Cubes cut out spheres as catalyst particles

Until now, using nanoparticles as catalysts for green hydrogen was like using rowers in a figure of eight: you could only measure the average performance, but not find out who was the best. A new method developed by the group led by Prof Dr Kristina Tschulik, Head of the Chair of Electrochemistry and Nanoscale Materials at Ruhr University Bochum, has changed this. In cooperation with researchers from the University of Duisburg-Essen, she was able to prove that spherical nanoparticles work more efficiently than spherical ones. This paves the way for the targeted design of favourable and efficient catalysts for green hydrogen. The researchers report in the journal Advanced Functional Materials from 3 January 2023.

Making electrolysis competitive

The world must reduce CO₂ emissions in order to combat climate change. To this end, the so-called grey hydrogen that is commonly used today, which is obtained from crude oil and natural gas, is to be replaced by green hydrogen that comes from renewable sources. Green hydrogen can be produced by electrolysis, whereby water is split into hydrogen and oxygen using electricity. However, a number of challenges still need to be overcome in order to make electrolysis competitive. Currently, the efficiency of the water splitting process is limited and there is a lack of powerful, durable and cost-effective catalysts for it. „The currently most active electrocatalysts are based on the rare and expensive precious metals iridium, ruthenium or platinum“, says Kristina Tschulik. „In science, we therefore have the task of developing new, highly active, precious metal-free electrocatalysts“

Your group is investigating catalysts in the form of base metal oxide nanoparticles – a million times smaller than a human hair. Industrially produced, they vary in shape, size and chemical composition. „In measurements, so-called catalyser inks are examined in which billions of particles are mixed with binders and additives“, explains Kristina Tschulik. You can only measure an average performance, but not the activity of individual particles - and that is what matters. „If we knew which particle shape or crystal facet – these are the surfaces that point outwards – is particularly active, we could specifically produce particles with precisely this shape“, says Dr Hatem Amin, research associate in analytical chemistry at Ruhr-Universität Bochum.

Winner of the race among nanoparticles

The working group has developed a method with which individual particles can be analysed directly in solution. This makes it possible to compare the activity of different nanomaterials with each other and thus clarify the influence of particle properties such as their shape and composition on water splitting. Our results show that cobalt oxide particles in the form of individual cubes are more active than spheres, which always have several facets.

Theory confirms the experiment

This experimental finding by the Bochum group was confirmed by the cooperation partners led by Prof Dr Rossitza Pentcheva from the University of Duisburg-Essen as part of the Collaborative Research Centre/Transregios 247. Their theoretical investigations indicate a change in the active catalyser areas: from cobalt atoms surrounded octahedrally by oxygen atoms to cobalt atoms surrounded tetrahedrally. The findings on the relationship between particle shape and activity lay the foundation for the knowledge-based design of suitable catalyst materials and thus for the transformation of our fossil energy and chemical industry towards a circular economy based on renewable energy sources and highly active, long-lasting catalysts," says Kristina Tschulik.

Press release from 10 January 2023