In today’s global energy and climate crisis, finding sustainable and clean fuels is critical. One potential candidate that is largely achieving relevance is hydrogen. Nevertheless, today’s industrial hydrogen generation still has a significant carbon dioxide footprint, particularly while considering approaches such as non-sustainable electrolysis or steam reforming.
Group leader Prof. Dominik Eder (left) and first author of the study Pablo Ayala (right). Image Credit: Vienna University of Technology
A research group headed by Prof. Dominik Eder from the Institute of Materials Chemistry (TU Wien) is, hence, concentrating on developing eco-friendly methods for attaining hydrogen, such as photocatalysis. This method allows the conversion of water molecules to hydrogen supported by just light and a catalyst. Via this process, the sun’s rich and clean energy can be stored in the chemical bonds of this alleged solar fuel. Recently, the outcomes have been published in the scientific journal Advanced Energy Materials.
Novel Photocatalysts
The catalyst plays a critical role when generating green hydrogen by photocatalysis. Unlike industrial catalysts, a photocatalyst uses light energy to enable the water to split at ambient pressures and room temperature. Among the most potential candidates are metal-organic frameworks (MOFs). They are composed of molecular inorganic building units held united through organic linker molecules. Jointly, they make highly porous 3D networks with an outstanding huge surface area and exceptional charge separation properties.
Nevertheless, most MOFs are only active in UV light irradiation, which is why the community changes the organic compounds to turn them capable of absorbing visible light. But, these alterations have a negative effect on the electrons’ mobility. Another drawback concerns charge extraction, where the electrons are expelled from the material: “While MOFs are indeed great at separating charge carriers at the organic-inorganic interfaces, their efficient extraction for catalytic use remains a challenge,” Dominik Eder explains.
In recent times, MOFs with layered structures have gained a lot of focus for application in optoelectronic applications, as they show highly enhanced charge extraction properties.
You can picture these layered structures as a Manner Schnitte, where the waffle is the inorganic part and the chocolate is the organic ligand holding them together. You just need to make the waffle part conductive.
Pablo Ayala, Study Lead Author, TU Wien
Challenges in Water Splitting
Unlike 3D-MOFs, a layered MOF is normally non-porous, minimizing the catalytically active surface to the external surface of the particles. “Hence, we had to find a way to make these particles as small as possible,” Eder describes. Nevertheless, materials’ nanostructuring usually comes together with the introduction of structural flaws. These can serve as charge traps and slow down the extraction of charges. “Nobody likes a Manner Schnitte with missing chocolate,” Ayala continues with his comparison. “In the case of photocatalysis, we also need the best possible material that can be produced.”
Thus, the team of Dominik Eder came up with a new synthesis path in which even the tiniest crystalline structures can be generated without any defects. This was attained in collaboration with local and international universities. The new, layered MOFs are founded on titanium and have a cubic form of only a few nanometers in size. The material has already attained record values in photocatalytic hydrogen generation in the visible light’s effect.
Supported by computer simulations conducted at Technion in Israel, the research group could show the fundamental reaction mechanism and establish two things: Firstly, the layered nature of the MOF is actually the key to successful charge extraction and separation. Secondly, missing-ligand defects function as unnecessary charge traps that have to be eliminated as much as possible to improve the photocatalytic performance of the material.
Currently, the team is developing new layered MOFs and analyzing them for different energy applications.
Journal Reference:
Ayala, P., et al. (2023) The Emergence of 2D Building Units in Metal‐Organic Frameworks for Photocatalytic Hydrogen Evolution: A Case Study with COK‐47. Advanced Energy Materials. doi.org/10.1002/aenm.202300961.