Using solar energy to split water into its component parts, thereby allowing the solar energy to be stored as hydrogen fuel, generally involves one of two methods: using photoelectrochemical cells to directly split the water, or using solar cells to produce electricity to power an electrolyzer that separates the water molecules. One problem associated with the latter method is that it currently relies on rare metals. But now scientists from Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland have managed to do so using common materials, and have achieved a record solar energy to hydrogen conversion efficiency in the process.
In addition to the nickel and iron catalysts used for the electrodes in their electrolyzer, the researchers are using solar absorbers made of perovskite – another abundant material – in the solar cells. Perovskite has been generating interest in recent years for its use in different kinds of solar cells as well as in solar water splitting.
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The material, which could be a cheaper alternative to the silicon more generally used in photovoltaic cells, is not a new discovery, having been first found in the Ural mountains in 1839. Originally only the oxide mineral itself was called perovskite but the name now takes in other elements with the same crystalline structure and is often found in items as workaday as car batteries.
The team from EPFL's Laboratory of Photonics and Interfaces, led by postdoctoral student Jingshan Luo, achieved a solar energy to hydrogen conversion efficiency of 12.3 percent. Any conversion rate over 10 percent is considered exceptional and only once have researchers achieved a higher rate, with a 12.4% efficiency achieved in 1998 according to the journal Science. The difference between the two was that the solar cells used in the 1998 endeavor used far more expensive materials, making any wider scaling up tricky.
"Our electrodes work just as well as the expensive platinum-based models customarily used," said Luo. The other advantage of these perovskite cells is that they can generate higher-than-average open circuit voltages – over 1 V compared with the 0.7 V of a silicon cell, meaning only two perovskite cells, not three silicon ones, are needed to generate the 1.7 V needed for water electrolysis.
“This is the first time we have been able to get hydrogen through electrolysis with only two cells!" Luo said.
Is there a downside to record efficiency and the possibilities of cheap hydrogen fuel production? Unfortunately, yes. Science notes that, "One major drawback of this system is the instability of the perovskite PVs, which results in a degradation of the photocurrent over a period of hours. The cause of the instability is not yet fully understood." The scientists hope that this problem, which very obviously prevents any scaling up or production, may soon be solved and efficiency improved even more.
The team's paper appears in Science.
Luo outlines the achievement in the video below.