Fully Healable, Liquid-free Osmotic Power Generator Using Nanofluidic Membrane of 2D Vanadium Pentoxide Nanosheet

Osmotic power generators or salinity gradient-powered generators rely on the fundamental principle of reverse osmosis, coupled with electrochemical processes, to produce electricity. The power generator consists of two chambers, one containing a high concentration (HC) of ions and the other containing a low concentration (LC) of ions, separated by a permselective membrane (anion selective membrane, AEM or cationic selective membranes, CEM), which allows passage of co-ions and restricts the counter ions. This selective movement of ions through the perm-selective membrane results in electric power generation which is harnessed at the electrodes placed in the chambers. Despite decades of improvement in harnessing energy from salinity gradients, its practical application remains rare. This is primarily due to the lack of high-throughput fabrication techniques for nanopores and the need for expensive techniques. More importantly, the use of liquid electrolytes limits its use in portable devices.<br/>We report an osmotic power generator (OPG) utilizing a restacked membrane of vanadium pentoxide (V₂O₅) nanosheets and ion-infused gelatin hydrogel. V₂O₅ nanosheet membrane is the choice owing to its high cation transport property and easy fabrication process. While gelatin hydrogel is used owing to its exceptional biocompatibility, low cost of synthesis, easy availability, and high-water retention. Moreover, gelatin hydrogel allows for efficient diffusion of ions across its matrix to facilitate osmosis and ensure a steady ionic flow. This gel-based osmotic power generator (GOPG) offers significant advantages over traditional liquid-based OPGs, including enhanced portability, higher power output, and improved spillage safety. The highly cation-selective nanofluidic membrane was fabricated by vacuum-assisted assembly of 2D V₂O₅ nanosheets which was in turn obtained by exfoliation of bulk V₂O₅ crystals with hydrogen peroxide (H₂O₂). To prepare the hydrogel, 1 gram of gelatin was stirred with 10 ml of the aqueous KCl solution of desired molarity at 80 °C for 2 hours. As the gelatin is completely dissolved, 400 µl of the solution is poured on cylindrical molds of diameter 1 cm and depth 0.3 cm to be cooled at 4 °C for 30 minutes for gelation. Gels of two different KCl concentrations (1M and 0.001M) was placed on two individual Ag/AgCl coated PET, which is used as the current collector. For the fabrication of the GOPG, a circular piece of the V₂O₅ membrane was sandwiched between the gels of higher and lower concentrations. As the gels of two different concentrations come in contact with the perm-selective V₂O₅ membrane, the positive ion from the high-concentration gel migrates toward the low concentration and electron flows through the external electrode. This gives rise to a stable open circuit voltage of 0.27 V and a short circuit current of 0.35 milliamperes, accounting for a power density of 0.13 Wm^-2. We have systematically investigated the effects of various ions, concentration gradients, and membrane thicknesses to optimize power output. On comparing with power produced using KCl, NaCl, and LiCl-infused hydrogels, KCl shows the highest power density due to its smaller hydration radius and higher mobility through V₂O₅ nanochannels. Furthermore, it was observed that thinner V₂O₅ membranes give high power density owing to the reduced ionic resistance through the membrane. Remarkably, physical damage to theV₂O₅ membrane can be easily repaired with a drop of water, while cracks in the gelatin gel can be healed by reheating and cooling the gel, making it a fully restorable osmotic power generator. The GOPGs can be connected in series or parallel to increase voltage and current, facilitating practical applications. We demonstrated the capability of the GOPG to power an LED and a humidity meter, highlighting its potential for real-world applications.