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plit a water molecule, and you
get hydrogen and oxygen. Burn
that hydrogen as fuel and you
get water. This straightforward,
pollution-free cycle is part of what makes
hydrogen so tantalizing as a potential
renewable fuel. Unfortunately, splitting
water molecules to generate
hydrogen is not currently
very energy efficient. In
fact, more than 95 percent
of hydrogen currently used
in industry is produced from
fossil fuels, not water.
But, in a recent study,
researchers have combined
two compounds — nine parts
sulfur-rich molybdenum
sulfide to one part titanium
dioxide — to synthesize
a water-vapor adsorbing
paint that improves exist-
ing water-splitting methods.
Their results, published in ACS Nano,
raise hope that such materials could even-
tually decrease reliance on carbon-based
sources of hydrogen.
“The traditional approach [to gen-
erating hydrogen by splitting water
molecules] has been to use purified water
with specific catalysts added,” says Tor-
ben Daeneke, a research fellow at RMIT
University in Melbourne, Australia, and
co-author of the new study. But that
requires energy for the purification steps
and consumes water that could be used
for other purposes. “The system we have
developed is water neutral; no water
purification is necessary and no drinking
water is used.”
In the paint, which can be used on
insulating surfaces like glass or brick
walls, molecules of molybdenum sul-
fides form highly porous networks. The
high porosity and chemical structure of
these networks allow the materials to
adsorb large amounts of water vapor
from the atmosphere. Molybdenum sul-
fides are semiconductors, and pairing
them with titanium dioxide — a photo-
catalyst that generates electric currentwhen exposed to certain wavelengths of
light — allows water molecules adsorbed
by the molybdenum sulfides to be split
into hydrogen and oxygen by the energy
in ambient sunlight.
To test the process, the researchers
placed glass chips coated with the paintin a sealed 500-milliliter glass vessel sat-
urated with moisture. The vessels had
detachable sensor chips to measure the
hydrogen and oxygen produced. When
illuminated, hydrogen and oxygen were
generated in a 2:1 ratio, indicating that
water molecules were indeed split to
produce the gases.
“Traditionally, electro-catalysis [of
water] has been very expensive,” says
co-author Kourosh Kalantar-zadeh, a
professor of engineering at RMIT. “This
new process should be cheaper because
molybdenum is a bulk commodity and
titanium dioxide is cheap and widely used.”
How to best collect the hydrogen pro-
duced through this process remains a
tricky problem that the team is working
to resolve.
“There are challenges related to hydro-
gen storage, and the efficiency of the
conversion from water to hydrogen is
not yet economical,” says Brian Kor-
gel, a professor of engineering at the
University of Texas at Austin, who was
not involved in the study. “I would have
liked to have seen the authors calculate
the quantum efficiency for the watersplitting [essentially, the number of water
molecules split per photon absorbed]
reported in the paper.” Quantum effi-
ciency numbers would allow researchers
to determine the efficiency of this process
compared to existing methods of gener-
ating hydrogen by splitting water.
Additionally, titanium
dioxide only absorbs UV
light, which decreases its effi-
ciency, Korgel says. Using a
material “that carries out the
[photolytic] conversion using
longer wavelength photons
would be helpful,” he says.
In the current setup, some
of the incident light doesn’t
contribute to the photocat-
alytic process and instead
contributes heat energy,
Daeneke says. That heat
“raises the temperature of the
mixture to above 60 degrees
Celsius, at which point the molybdenum
sulfide can no longer hold moisture effec-
tively.” And without moisture, there is
nothing for the electric current to split,
and no hydrogen production.
In their laboratory tests, the research-
ers found that when they coated a glass
substrate with the molybdenum sulfide-ti-
tanium dioxide mixture and illuminated
it, hydrogen production ceased after pro-
longed exposure to light. “Recharging” the
paint in a dark and humid environment
for 30 minutes led to almost three times
more hydrogen production compared to
continuous illumination.
Although difficulties remain in gener-
ating hydrogen with the new technique,
Korgel thinks it’s a useful step. This is
“interesting work that will probably
inspire more researchers to work on the
problem of photocatalytic splitting of
water vapor,” he says.
Daeneke and Kalantar-zadeh next plan
to test other photocatalysts that could
be more efficient than titanium diox-
ide at absorbing energy at wavelengths
throughout the light spectrum.
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