A pH gradient induced rectification inversion in asymmetric nanochannels leads to remarkably improved osmotic power

Biological channel membranes regulate direction and degree of ion transport in response to external stimuli, yet it is a challenge to develop artificial nanofluidics with such functions without any chemical modification of the channel walls. Herein, we report the asymmetric branch-type alumina nanochannel (BAN) membrane, that comprises large-pore stem channels and small-pore branched channels and exhibits reversed and significantly enhanced ionic rectification when subjected to a pH gradient. The anomalous transport properties are supported by our rigorous model that takes into account the equilibrium surface reactions of alumina hydroxyl groups on the nanochannel walls with surface protons. The simulation results indicated that the pH gradient-induced bipolar surface charge distributions on the BAN walls lead to the reversal and enhancement of ion transport properties. This work highlights the link between surface chemistry, concentration polarization and ion current rectification, significantly extending the knowledge of how asymmetric pH environments influence transport properties of asymmetric nanofluidics. The application of remarkably amplified osmotic power is also demonstrated, highlighting that the BAN membrane achieves a jump of power density output from -2.1 to -5.9 W/m2 in the presence of a pH gradient, thus exceeding previously reported values.