Nanotubes for Osmotic Energy Harvesting

New paradigms for fluid transport are expected to emerge from the confinement of liquids at the nanoscales [1- 3], with potential breakthroughs in ultra-filtration, desalination, and energy conversion [4]. Nevertheless, advancing the fundamental understanding of fluid transport at the smallest scales requires mass and ion dynamics to be ultimately characterized across an individual channel to avoid averaging over many pores. To this end, a major challenge for nanofluidics consists in building distinctive and well-controlled nano-channels, amenable for systematic exploration of their properties. In this work [5] we describe the elaboration and exploitation of a new hierarchical nanofluidic device, made of a unique boron-nitride (BN) nanotube that transpierces an ultrathin membrane and connects two fluid reservoirs. Such a transmembrane geometry allows the versatile exploration of fluidic transport through a single nanotube under diverse forcing, including electric fields, pressure drops, and chemical gradients. Using this device, we discover huge osmotically-induced electric currents generated by salinity gradients, exceeding by two orders of magnitude their pressure-driven counterpart. We show that this result originates from the anomalously high surface charge carried by the BN internal surface in water at large pH, which was independently quantified from conductance measurements. The nano-assembling route using nanostructures as building blocks opens new perspectives to explore fluid, ionic and molecule transport at nanoscales, paving the way towards biomimetic functionalities. Our results furthermore suggests that boron-nitride nanotubes could be advantageously exploited as membranes for osmotic power harvesting under salinity gradients, making it a prime renewable source of energy. [1] W. Sparreboom, et al., Nature Nano, 4, 713-720 (2009). [2] L. Bocquet and E. Charlaix, Chem. Soc. Rev. 39, 1073-1095 (2010). [3] J. C. Rasaih, et al., Annu. Rev. Phys. Chem. 59 713-740 (2008). [4] B. E. Logan and M. Elimelech, Nature, 488 313-319 (2012). [5] A. Siria, et al. Nature, 494, 455-458, (2013).