Single symmetric nanopores in ultrathin crystalline ferroelectric BiFeO₃ as polarization-switchable ionic diodes

Artificial ionic diodes, inspired by biological ion channels, are typically realized by creating asymmetries in pore geometry or surface charge. Ferroelectric polarization, arising from the spontaneous alignment of electric dipoles within materials, provides an alternative strategy to achieve directional ion transport without additional geometry modification or surface functionalization. This work demonstrates that ultrathin freestanding ferroelectric crystalline bismuth ferrite symmetric-type single-nanopore membranes, with a thickness down to 30 nm, exhibit rectified ion currents driven by naturally spontaneous polarization. These nanopores exhibit reversible polarization switching with hysteretic ion current rectification in response to external pH stimuli, offering additional tunability in emulating the functions of biological ion channels. Furthermore, significant osmotic power generation is observed, enabled by current amplification through directional ion transport. These findings highlight the potential of utilizing ultrathin ferroelectric oxide membrane materials as ionic diodes with directional preferences and switchable polarizations for developing ionic circuits, mimicking biological membranes, and facilitating osmotic power conversion.