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

Abstract

ABSTRACT 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 cm 2 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.