Metal oxides such as rust are suitable as photo electrodes to generate “green” hydrogen with sunlight. However, despite decades of research on this inexpensive material, progress has been limited.
A team at the HZB, together with partners from Ben Gurion University and Technion, Israel, has now analyzed the optoelectronic properties of rust (hematite) and other metal oxides in previously unknown detail.
Their results show that the maximum achievable efficiency of hematite electrodes is significantly lower than previously assumed. The study also provides specific information on how new materials for photoelectrodes can be assessed more realistically.
In the energy system of the future, hydrogen will be required in large quantities as an energy carrier and raw material. To do this, however, hydrogen has to be generated in a climate-neutral way, for example via so-called photo-electrolysis, in which water is split into its elements hydrogen and oxygen. The necessary energy is provided by sunlight. Semiconducting materials that convert sunlight into electricity and remain stable in water can be used as photoelectrodes. Metal oxides are among the best candidates for inexpensive and stable photoelectrodes. Some of these metal oxides also have catalytically active surfaces that accelerate the formation of hydrogen at the cathode and oxygen at the anode.
Why does rust stay below the calculated possibilities?
Research has long focused on hematite (α-Fe2O3), which is widely known as rust. Hematite is stable in water, extremely inexpensive and is well suited as a photoanode with proven catalytic activity for the development of oxygen. Although hematite photoanodes have been researched for about 50 years, the photocurrent conversion efficiency is less than 50 percent of the theoretical maximum. For comparison: The efficiency of the semiconductor material silicon, which today dominates almost 90 percent of the photovoltaic market, is around 90 percent of the theoretical maximum value. The reasons have been puzzled for a long time. What exactly was overlooked? Why is it that, despite long research, only modest increases in efficiency could be achieved?
Israeli-German cooperation solves the rust puzzle
But now a team led by Dr. Daniel Grave (Ben Gurion University), Dr. Dennis Friedrich (HZB) and Prof. Dr. Avner Rothschild (Technion) provided an explanation for this and published it in Nature Materials. The group at the Technion investigated how the wavelength of the absorbed light influences the photoelectrochemical properties of the hematite thin layers, while the HZB team determined the mobility of the charge carriers in thin rust layers using time-resolved microwave measurements.
Important physical property of the material
By combining their results, the researchers succeeded in extracting a fundamental physical property of the material that was previously neglected when considering inorganic solar absorbers: the spectrum of the photogeneration yield. “Roughly speaking, this means that only part of the light energy absorbed in the hematite also generates mobile charge carriers, the rest rather generates localized charge carriers and is thus lost,” explains Grave.
Upper limit recalculated
“This new approach gives an experimental insight into the interaction between light and material in hematite and allows the spectrum to be differentiated into productive and non-productive absorption,” explains Rothschild. “We were able to show that the effective upper limit for the conversion efficiency of hematite photoanodes is significantly lower than previously calculated,” says Grave. According to the new calculation, today’s “champions” among the hematite photoanodes come quite close to the theoretically possible maximum. So it couldn’t be much better.
Realistic assessment of new materials
The approach has also been successfully applied to the model material TiO2 and the currently best metal oxide photoanode material BiVO4. “With this new approach, we have added a powerful tool to our arsenal that enables us to determine the true potential of photoelectrode materials. Hopefully, if we apply this to novel materials, it will accelerate the discovery and development of the ideal photoelectrode for solar water splitting. It would also allow us to ‘fail quickly’, which is probably just as important when developing new absorber materials, ”says Friedrich.
publication : Nature Materials (2021): Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin-film metal oxide photoanodes for solar water splitting
Daniel A. Grave, David S. Ellis, Yifat Piekner, Moritz Kölbach, Hen Dotan, Asaf Kay, Patrick Schnell, Roel van de Krol, Fatwa F. Abdi, Dennis Friedrich and Avner Rothschild
DOI: 10.1038 / s41563-021-00955-y
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