In the study published in Science Advances, researchers from the School of Chemistry at the University of New South Wales (UNSW) show that it is possible to ‘grow’ interconnected hierarchical 3D nanoscale structures in sequence that have unique chemical and physical properties to sustain the energy conversion reactions.
In chemistry, hierarchical structures are configurations of units such as molecules within an organization of other units which themselves can be ordered.
Similar phenomena can be seen in the natural world, such as in 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 difficult to replicate these 3D structures with nanoscale metallic components.
‘To date, scientists have been able to assemble hierarchical-like 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 achieve the level of precision needed for nanoscale assembly, we needed to develop an entirely new bottom-up methodology.’
The researchers used chemical synthesis, an approach that builds complex chemical compounds from simpler ones. They were able to carefully grow hexagonal crystal structure nickel branches on cubic crystal structure cores to create 3D hierarchical structures with dimensions of approximately 10-20 nanometers.
The resulting interconnected 3D nanostructure has a high surface area, high conductivity due to the direct connection of a metal core and branches, and has surfaces that can be chemically modified.
These properties make it an ideal electrocatalyst support – a substance that helps speed up the rate of reactions – in the oxygen evolution reaction, a crucial 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 in contrast to what we do by assembling micrometre-level structures, which is to start with bulk material and etch it,’ says the study’s lead author, Dr Lucy Gloag, post- doctorate from the School of Chemistry, UNSW Science.
“This new method allows us to have excellent control over the conditions, which allows us to keep all the ultra-small – nanoscale – components where the 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 wasted as it cannot take part in the reaction environment.
These new 3D nanostructures are designed to expose more atoms to the reaction environment, which can facilitate more efficient and effective catalysis for energy conversion, Tilley says.
“Whether this is used in a fuel cell or battery, having more surface area for the catalyst means the reaction will be more efficient when converting hydrogen to electricity,” Tilley explained.
Dr. Gloag says this means less material needs to be used for the reaction.
“It will also ultimately reduce costs, making energy production more sustainable and ultimately further moving away from our dependence on fossil fuels.”
In the next phase of the research, the scientists will look at modifying the material’s surface with platinum, which is a superior albeit more expensive catalytic metal. About one-sixth of the cost of an electric car alone is made up of the platinum that powers the fuel cell.
“These exceptionally high surface areas would support a material like platinum to be layered into individual atoms, so we have the absolute best use of these expensive metals in a reaction environment,” Tilley concluded.