Intermittent energy sources, including solar and wind, require scalable, low-cost, multi-hour
energy storage solutions to be effectively incorporated into the grid.
Redox-flow batteries offer a solution, but suffer from rapid capacity fade and low Coulombic efficiency due to the high permeability of
redox-active species across the battery's membrane. Here we show that active-species
crossover can be arrested by scaling the membrane's pore size to
molecular dimensions and in turn increasing the size of the active material to be above the membrane's pore-size exclusion limit. When oligomeric
redox-active
organic molecules were paired with microporous
polymer membranes, the rate of active-material
crossover was either completely blocked or slowed more than 9,000-fold compared to traditional separators at minimal cost to
ionic conductivity. In the case of the latter, this corresponds to an absolute rate of ROM
crossover of less than 3 μmol cm−2 day−1 (for a 1.0 M
concentration gradient), which exceeds performance targets recently set forth by the battery industry. This strategy was generalizable to both high and low-potential ROMs in a variety of electrolytes, highlighting the importance of macromolecular design in implementing next-generation
redox-flow batteries.