HAADF-STEM image of PtNPs/2H-MoS2. Image: Zhang et al, Nature

Hydrogen catalyst is an important step towards a clean future

2 October 2023

Renewable energy generation, from sources like wind and solar, is rapidly growing. However, some of the energy generated needs to be stored for when weather conditions are unfavourable for wind and sun. One promising way to do this is to save the energy in the form of hydrogen, which can be stored and transported for later use.

To do this, the renewable energy is used to split water molecules into hydrogen and oxygen, with the energy stored in the hydrogen atoms. This uses platinum catalysts to spur a reaction that splits the water molecule, which is called electrolysis. However, although platinum is an excellent catalyst for this reaction, it is expensive and rare, so minimising its use is important to reduce system cost and limit platinum extraction.

Now, an international team led by City University of Hong Kong (CityU) researchers have designed and tested a catalyst that uses as little platinum as possible to produce an efficient but cost-effective platform for water splitting. This means the catalyst could be cheaply scaled up for mass use. The study was published in the journal Nature.

Lead researcher Professor Zhang Hua, Herman Hu Chair Professor of Nanomaterials at CityU, said: “Hydrogen generated by electrocatalytic water splitting is regarded as one of the most promising clean energies for replacing fossil fuels in the near future, reducing environmental pollution and the greenhouse effect.”

Highly efficient catalyst

The CityU team has been experimenting with ultrathin transition-metal dichalcogenide (TMD) nanosheets for some time, as they can serve as templates for constructing nanomaterials and supporting metal catalysts. Although these materials hold great promise, it has been hard for researchers to determine how the crystal structure of the TMD nanosheet affects the growth of the material it supports.

To create the platinum catalysts, the team used TMD nanosheets made of molybdenum disulphide (MoS2). MoS2 has two different crystal configurations: a semi-metallic phase and a semiconducting phase. The synthesis at a high purity of the semi-metallic phase has traditionally been difficult, however the CityU team was able to create semi-metallic nanosheets with high purity, and also semiconducting sheets, and compare their suitability for platinum deposition.

They found that the metallic phase allowed platinum to grow as dispersed single atoms, providing a catalyst that uses much less of the metal than existing catalysts and with improved performance.

The catalyst was tested in the labs of Professor Anthony Kucernak from Imperial College London (UK), where it showed showed superior efficiency and high stability during the electrocatalytic hydrogen evolution reaction (HER), a vital step in electrocatalytic water-splitting for hydrogen production.

Beyond this catalyst, the CityU team’s insights into how the MoS2 crystal phase affects the growth of the supported metal could point to to new ways to engineer nanomaterials. This would then help in the design and synthesis of highly efficient catalysts, contributing to cleaner energies and sustainable development.

“We will develop more efficient catalysts based on this finding and explore their applications in various catalytic reactions,” said Dr Shi Zhenyu, a postdoctoral researcher in CityU’s Department of Chemistry and the first author of the paper.

Towards a hydrogen economy

The Imperial team have the tools for stringent testing of new catalysts because they have developed technologies that are designed to make use of them. Professor Kucernak and colleagues have set up several companies based on these technologies, including RFC Power that specialises in hydrogen flow batteries, which could be improved by using the new single-atom platinum catalysts.

This covers the stage of turning renewable energy into hydrogen. But to use the hydrogen again as energy, such as for powering cars or other vehicles, it needs to be transformed again using fuel cells. These perform the reverse reactions, producing only water vapour as a by-product of an oxygen-splitting reaction. Recently, Professor Kucernak and colleagues revealed a single-atom catalyst for this reaction that is based on iron, instead of platinum, which will also reduce the cost of this technology.

Bramble Energy, another spinout led by Professor Kucernak, will test this technology in their fuel cells. Both single-atoms catalysts – one helping turn renewable energy into hydrogen storage, and the other helping that energy be released as electricity later – therefore have the power to bring a hydrogen economy closer to reality.

Professor Kucernak said: “The UK Hydrogen Strategy sets out an ambition to reach 10GW of low-carbon hydrogen production capacity by 2030. To facilitate that goal, we need to ramp up the production of cheap, easy-to-produce and efficient hydrogen storage. The new electrocatalyst could be a major contributor to this, ultimately helping the UK meet its net-zero goals by 2050.”