Realized potential as neutral pH flow batteries achieve high power densities

High power density operation of redox flow batteries (RFBs) is essential for lowering system costs, but until now, only acid-based chemistries have achieved such performance, primarily due to rapid membrane proton (H+) transport. Here, we report a neutral pH RFB using the highly reducing Cr-(1,3-propylenediaminetetraacetate) (CrPDTA) complex that achieves acid-like power performance while utilizing potassium ion (K+) transport. We investigate RFB resistance components and demonstrate the high and consistent K+ conductivity of the Fumasep E-620(K) membrane. When combined with a robust bismuth electrocatalyst, this membrane enables constant voltage efficiency operation of a CrPDTA|Fe(CN)6 RFB for 200 cycles. An optimized CrPDTA|Fe(CN)6 RFB, which combines a high cell potential with a low area-specific resistance (0.46 Ω cm2), demonstrates a maximum discharge power density of 1.63 W cm−2 and an average discharge power density over 500 mW cm−2 while maintaining 80% round-trip energy efficiency cycling, which are records for non-acid-based RFBs.