Berlin - Hydrogen can be produced in a climate-neutral manner if water is electrolytically split into hydrogen and oxygen using solar energy. This approach requires inexpensive photoelectrodes that deliver a specific photovoltage under illumination and remain stable in aqueous electrolytes, according to a statement from the Helmholtz Center Berlin for Materials and Energy GmbH (HZB). However, “conventional semiconductors corrode very quickly in water,” according to the researchers. Although metal oxide thin films are more stable, they still corrode over time. One of the most successful photoanode materials is bismuth vanadate (BiVO4), a metal oxide in which the photocurrents are already close to the theoretical limit. “The main challenge for commercially viable PEC water splitting is to evaluate and improve the stability of photoelectrode materials during their PEC operation,” the scientists explain.
In order to understand the corrosion processes of high-quality BiVO4 photoelectrodes, an international team at the HZB Institute for Solar Fuels led by Roel van de Krol investigated the processes. “So far we have only been able to examine photoelectrodes before and after photoelectrochemical corrosion,” says Ibbi Ahmet (HZB), who initiated the study together with Siyuan Zhang from the Max Planck Institute. “It was a bit like reading only the first and last chapters of a book and not knowing how all the characters died.” Therefore, the processes were now “in operando” in the electrolytic water splitting during the oxygen evolution reaction ( OER) was observed and the stability of photoelectrodes and catalysts was compared.
The researchers were able to “determine in real time which elements were dissolved from the surface of the BiVO4 photoanodes during the photoelectrochemical reaction.” One result of the work is the determination of a parameter, the stability number (S), says Ahmet. This is calculated from the ratio between the oxygen molecules produced and the number of dissolved metal atoms in the electrolyte and is “a perfectly comparable measure of photoelectrode stability.” Stability is high when the splitting of water proceeds quickly and only a few metal atoms get into the electrolyte. In a next step, the scientists want to “design practical solutions to improve the stability of BiVO4 photoanodes and enable their use in long-term practical applications.”
The work was published in: ACS Applied Energy Materials (2020): “Different Photostability of BiVO4 in Near-pH-Neutral Electrolytes”. Siyuan Zhang, Ibbi Ahmet, Se-Ho Kim, Olga Kasian, Andrea M. Mingers, Patrick Schnell, Moritz Kölbach, Joohyun Lim, Anna Fischer, Karl JJ Mayrhofer, Serhiy Cherevko, Baptiste Gault, Roel van de Krol, and Christina Scheu. (DOI: 10.1021/acsaem.0c01904)
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Scalable large-area BiVO4 photoanode: Researchers at HZB want to design practical solutions to improve stability / © HZB
