From cages to sponge: Transform morphs of clathrate materials into powerful green hydrogen catalysts

Clathrates are characterized by complex cage structures that also provide space for guest ions. Now, the team is the first to investigate the compatibility of clathrates as catalysts for electrolytic hydrogen production with impressive results. The clathrate samples were even more efficient and robust than the currently used nickel-based catalysts. They also found a reason for this performance improvement.

Measurements on the Bessy II showed that the clathrate undergoes structural changes during the catalytic reaction. The three-dimensional cage structure collapsed into ultra-thin nanosheets that allow for maximum contact with the active catalyst center. This study is published in the Journal Angewandte Chemie International Edition.

Hydrogen can be produced by electrolysis of water. If the electrical energy required for this process is from a renewable energy source, this hydrogen is carbon neutral. This “green” hydrogen is considered an important building block for future energy systems and is also needed in large quantities as a raw material for the chemical industry.

Two reactions are important in electrolysis: hydrogen evolution at the cathode and oxygen evolution at the anode (OER). However, oxygen evolution reactions in particular slow down the desired process. To speed up hydrogen production, more efficient and robust catalysts must be developed for the OER process.

Clasrate, cage-made structure

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 are coming.

“The contact between the active nickel center and the electrolyte plays a key role in the efficiency of the catalyst,” says the chemist. This surface area is limited in traditional nickel compounds. “We therefore wanted to test whether we could use nickel-containing samples from an attractive class of materials known as clathrates as catalysts.”

The material is made of Ba8ni6ge40 and was produced at the Institute of Technology Munich. Like all clathrates, they are characterized by the complex crystal structure of polyhedral cages formed by germanium and nickel surrounding barium. This structure provides clathrates with interesting properties as thermoelectric, superconductors, or battery electrodes. However, up until now, no research group considered investigating clathrate studies as electrocatalysts.

Electrochemical measurements showed that Ba8ni6ge40 as a catalyst outweighs the efficiency of nickel-based catalysts at a current density of 550 mAcm⁻². The stability was also very high. Even after 10 days of continuous operation, activity did not decrease significantly.

The team used a combination of experiments to find out why the materials are very suitable. In Bessy II, they studied the samples using in situ X-ray absorption spectroscopy (XAS), while basic structural characterizations were performed at Berlin and Technisch University.

From cages to sponges

Their analysis showed that the aqueous electrolyte Ba8ni6ge40 particles undergo structural transformation under an electric field.

“Germanium and barium atoms make up almost 90% of the starting material of the clathrate, leaving behind a very porous sponge-like nanolayer of 10% nickel that is completely washed away and providing the maximum surface area,” says Dr. Niklas Hausman, Ph.D. of the Menes team. This conversion causes more catalytically active nickel centers to contact the electrolyte.

“We were actually surprised at how well these samples worked as OER catalysts. We hope that we can observe similar results with other transition metal clathrates, and discover a very interesting class of materials for electrocatalysts,” says Menesez.