High-resolution TEM and HAADF-STEM images showing Turing-type structures, respectively. The Turing stripes consisted of nanograins that met at the Y-type bifurcations. Image: Lu et al, Nature

Nature-inspired catalysis

10 January 2024

Scientists at the City University of Hong Kong have broken new ground in the field of hydrogen energy by creating a catalyst that draws inspiration from patterns found in nature.

The Turing pattern is a concept introduced by English mathematician Alan Turing in a 1952 paper which describes how patterns in nature, such as stripes and spots, can arise naturally and autonomously from a homogeneous, uniform state. Turing patterns arise in reaction-diffusion systems, which involve two or more interacting chemical substances that diffuse through a medium at different rates. According to Turing's theory, one substance acts as an activator with a short-range effect, while the other acts as an inhibitor with a long-range effect. In nature, Turing patterns are believed to be responsible for various features, such as the stripes on a zebra, the spots on a leopard, and the arrangement of leaves around a stem. Turing patterns may also be responsible for the formation of human fingerprints.

In the context of material science and electrocatalysts, the concept of Turing patterns can be applied to the design of materials with specific nanostructures. By controlling the reaction and diffusion of chemical species during the synthesis of nanomaterials, scientists can create materials with desired patterns and properties that are beneficial for catalytic processes.

The CityU research team led by Professor Lu Jian (recipient of a Croucher CAS Joint Funding Scheme grant) has applied this principle to improve the catalysts used in producing hydrogen fuel. Catalysts are essential for the water electrolysis process through which hydrogen is produced. Unfortunately, current catalysts degrade over time, making them less effective and increasing the cost of hydrogen production. The findings were published in Nature Communications.

Lu's team tackled this problem by engineering a new type of catalyst using what they call a Turing structuring strategy. They focused on creating a stable and efficient catalyst that resists wear and tear over time, something that previous catalysts have struggled to achieve.

The team's innovation lies in their creation of ultra-thin nanosheet catalysts that incorporate multiple nanotwin crystals. These are tiny, paired crystals that share some of their lattice structure. This structure is what makes the catalysts more stable. The scientists created these nanosheets using a two-step process that resulted in platinum-nickel-niobium (PtNiNb) nanosheets with a unique pattern that resembles Turing patterns. The specific arrangement of these patterns on the nanosheets is what helps the catalyst remain stable.

This stability is crucial for the catalyst's ability to split water into hydrogen and oxygen efficiently over a long period. The team showed that their new catalyst could significantly outperform existing ones. In practice, the PtNiNb catalyst produced 23.5 times more activity and was over three times more stable than the commercial platinum-carbon (Pt/C) catalysts commonly used today.

The catalysts were tested in a real-world scenario—an anion-exchange-membrane water electrolyser—and demonstrated remarkable endurance running continuously for over 500 hours without losing efficiency.