Researchers at Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) are asking why some catalysts increase their performance when used. As a general rule, most catalyst materials deteriorate during repeated catalytic cycles – they age.

One example increasing its performance is the mineral erythrite, a mineral compound comprising cobalt and arsenic oxides. Erythrite lends itself to accelerating oxygen generation at the anode during electrolytic splitting of water into hydrogen and oxygen.

Erythrite Crystals. Image Credit: Wikipedia. Click image for the largest view.

The mineral, with a molecular formula of (Co3(AsO4)2.8H2O), stands out because of its purple color. Erythrite lends itself to accelerating oxygen generation at the anode during electrolytic splitting of water into hydrogen and oxygen.

The young investigator group headed by Dr. Marcel Risch at the HZB together with groups from Costa Rica has now analyzed these catalyzing mineral materials in detail at BESSY II and made an interesting discovery. The paper describing the study and results has been published in the journal Advanced Energy Materials.

Using samples produced by colleagues in Costa Rica consisting of tiny erythrite crystals in powder form, Javier Villalobos, a doctoral student in Risch’s group at the HZB, coated the electrodes with this powder. He then examined them before, during, and after hundreds of electrolysis cycles in four different pH-neutral electrolytes, including ordinary soda water (carbonated water).

Graphic Illustration of erythrite catalyst restructuring. Image Credit: Helmholtz-Zentrum Berlin. Click image for the largest view.

Over time, the surface of each catalytically active layer exhibited clear changes in all the electrolytes. The original crystalline structure was lost, as shown by images from the scanning electron microscope, and more cobalt ions changed their oxidation number due to the applied voltage, which was determined electrochemically. Increased oxygen yield was also demonstrated over time in soda water (carbonated water), though only in that electrolyte. The catalyst clearly improved.

With analyses at BESSY II, the researchers are now able to explain why this was the case: using X-ray absorption spectroscopy, they scanned the atomic and chemical environment around the cobalt ions. The more active samples lost their original erythrite crystal structure and were transformed into a less ordered structure that can be described as platelets just two atoms thick. The larger these platelets became, the more active the sample became.

The data over the course of the catalysis cycles showed that the oxidation number of the cobalt in these platelets increased the most in soda water, from 2.0 to 2.8. Since oxides with an oxidation number of 3 are known to be very good catalysts, this explains the improvement relative to the catalysts that formed in the other electrolytes.

In soda water, the oxygen yield per cobalt ion decreased by a factor of 28 over 800 cycles, but at the same time 56 times as many cobalt atoms changed their oxidation number electrochemically. Macroscopically, the electrical current generation and thus the oxygen yield of the electrode doubled.

Risch explained concisely, “Over time, the material becomes like Swiss cheese with many holes and a larger surface area where many more reactions can take place. Even if the individual catalytically active centers become somewhat weaker over time, the larger surface area means that many more potential catalytically active centers come into contact with the electrolyte and increase the yield.”

Risch suggested that such mechanisms can also be found in many other classes of materials consisting of non-toxic compounds, which can be developed into suitable catalysts.


This is just enough of a basic research result to make your humble writer suspect that a new rush to research in catalysts is about to take off. While the oxygen part of water electrolysis is at issue in this post, there is likely a big array of other fuel and chemical opportunities ripe for discoveries. One can reasonably expect over time that there are big changes coming in the fuel and chemical fields.


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