Nonsolvent induced phase separation (NIPS) is a classical method for preparing porous polymeric membranes, which has been widely applied in industrial membrane manufacturing for more than six decades. However, because multiple pore structures form simultaneously during the NIPS process, the underlying microstructure formation mechanism remains incompletely understood. This knowledge gap has hindered the rational design of membrane structures and the precise regulation of membrane performance.
In a recent study published in National Science Review, a research team led by Prof. LI Xianfeng from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) systematically elucidated the microstructure formation mechanism of NIPS by decoupling the formation of different pores structures, including macrovoids and cellular pores. Based on these findings, the team designed a free-standing ultrathin porous polymeric membrane with a thickness of only 2.7 μm that simultaneously delivers high selectivity and high conductivity. When applied in a vanadium flow battery, the membranes demonstrated outstanding eletrochemical performance.
Decoupled formation of macrovoids and cellular pores and the underlying NIPS mechanism (Image by JIA Chaoyang and LU Wenjing)
"We developed a novel observation cell for NIPS process, which enabled us to decoupled the formation of macrovoids from that of cellular pores by modulating the flow geometry at the nonsolvent–polymer solution interface," said Prof. LI.
Using this experimental setup, the researchers revealed that macrovoid formation originates from hydrodynamic instability and demonstrated that macrovoids growth can be precisely controlled by tuning the interface geometry between the nonsolvent and polymer solution. They further clarified the thermodynamic origin of cellular pores formation and established a quantitative model linking cellular pores area density to key thermodynamic parameters.
By eliminating the mass-transfer interference and spatial heterogeneity introduced by macrovoids, the researchers uncovered the intrinsic relationship between membrane formation kinetics and solvent–nonsolvent interdiffusion during the NIPS process.
The researchers then investigated the structure–property relationship of porous membranes fabricated through NIPS. Guided by this relationship, they developed a free-standing porous membrane only 2.7 μm thick. When applied in a vanadium flow battery, the membrane achieved an energy efficiency exceeding 80% at a current density of 220 mA/cm2.
"Our study represents an important advance in the fundamental understanding of NIPS and provides valuable theoretical guidance for the rational design of porous membranes with precisely tunable structures and properties," said Prof. LI.