Graphene has been considered a miracle material since its discovery because it combines several exceptional properties. It is thin and light while being stable and flexible, and also has high electrical conductivity. Many applications are already possible, while many remain to be explored. Graphene also holds promise for the development of “reservoirs” that can be used to store larger amounts of hydrogen.
Graphene is a modification of carbon and has a two-dimensional structure. Hydrogen atoms can be temporarily stored on its surfaces and then reused for various processes. But to store the largest possible quantities of hydrogen, large areas are needed. Indeed, the properties of the graphene layer can only be used optimally if there is a maximum amount of active surface in a minimum volume.
But to achieve maximum surface area in a compact form, graphene must be transferred from a usual two-dimensional arrangement on a substrate surface to a three-dimensional structure. This was the challenge taken up by Dr. Stefan Heun at the Istituto Nanoscienze of the Consiglio Nazionale delle Ricerche (CNR) in Pisa, Italy. To achieve economic relevance, a “tank” must be able to store at least five kilograms of hydrogen, without exceeding a weight of 100 kilograms or a volume of 100 liters. If such a large amount of hydrogen has to be stored, more than 10 km2 of graphene is needed. A three-dimensional arrangement of graphene is therefore inevitable.
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He found the relevant expertise at the Institute for Sensor and Actuator Systems at the Technical University of Vienna, Austria. Here, Prof. Ulrich Schmid’s group has been studying processes for years to integrate extremely fine porous structures into dense materials in a controlled manner. Indeed, many material properties can be influenced over a wide range by selectively controlling porosity. In the process, the research group managed to develop an electrochemical process that can etch tiny holes and channels in certain materials, such as semiconductor silicon carbide. The process consists of several steps using very specific solvents, electric current and UV irradiation.
Photo: The electron microscope preparation chamber where graphene is produced.
Dr. Stefano Veronesi, a member of Dr. Heun’s research group at the Istituto Nanoscienze, explains what the application of this process means in terms of hydrogen storage: “Graphene can bind (store) molecular hydrogen and elementary on the surface. At room temperature, however, only elemental hydrogen binds well to graphene. Molecular hydrogen, on the other hand, only forms a very weak bond with the surface of graphene. By selective functionalization (“grafting”) of the graphene surface, the “storage” capacity of the graphene surface can be significantly increased even at room temperature. The amount of hydrogen that can be stored is determined by the surface area of graphene present – the more graphene there is, the more hydrogen can be stored.
There are different ways to make graphene. In the research consortium with the Istituto Nanoscienze and the University of Antwerp, Belgium, the team from Vienna University of Technology worked with silicon carbide (SiC) – a crystal composed of silicon and carbon. The goal of the research was to show that it is possible to create the two-dimensional material graphene on a three-dimensional substrate. To do this, the silicon carbide was made porous in a targeted manner and its surface was then transformed into graphene.
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If the surface of silicon carbide is heated to high temperatures and ultra-low ambient pressure, the silicon evaporates and the carbon remains. To then obtain a layer of graphene on a 3D surface, the researchers developed an electrochemical etching process that transforms solid silicon carbide into the desired porous nanostructure. This process removes approximately 42% of the volume. The remaining nanostructure is then heated under a high vacuum by the Pisa researchers to trigger the formation of graphene on the surface.
Photo: The image on the right shows a schematic representation of the structure. The yellow areas show the porous structure. The black hexagonal lattices show the formation of graphene on the surface of the porous structure.
The success of the experiment was investigated by experts from the Institute of Electron Microscopy for Materials Science (EMAT) at the University of Antwerp, Belgium. There, it was shown that many graphene sections had indeed formed on the complex-shaped surface of the 3D nanostructure. Thus, it was possible to prove that graphene can also be generated in a 3D structure – a breakthrough discovery for the development of a hydrogen “reservoir” capable of storing a few kilograms of hydrogen at low pressure and room temperature. . Functionalized graphene is extremely promising for this application, says Professor Schmid.
Another field of application for large graphene surfaces is that of chemical sensors, which can be used, for example, to detect rare gas constituents. “When gas molecules dock on the surface of graphene-based gas sensors, they detect changes in the electrical conductivity of the graphene layer. Depending on the gas molecule, it then donates electrons to the graphene layer (donor) or accepts electrons from the graphene layer (acceptor). This exchange of electrons modifies the conductivity of the graphene layer. Due to the ultra-thin graphene layer, this measuring principle is extremely sensitive and allows the detection of single molecules, explains Dr. Georg Pfusterschmied, member of Professor Schmid’s research group and co-author of the study.
Gas sensors have applications in a variety of industries, including fire detection, leak location, emissions measurement, detection of warfare agents such as explosives or poisonous gases, and determination of air quality. indoor air.
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S. Veronesi et al., 3D Arrangement of Conformally Grown Epitaxial Graphene on Porous Crystalline SiC, Carbon 189, 210 (2022). https://www.sciencedirect.com/science/article/abs/pii/S000862232101201X?via%3Dihub