(Berlin) - Typically, most catalyst materials deteriorate over time because they age. “But there are also compounds that can increase their performance during catalysis.” This is what a group of scientists discovered with the purple mineral erythrin, a compound of cobalt and arsenic oxides. Erythrin is suitable for accelerating oxygen development at the anode during the electrolytic splitting of water into hydrogen and oxygen.
Marcel Risch's team at the Helmholtz Center Berlin for Materials and Energy GmbH (HZB) analyzed these mineral catalyst materials with a research group from Costa Rica. They made a powder of tiny erythrin crystals and used it to coat the electrodes. This was followed by investigations before, during and after hundreds of electrolysis cycles in four different PH-neutral electrolytes, including soda water. 
Over time, the surfaces of the catalytically active layer in all electrolytes would have shown significant changes. According to the information, they lost their original crystal form. In addition, more cobalt ions changed their oxidation number due to the applied voltage, which was determined electrochemically. “In soda water, and only in this electrolyte, there was also a higher oxygen production over time,” said the scientists: “The catalyst became significantly better.”
The scientists used X-ray absorption spectroscopy to scan the atomic and chemical environment around the cobalt ions. Result: “The more active samples lost their original erythritol crystal structure and transformed into a less ordered structure that can be described as platelets with a thickness of only two atoms,” it says. The larger these platelets became, the more active the sample was. The measurements showed that over the course of the catalytic cycles, the oxidation number of the cobalt in these platelets in soda water increased the most, namely from 2 to 2,8. “Since oxides with oxidation number 3 are known to be good catalysts, this explains the improvement relative to the catalysts that form in the other electrolytes.”
Oxygen yield doubled
In soda water, the oxygen yield per cobalt ion fell by a factor of 28 over 800 cycles - but at the same time, 56 times more cobalt atoms changed their oxidation number electrochemically, the scientists found. “Macroscopically, the electricity generation and thus the oxygen yield of the electrode doubled.”
“Over time, the material becomes a Swiss cheese with lots of holes and a large surface on which many more reactions can take place,” says Marcel Risch, explaining the effect. Even if the individual catalytically active centers become slightly weaker over time, the larger surface area would mean that many more potential catalytically active centers would come into contact with the electrolyte and increase the yield. The team leader estimates that “such mechanisms can also be found in many other classes of materials that consist of non-toxic compounds and that can be developed into suitable catalysts.”
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Photo above
Helmholtz Center Berlin / © HZB, M.Setzpfandt
Photo middle
It was not until 1719 that various samples of the mineral, also known at the time as “goblin blossom,” were cataloged and assigned to locations. They came from various regions, some of them pits, including the Erzgebirge, Schneeberg, Freiberg and St. Veit near Wolkenstein in Saxony; Blankenburg and Saalfeld in Thuringia as well as from the Czech Republic. The picture shows erytrine on quartz from the art collection of the city of Zwickau / © www.kunstsammlungen-zwickau.de
