Green hydrogen: Perovskite oxide catalysts analysed in an X-ray beam

Schematische Ansicht der transformierten Schicht (hellgrau) auf dem LaNiO<sub>3</sub> Perowskitfilm (gr&uuml;n), aufgewachsen auf einem Substrat (braun). Rechts ist die vergr&ouml;&szlig;erte Seitenansicht der transformierten Oxyhydroxid-Schicht (mit Spindichte an den Ni-Pl&auml;tzen) aus Simulationen dargestellt.

Schematische Ansicht der transformierten Schicht (hellgrau) auf dem LaNiO3 Perowskitfilm (grün), aufgewachsen auf einem Substrat (braun). Rechts ist die vergrößerte Seitenansicht der transformierten Oxyhydroxid-Schicht (mit Spindichte an den Ni-Plätzen) aus Simulationen dargestellt. © UDE/AG Pentcheva

The production of green hydrogen requires catalysts that control the process of splitting water into oxygen and hydrogen. However, the structure of the catalyst changes under electrical tension, which also influences the catalytic activity. A team from the universities of Duisburg-Essen and Twente has investigated at BESSY II and elsewhere how the transformation of surfaces in perovskite oxide catalysts controls the activity of the oxygen evolution reaction. 

In a climate-neutral energy system of the future, the sun and wind will be the main sources of electricity. Some of the "green" electricity can be used for the electrolytic splitting of water to produce "green" hydrogen. Hydrogen is an efficient energy storage medium and a valuable raw material for industry. Catalysts are used in electrolysis to accelerate the desired reaction and make the process more efficient. Different catalysts are used for hydrogen separation than for oxygen evolution, but both are necessary.

Perovskite oxide catalysts: inexpensive and with great potential

An interdisciplinary and international group of scientists from the University of Essen-Duisburg, the University of Twente, Forschungszentrum Jülich and HZB has now investigated the class of perovskite oxide catalysts for the oxygen evolution reaction in detail. Perovskite oxide catalysts have been significantly further developed in recent years, they are inexpensive and have the potential for further increases in catalytic efficiency. However, within a short time, changes appear on the surfaces of these materials which reduce the catalytic effect.

Spectroscopy at BESSY II

For this reason, the group has now analysed the surface structure in particular and compared the experimental data with density functional calculations. And spectroscopic analyses at the X-ray source BESSY II were performed. "We were able to determine that a certain surface facet is significantly more active and at the same time more stable than others. X-ray analyses allow us to understand how to overcome the traditional trade-off between activity and stability," says HZB scientist Dr Marcel Risch. The results also show how certain surface facets transform and where, for example, hydrogen atoms (or protons) accumulate.

These insights into transformation processes and structural transformations and chemical processes on the different facets of the samples studied are valuable: they contribute to the knowledge-based design of materials as electrocatalysts. After all, electrocatalysts are the key to many applications in green chemistry.

 

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