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A technique to measure the cation-transference number of salts in fully hydrated ion-selective membranes has been developed and demonstrated on Nafion 117 for LiCl and Li 2 SO 4 . Dilute solution theory is used to identify experimental conditions that reduce the propagation of uncertainties in membrane properties to transference number estimates. This technique has advantages over commonly used methods, including the elimination of the need for the analysis of electrode potentials in approaches that exploit electroanalytical methods or the need for additional information required to reconcile NMR-based methods with the bulk transport property. It additionally allows for numerous measurements per day and offers the possibility to relate trace measurements of either cations or anions to values of transference number. For LiCl both modes of the technique were employed; the anion-tracer method is more precise and gives t + = 0.936 ± 0.010. The experimental procedure was repeated using the cation-tracer method for Li 2 SO 4 , and t + = 0.95 ± 0.06 was estimated. The 21 st century presents a key challenge in energy storage due to the intermittency of wind and solar energy sources. Lithium ion batteries, fuel cells, and redox flow batteries are promising technologies for energy storage since they have high energy densities, are environmentally friendly, and have an array of other desirable properties. [1][2][3][4][5][6] Solid electrolytes are ubiquitous for these energy storage systems since they serve the important function of mechanical separation between electrodes. In addition, solid electrolytes circumvent several problems associated with liquid electrolytes such as the leakage of electrolyte solution and the reaction of volatile organic solvents.7 Of the solid electrolytes, polymer electrolytes have garnered the most interest in recent years. [8][9][10][11] These membranes facilitate ionic transport between electrodes, inhibit electron flow between electrodes, prevent direct contact between electrodes, and minimize mixing of the anolyte and catholyte.In recent decades, researchers have sought to understand the relationship between the molecular structure of polymer electrolytes and their performance. These structure-property relationships have been developed for two major types of polymer electrolytes: (I) mixtures of salts in high molecular weight polymers and (II) polymerized ionic liquids (single ion conductors). Polyethylene oxide (PEO), polypropylene oxide (PPO), and 4 poly[bis(methoxy-ethoxyethoxy) phosphazene] (MEEP) are all promising Type I polymer electrolytes.12-16 The electron-donating groups incorporated into the polymer architecture are responsible for solvating the lithium ion while the fast segmental dynamics promote high ionic conductivities through fluctuation-driven diffusion. [17][18][19] However, Type I polymer electrolytes typically suffer from poor mechanical properties, which is an unfortunate compromise for the fast segmental dynamics. Furthermore, Type I polymer electrolytes have relative...

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