Photoanodes convert light into electrons that combine with protons to form hydrogen gas. (Illustration: Berkeley Lab)
Artificial photosynthesis it could be called: making hydrogen from sunshine. “Five years ago we were a factor thousand removed from commercial viability, now only a factor three.”
Imagine pouring water into a tank with a partition plate in the middle. You close the lid, put the tank in the sunlight and sit back to watch the miracle happen: on both sides bubbles emerge on the plate. They grow, detach and bubble upwards. Hydrogen for free as long as the sun shines. The tank will also produce oxygen, but that is of less importance.
Until now, this sun-powered fuel production was mostly mythical. Nonetheless, important advances have been made. Last Monday, Dr Christina Enache defended her thesis, in which she tested a number of materials on their suitability as photoanodes (that produce electrons under sunlight). And although she didn’t find a material that was suitable enough (she tested titanium dioxide, indium-vanadium oxide and iron oxide), her work has been most valuable to other researchers at ChemE (faculty of Applied Sciences), says her PhD supervisor, Dr Roel van de Krol: “Thanks to Christina’s work we now have a far better idea of which oxides may work as a photoanode and what the challenges are.”
For example, Dr Enache found out that in indium-vanadium oxide (InVO4), absorption of visible light only takes place in the first few nanometres of the material. Moreover, when the amount of indium and vanadium are not exactly equal, the oxide has no photoelectric activity.
Since then, the group has targeted bismuth vanadium oxide (BiVO4) as candidate material. “It’s mostly known as a bright yellow pigment,” says Van de Krol. This oxide is less critical to its exact chemical composition than InVO4, plus it allows a far deeper penetration of light. On top of that, manufacture of an active layer is simple and direct. A solution of bismuth and vanadium is sprayed onto a hot surface where the solvent vaporises and the metals oxidise. With this method, solar-to-hydrogen conversion efficiencies of about 3 percent are within reach, as Van de Krol and colleagues recently reported in the Journal of Physical Chemistry C. For comparison: electrolysis in combination with a PV installation has about 10 percent efficiency, but is more expensive. “We have to improve the efficiency by another factor of three,” Van de Krol says, “to make the technology interesting for investors.”
Christina Simona Enache, ‘Characterization of Thin Film Photoanodes for Solar Water Splitting’, PhD supervisors Prof. Joop Schoonman and Dr Roel van de Krol, 23 April 2012
