flow reactor design equation. Results were compared to well-characterised electrochemical flow reactors found in the literature. The significance of characterisation techniques and basic reactor models during the development of new processes is emphasised.