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Clathrates, known for their intricate cage-like structure, provide an environment that can accommodate guest ions. A recent study has successfully examined their potential as catalysts for electrolytic hydrogen production, revealing that these clathrate samples surpassed the efficiency and durability of conventional nickel-based catalysts. Notably, researchers identified a specific mechanism behind this improved performance: analysis conducted at BESSY II demonstrated that during the catalytic reaction, the clathrates undergo structural changes, transitioning from their three-dimensional cage form into ultra-thin nanosheets. This transformation facilitates increased interaction with the active catalytic centers. The findings are detailed in the journal Angewandte Chemie.
Hydrogen production via water electrolysis can lead to a carbon-neutral energy source when the required electrical energy is derived from renewable resources. This ‘green’ hydrogen is projected to play a critical role in future energy systems and is also essential as a raw material in the chemical industry. Two pivotal reactions occur during electrolysis: hydrogen evolution at the cathode and oxygen evolution at the anode — the latter of which poses challenges by slowing down the overall process. Thus, there is a pressing need for the development of more efficient and robust catalysts, particularly for the oxygen evolution reaction (OER).
Clathrates, a structure built of cages
Currently, nickel-based compounds represent a reliable and cost-effective option for catalyzing the alkaline OER. Dr. Prashanth Menezes and his research team are exploring alternatives. “The efficiency of a catalyst significantly depends on the interaction between active nickel centers and the electrolyte,” explains Menezes. Traditional nickel compounds often have restricted surface areas, prompting the team to investigate whether nickel-containing clathrates could serve as superior catalysts.
The clathrate material under study, Ba₈Ni₆Ge₄₀, was synthesized at the Technical University of Munich. Clathrates are distinctive for their complex crystalline structures formed by polyhedral cages, consisting of germanium and nickel, surrounding barium atoms. This unique architecture endows clathrates with remarkable properties, making them appealing for applications in thermoelectrics, superconductors, and battery electrodes. Until now, however, clathrates had not been targeted as candidates for electrocatalysis.
Experiments at universities and BESSY II
Electrochemical assessments revealed that Ba₈Ni₆Ge₄₀ outperformed nickel-based catalysts at an industrially relevant current density of 550 mA cm⁻². The catalyst also demonstrated exceptional durability, maintaining its activity over a 10-day continuous operational period without significant degradation.
To uncover the reasons for this impressive performance, the team employed a multi-faceted experimental approach. At BESSY II, they utilized in situ X-ray absorption spectroscopy (XAS) to analyze the samples, while foundational structural characterization was carried out at the Freie and Technische Universität Berlin.
From cage to sponge
The analysis disclosed that the particles of Ba₈Ni₆Ge₄₀ underwent a structural reconfiguration in an aqueous electrolyte when subjected to an electric field: germanium and barium atoms were gradually expelled from the original three-dimensional network. “Approximately 90% of the clathrate’s initial composition is made up of germanium and barium, and they are entirely dissolved, resulting in highly porous, sponge-like nanolayers of the remaining 10% nickel that greatly enhance the surface area,” notes Dr. Niklas Hausmann, a member of Menezes’ team. This transformation maximizes the exposure of catalytically active nickel centers to the electrolyte.
“We were genuinely surprised by the effectiveness of these samples as OER catalysts. We believe that similar outcomes could be observed with other transition metal clathrates, revealing a promising class of materials for electrocatalysis,” Menezes concludes.
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