Anomalous Diffusive and Electric-field-driven Ion Transport in Boron-Nitride Nanotubes

We report on anomalous transport of ions in solution through macroscopic arrays of vertically aligned, surface-charged boron-nitride nanotubes (BNNTs). The 3- and 12nm-diameter BNNTs revealed two atypical transport phenomena: (i) ultra-fast cation-selective diffusion under concentration gradients that exceeded Fickian diffusion by up to 31-fold and (ii) anomalous relative diffusion rates and ionic conductances for alkali-metal ions (K+, Na+, and Li+) that were opposite to the ordering of their bulk mobilities in solution, and also different from that observed in carbon nanotubes and 2D boron-nitride (BN) nanochannels. The enhanced diffusive transport is shown to result from diffusio-osmosis at the charged BN walls, with the enhancement scaling inversely with pore diameter. The anomalous relative diffusivities for different cations is seen to arise from ion-specific interactions with the charged surface of the BNNTs. These unusual transport phenomena, as well as the flexible and scalable membrane-fabrication process, may enable ion-selective membranes optimized for “blue” energy conversion, lithium recovery, and other molecular separations. As an example, macroscopic arrays of aligned BNNTs in a salt-concentration gradient produced per-pore osmotic-power densities up 15,300 W/m2, with energy-conversion efficiencies approaching the theoretical maximum of 50%.