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.