Faster progress with modern X-ray sources

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Green hydrogen is an energy carrier of the future. It is obtained by electrolytically splitting water with the energy of the wind or the sun and stores this energy in chemical form. To facilitate the separation of water molecules (and reduce the energy input), the electrodes are coated with catalytically active materials. Dr. Marcel Risch and his group of young researchers Engineering mechanisms of oxygen evolution study the evolution of oxygen in the electrocatalysis of water. Indeed, oxygen evolution in particular must operate more efficiently for economical hydrogen production.

In order to produce green hydrogen, water can be fractionated by electrocatalysis, powered by renewable sources such as the sun or the wind. A review article in the journal Angewandte Chemie Int. Ed. shows how modern X-ray sources such as BESSY II can advance the development of suitable electrocatalysts. In particular, X-ray absorption spectroscopy can be used to determine the active states of catalytically active materials for the oxygen evolution reaction. This is an important contribution to the development of efficient catalysts from inexpensive and widely available elements.

An exciting class of materials

An exciting class of materials for electrocatalysts are manganese oxides, which come in many different structural variants. “A decisive criterion for suitability as an electrocatalyst is the degree of oxidation of the material and its evolution during the reaction,” explains Risch. In the case of manganese oxides, there is also a wide diversity of possible oxidation states. X-ray absorption spectroscopy (XAS) provides information about oxidation states: X-ray quanta with appropriate energy excite electrons on the innermost shells, which absorb these quanta. Depending on the oxidation number, this absorption can be observed at different excitation energies. Risch’s team built an electrolysis cell that allows XAS measurements during electrolysis.

X-ray absorption spectroscopy

“With X-ray absorption spectroscopy, we can not only determine oxidation numbers, but also observe corrosion processes or phase changes in the material,” says Risch. Combined with electrochemical measurements, the measurement data thus allow a much better understanding of the material during electrocatalysis. However, the required high intensity of X-rays is only available with modern synchrotron light sources. In Berlin, HZB operates BESSY II for this purpose. There are about 50 such light sources for research around the world.

Time scales from short to long

Risch still sees great potential for the application of X-ray absorption spectroscopy, especially with respect to observational time scales. Indeed, typical measurement times are a few minutes per measurement. Electrocatalytic reactions, however, take place on shorter time scales. “If we could observe electrocatalysis in real time, we could better understand important details,” Risch says. With this knowledge, cheap and environmentally friendly catalysts could be developed more quickly. On the other hand, many “aging” processes take place over weeks or months. “We could, for example, examine the same sample over and over again at regular intervals to understand these processes,” advises Risch. This would also allow the development of long-term stable electrocatalysts.

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Material provided by Helmholtz-Zentrum Berlin for Materials and Energy. Note: Content may be edited for style and length.

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