Robust and High performance hollow fiber membranes for energy harvesting from salinity gradients by pressure retarded osmosis

• Robust and high performance PRO hollow fiber membranes have been fabricated.
• A power density of 20.9 W/m2 was achieved using synthetic seawater brine as draw and river water as feed.
• The simultaneous specific reverse salt flux was found to be 0.03 mol/L at 15 bar operating pressure.
• Water permeability and S value varied noticeably corresponding to the change in imposed pressure.
• Over a certain range, such variations proved to have positive impacts on membrane performance.

Pressure retarded osmosis (PRO) is a promising membrane process to harvest the vast amount of free energy from the mixing of fresh and salty waters without adverse environmental impacts. A competent membrane for PRO should possess the features of high water permeability to allow water permeation, excellent salt rejection to maintain the osmotic driving force, low structure parameter to minimize the internal concentration polarization (ICP), and robust strength to withstand the required hydraulic pressure. In this work, we have developed a novel hollow fiber membrane by balancing these competing factors based on our prior experience in the thin-film composite (TFC) membranes. The newly developed TFC hollow fiber membrane can achieve a power density of 20.9 W/m2 at a pressure of 15 bar, using synthetic seawater brine (1.0 M NaCl) as the draw solution and synthetic river water (1 mM NaCl) as the feed water, respectively. The simultaneous specific reverse salt flux was found to be 0.03 mol/L, which is much lower than reported flat-sheet membranes, as the self-supported hollow fiber membrane can eliminate the deformation-enhanced reverse salt diffusion that is typical for flat-sheet membranes. However it was observed that the water permeability and structure parameter of the TFC hollow fiber membrane varied noticeably corresponding to the change in the hydraulic pressure imposed in the fiber lumen. Over a certain range (ΔP<15 bar), such variations of increasing A value and decreasing S value proved to have positive impacts, as they facilitate the water permeation and reduce the ICP in the PRO process.