Bipotentiostatic Control Unlocks Flashing Ratchet Features in Ion Pumps

TL;DR

The paper tackles the challenge of separating ions with identical charge using ratchet-based ion pumps (RBIPs). It demonstrates a bipotentiostat-driven RBIP that imposes complementary electrical signals to realize flashing ratchet behavior. Key contributions include frequency-dependent current reversals, tunable amplitude asymmetry via contact offsets, and up to a 1000% performance gain over the prior floating-drive RBIP, enabling more effective ion pumping. This work advances ratchet-driven selective ion separation toward real-time, tunable control with potential applications in water treatment, resource recovery, and battery recycling.

Abstract

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.

Figures (14)

• Figure 1: A schematic illustration of the experimental system: (a) $V_{out}$ measurement configuration. (b) $I_{out}$ measurement configuration
• Figure 2: (a) An example for complementary $V_{in,f}(t)$ and $V_{in,n}(t)$ input signals with $d_{c,f}=0.25$, $d_{c,n}=0.75$, and phase shift $\theta=270^\circ$. (b) The RBIP output voltage as a function of $d_{c,f}$, $d_{c,n}$, and $\theta$, at a frequency of 100 Hz.
• Figure 3: The RBIP output current as a function of the input signal duty cycle, when driven with the bipotentiostat (solid lines) and with a floating drive (dashed lines). Traces show the mean of duplicate measurements; error bars indicate maximum and minimum values.
• Figure 4: The RBIP output voltage (a) and current (b) as a function of the input signal duty cycle and frequency. (c) The output current density and voltage as a function of input signal frequency at $d_c=0.1$. (d) Cyclic voltammograms measured between the Ag/AgCl wires while the RBIP was operated and when it as OFF. During the ON state, a square wave signal was applied with $f = 100$ Hz, $d_c = 0.7$. In OFF1 and OFF2, the device contacts were not connected (open-circuit contacts). In OFF3, DC inputs of $V_{\text{in,f}} = 0.3$ V and $V_{\text{in,n}} = 0.1$ V were applied, corresponding to the time-averaged values of the ON signal. The chronological order of the measurements is as they appear in the legend.
• Figure 5: (a) Output current density as a function of duty cycle for several input signal offsets with $V_{\text{offset,f}} = V_{\text{offset,n}}$. (b) Stopping duty cycle as a function of $V_{\text{offset,n}}$ and several values of $V_{\text{offset,f}}$. The input signal frequency is 7Hz, and $V_{\text{p-p}} = 0.3$V for both figures.