Assessing the Effects of Crowding, Pore Size, and Interactions on Electro-Osmotic Drag Coefficients
LC Jacobson and XM Ren and V Molinero, JOURNAL OF PHYSICAL CHEMISTRY C, 118, 2093-2103 (2014).
Water flow coupled to the migration of ions is an important aspect of the performance of polymer electrolyte membrane fuel cells. The water gradients arising from the operation of fuel cells can result in flooding and drying-out of the electrodes and drying of regions of the membrane, with concomitant losses in conductivity and efficiency. The electro-osmotic drag coefficient measures the ratio between the flow of solvent molecules to that of a charged species toward an electrode in the presence of an applied electric field. The effects of variables such as pore radius, crowding, temperature, electric field strength, and ion concentration on the mobility of ions and accompanying water molecules in an applied electric field are still not well understood. Here, we investigate these factors with coarse-grained molecular simulations using an efficient model of water and sodium chloride ions and compare these results with those from previous experiments on proton exchange membranes as well as new experimental results for an anion exchange membrane. The anion exchange membranes have a smaller value of K-drag than the proton-exchange membranes, which may be attributed to smaller water domains and a different charge carrier (hydroxide instead of protons). We directly determine the role of pore size on K-drag and confirm that narrower pores result in less electro-osmotic drag. Our simulations show that K-drag is sensitive to the interaction of the charge carrier with water molecules. The results of this work suggest that the most promising approach to minimize electro-osmotic drag while maintaining adequate ion conductivity is to control the morphology of the membrane structure at the microscopic level.
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