Ionic Solar Cell Desalinates Water

Artist’s rendition of bipolar-membrane design for ionic electricity generation. (Source: W. White)

Modern solar cells have existed in one form or another for over 60 years. Little atten­tion has been paid, however, to the promise of using light to drive another elec­tricity-generating process – the transport of oppo­sitely charged protons and hydroxides obtained by disso­ciating water molecules. American researchers a report such a design, which has promising appli­cation in producing electricity to turn brackish water drinkable.

The researchers, led by Shane Ardo, an Assistant Professor of Chemistry, Chemical Engi­neering, and Materials Science at the Univer­sity of California, Irvine, report that they have crafted an ionic analog to the elec­tronic pn-junction solar cell, harnessing light to exploit the semi­conductor-like behavior of water and generate ionic elec­tricity. They hope to use such a mechanism to manu­facture a device that would directly desa­linate saltwater upon exposure to sunlight. “There had been other experi­ments dating back to the 1980s that photo­excited materials so as to pass an ionic current through them, and theo­retical studies said that those currents should be able to reach the same levels as their electronic analogs, but none of them worked all that well,” says William White, a graduate student in Ardo’s research group.

In this case, the researchers attained more success by allowing water to permeate through two ion-exchange membranes, one that mostly trans­ported cations like protons and one that mostly trans­ported anions like hydro­xides, functioning as a pair of chemical gates to attain charge sepa­ration. Shining a laser on the system prompted light-sensitive organic dye molecules bound to the membrane to liberate protons, which then trans­ported to the more acidic side of the membrane and produced a measurable ionic current and voltages of over 100 mV in some instances (60 mV on average).

Despite crossing the 100 mV photo­voltage threshold at times, the level of electric current that the double-membrane system can achieve remains its chief limi­tation. The photo­voltage would need to be magnified by more than another factor of two to reach the ~200 mV mark necessary to desa­linate seawater, a target that the researchers are opti­mistic about hitting. “It all comes down to the funda­mental physics of how long the charge-carriers persist before recom­bining to form water,” Ardo says. “Knowing the proper­ties of water, we are able to more intelli­gently design one of these bipolar-membrane inter­faces so that we can maximize the voltage and the current.”

In the long run, desali­nation is just one possible appli­cation of the synthetic light-driven proton pump developed by the researchers. It could also have potential for inter­facing with electronic devices, or even for powering signaling in brain-machine inter­faces that combine living tissue and arti­ficial circuity, a role that cannot be filled by tradi­tional solar cells, which are unstable in biolo­gical systems. “We have had a lot of ideas about what this tech­nology could be used for; it’s just a question of learning enough to cross between fields and make the device work for those intended appli­cations,” says Ardo. “I think this is just another example of what you can do when you have scien­tists who are trained across many disci­plines and think outside the box.” (Source: Cell Press)

Reference: W. White et al.: Conversion of Visible Light into Ionic Power Using Photoacid-Dye-Sensitized Bipolar Ion-Exchange Membranes, Joule, online 15 November 2017; DOI: 10.1016/j.joule.2017.10.015

Link: Dept. of Chemistry, University of California, Irvine, USA

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