Rusty photoelectrodes better in theory than practice

Hydrogen will be needed in large quantities as an energy carrier and raw material in the energy systems of the future. To achieve this, however, hydrogen must be produced in a climate-neutral way, for example through so-called photoelectrolysis, whereby sunlight splits water into hydrogen and oxygen.

As photoelectrodes, semiconducting materials are needed that can convert sunlight into electricity and remain stable in water. Metal oxides are among the best candidates for stable and inexpensive photoelectrodes. Some of these metal oxides also have catalytically active surfaces that accelerate the formation of hydrogen at the cathode or oxygen at the anode.

Research has long focused on hematite (α-Fe₂O₃), which is better known as rust. Hematite is stable in water, extremely inexpensive and has a demonstrated catalytic activity for oxygen evolution. But although research on hematite photoanodes has been going on for about 50 years, their photocurrent conversion efficiency is still less than 50% of the theoretical maximum value. By comparison, the photocurrent efficiency of the semiconductor silicon, which currently dominates almost 90% of the photovoltaic market, is about 90% of the theoretical maximum value.

Scientists have puzzled over this for a long time. What exactly has been overlooked? Why have only modest increases in efficiency been achieved with hematite photoelectrodes?

Now, in a paper in Nature Materials, a team led by Dennis Friedrich at Helmholtz-Zentrum Berlin in Germany, Daniel Grave at Ben Gurion University in Israel and Avner Rothschild at Technion – Israel Institute of Technology provides an explanation for why hematite falls so far short of the calculated maximum value. The group at Technion investigated how the wavelength of absorbed light in hematite thin films affects their photoelectrochemical properties, while the HZB team used time-resolved microwave measurements to determine the wavelength-dependent charge-carrier properties of thin films of rust.

By combining their results, the researchers succeeded in extracting a fundamental physical property that had generally been neglected when considering inorganic solar absorbers – the photogeneration yield spectrum. "Roughly speaking, this means that only part of the energy of the light absorbed by hematite generates mobile charge carriers, the rest generates rather localized excited states and is thus lost," explains Grave.

"This new approach provides experimental insight into light-matter interaction in hematite and allows distinguishing its optical absorption spectrum into productive absorption and non-productive absorption," says Rothschild.

"We could show that the effective upper limit for the conversion efficiency of hematite photoanodes is significantly lower than that expected based on above band-gap absorption," adds Grave. According to the new calculations, today's 'champion' hematite photoanodes have already come quite close to the theoretically possible maximum. So it doesn't get much better than that.

This approach has also been successfully applied to TiO₂, a model material, and BiVO₄, which is currently the best-performing metal oxide photoanode material. "With this new approach, we have added a powerful tool to our arsenal that enables us to identify the realizable potential of photoelectrode materials," says Friedrich. "Implementing this to novel materials will hopefully expedite the discovery and development of the ideal photoelectrode for solar water splitting. It would also allow us to 'fail quickly', which is arguably just as important when developing new absorber materials."

This story is adapted from material from Helmholtz-Zentrum Berlin, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.