Green hydrogen: Why do certain catalysts improve in operation?
Crystalline cobalt arsenide is a catalyst that generates oxygen during electrolytic water splitting in the production of hydrogen.

Date:
August 9, 2021
Source:
Helmholtz-Zentrum Berlin fu"r Materialien und Energie
Summary:
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 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.



FULL STORY ==========================================================================
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 the mineral
erythrite, a mineral compound comprising cobalt and arsenic oxides
with a molecular formula of (Co3 (AsO4)2?8H2O). The mineral stands out
because of its purple colour. Erythrite lends itself to accelerating
oxygen generation at the anode during electrolytic splitting of water
into hydrogen and oxygen.


========================================================================== Samples from Costa Rica The young investigator group headed by Dr. Marcel
Risch at the HZB together with groups from Costa Rica has now analysed
these catalysing mineral materials in detail at BESSY II and made an interesting discovery.

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).

Loss of original structure 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.



========================================================================== Observations at BESSY II 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 was. 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.

Oxygen yield doubled 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.

From needles to Swiss cheese In a nutshell, Risch explains: "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 centres 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 can be developed into suitable catalysts.

========================================================================== Story Source: Materials provided by Helmholtz-Zentrum_Berlin_fu"r_Materialien_und_Energie.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Javier Villalobos, Diego Gonza'lez‐Flores, Roberto Urcuyo,
Mavis L.

Montero, Go"tz Schuck, Paul Beyer, Marcel Risch. Requirements
for Beneficial Electrochemical Restructuring: A Model Study on a
Cobalt Oxide in Selected Electrolytes. Advanced Energy Materials,
2021; 2101737 DOI: 10.1002/aenm.202101737 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210809144025.htm

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