Solar Hydrogen Made Easy

Tuesday, July 30, 2013 @ 02:07 PM gHale

Using a simple solar cell and a photo anode made of a metal oxide, it is possible to store nearly five percent of solar energy chemically in the form of hydrogen, researchers said.

This is could be a boost for the solar energy arena because as the design of the solar cell is much simpler than that of the high-efficiency triple-junction cells based on amorphous silicon or expensive III-V semiconductors traditionally used, said researchers at HZB and TU Delft.

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The photo anode, made from the metal oxide bismuth vanadate (BiVO4) to which researchers added in a small amount of tungsten atoms, ended up sprayed onto a piece of conducting glass and coated with an inexpensive cobalt phosphate catalyst.

“Basically, we combined the best of both worlds,” said Professor Dr. Roel van de Krol, head of the HZB Institute for Solar Fuels. “We start with a chemically stable, low cost metal oxide, add a really good but simple silicon-based thin film solar cell, and – voilà – we’ve just created a cost-effective, highly stable, and highly efficient solar fuel device.”

Researchers were able to develop a simple system for using sunlight to split water into hydrogen and oxygen. This process, artificial photosynthesis, allows solar energy to end up stored in the form of hydrogen. The hydrogen can then work as fuel either directly or in the form of methane, or it can generate electricity in a fuel cell. One estimate shows the potential inherent in this technology: At a solar performance in Germany of 600 Watts per square meter, 100 square meters of this type of system is theoretically capable of storing 3 kilowatt hours of energy in the form of hydrogen in one hour of sunshine. This energy could then be available at night or on cloudy days.

Van de Krol and his team essentially started with a relatively simple silicon-based thin film cell to which they added a metal oxide layer.

This layer is the only part of the cell that is in contact with the water, and acts as a photo anode for oxygen formation. At the same time, it helps to prevent corrosion of the sensitive silicon cell.

The researchers systematically examined and optimized processes such as light absorption, separation of charges, and splitting of water molecules. Theoretically, a solar-to-chemical efficiency of up to nine percent is possible when you use a photo anode made from bismuth vanadate, said van de Krol. While this is just the beginning of the researcher they were already able to solve one problem: Using an inexpensive cobalt phosphate catalyst, they managed to substantially accelerate the process of oxygen formation at the photo anode.

The biggest challenge, however, was the efficient separation of electrical charges within the bismuth vanadate film.

Metal oxides may be stable and cheap, but the charge carriers have a tendency to quickly recombine. This means they are no longer available for the water splitting reaction.

That all changed when van de Krol and his team figured out that it helps to add wolfram atoms to the bismuth vanadate film.

“What’s important is that we distribute these wolfram atoms in a very specific way so that they can set up an internal electric field, which helps to prevent recombination,” van de Krol said.

For this to work, the scientists took a bismuth vanadium wolfram solution and sprayed it onto a heated glass substrate. This caused the solution to evaporate. By repeatedly spraying different wolfram concentrations onto the glass, they were able to create a highly efficient photo-active metal oxide film some 300 nanometers thick.

“We don’t really understand quite yet why bismuth vanadate works so much better than other metal oxides,” van de Krol said. “We found that more than 80 percent of the incident photons contribute to the current, an unexpectedly high value that sets a new record for metal oxides.”

The next challenge is scaling these kinds of systems to several square meters so they can yield relevant amounts of hydrogen.

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