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.
Grätzel’s new iron-oxide films can convert an impressive and, according to the researchers, “unprecedented” 42 percent of ultraviolet photons in sunlight into electrons and holes. But the system’s overall efficiency is only about 4 percent, in part because iron oxide doesn’t absorb all the parts of the solar spectrum.
The main achievement of Grätzel’s new research, which appears in the current issue of the Journal of the American Chemical Society, is that it examines the interactions at work in the system in great detail, says Brian Holcroft, CEO of Hydrogen Solar, a company based in Guildford, UK, that is developing ways to mass-produce panels inspired by Grätzel’s materials. The findings suggest several strategies that could help the iron-oxide-based panel reach the 10 percent efficiency level that would make the technology competitive with current ways of creating hydrogen, Holcroft says. (Iron oxide could theoretically be as much as 20 percent efficient.) These include adjusting the amount and arrangement of silicon and cobalt, and improving the structure of the films.
If this level of efficiency can be met, hydrogen-generating solar energy could mitigate some of the challenges that threaten to make hydrogen fuel-cell vehicles impractical, says George Sverdrup, hydrogen technology manager at the National Renewable Energy Laboratory (NREL), in Golden, CO. For example, if consumers and businesses used these panels to make hydrogen, rather than getting hydrogen from a large facility, it would cut out the cost of shipping hydrogen, making hydrogen more affordable. Solar-to-hydrogen panels would be more efficient than small electrolysis machines, and they would ensure that the hydrogen comes from a renewable source.
But challenges remain. Researchers at Hydrogen Solar, for example, are looking for a replacement for the expensive platinum now used in one of the cell’s electrodes, which will be important for keeping down costs, especially as demand increases for platinum in this and other applications, such as fuel cells. Meanwhile, Sverdrup says other researchers, including those at NREL, are working with materials that are much more efficient than iron oxide but so far have lasted only hours. If researchers can make them last longer, the materials could challenge iron oxide.
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