Ion-hydration-controlled large osmotic power with arrays of angstrom scale capillaries of vermiculite

In the osmotic power generation field, reaching the industrial benchmark has been challenging because of the need for capillaries close to the sizes of ions and molecules. Here, we fabricate well-controlled “along-the-capillary” membranes of Na-vermiculite with a capillary size of ∼ 5 Å. They exhibit 1,600 times enhanced conductivity compared with commonly studied “across-the-capillary” membranes. Interestingly, they show a very high cation selectivity of 0.83 for NaCl solutions, which results in large power densities of 9.6 W/m2 and 12.2 W/m2 at concentration gradients of 50 and 1,000, respectively, at 296 K, for a large membrane length of 100 μm. The power density shows an exponential increase with temperature, reaching 65.1 W/m2 for a concentration gradient of 50 at 333 K. This markedly differs from the classical behavior and indicates the role of ion (de)hydration in enhancing power density, opening possibilities for exploiting such membranes for energy harvesting applications.