  Directed ion transport in liquid electrolyte solutions underlies numerous
phenomena in nature and industry including neuronal signaling, photosynthesis
and respiration, electrodialysis for desalination, and recovery of critical
materials. Here, we report the first demonstration of an ion pump that drives
ions in aqueous electrolytes against a force using a capacitive ratchet
mechanism. Our ratchet-based ion pumps utilize the non-linear capacitive nature
of electric double layers for symmetry breaking which drives a net
time-averaged ion flux in response to a time varying input signal. Since the
devices are driven by a non-linear charging and discharging of double layers,
they do not require redox reactions for continual operation. Ratchet-based ion
pumps were fabricated by depositing thin gold layers on the two surfaces of
anodized alumina wafers, forming nanoporous capacitor-like structures. Pumping
occurs when a wafer is placed between two compartments of aqueous electrolyte
and the electric potential across it is modulated. In response to various input
signals, persistent ionic voltages and sustained currents were observed,
consistent with net unidirectional ion transport, even though conduction
through the membrane was non-rectifying. The generated ionic power was used in
conjunction with an additional shunt pathway to demonstrate electrolyte
demixing.
