University Oregon scientists explains solar water-splitting cells operations using photoelectrochemistry

New study illustrates the working of solar water-splitting cells

11:51 AM, 3rd December 2013
University of Oregon research news
Shannon Boettcher, left, and Fuding Lin of the University of Oregon pursued a better understand the basic fundamentals involved in how solar water-splitting devices work.

EUGENE, US: University of Oregon scientists have provided new insight into how solar water-splitting cells work, with the help of a new method called “dual-electrode photoelectrochemistry.” The most important parameter reported was the ion-permeability of electrocatalysts used in water-splitting devices.

According to Shannon W Boettcher, Professor, University of Oregon, the 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.

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. We fabricated devices which have separate metal contacts to the semiconductor and electrocatalyst,” said Boettcher.

Fuding Lin, Postdoctoral researcher, electrically contacted a single-crystal of semiconducting titanium dioxide and coated it with various electrocatalyst films. A film of gold only 10 nanometre 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,” said Boettcher.

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. When these catalysts are in solution the catalyst’s energy can move up and down as its oxidation state changes,” said Lin.

Catalysts with non-porous structures in semiconductor-catalytic junctions don’t show this behavior and typically don’t work as well, said Boettcher.

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. This lack of understanding makes improving water-splitting devices difficult, as researchers have been relying on trial-and-error instead of rational design,” said Boettcher.

“The system used in the study was not efficient. That wasn’t our goal. 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,” added Boettcher.

 

© University of Oregon News

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