RESEARCH HIGHLIGHTS

WATER OXIDATION

Using an overlooked catalyst to produce hydrogen
An Earth-abundant catalyst promises to make the production of hydrogen fuel more feasible© JIM WEST/SCIENCE PHOTO LIBRARYA

low-cost, stable catalyst
for generating hydrogenfuel from water has been foundby RIKEN researchers1. Thisdevelopment could makethe production of hydrogenfrom water and sunlight more
economically viable.
Hydrogen is a highly attractive,
energy-efficient renewable fuelsource that produces only water
when burned. It can be generated
from sunlight using protonexchange membrane electrolysis
cells. But because the conditionsin these electrolysis cells become
acidic, most catalysts corrodein them.
The only catalysts that havebeen found to be stable undersuch conditions are those basedon iridium oxide, but iridium isscarce and expensive.
“Iridium is rarer than gold,”says Hideshi Ooka of theRIKEN Center for Sustainable Resource Science (CSRS).
“One study found that there
simply isn’t enough iridium inthe world to meet the global
energy demand.”Now, Ailong Li, also of theCSRS, Ooka and their coworkers have discovered a stable
catalyst, gamma manganeseoxide (γ-MnO2), for protonexchange membrane electrolysis
cells that is about a million times
more abundant than iridium.
They found that γ-MnO2 isstable in acid in a specific rangeof potentials.
The material had been
overlooked previously
because predictions based onthermodynamics indicatedthat it should be unstable inacid. But it dawned on Li thatsince γ-MnO2 is generally
FALL 2019

Hydrogen fuel could be used to power the cars of the future.

made using concentrated acid,it may be stable in acid undercertain conditions.
“Li’s knowledge of the synthetic
procedures in acid was criticalfor us to test this material,”
explains Ooka. “Otherwise, wewould have ignored this material,
just as many other groups havein the past.”
To check the effect of thepotential on the catalyst’sstability, the team analyzed
the catalyst and the water it
was immersed in using ultraviolet–visible spectroscopy.
They found that in a narrowpotential window of 0.1 volts
it was so stable that it has notdegraded significantly evenafter more than a year of continuous electrolysis.
Ooka explains that the lesson
from the study is not to rely on
oversimplified analyses basedon thermodynamics. “Thermodynamics just tells you that thismaterial could dissolve,” he says.
“But it doesn’t mean it will.”
The team is now optimizing
their catalyst toward industrialhydrogen production. They
also plan to use their spectroscopic technique to see whether
any other materials have
similar windows of stability.
Looking further ahead, Ryuhei
Nakamura, who led the team,says: “We will seek to combine
this technique with carbondioxide fixation. That would
allow us to produce chemicals
and fuels from water andcarbon dioxide, mimicking
photosynthesis, which would
truly bolster the sustainabilityof human society.”
Reference

1. Li, A., Ooka, H., Bonnet,
N., Hayashi, T., Sun, Y.,
Jiang, Q., Li, C., Han, H.
& Nakamura, R. Stablepotential windows forlong-term electrocatalysis
by manganese oxides
under acidic conditions.
Angewandte ChemieInternational Edition 58,
5054–5058 (2019).

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