Nernst-Planck transport theory for (reverse) electrodialysis: III. Optimal membrane thickness for enhanced process performance

Abstract

The effect of the thickness of ion exchange membranes has been investigated for electrodialysis (ED) and reverse electrodialysis (RED), both experimentally and through theoretical modeling. By developing a two-dimensional model based on Nernst-Planck theory, we theoretically find that reducing the membrane thickness benefits process performance only until a certain value, below which performance drops. For ED, an optimum thickness can be identified in the range of 10-20 µm, while for RED the maximum power density is found for membranes that are three times as thick. Model calculations compare well with experimental data collected with a series of homogeneous membranes with the same chemical composition and a thickness in the range of 10-75 µm. Our results show that the classical picture that membranes should be as thin as possible (as long as they remain pinhole-free and structurally stable) is insufficient, and must be replaced by a more accurate theoretical framework.

Energy Harvesting from Brines by Reverse Electrodialysis Using Nafion Membranes

“…Even though the thickness and the ionic resistance are proportional, thinner membrane does not necessarily perform better. Tedesco et al (2018) carried out experiments with FAS and FKS Fumasep (FUMATECH BWT GmbH, Bietigheim-Bissingen, Germany) membranes with varying thickness between 14–90 µm, and concluded having thinner membranes was not beneficial for maximum power density [ 43 ].…”

Capacitive processes for carbon capture and energy recovery from CO2 emissions : Shaping a new technology going from water to gas applications

“…As explained in section 6.3.2, improved H2CO3 * permeability through the ionomer layer could increase the OCV values. MEA cell designs have been optimized for many years in fuel cells, for instance, by selecting suitable ionomer material 215,216 , optimizing the electrodes and membrane thickness 205,214 , as well as optimizing the loading and structure of the coating in the electrodes.…”

“…The non-homogenous ionomer coating leads to a non-homogenous ion selectivity through the electrodes. Thinner ionomer layers (e.g., IEM) are prone to higher co-ion expulsion 205 , which reduces the ion selectivity of the ionomer layer. In this regard, future studies should be focused on testing different coating methods (e.g., slurry casting 18,23 , electrospinning 206,207 , electrospraying 208 , or spraying with a spray gun 209,210 ) to better tune the coating homogeneity, loading, and content.…”

“…Nevertheless, the thickness of the coated electrodes is thicker than the uncoated electrodes, which could result in a higher pressure applied between the coated electrode and the current collector. Therefore, Rohmic can be further improved by increasing the ionic conductivity of the CEM (e.g., decreasing thickness 205,214 ) but also by adapting the force applied to the electrodes (cell construction design). Fig.…”

“…An extensive range of theoretical models has been developed to predict salt electrosorption in CDI cells 70,84,86,152,221 over the years. Besides, salt ion transport through ion exchange membranes has been investigated not only for Membrane-CDI (MCDI) but also for other technologies, e.g., capacitive mixing (CAPMIX) 21,22,132,137 , electrodialysis (ED) 205,[222][223][224] and reverse electrodialysis (RED) 88,225 . However, understanding CO2-sparged solutions require the incorporation of additional physical phenomena (e.g., gas absorption, dissociation of amphoteric ions), which is not the case for salt solutions.…”