Constructing Integrated Dual‐Skin‐Layer Porous Membranes via a Bottom‐Up Phase Inversion Relay Strategy for Flow Batteries

ABSTRACT A bottom‐up relay strategy is first proposed to fabricate integrated dual‐skin‐layer porous membranes (macropore layer between bilateral skin layers) by ingeniously introducing a non‐solvent carrier. Benefiting from the phase inversion relay process, bilateral skin layers can be tailored freely, where ethylene glycol in the non‐solvent carrier effectively regulates the structure of the bottom skin layer by adjusting the ratio of free water to bound water. Thus, a dual‐skin‐layer membrane is designed and applied in a vanadium flow battery (VFB), that features symmetric skin layers (∼600 nm) with sub‐nanometer (∼4.5 Å) pores. The symmetric structure reduces the pressure required for proton migration via the Vehicle mechanism, and the sub‐nanometer pores effectively separate vanadium ions from protons by size exclusion, breaking the trade‐off in traditional membranes. Moreover, the bilateral skin layers protect the macropore layer against the vanadium fouling and mechanical damage. Furthermore, the formation mechanism and ion transport dynamics of the dual‐skin‐layer membrane are analyzed by molecular dynamics and multi‐physical field simulation. As a result, the optimized membrane enables stable VFB operation for over 6000 cycles at 200 mA cm ^{− 2} with energy efficiency of ∼83.6%. This work provides an effective approach to prepare integrated symmetric membranes with bilateral skin layers.