Anomalous ultrafast lithium-ion transport through boron nitride nanotube membranes

Here we studied the aqueous transport of different alkali-metal ions in charged boron nitride nanotubes (BNNTs) and compared the results with those obtained in carbon nanotubes, using macroscopic, vertically aligned nanotube membranes at densities up to 10⁷ pores cm^-2. Our study reveals that ion transport in 3- and 12-nm-diameter charged BNNTs is fundamentally different from that in either carbon nanotubes of a similar size, or two-dimensional boron nitride nanochannels. We find two unexpected transport phenomena: ultrafast, cation-selective diffusion that exceeds Fickian diffusion up to 31-fold; and preferentially enhanced transport rates for Li⁺ over other alkali-metal ions (K⁺ and Na⁺) that are opposite to the ordering of their mobilities in bulk solution. We show that the overall fast transport of cations is due to diffusio-osmotic surface transport, while the preferentially enhanced transport of Li⁺ is believed to result from ion-specific interactions with the charged BNNTs. As a result of enhanced and cation-selective transport, the BNNT membranes produced per-pore osmotic-power densities up to 15,300 W m^-2 in a 1 mM:1 M LiCl concentration gradient at pH 11. The energy-conversion efficiency approached the theoretical limit of 50% at pH 5.5. As a demonstration, we power a calculator, watch and light-emitting diode using 1-cm² BNNT membranes in a salinity gradient. The unusual transport phenomena in BNNTs, as well as the flexible and scalable membrane-fabrication process, may enable ion-selective nanotube membranes optimized for lithium recovery, 'blue' osmotic energy and other separation and energy-conversion processes.