Finding clean, sustainable fuels is critical in today’s global energy and climate crisis. Hydrogen is a promising candidate that is becoming increasingly important. However, today’s industrial hydrogen production still contains a significant amount of carbon dioxide2 footprint, especially considering processes such as steam reforming or unsustainable electrolysis.
A team led by Professor Dominique Eder of the Institute for Materials Chemistry (TU Wien) is therefore focusing on developing environmentally friendly processes for obtaining hydrogen, for example by photocatalysis. This process allows water molecules to be converted into hydrogen with the help of light and a catalyst. Through this process, the sun’s abundant and clean energy can be stored in the chemical bonds of so-called solar fuels. The results were recently published in the Scientific Journall Advanced Energy Materials.
New photocatalysts
When producing green hydrogen by photocatalysis, the catalyst plays a crucial role. Unlike industrial catalysts, the photocatalyst uses the energy of light to facilitate the separation of water at room temperature and ambient pressures. Among the promising candidates are MOFs, which are called MOFs. They consist of inorganic molecular building blocks linked together by organic linker molecules. Together, they form highly porous 3D networks with exceptionally large surface area and excellent charge separation properties.
However, most MOFs are only active under ultraviolet light irradiation, which is why the community alters organic compounds to make them able to absorb visible light. However, these modifications have a negative effect on the movement of electrons. Another limitation relates to charge extraction, as electrons are released from the material: “While MOFs are already remarkable at separating charge carriers at organic-inorganic interfaces, their efficient extraction for catalytic use remains challenging,” explains Dominic Eder.
Recently, MOFs with multilayered structures have received significant attention for their use in optoelectronic applications, as they display significantly improved charge extraction properties. “You can picture these layered structures like a Manner Schnitte, where the waffle is the inorganic part and the chocolate is the organic bond that holds them together,” explains Pablo Ayala, lead author of the study. “You just need to make the waffle part conductive.”
Challenges in water division
In contrast to 3D MOFs, layered MOFs are typically non-porous, which reduces the catalytically active area to the particle outer surface. “So, we had to find a way to make these particles as small as possible,” explains Eder. However, the nanostructure of materials is often accompanied by the introduction of structural defects. These can act as charge traps and slow cargo extraction. “Nobody likes a Manner Schnitte with missing chocolate,” Ayala continues to compare. “In the case of photocatalysis, we also need the best material that can be produced.”
Therefore, Dominik Eder’s team has developed a new synthetic route by which the smallest crystal structures can be produced free of defects. This was achieved in cooperation with local and international universities. New layered MOFs are based on titanium and have a cubic shape only a few nanometers in size. The material was already able to achieve record values in photocatalytic hydrogen production under the influence of visible light.
With the help of computer simulations performed at the Technion in Israel, the team was able to unravel the underlying reaction mechanism and show two things: First, that the layered nature of MOFs is indeed key to efficient charge separation and extraction. Second, the defects of missing bonds act as unwanted charge traps that should be avoided as much as possible to improve the photocatalytic performance of the material.
The research group is currently designing and exploring new layered organic frameworks for different energy applications.