In a study published in Science Advances, researchers from the Faculty of Chemistry at the University of New South Wales (UNSW) show that it is possible to sequentially ‘grow’ interconnected hierarchical structures in 3D at the nanoscale that have unique chemical and physical properties to support the energy conversion reaction.
In chemistry, hierarchical structures are configurations of units such as molecules within an organization of other units that themselves can be ordered.
Similar phenomena can be seen in the natural world, such as flower petals and tree branches. But where these structures have extraordinary potential is at a level beyond the visibility of the human eye – at the nanoscale.
Using conventional methods, scientists have found it challenging to replicate these 3D structures with nanoscale metallic components.
“To date, scientists have been able to assemble hierarchical-type structures at the micrometre or molecular scale,” says Professor Richard Tilley, director of the Electron Microscope Unit at UNSW and senior author of the study.
“But to get the level of precision needed for nanoscale assembly, we had to develop a completely new bottom-up methodology.”
The researchers used chemical synthesis, an approach that constructs complex chemical compounds from simpler ones. They were able to carefully grow hexagonal crystal-structured nickel branches on cubic crystal-structured cores to create 3D hierarchical structures with dimensions of about 10-20 nanometers.
The resulting interconnected 3D nanostructure has a large surface area, high conductivity due to the direct connection of the metal core and branches, and has chemically modifiable surfaces.
These properties make it an ideal support for an electrocatalyst – a substance that helps speed up the rate of reactions – in the oxygen release reaction, a key process in energy conversion. The properties of the nanostructure were examined using electrochemical analysis from state-of-the-art electron microscopes provided by the Electron Microscope Unit.
“Growing the material step by step is a contrast to what we do by assembling structures at the micrometer scale, which is to start with a bulk material and etch it,” says lead study author Dr Lucy Gloag, a postdoctoral fellow in the School of Chemistry, UNSW Science.
“This new method gives us excellent control over the conditions, which allows us to keep all the components ultra-small – at the nanoscale – where unique catalytic properties exist.”
Nanocatalysts in fuel cells
In conventional catalysts, which are often spherical, most of the atoms are stuck in the center of the sphere. There are very few atoms on the surface, which means that most of the material is lost because it cannot participate in the reaction environment.
These new 3D nanostructures are engineered to expose more atoms to the reaction environment, which can facilitate more efficient and effective catalysis for energy conversion, Tilley says.
“If this is used in a fuel cell or battery, the larger surface area for the catalyst means the reaction will be more efficient at converting hydrogen into electricity,” Tilley explained.
dr. Gloag says this means that less material needs to be used for the reaction.
“This will ultimately also reduce costs, making energy production more sustainable and ultimately moving us even further away from fossil fuel dependence.”
In the next phase of the research, the scientists will try to modify the surface of the material with platinum, which is a better catalytic metal even though it is more expensive. About one-sixth the price of an electric-only car is the platinum that powers the fuel cell.
“These extremely large surfaces would support layers of materials like platinum in individual atoms, so we have the absolute best use of these expensive metals in a reaction environment,” Tilley concluded.