Episode 8: Electrochemical device driven with a capacitive ratchet mechanism

In this podcast episode, MRS Bulletin’s Laura Leay interviews Gideon Segev from Tel Aviv University in Israel and Lawrence Berkeley National Laboratory and Shane Ardo from the University of California, Irvine about their ratchet-based ion pumps (RBIPs). Consisting of a nanoporous capacitor-like structure, the RBIP drives a flux of charged particles at voltages as low as 50 mV, while redox reactions need at least 1.23 V. Furthermore, the ratchet is selective where ions can be sorted based on their diffusion coefficient. This opens doors for efficient devices for desalination and selective ion separation. This work was published in a recent issue of Nature Materials. 

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay. As science uncovers more about materials, it can become harder to do something brand new. One avenue is to transfer concepts from one area of science to another, with some surprising results.

GIDEON SEGEV: We’d been talking about how to transfer concepts from semiconductor devices to electrochemical systems. The thought is to just take tricks people do with semiconductors, but instead of conducting electrons we’re going to do the same stuff with ions.

LAURA LEAY: That was Professor Gideon Segev from Tel Aviv University in Israel and Lawrence Berkeley National Lab. The concept that Gideon’s team developed looked a lot like a ratchet – a nonequilibrium device that utilizes temporally modulated input signals and spatial asymmetries to drive a steady state particle flux. To drive a flux of charged particles, a structure resembling a capacitor was developed. The capacitive ratchet mechanism has advantages over utilizing redox reactions, including a lower voltage: where redox reactions need at least 1.23 V, the ratchet can work using voltages as low as 50 mV. Importantly, the ratchet is also selective – ions can be sorted based on their diffusion coefficient by selecting the appropriate frequency for the input signal. This opens doors for efficient devices for desalination and selective ion separation. As Professor Shane Ardo from the University of California puts it:

SHANE ARDO: If the goal is to move ions from one side of the device to another, you wouldn’t draw a chemical reaction – necessarily – that involves doing a redox reaction. So it’s advantageous if you can find a mechanism by which you didn’t have to use the redox reaction because that’s not necessary for the thing you’re looking to do. 

LAURA LEAY: The most recent work demonstrates the ratchet-based ion pump experimentally using off-the-shelf nanoporous anodized aluminum oxide wafers which had the planar surfaces of the wafers coated with a thin layer of metal. 

GIDEON SEGEV: Essentially the device is a membrane that has two metal layers – thin films – on the two sides. And so we put it between two compartments of electrolyte and basically applied different signals between the two surfaces, so basically applying different square waves across. And what is does is the square wave induces what we call a capacitive ratchet mechanism that drives ions from one compartment to another. In the paper one of the things we did is we used this driving mechanism inside an electrodialysis-like setup in which we also have an anion exchange membrane – which transports only anions – and a cation exchange membrane which transports only cations, and we use this asymmetry to desalinate water. 

LAURA LEAY: The device is based on a previously published theory. Further modeling work to describe the device led to new insight and uncovered some counterintuitive behavior.

SHANE ARDO: You’re modeling things and simulating, and something came out of it that was unexpected that presented itself as almost a different architecture than one would typically design for an electrochemical device. In electrochemistry you need two pathways for the current to flow; you need one pathway where the electrons flow in the external circuit and then the ions are flowing to match that current so you have a net cyclical pathway for the current. What Gideon’s model had shown was that you didn’t need necessarily a second pathway; you could have two different ions moving through the same pathway and that would be able to constitute this kind of net two pathways of current. It’s called an ambipolar transport. And the ambipolar transport could mean the ratchet could be pumping, for example, a sodium ion one direction and because of the oscillatory nature of the input voltage you could kind of walk the chloride with it.

LAURA LEAY: The modeling suggests instead of using two membranes to transport different ions, only one membrane may be required which simplifies the electrodialysis setup. Although this proof-of-concept has used an off-the-shelf component to demonstrate the ratchet mechanism, future work will focus on designing a new materials system from the bottom up, based on the modeling work. Like many breakthroughs, demonstrating this ratchet mechanism is the result of collaboration between people with different expertise who bring different schools of thought to tackle a challenge. This work was published in a recent issue of Nature Materials. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.