Solvation-Gradient Thin-Film Composite Membranes Decouple Ferricyanide Rejection From Hydroxide Conduction in Alkaline Zinc-Iron Flow Batteries.

The long-standing trade-off between ion selectivity and ionic conductivity in membrane materials stems from the difficulty in rapidly transporting ions through chemically homogeneous channels while simultaneously excluding large ions. Here, we report a solvation-gradient membrane prepared via reaction-diffusion-coupled interfacial polymerization between piperazine and 1,3,5-tris(bromomethyl)benzene. The resulting membrane comprised a dense outer selective layer approximately 35 nm thick (with a transport pore size of approximately 6 Å) and an NH-rich inner layer approximately 400 nm thick. The outer layer imposes a high entry/desolvation barrier on large ions, thereby inhibiting their transport, whereas the NH-rich region supports rapid OH- transport through re-solvation and hydrogen-bond reorganization. This depth-dependent architecture spatially separates ferricyanide rejection from hydroxide conduction. Depth-resolved spectroscopy and molecular dynamics simulations supported a depth-dependent solvation environment and revealed stronger water/OH- interactions in the NH-rich region. Optimization of poly(tertiary amine) (PTA) membranes achieved low Fe(CN)6 3- permeability (8.07 × 10-7 cm2 h-1) and OH- conductivity (41.08 mS cm-1), enabling an alkaline zinc-iron flow battery to achieve an energy efficiency of 89.94% at 80 mA cm-2 and stable cycling for over 800 h. This study suggests depth-dependent solvation regulation as a useful design principle for alleviating the selectivity-conductivity trade-off in electrochemical membranes.