UNSW Sydney scientists have developed a revolutionary way for producing microscopic 3D materials, which might potentially make fuel cells such as hydrogen batteries less expensive and more sustainable. Researchers from UNSW Science’s School of Chemistry explain how to progressively “build” linked hierarchical structures in 3D at the nanoscale with unique chemical and physical characteristics to facilitate energy conversion activities in a work published in Science Advances.
Hierarchical structures in chemistry are arrangements of units such as molecules inside an organisation of other units that may be arranged. Similar phenomena may be seen in nature, such as flower petals and tree branches. However, where these structures have tremendous promise is at a level beyond the human eye’s visibility—at the nanoscale.
Scientists have found it difficult to construct these 3D structures with metal components on the nanoscale using traditional techniques. To grasp how thin these tiny 3D materials must be, consider that one centimetre equals 10 millimetres. If you counted one million small segments in one of those millimetres, each one would be one nanometre (nm).
“To far, scientists have been able to create hierarchical-type structures on the micrometre or molecular size,” says Professor Richard Tilley, Director of UNSW’s Electron Microscope Unit and research senior author. “However, to achieve the degree of accuracy required for nanoscale assembly, we needed to build an altogether new bottom-up process.”
Chemical synthesis, a method for creating complex chemical molecules from simpler ones, was applied by the researchers. They were able to produce 3D hierarchical structures with dimensions ranging from 10 to 20 nanometers by carefully growing hexagonal crystal-structured nickel branches on cubic crystal-structured cores.
The resultant linked 3D nanostructure features a large surface area, a high conductivity owing to the direct connection of a metallic core and branches, and chemically modifiable surfaces. These characteristics make it a good electrocatalyst support—a material that aids in the acceleration of reactions—in the oxygen evolution reaction, a critical step in energy conversion. The characteristics of the nanostructure were investigated utilising electrochemical analyses using state-of-the-art electron microscopes supplied by the Electron Microscope Unit.
“Growing the material step by step contrasts with what we do building structures at the micrometre level, which is beginning with bulk material and etching it down,” said the study’s main author, Dr. Lucy Gloag, a Postdoctoral Fellow at UNSW Science’s School of Chemistry. “We have good control over the settings with this new approach, which enables us to maintain all of the components ultra-small—on the nanoscale—where the special catalytic capabilities occur.”
Fuel cells using nanocatalysts
Most atoms in typical catalysts, which are often spherical, are trapped at the centre of the sphere. Because there are so few atoms on the surface, much of the material is wasted because it cannot participate in the reaction environment.
According to Prof. Tilley, these novel 3D nanostructures are built to expose more atoms to the reaction environment, which may permit more efficient and effective catalysis for energy conversion.
“Having a bigger surface area for the catalyst implies the process will be more effective when turning hydrogen into energy if this is utilised in a fuel cell or battery,” Prof. Tilley explains.
According to Dr. Gloag, this indicates that less material is required for the reaction.
“It will gradually lower prices as well, making energy production more sustainable and eventually changing our reliance away from fossil fuels.”
The scientists will next try to change the surface of the material using platinum, a better catalytic metal that is more costly. The platinum used to power the fuel cell costs about a sixth of the price of an electric automobile.
“These very large surface areas would allow a substance like platinum to be put on in individual atoms,” Prof. Tilley explains.
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