To harness the power of the sun, look at a material’s surface

With their ability to turn the sun’s energy into fuel, photoelectrochemical cells hold promise for the next generation of renewable energy.

The cells, which work by using solar energy to split water into hydrogen and oxygen, essentially create an artificial photosynthesis system. But designing and developing them remains a challenge.

One big issue lies with materials—what materials to use to create the photoelectrodes that power these cells, and how to optimize them to best enable these reactions.

Giulia Galli, Liew Family Professor of Molecular Engineering at the Institute for Molecular Engineering (IME) at the University of Chicago, and her collaborators have used theory and computation to uncover three major factors that determine how effectively a material enables these reactions when it interfaces with water.

By examining a prototypical oxide material, tungsten trioxide (WO3), the researchers found that the surface’s defects, the excess electronic charge present at the interface, and temperature fluctuations were the most important factors in determining how effective the material would be at splitting water.

While most previous studies have examined the material as a whole, Galli and her group looked at the surface properties of the material and its interface with water.

“We found it made a key difference to look at the interface of the material with water. That’s where the action happens,” said Galli, who is also a professor of chemistry and a senior scientist at Argonne National Laboratory.

The results were recently published in Nature Materials. The research was part of 10 years of work among theorists and experimentalists involved in the Center for Chemical Innovation, a National Science Foundation (NSF)-funded center.

“The design and optimization of artificial photosynthesis systems are critical to developing sustainable solar-to-fuel conversion technologies,” said Kathy Covert, NSF program director for the Centers for Chemical Innovation program. “This theoretical and computational effort by Dr. Galli demonstrates an important step forward in overcoming the challenges of designing and optimizing the photoelectrodes needed for artificial photosynthesis processes.”

Galli, a theorist, and her group develop computational models to study the properties of materials to understand how to best design and optimize them for certain uses. Over the last decade, she and her team have developed new theoretical and computational tools to examine which properties were most important among materials used to develop photoelectrochemical cells.

To do so, they examined tungsten trioxide, a material that has been well-characterized and which has properties similar to those of other possible material candidates for these cells.

They ran calculations on what happens to the surface of the material at finite temperature, meaning at a temperature above zero, and found that the presence of surface defects—when oxygen atoms are lost from the uppermost surface layer—is critical to understand what happens to the material when it interfaces with water. Those atoms are responsible for creating active sites for chemical reactions, like splitting water.

“We can work with experimentalists to optimize interfaces now that we have identified important descriptors,” Galli said. “These statements can also be translated to other oxides."

“The Center for Chemical Innovation has made tremendous progress in this area,” she continued. “We are excited.”

Additional authors on the paper include Matteo Gerosa, a former postdoctoral fellow in the Galli group; Marco Govoni, an assistant scientist at Argonne National Laboratory; and Francois Gygi, a professor of computer science at the University of California, Davis.

Funding was provided by the National Science Foundation. The research was conducted using methods and codes developed by MICCoM (Midwest Integrated Center for Computational Materials), funded by the Department of Energy.

Citation: “The role of defects and excess surface charges at finite temperature for optimizing oxide photoabsorbers,” Matteo Gerosa, Francois Gygi, Marco Govoni, and Giulia Galli, Nature Materials, October 29, 2018. doi: 10.1038/s41563-018-0192-4