image: CP formation is a typical result of the nucleation-driven phase separation. Hydrodynamic instability causes macrovoid formation. The coupling of CPs and macrovoids forms the resultant porous membrane microstructures.
Credit: ©Science China Press
For decades, a notable phenomenon exists in materials science: many well-established classic techniques boast mature processes and widespread applications, yet lack a complete and rigorous fundamental theoretical framework. Porous polymeric separation membranes are typical functional materials of this kind, which act as irreplaceable core components in water treatment, large-scale energy storage, material separation and many other key fields.
Nonsolvent induced phase separation (NIPS) is the dominant classic technology for fabricating such porous membranes. With a history of over 60 years, this fabrication method stands out for its simple operation, broad material compatibility and scalable production capacity, and has long been recognized as an efficient route to prepare porous membranes. Nevertheless, the detailed pore formation mechanism of NIPS has remained controversial without a unified and comprehensive scientific interpretation. This theoretical gap has greatly hindered the rational design and performance optimization of high-performance membranes, and restricted the large-scale deployment of membrane materials in high-end energy storage and precision separation scenarios. A mature fabrication technology paired with an ambiguous underlying mechanism has become a long-standing scientific puzzle in the field of porous membrane research. Hence, researchers have long sought to unravel the core mysteries of NIPS pore formation, build a sound theoretical system, and realize the transformation of membrane fabrication from empirical trial-and-error to precise rational design.
Recently, the research team led by Prof. Xianfeng Li at Dalian Institute of Chemical Physics, Chinese Academy of Sciences delivers brand-new solutions to renovate the classic NIPS theory. Breaking the limitations of conventional empirical experiments, the team advanced both experimental devices and theoretical interpretations to achieve a systematic breakthrough in understanding NIPS pore formation mechanisms. The researchers optimized the traditional Hele–Shaw fluid observation cell and precisely regulated the interfacial contact state between nonsolvent and polymer solution. For the first time, they fully decoupled the formation processes of two major pore structures—macrovoids and cellular pores, overcoming the long-term research bottleneck caused by overlapping formations of different pore.
Core Experiments and Mechanistic Discoveries
Combining in-suit optical microscopy observation, fluorescence imaging characterization and hydrodynamic theoretical analysis, the team clarified the distinct formation rules of the two pore structures. The study confirms that macrovoids and cellular pores follow completely different evolutionary pathways. Contrary to the traditional view that macrovoid formation is governed by phase separation rate, these large pores originate from viscous fingering driven by hydrodynamic instability. Their size and morphology are mainly modulated by fluid parameters such as flow velocity and interfacial tension. In contrast, ubiquitous cellular pores are formed via the classic nucleation-growth mechanism driven by Fick diffusion between solvent and nonsolvent. Furthermore, the team proposed an innovative thermodynamic parameter EV, which integrates two key factors: molecular interaction energy and molecular volume. This new parameter enables accurate prediction of the areal density of cellular pores, with prediction accuracy far exceeding that of conventional theoretical models, providing a quantitative guideline for precise regulation of membrane pore structures. The updated mechanistic framework fully clarifies the core principles of the NIPS fabrication process.
Practical Application and Research Outcomes
Guided by the newly established pore formation mechanisms, the team designed a series of ultrathin porous membranes, and systematically verified the intrinsic correlation between microstructure and practical performance. Among them, the membrane (PBInHex) prepared with n-hexane as the nonsolvent has an ultra-low thickness of merely 2.7 μm. Its well-controlled internal angstrom-scale free-volume size enables highly efficient and precise ion sieving.
To validate the practical application potential of the membrane, the researchers assembled a vanadium flow battery with PBInHex. At a high current density of 220 mA cm–², the assembled battery achieves an energy efficiency of 80.6%, and maintains stable operation for more than 500 charge-discharge cycles. The performance outperforms the mainstream commercial Nafion 212 membrane, demonstrating great potential for industrialization.
This study significantly complements and extends the decades-old classic NIPS phase separation theory, filling the long-standing theoretical gap in membrane pore formation mechanisms. The integrated mechanism-structure-performance design framework established in this work provides solid theoretical support for the precise control of fabrication processes and directional performance optimization of membrane materials.