arXiv subjects

Alon Herman

Publications and source records attributed to Alon Herman.

5 recordsLinked to original sources

arXiv2025

Bipotentiostatic Control Unlocks Flashing Ratchet Features in Ion Pumps

The selective separation of same-charge ions is a longstanding challenge in resource recovery, battery recycling, and water treatment. Theoretical studies have shown that ratchet-based ion pumps (RBIPs) can separate ions with the same charge and valance by driving them in opposite directions according to their diffusion coefficients. This process relies on frequency dependent current reversal, a unique feature of ratchets in which the particle current direction is inverted with the input signal frequency. Previous experimental demonstrations of RBIPs achieved ion pumping against electrostatic forces and water deionization, but lacked frequency-dependent current reversal and control of the asymmetry of the device. Here, we report the first experimental realization of these key functionalities by driving RBIPs with a bipotentiostat. Complementary input signals applied to RBIP contacts unlock a flashing ratchet-like behavior, and enhances the device performance by an order of magnitude compared to the prior floating-drive approach. The enhanced control of the electrostatic potential at the RBIP surfaces leads to frequency dependent current reversals, and the addition of a potential offset to the input signal enables tuning the amplitude asymmetry of the device. This flashing ratchet functionality provides a significant step towards the realization of ratchet driven selective ion separation systems.

physics.chem-ph↗

arXiv2025

Tuning electrochemical reactions with ratchet-based ion pumps

Electrochemical reactions are highly sensitive to the physical and chemical environment near the electrodes. Thus, controlling the electrolyte ionic composition and the electrochemical potential of specific ions can modify the overpotential of electrochemical reactions and enhance their selectivity toward the desired products. Ratchet-based ion pumps (RBIPs) are membrane-like devices that utilize temporal potential modulation to drive a net ionic flux with no associated electrochemical reactions. RBIPs were fabricated by coating the surfaces of nanoporous alumina wafers with metals, forming nanoporous capacitors. Placing the RBIP between two electrolyte compartments and applying an alternating signal between the metal layers resulted in a voltage buildup across the membrane, leading to ion pumping. Here, we demonstrate that by modifying the electrochemical potential of ions, RBIPs can accelerate or inhibit electrochemical reactions on the surface of adjacent water-splitting electrodes according to the RBIP input signal. Proton pumping towards a water-splitting cathode prevented proton depletion due to the hydrogen evolution reaction and maintained the pH in the cathode compartment. The combination of ion pumping and ion selectivity can enable the electrolyte composition to be tuned near the electrodes, providing greater control over the electrochemical process.

physics.app-ph↗

arXiv2024

Ambipolar Ion Pumping with Ratchet Driven Active Membranes

In recent years there has been significant progress in the development of artificial ion pumping membranes. Ion pumps based on asymmetric nano-pores have been shown to operate as ionic current rectifiers, thus pumping a net ion flux against a concentration gradient even when driven with unbiased ac signals. However, since ion transport relies on charged nano-pores, it is selective to either cations or anions, and thus cannot pump both cations and anions simultaneously. In this paper, we present a model for an electronically active membrane which is based on a flashing ratchet mechanism. The model includes adjacent electrolyte reservoirs and ion-ion interactions, which were not accounted for in prior similar models, and thus provides a better understanding of the driving mechanism and potential capabilities and limitations. It is shown that unlike most other proposed ion pumps, the ratchet-based ion pump (RBIP) drives both cations and anions in the same direction and up a concentration gradient. This process, referred to as ambipolar ion pumping, is shown to be highly robust for many electrolyte compositions and input signals. The membrane is composed of alternating conductive thin layers (electrodes), separated by insulating layers in an asymmetric design. With insulating layers thickness of 70 and 30 nm and an input signal amplitude of 0.5 V, the device drives a salt flux of 0.03 mol/m^2 s in a mildly saline solution (10 mM). Thus, RBIPs may pave the way for many exciting future applications involving ambipolar ion pumping, most notably for desalination.

physics.app-ph↗

arXiv2023

A nanoporous capacitive electrochemical ratchet for continuous ion separations

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.

physics.app-ph↗

arXiv2022

Ratchet based ion pumps for selective ion separations

The development of a selective, membrane-based ion separation technology may prove useful in a wide range of applications such as water treatment, battery recycling, ion specific chemical sensors, extraction of valuable metals from sea water, and bio-medical devices. In this work we show that a flashing ratchet mechanism can be used for high precision ion separation. The suggested ratchet-based ion pumps utilize a unique feature of ratchets, the frequency dependent current reversals, to drive ions with the same charge but different diffusion coefficients in opposite directions. We show that for the prevalent ions in water, ions with a relative diffusion coefficient difference as small as 1% can be separated by driving them in opposite directions with a velocity difference as high as 1.2 mm/s. Since the pumping properties of the ratchet are determined by an electric input signal, the proposed ion pumps can pave the way for simple large-scale, fit-to-purpose selective ion separation systems.

physics.app-ph↗