Two American researchers have found a more efficient way of splitting water into hydrogen and oxygen gas using solar energy. If such a process can be performed efficiently on a large scale, it could be the ultimate clean energy source.
Giulia Galli from the University of Chicago’s Institute for Molecular Engineering and Kyoung-Shin Choi from the University of Wisconsin, Madison found a way to increase the efficiency with which an electrode used for splitting water absorbs solar photons while at the same time improving the flow of electrons from one electrode to another.
Choi and postdoctoral researcher Tae Woo Kim found that if they heated an electrode made of the semiconducting compound bismuth vanadate to 350 degrees Celsius while flowing nitrogen gas over it, some of the nitrogen was incorporated into the compound.
The result was a notable increase in both photon absorption and electron transport. What was not clear was exactly how the nitrogen was facilitating the observed changes. So Choi turned to Galli, a theorist, to see if her simulations of the system could provide insight into what was going on.
Galli and former graduate student Yuan Ping found that the nitrogen was acting on the electrode in several ways. Heating the sample while flowing nitrogen gas is known to extract oxygen atoms from the bismuth vanadate, creating “defects.” Galli’s team found that these defects enhanced the transport of electrons.
At the same time, the nitrogen that had been incorporated into the compound increased the transport of electrons independent of the defects. This lowered the energy needed to kick electrons into the state in which they were available to split water meaning more of the solar energy could be used by the electrode.
”Now we understand what’s going on at the microscopic level,” said Galli. “So people can use these concepts —incorporation of a new element and new defects into the material — in other systems to try to improve their efficiency. These are very general concepts that could also be applied to other materials.”
Choi said their study will encourage researchers in the field to develop ways to improve multiple processes using a single treatment.
“It’s not just about achieving higher efficiency, it’s about providing a strategy for the field,” she said.