The Performance of All Iron-Based Redox Flow Batteries Enhanced by Carbon Nanotube Catalysts

Carbon nanotubes (CNTs) are applied as catalysts to improve redox reaction of iron and 2,2-bis(hydroxymethyl)-2,2',2''-nitrilotriethanol (Fe(BIS–TRIS)) complex as negolyte of iron-based flow batteries (FBs). Especially, multi-walled CNT (MWCNT) and carboxylic acid-functionalized MWCNT (CACNT) are adopted as the catalysts. Effects of the catalysts are electrochemically analyzed. CACNT is expected to have a better performance than MWCNT due to its abundant hydrophilic functional groups. However, MWCNT is more effective catalyst than CACNT for improving redox reactivity of Fe(BIS–TRIS). This is because carboxylic acid groups of CACNT are deprotonated to form COO⁻ ions in aqueous electrolytes. FB single cell tests show that energy efficiency (EE) is 80.2% without catalyst at 80 mA cm⁻², but the EE increases when catalyst is doped at negative electrode. When MWCNT and CACNT are doped, EE is 84.0 and 83.5%. The lower performance of CACNT is because of electrostatic repulsion occurring between anionic Fe(BIS–TRIS) molecules and COO⁻ ions. Therefore, a larger overpotential is formed in the case of CACNT than MWCNT. This means when electrode is treated by catalyst, interaction of active material and catalyst should be considered.