THE sun provides enough energy in an hour to meet the world’s demands for a year, yet solar energy accounts for barely 1% of global power consumption. Plenty of researchers are working on making solar cells turn sunlight into electricity more efficiently. Some, though, are trying instead to turn it into fuel, using so-called photoelectrochemical (PEC) cells. Unfortunately, most processes designed to do this have proved complex and inefficient. But Florent Boudoire and Artur Braun of the Swiss Federal Laboratories for Materials Science and Technology think they have found a way to improve things.
The PEC cells which Mr Boudoire and Dr Braun are interested in use sunlight to split water into hydrogen and oxygen. They do this by employing a photoelectrode to convert the light into electricity and thus create a circuit that runs through the water. The gases are then generated by electrolysis.
The researchers’ cells use iron oxide and tungsten oxide to do the light-capturing. Iron oxide absorbs visible light and tungsten oxide absorbs ultraviolet. Also, tungsten oxide has a high refractive index. This means that light finds it hard to escape once it is inside a piece of tungsten oxide because it is bounced back and forth by a phenomenon called total internal reflection, increasing the chance it will be absorbed. Thus, in combination, iron and tungsten oxides can mop up as much as 35% of incident sunlight.
The theory therefore sounds good. But turning it into a practical device has proved hard because iron oxide is a poor electrical conductor unless it is in the form of an extremely thin film. Such a film, unfortunately, does not absorb enough sunlight for good water-splitting.
One way around this is to boost a cell’s light-absorbing abilities by using lithography both to shape its components and to dope them with materials that change their electrical properties. That, though, is expensive, so Mr Boudoire and Dr Braun sought a cheaper method.
Their trick, which they describe in a forthcoming paper in Energy & Environmental Science, forms the tungsten oxide into spheres a few hundred nanometres across and then covers them with a thin layer of iron oxide. That arrangement maximises internal reflection but also makes the reflected light available for absorption by the iron oxide at the junction between it and the tungsten oxide. Crucially, the researchers have found out how to create this arrangement cheaply.
First, they mix a solution of ammonium tungstate with a polymer to create a suspension of plastic droplets, each of which contains ammonium tungstate. Then they spray this mixture onto a sheet of glass and let it dry. Next, they heat the glass in an oven to burn the plastic away and thus transform the droplets into tungsten-oxide microspheres. Finally, they spray the whole lot with a solution of ferric nitrate and heat it again to create the coverings.
The result is an arrangement good both for absorbing light and for transforming it into electricity. And the process should be easy to scale up, meaning that PEC cells made this way could be deployed industrially. The hydrogen thus produced might be sold for use as fuel or stored locally and burned to generate electricity at night, when standard solar energy is unavailable.
Curiously, the researchers found after they had finished their experiments that nature had beaten them to it. The microstructure of their cells, with its spherical light collectors, resembles what goes on inside a moth’s eyes. These, which have evolved the ability to collect as much light as possible in order to see in the dark, and to reflect as little of it as possible to avoid being detected by predators, also contain tiny light-absorbing spheres. Nothing under the sun, as it were, is new.
(Photo credit: GUILLAUME SOUVANT / AFP)
This article appeared in the Science & technology section of the print edition under the headline "Tiny balls of fire"
