Resonant osmotic diodes for voltage-induced water filtration across composite membranes

Nanofluidics have led to the discovery of unconventional properties for water and ion transport at the nanoscale, but key challenges remain in their large-scale implementation. Here we report an osmotic resonance across macroscopic composite membranes made by the assembly of microporous and mesoporous layers, taking root from the rectified osmotic transport in nanopores. This osmotic diode induces ionic sieving and continuous fast macroscopic electro-osmotic transport. This is the basis for a versatile approach for water purification, by which fresh water is driven across a composite material under an a.c. electric field. Water flow is driven within the mesoporous layer, while selectivity is achieved within the microporous layer. The maximal rectified, diode-like water flow is found to be in the hertz range. Building on analytical predictions, we show that a conversion factor of up to ~15 equivalent bars per applied volt can be reached using appropriate materials.