A nanoporous capacitive electrochemical ratchet for continuous ion separations

Abstract

Directed ion transport in liquid electrolyte solutions underlies many phenomena in Nature and industry. While Nature has devised structures that drive continuous ion flow without Faradaic redox reactions, artificial analogs do not exist. Here we report the first demonstration of an ion pump that drives aqueous ions against a force using a capacitive ratchet mechanism that does not require redox reactions. Modulation of an electric potential between thin metallic layers on either face of a nanoporous alumina wafer immersed in solution resulted in persistent voltages and ionic currents. This occurs due to the non-linear capacitive nature of electric double layers, whose repeated charging and discharging sustains a continuous ion flux. Ratchet driven electrodialysis was demonstrated reaching a 50% decrease in the conductivity of the solution in a dilution cell. These ratchet-based ion pumps can enable continuous desalination and selective ion separation using an electrically powered device with no moving parts.