Asymmetric Doping of a Polyelectrolyte Network Into a Tough Slide-Ring Hydrogel Membrane to Enhance Sustainable...

Asymmetric ion-selective membranes are beneficial for harvesting osmotic energy from salinity gradients such as the seawater-freshwater interface by reverse electrodialysis (RED). Among the various RED membrane technologies, gradient hydrogel membranes can exhibit exceptional performance due to structural features that facilitate ion-selective transport, but often suffer from poor mechanical properties and/or high internal resistance. Here, a high-entanglement slide-ring asymmetric polyelectrolyte double-network (SRAP-DN) hydrogel membrane has been prepared by unilateral photopolymerization for osmotic energy harvesting by RED. The membrane can achieve high output power densities >10 Wm^-2 under the neutral 50-fold salinity gradient, which can be boosted to >16 Wm^-2 at pH 12 with a 200-fold KCl gradient. Unlike most hydrogel membranes, SRAP-DN is remarkably tough yet extremely high in water content (93%), which lowers internal resistance while benefiting cost and scalability by minimizing polymer content. SRAP-DN was leveraged to prototype miniature and flexible thin-film power supplies made of 24 cells connected in series that can harvest osmotic energy from natural seawater and river water to produce >2 V of stable potential. The excellent performance of these tough, water-rich membranes in blue energy harvesting bodes well for the prospect of low-cost, eco-, and bio-friendly lightweight power supplies for wearable, implantable, or clean energy technologies.