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Researchers in Switzerland have demonstrated more-efficient water-splitting solar cells based on a cheap, abundant, and long-lasting material: rust. The advance could lead to a cheap and energy-efficient way to generate hydrogen for fuel-cell vehicles using solar energy.

Water-splitting solar panels would have important advantages over existing technologies in terms of hydrogen production. Right now, the primary way to make hydrogen is to separate it from natural gas, a process that generates carbon dioxide and undercuts the main motivation for moving to hydrogen fuel-cell vehicles: ending dependence on fossil fuels. The current alternative is electrolysis, which uses electricity to break water into hydrogen and oxygen, with the two gases forming at opposite electrodes. Although electrolysis is costly, it can be cleaner if the source of the electricity is wind, sun, or some other carbon-free source.

But if the source of the electricity is the sun, it would be much more efficient to use solar energy to produce hydrogen by a photochemical process inside the cell itself. By improving the efficiency of such solar panels, Michael Grätzel, chemistry professor at the Ecole Polytechnique Fédérale de Lausanne, in Switzerland, and his colleagues have taken an important step toward this goal.

The researchers have shown that by including small amounts of silicon and cobalt, they can grow nanostructured thin films of iron oxide that convert sunlight into the electrons needed to form hydrogen from water. And the iron oxide films do this more efficiently than ever before with this material.

Iron oxide has long been an appealing material for such solar panels, in part because it holds up well in contact with water. But although it can absorb sunlight, the resulting charge carriers could not easily escape the material, so they recombined, canceling each other out before they could split any water. By doping the rust with silicon, the researchers coaxed the material to form cauliflower-like structures with extremely high surface area, ensuring that a large part of the atoms in the material were in contact with the water, or very close to it. That way, holes could easily escape into the water, where they prompt the generation of oxygen gas. The silicon also improves electron conductivity in the material, which is important for generating hydrogen gas at an opposite electrode. The researchers further improved the process by adding cobalt, which acts as a catalyst for the reactions.

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