Green hydrogen: A cage structured material transforms into a performant catalyst
The illustration shows schematically how nanothin sheets of nickel compounds are released from the clathrate structure, providing an extremely large surface area for the oxygen evolution reaction. © Hongyuan Yang/HZB/TUB
Ba8Ni6Ge40 consists of polyhedral cages made of nickel (gray) and germanium (purple), each containing a barium atom. © Hongyuan Yang/HZB/TUB
Investigations under the transmission electron microscope of the Ba8Ni6Ge40 after 24 h as an OER catalyst show that the material has changed into a spongy Nickel network with a high surface area. The elemental analysis reveals that nearly all Germanium and Barium atoms have been washed out during this transformation.
© 10.1002/anie.202424743
Clathrates are characterised by a complex cage structure that provides space for guest ions too. Now, for the first time, a team has investigated the suitability of clathrates as catalysts for electrolytic hydrogen production with impressive results: the clathrate sample was even more efficient and robust than currently used nickel-based catalysts. They also found a reason for this enhanced performance. Measurements at BESSY II showed that the clathrates undergo structural changes during the catalytic reaction: the three-dimensional cage structure decays into ultra-thin nanosheets that allow maximum contact with active catalytic centres. The study has been published in the journal ‘Angewandte Chemie’.
Hydrogen can be produced by electrolysis of water. If the electrical energy required for this process comes from renewable sources, this hydrogen is even carbon neutral. This 'green' hydrogen is seen as an important building block for the energy system of the future and is also needed in large quantities as a raw material for the chemical industry. Two reactions are crucial in electrolysis: hydrogen evolution at the cathode and oxygen evolution at the anode (OER). However, the oxygen evolution reaction in particular slows down the desired process. To speed up hydrogen production, more efficient and robust catalysts for the OER process need to be developed.
Clathrates, a structure build of cages
Currently, nickel-based compounds are considered to be good and inexpensive catalysts for the alkaline oxygen evolution reaction. This is where Dr. Prashanth Menezes and his team come in. ‘The contact between the active nickel centres and the electrolyte plays a crucial role in the efficiency of a catalyst,' says the chemist. In conventional nickel compounds, this surface area is limited. ‘We therefore wanted to test whether nickel-containing samples from the fascinating class of materials known as clathrates could be used as catalysts'.
The materials are made of Ba8Ni6Ge40 and were produced at the Technical University of Munich. Like all clathrates, they are characterised by a complex crystalline structure of polyhedral cages, in this case, formed by germanium and nickel, enclosing barium. This structure gives clathrates special properties that make them interesting as thermoelectrics, superconductors or battery electrodes. However, until now, no research group had considered of investigating clathrates as electrocatalysts.
Experiments at universities and BESSY II
The electrochemical measurements showed that the Ba₈Ni₆Ge₄₀ as a catalyst exceeded the efficiency of nickel based catalysts at a current density of 550 mA cm⁻², a value also used in industrial electrolysis. The stability was also remarkably high: even after 10 days of continuous operation, the activity did not decrease significantly.
The team used a combination of experiments to find out why the material is so remarkably well suited. At BESSY II, they studied the samples using in situ X-ray absorption spectroscopy (XAS), while basic structural characterisation were carried out at the Freie and Technische Universität Berlin.
From cage to sponge
Their analysis showed that the Ba8Ni6Ge40 particles in the aqueous electrolyte undergo a structural transformation under an electric field: germanium and barium atoms dissolve out of the former three-dimensional framework. ‘The germanium and barium atoms make up almost 90 % of the clathrate starting material and they are completely washed out, leaving behind highly porous, sponge-like nanolayers of the remaining 10 % nickel that offer a maximum surface area,‘ says Dr. Niklas Hausmann from Menezes’ team. This transformation brings more and more catalytically active nickel centres into contact with the electrolyte.
‘We were actually surprised by how well these samples work as OER catalysts. We expect that we can observe similar results with other transition metal clathrates and that we have discovered a very interesting class of materials for electrocatalysts,’ says Menezes.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.
- Successful master's degree in IR thermography on solar facadesWe are delighted to congratulate our student employee Luca Raschke on successfully completing her Master's degree in Renewable Energies at the Hochschule für Technik und Wirtschaft Berlin - and with distinction!
- BESSY II: Phosphorus chains – a 1D material with 1D electronic propertiesFor the first time, a team at BESSY II has succeeded in demonstrating the one-dimensional electronic properties in phosphorus. The samples consisted of short chains of phosphorus atoms that self-organise at specific angles on a silver substrate. Through sophisticated analysis, the team was able to disentangle the contributions of these differently aligned chains. This revealed that the electronic properties of each chain are indeed one-dimensional. Calculations predict an exciting phase transition to be expected as soon as these chains are more closely packed. While material consisting of individual chains with longer distances is semiconducting, a very dense chain structure would be metallic.