Multicomponent space‐charge transport model for ion‐exchange membranes

Yang, Yahan; Pintauro, Peter N.

doi:10.1002/aic.690460610

Cited by 65 publications

(75 citation statements)

References 27 publications

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“…23 presents experimental data for transient transport of H + , Na + , and Cs + ions across a Nafion cation exchange membrane, separating acid and salt compartments. 37 The HSO 4 À and SO 4 2À ions cannot cross the membrane. The H + ions transfer from the acid to the salt compartment.…”

Section: Ionic Diffusion and The Influence Of Electrostatic Potentialmentioning

confidence: 99%

Uphill diffusion in multicomponent mixtures

2015

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Molecular diffusion is an omnipresent phenomena that is important in a wide variety of contexts in chemical, physical, and biological processes. In the majority of cases, the diffusion process can be adequately described by Fick's law that postulates a linear relationship between the flux of any species and its own concentration gradient. Most commonly, a component diffuses down the concentration gradient. The major objective of this review is to highlight a very wide variety of situations that cause the uphill transport of one constituent in the mixture. Uphill diffusion may occur in multicomponent mixtures in which the diffusion flux of any species is strongly coupled to that of its partner species. Such coupling effects often arise from strong thermodynamic non-idealities. For a quantitative description we need to use chemical potential gradients as driving forces. The transport of ionic species in aqueous solutions is coupled with its partner ions because of the electro-neutrality constraints; such constraints may accelerate or decelerate a specific ion. When uphill diffusion occurs, we observe transient overshoots during equilibration; the equilibration process follows serpentine trajectories in composition space. For mixtures of liquids, alloys, ceramics and glasses the serpentine trajectories could cause entry into meta-stable composition zones; such entry could result in phenomena such as spinodal decomposition, spontaneous emulsification, and the Ouzo effect. For distillation of multicomponent mixtures that form azeotropes, uphill diffusion may allow crossing of distillation boundaries that are normally forbidden. For mixture separations with microporous adsorbents, uphill diffusion can cause supra-equilibrium loadings to be achieved during transient uptake within crystals; this allows the possibility of over-riding adsorption equilibrium for achieving difficult separations. Key learning points(1) Coupling effects in mixture diffusion may cause uphill transport of a component (2) Uphill diffusion results in transient overshoots and serpentine equilibration trajectories (3) Non-ideal phase equilibrium thermodynamics is often the major source of diffusional coupling effects (4) Chemical potential gradients are the proper driving forces for diffusion (5) Uphill transport can be exploited to achieve difficult and unusual separations

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Section: Ionic Diffusion and The Influence Of Electrostatic Potentialmentioning

confidence: 99%

Uphill diffusion in multicomponent mixtures

2015

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“…Various theoretical frameworks have been applied to investigate such systems. [30][31][32][33][34][35][36] Concentratedsolution theory accounts for all interactions between the various components in principle. However, the necessary transport coefficients are rarely available and are not easily probed experimentally.…”

Section: A5016mentioning

confidence: 99%

The Influence of Electric Field on Crossover in Redox-Flow Batteries

Darling

Weber

Tucker

et al. 2015

J. Electrochem. Soc.

152

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Transport of active species through the ion-exchange membrane separating the electrodes in a redox-flow battery is an important source of inefficiency. Migration and electro-osmosis have significant impacts on the crossover of reactive anions, cations, and neutral species. In this paper, these phenomena are theoretically and experimentally explored for commercial cation-exchange membranes. The theoretical analysis indicates that plotting the cumulative Coulombic mismatch between charge and discharge as a function of time can be used to assess crossover rates. The relative importance of migration and electro-osmosis over diffusion is quantified and shown to increase with increasing current density and membrane thickness because the contributions of migration and electro-osmosis to ionic flux are independent of membrane thickness and proportional to current density, while diffusion is inversely proportional to membrane thickness and independent of current density. Redox-flow batteries (RFBs) possess compelling attributes for stationary energy storage. In particular, the inherent decoupling of energy and power in RFBs enables the cost effective use of active materials with low energy density.1,2 Energy is stored in redox-active molecules and power is generated by oxidizing and reducing different redox couples at the positive and negative electrodes.3-5 Two promising technologies are the all-vanadium RFB (VRB) and the hydrogen/bromine RFB, where good performances have been demonstrated recently. [6][7][8][9][10][11][12][13][14] Protons in a cation-exchange membrane (CEM) typically carry charge between the two electrodes in these two RFBs. In both systems, movement of reactant or product species from one electrode to the other through the membrane results in inefficiency and reversible capacity loss. 8,15,16 For these reasons it is worthwhile to investigate the causes of crossover, and examination of the two chosen RFBs allows for the study of transport of active cations, anions, and neutral species.The reactions at the negative and positive electrodes in a VRB are:andrespectively. Protons are the primary ionic charge carriers. Because the active species at both electrodes are vanadium ions, transfer from one electrode to the other is less detrimental than it is in other RFBs that have dissimilar active materials at the two electrodes. Vanadium that traverses the separator reacts to form a discharged ion at the opposite electrode. 17 Although imbalances that occur after repeated cycling can be recovered by mixing the electrolytes, vanadium that crosses the membrane represents an inefficiency that is detrimental in applications like grid-scale energy storage where high efficiency is required for economic success.For the hydrogen/bromine RFB, the reactions at the negative and positive electrodes areBr 2 (aq) + 2e 18-21 Br species that enter the negative electrode can be separated from the negative electrolyte and returned to the positive electrolyte without causing permanent decay, in a similar fashion to vanadium in V...

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“…This transport is seen in dissolution studies where one finds platinum and alloy catalyst movement through the membrane after operation [130,[216][217][218][219]. To model such effects, the same basic approaches and equations as described in section 3 have been used [92,216,[220][221][222].…”

Section: Impurity Ion and Electrolyte Effectsmentioning

confidence: 99%