Oregon researchers shed new light on solar water-splitting process

December 01, 2013

EUGENE, Ore. -- With the help of a new method called "dual-electrode photoelectrochemistry," University of Oregon scientists have provided new insight into how solar water-splitting cells work. An important and overlooked parameter, they report, is the ion-permeability of electrocatalysts used in water-splitting devices.

Their discovery could help replace a trial-and-error approach to paring electrocatalysts with semiconductors with an efficient method for using sunlight to separate hydrogen and oxygen from water to generate renewable energy, says Shannon W. Boettcher, professor of chemistry and head of the Solar Materials and Electrochemistry Laboratory in the UO's Materials Science Institute.

The research is described in a paper placed online Dec. 1 in advance of regular publication in the journal Nature Materials.

Solar water-splitting cells, which mimic photosynthesis, require at least two different types of materials: a semiconductor that absorbs sunlight and generates excited electrons and an electrocatalyst, typically a very thin film of a metal oxide that contains elements such as nickel, iron and oxygen, which serves to accelerate the rate at which electrons move on and off water molecules that are getting split into hydrogen and oxygen.

"We developed a new way to study the flow of electrons at the interface between semiconductors and electrocatalysts," Boettcher said. "We fabricated devices which have separate metal contacts to the semiconductor and electrocatalyst."

To do so, lead author Fuding Lin, a postdoctoral researcher, electrically contacted a single-crystal of semiconducting titanium dioxide and coated it with various electrocatalyst films. A film of gold only 10 nanometers thick was used to electrically contact the top of the electrocatalysts. Both contacts were used as probes to independently monitor and control the voltage and current at semiconductor-electrocatalyst junctions with a device known as a bipotentiostat. Lin focused on oxygen-evolution reaction -- the most difficult and inefficient step in the water-splitting process.

"This experiment allowed us to watch charge accumulate in the catalyst and change the catalyst's voltage," Boettcher said. It turns out, Lin said, that a thin layer of ion-porous electrocatalyst material works best, because the properties of the interface with the semiconductor adapt during operation as the charges excited by sunlight flow from the semiconductor onto the catalyst.

The research was designed to understand how maximum energy might be extracted from excited electrons in a semiconductor when the electrons enter the catalyst, where a chemical reaction separates oxygen and hydrogen. To date, Lin said, researchers have been experimenting with materials for creating efficient and cost-effective devices, but minimizing the energy loss associated with the catalyst-semiconductor interface has been a major hurdle.

In the study, Lin compared the movement of electrons between semiconductors coated with porous nickel oxyhydroxide -- a film previously shown by Boettcher's lab to yield excellent electrocatalytic efficiency for separating oxygen from water -- with semiconductors modified with non-permeable films of iridium oxide.

"The ion porous material allows water and ions to permeate the catalyst material," Lin said. "When these catalysts are in solution the catalyst's energy can move up and down as its oxidation state changes."

Catalysts with non-porous structures in semiconductor-catalytic junctions don't show this behavior and typically don't work as well, said Boettcher, who also is a member of Oregon BEST (Oregon Built Environment & Sustainable Technologies Center), a state signature initiative.

Converting sunlight into energy and storing it for later use in an economically viable way is a major challenge in the quest to replace fossil fuels with renewable energy. Traditional solar photovoltaic cells absorb sunlight to form excited electrons that are funneled into wires as electricity but storing energy as electricity, for example in batteries, is expensive.

Details about how excited electrons move from semiconductors to catalysts have been poorly understood, Boettcher said. "This lack of understanding makes improving water-splitting devices difficult, as researchers have been relying on trial-and-error instead of rational design."

The system used in the study, Boettcher added, was not efficient. "That wasn't our goal," he said. "We wanted to understand what's happening at a basic level with well-defined materials. This will facilitate the design of systems that are more efficient using other materials."

"Researchers at the University of Oregon are reengineering the science, manufacturing and business processes related to critical products," said Kimberly Andrews Espy, vice president for research and innovation and dean of the UO Graduate School. "This important discovery by Dr. Boettcher and his team could lead to more efficient systems that help foster a sustainable future."
-end-
The researchers used UO's Center for Advanced Materials Characterization in Oregon (CAMCOR) and the Support Network for Research and Innovation in Solar Energy (SuNRISE) to study the interfaces.

The U.S. Department of Energy (DE-FG0212ER16323) funded the research. The DuPont Young Professor Program also supports Boettcher's work.

About the University of Oregon

The University of Oregon is among the 108 institutions chosen from 4,633 U.S. universities for top-tier designation of "Very High Research Activity" in the 2010 Carnegie Classification of Institutions of Higher Education. The UO also is one of two Pacific Northwest members of the Association of American Universities.

Sources: Shannon Boettcher, assistant professor of chemistry and biochemistry, 541-346-2543, swb@uoregon.edu, and Fuding Lin, postdoctoral research associate, 541-346-8075, flin@uoregon.edu

Links:

Boettcher faculty page: http://chemistry.uoregon.edu/fac.html?boettcher

Solar Materials and Electrochemistry Laboratory: https://wiki.uoregon.edu//display/BOETTCHERLAB/Boettcher+Group+Website+and+Wiki

UO Chemistry and Biochemistry Department: http://pages.uoregon.edu/chem/

Materials Science Institute: http://materialscience.uoregon.edu/

CAMCOR: http://camcor.uoregon.edu/

SuNRISE: http://camcor.uoregon.edu/labs/sunrise

Oregon BEST: http://oregonbest.org/

Follow UO Science on Facebook: http://www.facebook.com/UniversityOfOregonScience

UO Science on Twitter: http://twitter.com/UO_Research

More UO Science/Research News: http://uoresearch.uoregon.edu

Note: The University of Oregon is equipped with an on-campus television studio with a point-of-origin Vyvx connection, which provides broadcast-quality video to networks worldwide via fiber optic network. In addition, there is video access to satellite uplink, and audio access to an ISDN codec for broadcast-quality radio interviews.

University of Oregon

Related Renewable Energy Articles from Brightsurf:

Creating higher energy density lithium-ion batteries for renewable energy applications
Lithium-ion batteries that function as high-performance power sources for renewable applications, such as electric vehicles and consumer electronics, require electrodes that deliver high energy density without compromising cell lifetimes.

Renewable energy targets can undermine sustainable intentions
Renewable energy targets (RETs) may be too blunt a tool for ensuring a sustainable future, according to University of Queensland-led research.

Intelligent software for district renewable energy management
CSEM has developed Maestro, an intelligent software application that can manage and schedule the production and use of renewable energies for an entire neighborhood.

Renewable energy transition makes dollars and sense
New UNSW research has disproved the claim that the transition to renewable electricity systems will harm the global economy.

Renewable energy advance
In order to identify materials that can improve storage technologies for fuel cells and batteries, you need to be able to visualize the actual three-dimensional structure of a particular material up close and in context.

Illuminating the future of renewable energy
A new chemical compound created by researchers at West Virginia University is lighting the way for renewable energy.

Using fiber optics to advance safe and renewable energy
Fiber optic cables, it turns out, can be incredibly useful scientific sensors.

Renewable energy developments threaten biodiverse areas
More than 2000 renewable energy facilities are built in areas of environmental significance and threaten the natural habitats of plant and animal species across the globe.

Could water solve the renewable energy storage challenge?
Seasonally pumped hydropower storage could provide an affordable way to store renewable energy over the long-term, filling a much needed gap to support the transition to renewable energy, according to a new study from IIASA scientists.

Scientists take strides towards entirely renewable energy
Researchers have made a major discovery that will make it immeasurably easier for people (or super-computers) to search for an elusive 'green bullet' catalyst that could ultimately provide entirely renewable energy.

Read More: Renewable Energy News and Renewable Energy Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.