In a reversal of momentum in research back to inexpensive materials and easily scalable production processes, Kevin Sivula and his colleagues at Ecole Polytechnique Fédérale de Lausanne (EPFL) are enabling an economically viable method for solar hydrogen production.
There new device, while still in an experimental stage is the focus of a description in an article published in the journal Nature Photonics.
A little background, the idea of converting solar energy into hydrogen is not a new one. Researchers have been working on the idea for more than four decades. During the 1990s, EPFL joined the hunt, with the research of Michaël Grätzel. With a colleague from University of Geneva, he invented the photoelectrochemical (PEC) tandem solar cell, a technique for producing hydrogen directly from water. Their prototypes shared the same basic principle: a dye-sensitized solar cell, which was also invented by Michael Grätzel, combined with an oxide-based semiconductor.
Grätzel’s device is completely self-contained. The electrons produced are used to break up water molecules and reform the pieces into oxygen and hydrogen. In the same liquid, two distinct layers in the device have the job of generating electrons when stimulated by light, an oxide semiconductor, which performs the oxygen evolution reaction, and a dye-sensitized cell, which liberates the hydrogen.
The Grätzel device while elegant and utopian is simply way, way too expensive.
The Sivula team’s latest prototype focused on resolving the main outstanding problem with PEC technology: its cost.
Sivula points out a relevant comparison, “A U.S. team managed to attain an impressive efficiency of 12.4%. The system is very interesting from a theoretical perspective, but with their method it would cost $10,000 to produce a 10 square centimeter surface.”
Sivula and his team let reason rule: setting themselves a limitation from the start – to use only affordable materials and techniques. It wasn’t an easy task, but they managed.
Sivula explains, “The most expensive material in our device is the glass plate.” The efficiency is still low – between 1.4% and 3.6%, depending on the prototype used. But the technology has great potential. “With our less expensive concept based on iron oxide, we hope to be able to attain efficiencies of 10% in a few years, for less than $80 per square meter. At that price, we’ll be competitive with traditional methods of hydrogen production,” asserts Sivula.
The semiconductor, which performs the oxygen evolution reaction, is just iron oxide. “It’s a stable and abundant material. There’s no way it will rust any further! But it’s one of the worst semiconductors available,” Sivula allows.
This is where the high tech and ingenuity comes in. The iron oxide used by the team is a bit more developed than what you’d find on an old nail. The iron oxide is nanostructured, enhanced with silicon oxide, covered with a nanometer-thin layer of aluminum oxide and cobalt oxide. These treatments optimize the electrochemical properties of the material, but are nonetheless simple to apply. “We needed to develop easy preparation methods, like ones in which you could just dip or paint the material,” explained Silvula.
The second part of the device is composed of a dye and low cost titanium dioxide, the basic ingredients of a dye-sensitized solar cell. This second layer lets the electrons transferred by the iron oxide gain enough energy to extract hydrogen from water.
The result is a breakthrough in performance that has been enabled by recent advances in the study of both the iron oxide and dye-sensitized titanium dioxide. Both of these technologies are rapidly advancing.
Sivula predicts that the tandem cell technology will eventually be able to attain an efficiency of 16% with iron oxide, while still remaining low cost, which is, after all, the attractiveness of the approach.
$80 per square meter, even at just 12% efficiency would be a boon to solar applications supplying a ready fuel. That’s just a low enough cost to be very attractive, indeed. EPFL’s effort to make stored solar energy inexpensive could considerably increase the potential of solar energy to serve as a viable renewable energy source for the future.
This approach might make the difference. It’s very smart to start cheap and work up than start wildly expensive and work down. Might want to keep that “Photoelectrochemical” technology on the watch list.