These transmission electron microscope images show a cross-section of pristine (left) and damaged (right) bismuth vanadate, a thin-film semiconductor. The bismuth vanadate is colored yellow to highlight the contrast with the layer below; the yellow layer is intact on the left, but fragmented on the right after exposure to an alkaline bath. Images: Matthew McDowell/Caltech.One of the major challenges for scientists working to create artificial photosynthesis systems that can efficiently convert sunlight, water and carbon dioxide into fuel is finding materials that can both do the work and also survive being exposed to harsh environments. Existing methods to determine material stability have been hit and miss, but a Berkeley Lab-led research team has now employed a combination of experimental and theoretical tools to rigorously determine how well a material will weather such harsh environments.
The researchers, part of the Joint Center for Artificial Photosynthesis (JCAP) at the Lawrence Berkeley National Laboratory (Berkeley Lab), describe their work in a paper published in Nature Communications.
"None of the existing methods to predict material stability were working," said study lead author Francesca Toma, a Berkeley Lab staff scientist in the Chemical Sciences Division. "We need to develop a set of techniques that could give us a more accurate assessment of how a material will behave in real-world applications. How can we figure out if this material is going to last 10 years? Having methods that allow us to understand how a material degrades and to predict its stability over the years is an important advance."
Artificial photosynthesis has a way to go to achieve the controlled, stable processes of its natural counterpart. A key step in both natural and artificial photosynthesis is the splitting of water into its constituent elements, hydrogen and oxygen. In natural systems, ensuring the stability of the components that perform this function is not required, since they can self-heal in living cells.
Unlike plants, practical solar fuel generators demand stable materials that do not need to be continuously replenished. Another consideration is that these devices need to operate in highly corrosive conditions that exacerbate the wear and tear on sensitive components. Unfortunately, most materials do not survive in these harsh environments, causing their performance to degrade over time.
In this study, the researchers focused on bismuth vanadate, a thin-film semiconductor that has emerged as a leading candidate for use as the positively-charged electrode, or photoanode, in a photoelectric cell that absorbs sunlight to split water. Going by traditional approaches for predicting material characteristics, bismuth vanadate should be resistant to chemical attack, but it is not.
In reality, bismuth vanadate exhibits complex chemical instabilities that originate from kinetic limitations. These are related to the inability of bismuth vanadate to structurally reorganize its surface phase to reach a stable configuration under the operating conditions.
The scientists used carefully selected experimental methods to analyze bismuth vanadate before and after its use, as well as directly under operational conditions. This revealed an accumulation of light-generated charge at the surface of the film, leading to structural destabilization of the metal oxide semiconductor and chemical attacks.
"For complex metal oxides, a significant structural reorganization is required to create a thin layer on the surface that can be thermodynamically stable, and that process can be very slow," said Toma.
"Today, bismuth vanadate is one of the best materials available for constructing photoanodes," said Ian Sharp, a staff scientist in the Chemical Sciences Division. "Ultimately, though, we need to discover new semiconductors that can more efficiently absorb light and help drive the reactions that allow us to store energy from the sun in chemical bonds."
The researchers added that one of the next steps in understanding these materials is to study the relation between the local chemical composition and performance over different length and time scales under operating conditions.
"Understanding the origin of the degradation process is crucial to designing materials that are more resistant," said Kristin Persson, a staff scientist in Materials Science and Engineering and in the Energy Technologies Area at Berkeley Lab. "It is our hope that this study will spark further improvements in the screening and development of new materials with enhanced stability under operating conditions."
This story is adapted from material from the Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.