Membrane-free osmotic power generators with high-performance energy conversion

Abstract

Osmotic energy between two solutions with salinity gradients is sustainable and can be generally converted to electricity through membrane-based processes. However, high membrane resistance limits the maximum output power density to 10 W m⁻² at a large scale. Herein, we developed a membrane-free osmotic power generator (OPG) using immiscible polycation and polyanion aqueous sols. With the stacking of two sols, polyelectrolyte bilayers assemble spontaneously at the interphase, thereby enabling bidirectional cation/anion selective transport. Under a 50-fold NaCl salinity gradient, a single membrane-free OPG unit exhibited optimal output power densities of 39.4 (Na⁺ gradients) or 74.9 (Cl⁻ gradients) W m⁻² due to its low internal resistance; these values were far higher than those reported in other works (<10 W m⁻²) under similar conditions. Moreover, these polyelectrolyte bilayers possessed self-healing ability and maintained a stable output current after puncturing. Integrated OPGs were connected in series by alternating liquid–liquid stacking, thereby delivering a desirable voltage output (i.e., ∼0.035 V pair⁻¹). As a proof-of-concept, integrated OPGs could be packed as a flexible power supply for other electronics. This work proposes a new approach to design high-performance OPGs for practical applications.