Charge-Gradient Sulfonated Poly(ether ether ketone) Membrane with Enhanced Ion Selectivity for Osmotic Energy Conversion

Engineered asymmetric heterogeneous ion-selective membranes have become a focal point for their improved efficiency in harnessing osmotic energy from ionic solutions with varying salinity. However, achieving both energy conversion efficiency and excellent chemical stability necessitates effectively mitigating the formation of detrimental interface cracks between two different layers. We develop a charge-gradient sulfonated poly(ether ether ketone) (SPEEK) membrane (CG-SPEEK) on a large-scale using a straightforward coating method. As an osmotic energy generator, CG-SPEEK membrane achieves an impressive output power density of 9.2 W m^-2 and exhibits ultrahigh cation selectivity (0.99), with an energy conversion efficiency of 48% at a 50-fold NaCl concentration gradient. The results highlight the ion diode effects of CG-SPEEK, driven by a charge density gradient that accelerates cation transport while suppressing ion concentration polarization. Density functional theory simulations provide further insights, revealing that the energy barrier for Na⁺ ion transport through CG-SPEEK membrane is lower than that through a homogeneous SPEEK membrane. This work not only enhances our understanding of ion transport dynamics but also establishes the CG-SPEEK membrane as a promising candidate for efficient osmotic energy conversion applications.