Continuous Ion Separations Using Non-Faradaic Capacitive AC Ratcheting

Traditional electrochemical separations processes require Faradaic reactions for sustained currents. We discovered that this limitation can be overcome by oscillating the applied potential across an ion-permeable material that has an asymmetric electric potential profile. We demonstrated this phenomenon for the first time using a flashing ratchet consisting of a nanoporous anodized aluminum oxide membrane infiltrated with salt water and containing metallic contacts on either side. When a symmetric +/-300 mV square-wave potential was applied to the metallic contacts at a frequency of ~100 Hz, an open-circuit potential as large as ~50 mV was observed between Ag/AgCl electrodes immersed in the chloride-containing electrolyte and positioned across the membrane. While this open-circuit potential was determined to be a consequence of net ionic polarization, additional electrochemical data were also consistent with transport of neutral salt across the membrane via a proposed ambipolar transport mechanism. In comparison, application of a DC potential bias resulted in non-Faradaic charging, and a near-zero long-time open-circuit potential. Moreover, high ionic strengths and large pore sizes diminished ratcheting behavior, consistent will more complete screening of surface charges in the nanopores. Collectively, this work represents a new paradigm for direct ion pumping and salt separations that requires no Faradaic reactions or additional transport pathway for ions or electrons.