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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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Here, the phase behavior of water in hydrogels has a broad impact on many health and energy applications. Our previous study showed that polyampholyte hydrogel has the potential to be used as an aqueous gel electrolyte in electrochemical storage devices at -30 °C due to enhanced low-temperature conductivity. In this study, we detail the impact polymer structure has on this enhanced conductivity, explaining this finding with a model charge-balanced polyampholyte, poly(4-vinylbenzenesulfonate- co -[3-(methacryloylam ino)propyl] trimethylammonium chloride), a hydrogel whose polymer and water structures are probed by variable-temperature SAXS, WAXS, and solid-state NMR spectroscopy. SAXS results at room temperature indicate a networked globule structure in the charge-balanced polyampholyte hydrogels. The globular radius of gyration is ~ 2.5 nm, whereas the globular size and its clustering structure are dependent on synthesis parameters. Variable-temperature SAXS data reveal a temperature-dependent structure evolution of the polyampholyte hydrogel. An interconnected globular network structure of polymer-rich phase at low temperature, observed by electron microscopy, is suggested to preserve ion-conducting channels of nonfrozen water molecules at low temperatures. This hypothesis is further supported by solid-state NMR spectroscopy. Together these findings provide macromolecular- and molecular-level insight that may be used to design gel electrolytes for enhanced low-temperature performance.
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Membrane transport properties are crucial for electrochemical devices, and these properties are influenced by the composition and concentration of the electrolyte in contact with the membrane. We apply this general membrane-electrolyte system approach to alkaline flow batteries, studying conductivity and ferricyanide crossover of Nafion and E-620. We report undetectable crossover for as-received Nafion and E-620 after both sodium and potassium exchange, but high ferricyanide permeability of 10 –7 to 10 –8 cm2 s –1 for Nafion subjected to pre-treatment prevalent in the flow battery literature. We show how the electrolyte mass fraction in hydrated membranes regulates the influence of ion concentration on membrane conductivity, identifying that increasing electrolyte concentration may not increase 1 membrane conductivity even when it increases electrolyte conductivity. To illustrate this behavior we introduce a new metric, the membrane penalty, as the ratio of the conductivity of the electrolyte to that of the membrane equilibrated with the electrolyte. We discuss the tradeoff between flow battery volumetric capacity and areal power density that arises from these findings. Finally, we apply insights from this approach to provide recommendations for use of membranes in alkaline flow cells, and electrochemical reactors in general.
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