Advances in membrane technology for the NASA redox energy storage system

Anion exchange membranes used in the NASA-Redox Energy Storage System based on the soluble iron and chromium chloride redox couples, have been under development by Ionics, Inc. for the past five years. The membrane serves as a charge transferring medium as well as a reactant separator and is the key enabling component in this storage technology. Each membrane formulation undergoes a series of screening tests for area-resistivity, static (non-flow) diffusion rate determination, and performance in Redox systems. The CDIL series of membranes (a copolymer of vinylbenzyl chloride and dimethylaminoethyl methacrylate) has, by virtue of its chemical stability and high ion exchange capacity, demonstrated superior properties in the Redox environment. Recent improvements made in the area of ion selectivity and resistivity within the last year have resulted from minor variations in the membrane formulations. Results of these screening tests are as follows: area-resistivity = 5.4 ..cap omega.. . cm/sup 2/ when equilibrated in 1.0 M FeCl/sub 3/; static diffusion/selectivity = 20 ..mu..g Fe/sup +3//hr/cm/sup 2/. Additional resistivity results at several acid and iron solution concentrations, iron diffusion rates, and time dependent iron fouling of the various membrane formulations are presented in comparison to past standard formulations.