Tiny 3D structures to make fuel cells more efficient

In a study published in Science Advances, researchers from the School of Chemistry at the University of New South Wales (UNSW) demonstrated a novel technique for creating 3D materials that could eventually make hydrogen batteries and other fuel cells more sustainable. The study showed it is possible to sequentially ‘grow’ interconnected hierarchical structures in 3D at the nanoscale, creating unique chemical and physical properties to support energy conversion reactions.

Hierarchical structures are configurations of units like molecules within an organisation of other units that themselves may be ordered. In the natural world, similar phenomena can be seen in flower petals and tree branches. These structures have extraordinary potential at a level beyond the visibility of the human eye — at the nanoscale.

Scientists found it challenging to replicate these 3D structures with metal components on the nanoscale using conventional methods. Each of these materials must be small enough to measure in nanometres, and there are 1,000,000 nm in a mm.

“To date, scientists have been able to assemble hierarchical-type structures on the micrometre or molecular scale,” said Prof Richard Tilley, Director of the Electron Microscope Unit at UNSW and senior author of the study. “But to get the level of precision needed to assemble on the nanoscale, we needed to develop an entirely new bottom-up methodology.”

The researchers were able to use chemical synthesis to grow hexagonal crystal-structured nickel branches on cubic crystal-structured cores to create 3D hierarchical structures with dimensions of around 10–20 nanometres.

The resulting 3D nanostructure has a high surface area, high conductivity due to the direct connection of a metallic core and branches and surfaces that can be chemically modified. These properties make it a suitable electrocatalyst support, helping to speed up the rate of reactions in the oxygen evolution reaction, which is a crucial process in energy conversion. Its properties were examined using electrochemical analysis from electron microscopes provided by the Electron Microscope Unit.

“Growing the material step by step is a contrast to what we do assembling structures at the micrometre level, which is starting with bulk material and etching it down,” said Dr Lucy Gloag, lead author of the study and Postdoctoral Fellow at the School of Chemistry, UNSW Science. “This new method allows us to have excellent control over the conditions, which lets us keep all of the components ultrasmall — on the nanoscale — where the unique catalytic properties exist.”

In conventional catalysts, which are often spherical, most atoms remain stuck in the middle of the sphere and very few on the surface, meaning most material would be wasted as it cannot take part in the reaction environment. The new 3D nanostructures are engineered to expose more atoms to the reaction environment, which, according to Tilley, may facilitate more efficient and effective catalysis for energy conversion.

“If this is used in a fuel cell or battery, having a higher surface area for the catalyst means the reaction will be more efficient when converting hydrogen into electricity,” Tilley said.

Gloag said it means that less of the material needs to be used for the reaction.

“It will eventually decrease the costs as well, making energy production more sustainable and ultimately shifting our dependence further away from fossil fuels.”

In the next research stage, the scientists will aim to modify the surface of the material with platinum, which, though more expensive, is a superior catalytic metal.

“These exceptionally high surface areas would support a material like platinum to be layered on in individual atoms, so we have the absolute best use of these expensive metals in a reaction environment,” Tilley said.

Image caption: Authors of the study, Professor Richard Tilley and Dr Lucy Gloag. Photo: UNSW.