Bleach-free spin cycle

Splitting water using a renewable energy source has been mooted as the way forward towards "zero-carbon" electricity as the process releases hydrogen which can be used to drive fuel cells. Unfortunately, as with any chemical process, photochemical, thermal, or electrochemical, efficiency is never 100 percent. In the case of solar-powered electrolysis of water itself, not only are oxygen and hydrogen gas released, as you would hope, but a side product in the form of hydrogen peroxide is generated. Now, researchers in Israel and The Netherlands based at the Weizmann Institute of Science, in Rehovot and Eindhoven University of Technology, respectively, have found a way to almost complete suppress the generation of said byproduct by taking control of the spin of electrons in the reaction. [R Naaman et al., J Am Chem Soc (2017) 139(7), 2794-2798; DOI: 10.1021/jacs.6b12971].

Releasing hydrogen from water using solar power could jumpstart the hydrogen economy if it were not for the corrosive power of hydrogen peroxide, which eats away at electrodes in the system lower efficiency and making the process not so viable. Weizmann scientist Ron Naaman and his team working with and Eindhoven's Bert Meijer and his research group have investigated the role electron spin plays in the reactions involving oxygen that can generate byproducts rather than pure oxygen gas as the second substance released from the electrolysis of water. The team had hypothesized that if they could align both spins then the formation of hydrogen peroxide would be blocked entirely, because the ground state of this molecule requires two electrons to have opposite spins. Oxygen, by contrast, forms when the electrons have parallel spins.

To achieve spin alignment the team first coated their titanium dioxide anode with chiral organic semiconductors from helically aggregated dyes as sensitizers; zinc porphyrins and triarylamines. These materials allow the team to inject only electrons with their spins aligned into the reaction, a phenomenon based on earlier work by Naaman's team demonstrating that electron transmission is selective through a chiral film. "The effect on water splitting exceeded our expectations," Naaman says. "The formation of hydrogen peroxide was almost entirely suppressed. We also saw a significant increase in the cell's current. And because chiral molecules are very common in nature, we expect this finding may have significance in many areas of research."

The team cannot yet say by how much this approach will improve efficiency in a real-world system. "Our goal was to be able to control the reaction and to understand what exactly was going on," Meijer adds. "In some ways, this was a stroke of luck because the supramolecular structures had not originally been intended for this purpose…we're very busy optimizing the process."

"The next step in the work is working towards increasing the current density in the process," Naaman told Materials Today. We did reduce the overpotential required for the process and now we need to increase the current density to make the system useful."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".