A 2.2 V Aqueous Redox Flow Battery Enabled by Passive pH Decoupling

Aqueous redox flow batteries (ARFBs) comprising earth-abundant charge carriers could cheaply store grid-scale renewable electricity, but most lab-scale systems have a low open-circuit potential (OCP) that inflates system cost. We report a non-hybrid aqueous flow cell with an OCP of 2.2 V, among the highest reported for this class, pairing an alkaline chromium-1,3-propylenediaminetetraacetate negolyte with an acidic bromide posolyte. The two electrolytes maintain an approximately seven-unit pH gradient across a single cation-exchange membrane, without external oxygen or a bipolar-membrane recovery sub-cell: operando pH measurements and a zero-dimensional model show that the difference is sustained passively by balancing slow proton crossover against hydrogen evolution and positive-side acid generation. The cell cycles for 240 h with a Coulombic efficiency of up to 99.5%; tuning the posolyte additive, electrode, and membrane raises the peak power density to 534 mW cm–2 at 43 °C.