A Study on the Conductivity and Selectivity of Lithiated Nafion Membranes in Non-Aqueous Electrolytes
In a range of electrical energy storage technologies, redox flow batteries (RFBs), which reversibly convert chemical energy to electrical energy, have shown a favorable balance of cost, safety, and performance for stationary applications. Shifting from aqueous to non-aqueous electrolytes may promise a higher cell voltage due to the extended electrochemically stable window (~ 4 V) and an enriched selection of redox materials due to the broader variety of organic solvents . As a key component in RFBs, an ideal membrane should have negligible electronic conductivity, high ionic conductivity, good chemical stability and mechanical strength, and low active species crossover. However, to date, no non-aqueous membrane has been identified that simultaneously fulfills these requirements, especially high ionic conductivity and low crossover rate.
To meet aggressive established cost targets ($120/kWh), our recent work suggests the maximum area-specific resistance for non-aqueous RFBs is 2.3 Ω ∙ cm2, which corresponds to the ionic conductivity of 1.1 mS/cm and 7.7 mS/cm for a membrane with a thickness of 25.4 µm (such as Nafion® 211) and 177.8 µm (such as Nafion® 117), respectively . However, there is little data on the effectiveness of successful aqueous membranes under non-aqueous conditions. To this end, we explore the performance of lithiated Nafion 117 (Li N117) in non-aqueous electrolytes with a focus on the relationships between ionic conductivity, species selectivity, and membrane microstructure. Key membrane properties and performance metrics are quantified in electrolytes consisting of different salt and solvent combinations. Species crossover is studied using model compounds with positive, neutral, and negative charge. Membrane microstructure is probed using Small Angle X-ray Scattering and solvent permeability measurements. Results indicate a critical relationship between pore size, ionic conductivity and species selectivity and provide guidance for future research directions in non-aqueous separations.
This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.
. R. Darling, K. Gallagher, J. Kowalski, S. Ha and F. Brushett. Energy & Environmental Science, vol. 7, no. 11, pp. 3459-3477, 2014.
. R. Darling, K. Gallagher, W. Xie, L. Su and F. Brushett. submitted manuscript