Ion-Selective Transport and Osmotic Power Generation in Boron-Nitride Membranes with Non-overlapping Double Layers

Hexagonal boron nitride (h-BN) membranes, which develop high surface charge in basic solutions, are of interest for various potential applications, including osmotic power generation, highly efficient filtering and separation processes. Here, we investigate ion transport and power generation in macroscopic h-BN nanopore membranes (BN-AAO) fabricated from anodized alumina (AAO) templates. Despite the relatively large pore diameter of 30 nm, the BN-AAO membranes show highly selective transport, with electrophoretic transport rates differing by a factor of ~74.5 between positively and negatively charged fluorescent ions (NDS^(-2) vs Ru(bpy)⁽⁺²⁾ ) at pH=10. The membranes also show high osmotic power densities in the range of 27-99 W/m², based on the open pore area, for a KCl molarity difference across the membrane of C_H/C_L = 1000/1 mM at pH=11. The ion selectivity and osmotic power generation are correlated to each other, and both drop with decreasing surface-charge density at lower pH. However, conductance tests suggest that an improved theoretical model, considering the concentration-dependent surface charge, may be needed to correctly estimate the surface charge of h-BN nanochannels. We conclude by comparing the BN-AAO membranes with similar selective ion-transport systems.