Transport of Uncharged and Oxidized Redox Molecules in a Cation Exchange Membrane Solvated with a Nonaqueous Electrolyte

High Resolution Image Download MS PowerPoint Slide Nonaqueous redox flow batteries (RFBs) are a promising technology to meet the growing need for grid-scale energy storage. RFBs rely on a membrane separator to facilitate desired ion transport while restricting or preventing soluble electroactive redox molecule transport, and engineering such a membrane is essential for efficient RFB operation. The impact of oxidation of redox molecules, which changes the redox molecule from uncharged to net positively charged, on membrane selectivity during nonaqueous RFB operation has not been extensively studied. Upon oxidation, the positively charged redox molecule can interact differently with the cation exchange membrane (CEM) separator and potentially compete with desired transport of electrolyte cations, thereby compromising selectivity. To better understand uncharged and charged redox molecule transport in a sulfonated CEM, the permeability of several redox molecules (both uncharged and positively charged) was evaluated using a nonaqueous electrolyte surrounding a CEM material previously reported as promising for nonaqueous electrochemical applications. Redox molecule derivatives were chosen to investigate the influence of electron withdrawing and donating functional groups, which have been used to engineer redox molecule solubility in electrolytes, on redox molecule permeability in the membrane. For uncharged redox molecules, electron donating groups reduced the polarity of the redox molecule and led to a decrease in redox molecule permeability. Electron withdrawing functional groups had the opposite effect. Upon electrochemical oxidation, the permeability of the positively charged redox molecules generally decreased relative to that of the uncharged state. As such, electron donating groups may reduce permeation of a given redox molecule in CEMs, and oxidation of redox molecules to form positively charged species may reduce permeability relative to that of uncharged redox molecules in nonaqueous solvated membrane systems. For such situations, uncharged redox molecule permeability values may be sufficient for investigating the worst-case scenario for redox molecule crossover in RFBs.