Cation exchange membranes based on the Donnan exclusion effect for aqueous organic redox flow batteries

Abstract

Ion exchange membranes are a key parameter determining the energy efficiency of redox flow battery (RFB) systems as they facilitate the rapid transport of charge carrier ions while preventing the cross-diffusion of redox-active species. Cation exchange membranes (CEMs) possess higher chemical stability, more established fabrication processes, and lower manufacturing costs compared to anion exchange membranes (AEMs). However, a significant challenge arises with CEMs, such as Nafion, due to the Donnan effect, inhibiting compatibility with positively charged molecules. Here we use a simple strategy for modifying CEMs, utilising the Donnan effect produced by quaternary ammonium salt molecules. The modified CEMs enable performance comparable to that of AEMs, allowing for broader application in systems with cationic electrolytes. Using N¹-ferrocenylmethyl-N¹,N¹,N²,N²,N²-pentamethyl-1,2-propanediamine dichloride (Fc-N₂) as the catholyte, in conjunction with bis(3-trimethylammonio)propyl viologen tetrachloride (N₂-Vi) as the anolyte, an impressive coulombic efficiency (CE) of approximately 99.7% was achieved at a current density of 40 mA cm⁻², along with an energy efficiency (EE) of nearly 71.1%, which is comparable to the 75.75% observed with AEMs. This strategy demonstrates broad applicability across various positively charged reactive substances, confirming its universality and potential for improving RFB performance.