Sorption and permeation of solutions of chloride salts, water and methanol in a Nafion membrane

Predicting Diffusion Coefficients in Nafion Membranes during the Soaking Process Using a Machine Learning Approach

Nafion, a versatile polymer used in electrochemistry and membrane technologies, exhibits complex behaviors in saline environments. This study explores Nafion membrane's IR spectra during soaking and subsequent drying processes in salt solutions at various concentrations. Utilizing the principles of Fick's second law, diffusion coefficients for these processes are derived via exponential approximation. By harnessing machine learning (ML) techniques, including the optimization of neural network hyperparameters via a genetic algorithm (GA) and leveraging various regressors, we effectively pinpointed the optimal model for predicting diffusion coefficients. Notably, for the prediction of soaking coefficients, our model is composed of layers with 64, 64, 32, and 16 neurons, employing ReLU, ELU, sigmoid, and ELU activation functions, respectively. Conversely, for drying coefficients, our model features two hidden layers with 16 and 12 neurons, utilizing sigmoid and ELU activation functions, respectively.

Role of ionic interactions in the deformation and fracture behavior of perfluorosulfonic-acid membranes

The stability of ion-conductive membranes, such as perfluorosulfonic-acid (PFSA) membranes, as a solid-electrolyte separator in energy devices is strongly linked to their mechanical properties, the characterization of which presents challenges, especially in the presence of ionic interactions. Ionomer membranes' elastic properties are affected by cations; however, their influence on deformation at small and large strains is relatively unexplored. In this paper, we report the stress-strain response and fracture behavior of Nafion membranes exchanged with various cations examined in three deformation regimes. In the small-strain regime, the Young's modulus is strongly dependent on cation size, due to the reduced mobility and local stiffening of polymer chains. The Young's modulus, yield limit and strain-hardening modulus all increase with monovalent cation size in the order H+ < Li+ < Na+ < K+ < Cs+, but with varying dependence. In the failure regime, however, the break strain and fracture energy of the membrane decrease in the presence of larger cations, which promote deformation instability while decreasing plastic dissipation energy during crack propagation, thereby leading to more brittle fracture. These results not only demonstrate the trade-off between strength and fracture toughness, but also reveal how it is altered by the ionic interactions, which also dictate the inverse relationship between stretchability and stiffness. Moreover, the measured stress-strain data are reproduced by the constitutive relations to extract parameters that are correlated to the fracture energy through craze instability. Such relationships provide insight into how parameters extracted from tensile testing can be used to assess membrane stability and the role of ionic interactions.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradation phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chemistry and side-chain is also discussed to present a broader perspective.

Water permeation through Nafion membranes: the role of water activity

The permeation of water through 1100 equivalent weight Nation membranes has been measured for film thicknesses of 51-254 microm, temperatures of 30-80 degrees C, and water activities (a(w)) from 0.3 to 1 (liquid water). Water permeation coefficients increased with water content in Nafion. For feed side water activity in the range 0 < a(w) < 0.8, permeation coefficients increased linearly with water activity and scaled inversely with membrane thickness. The permeation coefficients were independent of membrane thickness when the feed side of the membrane was in contact with liquid water (a(w) = 1). The permeation coefficient for a 127 microm thick membrane increased by a factor of 10 between contacting the feed side of the membrane to water vapor (a(w) = 0.9) compared to liquid water (a(w) = 1). Water permeation couples interfacial transport across the fluid membrane interface with water transport through the hydrophilic phase of Nafion. At low water activity the hydrophilic volume fraction is small and permeation is limited by water diffusion. The volume fraction of the hydrophilic phase increases with water activity, increasing water transport. As a(w) --> 1, the effective transport rate increased by almost an order of magnitude, resulting in a change of the limiting transport resistance from water permeation across the membrane to interfacial mass transport at the gas/membrane interface.

Experimental estimation of equilibrium and transport properties of sulfonated cation-exchange membranes with different morphologies

Solvent uptake, hydraulic and electroosmotic permeabilities, true cation transport number, effective fixed charge concentration, and limiting current values have been determined in aqueous LiCl solutions for three commercial cation-exchange membranes with different morphologies and similar electric properties. The differences found in the equilibrium and transport properties of the membranes have been analyzed on the basis of their different structures. The experimental results show that the membrane morphology has an influence on the effect that the presence of an electrolyte has in the solvent uptake and in the liquid permeation. Differences have also been found in the polarization concentration effects, and on the loss of the membrane selectivity with the increase of the electrolyte concentration.