Scientists at EPFL have come up with an inexpensive way to store solar energy as hydrogen. It's a concept that has been around for a while; a tandem cell consisting of a dye-sensitized solar cell coupled with an oxide-based semiconductor to convert solar energy into hydrogen, which can then be stored. That way the energy can be used either day or night instead of the "use it or lose it" set up of current solar panels.
So far, the most successful iteration of a device like this was able to attain an efficiency of 12.4 percent, but the materials used were expensive and would cost about $10,000 to cover a 10 square centimeter surface.
The EPFL researchers, led by Kevin Sivula, wanted to make something that could actually be produced and so they limited themselves to inexpensive materials and to production techniques that could easily be scaled up. According to the team, the most expensive part of this new tandem solar cell is the glass plate.
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 admits.
That's why the iron oxide used by the team is a bit more developed than what you'd find on an old nail. 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."
The second part of the device is composed of a dye and 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.
So far the efficiency is low -- between 1.4 and 3.6 percent -- but because of the low cost it's possible to keep experimenting with it and tweaking it until improvements are made. The team believes that they will be able to reach an efficiency of 10 percent in just a few years and that the device should cost only $80 per square meter. Ultimately, the researchers think the tandem cell technology could hit an efficiency of 16 percent with the iron oxide, while still keeping its cost low.
You can see the tandem cell in the video below.