Investigation of symmetric and non-symmetric cell designs for redox flow batteries utilizing indigo carmine as anolyte

Abstract

Aqueous redox flow batteries (RFB) based on all-organic and organometallic compounds are promising systems for energy storage from intermittent renewable energy sources. Here we report a water based RFB using indigo carmine (IC), a water-soluble organic material, as anolyte in an all-organic and organometallic RFB. IC was paired with 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt monohydrate (BQDS) and potassium ferrocyanide (K₄[Fe(CN)₆]·3H₂O) in sulfuric acid aqueous solution (1 M) and sodium hydroxide aqueous solution (1 M), respectively. The impact of varying the concentration of IC and the active area dimensions of the electrochemical cell (4 cm² and 16 cm²) on the performance of both RFBs was investigated. The all-organic IC/BQDS 4 cm²-RFB showed an increase in storage capacity from 22.3 mWh/L to 72.5 mWh/L with an increase in IC concentration from 5 mM to 10 mM. This was accompanied by a significant increase in capacity retention from 72 % to 97 %. For the organometallic IC/K₄[Fe(CN)₆]·3H₂O] 4 cm²-RFB, the storage capacity increases (23.5 mWh/L vs. 49.2 mWh/L) and almost no changes were observed in capacity retention (27 % vs. 21 %) with increasing concentration. However, the capacity retention was significantly lower compared to the purely organic RFB (21 % vs. 72 %). Increasing the active area of the electrochemical cell from 4 cm² to 16 cm² positively influenced the performance of all-organic RFBs. This was particularly evident in the increased average discharge energy density and storage capacity. Symmetrical IC-RFBs were tested with a balanced and over-balanced cell configuration. The formation of isatin-5-sulphonic acid sodium salt by cleavage of the CC double bond causes a decrease in Coulombic efficiency and capacity fade rate. This study highlights the potential of IC as anolyte, the effect of the active area size of the electrochemical cell on the performance of all-organic redox flow battery systems, and the need to fine-tune the chemical structure of IC for long-term and large-scale applications.