Schematic of the electrochemical restructuring of erythrite during electrolysis. The fine needle-like structure melts during the conversion from a crystalline material to an amorphous one, which is porous like Swiss cheese. Image: HZB.As a rule, most catalyst materials deteriorate during repeated catalytic cycles – they age. But there are also compounds that increase their performance over the course of catalysis. One example is erythrite, a mineral compound made of cobalt and arsenic oxides with a molecular formula of (Co3(AsO4)2·8H2O). Purple in color, erythrite has proven to be effective at accelerating the generation of oxygen at the anode during electrolytic splitting of water into hydrogen and oxygen.
Now, a group headed by Marcel Risch at Helmholtz–Zentrum Berlin (HZB) in Germany, together with groups from Costa Rica, has analyzed these catalyzing mineral materials in detail at BESSY II and made an interesting discovery, which they report in a paper in Advanced Energy Materials.
To do this, Javier Villalobos, a doctoral student in Risch's group at HZB, coated electrolysis anodes with tiny erythrite crystals in powder form, produced by the colleagues in Costa Rica. He then examined the crystals before, during and after hundreds of electrolysis cycles in four different pH-neutral electrolytes, including ordinary soda water (carbonated water).
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 an increasing number of 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), but not in any of the other electrolytes.
Thanks to the 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 in the crystals. The more active samples lost their original erythrite crystal structure and were transformed into a less ordered structure comprising platelets just two atoms thick. The larger these platelets became, the more active the sample was.
Over the course of the catalysis cycles, 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 three 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.
"Over time, the material becomes like Swiss cheese, with many holes and a larger surface area where many more reactions can take place," explains Risch. "Even if the individual catalytically active centers become somewhat weaker over time, the larger surface area means that many more potential catalytically active centres come into contact with the electrolyte and increase the yield."
Risch suggests that such mechanisms can also be found in many other classes of materials consisting of non-toxic compounds, which could be developed into suitable catalysts.
This story is adapted from material from Helmholtz–Zentrum Berlin, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.