SCIENCE sciencemag.org 13 MARCH 2020 • VOL 367 ISSUE 6483 1181PHOTO: YU SEUNG KIM/LOS ALAMOS NATIONAL LABORATORY
R
unning the world on renewable en-
ergy is simple, in principle: Harvest
solar and wind energy, and use any
extra to power devices called elec-
trolyzers that split water into oxygen
(O 2 ) and hydrogen gas. Hydrogen
(H 2 ) can serve as a fuel; it is also a staple of
the chemical industry. The trouble is that
current electrolyzers are costly, requiring
either expensive catalysts or pricey metal
housings. Now, researchers report combin-
ing the best of both approaches to make
a version that needs only cheap materials.
“I consider this a great breakthrough,”
says Hui Xu, a chemical engi-
neer at Giner Inc., an electro-
chemistry company. Xu says he
and his colleagues presented
similar results at a Department
of Energy meeting last year, but
have not yet published them.
Their work and another team’s
new device, described this
week in Nature Energy, could
bolster the global embrace of
renewable energy, if the new
electrolyzers prove to be cheap
and stable during many years
of operation. “We are on the
cusp of getting that done,” says
Yushan Yan, a chemical engi-
neer at the University of Dela-
ware, Newark, who is working
on similar technology. A hand-
ful of small companies, including one he
founded, have formed to commercialize it.
Scientists have known how to split water
into H 2 and O 2 for more than 200 years:
Put two metal electrodes in a jar of water,
apply an electrical voltage between them,
and H 2 and O 2 will bubble up at separate
electrodes. Because a mix of the gases
can explode, today’s most common setups
separate the anode and cathode with a
thick, porous plastic sheet. They also use
metal catalysts—most often inexpensive
ones such as nickel and iron—to speed
the reactions.
To make the water able to better conduct
ions that move through the devices, today’s
most common electrolyzers add high levels
of potassium hydroxide (KOH) to the water.
At the cathode, or negative electrode, wa-
ter molecules split into H+ and OH– ions.The H+ ions combine with electrons from
the cathode to make H 2. The OH– ions dif-
fuse through the membrane to the anode,
or positive electrode, where they react to
generate O 2 and water.
But KOH is highly caustic, so engineers
have to build their devices out of expensive
inert metals such as titanium, says Yu Seung
Kim, a chemist at Los Alamos National Lab-
oratory. That drawback prompted research-
ers in the 1960s to develop a version of the
technology known as a proton-exchange
membrane (PEM) electrolyzer, in which the
dividing membrane is designed to selec-
tively allow H+ ions through. A PEM cell’s
catalysts aren’t on the electrodes them-selves, but are tethered to opposite sides of
the membrane. In this setup, catalysts on
the anode side split water molecules into
H+ and OH– ions, with the latter instantly
reacting at the catalysts to form O 2 mol-
ecules. The H+ ions then migrate through
the plastic membrane to the cathode side,
where catalysts tethered to the membrane
turn the H+ ions into H 2.
Because OH– ions don’t migrate through
PEM cells, there’s no need for highly alka-
line conditions. The devices also typically
produce hydrogen at five times the rate of
the alkaline version. But these membrane
cells have their own downsides: They still
need some expensive corrosion-resistant
metals to withstand acidic conditions
produced by the proton-conducting mem-
brane. They also require catalysts made
from platinum and iridium. Those metalsare expensive and rare. For example, the
global production of iridium is only 7 tons.
“There is simply not enough [precious
metals] for large-scale hydrogen produc-
tion,” Xu says.
Now, Kim and his colleagues at Los Ala-
mos, along with researchers at Washing-
ton State University, say they’ve combined
the best of both approaches. Their new
device creates a highly alkaline environ-
ment to encourage water splitting. But it
does so with the PEM approach of tether-
ing catalysts to opposite faces of an ion-
conducting membrane. As with the KOH
setup, catalysts on the cathode side split
water molecules into H+ and OH– ions.
The former converts to H 2 , and
the latter travels through the
membrane, known as an anion
exchange membrane (AEM). It
is designed to create a highly
alkaline local environment that
speeds the travel of OH– ions to
the anode side, where tethered
catalysts prompt them to react
to make O 2.
The upshot is that alkaline
conditions near the membrane
allow the electrolyzer to rely on
cheap and abundant nickel-,
iron-, and molybdenum-based
catalysts to split water. Yet,
because the alkalinity is local-
ized, the electrolyzer can be
built from stainless steel. The
new device generates hydrogen
about three times faster than conventional
alkaline devices, though still more slowly
than commercial PEM electrolyzers, Kim
and his colleagues report. “The combina-
tion of the older alkaline technology and
membrane PEM technology is the path for-
ward,” Xu says.
The new setup needs to prove its dura-
bility. Initial indications suggest the mem-
brane begins to break down after only about
10 hours of operation. Kim says the main
problem is likely that the polymer mem-
brane readily absorbs water. Over time, this
may cause the catalyst particles to come un-
glued and drift away. The team hopes that
adding fluorine to the membrane will repel
the water. With that and other fixes, Kim
hopes, AEM electrolyzers could join solar
cells and windmills as a key technology for
a carbon-free world. jBy Robert F. ServiceMATERIALS SCIENCENew electrolyzer splits water on the cheap
Devices that forgo expensive metals could turn renewable electricity into hydrogen
NEWS | IN DEPTHIn this new electrolyzer, an improved ion-conducting membrane (yellow film, right)
enables hydrogen generation from water without expensive catalysts.Published by AAAS