  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 fabricated well-controlled 'along-the-capillary'
membranes of Na-vermiculite with a capillary size of ~5 Angstrom. They exhibit
1600 times enhanced conductivity compared to commonly studied
'across-the-capillary' membranes. Interestingly, they show a very high cation
selectivity of 0.83 for NaCl solutions, which resulted in large power densities
of 9.6 W.m^-2 and 12.2 W.m^-2 at concentration gradients of 50 and 1000,
respectively, at 296 K, for an unusually large membrane length of 100 micron.
The power density shows an exponential increase with temperature, reaching 65.1
W.m-2 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 new possibilities for exploiting such membranes for
energy harvesting applications.
