Utilization of novel alginate membranes developed for quinone based aqueous redox flow batteries

Alginate (Alg)-based mambranes are explored for use in aqueous redox flow batteries (ARFBs). Alg is a cheap natural material of low toxicity. To improve the properties of pristine Alg membranes, they are ionically crosslinked with calcium chloride (CaCl₂) or alternatively treated with organic solvents. While the effect of CaCl₂ is negligible, membranes treated with tetrahydrofuran (THF) or chloroform (THF-Alg and Chlor-Alg) show higher tensile strength and Young's modulus than Alg and Ca-Alg membranes. All four membranes are tested in ARFBs including TironA and 2,7-anthraquinone disulfonic acid as redox couple. A significant increase in voltage efficiency (VE) is observed with the organic solvent-treated membranes (70.9, 70.6, 73.0 and 74.3 % for Alg, Ca-Alg, Chlor-Alg and THF-Alg membranes), while VE of ARFB including Nafion 117 is 73.0 %, meaning that ARFB including THF-Alg shows a better VE. This is partly due to the area specific resistance (ASR). More specifically, when ASR of 70 μm thick THF-Alg and 200 μm thick Nafion 117 membranes submerged in 2 M sulfuric acid is measured, they show 76.7 and 100.6 mΩ cm². This difference in ASR promotes the rapid increase in VE of ARFB including THF-Alg membrane.