Extending the theory of classical nonsolvent-induced phase separation to regulate membrane pores

Nonsolvent-induced phase separation, employed for over 60 years to prepare porous membranes, still has unclear pore formation mechanisms due to coupled variables. Classical theory links the distinct pore morphologies, i.e. macrovoids or cellular pores, to instantaneous and delayed phase separation, respectively. However, when the formations of macrovoids and cellular pores were decoupled in a tunable device that regulates the nonsolvent hydrodynamics, it was proven that hydrodynamic instability drives macrovoid formation, while cellular pores form via a nucleation-growth mechanism. By establishing a quantitative relationship between nonsolvent and area density of cellular pores, we achieved further optimization of the membrane morphology, enabling its application in vanadium flow batteries with significantly enhanced performance. This work extends the theory of phase separation and provides a causality-driven framework for precision membrane design.