Artificial photosynthesis, a process that uses sunlight to split water into hydrogen and oxygen, allows the storage of solar energy as hydrogen. The hydrogen can then be used as a fuel, either directly or in the form of methane, or it can generate electricity in a fuel cell.
Scientists at the Helmholtz-Zentrum Berlin (HZB) and the Delft University of Technology (TU Delft) have been working on the development of such a device using a simple solar cell and a photo anode made of metal oxide bismuth vanadate, or BiVO4, to successfully store nearly 5% of solar energy as hydrogen.
The photo anode BiVO4, to which a small amount of tungsten atoms were added, was sprayed onto a piece of conducting glass and coated with an inexpensive cobalt phosphate catalyst.
"Basically, we combined the best of both worlds," said 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 weve just created a cost-effective, highly stable, and highly efficient solar fuel device."
The potential seen in such a research is very high. Taking Germany as an example the researchers state that with a solar performance of 600 Watts per square meter, 100 square meters of such a system can theoretically store 3 kWh of energy in the form of hydrogen within one sunshine hour. This energy can then be made available at night or when needed.
The team used a relatively simple silicon-based thin film cell and added the metal oxide layer to it. This layer is the only part of the cell that is in contact with the water as the research states, and acts as a photo anode for oxygen formation. Simultaneously, it helps to prevent corrosion of the silicon cell. The researchers systematically examined and optimized processes such as light absorption, separation of charges, and splitting of water molecules. According to van de Krol, a solar-to-chemical efficiency of up to 9% is possible when a photo anode made from bismuth vanadate is used. With the cobalt phosphate catalyst the team managed to substantially accelerate the process of oxygen formation at the photo anode.
The biggest hurdle, however, was to separate the electrical charges within the bismuth vanadate film according to the team. Even though metal oxides are cheap and stable, the charge carriers have the tendency to quickly recombine thus rendering them unavailable for water splitting. Van de Krol and his team managed to solve this issue by adding wolfram atoms to the bismuth vanadate film in a very specific way.
This sets up an internal electric field that helps prevent recombination. Bismuth vanadium wolfram solution was sprayed onto a heated glass substrate which caused the solution to evaporate. By repeatedly spraying different wolfram concentrations onto the glass, a highly efficient photo-active metal oxide film some 300 nanometers thick was created.
"We don't really understand quite yet why bismuth vanadate works so much better than other metal oxides. We found that more than 80% of the incident photons contribute to the current, an unexpectedly high value that sets a new record for metal oxides," says van de Krol.
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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