Thin-film composite pressure retarded osmosis membranes for sustainable power generation from salinity gradients

Pressure retarded osmosis has the potential to produce renewable energy from natural salinity gradients. This work presents the fabrication of thin-film composite membranes customized for high performance in pressure retarded osmosis. We also present the development of a theoretical model to predict the water flux in pressure retarded osmosis, from which we can predict the power density that can be achieved by a membrane. The model is the first to incorporate external concentration polarization, a performance limiting phenomenon that becomes significant for high-performance membranes. The fabricated membranes consist of a selective polyamide layer formed by interfacial polymerization on top of a polysulfone support layer made by phase separation. The highly porous support layer (structural parameter S = 349 μm), which minimizes internal concentration polarization, allows the transport properties of the active layer to be customized to enhance PRO performance. It is shown that a hand-cast membrane that balances permeability and selectivity (A = 5.81 L m^-2 h^-1 bar^-1, B = 0.88 L m^-2 h^-1) is projected to achieve the highest potential peak power density of 10.0 W/m² for a river water feed solution and seawater draw solution. The outstanding performance of this membrane is attributed to the high water permeability of the active layer, coupled with a moderate salt permeability and the ability of the support layer to suppress the undesirable accumulation of leaked salt in the porous support. Membranes with greater selectivity (i.e., lower salt permeability, B = 0.16 L m^-2 h^-1) suffered from a lower water permeability (A = 1.74 L m^-2 h^-1 bar^-1) and would yield a lower peak power density of 6.1 W/m², while membranes with a higher permeability and lower selectivity (A = 7.55 L m^-2 h^-1 bar^-1, B = 5.45 L m^-2 h^-1) performed poorly due to severe reverse salt permeation, resulting in a similar projected peak power density of 6.1 W/m².