Solar hydrogen technology is an emerging field that has huge potential for low-cost regenerative energy generation in future. Directly inspired by how plants produce energy from sunlight, synthetic photoelectrodes absorb solar rays to produce electric current, which then catalyses the splitting of water into oxygen and hydrogen, a highly versatile fuel.

The Helmholtz Centre Berlin’s Institute for Solar Fuels (HBZ) has been focusing efforts on developing suitable photoelectrodes – semiconductors that remain stable in aqueous environments – and is getting promising results with the compound bismuth vanadate (BiVO4). "Basically, we know that just by immersing bismuth vanadate in the aqueous solution the chemical composition of the surface changes," says the institute’s Dr David Starr in a press release. But until now, it has not been clear exactly how the material’s electronic properties are affected when it’s brought into contact with water.

A new study in the Journal of the American Chemical Society, however, has now thrown light on this question. Following extensive experiments at the Advanced Light Source at Lawrence Berkeley National Laboratory, it was found “excess electrons from dopants or defects aid the dissociation of water which in turn stabilises so-called polarons at the surface.”

While this might not sound like a major breakthrough, the discovery is highly significant for HBZ’s work on solar H2 photoelectrodes. The study provides valuable insights into processes that modify the surface chemical composition and electronic structure and informs design improvements for photoanodes.

But there’s still a lot of work to be done: "What we can't yet assess for sure is what role the polarons play in charge transfer. Whether they promote it and thus increase efficiency or, on the contrary, are an obstacle, we still need to figure that out," Starr says.